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Keith A Crutcher

Publications and source records attributed to Keith A Crutcher.

11 recordsLinked to original sources
PubMed2006

Global expression of NGF promotes sympathetic axonal growth in CNS white matter but does not alter its parallel orientation.

Axonal regeneration is normally limited after injuries to CNS white matter. Infusion of neurotrophins has been successful in promoting regenerative growth through injured white matter but this growth generally fails to extend beyond the infusion site. These observations are consistent with a chemotropic effect of these factors on axonal growth and support the prevailing view that neurotrophin-induced axonal regeneration requires the use of gradients, i.e., gradually increasing neurotrophin levels along the target fiber tract. To examine the potential of global overexpression of neurotrophins to promote, and/or modify the orientation of, regenerative axonal growth within white matter, we grafted nerve growth factor (NGF) responsive neurons into the corpus callosum of transgenic mice overexpressing NGF throughout the CNS under control of the promoter for glial fibrillary acidic protein. One week later, glial fibrillary acidic protein and chondroitin sulfate proteoglycan immunoreactivity increased within injured white matter around the grafts. NGF levels were significantly higher in the brains of transgenic compared with non-transgenic mice and further elevated within injury sites compared with the homotypic region of the non-injured side. Although there was minimal outgrowth from neurons grafted into non-transgenic mice, extensive parallel axonal regeneration had occurred within the corpus callosum up to 1.5 mm beyond the astrogliotic scar (the site of maximum NGF expression) in transgenic mice. These results demonstrate that global overexpression of neurotrophins does not override the constraints limiting regenerative growth to parallel orientations and suggest that such factors need not be presented as positive gradients to promote axonal regeneration within white matter.

Animals↗
PubMed2006

Null mutations for exon III and exon IV of the p75 neurotrophin receptor gene enhance sympathetic sprouting in response to elevated levels of nerve growth factor in transgenic mice.

Under normal conditions, expression of the p75 neurotrophin receptor (p75NTR) by sympathetic neurons can increase the affinity of the signaling receptor, trkA, to target-derived nerve growth factor (NGF) at distal axons. We have previously reported that sprouting of sympathetic axons into NGF-rich target tissues is enhanced when p75NTR expression is perturbed, leading to the postulate that p75NTR may restrain sympathetic sprouting in response to elevated NGF levels. These observations were made using mice having a null mutation of the third p75NTR exon, a line that may express a hypomorphic form of this receptor. Since mice carrying a null mutation of the fourth p75NTR exon may not express a similar splice variant, we sought to determine whether these animals possess the same phenotype of enhanced sympathetic sprouting in response to elevated levels of NGF. Both lines of transgenic mice lacking p75NTR displayed similar degrees of sympathetic axonal sprouting into the cerebellum and trigeminal ganglia, two target tissues having elevated levels of NGF protein. Furthermore, the densities of sympathetic axons in both targets were significantly greater than those observed in age-matched NGF transgenic siblings expressing full-length p75NTR. Our new findings provide a comparative analysis of the phenotype in two independent mutations of the same neurotrophin receptor, revealing that p75NTR plays an important role in restricting sympathetic sprouting in response to higher NGF levels.

Animals↗
PubMed2005

The receptor-binding region of human apolipoprotein E has direct anti-infective activity.

BACKGROUND: The APOE genotype has a uniquely strong influence on the outcome of viral infection. The mechanism is unknown, although one possibility is direct inhibition of viral entry into cells. METHODS: We have examined the direct anti-infective activity of a peptide analogue of the receptor-binding region of apolipoprotein E (apoE) that is known as "apoE dimer tandem repeat peptide" (apoEdp) and has previously been shown to mimic some of the biological effects of apoE and that recently was shown to bind low-density lipoprotein receptor-related protein. RESULTS: apoEdp has activity against herpes simplex virus types 1 and 2, human immunodeficiency virus, Pseudomonas aeruginosa, and Staphylococcus aureus; concentrations in the range of 1-20 micromol/L inhibit infection by 50%. These biological actions depend on adoption of an alpha -helical structure, as has been found for other biological effects of apoE peptides. The peptide interferes with the earliest stages of viral infection, preventing viral attachment and exerting a mild virucidal action. In addition, an N-terminal fragment of apoE that also contains this binding domain has antiviral activity. CONCLUSIONS: These data suggest that human apoE or fragments containing the receptor-binding domain may contribute to innate immunity to viral infection by direct disruption of viral particles and/or inhibition of viral attachment, thus reducing viral entry.

Animals↗
PubMed2005

'Mature' nerve growth factor is a minor species in most peripheral tissues.

The classic neurotrophin hypothesis is based on the idea that innervating neurons derive 'mature' neurotrophin provided by the target for their survival. Yet large precursor forms of the neurotrophin nerve growth factor (NGF) have been reported in both central and peripheral tissues. In the present study, immunoblotting was used to survey peripheral tissues containing NGF-responsive neurons and to characterize various NGF species. These results demonstrate that 'mature' forms of NGF, i.e., the 13 and 16kDa species, are rare in sympathetic and sensory ganglia and in their peripheral targets, and that large molecular weight NGF precursors are abundant. In addition, certain NGF forms predominate in a given tissue, with each tissue exhibiting a characteristic NGF expression pattern. These findings suggest that NGF processing in peripheral tissues and in NGF-responsive ganglia may involve a variety of NGF species.

Animals↗
PubMed2004

Apolipoprotein E is a prime suspect, not just an accomplice, in Alzheimer's disease.

