Dose and age-dependent axonal responses of embryonic trigeminal neurons to localized NGF via p75NTR receptor

Nerve growth factor (NGF) and related neurotrophins are target-derived survival factors for sensory neurons. In addition, these peptides modulate neuronal differentiation, axon guidance, and synaptic plasticity. We tested axonal behavior of embryonic trigeminal neurons towards localized sources of NGF in collagen gel assays. Trigeminal axons preferentially grow towards lower doses of localized NGF and grow away from higher concentrations at earlier stages of development, but do not show this response later. Dorsal root ganglion axons also show similar responses to NGF, but NGF-dependent superior cervical ganglion axons do not. Such axonal responses to localized NGF sources were also observed in Bax-/- mice, suggesting that the axonal effects are largely independent of cell survival. Immunocytochemical studies indicated that axons, which grow towards or away from localized NGF are TrkA-positive, and TrkA-/- TG axons do not respond to any dose of NGF. We further show that axonal responses to NGF are absent in TG derived from mice that lack the p75 neurotrophin receptor (p75NTR). Collectively, our results suggest that localized sources of NGF can direct axon outgrowth from trigeminal ganglion in a dose- and age-dependent fashion, mediated by p75NTR signaling through TrkA expressing axons.     

Injury‐induced spinal motor neuron apoptosis is preceded by DNA single‐strand breaks and is p53‐ and Bax‐dependent

The mechanisms of injury‐induced apoptosis of neurons within the spinal cord are not understood. We used a model of peripheral nerve‐spinal cord injury in the rat and mouse to induce motor neuron degeneration. In this animal model, unilateral avulsion of the sciatic nerve causes apoptosis of motor neurons. We tested the hypothesis that p53 and Bax regulate this neuronal apoptosis, and that DNA damage is an early upstream signal. Adult mice and rats received unilateral avulsions causing lumbar motor neurons to achieve endstage apoptosis at 7–14 days postlesion. This motor neuron apoptosis is blocked in bax^?/? and p53^?/? mice. Single‐cell gel electrophoresis (comet assay), immunocytochemistry, and quantitative immunogold electron microscopy were used to measure molecular changes in motor neurons during the progression of apoptosis. Injured motor neurons accumulate single‐strand breaks in DNA by 5 days. p53 accumulates in nuclei of motor neurons destined to undergo apoptosis. p53 is functionally activated by 4–5 days postlesion, as revealed by immunodetection of phosphorylated p53. Preapoptotically, Bax translocates to mitochondria, cytochrome c accumulates in the cytoplasm, and caspase‐3 is activated. These results demonstrate that motor neuron apoptosis in the adult spinal cord is controlled by upstream mechanisms involving DNA damage and activation of p53 and downstream mechanisms involving upregulated Bax and cytochrome c and their translocation, accumulation of mitochondria, and activation of caspase‐3. We conclude that adult motor neuron death after nerve avulsion is DNA damage‐induced, p53‐ and Bax‐dependent apoptosis. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 181–197, 2002; DOI 10.1002/neu.10026     

BMP7‐induced dendritic growth in sympathetic neurons requires p75NTR signaling

Dendritic morphology is a critical determinant of neuronal connectivity, and in postganglionic sympathetic neurons, tonic activity correlates directly with the size of the dendritic arbor. Thus, identifying signaling mechanisms that regulate dendritic arborization of sympathetic neurons is important to understanding how functional neural circuitry is established and maintained in the sympathetic nervous system. Bone morphogenetic proteins (BMPs) promote dendritic growth in sympathetic neurons; however, downstream signaling events that link BMP receptor activation to dendritic growth are poorly characterized. We previously reported that BMP7 upregulates p75^NTR mRNA in cultured sympathetic neurons. This receptor is implicated in controlling dendritic growth in central neurons but whether p75^NTR regulates dendritic growth in peripheral neurons is not known. Here, we demonstrate that BMP7 increases p75^NTR protein in cultured sympathetic neurons, and this effect is blocked by pharmacologic inhibition of signaling via BMP type I receptor. BMP7 does not trigger dendritic growth in sympathetic neurons dissociated from superior cervical ganglia (SCG) of p75^NTR nullizygous mice, and overexpression of p75^NTR in p75^NTR?/? neurons is sufficient to cause dendritic growth even in the absence of BMP7. Morphometric analyses of SCG from wild‐type versus p75^NTR nullizygous mice at 3, 6, and 12 to 16 weeks of age indicated that genetic deletion of p75^NTR does not prevent dendritic growth but does stunt dendritic maturation in sympathetic neurons. These data support the hypotheses that p75^NTR is involved in downstream signaling events that mediate BMP7‐induced dendritic growth in sympathetic neurons, and suggest that p75^NTR signaling positively modulates dendritic complexity in sympathetic neurons in vivo. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 76: 1003–1013, 2016     

