A pro-apoptotic fragment of the p75 neurotrophin receptor is expressed in p75NTRExonIV null mice

Deletion of p75NTR rescues behavioral and cognitive dysfunction in SPS-induced PTSD mice through hippocampal PI3K/Akt/mTOR pathway

Post-traumatic stress disorder (PTSD) is a persistent mental illness caused by severe traumatic events, and its pathogenesis is still unclear. Recent studies indicate that p75 neurotrophic factor receptor (p75NTR) plays a crucial role in neurological diseases, but the role of p75NTR in PTSD is currently unknown. To investigate the effects and mechanisms of p75NTR in PTSD, in this study, a functional p75NTR-deficient mouse was used to establish a PTSD model by single prolonged stress (SPS) paradigm, then the behavioral effects and underlying mechanisms were further investigated. The results demonstrated that p75NTR deletion alleviated anxiety-like behavior and spatial learning and memory impairment in SPS-induced PTSD mice. Further study indicated that deletion of p75NTR downregulated the expression of apoptosis (Bax) and autophagy (Beclin-1) related proteins in the hippocampus of PTSD mice, protected against hippocampal neuronal damage, upregulated the expression of synaptic-related proteins (PSD95 and Synapsin I), increased dendritic complexity and dendritic spine density, and improved synaptic plasticity through the PI3K/Akt/mTOR pathway. In conclusion, deletion of p75NTR rescues behavioral and cognitive dysfunction through PI3K/Akt/mTOR pathway mediated regulation of hippocampal autophagy, apoptosis and synaptic plasticity in SPS-induced PTSD mice, which provides a potential therapeutic target for the treatment of PTSD.

Redundancy of p75NTR neurotrophin receptor function in development, growth and fertility in the rat

The p75NTR neurotrophin receptor has positive and negative roles regulating cell survival in the nervous system. Unambiguous interpretation of p75NTR function in vivo has been complicated, however, by residual expression of alternate forms of p75NTR protein in initial p75NTR knock-out mouse models. As rats are the preferred rodent for studying brain and behaviour, and to simplify interpretation of the knock-out phenotype, we report here the generation of a mutant rat devoid of the p75NTR protein. TALEN-mediated recombination in embryonic stem cells (ESCs) was used to flank exon 2 of p75NTR with Lox P sites and produce transgenic rats carrying either un-recombined floxed p75NTREx2-fl, or recombined, exon-2 deleted p75NTREx2-Δ alleles. Crossing p75NTREx2-fl rats with a Cre-deleter strain efficiently removed exon 2 in vivo. Excision of exon 2 causes a frameshift after p75NTR Gly23 and eliminated p75NTR protein expression. Rats lacking p75NTR were healthy, fertile, and histological analysis did not reveal significant changes in cellular density or overall structure in their brains. p75NTR function is therefore largely dispensable for normal development, growth and basal homeostasis in the rat. However, the availability of constitutive and conditional p75NTREx2-Δ rats provides new opportunities to investigate specific roles of p75NTR upon injury and during tissue repair.

Deletion of p75NTR rescues the synaptic but not the inflammatory status in the brain of a mouse model for Alzheimer's disease

Introduction

Alzheimer's disease (AD), is characterized by a gradual cognitive decline associated with the accumulation of Amyloid beta (Aβ)-oligomers, progressive neuronal degeneration and chronic neuroinflammation. Among the receptors shown to bind and possibly transduce the toxic effects of Aβ-oligomers is the p75 neurotrophin receptor (p75^NTR). Interestingly, p75^NTR mediates several crucial processes in the nervous system, including neuronal survival and apoptosis, maintenance of the neuronal architecture, and plasticity. Furthermore, p75^NTR is also expressed in microglia, the resident immune cells of the brain, where it is markedly increased under pathological conditions. These observations indicate p75^NTR as a potential candidate for mediating Aβ-induced toxic effects at the interface between the nervous and the immune system, thereby potentially participating in the crosstalk between these two systems.

Methods

Here we used APP/PS1 transgenic mice (APP/PS1tg) and compared the Aβ-induced alterations in neuronal function, chronic inflammation as well as their cognitive consequences between 10 months old APP/PS1tg and APP/PS1tg x p75^NTRexonIV knockout mice.

Results

Electrophysiological recordings show that a loss of p75^NTR rescues the impairment in long-term potentiation at the Schaffer collaterals in the hippocampus of APP/PS1tg mice. Interestingly, however loss of p75^NTR does not influence the severity of neuroinflammation, microglia activation or the decline in spatial learning and memory processes observed in APP/PS1tg mice.

Conclusion

Together these results indicate that while a deletion of p75^NTR rescues the synaptic defect and the impairment in synaptic plasticity, it does not affect the progression of the neuroinflammation and the cognitive decline in a mouse model for AD.

