Cerebellar GABAergic progenitors adopt an external granule cell-like phenotype in the absence of Ptf1a transcription factor expression

Phenotypic and genetic analysis of the cerebellar mutant tmgc26, a new ENU-induced ROR-alpha allele

ROR-alpha is an orphan nuclear receptor, inactivation of which cell-autonomously blocks differentiation of cerebellar Purkinje cells with a secondary loss of granule neurons. As part of our ENU mutagenesis screen we isolated the recessive tmgc26 mouse mutant, characterized by early-onset progressive ataxia, cerebellar degeneration and juvenile lethality. Detailed analysis of the tmgc26-/- cerebella revealed Purkinje cell and granule cell abnormalities, and defects in molecular layer interneurons and radial glia. Chimera studies suggested a cell-autonomous effect of the tmgc26 mutation in Purkinje cells and molecular layer interneurons, and a non-cell-autonomous effect in granule cells. The mutation was mapped to a 13-Mb interval on chromosome 9, a region that contains the ROR-alpha gene. Sequencing of genomic DNA revealed a T-to-A transition in exon 5 of the ROR-alpha gene, resulting in a nonsense mutation C257X and severe truncation of the ROR-alpha protein. Together, our data identify new roles for ROR-alpha in molecular layer interneurons and radial glia development and suggest tmgc26 as a novel ROR-alpha allele that may be used to further delineate the molecular mechanisms of ROR-alpha action.

Transventricular delivery of Sonic hedgehog is essential to cerebellar ventricular zone development

Cerebellar neurons are generated from two germinal neuroepithelia: the ventricular zone (VZ) and rhombic lip. Signaling mechanisms that maintain the proliferative capacity of VZ resident progenitors remain elusive. We reveal that Sonic hedgehog (Shh) signaling is active in the cerebellar VZ and essential to radial glial cell proliferation and expansion of GABAergic interneurons. We demonstrate that the cerebellum is not the source of Shh that signals to the early VZ, and suggest a transventricular path for Shh ligand delivery. In agreement, we detected the presence of Shh protein in the circulating embryonic cerebrospinal fluid. This study identifies Shh as an essential proliferative signal for the cerebellar ventricular germinal zone, underscoring the potential contribution of VZ progenitors in the pathogenesis of cerebellar diseases associated with deregulated Shh signaling, and reveals a transventricular source of Shh in regulating neural development.

Neurogenin1 expression in cell lineages of the cerebellar cortex in embryonic and postnatal mice

The basic helix-loop-helix (bHLH) transcription factors Ptf1a and Math1 are necessary for the specification of gamma-aminobutyric acid-ergic and glutamatergic cell lineages in the cerebellum, respectively. Recent evidence suggests cascades of bHLH factor activities drive cell type specificity in Ptf1a(+ve) and Math1(+ve) lineages. In this manuscript, we reveal cell lineages in the cerebellar cortex but not deep cerebellar nuclei express the pro-neural bHLH factor Neurogenin1 (Ngn1). Ngn1 is expressed in ventricular zone progenitors and in newly generated neurons in the caudal cerebellar primordium. In later embryonic and postnatal developmental stages, Ngn1 is expressed in progenitors and in migrating interneurons in the prospective white matter. Transgenic fate-mapping reveals Ngn1 reporter-gene expression in Purkinje cells, multiple inhibitory interneuron cell types, and in unipolar brush cells of the cortex. The data suggest Ngn1 is a component of the bHLH factor code regulating cell type specification in the cerebellar cortex.