Signaling of the neurotrophin receptor p75 in relation to Alzheimer’s disease

The cellular mechanism of neuronal apoptosis in Alzheimer’s disease (AD) is poorly understood. Many hypotheses have been put fourth to explain the underlying reason for neuro-degeneration in AD. Here, it is demonstrated that all neurotrophins that activated p75, without co-activation of the relevant Trk co-receptor, mediated apoptosis in hippocampal neurons. Thus, proneurotrophins and amyloid β peptides (Aβ) can induce p75-mediated apoptosis in hippocampal neurons since they do not bind or activate Trk receptors. Based on the combined effects of aging, proneurotrophins, neurotrophins, and Aβ, a novel model of pathogenesis in AD is proposed. This mini-review explores the ligand and cell type based signaling pathways of the neurotrophin receptor p75 relating to Alzheimer’s disease.     

Necdin and TrkA contribute to modulation by p75NTR of resistance to oxidant stress

The neurotrophin receptor p75NTR provides protection from oxidant stress induced by 6-hydroxydopamine (6-OHDA) and resultant cell death. In the absence of p75NTR, TrkA is upregulated and its signaling pathway effectors are increasingly activated. Necdin, a MAGE protein and known interactor of p75NTR and TrkA, is a potential mediator of this phenomenon. Decreased expression of necdin protein in p75NTR-deficient PC12 cells decreased TrkA expression and increased PC12 cell resistance to 6-OHDA. Inhibition of JNK phosphorylation by SP600125 also resulted in increased resistance to 6-OHDA, suggesting that TrkA signaling underlies the susceptibility of these cells to oxidant stress.     

Identification and kainic acid-induced up-regulation of low-affinity p75 neurotrophin receptor (p75NTR) in the nigral dopamine neurons of adult rats

Parkinson's disease is a common and severe debilitating neurological disease that results from massive and progressive degenerative death of dopamine neurons in the substantia nigra, but the mechanisms of neuronal degeneration and disease progression remains largely obscure. We are interested in possible implications of low-affinity p75 neurotrophin receptor (p75NTR), which may mediate neuronal apoptosis in the central nervous system, in triggering cell death of the nigral dopamine neurons. The RT-PCR and immunohistochemistry were carried out to detect if p75NTR is expressed in these nigral neurons and up-regulated by kainic acid (KA) insult in adult rats. It revealed p75NTR-positive immunoreactivity in the substantia nigra, and co-localization of p75NTR and tyrosine hydroxylase (TH) was found in a large number of substantia nigra neurons beside confirmation of p75NTR in the choline acetyltransferase (ChAT)-positive forebrain neurons. Cell count data further indicated that about 47-100% of TH-positive nigral neurons and 98-100% of ChAT-positive forebrain neurons express p75NTR. More interestingly, significant increasing in both p75NTR mRNA and p75NTR-positive neurons occurred rapidly following KA insult in the substantia nigra of animal model. The present study has provided first evidence on p75NTR expression and KA-inducing p75NTR up-regulation in substantia nigra neurons in rodent animals. Taken together with previous data on p75NTR functions in neuronal apoptosis, this study also suggests that p75NTR may play important roles in neuronal cell survival or excitotoxic degeneration of dopamine neurons in the substantia nigra in pathogenesis of Parkinson's disease in human beings.     

p75 Neurotrophin receptor mediates neurotrophin activation of NF‐kappa B and induction of iNOS expression in P19 neurons