Disentangling the signaling complexity of nerve growth factor receptors by CRISPR /Cas9

The binding of nerve growth factor (NGF) to the tropomyosin-related kinase A (TrkA) and p75^NTR receptors activates a large variety of pathways regulating critical processes as diverse as proliferation, differentiation, membrane potential, synaptic plasticity, and pain. To ascertain the details of TrkA-p75^NTR interaction and cooperation, a plethora of experiments, mostly based on receptor overexpression or downregulation, have been performed. Among the heterogeneous cellular systems used for studying NGF signaling, the PC12 pheochromocytoma-derived cell line is a widely used model. By means of CRISPR/Cas9 genome editing, we created PC12 cells lacking TrkA, p75^NTR , or both. We found that TrkA-null cells become unresponsive to NGF. Conversely, the absence of p75^NTR enhances the phosphorylation of TrkA and its effectors. Using a patch-clamp, we demonstrated that the individual activation of TrkA and p75^NTR by NGF results in antagonizing effects on the membrane potential. These newly developed PC12 cell lines can be used to investigate the specific roles of TrkA and p75^NTR in a genetically defined cellular model, thus providing a useful platform for future studies and further gene editing.

Sexual Dimorphism in Balance and Coordination in p75NTRexonIII Knock-Out Mice

The p75 neurotrophin receptor (p75NTR) is implicated in various biological functions during development and adulthood. Several animal models have been developed to identify the roles of p75NTR in vivo and in vitro. P75NTR^ExonIII knock-out mice are widely used to study the neurotrophin receptor and its signaling pathways. Similar to other models of p75NTR knock-out (p75NTR^{Exon IV} KO) or conditional knock-out (p75NTR^fl/fl) mice, p75NTR^ExonIII knock-out mice present severe abnormalities in walking, gait, balance and strength. The present study identifies a sexual dimorphism in the p75NTR^ExonIII knock-out strain regarding balance and coordination. Using Kondziela’s inverted grid test, we observed that p75NTR^ExonIII knock-out males performed poorly at the task, whereas p75NTR^ExonIII knock-out females did not exhibit any defects. We also observed that female p75NTR^ExonIII knock-out mice performed significantly better than male p75NTR^ExonIII knock-out mice at the beam balance test. There were no differences in strength, skin innervation, or the number of ulcers on the toes between p75NTR^ExonIII knock-out males and females. The literature regarding the role of p75NTR in behavior is controversial; our results suggest that studies investigating the role of p75NTR in vivo using p75NTR knock-out mice should systematically report data from males and females.

Brain-derived neurotrophic factor precursor in the immune system is a novel target for treating multiple sclerosis

Rationale: Brain-derived neurotrophic factor precursor (proBDNF) is expressed in the central nervous system (CNS) and the immune system. However, the role of proBDNF in the pathogenesis of multiple sclerosis (MS) is unknown.

Methods: Peripheral blood and post-mortem brain and spinal cord specimens were obtained from multiple sclerosis patients to analyze proBDNF expression in peripheral lymphocytes and infiltrating immune cells in the lesion site. The proBDNF expression profile was also examined in the experimental autoimmune encephalomyelitis (EAE) mouse model, and polyclonal and monoclonal anti-proBDNF antibodies were used to explore their therapeutic effect in EAE. Finally, the role of proBDNF in the inflammatory immune activity of peripheral blood mononuclear cells (PBMCs) was verified in vitro experiments.

Results: High proBDNF expression was detected in the circulating lymphocytes and infiltrated inflammatory cells at the lesion sites of the brain and spinal cord in MS patients. In the EAE mouse model, proBDNF was upregulated in CNS and in circulating and splenic lymphocytes. Systemic but not intracranial administration of anti-proBDNF blocking antibodies attenuated clinical scores, limited demyelination, and inhibited proinflammatory cytokines in EAE mice. Immuno-stimulants treatment increased the proBDNF release and upregulated the expression of p75 neurotrophic receptors (p75^NTR) in lymphocytes. The monoclonal antibody against proBDNF inhibited the inflammatory response of PBMCs upon stimulations.

Conclusion: The findings suggest that proBDNF from immune cells promotes the immunopathogenesis of MS. Monoclonal Ab-proB may be a promising therapeutic agent for treating MS.

Saikosaponin-d impedes hippocampal neurogenesis and causes cognitive deficits by inhibiting the survival of neural stem/progenitor cells via neurotrophin receptor signaling in mice

