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Biomedical subjects

John G Parnavelas

Publications and source records attributed to John G Parnavelas.

At least 19 recordsLinked to original sources
PubMed2006

Role of p35/Cdk5 in preplate splitting in the developing cerebral cortex.

The earliest generated cells of the mammalian cerebral cortex form the preplate layer (PPL). The subsequently born cortical plate (CP) cells split this layer into the superficial layer I (LI) and the deep subplate (SP). The cellular and molecular mechanisms that underlie this event are unclear. To investigate the role of the cyclin-dependent kinase 5 (Cdk5) and its activator p35 in preplate splitting, we used Nissl staining, carbocyanine dye tracing, cell birthdating, and immunohistochemistry for calretinin (CalR) in p35 and Cdk5 knockout mice. Our data demonstrated changes in early cortical lamination and aberrant thalamic axon trajectories in these mice. Specifically, LI was thicker, and cell-dense and thalamic axons did not accumulate in the SP layer before invading the CP. Instead, they grew past the SP and more superficial cortical layers and coursed obliquely toward the pial surface. This behavior has been previously observed in reeler mice and suggests a defect in PPL splitting. CalR immunohistochemistry and bromo-deoxyuridine birthdating confirmed the abnormality in position of the earliest generated cortical cells of mutants. These observations suggest that the p35/Cdk5 pathway plays a role in preplate splitting in addition to regulating layer formation.

Aging↗
PubMed2006

The role of ARX in cortical development.

The ARX protein (encoded by the aristaless-related homeobox gene) is a member of the paired class of homeoproteins. More precisely, it is a member of the Aristaless subclass of proteins with a glutamine residue (Q) at the critical position 50 of the homeodomain (Q50). Through identification of diverse inherited or de novo mutations, genetic investigations of X-linked mental retardation conditions have demonstrated the implication of ARX in a wide spectrum of disorders extending from phenotypes with severe neuronal migration defects, such as lissencephaly, to mild forms of X-linked mental retardation without apparent brain abnormalities. These investigations have recently directed attention to the role of this gene in brain development. Analysis of its spatiotemporal localization profile have revealed expression mainly in telencephalic structures at all stages of development. Interestingly, in adult, ARX expression becomes restricted to a population of GABAergic neurons. Although the identification of the target genes regulated by ARX remains a crucial step to better understanding its role during brain development, studies of the role of ARX orthologs in different models have indicated that it is essential for important developmental processes such as proliferation, cell differentiation and neuronal migration.

Animals↗
PubMed2006

Cell and molecular mechanisms involved in the migration of cortical interneurons.

Since the discovery that the vast majority of the GABA-containing interneurons of the cerebral cortex arise in the subpallium, considerable effort has been put into the description of the precise origin of these neurons in subdivisions of the ganglionic eminence and in the migratory routes they follow on their way to the developing cortex. More recently, studies have focused on the molecular and cellular mechanisms that guide their migration. Investigations of the molecular mechanisms involved have demonstrated important roles for numerous transcription factors, motogenic factors and guidance molecules. Here, we review results of very recent analyses of the underlying cellular mechanisms and specifically of the movement of the nucleus, cytoplasmic components and neuritic processes during interneuron migration.

Animals↗
PubMed2006

Apoptosis in the rat basal forebrain during development and following lesions of connections.

Evidence suggests that neurotrophins are essential for the survival and phenotypic maintenance of cholinergic basal forebrain (BF) neurons. We evaluated the pattern of programmed cell death in the BF of the rat during development and after ablations of the cerebral cortex, a major target area and source of neurotrophins for BF neurons. We identified dying cells using the TUNEL (terminal deoxynucleotidyl-transferase-mediated dUTP-biotin nick end labelling) method and confirmed their apoptotic morphology with electron microscopy. Moreover, we demonstrated the expression of the apoptotic marker active caspase-3 in cells with features of apoptosis. TUNEL(+) cells were present in the developing BF during the first two postnatal weeks. Their frequency peaked at postnatal day (P)1 and at P5. TUNEL used in conjunction with immunofluorescence for neuronal nuclear protein (NeuN) showed that, at both peak stages, the majority of apoptotic cells were neurons. Extensive lesions of the cerebral cortex at different ages (P0, P7 and P14) did not induce significant changes in the frequency of apoptotic BF neurons. However, they resulted in alterations in the morphological phenotype of choline acetyltransferase (ChAT)-immunoreactive neurons in the BF, and a reduction in their number which was inversely proportional to the age at which the lesions were performed. We suggest that: (i) apoptosis is temporally coordinated with the morphological and neurochemical differentiation of BF neurons and the establishment of connections with their target areas; and (ii) cortical ablations do not affect the survival of BF neurons, but they influence the phenotype of cholinergic BF neurons.

