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Oswald Steward

Publications and source records attributed to Oswald Steward.

At least 37 records · Page 2Linked to original sources
PubMed2004

The dorsolateral corticospinal tract in mice: an alternative route for corticospinal input to caudal segments following dorsal column lesions.

In rodents, the main contingent of corticospinal tract (CST) axons descends in the ventral part of the dorsal column. There is, however, a contingent of CST axons that descends in the dorsolateral column (the "dorsolateral corticospinal tract," or DLCST). Here, we define some of the features of the DLCST by tracing CST projections following injections of biotinylated dextran amine into the sensorimotor cortex, assessing the distribution of DLCST axons and terminal arborizations in intact mice and in mice in which the main contingent of CST axons in the dorsal column had been transected. Axons of the DLCST diverge from the main tract at the pyramidal decussation, gather in fascicles in the dorsolateral gray matter below the spinomedullary junction, and project in a gradual trajectory laterally toward the dorsolateral column over the first few cervical segments. DLCST axons then project along the dorsolateral column to sacral levels, giving rise to collaterals that project into the gray matter. Labeled DLCST axons were most abundant in cervical segments, where they were often collected in fascicles, and progressively decreased in number in more caudal segments. Tracing of DLCST axons in mice with selective lesions of the dorsal column revealed that DLCST axons arborize extensively throughout the dorsal and ventral horns and that the overall territory that the DLCST axons invade is similar to the territory innervated by the CST axons in the main tract. Some DLCST axon arbors with varicosities are seen near large neurons in the ventral horn (presumed motoneurons). Substantial numbers of DLCST axons project across the midline to the gray matter on the contralateral side. Thus, the DLCST provides an alternate route for CST input to caudal segments, which is of particular relevance for studies of CST distribution and function following partial spinal cord injuries.

Animals↗
PubMed2004

Chronic nerve compression induces local demyelination and remyelination in a rat model of carpal tunnel syndrome.

In a previous study, we demonstrated that chronic compression of rat sciatic nerve, a model of compressive neuropathies, triggered dramatic Schwann cell proliferation and concurrent apoptosis. Importantly, this Schwann cell response occurred before there are signs of overt axonal pathology, raising the question of whether there are alterations in axonal myelination in the areas of the nerve in which Schwann cell apoptosis and proliferation occur. Here, we use nerve teasing techniques and unbiased stereology to assess myelination in nerves after 1 and 8 months of compression. Evaluations of myelin thickness and axonal diameter (AD) using design-based, unbiased stereology revealed alterations in myelin structure that indicate remyelination, specifically a dramatic decrease in the average internodal length (IL) and an increase in the proportion of axons with thin myelin sheaths. The mean IL was reduced after 1 month of chronic nerve injury with no further decrease in IL at 8 months. There was limited change in average axonal diameter at both 1 and 8 months. Measures of myelin thickness revealed not only a greater than 6-fold increase in the number of axons with very thin (<5 microm thickness) myelin sheaths, but also a proportional decrease in the number of axons with the thick myelin sheaths characteristic of normal nerve. These results confirm that an early consequence of chronic nerve compression (CNC) is local demyelination and remyelination, which may be the primary cause of alterations in nerve function during the early period post-compression.

Animals↗
PubMed2004

Quantitative assessment of deficits and recovery of forelimb motor function after cervical spinal cord injury in mice.

A large proportion of spinal cord injuries (SCIs) in humans are at the cervical (C) level, but there are few tests to quantitatively assess forelimb motor function after cervical spinal cord injury in rodents. Here, we describe a simple and reliable technique for assessing forelimb grip strength over time. Female C57Bl/6 mice were trained on the Grip Strength Meter (GSM, TSE-Systems), then received a lateral hemisection of the spinal cord at level C5, C6, C7, or T1. Gripping ability by each forepaw was then tested for 4 weeks postinjury. Before injury, there was no significant difference in the force exerted by either forepaw. After hemisections at C5, C6, or C7, the forepaw ipsilateral to the injury was initially completely unable to grip (day 2 postinjury), and there was a slight transient decrease in the strength of the contralateral paw compared to presurgical levels. The ipsilateral forepaw exhibited no ability to grip until about 10-14 days postlesion, at which time grip reappeared and strength then recovered over a period of a few days to a level that was about 50% of preinjury levels. Grip strength was minimally and transiently affected by hemisection at T1. The grip strength analysis provides a convenient, quantitative measure of the loss and recovery of forelimb function after cervical injury.

