Comparative analysis of proneural gene expression in the embryonic cerebellum

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Revisiting the development of cerebellar inhibitory interneurons in the light of single-cell genetic analyses

The present review aims to provide a short update of our understanding of the inhibitory interneurons of the cerebellum. While these cells constitute but a minority of all cerebellar neurons, their functional significance is increasingly being recognized. For one, inhibitory interneurons of the cerebellar cortex are now known to constitute a clearly more diverse group than their traditional grouping as stellate, basket, and Golgi cells suggests, and this diversity is now substantiated by single-cell genetic data. The past decade or so has also provided important information about interneurons in cerebellar nuclei. Significantly, developmental studies have revealed that the specification and formation of cerebellar inhibitory interneurons fundamentally differ from, say, the cortical interneurons, and define a mode of diversification critically dependent on spatiotemporally patterned external signals. Last, but not least, in the past years, dysfunction of cerebellar inhibitory interneurons could also be linked with clinically defined deficits. I hope that this review, however fragmentary, may stimulate interest and help focus research towards understanding the cerebellum.

Integrated single-cell transcriptomic and epigenetic study of cell state transition and lineage commitment in embryonic mouse cerebellum

Recent studies using single-cell RNA-sequencing have revealed cellular heterogeneity in the developing mammalian cerebellum, yet the regulatory logic underlying this cellular diversity remains to be elucidated. Using integrated single-cell RNA and ATAC analyses, we resolved developmental trajectories of cerebellar progenitors and identified putative trans- and cis-elements that control cell state transition. We reverse engineered gene regulatory networks (GRNs) of each cerebellar cell type. Through in silico simulations and in vivo experiments, we validated the efficacy of GRN analyses and uncovered the molecular control of a posterior transitory zone (PTZ), a distinct progenitor zone residing immediately anterior to the morphologically defined rhombic lip (RL). We showed that perturbing cell fate specification in the PTZ and RL causes posterior cerebellar vermis hypoplasia, the most common cerebellar birth defect in humans. Our study provides a foundation for comprehensive studies of developmental programs of the mammalian cerebellum.

Cerebellar Patterning Defects in Mutant Mice

The cerebellar cortex is highly compartmentalized and serves as a remarkable model for pattern formation throughout the brain. In brief, the adult cerebellar cortex is subdivided into five anteroposterior units—transverse zones—and subsequently, each zone is divided into ∼20 parasagittal stripes. Zone-and-stripe pattern formation involves the interplay of two parallel developmental pathways—one for inhibitory neurons, the second for excitatory. In the inhibitory pathway, progenitor cells of the 4th ventricle generate the Purkinje cells and inhibitory interneurons. In the excitatory pathway, progenitor cells in the upper rhombic lip give rise to the external granular layer, and subsequently to the granular layer of the adult. Both the excitatory and inhibitory developmental pathways are spatially patterned and the interactions of the two generate the complex topography of the adult. This review briefly describes the cellular and molecular mechanisms that underly zone-and-stripe development with a particular focus on mutations known to interfere with normal cerebellar development and the light they cast on the mechanisms of pattern formation.

A recessive PRDM13 mutation results in congenital hypogonadotropic hypogonadism and cerebellar hypoplasia

The positive regulatory (PR) domain containing 13 (PRDM13) putative chromatin modifier and transcriptional regulator functions downstream of the transcription factor PTF1A, which controls GABAergic fate in the spinal cord and neurogenesis in the hypothalamus. Here, we report a recessive syndrome associated with PRDM13 mutation. Patients exhibited intellectual disability, ataxia with cerebellar hypoplasia, scoliosis, and delayed puberty with congenital hypogonadotropic hypogonadism (CHH). Expression studies revealed Prdm13/PRDM13 transcripts in the developing hypothalamus and cerebellum in mouse and human. An analysis of hypothalamus and cerebellum development in mice homozygous for a Prdm13 mutant allele revealed a significant reduction in the number of Kisspeptin (Kiss1) neurons in the hypothalamus and PAX2+ progenitors emerging from the cerebellar ventricular zone. The latter was accompanied by ectopic expression of the glutamatergic lineage marker TLX3. Prdm13-deficient mice displayed cerebellar hypoplasia and normal gonadal structure, but delayed pubertal onset. Together, these findings identify PRDM13 as a critical regulator of GABAergic cell fate in the cerebellum and of hypothalamic kisspeptin neuron development, providing a mechanistic explanation for the cooccurrence of CHH and cerebellar hypoplasia in this syndrome. To our knowledge, this is the first evidence linking disrupted PRDM13-mediated regulation of Kiss1 neurons to CHH in humans.

Common Origin of the Cerebellar Dual Somatotopic Areas Revealed by Tracking Embryonic Purkinje Cell Clusters with Birthdate Tagging

One of the notable characteristics of the functional localization in the cerebellar cortex is the dual representation of the body (somatotopy) on its anterior-posterior axis. This somatotopy is conspicuous in the C1/C3 module, which is demarcated as the multiple zebrin-negative and weekly-positive stripes in dual paravermal areas in anterior and posterior lobules within the cerebellar compartments. In this report, we describe the early formation process of the cerebellar compartmentalization, particularly in the C1/C3 module. As developing PCs guide formation of the module-specific proper neuronal circuits in the cerebellum, we hypothesized that the rearrangement of embryonic Purkinje cell (PC) clusters shapes the adult cerebellar compartmentalization. By identifying PC clusters with immunostaining of marker molecules and genetical birthdate-tagging with Neurog2-CreER (G2A) mice, we clarified the three-dimensional spatial organization of the PC clusters and tracked the lineage relationships among the PC clusters from embryonic day 14.5 (E14.5) till E17.5. The number of recognized clusters increased from 9 at E14.5 to 37 at E17.5. Among E14.5 PC clusters, the c-l (central-lateral) cluster which lacked E10.5-born PCs divided into six c-l lineage clusters. They separately migrated underneath other clusters and positioned far apart mediolaterally as well as rostrocaudally by E17.5. They were eventually transformed mainly into multiple separate zebrin-negative and weakly-positive stripes, which together configured the adult C1/C3 module, in the anterior and posterior paravermal lobules. The results indicate that the spatial rearrangement of embryonic PC clusters is involved in forming the dual somatotopic areas in the adult mouse paravermal cerebellar cortex.