Acetylcholine↗
PubMed2006

Robo1 regulates the development of major axon tracts and interneuron migration in the forebrain.

The Slit genes encode secreted ligands that regulate axon branching, commissural axon pathfinding and neuronal migration. The principal identified receptor for Slit is Robo (Roundabout in Drosophila). To investigate Slit signalling in forebrain development, we generated Robo1 knockout mice by targeted deletion of exon 5 of the Robo1 gene. Homozygote knockout mice died at birth, but prenatally displayed major defects in axon pathfinding and cortical interneuron migration. Axon pathfinding defects included dysgenesis of the corpus callosum and hippocampal commissure, and abnormalities in corticothalamic and thalamocortical targeting. Slit2 and Slit1/2 double mutants display malformations in callosal development, and in corticothalamic and thalamocortical targeting, as well as optic tract defects. In these animals, corticothalamic axons form large fasciculated bundles that aberrantly cross the midline at the level of the hippocampal and anterior commissures, and more caudally at the medial preoptic area. Such phenotypes of corticothalamic targeting were not observed in Robo1 knockout mice but, instead, both corticothalamic and thalamocortical axons aberrantly arrived at their respective targets at least 1 day earlier than controls. By contrast, in Slit mutants, fewer thalamic axons actually arrive in the cortex during development. Finally, significantly more interneurons (up to twice as many at E12.5 and E15.5) migrated into the cortex of Robo1 knockout mice, particularly in both rostral and parietal regions, but not caudal cortex. These results indicate that Robo1 mutants have distinct phenotypes, some of which are different from those described in Slit mutants, suggesting that additional ligands, receptors or receptor partners are likely to be involved in Slit/Robo signalling.

Animals↗
PubMed2005

Reelin provides an inhibitory signal in the migration of gonadotropin-releasing hormone neurons.

Gonadotropin-releasing hormone (GnRH) neurons, a small number of cells scattered in the hypothalamic region of the basal forebrain, play an important role in reproductive function. These cells originate in the olfactory placode and migrate into the basal forebrain in late embryonic life. Here, we show that reelin, which is expressed along the route of the migrating cells, has an inhibitory role in guiding GnRH neurons to the basal forebrain. Only a small (approximately 5%) subpopulation of these neurons expresses one of the reelin receptors (ApoER2/Lrp8), and all GnRH neurons appear to lack the intracellular adaptor protein Dab1, suggesting that the function of reelin is not mediated by the conventional signal transduction pathway. The importance of reelin in the establishment of GnRH neurons in the hypothalamus was confirmed by our finding that the brains of developing and adult reeler mice of both sexes contained a markedly reduced number of these neuroendocrine neurons. Furthermore, the testes of adult males showed dilation of seminiferous tubules and reduction in their density when compared with controls. Mutants lacking the reelin receptors ApoER2 and Vldlr, and scrambler mice lacking Dab1, showed a normal complement of GnRH neurons in the hypothalamus, confirming that the effect of reelin in their migration is independent of Dab1.

Animals↗
PubMed2005

Development of the dopaminergic neurons in the rodent brainstem.

The loss of dopaminergic (DA) neurons in the ventral midbrain is the principal cause of Parkinson's disease. The search for candidate molecules that promote the genesis and survival capacities of DA neurons is a major area of investigation and hope. A better characterization of the developmental pathways that govern the specification, differentiation, and survival of these neurons will be essential in devising therapies aimed to rescue or replace midbrain DA neurons in Parkinson's patients. In this brief review, we will discuss the major steps in the normal development of midbrain DA neurons.

Animals↗
PubMed2004

Noradrenergic innervation of the developing and mature septal area of the rat.

