Phosphorylation of Neurogenin2 specifies the migration properties and the dendritic morphology of pyramidal neurons in the neocortex

Hippocampus-like corticoneurogenesis induced by two isoforms of the BTB-zinc finger gene Zbtb20 in mice

Hippocampus-associated genes that orchestrate the formation of the compact stratum pyramidale are largely unknown. The BTB (broad complex, tramtrack, bric-a-brac)-zinc finger gene Zbtb20 (also known as HOF, Znf288, Zfp288) encodes two protein isoforms, designated Zbtb20(S) and Zbtb20(L), which are expressed in newborn pyramidal neurons of the presumptive hippocampus in mice. Here, we have generated transgenic mice with ectopic expression of Zbtb20(S) and Zbtb20(L) in immature pyramidal neurons differentiated from multipotent non-hippocampal precursors. The subiculum and posterior retrosplenial areas in these mice were transformed into a three-layered hippocampus-like cortex with a compact homogenous pyramidal cell layer. Severe malformations of lamination occur in neocortical areas, which coincide with a deficiency in expression of cortical lamination markers. The alterations in cortical cytoarchitecture result in behavioral abnormalities suggestive of a deficient processing of visual and spatial memory cues in the cerebral cortex of adult Zbtb20 transgenic mice. Overall, our in vivo data suggest that Zbtb20 functions as a molecular switch for a pathway that induces invariant pyramidal neuron morphogenesis and suppression of cell fate transitions in newborn neurons.

Asymmetric cell division during brain morphogenesis

The division patterns of neural progenitor cells in developing vertebrate brains have traditionally been classified into three types: (i) "symmetric" divisions producing two progenitor cells (P/P division), (ii) "symmetric" divisions producing two neurons (N/N division), and (iii) "asymmetric" divisions producing one progenitor cell and one neuron (P/N division). Many studies examining the mechanism(s) regulating P/N divisions have focused on mitotic cleavage orientation and the possible uneven distribution of cell-fate determining molecules such as Numb. Although these two factors may intrinsically determine daughter cell fate arising from M-phase progenitor cells, no unified explanations have yet to be put forth incorporating all available data. In this review, I will discuss recent advances in techniques allowing the more detailed monitoring of daughter cell behavior in a heterogeneously pseudostratified neuroepithelium that demonstrate previously unrecognized asymmetries in P/P divisions. Careful observations of daughter cell behavior suggest that, immediately after their birth at the apical surface of the neuroepithelium, generated cells may not yet be fate committed but rather integrate extrinsic and intrinsic signals during GI phase before continuing down a developmental pathway.

Pax6 is required for delta-catenin/neurojugin expression during retinal, cerebellar and cortical development in mice

The transcription factor Pax6 regulates multiple aspects of central nervous system (CNS) development. At the cellular level, the Pax6 mutation was reported to affect homophilic and heterophilic cellular adhesion, neuron polarity and neurite outgrowth. These abnormalities were observed in multiple regions of Pax6-mutant CNS, suggesting a common function for Pax6 in regulating cytoskeletal dynamics. However, target genes mediating Pax6 function in cytoskeletal dynamics remain largely unknown. Using DNA microarrays, we identified delta-catenin (delta-catenin /neurojugin) as a potential direct target of Pax6 in the CNS. delta-catenin encodes a large cytoskeletal protein that localizes at adherens junction in the CNS and that can modulate neurite outgrowth and N-cadherin turnover. delta-catenin was found to be co-expressed with Pax6 in several regions of the developing CNS. In Pax6 mutant embryos, delta-catenin expression was severely reduced in the optic vesicle neural ectoderm, in the ventricular zone of the neocortex and in the external granule layer of the cerebellum. We identified a Pax6 binding site in delta-catenin promoter that is conserved between mice and humans and which is effectively bound by Pax6 in vitro. Our results suggest that Pax6 regulates delta-catenin expression during CNS development in mice.

Proneural bHLH and Brn Proteins Coregulate a Neurogenic Program through Cooperative Binding to a Conserved DNA Motif

Proneural proteins play a central role in vertebrate neurogenesis, but little is known of the genes that they regulate and of the factors that interact with proneural proteins to activate a neurogenic program. Here, we demonstrate that the proneural protein Mash1 and the POU proteins Brn1 and Brn2 interact on the promoter of the Notch ligand Delta1 and synergistically activate Delta1 transcription, a key step in neurogenesis. Overexpression experiments in vivo indicate that Brn2, like Mash1, regulates additional aspects of neurogenesis, including the division of progenitors and the differentiation and migration of neurons. We identify by in silico screening a number of additional candidate target genes, which are recognized by Mash1 and Brn proteins through a DNA-binding motif similar to that found in the Delta1 gene and present a broad range of activities. We thus propose that Mash1 synergizes with Brn factors to regulate multiple steps of neurogenesis.