Gsx2 is required for specification of neurons in the inferior olivary nuclei from Ptf1a-expressing neural progenitors in zebrafish

Neurons in the inferior olivary nuclei (IO neurons) send climbing fibers to Purkinje cells to elicit functions of the cerebellum. IO neurons and Purkinje cells are derived from neural progenitors expressing the proneural gene ptf1a In this study, we found that the homeobox gene gsx2 was co-expressed with ptf1a in IO progenitors in zebrafish. Both gsx2 and ptf1a zebrafish mutants showed a strong reduction or loss of IO neurons. The expression of ptf1a was not affected in gsx2 mutants, and vice versa. In IO progenitors, the ptf1a mutation increased apoptosis whereas the gsx2 mutation did not, suggesting that ptf1a and gsx2 are regulated independently of each other and have distinct roles. The fibroblast growth factors (Fgf) 3 and 8a, and retinoic acid signals negatively and positively, respectively, regulated gsx2 expression and thereby the development of IO neurons. mafba and Hox genes are at least partly involved in the Fgf- and retinoic acid-dependent regulation of IO neuronal development. Our results indicate that gsx2 mediates the rostro-caudal positional signals to specify the identity of IO neurons from ptf1a-expressing neural progenitors.

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.

Deconstructing cerebellar development cell by cell

The cerebellum is a pivotal centre for the integration and processing of motor and sensory information. Its extended development into the postnatal period makes this structure vulnerable to a variety of pathologies, including neoplasia. These properties have prompted intensive investigations that reveal not only developmental mechanisms in common with other regions of the neuraxis but also unique strategies to generate neuronal diversity. How the phenotypically distinct cell types of the cerebellum emerge rests on understanding how gene expression differences arise in a spatially and temporally coordinated manner from initially homogeneous cell populations. Increasingly sophisticated fate mapping approaches, culminating in genetic-induced fate mapping, have furthered the understanding of lineage relationships between early- versus later-born cells. Tracing the developmental histories of cells in this way coupled with analysis of gene expression patterns has provided insight into the developmental genetic programmes that instruct cellular heterogeneity. A limitation to date has been the bulk analysis of cells, which blurs lineage relationships and obscures gene expression differences between cells that underpin the cellular taxonomy of the cerebellum. This review emphasises recent discoveries, focusing mainly on single-cell sequencing in mouse and parallel human studies that elucidate neural progenitor developmental trajectories with unprecedented resolution. Complementary functional studies of neural repair after cerebellar injury are challenging assumptions about the stability of postnatal cellular identities. The result is a wealth of new information about the developmental mechanisms that generate cerebellar neural diversity, with implications for human evolution.

Cellular Mechanisms Involved in Cerebellar Microzonation

In the last 50 years, our vision of the cerebellum has vastly evolved starting with Voogd's (1967) description of extracerebellar projections' terminations and how the projection maps transformed the presumptive homogeneity of the cerebellar cortex into a more complex center subdivided into transverse and longitudinal distinct functional zones. The picture became still more complex with Richard Hawkes and colleagues' (Gravel et al., 1987) discovery of the biochemical heterogeneity of Purkinje cells (PCs), by screening their molecular identities with monoclonal antibodies. Antigens were expressed in a parasagittal pattern with subsets of PCs either possessing or lacking the respective antigens, which divided the cerebellar cortex into precise longitudinal compartments that are congruent with the projection maps. The correlation of these two maps in adult cerebellum shows a perfect matching of developmental mechanisms. This review discusses a series of arguments in favor of the essential role played by PCs in organizing the microzonation of the cerebellum during development (the "matching" hypothesis).

Diverse Neuron Properties and Complex Network Dynamics in the Cerebellar Cortical Inhibitory Circuit

Neuronal inhibition can be defined as a spatiotemporal restriction or suppression of local microcircuit activity. The importance of inhibition relies in its fundamental role in shaping signal processing in single neurons and neuronal circuits. In this context, the activity of inhibitory interneurons proved the key to endow networks with complex computational and dynamic properties. In the last 50 years, the prevailing view on the functional role of cerebellar cortical inhibitory circuits was that excitatory and inhibitory inputs sum spatially and temporally in order to determine the motor output through Purkinje cells (PCs). Consequently, cerebellar inhibition has traditionally been conceived in terms of restricting or blocking excitation. This assumption has been challenged, in particular in the cerebellar cortex where all neurons except granule cells (and unipolar brush cells in specific lobules) are inhibitory and fire spontaneously at high rates. Recently, a combination of electrophysiological recordings in vitro and in vivo, imaging, optogenetics and computational modeling, has revealed that inhibitory interneurons play a much more complex role in regulating cerebellar microcircuit functions: inhibition shapes neuronal response dynamics in the whole circuit and eventually regulate the PC output. This review elaborates current knowledge on cerebellar inhibitory interneurons [Golgi cells, Lugaro cells (LCs), basket cells (BCs) and stellate cells (SCs)], starting from their ontogenesis and moving up to their morphological, physiological and plastic properties, and integrates this knowledge with that on the more renown granule cells and PCs. We will focus on the circuit loops in which these interneurons are involved and on the way they generate feed-forward, feedback and lateral inhibition along with complex spatio-temporal response dynamics. In this perspective, inhibitory interneurons emerge as the real controllers of cerebellar functioning.

Functional Outcomes of Cerebellar Malformations

The cerebellum is well-established as a primary center for controlling sensorimotor functions. However, recent experiments have demonstrated additional roles for the cerebellum in higher-order cognitive functions such as language, emotion, reward, social behavior, and working memory. Based on the diversity of behaviors that it can influence, it is therefore not surprising that cerebellar dysfunction is linked to motor diseases such as ataxia, dystonia, tremor, and Parkinson's disease as well to non-motor disorders including autism spectrum disorders (ASD), schizophrenia, depression, and anxiety. Regardless of the condition, there is a growing consensus that developmental disturbances of the cerebellum may be a central culprit in triggering a number of distinct pathophysiological processes. Here, we consider how cerebellar malformations and neuronal circuit wiring impact brain function and behavior during development. We use the cerebellum as a model to discuss the expanding view that local integrated brain circuits function within the context of distributed global networks to communicate the computations that drive complex behavior. We highlight growing concerns that neurological and neuropsychiatric diseases with severe behavioral outcomes originate from developmental insults to the cerebellum.

