Absence of p75(NTR) expression reduces nerve growth factor immunolocalization in cholinergic septal neurons

The p75 neurotrophin receptor has nonapoptotic antineurotrophic actions in the basal forebrain

Because of controversy about the role of the p75 neurotrophin receptor (p75(NTR) ) in the cholinergic basal forebrain (CBF), we investigated this region in p75(NTR) third exon knockout mice that were congenic with 129/Sv controls. They express a shortened intracellular form of p75(NTR) , permitting detection of p75(NTR) -expressing cells. We performed separate counts of choline acetyltransferase (ChAT)-expressing and p75(NTR) -expressing neurons. In agreement with past reports, the number of ChAT-immunoreactive neurons in knockout mice was greater than in wild-type mice, and this was evident in each of the main anatomical divisions of the CBF. In contrast, the number of p75(NTR) -immunoreactive neurons did not differ between genotypes. The biggest increase in ChAT neurons (27%) was in the horizontal limb of the diagonal band of Broca (HDB), in which region the number of p75(NTR) -positive neurons was unchanged. Double staining revealed that some neurons in wild-type mice expressed p75(NTR) but not ChAT. In the knockout mice, all p75(NTR) -expressing neurons expressed ChAT. The increase in cholinergic neurons, therefore, was at least partially attributable to a higher proportion of ChAT immunoreactivity within the population of p75(NTR) -expressing neurons. Cholinergic neurons were also larger in knockout mice than in controls. In the hippocampal CA1 region, knockout mice had a greater number of cholinergic fibers. There was a 77% increase in hippocampal ChAT activity in knockout mice and a 38% increase in heterozygotes. The data do not support an apoptotic role but indicate a broad antineurotrophic role of p75(NTR) in the cholinergic basal forebrain.

Dissecting the involvement of tropomyosin-related kinase A and p75 neurotrophin receptor signaling in NGF deficit-induced neurodegeneration

NGF, the principal neurotrophic factor for basal forebrain cholinergic neurons (BFCNs), has been correlated to Alzheimer's disease (AD) because of the selective vulnerability of BFCNs in AD. These correlative links do not substantiate a comprehensive cause-effect mechanism connecting NGF deficit to overall AD neurodegeneration. A demonstration that neutralizing NGF activity could have consequences beyond a direct interference with the cholinergic system came from studies in the AD11 mouse model, in which the expression of a highly specific anti-NGF antibody determines a neurodegeneration that encompasses several features of human AD. Because the transgenic antibody binds to mature NGF much more strongly than to proNGF and prevents binding of mature NGF to the tropomyosin-related kinase A (TrkA) receptor and to p75 neurotrophin receptor (p75NTR), we postulated that neurodegeneration in AD11 mice is provoked by an imbalance of proNGF/NGF signaling and, consequently, of TrkA/p75NTR signaling. To test this hypothesis, in this study we characterize the phenotype of two lines of transgenic mice, one in which TrkA signaling is inhibited by neutralizing anti-TrkA antibodies and a second one in which anti-NGF mice were crossed to p75NTR(exonIII(-/-)) mice to abrogate p75NTR signaling. TrkA neutralization determines a strong cholinergic deficit and the appearance of beta-amyloid peptide (Abeta) but no tau-related pathology. In contrast, abrogating p75NTR signaling determines a full rescue of the cholinergic and Abeta phenotype of anti-NGF mice, but tau hyperphosphorylation is exacerbated. Thus, we demonstrate that inhibiting TrkA signaling activates Abeta accumulation and that different streams of AD neurodegeneration are related in complex ways to TrkA versus p75NTR signaling.

