Search PubMed⌕ Search

Biomedical subjects

W D Snider

Publications and source records attributed to W D Snider.

At least 55 records · Page 3Linked to original sources
PubMed1995

Parvalbumin is a marker of ALS-resistant motor neurons.

The selective vulnerability of limb and bulbar motor neurons is a hallmark of degenerative human motor neuron diseases such as amyotrophic lateral sclerosis (ALS). Currently, there are no known molecular characteristics to distinguish between motor neuron pools which are highly susceptible to degeneration in ALS and those populations which are resistant. Using in situ hybridization on adult rat tissue, we demonstrated that ALS-resistant motor pools robustly express mRNA for the calcium binding protein parvalbumin, while no measurable parvalbumin expression is found in ALS-sensitive motor neuron populations. In contrast, mRNA expression for each of several other calcium binding proteins such as calbindin-D28K, calretinin and calmodulin appears similar in the various motor pools. Thus, parvalbumin represents a biochemical marker of ALS-resistant motor neurons, and may provide insight into the mechanisms of resistance of certain motor neurons to disease.

Amyotrophic Lateral Sclerosis↗
PubMed1995

Neurotrophin receptor mRNA expression defines distinct populations of neurons in rat dorsal root ganglia.

The biological actions of neurotrophins are mediated by specific neurotrophin receptor tyrosine kinases (Trks). A low-affinity nerve growth factor (NGF) receptor, p75, appears to modulate sensitivity to neurotrophins in some neuronal populations. It has been recently demonstrated that genes encoding members of the Trk family are expressed in distinct patterns in the dorsal root ganglia (DRG; Mu et al. [1993] (J. Neurosci. 13:4029- 4041). However, the extent to which different neurotrophin receptor genes are coexpressed by individual DRG neurons is unknown. The question of coexpression is important since the expression of more than one member of the trk family by DRG neurons would suggest the potential for regulation by multiple neurotrophins. To address this question, a combination of isotopic and colorimetric in situ hybridization was performed on rat thoracic DRG using riboprobes specific for trkA, trkB, trkC, and p75. We show here that neurons that express trkA are largely distinct from those that express trkC, although there is a small subpopulation that expresses both of these genes. We also show that there is a distinct population of DRG neurons that expresses trkB and does not coexpress either trkA or trkC. P75 is expressed in almost all neurons that express trkA or trkB, but is coexpressed in only 50% of trkC-expressing neurons. Importantly, p75 is not expressed in DRG neurons independent of trk expression. Finally, a subpopulation of DRG neurons does not express any of the neurotrophin receptor mRNAs. Our results demonstrate that there are distinct populations of DRG neurons that express each member of the neurotrophin receptor tyrosine kinase family. Our findings of extensive colocalization of p75 with trkA and trkB lend support to the idea that p75 is important in mediating the actions of NGF and brain-derived neurotrophic factor on DRG neurons. Interestingly, however, p75 expression is clearly unimportant for a subpopulation of neurons that require neurotrophin-3. The fact that p75 is not expressed in the absence of trkA, trkB, or trkC suggests that the function of p75 is closely related to functions of the known neurotrophin-receptor tyrosine kinases. Finally, our results suggest that a significant percentage of DRG neurons may be regulated by non-neurotrophin neuronal growth factors.

Animals↗
PubMed1995

Pathologic alterations in pre- and postsynaptic elements in aged mouse sympathetic ganglia.

