Efficient gene delivery into multiple CNS territories using in utero electroporation

Microglia morphophysiological diversity and its implications for the CNS

Microglia are mononuclear phagocytes of mesodermal origin that migrate to the central nervous system (CNS) during the early stages of embryonic development. After colonizing the CNS, they proliferate and remain able to self-renew throughout life, maintaining the number of microglia around 5-12% of the cells in the CNS parenchyma. They are considered to play key roles in development, homeostasis and innate immunity of the CNS. Microglia are exceptionally diverse in their morphological characteristics, actively modifying the shape of their processes and soma in response to different stimuli. This broad morphological spectrum of microglia responses is considered to be closely correlated to their diverse range of functions in health and disease. However, the morphophysiological attributes of microglia, and the structural and functional features of microglia-neuron interactions, remain largely unknown. Here, we assess the current knowledge of the diverse microglial morphologies, with a focus on the correlation between microglial shape and function. We also outline some of the current challenges, opportunities, and future directions that will help us to tackle unanswered questions about microglia, and to continue unravelling the mysteries of microglia, in all its shapes.

Live-cell imaging of microglial interactions with radial glia in transgenic embryonic mouse brains using slice culture

Microglial dynamics and interactions with nearby radial glia can be visualized in real time in embryonic mouse brain tissue using time-lapse imaging in slice culture. This live-cell imaging protocol can be used to study the morphology and activities of a number of cell types across a variety of brain regions and developmental time points. The advantage of this brain slice culture model is that it allows for the visualization of cellular interactions and movements in real time, especially across embryogenesis.

For complete details on the use and execution of this protocol, please refer to Rosin et al. (2021).

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Microglia and radial glia interactions can be captured in embryonic brain slices

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Microglial dynamics and interactions with nearby cells can be visualized in real time

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Live-cell imaging can be used to study various cell types in the developing brain

A subpopulation of embryonic microglia respond to maternal stress and influence nearby neural progenitors

The interplay between hypothalamic neurons and microglia as they integrate stressors to regulate homeostasis is of growing interest. We asked if microglia in the embryonic hypothalamus were likewise stress responsive and, if so, whether their precocious activation perturbs nearby neural stem cell (NSC) programs. We performed single-cell transcriptomics to define embryonic hypothalamic microglia heterogeneity and identified four microglial subsets, including a subpopulation adjacent to NSCs that was responsive to gestational cold stress. Stress exposure elevated CCL3 and CCL4 secretion, but only in male brains, and ex vivo CCL4 treatment of hypothalamic NSCs altered proliferation and differentiation. Concomitantly, gestational stress decreased PVN oxytocin neurons only in male embryos, which was reversed by microglia depletion. Adult offspring exposed to gestational stress displayed altered social behaviors, which was likewise microglia dependent, but only in males. Collectively, immature hypothalamic microglia play an unappreciated role in translating maternal stressors to sexually dimorphic perturbation of neurodevelopmental programs.

Ascl1 is required to specify a subset of ventromedial hypothalamic neurons

Despite clear physiological roles, the ventromedial hypothalamus (VMH) developmental programs are poorly understood. Here, we asked whether the proneural gene achaete-scute homolog 1 (Ascl1) contributes to VMH development. Ascl1 transcripts were detected in embryonic day (E) 10.5 to postnatal day 0 VMH neural progenitors. The elimination of Ascl1 reduced the number of VMH neurons at E12.5 and E15.5, particularly within the VMH-central (VMHC) and -dorsomedial (VMHDM) subdomains, and resulted in a VMH cell fate change from glutamatergic to GABAergic. We observed a loss of Neurog3 expression in Ascl1-/- hypothalamic progenitors and an upregulation of Neurog3 when Ascl1 was overexpressed. We also demonstrated a glutamatergic to GABAergic fate switch in Neurog3-null mutant mice, suggesting that Ascl1 might act via Neurog3 to drive VMH cell fate decisions. We also showed a concomitant increase in expression of the central GABAergic fate determinant Dlx1/2 in the Ascl1-null hypothalamus. However, Ascl1 was not sufficient to induce an ectopic VMH fate when overexpressed outside the normal window of competency. Combined, Ascl1 is required but not sufficient to specify the neurotransmitter identity of VMH neurons, acting in a transcriptional cascade with Neurog3.

