This research was supported by the National Institutes of Health (K99MH104595) and the NICHD intramural research program to T.J.P. We thank Gord Fishell, in whose lab this approach was originally established.

<ol>
	<li>Bozzi, Y., Casarosa, S., Caleo, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epilepsy+as+a+neurodevelopmental+disorder.">Epilepsy as a neurodevelopmental disorder.</a> Front Psychiatry. 3 (19), (2012).</li><li>Takano, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interneuron+Dysfunction+in+Syndromic+Autism:+Recent+Advances.">Interneuron Dysfunction in Syndromic Autism: Recent Advances.</a> Dev Neurosci. , (2015).</li><li>Inan, M., Petros, T. J., Anderson, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Losing+your+inhibition:+linking+cortical+GABAergic+interneurons+to+schizophrenia.">Losing your inhibition: linking cortical GABAergic interneurons to schizophrenia.</a> Neurobiol Dis. 53, 36-48 (2013).</li><li>Batista-Brito, R., Machold, R., Klein, C., Fishell, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gene+expression+in+cortical+interneuron+precursors+is+prescient+of+their+mature+function.">Gene expression in cortical interneuron precursors is prescient of their mature function.</a> Cereb Cortex. 18 (10), 2306-2317 (2008).</li><li>Flames, N., 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=Delineation+of+multiple+subpallial+progenitor+domains+by+the+combinatorial+expression+of+transcriptional+codes.">Delineation of multiple subpallial progenitor domains by the combinatorial expression of transcriptional codes.</a> J Neurosci. 27 (36), 9682-9695 (2007).</li><li>Wonders, C. P., 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+spatial+bias+for+the+origins+of+interneuron+subgroups+within+the+medial+ganglionic+eminence.">A spatial bias for the origins of interneuron subgroups within the medial ganglionic eminence.</a> Dev Biol. 314 (1), 127-136 (2008).</li><li>Inan, M., Welagen, J., Anderson, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spatial+and+temporal+bias+in+the+mitotic+origins+of+somatostatin-+and+parvalbumin-expressing+interneuron+subgroups+and+the+chandelier+subtype+in+the+medial+ganglionic+eminence.">Spatial and temporal bias in the mitotic origins of somatostatin- and parvalbumin-expressing interneuron subgroups and the chandelier subtype in the medial ganglionic eminence.</a> Cereb Cortex. 22 (4), 820-827 (2012).</li><li>Petros, T. J., Bultje, R. S., Ross, M. E., Fishell, G., Anderson, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Apical+versus+Basal+Neurogenesis+Directs+Cortical+Interneuron+Subclass+Fate.">Apical versus Basal Neurogenesis Directs Cortical Interneuron Subclass Fate.</a> Cell Rep. 13 (6), 1090-1095 (2015).</li><li>Taniguchi, H., Lu, J., Huang, Z. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+spatial+and+temporal+origin+of+chandelier+cells+in+mouse+neocortex.">The spatial and temporal origin of chandelier cells in mouse neocortex.</a> Science. 339 (6115), 70-74 (2013).</li><li>Bandler, R. C., Mayer, C., Fishell, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cortical+interneuron+specification:+the+juncture+of+genes,+time+and+geometry.">Cortical interneuron specification: the juncture of genes, time and geometry.</a> Curr Opin Neurobiol. 42, 17-24 (2017).</li><li>Kepecs, A., Fishell, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Interneuron+cell+types+are+fit+to+function.">Interneuron cell types are fit to function.</a> Nature. 505 (7483), 318-326 (2014).</li><li>Mayer, C., 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=Developmental+diversification+of+cortical+inhibitory+interneurons.">Developmental diversification of cortical inhibitory interneurons.</a> Nature. 555 (7697), 457-462 (2018).</li><li>Mi, D., 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=Early+emergence+of+cortical+interneuron+diversity+in+the+mouse+embryo.">Early emergence of cortical interneuron diversity in the mouse embryo.</a> Science. 360 (6384), 81-85 (2018).</li><li>Alvarez-Dolado, M., 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=Cortical+inhibition+modified+by+embryonic+neural+precursors+grafted+into+the+postnatal+brain.">Cortical inhibition modified by embryonic neural precursors grafted into the postnatal brain.</a> J Neurosci. 26 (28), 7380-7389 (2006).</li><li>Baraban, S. C., 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=Reduction+of+seizures+by+transplantation+of+cortical+GABAergic+interneuron+precursors+into+Kv1.1+mutant+mice.">Reduction of seizures by transplantation of cortical GABAergic interneuron precursors into Kv1.1 mutant mice.</a> Proc Natl Acad Sci U S A. 106 (36), 15472-15477 (2009).</li><li>De la Cruz, 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=Interneuron+progenitors+attenuate+the+power+of+acute+focal+ictal+discharges.">Interneuron progenitors attenuate the power of acute focal ictal discharges.</a> Neurotherapeutics. 8 (4), 763-773 (2011).</li><li>Gilani, A. I., 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=Interneuron+precursor+transplants+in+adult+hippocampus+reverse+psychosis-relevant+features+in+a+mouse+model+of+hippocampal+disinhibition.">Interneuron precursor transplants in adult hippocampus reverse psychosis-relevant features in a mouse model of hippocampal disinhibition.</a> Proc Natl Acad Sci U S A. 111 (20), 7450-7455 (2014).</li><li>Larimer, P., 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=Caudal+Ganglionic+Eminence+Precursor+Transplants+Disperse+and+Integrate+as+Lineage-Specific+Interneurons+but+Do+Not+Induce+Cortical+Plasticity.">Caudal Ganglionic Eminence Precursor Transplants Disperse and Integrate as Lineage-Specific Interneurons but Do Not Induce Cortical Plasticity.</a> Cell Rep. 16 (5), 1391-1404 (2016).</li><li>Martinez-Cerdeno, V., 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=Embryonic+MGE+precursor+cells+grafted+into+adult+rat+striatum+integrate+and+ameliorate+motor+symptoms+in+6-OHDA-lesioned+rats.">Embryonic MGE precursor cells grafted into adult rat striatum integrate and ameliorate motor symptoms in 6-OHDA-lesioned rats.</a> Cell Stem Cell. 6 (3), 238-250 (2010).</li><li>Quattrocolo, G., Fishell, G., Petros, T. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Heterotopic+Transplantations+Reveal+Environmental+Influences+on+Interneuron+Diversity+and+Maturation.">Heterotopic Transplantations Reveal Environmental Influences on Interneuron Diversity and Maturation.</a> Cell Rep. 21 (3), 721-731 (2017).</li><li>Xu, Q., Tam, M., Anderson, S. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Fate+mapping+Nkx2.1-lineage+cells+in+the+mouse+telencephalon.">Fate mapping Nkx2.1-lineage cells in the mouse telencephalon.</a> J Comp Neurol. 506 (1), 16-29 (2008).</li><li>Thompson, L., Bjorklund, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Survival,+differentiation,+and+connectivity+of+ventral+mesencephalic+dopamine+neurons+following+transplantation.">Survival, differentiation, and connectivity of ventral mesencephalic dopamine neurons following transplantation.</a> Prog Brain Res. 200, 61-95 (2012).</li><li>Liang, Y., Agren, L., Lyczek, A., Walczak, P., Bulte, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neural+progenitor+cell+survival+in+mouse+brain+can+be+improved+by+co-transplantation+of+helper+cells+expressing+bFGF+under+doxycycline+control.">Neural progenitor cell survival in mouse brain can be improved by co-transplantation of helper cells expressing bFGF under doxycycline control.</a> Exp Neurol. 247, 73-79 (2013).</li></ol>

