Induction and patterning of neuronal development, and its connection to cell cycle control

A genome-wide in situ hybridization map of RNA-binding proteins reveals anatomically restricted expression in the developing mouse brain

Background

In eukaryotic cells, RNA-binding proteins (RBPs) contribute to gene expression by regulating the form, abundance, and stability of both coding and non-coding RNA. In the vertebrate brain, RBPs account for many distinctive features of RNA processing such as activity-dependent transcript localization and localized protein synthesis. Several RBPs with activities that are important for the proper function of adult brain have been identified, but how many RBPs exist and where these genes are expressed in the developing brain is uncharacterized.

Results

Here we describe a comprehensive catalogue of the unique RBPs encoded in the mouse genome and provide an online database of RBP expression in developing brain. We identified 380 putative RBPs in the mouse genome. Using in situ hybridization, we visualized the expression of 323 of these RBP genes in the brains of developing mice at embryonic day 13.5, when critical fate choice decisions are made and at P0, when major structural components of the adult brain are apparent. We demonstrate i) that 16 of the 323 RBPs examined show neural-specific expression at the stages we examined, and ii) that a far larger subset (221) shows regionally restricted expression in the brain. Of the regionally restricted RBPs, we describe one group that is preferentially expressed in the E13.5 ventricular areas and a second group that shows spatially restricted expression in post-mitotic regions of the embryonic brain. Additionally, we find a subset of RBPs that share the same complex pattern of expression, in proliferating regions of the embryonic and postnatal NS and peripheral tissues.

Conclusion

Our data show that, in contrast to their proposed ubiquitous involvement in gene regulation, most RBPs are not uniformly expressed. Here we demonstrate the region-specific expression of RBPs in proliferating vs. post-mitotic brain regions as well as cell-type-specific RBP expression. We identify uncharacterized RBPs that exhibit neural-specific expression as well as novel RBPs that show expression in non-neural tissues. The data presented here and in an online database provide a visual filter for the functional analysis of individual RBPs.

Six3 functions in anterior neural plate specification by promoting cell proliferation and inhibiting Bmp4 expression

Although it is well established that Six3 is a crucial regulator of vertebrate eye and forebrain development, it is unknown whether this homeodomain protein has a role in the initial specification of the anterior neural plate. In this study, we show that exogenous Six3 can expand the anterior neural plate in both Xenopus and zebrafish, and that this occurs in part through Six3-dependent transcriptional regulation of the cell cycle regulators cyclinD1 and p27Xic1, as well as the anti-neurogenic genes Zic2 and Xhairy2. However, Six3 can still expand the neural plate in the presence of cell cycle inhibitors and we show that this is likely to be due to its ability to repress the expression of Bmp4 in ectoderm adjacent to the anterior neural plate. Furthermore, exogenous Six3 is able to restore the size of the anterior neural plate in chordino mutant zebrafish, indicating that it has the ability to promote anterior neural development by antagonising the activity of the BMP pathway. On its own, Six3 is unable to induce neural tissue in animal caps, but it can do so in combination with Otx2. These results suggest a very early role for Six3 in specification of the anterior neural plate, through the regulation of cell proliferation and the inhibition of BMP signalling.

dlx3b and dlx4b function in the development of Rohon-Beard sensory neurons and trigeminal placode in the zebrafish neurula