Dysfunction of the sympathetic autonomic nervous system is an increasingly recognized, although poorly understood, complication of increasing age in experimental animals and man. In this study of young adult (4-6 months old) and aged (12-24 months old) mice we have examined the ultrastructural appearance of perikarya, dendritic processes, preterminal axons, and synapses in selected sympathetic ganglia as well as the three-dimensional structure of the dendritic arborizations of principal sympathetic neurons using intracellular injections of Lucifer Yellow. Ultrastructural examination demonstrated numerous markedly enlarged presynaptic terminal axons and synapses which distorted the contours of perikarya and dendrites of neurons within the prevertebral celiac/superior mesenteric and paravertebral superior cervical and stellate sympathetic ganglia of aged mice. Dilated preterminal axons had the distinctive ultrastructural appearance of neuroaxonal dystrophy, a pathologic process described in a wide variety of clinical and experimental entities. Dystrophic axons were identical in ultrastructural appearance in young and old animals, differing only in frequency. A distinctive type of ultrastructural alteration, characterized by markedly distended neurites containing numerous vacuoles, was confined to the superior cervical ganglia and also increased in frequency with aging. Although many intraganglionic vacuolated processes disappeared with surgical interruption of the cervical sympathetic trunk, which contains the preganglionic axons innervating the superior cervical ganglia, others persisted. In addition, the presence in some processes of admixed ribosomes, lipofuscin, or continuity with the cell body indicated that numerous neuritic alterations within aged sympathetic ganglia were likely of dendritic origin. Intracellular injections of Lucifer Yellow into principal sympathetic neurons demonstrated that the dendritic arborizations of the celiac/superior mesenteric ganglia neurons of young adult mice were significantly more complex and extensive than those of the superior cervical ganglia. Sympathetic neurons of aged superior cervical ganglia, but not superior mesenteric ganglia, appeared significantly smaller with regard to total dendritic length, extent, and branching when compared to those of young animals. In the aged superior cervical ganglia, short, stunted dendritic processes also exhibited large, focal, often multiple, swellings, a phenomenon infrequently observed in the superior cervical ganglia of young animals. The celiac/superior mesenteric ganglia of aged or young adult mouse failed to exhibit comparable dendritic swellings.

Aging↗
PubMed1995

Molecular genetics of neuronal survival.

The past year has witnessed remarkable progress towards understanding the molecular genetics of neuronal survival. Gene-targeting experiments in mice have confirmed the long-standing idea that the nerve growth factor model of neuronal survival--that is, neuronal dependence on target-derived molecules during a critical period in development--is broadly applicable. Furthermore, a variety of biochemical and genetic techniques applied to both mammals and invertebrates have identified new genes involved in regulating cell survival during development.

Animals↗
PubMed1995

Expression of the neurofibromatosis 2 (NF2) gene isoforms during rat embryonic development.

The neurofibromatosis 2 (NF2) gene product, merlin, encodes a 595 amino acid protein with sequence similarity to a family of proteins linking cell membrane proteins to the cytoskeleton. Two isoforms of merlin have been described which differ by the presence (type 2 merlin) or absence (type 1 merlin) of exon 16 sequences inserted into the extreme carboxyl terminus of the protein. To determine the role of this important negative growth regulator during normal embryonic development, the expression of these two merlin isoforms was examined at representative stages of rat embryogenesis and in adult tissues. Partial sequence analysis of the rat merlin gene demonstrated striking amino acid identity to the published mouse and human merlin gene sequences. In situ hybridization and RT-PCR analyses demonstrated that rat merlin is widely expressed during embryogenesis and early postnatal life in most tissues but becomes restricted to the brainstem, cerebellum, dorsal root ganglia, spinal cord, adrenal gland and testis in adult animals. The elucidation of the pattern of merlin gene expression in adult and embryonic tissues provides the foundations for future studies aimed at determining the function(s) of this protein during cell differentiation and embryonic development.

Adrenal Glands↗
PubMed1995

Non-TrkA-expressing small DRG neurons are lost in TrkA deficient mice.

Experiments over the past decade in which NGF/TrkA signaling has been abolished by antibodies or targeted gene mutations have shown that 70-85% of dorsal root ganglion (DRG) neurons require NGF for survival during development. There is consensus that many of the NGF-dependent neurons are small-diameter, peptidergic neurons subserving nociception. These neurons express the signaling receptor for NGF, TrkA. There is a major discrepancy, however, between the percentage of DRG neurons which require NGF for survival (70-85%) and percentage of DRG neurons expressing TrkA receptors (40-50%). The identity of these non-TrkA expressing, NGF-dependent neurons has not been established. A candidate group is a population of small DRG neurons with unmyelinated axons which bind BSI isolectins from the plant, Bandeiraea simplicifolia. We show here that most of these BSI-binding DRG neurons do not express TrkA in adult mice. However, in mutant mice in which NGF/TrkA signaling has been abolished by inactivation of the trkA gene, BSI-staining in the DRG and dorsal horn is completely eliminated. BSI-binding DRG cells are thus the first identified neuronal population in which cells do not express TrkA in maturity, but require NGF/TrkA signaling for survival during embryonic development. These neurons must either depend on NGF via a novel, indirect mechanism or alternatively, downregulate TrkA expression during development.