The authors have nothing to disclose.

One critical aspect of this protocol is maximizing the survivability of the cells. Ensuring that the tissue and cells are always in ice cold carboxygenated sACSF is necessary to promote cell survival. This requires an efficient dissection and dissociation strategy to minimize the length of time that the cells spend in various solution and outside of the brain environment. Depending on the number of brain regions being dissected and transplanted, it can be beneficial to have a partner aid in the dissection and/or transplantation steps to decrease the length of the experiment.&#160;To ensure the health and survival of the pup, it is also critical to minimize the ice anesthesia time (&#60; 4 min) and keep the pup heated with light during the transplantation procedure.
There can be significant variability in the transplanted brains, some may not contain any grafted cells while other brains will contain thousands. Combining several P0-P2 litters for dissection can increase the number of cells harvested for grafting, which allows one to either increase the transplant cell density and/or increase the number of grafted pups, both of which will improve the likelihood of successful transplants. To target the proper brain region, it is critical that the pup&#39;s head is stabilized and does not move when lowering the micropipette into the brain. Any shifting of the head or bending of the micropipette shaft can result in mistargeting and inaccurate injections (such as loss of cells into the lateral ventricle). Also, performing multiple injection sites per pup (either unilaterally or bilaterally) can improve success rates in case one injection site is mistargeted. In the future, it would be optimal to develop a more efficient stereotaxic strategy to stabilize the pup and provide more accurate and consistent injections.
Th P0-P2 time frame was chosen for both scientific and technical reasons. At this age, most interneuron precursors have migrated to their terminal sites but have minimal environmental interactions. This time frame may not hold true for other neuronal cell types and brain regions, so one may want to adjust these timepoints for their specific purposes. For example, it would be interesting to compare the therapeutic potential of these P0-P2 harvested interneuron precursors with MGE-harvested cells: one age may have more benefits and/or less unwanted side effects than the other. Additionally, it could be interesting to perform heterochronic transplants (e.g., collecting cells at P0-P2 and transplanting into P7-10 brains, or vice versa) to identify how temporal changes in the environment could affect cell fate and maturation. The advantage of performing injections at P0-2 is that the skull is relatively thin and can be easily penetrated with a sharp micropipette. Any injections over P5 will require removing or thinning the skull.
The analysis described in this protocol is limited to specific characteristics of interneuron subtypes that are differentially expressed between brain regions. In the future, one could utilize the power of single cell sequencing to characterize the full transcriptome of grafted cells. This unbiased approach could identify specific genes (or larger signaling cascades) that are strongly enriched or depleted in transplanted cells compared to controls, which would provide insight into candidate environmental cues that would influence fate determination or maturation.

