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Biomedical subjects

W C Mobley

Publications and source records attributed to W C Mobley.

At least 37 records · Page 2Linked to original sources
PubMed1997

Absence of p75NTR causes increased basal forebrain cholinergic neuron size, choline acetyltransferase activity, and target innervation.

Emerging evidence suggests that the p75 neurotrophin receptor (p75NTR) mediates cell death; however, it is not known whether p75NTR negatively regulates other neuronal phenotypes. We found that mice null for p75NTR displayed highly significant increases in the size of basal forebrain cholinergic neurons, including those that are TrkA-positive. Cholinergic hippocampal target innervation also was increased significantly. Activity of the cholinergic neurotransmitter synthetic enzyme choline acetyltransferase (ChAT) was increased in both the medial septum and hippocampus. Upregulation of these cholinergic features was not associated with increased basal forebrain or hippocampal target NGF levels. In contrast, striatal cholinergic neurons, which do not express p75NTR, showed no difference in neuronal number, size, or ChAT activity between wild-type and p75NTR null mutant mice. These findings indicate that p75NTR negatively regulates cholinergic neuronal phenotype of the basal forebrain cholinergic neurons, including cell size, target innervation, and neurotransmitter synthesis.

Animals↗
PubMed1997

A signaling organelle containing the nerve growth factor-activated receptor tyrosine kinase, TrkA.

The topology of signal transduction is particularly important for neurons. Neurotrophic factors such as nerve growth factor (NGF) interact with receptors at distal axons and a signal is transduced by retrograde transport to the cell body to ensure survival of the neuron. We have discovered an organelle that may account for the retrograde transport of the neurotrophin signal. This organelle is derived from endocytosis of the receptor tyrosine kinase for NGF, TrkA. In vitro reactions containing semi-intact PC12 cells and ATP were used to enhance recovery of a novel organelle: small vesicles containing internalized NGF bound to activated TrkA. These vesicles were distinct from clathrin coated vesicles, uncoated primary endocytic vesicles, and synaptic vesicles, and resembled transport vesicles in their sedimentation velocity. They contained 10% of the total bound NGF and almost one-third of the total tyrosine phosphorylated TrkA. These small vesicles are compelling candidates for the organelles through which the neurotrophin signal is conveyed down the axon.

Animals↗
PubMed1997

A hypothalamic neuronal cell line persistently infected with scrapie prions exhibits apoptosis.

Neuronal death and vacuolation are characteristics of the CNS degeneration found in prion diseases. Relatively few cultured cell lines have been identified that can be persistently infected with scrapie prions, and none of these cells show cytopathologic changes reminiscent of prion neuropathology. The differentiated neuronal cell line GT1, established from gonadotropin hormone releasing-hormone neurons immortalized by genetically targeted tumorigenesis in transgenic mice (P. L. Mellon, JJ. Windle, P. C. Goldsmith, C. A. Padula, J. L. Roberts, and R. I. Weiner, Neuron 5:1-10, 1990), was examined for its ability to support prion formation. We found that GT1 cells could be persistently infected with mouse RML prions and that conditioned medium from infected cells could transfer prions to uninfected cells. In many but not all experiments, a subpopulation of cells showed reduced viability, morphological signs of neurodegeneration and vacuolation, and features of apoptosis. Subclones of GT1 cells that were stably transfected with the trk4 gene encoding the high-affinity nerve growth factor (NGF) receptor (GT1-trk) could also be persistently infected. NGF increased the viability of the scrapie-infected GT1-trk cells and reduced the morphological and biochemical signs of vacuolation and apoptosis. GT1 cells represent a novel system for studying the molecular mechanisms underlying prion infectivity and subsequent neurodegenerative changes.

Animals↗
PubMed1996

Endocytosis of activated TrkA: evidence that nerve growth factor induces formation of signaling endosomes.

The survival, differentiation, and maintenance of responsive neurons are regulated by nerve growth factor (NGF), which is secreted by the target and interacts with receptors on the axon tip. It is uncertain how the NGF signal is communicated retrogradely from distal axons to neuron cell bodies. Retrograde transport of activated receptors in endocytic vesicles could convey the signal. However, little is known about endocytosis of NGF receptors, and there is no evidence that NGF receptors continue to signal after endocytosis. We have examined early events in the membrane traffic of NGF and its receptor, gp140(TrkA) (TrkA), in PC12 cells. NGF induced rapid and extensive endocytosis of TrkA in these cells, and the receptor subsequently moved into small organelles located near the plasma membrane. Some of these organelles contained clathrin and alpha-adaptin, which implies that TrkA is internalized by clathrin-mediated endocytosis. Using mechanical permeabilization and fractionation, intracellular organelles derived from endocytosis were separated from the plasma membrane. After NGF treatment, NGF was bound to TrkA in endocytic organelles, and TrkA was tyrosine-phosphorylated and bound to PLC-gamma1, suggesting that these receptors were competent to initiate signal transduction. These studies raise the possibility that NGF induces formation of signaling endosomes containing activated TrkA. They are an important first step in elucidating the molecular mechanism of NGF retrograde signaling.

