Analysis of cerebellar development in math1 null embryos and chimeras

Regulation of cerebellar network development by granule cells and their molecules

The well-organized cerebellar structures and neuronal networks are likely crucial for their functions in motor coordination, motor learning, cognition, and emotion. Such cerebellar structures and neuronal networks are formed during developmental periods through orchestrated mechanisms, which include not only cell-autonomous programs but also interactions between the same or different types of neurons. Cerebellar granule cells (GCs) are the most numerous neurons in the brain and are generated through intensive cell division of GC precursors (GCPs) during postnatal developmental periods. While GCs go through their own developmental processes of proliferation, differentiation, migration, and maturation, they also play a crucial role in cerebellar development. One of the best-characterized contributions is the enlargement and foliation of the cerebellum through massive proliferation of GCPs. In addition to this contribution, studies have shown that immature GCs and GCPs regulate multiple factors in the developing cerebellum, such as the development of other types of cerebellar neurons or the establishment of afferent innervations. These studies have often found impairments of cerebellar development in animals lacking expression of certain molecules in GCs, suggesting that the regulations are mediated by molecules that are secreted from or present in GCs. Given the growing recognition of GCs as regulators of cerebellar development, this review will summarize our current understanding of cerebellar development regulated by GCs and molecules in GCs, based on accumulated studies and recent findings, and will discuss their potential further contributions.

Regulation of early cerebellar development

The study of cerebellar development has been at the forefront of neuroscience since the pioneering work of Wilhelm His Sr., Santiago Ramón y Cajal and many others since the 19th century. They laid the foundation to identify the circuitry of the cerebellum, already revealing its stereotypic three-layered cortex and discerning several of its neuronal components. Their work was fundamental in the acceptance of the neuron doctrine, which acknowledges the key role of individual neurons in forming the basic units of the nervous system. Increasing evidence shows that the cerebellum performs a variety of homeostatic and higher order neuronal functions beyond the mere control of motor behaviour. Over the last three decades, many studies have revealed the molecular machinery that regulates distinct aspects of cerebellar development, from the establishment of a cerebellar anlage in the posterior brain to the identification of cerebellar neuron diversity at the single cell level. In this review, we focus on summarizing our current knowledge on early cerebellar development with a particular emphasis on the molecular determinants that secure neuron specification and contribute to the diversity of cerebellar neurons.

The Transcription Factor Pou3f1 Sheds Light on the Development and Molecular Diversity of Glutamatergic Cerebellar Nuclear Neurons in the Mouse

The cerebellar nuclear (CN) neurons are a molecularly heterogeneous population whose specification into the different cerebellar nuclei is defined by the expression of varying sets of transcription factors. Here, we present a novel molecular marker, Pou3f1, that delineates specific sets of glutamatergic CN neurons. The glutamatergic identity of Pou3f1⁺ cells was confirmed by: (1) the co-expression of vGluT2, a cell marker of glutamatergic neurons; (2) the lack of co-expression between Pou3f1 and GAD67, a marker of GABAergic neurons; (3) the co-expression of Atoh1, the master regulator required for the production of all cerebellar glutamatergic lineages; and (4) the absence of Pou3f1-expressing cells in the Atoh1-null cerebellum. Furthermore, the lack of Pax6 and Tbr1 expression in Pou3f1⁺ cells reveals that Pou3f1-expressing CN neurons specifically settle in the interposed and dentate nuclei. In addition, the Pou3f1-labeled glutamatergic CN neurons can be further classified by the expression of Brn2 and Irx3. The results of the present study align with previous findings highlighting that the survival of the interposed and dentate CN neurons is largely independent of Pax6. More importantly, the present study extends the field’s collective knowledge of the molecular diversity of cerebellar nuclei.

Maturation of Purkinje cell firing properties relies on neurogenesis of excitatory neurons

Preterm infants that suffer cerebellar insults often develop motor disorders and cognitive difficulty. Excitatory granule cells, the most numerous neuron type in the brain, are especially vulnerable and likely instigate disease by impairing the function of their targets, the Purkinje cells. Here, we use regional genetic manipulations and in vivo electrophysiology to test whether excitatory neurons establish the firing properties of Purkinje cells during postnatal mouse development. We generated mutant mice that lack the majority of excitatory cerebellar neurons and tracked the structural and functional consequences on Purkinje cells. We reveal that Purkinje cells fail to acquire their typical morphology and connectivity, and that the concomitant transformation of Purkinje cell firing activity does not occur either. We also show that our mutant pups have impaired motor behaviors and vocal skills. These data argue that excitatory cerebellar neurons define the maturation time-window for postnatal Purkinje cell functions and refine cerebellar-dependent behaviors.

Interactions Between Purkinje Cells and Granule Cells Coordinate the Development of Functional Cerebellar Circuits

Cerebellar development has a remarkably protracted morphogenetic timeline that is coordinated by multiple cell types. Here, we discuss the intriguing cellular consequences of interactions between inhibitory Purkinje cells and excitatory granule cells during embryonic and postnatal development. Purkinje cells are central to all cerebellar circuits, they are the first cerebellar cortical neurons to be born, and based on their cellular and molecular signaling, they are considered the master regulators of cerebellar development. Although rudimentary Purkinje cell circuits are already present at birth, their connectivity is morphologically and functionally distinct from their mature counterparts. The establishment of the Purkinje cell circuit with its mature firing properties has a temporal dependence on cues provided by granule cells. Granule cells are the latest born, yet most populous, neuronal type in the cerebellar cortex. They provide a combination of mechanical, molecular and activity-based cues that shape the maturation of Purkinje cell structure, connectivity and function. We propose that the wiring of Purkinje cells for function falls into two developmental phases: an initial phase that is guided by intrinsic mechanisms and a later phase that is guided by dynamically-acting cues, some of which are provided by granule cells. In this review, we highlight the mechanisms that granule cells use to help establish the unique properties of Purkinje cell firing.