Proper cortical function requires a balance between excitatory projection neurons and inhibitory GABAergic interneurons, an extremely heterogeneous population with distinct morphologies, electrophysiological properties, connectivity and neurochemical markers. Abnormal development and function of interneurons (and specific interneuron subgroups) has been linked to the pathobiology of psychiatric disorders such as schizophrenia, autism and epilepsy1,2,3. Furthermore, many genes implicated in these brain disorders are strongly enriched in young interneurons4. Thus, a greater understanding of the mechanisms that regulate interneuron fate determination and maturation is needed to understand normal development and potential etiologies of numerous brain diseases.
Forebrain interneurons are born primarily from two transient embryonic structures, the medial and caudal ganglionic eminences (MGE and CGE, respectively). These postmitotic cells (interneuron precursors) then undergo a protracted tangential migration phase to disperse throughout the telencephalon where they integrate into a wide variety of circuits. MGE-derived interneurons consist of three largely non-overlapping, neurochemically defined subgroups: fast spiking parvalbumin (PV+) interneurons, non-fast spiking somatostatin (SST+) interneurons, and late spiking neuronal nitric oxide synthase (nNOS+) interneurons that constitute hippocampal neurogliaform and ivy cells. Numerous labs have identified several mechanisms within the MGE that regulate initial fate decisions into PV+ or SST+ interneurons, including spatial gradients of morphogens, birthdate of interneuron precursors, and the mode of neurogenic division5,6,7,8,9,10. It has been proposed that interneurons initially differentiate into &#39;cardinal classes&#39; and then progressively mature into &#39;definitive classes&#39; as they interact with their environment11. Recent evidence indicates that some mature interneuron subtypes may be genetically hardwired as these cells become postmitotic in the ganglionic eminences, indicating that early defined intrinsic genetic programs may play a larger role than previously appreciated12,13. However, the key question of how the intrinsic genetic programs interact with environmental cues to drive differentiation into distinct interneuron subtypes remains largely unexplored.
Numerous studies have transplanted embryonic MGE cells directly into a variety of brain regions, with the consensus results that grafted cells mature and release GABA to generally inhibit the local endogenous circuitry14,15,16,17,18,19. These promising observations have generated significant interest in using human induced pluripotent stem cell (hIPSC)-derived interneurons to treat a variety of brain diseases. However, very few of these studies assess if these grafted cells mature into the expected types of mature interneurons, a critical component when one thinks about translational approaches.
To address how environment influences interneuron differentiation and maturation, a strategy was devised to transplant immature interneuron precursors into new brain environments in order to examine whether grafted interneurons adopt features of the host environment or retain features from the donor environment20. MGE transplants are not suitable to address this question because the MGE contains a mixed population of interneuron and GABAergic projection cells that disperse throughout numerous brain regions21. Without knowing where these MGE cells would have migrated, one can not fully assess how these transplantations are affected by the brain environment. By harvesting interneuron precursors at early postnatal timepoints, this problem is circumvented by obtaining immature cells that have completed their migration and reached their target brain region but have minimal interaction with the environment. By focusing on specific features of interneurons that are differentially expressed between distinct brain regions, one can then determine how the host environment changes interneuron properties. The general approach outlined in this protocol should be applicable to any investigator that wants to examine how young neurons behave when challenged in a new environment.

<table border="0" cellpadding="0" cellspacing="0" style="width:776px;" width="777">
	<tbody>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Sodium chloride</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">S7653</td>
			<td style="width:191px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Sodium bicarbonate</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">S6297</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Potassium chloride</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">P9541</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Sodium phosphate monobasic</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">S0751</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Calcium chloride</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">C5080</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Magnesium chloride</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">M2670</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Glucose</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">G7528</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Sucrose</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">S7903</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Brain Matrices</td>
			<td style="width:203px;">Roboz</td>
			<td style="width:136px;">SA-2165</td>
			<td>Only needed if harvesting striatum</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Fine point Dumont Forceps</td>
			<td style="width:203px;">Roboz</td>
			<td style="width:136px;">RS-4978</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Microdissecting scissors</td>
			<td style="width:203px;">Roboz</td>
			<td style="width:136px;">RS-5940</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Razor blades</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">12-640</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Pasteur pipettes</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">1367820C</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Nanoject III</td>
			<td style="width:203px;">Drummond</td>
			<td style="width:136px;">3-000-207</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Manual Manipulator w/ stand</td>
			<td style="width:203px;">World Precision Instruments&#160;</td>
			<td style="width:136px;">M3301R/M10</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">5 ml round bottom plastic tubes</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">149591A</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">60 mm Petri dishes</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">12556001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">100 mm Petri dishes</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">12565100</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Pronase</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">10165921001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Fetal Bovine Serum (FBS)</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">16140063</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">DNase I</td>
			<td style="width:203px;">Sigma</td>
			<td style="width:136px;">4716728001</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Celltrics 50um filters</td>
			<td style="width:203px;">Sysmex</td>
			<td style="width:136px;">04-0042327</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Trypan blue</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">15-250-061</td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:247px;">Hemocytometer</td>
			<td style="width:203px;">ThermoFisher</td>
			<td style="width:136px;">02-671-6</td>
			<td></td>
		</tr>
	</tbody>
</table>

homochronic transplantation of interneuron precursors into early postnatal mouse brains