Early trigeminal ganglion afferents enter the cerebellum before the Purkinje cells are born and target the nuclear transitory zone

In the standard model for the development of climbing and mossy fiber afferent pathways to the cerebellum, the ingrowing axons target the embryonic Purkinje cell somata (around embryonic ages (E13-E16 in mice). In this report, we describe a novel earlier stage in afferent development. Immunostaining for a neurofilament-associated antigen (NAA) reveals the early axon distributions with remarkable clarity. Using a combination of DiI axon tract tracing, analysis of neurogenin1 null mice, which do not develop trigeminal ganglia, and mouse embryos maintained in vitro, we show that the first axons to innervate the cerebellar primordium as early as E9 arise from the trigeminal ganglion. Therefore, early trigeminal axons are in situ before the Purkinje cells are born. Double immunostaining for NAA and markers of the different domains in the cerebellar primordium reveal that afferents first target the nuclear transitory zone (E9-E10), and only later (E10-E11) are the axons, either collaterals from the trigeminal ganglion or a new afferent source (e.g., vestibular ganglia), seen in the Purkinje cell plate. The finding that the earliest axons to the cerebellum derive from the trigeminal ganglion and enter the cerebellar primordium before the Purkinje cells are born, where they seem to target the cerebellar nuclei, reveals a novel stage in the development of the cerebellar afferents.

Zebrin II Is Ectopically Expressed in Microglia in the Cerebellum of Neurogenin 2 Null Mice

Zebrin II/aldolase C expression in the normal cerebellum is restricted to a Purkinje cell subset and is the canonical marker for stripes and zones. This spatial restriction has been confirmed in over 30 species of mammals, birds, fish, etc. In a transgenic mouse model in which the Neurogenin 2 gene has been disrupted (Neurog2^-/-), the cerebellum is smaller than normal and Purkinje cell dendrites are disordered, but the basic zone and stripe architecture is preserved. Here, we show that in the Neurog2^-/- mouse, in addition to the normal Purkinje cell expression, zebrin II is also expressed in a population of cells with a morphology characteristic of microglia. This identity was confirmed by double immunohistochemistry for zebrin II and the microglial marker, Iba1. The expression of zebrin II in cerebellar microglia is not restricted by zone or stripe or lamina. A second zone and stripe marker, PLCβ4, does not show the same ectopic expression. When microglia are compared in control vs. Neurog2^-/- mice, no difference is seen in apparent number or distribution, suggesting that the ectopic zebrin II immunoreactivity in Neurog2^-/- cerebellum reflects an ectopic expression rather than the invasion of a new population of microglia from the periphery. This ectopic expression of zebrin II in microglia is unique as it is not seen in numerous other models of cerebellar disruption, such as in Acp2^-/- mice and in human pontocerebellar hypoplasia. The upregulation of zebrin II in microglia is thus specific to the disruption of Neurog2 downstream pathways, rather than a generic response to a cerebellar disruption.

Specification of diverse cell types during early neurogenesis of the mouse cerebellum

We applied single-cell RNA sequencing to profile genome-wide gene expression in about 9400 individual cerebellar cells from the mouse embryo at embryonic day 13.5. Reiterative clustering identified the major cerebellar cell types and subpopulations of different lineages. Through pseudotemporal ordering to reconstruct developmental trajectories, we identified novel transcriptional programs controlling cell fate specification of populations arising from the ventricular zone and the rhombic lip, two distinct germinal zones of the embryonic cerebellum. Together, our data revealed cell-specific markers for studying the cerebellum, gene-expression cascades underlying cell fate specification, and a number of previously unknown subpopulations that may play an integral role in the formation and function of the cerebellum. Our findings will facilitate new discovery by providing insights into the molecular and cell type diversity in the developing cerebellum.

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.

Sox14 Is Required for a Specific Subset of Cerebello-Olivary Projections

We identify Sox14 as an exclusive marker of inhibitory projection neurons in the lateral and interposed, but not the medial, cerebellar nuclei. Sox14⁺ neurons make up ∼80% of Gad1⁺ neurons in these nuclei and are indistinguishable by soma size from other inhibitory neurons. All Sox14⁺ neurons of the lateral and interposed cerebellar nuclei are generated at approximately E10/10.5 and extend long-range, predominantly contralateral projections to the inferior olive. A small Sox14⁺ population in the adjacent vestibular nucleus "Y" sends an ipsilateral projection to the oculomotor nucleus. Cerebellar Sox14⁺ and glutamatergic projection neurons assemble in non-overlapping populations at the nuclear transition zone, and their integration into a coherent nucleus depends on Sox14 function. Targeted ablation of Sox14⁺ cells by conditional viral expression of diphtheria toxin leads to significantly impaired motor learning. Contrary to expectations, associative learning is unaffected by unilateral Sox14⁺ neuron elimination in the interposed and lateral nuclei.SIGNIFICANCE STATEMENT The cerebellar nuclei are central to cerebellar function, yet how they modulate and process cerebellar inputs and outputs is still primarily unknown. Our study gives a direct insight into how nucleo-olivary projection neurons are generated, their projections, and their function in an intact behaving mouse. These neurons play a critical conceptual role in all models of cerebellar function, and this study represents the first specific analysis of their molecular identity and function and offers a powerful model for future investigation of cerebellar function in motor control and learning.

Insights into cerebellar development and connectivity

The cerebellum has a well-established role in controlling motor functions such coordination, balance, posture, and skilled learning. There is mounting evidence that it might also play a critical role in non-motor functions such as cognition and emotion. It is therefore not surprising that cerebellar defects are associated with a wide array of diseases including ataxia, dystonia, tremor, schizophrenia, dyslexia, and autism spectrum disorder. What is intriguing is that a seemingly uniform circuit that is often described as being "simple" should carry out all of these behaviors. Analyses of how cerebellar circuits develop have revealed that such descriptions massively underestimate the complexity of the cerebellum. The cerebellum is in fact highly patterned and organized around a series of parasagittal stripes and transverse zones. This topographic architecture partitions all cerebellar circuits into functional modules that are thought to enhance processing power during cerebellar dependent behaviors. What are arguably the most remarkable features of cerebellar topography are the developmental processes that produce them. This review is concerned with the genetic and cellular mechanisms that orchestrate cerebellar patterning. We place a major focus on how Purkinje cells control multiple aspects of cerebellar circuit assembly. Using this model, we discuss evidence for how "zebra-like" patterns in Purkinje cells sculpt the cerebellum, how specific genetic cues mediate the process, and how activity refines the patterns into an adult map that is capable of executing various functions. We also discuss how defective Purkinje cell patterning might impact the pathogenesis of neurological conditions.