Animals↗
PubMed2004

Assessment of factors regulating axon growth between the cortex and spinal cord in organotypic co-cultures: effects of age and neurotrophic factors.

Axon growth failure in the central nervous system (CNS) of adult animals is thought to be attributable to several factors, including an inadequate intrinsic growth response, the presence of inhibitory molecules, and a lack of adequate neurotrophic support. Here we use a new in vitro assay system to quantitatively assess growth of axons in cortex/spinal cord organotypic co-cultures from neonatal rats. Co-cultures of cortex and spinal cord were prepared from neonatal rats at P3 or P7, and by pairing cortex and spinal cords from different ages. Axon growth from the cortex to the spinal cord was assessed using DiI tract tracing techniques. Axons could be traced from the cortex to the spinal cord in co-cultures in which both tissues were obtained from P3 animals, whereas few axons crossed the cortex/spinal cord boundary in co-cultures from P7 animals. A larger number of axons could be traced across the boundary in co-cultures from P3 animals that were treated with neurotrophins (NGF, BDNF, or NT3), whereas neurotrophins produced minimal growth enhancement in P7 co-cultures. In mixed age co-cultures of P7 cortex with P3 spinal cord, moderate numbers of axons extended between the cortex and spinal cord when cultures were treated with neurotrophins, but few if any crossing axons were detected in co-cultures of P3 cortex with P7 spinal cords. These results indicate that successful growth of axons from the cortex to the spinal cord depends on the developmental age of the tissue terrain (the spinal cord and/or the interface between cortex and spinal cord explants), and to a lesser extent on the developmental state of the cortical neurons, and that axon growth between cortex and spinal cord can be enhanced by exogenous neurotrophins. These co-cultures provide a potentially useful assay for factors that affect axon growth that is intermediate between assays based on dissociated neurons and the intact tissue terrain.

Age Factors↗
PubMed2003

Compartmentalized synthesis and degradation of proteins in neurons.

An important aspect of gene expression in neurons involves the delivery of mRNAs to particular subcellular domains, where translation of the mRNAs is locally controlled. Local synthesis of protein within dendrites plays a key role in activity-dependent synaptic modifications. In growing axons, local synthesis in the growth cone is important for extension and guidance. Recent evidence also documents the existence of mechanisms permitting local protein degradation, providing bidirectional control of protein composition in local domains. Here, we summarize what is known about local synthesis and degradation of protein in dendrites and axons, highlighting key unresolved questions.

Animals↗
PubMed2003

Ascending sensory, but not other long-tract axons, regenerate into the connective tissue matrix that forms at the site of a spinal cord injury in mice.

Mice exhibit a unique wound healing response following spinal cord injury in which the lesion site fills in with a connective tissue matrix. Previous studies have revealed that axons grow into this matrix, but the source of the axons remained unknown. The present study assesses whether any of these axons were the result of long tract regeneration. C57Bl/6 mice received crush injuries and were allowed to survive for 6 weeks to 7 months. Biotinylated dextran amine (BDA) was injected into the somato-motor cortex to trace descending corticospinal tract (CST) axons, into the midbrain to label descending brainstem pathways including the rubrospinal and reticulospinal tracts, or into the L5 dorsal root ganglion to trace ascending projections of first-order sensory neurons. Spinal cords from other mice were prepared for immunocytochemistry using antibodies against neurofilament protein (NF), 5-HT to reveal descending serotonergic axons, calcitonin gene-related protein (CGRP) to reveal ascending sensory axons, and chondroitin sulfate proteoglycan (CSPG) to assess the distribution of molecules that are inhibitory to axon growth. NF immunostaining revealed axons in the connective tissue matrix at the lesion site, confirming previous studies that used protargol staining. CST axons did not enter the connective tissue matrix, but did sprout extensively in segments adjacent to the injury site. Rubrospinal and reticulospinal tract axons also did not grow into the lesion site. 5-HT-positive axons extended to the edge of the lesion, and a few axons followed astrocyte processes into the margins of the lesion site. In contrast to the other pathways, BDA-labeled ascending sensory axons did extend into and arborized extensively within the connective tissue matrix, although the subgroup of ascending axons that are positive for CGRP did not. These results indicate that the connective tissue matrix is permissive for regeneration of some classes of ascending sensory axons but not for other axonal systems.

