Neuronal Migration Process Research Articles

Decreased neural damage after spinal cord injury in tPA-deficient mice.

Tissue plasminogen activator (tPA) is a serine protease that converts plasminogen to plasmin. It plays an important role in the nervous system, including the processes of neuronal migration, neurite outgrowth, and neuronal plasticity. tPA has also been suggested to have a role in several neuropathological conditions, such as cerebral ischemia, seizures, and demyelinating diseases. To investigate the role of tPA in spinal cord injury, wild-type mice and mice with homozygous tPA deficiency (tPA(-/-) mice) were subjected to spinal cord contusion and the differences of hindlimb function, electrophysiological changes, and histopathological changes were assessed for 6 weeks. Functional recovery was greater in tPA(-/-) mice than in wild-type mice throughout the observation period. The time course of myoelectric motor-evoked potentials supported the hindlimb functional findings. Histological examination showed that injured areas were smaller in tPA(-/-) mice than wild-type mice on Luxol fast blue staining or myelin basic protein and neurofilament protein immunostaining at 6 weeks after contusion. Electron microscopy showed that the white matter was better preserved in tPA(-/-) mice than in wild-type mice. The expression of tPA protein was widespread on the first day after contusion and this expression was detected for at least a week. Activation of microglia/macrophages and apoptotic cell death were significantly reduced in tPA(-/-) mice after contusion. This study shows that neural damage is decreased in tPA(-/-) mice after spinal cord injury. Suppression of tPA production may help to decrease secondary injury after spinal cord contusion.

Hypotension in the Neonate

After completing this article, readers should be able to: 1. Describe the relationship between blood pressure and birthweight, gestational age, and postnatal age. 2. Delineate potential negative consequences of treating hypotension in the neonate. 3. Characterize the evaluation of a neonate prior to treating suspected hypotension. 4. Describe current and potential future therapeutic options for treatment of neonatal hypotension. Deciding which neonates require treatment for a low arterial blood pressure (BP) often is difficult. To some extent, this has been due to a lack of normative BP data, particularly among preterm neonates. However, we now have reliable data relating birthweight, gestational age, and postnatal age to BP in term and preterm neonates. Nevertheless, the management of suspected hypotension in various neonatal intensive care units differs. Although there are good studies comparing various modes of therapy, prospective trials relating long-term outcome to treatment versus no treatment at various BP values have not been performed. Concern about the deleterious effects of hypotension is greater for preterm neonates primarily because of the relative inability to regulate cerebral blood flow in preterm neonates compared with infants who are born at term. ### Preterm Neonates Clearly, the primary issue surrounding possible hypotension in preterm neonates is the concern that these infants may experience impaired central nervous system perfusion, leading to ischemic damage. Low arterial BP has been associated with a decrease in cerebral blood flow, and the apparent impairment of autoregulation of cerebral blood flow in distressed infants emphasizes the importance of preventing both hypotension and rapid increases in BP in preterm neonates. Impaired autoregulation, demonstrated by near-infrared spectroscopy, and BP have been shown to be predictive of germinal matrix-intraventricular hemorrhage, periventricular leukomalacia, or both. When mean blood pressure (MBP) was measured continuously in neonates at gestational ages 26 to 30 weeks, a highly significant relationship was demonstrated between MBP less than …

Expression of the α4 isoform of the nicotinic acetylcholine receptor in the fetal human cerebral cortex

Nicotinic acetylcholine receptors are likely to play an important role in neuronal migration during development. Furthermore, the α4 receptor subunit gene is related to a hereditary juvenile form of epilepsy. Only little information is available, however, on the expression of cerebrocortical nicotinic acetylcholine receptors during human fetal development. Using non-isotopic in situ hybridization and immunohistochemistry, we have studied the distribution of the α4 subunit of the nicotinic acetylcholine receptor mRNA and protein in the human frontal cortex at middle (17–24 weeks of gestation) and late (34–42 weeks of gestation) fetal stages. Both, α4 receptor mRNA and α4 receptor protein were observed beginning during week 17–18 of gestation. At this time of development, a few weakly labeled mRNA-containing cells were present mainly in the ventricular zone, the subplate and the cortical plate. A similar distribution pattern was found for the receptor protein. Around week 38 of gestation, the distribution in the cerebral cortex of α4 subunit-containing cells was similar to that of adult human cortices with the highest densities of labeled neurons found in layers II/III, followed by layers V and VI. Nicotinic acetylcholine receptor-containing neurons appear rather early in human fetal development. Given functional maturity, they may interact during cortical development with acetylcholine released from corticopetal fibers or other yet unknown sources subserving the process of neuronal migration and pathfinding.