Plag1 and Plagl2 have overlapping and distinct functions in telencephalic development

The Plag gene family has three members; Plagl1/Zac1, which is a tumor suppressor gene, and Plag1 and Plagl2, which are proto-oncogenes. All three genes are known to be expressed in embryonic neural progenitors, and Zac1 regulates proliferation, neuronal differentiation and migration in the developing neocortex. Here we examined the functions of Plag1 and Plagl2 in neocortical development. We first attempted, and were unable to generate, E12.5 Plag1;Plagl2 double mutants, indicating that at least one Plag1 or Plagl2 gene copy is required for embryonic survival. We therefore focused on single mutants, revealing a telencephalic patterning defect in E12.5 Plagl2 mutants and a proliferation/differentiation defect in Plag1 mutant neocortices. Specifically, the ventral pallium, a dorsal telencephalic territory, expands into the ventral telencephalon in Plagl2 mutants. In contrast, Plag1 mutants develop normal regional territories, but neocortical progenitors proliferate less and instead produce more neurons. Finally, in gain-of-function studies, both Plag1 and Plagl2 reduce neurogenesis and increase BrdU-uptake, indicative of enhanced proliferation, but while Plagl2 effects on proliferation are more immediate, Plag1 effects are delayed. Taken together, we found that the Plag proto-oncogenes genes are essential regulators of neocortical development and although Plag1 and Plagl2 functions are similar, they do not entirely overlap. This article has an associated First Person interview with the first author of the paper.

A non-canonical role for the proneural gene Neurog1 as a negative regulator of neocortical neurogenesis

Neural progenitors undergo temporal identity transitions to sequentially generate the neuronal and glial cells that make up the mature brain. Proneural genes have well-characterised roles in promoting neural cell differentiation and subtype specification, but they also regulate the timing of identity transitions through poorly understood mechanisms. Here, we investigated how the highly related proneural genes Neurog1 and Neurog2 interact to control the timing of neocortical neurogenesis. We found that Neurog1 acts in an atypical fashion as it is required to suppress rather than promote neuronal differentiation in early corticogenesis. In Neurog1-/- neocortices, early born neurons differentiate in excess, whereas, in vitro, Neurog1-/- progenitors have a decreased propensity to proliferate and form neurospheres. Instead, Neurog1-/- progenitors preferentially generate neurons, a phenotype restricted to the Neurog2+ progenitor pool. Mechanistically, Neurog1 and Neurog2 heterodimerise, and while Neurog1 and Neurog2 individually promote neurogenesis, misexpression together blocks this effect. Finally, Neurog1 is also required to induce the expression of neurogenic factors (Dll1 and Hes5) and to repress the expression of neuronal differentiation genes (Fezf2 and Neurod6). Neurog1 thus employs different mechanisms to temper the pace of early neocortical neurogenesis.

In utero electroporation induces cell death and alters embryonic microglia morphology and expression signatures in the developing hypothalamus

Background

Since its inception in 2001, in utero electroporation (IUE) has been widely used by the neuroscience community. IUE is a technique developed to introduce plasmid DNA into embryonic mouse brains without permanently removing the embryos from the uterus. Given that IUE labels cells that line the ventricles, including radial fibers and migrating neuroblasts, this technique is an excellent tool for studying factors that govern neural cell fate determination and migration in the developing mouse brain. Whether IUE has an effect on microglia, the immune cells of the central nervous system (CNS), has yet to be investigated.

Methods

We used IUE and the pCIG2, pCIC-Ascl1, or pRFP-C-RS expression vectors to label radial glia lining the ventricles of the embryonic cortex and/or hypothalamus. Specifically, we conducted IUE at E14.5 and harvested the brains at E15.5 or E17.5. Immunohistochemistry, along with cytokine and chemokine analyses, were performed on embryonic brains with or without IUE exposure.