P19 embryonic carcinoma cells can be differentiated into neurons that form synaptic connections and that produce a variety of neurotransmitters. Results of RT‐PCR indicate that P19 neurons express several neurotrophin receptors (p75^NTR, trkB, and trkC, but not trkA) but they do not express any of the four neurotrophins. Consistent with the presence of trkB but not trkA, BDNF causes rapid phosphorylation of MAP kinases ERK1 and ERK2, but NGF does not. Neurotrophins induce translocation of NF‐κB into the nucleus. All four neurotrophins induce activation of NF‐κB in a biphasic manner. This effect is apparently mediated by p75^NTR, because an inhibitor of trk receptors, K252a, does not inhibit activation of NF‐κB. Instead, K252a itself promotes activation of NF‐κB and this effect is additive with the effect of neurotrophins. Inhibition of reactive oxygen intermediates with PDTC completely abolishes basal activity of NF‐κB and strongly inhibits activation of NF‐κB by neurotrophins, indicating an important role of reactive oxygen intermediates in the pathway by which neurotrophins activate NF‐κB. NF‐κB is known to promote expression of the iNOS gene. We found that all four neurotrophins increased iNOS mRNA levels, resulting in increased accumulation of iNOS protein. In contrast, none of the neurotrophins stimulated nNOS mRNA or protein synthesis. PDTC abolishes constitutive and neurotrophin‐induced expression of iNOS mRNA and protein and abolishes constitutive expression of nNOS mRNA, suggesting that reactive oxygen intermediates promote expression of nNOS. © 2003 Wiley Periodicals, Inc. J Neurobiol 55: 191–203, 2003     

Analysis of retrograde transport in motor neurons reveals common endocytic carriers for tetanus toxin and neurotrophin receptor p75NTR.

Axonal retrograde transport is essential for neuronal growth and survival. However, the nature and dynamics of the membrane compartments involved in this process are poorly characterized. To shed light on this pathway, we established an experimental system for the visualization and the quantitative study of retrograde transport in living motor neurons based on a fluorescent fragment of tetanus toxin (TeNT HC). Morphological and kinetic analysis of TeNT HC retrograde carriers reveals two major groups of organelles: round vesicles and fast tubular structures. TeNT HC carriers lack markers of the classical endocytic pathway and are not acidified during axonal transport. Importantly, TeNT HC and NGF share the same retrograde transport organelles, which are characterized by the presence of the neurotrophin receptor p75NTR. Our results provide the first direct visualization of retrograde transport in living motor neurons, and reveal a novel retrograde route that could be used both by physiological ligands (i.e., neurotrophins) and TeNT to enter the central nervous system.     

Temporally restricted death and the role of p75NTR as a survival receptor in the developing sensory nervous system

The peripheral somatosensory system overproduces neurons early in development followed by a period of cell death during final target innervation. The decision to survive or die in somatosensory neurons of the dorsal root ganglion (DRG) is mediated by target‐derived neurotrophic factors and their cognate receptors. Subsets of peripheral somatosensory neurons can be crudely defined by the neurotrophic receptors that they express: peptidergic nociceptors (TrkA+), nonpeptidergic nociceptors (Ret+), mechanoreceptors (Ret+ or TrkB+), and proprioceptors (TrkC+). A direct comparison of early developmental timing between these subsets has not been performed. Here we characterized the accumulation and death of TrkA, B, C, and Ret+ neurons in the DRG as a function of developmental time. We find that TrkB, TrkC, and Ret‐expressing neurons in the DRG complete developmental cell death prior to TrkA‐expressing neurons. Given the broadly defined roles of the neurotrophin receptor p75NTR in augmenting neurotrophic signaling in sensory neurons, we investigated its role in supporting the survival of these distinct subpopulations. We find that TrkA+, TrkB+, and TrkC+ sensory neuron subpopulations require p75NTR for survival, but proliferating progenitors do not. These data demonstrate how diverging sensory neurons undergo successive waves of cell death and how p75NTR represses the magnitude, but not developmental window of this culling. © 2018 Wiley Periodicals, Inc. Develop Neurobiol 78: 701–717, 2018     