LIS1 and dynein motor function in neuronal migration and development.

Neuronal migration has been studied extensively for over 30 years in diverse mammalian species from the mouse to human. The sequence of events that occurs during cortical development is shared by all of these species (for reviews, see Gleeson and Walsh 2000; Walsh and Goffinet 2000). At the time of neurogenesis, neural precursors proliferate and differentiate into young postmitotic neurons. These postmitotic immature neurons migrate from the ventricular zone (VZ) to a layer called the preplate at the surface of the developing cerebral cortex. The first migrating neurons split the preplate and form the cortical plate, which develops into the cortex. As migration from the VZ continues, cortical lamination is established in an inside-out fashion. The earliest-born neurons end up deep in the cortex, as later-born neurons migrate past them toward the pial surface to establish more superficial layers of the cortex. The latest-born neurons reside near the pial surface. In the final stages of cortical development, synaptogenesis and apoptotic elimination of populations of neurons occur. Physically, the migration of neurons require the same three steps necessary for migration of any cell: The extension of the leading edge that explores its environment for attractive and repulsive signals; the movement of the nucleus into the leading process, called nucleokinesis; and the retraction of the trailing process. The understanding of the process of mammalian neuronal migration resulted primarily from neurobiological studies of brain development in normal mammals and mutant mice such as reeler. These studies provided important insight into the laminar development of the CNS. However, they did not identify the genetic and developmental pathways that regulate neuronal migration. Cloning of disease genes for human neuronal migration defects and mouse mutants have provided critical entry points into these pathways, whereas genetic and cell biological studies have guided our understanding of the function of these gene products in neuronal migration. For example, cloning of the gene mutated in reeler and the identification of its protein product RELN led to the elucidation of a RELN-dependent signal transduction pathway (for recent review, see Rice and Curran 1999). Other genes have been identified that are responsible for neuronal migration defects in the human (LIS1 and DCX) and mouse (Cdk5 and its required activators p35 and p39), but until recently, the relationship between these genes involved in migration were unknown. Several recent studies have demonstrated that LIS1, the protein product of a gene mutated in the human neuronal migration defect lissencephaly, binds to and regulates dynein motor function in the cell. These studies place LIS1 in the midst of a well-studied motor complex important for several critical cell functions and perhaps provide a context for integrating several neuronal migration pathways. In this review, we will provide some background on the human and mouse studies that provided the entry points to neuronal migration pathways, the investigation of gene function in model systems, and what these studies tell us about the mechanisms regulating neuronal migration. We will focus on the recent studies of LIS1 function and how they can potentially integrate other known pathways of neuronal migration.

Differing, Spatially Restricted Roles of Ionotropic Glutamate Receptors in Regulating the Migration of GnRH Neurons during Embryogenesis