Conditional deletion of Neurog1 in the cerebellum of postnatal mice delays inhibitory interneuron maturation

The transcriptional programs that drive the generation of diverse GABAergic neuron populations from their common progenitor pools in the developing cerebellum remain unclear. Neurog1 is a pro-neural basic helix-loop-helix transcription factor expressed in GABAergic progenitor cells in the ventricular zone (VZ) of embryos and subsequently in the presumptive white matter (pWM) tracts of developing postnatal mice. Genetic inducible fate-mapping labels Purkinje cells and all inhibitory interneuron cell types of the cerebellar cortex. As conventional Neurog1^Neo knockout (KO) mice are neonatal lethal, we generated Neurog1^loxP mutant mice to examine the effects of conditional Neurog1 deletion on the postnatal development of the cerebellum. Targeted Neurog1 loss-of-function in the developing cerebellum does not result in significant differences in cerebellar morphology or in the number of GABAergic neurons in the cerebellar cortex of mice at postnatal day 21 (P21). To determine the effects of Neurog1 deletion on GABAergic progenitors, we quantified rates of cell proliferation and cell cycle progression or re-entry in embryonic Neurog1^Neo and postnatal Neurog1^loxP mutants. The data revealed no significant effect of Neurog1 loss-of-function on embryonic day 12.5 (E12.5) VZ progenitors or on P5 and P6 progenitors in the pWM at P7. However, 4-5 day pulse-labeling of P5 and P6 progenitors revealed reductions in inhibitory interneuron dispersal from the pWM to the cerebellar cortex in P10 conditional Neurog1^loxP/loxP KO mice. Thus, our conditional Neurog1 KO approach reveals a requirement for Neurog1 activity in inhibitory interneuron cell dispersal from pWM tracts in the developing cerebellum of postnatal mice.

Instructing neuronal identity during CNS development and astroglial-lineage reprogramming: Roles of NEUROG2 and ASCL1

The adult mammalian brain contains an enormous variety of neuronal types, which are generally categorized in large groups, based on their neurochemical identity, hodological properties and molecular markers. This broad classification has allowed the correlation between individual neural progenitor populations and their neuronal progeny, thus contributing to probe the cellular and molecular mechanisms involved in neuronal identity determination during central nervous system (CNS) development. In this review, we discuss the contribution of the proneural genes Neurogenin2 (Neurog2) and Achaete-scute homolog 1 (Ascl1) for the specification of neuronal phenotypes in the developing neocortex, cerebellum and retina. Then, we revise recent data on astroglia cell lineage reprogramming into induced neurons using the same proneural proteins to compare the neuronal phenotypes obtained from astroglial cells originated in those CNS regions. We conclude that Ascl1 and Neurog2 have different contributions to determine neuronal fates, depending on the neural progenitor or astroglial population expressing those proneural factors. Finally, we discuss some possible explanations for these seemingly conflicting effects of Ascl1 and Neurog2 and propose future approaches to further dissect the molecular mechanisms of neuronal identity specification.

Zfp423/ZNF423 regulates cell cycle progression, the mode of cell division and the DNA-damage response in Purkinje neuron progenitors

The Zfp423/ZNF423 gene encodes a 30-zinc-finger transcription factor involved in key developmental pathways. Although null Zfp423 mutants develop cerebellar malformations, the underlying mechanism remains unknown. ZNF423 mutations are associated with Joubert Syndrome, a ciliopathy causing cerebellar vermis hypoplasia and ataxia. ZNF423 participates in the DNA-damage response (DDR), raising questions regarding its role as a regulator of neural progenitor cell cycle progression in cerebellar development. To characterize in vivo the function of ZFP423 in neurogenesis, we analyzed allelic murine mutants in which distinct functional domains are deleted. One deletion impairs mitotic spindle orientation, leading to premature cell cycle exit and Purkinje cell (PC) progenitor pool deletion. The other deletion impairs PC differentiation. In both mutants, cell cycle progression is remarkably delayed and DDR markers are upregulated in cerebellar ventricular zone progenitors. Our in vivo evidence sheds light on the domain-specific roles played by ZFP423 in different aspects of PC progenitor development, and at the same time strengthens the emerging notion that an impaired DDR may be a key factor in the pathogenesis of JS and other ciliopathies.

Distinct Activities of Tfap2A and Tfap2B in the Specification of GABAergic Interneurons in the Developing Cerebellum

GABAergic inhibitory neurons in the cerebellum are subdivided into Purkinje cells and distinct subtypes of interneurons from the same pool of progenitors, but the determinants of this diversification process are not well defined. To explore the transcriptional regulation of the development of cerebellar inhibitory neurons, we examined the role of Tfap2A and Tfap2B in the specification of GABAergic neuronal subtypes in mice. We show that Tfap2A and Tfap2B are expressed in inhibitory precursors during embryonic development and that their expression persists into adulthood. The onset of their expression follows Ptf1a and Olig2, key determinants of GABAergic neuronal fate in the cerebellum; and, their expression precedes Pax2, an interneuron-specific factor. Tfap2A is expressed by all GABAergic neurons, whereas Tfap2B is selectively expressed by interneurons. Genetic manipulation via in utero electroporation (IUE) reveals that Tfap2B is necessary for interneuron specification and is capable of suppressing the generation of excitatory cells. Tfap2A, but not Tfap2B, is capable of inducing the generation of interneurons when misexpressed in the ventricular neuroepithelium. Together, our results demonstrate that the differential expression of Tfap2A and Tfap2B defines subtypes of GABAergic neurons and plays specific, but complementary roles in the specification of interneurons in the developing cerebellum.

