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P Rakic

Publications and source records attributed to P Rakic.

At least 55 records · Page 3Linked to original sources
PubMed1999

The role of cell death in regulating the size and shape of the mammalian forebrain.

The size of the cerebral cortex is determined by the rate of production of neurons and glial cells in the proliferative ventricular and subventricular zones. Recent studies from targeted mutations of different death-effector gene families indicate that programmed cell death (PCD) plays an important role in cell production and early morphogenesis of the mammalian forebrain before the formation of neuronal connections. For example, disruption of the c/Jun N-kinase signaling pathway by double-targeted mutation of both Jnk1 and Jnk2 results in increased PCD in the forebrain leading to precocious degeneration of cerebral precursors. In contrast, disturbance of the caspase cascade by targeted disruption of either casp-9 or casp-3 leads to decreased PCD causing expansion and exencephaly of the forebrain as well as supernumerary neurons in the cerebral cortex. The supernumerary neurons in these knockout mice align radially and form an expanded cortical plate which begins to form cerebral convolutions. Thus, the precise coordination of different apoptotic signaling pathways during early stages of neurogenesis is crucial for regulation of the proper cortical size and shape.

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PubMed1998

Reduced apoptosis and cytochrome c-mediated caspase activation in mice lacking caspase 9.

Caspases are essential components of the mammalian cell death machinery. Here we test the hypothesis that Caspase 9 (Casp9) is a critical upstream activator of caspases through gene targeting in mice. The majority of Casp9 knockout mice die perinatally with a markedly enlarged and malformed cerebrum caused by reduced apoptosis during brain development. Casp9 deletion prevents activation of Casp3 in embryonic brains in vivo, and Casp9-deficient thymocytes show resistance to a subset of apoptotic stimuli, including absence of Casp3-like cleavage and delayed DNA fragmentation. Moreover, the cytochrome c-mediated cleavage of Casp3 is absent in the cytosolic extracts of Casp9-deficient cells but is restored after addition of in vitro-translated Casp9. Together, these results indicate that Casp9 is a critical upstream activator of the caspase cascade in vivo.

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PubMed1998

Distinct modes of neuronal migration in different domains of developing cerebellar cortex.

As postmitotic neurons migrate to their final destinations, they encounter different cellular microenvironments, but functional responses of migrating neurons to changes in local environmental cues have not been examined. In the present study, we used a confocal microscope on acute cerebellar slice preparations to examine real-time changes in the shape of granule cells, as well as the mode and rate of their migration as they transit different microenvironments. The rate of granule cell movement is fastest in the molecular layer, whereas their elongated somata and long leading processes remain in close contact with Bergmann glial fibers. Cell movement is slowest in the Purkinje cell layer after granule cells detach from the surface of Bergmann glia and the somata become transiently round, whereas the leading processes considerably shorten. Surprisingly, after entering the internal granular layer, granule cells re-extend both their somata and leading processes as they resume rapid movement independent of Bergmann glial fibers. In this last phase of migration, described here for the first time, most granule cells move radially for >100 micron (a distance comparable to that observed in the molecular layer) until they reach the deep strata of the internal granular layer, where they become rounded again and form synaptic contacts with mossy fiber terminals. These observations reveal that migrating neurons alter their shape, rate, and mode of movement in response to local environmental cues and open the possibility for testing the role of signaling molecules in cerebellar neurogenesis.

Aging↗
PubMed1998

Changes in cell-cycle kinetics during the development and evolution of primate neocortex.

The evolutionary expansion of neocortical size in mammals is particularly prominent in anthropoid primates (i.e., monkeys, apes, and humans) and reflects an increased number of cortical cells, yet the developmental basis for this increase remains undefined. Cortical cell production depends on the length of the cell-division cycle of progenitor cells during neurogenesis, which previously has been measured only in smaller-brained rodents. To investigate whether cortical expansion in primates reflects modification of cell-cycle kinetics, we determined cell-cycle length during neurogenesis in the proliferative cerebral ventricular zone of fetal rhesus monkeys, by using cumulative S-phase labeling with bromodeoxyuridine. Cell-cycle durations in monkeys were as much as 5 times longer than those reported in rodents. Nonetheless, substantially more total rounds of cell division elapsed during the prolonged neurogenetic period of the monkey cortex, providing a basis for increased cell production. Moreover, unlike the progressive slowing that occurs during cortical development in rodents, cell division accelerated during neurogenesis of the enlarged cortical layers in monkeys. These findings suggest that evolutionary modification of the duration and number of progenitor cell divisions contributed to both the expansion and laminar elaboration of the primate neocortex.

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PubMed1998

Peptidergic innervation and the nicotinic acetylcholine receptor in the primate basal nucleus.

