This work was supported by Grants from the National Institute of Mental Health (R00MH103531 and R01MH116901 to J.A.), a predoctoral training Grant from the National Institute of General Medicine (T32GM007635 to S.R.), and a Lyda Hill Gilliam Fellowship for Advanced Study (GT11021 to S.R.). We thank Dr. Kevin Woolfrey for help with the microscope, Dr. K Ulrich Bayer for the use of his epifluorescent microscope, and Thomas S&#252;dhof (Stanford University) for the LRRTM2 plasmid.

1. Cell culture and transfection
<ol>
	<li>Make HEK cell media with DMEM, 1x (Dulbecco&#39;s Modification of Eagle&#39;s Medium) supplemented with 4.5 g/L glucose, L-glutamine &#38; sodium pyruvate and 10% FBS. Sterile filter.</li>
	<li>Predetermine suitable ligands and receptors for aggregation assay. 
	NOTE: Neurexin3&#945; SS4+/- and one of its known ligands, LRRTM2, were used in this study. Ligands and receptors of interest were expressed from cDNAs in pcDNA3.1. A Gibson assembly was used to insert Neurexin3&#945; into pcDNA3.112. Neurexin3&#945; F/R: TTTAAACTTAAGCTTGGTACCGAGCTCGGATCCGCCACCATGAGCTTTACCCTCCACTC/ 
	GAGCGGCCGCCACTGTGCTGGATATCTGCAGAATTCTTACACATAATACTCCTTGTCCTT.</li>
	<li>Prepare HEK293T cells.
	<ol>
		<li>Grow HEK293T cells to confluency in one T-75 flask.</li>
		<li>Once confluent, use 2 mL of trypsin and place in 37 &#176;C incubator for 2 min. Add 6 mL of HEK media to the flask to resuspend cells and transfer all 8 mL to a 15 mL conical tube.</li>
		<li>Pellet at 500 x g for 5 min and resuspend in HEK cell media for a total of 8 mL.</li>
		<li>Count cells and add 735,000 cells into each well of a 6-well plate. Adjust final volume to 2 mL for each well using HEK cell media.</li>
		<li>Place in 37 &#176;C incubator and allow cells to grow overnight or until they reach 50-60% confluency.</li>
	</ol>
	</li>
	<li>Transfect HEK293T cells using the calcium phosphate method13.
	<ol>
		<li>Transfect well-1 with 3 &#181;g of the protein of interest and co-transfect with 1 &#181;g of fluorescent protein (3 &#181;g of pcDNA3.1-Neurexin3&#945;WT SS4- and 1 &#181;g of mCherry).</li>
		<li>Transfect well-2 as in step 1.4.1. but with the mutated protein of interest (pcDNA3.1-Neurexin3&#945;A687T SS4-).</li>
		<li>Transfect well-3 with 3 &#181;g of the ligand of interest and co-transfect with 1 &#181;g of another fluorescent protein (3 &#181;g of pcDNA3.1 LRRTM2 and 1 &#181;g of GFP).</li>
		<li>Transfect well-4 and well-5 to serve as negative controls: well-4 with 1 &#181;g of GFP and well-5 with 1 &#181;g of mCherry.</li>
		<li>Prepare another plate (as in step 1.4.1-1.4.4) if requiring additional conditions or controls (Neurexin3&#945;WT/A687T SS4+). 
		NOTE: Transfection efficiency is analyzed 24 h after transfection under an epifluorescence microscope and quantified as the number of cells expressing the fluorescent protein they were transfected with. A more streamlined approach would include the transfection of HEK cells with a bicistronic vector coding for a fluorescent protein and the ligand of interest and is highly recommended above co-transfection. In the case of this study, alpha Neurexins are ~4.3 kb and low fluorescence intensity was observed using a bicistronic system necessitating co-transfection.</li>
	</ol>
	</li>
	<li>48 hours after transfection, harvest cells for aggregation.
	<ol>
		<li>Wash each well twice with PBS.</li>
		<li>Add 1 mL of 10 mM EDTA in PBS into each well to gently dissociate cell-to-cell interactions and incubate plate at 37 &#176;C for 5 min. 
		NOTE: Trypsin is not recommended for step 1.5.2 due to potential proteolytic cleavage of adhesion molecules in study. Additionally, after EDTA addition the protocol may not be stopped until completion as cells will now be exposed to ambient conditions.</li>
		<li>Gently tap plate to detach the cells, and harvest each well into separate 15 mL conical tubes.</li>
		<li>Centrifuge conical tubes at 500 x g and room temperature for 5 min.</li>
	</ol>
	</li>
	<li>While cells are pelleting, prepare 6 incubation tubes by labeling the top of each microcentrifuge tube with each condition. 
	NOTE: Each permutation of GFP and mCherry conditions should be used to encompass all experimental conditions and proper controls. For example: 1. GFP/mCherry, 2. mCherry/LRRTM2-GFP 3. GFP/Neurexin3&#945;WT SS4-&#8212;mCherry, 4. GFP/Neurexin3&#945;A687T SS4- &#8211;mCherry, 5. Neurexin3&#945;WT SS4-&#8212;mCherry/LRRTM2&#8212;GFP, 6. Neurexin3&#945;A687T SS4- &#8211;mCherry/LRRTM2&#8212;GFP. Make additional tubes to accommodate further conditions and controls.</li>
	<li>Remove the supernatant and resuspend cells in 500 &#181;L of HEK media with 10 mM CaCl2 and 10 mM MgCl2 warmed to 37 &#176;C. 
	NOTE: The addition of CaCl2 and MgCl2 allows adhesion molecules to reestablish binding and is only required if the transcellular interaction partners in question require divalent cations for adhesion.</li>
	<li>Count the cells in each 15 mL conical tube using a hemocytometer and aliquot 200,000 cells of each condition into appropriate tube from step 1.6.1 for a 1:1 mix in a total volume of 500 &#181;L. 
	NOTE: It should only take 5 min per condition to count and aliquot amounts.</li>
	<li>Incubate tubes at room temperature in a slow tube rotator.</li>
</ol>
2. Image acquisition
<ol>
	<li>Optimize microscope acquisition parameters for specific samples. In this example, images were taken on a wide-field microscope. Use a 5x air objective (NA: 0.15; WD: 20000 &#956;m) to get a large enough field for analysis.</li>
	<li>Assess baseline aggregation immediately after mixing the two conditions of HEK cells in step 1.8. These are now the &#8216;time zero&#8217; images.
	<ol>
		<li>Pipette 40 &#181;L of each sample mixture onto a charged microscope slide and image under fluorescence in both the 488 and 561 channels.</li>
		<li>Acquire three different fields of view at one focus plane per sample drop.</li>
	</ol>
	</li>
	<li>Acquire final images at 60 min as the &#8216;time 60&#8217; image.
	<ol>
		<li>To obtain the &#8216;time 60&#8217; image of the mixture after a 60 min incubation, take another 40 &#181;L sample of each condition from rotating tubes and pipette each sample onto a charged slide. Image as in step 2.2.2. 
		NOTE: Cell aggregation should be checked every 15 min until saturation occurs. Timing of aggregation will depend on the proteins being tested.</li>
	</ol>
	</li>
</ol>
3. ImageJ/Fiji Analysis
<ol>
	<li>To quantify the extent of aggregation using Fiji/ImageJ, save analysis files.
	<ol>
		<li>Save the provided Supplemental coding files into the imageJ macros folder on the computer.</li>
		<li>Install the aggregation macro provided (Plugins, Macros, Install, and select the &#8220;AggregationAssay.txt&#8221; file).</li>
	</ol>
	</li>
	<li>Determine thresholds.
	<ol>
		<li>Load a &#8216;time zero&#8217; .tif file into imageJ and split the channels (Image | Color | Split Channels). 
		NOTE: The &#8216;time zero&#8217; image is used to determine the thresholding and smallest puncta size for the whole experiment.</li>
		<li>Mask each channel (Plugins | Macros | AggregationAssay_MakeMask). Make Mask From Image window will appear. Check boxes next to Determine Threshold for Image and Determine Cluster Params from Histogram and click OK.</li>
		<li>Determine the threshold of the image using the slide bar, record the number to the right of the slide bar and click OK.</li>
		<li>A Histogram of Cluster Size will appear. Select a cluster size from the histogram that suits the experiment, type this number in the Min Cluster Size: box, and click OK. Clusters below this size will not be analyzed.</li>
	</ol>
	</li>
	<li>Run the analysis.
	<ol>
		<li>Open the &#8216;time 60&#8217; image of condition 1 in imageJ and split the channels as in step 3.2.1.</li>
		<li>Mask each channel (Plugins, Macros, AggregationAssay_MakeMask). Use the same threshold and size determined in step 3.2.3 and step 3.2.4. Unselect the boxes next to Determine Threshold for Image and Determine Cluster Params from Histogram and manually type the size and thresholds into the appropriate fields then click OK.</li>
		<li>Calculate the aggregation index (Plugins | Macros | AggregationAssay_CalculateOverlap). Select the masked channels to be compared and directory into which the resulting files will save.</li>
		<li>Repeat steps 3.3.1&#8211;3.3.3 for every &#8216;time 60&#8217; image in every condition. 
		NOTE: The aggregation index is defined as the total overlap area divided by the sum of the two channel areas minus the overlap area multiplied by 100 (Aggregation index = overlap area/[area of channel 1 + area of channel 2 &#8211; overlap area] x 100). This normalization is an &#8216;OR&#8217; operation between the two masked channels representing the total pixels in either mask.</li>
	</ol>
	</li>
</ol>