There is now a large body of evidence suggesting that apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease. Yet in proportion to the total number of investigations in this field, relatively few groups are studying the contribution of this cholesterol-binding protein to disease risk and severity. Of those that are, a major focus is on the impact of apoE on amyloid-related mechanisms of disease. I argue here that apoE should be considered a major culprit in its own right, not simply in a supporting role. The argument is based on several lines of evidence, including the fact that apoE is associated with both plaques and tangles, the overwhelming evidence for genetic risk of the disease attributed to apoE, increasing evidence that apoE might also modify risk of other nonamyloidogenic neurological diseases, neurotoxicity attributed to apoE and/or proteolytic fragments of apoE, negative consequences of transgenic expression of apoE4 in mice, and genetic evidence for polymorphisms that increase both apoE expression and disease risk, regardless of isoform.

Alzheimer Disease↗
PubMed2004

Progress toward identification of protease activity involved in proteolysis of apolipoprotein e in human brain.

Apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease (AD). Increasing evidence supports the hypothesis that the presence of the E4 isoform of this cholesterol-binding protein contributes directly to disease risk, age of onset, and severity of the neuropathology. For example, studies in transgenic mice demonstrate that apoE is necessary for the formation of plaques with neuritic pathology. The precise mechanism by which apoE contributes to the disease remains unknown. However, several lines of investigation from a number of laboratories now point to a role for proteolytic fragments of apoE in the formation of both plaques and tangles, the two pathological hallmarks of the disease. In particular, the C-terminal portion of apoE has been implicated in binding to amyloid and is localized to plaques. The N-terminal domain, on the other hand, is neurotoxic in culture and has been localized to, and implicated in the formation of, neurofibrillary tangles. These results suggest that inhibition of apoE proteolysis is a potential therapeutic strategy for AD. Using human brain homogenates, we have determined that proteolysis of apoE is greatest at acidic pH and can be inhibited by compounds targeting aspartic proteases. The feasibility of screening candidate inhibitors is supported by both ELISA and immunoblotting methods. Future studies will use a combination of in vitro and in vivo assays to test the efficacy of the most effective compounds for their ability to inhibit apoE proteolysis in human brain and apoE transgenic mouse brain tissue.

Alzheimer Disease↗
PubMed2003

Apolipoprotein E-related neurotoxicity as a therapeutic target for Alzheimer's disease.

Apolipoprotein E (apoE) remains the most important genetic risk factor for the development of Alzheimer's disease (AD). Still elusive, the role of apoE is under intense investigation. We propose that proteolysis of apoE in the brain leads to two major fragments, N- and C-terminal apoE, each of which would drive a different neuropathological pathway. N-terminal fragments of apoE are implicated in neurotoxicity, and C-terminal fragments might play a role in amyloid deposition and plaque formation. The greater risk of AD associated with the E4 isoform might relate to its greater neurotoxicity. Drugs that either directly inhibit the toxic effects of apoE or prevent the production of apoE fragments may provide novel therapeutic approaches to the treatment of AD and other disorders in which apoE is implicated.

Alzheimer Disease↗
PubMed2002

Inhibition of apolipoprotein E-related neurotoxicity by glycosaminoglycans and their oligosaccharides.

Apolipoprotein E (apoE) has been genetically linked to late-onset Alzheimer's disease (AD). The role of this lipid-transport protein in AD remains to be established. One hypothesis is that apoE, particularly the apoE4 isoform, may have neurotoxic effects as demonstrated using apoE-related synthetic peptides and the N-terminal fragment of apoE. ApoE is a heparan-sulfate binding protein, and apoE peptide neurotoxicity can be blocked by heparin and prevented by degrading heparan sulfate or inhibiting its biosynthesis. The possibility that heparin inhibition of toxicity is mediated by a specific oligosaccharide sequence was investigated using a bioassay to determine the inhibition of apoE peptide toxicity by glycosaminoglycans and purified glycosaminoglycan oligosaccharides. Studies on modified heparins showed that the presence of N-sulfo groups and either 2- or 6-O sulfo groups were required for inhibition of toxicity. Heparin oligosaccharides with eight or more saccharide residues with seven O-sulfo groups and four N-sulfo groups exhibited potent inhibition. Larger oligosaccharides, and heparin and heparan sulfate polymers, afforded comparable, or somewhat better, protective effects but also caused clumping and detachment of cells when administrated alone.

Animals↗
PubMed2002

Aging and neuronal plasticity: lessons from a model.

In spite of many well-documented examples of age-related reductions in neuronal plasticity, the causes of such changes remain largely unknown. One example of age-reduced plasticity involves an aberrant sprouting response of mature rat sympathetic neurons into the CNS (hippocampal formation). This phenomenon has proven to be useful for exploring the relative contribution of target aging (extrinsic influences) versus neuronal aging (intrinsic influences) to reduced sprouting. Aged sympathetic neurons mount a robust growth response when confronted with young target tissue or when exposed to exogenous trophic factor in vivo. In contrast, the aged target tissue (the hippocampal formation in this example) exhibits reduced receptivity for sympathetic sprouting. This change in the target does not appear to be due to alterations in baseline levels of trophic or substrate support for axonal growth. Rather, aging appears to dampen the consequences of target denervation so that the aged target elicits less sprouting. Age-related reductions in neuronal sprouting are speculated to reflect increasing commitment to information storage at the expense of neuronal plasticity.

Aging↗