Peptidergic innervation and localization of the neuronal nicotinic acetylcholine receptor (nAChR) was studied in the basal forebrain of Macaca fascicularis in order to provide microstructural proofs for the theory (Changeux et al., 1992) that calcitonin gene-related peptide (CGRP) is responsible for the maintenance of the acetylcholine receptor. Distribution and localization of five neuropeptides, namely substance P (SP), CGRP, neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP) neurotensin (NT), and the neuropeptides parvalbumin (PV) and the alpha-bungarotoxin- (alpha-BTX-) binding protein was studied by means of light- and electron microscopic pre-embedding immunocytochemistry. Immunohistochemical double staining revealed that large cholinergic principal nerve cells in the basal forebrain, corresponding to cell group Ch4 constituting Meynert's basal nucleus (BNM), and exerting intense choline acetyltransferase (ChAT) immunoreactivity, are synaptically innervated by axons displaying CGRP immunoreactivity. While SP, NPY, PV and CGRP establish dense networks in BNM, innervation by NT and VIP is sparse. Biotinylated alpha-BTX visualizes beaded axons that surround dendrites and perikarya of cholinergic principal cells. Electron microscopic organization of the neuropil in BNM is characterized by a glomerular (or rather cartridge-like) arrangement of axons surrounding dendrites of non-cholinergic principal nerve cells. At least one of the axons establishing the glomerulus (cartridge) exerts CGRP immunopositivity while alpha-BTX-immunopositive axons, presynaptic to dendrites of principal cells, are attached to the glomeruli (cartridges) from outside. As alpha-BTX-binding indicates localization of the alpha7 subunit of the neuronal nAChR, the microtopographical arrangement supports the idea that, in a manner similar to that in the neuromuscular junction, CGRP might contribute to the maintenance of nAChR also in BNM. Our results suggest that presynaptic nAChR-s are involved in the regulation of acetylcholine release from a feed-forward amplification mechanism of cholinergic principal cells of BNM.

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PubMed1998

Orchestration of neuronal migration by activity of ion channels, neurotransmitter receptors, and intracellular Ca2+ fluctuations.

The real-time observation of cell movement in acute cerebellar slices reveals that granule cells alter their shape concomitantly with changes in the mode and rate of migration as they traverse different cortical layers. Although the origin of local environmental cues responsible for these position-specific changes in migratory behavior remains unclear, several signaling mechanisms involved in controlling granule cell movement have emerged. The onset of one such mechanism is marked by the expression of voltage-gated ion channels and neurotransmitter receptors in postmitotic cells prior to the initiation of their migration. Granule cells start their radial migration after the expression of N-type Ca2+ channels and the N-methyl-D-aspartate subtype of glutamate receptors on the plasmalemmal surface. Blockade of the channel or receptor activity significantly decreases the rate of cell movement, indicating that the activation of these membrane constituents provides an essential signal for the translocation of granule cells. Another signal that controls the rate of cell migration is embedded in the combined amplitude and frequency components of Ca2+ fluctuations in the somata of migrating granule cells. Interestingly, each phase of Ca2+ fluctuation controls a separate phase of saltatory movement in the granule cells: The cells move forward during the phase of transient Ca2+ elevation and remain stationary during the troughs. Consequently, the changes in the amplitude and frequency components of Ca2+ fluctuations directly affect granule cell movement: Reducing the amplitude or frequency of Ca2+ fluctuations slows down the speed of cell movement, while the enhancement of these components accelerates migration. These findings suggest that signaling molecules present in the local cellular milieu encountered on the migratory route control the shape and motility of granule cells by modifying Ca2+ fluctuations in the soma through the activation of specific ion channels and neurotransmitter receptors.

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PubMed1997

Developmental expression, pattern of distribution, and effect on cell aggregation implicate a neuron-glial junctional domain protein in neuronal migration.

We developed a panel of monoclonal antibodies to cerebellar astroglial cells and selected for study those that revealed microdomain structures on the cell surface of neocortical and cerebellar astrocytes. One antibody, 15D7-AD7, recognized the approximately 72 kDa polypeptide doublet that was identified previously by the polyclonal antibody D4 as a component of the microdomain structure formed between migrating neurons and radial glial cell processes (Cameron and Rakic [1994] J. Neurosci. 14:3139-3155). Immunofluorescent localization studies reveal a spatial and temporal pattern of 15D7 immunoreactivity in multiple brain regions that correlates well with time periods when neuronal cell migration is a prominent morphogenetic event. In areas where the process of migration is underway, 15D7 immunoreactivity is detected simultaneously in both radial glial cells and cells that have the positional and morphologic features characteristic of migrating neurons. Subsequent to the completion of migration, immunoreactivity is detected in the transitional forms of radial glial cells and mature astrocytes, but not in neurons. Cell aggregation analyses reveal that 15D7 antibodies perturb the rate of aggregation for astrocyte-astrocyte, neuron-neuron, and mixed cell-cell combinations. Taken together, the present studies suggest that the polypeptides recognized by the 15D7 antibodies likely participate in an adhesive process, principally within the ventricular and subventricular zones, that is essential at the onset of the cell migration process.