Animals↗
PubMed1996

Developmental abnormalities and age-related neurodegeneration in a mouse model of Down syndrome.

To study the pathogenesis of central nervous system abnormalities in Down syndrome (DS), we have analyzed a new genetic model of DS, the partial trisomy 16 (Ts65Dn) mouse. Ts65Dn mice have an extra copy of the distal aspect of mouse chromosome 16, a segment homologous to human chromosome 21 that contains much of the genetic material responsible for the DS phenotype. Ts65Dn mice show developmental delay during the postnatal period as well as abnormal behaviors in both young and adult animals that may be analogous to mental retardation. Though the Ts65Dn brain is normal on gross examination, there is age-related degeneration of septohippocampal cholinergic neurons and astrocytic hypertrophy, markers of the Alzheimer disease pathology that is present in elderly DS individuals. These findings suggest that Ts65Dn mice may be used to study certain developmental and degenerative abnormalities in the DS brain.

Aging↗
PubMed1996

Expression of neuronal-NOS in developing basal forebrain cholinergic neurons: regulation by NGF.

Nerve growth factor (NGF) acts through the receptor tyrosine kinase trkA to serve as a trophic factor for cholinergic neurons in the medial septal nucleus and vertical limb of the diagonal band. We have previously shown that the neuronal isoform of nitric oxide synthase (NOS) is selectively expressed in a large fraction of trkA-expressing cholinergic neurons in these brain regions in the adult rat, and that NGF induces the expression of neuronal-NOS in these cells. Herein, we show that: 1) neuronal-NOS is also localized to these neurons in the developing septum; 2) the expression of neuronal-NOS is regulated in the developing medial septal nucleus and vertical limb of the diagonal band; 3) neuronal-NOS regulation parallels that for other markers of basal forebrain cholinergic neuron differentiation, such as cholineacetyltransferase; and 4) NGF infusion in the postnatal period induces robust increases in neuronal-NOS mRNA and in NOS activity in the basal forebrain. Taken together with earlier findings, our results suggest that neuronal-NOS has a role in the differentiation and mature function of septal cholinergic neurons. Through enhancing neuronal-NOS synthesis, endogenous NGF is likely to regulate NO functions in vivo.

Acetylcholinesterase↗
PubMed1996

A comprehensive study of the spatiotemporal pattern of beta-amyloid precursor protein mRNA and protein in the rat brain: lack of modulation by exogenously applied nerve growth factor.

Nerve growth factor (NGF) is a neurotrophic factor for basal forebrain cholinergic neurons, a population that degenerates and dies in Alzheimer's disease (AD). It has been suggested that NGF be used to treat AD patients. However, in vivo administration of NGF to the developing hamster brain was shown to induce the expression of the beta-amyloid precursor protein (beta APP) gene. The association of alterations in beta APP gene expression with AD-like neuropathological changes and cognitive impairment in animals, and with AD-like neurodegeneration in Down syndrome patients suggests that NGF-mediated increases in beta APP expression could negate or attenuate NGF's neurotrophic activity in AD treatment trials. The present study was undertaken to explore further the influence of NGF on beta APP expression, and to determine which, if any, of the beta APP mRNAs is altered in response to NGF treatment. We first examined the spatiotemporal pattern of beta APP-695 and Kunitz protease inhibitor (KPI)-containing beta APP mRNA expression in the rat brain. Specific oligonucleotide probes were used to show that these mRNAs are present during embryonic development. In addition, we evaluated postnatal expression in nine brain regions and showed that beta APP mRNAs were readily detected in all regions at postnatal day 2. In human brain, the relative levels of beta APP-695 and beta APP-KPI mRNA and their protein are discordant, in that the level of beta APP-695 mRNA is slightly higher than that of beta APP-KPI, but beta APP-KPI protein predominates. In contrast, the several-fold excess of beta APP-695 mRNA relative to beta APP-KPI mRNA in the rat brain was also reflected at the protein level. Surprisingly, administration of exogenous NGF failed to affect rat beta APP mRNA levels either in vitro or during postnatal development in vivo.

Amyloid beta-Protein Precursor↗
PubMed1996

Abnormal plasma membrane properties and functions in prion-infected cell lines.