Ptf1a directly controls expression of immunoglobulin superfamily molecules Nephrin and Neph3 in the developing central nervous system

Ptf1a, a basic helix-loop-helix transcription factor, plays an indispensable role for cell fate specification of subsets of neurons in the developing central nervous system. However, downstream molecules induced by Ptf1a during neural development have not been well characterized. In the present study, we identified immunoglobulin superfamily molecules, Nephrin and Neph3, as direct downstream targets of Ptf1a. First, the expression domains of Nephrin and Neph3 closely resembled those of Ptf1a in the developing retina, hypothalamus, cerebellum, hindbrain, and spinal cord. Second, Ptf1a bound directly to a PTF-binding motif in the 5'-flanking region of Nephrin and Neph3 genes. Third, Ptf1a activated transcription driven by the 5'-flanking region of these genes. Finally, the expression of Nephrin and Neph3 was lost in Ptf1a-null mice, whereas ectopic expression of Nephrin and Neph3 was induced by forced expression of Ptf1a. We provided further evidence that Nephrin and Neph3 could interact homophilically and heterophilically, suggesting that Nephrin and Neph3 might regulate certain developmental aspects of Ptf1a-positive neurons as homo- or heterooligomers.

Rautenlippe Redux -- toward a unified view of the precerebellar rhombic lip

The rhombic lip (aka rautenlippe) is a germinative neuroepithelium rimming the opening of the hindbrain fourth ventricle during development. Studies spanning more than a century have shown that the rhombic lip produces numerous brainstem neuronal populations unique in their development and functions. While these studies have largely been anatomical in nature, recent applications of newer techniques such as genetic fate mapping and conditional mutagenesis have resolved the rhombic lip into numerous molecularly distinct progenitor domains along spatial and temporal axes that give rise to specific neuron subtypes and systems. This exciting convergence between anatomical and molecular definitions of the rhombic lip and its constituent progenitor populations provides now an important framework for further studies into the genetic basis of development and function of numerous hindbrain neuron types crucial to life.

Multiple transcriptional mechanisms control Ptf1a levels during neural development including autoregulation by the PTF1-J complex

Ptf1a, along with an E protein and Rbpj, forms the transcription factor complex PTF1-J that is essential for proper specification of inhibitory neurons in the spinal cord, retina, and cerebellum. Here we show that two highly conserved noncoding genomic regions, a distal 2.3 kb sequence located 13.4 kb 5' and a 12.4 kb sequence located immediately 3' of the Ptf1a coding region, have distinct activity in controlling Ptf1a expression in all of these domains. The 5' 2.3 kb sequence functions as an autoregulatory element and directs reporter gene expression to all Ptf1a domains in the developing nervous system. The autoregulatory activity of this element was demonstrated by binding of the PTF1-J complex in vitro, Ptf1a localization to this genomic region in vivo, and the in vivo requirement of Ptf1a for the activity of the regulatory element in transgenic mice. In contrast, the 12.4 kb 3' regulatory region does not contain any conserved PTF1 sites, and its expression in transgenic mice is independent of Ptf1a. Thus, regulatory information for initiation of Ptf1a expression in the developing nervous system is located within the 12.4 kb sequence 3' of the Ptf1a gene. Together, these results identify multiple transcriptional mechanisms that control Ptf1a levels, one modulating levels by autoregulation through the PTF1-J complex, and the other a Ptf1a-independent mechanism for initial activation.

Ptf1a is expressed transiently in all types of amacrine cells in the embryonic zebrafish retina

Background

The vertebrate retina is composed of five major types of neurons: three excitatory (photoreceptors, bipolar cells and ganglion cells) and two inhibitory (horizontal and amacrine cells). The transcription factor Ptf1a (pancreas transcription factor 1a) is important for the normal development of the inhibitory retinal neurons.

Results

Using a transgenic Ptf1a:GFP reporter and in situ hybridization in the zebrafish retina, we show that ptf1a message is transiently expressed in all amacrine and horizontal cells within hours after the terminal division of multipotent progenitors at the apical surface of the retinal neuroepithelium, and remains on as these cells migrate to their final laminar location. The message then shuts off, but we can follow the stable Ptf1a:GFP protein for up to 120 hours post-fertilization. A variety of anatomically and neurochemically distinct subtypes of amacrine cells can already be distinguished at this embryonic time point.

Conclusion

The timing of Ptf1a expression suggests that it is involved in the very early stages or steps in the differentiation of amacrine cells, which, due to the perdurance of the Ptf1a:GFP, can be seen to rapidly diversify into a large number of subtypes. This work sets the stage for future studies looking at genetic specification of amacrine subtypes.