Aging↗
PubMed1997

Absence of excitotoxicity-induced apoptosis in the hippocampus of mice lacking the Jnk3 gene.

Excitatory amino acids induce both acute membrane depolarization and latent cellular toxicity, which often leads to apoptosis in many neurological disorders. Recent studies indicate that glutamate toxicity may involve the c-Jun amino-terminal kinase (JNK) group of mitogen-activated protein (MAP) kinases. One member of the JNK family, Jnk3, may be required for stress-induced neuronal apoptosis, as it is selectively expressed in the nervous system. Here we report that disruption of the gene encoding Jnk3 in mice caused the mice to be resistant to the excitotoxic glutamate-receptor agonist kainic acid: they showed a reduction in seizure activity and hippocampal neuron apoptosis was prevented. Although application of kainic acid imposed the same level of noxious stress, the phosphorylation of c-Jun and the transcriptional activity of the AP-1 transcription factor complex were markedly reduced in the mutant mice. These data indicate that the observed neuroprotection is due to the extinction of a Jnk3-mediated signalling pathway, which is an important component in the pathogenesis of glutamate neurotoxicity.

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PubMed1997

Early divergence of magnocellular and parvocellular functional subsystems in the embryonic primate visual system.

In both human and Old World primates visual information is conveyed by two parallel pathways: the magnocellular (M) and parvocellular (P) streams that project to separate layers of the lateral geniculate nucleus and are involved primarily in motion and color/form discrimination. The present study provides evidence that retinal ganglion cells in the macaque monkey embryo diverge into M and P subtypes soon after their last mitotic division and that optic axons project directly and selectively to either the M or P moieties of the developing lateral geniculate nucleus. Thus, initial M projections from the eyes overlap only in prospective layers 1 and 2, whereas initial P projections overlap within prospective layers 3-6. We suggest that the divergence of the M and P pathways requires developmental mechanisms different from those underlying competition-driven segregation of initially intermixed eye-specific domains in the primate visual system.

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PubMed1997

Radiation-induced, lamina-specific deletion of neurons in the primate visual cortex.

To examine the early determinants of cortical cytoarchitecture, we deleted specific neuronal classes in the primate visual cortex by ionizing irradiation at selected prenatal stages. Multiple doses of X-rays were delivered to the macaque monkey brain between embryonic day (E) 80 and E90 to block the division of cells destined to populate the superficial cortical layers, between E70 and E79 to eliminate neurons destined for the middle layers; and between E33 and E40 to delete neurons destined for the lateral geniculate nucleus (LGN) that project to the cortex. All animals were killed after birth, and their brains were processed for histological and electron microscopic analyses. Cell density and number in the LGN and visual cortex were determined by using three-dimensional, computer-aided morphometry. In animals irradiated with low doses (total of approximately 200 cGy) during the genesis of the LGN but before the onset of corticogenesis (E33-40), the LGN was reduced in both volume and number of neurons. Area 17 in these animals displayed only slight changes in cortical thickness, cell density, and area-specific cytoarchitectonic features, whereas the total surface devoted to area 17 was significantly diminished. In contrast, animals irradiated with low doses during the period of corticogenesis, after the completion of the LGN genesis, showed no significant change in the volume of the LGN or in the number of its cells. Moreover, in these animals, the surface of area 17 was not significantly altered, although the cortical layers generated at the time of irradiation had a significantly lower density and total number of cells, whereas the layers generated before and after the period of irradiation were spared. In contrast, cases exposed to high doses of X-ray (total > 300 cGy) showed more severe effects, including all layers. However, layers normally generated during irradiation were depleted and consisted of cell-sparse strata populated by densely packed neuropil (axons, small dendrites, dendritic spines, and synaptic boutons). These cell-sparse strata were situated deeper in the early irradiated animals than in the later irradiated animals, and their laminar position changed abruptly at the area 17/18 border. These results show that low doses of irradiation in a slowly developing primate brain can be used effectively to eliminate targeted classes of neurons before they reach their final position, providing an opportunity to examine the role of cell-cell interactions in the formation of circuitry and the role of specific cell classes in cortical development.

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PubMed1997

Restrictive clonal allocation in the chimeric mouse brain.