<ol>
	<li>S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neuroligins+and+neurexins+link+synaptic+function+to+cognitive+disease.">Neuroligins and neurexins link synaptic function to cognitive disease.</a> Nature. 455 (7215), 903-911 (2008).</li><li>Lakey, J. H., Raggett, E. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measuring+protein-protein+interactions.">Measuring protein-protein interactions.</a> Current Opinion in Structural Biology. 8 (1), 119-123 (1998).</li><li>Aoto, J., Martinelli, D. C., Malenka, R. C., Tabuchi, K., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Presynaptic+neurexin-3+alternative+splicing+trans-synaptically+controls+postsynaptic+AMPA+receptor+trafficking.">Presynaptic neurexin-3 alternative splicing trans-synaptically controls postsynaptic AMPA receptor trafficking.</a> Cell. 154 (1), 75-88 (2013).</li><li>Aoto, J., F&#246;ldy, C., Ilcus, S. M. C., Tabuchi, K., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Distinct+circuit-dependent+functions+of+presynaptic+neurexin-3+at+GABAergic+and+glutamatergic+synapses.">Distinct circuit-dependent functions of presynaptic neurexin-3 at GABAergic and glutamatergic synapses.</a> Nature Neuroscience. 18 (7), 997-1007 (2015).</li><li>S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synaptic+Neurexin+Complexes:+A+Molecular+Code+for+the+Logic+of+Neural+Circuits.">Synaptic Neurexin Complexes: A Molecular Code for the Logic of Neural Circuits.</a> Cell. 171 (4), 745-769 (2017).</li><li>Dai, J., Aoto, J., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alternative+Splicing+of+Presynaptic+Neurexins+Differentially+Controls+Postsynaptic+NMDA+and+AMPA+Receptor+Responses.">Alternative Splicing of Presynaptic Neurexins Differentially Controls Postsynaptic NMDA and AMPA Receptor Responses.</a> Neuron. 102 (5), 993-1008 (2019).</li><li>Restrepo, S., Langer, N. J., Nelson, K. A., Aoto, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modeling+a+Neurexin-3&#945;+Human+Mutation+in+Mouse+Neurons+Identifies+a+Novel+Role+in+the+Regulation+of+Transsynaptic+Signaling+and+Neurotransmitter+Release+at+Excitatory+Synapses.">Modeling a Neurexin-3&#945; Human Mutation in Mouse Neurons Identifies a Novel Role in the Regulation of Transsynaptic Signaling and Neurotransmitter Release at Excitatory Synapses.</a> The Journal of Neuroscience. 39 (46), 9065-9082 (2019).</li><li>Ko, J., Fuccillo, M. V., Malenka, R. C., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=LRRTM2+Functions+as+a+Neurexin+Ligand+in+Promoting+Excitatory+Synapse+Formation.">LRRTM2 Functions as a Neurexin Ligand in Promoting Excitatory Synapse Formation.</a> Neuron. 64 (6), 791-798 (2009).</li><li>de Wit, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=LRRTM2+Interacts+with+Neurexin1+and+Regulates+Excitatory+Synapse+Formation.">LRRTM2 Interacts with Neurexin1 and Regulates Excitatory Synapse Formation.</a> Neuron. 64 (6), 799-806 (2009).</li><li>Linhoff, M. W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=An+Unbiased+Expression+Screen+for+Synaptogenic+Proteins+Identifies+the+LRRTM+Protein+Family+as+Synaptic+Organizers.">An Unbiased Expression Screen for Synaptogenic Proteins Identifies the LRRTM Protein Family as Synaptic Organizers.</a> Neuron. 61 (5), 734-749 (2009).</li><li>Siddiqui, T. J., Pancaroglu, R., Kang, Y., Rooyakkers, A., Craig, A. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=LRRTMs+and+Neuroligins+Bind+Neurexins+with+a+Differential+Code+to+Cooperate+in+Glutamate+Synapse+Development.">LRRTMs and Neuroligins Bind Neurexins with a Differential Code to Cooperate in Glutamate Synapse Development.</a> Journal of Neuroscience. 30 (22), 7495-7506 (2010).</li><li>Gibson, D. G., Young, L., Chuang, R. -. Y., Venter, J. C., Hutchison, C. A., Smith, H. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Enzymatic+assembly+of+DNA+molecules+up+to+several+hundred+kilobases.">Enzymatic assembly of DNA molecules up to several hundred kilobases.</a> Nature Methods. 6 (5), 343-345 (2009).</li><li>Bacchetti, S., Graham, F. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transfer+of+the+gene+for+thymidine+kinase+to+thymidine+kinase-deficient+human+cells+by+purified+herpes+simplex+viral+DNA.">Transfer of the gene for thymidine kinase to thymidine kinase-deficient human cells by purified herpes simplex viral DNA.</a> Proceedings of the National Academy of Sciences of the United States of America. 74 (4), 1590-1594 (1977).</li><li>Cerchiari, A. E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+strategy+for+tissue+self-organization+that+is+robust+to+cellular+heterogeneity+and+plasticity.">A strategy for tissue self-organization that is robust to cellular heterogeneity and plasticity.</a> Proceedings of the National Academy of Sciences of the United States of America. 112 (7), 2287-2292 (2015).</li><li>Burdick, M. M., McCarty, O. J. T., Jadhav, S., Konstantopoulos, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell-cell+interactions+in+inflammation+and+cancer+metastasis.">Cell-cell interactions in inflammation and cancer metastasis.</a> IEEE Engineering in Medicine and Biology Magazine. 20 (3), 86-91 (2001).</li><li>Fox, D. A., Gizinski, A., Morgan, R., Lundy, S. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cell-cell+interactions+in+rheumatoid+arthritis+synovium.">Cell-cell interactions in rheumatoid arthritis synovium.</a> Rheumatic Diseases Clinics of North America. 36 (2), 311-323 (2010).</li><li>Yamagata, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+insights+into+modulation+and+selectivity+of+transsynaptic+neurexin-LRRTM+interaction.">Structural insights into modulation and selectivity of transsynaptic neurexin-LRRTM interaction.</a> Nature Communications. 9 (1), 3964 (2018).</li><li>Nguyen, T., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Binding+properties+of+neuroligin+1+and+neurexin+1beta+reveal+function+as+heterophilic+cell+adhesion+molecules.">Binding properties of neuroligin 1 and neurexin 1beta reveal function as heterophilic cell adhesion molecules.</a> The Journal of Biological Chemistry. 272 (41), 26032-26039 (1997).</li><li>Boucard, A. A., Ko, J., S&#252;dhof, T. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High+Affinity+Neurexin+Binding+to+Cell+Adhesion+G-protein-coupled+Receptor+CIRL1/Latrophilin-1+Produces+an+Intercellular+Adhesion+Complex.">High Affinity Neurexin Binding to Cell Adhesion G-protein-coupled Receptor CIRL1/Latrophilin-1 Produces an Intercellular Adhesion Complex.</a> Journal of Biological Chemistry. 287 (12), 9399-9413 (2012).</li></ol>

