Protooncogene expression identifies a transient columnar organization of the forebrain within the late embryonic ventricular zone

Inactivation of the 3-phosphoglycerate dehydrogenase gene in mice: changes in gene expression and associated regulatory networks resulting from serine deficiency

D-3-Phosphoglycerate dehydrogenase (Phgdh) is a necessary enzyme for de novo L-serine biosynthesis. Mutations in the human PHGDH cause serine deficiency disorders characterized by severe neurological symptoms including congenital microcephaly and psychomotor retardation. We showed previously that targeted disruption of Phgdh in mice causes overall growth retardation with severe brain microcephaly and leads to embryonic lethality. Here, amino acid analysis of Phgdh knockout (KO) mouse embryos demonstrates that free serine and glycine concentrations are decreased markedly in head samples, reflecting the metabolic changes of serine deficiency found in human patients. To understand the pathogenesis of serine deficiency disorders at the molecular level, we have exploited this animal model to identify altered gene expression patterns using a microarray technology. Comparative microarray analysis of the Phgdh KO and wild-type head at gestational day 13.5 revealed an upregulation of genes involved in transfer RNA aminoacylation, amino acid metabolism, amino acid transport, transcriptional regulation, and translation, and a downregulation of genes involved in transcription in neuronal progenitors and muscle and cartilage development. A computational network analysis software was used to construct transcriptional regulatory networks operative in the Phgdh KO embryos in vivo. These observations suggest that Phgdh inactivation alters transcriptional programs in several regulatory networks.

Gliogenesis in the central nervous system

Multipotential neuroepithelial stem cells are thought to give rise to all the differentiated cells of the central nervous system (CNS). The developmental potential of these multipotent stem cells becomes more restricted as they differentiate into progressively more committed cells and ultimately into mature neurons and glia. In studying gliogenesis, the optic nerve and spinal cord have become invaluable models and the progressive stages of differentiation are being clarified. Multiple classes of glial precursors termed glial restricted precursors (GRP), oligospheres, oligodendrocyte-type2 astrocyte (O-2A) and astrocyte precursor cells (APC) have been identified. Similar classes of precursor cells can be isolated from human neural stem cell cultures and from embryonic stem (ES) cell cultures providing a non-fetal source of such cells. In this review, we discuss gliogenesis, glial stem cells, putative relationships of these cells to each other, factors implicated in gliogenesis, and therapeutic applications of glial precursors.

Cellular localization of PDGF mRNAs in developing human forebrain

Platelet-derived growth factor (PDGF) has been implicated in the processes regulating gliogenesis in the CNS. Conflicting in vivo data in rodents have variously implicated either glia or neurons as being the primary source of PDGF. We have used in situ hybridization and immunocytochemical analysis to study the in vivo expression and cellular localization of PDGF-A, sis/PDGF-B, together with the two PDGF receptors alpha and beta, in developing human forebrain. In this study we demonstrate the strong expression of mRNA and protein of both PDGF chains, A and B, and their receptors, alpha and beta, in human embryonic glial cells. The neurons, in contrast to glial cells, expressed lower levels of PDGF and PDGF-receptor mRNAs and protein. Identification of the cell types expressing the PDGF and PDGF-receptor mRNAs was achieved by counterstaining with antibodies specific for glial cells (GFAP) and neurons (NF). The predominant glial-specific expression of both PDGF-A and PDGF-B, together with the coexpression of their receptors alpha and beta, suggests an important role for the PDGF isoforms in the development of human embryonic glial cells and neurons in vivo.

