Roof plate-dependent patterning of the vertebrate dorsal central nervous system

In the vertebrate central nervous system (CNS), diverse cellular types are generated in response to inductive signals provided by specialized cellular groups that act as organizing centers. The roof plate is a critical dorsal signaling center that occupies the dorsal midline of the developing CNS along its entire anterior-posterior axis. During caudal neural tube development, the roof plate produces proteins of the Bmp and Wnt families controlling proliferation, specification, migration, and axon guidance of adjacent dorsal interneurons. Although primarily investigated in the developing spinal cord, a growing number of studies indicate that roof plate-derived signals are also critical for the patterning of dorsal structures in more rostral regions of CNS including the hindbrain, diencephalon and telencephalon. In this review, we discuss recent progress towards understanding the molecular and cellular mechanisms of roof plate-dependent patterning of the dorsal CNS.     

An early role for WNT signaling in specifying neural patterns of Cdx and Hox gene expression and motor neuron subtype identity

The link between extrinsic signaling, progenitor cell specification and neuronal subtype identity is central to the developmental organization of the vertebrate central nervous system. In the hindbrain and spinal cord, distinctions in the rostrocaudal identity of progenitor cells are associated with the generation of different motor neuron subtypes. Two fundamental classes of motor neurons, those with dorsal (dMN) and ventral (vMN) exit points, are generated over largely non-overlapping rostrocaudal domains of the caudal neural tube. Cdx and Hox genes are important determinants of the rostrocaudal identity of neural progenitor cells, but the link between early patterning signals, neural Cdx and Hox gene expression, and the generation of dMN and vMN subtypes, is unclear. Using an in vitro assay of neural differentiation, we provide evidence that an early Wnt-based program is required to interact with a later retinoic acid- and fibroblast growth factor-mediated mechanism to generate a pattern of Cdx and Hox profiles characteristic of hindbrain and spinal cord progenitor cells that prefigure the generation of vMNs and dMNs.     

Segmental development of reticulospinal and branchiomotor neurons in lamprey: insights into the evolution of the vertebrate hindbrain

During development, the vertebrate hindbrain is subdivided along its anteroposterior axis into a series of segmental bulges called rhombomeres. These segments in turn generate a repeated pattern of rhombomere-specific neurons, including reticular and branchiomotor neurons. In amphioxus (Cephalochordata), the sister group of the vertebrates, a bona fide segmented hindbrain is lacking, although the embryonic brain vesicle shows molecular anteroposterior regionalization. Therefore, evaluation of the segmental patterning of the central nervous system of agnathan embryos is relevant to our understanding of the origin of the developmental plan of the vertebrate hindbrain. To investigate the neuronal organization of the hindbrain of the Japanese lamprey, Lethenteron japonicum, we retrogradely labeled the reticulospinal and branchial motoneurons. By combining this analysis with a study of the expression patterns of genes identifying specific rhombomeric territories such as LjKrox20, LjPax6, LjEphC and LjHox3, we found that the reticular neurons in the lamprey hindbrain, including isthmic, bulbar and Mauthner cells, develop in conserved rhombomere-specific positions, similar to those in the zebrafish. By contrast, lamprey trigeminal and facial motor nuclei are not in register with rhombomere boundaries, unlike those of gnathostomes. The trigeminal-facial boundary corresponds to the rostral border of LjHox3 expression in the middle of rhombomere 4. Exogenous application of retinoic acid (RA) induced a rostral shift of both the LjHox3 expression domain and branchiomotor nuclei with no obvious repatterning of rhombomeric segmentation and reticular neurons. Therefore, whereas subtype variations of motoneuron identity along the anteroposterior axis may rely on Hox-dependent positional values, as in gnathostomes, such variations in the lamprey are not constrained by hindbrain segmentation. We hypothesize that the registering of hindbrain segmentation and neuronal patterning may have been acquired through successive and independent stepwise patterning changes during evolution.     

Par proteins and neuronal polarity

A hallmark of neurons is their ability to polarize with dendrite and axon specification to allow the proper flow of information through the nervous system. Over the past decade, extensive research has been performed in an attempt to understand the molecular and cellular machinery mediating this neuronal polarization process. It has become evident that many of the critical regulators involved in establishing neuronal polarity are evolutionarily conserved proteins that had previously been implicated in controlling the polarization of other cell types. At the forefront of this research are the partition defective (Par) proteins. In this review,we will provide a commentary on the progress of work regarding the central importance of Parproteins in the establishment of neuronal polarity.     