Neuronal fate determination and maturation requires an intricate interplay between genetic programs and environmental signals. However, disentangling the roles of intrinsic vs. extrinsic mechanisms that regulate this differentiation process is a conundrum for all developmental neurobiologists. This issue is magnified for GABAergic interneurons, an incredibly heterogeneous cell population that is born from transient embryonic structures and undergo a protracted migratory phase to disperse throughout the telencephalon. To explore how different brain environments affect interneuron fate and maturation, we developed a protocol for harvesting fluorescently labeled immature interneuron precursors from specific brain regions in newborn mice (P0-P2). At this age, interneuron migration is nearly complete and these cells are residing in their final resting environments with relatively little synaptic integration. Following collection of single cell solutions via flow cytometry, these interneuron precursors are transplanted into P0-P2 wildtype postnatal pups. By performing both homotopic (e.g., cortex-to-cortex) or heterotopic (e.g., cortex-to-hippocampus) transplantations, one can assess how challenging immature interneurons in new brain environments affects their fate, maturation, and circuit integration. Brains can be harvested in adult mice and assayed with a wide variety of posthoc analysis on grafted cells, including immunohistochemical, electrophysiological and transcriptional profiling. This general approach provides investigators with a strategy to assay how distinct brain environments can influence numerous aspects of neuron development and identify if specific neuronal characteristics are primarily driven by hardwired genetic programs or environmental cues.

This protocol demonstrates how to harvest specific brain regions from early postnatal brains (Figure 1-2), collect single cell dissociations of interneuron precursors, and transplant these cells into various brain regions in naive WT postnatal pups (Figure 3). For posthoc analysis, brains that received interneuron precursor grafts were harvested between P30-35 to characterize cell morphology, neurochemical markers and electrophysiological properties. These types of assays are often carried out between P21-P30 in normal mice, but since the maturation of transplanted cells might be slightly delayed due to the dissection/dissociation procedure, waiting an additional 5-10 days is recommended to compensate for this delayed maturation. The type of analysis to be performed will dictate the proper strategy to harvest the brain. Notably, we did not observe preferential cell death of specific interneuron subgroups that could bias for or against certain subtypes20.
For immunohistochemical analysis, mice were perfused with 4% paraformaldehyde and the brains were removed. 50 &#956;m vibratome slices were prepared through the targeted brain region and stored in antifreeze solution and/or processed for immunostaining as previously described20. Some brains did not contain any tomato+ cells, which could be due to improper targeting (e.g., injection too deep into the ventricle), cells lost or undergoing apoptosis during the grafting procedure, or rejection of transplanted cells by the host. Based on final cell counts, it is estimated that only 2-5% of grafted cells survive20, which is in line with other transplantation procedures22,23.
Not surprisingly, there was significant variability in the total number of tomato+ cells in successful transplants, ranging from dozens to several thousand tomato+ cells (Figure 4A). Grafted cells were localized in the correct regions, with many displaying interneuron morphologies and well-characterized interneuron neurochemical markers (Figure 4B). Similar cell survival numbers and maturation profiles were observed even when cells were grafted into new environments in heterotopic transplantations (Figure 4C).
In addition to immunohistochemical analysis, electrophysiological analysis on grafted cells was performed to confirm that they have integrated into brain circuitry and display expected intrinsic and firing properties. Brains were harvested from P30-35 mice and slices prepared for physiological recordings as previously described20. The grafted interneurons presented adult-like physiological properties and distinct firing patterns could be characterized that were representative of well-characterized interneuron subtypes (Figure 5A), suggesting that grafted interneurons were able to properly mature in the host environment. To verify that transplanted cells were integrated in the neuronal network, sEPSCs were also recorded (Figure 5B). In addition, a subset of transplants were performed with interneurons expressing ChR2 followed by recording from pyramidal cells localized near transplanted interneurons. These data demonstrated that postsynaptic GABAergic currents are evoked by blue light (Figure 5C-D).
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/57723/57723fig4.jpg" /> 
Figure 4: Grafted interneuron precursors populate host brain regions Representative sections from P30 WT mice that were transplanted with tomato+ interneuron precursors at P1. (A) In homotopic cortex-to-cortex transplantations, the grafted cells populate all cortical layers and display morphologies that mimic endogenous interneurons. Selected images highlight the variability in cell numbers from different transplants, with the left image having a much greater number of tomato+ cells per section compared to the transplant on the right side. (B) Low magnification (left) and high magnification (right) representative sections from homotopic hippocampus-to-hippocampus grafts. Note that the majority of tomato+ cells in the stratum oriens (SO) express SST (likely O-LM cells) whereas many tomato+ cells in the stratum pyramidale (SP) express PV (likely basket cells), similar to endogenous hippocampal interneurons. (C) Example of a heterotopic transplantation (Cortex-to-Striatum) with tomato+ present in the striatum. Scale bars = 200 &#956;m in A in low mag panel in B, 50 &#956;m in C and high power mag in B. <a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57723/57723fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/57723/57723fig5.jpg" /> 
Figure 5: Grafted interneurons are electrophysiologically mature and integrate in the host neuronal network&#160; 
(A) Representative examples of the highest firing frequencies recorded from grafted interneurons. Left, Fast Spiking interneuron from a Hip-to-Ctx graft, injected current steps: -100 pA and 520 pA; right, Non-Fast Spiking interneuron from a Ctx-to-Ctx graft, injected current steps: -100 pA and 360 pA. (B) Example of sEPSCs recorded in a Late Spiking interneuron from a Hip-to-Hip transplant. (C) Representative image displaying Nkx2.1-Cre;Ai32 cells (YFP) from a Ctx-to-Ctx transplant with a biocytin-filled (red) pyramidal cell. Scale bar = 50 &#956;m. (D) Example of a GABAergic postsynaptic currents evoked by blue light pulses recorded in pyramidal cells, recorded with a [145 mM] Cl-. In black, average traces; in red, average response recorded in the presence of Gabazine.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/57723/57723fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Homochronic Transplantation of Interneuron Precursors into Early Postnatal Mouse Brains at JoVE.com