Results

IUE using the pCIG2, pCIC-Ascl1, or pRFP-C-RS vectors alone altered microglia morphology, where the majority of microglia near the ventricles were amoeboid and displayed altered expression signatures, including the upregulation of Cd45 and downregulation of P2ry12. Moreover, IUE led to increases in P2ry12⁻ cells that were Iba1⁺/IgG⁺ double-positive in the brain parenchyma and resembled macrophages infiltrating the brain proper from the periphery. Furthermore, IUE resulted in a significant increase in cell death in the developing hypothalamus, with concomitant increases in cytokines and chemokines known to be released during pro-inflammatory states (IL-1β, IL-6, MIP-2, RANTES, MCP-1). Interestingly, the cortex was protected from elevated cell death following IUE, implying that microglia that reside in the hypothalamus might be particularly sensitive during embryonic development.

Conclusions

Our results suggest that IUE might have unintended consequences of activating microglia in the embryonic brain, which could have long-term effects, particularly within the hypothalamus.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1213-6) contains supplementary material, which is available to authorized users.

Inducing Cre-lox Recombination in Mouse Cerebral Cortex Through In Utero Electroporation

Cell-autonomous neuronal functions of genes can be revealed by causing loss or gain of function of a gene in a small and sparse population of neurons. To do so requires generating a mosaic in which neurons with loss or gain of function of a gene are surrounded by genetically unperturbed tissue. Here, we combine the Cre-lox recombination system with in utero electroporation in order to generate mosaic brain tissue that can be used to study the cell-autonomous function of genes in neurons. DNA constructs (available through repositories), coding for a fluorescent label and Cre recombinase, are introduced into developing cortical neurons containing genes flanked with loxP sites in the brains of mouse embryos using in utero electroporation. Additionally, we describe various adaptations to the in utero electroporation method that increase survivability and reproducibility. This method also involves establishing a titer for Cre-mediated recombination in a sparse or dense population of neurons. Histological preparations of labeled brain tissue do not require (but can be adapted to) immunohistochemistry. The constructs used guarantee that fluorescently labeled neurons carry the gene for Cre recombinase. Histological preparations allow morphological analysis of neurons through confocal imaging of dendritic and axonal arbors and dendritic spines. Because loss or gain of function is achieved in sparse mosaic tissue, this method permits the study of cell-autonomous necessity and sufficiency of gene products in vivo.

Neurog2 and Ascl1 together regulate a postmitotic derepression circuit to govern laminar fate specification in the murine neocortex

A derepression mode of cell-fate specification involving the transcriptional repressors Tbr1, Fezf2, Satb2, and Ctip2 operates in neocortical projection neurons to specify six layer identities in sequence. Less well understood is how laminar fate transitions are regulated in cortical progenitors. The proneural genes Neurog2 and Ascl1 cooperate in progenitors to control the temporal switch from neurogenesis to gliogenesis. Here we asked whether these proneural genes also regulate laminar fate transitions. Several defects were observed in the derepression circuit in Neurog2^-/-;Ascl1^-/- mutants: an inability to repress expression of Tbr1 (a deep layer VI marker) during upper-layer neurogenesis, a loss of Fezf2⁺/Ctip2⁺ layer V neurons, and precocious differentiation of normally late-born, Satb2⁺ layer II-IV neurons. Conversely, in stable gain-of-function transgenics, Neurog2 promoted differentiative divisions and extended the period of Tbr1⁺/Ctip2⁺ deep-layer neurogenesis while reducing Satb2⁺ upper-layer neurogenesis. Similarly, acute misexpression of Neurog2 in early cortical progenitors promoted Tbr1 expression, whereas both Neurog2 and Ascl1 induced Ctip2. However, Neurog2 was unable to influence the derepression circuit when misexpressed in late cortical progenitors, and Ascl1 repressed only Satb2. Nevertheless, neurons derived from late misexpression of Neurog2 and, to a lesser extent, Ascl1, extended aberrant subcortical axon projections characteristic of early-born neurons. Finally, Neurog2 and Ascl1 altered the expression of Ikaros and Foxg1, known temporal regulators. Proneural genes thus act in a context-dependent fashion as early determinants, promoting deep-layer neurogenesis in early cortical progenitors via input into the derepression circuit while also influencing other temporal regulators.