Animals↗
PubMed2003

Chronic nerve compression induces concurrent apoptosis and proliferation of Schwann cells.

Chronic nerve compression (CNC), as in carpal tunnel syndrome, is a common cause of peripheral nerve dysfunction in humans. Previous studies using animal models have demonstrated progressive demyelination and a slowing of nerve conduction velocity. To characterize the Schwann cell response to CNC, we evaluated total Schwann cell number, apoptosis, and proliferation in an animal model of CNC. Design-based stereologic techniques revealed a striking transient increase in Schwann cell number following CNC. Schwann cell number increased sixfold relative to the normal nerve at the site of compression at 1 month and then slowly declined toward control levels. Nevertheless, assays of apoptosis (TUNEL and an antipoly-ADP-ribose polymerase labeling assays) revealed extensive Schwann cell apoptosis at 2 weeks postcompression, which is during the time when Schwann cell number was increasing. Electron microscopic analysis confirmed that these dramatic changes in Schwann cells occurred in the absence of axon degeneration and axonal swelling and before there were any detectable alterations in nerve conduction velocity. Counts of bromodeoxyuridine-labeled Schwann cells revealed that proliferation occurred concurrently with ongoing apoptosis. To define further the possible mitogenic properties of mechanical stimuli on Schwann cells, we used an in-vitro model to deliver shear stress in the form of laminar fluid flow to pure populations of Schwann cells and confirmed that mechanical stimuli induce Schwann cell proliferation. Our findings indicate that chronic nerve compression induces Schwann cell turnover with minimal axonal injury and support the idea that mechanical stimuli have a direct mitogenic effect on Schwann cells.

Animals↗
PubMed2003

Lack of enhanced spinal regeneration in Nogo-deficient mice.

The failure of regeneration of severed axons in the adult mammalian central nervous system is thought to be due partly to the presence of endogenous inhibitors of axon regeneration. The nogo gene encodes three proteins (Nogo-A, -B, and -C) that have been proposed to contribute to this inhibition. To determine whether deletion of nogo enhances regenerative ability, we generated two lines of mutant mice, one lacking Nogo-A and -B but not -C (Nogo-A/B mutant), and one deficient in all three isoforms (Nogo-A/B/C mutant). Although Nogo-A/B-deficient myelin has reduced inhibitory activity in a neurite outgrowth assay in vitro, tracing of corticospinal tract fibers after dorsal hemisection of the spinal cord did not reveal an obvious increase in regeneration or sprouting of these fibers in either mouse line, suggesting that elimination of Nogo alone is not sufficient to induce extensive axon regeneration.

Animals↗
PubMed2003

False resurrections: distinguishing regenerated from spared axons in the injured central nervous system.

Several recent studies report that axon regeneration can be induced in the mature mammalian nervous system by novel treatments or genetic manipulations. In assessing these reports, it is important to be mindful of the history of regeneration research, which is littered with the corpses of studies that reported regeneration that later proved incorrect. One important reason is the "spared axon conundrum," in which axons that survive a lesion are mistakenly identified as having regenerated. Here, we illustrate the problem and propose criteria that may be used to identify regenerated vs. spared axons, focusing on the injured spinal cord.

Animals↗
PubMed2003

Visualizing changes in circuit activity resulting from denervation and reinnervation using immediate early gene expression.