We have examined here the role of glutamate in regulating the process of tangential neuronal migration during embryogenesis by investigating the roles of AMPA and NMDA receptors in the migration of the gonadotropin-releasing hormone (GnRH) neurons from the nose to the hypothalamus. We first determined that GluR1-4 subunit mRNAs were present from embryonic day (E) 12.5 along the complete nose-brain migratory pathway of the GnRH neurons, whereas that of the obligatory NMDAR1 transcript was present only in brain regions of GnRH migration. In vivo studies revealed that AMPA receptor antagonism between E12.5 and E16.5 resulted in a significant (p < 0.05) accumulation of GnRH neurons in the nose adjacent to the cribiform plate. In contrast, NMDA receptor antagonism over E12.5-E16.5 or E13.5-E16.5 caused a selective increase (p < 0.05) in the number of GnRH neurons located in their final resting place within the diagonal band of Broca and preoptic area. Dual-labeling studies using GnRH promoter-LacZ transgenic mice, which facilitate the identification of receptors in GnRH neurons, identified the presence of NMDAR1 receptors in approximately 6% of embryonic GnRH neurons located throughout the migratory pathway. Postnatally, the percentage of GnRH neurons expressing NMDAR1 increased to 50%. These results indicate that tonic AMPA receptor activation enhances the migration of GnRH neurons from the nose into the brain, whereas that of NMDA receptor activation slows the final phase of GnRH migration within the forebrain. These in vivo observations demonstrate differing, spatially restricted roles for AMPA and NMDA receptor activation in the process of tangential neuronal migration.

Adverse effects of fetal cocaine exposure on neonatal auditory information processing.

Studies with animals have shown that in utero exposure to cocaine interferes with fetal brain development by disrupting the processes of neuronal proliferation, differentiation, and migration, often leading to subsequent neurobehavioral deficits. However, studies with humans have produced inconsistent findings. Although neurobehavioral abnormalities have been observed among cocaine-exposed infants in several studies and in some cases dose-response effects have been found, the specific neurobehaviors affected vary from one study to the next. Researchers studying the effects of fetal cocaine-exposure are faced with many difficult challenges. For example, women who use cocaine typically use other substances in addition to cocaine, many of the methods available for identifying cocaine-exposed neonates are not reliable, and the available methods for assessing cocaine-exposed newborns may not be sufficiently sensitive to detect the subtle effects of cocaine on the developing central nervous system. Despite these difficulties, there is a growing body of research that suggests that fetal cocaine exposure is associated with subsequent language deficits among children exposed in utero. However, it is virtually impossible to disentangle the effects of the impoverished environments in which these children are often raised from the effect, if any, of fetal cocaine exposure. To determine the effects of fetal cocaine exposure independent of postnatal environmental effects, cocaine-exposed neonates would ideally be tested within the first few weeks of birth, and to identify early risks for subsequent language delay, well-researched auditory information processing measures could be used. The purpose of the present study was to assess the effects of fetal cocaine exposure on neonatal auditory information processing ability. To overcome limitations of some previous studies on the neuroteratogenic effects of cocaine, such as unreliable subject identification techniques, inadequate control over confounding variables, and questionable measures of central nervous system integrity, a valid measure of auditory information processing was used in a rigorous, case-control design. Newborn information processing was assessed using habituation and recovery of head-turning toward an auditory stimulus across the 3 phases of the procedure: familiarization, novelty, and dishabituation. During the familiarization phase, the infant orients and habituates to a repeated word; during the novelty phase, the infant recovers head-turning to a novel word and subsequently habituates to this word; and during the dishabituation phase the infant displays renewed head-turning to the return of the original stimulus. Testing takes approximately 20 minutes. This procedure has been shown previously to discriminate among infants at high-, moderate-, and low-risk for subsequent developmental delay. Twenty-five cocaine-exposed and 25 nonexposed control neonates, identified by meconium analysis, urine analysis, and/or maternal self-report, were tested on the auditory information processing procedure. The majority of infants were tested within the first few days of birth. Cocaine-exposed and control neonates were matched on birth weight, gestational age, Apgar scores, age at testing, and socioeconomic status as reflected by household income. Mothers were matched on age, weight gain, cigarette smoking, and alcohol consumption. Fetal cocaine exposure was associated with impaired auditory information processing. Both cocaine-exposed and nonexposed control neonates oriented to the familiarization stimulus, but cocaine-exposed neonates displayed impaired habituation. Moreover, cocaine-exposed neonates did not recover or habituate to the novel stimulus or dishabituate to the return of the familiarization stimulus. (ABSTRACT TRUNCATED)

Current concepts of cerebral malformation syndromes.