Homochronic Transplantation of Interneuron Precursors into Early Postnatal Mouse Brains

Challenging young neurons in new brain regions can reveal important insights into how the environment sculpts neuronal fate and maturation. This protocol describes a procedure to harvest interneuron precursors from specific brain regions and transplant them either homotopically or heterotopically into the brain of postnatal pups.

Neuronal fate determination and maturation requires an intricate interplay between genetic programs and environmental signals. However, disentangling the ...

homochronic-transplantation-interneuron-precursors-into-early

Nanyang Technological University

Research

JoVE Journal

Neuroscience

7.8K Views.  Norwegian University of Science and Technology. Challenging young neurons in new brain regions can reveal important insights into how the environment sculpts neuronal fate and maturation. This protocol describes a procedure to harvest interneuron precursors from specific brain regions and transplant them either homotopically or heterotopically into the brain of postnatal pups.

Video: Homochronic Transplantation of Interneuron Precursors into Early Postnatal Mouse Brains

Mosaic Analysis of Gene Function in Postnatal Mouse Brain Development by Using Virus-based Cre Recombination

Normal brain function relies not only on embryonic development when major neuronal pathways are established, but also on postnatal 
development when neural circuits are matured and refined. Misregulation at this stage may lead to neurological and psychiatric disorders such as autism 
and schizophrenia1,2. Many genes have been studied in the prenatal brain and found crucial to many developmental processes3-5. However, their 
function in the postnatal brain is largely unknown, partly because their deletion in mice often leads to lethality during neonatal development, and partly because their requirement in early development hampers the postnatal analysis. To overcome these obstacles, floxed alleles of these genes are currently being generated in mice 6. When combined with transgenic alleles that express Cre recombinase in specific cell types, conditional deletion can be achieved to study gene function in the postnatal brain. However, this method requires additional alleles and extra time (3-6 months) to generate the mice with appropriate genotypes, thereby limiting the expansion of the genetic analysis to a large scale in the mouse brain.
Here we demonstrate a complementary approach that uses virally-expressed Cre to study these floxed alleles rapidly and 
systematically in postnatal brain development. By injecting recombinant adeno-associated viruses (rAAVs)7,8 encoding Cre into the neonatal brain, 
we are able to delete the gene of interest in different regions of the brain. By controlling the viral titer and coexpressing a fluorescent 
protein marker, we can simultaneously achieve mosaic gene inactivation and sparse neuronal labeling. This method bypasses the requirement of 
many genes in early development, and allows us to study their cell autonomous function in many critical processes in postnatal brain development,
 including axonal and dendritic growth, branching, and tiling, as well as synapse formation and refinement. This method has been used successfully 
in our own lab (unpublished results) and others8,9, and can be extended to other viruses, such as lentivirus 9, as well as to the expression of 
shRNA or dominant active proteins 10. Furthermore, by combining this technique with electrophysiology as well as recently-developed optical 
imaging tools 11, this method provides a new strategy to study how genetic pathways influence neural circuit development and function in mice 
and rats.

An in vivo method to test gene function in postnatal brain is described. Recombinant AAVs expressing Cre and/or a fluorescent protein are injected into neonatal mouse brain. Mosaic gene inactivation and sparse neuronal labeling are achieved, allowing rapid analysis of gene function in processes critical to neural circuit development.