Lineage Reprogramming of Astroglial Cells from Different Origins into Distinct Neuronal Subtypes

Astroglial cells isolated from the rodent postnatal cerebral cortex are particularly susceptible to lineage reprogramming into neurons. However, it remains unknown whether other astroglial populations retain the same potential. Likewise, little is known about the fate of induced neurons (iNs) in vivo. In this study we addressed these questions using two different astroglial populations isolated from the postnatal brain reprogrammed either with Neurogenin-2 (Neurog2) or Achaete scute homolog-1 (Ascl1). We show that cerebellum (CerebAstro) and cerebral cortex astroglia (CtxAstro) generates iNs with distinctive neurochemical and morphological properties. Both astroglial populations contribute iNs to the olfactory bulb following transplantation in the postnatal and adult mouse subventricular zone. However, only CtxAstro transfected with Neurog2 differentiate into pyramidal-like iNs after transplantation in the postnatal cerebral cortex. Altogether, our data indicate that the origin of the astroglial population and transcription factors used for reprogramming, as well as the region of integration, affect the fate of iNs.

Wnt5a is a crucial regulator of neurogenesis during cerebellum development

The role of Wnt5a has been extensively explored in various aspects of development but its role in cerebellar development remains elusive. Here, for the first time we unravel the expression pattern and functional significance of Wnt5a in cerebellar development using Wnt5a^−/− and Nestin-Cre mediated conditional knockout mouse models. We demonstrate that loss of Wnt5a results in cerebellar hypoplasia and depletion of GABAergic and glutamatergic neurons. Besides, Purkinje cells of the mutants displayed stunted, poorly branched dendritic arbors. Furthermore, we show that the overall reduction is due to decreased radial glial and granule neuron progenitor cell proliferation. At molecular level we provide evidence for non-canonical mode of action of Wnt5a and its regulation over genes associated with progenitor proliferation. Altogether our findings imply that Wnt5a signaling is a crucial regulator of cerebellar development and would aid in better understanding of cerebellar disease pathogenesis caused due to deregulation of Wnt signaling.

Consensus Paper: Cerebellar Development

The development of the mammalian cerebellum is orchestrated by both cell-autonomous programs and inductive environmental influences. Here, we describe the main processes of cerebellar ontogenesis, highlighting the neurogenic strategies used by developing progenitors, the genetic programs involved in cell fate specification, the progressive changes of structural organization, and some of the better-known abnormalities associated with developmental disorders of the cerebellum.

Olig2 regulates Purkinje cell generation in the early developing mouse cerebellum

The oligodendrocyte transcription factor Olig2 plays a crucial role in the neurogenesis of both spinal cord and brain. In the cerebellum, deletion of both Olig2 and Olig1 results in impaired genesis of Purkinje cells (PCs) and Pax2⁺ interneurons. Here, we perform an independent study to show that Olig2 protein is transiently expressed in the cerebellar ventricular zone (VZ) during a period when PCs are specified. Further analyses demonstrate that Olig2 is expressed in both cerebellar VZ progenitors and early-born neurons. In addition, unlike in the ganglionic eminence of the embryonic forebrain where Olig2 is mostly expressed in proliferating progenitors, Olig2⁺ cells in the cerebellar VZ are in the process of leaving the cell cycle and differentiating into postmitotic neurons. Functionally, deletion of Olig2 alone results in a preferential reduction of PCs in the cerebellum, which is likely mediated by decreased neuronal generation from their cerebellar VZ progenitors. Furthermore, our long-term lineage tracing experiments show that cerebellar Olig gene-expressing progenitors produce PCs but rarely Pax2⁺ interneurons in the developing cerebellum, which opposes the “temporal identity transition” model of the cerebellar VZ progenitors stating that majority of Pax2⁺ interneuron progenitors are transitioned from Olig2⁺ PC progenitors.

Neurog1 Genetic Inducible Fate Mapping (GIFM) Reveals the Existence of Complex Spatiotemporal Cyto-Architectures in the Developing Cerebellum

Neurog1 is a pro-neural basic helix-loop-helix (bHLH) transcription factor expressed in progenitor cells located in the ventricular zone and subsequently the presumptive white matter tracts of the developing mouse cerebellum. We used genetic inducible fate mapping (GIFM) with a transgenic Neurog1-CreER allele to characterize the contributions of Neurog1 lineages to cerebellar circuit formation in mice. GIFM reveals Neurog1-expressing progenitors are fate-mapped to become Purkinje cells and all GABAergic interneuron cell types of the cerebellar cortex but not glia. The spatiotemporal sequence of GIFM is unique to each neuronal cell type. GIFM on embryonic days (E) 10.5 to E12.5 labels Purkinje cells with different medial-lateral settling patterns depending on the day of tamoxifen delivery. GIFM on E11.5 to P7 labels interneurons and the timing of tamoxifen administration correlates with the final inside-to-outside resting position of GABAergic interneurons in the cerebellar cortex. Proliferative status and long-term BrdU retention of GIFM lineages reveals Purkinje cells express Neurog1 around the time they become post-mitotic. In contrast, GIFM labels mitotic and post-mitotic interneurons. Neurog1-CreER GIFM reveals a correlation between the timing of Neurog1 expression and the spatial organization of GABAergic neurons in the cerebellar cortex with possible implications for cerebellar circuit assembly.

Pax2-Islet1 Transgenic Mice Are Hyperactive and Have Altered Cerebellar Foliation

The programming of cell fate by transcription factors requires precise regulation of their time and level of expression. The LIM-homeodomain transcription factor Islet1 (Isl1) is involved in cell-fate specification of motor neurons, and it may play a similar role in the inner ear. In order to study its role in the regulation of vestibulo-motor development, we investigated a transgenic mouse expressing Isl1 under the Pax2 promoter control (Tg^+/−). The transgenic mice show altered level, time, and place of expression of Isl1 but are viable. However, Tg^+/− mice exhibit hyperactivity, including circling behavior, and progressive age-related decline in hearing, which has been reported previously. Here, we describe the molecular and morphological changes in the cerebellum and vestibular system that may cause the hyperactivity of Tg^+/− mice. The transgene altered the formation of folia in the cerebellum, the distribution of calretinin labeled unipolar brush cells, and reduced the size of the cerebellum, inferior colliculus, and saccule. Age-related progressive reduction of calbindin expression was detected in Purkinje cells in the transgenic cerebella. The hyperactivity of Tg^+/− mice is reduced upon the administration of picrotoxin, a non-competitive channel blocker for the γ-aminobutyric acid (GABA) receptor chloride channels. This suggests that the overexpression of Isl1 significantly affects the functions of GABAergic neurons. We demonstrate that the overexpression of Isl1 affects the development and function of the cerebello-vestibular system, resulting in hyperactivity.