The autism susceptibility gene met regulates zebrafish cerebellar development and facial motor neuron migration

During development, Met signaling regulates a range of cellular processes including growth, differentiation, survival and migration. The Met gene encodes a tyrosine kinase receptor, which is activated by Hgf (hepatocyte growth factor) ligand. Altered regulation of human MET expression has been implicated in autism. In mouse, Met signaling has been shown to regulate cerebellum development. Since abnormalities in cerebellar structure have been reported in some autistic patients, we have used the zebrafish to address the role of Met signaling during cerebellar development and thus further our understanding of the molecular basis of autism. We find that zebrafish met is expressed in the cerebellar primordium, later localizing to the ventricular zone (VZ), with the hgf1 and hgf2 ligand genes expressed in surrounding tissues. Morpholino knockdown of either Met or its Hgf ligands leads to a significant reduction in the size of the cerebellum, primarily as a consequence of reduced proliferation. Met signaling knockdown disrupts specification of VZ-derived cell types, and also reduces granule cell numbers, due to an early effect on cerebellar proliferation and/or as an indirect consequence of loss of signals from VZ-derived cells later in development. These patterning defects preclude analysis of cerebellar neuronal migration, but we have found that Met signaling is necessary for migration of hindbrain facial motor neurons. In summary, we have described roles for Met signaling in coordinating growth and cell type specification within the developing cerebellum, and in migration of hindbrain neurons. These functions may underlie the correlation between altered MET regulation and autism spectrum disorders.

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.

Neurog2 is a direct downstream target of the Ptf1a-Rbpj transcription complex in dorsal spinal cord

PTF1-J is a trimeric transcription factor complex essential for generating the correct balance of GABAergic and glutamatergic interneurons in multiple regions of the nervous system, including the dorsal horn of the spinal cord and the cerebellum. Although the components of PTF1-J have been identified as the basic helix-loop-helix (bHLH) factor Ptf1a, its heterodimeric E-protein partner, and Rbpj, no neural targets are known for this transcription factor complex. Here we identify the neuronal differentiation gene Neurog2 (Ngn2, Math4A, neurogenin 2) as a direct target of PTF1-J. A Neurog2 dorsal neural tube enhancer localized 3' of the Neurog2 coding sequence was identified that requires a PTF1-J binding site for dorsal activity in mouse and chick neural tube. Gain and loss of Ptf1a function in vivo demonstrate its role in Neurog2 enhancer activity. Furthermore, chromatin immunoprecipitation from neural tube tissue demonstrates that Ptf1a is bound to the Neurog2 enhancer. Thus, Neurog2 expression is directly regulated by the PTF1-J complex, identifying Neurog2 as the first neural target of Ptf1a and revealing a bHLH transcription factor cascade functioning in the specification of GABAergic neurons in the dorsal spinal cord and cerebellum.

Development of cerebellar GABAergic interneurons: origin and shaping of the "minibrain" local connections

The cerebellar circuits comprise a limited number of neuronal phenotypes embedded in a defined cytoarchitecture and generated according to specific spatio-temporal patterns. The local GABAergic network is composed of several interneuron phenotypes that play essential roles in information processing by modulating the activity of cerebellar cortical inputs and outputs. A major issue in the study of cerebellar development is to understand the mechanisms that underlie the generation of different interneuron classes and regulate their placement in the cerebellar architecture and integration in the cortico-nuclear network. Recent findings indicate that the variety of cerebellar interneurons derives from a single population of multipotent progenitors whose fate choices are determined by instructive environmental information. Such a strategy, which is unique for the cerebellum along the neuraxis, allows great flexibility in the control of the quality and quantity of GABAergic interneurons that are produced, thus facilitating the adaptive shaping of the cerebellar network to specific functional demands.