The authors have nothing to disclose.

Dissecting the protein-protein interactions that occur in trans during cell adhesion can lead to a better understanding of the molecular mechanisms underlying basic cellular processes including the formation, function and maintenance of synapses during maturation and remodeling. The implications of cell-to-cell interactions expand beyond neurobiology and have broader roles in signal transduction, cell migration and tissue development14. Aberrations in cell adhesion can disrupt cellular processes imperative for proper cell function and can underly a variety of etiologies such as cancer, arthritis, addiction, autism and schizophrenia1,15,16. Here, we provide an optimized detailed protocol involving HEK cell aggregation that allows for testing cell-adhesion interactions in trans. 
With this HEK cell aggregation protocol, we can dissect differences in aggregation to approximate binding capacity between a presynaptic protein and its postsynaptic ligand. As such, we can ask questions such as: is the interaction between two essential synaptic proteins affected by a point mutation? More specifically, is the trans interaction of Neurexin3&#945; with LRRTM2 affected by A687T? The A687T missense mutation studied here is located in a previously unstudied linker region of the extracellular domain of Neurexin3&#945;7. The results illustrate that the A687T mutation enhances the aggregation of cells containing Neurexin3&#945;A687T to cells containing LRRTM2 (Figure 2C). This finding is significant because it was previously shown that LRRTMs only bind to Neurexins via a Neurexin domain called LNS617, and further suggests that sequences upstream of LNS6 can exert an independent effect on the trans interactions between Neurexin3&#945; SS4- and LRRTM27.
The results in Figure 2 exhibit the specificity of this assay as all negative controls (GFP/mCherry; Neurexin3&#945;WT/A687T SS4- &#8212;mCherry/GFP or LRRTM-GFP/mCherry) had no observable aggregation after 60 minutes. A critical control used in this study, Neurexin3&#945;WT/A687T SS4+ &#8212;mCherry/LRRTM-GFP, also exhibited no aggregation after 60 min because LRRTM2 is known to bind exclusively to SS4 lacking (SS4-) isoforms of Neurexin. This control demonstrates that simple overexpression of membrane-bound molecules is insufficient to force aggregation in this system, which further illustrates the specificity of this assay.
The cell adhesion assay described here has been widely used to test the interactions of transsynaptic cell-adhesion molecules8,18,19; however, it may be used to test adhesive cell-to-cell interactions of membrane tethered proteins. This protocol, which has evolved over time, was optimized from the original published protocol and subsequent variations of the protocol by changing three experimental parameters. One, the incubation temperature for the cells was changed. The original protocol calls for the incubation of cells in step 1.9 at 4 &#176;C. This low temperature can act as an environmental stressor and can decrease cell viability leading to cell death and lysis. During cell lysis, the cells secrete genomic DNA and cell debris into media that causes cells to clump in solution leading to false positives during imaging. Two, the number of cells per condition were increased to 200,000 per population and the objective size was changed to a 5x; this allows the researcher to image a greater number of interactions per field of view in order to increase statistical power within each n. Three, the aggregation index was measured differently. Previous aggregation indices were limited by the selection of cluster size by the experimenter leading to the exclusion of &#8220;subthreshold&#8221; clusters. By contrast thresholds are now set for individual cell size at &#8216;time zero&#8217;, and the aggregation is now calculated as the overlapping area divided by the sum of the two channel areas minus the overlap area multiplied by 100 at &#8216;time 60&#8217; which allows for the inclusion of smaller positive clusters making the assay more sensitive to other protein-ligand pairs (Figure 2B,C).
Troubleshooting and optimization may be needed depending on the interacting proteins tested. Although the incubation period until final image acquisition will differ depending on the tested proteins, it is critical that no aggregation is observed at &#8216;time zero&#8217;. If aggregation is seen at time zero, it may imply that the cells were not healthy to begin with. This may be due to several factors including sudden exposure to temperatures below 37 &#176;C or slow experimental procedure. Importantly, the resuspension media for step 1.7 should be at 37 &#176;C, which will allow the cells to gradually reach room temperature without a sudden harsh drop in temperature. One way to have optimal cell health throughout the experiment is to ensure that steps 1.7 through 1.9 are completed within a 25 min timeframe with no breaks in between. It is recommended that the microscope is setup and ready to use before cell harvest cells in step 1.5; this ensures that a true &#8216;time zero&#8217; image can be taken immediately in step 2.2.
If aggregation is not observed after 60 minutes of incubation, several factors may be contributing to this lack of adhesion. First, the proteins in question may not be binding partners or may engage in low affinity interactions not detectable by this assay. In this case, a more sensitive assay may be required.&#160; Second, the protein(s) of interest may not localize to the membrane when expressed alone in HEK cells. In this case, confirm that the protein is membrane localized by using biochemical techniques (surface biotinylation) or directly tag the protein of interest with a fluorescent tag and image HEK293T cells at 40x or higher magnification to assess surface expression. However, after membrane localization is confirmed, we recommend using untagged variants for this HEK cell aggregation assay in order to more accurately assess the binding capacity of the native protein. Third, in this protocol we allowed Neurexin3&#945; to express for 48 h; however, protein expression time may vary depending on the proteins tested. Fourth, it is critical to supplement the media in step 1.7 with MgCl2 and CaCl2 if the proteins in question are dependent on divalent cations as is the case with the example of Neurexins and LRRTM211. If it is unknown whether the interaction partners require divalent cations for adhesion, add EDTA into one condition. The EDTA should chelate remining cations naturally present in DMEM ensuring a calcium and magnesium free solution. If adhesion is observed after EDTA addition, the proteins in question do not require divalent cations for adhesion.
Many methods exist to test protein interactions; however, most are not specific for testing only trans interactions that occur from cell to cell and require tedious protein purification steps. Although the HEK cell aggregation assay is not a direct measure of affinity and should not be used to replace a more quantitative approach to recover dissociation constants, to the best of our knowledge, this assay represents an approach to test true trans interactions in a semi-quantitative and relatively efficient manner. Moreover, due to the relative ease of this assay, the HEK cell aggregation assay can be used in conjunction with these more quantitative approaches to obtain a broader and more complete characterization of the interactions taking place.