Cellular commitment in the developing cerebellum

The mammalian cerebellum is located in the posterior cranial fossa and is critical for motor coordination and non-motor functions including cognitive and emotional processes. The anatomical structure of cerebellum is distinct with a three-layered cortex. During development, neurogenesis and fate decisions of cerebellar primordium cells are orchestrated through tightly controlled molecular events involving multiple genetic pathways. In this review, we will highlight the anatomical structure of human and mouse cerebellum, the cellular composition of developing cerebellum, and the underlying gene expression programs involved in cell fate commitments in the cerebellum. A critical evaluation of the cell death literature suggests that apoptosis occurs in ~5% of cerebellar cells, most shortly after mitosis. Apoptosis and cellular autophagy likely play significant roles in cerebellar development, we provide a comprehensive discussion of their role in cerebellar development and organization. We also address the possible function of unfolded protein response in regulation of cerebellar neurogenesis. We discuss recent advancements in understanding the epigenetic signature of cerebellar compartments and possible connections between DNA methylation, microRNAs and cerebellar neurodegeneration. Finally, we discuss genetic diseases associated with cerebellar dysfunction and their role in the aging cerebellum.

Development of the cerebellum: simple steps to make a 'little brain'

The cerebellum is a pre-eminent model for the study of neurogenesis and circuit assembly. Increasing interest in the cerebellum as a participant in higher cognitive processes and as a locus for a range of disorders and diseases make this simple yet elusive structure an important model in a number of fields. In recent years, our understanding of some of the more familiar aspects of cerebellar growth, such as its territorial allocation and the origin of its various cell types, has undergone major recalibration. Furthermore, owing to its stereotyped circuitry across a range of species, insights from a variety of species have contributed to an increasingly rich picture of how this system develops. Here, we review these recent advances and explore three distinct aspects of cerebellar development - allocation of the cerebellar anlage, the significance of transit amplification and the generation of neuronal diversity - each defined by distinct regulatory mechanisms and each with special significance for health and disease.

Mechanisms regulating GABAergic neuron development

Neurons using gamma-aminobutyric acid (GABA) as their neurotransmitter are the main inhibitory neurons in the mature central nervous system (CNS) and show great variation in their form and function. GABAergic neurons are produced in all of the main domains of the CNS, where they develop from discrete regions of the neuroepithelium. Here, we review the gene expression and regulatory mechanisms controlling the main steps of GABAergic neuron development: early patterning of the proliferative neuroepithelium, production of postmitotic neural precursors, establishment of their identity and migration. By comparing the molecular regulation of these events across CNS, we broadly identify three regions utilizing distinct molecular toolkits for GABAergic fate determination: telencephalon-anterior diencephalon (DLX2 type), posterior diencephalon-midbrain (GATA2 type) and hindbrain-spinal cord (PTF1A and TAL1 types). Similarities and differences in the molecular regulatory mechanisms reveal the core determinants of a GABAergic neuron as well as provide insights into generation of the vast diversity of these neurons.

Ascl1 as a novel player in the Ptf1a transcriptional network for GABAergic cell specification in the retina

In contrast with the wealth of data involving bHLH and homeodomain transcription factors in retinal cell type determination, the molecular bases underlying neurotransmitter subtype specification is far less understood. Using both gain and loss of function analyses in Xenopus, we investigated the putative implication of the bHLH factor Ascl1 in this process. We found that in addition to its previously characterized proneural function, Ascl1 also contributes to the specification of the GABAergic phenotype. We showed that it is necessary for retinal GABAergic cell genesis and sufficient in overexpression experiments to bias a subset of retinal precursor cells towards a GABAergic fate. We also analysed the relationships between Ascl1 and a set of other bHLH factors using an in vivo ectopic neurogenic assay. We demonstrated that Ascl1 has unique features as a GABAergic inducer and is epistatic over factors endowed with glutamatergic potentialities such as Neurog2, NeuroD1 or Atoh7. This functional specificity is conferred by the basic DNA binding domain of Ascl1 and involves a specific genetic network, distinct from that underlying its previously demonstrated effects on catecholaminergic differentiation. Our data show that GABAergic inducing activity of Ascl1 requires the direct transcriptional regulation of Ptf1a, providing therefore a new piece of the network governing neurotransmitter subtype specification during retinogenesis.

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

Cellular and molecular basis of cerebellar development

Historically, the molecular and cellular mechanisms of cerebellar development were investigated through structural descriptions and studying spontaneous mutations in animal models and humans. Advances in experimental embryology, genetic engineering, and neuroimaging techniques render today the possibility to approach the analysis of molecular mechanisms underlying histogenesis and morphogenesis of the cerebellum by experimental designs. Several genes and molecules were identified to be involved in the cerebellar plate regionalization, specification, and differentiation of cerebellar neurons, as well as the establishment of cellular migratory routes and the subsequent neuronal connectivity. Indeed, pattern formation of the cerebellum requires the adequate orchestration of both key morphogenetic signals, arising from distinct brain regions, and local expression of specific transcription factors. Thus, the present review wants to revisit and discuss these morphogenetic and molecular mechanisms taking place during cerebellar development in order to understand causal processes regulating cerebellar cytoarchitecture, its highly topographically ordered circuitry and its role in brain function.

The compartmental restriction of cerebellar interneurons

The Purkinje cells (PC's) of the cerebellar cortex are subdivided into multiple different molecular phenotypes that form an elaborate array of parasagittal stripes. This array serves as a scaffold around which afferent topography is organized. The ways in which cerebellar interneurons may be restricted by this scaffolding are less well-understood. This review begins with a brief survey of cerebellar topography. Next, it reviews the development of stripes in the cerebellum with a particular emphasis on the embryological origins of cerebellar interneurons. These data serve as a foundation to discuss the hypothesis that cerebellar compartment boundaries also restrict cerebellar interneurons, both excitatory [granule cells, unipolar brush cells (UBCs)] and inhibitory (e.g., Golgi cells, basket cells). Finally, it is proposed that the same PC scaffold that restricts afferent terminal fields to stripes may also act to organize cerebellar interneurons.

