Changes in truncated trkB and p75 receptor expression in the rat spinal cord following spinal cord hemisection and spinal cord hemisection plus neurotrophin treatment

Although numerous studies have examined the effects of neurotrophin treatment following spinal cord injury, few have examined the changes that occur in the neurotrophin receptors following either such damage or neurotrophin treatment. To determine what changes occur in neurotrophin receptor expression following spinal cord damage, adult rats received a midthoracic spinal cord hemisection alone or in combination with intrathecal application of brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). Using immunohistochemical and in situ hybridization techniques, p75, trkA, trkB, and trkC receptor expression was examined throughout the spinal cord. Results showed that trkA, full-length trkB, and trkC receptors were not present in the lesion site but had a normal expression pattern in uninjured parts of the spinal cord. In contrast, p75 receptor expression occurred on Schwann cells throughout the lesion site. BDNF and NT-3 (but not saline) applied to the lesion site increased this expression. In addition, the truncated trkB receptor was expressed in the border between the lesion and intact spinal cord. Truncated trkB receptor expression was also increased throughout the white matter ipsilateral to the lesion and BDNF (but not NT-3 or saline) prevented this increase. The study is the first to show changes in truncated trkB receptor expression that extend beyond the site of a spinal cord lesion and is one of the first to show that BDNF and NT-3 affect Schwann cells and/or p75 expression following spinal cord damage. These results indicate that changes in neurotrophin receptor expression following spinal cord injury could influence the availability of neurotrophins at the lesion site. In addition, neurotrophins may affect their own availability to damaged neurons by altering the expression of the p75 and truncated trkB receptor.     

p75 neurotrophin receptor signal pathway influence on apoptosis in anterior horn neurons of the spinal cord in a rat model of cauda equina compression injury

BACKGROUND： Studies have demonstrated that cauda equina compression results in apoptosis of motor neurons in the spinal cord. The combination of p75 neurotrophin receptor （p75NTR） and precursor of nerve growth factor （pro-NGF） expression initiates the apoptotic pathway and induces neuronal apoptosis. However, few reports have focused on the p75-mediated mechanism of neuronal apoptosis following cauda equine compression injury OBJECTIVE： To determine apoptosis of spinal cord neurons and activation of the pro-NGF-p75NTR-JNK（c-Jun N-terminal kinase） signal pathway in rats following cauda equina compression, and to verify experimental outcomes. DESIGN, TIME AND SETTING： A randomized, controlled, in vivo experiment was performed at the Medical Experimental Center of Xi＇an Jiaotong University between April and November in 2008. MATERIALS： Streptavidin-perosidase kit was purchased from Wuhan Boster, China; in situ end labeling detection kit was provided by Promega, USA; type AEG-220G electron microscope was purchased from Hitachi, Japan. METHODS： A total of 48 healthy, adult, female, Sprague Dawley rats were randomly assigned to three groups： normal （n = 6）, sham-surgery （n = 6）, and compression （n = 36）. The compression group was randomly assigned to six subsets at 1,3, 5, 7, 14, and 28 days, respectively, with 6 rats in each subset. A cylindrical silica gel stick was implanted into the rats to compress 75% of the vertebral canal in the compression group; in the sham-surgery group, only vertebral resection was performed; and no procedures were performed in the normal group. MAIN OUTCOME MEASURES： At 1,3, 5, 7, 14, and 28 days following compression, L2-3 spinal cord segments were processed for immunohistochemistry, in situ cell apoptosis detection, and transmission electron microscopy observation. Nissl staining was used to observe neuronal survival in the L2 spinal cord segment. Immunohistochemistry was applied to detect expressions of pro-NGF, p75NTR, and JNK in the L2 segment. TUNEL fluorometric method was used to observe apoptosis of neurons in the L2 segment. RESULTS： In the normal and sham-surgery groups, little neuronal apoptosis was observed in the L2-3 spinal cord segment. At 3 days after compression injury, pro-NGF, p75NTR and JNK expression was observed in the spinal cord. Expression levels reached a peak at 7 days, and then gradually decreased. In the compression and sham-surgery groups, neurons primarily expressed pro-NGF and p75NTR. The number of JNK-positive neurons in the compression group was dramatically increased compared with the sham-surgery group （P〈 0.05）. A few neurons were apoptotic in the spinal cord 1 day after compression injury. The number of apoptotic neurons gradually increased and reached a peak at 7 days, and subsequently decreased. Apoptosis was still detectable at 28 days. There was a positive correlation between p75NTR expression and neuronal apoptosis （r= 0.75, P〈 0.05）. CONCLUSION： Following cauda equina compression injury, apoptosis of spinal cord neurons was observed. The compression-induced neuronal apoptosis was associated with p75NTR expression in the L2-3 spinal cord segment.     