Neural stem/progenitor cells (NPCs) are multipotent stem cells in the central nervous system. Damage to NPCs has been demonstrated to cause adverse effects on neurogenesis and to contribute to neurological diseases. Our previous research suggested that saikosaponin-d (SSd), a cytostatic drug belonging to the bioactive triterpenoid saponins, exhibited neurotoxicity by inhibiting hippocampal neurogenesis, but the underlying mechanism remained elusive. This study was performed to clarify the role of SSd in cognitive function and the mechanism by which SSd induced damage to hippocampal neurogenesis and NPCs. Our results indicated that SSd caused hippocampus-dependent cognitive deficits and inhibited hippocampal neurogenesis by reducing the numbers of newborn neurons in mice. RNA sequencing analysis revealed that SSd-induced neurotoxicity in the hippocampus involved neurotrophin receptor-interacting MAGE (NRAGE)/neurotrophin receptor interacting factor (NRIF)/p75NTR -associated cell death executor (NADE) cell signaling activated by the p75 neurotrophin receptor (p75NTR ). Mechanistic studies showed that a short hairpin RNA targeting p75NTR intracellular domain reversed SSd-increased NRAGE/NRIF/NADE signaling and the c-Jun N-terminal kinase/caspase apoptotic pathway, subsequently contributing to the survival of NPCs, as well as cell proliferation and differentiation. The addition of recombinant brain-derived neurotrophic factor (BDNF) ameliorated the SSd-induced inhibition of BDNF/Tyrosine kinase receptor B (TrkB) neurotrophic signaling, but did not affect SSd-activated pro-BDNF/p75NTR signaling. Moreover, the SSd-induced elevation of cytosolic Ca2+ concentration was responsible for damage to NPCs. The extracellular Ca2+ chelator ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid (EGTA), rather than the intracellular Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester) (BAPTA/AM), attenuated SSd-induced cytosolic Ca2+ dysfunction and SSd-disordered TrkB/p75NTR signaling. Overall, this study demonstrated a new mechanism for the neurotoxic effect of SSd, which has emerging implications for pharmacological research of SSd and provides a better understanding of neurotoxicity induced by cytostatic drugs.

Partial deletion of p75NTR in large-diameter DRG neurons exerts no influence upon the survival of peripheral sensory neurons in vivo

The p75 neurotrophin receptor (p75NTR ) is required for maintaining peripheral sensory neuron survival and function; however, the underlying cellular mechanism remains unclear. The general view is that expression of p75NTR by the neuron itself is required for maintaining sensory neuron survival and myelination in the peripheral nervous system (PNS). Adopting a neuronal-specific conditional knockout strategy, we demonstrate the partial depletion of p75NTR in neurons exerts little influence upon maintaining sensory neuron survival and peripheral nerve myelination in health and after demyelinating neuropathy. Our data show that the density and total number of dorsal root ganglion (DRG) neurons in 2-month-old mice is not affected following the deletion of p75NTR in large-diameter myelinating neurons, as assessed by stereology. Adopting experimental autoimmune neuritis induced in adult male mice, an animal model of demyelinating peripheral neuropathy, we identify that deleting p75NTR in myelinating neurons exerts no influence upon the disease progression, the total number of DRG neurons, and the extent of myelin damage in the sciatic nerve, indicating that the expression of neuronal p75NTR is not essential for maintaining peripheral neuron survival and myelination after a demyelinating insult in vivo. Together, results of this study suggest that the survival and myelination of peripheral sensory neurons is independent of p75NTR expressed by a subtype of neurons in vivo. Thus, our findings provide new insights into the mechanism underpinning p75NTR -mediated neuronal survival in the PNS.

Peripheral Nerve Regeneration Is Independent From Schwann Cell p75NTR Expression

Schwann cell reprogramming and differentiation are crucial prerequisites for neuronal regeneration and re-myelination to occur following injury to peripheral nerves. The neurotrophin receptor p75NTR has been identified as a positive modulator for Schwann cell myelination during development and implicated in promoting nerve regeneration after injury. However, most studies base this conclusion on results obtained from complete p75NTR knockout mouse models and cannot dissect the specific role of p75NTR expressed by Schwann cells. In this present study, a conditional knockout model selectively deleting p75NTR expression in Schwann cells was generated, where p75NTR expression is replaced with that of an mCherry reporter. Silencing of Schwann cell p75NTR expression was confirmed in the sciatic nerve in vivo and in vitro, without altering axonal expression of p75NTR. No difference in sciatic nerve myelination during development or following sciatic nerve crush injury was observed, as determined by quantification of both myelinated and unmyelinated nerve fiber densities, myelinated axonal diameter and myelin thickness. However, the absence of Schwann cell p75NTR reduced motor nerve conduction velocity after crush injury. Our data indicate that the absence of Schwann cell p75NTR expression in vivo is not critical for axonal regrowth or remyelination following sciatic nerve crush injury, but does play a key role in functional recovery. Overall, this represents the first step in redefining the role of p75NTR in the peripheral nervous system, suggesting that the Schwann cell-axon unit functions as a syncytium, with the previous published involvement of p75NTR in remyelination most likely depending on axonal/neuronal p75NTR and/or mutual glial-axonal interactions.

Regulation of Neurogenesis by Neurotrophins during Adulthood: Expected and Unexpected Roles

The subventricular zone (SVZ) of the anterolateral ventricle and the subgranular zone (SGZ) of the hippocampal dentate gyrus are the two main regions of the adult mammalian brain in which neurogenesis is maintained throughout life. Because alterations in adult neurogenesis appear to be a common hallmark of different neurodegenerative diseases, understanding the molecular mechanisms controlling adult neurogenesis is a focus of active research. Neurotrophic factors are a family of molecules that play critical roles in the survival and differentiation of neurons during development and in the control of neural plasticity in the adult. Several neurotrophins and neurotrophin receptors have been implicated in the regulation of adult neurogenesis at different levels. Here, we review the current understanding of neurotrophin modulation of adult neurogenesis in both the SVZ and SGZ. We compile data supporting a variety of roles for neurotrophins/neurotrophin receptors in different scenarios, including both expected and unexpected functions.