Synaptic interactions facilitated by synaptic adhesion molecules are foundational for the development, organization, specification, maintenance and function of synapses and the generation of neural networks. The identification of these transsynaptic cell adhesion molecules is rapidly increasing; thus, it is fundamentally important to identify binding partners and understand how these new adhesion molecules interact with each other. Additionally, genome sequencing has identified mutations in many of these adhesion molecules that are commonly linked to a multitude of neurodevelopmental, neuropsychiatric, and addiction disorders1. Mutations in genes that code for synaptic cell-adhesion molecules may detrimentally alter trans interactions and may contribute to pathophysiological alterations in synapse formation and or maintenance.
Multiple assays exist to quantitatively assess protein-protein interactions such as isothermal calorimetry, circular dichroism, surface plasmon resonance2 and although quantitative in nature, they have several limitations. First, they require recombinant protein, sometimes demanding lengthy and tedious purification steps. Second, they require sophisticated specialized equipment and technical expertise. Third, they can overestimate the extent of binding as they allow for both cis and trans interactions between proteins that are naturally tethered to a membrane in vivo. Here we propose a simple and relatively rapid assay that exclusively tests trans interactions.
To circumvent many of the complications associated with purified protein assays, we have optimized a cell-based protein interaction assay that recapitulates trans interactions in a reduced heterologous cell system. This assay has been previously used in various forms to study transcellular interactions. In this approach, candidate cell adhesion molecules are transfected into HEK293T cells. At physiological conditions, HEK293T cells do not exhibit self-aggregation, making them exemplary models for this assay. However, when individual populations of HEK cells expressing receptor and ligand are combined, the binding of the receptor and the ligand forces aggregation of HEK cells to occur. This aggregation is mediated exclusively by trans interactions and is usually observable in tens of minutes. No protein purification steps are required in this method, and the efficiency of the method relies on the paradigm that populations of HEK cells expressing cognate adhesion molecules are being combined and then imaged only tens of minutes later. Additionally, this method is relatively inexpensive, as neither antibodies nor costly equipment are required. The only equipment required for the acquisition of data is a standard fluorescent microscope. An additional advantage to this cell-based assay is the ability to quickly screen the effect of disease relevant point mutations on trans interactions. This can be performed by transfecting HEK cells with cDNAs of the mutant variants of the protein of interest.
In this protocol, we present an example in which we investigate whether a missense mutation in Neurexin3&#945; (Neurexin3&#945;A687T), identified in a patient diagnosed with profound intellectual disability and epilepsy, alters interactions in trans with leucine-rich repeat transmembrane protein 2 (LRRTM2). Neurexin3&#945; is a member of the evolutionarily conserved family of presynaptic cell-adhesion molecules and while recent work has identified multiple roles at the synapse3,4,5,6,7, our synaptic understanding of this molecule and all members of the neurexin family remains incomplete. LRRTM2 is an excitatory postsynaptic cell adhesion protein that participates in synapse formation and maintenance8,9,10. Importantly, LRRTM2 exclusively interacts with neurexin isoforms that lack the splice site 4 alternative exon (SS4-) but not with neurexin isoforms containing the splice site 4 alternative exon (SS4+). The human missense mutation (A687T) identified in Neurexin3&#945; is located in an unstudied extracellular region that is evolutionarily conserved and is conserved between all alpha neurexins7. As the interaction between these two molecules has been established8,9,11, we posed the question: is the binding capability of Neurexin3&#945; SS4- to LRRTM2 altered by an A687T point mutation? This assay revealed that the A687T point mutation unexpectedly enhanced the aggregation of Neurexin3&#945; to LRRTM2 suggesting that the extracellular region in which the point mutation is located, plays a role in mediating transsynaptic interactions.