Whether, and to what extent, lineage restriction contributes to the organization of the mammalian brain remains unclear. Here we address this issue by examining the distribution of clonally related cells in chimeric mice generated by injecting genetically tagged embryonic stem (ES) cells into blastocyst embryos. Our examination of postnatal chimeric brains revealed that the vast majority of labeled ES cell descendents were confined within a different subset of brain regions in each animal. Moreover, the deployment of labeled cells in different brain regions was distinctive. The pattern of ordered and binomial colonization suggested that early diversified founder cells may constrain the fates of their descendants through a restriction of dispersion. In addition, the symmetrical distribution of ES cell descendants suggests that bilaterally corresponding structures may arise from a common set of progenitor cells. Finally, clones of cells formed a continuous band within the deep strata of the neocortex. This later finding in conjunction with the radial distribution of clones in remaining layers observed in previous studies indicates that the cerebral neocortex may derive from two groups of founder cells, which is consistent with the hypothesis of dual phylogenetic origins of the mammalian cerebral cortex.

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PubMed1997

Early emergence of photoreceptor mosaicism in the primate retina revealed by a novel cone-specific monoclonal antibody.

We have examined the emergence of the photoreceptor mosaic in fetal macaque monkeys by using a novel monoclonal antibody, 7G6, that recognizes all cones in the adult primate retina. In the fetal retina, however, between embryonic (E) day 80 and E130, some opsin-positive cones were not labeled by 7G6. Double-labeling experiments revealed that though long and middle wavelength-sensitive fetal cones are 7G6-positive, a subset of short wavelength-sensitive cones are delayed in their acquisition of 7G6 immunoreactivity. Heterogeneity in cone labeling with 7G6 was evident at all fetal ages. The onset of 7G6 immunoreactivity, surprisingly, precedes both the expression of the cone opsins and the formation of synaptic contacts in the outer plexiform layer. Specifically, a population of cones was labeled in the periphery of the E65 retina, concomitant with ongoing cone genesis. Moreover, early-differentiating 7G6-positive cones are organized into a regular array in immature, peripheral regions of the fetal retina indicating that the spatial arrangement of cones is initiated either during active cone proliferation or the initial differentiation of these cells. These results suggest that the periodic spacing of cones in the primate retina emerges autonomously within the photoreceptor layer, prior to the formation of synaptic connections within the retina or with the brain.

Animals↗
PubMed1997

Role of GGF/neuregulin signaling in interactions between migrating neurons and radial glia in the developing cerebral cortex.

During neuronal migration to the developing cerebral cortex, neurons regulate radial glial cell function and radial glial cells, in turn, support neuronal cell migration and differentiation. To study how migrating neurons and radial glial cells influence each others' function in the developing cerebral cortex, we examined the role of glial growth factor (a soluble form of neuregulin), in neuron-radial glial interactions. Here, we show that GGF is expressed by migrating cortical neurons and promotes their migration along radial glial fibers. Concurrently, GGF also promotes the maintenance and elongation of radial glial cells, which are essential for guiding neuronal migration to the cortex. In the absence of GGF signaling via erbB2 receptors, radial glial development is abnormal. Furthermore, GGF's regulation of radial glial development is mediated in part by brain lipid-binding protein (BLBP), a neuronally induced, radial glial molecule, previously shown to be essential for the establishment and maintenance of radial glial fiber system. The ability of GGF to influence both neuronal migration and radial glial development in a mutually dependent manner suggests that it functions as a mediator of interactions between migrating neurons and radial glial cells in the developing cerebral cortex.

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PubMed1996

Decreased apoptosis in the brain and premature lethality in CPP32-deficient mice.

Programmed cell death (apoptosis) is a prominent feature of the development of the immune and nervous systems. The identification of the Caenorhabditis elegans cell death gene, ced-3, as a prototype of the interleukin-1beta converting enzyme (ICE) protease family has led to extensive evidence implicating these enzymes in apoptosis. Among the ten or more members of the ICE protease family, CPP32/yama/apopain exhibits the highest similarity to CED-3 in both sequence homology and substrate specificity. To analyse its function in vivo, we generated CPP32-deficient mice by homologous recombination. These mice, born at a frequency lower than expected by mendelian genetics, were smaller than their littermates and died at 1-3 weeks of age. Although their thymocytes retained normal susceptibility to various apoptotic stimuli, brain development in CPP32-deficient mice was profoundly affected, and discernible by embryonic day 12, resulting in a variety of hyperplasias and disorganized cell deployment. These supernumerary cells were postmitotic and terminally differentiated by the postnatal stage. Pyknotic clusters at sites of major morphogenetic change during normal brain development were not observed in the mutant embryos, indicating decreased apoptosis in the absence of CPP32. Thus CPP32 is shown to play a critical role during morphogenetic cell death in the mammalian brain.

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