Normal brain function relies not only on embryonic development when major neuronal pathways are established, but also on postnatal 
development when ...

Surgical Transplantation of Mouse Neural Stem Cells into the Spinal Cords of Mice Infected with Neurotropic Mouse Hepatitis Virus

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (MHV) develop pathological and clinical outcomes similar to patients with the demyelinating disease Multiple Sclerosis (MS). We have shown that transplantation of NSCs into the spinal cords of sick mice results in a significant improvement in both remyelination and in clinical outcome. Cell replacement therapies for the treatment of chronic neurologic diseases are now a reality and in vivo models are vital in understanding the interactions between the engrafted cells and host tissue microenvironment. This presentation provides an adapted method for transplanting cells into the spinal cord of JHMV-infected mice. In brief, we provide a procedure for i) preparation of NSCs prior to transplant, ii) pre-operative care of mice, iii) exposure of the spinal cord via laminectomy, iv) stereotactic injection of NSCs, and iv) post-operative care.

The transplantation of mouse neural stem cells (NSCs) into the spinal cords of mice with established demyelination is detailed. The preparation of NSCs, the laminectomy of thoracic vertebra 9 (T9), and transplantation of NSCs is outlined along with the pre- and post-operative care of the mice.

Mice infected with the neurotropic JHM strain of mouse hepatitis virus (MHV) develop pathological and clinical outcomes similar to patients with the ...

Micron-scale Resolution Optical Tomography of Entire Mouse Brains with Confocal Light Sheet Microscopy

Understanding the architecture of mammalian brain at single-cell resolution is one of the key issues of neuroscience. However, mapping neuronal soma and projections throughout the whole brain is still challenging for imaging and data management technologies. Indeed, macroscopic volumes need to be reconstructed with high resolution and contrast in a reasonable time, producing datasets in the TeraByte range. We recently demonstrated an optical method (confocal light sheet microscopy, CLSM) capable of obtaining micron-scale reconstruction of entire mouse brains labeled with enhanced green fluorescent protein (EGFP). Combining light sheet illumination and confocal detection, CLSM allows deep imaging inside macroscopic cleared specimens with high contrast and speed. Here we describe the complete experimental pipeline to obtain comprehensive and human-readable images of entire mouse brains labeled with fluorescent proteins. The clearing and the mounting procedures are described, together with the steps to perform an optical tomography on its whole volume by acquiring many parallel adjacent stacks. We showed the usage of open-source custom-made software tools enabling stitching of the multiple stacks and multi-resolution data navigation. Finally, we illustrated some example of brain maps: the cerebellum from an L7-GFP transgenic mouse, in which all Purkinje cells are selectively labeled, and the whole brain from a thy1-GFP-M mouse, characterized by a random sparse neuronal labeling.

In this article we describe the full experimental procedure to reconstruct, with high resolution, the fine brain anatomy of fluorescently labeled mouse brains. The described protocol includes sample preparation and clearing, specimen mounting for imaging, data post-processing and multi-scale visualization.

Understanding the architecture of mammalian  brain at single-cell resolution is one of the key issues of neuroscience.  However, mapping neuronal soma and ...

In vivo Postnatal Electroporation and Time-lapse Imaging of Neuroblast Migration in Mouse Acute Brain Slices

The subventricular zone (SVZ) is one of the main neurogenic niches in the postnatal brain. Here, neural progenitors proliferate and give rise to neuroblasts able to move along the rostral migratory stream (RMS) towards the olfactory bulb (OB). This long-distance migration is required for the subsequent maturation of newborn neurons in the OB, but the molecular mechanisms regulating this process are still unclear. Investigating the signaling pathways controlling neuroblast motility may not only help understand a fundamental step in neurogenesis, but also have therapeutic regenerative potential, given the ability of these neuroblasts to target brain sites affected by injury, stroke, or degeneration.
In this manuscript we describe a detailed protocol for in vivo postnatal electroporation and subsequent time-lapse imaging of neuroblast migration in the mouse RMS. Postnatal electroporation can efficiently transfect SVZ progenitor cells, which in turn generate neuroblasts migrating along the RMS. Using confocal spinning disk time-lapse microscopy on acute brain slice cultures, neuroblast migration can be monitored in an environment closely resembling the in vivo condition. Moreover, neuroblast motility can be tracked and quantitatively analyzed. As an example, we describe how to use in vivo postnatal electroporation of a GFP-expressing plasmid to label and visualize neuroblasts migrating along the RMS. Electroporation of shRNA or CRE recombinase-expressing plasmids in conditional knockout mice employing the LoxP system can also be used to target genes of interest. Pharmacological manipulation of acute brain slice cultures can be performed to investigate the role of different signaling molecules in neuroblast migration. By coupling in vivo electroporation with time-lapse imaging, we hope to understand the molecular mechanisms controlling neuroblast motility and contribute to the development of novel approaches to promote brain repair.