3,4-methylenedioxyamphetamine upregulates p75 neurotrophin receptor protein expression in the rat brain

The p75 neurotrophin receptor,which is a member of the tumor necrosis factor receptor superfamily,facilitates apoptosis during development and following central nervous system injury.Previous studies have shown that programmed cell death is likely involved in the neurotoxic effects of 3,4-methylenedioxy-N-methylamphetamine (MDMA),because MDMA induces apoptosis of immortalized neurons through regulation of proteins belonging to the Bcl-2 family.In the present study,intraperitoneal injection of different doses of MDMA (20,50,and 100 mg/kg) induced significant behavioral changes,such as increased excitability,increased activity,and irritability in rats.Moreover,changes exhibited dose-dependent adaptation.Following MDMA injection in rat brain tissue,the number of apoptotic cells dose-dependently increased and p75 neurotrophin receptor expression significantly increased in the prefrontal cortex,cerebellum,and hippocampus.These findings confirmed that MDMA induced neuronal apoptosis,and results suggested that this effect was related by upregulated protein expression of the p75 neurotrophin receptor.     

Association between p75 neurotrophin receptor gene expression and cell apoptosis in tissues surrounding hematomas in rat models of intracerebral hemorrhage

Animal models of intracerebral hemorrhage were established by injection of autologous blood into the caudate nucleus in rats. Cell apoptosis was measured by flow cytometry and immunohistochemical staining of the p75 neurotrophin receptor. p75 neurotrophin receptor protein was detected by immunohistochemistry. p75 neurotrophin receptor mRNA was examined by quantitative real-time polymerase chain reactions. At 24 hours after modeling, cellular apoptosis occured around hematoma with upregulation of p75 neurotrophin receptor protein and mRNA was observed, which directly correlated to apoptosis. This observation indicated that p75 neurotrophin receptor upregulation was associated with cell apoptosis around hematomas after intracerebral hemorrhage.     

Small molecule modulation of p75 neurotrophin receptor functions

Ligand-independent and/or proNGF-induced p75(NTR) signaling has emerged as a potential major contributor to a number of pathological states, including axotomy-induced death, motor neuron degeneration, neuronal degeneration in Alzheimer's disease and oligodendrocyte death following spinal cord injury. A long standing goal in the neurotrophin field has been the development of non-peptide, small molecules capable of functioning as specific ligands at neurotrophin receptors such as p75(NTR) to promote desired biological outcomes. Synthetic peptides modeled on neurotrophin protein domains have been found to bind to and activate various neurotrophin receptors, raising the possibility that active, non-peptide, small molecule ligands might also be identified; however, traditional high-throughput screening approaches have been largely ineffective in identifying such compounds. Using pharmacophores derived from the structure of loop 1 of nerve growth factor, non-peptide, small molecules that function as p75(NTR) ligands to promote survival and block proNGF-induced death have recently been identified. Small molecule p75(NTR) ligands, with high potency and specificity, may provide novel therapeutic approaches for neurodegenerative diseases, neurotrauma and other pathologic states.     

Inactivation and activation of Ras by the neurotrophin receptor p75

The neurotrophin receptor p75 induces neurotrophic and/or apoptotic signalling pathways and can also cooperate with the neurotrophic Trk receptor tyrosine kinases. Its intracellular part encloses a so-called 'death domain' with a segment similar to the wasp venom mastoparan which binds small GTPases such as Rho. To study possible interactions of p75 and Ras (and Rho) we used wild-type and mutant genes of p75 stably expressed by MDCK cells which normally have neither Trk nor p75. We found that p75 can directly bind the GTPases Ras and Rho and that the unstimulated p75 inactivates total cellular Ras through a differential influence on the dissociation of GDP and GTP from Ras and an exchange of bound Ras.GDP for free Ras.GTP. These properties of p75 could also be demonstrated in vitro and should therefore be cell type-independent. Stimulation of p75 with nerve growth factor causes Ras activation via adapter proteins known from Trk signalling and induces rapid outgrowth of cellular processes. Both inactivation and activation of Ras by p75 are controlled by the phosphorylation state of the receptor's two intracellular tyrosines. p75 also influences Rho activation and inactivation, and the combined interactions of the receptor with the two GTPases Ras and Rho can regulate neurite formation in an efficient, synergistic way.     