Deletion of p75NTR enhances the cholinergic innervation pattern of the visual cortex

The cholinergic system is involved in cortical plasticity, attention, and learning. Within the visual cortex the cholinergic system seems to play a role in visual perception. The cholinergic neurons which project into the visual cortex are located in the basal forebrain. It has been shown that mice deficient for the low-affinity neurotrophin receptor p75NTR display increased numbers of cholinergic neurons in the basal forebrain and a denser cholinergic innervation of the hippocampus. This prompted us to analyze whether the cholinergic system is altered in adult p75NTR deficient mice. By analyzing the densities of cholinergic fibers within layer IV as well as within layer V of the visual cortex, we found that adult p75NTR deficient mice display increased cholinergic fiber densities. However, this increase was not accompanied by an increase in the density of local cholinergic neurons within the visual cortex. This indicates that the enhanced cholinergic innervation of the visual cortex is due to alteration of the cholinergic neurons located in the basal forebrain, projecting to the visual cortex. The increased cholinergic innervation of the visual cortex makes the p75NTR deficient mice an attractive model to study the necessity of the cholinergic system for the visual cortex.

L-type Calcium Channel Cav1.2 Is Required for Maintenance of Auditory Brainstem Nuclei

Cav1.2 and Cav1.3 are the major L-type voltage-gated Ca(2+) channels in the CNS. Yet, their individual in vivo functions are largely unknown. Both channel subunits are expressed in the auditory brainstem, where Cav1.3 is essential for proper maturation. Here, we investigated the role of Cav1.2 by targeted deletion in the mouse embryonic auditory brainstem. Similar to Cav1.3, loss of Cav1.2 resulted in a significant decrease in the volume and cell number of auditory nuclei. Contrary to the deletion of Cav1.3, the action potentials of lateral superior olive (LSO) neurons were narrower compared with controls, whereas the firing behavior and neurotransmission appeared unchanged. Furthermore, auditory brainstem responses were nearly normal in mice lacking Cav1.2. Perineuronal nets were also unaffected. The medial nucleus of the trapezoid body underwent a rapid cell loss between postnatal days P0 and P4, shortly after circuit formation. Phosphorylated cAMP response element-binding protein (CREB), nuclear NFATc4, and the expression levels of p75NTR, Fas, and FasL did not correlate with cell death. These data demonstrate for the first time that both Cav1.2 and Cav1.3 are necessary for neuronal survival but are differentially required for the biophysical properties of neurons. Thus, they perform common as well as distinct functions in the same tissue.

The role of p75NTR in cholinergic basal forebrain structure and function

The role of the p75 neurotrophin receptor (p75(NTR)) in adult cholinergic basal forebrain (cBF) neurons is unclear due to conflicting results from previous studies and to limitations of existing p75(NTR)-knock-out mouse models. In the present study we used a novel conditional knock-out line (ChAT-cre p75(in/in)) to assess the role of p75(NTR) in the cBF by eliminating p75(NTR) in choline acetyl-transferase-expressing cells. We show that the absence of p75(NTR) results in a lasting increase in cBF cell number, cell size, and cholinergic innervation to the cortex. Analysis of adult ChAT-cre p75(in/in) mice revealed that mutant animals show a similar loss of cBF neurons with age to that observed in wild-type animals, indicating that p75(NTR) does not play a significant role in mediating this age-related decline in cBF neuronal number. However, the increased cholinergic axonal innervation of the cortex, but not the hippocampus, corresponded to alterations in idiothetic but not allothetic navigation. These findings support a role for p75(NTR)-mediated regulation of cholinergic-dependent cognitive function, and suggest that the variability in previous reports of cBF neuron number may stem from limited spatial and temporal control of p75(NTR) expression in existing knock-out models.

The biology of neurotrophins: cardiovascular function

This chapter addresses the role of neurotrophins in the development of the heart, blood vessels, and neural circuits that control cardiovascular function, as well as the role of neurotrophins in the mature cardiovascular system. The cardiovascular system includes the heart and vasculature whose functions are tightly controlled by the nervous system. Neurons, cardiomyocytes, endothelial cells, vascular smooth muscle cells, and pericytes are all targets for neurotrophin action during development. Neurotrophin expression continues throughout life, and several common pathologies that impact cardiovascular function involve changes in the expression or activity of neurotrophins. These include atherosclerosis, hypertension, diabetes, acute myocardial infarction, and heart failure. In many of these conditions, altered expression of neurotrophins and/or neurotrophin receptors has direct effects on vascular endothelial and smooth muscle cells in addition to effects on nerves that modulate vascular resistance and cardiac function. This chapter summarizes the effects of neurotrophins in cardiovascular physiology and pathophysiology.