p/CAF modulates the activity of the transcription factor p48/Ptf1a involved in pancreatic acinar differentiation

p48, also called Ptf1a (pancreas-specific transcription factor 1a), is a tissue-restricted bHLH (basic helix loop helix) transcription factor which is critical for pancreatic commitment during development and for the activation and maintenance of the acinar differentiation programme in the exocrine pancreas. High-level expression of exocrine digestive enzymes, a hallmark of mature acinar cells, depends largely on the trimeric complex PTF1, formed by p48, RBP-L (recombination signal-binding protein 1-like) and a class A bHLH protein. In addition, p48 induces cell-cycle exit by controlling G1/S-phase progression. However, the mechanisms that mediate PTF1-dependent gene activation are poorly understood. In the present study, we report that p48 increases transcription through two activation domains located in its N-terminal region by recruiting transcriptional co-activators. The histone acetyltransferase cofactor p/CAF {p300/CBP [CREB (cAMP-response-element-binding protein)-binding protein]-associated factor} interacts with p48 in acinar cells in vivo and is associated with the promoter region of acinar genes targeted by the PTF1 complex. p/CAF potentiates PTF1 transcriptional activity by enhancing selectively the p48 transactivation activity. p/CAF promotes the nuclear accumulation of p48 and its in vivo acetylation in Lys200. The K200R mutation abolishes the transcriptional activity of p48, as well as its capacity to functionally co-operate with RBP-L to ensure effective PTF1-driven transcription, indicating that p/CAF-mediated acetylation of p48 is required for the full transcriptional activity of PTF1. In contrast, p/CAF did not co-operate with p48 in its growth regulatory effects. These results support a critical and selective role of p/CAF in PTF1-dependent gene activation during acinar differentiation.

Transcriptional cascade from Math1 to Mbh1 and Mbh2 is required for cerebellar granule cell differentiation

Cerebellar granule cells (CGCs) are the most abundant neuronal type in the mammalian brain, and their differentiation is regulated by the basic helix-loop-helix gene, Math1. However, little is known about downstream genes of Math1 and their functions in the cerebellum. To investigate them, we have here established an electroporation-based in vivo gene transfer method in the developing mouse cerebellum. Misexpression of Math1 ectopically induced expression of Bar-class homeobox genes, Mbh1 and Mbh2, which are expressed by CGCs. Conversely, their expression was repressed in CGCs by knockdown of Math1. These findings, taken together with chromatin immunoprecipitation assays, suggest that Math1 directly regulates the Mbh genes in CGCs. Furthermore, a dominant-negative form of the Mbh proteins disrupted proper formation of the external granule layer and differentiation of CGCs, whereas misexpression of the Mbh genes ectopically induced expression of a CGC marker in nonneuronal cells, indicating that the Mbh proteins are required for the differentiation of CGCs.

Comparative analysis of proneural gene expression in the embryonic cerebellum

The embryonic cerebellum contains two germinative epithelia: the rhombic lip and the ventricular zone. While the lineage of glutamatergic neurons arising from the rhombic lip has been characterized, plenty remains to be learned about the factors giving rise to the array of ventricular zone-derived gamma-aminobutyric acid (GABA)ergic neurons. In the present study, we describe the expression of proneural genes Mash1/Ascl1, Ngn1/Neurog1, and Ngn2/Neurog2 in the cerebellar primordium at key stages of Purkinje cell and interneuron development, and compare them with the expression of other genes active in the same context. Our results indicate that Ngn1, Ngn2 and Mash1 are expressed at relevant stages of cerebellar neurogenesis in the prospective cerebellar nuclei and in the ventricular zone, excluding the Math1/Atoh1-positive rhombic lip. Their expression domains are only partially overlapping, suggesting that they may contribute selectively to ventricular zone regionalization, giving rise to the diversity of cerebellar GABA neurons and, possibly, Purkinje cell subtypes.