In vivo Postnatal Electroporation and Time-lapse Imaging of Neuroblast Migration in Mouse Acute Brain Slices

Neuroblast migration is a fundamental event in postnatal neurogenesis. We describe a protocol for efficient labeling of neuroblasts by in vivo postnatal electroporation and subsequent visualization of their migration using time-lapse imaging of acute brain slices. We include a description for the quantitative analysis of neuroblast dynamics by video tracking.

The subventricular zone (SVZ) is one of the main neurogenic niches in the postnatal brain. Here, neural progenitors proliferate and give rise to ...

Differentiation of Mouse Embryonic Stem Cells into Cortical Interneuron Precursors

GABAergic cortical interneurons are a heterogeneous population of cells that play critical roles in regulating the output of excitatory pyramidal neurons as well as synchronizing the outputs of pyramidal neuron ensembles. Deficits in interneuron function have been implicated in a variety of neuropsychiatric disorders, including schizophrenia, autism, and epilepsy. The derivation of cortical interneurons from embryonic stem cells not only allows for the study of their development and function, but provides insight into the molecular mechanisms underlying the pathogenesis of cortical interneuron-related disorders. Interneurons also have the remarkable capacity to survive, migrate, and integrate into host cortical circuitry post-transplantation, making them ideal candidates for use in cell-based therapies. Here, we present a scalable, highly efficient, modified embryoid body-to-monolayer method for the derivation of Nkx2.1-expressing interneuron progenitors and their progeny from mouse embryonic stem cells (mESCs). Using a Nkx2.1::mCherry:Lhx6::GFP dual reporter mESC line, Nkx2.1 progenitors or their Lhx6-expressing post-mitotic progeny can be isolated via fluorescence-activated cell sorting (FACS) and subsequently used in a number of downstream applications. We also provide methods to enrich for parvalbumin (PV) or somatostatin (SST) interneuron subgroups, which may be helpful for studying aspects of fate determination or for use in therapeutic applications that would benefit from interneuron subgroup-enriched transplantations.

This protocol describes a method for generating cortical interneuron progenitors and post-mitotic interneuron precursors from mouse embryonic stem cells using a modified embryoid body-to-monolayer method. These progenitors/precursors can be used in vitro or fluorescently sorted and transplanted into neonatal neocortex for studying fate determination, or used in therapeutic applications.

GABAergic cortical interneurons are a heterogeneous population of cells that play critical roles in regulating the output of excitatory pyramidal neurons ...

High-resolution Confocal Imaging of the Blood-brain Barrier: Imaging, 3D Reconstruction, and Quantification of Transcytosis

The blood-brain barrier (BBB) is a dynamic multicellular interface that regulates the transport of molecules between the circulation and the brain. Transcytosis across the BBB regulates the delivery of hormones, metabolites, and therapeutic antibodies to the brain parenchyma. Here, we present a protocol that combines immunofluorescence of free-floating sections with laser scanning confocal microscopy and image analysis to visualize subcellular organelles within endothelial cells at the BBB. Combining this data-set with 3D image analysis software allows for the semi-automated segmentation and quantification of capillary volume and surface area, as well as the number and intensity of intracellular organelles at the BBB. The detection of mouse endogenous immunoglobulin (IgG) within intracellular vesicles and their quantification at the BBB is used to illustrate the method. This protocol can potentially be applied to the investigation of the mechanisms controlling BBB transcytosis of different molecules in vivo.

Here we present a microscopy-based protocol for high-resolution imaging and a three-dimensional reconstruction of the mouse neurovascular unit and blood-brain barrier using brain free-floating sections. This method allows for the visualization, analysis, and quantification of intracellular organelles at the BBB.

The blood-brain barrier (BBB) is a dynamic multicellular interface that regulates the transport of molecules between the circulation and the brain. ...

Stereotaxic Surgery for Genetic Manipulation in Striatal Cells of Neonatal Mouse Brains

Many genes are expressed in embryonic brains, and some of them are continuously expressed in the brain after birth. For such persistently expressed genes, they may function to regulate the developmental process and/or physiological function in neonatal brains. To investigate neurobiological functions of specific genes in the brain, it is essential to inactivate genes in the brain. Here, we describe a simple stereotaxic method to inactivate gene expression in the striatum of transgenic mice at neonatal time windows. AAV-eGFP-Cre viruses were microinjected into the striatum of Ai14 reporter gene mice at postnatal day (P) 2 by stereotaxic brain surgery. The tdTomato reporter gene expression was detected in P14 striatum, suggesting a successful Cre-loxP mediated DNA recombination in AAV-transduced striatal cells. We further validated this technique by microinjecting AAV-eGFP-Cre viruses into P2Foxp2fl/fl mice. Double labeling of GFP and Foxp2 showed that GFP-positive cells lacked Foxp2 immunoreactivity in P9 striatum, suggesting the loss of Foxp2 protein in AAV-eGFP-Cre transduced striatal cells. Taken together, these results demonstrate an effective genetic deletion by stereotaxically microinjected AAV-eGFP-Cre viruses in specific neuronal populations in the neonatal brains of floxed transgenic mice. In conclusion, our stereotaxic technique provides an easy and simple platform for genetic manipulation in neonatal mouse brains. The technique can not only be used to delete genes in specific regions of neonatal brains, but it also can be used to inject pharmacological drugs, neuronal tracers, genetically modified optogenetics and chemogenetics proteins, neuronal activity indicators and other reagents into the striatum of neonatal mouse brains.