HIC1 Represses Atoh1 Transcription and Hair Cell Differentiation in the Cochlea

Across species, expression of the basic helix-loop-helix transcription factor ATOH1 promotes differentiation of cochlear supporting cells to sensory hair cells required for hearing. In mammals, this process is limited to development, whereas nonmammalian vertebrates can also regenerate hair cells after injury. The mechanistic basis for this difference is not fully understood. Hypermethylated in cancer 1 (HIC1) is a transcriptional repressor known to inhibit Atoh1 in the cerebellum. We therefore investigated its potential role in cochlear hair cell differentiation. We find that Hic1 is expressed throughout the postnatal murine cochlear sensory epithelium. In cochlear organoids, Hic1 knockdown induces Atoh1 expression and promotes hair cell differentiation, while Hic1 overexpression hinders differentiation. Wild-type HIC1, but not the DNA-binding mutant C521S, suppresses activity of the Atoh1 autoregulatory enhancer and blocks its responsiveness to β-catenin activation. Our findings reveal the importance of HIC1 repression of Atoh1 in the cochlea, which may be targeted to promote hair cell regeneration.

Olig3 regulates early cerebellar development

The mature cerebellum controls motor skill precision and participates in other sophisticated brain functions that include learning, cognition, and speech. Different types of GABAergic and glutamatergic cerebellar neurons originate in temporal order from two progenitor niches, the ventricular zone and rhombic lip, which express the transcription factors Ptf1a and Atoh1, respectively. However, the molecular machinery required to specify the distinct neuronal types emanating from these progenitor zones is still unclear. Here, we uncover the transcription factor Olig3 as a major determinant in generating the earliest neuronal derivatives emanating from both progenitor zones in mice. In the rhombic lip, Olig3 regulates progenitor cell proliferation. In the ventricular zone, Olig3 safeguards Purkinje cell specification by curtailing the expression of Pax2, a transcription factor that suppresses the Purkinje cell differentiation program. Our work thus defines Olig3 as a key factor in early cerebellar development.

Origins, Development, and Compartmentation of the Granule Cells of the Cerebellum

Granule cells (GCs) are the most numerous cell type in the cerebellum and indeed, in the brain: at least 99% of all cerebellar neurons are granule cells. In this review article, we first consider the formation of the upper rhombic lip, from which all granule cell precursors arise, and the way by which the upper rhombic lip generates the external granular layer, a secondary germinal epithelium that serves to amplify the upper rhombic lip precursors. Next, we review the mechanisms by which postmitotic granule cells are generated in the external granular layer and migrate radially to settle in the granular layer. In addition, we review the evidence that far from being a homogeneous population, granule cells come in multiple phenotypes with distinct topographical distributions and consider ways in which the heterogeneity of granule cells might arise during development.

Heterochronic Developmental Shifts Underlying Squamate Cerebellar Diversity Unveil the Key Features of Amniote Cerebellogenesis

Despite a remarkable conservation of architecture and function, the cerebellum of vertebrates shows extensive variation in morphology, size, and foliation pattern. These features make this brain subdivision a powerful model to investigate the evolutionary developmental mechanisms underlying neuroanatomical complexity both within and between anamniote and amniote species. Here, we fill a major evolutionary gap by characterizing the developing cerebellum in two non-avian reptile species-bearded dragon lizard and African house snake-representative of extreme cerebellar morphologies and neuronal arrangement patterns found in squamates. Our data suggest that developmental strategies regarded as exclusive hallmark of birds and mammals, including transit amplification in an external granule layer (EGL) and Sonic hedgehog expression by underlying Purkinje cells (PCs), contribute to squamate cerebellogenesis independently from foliation pattern. Furthermore, direct comparison of our models suggests the key importance of spatiotemporal patterning and dynamic interaction between granule cells and PCs in defining cortical organization. Especially, the observed heterochronic shifts in early cerebellogenesis events, including upper rhombic lip progenitor activity and EGL maintenance, are strongly expected to affect the dynamics of molecular interaction between neuronal cell types in snakes. Altogether, these findings help clarifying some of the morphogenetic and molecular underpinnings of amniote cerebellar corticogenesis, but also suggest new potential molecular mechanisms underlying cerebellar complexity in squamates. Furthermore, squamate models analyzed here are revealed as key animal models to further understand mechanisms of brain organization.

The Paneth Cell: The Curator and Defender of the Immature Small Intestine

Paneth cells were first described in the late 19th century by Gustav Schwalbe and Josef Paneth as columnar epithelial cells possessing prominent eosinophilic granules in their cytoplasm. Decades later there is continued interest in Paneth cells as they play an integral role in maintaining intestinal homeostasis and modulating the physiology of the small intestine and its associated microbial flora. Paneth cells are highly specialized secretory epithelial cells located in the small intestinal crypts of Lieberkühn. The dense granules produced by Paneth cells contain an abundance of antimicrobial peptides and immunomodulating proteins that function to regulate the composition of the intestinal flora. This in turn plays a significant role in secondary regulation of the host microvasculature, the normal injury and repair mechanisms of the intestinal epithelial layer, and the levels of intestinal inflammation. These critical functions may have even more importance in the immature intestine of premature infants. While Paneth cells begin to develop in the middle of human gestation, they do not become immune competent or reach their adult density until closer to term gestation. This leaves preterm infants deficient in normal Paneth cell biology during the greatest window of susceptibility to develop intestinal pathology such as necrotizing enterocolitis (NEC). As 10% of infants worldwide are currently born prematurely, there is a significant population of infants contending with an inadequate cohort of Paneth cells. Infants who have developed NEC have decreased Paneth cell numbers compared to age-matched controls, and ablation of murine Paneth cells results in a NEC-like phenotype suggesting again that Paneth cell function is critical to homeostasis to the immature intestine. This review will provide an up to date and comprehensive look at Paneth cell ontogeny, the impact Paneth cells have on the host-microbial axis in the immature intestine, and the repercussions of Paneth cell dysfunction or loss on injury and repair mechanisms in the immature gut.

The generation of granule cells during the development and evolution of the cerebellum

The cerebellum coordinates vestibular input into the hindbrain to control balance and movement, and its anatomical complexity is increasingly viewed as a high-throughput processing center for sensory and cognitive functions. Cerebellum development however is relatively simple, and arises from a specialized structure in the anterior hindbrain called the rhombic lip, which along with the ventricular zone of the rostral-most dorsal hindbrain region, give rise to the distinct cell types that constitute the cerebellum. Granule cells, being the most numerous cell types, arise from the rhombic lip and form a dense and distinct layer of the cerebellar cortex. In this short review, we describe the various strategies used by amniotes and anamniotes to generate and diversify granule cell types during cerebellar development.