We describe a novel strategy to evaluate circuit function after brain injury that takes advantage of experience-dependent immediate early gene (IEG) expression. When normal rats undergo training or are exposed to a novel environment, there is a strong induction of IEG expression in forebrain regions, including the hippocampus. This gene induction identifies the neurons that are engaged during the experience. Here, we demonstrate that experience-dependent IEG induction is diminished after brain injury in young adult rats (120-200 gm), specifically after unilateral lesions of the entorhinal cortex (EC), and then recovers with a time course consistent with reinnervation. In situ hybridization techniques were used to assess the expression of the activity-regulated cytoskeleton-associated protein Arc at various times after the lesion (4, 8, 12, 16, or 30 d). One group of rats was allowed to explore a complex novel environment for 1 hr; control operated animals remained in their home cage. In unoperated animals, exposure to the novel environment induced Arc mRNA levels in most pyramidal neurons in CA1, in many pyramidal neurons in CA3, and in a small number of dentate granule cells. This characteristic pattern of induction was absent at early time points after unilateral EC lesions (4 and 8 d) but recovered progressively at later time points. The recovery of Arc expression occurred with approximately the same time course as the reinnervation of the dentate gyrus as a result of postlesion sprouting. These results document a novel approach for quantitatively assessing activity-regulated gene expression in polysynaptic circuits after trauma.

Afferent Pathways↗
PubMed2003

Mitochondrial uncoupling protein-2 protects the immature brain from excitotoxic neuronal death.

Excitotoxic cell death is the fundamental process responsible for many human neurodegenerative disorders, yet the basic mechanisms involved are not fully understood. Here, we exploited the fact that the immature brain is remarkably resistant to seizure-induced excitotoxic cell death and examined the underlying protective mechanisms. We found that, unlike in the adult, seizures do not increase the formation of reactive oxygen species or result in mitochondrial dysfunction in neonatal brain, because of high levels of the mitochondrial uncoupling protein (UCP2). UCP2 expression and function were basally increased in neonatal brain by the fat-rich diet of maternal milk, and substituting a low-fat diet reduced UCP2, restored mitochondrial coupling, and permitted seizure-induced neuronal injury. Thus, modulation of UCP2 expression and function by dietary fat protects neonatal neurons from excitotoxicity by preventing mitochondrial dysfunction. This mechanism offers novel neuroprotective strategies for individuals, greater than 1% of the world's population, who are affected by seizures.

Age Factors↗
PubMed2003

Physical size does not determine the unique histopathological response seen in the injured mouse spinal cord.

Mice display a wound healing process after spinal cord injury that has not been seen in any other species. Rather than exhibiting progressive necrosis and cavitation at the injury site, the mouse lesion site fills in with connective tissue. The connective tissue matrix then undergoes a remodeling in which it contracts, drawing the two ends of the injured spinal cord closer together. One possible explanation for this unique wound healing response is that the spinal cord is much smaller in mice than in rats or other species that have been analyzed. To evaluate this possibility, we compared the histopathological response to spinal cord injury in mice, hamsters (in which the spinal cord is similar in size to that in mice), and rats. Crush injuries were produced at T9, and lesion area and cavitation were assessed using quantitative morphometry at 1, 3, and 8 weeks post-injury. Both hamsters and rats exhibited large lesions and areas of cavitation that increased from 1 to 3 weeks and then stabilized. In mice, the lesion site contained compact meshworks of cells and connective tissue that decreased in size over time. In rats, the cavities contained substantial degeneration debris, whereas in hamsters, cavities were fluid-filled cysts with minimal debris. Hamsters had the largest lesions relative to the cross-sectional area of the spinal cord, despite having a spinal cord similar in size to the mouse. These results indicate that the physical size of the spinal cord does not determine the unique histopathological responses after spinal cord injury in mice.

Animals↗
PubMed2002

mRNA at synapses, synaptic plasticity, and memory consolidation.

Miller et al. (this issue of Neuron) report that deletion of the 3'UTR of alpha-CaMKII mRNA prevents dendritic delivery of the mRNA in transgenic mice and thus local synthesis of alpha-CaMKII protein in dendrites. 3'UTR mutant mice exhibit decreases in alpha-CaMKII protein in postsynaptic densities, and deficits in late phase LTP and in memory consolidation.

3' Untranslated Regions↗
PubMed2002

Genetic influences on secondary degeneration and wound healing following spinal cord injury in various strains of mice.