Rohon-Beard sensory neurons, neural crest cells, and sensory placodes can be distinguished at the boundary of the embryonic epidermis (skin) and the neural plate. The inductive signals at the neural plate border region are likely to involve a gradient of bone morphogenic protein (BMP) in conjunction with FGF and Wnts and other signals. However, how these signals are transduced to produce the final cell fate remains to be determined. Recent evidence from Xenopus and chick suggest that Dlx genes are required for the generation of cell fates at the neural plate border (McLarren, K.W., Litsiou, A., Streit, A., 2003. DLX5 positions the neural crest and preplacode region at the border of the neural plate. Dev. Biol. 259, 34-47; Woda, J.M., Pastagia, J., Mercola, M., Artinger, K.B., 2003. Dlx proteins position the neural plate border and determine adjacent cell fates. Development 130, 331-342). In the present study, we extend these findings to zebrafish, where we unequivocally demonstrate that dlx3b and dlx4b function in a dose-dependent manner to specify cell fates such as Rohon-Beard sensory neurons and trigeminal sensory placodes. dlx function was examined by inhibiting: (1) protein levels with antisense morpholino oligonucleotides (MOs), and (2) activity by repressing the ability of dlx-homeodomain to bind to downstream targets (EnR-dlx3bhd mRNA; dlx3b homeodomain fused to Engrailed transcriptional repressor domain). Inhibition of dlx3b and dlx4b protein and activity resulted in the reduction or complete loss of Rohon-Beard (RB) sensory neurons and trigeminal (TG) sensory placodes. These data suggest that dlx3b and dlx4b function in the specification of RB neurons and trigeminal sensory placodes in zebrafish. Further, we have shown that dlx3b and dlx4b function in a non-cell-autonomous manner for RB neuron development; dlx3b and dlx4b act to regulate bmp2b expression at the non-neural ectodermal border. These data suggest that the contribution of dlx3b and dlx4b to neural plate border formation is partially non-cell-autonomous acting via BMP activity.

Activating the PARP-1 sensor component of the groucho/ TLE1 corepressor complex mediates a CaMKinase IIdelta-dependent neurogenic gene activation pathway

Switching specific patterns of gene repression and activation in response to precise temporal/spatial signals is critical for normal development. Here we report a pathway in which induction of CaMKIIdelta triggers an unexpected switch in the function of the HES1 transcription factor from a TLE-dependent repressor to an activator required for neuronal differentiation. These events are based on activation of the poly(ADP-ribose) polymerase1 (PARP-1) sensor component of the groucho/TLE-corepressor complex mediating dismissal of the corepressor complex from HES1-regulated promoters. In parallel, CaMKIIdelta mediates a required phosphorylation of HES1 to permit neurogenic gene activation, revealing the ability of a specific signaling pathway to modulate both the derepression and the subsequent coactivator recruitment events required for transcriptional activation of a neurogenic program. The identification of PARP-1 as a regulated promoter-specific exchange factor required for activation of specific neurogenic gene programs is likely to be prototypic of similar molecular mechanisms.

Inhibition of neurogenesis at the zebrafish midbrain-hindbrain boundary by the combined and dose-dependent activity of a new hairy/E(spl) gene pair

The intervening zone (IZ) is a pool of progenitor cells located at the midbrain-hindbrain boundary (MHB) and important for MHB maintenance, midbrain-hindbrain growth and the generation of midbrain-hindbrain neurons. Recently, we implicated the Hairy/E(spl) transcription factor Her5 in the formation of the medial (most basal) part of the IZ (MIZ) in zebrafish; the molecular bases for lateral IZ (LIZ) formation, however, remain unknown. We now demonstrate that her5 is physically linked to a new family member, him, displaying an identical MHB expression pattern. Using single and double knockdowns of him and her5, as well as a him+her5 deletion mutant background (b404), we demonstrate that Him and Her5 are equally necessary for MIZ formation, and that they act redundantly in LIZ formation in vivo. We show that these processes do not involve cross-regulation between Him and Her5 expression or activities, although Him and Her5 can heterodimerize with high affinity. Increasing the function of one factor when the other is depleted further shows that Him and Her5 are functionally interchangeable. Together, our results demonstrate that patterning and neurogenesis are integrated by the her5-him gene pair to maintain a progenitor pool at the embryonic MHB. We propose a molecular mechanism for this process where the global 'Him+Her5' activity inhibits ngn1 expression in a dose-dependent manner and through different sensitivity thresholds along the medio-lateral axis of the neural plate.