A long trail of evidence indicates that the formation of PrPSc or its accumulation causes the neuronal dysfunction and clinical features of prion diseases. The results of our current line of studies argue that the main neuropathological and clinical features of prion diseases are explained by altered ion channel function secondary to decreased plasma membrane fluidity. This kind of mechanism has the potential to functionally disconnect neuronal networks and cause neuronal vacuolation. Our laboratory is currently focusing its investigations on pathogenic mechanisms that have the potential to link the formation of PrPSc with plasma membrane abnormalities in prion diseases. In summary, the first hypothesis suggests that the conversion of PrPC to PrPSc affects plasma membrane fluidity directly, which secondarily alters the properties and functions of its components. In contrast, the second hypothesis argues that PrPSc accumulation alters the ability of chaperones to correctly fold plasma-membrane proteins during their synthesis, which directly affects the properties of nascent proteins and secondarily affects membrane fluidity. Our current investigations are attempting to determine which of these mechanisms are plausible and, then, which is primary.

Animals↗
PubMed1995

Therapeutic applications of neurotrophic factors in disorders of motor neurons and peripheral nerves.

Research in the past few years has produced exciting progress in our understanding of neurotrophic factors. Robust effects of neurotrophic factors on neuronal survival and differentiation in animal studies have encouraged initiation of clinical trials for diseases of the human nervous system. In this article, the data for the actions of neurotrophic factors and the rationale for their use in clinical trials are reviewed. Recent data demonstrating efficacy of insulin-like growth factor 1 in amyotrophic lateral sclerosis suggest that neurotrophic factors can be used to treat neurological disease.

Amyotrophic Lateral Sclerosis↗
PubMed1995

Multiple levels for regulation of TrkA in PC12 cells by nerve growth factor.

TrkA is a receptor tyrosine kinase for nerve growth factor (NGF). Recent studies indicate that NGF regulates not only activation of trkA kinase but also expression of the trkA gene. To further define NGF actions on trkA, we examined binding and signaling through trkA after both short and long intervals of NGF treatment. Induction of tyrosine phosphorylation on gp140trkA was rapidly followed by down-regulation of cell surface and total cellular gp140trkA. At later intervals, increased expression of trkA was evident in increased mRNA and protein levels. At 7 days, there was increased binding to gp140trkA and increased signaling through this receptor. NGF appears to regulate trkA at several levels. In neurons persistently exposed to NGF, maintenance of NGF signaling may require increased trkA gene expression.

Amino Acid Sequence↗
PubMed1995

Regulation of TrkA and ChAT expression in developing rat basal forebrain: evidence that both exogenous and endogenous NGF regulate differentiation of cholinergic neurons.

TrkA is a receptor tyrosine kinase whose activation transduces NGF signaling. TrkA expression has been demonstrated in NGF-responsive adult basal forebrain cholinergic neurons (BFCNs). Several lines of evidence have suggested that endogenous NGF plays a role in the development and differentiation of these neurons. We examined TrkA expression during development. TrkA mRNA and protein were present in basal forebrain neurons during the entire postnatal period; the distribution of neurons bearing these markers was identical to that for those containing choline acetyltransferase (ChAT) mRNA, suggesting that, as in the adult, TrkA gene expression is localized to BFCNs. The expression of TrkA and ChAT followed a very similar temporal pattern, suggesting regulation by the same factor(s). We discovered that NGF administration in vivo activated TrkA receptors, and increased both TrkA and ChAT mRNA; conversely, anti-NGF infusions suppressed expression of both genes. These results suggest that endogenous NGF regulates expression of TrkA and ChAT. Finally, while NGF infusion increased the size of developing BFCNs, NGF antibodies inhibited the normal developmental increase. The results are evidence that endogenous NGF acts on developing BFCNs to enhance gene expression and cellular differentiation.

Aging↗
PubMed1995

TrkA expression in the CNS: evidence for the existence of several novel NGF-responsive CNS neurons.

NGF acts as a neurotrophic factor by binding and activating its receptor on certain neuronal populations in the CNS and PNS. TrkA is a receptor for NGF. Recent findings in vitro indicate that this NGF-activated receptor tyrosine kinase transduces the NGF signal. To further define NGF actions in the CNS, we examined trkA expression in the adult rat brain. We found that trkA mRNA and immunoreactivity (IR) coincided in specific, defined neuronal populations in the forebrain and brainstem. In addition to cholinergic neurons in the basal forebrain and neostriatum, trkA expression was found in noncholinergic neurons in (1) the paraventricular anterior and reuniens thalamic nuclei, (2) the rostral and intermediate subnuclei of the interpeduncular nucleus (IPN), (3) scattered neurons in the ventrolateral and paramedian medulla, (4) the prepositus hypoglossal nucleus, and (5) the area postrema. NGF responsiveness was demonstrated for each of these populations. In contrast to trkA, p75NGFR was found only in a minority of NGF-responsive populations. Our data provide further evidence that expression of trkA marks NGF-responsive CNS neurons and suggests novel roles for NGF in the brain.

Animals↗