Enhanced spatial memory and hippocampal long-term potentiation in p75 neurotrophin receptor knockout mice

Previous reports have described increases in the size and number of cholinergic neurons in the basal forebrain in p75 neurotrophin receptor (p75(NTR)) knockout mice. In an earlier study, we also found improved spatial memory in these mice, raising the possibility that p75(NTR) regulates hippocampal function by its effects on the cholinergic basal forebrain. We therefore investigated hippocampal long-term potentiation in p75(NTR) knockout mice that shared the same genetic background as control 129/Sv mice. We also investigated heterozygous mice, carrying just one functional p75(NTR) allele. The p75(NTR) knockout mice had enhanced long-term potentiation in the Schafer collateral fiber synapses of the hippocampus. Heterozygous mice had an intermediate level, greater than controls but less than knockout mice. Hippocampal choline acetyltransferase activity was also markedly elevated in p75(NTR) knockout mice, with a smaller increase in heterozygous mice. In the Barnes maze, p75(NTR) knockout mice displayed markedly superior learning to controls, and this was evident over the three age brackets tested. At each age, the performance of heterozygous mice was intermediate to the other groups. In the open field test, p75(NTR) knockout mice exhibited greater stress-related behavioral responses, including freezing, than did control animals. There were no differences between the three groups in a test of olfactory function. The dose-dependent effects of p75(NTR) gene copy number on hippocampal plasticity and spatial memory indicate that p75(NTR) has profound effects on hippocampal function. Bearing in mind that p75(NTR) is very sparsely expressed in the adult hippocampus and has a potent effect on hippocampal choline acetyltransferase activity, the effects of p75(NTR) on hippocampal function are likely to be mediated indirectly, by its actions on basal forebrain cholinergic neurons.

Proteomic assessment of sympathetic ganglia from adult mice that possess null mutations of ExonIII or ExonIV in the p75 neurotrophin receptor gene

Neurotrophins, such as nerve growth factor (NGF), are capable of binding to the transmembrane p75 neurotrophin receptor (p75NTR), which regulates a variety of cellular responses including apoptosis and axonal elongation. While the development of mutant mouse strains that lack functional p75NTR expression has provided further insight into the importance of this neurotrophin receptor, there remains a paucity of information concerning how the loss of p75NTR expression may alter neural phenotypes. To address this issue, we assessed the proteome of the cervical sympathetic ganglia from two mutant lines of mice, which were compared to the ganglionic proteome of age-matched wild type mice. The ganglionic proteome of mice possessing two mutant alleles of either exonIII or exonIV for the p75NTR gene displayed detectable alterations in levels of Lamin A, tyrosine hydroxylase, and Annexin V, as compared to ganglionic proteome of wild type mice. Decreased expression of the basic isoform of tyrosine hydroxylase may be linked to perturbed NGF signaling in the absence of p75NTR in mutant mice. Stereological measurement showed significant increases in the number of sympathetic neurons in both lines of p75NTR-deficient mice, relative to wild type mice. This enhanced survival of sympathetic neurons coincides with shifts toward the more basic isoforms of Annexin V in mutant mice. This study, in addition to providing the first comparative proteomic assessment of sympathetic ganglia, sheds new light onto the phenotypic changes that occur as a consequence of a loss of p75NTR expression in adult mice.

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.

Structural and neurochemical features of postganglionic sympathetic neurons in the superior mesenteric ganglion of spontaneously hypertensive rats

Postganglionic sympathetic neurons, which are exquisitely sensitive to small changes in levels of target-derived nerve growth factor (NGF), express two transmembrane receptors: 1) the trkA receptor mediates neuron survival and neurite outgrowth; and 2) the p75 neurotrophin receptor (p75NTR) enhances neuronal responsiveness of trkA to NGF. Elevating levels of NGF induces several morphological and neurochemical alterations in sympathetic neurons, including axonal sprouting, increased levels of p75NTR mRNA relative to trkA mRNA, and increased accumulations of NGF in hypertrophied somata. Spontaneously hypertensive rats (SHR) display both elevated NGF levels and increased sympathetic axonal innervation of the mesenteric vasculature. In this investigation we assessed whether sympathetic neurons innervating the mesenteric vasculature of SHR display other features indicative of increased levels of target-derived NGF. In 5-week-old SHR, levels of both p75NTR and trkA mRNA in mesenteric sympathetic neurons were significantly elevated compared to levels in age-matched control rats. By 15 and 30 weeks of age, levels of p75NTR mRNA expression in mesenteric sympathetic neurons were similar between SHR and control rats. Accumulations of NGF were depleted in the sympathetic somata of 15- and 30-week-old SHR compared to age-matched control rats. Moreover, sympathetic neurons in SHR were not hypertrophied, as the sizes of somata were comparable between SHR and control rats. Our data illustrate that despite having augmented levels of NGF in the mesenteric vasculature, SHR do not display many of the morphological and neurochemical features that are associated with an enhanced responsiveness by sympathetic neurons to elevated levels of target-derived NGF.