Animals↗
PubMed1995

Expression of the neurofibromatosis 1 (NF1) isoforms in developing and adult rat tissues.

The neurofibromatosis 1 (NF1) gene encodes a large M(r) approximately 250,000 phosphoprotein, the expression of which in adult tissues is limited to neurons, Schwann cells, oligodendrocytes, adrenal medulla, and leukocytes. The presence of two alternatively spliced exons (23a and 48a) in the NF1 gene allow for the generation of four possible neurofibromin isoforms. Type 1 neurofibromin contains neither 23a or 48a exon sequences, while type 2 neurofibromin contains only the 23a exon insertion. Previous studies have demonstrated that types 1 and 2 neurofibromin might have different functional properties relative to microtubule association and GTPase-activating protein activity towards p21-ras. To determine the normal pattern of expression of these NF1 isoforms, the adult and developmental expression of types 1 and 2 NF1 was examined. Herein, we demonstrate that NF1 mRNA is expressed at varying levels in adult tissues and is developmentally regulated during embryogenesis. Neurons in the central nervous system express predominantly type 1 NF1. Using mouse neocortical cultures enriched for neurons or glial cells, type 1 NF1 predominance was demonstrated in neurons, while type 2 NF1 predominated in glial cells. In contrast to central nervous system neurons, neurons expressing the type 2 NF1 isoform were identified in the developing dorsal root ganglia and spinal cord by in situ hybridization using a type 2-specific oligonucleotide probe. The elucidation of the differential expression pattern of these two NF1 isoforms during development and in adult life provides the foundations for future studies aimed at determining the functions of these neurofibromin isoforms.

Animals↗
PubMed1994

Development of interneurons with ipsilateral projections in embryonic rat spinal cord.

Considerable progress has been made in recent years in identifying molecules with restricted expression in mammalian spinal cord at early developmental stages. However, the significance of the different expression patterns for most of these molecules is unclear because so little is known about the development of various classes of spinal interneurons. Recently, we have characterized the development of rat spinal cord interneurons with an axon that crosses in the ventral commissure (Silos-Santiago and Snider, J. Comp. Neurol., 325:514, 1992). In the current study, we describe the morphological development of ipsilaterally projecting spinal interneurons in laminae V-VIII of the thoracic spinal cord. These neurons were labelled by retrograde lateral diffusion of DiI after crystals were placed in various locations in the embryonic thoracic cord. By E14, approximately 48 hours after the first interneurons are generated, eight different groups of ipsilateral interneurons are present in the spinal cord. By E15, these groups of ipsilateral interneurons have reached distinct locations within the gray matter. Even at this early stage, different groups of cells have elaborated characteristic dendritic arborizations. By E19, at least 17 different types of ipsilateral interneurons can be identified on the basis of location and dendritic morphology. In general, ipsilateral interneurons are located more dorsally and laterally than commissural interneurons at all stages of embryonic development. Furthermore, in comparison with commissural neurons, fewer ipsilateral interneurons have dendritic arbors with a mediolateral orientation in the transverse plane. This work demonstrates that rat embryonic spinal cord contains a large number of morphologically distinct classes of interneurons that extend axons into the ipsilateral lateral funiculus. These neurons can be distinguished from commissural neurons on the basis of location and morphology. These results, taken together with those from our previous study, provide a framework for the localization of gene expression to different classes of spinal interneurons at early developmental stages.

Animals↗
PubMed1994

Disruption of the neurotrophin-3 receptor gene trkC eliminates la muscle afferents and results in abnormal movements.

The trkC gene is expressed throughout the mammalian nervous system and encodes a series of tyrosine protein kinase isoforms that serve as receptors for neurotrophin-3 (NT3), a member of the nerve growth factor (NGF) family of neurotrophic factors. One of these isoforms, gp145trkC/TrkC K1, mediates the trophic properties of NT3 in cultured cells. Here we show that homozygous mice defective for TrkC tyrosine protein kinase receptors lack Ia muscle afferent projections to spinal motor neurons and have fewer large myelinated axons in the dorsal root and posterior columns of the spinal cord. These mice display abnormal movements and postures, indicating that NT3/TrkC-dependent sensor; neurons may play a primary role in proprioception, the sense of position and movement of the limbs.