TRK and p75 neurotrophin receptor systems in the developing human brain

The prenatal development of the neurons immunoreactive for high-affinity tropomycin-related kinase (trk) receptor (pan trk which recognizes trkA, trkB, and trkC) and low-affinity p75 neurotrophin receptor (p75^NTR) was examined in the human brain from embryonic weeks 10 to 34 of gestation. In the embryonic week 10 specimen in which only brainstem regions were available for evaluation, trk immunoreactivity (trk-ir) was observed in the ventral cochlear, solitary, raphe, spinal trigeminal, and hypoglossal nuclei, as well as the vestibular complex and medullary reticular formation. At this time point of gestation, p75^NTR-immunoreactive (p75^NTR-ir) staining was observed within these same regions plus the inferior olivary and ambiguus nuclei. At embryonic week 14, trk-ir neurons were seen within the subplate zone of the entorhinal cortex, basal forebrain, caudate nucleus, putamen, external segment of the globus pallidus, specific thalamic nuclei, lateral mammillary nucleus, habenula nucleus, select brainstem nuclei, and the dentate nucleus of cerebellum. At this gestational time point, p75^NTR-ir neurons were observed in each of these structures, with the exception of the caudate nucleus, specific thalamic nuclei, lateral mammillary nucleus, and habenula nucleus. Additionally, p75^NTR-ir neurons were observed within the corpus callosum. The staining pattern for both trk and p75^NTR remained unchanged at embryonic weeks 15 to 16 except for the addition of trk-ir and p75^NTR-ir within the cortical subplate zone, hippocampus, and subthalamic nucleus. By embryonic week 18, trk-ir neurons were widely expressed within mostly all thalamic nuclei. In contrast, trk-ir was no longer seen within the hypoglossal, cuneate, and gracile nuclei at this time point. This staining pattern for trk and p75^NTR remained virtually unchanged from embryonic weeks 19 to 20 and embryonic weeks 16 to 20, respectively. From embryonic weeks 22 to 34, the distribution of both trk-ir and p75^NTR-ir neurons changed gradually. During this period, neurons in most thalamic and some brainstem nuclei became progressively immunonegative for trk, whereas neurons in the neocortical subplate zone, corpus callosum, and hilar region of dentate gyrus gradually lost immunoreactivity for p75^NTR. These data demonstrate an important and complex role for both the high- (trk) and low-(p75) affinity neurotrophin receptors during the development of multiple neuronal systems in the human brain. © 1996 Wiley-Liss, Inc.     

Spatial and temporal gene expression profiling of the contused rat spinal cord

Microarray technology was used to examine gene expression changes following contusive injury of the adult rat spinal cord. To obtain a global understanding of the changes triggered by the injury, differential gene expression was examined spatially, using tissue samples from the epicenter of injury as well as 1 cm rostral and 1 cm caudal to the epicenter, and temporally, at 3 h, 24 h, 7 days, and 35 days post-injury. To filter out gene expression changes that were due to the laminectomy, samples of contused tissue were compared to laminectomy-only controls. We took advantage of four different, complementary methods of data analysis to detect differentially expressed genes. We have identified functional groups of genes that are differentially regulated in our model, including those associated with apoptosis, cell cycle, inflammation, and cholesterol metabolism. Our analysis has led to the identification of novel potential therapeutic targets within each group of genes that is discussed.     

Regulation of cardiac innervation and function via the p75 neurotrophin receptor

Homeostatic regulation of cardiac function is dependent on the balance of inputs from the sympathetic and parasympathetic nervous systems. We investigated whether the p75 neurotrophin receptor plays a developmental role in cardiac innervation by analyzing sympathetic and parasympathetic fibers in the atria of p75 knockout and wildtype mice at several stages of postnatal development, and examining the effect on control of heart rate. We found that parasympathetic innervation of the atria in p75-/- mice was similar to wildtype at all time points, but that the density of sympathetic innervation was dynamically regulated. Compared to wildtype mice, the p75-/- mice had less innervation at postnatal day 4, an increase at day 28, and decreased innervation in adult mice. These changes reflect defects in initial fiber in-growth and the timing of the normal developmental decrease in sympathetic innervation density in the atria. Thus, p75 regulates both the growth and stability of cardiac sympathetic fibers. The distribution of sympathetic fibers was also altered, so that many regions lacked innervation. Basal heart rate was depressed in adult p75-/- mice, and these mice exhibited a diminished heart rate response to restraint stress. This resulted from the lack of sympathetic innervation rather than increased parasympathetic transmission or a direct effect of p75 in cardiac cells. Norepinephrine was elevated in p75-/- atria, but stimulating norepinephrine release with tyramine produced less tachycardia in p75-/- mice than wild type mice. This suggests that altered density and distribution of sympathetic fibers in p75-/- atria impairs the control of heart rate.     