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		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="60">
			<td height="60" style="height:60px;width:339px;">1.5 mL disposable microtubes with snap caps</td>
			<td style="width:121px;">VWR</td>
			<td style="width:161px;">89000-028</td>
			<td style="width:167px;">Incubation of mixed population of HEK cells</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">1000 mL Rapid&#8212;Flow Filter Unit, 0.2 um aPES membrane</td>
			<td style="width:121px;">Thermo Fisher</td>
			<td style="width:161px;">567-0020</td>
			<td style="width:167px;">Sterilization of HEK media</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">15 mL SpectraTube centrifuge tubes</td>
			<td style="width:121px;">Ward&#8217;s Science</td>
			<td style="width:161px;">470224-998</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">6 well sterile tissue culture plates</td>
			<td style="width:121px;">VWR</td>
			<td style="width:161px;">100062-892</td>
			<td style="width:167px;">culturing HEK cells</td>
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:339px;">Calcium Chloride</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">223506-500G</td>
			<td style="width:167px;">Calcium phosphate transfection, HEK cell resuspension</td>
		</tr>
		<tr height="44">
			<td height="44" style="height:45px;width:339px;">Centrifuge- Sorvall Legend RT</td>
			<td style="width:121px;">Kendro Laboratory Products</td>
			<td align="right" style="width:161px;">75004377</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:22px;width:339px;">CO2 cell incubator</td>
			<td style="width:121px;">Thermo Scientific</td>
			<td style="width:161px;">HERACELL 150i</td>
			<td style="width:167px;">Incubation of HEK cells during growth</td>
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:339px;">DMEM, 1x (Dulbecco&#39;s Modification of Eagle&#39;s Medium) with 4.5 g/L glucose, L-glutamine &#38; sodium pyruvate</td>
			<td style="width:121px;">Corning</td>
			<td style="width:161px;">10-013-CV</td>
			<td style="width:167px;">HEK cell maintenance</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Dulbecco&#8217;s Phosphate Buffered Saline PBS (1X)</td>
			<td style="width:121px;">Gibco</td>
			<td style="width:161px;">14190-144</td>
			<td style="width:167px;">Passaging/harvesting HEK cell</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">Ethylenediaminetetraacetic acid</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">ED-500G</td>
			<td style="width:167px;">Harvesting HEK cells</td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:339px;">Falcon Vented culture flasks, 75cm2 growth area</td>
			<td style="width:121px;">Corning</td>
			<td style="width:161px;">9381M26</td>
			<td style="width:167px;">Culturing HEK cells</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Fetal Bovine Serum</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">17L184</td>
			<td style="width:167px;">HEK cell maintenance</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">HEK293T cells</td>
			<td style="width:121px;">ATCC</td>
			<td style="width:161px;"></td>
			<td style="width:167px;">Model system</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">ImageJ</td>
			<td style="width:121px;">NIH</td>
			<td style="width:161px;">V: 2.0.0-rc-69/1.52p</td>
			<td style="width:167px;">Image analysis</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:43px;width:339px;">Magnesium Chloride hexahydrate</td>
			<td style="width:121px;">Sigma</td>
			<td style="width:161px;">M9272-500G</td>
			<td style="width:167px;">HEK cell resuspension</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Sodium phosphate dibasic anhydrous</td>
			<td style="width:121px;">Fisher BioReagents</td>
			<td style="width:161px;">BP332-500</td>
			<td style="width:167px;">Calcium phosphate transfection</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Trypsin 0.25% (1X) Solution</td>
			<td style="width:121px;">GE Healthcare Life Sciences</td>
			<td style="width:161px;">SH30042.01</td>
			<td style="width:167px;">Passaging HEK cells</td>
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:339px;">Tube rotator</td>
			<td style="width:121px;"></td>
			<td style="width:161px;"></td>
			<td style="width:167px;">Incubation of mixed population of HEK cells</td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:339px;">UltraClear Microscope slides. White Frosted, Positive Charged</td>
			<td style="width:121px;">Denville Scientific Inc.</td>
			<td style="width:161px;">M1021</td>
			<td style="width:167px;">Image acquisition</td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;width:339px;">Wide-field microscope</td>
			<td style="width:121px;">Zeiss</td>
			<td style="width:161px;">Axio Vert 200M</td>
			<td style="width:167px;">Image acquisition</td>
		</tr>
	</tbody>
</table>

measuring transcellular interactions through protein aggregation in a heterologous cell system

Protein interactions at cellular interfaces dictate a multitude of biological outcomes ranging from tissue development and cancer progression to synapse formation and maintenance. Many of these fundamental interactions occur in trans and are typically induced by heterophilic or homophilic interactions between cells expressing membrane anchored binding pairs. Elucidating how disease relevant mutations disrupt these fundamental protein interactions can provide insight into a myriad of cell biology fields. Many protein-protein interaction assays do not typically disambiguate between cis and trans interactions, which potentially leads to an overestimation of the extent of binding that is occurring in vivo and involve labor intensive purification of protein and/or specialized monitoring equipment. Here, we present an optimized simple protocol that allows for the observation and quantification of only trans interactions without the need for lengthy protein purifications or specialized equipment. The HEK cell aggregation assay involves the mixing of two independent populations of HEK cells, each expressing membrane-bound cognate ligands. After a short incubation period, samples are imaged and the resulting aggregates are quantified.

The A687T mutation increases Neurexin3&#945; SS4- binding to LRRTM27 
To investigate how intercellular interactions of two known synaptic proteins are affected by the introduction of a point mutation found in a patient with intellectual disability and epilepsy, we used the above HEK cell aggregation assay (Figure 1). Cells were transfected according to section 1 and prepared for imaging according to sections 1 and 2 of the protocol. Cells were imaged at baseline where no aggregation was observed as expected (not shown). Images acquired at 60 minutes were analyzed as in section 3 of the protocol. To minimize selection bias, conditions were randomized to blind the experimenter. For similar reasons the whole field of view of every image was selected as an ROI.
Conditions in which cells were not expressing any synaptic ligands (GFP/mCherry) showed minimal aggregation after a 60-minute incubation (Figure 2). Equally, conditions in which only one of the two populations of cells were expressing synaptic ligands (mCherry/LRRTM2-GFP or GFP/Neurexin3&#945;WT/A687T SS4- &#8212;mCherry) exhibited minimal aggregation after a 60 min incubation (Figure 2). As expected, conditions that contained two populations of cells with incompatible adhesions molecules (Neurexin3&#945;WT/A687T SS4+&#8212;mCherry/LRRTM2-GFP) exhibited no aggregation at 60 minutes (Figure 2B). These conditions served as critical negative controls as LRRTM2 is known to bind exclusively to SS4 lacking isoforms (SS4-) of Neurexins and highlights the specificity of this aggregation assay.
Conditions with compatible adhesion molecules8,9,10 (Neurexin3&#945;WT SS4- &#8212;mCherry/LRRTM2-GFP) exhibited significant aggregation after a 60 min incubation (Figure 2C). Aggregation can be visualized as yellow and is present in the overlap between cells (Figure 1 and Figure 2). Surprisingly, the condition where Neurexin3&#945; A687T SS4- was co-incubated with LRRTM2 (Neurexin3&#945;A687T SS4- &#8212;mCherry/LRRTM2-GFP) exhibited significantly more aggregation as compared to its wildtype counterpart (Neurexin3&#945;WT SS4- &#8212;mCherry/LRRTM2-GFP; positive control). This suggests that the A687T point mutation in Neurexin3&#945; enhances the binding capabilities of Neurexin3&#945; SS4- to LRRTM2.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/61237/61237fig1.jpg" /> 
Figure 1. Workflow of aggregation assay. (A) HEK293T cells are transfected using a protein-1/mCherry, or a protein-2/GFP. (B) Expression of Neurexin3&#945; takes 48 h. (C) Cells are harvested using 10 mM EDTA and then pelleted (D). (E) Cells are mixed in a 1:1 ratio and resuspended in 500 &#181;L of HEK media containing 10 mM CaCl2 and MgCl2. (F) Cells are incubated at room temperature until aggregation occurs and a final image is acquired at &#8216;time 60&#8217;. Aggregation is visualized as the number of yellow puncta visible in the field of view. No aggregation is observed when cells are not expressing the correct receptor ligand pairs and are thus seen as individual red or green puncta. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61237/61237fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/61237/61237fig2.jpg" /> 
Figure 2. Neurexin3&#945;A687T SS4- has enhanced aggregation with excitatory ligand LRRTM2 compared to Neurexin3&#945;WT SS4-. (A) Representative images of negative controls after a 60 min incubation of mCherry with GFP, mCherry with LRRTM2, GFP with Neurexin3&#945;WT SS4+/-, and GFP with Neurexin3&#945;A687T SS4+/-. Aggregation was observed after 60 minutes in both LRRTM2 with Neurexin3&#945;WT SS4-, and LRRTM2 with Neurexin3&#945;A687T SS4-. (B) Quantification of the aggregation index in all SS4+ conditions after 60 minutes. Aggregation index = overlap area/(area of channel 1 + area of channel 2 &#8211; overlap area) x 100. (C) Same as (B) but SS4-. Data shown represent the mean &#177; SEM of the number of experiments (SEM: GFP/mCherry &#177; 0.0245, mCherry/LRRTM2 &#177; 0.02465, GFP/Neurexin3&#945;WT SS4- &#177; 0.02453, GFP/Neurexin3&#945;A687T SS4- &#177; 0.0109, Neurexin3&#945;WT SS4-/LRRTM2 &#177; 0.0538, Neurexin3&#945;A687T SS4-/LRRTM2 &#177; 0.0174; p= 0.0136). Numbers presented in bars represent the number of independent experiments carried out. Dotted lines represent baseline or average minimal aggregation of control conditions. Statistical significance was determined by one-way ANOVA, multiple comparisons; *p&#60;0.05; ****p&#60;0.0001.This figure has been modified from Restrepo et al.7. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61237/61237fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
Supplemental Coding Files. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/61237/Supplemental_Coding_Files.zip" target="_blank">Please click here to download these files.</a>