Regulation of Brn3b by DLX1 and DLX2 is required for retinal ganglion cell differentiation in the vertebrate retina

Regulated retinal ganglion cell (RGC) differentiation and axonal guidance is required for a functional visual system. Homeodomain and basic helix-loop-helix transcription factors are required for retinogenesis, as well as patterning, differentiation and maintenance of specific retinal cell types. We hypothesized that Dlx1, Dlx2 and Brn3b homeobox genes function in parallel intrinsic pathways to determine RGC fate and therefore generated Dlx1/Dlx2/Brn3b triple-knockout mice. A more severe retinal phenotype was found in the Dlx1/Dlx2/Brn3b-null retinas than was predicted by combining features of the Brn3b single- and Dlx1/Dlx2 double-knockout retinas, including near total RGC loss with a marked increase in amacrine cells in the ganglion cell layer. Furthermore, we discovered that DLX1 and DLX2 function as direct transcriptional activators of Brn3b expression. Knockdown of Dlx2 expression in primary embryonic retinal cultures and Dlx2 gain of function in utero strongly support that DLX2 is both necessary and sufficient for Brn3b expression in vivo. We suggest that ATOH7 specifies RGC-committed progenitors and that Dlx1 and Dlx2 function both downstream of ATOH7 and in parallel, but cooperative, pathways that involve regulation of Brn3b expression to determine RGC fate.

Summary:Dlx1/2 homeobox genes regulate retinal ganglion cell (RGC) differentiation by directly activating Brn3b expression; accordingly, Dlx1/Dlx2/Brn3b triple-knockout mice exhibit near complete RGC loss.

Zac1 Regulates the Differentiation and Migration of Neocortical Neurons via Pac1

Imprinted genes are dosage sensitive, and their dysregulated expression is linked to disorders of growth and proliferation, including fetal and postnatal growth restriction. Common sequelae of growth disorders include neurodevelopmental defects, some of which are indirectly related to placental insufficiency. However, several growth-associated imprinted genes are also expressed in the embryonic CNS, in which their aberrant expression may more directly affect neurodevelopment. To test whether growth-associated genes influence neural lineage progression, we focused on the maternally imprinted gene Zac1. In humans, either loss or gain of ZAC1 expression is associated with reduced growth rates and intellectual disability. To test whether increased Zac1 expression directly perturbs neurodevelopment, we misexpressed Zac1 in murine neocortical progenitors. The effects were striking: Zac1 delayed the transition of apical radial glial cells to basal intermediate neuronal progenitors and postponed their subsequent differentiation into neurons. Zac1 misexpression also blocked neuronal migration, with Zac1-overexpressing neurons pausing more frequently and forming fewer neurite branches during the period when locomoting neurons undergo dynamic morphological transitions. Similar, albeit less striking, neuronal migration and morphological defects were observed on Zac1 knockdown, indicating that Zac1 levels must be regulated precisely. Finally, Zac1 controlled neuronal migration by regulating Pac1 transcription, a receptor for the neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP). Pac1 and Zac1 loss- and gain-of-function presented as phenocopies, and overexpression of Pac1 rescued the Zac1 knockdown neuronal migration phenotype. Thus, dysregulated Zac1 expression has striking consequences on neocortical development, suggesting that misexpression of this transcription factor in the brain in certain growth disorders may contribute to neurocognitive deficits. Significance statement: Altered expression of imprinted genes is linked to cognitive dysfunction and neuropsychological disorders, such as Angelman and Prader-Willi syndromes, and autism spectrum disorder. Mouse models have also revealed the importance of imprinting for brain development, with chimeras generated with parthenogenetic (two maternal chromosomes) or androgenetic (two paternal chromosomes) cells displaying altered brain sizes and cellular defects. Despite these striking phenotypes, only a handful of imprinted genes are known or suspected to regulate brain development (e.g., Dlk1, Peg3, Ube3a, necdin, and Grb10). Herein we show that the maternally imprinted gene Zac1 is a critical regulator of neocortical development. Our studies are relevant because loss of 6q24 maternal imprinting in humans results in elevated ZAC1 expression, which has been associated with neurocognitive defects.