Glycyrrhizin improves p75NTR-associated sciatic nerve regeneration in a BALB/c mouse model

Glycyrrhizin has a role in immune regulation in the central nervous system, but its impact on sciatic nerve injury had not previously been reported. In this study, a BALB/c mouse model of sciatic nerve injury was used to explore the role of glycyrrhizin in sciatic nerve repair and its underlying mechanism. Glycyrrhizin with intragastric gavage of 10 and 20 mg/kg weight per day (mid- and high-dose, respectively) inhibited p75 neurotrophin receptor (p75NTR) expression at the protein and mRNA levels versus the 5 mg/kg (low-dose) group and control (0.9% NaCl solution) at one, two, four and eight weeks following sciatic nerve injury, and simultaneously improved the action potential amplitude and motor nerve conductive velocity. Combined Marsland, Glees and Erikson’s silver stain and Luxol fast blue staining results indicated that high- and mid-dose glycyrrhizin promoted improved sciatic nerve myelination compared with the low-dose or control groups eight weeks after injury. Immunofluorescence staining demonstrated that glycyrrhizin had an inhibitory effect to a certain degree on local hypertrophic scar and inflammatory responses in the mouse model. In conclusion, glycyrrhizin can promote sciatic nerve regeneration and functional repair, in which doses of 10 and 20 mg/kg per day are more effective than lower doses, and such regeneration is associated with the downregulation of p75NTR.

The biological functions and signaling mechanisms of the p75 neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including programmed cell death, axonal growth and degeneration, cell proliferation, myelination, and synaptic plasticity. The multiplicity of cellular functions governed by the receptor arises from the variety of ligands and co-receptors which associate with p75(NTR) and regulate its signaling. P75(NTR) promotes survival through interactions with Trk receptors, inhibits axonal regeneration via partnerships with Nogo receptor (Nogo-R) and Lingo-1, and promotes apoptosis through association with Sortilin. Signals downstream of these interactions are further modulated through regulated intramembrane proteolysis (RIP) of p75(NTR) and by interactions with numerous cytosolic partners. In this chapter, we discuss the intricate signaling mechanisms of p75(NTR), emphasizing how these signals are differentially regulated to mediate these diverse cellular functions.

Nerve growth factor-induced cell cycle reentry in newborn neurons is triggered by p38MAPK-dependent E2F4 phosphorylation

Cumulative evidence indicates that activation of cyclin D-dependent kinase 4/6 (cdk4/6) represents a major trigger of cell cycle reentry and apoptosis in vertebrate neurons. We show here the existence of another mechanism triggering cell cycle reentry in differentiating chick retinal neurons (DCRNs), based on phosphorylation of E2F4 by p38(MAPK). We demonstrate that the activation of p75(NTR) by nerve growth factor (NGF) induces nuclear p38(MAPK) kinase activity, which leads to Thr phosphorylation and subsequent recruitment of E2F4 to the E2F-responsive cdc2 promoter. Inhibition of p38(MAPK), but not of cdk4/6, specifically prevents NGF-dependent cell cycle reentry and apoptosis in DCRNs. Moreover, a constitutively active form of chick E2F4 (Thr261Glu/Thr263Glu) stimulates G(1)/S transition and apoptosis, even after inhibition of p38(MAPK) activity. In contrast, a dominant-negative E2F4 form (Thr261Ala/Thr263Ala) prevents NGF-induced cell cycle reactivation and cell death in DCRNs. These results indicate that NGF-induced cell cycle reentry in neurons depends on the activation of a novel, cdk4/6-independent pathway that may participate in neurodegeneration.

Suppression of p75 neurotrophin receptor surface expression with intrabodies influences Bcl-xL mRNA expression and neurite outgrowth in PC12 cells

BACKGROUND

Although p75 neurotrophin receptor (p75NTR) is the first neurotrophin receptor isolated, its diverse physiological functions and signaling have remained elusive for many years. Loss-of-function phenotypic analyses for p75NTR were mainly focused at the genetic level; however these approaches were impacted by off-target effect, insufficient stability, unspecific stress response or alternative active splicing products. In this study, p75NTR surface expression was suppressed for the first time at the protein level by endoplasmic reticulum (ER) retained intrabodies.

RESULTS

Three monoclonal recombinant antibody fragments (scFv) with affinities in the low nanomolar range to murine p75NTR were isolated by antibody phage display. To suppress p75NTR cell surface expression, the encoding genes of these scFvs extended by the ER retention peptide KDEL were transiently transfected into the neuron-like rat pheochromocytoma cell line PC12 and the mouse neuroblastoma x mouse spinal cord hybrid cell line NSC19. The ER retained intrabody construct, SH325-G7-KDEL, mediated a downregulation of p75NTR cell surface expression as shown by flow cytometry. This effect was maintained over a period of at least eight days without activating an unfolded protein response (UPR). Moreover, the ER retention of p75NTR resulted in downregulation of mRNA levels of the anti-apoptotic protein Bcl-xL as well as in strong inhibition of NGF-induced neurite outgrowth in PC12 cells.

CONCLUSION

The ER retained intrabody SH325-G7-KDEL not only induces phenotypic knockdown of this p75NTR but also p75NTR-associated cellular responses in PC12 cells.