Retinoic acid and development of the central nervous system.

We consider the evidence that RA, the vitamin A metabolite, is involved in three fundamental aspects of the development of the CNS: 1) the stimulation of axon outgrowth in particular neuronal sub-types; 2) the migration of the neural crest; and 3) the specification of rostrocaudal position in the developing CNS (forebrain, midbrain, hindbrain, spinal cord). The evidence we discuss involves RA-induction of neurites in cell cultures and explants of neural tissue; the teratological effects of RA on the embryo's nervous system; the observation that RA can be detected endogenously in the spinal cord; and the fact that the receptors and binding proteins for RA are expressed in precise domains and neuronal cell types within the nervous system.     

A retinoic acid-Hox hierarchy controls both anterior/posterior patterning and neuronal specification in the developing central nervous system of the cephalochordate amphioxus

Retinoic acid (RA) mediates both anterior/posterior patterning and neuronal specification in the vertebrate central nervous system (CNS). However, the molecular mechanisms downstream of RA are not well understood. To investigate these mechanisms, we used the invertebrate chordate amphioxus, in which the CNS, although containing only about 20,000 neurons in adults, like the vertebrate CNS, has a forebrain, midbrain, hindbrain, and spinal cord and is regionalized by RA-signaling. Here we show, first, that domains of genes with expression normally limited to diencephalon and midbrain are generally not affected by altered RA-signaling, second, that contrary to previous reports, not only Hox1, 3, and 4, but also Hox2 and Hox6 are collinearly expressed in the amphioxus CNS, and third, that collinear expression of all these Hox genes is controlled by RA-signaling. Finally, we show that Hox1 is involved in mediating both the role of RA-signaling in regionalization of the hindbrain and in specification of hindbrain motor neurons. Thus, morpholino knock-down of the single amphioxus Hox1 mimics the effects of treatments with an RA-antagonist. This analysis establishes RA-dependent regulation of collinear Hox expression as a feature common to the chordate CNS and indicates that the RA-Hox hierarchy functions both in proper anterior/posterior patterning of the developing CNS and in specification of neuronal identity.     

On the evolution of central nervous systems: Implications from polyclad turbellarian neurobiology

The nervous system of the polyclad turbellarian Notoplana acticola consists of a series of nerve plexuses and a central ganglion, the brain. The brain contains a variety of cell types including multipolar heteropolar and bipolar neurons. These cell types are rare in other invertebrate ganglia. Individual neurons also contain a variety of different ion channels. both spiking and nonspiking neurons are found. Some neurons are multimodal interneurons. Habituation appears to be a postsynaptic phenomenon. Sensitization and long-term potentiation have not been demonstrated. Polyclads appear to represent a stage in the evolution of centralized nervous systems where much of the neuronal machinery underlying behavior occurs in the peripheral nervous system and the brain's main functions are the coordination and sequencing of peripherally placed reflexes. Even at this stage, however, the brain already contains cells that seem as advanced as those found in higher organisms.     

Multiple dorsoventral origins of oligodendrocyte generation in the spinal cord and hindbrain

Studies have indicated that oligodendrocytes in the spinal cord originate from a ventral progenitor domain defined by expression of the oligodendrocyte-determining bHLH proteins Olig1 and Olig2. Here, we provide evidence that progenitors in the dorsal spinal cord and hindbrain also produce oligodendrocytes and that the specification of these cells may result from a dorsal evasion of BMP signaling over time. Moreover, we show that the generation of ventral oligodendrocytes in the spinal cord depends on Nkx6.1 and Nkx6.2 function, while these homeodomain proteins in the anterior hindbrain instead suppress oligodendrocyte specification. The opposing roles for Nkx6 proteins in the spinal cord and hindbrain, in turn, appear to reflect that oligodendrocytes are produced by distinct ventral progenitor domains at these axial levels. Based on these findings, we propose that oligodendrocytes derive from several distinct positional origins and that the activation of Olig1/2 at different positions is controlled by distinct genetic programs.     