Watch this Scientific Journal Video about Measuring Transcellular Interactions through Protein Aggregation in a Heterologous Cell System at JoVE.com

Measuring Transcellular Interactions through Protein Aggregation in a Heterologous Cell System

Here, we present an optimized protocol to rapidly and semiquantitatively measure ligand-receptor interactions in trans in a heterologous cell system using fluorescence microscopy.

Protein interactions at cellular interfaces dictate a multitude of biological outcomes ranging from tissue development and cancer progression to synapse ...

measuring-transcellular-interactions-through-protein-aggregation

Nanyang Technological University

Research

JoVE Journal

Neuroscience

3.4K Views.  University of Colorado Denver School of Medicine. Here, we present an optimized protocol to rapidly and semiquantitatively measure ligand-receptor interactions in trans in a heterologous cell system using fluorescence microscopy.

Video: Measuring Transcellular Interactions through Protein Aggregation in a Heterologous Cell System

Assaying Surface Expression of Chemosensory Receptors in Heterologous Cells

The vivid world of odors is recognized by the sense of olfaction. Olfaction in mice is mediated by a repertoire of about 1200 G Protein Coupled Receptors (GPCRs) 1 that are postulated to bind volatile odorant molecules and converting the extracellular signal into an intracellular signal by coupling with G protein G&alpha;olf. Binding of the odorants to the receptors is thought to follow a combinatorial rule, that is, one odorant may bind several receptors and one receptor may bind several odorants to varying degrees 2. Biochemical, signaling and ligand binding studies have been conveniently carried out for most GPCRs using heterologous cells. However use of heterologous cells for study of odorant receptors, was precluded for a long time since on transfection they failed to export to the surface. Saito et al have demonstrated single membrane pass Receptor Transporting Protein (RTP) family chaperones show enhanced expression in the olfactory sensory neurons and act as chaperones to traffic odorant receptors to the surface in heterologous cells, when co transfected together 3. To carry out biochemical assays for receptors using heterologous cells, one must first determine if the receptor shows robust surface expression in the cell line. This can be assayed by overexpressing the receptors with the chaperone RTP1S followed by live cell staining to fluorescently label the extracellular domain or a tag in the extracellular domain exclusively. Here we demonstrate a protocol to carry out live cell staining that can be used to detect odorant receptors on the surface of HEK293T cells conveniently. In addition, it may also be used to assay for surface expression of other chemosensory receptors or GPCRs.

Here we demonstrate a protocol to carry out live cell staining that can be used to detect odorant receptors on the surface of HEK293T cells conveniently. In addition, it may also be used to assay for surface expression of other chemosensory receptors or GPCRs.

The vivid world of odors is recognized by the sense of olfaction. Olfaction in mice is mediated by a repertoire of about 1200 G Protein Coupled Receptors ...

Efficient Derivation of Human Neuronal Progenitors and Neurons from Pluripotent Human Embryonic Stem Cells with Small Molecule Induction

There is a large unfulfilled need for a clinically-suitable human neuronal cell source for repair or regeneration of the damaged central nervous system (CNS) structure and circuitry in today's healthcare industry. Cell-based therapies hold great promise to restore the lost nerve tissue and function for CNS disorders. However, cell therapies based on CNS-derived neural stem cells have encountered supply restriction and difficulty to use in the clinical setting due to their limited expansion ability in culture and failing plasticity after extensive passaging1-3. Despite some beneficial outcomes, the CNS-derived human neural stem cells (hNSCs) appear to exert their therapeutic effects primarily by their non-neuronal progenies through producing trophic and neuroprotective molecules to rescue the endogenous cells1-3. Alternatively, pluripotent human embryonic stem cells (hESCs) proffer cures for a wide range of neurological disorders by supplying the diversity of human neuronal cell types in the developing CNS for regeneration1,4-7. However, how to channel the wide differentiation potential of pluripotent hESCs efficiently and predictably to a desired phenotype has been a major challenge for both developmental study and clinical translation. Conventional approaches rely on multi-lineage inclination of pluripotent cells through spontaneous germ layer differentiation, resulting in inefficient and uncontrollable lineage-commitment that is often followed by phenotypic heterogeneity and instability, hence, a high risk of tumorigenicity7-10. In addition, undefined foreign/animal biological supplements and/or feeders that have typically been used for the isolation, expansion, and differentiation of hESCs may make direct use of such cell-specialized grafts in patients problematic11-13. To overcome these obstacles, we have resolved the elements of a defined culture system necessary and sufficient for sustaining the epiblast pluripotence of hESCs, serving as a platform for de novo derivation of clinically-suitable hESCs and effectively directing such hESCs uniformly towards clinically-relevant lineages by small molecules14 (please see a schematic in Fig. 1). Retinoic acid (RA) does not induce neuronal differentiation of undifferentiated hESCs maintained on feeders1, 14. And unlike mouse ESCs, treating hESC-differentiated embryoid bodies (EBs) only slightly increases the low yield of neurons1, 14, 15. However, after screening a variety of small molecules and growth factors, we found that such defined conditions rendered retinoic acid (RA) sufficient to induce the specification of neuroectoderm direct from pluripotent hESCs that further progressed to neuroblasts that generated human neuronal progenitors and neurons in the developing CNS with high efficiency (Fig. 2). We defined conditions for induction of neuroblasts direct from pluripotent hESCs without an intervening multi-lineage embryoid body stage, enabling well-controlled efficient derivation of a large supply of human neuronal cells across the spectrum of developmental stages for cell-based therapeutics.