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.

The role of the ubiquitin proteasome system in cerebellar development and medulloblastoma

Cerebellar granule cells precursors are derived from the upper rhombic lip and migrate tangentially independent of glia along the subpial stream pathway to form the external germinal zone. Postnatally, granule cells migrate from the external germinal zone radially through the Purkinje cell layer, guided by Bergmann glia fibers, to the internal granular cell layer.

Medulloblastomas (MBs) are the most common malignant childhood brain tumor. Many of these tumors develop from precursor cells of the embryonic rhombic lips. Four main groups of MB are recognized. The WNT group of MBs arise primarily from the lower rhombic lip and embryonic brainstem. The SHH group of MBs originate from cerebellar granule cell precursors in the external germinal zone of the embryonic cerebellum. The cellular origins of type 3 and type 4 MBs are not clear.

Several ubiquitin ligases are revealed to be significant factors in development of the cerebellum as well as in the initiation and maintenance of MBs. Proteasome dysfunction at a critical stage of development may be a major factor in determining whether progenitor cells which are destined to become granule cells differentiate normally or become MB cells. We propose the hypothesis that proteasomal activity is essential to regulate the critical transition between proliferating granule cells and differentiated granule cells and that proteasome dysfunction may lead to MB. Proteasome dysfunction could also account for various mutations in MBs resulting from deficiencies in DNA checkpoint and repair mechanisms prior to development of MBs.

Data showing a role for the ubiquitin ligases β-TrCP, FBW7, Huwe1, and SKP2 in MBs suggest the possibility of a classification of MBs based on the expression (over expression or under expression) of specific ubiquitin ligases which function as oncogenes, tumor suppressors or cell cycle regulators.

Wls provides a new compartmental view of the rhombic lip in mouse cerebellar development

Math1 is the defining molecule of the cerebellar rhombic lip and Pax6 is downstream in the Math1 pathway. In the present study, we discover that Wntless (Wls) is a novel molecular marker of the cells in the interior face of the rhombic lip throughout normal mouse cerebellar development. Wls expression is found complementary to the expression of Math1 and Pax6, which are localized to the exterior face of the rhombic lip. To determine the interaction between these molecules, we examine the loss-of-Math1 or loss-of-Pax6 in the cerebellum, i.e., the Math1-null and Pax6-null (Sey) mutant cerebella. The presence of Wls-positive cells in the Math1-null rhombic lip indicates that Wls expression is independent of Math1. In the Sey mutant cerebellum, there is an expansion of Wls-expressing cells into regions that are normally colonized by Pax6-expressing cells. The ectopic expression of Wls in the Pax6-null cerebellum suggests a negative interaction between Wls-expressing cells and Pax6-positive cells. These findings suggest that the rhombic lip is dynamically patterned by the expression of Wls, Math1, and Pax6. We also examine five rhombic lip cell markers (Wls, Math1, Pax6, Lmx1a, and Tbr2) to identify four molecularly distinct compartments in the rhombic lip during cerebellar development. The existence of spatial compartmentation in the rhombic lip and the interplay between Wls, Math1, and Pax6 in the rhombic lip provides novel views of early cerebellar development.

Wls provides a new compartmental view of the rhombic lip in mouse cerebellar development

Math1 is the defining molecule of the cerebellar rhombic lip and Pax6 is downstream in the Math1 pathway. In the present study, we discover that Wntless (Wls) is a novel molecular marker of the cells in the interior face of the rhombic lip throughout normal mouse cerebellar development. Wls expression is found complementary to the expression of Math1 and Pax6, which are localized to the exterior face of the rhombic lip. To determine the interaction between these molecules, we examine the loss-of-Math1 or loss-of-Pax6 in the cerebellum, i.e., the Math1-null and Pax6-null (Sey) mutant cerebella. The presence of Wls-positive cells in the Math1-null rhombic lip indicates that Wls expression is independent of Math1. In the Sey mutant cerebellum, there is an expansion of Wls-expressing cells into regions that are normally colonized by Pax6-expressing cells. The ectopic expression of Wls in the Pax6-null cerebellum suggests a negative interaction between Wls-expressing cells and Pax6-positive cells. These findings suggest that the rhombic lip is dynamically patterned by the expression of Wls, Math1, and Pax6. We also examine five rhombic lip cell markers (Wls, Math1, Pax6, Lmx1a, and Tbr2) to identify four molecularly distinct compartments in the rhombic lip during cerebellar development. The existence of spatial compartmentation in the rhombic lip and the interplay between Wls, Math1, and Pax6 in the rhombic lip provides novel views of early cerebellar development.

Growth and differentiation factor 10 (Gdf10) is involved in Bergmann glial cell development under Shh regulation

Growth differentiation factor 10 (Gdf10), also known as Bmp3b, is a member of the transforming growth factor (TGF)-ß superfamily. Gdf10 is expressed in Bergmann glial cells, which was investigated by single-cell transcriptional profiling (Koirala and Corfas, (2010) PLoS ONE 5: e9198). Here we provide a detailed characterization of Gdf10 expression from E14, the stage at which Gdf10 is expressed for the first time in the cerebellum, until P28. We detected Gdf10 expression in both germinal zones: in the ventricular zone (VZ) of the 4th ventricle as well as in the rhombic lip (RL). The VZ has been postulated to give rise to GABAergic neurons and glial cells, whereas the RL gives rise to glutamatergic neurons. Thus, it was very surprising to discover a gene that is expressed exclusively in glial cells and is not restricted to an expression in the VZ, but is also present in the RL. At postnatal stages Gdf10 was distributed equally in Bergmann glial cells of the cerebellum. Furthermore, we found Gdf10 to be regulated by Sonic hedgehog (Shh), which is secreted by Purkinje cells of the cerebellum. In the conditional Shh mutants, glial cells showed a reduced expression of Gdf10, whereas the expression of Nestin and Vimentin was unchanged. Thus, we show for the first time, that Gdf10, expressed in Bergmann glial cells, is affected by the loss of Shh as early as E18.5, suggesting a regulation of glial development by Shh.

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.

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.