Besides Purkinje cells and granule neurons: an appraisal of the cell biology of the interneurons of the cerebellar cortex

Ever since the groundbreaking work of Ramon y Cajal, the cerebellar cortex has been recognized as one of the most regularly structured and wired parts of the brain formed by a rather limited set of distinct cells. Its rather protracted course of development, which persists well into postnatal life, the availability of multiple natural mutants, and, more recently, the availability of distinct molecular genetic tools to identify and manipulate discrete cell types have suggested the cerebellar cortex as an excellent model to understand the formation and working of the central nervous system. However, the formulation of a unifying model of cerebellar function has so far proven to be a most cantankerous problem, not least because our understanding of the internal cerebellar cortical circuitry is clearly spotty. Recent research has highlighted the fact that cerebellar cortical interneurons are a quite more diverse and heterogeneous class of cells than generally appreciated, and have provided novel insights into the mechanisms that underpin the development and histogenetic integration of these cells. Here, we provide a short overview of cerebellar cortical interneuron diversity, and we summarize some recent results that are hoped to provide a primer on current understanding of cerebellar biology.

Cerebellar development and disease

The molecular control of cell-type specification within the developing cerebellum as well as the genetic causes of the most common human developmental cerebellar disorders have long remained mysterious. Recent genetic lineage and loss-of-function data from mice have revealed unique and nonoverlapping anatomical origins for GABAergic neurons from ventricular zone precursors and glutamatergic cell from rhombic lip precursors, mirroring distinct origins for these neurotransmitter-specific cell types in the cerebral cortex. Mouse studies elucidating the role of Ptf1a as a cerebellar ventricular zone GABerigic fate switch were actually preceded by the recognition that PTF1A mutations in humans cause cerebellar agenesis, a birth defect of the human cerebellum. Indeed, several genes for congenital human cerebellar malformations have recently been identified, including genes causing Joubert syndrome, Dandy-Walker malformation, and pontocerebellar hypoplasia. These studies have pointed to surprisingly complex roles for transcriptional regulation, mitochondrial function, and neuronal cilia in patterning, homeostasis, and cell proliferation during cerebellar development. Together, mouse and human studies are synergistically advancing our understanding of the developmental mechanisms that generate the uniquely complex mature cerebellum.

A nonclassical bHLH Rbpj transcription factor complex is required for specification of GABAergic neurons independent of Notch signaling

Neural networks are balanced by inhibitory and excitatory neuronal activity. The formation of these networks is initially generated through neuronal subtype specification controlled by transcription factors. The basic helix-loop-helix (bHLH) transcription factor Ptf1a is essential for the generation of GABAergic inhibitory neurons in the dorsal spinal cord, cerebellum, and retina. The transcription factor Rbpj is a transducer of the Notch signaling pathway that functions to maintain neural progenitor cells. Here we demonstrate Ptf1a and Rbpj interact in a complex that is required in vivo for specification of the GABAergic neurons, a function that cannot be substituted by the classical form of the bHLH heterodimer with E-protein or Notch signaling through Rbpj. We show that a mutant form of Ptf1a without the ability to bind Rbpj, while retaining its ability to interact with E-protein, is incapable of inducing GABAergic (Pax2)- and suppressing glutamatergic (Tlx3)-expressing cells in the chick and mouse neural tube. Moreover, we use an Rbpj conditional mutation to demonstrate that Rbpj function is essential for GABAergic specification, and that this function is independent of the Notch signaling pathway. Together, these findings demonstrate the requirement for a Ptf1a-Rbpj complex in controlling the balanced formation of inhibitory and excitatory neurons in the developing spinal cord, and point to a novel Notch-independent function for Rbpj in nervous system development.

Origin of climbing fiber neurons and their developmental dependence on Ptf1a

Climbing fiber (CF) neurons in the inferior olivary nucleus (ION) extend their axons to Purkinje cells, playing a crucial role in regulating cerebellar function. However, little is known about their precise place of birth and developmental molecular machinery. Here, we describe the origin of the CF neuron lineage and the involvement of Ptf1a (pancreatic transcription factor 1a) in CF neuron development. Ptf1a protein was found to be expressed in a discrete dorsolateral region of the embryonic caudal hindbrain neuroepithelium. Because expression of Ptf1a is not overlapping other transcription factors such as Math1 (mouse atonal homolog 1) and Neurogenin1, which are suggested to define domains within caudal hindbrain neuroepithelium (Landsberg et al., 2005), we named the neuroepithelial region the Ptf1a domain. Analysis of mice that express beta-galactosidase from the Ptf1a locus revealed that CF neurons are derived from the Ptf1a domain. In contrast, retrograde labeling of precerebellar neurons indicated that mossy fiber neurons are not derived from Ptf1a-expressing progenitors. We could observe a detailed migratory path of CF neurons from the Ptf1a domain to the ION during embryogenesis. In Ptf1a null mutants, putative immature CF neurons produced from this domain were unable to migrate or differentiate appropriately, resulting in a failure of ION formation. Apoptotic cells were observed in the mutant hindbrain. Furthermore, the fate of some cells in the Ptf1a lineage were changed to mossy fiber neurons in Ptf1a null mutants. These findings clarify the precise origin of CF neurons and suggest that Ptf1a controls their fate, survival, differentiation, and migration during development.