The functions of the preplate in development and evolution of the neocortex and hippocampus

Recently, it has been shown that the early developmental organization of the archicortical hippocampus resembles that of the neocortex. In both cortices at embryonic stages, a preplate is present, which is split by the formation of the cortical plate into a marginal zone and a subplate layer. The pioneer neurons of the preplate are believed to form a phylogenetically ancient cortical structure. Neurons in these preplate layers are the first postmitotic neurons and have important roles in the development of the cerebral cortex. Cajal-Retzius cells in the marginal zone regulate the phenotype of radial glial cells and may direct neuronal migration establishing the inside-out gradient of corticogenesis. Furthermore, pioneer neurons form the initial axonal connections with other (sub)cortical structures. A significant difference between the hippocampus and neocortex, however, is that in the hippocampus, most afferents are guided by the pioneer neurons in the prominent marginal zone, while in the neocortex most ingrowing afferent axons enter via the subplate. At later developmental periods, most pioneer neurons disappear by cell death or transform into other neuronal shapes. Here, we review the early developmental organization of the mammalian cerebral cortex (both neocortex and hippocampus) and discuss the functions and fate of pioneer neurons in cortical development, in particular that of Cajal-Retzius cells. Evaluating the developmental properties of the hippocampus and neocortex, we present the hypothesis that the distribution of the main ingrowing afferent systems in the developing neocortex, which differs from the one in the hippocampal region, may have enabled the specific evolution of the neocortex.

Regulation of neuroblast cell-cycle kinetics plays a crucial role in the generation of unique features of neocortical areas

Cortical neurons are generated in the germinal zones lining the ventricles before migrating predominantly radially. To investigate regional differences in the cell-cycle kinetics of neuroblasts, pulse [3H]-thymidine injections were made throughout corticogenesis, and labeled neuron counts were compared in areas 3, 6, 17, and 18a in the adult mouse. The relationship between height in the cortex and intensity of autoradiographic signal distinguishes first generation and subsequent generations of neurons. This provides the mitotic history of defined sets of neurons and is a powerful tool for analyzing areal differences in cell-cycle kinetics. The infragranular laminar labeling indices of different generations show significant differences in areas 3 and 6. The labeling index of first generation neurons shows that the rate of neuron production is higher in area 3 than in area 6. This increased generation rate in area 3 was accompanied by two major changes. First, computation of the labeling index of the subsequent generation neurons (which reflects percentages of precursors in S-phase at the moment of the pulse) indicates a shorter cell cycle in area 3. Second, the total population of labeled neurons contains a higher proportion of first generation neurons in area 3, implying a higher leaving fraction in this area. Computer simulations of these areal differences of cell-cycle kinetics generate neuron numbers that are in close agreement with published data. Altogether these findings reveal an early regionalization of the ventricular zone that serves to generate unique features of future cortical areas.

Regulation of neuroblast cell-cycle kinetics plays a crucial role in the generation of unique features of neocortical areas

Cortical neurons are generated in the germinal zones lining the ventricles before migrating predominantly radially. To investigate regional differences in the cell-cycle kinetics of neuroblasts, pulse [3H]-thymidine injections were made throughout corticogenesis, and labeled neuron counts were compared in areas 3, 6, 17, and 18a in the adult mouse. The relationship between height in the cortex and intensity of autoradiographic signal distinguishes first generation and subsequent generations of neurons. This provides the mitotic history of defined sets of neurons and is a powerful tool for analyzing areal differences in cell-cycle kinetics. The infragranular laminar labeling indices of different generations show significant differences in areas 3 and 6. The labeling index of first generation neurons shows that the rate of neuron production is higher in area 3 than in area 6. This increased generation rate in area 3 was accompanied by two major changes. First, computation of the labeling index of the subsequent generation neurons (which reflects percentages of precursors in S-phase at the moment of the pulse) indicates a shorter cell cycle in area 3. Second, the total population of labeled neurons contains a higher proportion of first generation neurons in area 3, implying a higher leaving fraction in this area. Computer simulations of these areal differences of cell-cycle kinetics generate neuron numbers that are in close agreement with published data. Altogether these findings reveal an early regionalization of the ventricular zone that serves to generate unique features of future cortical areas.