Kainic acid-induced neuronal degeneration in hippocampal pyramidal neurons is driven by both intrinsic and extrinsic factors: analysis of FVB/N↔C57BL/6 chimeras

The excitotoxic effects of kainic acid (KA) in the mouse hippocampus is strain dependent. Following KA administration, the large majority of hippocampal pyramidal cells die in the FVB/N (FVB) mouse, while the pyramidal cells of the C57BL/6 (B6) strain are largely spared. We generated aggregation chimeras between the sensitive FVB and the resistant B6 strains to investigate whether intrinsic or extrinsic features of a neuron confer cell vulnerability or resistance to KA. The constitutive expression of transgenic green fluorescence protein (GFP) or β-galactosidase expressed from the ROSA26 locus was used to mark cells in FVB or B6 mice, respectively. These makers enable the identification of cells from each parental genotype while TUNEL (terminal deoxynucleotidyl transferase-mediated biotinylated dUTP nick end labeling)-staining labeled dying cells. The analysis of the percentage of dying cells in FVB-GFP ↔ B6-ROSA chimeras yielded an intriguing mix of both intrinsic and extrinsic factors in the readout of cell phenotype. Thus, normally resistant B6-ROSA pyramidal neurons demonstrated an increasing sensitivity to KA, in a linear fashion, when the percentage of FVB-GFP cells was increased, either across chimeras or in different regions of the same chimera. However, the death of B6-ROSA pyramidal cells never exceeded ∼70% of the total amount of B6 neurons regardless of the amount of FVB cells in the chimeric hippocampus. In a similar manner, FVB-GFP cells show lower amounts of cell death in chimeras that are colonized by B6-ROSA cells, but again, are never fully rescued. These data indicate that both intrinsic and extrinsic factors modulate the sensitivity of hippocampal pyramidal cells to kainic acid.

Inflammatory effects of highly pathogenic H5N1 influenza virus infection in the CNS of mice

The A/VN/1203/04 strain of the H5N1 influenza virus is capable of infecting the CNS of mice and inducing a number of neurodegenerative pathologies. Here, we examined the effects of H5N1 on several pathological aspects affected in parkinsonism, including loss of the phenotype of dopaminergic neurons located in the substantia nigra pars compacta (SNpc), expression of monoamines and indolamines in brain, alterations in SNpc microglia number and morphology, and expression of cytokines, chemokines, and growth factors. We find that H5N1 induces a transient loss of the dopaminergic phenotype in SNpc and now report that this loss recovers by 90 d after infection. A similar pattern of loss and recovery was seen in monoamine levels of the basal ganglia. The inflammatory response in lung and different regions of the brain known to be targets of the H5N1 virus (brainstem, substantia nigra, striatum, and cortex) were examined at 3, 10, 21, 60, and 90 d after infection. In each of these brain regions, we found a significant increase in the number of activated microglia that lasted at least 90 d. We also quantified expression of IL-1α, IL-1β, IL-2, IL-6, IL-9, IL-10, IL-12(p70), IL-13, TNF-α, IFN-γ, granulocyte-macrophage colony-stimulating factor, granulocyte colony-stimulating factor, macrophage colony-stimulating factor, eotaxin, interferon-inducible protein 10, cytokine-induced neutrophil chemoattractant, monocyte chemotactic protein-1, macrophage inflammatory protein (MIP) 1α, MIP-1β, and VEGF, and found that the pattern and levels of expression are dependent on both brain region and time after infection. We conclude that H5N1 infection in mice induces a long-lasting inflammatory response in brain and may play a contributing factor in the development of pathologies in neurodegenerative disorders.

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.

Astroglial cells in the external granular layer are precursors of cerebellar granule neurons in neonates

It is well established that cerebellar granule cell precursors (GCPs) initially derive from progenitors in the rhombic lip of the embryonic cerebellar primordium. GCPs proliferate and migrate tangentially across the cerebellum to form the external granule cell layer (EGL) in late embryogenesis and early postnatal development. It is unclear whether GCPs are specified exclusively in the embryonic rhombic lip or whether their precursor persists in the neonate. Using transgenic mice expressing DsRed under the human glial fibrillary acidic protein (hGFAP) promoter, we found 2 populations of DsRed(+) cells in the EGL in the first postnatal week defined by bright and faint DsRed-fluorescent signal. Bright DsRed(+) cells have a protein expression profile and electrophysiological characteristics typical of astrocytes, but faint DsRed(+) cells in the EGL and internal granule cell layer (IGL) express markers and physiological properties of immature neurons. To determine if these astroglial cells gave rise to GCPs, we genetically tagged them with EGFP or betagal reporter genes at postnatal day (P)3-P5 using a hGFAP promoter driven inducible Cre recombinase. We found that GFAP promoter(+) cells in the EGL are proliferative and express glial and neural stem cell markers. In addition, immature granule cells (GCs) en route to the IGL at P12 as well as GCs in the mature cerebellum, 30days after recombination, express the reporter protein, suggesting that GFAP promoter(+) cells in the EGL generate a subset of granule cells. The identification of glial cells which function as neuronal progenitor cells profoundly impacts our understanding of cellular plasticity in the developing cerebellum.

Early cerebellar granule cell migration in the mouse embryonic development

Pax6, a paired homeobox DNA binding protein, has been found to be expressed in the cerebellum in both granule cells and their precursors in the external granular layer (EGL). In this study we have traced Pax6 expression through embryonic development in mice by using a polyclonal antibody against Pax6 and used it to study the cellular dispersal pattern of the EGL. During dispersal the EGL was thicker and Pax6 expression was more intense on the rostral side of the lateral corners of the cerebellum. Pax6 immunoreactive cells were found to be migrating from the EGL during the early stage of EGL dispersal, which suggested the early inward migration of granule cells. Double staining with various markers confirmed that the early-migrating cells are not Purkinje cells, interneurons or glia. Although the Pax6 immunoreactive cells within the cerebellum were not apparently proliferating, NeuN, a marker for postmitotic granule cells, was not expressed in these cells until E16. Furthermore, granule cells were observed migrating inwards from the EGL both during and after EGL dispersal. These early migrating granule cells populated the whole cerebellum. These findings offer novel views on specific stages of granule cell dispersal and migration.