Specification of motoneurons from human embryonic stem cells

An understanding of how mammalian stem cells produce specific neuronal subtypes remains elusive. Here we show that human embryonic stem cells generated early neuroectodermal cells, which organized into rosettes and expressed Pax6 but not Sox1, and then late neuroectodermal cells, which formed neural tube-like structures and expressed both Pax6 and Sox1. Only the early, but not the late, neuroectodermal cells were efficiently posteriorized by retinoic acid and, in the presence of sonic hedgehog, differentiated into spinal motoneurons. The in vitro-generated motoneurons expressed HB9, HoxC8, choline acetyltransferase and vesicular acetylcholine transporter, induced clustering of acetylcholine receptors in myotubes, and were electrophysiologically active. These findings indicate that retinoic acid action is required during neuroectoderm induction for motoneuron specification and suggest that stem cells have restricted capacity to generate region-specific projection neurons even at an early developmental stage.

Divide et Impera – the midbrain–hindbrain boundary and its organizer

The midbrain-hindbrain organizer (MHO) is a signalling centre that orchestrates development of the mesencephalic and anterior metencephalic primordia. In recent years, details have been revealed about the molecular nature of these signals, their transmission and reception, and the regulatory processes associated with MHO function. This article reviews recent progress in understanding the genetic and molecular components of the MHO, and how they synergize to control brain development.

Conserved and acquired features of neurogenin1 regulation

The telencephalon shows vast morphological variations among different vertebrate groups. The transcription factor neurogenin1(ngn1) controls neurogenesis in the mouse pallium and is also expressed in the dorsal telencephalon of the evolutionary distant zebrafish. The upstream regions of the zebrafish and mammalian ngn1 loci harbour several stretches of conserved sequences. Here, we show that the upstream region of zebrafish ngn1 is capable of faithfully recapitulating endogenous expression in the zebrafish and mouse telencephalon. A single conserved regulatory region is essential for dorsal telencephalic expression in the zebrafish, and for expression in the dorsal pallium of the mouse. However, a second conserved region that is inactive in the fish telencephalon is necessary for expression in the lateral pallium of mouse embryos. This regulatory region, which drives expression in the zebrafish diencephalon and hindbrain, is dependent on Pax6 activity and binds recombinant Pax6 in vitro. Thus, the regulatory elements of ngn1 appear to be conserved among vertebrates, with certain differences being incorporated in the utilisation of these enhancers, for the acquisition of more advanced features in amniotes. Our data provide evidence for the co-option of regulatory regions as a mechanism of evolutionary diversification of expression patterns, and suggest that an alteration in Pax6expression was crucial in neocortex evolution.

Induction of bone marrow stromal cells to neurons: differentiation, transdifferentiation, or artifact?

Differentiation of stem cells toward a neuronal lineage normally involves a gradually progressive restriction in developmental potential and is regulated by a diverse set of specific and temporally precise genetic events. However, recent studies have indicated that both rodent and human bone marrow stromal cells (MSCs) can be rapidly (within minutes to hours) induced to differentiate into neurons in vitro by relatively simple chemical means (using beta-mercaptoethanol [BME] or dimethylsulfoxide [DMSO] and butylated hydroxyanisol [BHA]; Woodbury et al. [ 2000] J. Neurosci. Res. 61:364-370). The ability to transdifferentiate an easily accessible cell source into neurons could have substantial potential for promoting neural repair. We therefore explored the potential of simple chemical methods to transdifferentiate other cell types, including primary rat fibroblasts, primary human keratinocytes, HEK293 cells, rat PC-12 cells, and as positive control rat bone marrow stromal (BMS) cells. Surprisingly, all cells except for keratinocytes adopted at least partial "neuron-like" pyramidal cell morphology with fine-cellular extensions resembling neurites upon stimulation with BME or DMSO/BHA. However, time-lapse microscopy indicated that the chemical exposure of MSCs did not result in new neurite growth but rather cellular shrinkage, with retraction of the majority of existing cell extensions, leaving only few, fine neurite-like processes. To determine whether the chemically induced transdifferentiation resulted from simple cellular toxicity, MSCs were exposed to various stressors, including detergents, high-molarity sodium chloride, and extremes of pH. In all cases, cellular shrinkage and adoption of pseudoneuronal morphology were observed. Concomitantly with cellular shrinkage, apparent increases in immunolabeling for the neuronal markers NSE and NeuN were detected in the cell soma that could not be confirmed by RT-PCR. Furthermore, blockade of protein synthesis with cycloheximide did not prevent cells from adopting "neuron-like" morphology after chemical induction. Thus, morphological changes and increases in immunolabeling for certain cellular markers upon "chemical induction" of MSCs are likely the result of cellular toxicity, cell shrinkage, and changes in the cytoskeleton and do not represent regulated steps in a complicated cellular differentiation process.

