Expression domains suggest cell-cycle independent roles of growth-arrest molecules in the adult brain of the medaka, Oryzias latipes

Gadd45 in Neuronal Development, Function, and Injury

The growth arrest and DNA damage-inducible (Gadd) 45 proteins have been associated with numerous cellular mechanisms including cell cycle control, DNA damage sensation and repair, genotoxic stress, neoplasia, and molecular epigenetics. The genes were originally identified in in vitro screens of irradiation- and interleukin-induced transcription and have since been implicated in a host of normal and aberrant central nervous system processes. These include early and postnatal development, injury, cancer, memory, aging, and neurodegenerative and psychiatric disease states. The proteins act through a variety of molecular signaling cascades including the MAPK cascade, cell cycle control mechanisms, histone regulation, and epigenetic DNA demethylation. In this review, we provide a comprehensive discussion of the literature implicating each of the three members of the Gadd45 family in these processes.

Characterization of neurogenic niches in the telencephalon of juvenile and adult sharks

Neurogenesis is a multistep process by which progenitor cells become terminally differentiated neurons. Adult neurogenesis has gathered increasing interest with the aim of developing new cell-based treatments for neurodegenerative diseases in humans. Active sites of adult neurogenesis exist from fish to mammals, although in the adult mammalian brain the number and extension of neurogenic areas is considerably reduced in comparison to non-mammalian vertebrates and they become mostly reduced to the telencephalon. Much of our understanding in this field is based in studies on mammals and zebrafish, a modern bony fish. The use of the cartilaginous fish Scyliorhinus canicula (representative of basal gnathostomes) as a model expands the comparative framework to a species that shows highly neurogenic activity in the adult brain. In this work, we studied the proliferation pattern in the telencephalon of juvenile and adult specimens of S. canicula using antibodies against the proliferation marker proliferating cell nuclear antigen (PCNA). We have characterized proliferating niches using stem cell markers (Sex determining region Y-box 2), glial markers (glial fibrillary acidic protein, brain lipid binding protein and glutamine synthase), intermediate progenitor cell markers (Dlx2 and Tbr2) and markers for migrating neuroblasts (Doublecortin). Based in the expression pattern of these markers, we demonstrate the existence of different cell subtypes within the PCNA immunoreactive zones including non-glial stem cells, glial progenitors, intermediate progenitor-like cells and migratory neuroblasts, which were widely distributed in the ventricular zone of the pallium, suggesting that the main progenitor types that constitute the neurogenic niche in mammals are already present in cartilaginous fishes.

A Conserved Developmental Mechanism Builds Complex Visual Systems in Insects and Vertebrates

The visual systems of vertebrates and many other bilaterian clades consist of complex neural structures guiding a wide spectrum of behaviors. Homologies at the level of cell types and even discrete neural circuits have been proposed, but many questions of how the architecture of visual neuropils evolved among different phyla remain open. In this review we argue that the profound conservation of genetic and developmental steps generating the eye and its target neuropils in fish and fruit flies supports a homology between some core elements of bilaterian visual circuitries. Fish retina and tectum, and fly optic lobe, develop from a partitioned, unidirectionally proliferating neurectodermal domain that combines slowly dividing neuroepithelial stem cells and rapidly amplifying progenitors with shared genetic signatures to generate large numbers and different types of neurons in a temporally ordered way. This peculiar 'conveyor belt neurogenesis' could play an essential role in generating the topographically ordered circuitry of the visual system.

Transcriptional mechanisms of developmental cell cycle arrest: problems and models

Metazoans begin their life as a single cell. Then, this cell enters a more or less protracted period of active cell proliferation, which can be considered as the default cellular state. A crucial event, the developmental cell cycle exit, occurs thereafter. This phenomenon allows for differentiation to happen and regulates the final size of organs and organisms. Its control is still poorly understood. Herein, we review some transcriptional mechanisms of cell cycle exit in animals, and propose to use cellular conveyor belts as model systems for its study. We finally point to evidence that suggests that the mechanisms of developmental cell cycle arrest may have to be maintained in adult tissues.

