Math1 Is Expressed in Temporally Discrete Pools of Cerebellar Rhombic-Lip Neural Progenitors

Molecular machinery governing GABAergic neuron specification in the cerebellum

Although the cerebellum contains a relatively small variety of neurons, the molecular machinery governing neuronal generation and/or subtype specification is still poorly understood. We identified a novel mutant mouse, cerebelless, which lacks the entire cerebellar cortex but survives up to the adult stages. Analyses of its phenotypes and identification of its responsible gene clarified that Ptf1a (pancreas transcription factor 1a), which encodes a bHLH transcription factor, is involved in cerebellar GABAergic neuron production. Together with recently published papers describing another bHLH gene, Math1, our study proposes that two bHLH transcription factors, PTF1A and MATH1, may participate in regionalization of cerebellar neuroepithelium, defining two distinct areas, the ventricular zone and the rhombic lip, which generate GABAergic and glutamatergic neurons, respectively. Here I will describe a novel cerebellar mutant, cerebelless, review the role of Ptf1a in GABAergic neuron production, and discuss new insights into cerebellar development from the vantage point of regulation by bHLH transcription factors.

Molecular markers of neuronal progenitors in the embryonic cerebellar anlage

The cerebellum, like the cerebrum, includes a nuclear structure and an overlying cortical structure. Experiments in the past decade have expanded knowledge beyond the traditional function of the cerebellum to include critical roles in motor learning and memory and sensory discrimination. The initial steps in cerebellar development depend on inductive signaling involving FGF and Wnt proteins produced at the mesencephalic/metencephalic boundary. To address the issue of how individual cerebellar cell fates within the cerebellar territory are specified, we examined the expression of transcription factors, including mammalian homologues of LIM homeodomain-containing proteins, basic helix-loop-helix proteins, and three amino acid loop-containing proteins. The results of these studies show that combinatorial codes of transcription factors define precursors of the cerebellar nuclei, and both Purkinje cells and granule neurons of the cerebellar cortex. Examination of gene expression patterns in several hundred lines of Egfp-BAC (bacterial artificial chromosome) transgenic mice in the GENSAT Project revealed numerous genes with restricted expression in cerebellar progenitor populations, including genes specific for cerebellar nuclear precursors and Purkinje cell precursors. In addition, we identified patterns of gene expression that link granule and Purkinje cells to their precerebellar nuclei. These results identify molecular pathways that offer new insights on the development of the nuclear and cortical structures of the cerebellum, as well as components of the cerebellar circuitry.

Genetic inducible fate mapping in mouse: establishing genetic lineages and defining genetic neuroanatomy in the nervous system

A fascinating aspect of developmental biology is how organs are assembled in three dimensions over time. Fundamental to understanding organogenesis is the ability to determine when and where specific cell types are generated, the lineage of each cell, and how cells move to reside in their final position. Numerous methods have been developed to mark and follow the fate of cells in various model organisms used by developmental biologists, but most are not readily applicable to mouse embryos in utero because they involve physical marking of cells through injection of tracers. The advent of sophisticated transgenic and gene targeting techniques, combined with the use of site-specific recombinases, has revolutionized fate mapping studies in mouse. Furthermore, using genetic fate mapping to mark cells has opened up the possibility of addressing fundamental questions that cannot be studied with traditional methods of fate mapping in other organisms. Specifically, genetic fate mapping allows both the relationship between embryonic gene expression and cell fate (genetic lineage) to be determined, as well as the link between gene expression domains and anatomy (genetic anatomy) to be established. In this review, we present the ever-evolving development of genetic fate mapping techniques in mouse, especially the recent advance of Genetic Inducible Fate Mapping. We then review recent studies in the nervous system (focusing on the anterior hindbrain) as well as in the limb and with adult stem cells to highlight fundamental developmental processes that can be discovered using genetic fate mapping approaches. We end with a look toward the future at a powerful new approach that combines genetic fate mapping with cellular phenotyping alleles to study cell morphology, physiology, and function using examples from the nervous system.