Amelioration of cholinergic neurons dysfunction in aged rats depends on the continuous supply of NGF

The present study was designed to examine whether NGF-induced improvement in morphology of senile basal forebrain cholinergic neurons persist after discontinuation of NGF treatment. Trophic effect of continuous intraventricular infusion of NGF was tested in the 4- and 28 months old male Wistar rats immediately after cessation of NGF and 3 or 6 weeks after termination of treatment. Immunohistochemical procedure for ChAT, TrkA, and p75(NTR) receptor has been applied to identify cholinergic cells in the basal forebrain structures. Using the quantitative image analyzer, morphometric and densitometric parameters of cholinergic cells were measured. In untreated 28-month-old rats a reduction in the number, size and intensity of staining of cholinergic neurons was observed in all basal forebrain structures. NGF significantly improved morphological parameters of ChAT- and TrkA-positive cells in aged rats. In 28-month-old rats tested within 3 and 6 weeks after discontinuation of infusion a renewed progressive deterioration of cholinergic phenotype of basal forebrain neurons was observed when compared with the NGF-treated immediately tested group. The parallel staining for p75(NTR) revealed normal morphology of the basal forebrain neurons, despite of the age of rats or the NGF treatment. Analysis of Nissl stained sections also showed that 28-month-old rats did not display significant losses of neurons in the basal forebrain when compared with the young animals. These findings demonstrate that senile impairment of cholinergic neurons is induced by a loss of cholinergic phenotype rather than an acute degeneration of cell bodies. NGF may be beneficial in enhancing cholinergic neurochemical parameters, but the protective effects seem to be dependent on the continuous supply of NGF.

Estrogen treatment suppresses forebrain p75 neurotrophin receptor expression in aged, noncycling female rats

There is increasing evidence that estrogen has beneficial effects on cognition, both in humans and in rodents, and may delay Alzheimer's disease onset in postmenopausal women. Several rodent studies have utilised the ovariectomy model to show estrogen regulation of the p75 neurotrophin receptor, TrkA, and markers of acetylcholine synthesis in the cholinergic basal forebrain. We studied estrogenic effects in aged (16-17-month-old), noncycling rats. Estrogen treatment for 10 days drastically reduced p75(NTR) immunoreactivity in the rostral parts of the basal forebrain. The number of p75(NTR)-immunoreactive neurons was decreased, and those neurons remaining positive for p75(NTR) showed reduced p75(NTR) staining intensity. In vehicle-treated rats, almost all choline acetyltransferase-immunoreactive neurons were p75(NTR) positive (and vice versa), but, in estrogen treated rats, large numbers of choline acetyltransferase-immunoreactive cells were negative for p75(NTR). Similar levels of p75(NTR) down-regulation in the rostral basal forebrain were found when estrogen treatment was extended to 6 weeks. There was no reduction in the number of p75(NTR)-immunoreactive neurons in the caudal basal forebrain after 10 days of treatment. After 6 weeks of treatment, however, there was evidence of p75(NTR) down-regulation in the caudal basal forebrain. There was no evidence of hypertrophy or atrophy of cholinergic neurons even after 6 weeks of estrogen treatment. Considering the evidence for the role of p75(NTR) in regulating survival, growth and nerve growth factor responsiveness of cholinergic basal forebrain neurons, the results indicate an important aspect of estrogen's effects on the nervous system.