The noradrenergic innervation of the developing and mature septal area of the rat was examined with light and electron microscopic immunocytochemistry using an antibody against dopamine-beta-hydroxylase. At birth, a small number of relatively thick noradrenergic fibers were found to innervate the lateral septum (mainly its intermediate part) and the nuclei of the vertical and horizontal limbs of the diagonal band of Broca. By postnatal day 7, a substantial increase in their density was observed. At this age some labeled fibers left the medial forebrain bundle and invaded the nucleus of the horizontal limb of the diagonal band. These fibers then ran in a ventrodorsal direction and innervated the nucleus of the vertical limb before entering the medial septum. Immunoreactive fibers were finer and more varicose than at birth. In the subsequent 2 weeks, the density of labeled fibers in the septal area was further increased. By postnatal day 21, the distribution pattern and density of the noradrenergic innervation appeared similar to the adult. In the adult, noradrenergic fibers exhibited more varicosities than in younger rats. Electron microscopic analysis revealed a low proportion (peaked at P7) of noradrenergic varicosities engaged in synaptic contacts throughout development. The overwhelming majority of these synapses were symmetrical, predominantly with small or medium-sized dendrites. The present findings provide the morphological basis for the functional interactions between noradrenergic afferents and neuronal elements in the septal area. The low proportion of synaptic contacts found in this study suggests that noradrenaline may exert its action in the septal area mainly through transmission by diffusion (volume transmission), as has been suggested for other areas of the developing and adult brain.

Animals↗
PubMed2004

Lhx6 regulates the migration of cortical interneurons from the ventral telencephalon but does not specify their GABA phenotype.

The LIM homeodomain family of transcription factors is involved in many processes in the developing CNS, ranging from cell fate specification to connectivity. A member of this family of transcription factors, lhx6, is expressed in the medial ganglionic eminence (MGE) of the ventral telencephalon, where the vast majority of cortical interneurons are generated. Its expression in the GABA-containing MGE cells that migrate to the cortex suggests that this gene uniquely or in combination with other transcription factors may play a role in the neurochemical identity and migration of these neurons. We performed loss of function studies for lhx6 in mouse embryonic day 13.5 brain slices and dissociated MGE neuronal cultures using Lhx6-targeted small interfering RNA produced by a U6 promoter-driven vector. We found that silencing lhx6 impeded the tangential migration of interneurons into the cortex, although it did not obstruct their dispersion within the ganglionic eminence. Blocking lhx6 expression in dissociated MGE cultured neurons did not interfere with the production of GABA or its synthesizing enzyme. These results indicate that lhx6, unlike the closely related member lhx7, does not regulate neurotransmitter choice in interneurons but plays an important role in their migration from the ventral telencephalon to the neocortex.

Animals↗
PubMed2004

Distribution of cortical interneurons in grey matter heterotopia in patients with epilepsy.

PURPOSE: Grey matter heterotopia are well-defined malformations of the cortex often associated with severe epilepsy. Defects have been identified in genes, including DCX and FLN1, that influence radial migration of postmitotic cells from the ventricular zone to the cortical plate. A proportion of cortical gamma-aminobutyric acid (GABA)-containing interneurons may arise from the ganglionic eminence of the ventral telencephalon. We aimed to identify the subtypes and localisation of interneurons within grey matter heterotopia relative to cortex. METHODS: By using quantitative immunohistochemistry, we studied the density and distribution of interneurons within six cases of grey matter heterotopia in postmortem tissue from patients with epilepsy. RESULTS: In many cases, a suggestion of focal rudimentary laminar arrangement and small reelin-positive cells was identified within the heterotopia. Immunohistochemistry for glutamic acid decarboxylase(65/57), parvalbumin, calbindin, and calretinin showed inhibitory neurons of all subtypes represented within the heterotopia, and of normal morphology. The mean densities of interneurons were overall similar to those of the overlying cortex, but the interneurons showed less organisation and were more randomly orientated compared with cortex. CONCLUSIONS: Interneurons within heterotopia probably arise from the ventricular zone, but their abnormal local organization may influence the epileptogenicity of these lesions.

Adolescent↗
PubMed2003

Restricted expression of Slap-1 in the rodent cerebral cortex.