Migration and nucleogenesis of mouse precerebellar neurons visualized by in utero electroporation of a green fluorescent protein gene

Neural migration is a critical step for accurate CNS development, but the molecular mechanisms that regulate migration, settlement and nucleogenesis remain largely unknown. The precerebellar neurons (PCNs), generated in the lower rhombic lip (LRL), migrate towards their destinations: some neurons form the pontine gray nucleus (PGN) and reticulotegmental nucleus (RTN) in the ipsilateral pons, while others form the lateral reticular and external cuneate nuclei in the contralateral medulla after crossing the midline. Here, by introducing an EGFP gene into a unilateral LRL of mouse embryos by in utero electroporation, we specifically labeled and tracked the PCNs in vivo. We found that a substantial number of the labeled neurons crossed the midline and formed PGN/RTN on the contralateral side. In addition, we found that a subpopulation of the interpolar subnucleus of the spinal trigeminal nucleus, which projects the axons to the cerebellum, was one of the PCNs derived from the LRL. Furthermore, because the electroporated mice were born and grew up healthy, we could visualize the PCNs and their mossy fibers in the adult brain. Therefore, the EGFP labeling of PCNs can be applied to studying the physiology of the mossy fiber system as well as PCN development in embryos.

Unipolar brush cells of the cerebellum are produced in the rhombic lip and migrate through developing white matter

Unipolar brush cells (UBCs) are glutamatergic interneurons in the cerebellar cortex and dorsal cochlear nucleus. We studied the development of UBCs, using transcription factor Tbr2/Eomes as a marker for UBCs and their progenitors in embryonic and postnatal mouse cerebellum. Tbr2+ UBCs appeared to migrate out of the upper rhombic lip via two cellular streams: a dorsal pathway into developing cerebellar white matter, where the migrating cells dispersed widely before entering the internal granular layer, and a rostral pathway along the cerebellar ventricular zone toward the brainstem. Ablation of the rhombic lip in organotypic slice cultures substantially reduced the production of Tbr2+ UBCs. In coculture experiments, Tbr2+ UBCs migrated from rhombic lip explants directly into the developing white matter of adjacent cerebellar slices. The origin of Tbr2+ UBCs was confirmed by colocalization with beta-galactosidase expressed from the Math1 locus, a molecular marker of rhombic lip lineages. Moreover, the production of Tbr2+ UBCs was Math1 dependent, as Tbr2+ UBCs were severely reduced in Math1-null cerebellum. In reeler mutant mice, Tbr2+ UBCs accumulated near the rhombic lip, consistent with impaired migration through developing white matter. Our results suggest that UBCs arise from the rhombic lip and migrate via novel pathways to their final destinations in the cerebellum and dorsal cochlear nucleus. Our findings support a model of cerebellar neurogenesis, in which glutamatergic and GABAergic neurons are produced from separate progenitor pools located mainly in the rhombic lip and the cerebellar ventricular zone, respectively.

Directing neuron-specific transgene expression in the mouse CNS

Recent advances in molecular genetics have produced many novel strategies for directing the expression of both functional and regulatory elements in transgenic mice. With the application of such approaches, the specific populations that comprise CNS networks can be both visualized and manipulated. Transgenic methods now range from the use of specific enhancer elements and large genomic regions assembled using BACs and PACs, to the use of gene targeting to a specific locus. In addition, the advent of transactivators and site-specific recombinases has provided unprecedented spatial and temporal control for directing expression in the CNS using a combination of appropriate alleles. As a result, the promise of being able to use transgenics to target specific neuronal populations is now being realized.

The roof plate regulates cerebellar cell-type specification and proliferation

During embryogenesis, the isthmic organizer, a well-described signaling center at the junction of the mid-hindbrain, establishes the cerebellar territory along the anterior/posterior axis of the neural tube. Mechanisms specifying distinct populations within the early cerebellar anlage are less defined. Using a newly developed gene expression map of the early cerebellar anlage, we demonstrate that secreted signals from the rhombomere 1 roof plate are both necessary and sufficient for specification of the adjacent cerebellar rhombic lip and its derivative fates. Surprisingly, we show that the roof plate is not absolutely required for initial specification of more distal cerebellar cell fates, but rather regulates progenitor proliferation and cell position within the cerebellar anlage. Thus, in addition to the isthmus, the roof plate represents an important signaling center controlling multiple aspects of cerebellar patterning.