In the past, children with many brain malformations were classified as having static encephalopathies (cerebral palsy), often attributed to perinatal or prenatal distress. Understanding of the frequency and clinical manifestations of brain malformations, however, has increased dramatically in the past 10 to 15 years. During this time, it has become apparent that many static encephalopathies in children have a brain malformation as their substrate. Most of the increase in our knowledge can be attributed to advances in neuroimaging and in molecular biology. In general, radiologic analysis of the brain allows similar malformations to be classified together. Subsequent genetic analysis of the affected children often reveals the affected gene, leading to identification of the gene product and, ideally, an ultimate understanding of the molecular mechanism of malformation. Currently, many genes involved in the complicated process of neuronal proliferation, migration, and organization are being identified. Knowledge of these genes and a better radiologic classification system enable the referring physician to give better care, more sophisticated genetic counseling, and a more precise prognosis for the child. To illustrate this mechanism of classification, three groups of malformations are discussed, in which a combination of neuroimaging analysis and molecular biologic analysis have led to a new understanding of the malformation syndromes.

Neurodevelopmental abnormalities in schizophrenia: insights from neuropathology.

Growing epidemiological, genetic, and clinical neurobiological evidence indicates that abnormalities in brain development play determining roles in the pathobiology of schizophrenia. Neuropathological research has made significant progress in delineating cellular and molecular abnormalities in schizophrenia that have relevance to neurodevelopment. This paper reviews the neurodevelopmental processes of neurogenesis, neuronal migration, differentiation, synaptogenesis, neuron and synaptic pruning, and myelination and the reported neuropathological findings in schizophrenia that may be a consequence of disturbances in these processes. While many neuropathological findings in schizophrenia are controversial or await confirmation, reported abnormalities in neuron density, number and morphology, cytoarchitecture, dendritic arbors and spines, synapse-related proteins, and the well-established absence of gliosis or any other evidence of neurodegeneration or neural injury all provide support for the neurodevelopmental model of schizophrenia.

Intracellular levels of the LIS1 protein correlate with clinical and neuroradiological findings in patients with classical lissencephaly

We report on the genotype-phenotype correlation in 7 patients with classical lissencephaly carrying a heterozygous subtle mutation in the LIS1 gene. Six patients, showed a mutation predicted to encode for a truncated protein, and one mutation altered a splicing site, resulting in skipping of exon 4. Western blot analysis performed on the lymphoblastoid cell line of 2 patients bearing truncating mutations indicated that the mutated allele did not produce a detectable amount of the LIS1 protein; whereas the analysis performed on the fibroblasts from the patient with a splice-site mutation was suggestive of partial protein synthesis from the mutated allele. Although clinical and magnetic resonance imaging findings of patients with truncating mutations did not differ from those observed in patients with a heterozygous deletion, the patient bearing the exon-skipping mutation had less severe clinical and brain involvement. Our data suggest that truncating mutations in the LIS1 gene are relatively common among patients with classical lissencephaly not bearing a heterozygous deletion at 17p13.3, and strengthen the relevance of correct intracellular dosage of the LIS1 protein in the neuronal migration process.

Temporal pattern of plasminogen activator activity in the developing chick cerebellum

Plasminogen activators are considered to be involved in several developmental events. The present work aims at characterizing the developmental pattern of expression of plasminogen activators in the chick cerebellum.Soluble fractions derived by ultracentrifugation from Triton X-100 treated membrane fractions were used for determination of the enzyme activity with a radial fibrinolytic assay. By using specific inhibitors and different anti-plasminogen activators antibodies it is shown that only one type of the enzyme, the urokinase-type plasminogen activator, is expressed during the cerebellum ontogeny. Our results show the existence of a bimodal pattern of enzyme activity with two peaks that temporally coincide with the processes of massive neuronal migration, neurite outgrowth and synapse formation and plasticity. It is proposed that plasminogen activator could play a role in these developmental events and that its pattern of variability is developmentally regulated.