Distribution and functions of platelet-derived growth factors and their receptors during embryogenesis

Platelet-derived growth factors (PDGFs) are soluble proteins that mediate intercellular signaling via receptor tyrosine kinases. The patterns of PDGF and PDGF receptor expression during embryogenesis are complex and dynamic and suggest that signaling can be autocrine or paracrine, depending on the particular tissue and the stage of development. Mesenchymal cells throughout the embryo and within some developing organs produce PDGF receptors, whereas their ligands are often produced by adjacent epithelial or endothelial cells. Disruption of PDGF signaling in the embryo leads to morphogenetic defects and embryonic or perinatal lethality. Tissues that are particularly susceptible to the absence of PDGF signaling are migrating mesoderm cells during gastrulation, nonneuronal neural crest cell derivatives, and kidney mesangial cells. These tissues share the common feature of undergoing epithelial-mesenchymal transitions. We review current knowledge of the distribution of PDGF ligands and receptors and discuss how this distribution may relate to several roles for PDGF during embryogenesis, particularly the regulation of mesenchymal cell behavior.

Schizophrenia, the heteromodal association neocortex and development: potential for a neurogenetic approach

The heteromodal association neocortex is believed to be a major site of involvement in schizophrenia. This system includes the prefrontal cortex and portions of the superior temporal and inferior parietal cortices, which are linked in cognitive networks observing complex executive functions. The heteromodal cortex is highly elaborated in humans and is believed to continue to develop past birth. The neuropathology of schizophrenia is likely to be heterogeneous and appears to involve developmental abnormalities, with a prominent genetic component. However, the genes involved in the development of the neocortex, and particularly the heteromodal cortex, are not well understood. A candidate-gene approach to schizophrenia using techniques of differential expression might now be feasible and could illuminate the basic neurobiology of the heteromodal cortical network.

Getting there and being there in the cerebral cortex

The mammalian neocortex is composed of functional areas that are specified to process particular aspects of information. How is this specification achieved during development? Since cells migrate to their final positions in the developing nervous system, a central issue is the relation between cellular migration and positional information. This review combines evidence for early positional specification in the developing cortex with evidence for cellular dispersion during migration. A model is suggested whereby stable cues provide positional information and minorities of 'displaced' cells are respecified accordingly. Comparison with other parts of the CNS reveals that cellular dispersal is ubiquitous and has to be included in any mechanism relaying positional specification. Ontogenetic and phylogenetic considerations suggest that radial glial cells might provide the positional information in the developing nervous system.

Modulation of the cell cycle contributes to the parcellation of the primate visual cortex

An as-yet unresolved issue in developmental neurobiology is whether the discrete areas that form the mammalian cortex emerge from a uniform cortical plate or whether they are already specified in the germinal zone. A feature of the primate striate cortex is that the number of neurons per unit area is twice that of anywhere else in the cerebral cortex. Here we take advantage of this unique structural feature to investigate whether the extra striate cortical cells are due to increased neuron production during neurogenesis. We labelled precursors undergoing terminal cell division with 3H-thymidine and allowed them to migrate to the cortical plate. Cell counts revealed that their rate of production in the germinal zone of striate cortex is higher than in that given rise to extrastriate cortex. Also, we used 3H-thymidine pulse injections to investigate cell cycle dynamics and found that this phase of increased production of striate cortical cells is associated with changes in the parameters of the cell cycle. These results show that cortical area identity is at least partially determined at the level of the ventricular zone.

Clonal heterogeneity in the germinal zone of the developing rat telencephalon

A double-labeling technique, combining retroviral tagging of individual cell lines (one clone per brain hemisphere) with the simultaneous [3H]thymidine-labeling of dividing cells in S phase, was used to study proliferation characteristics of individual precursor cell lines in the germinal zone of the developing rat forebrain. The cortical germinal zone was found to be segregated into three spatially distinct horizontal populations of precursor cell lineages, which differed in cell cycle kinetics, amount of cell death, and synchronous versus asynchronous mode of proliferation. The striatal germinal zone demonstrated a similar heterogeneity in the cell cycle characteristics of proliferating clones, but did not show nearly as distinct a spatial segregation of these different populations. The results demonstrate the clonal heterogeneity among precursor populations in the telencephalon and the differential spatial organization of the cortical and the striatal germinal zones. This germinal zone heterogeneity may predict some of the differences found among cellular phenotypes in the adult forebrain.