Development of the human cerebellum and its disorders

The cerebellum arises from two anatomically and molecularly different proliferative compartments: the cerebellar ventricular zone and the rhombic lip. The protracted development makes the cerebellum vulnerable to a broad spectrum of developmental disorders, of which the more frequent (the Dandy-Walker and related malformations and the pontocerebellar hypoplasias) are discussed in this article. Several genes for congenital malformations of the human cerebellum have recently been identified, including genes causing Joubert syndrome, the Dandy-Walker malformation, and pontocerebellar hypoplasias.

Insulin-like growth factor 2 is required for progression to advanced medulloblastoma in patched1 heterozygous mice

Medulloblastoma (MB) can arise in the cerebellum due to genetic activation of the Sonic Hedgehog (Shh) signaling pathway. During normal cerebellum development, Shh spurs the proliferation of granule neuron precursors (GNP), the precursor cells of MB. Mutations in the Shh receptor gene patched1 (ptc1+/-) lead to increased MB incidence in humans and mice. MB tumorigenesis in mice heterozygous for ptc1+/- shows distinct steps of progression. Most ptc1+/- mice form clusters of preneoplastic cells on the surface of the mature cerebellum that actively transcribe Shh target genes. In approximately 15% of mice, these preneoplastic cells will become fast-growing, lethal tumors. It was previously shown that the loss of function of insulin-like growth factor 2 (igf2) suppresses MB formation in ptc1+/- mice. We found that igf2 is not expressed in preneoplastic lesions but is induced as these lesions progress to more advanced MB tumors. Igf2 is not required for formation of preneoplastic lesions but is necessary for progression to advanced tumors. Exogenous Igf2 protein promoted proliferation of MB precursor cells (GNP) and a MB cell line, PZp53(MED). Blocking igf2 signaling inhibited growth of PZp53(MED) cells, implicating igf2 as a potential clinical target.

Just say no to ATOH: how HIC1 methylation might predispose medulloblastoma to lineage addiction

Hypermethylated in cancer-1 (HIC1) is a tumor suppressor frequently targeted for promoter hypermethylation in medulloblastoma, an embryonal tumor of the cerebellum. Recently, we showed that HIC1 is a direct transcriptional repressor of ATOH1, a proneural transcription factor required for normal cerebellar development, as well as for medulloblastoma cell viability. Because demethylating agents can induce reexpression of silenced tumor suppressors, restoring HIC1 function may present an attractive therapeutic avenue in medulloblastoma by exploiting an apparent addiction to ATOH1.

Interactions of PACAP and ceramides in the control of granule cell apoptosis during cerebellar development

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide that belongs to the secretin/glucagon/vasoactive intestinal polypeptide superfamily. The PACAPergic system is actively expressed in the developing cerebellum of mammals. In particular, PACAP receptors are expressed by granule cell precursors suggesting a role of the peptide in neurogenesis of this cell type. Consistent with this hypothesis, several studies reported antiapoptotic effects of PACAP in the developing cerebellum. On the other hand, the sphingomyelin metabolites ceramides are recognized as important signaling molecules that play pivotal roles during neuronal development. Ceramides, which production can be induced by death factors such as FasL or TNFalpha, are involved in the control of cell survival during brain development through activation of caspase-dependent mechanisms. The present review focuses on the interactions between PACAP and ceramides in the control of granule cell survival and on the transduction mechanisms associated with the anti- and proapoptotic effects of PACAP and ceramides, respectively.

Cooperation between the Hic1 and Ptch1 tumor suppressors in medulloblastoma

Medulloblastoma is an embryonal tumor thought to arise from the granule cell precursors (GCPs) of the cerebellum. PATCHED (PTCH), an inhibitor of Hedgehog signaling, is the best-characterized tumor suppressor in medulloblastoma. However, <20% of medulloblastomas have mutations in PTCH. In the search for other tumor suppressors, interest has focused on the deletion events at the 17p13.3 locus, the most common genetic defect in medulloblastoma. This chromosomal region contains HYPERMETHYLATED IN CANCER 1 (HIC1), a transcriptional repressor that is a frequent target of epigenetic gene silencing in medulloblastoma. Here we use a mouse model of Ptch1 heterozygosity to reveal a critical tumor suppressor function for Hic1 in medulloblastoma. When compared with Ptch1 heterozygous mutants, compound Ptch1/Hic1 heterozygotes display a fourfold increased incidence of medulloblastoma. We show that Hic1 is a direct transcriptional repressor of Atonal Homolog 1 (Atoh1), a proneural transcription factor essential for cerebellar development, and show that ATOH1 expression is required for human medulloblastoma cell growth in vitro. Given that Atoh1 is also a putative target of Hh signaling, we conclude that the Hic1 and Ptch1 tumor suppressors cooperate to silence Atoh1 expression during a critical phase in GCP differentiation in which malignant transformation may lead to medulloblastoma.

Mapping of Cbln1-like immunoreactivity in adult and developing mouse brain and its localization to the endolysosomal compartment of neurons

Cbln1 is a secreted glycoprotein essential for synapse structure and function in cerebellum that is also expressed in extracerebellar structures where its function is unknown. Furthermore, Cbln1 assembles into homomeric complexes and heteromeric complexes with three family members (Cbln2-Cbln4), thereby influencing each other's degradation and secretion. Therefore, to understand its function, it is essential to establish the location of Cbln1 relative to other family members. The localization of Cbln1 in brain was determined using immunohistochemistry and cbln1-lacZ transgenic mice. Cbln1-like immunoreactivity (CLI) was always punctate and localized to the cytoplasm of neurons. The punctate CLI colocalized with cathepsin D, a lysosomal marker, but not with markers of endoplasmic reticulum or Golgi, indicating that Cbln1 is present in neuronal endosomes/lysosomes. This may represent the cellular mechanism underlying the regulated degradation of Cbln1 observed in vivo. Outside the cerebellum, CLI mapped to multiple brain regions that were frequently synaptically interconnected, warranting their analysis in cbln1-null mice. Furthermore, whereas CLI increased dramatically in the cerebellum of cbln3-null mice it was unchanged in extracerebellar neurons. This opens the possibility that other family members that are coexpressed in these areas control Cbln1 levels, potentially by modulating processing in the endolysosomal pathway. During development of cbln1-lacZ mice, beta-galactosidase staining was first observed in proliferating granule cell precursors prior to synaptogenesis and thereafter in maturing and adult granule cells. As cbln3 is only expressed in post-mitotic, post-migratory granule cells, Cbln1 homomeric complexes in precursors and Cbln1-Cbln3 heteromeric complexes in mature granule cells may have distinct functions and turnover.