Generation of mice with a conditional allele for the p75(NTR) neurotrophin receptor gene

The p75(NTR) neurotrophin receptor has been implicated in multiple biological and pathological processes. While significant advances have recently been made in understanding the physiologic role of p75(NTR) , many details and aspects remain to be determined. This is in part because the two existing knockout mouse models (Exons 3 or 4 deleted, respectively), both display features that defy definitive conclusions. Here we describe the generation of mice that carry a conditional p75(NTR) (p75(NTR-FX) ) allele made by flanking Exons 4-6, which encode the transmembrane and all cytoplasmic domains, by loxP sites. To validate this novel conditional allele, both neural crest-specific p75(NTR) /Wnt1-Cre mutants and conventional p75(NTR) null mutants were generated. Both mutants displayed abnormal hind limb reflexes, implying that loss of p75(NTR) in neural crest-derived cells causes a peripheral neuropathy similar to that seen in conventional p75(NTR) mutants. This novel conditional p75(NTR) allele will offer new opportunities to investigate the role of p75(NTR) in specific tissues and cells.

Proteolytic processing of the p75 neurotrophin receptor: A prerequisite for signalling?: Neuronal life, growth and death signalling are crucially regulated by intra-membrane proteolysis and trafficking of p75(NTR)

The common neurotrophin receptor (p75(NTR) ) regulates various functions in the developing and adult nervous system. Cell survival, cell death, axonal and growth cone retraction, and regulation of the cell cycle can be regulated by p75(NTR) -mediated signals following activation by either mature or pro-neurotrophins and in combination with various co-receptors, including Trk receptors and sortilin. Here, we review the known functions of p75(NTR) by cell type, receptor-ligand combination, and whether regulated intra-membrane proteolysis of p75(NTR) is required for signalling. We highlight that the generation of the intracellular domain fragment of p75(NTR) is associated with many of the receptor functions, regardless of its ligand and co-receptor interactions.

Stress-induced expression of the p75 neurotrophin receptor is regulated by O-GlcNAcylation of the Sp1 transcription factor

Injury-induced expression of p75 neurotrophin receptor (p75NTR) in the CNS induces neuronal apoptosis and prevents neuronal regrowth. The mechanisms regulating injury-induced p75NTR expression are poorly characterized but previous studies have established that reductions in extracellular osmolarity which mimic cytotoxic edema induce p75NTR gene expression through pathways that activate the Sp1 transcription factor. In this report, we examined how extracellular osmolarity converges on Sp1 to regulate p75NTR expression. We report that levels of O-linked N-acetylglucosamine (O-GlcNAc) transferase (OGT), the enzyme that mediates O-linked attachment of GlcNAc, are reduced by extracellular hypo-osmolarity and that global levels of protein O-GlcNAcylation and of Sp1 show a corresponding decline. We demonstrate that chemical and RNAi-based treatments that reduce cellular O-GlcNAcylation facilitate p75NTR induction by hypo-osmolarity, directly linking protein O-GlcNAcylation to p75NTR induction. To determine if Sp1 O-GlcNAc content regulates p75NTR expression, we replaced endogenous Sp1 with a Sp1 mutated at O-GlcNAc target residues. This O-GlcNAc-deficient form of Sp1-enhanced p75NTR expression, demonstrating that O-GlcNAcylation of Sp1 negatively regulates p75NTR expression. We conclude that a stress-induced decline in the O-GlcNAc content of Sp1 drives expression of p75NTR.

Common factors regulating patterning of the nervous and vascular systems

The vascular and the nervous systems of vertebrates share many features with similar and often overlapping anatomy. The parallels between these two systems extend to the molecular level, where recent work has identified ever-increasing similarities between the molecular mechanisms employed in the specification, differentiation, and patterning of both systems. This review discusses some of the most recent literature on this subject, with particular emphasis on the roles that the Ephrin, Semaphorin, Netrin, and Slit signaling pathways play in vascular development.

Axotomy-induced neurotrophic withdrawal causes the loss of phenotypic differentiation and downregulation of NGF signalling, but not death of septal cholinergic neurons

BACKGROUND

Septal cholinergic neurons account for most of the cholinergic innervations of the hippocampus, playing a key role in the regulation of hippocampal synaptic activity. Disruption of the septo-hippocampal pathway by an experimental transection of the fimbria-fornix drastically reduces the target-derived trophic support received by cholinergic septal neurons, mainly nerve growth factor (NGF) from the hippocampus. Axotomy of cholinergic neurons induces a reduction in the number of neurons positive for cholinergic markers in the medial septum. In several studies, the reduction of cholinergic markers has been interpreted as analogous to the neurodegeneration of cholinergic cells, ruling out the possibility that neurons lose their cholinergic phenotype without dying. Understanding the mechanism of cholinergic neurodegeneration after axotomy is relevant, since this paradigm has been extensively explored as an animal model of the cholinergic impairment observed in neuropathologies such as Alzheimer's disease.The principal aim of this study was to evaluate, using modern quantitative confocal microscopy, neurodegenerative changes in septal cholinergic neurons after axotomy and to assess their response to delayed infusion of NGF in rats.