The new wave of p75 neurotrophin receptor targeted therapies

Neurotrophins have been recognized for decades for their beneficial effects on growth,survival,and maintenance in the central nervous system,all of which suggest potential therapeutic utility.Although understanding and harnessing the activity of neurotrophins has proven difficult,the past several years have seen significant strides in the development of deliverable therapies that modulate neurotrophin activity(Shen et al.,2019;Yang et al.,2020;Xie et al.,2021).These recent studies have primarily focused on the multifunctional p75 neurotrophin receptor(p75NTR)which is upregulated in central nervous system disease and injury,thus offering a unique target for intervention.     

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.     

The p75 neurotrophin receptor regulates hippocampal neurogenesis and related behaviours

Although changes to neural circuitry are believed to underlie behavioural characteristics mediated by the hippocampus, the contribution of neurogenesis to this process remains controversial. This is partially because the molecular regulators of neurogenesis remain to be fully elucidated, and experiments generically preventing neurogenesis have, for the most part, depended on paradigms involving irradiation. Here we show that mice lacking the p75 neurotrophin receptor (p75^NTR−/−) have 25% fewer neuroblasts and 50% fewer newborn neurons in the dentate gyrus, coincident with increased rates of cell death of newly born cells and a significantly smaller granular cell layer and dentate gyrus, than those of p75^NTR+/+ mice. Whereas p75^NTR−/− mice had increased latency to feed in a novelty-suppressed feeding paradigm they had increased mobility in another test of "depression", the tail-suspension test. p75^NTR−/− mice also had subtle behavioural impairment in Morris water maze tasks compared to wild-type animals. No difference between genotypes was found in relation to anxiety or exploration behaviour based on the elevated-plus maze, light-dark, hole-board, T-maze or forced-swim tests. Overall, this study demonstrates that p75^NTR is an important regulator of hippocampal neurogenesis, with concomitant effects on associated behaviours. However, the behavioural attributes of the p75^NTR−/− mice may be better explained by altered circuitry driven by the loss of p75^NTR in the basal forebrain, rather than direct changes to neurogenesis.     

Expression of p75 neurotrophin receptor in the injured and regenerating rat retina.

The low affinity neurotrophin receptor (p75) has been suggested to be involved in apoptosis after neuronal injury. The tempo-spatial expression of p75 in the axotomized and regenerating retinas has not yet been determined. We examined the expression of p75 in the RGCs of these retinas using 192-IgG immunohistochemistry. The failure to detect p75 immunoreactivity on retrogradely fluorogold (FG)-labeled retinal ganglion cells (RGCs) in axotomized and regenerating adult retinas indicated that p75 was not expressed on RGCs. The low affinity p75 receptor was mainly localized on the Muller cell processes identified with vimentin antibody. Since p75 is not present in RGCs, the proposed pro-apoptotic role of p75 involved in the RGC death after optic nerve injury is unlikely to occur in rats.     

Co-expression of trkA and p75 neurotrophin receptor in extracranial olfactory neuroblastoma cells

Olfactory neuroblastoma (ON, esthesioneuroblastoma) is a high-grade malignant tumour of neuronal origin. Little is known about the neurobiological behaviour of this tumour. Ten cases of ON and five cases of nasopharyngeal carcinoma were examined for expression of trkA and p75 neurotrophin receptor (p75NTR) using immunohistochemistry and double labelling fluorescence. We found that all ON tissues from 10 cases expressed both trkA and p75NTR at different levels. Double staining revealed that almost all trkA-immunoreactive ON cells also contained p75NTR immunoreactivity. By contrast, no trkA or p75NTR immunoreactivity was detected in nasopharyngeal carcinoma cells from five patients. These results suggest that nerve growth factor may play a role in the generation of ON and staining of trkA and p75NTR may assist in the diagnosis of ON.     

Photic regulation of circadian rhythms and the expression of p75 neurotrophin receptor immunoreactivity in the suprachiasmatic nucleus in rats

Neurotrophic factors have been implicated in the mechanism underlying photic regulation of circadian rhythms in mammals. In rats, the most abundant neurotrophin receptor found in the suprachiasmatic nucleus (SCN), the circadian clock, is the low affinity p75 neurotrophin receptor (p75NTR). This receptor is expressed by retinal afferents of the SCN, but nothing is known about its role in photic regulation of circadian rhythms. We show here that neonatal treatment with the retinal neurotoxin, monosodium glutamate (MSG), which has no effect on photic entrainment of circadian rhythms, nearly completely abolished p75NTR immunoreactivity in the SCN in rats. These findings suggest that p75NTR from retinal sources do not play an essential role in the mechanism mediating photic entrainment of circadian rhythms in rats.     

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.