Altered development of the brain after focal herpesvirus infection of the central nervous system

Human cytomegalovirus infection of the developing central nervous system (CNS) is a major cause of neurological damage in newborn infants and children. To investigate the pathogenesis of this human infection, we developed a mouse model of infection in the developing CNS. Intraperitoneal inoculation of newborn animals with murine cytomegalovirus resulted in virus replication in the liver followed by virus spread to the brain. Virus infection of the CNS was associated with the induction of inflammatory responses, including the induction of a large number of interferon-stimulated genes and histological evidence of focal encephalitis with recruitment of mononuclear cells to foci containing virus-infected cells. The morphogenesis of the cerebellum was delayed in infected animals. The defects in cerebellar development in infected animals were generalized and, although correlated temporally with virus replication and CNS inflammation, spatially unrelated to foci of virus-infected cells. Specific defects included decreased granular neuron proliferation and migration, expression of differentiation markers, and activation of neurotrophin receptors. These findings suggested that in the developing CNS, focal virus infection and induction of inflammatory responses in resident and infiltrating mononuclear cells resulted in delayed cerebellar morphogenesis.

The derivatives of the Wnt3a lineage in the central nervous system

The dorsal midline of the vertebrate neural tube is a source of signals that direct cell fate specification and proliferation. Using genetic fate mapping in the mouse and a previously generated Wnt3aCre line, we report here that genetically labeled cells of the Wnt3a lineage migrate widely from the dorsal midline into the dorsal half of the adult brain and spinal cord, contributing to diverse structures in the diencephalon, midbrain, and brainstem and extensively populating the rostral spinal cord. Conspicuously, many of these structures are linked in specific functional networks. Wnt3a lineage cells populate nuclei of the central auditory system from the medulla to thalamus, and the trigeminal sensory system from the cervical spinal cord to the midbrain. Our findings reveal the rich contributions of the Wnt3a lineage to a variety of brain structures and show that functionally integrated nuclei can share a molecular identity, provided by transient gene expression early in their development.

Nuclear factor I coordinates multiple phases of cerebellar granule cell development via regulation of cell adhesion molecules

A central question is how various stages of neuronal development are integrated as a differentiation program. Here we show that the nuclear factor I (NFI) family of transcriptional regulators is expressed and functions throughout the postmitotic development of cerebellar granule neurons (CGNs). Expression of an NFI dominant repressor in CGN cultures blocked axon outgrowth and dendrite formation and decreased CGN migration. Inhibition of NFI transactivation also disrupted extension and fasciculation of parallel fibers as well as CGN migration to the internal granule cell layer in cerebellar slices. In postnatal day 17 Nfia-deficient mice, parallel fibers were greatly diminished and disoriented, CGN dendrite formation was dramatically impaired, and migration from the external germinal layer (EGL) was retarded. Axonal marker expression also was disrupted within the EGL of embryonic day 18 Nfib-null mice. NFI regulation of axon extension was observed under conditions of homotypic cell contact, implicating cell surface proteins as downstream mediators of its actions in CGNs. Consistent with this, the cell adhesion molecules ephrin B1 and N-cadherin were identified as NFI gene targets in CGNs using inhibitor and Nfi mutant analysis as well as chromatin immunoprecipitation. Functional inhibition of ephrin B1 or N-cadherin interfered with CGN axon extension and guidance, migration, and dendritogenesis in cell culture as well as in situ. These studies define NFI as a key regulator of postmitotic CGN development, in particular of axon formation, dendritogenesis, and migratory behavior. Furthermore, they reveal how a single transcription factor family can control and integrate multiple aspects of neuronal differentiation through the regulation of cell adhesion molecules.

Intestine-specific ablation of mouse atonal homolog 1 (Math1) reveals a role in cellular homeostasis

BACKGROUND & AIMS

Math1 (Atoh1) is a basic helix-loop-helix transcription factor important for intestinal secretory cell differentiation. We hypothesized that Math1 is important in cell fate commitment, and therefore mediates proliferative homeostasis and the adaptive response following intestinal resection in the adult intestine.

METHODS

We generated mice with an intestine-specific mosaic deletion of Math1 (Math1(Delta intestine)) using the Cre/loxP system. Histologic analysis in adult Math1(Delta intestine) and wild-type littermates at baseline and following small bowel resection or sham surgery was performed.

RESULTS

We observed loss of Paneth, goblet, and enteroendocrine cells in Math1-null crypts. In addition, aberrant activation of the Math1 promoter occurred in absorptive enterocytes derived from Math1-null crypts, suggesting a change in cell fate. Proliferation was increased but apoptosis unchanged in Math1-mutant crypts compared to adjacent wild-type crypts. Math1(Delta intestine) mice and wild-type littermates displayed similar physiologic adaptive responses to small bowel resection as measured by changes in body weight and ileal wet weight. In contrast, Math1-mutant crypts displayed a blunted adaptive response compared to adjacent wild-type crypts.

CONCLUSIONS

We show that Math1 is essential for adult intestinal secretory cell production, and in its absence cells destined to a secretory phenotype instead adopt an absorptive phenotype. Subtle abnormalities of proliferation within Math1-null crypts in Math1(Delta intestine) mice were identified, together with a substantial defect in the adaptive response of Math1-null crypts following small bowel resection. Our results suggest that Math1 is critical for both cell fate determination within the intestinal epithelium and for regulation of the response to intestinal resection.

Expression of S100B during embryonic development of the mouse cerebellum

BACKGROUND

In the cerebellum of newborn S100B-EGFP mice, we had previously noted the presence of a large population of S100B-expressing cells, which we assumed to be immature Bergmann glial cells. In the present study, we have drawn on this observation to establish the precise spatio-temporal pattern of S100B gene expression in the embryonic cerebellum.

RESULTS

From E12.5 until E17.5, S100B was expressed in the primary radial glial scaffold involved in Purkinje progenitor exit from the ventricular zone and in the Sox9+ glial progenitors derived from it. During the same period coinciding with the primary phase of granule neuron precursor genesis, transient EGFP expression tagged the Pax6+ forerunners of granule precursors born in the cerebellar rhombic lip.