Development and evolution of cerebellar neural circuits

The cerebellum controls smooth and skillful movements and it is also involved in higher cognitive and emotional functions. The cerebellum is derived from the dorsal part of the anterior hindbrain and contains two groups of cerebellar neurons: glutamatergic and gamma-aminobutyric acid (GABA)ergic neurons. Purkinje cells are GABAergic and granule cells are glutamatergic. Granule and Purkinje cells receive input from outside of the cerebellum from mossy and climbing fibers. Genetic analysis of mice and zebrafish has revealed genetic cascades that control the development of the cerebellum and cerebellar neural circuits. During early neurogenesis, rostrocaudal patterning by intrinsic and extrinsic factors, such as Otx2, Gbx2 and Fgf8, plays an important role in the positioning and formation of the cerebellar primordium. The cerebellar glutamatergic neurons are derived from progenitors in the cerebellar rhombic lip, which express the proneural gene Atoh1. The GABAergic neurons are derived from progenitors in the ventricular zone, which express the proneural gene Ptf1a. The mossy and climbing fiber neurons originate from progenitors in the hindbrain rhombic lip that express Atoh1 or Ptf1a. Purkinje cells exhibit mediolateral compartmentalization determined on the birthdate of Purkinje cells, and linked to the precise neural circuitry formation. Recent studies have shown that anatomy and development of the cerebellum is conserved between mammals and bony fish (teleost species). In this review, we describe the development of cerebellar neurons and neural circuitry, and discuss their evolution by comparing developmental processes of mammalian and teleost cerebellum.

Neurogenin 2 regulates progenitor cell-cycle progression and Purkinje cell dendritogenesis in cerebellar development

By serving as the sole output of the cerebellar cortex, integrating a myriad of afferent stimuli, Purkinje cells (PCs) constitute the principal neuron in cerebellar circuits. Several neurodegenerative cerebellar ataxias feature a selective cell-autonomous loss of PCs, warranting the development of regenerative strategies. To date, very little is known as to the regulatory cascades controlling PC development. During central nervous system development, the proneural gene neurogenin 2 (Neurog2) contributes to many distinct neuronal types by specifying their fate and/or dictating development of their morphological features. By analyzing a mouse knock-in line expressing Cre recombinase under the control of Neurog2 cis-acting sequences we show that, in the cerebellar primordium, Neurog2 is expressed by cycling progenitors cell-autonomously fated to become PCs, even when transplanted heterochronically. During cerebellar development, Neurog2 is expressed in G1 phase by progenitors poised to exit the cell cycle. We demonstrate that, in the absence of Neurog2, both cell-cycle progression and neuronal output are significantly affected, leading to an overall reduction of the mature cerebellar volume. Although PC fate identity is correctly specified, the maturation of their dendritic arbor is severely affected in the absence of Neurog2, as null PCs develop stunted and poorly branched dendrites, a defect evident from the early stages of dendritogenesis. Thus, Neurog2 represents a key regulator of PC development and maturation.

GABAergic neuron specification in the spinal cord, the cerebellum, and the cochlear nucleus

In the nervous system, there are a wide variety of neuronal cell types that have morphologically, physiologically, and histochemically different characteristics. These various types of neurons can be classified into two groups: excitatory and inhibitory neurons. The elaborate balance of the activities of the two types is very important to elicit higher brain function, because its imbalance may cause neurological disorders, such as epilepsy and hyperalgesia. In the central nervous system, inhibitory neurons are mainly represented by GABAergic ones with some exceptions such as glycinergic. Although the machinery to specify GABAergic neurons was first studied in the telencephalon, identification of key molecules, such as pancreatic transcription factor 1a (Ptf1a), as well as recently developed genetic lineage-tracing methods led to the better understanding of GABAergic specification in other brain regions, such as the spinal cord, the cerebellum, and the cochlear nucleus.

Pattern formation during development of the embryonic cerebellum

The patterning of the embryonic cerebellum is vital to establish the elaborate zone and stripe architecture of the adult. This review considers early stages in cerebellar Purkinje cell patterning, from the organization of the ventricular zone to the development of Purkinje cell clusters—the precursors of the adult stripes.

Neuronal subtype specification in the cerebellum and dorsal hindbrain

In the nervous system, there are hundreds to thousands of neuronal cell types that have morphologically, physiologically, and histochemically different characteristics and this diversity may enable us to elicit higher brain function. A better understanding of the molecular machinery by which neuron subtype specification occurs is thus one of the most important issues in brain science. The dorsal hindbrain, including the cerebellum, is a good model system to study this issue because a variety of types of neurons are produced from this region. Recently developed genetic lineage-tracing methods in addition to gene-transfer technologies have clarified a fate map of neurons produced from the dorsal hindbrain and accelerated our understanding of the molecular machinery of neuronal subtype specification in the nervous system.

The genesis of cerebellar GABAergic neurons: fate potential and specification mechanisms

All cerebellar neurons derive from progenitors that proliferate in two germinal neuroepithelia: the ventricular zone (VZ) generates GABAergic neurons, whereas the rhombic lip is the origin of glutamatergic types. Among VZ-derivatives, GABAergic projection neurons, and interneurons are generated according to distinct strategies. Projection neurons (Purkinje cells and nucleo-olivary neurons) are produced at the onset of cerebellar neurogenesis by discrete progenitor pools located in distinct VZ microdomains. These cells are specified within the VZ and acquire mature phenotypes according to cell-autonomous developmental programs. On the other hand, the different categories of inhibitory interneurons derive from a single population of Pax-2-positive precursors that delaminate into the prospective white matter (PWM), where they continue to divide up to postnatal development. Heterotopic/heterochronic transplantation experiments indicate that interneuron progenitors maintain full developmental potentialities up to the end of cerebellar development and acquire mature phenotypes under the influence of environmental cues present in the PWM. Furthermore, the final fate choice occurs in postmitotic cells, rather than dividing progenitors. Extracerebellar cells grafted to the prospective cerebellar white matter are not responsive to local neurogenic cues and fail to adopt clear cerebellar identities. Conversely, cerebellar cells grafted to extracerebellar regions retain typical phenotypes of cerebellar GABAergic interneurons, but acquire type-specific traits under the influence of local cues. These findings indicate that interneuron progenitors are multipotent and sensitive to spatio-temporally patterned environmental signals that regulate the genesis of different categories of interneurons, in precise quantities and at defined times and places.