Early regional specification for a molecular neuronal phenotype in the rat neocortex

The timing of neocortical regional specification was examined using a monoclonal antibody, designated PC3.1, that binds a 29-kDa polypeptide and recognizes a neuronal subpopulation located in the lateral but not dorsomedial neocortex in the rat. When lateral cortical tissue fragments at embryonic days 12 and 16 were maintained in an organotypic culture system, a substantial number of neurons became PC3.1-immunopositive. In marked contrast, considerably fewer, if any, PC3.1-positive neurons were observed in cultures of dorsal cortical tissue. The selective appearance of PC3.1-immunopositive neurons was also observed in dissociated cultures derived from the lateral, but not dorsal, cortical primordium at embryonic day 13 and later. In light of previous reports showing that the interactions between developing neocortical neurons and cortical afferents begin at embryonic day 14 or later, our findings imply that some regional specification occurs well before these interactions and suggest the importance of elements intrinsic to the neocortex in establishing neocortical regional specificity. Furthermore, [3H]thymidine birth-dating experiments revealed that the majority of presumptive PC3.1-immunopositive neurons underwent their final mitosis around embryonic day 15, suggesting that the regional specification events for these neurons occur before their neurogenesis.

Expression of the Raf-1 protein in rat brain during development and its hormonal regulation in hypothalamus

To study mechanisms involved in the sexual differentiation of the rat brain, the expression of the protein product of the proto-oncogene c-raf-1 (Raf-1) was examined. Biochemical and immunocytochemical analyses localized Raf-1 in embryonic rat brain regions and demonstrated hormonally induced changes in Raf-1 expression. For this study an affinity-purified anti-peptide antiserum specific for Raf-1 (NH-44) was used. Western blots revealed an approximately 77 kD polypeptide isolated in the cytosol of developing rat brains. Raf-1 levels were highest in the embryonic (E) day 22 female hypothalamus (HYP), and approximately twofold higher than levels detected in male HYP at E22 as determined by quantitative protein dot blot and semiquantitative Western blot analyses. Raf-1 levels in HYP were greater than those in either brain stem (BS) or cortex. Immunocytochemical analysis revealed high levels of Raf-1 in selective brain regions (e.g., the ventromedial nucleus in the HYP, the mitral cell layers in the main and accessory olfactory bulbs (OB), and the locus coeruleus) at E22 and postnatal (P) day 1. Lower levels of immunoreactivity were observed in many areas of the perinatal neuraxis. To test hormonal regulation of Raf-1, testosterone propionate (TP) was administered to pregnant rats on E17; male and female fetuses were examined on E22. This treatment significantly decreased Raf-1 levels in female HYP, but not in male HYP, as determined by Western blot analysis. No significant sex difference or response to prenatal hormone treatments were observed in either brain stem or cortex. No significant sex difference was noted postnatally, and administration of TP 3 h after birth did not change Raf-1 levels examined 24 h later. In summary, Raf-1 was localized within selective regions of the rat brain, and its expression was altered by exogenous prenatal hormonal stimulation. One role for Raf-1 in signal transduction may be to delimit hormonal critical periods in sexual differentiation of the brain.

Principles of neural cell migration

A basic property of immature neurons is their ability to change position from the place of their final mitotic division in proliferative centers of the developing brain to the specific positions they will occupy in a given structure of the adult nervous system. Proper acquisition of neuron position, attained through the process of active migration, ultimately affects a cell's morphology, synaptic connectivity and function. Although various classes of neurons may use different molecular cues to guide their migration to distant structures, a surface-mediated interaction between neighboring cells is considered essential for all types of migration. Disturbance of this cell-cell interaction may be important in several congenital and/or acquired brain abnormalities. The present article considers the basic mechanisms and principles of neuronal cell migration in the mammalian central nervous system.