Induction of protein deletion through in utero electroporation to define deficits in neuronal migration in transgenic models

Genetic deletion using the Cre-Lox system in transgenic mouse lines is a powerful tool used to study protein function. However, except in very specific Cre models, deletion of a protein throughout a tissue or cell population often leads to complex phenotypes resulting from multiple interacting mechanisms. Determining whether a phenotype results from disruption of a cell autonomous mechanism, which is intrinsic to the cell in question, or from a non-cell autonomous mechanism, which would result from impairment of that cell's environment, can be difficult to discern. To gain insight into protein function in an in vivo context, in utero electroporation (IUE) enables gene deletion in a small subset of cells within the developing cortex or some other selected brain region. IUE can be used to target specific brain areas, including the dorsal telencephalon, medial telencephalon, hippocampus, or ganglionic eminence. This facilitates observation of the consequences of cell autonomous gene deletion in the context of a healthy environment. The goal of this protocol is to show how IUE can be used to analyze a defect in radial migration in a floxed transgenic mouse line, with an emphasis on distinguishing between the cell autonomous and non-cell autonomous effects of protein deletion. By comparing the phenotype resulting from gene deletion within the entire cortex versus IUE-mediated gene deletion in a limited cell population, greater insight into protein function in brain development can be obtained than by using either technique in isolation.

Two-photon excitation microscopy and its applications in neuroscience

Two-photon excitation (2PE) overcomes many challenges in fluorescence microscopy. Compared to confocal microscopy, 2PE microscopy improves depth penetration, owing to the longer excitation wavelength required and to the ability to collect scattered emission photons as a useful signal. It also minimizes photodamage because lower energy photons are used and because fluorescence is confined to the geometrical focus of the laser spot. 2PE is therefore ideal for high-resolution, deep-tissue, time-lapse imaging of dynamic processes in cell biology. Here, we provide examples of important applications of 2PE for in vivo imaging of neuronal structure and signals; we also describe how it can be combined with optogenetics or photolysis of caged molecules to simultaneously probe and control neuronal activity.

Gene delivery in mouse auditory brainstem and hindbrain using in utero electroporation

BACKGROUND

Manipulation of gene expression via recombinant viral vectors and creation of transgenic knock-out/in animals has revolutionized our understanding of genes that play critical roles during neuronal development and pathophysiology of neurological disorders. Recently, target-specific genetic manipulations are made possible to perform in combination with specific Cre-lines, albeit costly, labor-intensive and time consuming. Thus, alternative methods of gene manipulations to address important biological questions are highly desirable. In this study, we utilized in utero electroporation technique which involves efficient delivery of hindbrain-specific enhancer/promoter construct, Krox20 into the third ventricle of live mouse embryo to investigate green fluorescent protein (GFP) expression pattern in mouse auditory brainstem and other hindbrain neurons.

RESULTS

We created a GFP/DNA construct containing a Krox20 B enhancer and β-globin promoter to drive GFP expression in the hindbrain via injection into the third ventricle of E12 to E13.5 mice. Electrical currents were applied directly to the embryonic hindbrain to allow DNA uptake into the cell. Confocal images were then acquired from fixed brain slices to analyze GFP expression in mouse whole brain at different postnatal stages (P6-P21). By using a cell-type specific enhancer as well as region specific injection and electroporation, robust GFP expression in the cerebellum and auditory brainstem but not in the forebrain was observed. GFP expression in calyx of Held terminals was more robust in <P15 compared to >P15 mice. In contrast, GFP expression in MNTB neurons was more prevalent in >P15 compared to <P15. In regards to the relative expression of GFP versus the synaptic marker Vglut1, percentage fluorescence GFP intensity in the calyx was higher in P11 to P15 than P6 to P10 and P16 to P21 groups.

CONCLUSIONS

Taken together, this technique would potentially allow hindbrain-specific genetic manipulations such as knock-down, knock-in and rescue experiments to unravel critical molecular substrates underpinning functional and morphological remodeling of synapses as well as understanding the pathophysiology of certain neurological disorders targeting not only the auditory brainstem but also other parts of hindbrain, most notably the cerebellum.