We describe a protocol of stereotaxic surgery with a homemade head-fixed device for microinjecting reagents into the striatum of neonatal mouse brains. This technique allows genetic manipulation in neuronal cells of specific regions of neonatal mouse brains.

Many genes are expressed in embryonic brains, and some of them are continuously expressed in the brain after birth. For such persistently expressed genes, ...

Methods for the Discovery of Novel Compounds Modulating a Gamma-Aminobutyric Acid Receptor Type A Neurotransmission

This manuscript presents a step-by-step protocol for screening compounds at gamma-aminobutyric acid type A (GABAA) receptors and its use towards the identification of novel molecules active in preclinical assays from an in vitro recombinant receptor to their pharmacological effects at native receptors in rodent brain slices. For compounds binding at the benzodiazepine site of the receptor, the first step is to set up a primary screen that consists of developing radioligand binding assays on cell membranes expressing the major GABAA subtypes. Then, taking advantage of the heterologous expression of rodent and human GABAA receptors in Xenopus oocytes or HEK 293 cells, it is possible to explore, in electrophysiological assays, the physiological properties of the different receptor subtypes and the pharmacological properties of the identified compounds. The Xenopus oocyte system will be presented here, starting with the isolation of the oocytes and their microinjection with different mRNAs, up to the pharmacological characterization using two-electrode voltage clamps. Finally, recordings conducted in rodent brain slices will be described that are used as a secondary physiological test to assess the activity of molecules at their native receptors in a well-defined neuronal circuit. Extracellular recordings using population responses of multiple neurons are demonstrated together with the drug application.

Here, we present protocols to discover compounds active at GABAA receptors, from the binding to the physiology and pharmacology.

This manuscript presents a step-by-step protocol for screening compounds at gamma-aminobutyric acid type A (GABAA) receptors and its use towards the ...

Simultaneous Cryosectioning of Multiple Rodent Brains

Histology and immunohistochemistry are routine methods of analysis to visualize microscopic anatomy and localize proteins within biological tissue. In neuroscience, as well as a plethora of other scientific fields, these techniques are used. Immunohistochemistry can be done on slide mounted tissue or free-floating sections. Preparing slide-mounted samples is a time intensive process. The following protocol for a technique, called the Megabrain, reduced the time taken to cryosection and mount brain tissue by up to 90% by combining multiple brains into a single frozen block. Furthermore, this technique reduced variability seen between staining rounds, in a large histochemical study. The current technique has been optimized for using rodent brain tissue in downstream immunohistochemical analyses; however, it can be applied to different scientific fields that use cryosectioning.

Here, we present a protocol to freeze and section brain tissue from multiple animals as a timesaving alternative to processing single brains. This reduces staining variability during immunohistochemistry and reduces time cryosectioning and imaging.

Histology and immunohistochemistry are routine methods of analysis to visualize microscopic anatomy and localize proteins within biological tissue. In ...

Transplantation of Chemogenetically Engineered Cortical Interneuron Progenitors into Early Postnatal Mouse Brains

Neuronal development is regulated by a complex combination of environmental and genetic factors. Assessing the relative contribution of each component is a complicated task, which is particularly difficult in regards to the development of &#947;-aminobutyric acid (GABA)ergic cortical interneurons (CIs). CIs are the main inhibitory neurons in the cerebral cortex, and they play key roles in neuronal networks, by regulating both the activity of individual pyramidal neurons, as well as the oscillatory behavior of neuronal ensembles. They are generated in transient embryonic structures (medial and caudal ganglionic eminences -&#160;MGE and CGE) that are very difficult to efficiently target using in utero electroporation approaches. Interneuron progenitors migrate long distances during normal embryonic development, before they integrate in the cortical circuit. This remarkable ability to disperse and integrate into a developing network can be hijacked by transplanting embryonic interneuron precursors into early post-natal host cortices. Here, we present a protocol that allows genetic modification of embryonic interneuron progenitors using focal ex vivo electroporation. These engineered interneuron precursors are then transplanted into early post-natal host cortices, where they will mature into easily identifiable CIs. This protocol allows the use of multiple genetically encoded tools, or the ability to regulate the expression of specific genes in interneuron progenitors, in order to investigate the impact of either genetic or environmental variables on the maturation and integration of CIs.

Here we present a protocol, designed to use chemogenetic tools to manipulate the activity of cortical interneuron progenitors transplanted into the cortex of early postnatal mice.

Neuronal development is regulated by a complex combination of environmental and genetic factors. Assessing the relative contribution of each component is ...