Role of neuron-glial junctional domain proteins in the maintenance and termination of neuronal migration across the embryonic cerebral wall

To identify glial membrane proteins that contribute to the process of neuronal migration in the developing brain, we developed a polyclonal antiserum (D4) and a monoclonal antibody (NJPA1: neuron-glial junctional polypeptide antibody) that recognize membrane proteins localized to the plasmalemmal junction between migrating neurons and adjacent radial glial fibers (Cameron and Rakic, 1994). Here, we show that in the developing cerebral cortex, immunoreactivity for these junctional polypeptides is present throughout the neuronal migratory pathway but becomes minimal or absent where radial glial cell processes enter the marginal zone region, the barrier at which newly arrived neurons normally stop their migration and detach from their glial fiber substrate. We thus tested, using imprints of embryonic cerebral wall and slice preparations, whether the junctional membrane proteins detected by our antibodies contribute to the regulation of neuronal migration in the cerebral cortex. The rate of neuronal migration on glial cell substrates was reduced significantly in the presence of D4 or NJPA1 antibodies. Antibody exposure typically led to the withdrawal of leading processes, changes in microtubular organization and, in some instances, to detachment of neurons from their glial cell substrates. These results suggest that the polypeptides recognized by the D4 and NJPA1 antibodies are essential for the maintenance of normal neuronal migration. Dismantling of neuron-glial cell junctional domains formed by these membrane proteins may underlie neuronal cell detachment from glial migratory substrates at the interface between cortical plate and marginal zone in the developing cerebral wall.

Cell-Cell Signaling in Early Cortical Development

The remarkable process of neuronal migration has long been viewed as the key event of corticogenesis. Before neurons complete their final cell division and begin to migrate out of the proliferative zones, however, many important decisions concerning their ultimate fate have already been made. These critical decisions are, in part, under local environmental control. The signals by which proliferating cortical precursor cells interact with each other and with their environment are still largely unknown, but, as these factors are dis covered, they will alter our view of corticogenesis and most likely provide new insights into causes of cerebral malformations. This review is intended to describe the repertoire of cellular interactions detected thus far among proliferating cortical cells and to discuss possible roles of cell-cell signaling pathways. The Neurosci entist 1:268-276, 1995

Motor cortex excitability is reduced in patients with cerebellar lesions

The tubulin gene family is mainly expressed in post-mitotic neurons during cortical development with a specific spatial and temporal expression pattern. Members of this family encode dimeric proteins consisting of two closely related subunits (α and β), representing the major constituents of microtubules. Tubulin genes play a crucial role in the mechanisms of the Central Nervous System development such as neuronal migration and axonal guidance (axon outgrowth and maintenance). Different mutations in α/β-tubulin genes (TUBA1A, TUBA8, TUBB2A, TUBB4A, TUBB2B, TUBB3, and TUBB) might alter the dynamic properties and functions of microtubules in several ways, effecting a reduction in the number of functional tubulin heterodimers and causing alterations in GTP binding and disruptions of the binding of other proteins to microtubules (motor proteins and other microtubule interacting proteins).In recent years an increasing number of brain malformations has been associated with mutations in tubulin genes: malformations of cortical development such as lissencephaly and various grades of gyral disorganization, focal or diffuse polymicrogyria and open or closed-lips schizencephaly as likely consequences of an altered neuronal migration process; abnormalities or agenesis of the midline commissural structures (anterior commissure, corpus callosum and fornix), hypoplasia of the oculomotor and optic nerves, dysmorphisms of the hind-brain as expression of axon guidance disorders. Dysmorphisms of the basal ganglia (fusion between the caudate nucleus and putamen with absence of the anterior limb of the internal capsule) and hippocampi were also observed. A rare form of leukoencephalopathy characterized by hypomyelination with atrophy of the basal ganglia an cerebellum (H-ABC) was also recently described.The present review, describing the structural and functional features of tubulin genes, aims to revise the main cerebral associated malformations and related clinical aspects, suggesting a genotype–phenotype correlation.