RAS/ERK signaling controls proneural genetic programs in cortical development and gliomagenesis

Neural cell fate specification is well understood in the embryonic cerebral cortex, where the proneural genes Neurog2 and Ascl1 are key cell fate determinants. What is less well understood is how cellular diversity is generated in brain tumors. Gliomas and glioneuronal tumors, which are often localized in the cerebrum, are both characterized by a neoplastic glial component, but glioneuronal tumors also have an intermixed neuronal component. A core abnormality in both tumor groups is overactive RAS/ERK signaling, a pro-proliferative signal whose contributions to cell differentiation in oncogenesis are largely unexplored. We found that RAS/ERK activation levels differ in two distinct human tumors associated with constitutively active BRAF. Pilocytic astrocytomas, which contain abnormal glial cells, have higher ERK activation levels than gangliogliomas, which contain abnormal neuronal and glial cells. Using in vivo gain of function and loss of function in the mouse embryonic neocortex, we found that RAS/ERK signals control a proneural genetic switch, inhibiting Neurog2 expression while inducing Ascl1, a competing lineage determinant. Furthermore, we found that RAS/ERK levels control Ascl1's fate specification properties in murine cortical progenitors--at higher RAS/ERK levels, Ascl1(+) progenitors are biased toward proliferative glial programs, initiating astrocytomas, while at moderate RAS/ERK levels, Ascl1 promotes GABAergic neuronal and less glial differentiation, generating glioneuronal tumors. Mechanistically, Ascl1 is phosphorylated by ERK, and ERK phosphoacceptor sites are necessary for Ascl1's GABAergic neuronal and gliogenic potential. RAS/ERK signaling thus acts as a rheostat to influence neural cell fate selection in both normal cortical development and gliomagenesis, controlling Neurog2-Ascl1 expression and Ascl1 function.

Neurog1 and Neurog2 control two waves of neuronal differentiation in the piriform cortex

The three-layered piriform cortex, an integral part of the olfactory system, processes odor information relayed by olfactory bulb mitral cells. Specifically, mitral cell axons form the lateral olfactory tract (LOT) by targeting lateral olfactory tract (lot) guidepost cells in the piriform cortex. While lot cells and other piriform cortical neurons share a pallial origin, the factors that specify their precise phenotypes are poorly understood. Here we show that in mouse, the proneural genes Neurog1 and Neurog2 are coexpressed in the ventral pallium, a progenitor pool that first gives rise to Cajal-Retzius (CR) cells, which populate layer I of all cortical domains, and later to layer II/III neurons of the piriform cortex. Using loss-of-function and gain-of-function approaches, we find that Neurog1 has a unique early role in reducing CR cell neurogenesis by tempering Neurog2's proneural activity. In addition, Neurog1 and Neurog2 have redundant functions in the ventral pallium, acting in two phases to first specify a CR cell fate and later to specify layer II/III piriform cortex neuronal identities. In the early phase, Neurog1 and Neurog2 are also required for lot cell differentiation, which we reveal are a subset of CR neurons, the loss of which prevents mitral cell axon innervation and LOT formation. Consequently, mutation of Trp73, a CR-specific cortical gene, results in lot cell and LOT axon displacement. Neurog1 and Neurog2 thus have unique and redundant functions in the piriform cortex, controlling the timing of differentiation of early-born CR/lot cells and specifying the identities of later-born layer II/III neurons.

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.

Pax6 regulates Tbr1 and Tbr2 expressions in olfactory bulb mitral cells

Tracking olfactory bulb mitral cell development with BrdU labeling, we find that mitral cells are generated from Pax6+ radial glial cells in the ventricular zone of the embryonic olfactory bulb. Unlike cortical projection neurons, postmitotic mitral cell precursors express both Tbr1 and Tbr2. Our tracking experiments revealed that down-regulation of Pax6 preceded up-regulation of Tbrs, and that Tbr1 emerged earlier than Tbr2. Using in utero electroporation, we also show that Pax6 negatively regulates the expression of Tbr1 and Tbr2 in postmitotic mitral cell precursors. Exogenous expression of Pax6 in embryonic olfactory bulb postmitotic precursors decreased the number of cells that progressed to a mitral cell fate. In contrast, exogenous expression of Pax6 resulted in an increase of GABAergic and/or dopaminergic interneurons. These results indicate that Pax6 is a regulator of fate determination of precursor cells.