Amino Acid Sequence↗
PubMed1994

Upregulation of NMDARI mRNA induced by MK-801 is associated with massive death of axotomized motor neurones in adult rats.

Studies on the pathogenesis of human motor neurone disease have suffered from the absence of models of motor neurone degeneration in adult animals. Normally in adult rodents, transection of motor neurone axons results in only a modest degree of neuronal death. We reasoned that axotomy-induced motor neurone death might be enhanced by modulating glutamatergic transmission. By axotomizing the facial nerve in adult rats and then administering MK-801 for the first week of a 4-week or 8-week post-lesion survival period, we induced a 67% motor neurone loss by 8 weeks as compared with a 19% loss in controls. A possible explanation for the increased motor neurone loss after MK-801 treatment is that transient blockade of NMDA receptors may upregulate synthesis of NMDA receptor components. In order to test this idea, we employed quantitative in situ hybridization to determine the response of NMDAR1 mRNA to axotomy and axotomy + MK-801 treatment. Quantification of the percentage of area occupied by NMDAR1 silver grains per motor neurone somata indicated that axotomy alone did not provoke a change in NMDAR1 mRNA. However, axotomy and MK-801 combined treatment resulted in a highly significant upregulation of NMDAR1 mRNA when compared with controls or animals treated solely with axotomy. Our results suggest that motor neurone death in adult animals can be enhanced after axotomy in association with the upregulation of NMDA receptor mRNA. Thus, abnormalities in glutamate receptor signalling may lead to subacute motor neurone death in vivo. Furthermore these results indicate that transient treatment with MK-801 is a convenient method for enhancing the degree of motor neurone death after axotomy in adult animals.

Animals↗
PubMed1994

NGF and NT-3 have differing effects on the growth of dorsal root axons in developing mammalian spinal cord.

The functions of neurotrophins in relation to axon growth and branching during development of the nervous system are unknown. In order to address this question, we have investigated the influences of systemically administered mouse nerve growth factor (mNGF) and human recombinant neurotrophin-3 (hrNT-3) on dorsal root axon growth in the spinal cord of embryonic rats. As anticipated, mNGF has a marked influence on growth of dorsal root axons. In mNGF-treated animals, dorsal root axons in the developing dorsal funiculi and axon collaterals in developing gray matter are substantially longer than those of age-matched controls. Furthermore, growth cones of some dorsal root axons have more than twice the surface area of controls. These effects of NGF are highly selective. Dorsal root axons that occupy a lateral position in white matter and that normally give off collaterals to superficial dorsal horn are prominently affected. Axons that run medially in dorsal columns and that give off collaterals to laminae III and IV and the ventral horn are not demonstrably influenced by treatment with exogenous mNGF. In contrast to the striking effects of mNGF on dorsal root axon growth, the influences of hrNT-3 were considerably more complex. Administration of hrNT-3 increased the mean soma area of DRG neurons, particularly those at the larger end of the size spectrum, consistent with its hypothesized role as a growth factor for proprioceptive sensory neurons. However, in striking contrast to the actions of mNGF, hrNT-3 consistently inhibited axon collateral growth in gray matter at early developmental stages. At later stages, we could not discern a clear-cut influence of hrNT-3 on dorsal root axon growth and branching. We conclude that the ability of mNGF to stimulate axon growth in both white and gray matter is consistent with the idea that mNGF regulates the developing axonal projections of DRG neurons in vivo. In contrast, systemically administered hrNT-3 inhibits the axon collateralizations of DRG neurons in gray matter at early developmental stages. We hypothesize that this inhibitory effect may be related to disruption of a chemotropic gradient of NT-3, or to the widespread expression of the NT-3 receptor trkC, on non-neuronal cells.

Animals↗
PubMed1993

Localization of the mRNA for a chicken prion protein by in situ hybridization.