We have established a protocol for induction of neuroblasts direct from pluripotent human embryonic stem cells maintained under defined conditions with small molecules, which enables derivation of a large supply of human neuronal progenitors and neuronal cell types in the developing CNS for neural repair.

There is a large unfulfilled need for a clinically-suitable human neuronal cell source for repair or regeneration of the damaged central nervous system ...

Peering into the Dynamics of Social Interactions: Measuring Play Fighting in Rats

Play fighting in the rat involves attack and defense of the nape of the neck, which if contacted, is gently nuzzled with the snout. Because the movements of one animal are countered by the actions of its partner, play fighting is a complex, dynamic interaction. This dynamic complexity raises methodological problems about what to score for experimental studies. We present a scoring schema that is sensitive to the correlated nature of the actions performed. The frequency of play fighting can be measured by counting the number of playful nape attacks occurring per unit time. However, playful defense, as it can only occur in response to attack, is necessarily a contingent measure that is best measured as a percentage (#attacks defended/total # attacks X 100%). How a particular attack is defended against can involve one of several tactics, and these are contingent on defense having taken place; consequently, the type of defense is also best expressed contingently as a percentage. Two experiments illustrate how these measurements can be used to detect the effect of brain damage on play fighting even when there is no effect on overall playfulness. That is, the schema presented here is designed to detect and evaluate changes in the content of play following an experimental treatment.

Play fighting in the rat involves attack and defense of the nape of the neck, which if contacted, is gently nuzzled with the snout. Because the movements of one animal are countered by the actions of its partner, play fighting is a complex, dynamic interaction. This dynamic complexity raises methodological problems about what to score for experimental studies. We present a scoring schema that is sensitive to the correlated nature of the actions performed. Two experiments illustrate how these measurements can be used to detect the effect of brain damage on play fighting even when there is no effect on overall playfulness. That is, the schema presented here is designed to detect and evaluate changes in the content of play following an experimental treatment.

Play fighting in the rat involves attack and defense of the nape of the neck, which if contacted, is gently nuzzled with the snout. Because the movements ...

Dual Electrophysiological Recordings of Synaptically-evoked Astroglial and Neuronal Responses in Acute Hippocampal Slices

Astrocytes form together with neurons tripartite synapses, where they integrate and modulate neuronal activity. Indeed, astrocytes sense neuronal inputs through activation of their ion channels and neurotransmitter receptors, and process information in part through activity-dependent release of gliotransmitters. Furthermore, astrocytes constitute the main uptake system for glutamate, contribute to potassium spatial buffering, as well as to GABA clearance. These cells therefore constantly monitor synaptic activity, and are thereby sensitive indicators for alterations in synaptically-released glutamate, GABA and extracellular potassium levels. Additionally, alterations in astroglial uptake activity or buffering capacity can have severe effects on neuronal functions, and might be overlooked when characterizing physiopathological situations or knockout mice. Dual recording of neuronal and astroglial activities is therefore an important method to study alterations in synaptic strength associated to concomitant changes in astroglial uptake and buffering capacities. Here we describe how to prepare hippocampal slices, how to identify stratum radiatum astrocytes, and how to record simultaneously neuronal and astroglial electrophysiological responses. Furthermore, we describe how to isolate pharmacologically the synaptically-evoked astroglial currents.

The preparation of acute brain slices from isolated hippocampi, as well as the simultaneous electrophysiological recordings of astrocytes and neurons in stratum radiatum during stimulation of schaffer collaterals is described. The pharmacological isolation of astroglial potassium and glutamate transporter currents is demonstrated.

Astrocytes form together with neurons tripartite synapses, where they integrate and modulate neuronal activity. Indeed, astrocytes sense neuronal inputs ...

A Lightweight, Headphones-based System for Manipulating Auditory Feedback in Songbirds

Experimental manipulations of sensory feedback during complex behavior have provided valuable insights into the computations underlying motor control and sensorimotor plasticity1. Consistent sensory perturbations result in compensatory changes in motor output, reflecting changes in feedforward motor control that reduce the experienced feedback error. By quantifying how different sensory feedback errors affect human behavior, prior studies have explored how visual signals are used to recalibrate arm movements2,3 and auditory feedback is used to modify speech production4-7. The strength of this approach rests on the ability to mimic naturalistic errors in behavior, allowing the experimenter to observe how experienced errors in production are used to recalibrate motor output.
Songbirds provide an excellent animal model for investigating the neural basis of sensorimotor control and plasticity8,9. The songbird brain provides a well-defined circuit in which the areas necessary for song learning are spatially separated from those required for song production, and neural recording and lesion studies have made significant advances in understanding how different brain areas contribute to vocal behavior9-12. However, the lack of a naturalistic error-correction paradigm - in which a known acoustic parameter is perturbed by the experimenter and then corrected by the songbird - has made it difficult to understand the computations underlying vocal learning or how different elements of the neural circuit contribute to the correction of vocal errors13.
The technique described here gives the experimenter precise control over auditory feedback errors in singing birds, allowing the introduction of arbitrary sensory errors that can be used to drive vocal learning. Online sound-processing equipment is used to introduce a known perturbation to the acoustics of song, and a miniaturized headphones apparatus is used to replace a songbird's natural auditory feedback with the perturbed signal in real time. We have used this paradigm to perturb the fundamental frequency (pitch) of auditory feedback in adult songbirds, providing the first demonstration that adult birds maintain vocal performance using error correction14. The present protocol can be used to implement a wide range of sensory feedback perturbations (including but not limited to pitch shifts) to investigate the computational and neurophysiological basis of vocal learning.

We describe the design and assembly of miniaturized headphones suitable for replacing a songbird&#x2019;s natural auditory feedback with a manipulated acoustic signal. Online sound processing hardware is used to manipulate song output, introduce real-time errors in auditory feedback via the headphones, and drive vocal motor learning.

Experimental manipulations of sensory feedback during complex behavior have provided valuable insights into the computations underlying motor control and ...

Genetic Manipulation of the Mouse Developing Hypothalamus through In utero Electroporation

Genetic modification of specific regions of the developing mammalian brain is a very powerful experimental approach. However, generating novel mouse mutants is often frustratingly slow. It has been shown that access to the mouse brain developing in utero with reasonable post-operatory survival is possible. Still, results with this procedure have been reported almost exclusively for the most superficial and easily accessible part of the developing brain, i.e. the cortex. The thalamus, a narrower and more medial region, has proven more difficult to target. Transfection into deeper nuclei, especially those of the hypothalamus, is perhaps the most challenging and therefore very few results have been reported. Here we demonstrate a procedure to target the entire hypothalamic neuroepithelium or part of it (hypothalamic regions) for transfection through electroporation. The keys to our approach are longer narcosis times, injection in the third ventricle, and appropriate kind and positioning of the electrodes. Additionally, we show results of targeting and subsequent histological analysis of the most recessed hypothalamic nucleus, the mammillary body.