Ectopic Wnt/beta-catenin signaling induces neurogenesis in the spinal cord and hindbrain floor plate

The most ventral structure of the developing neural tube, the floor plate (FP), differs in neurogenic capacity along the neuraxis. The FP is largely non-neurogenic at the hindbrain and spinal cord levels, but generates large numbers of dopamine (mDA) neurons at the midbrain levels. Wnt1, and other Wnts are expressed in the ventral midbrain, and Wnt/beta catenin signaling can at least in part account for the difference in neurogenic capacity of the FP between midbrain and hindbrain levels. To further develop the hypothesis that canonical Wnt signaling promotes mDA specification and FP neurogenesis, we have generated a model wherein beta-catenin is conditionally stabilized throughout the FP. Here, we unambiguously show by fate mapping FP cells in this mutant, that the hindbrain and spinal cord FP are rendered highly neurogenic, producing large numbers of neurons. We reveal that a neurogenic hindbrain FP results in the altered settling pattern of neighboring precerebellar neuronal clusters. Moreover, in this mutant, mDA progenitor markers are induced throughout the rostrocaudal axis of the hindbrain FP, although TH+ mDA neurons are produced only in the rostral aspect of rhombomere (r)1. This is, at least in part, due to depressed Lmx1b levels by Wnt/beta catenin signaling; indeed, when Lmx1b levels are restored in this mutant, mDA are observed not only in rostral r1, but also at more caudal axial levels in the hindbrain, but not in the spinal cord. Taken together, these data elucidate both patterning and neurogenic functions of Wnt/beta catenin signaling in the FP, and thereby add to our understanding of the molecular logic of mDA specification and neurogenesis.     

Ptf1a, a bHLH transcriptional gene, defines GABAergic neuronal fates in cerebellum

The molecular machinery governing glutamatergic-GABAergic neuronal subtype specification is unclear. Here we describe a cerebellar mutant, cerebelless, which lacks the entire cerebellar cortex in adults. The primary defect of the mutant brains was a specific inhibition of GABAergic neuron production from the cerebellar ventricular zone (VZ), resulting in secondary and complete loss of external germinal layer, pontine, and olivary nuclei during development. We identified the responsible gene, Ptf1a, whose expression was lost in the cerebellar VZ but was maintained in the pancreas in cerebelless. Lineage tracing revealed that two types of neural precursors exist in the cerebellar VZ: Ptf1a-expressing and -nonexpressing precursors, which generate GABAergic and glutamatergic neurons, respectively. Introduction of Ptf1a into glutamatergic neuron precursors in the dorsal telencephalon generated GABAergic neurons with representative morphological and migratory features. Our results suggest that Ptf1a is involved in driving neural precursors to differentiate into GABAergic neurons in the cerebellum.     

Dynamic expression patterns of the pudgy/spondylocostal dysostosis gene Dll3 in the developing nervous system

Defects in the Notch pathway ligand Dll3 have been identified in the mouse pudgy (Dll3(pu)) and human spondylocostal dysostosis (SD, MIM 277300) mutations. Although these mutations are primarily associated with segmental defects in the axial skeleton and somitic patterning, they also exhibit cranial neurological defects. Therefore we have looked at the expression of Dll3 in the developing mouse nervous system. The expression of Notch ligands and receptors shares common features at 10.75 dpc in the rhombic lips and dorsal hindbrain. Temporal analysis of Dll3 expression from 9.0 to 11.0 dpc reveals that it is strongly expressed in laminar columns linked with regions of neuronal differentiation and hindbrain segmentation. Transverse sections show that Dll3 is expressed in territories where commissural neurons are formed. We have also looked at neuronal patterning in the mid-hindbrain region in Dll3(pu) mutants.     

The genetic control aspects of the development of the autonomic nervous system

This article provides a review of current views about the role of cell genetic machinery in the control of development of neurons of the autonomous nervous system. Some of the genes defining migration and specification of these neurons are described. We give a schematic presentation of the genetically determined organization of the neuronal networks, which are a basis of the intramural nervous machinery and sympathetic ganglia. We describe the distribution of neurons with different transmitter specificity in the cell populations comprising the neuronal networks.