The infectious agent (prion) responsible for transmissible spongiform encephalopathies in humans and animals is composed primarily of a 33- to 35-kDa glycoprotein called PrPSc (scrapie isoform of prion protein), which is a posttranslationally modified form of the normal cell-surface protein PrPC. Little is known about the function of PrPC. Interestingly, chPrP, the chicken homologue of PrPC, copurifies with a factor from brain that stimulates synthesis of acetylcholine receptors on skeletal muscle cells. Using in situ hybridization, we report here that chPrP mRNA is widely distributed in cholinergic and noncholinergic neurons throughout the adult central nervous system, including those in the telencephalic striata, thalamus and hypothalamus, optic tectum, medulla, cerebellum, and spinal cord. The mRNA is present in the brain and spinal cord as early as embryonic day 6 and is also found in dorsal root ganglia, retina, intestine, and heart. Our data suggest that if chPrP serves to regulate acetylcholine receptor number on postsynaptic targets, this is not its only function. It is likely that the protein plays a more widespread role in the central nervous system and perhaps elsewhere, possibly one related to intercellular communication, adhesion, or recognition. The chicken embryo represents an attractive experimental system in which to investigate the normal developmental function of PrPC.

Animals↗
PubMed1993

Developmental expression of the platelet-derived growth factor alpha-receptor gene in mammalian central nervous system.

We recently reported that the platelet-derived growth factor (PDGF) A-chain gene is highly expressed in neurons of embryonic and adult mouse central nervous system and suggested that its secretion by neurons may support development and maintenance of glia. We have now analyzed the levels and sites of expression of the cognate PDGF alpha-receptor gene in brain and spinal cord of embryonic and adult mice by in situ hybridization. The predominant cell populations in both gray and white matter expressing transcripts of the PDGF alpha-receptor gene are glial cells or their precursors. Transcripts consistently were not detected in neurons. Expression of the PDGF alpha-receptor gene was first observed at embryonic day 15, increased through postnatal day 14, and fell to lower levels in adults. Expression of the alpha-receptor gene corresponds in temporal sequence to the developmental period of glial migration and proliferation and to the expression of PDGF A by neurons. The results indicate that glia but not neurons have the potential to respond to PDGF A and suggest that neurons influence glial cell development through paracrine regulation.

Age Factors↗
PubMed1993

Influences of neurotrophins on mammalian motoneurons in vivo.

Several recently reported investigations have shown that a member of the neurotrophin family of neuronal growth factors, brain-derived neurotrophic factor (BDNF), supports motoneurons in vitro and rescues motoneurons from naturally occurring and axotomy-induced cell death (Oppenheim et al., 1992b; Sendtner et al., 1992b; Yan et al., 1992; Koliatsos et al., 1993; Henderson et al., 1993). In the current study, we have explored the issue of whether BDNF and other neurotrophins act to regulate motoneuron survival during development and asked whether synthesis of motoneuron transmitter enzymes is also regulated. We first examined whether spinal motoneurons in newborn animals could retrogradely transport iodinated neurotrophins from their targets in a specific, receptor-mediated manner. We found that motoneurons readily transported NGF, BDNF, and neurotrophin-3 (NT-3). The retrograde transport of one factor could be completely or largely blocked by excess of unlabeled homologous factor, but only partially blocked by excess of unlabeled heterologous factors. Since previous studies have shown that these three neurotrophins bind to the low-affinity NGF receptor, p75NGFR, with similar affinity, our data suggest that the retrograde transport of neurotrophins by motoneurons may be mediated by additional components, such as the trk family of proto-oncogenes. Consistent with this hypothesis, we demonstrate here that motoneurons express mRNA for two members of the trk family, trkB and trkC. Furthermore, both trkB and trkC were expressed by E13, consistent with a role for BDNF and NT-3 in regulating important developmental events involving motoneurons such as naturally occurring cell death. In order to determine which members of the neurotrophin family influence motoneuron survival and to assess the generality of their effects, we evaluated the abilities of NGF, BDNF, and NT-3 to save both spinal and cranial motoneurons after neonatal axotomy. Locally applied BDNF saved 40-70% of motoneurons which would ordinarily die after axotomy in lumbar and cranial motor pools, depending on the treatment protocol employed. NT-3 also exhibited some ability to rescue motoneurons and saved 20-25% of motoneurons which would die in the absence of treatment. Finally, we asked whether neurotrophins could influence synthesis of transmitter enzymes by motoneurons as well as their survival after axotomy. Locally applied BDNF and NT-3 could partially prevent the decrease of protein contents in L4 and L5 ventral roots which normally follows sciatic nerve transection. However, treatment with these neurotrophins did not prevent the decrease in choline acetyltransferase (ChAT) activity in L4 and L5 ventral roots which results from this procedure.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Neurotrophin receptor genes are expressed in distinct patterns in developing dorsal root ganglia.