Various inbred strains of mice exhibit dramatic differences in sensitivity to excitotoxic cell death induced by systemic injections of kainic acid (KA). The present study evaluates whether the same strains are also differentially sensitive to secondary degeneration after spinal cord injury, in which excitotoxic cell death is thought to play a pathogenic role. Spinal cord crush injuries were produced at T9 in two inbred strains that are resistant to KA-induced excitotoxic cell death (C57Bl/6 and Balb/c) and four strains that are sensitive (CD-1, FVB/N, 129T2 Sv/EMS, and C57Bl/10). The spinal cord was prepared for light microscopy at intervals from 1 to 56 days postinjury, and the area of damaged tissue (termed lesion size) and amount of cavitation were determined by quantitative image analysis. Lesion size increased between 1 and 7 days in all strains and then decreased steadily in a wound-healing process that occurs uniquely in mice. The extent of cavitation also gradually decreased from 7 to 56 days in all strains. Although lesion area and cavitation decreased in all strains, there were significant differences in lesion size and cavitation across strains. Specifically, lesion areas in the KA-sensitive strains FVB/N, 129T2 Sv/EMS, and CD-1 were significantly larger at 56 days postinjury than in the KA-resistant strains C57Bl/6 and Balb/c. We conclude that the genetic differences that confer resistance and sensitivity to KA-induced neurotoxicity also modify the secondary degenerative processes that occur after spinal cord injury, so that resistance to excitotoxic injury leads to smaller overall lesions and a more effective wound-healing response.

Animals↗
PubMed2002

Translating axon guidance cues.

Recent studies suggest that local translation of mRNA within axons plays a key role in two aspects of growth cone navigation. One line of evidence implies a local synthesis of proteins that mediate growth cone collapse and turning in response to guidance cues; another indicates that growth cones possess the machinery that would allow the local synthesis of receptors for axon guidance cues.

Animals↗
PubMed2002

Spatial learning and memory is preserved in rats after early development in a microgravity environment.

This study evaluated the cognitive mapping abilities of rats that spent part of their early development in a microgravity environment. Litters of male and female Sprague-Dawley rat pups were launched into space aboard the National Aeronautics and Space Administration space shuttle Columbia on postnatal day 8 or 14 and remained in space for 16 days. These animals were designated as FLT groups. Two age-matched control groups remained on Earth: those in standard vivarium housing (VIV) and those in housing identical to that aboard the shuttle (AGC). On return to Earth, animals were tested in three different tasks that measure spatial learning ability, the Morris water maze (MWM), and a modified version of the radial arm maze (RAM). Animals were also tested in an open field apparatus to measure general activity and exploratory activity. Performance and search strategies were evaluated in each of these tasks using an automated tracking system. Despite the dramatic differences in early experience, there were remarkably few differences between the FLT groups and their Earth-bound controls in these tasks. FLT animals learned the MWM and RAM as quickly as did controls. Evaluation of search patterns suggested subtle differences in patterns of exploration and in the strategies used to solve the tasks during the first few days of testing, but these differences normalized rapidly. Together, these data suggest that development in an environment without gravity has minimal long-term impact on spatial learning and memory abilities. Any differences due to development in microgravity are quickly reversed after return to earth normal gravity.

Animals↗
PubMed2002

Local synthesis of proteins at synaptic sites on dendrites: role in synaptic plasticity and memory consolidation?

Long-lasting forms of memory are thought to be mediated by modifications in synapses that are induced by particular patterns of activity, take time to be established (consolidated), and involve new gene expression. The molecular mechanisms underlying such long-lasting synaptic modifications remain to be defined. Here, we focus on new findings regarding synapse-specific gene expression and especially the intracellular transport and synaptic targeting of the mRNA for a recently identified immediate early gene called activity-regulated cytoskeleton-associated protein (Arc). Arc mRNA is transported into dendrites after episodes of neuronal activation. The newly synthesized Arc mRNA localizes selectively at synapses that have experienced particular patterns of activity, and this targeting depends on NMDA receptor activation. Arc protein is assembled into the postsynaptic junction, and Arc expression is strongly induced by brief behavioral experiences in select populations of neurons. These features of Arc induction and trafficking reveal novel cellular mechanisms that are well suited to mediate long-term synapse-specific modifications. We will review findings from other laboratories that both LTP and memory are disrupted when Arc induction is blocked, and we will discuss the possibility that Arc protein may play a key role in activity-dependent synaptic modification.

Cytoskeletal Proteins↗