RESULTS

We found that there is a slow reduction of cholinergic cells labeled by ChAT and p75 after axotomy. However, this phenomenon is not accompanied by neurodegenerative changes or by a decrease in total neuronal number in the medial septum. Although the remaining axotomized-neurons appear healthy, they are unable to respond to delayed NGF infusion.

CONCLUSIONS

Our results demonstrate that at 3 weeks, axotomized cholinergic neurons lose their cholinergic phenotype without dying and down-regulate their NGF-receptors, precluding the possibility of a response to NGF. Therefore, the physiological role of NGF in the adult septal cholinergic system is to support phenotypic differentiation and not survival of neurons. This evidence raises questions about the relationship between transcriptional regulation of the cholinergic phenotype by retrograde-derived trophic signaling and the transcriptional changes experienced when retrograde transport is impaired due to neuropathological conditions.

NTRK1 and NTRK2 receptors facilitate follicle assembly and early follicular development in the mouse ovary

Recent studies have demonstrated that neurotrophins (NTs) and their NTRK tyrosine kinase receptors, thought to be exclusively required for the development of the nervous system, are also involved in controlling ovarian development. Here, we show that primordial follicle formation is decreased in the absence of nerve growth factor (NGF) or its receptor NTRK1, and in the absence of NTRK2, the receptor for neurotrophin-4 (NTF4) and brain-derived neurotrophic factor (BDNF). This deficiency is not due to premature oocyte loss, because the ovaries of Ntrk1(-/-) and Ntrk2(-/-) mice do not show an increased rate of oocyte death antedating the initiation of folliculogenesis. Moreover, exposure of NGF-deficient ovaries to NGF rescues the defect in follicular assembly, if NTRK1 receptors are present, suggesting that the absence of NTs causes a delay, and not an irretrievable loss, of follicle formation. Both the number of secondary follicles and FSH receptor (FSHR) expression are diminished in Ntrk1- and Ntrk2-null ovaries, but not in ovaries lacking the common NT receptor NGFR. Transient exposure of wild-type ovaries to NTF4 increases Fshr gene expression and enhances the ability of the ovary to respond to FSH with formation of cyclin D2, a cell cycle protein mediating the proliferative actions of FSH in the ovary. These results indicate that both NTRK1 and NTRK2 receptors are necessary for the timely assembly of primordial follicles and for sustaining early follicular development. They also suggest that a mechanism by which NTRK2 receptors facilitate subsequent follicle development is by inducing the formation of functional FSHR.

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.

Role of neurotrophic factors in early ovarian development

Much is known about the endocrine hormonal mechanisms controlling ovarian development. More recently, attention has focused on identifying regulatory pathways that, operating within the ovarian microenvironment, contribute to the acquisition of ovarian reproductive competence. Within this framework, the concept has developed that neurotrophins (NTs) and their Trk tyrosine kinase receptors, long thought to be exclusively required for the development of the nervous system, are also involved in the control of ovarian maturation. The ovary of several species, including rodents, sheep, cows, nonhuman primates, and humans, produce NTs and express both the high-affinity receptors and the common p75 (NTR) receptor required for signaling. Studies in humans and rodents have shown that this expression is initiated during fetal life, before the formation of primordial follicles. Gene targeting approaches have identified TrkB, the high-affinity receptor for neurotrophin-4/5 and brain-derived neurotrophic factor, as a signaling module required for follicular assembly, early follicular growth, and oocyte survival. A similar approach has shown that nerve growth factor contributes independently to the growth of primordial follicles into gonadotropin-responsive structures. Altogether, these observations indicate that NTs are important contributors to the gonadotropin-independent process underlying the formation and initiation of ovarian follicular growth.

Gamma-secretase represents a therapeutic target for the treatment of invasive glioma mediated by the p75 neurotrophin receptor

The multifunctional signaling protein p75 neurotrophin receptor (p75^NTR) is a central regulator and major contributor to the highly invasive nature of malignant gliomas. Here, we show that neurotrophin-dependent regulated intramembrane proteolysis (RIP) of p75^NTR is required for p75^NTR-mediated glioma invasion, and identify a previously unnamed process for targeted glioma therapy. Expression of cleavage-resistant chimeras of p75^NTR or treatment of animals bearing p75^NTR-positive intracranial tumors with clinically applicable γ-secretase inhibitors resulted in dramatically decreased glioma invasion and prolonged survival. Importantly, proteolytic processing of p75^NTR was observed in p75^NTR-positive patient tumor specimens and brain tumor initiating cells. This work highlights the importance of p75^NTR as a therapeutic target, suggesting that γ-secretase inhibitors may have direct clinical application for the treatment of malignant glioma.