The cyclin-dependent kinase inhibitors p57 and p27 regulate neuronal migration in the developing mouse neocortex

Neuronal precursors remain in the proliferative zone of the developing mammalian neocortex until after they have undergone neuronal differentiation and cell cycle arrest. The newborn neurons then migrate away from the proliferative zone and enter the cortical plate. The molecules that coordinate migration with neuronal differentiation have been unclear. We have proposed in this study that the cdk inhibitors p57 and p27 play a role in this coordination. We have found that p57 and p27 mRNA increase upon neuronal differentiation of neocortical neuroepithelial cells. Knockdown of p57 by RNA interference resulted in a significant delay in the migration of neurons that entered the cortical plate but did not affect neuronal differentiation. Knockdown of p27 also inhibits neuronal migration in the intermediate zone as well as in the cortical plate, as reported by others. We have also found that knockdown of p27 increases p57 mRNA levels. These results suggest that both p57 and p27 play essential roles in neuronal migration and may, in concert, coordinate the timing of neuronal differentiation, migration, and possibly cell cycle arrest in neocortical development.

Neurons in motion: same principles for different shapes?

The special conformation of the developing nervous system, in which progenitor zones are largely confined to the lumen of the neural tube, places neuronal migration as one of the most fundamental processes in brain development. Previous studies have shown that different neuronal types adopt distinct morphological modes of migration in the developing brain, indicating that neuronal migration might be a diverse process. Here, we review recent data on the molecular mechanisms underlying neuronal migration that suggest that similar signaling principles are responsible for the frequently variable morphology of different types of migrating neuron. According to this idea, the same basic molecular mechanisms found in other cell types, such as fibroblasts, might have been adapted to the special morphological needs of migrating neurons in different regions of the developing brain.

Transcriptional regulation of neuronal phenotype in mammals

Transcription factors (TFs) play pivotal roles in directing the formation of neurons and glia. Here I will review the recent genome-scale analysis of the expression of TFs in the developing mouse nervous system and discuss the logic by which TFs control the establishment of neuronal phenotype. Accumulating evidence suggests that while combinatorial action of TFs is able to define the basic framework of the nervous system, other control mechanisms, such as stochastic and epigenetic regulation of gene expression, also contribute to the generation of nerve cell diversity.

p27kip1 independently promotes neuronal differentiation and migration in the cerebral cortex

The generation of neurons by progenitor cells involves the tight coordination of multiple cellular activities, including cell cycle exit, initiation of neuronal differentiation, and cell migration. The mechanisms that integrate these different events into a coherent developmental program are not well understood. Here we show that the cyclin-dependent kinase inhibitor p27(Kip1) plays an important role in neurogenesis in the mouse cerebral cortex by promoting the differentiation and radial migration of cortical projection neurons. Importantly, these two functions of p27(Kip1) involve distinct activities, which are independent of its role in cell cycle regulation. p27(Kip1) promotes neuronal differentiation by stabilizing Neurogenin2 protein, an activity carried by the N-terminal half of the protein. p27(Kip1) promotes neuronal migration by blocking RhoA signaling, an activity that resides in its C-terminal half. Thus, p27(Kip1) plays a key role in cortical development, acting as a modular protein that independently regulates and couples multiple cellular pathways contributing to neurogenesis.

Molecular mechanisms of cortical differentiation

During development, several populations of progenitor cells in the dorsal telencephalon generate a large variety of neurons which acquire distinct morphologies and physiological properties and serve distinct functions in the mammalian cortex. This paper reviews recent work that has identified (i) key molecules involved in the specification and differentiation of cortical neurons, (ii) novel genes which distinguish distinct subsets of cortical progenitors and may be involved in the diversification of cortical neurons present in different cortical layers, and (iii) mechanisms involved in the generation of different projection neuronal subtypes in the well-studied model of layer 5 of the rodent cortex.