The deep layers of the mammalian cerebral cortex contain pyramidal neurons that project predominantly to subcortical targets. To understand the mechanisms that determine the identity of deeper layer neurons, a PCR based subtractive hybridisation was performed to isolate genes that are specifically expressed during the specification of these neurons. One of the genes we isolated was the rat homologue of the mouse Slap-1. SLAP-1 is an adaptor protein containing SH2-SH3 domains and it participates in the signalling of Receptor Tyrosine Kinases. In situ hybridisation studies have shown that Slap-1 is not substantially expressed before E17. At later stages, it is specifically and selectively expressed by deeper layer neurons and by neurons of layers II/III in the developing cortex. The specific timing and location of its expression, suggests that this gene may play a role in the differentiation of these neurons.

Adaptor Proteins, Signal Transducing↗
PubMed2002

Connexin expression in homotypic and heterotypic cell coupling in the developing cerebral cortex.

Intercellular communication through gap junction channels is a prominent feature of the developing cerebral cortex. In the first 2 weeks after birth, a time critical in the development of the rat neocortex, extensive cell coupling has been documented that diminishes as the cortex matures. Among the family of gap junction proteins, connexins 26, 36, and 43 are differentially expressed during cortical development. We used intracellular dye injections and connexin immunohistochemistry to investigate the coupling patterns and connexin expression between the different neuronal and glial cell types of the developing cortex of the rat. We found that neurons and glia couple homotypically and heterotypically at postnatal days 7 and 14. Although the prevalence of coupling was homotypic, there was considerable heterotypic coupling that involved pyramidal and nonpyramidal neurons, the principal neuronal cell types of the cortex, or neurons and astrocytes. Coupling between different cell types appeared to be mediated by differential expression of connexins 26, 36, and 43. It may be that coupling between cells in the developing neocortex is a function of the spatial and temporal expression of these and other connexin proteins.

Animals↗
PubMed2002

A novel method of labeling and characterizing migrating neurons in the developing central nervous system.

Neuronal migration, a discrete event in the developing nervous system, is currently being intensively investigated using a variety of anatomical and molecular approaches. Using 4-chloromethyl benzoyl amino tetramethyl rhodamine (CMTMR) coated particles, we describe here a novel and efficient method of tracer labeling to investigate cell migration in embryonic and postnatal brain. Further, we demonstrate that application of CMTMR facilitates the labeling of a large number of migrating cells and enables the characterization of their phenotypes with immunohistochemical and in situ hybridization techniques. We also illustrate that CMTMR labeling is ideally suited for time-lapse imaging of the behavior and dynamics of migrating cells.

Animals↗
PubMed2002

Ventricle-directed migration in the developing cerebral cortex.

It is believed that postmitotic neurons migrate away from their sites of origin in the germinal zones to populate distant targets. Contrary to this notion, we found, using time-lapse imaging of brain slices, populations of neurons positioned at various levels of the developing neocortex that migrate towards the cortical ventricular zone. After a pause in this proliferative zone, they migrate radially in the direction of the pial surface to take up positions in the cortical plate. Immunohistochemical analysis together with tracer labeling in brain slices showed that cells showing ventricle-directed migration in the developing cortex are GABAergic interneurons originating in the ganglionic eminence in the ventral telencephalon. We speculate that combinations of chemoattractant and chemorepellent molecules are involved in this ventricle-directed migration and that interneurons may seek the cortical ventricular zone to receive layer information.

Animals↗
PubMed2002

Modes of neuronal migration in the developing cerebral cortex.

The conventional scheme of cortical formation shows that postmitotic neurons migrate away from the germinal ventricular zone to their positions in the developing cortex, guided by the processes of radial glial cells. However, recent studies indicate that different neuronal types adopt distinct modes of migration in the developing cortex. Here, we review evidence for two modes of radial movement: somal translocation, which is adopted by the early-generated neurons; and glia-guided locomotion, which is used predominantly by pyramidal cells. Cortical interneurons, which originate in the ventral telencephalon, use a third mode of migration. They migrate tangentially into the cortex, then seek the ventricular zone before moving radially to take up their positions in the cortical anlage.

Animals↗