CONCLUSION

This study provides the first characterization of S100B-expressing cell types of the embryonic mouse cerebellum in a high-resolution map. The transient activation of the S100B gene distinguishes granule neuron precursors from all other types of precursors so far identified in the rhombic lip, whereas its activation in radial glial precursors is a feature of Bergmann cell gliogenesis.

Wild-type cells rescue genotypically Math1-null hair cells in the inner ears of chimeric mice

The transcription factor Math1 has been shown to be critical in the formation of hair cells (HCs) in the inner ear. However, the influence of environmental factors in HC specification suggests that cell extrinsic factors are also crucial to their development. To test whether extrinsic factors impact development of Math1-null (Math1(beta-Gal/beta-Gal)) HCs, we examined neonatal (postnatal ages P0-P4.5) Math1-null chimeric mice in which genotypically mutant and wild-type cells intermingle to form the inner ear. We provide the first direct evidence that Math1-null HCs are able to be generated and survive in the conducive chimeric environment. beta-Galactosidase expression was used to identify genetically mutant cells while cells were phenotypically defined as HCs by morphological characteristics notably the expression of HC-specific markers. Genotypically mutant HCs were found in all sensory epithelia of the inner ear at all ages examined. Comparable results were obtained irrespective of the wild-type component of the chimeric mice. Thus, genotypically mutant cells retain the competence to differentiate into HCs. The implication is that the lack of the Math1 gene in HC precursors can be overcome by environmental influences, such as cell-cell interactions with wild-type cells, to ultimately result in the formation of HCs.

Antagonism between Notch and bone morphogenetic protein receptor signaling regulates neurogenesis in the cerebellar rhombic lip

Background

During the embryonic development of the cerebellum, neurons are produced from progenitor cells located along a ventricular zone within dorsal rhombomere 1 that extends caudally to the roof plate of the fourth ventricle. The apposition of the caudal neuroepithelium and roof plate results in a unique inductive region termed the cerebellar rhombic lip, which gives rise to granule cell precursors and other glutamatergic neuronal lineages. Recently, we and others have shown that, at early embryonic stages prior to the emergence of granule cell precursors (E12), waves of neurogenesis in the cerebellar rhombic lip produce specific hindbrain nuclei followed by deep cerebellar neurons. How the induction of rhombic lip-derived neurons from cerebellar progenitors is regulated during this phase of cerebellar development to produce these temporally discrete neuronal populations while maintaining a progenitor pool for subsequent neurogenesis is not known.

Results

Employing both gain- and loss-of-function methods, we find that Notch1 signaling in the cerebellar primordium regulates the responsiveness of progenitor cells to bone morphogenetic proteins (BMPs) secreted from the roof plate that stimulate the production of rhombic lip-derived neurons. In the absence of Notch1, cerebellar progenitors are depleted during the early production of hindbrain neurons, resulting in a severe decrease in the deep cerebellar nuclei that are normally born subsequently. Mechanistically, we demonstrate that Notch1 activity prevents the induction of Math1 by antagonizing the BMP receptor-signaling pathway at the level of Msx2 expression.

Conclusion

Our results provide a mechanism by which a balance between neural induction and maintenance of neural progenitors is achieved in the rhombic lip throughout embryonic development.

Math1 target genes are enriched with evolutionarily conserved clustered E-box binding sites

The basic helix-loop-helix (bHLH) transcription factor Math1 and its orthologs are fundamental for proper development of various neuronal subpopulations, such as cerebellar granule cells, D1 interneurons in the spinal cord, and inner ear hair cells. Although crucial for neurogenesis, the mechanisms by which Math1 specifically recognizes its direct targets are not fully understood. To search for direct and indirect target genes and signaling pathways controlled by Math1, we analyzed the effect of Math1 knockout on the expression profile of multiple genes in the embryonic cerebellum. Eighteen differentially expressed transcripts were identified and found to belong to a few developmentally-related functional groups, such as transcriptional regulation, proliferation, organogenesis, signal transduction, and apoptosis. Importantly, genomic analysis of E-box motifs has identified a significant enrichment and clustering of MATH1-binding E-boxes only in a subset of differentially expressed genes (Nr2f6, Hras1, and Hes5) in both mouse and man. Moreover, Math1 was shown by chromatin immunoprecipitation (ChIP) to bind, and by a luciferase reporter assay to activate transcription, of an upstream genomic fragment of Nr2f6. Taken together, we propose that when putative direct targets of Math1 are being selected for detailed studies on DNA microarray hybridization, the enrichment and clustering of binding E-boxes in multiple species may be helpful criteria. Our findings may be useful to the study of other bHLH transcription factors, many of which control the development of the nervous system.

Development and migration of GABAergic neurons in the mouse myelencephalon

GABAergic neurons are the major inhibitory interneurons that are widely distributed in the central nervous system. It is well established that they originate from a focal region in the embryonic forebrain during development, and then migrate to other regions such as the neocortex. However, the migration of GABAergic neurons remains obscure in other axial levels of the brain. We examined the early development of myelencephalic GABAergic neurons using glutamate decarboxylase 67 / green fluorescent protein (GAD67-GFP) knocking mice. Observation of fixed tissues in coronal sections and flat whole-mount preparations indicated that, while GFP-positive cells are restricted to the subpial region in the ventral aspect of the myelencephalon at an early stage, they spread dorsally and eventually occupy the entire region of the myelencephalon as development proceeds. We developed a flat-mount in vitro preparation in which these patterns of development could be recapitulated. Transplantation of dorsal myelencephalic tissue of a wildtype embryo to a corresponding region of GAD67-GFP mouse embryos clearly demonstrated invasion of dorsally oriented GABAergic neurons from host to donor tissue. These results indicate that ventral-to-dorsal tangential migration of GABAergic neurons takes place in the myelencephalon. Our results extend the observations in the forebrain that inhibitory and excitatory neurons in a specific brain compartment take distinct migratory paths.