Pathogenesis of migration disorders

PURPOSE OF REVIEW

Neocortical neurons have to migrate from their site of production in the periventricular germinative zone or in the ganglionic eminence towards the cortical plate. Our understanding of the underlying molecular mechanisms has advanced considerably in recent years due to the identification of genes involved in human migration disorders and experimental studies. This review will highlight some of the most recent findings in the deciphering of the molecular machinery controlling neuronal migration.

RECENT FINDINGS

Neuronal migration is a complex process which involves cytoskeletal molecules controlling the initiation of migration, leading edge extension and nucleokinesis; signalling molecules (the reelin pathway playing a central role) integrating external signals and linking them to the cytoskeleton; stop signals; and other molecular players including neurotrophins, glutamate receptors and peroxisome-derived factors. Emerging evidence supports the existence of cross-talk between these pathways.

SUMMARY

Identifying these mechanisms has shed light on typical human neuronal migration disorders such as periventricular heterotopias (disorder of migration initiation linked to filamin), type I lissencephaly (cytoskeletal abnormality linked to Lis1, a microtubule-associated protein), double cortex syndrome (cytoskeletal abnormality linked to doublecortin, a microtubule-associated protein), or lissencephaly plus cerebellar hypoplasia (reelin defect).

Coupling of cell migration with neurogenesis by proneural bHLH factors

After cell birth, almost all neurons in the mammalian central nervous system migrate. It is unclear whether and how cell migration is coupled with neurogenesis. Here we report that proneural basic helix-loop-helix (bHLH) transcription factors not only initiate neuronal differentiation but also potentiate cell migration. Mechanistically, proneural bHLH factors regulate the expression of genes critically involved in migration, including down-regulation of RhoA small GTPase and up-regulation of doublecortin and p35, which, in turn, modulate the actin and microtubule cytoskeleton assembly and enable newly generated neurons to migrate. In addition, we report that several DNA-binding-deficient proneural genes that fail to initiate neuronal differentiation still activate migration, whereas a different mutation of a proneural gene that causes a failure in initiating cell migration still leads to robust neuronal differentiation. Collectively, these data suggest that transcription programs for neurogenesis and migration are regulated by bHLH factors through partially distinct mechanisms.

Polarized secretory trafficking directs cargo for asymmetric dendrite growth and morphogenesis

Proper growth of dendrites is critical to the formation of neuronal circuits, but the cellular machinery that directs the addition of membrane components to generate dendritic architecture remains obscure. Here, we demonstrate that post-Golgi membrane trafficking is polarized toward longer dendrites of hippocampal pyramidal neurons in vitro and toward apical dendrites in vivo. Small Golgi outposts partition selectively into longer dendrites and are excluded from axons. In dendrites, Golgi outposts concentrate at branchpoints where they engage in post-Golgi trafficking. Within the cell body, the Golgi apparatus orients toward the longest dendrite, and this Golgi polarity precedes asymmetric dendrite growth. Manipulations that selectively block post-Golgi trafficking halt dendrite growth in developing neurons and cause a shrinkage of dendrites in mature pyramidal neurons. Further, disruption of Golgi polarity produces neurons with symmetric dendritic arbors lacking a single longest principal dendrite. These results define a novel polarized organization of neuronal secretory trafficking and demonstrate a mechanistic link between directed membrane trafficking and asymmetric dendrite growth.

Cellular and molecular control of neurogenesis in the mammalian telencephalon

The mammalian telencephalon exhibits an amazing diversity of neuronal types. The generation of this diversity relies on multiple developmental strategies, including the regional patterning of progenitors, their temporal specification, and the generation of intermediate progenitor populations. Progress has recently been made in characterizing some of the mechanisms involved. In particular, intermediate progenitors have been shown to play important roles in the generation of neurons in the cerebral cortex, and the properties and lineage relationships between radial glial cells and these intermediate progenitors have recently been examined by elegant time-lapse microscopic studies. Multiple pathways control the progression of neural lineages from multipotent stem cells to intermediate progenitors, postmitotic precursors and finally mature neurons. The regulation of two essential steps, neuronal commitment and specification of subtype identities, is increasingly well understood. These two steps are clearly distinct but co-ordinately regulated by common transcription factors such as neurogenins and Pax6. As our knowledge of the mechanisms of subtype specification of telencephalic neurons progresses, it will become possible to direct stem cells into generating particular telencephalic neuronal populations, opening the way to efficient neuronal replacement therapies.