FGF signaling mediates regeneration of the differentiating cerebellum through repatterning of the anterior hindbrain and reinitiation of neuronal migration

To address the regenerative capability of the differentiating hindbrain, we ablated the cerebellum in wild-type and transgenic zebrafish embryos. These larvae showed no obvious locomotive malfunction several days after the ablation. Expression analysis and in vivo time-lapse recording in GFP (green fluorescent protein)-transgenic embryos indicate that cerebellar neuronal cells can regenerate from the remaining anterior hindbrain. The onset of regeneration is accompanied by repatterning within the anterior hindbrain. Inhibition of FGF signaling immediately after cerebellar ablation results in the lack of regenerating cerebellar neuronal cells and the absence of cerebellar structures several days later. Moreover, impaired FGF signaling inhibits the repatterning of the anterior hindbrain and the reexpression of rhombic lip marker genes soon after cerebellar ablation. This demonstrates that the hindbrain is highly plastic in recapitulating early embryonic differentiation mechanisms during regeneration. Moreover, the regenerating system offers a means to uncouple cerebellar differentiation from complex morphogenetic tissue rearrangements.

From clusters to stripes: The developmental origins of adult cerebellar compartmentation

Many aspects of the adult cerebellum are organized into parasagittal stripes, including several types of neurons and prominent afferent and efferent projections. Purkinje cells are the best-studied example of parasagittal organization in the cerebellum and, in particular, zebrin II/aldolase C is the stereotypical molecular marker of Purkinje cell stripe heterogeneity in the adult. Zebrin II is a member of the so-called 'late-onset' class of parasagittal markers, which are first expressed shortly after the birth of the mouse and do not reach maturity until 2-3 weeks postnatal. In contrast, 'early-onset' pattern markers are expressed in ordered Purkinje cell clusters in the embryonic cerebellum but become expressed homogeneously shortly after birth. The approximately 10 day temporal gap between the patterned expression of early and late markers has impeded the identification of putative genealogical relationships between clusters and stripes. This review will describe Purkinje cell patterns and their transitions, and critically discuss the evidence for genealogical relationships between early and late patterns.

Temporal identity transition in the avian cerebellar rhombic lip

The rhombic lip is a discrete strip of neuroepithelium bordering the roofplate of the fourth ventricle, which gives rise to a defined sequence of migratory neuronal derivatives. In rhombomere 1 of the chick, early born cells give rise to post-mitotic hindbrain nuclei, while later derivatives comprise of cerebellar granule cell precursors, a unique proliferative, migratory precursor population that forms the external granule cell layer. We have examined the temporal specification of these two populations using a heterochronic grafting strategy, in ovo. When transplanted into younger neural tube, rhombic lip cells maintain their characteristic molecular markers and migrate into the hindbrain. Granule cell precursor derivatives of late grafts are, in addition, able to exploit neural crest streams to populate the branchial arches. Within the neural tube, derivatives of early and late rhombic lip progenitors display patterns of migration and process extension, characterised by specific trajectories and targets, which are consistent with their temporal origin. However, the normal temporal progression of cell production is disrupted in grafted progenitors: transplanted early rhombic lip fails to subsequently produce granule cell precursors. This indicates that, while the behaviour of derivatives is intrinsically specified at the rhombic lip, the orderly temporal transition in cell type production is dependent on extrinsic cues present only in the later embryo.

Development of the deep cerebellar nuclei: transcription factors and cell migration from the rhombic lip

The deep cerebellar nuclei (DCN) are the main output centers of the cerebellum, but little is known about their development. Using transcription factors as cell type-specific markers, we found that DCN neurons in mice are produced in the rhombic lip and migrate rostrally in a subpial stream to the nuclear transitory zone (NTZ). The rhombic lip-derived cells express transcription factors Pax6, Tbr2, and Tbr1 sequentially as they enter the NTZ. A subset of rhombic lip-derived cells also express reelin, a key regulator of Purkinje cell migrations. In organotypic slice cultures, the rhombic lip was necessary and sufficient to produce cells that migrate in the subpial stream, enter the NTZ, and express Pax6, Tbr2, Tbr1, and reelin. In later stages of development, the subpial stream is replaced by the external granular layer, and the NTZ organizes into distinct DCN nuclei. Tbr1 expression persists to adulthood in a subset of medial DCN projection neurons. In reeler mutant mice, which have a severe cerebellar malformation, rhombic lip-derived cells migrated to the NTZ, despite reelin deficiency. Studies in Tbr1 mutant mice suggested that Tbr1 plays a role in DCN morphogenesis but is not required for reelin expression, glutamatergic differentiation, or the initial formation of efferent axon pathways. Our findings reveal underlying similarities in the transcriptional programs for glutamatergic neuron production in the DCN and the cerebral cortex, and they support a model of cerebellar neurogenesis in which glutamatergic and GABAergic neurons are produced from separate progenitor compartments.