Combinatorial Fgf and Bmp signalling patterns the gastrula ectoderm into prospective neural and epidermal domains

Studies in fish and amphibia have shown that graded Bmp signalling activity regulates dorsal-to-ventral (DV) patterning of the gastrula embryo. In the ectoderm, it is thought that high levels of Bmp activity promote epidermal development ventrally, whereas secreted Bmp antagonists emanating from the organiser induce neural tissue dorsally. However, in zebrafish embryos, the domain of cells destined to contribute to the spinal cord extends all the way to the ventral side of the gastrula, a long way from the organiser. We show that in vegetal (trunk and tail) regions of the zebrafish gastrula, neural specification is initiated at all DV positions of the ectoderm in a manner that is unaffected by levels of Bmp activity and independent of organiser-derived signals. Instead, we find that Fgf activity is required to induce vegetal prospective neural markers and can do so without suppressing Bmp activity. We further show that Bmp signalling does occur within the vegetal prospective neural domain and that Bmp activity promotes the adoption of caudal fate by this tissue.

An inhibition of cyclin-dependent kinases that lengthens, but does not arrest, neuroepithelial cell cycle induces premature neurogenesis

The G1 phase of the cell cycle of neuroepithelial cells, the progenitors of all neurons of the mammalian central nervous system, has been known to lengthen concomitantly with the onset and progression of neurogenesis. We have investigated whether lengthening of the G1 phase of the neuroepithelial cell cycle is a cause, rather than a consequence, of neurogenesis. As an experimental system, we used whole mouse embryo culture, which was found to exactly reproduce the temporal and spatial gradients of the onset of neurogenesis occurring in utero. Olomoucine, a cell-permeable, highly specific inhibitor of cyclin-dependent kinases and G1 progression, was found to significantly lengthen, but not arrest, the cell cycle of neuroepithelial cells when used at 80 microM. This olomoucine treatment induced, in the telencephalic neuroepithelium of embryonic day 9.5 to 10.5 mouse embryos developing in whole embryo culture to embryonic day 10.5, (i) the premature up-regulation of TIS21, a marker identifying neuroepithelial cells that have switched from proliferative to neuron-generating divisions, and (ii) the premature generation of neurons. Our data indicate that lengthening G1 can alone be sufficient to induce neuroepithelial cell differentiation. We propose a model that links the effects of cell fate determinants and asymmetric cell division to the length of the cell cycle.

Her5 acts as a prepattern factor that blocks neurogenin1 and coe2 expression upstream of Notch to inhibit neurogenesis at the midbrain-hindbrain boundary

Neurogenesis in both vertebrates and invertebrates is tightly controlled in time and space involving both positive and negative regulators. We report here that the bHLH factor Her5 acts as a prepattern gene to prevent neurogenesis in the anlage of the midbrain/hindbrain boundary in the zebrafish neural plate. This involves selective suppression of both neurogenin1(ngn1) and coe2 mRNA expression in a process that is independent of Notch signalling, and where inhibition of either ngn1or coe2 expression is sufficient to prevent neuronal differentiation across the midbrain-hindbrain boundary. A ngn1 transgene faithfully responds to Her5 and deletion analysis of the transgene identifies an E-box in a ngn1 upstream enhancer to be required for repression by Her5. Together our data demonstrate a role of Her5 as a prepattern factor in the spatial definition of proneural domains in the zebrafish neural plate, in a manner similar to its Drosophila homologue Hairy.