All members of the neurotrophin family of neuronal growth factors promote survival and neurite outgrowth of dorsal root ganglion (DRG) neurons in vitro. The trk family of protooncogenes encodes receptors that are now thought to mediate the biological effects of neurotrophins. In order to learn more about the dependence of DRG neurons on neurotrophins in vivo, we have studied mRNA expression of members of the trk family in developing DRGs in embryonic and postnatal rats. We show here that neurotrophin receptors are expressed in thoracic and lumbar DRGs by embryonic day 13 (E13), which is only 24-48 hr after neurogenesis begins in these ganglia. Distinct patterns of expression of trkA, trkB, and trkC are readily apparent by E15. At this age, 40% of thoracic DRG neurons express trkA. In contrast, trkB and trkC are expressed by only 6% and 8%, respectively, of thoracic DRG neurons. These percentages change little between E15 and postnatal day 1. Although absolute numbers of DRG neurons expressing neurotrophin receptors are greater in lumbar than in thoracic ganglia, the ratios of DRG neurons expressing different members of the trk family are similar in the two regions. The different trks are expressed by distinct populations of DRG neurons from E15 onward. trkA is expressed predominantly by small neurons with darkly staining cytoplasm. trkB and trkC are expressed by large, lightly staining neurons. Size-frequency histograms show that trkA is expressed by neurons of variable sizes, but particularly by neurons at the smallest end of the spectrum. In contrast, trkC is expressed predominantly by large DRG neurons, including those with the largest soma areas. trkB is expressed by DRG neurons of intermediate size. Our results show that a majority of DRG neurons express mRNA for at least one member of the trk protooncogene family. Furthermore, trk expression occurs in a time frame consistent with the idea that trks mediate responses of DRG neurons to neurotrophins that are synthesized in both the periphery and spinal cord at early developmental stages. Finally, different populations of DRG neurons express different trks. We hypothesize that DRG neurons subserving different functions express different trks, and that trk expression of a particular class of DRG neurons determines its neurotrophin dependence during development.

Aging↗
PubMed1992

Development of commissural neurons in the embryonic rat spinal cord.

Little is known about the development of the various populations of interneurons in the mammalian spinal cord. We have utilized the lipid-soluble tracer DiI in fixed tissue to study the migration and dendritic arborization of spinal neurons with axons in the ventral commissure in embryonic rats. Crystals of DiI were placed in various locations in the thoracic spinal cord in order to label commissural neurons within the dorsal horn, intermediate zone, and ventral horn at E13.5, E15, E17, and E19. Seven different groups of commissural interneurons are present in the spinal cord by E13.5. Migration is relatively simple with groups occupying a position along the dorsoventral axis roughly corresponding to their position of origin along the neuroepithelium. By E15, commissural cells are near their final locations and exhibit characteristic morphology. One striking feature is the tendency of cells with similar morphology to cluster in distinct groups. By E19, at least 18 different types of commissural interneurons can be identified on morphological grounds. Although the situation is complex, some generalities about dendritic morphology are apparent. Commissural neurons located in the dorsal horn are small and have highly branched dendrites oriented along the dorsoventral axis. In more ventral regions, commissural neurons are larger and possess dendritic arbors oriented obliquely or parallel to the mediolateral axis with long dendrites extending toward the lateral and ventral funiculi. The number of primary dendrites of most groups is set by E15 and dendritic growth occurs in the transverse plane by lengthening and branching of these primary processes. This study demonstrates that a large number of classes of commissural interneurons can be recognized on the basis of characteristic morphologies and locations within the dorsal horn, intermediate zone and ventral horn of the embryonic rat spinal cord. This finding is consistent with the fact that commissural neurons project to many different targets and mediate a variety of different functions. The demonstration that dendritic arbors of spinal interneurons with characteristic morphologies can be conveniently labelled with DiI should prove useful in future studies on the development of specific circuits in the mammalian spinal cord.

Animals↗