Hypercolumns in primate visual cortex can develop in the absence of cues from photoreceptors

The visual cortex in primates consists of an array of anatomically and chemically identifiable cellular modules (hypercolumns) with distinct physiological properties. For example, layers II/III in the macaque monkey contain a regular array of cytochrome oxidase-rich blobs. Furthermore, the surrounding cytochrome oxidase-poor interblob regions have a higher density of neuropeptide Y-positive aspiny stellate cells. Neurons in the blobs are thought to mediate predominantly low spatial frequencies and color vision, while those in the interblobs appear to be engaged in pattern vision and high spatial frequency analysis. In this study we examined the role of the retina in the development of hypercolumns. A bilateral retinal ablation was performed in embryos at midgestation, before any photoreceptors had established contacts with other retinal neurons and before layers II/III of the cortex--or their synaptic connection--had been generated. We found that the cortex in operated animals had cytochrome oxidase blobs and that their size and spacing were normal. In addition, neuropeptide Y-containing neurons were preferentially distributed in the interblob region as in control animals. Our findings indicate that some basic aspects of the cyto- and chemoarchitectonic organization of the cerebral cortex, which presumably evolved for the analysis of form and color, can emerge in the absence of cues from the retinal photoreceptors that mediate these attributes of vision.

Characterization of biologically active, platelet-derived growth factor-like molecules produced by murine erythroid cells in vitro and in vivo

Platelet-derived growth factor (PDGF) is an important serum regulator of erythropoiesis in vitro. We have now obtained evidence suggesting that PDGF-like molecules may also modulate erythropoiesis in vivo. Western blot analysis of cytoplasmic extracts from Rauscher murine erythroleukemia cells and phenylhydrazine-treated mouse splenic erythroid cells revealed the presence of several PDGF-like proteins. The presence of PDGF-like proteins in the cytoplasm of these two erythroid cell types was confirmed by immunohistochemical staining. Using a serum-free biologic assay, PDGF-like biological activity was found in cell lysates and conditioned medium of both Rauscher cells and phenylhydrazine-treated mouse erythroid cells. Subcellular localization experiments revealed the biological activity to be concentrated in the cytosolic fraction. Using a series of antibodies to hematopoietic growth factors we demonstrated that PDGF-like biological activity was specifically immunoprecipitated by both monoclonal and polyclonal anti-human PDGF antibodies but not by antibodies to burst-promoting activity, granulocyte-macrophage colony-stimulating factor, IL-3, or erythropoietin. Taken together, the data are consistent with the hypothesis that PDGF-like molecules play a role in the regulation of mammalian erythropoiesis in vivo.

Phosphotyrosine antibodies specifically label ameboid microglia in vitro and ramified microglia in vivo

Using an affinity-purified, polyclonal antibody to phosphotyrosine (Wang: Molecular and Cellular Biology 5:3640-3643, 1985) we have previously demonstrated that phosphotyrosine immunoreactivity is restricted to a population of multipolar GFAP-negative neuroglia in adult rat brain (Tillotson and Wood: Journal of Comparative Neurology 282:133-141, 1989) and retina (Tillotson and Wood: Journal of Cell Biology 107:724a, 1988). In this study, we show that the phosphotyrosine-immunoreactive cells are microglia. This conclusion is supported by numerous morphological and ultrastructural similarities between the phosphotyrosine-immunoreactive cells and microglia. Furthermore, phosphotyrosine co-localizes with the microglial-specific B4 isolectin of Bandeiraea simplifolia-1 lectin. Phosphotyrosine antibodies also stain ameboid microglia in primary cultures of neonatal rat brain. In addition, after 7 days in vitro, microglia are the only phosphotyrosine-immunoreactive element in the cultures. This temporal pattern of staining in vitro mimics the developmental progression of phosphotyrosine immunoreactivity in situ, in which a variety of structures stain during postnatal neural development (Tillotson and Wood: Journal of Comparative Neurology 282:133-141, 1989), but only microglia stain in mature brain. The significance of phosphotyrosine-containing proteins potentially expressed in a microglial-specific manner is discussed.