Early steps in the development of the forebrain

The tremendous complexity of the adult forebrain makes it a challenging task to elucidate how this structure forms during embryonic development. Nevertheless, we are beginning to understand how a simple epithelial sheet of ectoderm gives rise to the labyrinthine network of cells that constitutes the functional forebrain. Here, we discuss early events in forebrain development--those that lead to the establishment of the anterior neural plate and the regional subdivision of this territory into the different domains of the prospective forebrain.

Hyzic, the Hydra homolog of the zic/odd-pairedgene, is involved in the early specification of the sensory nematocytes

Cnidaria are the first class of organisms in animal evolution with a nervous system. The cnidarian Hydra has two types of neuronal cell,nerve cells and nematocytes. Both differentiate from the same pool of pluripotent stem cells. Yet, the molecular regulation of neural differentiation in Hydra is largely unknown. Here, we report the identification of Hyzic, a homolog of the Zn-finger transcription factor gene zic/odd-paired, which acts as an early neural effector gene in vertebrates. We show, that Hyzic is expressed in the early nematocyte differentiation pathway, starting at the level of interstitial stem cells. Expression of Hyzic is restricted to the proliferative stages of nematoblasts. Hyzic acts before and possibly directly upstream of Cnash, a homolog of the proneural bHLH transcription factor gene achaete-scute, and of Nowa, an early nematocyte differentiation marker gene. Hyzic may determine stem cells to differentiate into nematocytes. Our data are consistent with a role of Hyzic in inhibiting nematocyte differentiation, by keeping committed nematoblast cells in the cell cycle. A similar role has been demonstrated for Zic genes in vertebrates. Our results suggest, that genetic cascades of neural development may be conserved from Hydra to vertebrates, indicating that the molecular regulation of neural development evolved only once.

Molecular conservation and novelties in vertebrate ear development

Evolution shaped the vertebrate ear into a complicated three-dimensional structure and positioned the sensory epithelia so that they can extract specific aspects of mechanical stimuli to govern vestibular and hearing-related responses of the whole organism. This information is conducted from the ear via specific neuronal connections to distinct areas of the hindbrain for proper processing. During development, the otic placode, a simple sheet of epidermal cells, transforms into a complicated system of ducts and recesses. This placode also generates the mechanoelectrical transducers, the hair cells, and sensory neurons of the vestibular and cochlear (spiral) ganglia of the ear. We argue that ear development can be broken down into dynamic processes that use a number of known and unknown genes to govern the formation of the three-dimensional labyrinth in an interactive fashion. Embedded in this process, but in large part independent of it, is an evolutionary conserved process that induces early the development of the neurosensory component of the ear. We present molecular data suggesting that this later process is, in its basic aspects, related to the mechanosensory cell formation across phyla and is extremely conserved at the molecular level. We suggest that sensory neuron development and maintenance are vertebrate or possibly chordate novelties and present the molecular data to support this notion.

Cell Proliferation during Early Hindbrain Development in Alligator

Cell proliferation, as determined by immunoreactivity to proliferating cell nuclear antigen (PCNA) was investigated during early hindbrain development in Alligator. At the earliest stage examined, stage 3, when five rhombomeres are present, PCNA immunoreactivity is more robust laterally towards the pial margin and interrhombomeric boundaries are clearly seen. Subsequently, PCNA immunoreactivity fills each respective rhombomere and increases in intensity but respects the boundaries between segments until stage 7. At this time, borders between rhombomeres become less distinct. At stage 8, when the full compliment of rhombomeres is present, two longitudinal columns of PCNA immunoreactivity are seen rostrally which coalesce into a single column caudally. Interrhombomeric borders are indistinct. Later, at stage 9/10, two longitudinal columns span the entire length of the hindbrain and interrhombomeric boundaries remain less clear. The more lateral column fades whereas the medial stripe persists through stage 12. Thereafter, immunoreactivity fades. These observations confirm the findings in other species that rhombomeres are centers of cell proliferation. This feature is most likely common to hindbrain development in all gnathostomes. It is hypothesized that these two longitudinal columns of cell proliferation might be important for future patterning of the hindbrain.