SMAD4 is essential for generating subtypes of neurons during cerebellar development

Cerebellum development involves the coordinated production of multiple neuronal cell types. The cerebellar primordium contains two germinative zones, the rhombic lip (RL) and the ventricular zone (VZ), which generate different types of glutamatergic and GABAergic neurons, respectively. What regulates the specification and production of glutamatergic and GABAergic neurons as well as the subtypes for each of these two broad classes remains largely unknown. Here we demonstrate with conditional genetic approaches in mice that SMAD4, a major mediator of BMP and TGFβ signaling, is required early in cerebellar development for maintaining the RL and generating subsets of RL-derived glutamatergic neurons, namely neurons of the deep cerebellar nuclei, unipolar brush cells, and the late cohort of granule cell precursors (GCPs). The early cohort of GCPs, despite being deficient for SMAD4, is still generated. In addition, the numbers of GABAergic neurons are reduced in the mutant and the distribution of Purkinje cells becomes abnormal. These studies demonstrate a temporally and spatially restricted requirement for SMAD4 in generating subtypes of cerebellar neurons.

Wnt1 expression temporally allocates upper rhombic lip progenitors and defines their terminal cell fate in the cerebellum

The cerebellum (Cb) controls movement related physiology using a diverse array of morphologically and biochemically distinct neurons. During development, the Cb is derived from rhombomere 1 (r1), an embryonic compartment patterned by a signaling center referred to as the isthmus organizer. The secreted glycoprotein WNT1 is expressed in the midbrain primordia (mesencephalon, mes) and at the posterior limit of the mes. WNT1 plays a pivotal role in maintaining the isthmus organizer and mutations in Wnt1 produce severe Cb defects that are generally attributed to aberrant organizer activity. Interestingly, Wnt1 is also expressed at the most posterior limit of dorsal r1, in a region known as the upper rhombic lip (URL). However, the distribution and molecular identity of Wnt1 expressing progenitors have not been carefully described in r1. We used Wnt1-Venus transgenic mice to generate a molecular map of Wnt1 expressing progenitors in relation to other well characterized Cb biomarkers such as MATH1 (ATOH1), LMX1a and OTX2. Our analysis validated Wnt1 expression in the URL and revealed molecularly-defined developmental zones in r1. We then used genetic inducible fate mapping (GIFM) to link transient Wnt1 expression in r1 to terminal cell fates in the mature Cb. Wnt1 expressing progenitors primarily contributed to neurons in deep cerebellar nuclei, granule cells, and unipolar brush cells in distinct but overlapping temporal windows and sparsely contributed to inhibitory neurons and Bergmann glia. We further demonstrate that the Wnt1 lineage does not follow a competency model of progressive lineage restriction to generate the Cb or the functionally related precerebellar system. Instead, progenitors initiate Wnt1 expression de novo to give rise to each Cb cell type and precerebellar nuclei. We also used GIFM to determine how the temporal control of Wnt1 expression is related to molecular identity and cell migration in Cb development. Our findings provide new insight into how lineage and timing establish cell diversity within the Cb system.

Ascl1 genetics reveals insights into cerebellum local circuit assembly

Two recently generated targeted mouse alleles of the neurogenic gene Ascl1 were used to characterize cerebellum circuit formation. First, genetic inducible fate mapping (GIFM) with an Ascl1(CreER) allele was found to specifically mark all glial and neuron cell types that arise from the ventricular zone (vz). Moreover, each cell type has a unique temporal profile of marking with Ascl1(CreER) GIFM. Of great utility, Purkinje cells (Pcs), an early cohort of Bergmann glia, and four classes of GABAergic interneurons can be genetically birth dated during embryogenesis using Ascl1(CreER) GIFM. Astrocytes and oligodendrocytes, in contrast, express Ascl1(CreER) throughout their proliferative phase in the white matter. Interestingly, the final position each neuron type acquires differs depending on when it expresses Ascl1. Interneurons (including candelabrum) attain a more outside position the later they express Ascl1, whereas Pcs have distinct settling patterns each day they express Ascl1. Second, using a conditional Ascl1 allele, we discovered that Ascl1 is differentially required for generation of most vz-derived cells. Mice lacking Ascl1 in the cerebellum have a major decrease in three types of interneurons with a tendency toward a loss of later-born interneurons, as well as an imbalance of oligodendrocytes and astrocytes. Double-mutant analysis indicates that a related helix-loop-helix protein, Ptf1a, functions with Ascl1 in generating interneurons and Pcs. By fate mapping vz-derived cells in Ascl1 mutants, we further discovered that Ascl1 plays a specific role during the time period when Pcs are generated in restricting vz progenitors from becoming rhombic lip progenitors.

Modulation of cell-cycle dynamics is required to regulate the number of cerebellar GABAergic interneurons and their rhythm of maturation

The progenitors of cerebellar GABAergic interneurons proliferate up to postnatal development in the prospective white matter, where they give rise to different neuronal subtypes, in defined quantities and according to precise spatiotemporal sequences. To investigate the mechanisms that regulate the specification of distinct interneuron phenotypes, we examined mice lacking the G1 phase-active cyclin D2. It has been reported that these mice show severe reduction of stellate cells, the last generated interneuron subtype. We found that loss of cyclin D2 actually impairs the whole process of interneuron genesis. In the mutant cerebella, progenitors of the prospective white matter show reduced proliferation rates and enhanced tendency to leave the cycle, whereas young postmitotic interneurons undergo severe delay of their maturation and migration. As a consequence, the progenitor pool is precociously exhausted and the number of interneurons is significantly reduced, although molecular layer interneurons are more affected than those of granular layer or deep nuclei. The characteristic inside-out sequence of interneuron placement in the cortical layers is also reversed, so that later born cells occupy deeper positions than earlier generated ones. Transplantation experiments show that the abnormalities of cyclin D2(-/-) interneurons are largely caused by cell-autonomous mechanisms. Therefore, cyclin D2 is not required for the specification of particular interneuron subtypes. Loss of this protein, however, disrupts regulatory mechanisms of cell cycle dynamics that are required to determine the numbers of interneurons of different types and impairs their rhythm of maturation and integration in the cerebellar circuitry.