Ptf1a triggers GABAergic neuronal cell fates in the retina

Background

In recent years, considerable knowledge has been gained on the molecular mechanisms underlying retinal cell fate specification. However, hitherto studies focused primarily on the six major retinal cell classes (five types of neurons of one type of glial cell), and paid little attention to the specification of different neuronal subtypes within the same cell class. In particular, the molecular machinery governing the specification of the two most abundant neurotransmitter phenotypes in the retina, GABAergic and glutamatergic, is largely unknown. In the spinal cord and cerebellum, the transcription factor Ptf1a is essential for GABAergic neuron production. In the mouse retina, Ptf1a has been shown to be involved in horizontal and most amacrine neurons differentiation.

Results

In this study, we examined the distribution of neurotransmitter subtypes following Ptf1a gain and loss of function in the Xenopus retina. We found cell-autonomous dramatic switches between GABAergic and glutamatergic neuron production, concomitant with profound defects in the genesis of amacrine and horizontal cells, which are mainly GABAergic. Therefore, we investigated whether Ptf1a promotes the fate of these two cell types or acts directly as a GABAergic subtype determination factor. In ectodermal explant assays, Ptf1a was found to be a potent inducer of the GABAergic subtype. Moreover, clonal analysis in the retina revealed that Ptf1a overexpression leads to an increased ratio of GABAergic subtypes among the whole amacrine and horizontal cell population, highlighting its instructive capacity to promote this specific subtype of inhibitory neurons. Finally, we also found that within bipolar cells, which are typically glutamatergic interneurons, Ptf1a is able to trigger a GABAergic fate.

Conclusion

Altogether, our results reveal for the first time in the retina a major player in the GABAergic versus glutamatergic cell specification genetic pathway.

Neurogenesis in the Cerebellum

In the past few years, genetic fate mapping experiments have changed our vision of cerebellar development, particularly in redefining the origin of gabaergic and glutamatergic neurons of the cerebellar cortex and highlighting the precise spatio-temporal sequence of their generation. Here the authors review cerebellar neurogenesis and discuss the fate mapping studies with other new information stemming from transplantation experiments, in an effort to link the developmental potential of neural progenitor populations of the cerebellum with their spatio-temporal origin. NEUROSCIENTIST 14(1):91—100, 2008.

LIM-homeodomain proteins Lhx1 and Lhx5, and their cofactor Ldb1, control Purkinje cell differentiation in the developing cerebellum

Purkinje cells are one of the major types of neurons that form the neural circuitry in the cerebellum essential for fine control of movement and posture. During development, Purkinje cells also are critically involved in the regulation of proliferation of progenitors of granule cells, the other major type of neurons in the cerebellum. The process that controls differentiation of Purkinje cells from their early precursors is poorly understood. Here we report that two closely related LIM-homeobox genes, Lhx1 and Lhx5, were expressed in the developing Purkinje cells soon after they exited the cell cycle and migrated out of the cerebellar ventricular zone. Double-mutant mice lacking function of both Lhx1 and Lhx5 showed a severe reduction in the number of Purkinje cells. In addition, targeted inactivation of Ldb1, which encodes a cofactor for all LIM-homeodomain proteins, resulted in a similar phenotype. Our studies thus provide evidence that these transcription regulators are essential for controlling Purkinje cell differentiation in the developing mammalian cerebellum.