Age‐related changes in hippocampal‐dependent synaptic plasticity and memory mediated by p75 neurotrophin receptor

The plasticity mechanisms in the nervous system that are important for learning and memory are greatly impacted during aging. Notably, hippocampal‐dependent long‐term plasticity and its associative plasticity, such as synaptic tagging and capture (STC), show considerable age‐related decline. The p75 neurotrophin receptor (p75^NTR) is a negative regulator of structural and functional plasticity in the brain and thus represents a potential candidate to mediate age‐related alterations. However, the mechanisms by which p75^NTR affects synaptic plasticity of aged neuronal networks and ultimately contribute to deficits in cognitive function have not been well characterized. Here, we report that mutant mice lacking the p75^NTR were resistant to age‐associated changes in long‐term plasticity, associative plasticity, and associative memory. Our study shows that p75^NTR is responsible for age‐dependent disruption of hippocampal homeostatic plasticity by modulating several signaling pathways, including BDNF, MAPK, Arc, and RhoA‐ROCK2‐LIMK1‐cofilin. p75^NTR may thus represent an important therapeutic target for limiting the age‐related memory and cognitive function deficits.     

The p75 neurotrophin receptor is localized to primary cilia in adult murine hippocampal dentate gyrus granule cells

The densely ciliated granule cell layer of the adult murine hippocampal dentate gyrus is one of two sites of adult neurogenesis. The granule cells have already been proven to localize their SSTR3 (somatostatin receptor 3) receptors to their so-called primary cilia. Here we show for the first time that 70-90% of these cells in 7-18 months-old wild-type and 3×Tg-AD (Alzheimer disease transgenic) mice also load p75^NTR receptors into the structures containing SSTR3, i.e., their primary cilia. On the other hand, p75^NTR’s TrkA co-receptors were not localized to cilia but conventionally distributed throughout the cell surface. Significantly fewer cells (20-40%) in the hippocampal CA1 and CA3 regions and cerebral cortex have p75^NTR containing cilia. While we don’t know what the impact of the cilial localization of p75^NTR on dentate gyral adult neurogenesis and memory encoding might be, the cilia’s amyloid β-activatable p75^NTR receptors could be damaging or lethal to the hippocampal functioning of amyloid β-accumulating Alzheimer brain.     

Distinct thyroid hormone‐dependent expression of trkB and p75NGFR in nonneuronal cells during the critical TH‐dependent period of the cochlea

Analyzing the thyroid hromone (TH)‐dependent period of the inner ear, we observed that the presence of triiodothyronine (T3) between postnatal day 3 (P3) and P12 is sufficient for functional maturation of the auditory system. Within this short time period, an unusual transient TH‐dependent expression of nonneuronal neurotrophin receptors (NT‐R) trkB and p75^NGFR was observed in correlation with neuronal and morphogenetic processes. The availability of thyroid hormone was revealed to be invariably correlated with (a) a transient expression of full‐length trkB in TRα1‐, TRα2‐ and TRβ1‐expressing hair cells concomitant to the segregation of afferent fibers and the synaptogenesis of efferent fibers; and (b) a transient expression of p75^NGFR in TRα1‐ and TRβ1‐expressing great epithelia ridge cells in direct spatiotemporal correlation with the appearance of apoptotic cells and morphogenetic maturation of the organ. For the first time, these data suggest a TH dependency of the expression of neurotrophin receptors in nonneuronal cells. A potential role of these peculiar neurotrophin receptor expression for the conversion of the biological function of TH on innervation patterning and morphogenesis during the critical TH‐dependent period of the inner ear may be considered. © 1999 John Wiley & Sons, Inc. J Neurobiol 38: 338–356, 1999     

Amyloid beta1–42 (Aβ42) up‐regulates the expression of sortilin via the p75NTR/RhoA signaling pathway