Transient expression of the conserved zinc finger gene INSM1 in progenitors and nascent neurons throughout embryonic and adult neurogenesis

INSM1 is a zinc-finger protein expressed in the developing nervous system and pancreas as well as in medulloblastomas and neuroendocrine tumors. With in situ hybridization combined with immunohistochemistry, we detected INSM1 mRNA in all embryonic to adult neuroproliferative areas examined: embryonic neocortex, ganglionic eminence, midbrain, retina, hindbrain, and spinal cord; autonomic, dorsal root, trigeminal and spiral ganglia; olfactory and vomeronasal organ epithelia; postnatal cerebellum; and juvenile to adult subgranular zone of dentate gyrus, subventricular zone, and rostral migratory stream leading to olfactory bulb. In most of these neurogenic areas, subsets of neuronal progenitors and nascent, but not mature, neurons express INSM1. For example, in developing cerebellum, INSM1 is present in proliferating progenitors of the outer external granule layer (EGL) and in postmitotic cells of the inner EGL, but not in mature granule cell neurons. Also, lining the neural tube from spinal cord to neocortex in mouse as well as human embryos, cells undergoing mitosis apically do not express INSM1. By contrast, nonsurface progenitors located in the basal ventricular and/or subventricular zones express INSM1. Whereas apical progenitors are proliferative and generate one or two additional progenitors, basal progenitors are thought to divide terminally and symmetrically to produce two neurons. The nematode ortholog of INSM1, EGL-46, is expressed during terminal symmetric neurogenic divisions and regulates the termination of proliferation. We propose that, in mice and humans, INSM1 is likewise expressed transiently during terminal neurogenic divisions, from late progenitors to nascent neurons, and particularly during symmetric neuronogenic divisions.

Morphogenesis in the retina of a slow-developing teleost: emergence of the GABAergic system in relation to cell proliferation and differentiation

Gamma-aminobutyric acid (GABA) has been implicated in cell proliferation and differentiation during development. In the present study, immunohistochemical techniques were used to investigate the development of the GABAergic system in the retina of the trout and its relation to markers of differentiation [calretinin (CR), and tyrosine hydroxylase (TH)]. The expression of Pax6, an eye-patterning protein involved in the proliferation and emergence of specific retinal cell types, was also studied. Retinal layering was observed to begin centrally in prehatching embryos, as the first GABAergic cells appeared in the ganglion cell layer (GCL) and inner part of the inner nuclear layer (INL). At hatching, GABAergic cells were also observed in the horizontal cell layer (HCL). In alevins, GABAergic cells and processes spread laterally following retinal growth although they did not invade neuroblastic retinal regions. CR- and Pax6-immunoreactive (ir) cells were first seen in the GCL and the inner part of the INL, whereas sparse TH-ir cells appeared in the INL. In juveniles, GABAergic cells were observed in the GCL, inner part of the INL and HCL, whereas CR-ir cells spread to the outer part of the INL and HCL. A subset of CR-ir in the GCL and of Pax6-ir cells in the GCL and INL showed colocalization with GABAergic markers. This study provides further comparative knowledge about the development of GABAergic system of the retina in teleosts and shows differences and similarities with that reported in fast-developing species such as zebrafish, in which retinal expression of GABA was transient in some populations.

Ol-insm1b, a SNAG family transcription factor involved in cell cycle arrest during medaka development

Through whole-mount in situ hybridisation screen on medaka (Oryzias latipes) brain, Ol-insm1b, a member of the Insm1/Mlt1 subfamily of SNAG-domain containing genes, has been isolated. It is strongly expressed during neurogenesis and pancreas organogenesis, with a pattern that suggests a role in cell cycle exit. Here, we describe Ol-insm1b expression pattern throughout development and in adult brain, and we report on its functional characterisation. Our data point to a previously unravelled role for Ol-insm1b as a down-regulator of cell proliferation during development, as it slows down the cycle without triggering apoptosis. Clonal analysis demonstrates that this effect is cell-autonomous, and, through molecular dissection studies, we demonstrate that it is likely to be non-transcriptional, albeit mediated by zinc-finger domains. Additionally, we report that Ol-insm1b mRNA, when injected in one cell of two-cell stage embryos, exhibits a surprising behaviour: it does not spread uniformly amongst daughter cells but remains cytoplasmically localised in the progeny of the injected blastomere. Our experiments suggest that Insm1 is a negative regulator of cell proliferation, possibly through mechanisms that do not involve modulation of transcription.

An automated in situ hybridization screen in the medaka to identify unknown neural genes

Despite the fact that a large body of factors that play important roles in development are known, there are still large gaps in understanding the genetic pathways that govern these processes. To find previously unknown genes that are expressed during embryonic development, we optimized and performed an automated whole-mount in situ hybridization screen on medaka embryos at the end of somitogenesis. Partial cDNA sequences were compared against public databases and identified according to similarities found to other genes and gene products. Among 321 isolated genes showing specific expression in the central nervous system in at least one of five stages of development, 55.14% represented genes whose functions are already documented (in fish or other model organisms). Additionally, 16.51% were identified as conserved unknown genes or genes with unknown function. We provide new data on eight of these genes that presented a restricted expression pattern that allowed for formulating testable hypotheses on their developmental roles, and that were homologous to mammalian molecules of unknown function. Thus, gene expression screening in medaka is an efficient tool for isolating new regulators of embryonic development, and can complement genome-sequencing projects that are producing a high number of genes without ascribed functions.