Genetic Manipulation of the Mouse Developing Hypothalamus through In utero Electroporation

Despite the functional and medical importance of the hypothalamus, in utero genetic manipulation of its development has rarely been attempted. We show a detailed procedure for in utero electroporation into the mouse hypothalamus and show representative results of total and partial (regional) hypothalamic transfection.

Genetic  modification of specific regions of the developing mammalian brain is a very  powerful experimental approach. However, generating novel mouse ...

A Caenorhabditis elegans Model System for Amylopathy Study

Amylopathy is a term that describes abnormal synthesis and accumulation of amyloid beta (A&beta;) in tissues with time. A&beta; is a hallmark of Alzheimer's disease (AD) and is found in Lewy body dementia, inclusion body myositis and cerebral amyloid angiopathy 1-4. Amylopathies progressively develop with time. For this reason simple organisms with short lifespans may help to elucidate molecular aspects of these conditions. Here, we describe experimental protocols to study A&beta;-mediated neurodegeneration using the worm Caenorhabditis elegans. Thus, we construct transgenic worms by injecting DNA encoding human A&beta;42 into the syncytial gonads of adult hermaphrodites. Transformant lines are stabilized by a mutagenesis-induced integration. Nematodes are age synchronized by collecting and seeding their eggs. The function of neurons expressing A&beta;42 is tested in opportune behavioral assays (chemotaxis assays). Primary neuronal cultures obtained from embryos are used to complement behavioral data and to test the neuroprotective effects of anti-apoptotic compounds.

A Caenorhabditis elegans Model System for Amylopathy Study

We describe methods to study aspects of amylopathies in the worm C. elegans. We show how to construct worms expressing human A&beta;42 in neurons and how to test their function in behavioral assays. We further show how to obtain primary neuronal cultures that can be used for pharmacological testing.

Amylopathy is a term that describes abnormal synthesis and accumulation of amyloid beta (A&beta;) in tissues with time. A&beta; is a hallmark of ...

Use of a Caspase Multiplexing Assay to Determine Apoptosis in a Hypothalamic Cell Model

The ability to multiplex assays in studies of complex cellular mechanisms eliminates the need for repetitive experiments, provides internal controls, and decreases waste in costs and reagents. Here we describe optimization of a multiplex assay to assess apoptosis following a palmitic acid (PA) challenge in an in vitro hypothalamic model, using both fluorescent and luminescent based assays to measure viable cell counts and caspase-3/7 activity in a 96-well microtiter plate format. Following PA challenge, viable cells were determined by a resazurin-based fluorescent assay. Caspase-3/7 activity was then determined using a luminogenic substrate, DEVD, and normalized to cell number. This multiplexing assay is a useful technique for determining change in caspase activity following an apoptotic stimulus, such as saturated fatty acid challenge. The saturated fatty acid PA can increase hypothalamic oxidative stress and apoptosis, indicating the potential importance of assays such as that described here in studying the relationship between saturated fatty acids and neuronal function.

Multiplex assays can provide beneficial information for basic cellular mechanisms&#160;and eliminate waste of reagents and unnecessary repetitive experiments. We describe here a multiplex caspase-3/7 activity assay, using fluorescent- and luminescent-based methods, to determine cell viability in an in vitro hypothalamic model following oxidative challenge with palmitic acid.

The ability to multiplex assays in studies of complex cellular mechanisms eliminates the need for repetitive experiments, provides internal controls, and ...

Inducing Plasticity of Astrocytic Receptors by Manipulation of Neuronal Firing Rates

Close to two decades of research has established that astrocytes in situ and in vivo express numerous G protein-coupled receptors (GPCRs) that can be stimulated by neuronally-released transmitter. However, the ability of astrocytic receptors to exhibit plasticity in response to changes in neuronal activity has received little attention. Here we describe a model system that can be used to globally scale up or down astrocytic group I metabotropic glutamate receptors (mGluRs) in acute brain slices. Included are methods on how to prepare parasagittal hippocampal slices, construct chambers suitable for long-term slice incubation, bidirectionally manipulate neuronal action potential frequency, load astrocytes and astrocyte processes with fluorescent Ca2+ indicator, and measure changes in astrocytic Gq GPCR activity by recording spontaneous and evoked astrocyte Ca2+ events using confocal microscopy. In essence, a &#8220;calcium roadmap&#8221; is provided for how to measure plasticity of astrocytic Gq GPCRs. Applications of the technique for study of astrocytes are discussed. Having an understanding of how astrocytic receptor signaling is affected by changes in neuronal activity has important implications for both normal synaptic function as well as processes underlying neurological disorders and neurodegenerative disease.

Here we describe an adaptation of protocols used to induce homeostatic plasticity in neurons for the study of plasticity of astrocytic G protein-coupled receptors. Recently used to examine changes in astrocytic group I mGluRs in juvenile mice, the method can be applied to measure scaling of various astrocytic GPCRs, in tissue from adult mice in situ and in vivo, and to gain a better appreciation of the sensitivity of astrocytic receptors to changes in neuronal activity.

Close to two decades of research has established that astrocytes in situ and in vivo express numerous G protein-coupled receptors (GPCRs) that can be ...

Quantitative Cell Biology of Neurodegeneration in Drosophila Through Unbiased Analysis of Fluorescently Tagged Proteins Using ImageJ

With the rising prevalence of neurodegenerative diseases, it is increasingly important to understand the underlying pathophysiology that leads to neuronal dysfunction and loss. Fluorescence-based imaging tools and technologies enable unprecedented analysis of subcellular neurobiological processes, yet there is still a need for unbiased, reproducible, and accessible approaches for extracting quantifiable data from imaging studies. We have developed a simple and adaptable workflow to extract quantitative data from fluorescence-based imaging studies using Drosophila models of neurodegeneration. Specifically, we describe an easy-to-follow, semi-automated approach using Fiji/ImageJ to analyze two cellular processes: first, we quantify protein aggregate content and profile in the Drosophila optic lobe using fluorescent-tagged mutant huntingtin proteins; and second, we assess autophagy-lysosome flux in the Drosophila visual system with ratiometric-based quantification of a tandem fluorescent reporter of autophagy. Importantly, the protocol outlined here includes a semi-automated segmentation step to ensure all fluorescent structures are analyzed to minimize selection bias and to increase resolution of subtle comparisons. This approach can be extended for the analysis of other cell biological structures and processes implicated in neurodegeneration, such as proteinaceous puncta (stress granules and synaptic complexes), as well as membrane-bound compartments (mitochondria and membrane trafficking vesicles). This method provides a standardized, yet adaptable reference point for image analysis and quantification, and could facilitate reliability and reproducibility across the field, and ultimately enhance mechanistic understanding of neurodegeneration.

Quantitative Cell Biology of Neurodegeneration in Drosophila Through Unbiased Analysis of Fluorescently Tagged Proteins Using ImageJ

We have developed a simple and adaptable workflow to extract quantitative data from fluorescence-imaging-based cell biological studies of protein aggregation and autophagic flux in the central nervous system of Drosophila models of neurodegeneration.

With the rising prevalence of neurodegenerative diseases, it is increasingly important to understand the underlying pathophysiology that leads to neuronal ...