Expansion of embryonic and adult neural stem cells by in utero electroporation or viral stereotaxic injection

Somatic stem cells can divide to generate additional stem cells (expansion) or more differentiated cell types (differentiation), which is fundamental for tissue formation during embryonic development and tissue homeostasis during adulthood ¹. Currently, great efforts are invested towards controlling the switch of somatic stem cells from expansion to differentiation because this is thought to be fundamental for developing novel strategies for regenerative medicine ^1,2. However, a major challenge in the study and use of somatic stem cell is that their expansion has been proven very difficult to control.

Here we describe a system that allows the control of neural stem/progenitor cell (altogether referred to as NSC) expansion in the mouse embryonic cortex or the adult hippocampus by manipulating the expression of the cdk4/cyclinD1 complex, a major regulator of the G1 phase of the cell cycle and somatic stem cell differentiation ^3,4. Specifically, two different approaches are described by which the cdk4/cyclinD1 complex is overexpressed in NSC in vivo. By the first approach, overexpression of the cell cycle regulators is obtained by injecting plasmids encoding for cdk4/cyclinD1 in the lumen of the mouse telencephalon followed by in utero electroporation to deliver them to NSC of the lateral cortex, thus, triggering episomal expression of the transgenes ^5-8. By the second approach, highly concentrated HIV-derived viruses are stereotaxically injected in the dentate gyrus of the adult mouse hippocampus, thus, triggering constitutive expression of the cell cycle regulators after integration of the viral construct in the genome of infected cells ⁹. Both approaches, whose basic principles were already described by other video protocols ^10-14, were here optimized to i) reduce tissue damage, ii) target wide portions of very specific brain regions, iii) obtain high numbers of manipulated cells within each region, and iv) trigger high expression levels of the transgenes within each cell. Transient overexpression of the transgenes using the two approaches is obtained by different means i.e. by natural dilution of the electroporated plasmids due to cell division or tamoxifen administration in Cre-expressing NSC infected with viruses carrying cdk4/cyclinD1 flanked by loxP sites, respectively ^9,15.

These methods provide a very powerful platform to acutely and tissue-specifically manipulate the expression of any gene in the mouse brain. In particular, by manipulating the expression of the cdk4/cyclinD1 complex, our system allows the temporal control of NSC expansion and their switch to differentiation, thus, ultimately increasing the number of neurons generated in the mammalian brain. Our approach may be critically important for basic research and using somatic stem cells for therapy of the mammalian central nervous system while providing a better understanding of i) stem cell contribution to tissue formation during development, ii) tissue homeostasis during adulthood, iii) the role of adult neurogenesis in cognitive functions, and perhaps, iv) better using somatic stem cells in models of neurodegenerative diseases.

GSK3 temporally regulates neurogenin 2 proneural activity in the neocortex

The neocortex is comprised of six neuronal layers that are generated in a defined temporal sequence. While extrinsic and intrinsic cues are known to regulate the sequential production of neocortical neurons, how these factors interact and function in a coordinated manner is poorly understood. The proneural gene Neurog2 is expressed in progenitors throughout corticogenesis, but is only required to specify early-born, deep-layer neuronal identities. Here, we examined how neuronal differentiation in general and Neurog2 function in particular are temporally controlled during murine neocortical development. We found that Neurog2 proneural activity declines in late corticogenesis, correlating with its phosphorylation by GSK3 kinase. Accordingly, GSK3 activity, which is negatively regulated by canonical Wnt signaling, increases over developmental time, while Wnt signaling correspondingly decreases. When ectopically activated, GSK3 inhibits Neurog2-mediated transcription in cultured cells and Neurog2 proneural activities in vivo. Conversely, a reduction in GSK3 activity promotes the precocious differentiation of later stage cortical progenitors without influencing laminar fate specification. Mechanistically, we show that GSK3 suppresses Neurog2 activity by influencing its choice of dimerization partner, promoting heterodimeric interactions with E47 (Tcfe2a), as opposed to Neurog2-Neurog2 homodimer formation, which occurs when GSK3 activity levels are low. At the functional level, Neurog2-E47 heterodimers have a reduced ability to transactivate neuronal differentiation genes compared with Neurog2-Neurog2 homodimers, both in vitro and in vivo. We thus conclude that the temporal regulation of Neurog2-E47 heterodimerization by GSK3 is a central component of the neuronal differentiation "clock" that coordinates the timing and tempo of neocortical neurogenesis in mouse.