Dendritic Development of Purkinje and Granule Cells in the Cerebellar Cortex of Rats Treated Postnatally with α-Difluoromethylomithine

Postnatal administration of alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of ornithine decarboxylase (ODC), is known to curtail replication of granule cells in the cerebellum of the rat, but its effects on post-proliferative neurons is unknown. This Golgi study assessed the influence of daily postnatal injections of DFMO on dendritic development of the Purkinje and granule cells. Although the overall appearance of the Purkinje cells was normal, DFMO affected morphogenesis by causing: 1) a reduction in somatic growth, 2) an elongation of the primary dendrite, 3) a lengthening of the distance between the soma and the pial surface, 4) a reduction in the number of tertiary dendrites, and 5) a disruption of the distribution of the tertiary branches. Many of these effects of DFMO may be mediated via the reduction of granule cells rather than representing a direct effect on dendritogenesis. In addition to reducing the number of granule cells, the distribution and morphology of these cells in the DFMO-treated rats were severely affected. Many granule cells did not complete their migration and remained trapped in the molecular layer, and many of those trapped had greatly elongated dendrites that extended into the internal granule cell layer. These results suggest that ornithine decarboxylase is involved in the postproliferative processes of neuronal migration and differentiation.

5′-Nucleotidase in postnatal ontogeny of rat cerebellum: a marker for migrating nerve cells?

The distribution of the ectoenzyme 5′-nucleotidase was studied in the developing rat cerebellum using enzyme- and immunocytochemical techniques in order to evaluate its appearance in the processes of cellular migration during nervous system development. 5′-Nucleotidase enzymatic activity appeared first in the external granular layer (postnatal day 6). Then, the inner portion of the external granular layer and the developing molecular layer became prominently stained (postnatal day 12). At 21 days the adult pattern was established with a diffuse labeling of the whole molecular layer. Additionally, 5′-nucleotidase enzymatic activity was found around Purkinje cells. At the ultrastructural level migrating nerve cells exhibited an intense surface labeling whereas resting neuroblasts of the external granular layer as well as all post-migratory nerve cells were negative. Immunocytochemistry differed from enzyme histochemistry in that immunoreactivity was found to be selectively associated with glial elements. It is suggested that 5′-nucleotidase may serve as a marker for processes of neuronal migration and neuron-glia interactions during postnatal ontogeny. Underlying mechanisms could be due to an interaction of 5′-nucleotidase with the extracellular matrix molecule, laminin, and cytoskeletal actin involved in locomotion.

A comparison of reading strategies in genetic dyslexics and children with right and left brain deficits

The reading levels and reading strategies of 43 children with genetic dyslexia, right-hemisphere lesions/dysfunction, and left-hemisphere lesions/dysfunction were retrospectively compared. Age and IQ were comparable. Only children who had a verbal or performance IQ over 90 were included in the study. Although the genetic dyslexics had a significantly higher Full Scale IQ than the other two groups, their reading level was significantly lower. This suggests that a disturbance of the normal processes of neuronal migration during fetal development may have profound effects on specific cognitive processes, without affecting others. In contrast, 50 percent of the boys with left-hemisphere deficits were disabled readers. One-third of the boys with right-hemisphere deficits were also poor readers. With one exception, the girls, who had comparable deficits, were good readers. A comparison was made between the groups in terms of reading strategies and patterns of errors.

Neuronal migration during the early development of the cerebral cortex: a scanning electron microscopic study.

Fixed cerebral vesicles of mouse foetuses were fractured and examined with the scanning electron microscope. This method provides a study of the three dimensional developmental features of the pseudostratified columnar epithelium up to the formation of the early cortex plate. Matrix cells are a cell population of homogeneous shape, however, mitotic cells are easily identified by their spherical form. The external surface of the brain is formed by the closely packed end feet of these cells covered by a basal membrane. The formation of the cortical plate is the result of a continuous cell migration in columnar arrangement towards the pia. Glioependymal cells extend along the whole brain wall and most likely provide guidance for the migrating cell cords. The formation of the so-called migratory zone is a consequence of the growth of the basal and the horizontal prolongations of emigrating cells. The significance of the cell to cell contacts for the neuronal migration processes is discussed.