Sortilin, a Golgi sorting protein and a member of the VPS10P family, is the co‐receptor for proneurotrophins, regulates protein trafficking, targets proteins to lysosomes, and regulates low density lipoprotein metabolism. The aim of this study was to investigate the expression and regulation of sortilin in Alzheimer's disease (AD). A significantly increased level of sortilin was found in human AD brain and in the brains of 6‐month‐old swedish‐amyloid precursor protein/PS1dE9 transgenic mice. Aβ₄₂ enhanced the protein and mRNA expression levels of sortilin in a dose‐ and time‐dependent manner in SH‐SY5Y cells, but had no effect on sorLA. In addition, proBDNF also significantly increased the protein and mRNA expression of sortilin in these cells. The recombinant extracellular domain of p75^NTR (P75ECD‐FC), or the antibody against the extracellular domain of p75^NTR, blocked the up‐regulation of sortilin induced by Amyloid‐β protein (Aβ), suggesting that Aβ₄₂ increased the expression level of sortilin and mRNA in SH‐SY5Y via the p75^NTR receptor. Inhibition of ROCK, but not Jun N‐terminal kinase, suppressed constitutive and Aβ₄₂‐induced expression of sortilin. In conclusion, this study shows that sortilin expression is increased in the AD brain in human and mice and that Aβ₄₂ oligomer increases sortilin gene and protein expression through p75^NTR and RhoA signaling pathways, suggesting a potential physiological interaction of Aβ₄₂ and sortilin in Alzheimer's disease.

     

p75 neurotrophin receptor is involved in proliferation of undifferentiated mouse embryonic stem cells

Neurotrophins and their receptors are known to play a role in the proliferation and survival of many different cell types of neuronal and non-neuronal lineages. In addition, there is much evidence in the literature showing that the p75 neurotrophin receptor (p75^NTR), alone or in association with members of the family of Trk receptors, is expressed in a wide variety of stem cells, although its role in such cells has not been completely elucidated. In the present work we have investigated the expression of p75^NTR and Trks in totipotent and pluripotent cells, the mouse pre-implantation embryo and embryonic stem and germ cells (ES and EG cells). p75^NTR and TrkA can be first detected in the blastocyst from which ES cell lines are derived. Mouse ES cells retain p75^NTR/TrkA expression. Nerve growth factor is the only neurotrophin able to stimulate ES cell growth in culture, without affecting the expression of stem cell markers, alkaline phosphatase, Oct4 and Nanog. Such proliferation effect was blocked by antagonizing either p75^NTR or TrkA. Interestingly, immunoreactivity to anti-p75^NTR antibodies is lost upon ES cell differentiation. The expression pattern of neurotrophin receptors in murine ES cells differs from human ES cells, that only express TrkB and C, and do not respond to NGF. In this paper we also show that, while primordial germ cells (PGC) do not express p75^NTR, when they are made to revert to an ES-like phenotype, becoming EG cells, expression of p75^NTR is turned on.     

Effect of p75NTR on the regulation of naturally occurring cell death and retinal ganglion cell number in the mouse eye

Neurotrophins induce neural cell survival and differentiation during retinal development and regeneration through the high-affinity tyrosine kinase (Trk) receptors. On the other hand, nerve growth factor (NGF) binding to the low-affinity neurotrophin receptor p75 (p75(NTR)) might induce programmed cell death (PCD) in the early phase of retinal development. In the present study, we examined the retinal cell types that experience p75(NTR)-induced PCD and identify them to be postmitotic retinal ganglion cells (RGCs). However, retinal morphology, RGC number, and BrdU-positive cell number in p75(NTR) knockout (KO) mouse were normal after embryonic day 15 (E15). In chick retina, migratory RGCs express p75(NTR), whereas layered RGCs express the high-affinity NGF receptor TrkA, which may switch the pro-apoptotic signaling of p75(NTR) into a neurotrophic one. In contrast to the chick model, migratory RGCs express TrkA, while stratified RGCs express p75(NTR) in mouse retina. However, RGC number in TrkA KO mouse was also normal at birth. We next examined the expression of transforming growth factor beta (TGFbeta) receptor, which modulates chick RGC number in combination with p75(NTR), but was absent in mouse RGCs. p75(NTR) and TrkA seem to be involved in the regulation of mouse RGC number in the early phase of retinal development, but the number may be later adjusted by other molecules. These results suggest the different mechanism of RGC number control between mouse and chick retina.