N-Myc and the cyclin-dependent kinase inhibitors p18Ink4c and p27Kip1 coordinately regulate cerebellar development

Conditional N-Myc deletion limits the proliferation of granule neuron progenitors (GNPs), perturbs foliation, and leads to reduced cerebellar mass. We show that c-Myc mRNA levels increase in N-Myc-null GNPs and that simultaneous deletion of both c- and N-Myc exacerbates defective cerebellar development. Moreover, N-Myc loss has been shown to trigger the precocious expression of two cyclin-dependent kinase inhibitors, Kip1 and Ink4c, in the cerebellar primordium. We now further demonstrate that the engineered disruption of the Kip1 and Ink4c genes in N-Myc-null cerebella partially rescues GNP cell proliferation and cerebellar foliation. These results provide definitive genetic evidence that expression of N-Myc and concomitant down-regulation of Ink4c and Kip1 contribute to the proper development of the cerebellum.

A key role for the HLH transcription factor EBF2COE2,O/E-3 in Purkinje neuron migration and cerebellar cortical topography

Early B-cell factor 2 (EBF2) is one of four mammalian members of an atypical helix-loop-helix transcription factor family (COE). COE proteins have been implicated in various aspects of nervous and immune system development. We and others have generated and described mice carrying a null mutation of Ebf2, a gene previously characterized in the context of Xenopus laevis primary neurogenesis and neuronal differentiation. In addition to deficits in neuroendocrine and olfactory development, and peripheral nerve maturation, Ebf2 null mice feature an ataxic gait and obvious motor deficits associated with clear-cut abnormalities of cerebellar development. The number of Purkinje cells (PCs) in the Ebf2 null is markedly decreased, resulting in a small cerebellum with notable foliation defects,particularly in the anterior vermis. We show that this stems from the defective migration of a molecularly defined PC subset that subsequently dies by apoptosis. Part of the striped cerebellar topography is disrupted due to cell death and, in addition, many of the surviving PCs, that would normally adopt a zebrin II-negative phenotype, transdifferentiate to Zebrin II-positive, an unprecedented finding suggesting that Ebf2 is required for the establishment of a proper cerebellar cortical map.

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.

Migratory routes and fates of cells transcribing the Wnt-1 gene in the murine hindbrain

To investigate the origins, migrations, and fates of Wnt-1-expressing cells in the murine hindbrain, mice carrying a Wnt-1 enhancer/lacZ transgene were observed from embryonic day (E) 8 through postnatal day 18. The transgene-stained ventricular layer waxed and waned prior to and following migrations from it. Stained cells migrated first external to the hindbrain as neural crest and then within it to form typical populations of the rhombic lip, as well as others not recognized as lip derivatives. Migrations originated in a temporally defined sequence, many from discrete rhombomeres. All moved first radially, then rostrally and/or ventrally, ipsi-, or contralaterally, in the mantle or marginal layers. These movements ultimately formed elements of several nuclei, aligned in four longitudinal bands: dorsal (including the gracile, cuneate, cochlear, and vestibular nuclei, plus cerebellar granular cells), dorsal intermediate (including trigeminal sensory, parvicellular reticular, and deep cerebellar nuclei), ventral intermediate (including lateral and intermediate reticular nuclei), and ventral (including the raphe obscurus and pontine nuclei). Transgene staining often persisted long enough to identify stained cells in their definitive, adult nuclei. However, staining was transient. The strength of the staining, however, was in its ability to reveal origins and migrations in both whole-mounts and sections, in single cell detail. The present results will permit analyses of the effects of genetic manipulations on Wnt-1 lineage cells.

Generation of cerebellar neuron precursors from embryonic stem cells

Here, we report in vitro generation of Math1+ cerebellar granule cell precursors and Purkinje cells from ES cells by using soluble patterning signals. When neural progenitors induced from ES cells in a serum-free suspension culture are subsequently treated with BMP4 and Wnt3a, a significant proportion of these neural cells become Math1+. The induced Math1+ cells are mitotically active and express markers characteristic of granule cell precursors (Pax6, Zic1, and Zipro1). After purification by FACS and coculture with postnatal cerebellar neurons, ES cell-derived Math1+ cells exhibit typical features of neurons of the external granule cell layer, including extensive motility and a T-shaped morphology. Interestingly, differentiation of L7+/Calbindin-D28K+ neurons (characteristic of Purkinje cells) is induced under similar culture conditions but exhibits a higher degree of enhancement by Fgf8 rather than by Wnt3a. This is the first report of in vitro recapitulation of early differentiation of cerebellar neurons by using the ES cell system.

A novel role for the choroid plexus in BMP-mediated inhibition of differentiation of cerebellar neural progenitors

Cerebellar granule cells, the most abundant neurons in the mammalian brain, arise in the rhombic lip located at the roof of the brain's fourth ventricle. Bordering the rhombic lip is the choroid plexus, a non-neuronal structure, composed of blood vessels enveloped by epithelial cells. Here, we show a striking decrease in neural differentiation of rhombic lip-derived cells, which failed to extend neuritic processes and attenuate Math1 promoter activity, when co-cultured with choroid plexus cells. Moreover, a blocking antibody against BMP7, a morphogenetic protein expressed in the choroid plexus, blocked the inhibitory effect of the choroid plexus, whereas purified BMP7 mimicked this effect, demonstrating causal involvement of BMP. On the other hand, the BMP antagonist NBL1 promoted neurogenesis in rhombic lip cultures from Math1 null mice displaying arrested differentiation. Our data indicate that besides its secretory and barrier functions, the choroid plexus has a novel role in attenuating the differentiation of adjacent neural progenitors.

The transmembrane semaphorin Sema6A controls cerebellar granule cell migration

The transmembrane semaphorin protein Sema6A is broadly expressed in the developing nervous system. Sema6A repels several classes of developing axons in vitro and contributes to thalamocortical axon guidance in vivo. Here we show that during cerebellum development, Sema6A is selectively expressed by postmitotic granule cells during their tangential migration in the deep external granule cell layer, but not during their radial migration. In Sema6A-deficient mice, many granule cells remain ectopic in the molecular layer where they differentiate and are contacted by mossy fibers. The analysis of ectopic granule cell morphology in Sema6a-/- mice, and of granule cell migration and neurite outgrowth in cerebellar explants, suggests that Sema6A controls the initiation of granule cell radial migration, probably through a modulation of nuclear and/or soma translocation. Finally, the analysis of mouse chimeras suggests that this function of Sema6A is primarily non-cell-autonomous.