Transcription factors in glutamatergic neurogenesis: conserved programs in neocortex, cerebellum, and adult hippocampus

Glutamatergic, pyramidal-projection neurons are produced in the embryonic cerebral cortex by a series of genetically programmed fate choices, implemented in large part by developmental transcription factors. Our work has focused on Pax6, Tbr2/Eomes, NeuroD, and Tbr1, which are expressed sequentially during the neurogenesis of pyramidal-projection neurons. Recently, we have found that the same transcription factors are expressed, in the same order, during glutamatergic neurogenesis in the adult dentate gyrus, and (with modifications) in the developing cerebellum. While the precise functional significance of this transcription factor expression sequence is unknown, its common appearance in embryonic and adult neurogenesis, and in different brain regions, suggests it is part of a conserved genetic program that specifies general properties of glutamatergic neurons in these regions. Subtypes of glutamatergic neurons (e.g., layer-specific fates in the cortex) are further determined by combinations of transcription factors, superimposed on general sequential programs. These new perspectives on neurogenesis add to the conceptual framework for strategies to engineer neural stem cells for the repair of specific brain circuits.

Assembly of the Brainstem Cochlear Nuclear Complex Is Revealed by Intersectional and Subtractive Genetic Fate Maps

The cochlear nuclear complex (CN) is the entry point for central auditory processing. Although constituent neurons have been studied physiologically, their embryological origins and molecular profiles remain obscure. Applying intersectional and subtractive genetic fate mapping approaches, we show that this complex develops modularly from genetically separable progenitor populations arrayed as rostrocaudal microdomains within and outside the hindbrain (lower) rhombic lip (LRL). The dorsal CN subdivision, structurally and topographically similar to the cerebellum, arises from microdomains unexpectedly caudal and noncontiguous to cerebellar primordium; ventral CN subdivisions arise from more rostral LRL. Magnocellular regions receive contributions from LRL and coaxial non-lip progenitors; contrastingly, ensheathing granule cells derive principally from LRL. Also LRL-derived and molecularly similar to CN granule cells are precerebellar mossy fiber neurons; surprisingly, these ostensibly intertwined populations have separable origins and adjacent but segregated migratory streams. Together, these findings provide new platforms for investigating the development and evolution of auditory and cerebellar systems.

Hindbrain Rhombic Lip Is Comprised of Discrete Progenitor Cell Populations Allocated by Pax6

The lower rhombic lip (LRL) is a germinal zone in the dorsal hindbrain productive of tangentially migrating neurons, streaming extramurally (mossy fiber neurons) or intramurally (climbing fiber neurons). Here we show that LRL territory, operationally defined by Wnt1 expression, is parceled into molecular subdomains predictive of cell fate. Progressing dorsoventrally, Lmx1a and Gdf7 expression identifies the primordium for hindbrain choroid plexus epithelial cells; Math1, for mossy fiber neurons; and immediately ventral to Math1 yet within Wnt1(+) territory, a climbing fiber primordium dominated by Ngn1-expressing cells. Elimination of Pax6 results in expansion of this Ngn1(+) progenitor pool and reduction in the Math1(+) pool, with accompanying later enlargement of the climbing fiber nucleus and reductions in mossy fiber nuclei. Pax6 loss also disrupts Msx expression cell-nonautonomously, suggesting Pax6 may influence LRL progenitor identity indirectly through potentiating BMP signaling. These studies suggest that underlying the diversity and proportions of fates produced by the LRL is a precise suborganization regulated by Pax6.

Math-Map(ic)s

In this issue of Neuron, Wang et al. and Machold and Fishell present contrasting molecular fate maps of Math1, which redefine the derivatives of the embryonic rhombic lip and offer a conceptual overhaul of cerebellar and precerebellar development. These fate maps identify a common developmental thread linking diverse, functionally associated neurons and reveal an exquisite temporal organization in cell production within a precise spatially defined region of neuroepithelium.