Despite technical advances, clinical prognosis of patients with malignant glioma, with an average survival of less than one year, has not changed. The highly invasive nature of these tumors, together with the recently identified brain tumor-initiating cells, provide disease reservoirs that render these tumors incurable by conventional therapies. Here, we present the first evidence to our knowledge that regulated intramembrane proteolysis of the neurotrophin receptor p75^NTR is a critical regulator of glioma invasion. Inhibition of this process by clinically relevant γ-secretase inhibitors dramatically impairs the highly invasive nature of genetically distinct glioblastomas and brain tumor-initiating cells and prolongs survival. These data highlight regulated intramembrane proteolysis as a therapeutic target of malignant glioma and implicate the application of γ-secretase inhibitors in the treatment of these devastating tumors.

Gamma-secretase inhibitors in clinical trials for patients with Alzheimer disease can be used to block the highly invasive behavior of malignant glioma and prolong survival.

Cardiovascular actions of neurotrophins

Neurotrophins were christened in consideration of their actions on the nervous system and, for a long time, they were the exclusive interest of neuroscientists. However, more recently, this family of proteins has been shown to possess essential cardiovascular functions. During cardiovascular development, neurotrophins and their receptors are essential factors in the formation of the heart and critical regulator of vascular development. Postnatally, neurotrophins control the survival of endothelial cells, vascular smooth muscle cells, and cardiomyocytes and regulate angiogenesis and vasculogenesis, by autocrine and paracrine mechanisms. Recent studies suggest the capacity of neurotrophins, via their tropomyosin-kinase receptors, to promote therapeutic neovascularization in animal models of hindlimb ischemia. Conversely, the neurotrophin low-affinity p75(NTR) receptor induces apoptosis of endothelial cells and vascular smooth muscle cells and impairs angiogenesis. Finally, nerve growth factor looks particularly promising in treating microvascular complications of diabetes or reducing cardiomyocyte apoptosis in the infarcted heart. These seminal discoveries have fuelled basic and translational research and thus opened a new field of investigation in cardiovascular medicine and therapeutics. Here, we review recent progress on the molecular signaling and roles played by neurotrophins in cardiovascular development, function, and pathology, and we discuss therapeutic potential of strategies based on neurotrophin manipulation.

Association between Presenilin-1 and TRAF6 modulates regulated intramembrane proteolysis of the p75NTR neurotrophin receptor

The p75 neurotrophin receptor (p75(NTR)) is a member of the tumour necrosis factor superfamily, which relies on the recruitment of cytosolic protein partners including the tumour necrosis factor receptor-associated factor 6 (TRAF6) E3 ubiquitin ligase to produce cellular responses. Recently, p75(NTR) was also shown to undergo presenilin-dependent, gamma-secretase-mediated regulated intramembrane proteolysis. In this study, we report the characterization of a highly conserved TRAF6-binding site (PxExxAr/Ac) in presenilin-1 (PS1) that mediates nerve growth factor (NGF)-induced association between PS1 and TRAF6. We demonstrate that disruption of this interaction between PS1 and TRAF6 inhibits TRAF6 autoubiquitination and gamma-secretase cleavage of p75(NTR). Additionally, we show that PS1-deficiency antagonizes NGF-induced I-kappaB degradation. Finally, we also show that p75(NTR) is a substrate for TRAF6-mediated ubiquitination and that TRAF6 E3 ligase activity is required for regulated intramembrane proteolysis of p75(NTR). In summary, our data suggest that an NGF-induced association between PS1 and TRAF6 influences regulated intramembrane proteolysis of p75(NTR).

Chronically increased ciliary neurotrophic factor and fibroblast growth factor-2 expression after spinal contusion in rats

Demyelination and oligodendrocyte loss following spinal cord injury (SCI) are well documented. Recently, we showed oligodendrocyte progenitor cell (OPC) accumulation and robust oligodendrocyte genesis occurring along SCI lesion borders. We have since begun investigating potential mechanisms for this endogenous repair response. Here, we examined ciliary neurotrophic factor (CNTF) and fibroblast growth factor-2 (FGF-2) expression, because both factors alter progenitor proliferation and differentiation and are increased in several CNS disorders. We hypothesized that CNTF and FGF-2 would increase after SCI, especially in regions of enhanced oligogenesis. First, CNTF protein was quantified using Western blots, which revealed that CNTF protein continually rose through 28 days post injury (dpi). Next, by using immunohistochemistry, we examined the spatiotemporal expression of CNTF in cross-sections spanning the injury site. CNTF immunoreactivity was observed on astrocytes and oligodendrocytes in naïve and contused spinal cords. Significantly increased CNTF was detected in spared white and gray matter between 5 and 28 dpi compared with uninjured controls. By 28 dpi, CNTF expression was significantly higher along lesion borders compared with outlying spared tissue; a similar distribution of phosphorylated STAT3, a transcription factor up-regulated by CNTF and to a lesser extent FGF-2, was also detected. Because CNTF can potentiate FGF-2 expression, we examined the distribution of FGF-2+ cells. Significantly more FGF-2+ cells were noted along lesion borders at 7 and 28 dpi. Thus, both CNTF and FGF-2 are present in regions of elevated OPC proliferation and oligodendrocyte generation after SCI and therefore may play a role in injury-induced gliogenesis.

Palmitoylation of the C-terminal fragment of p75(NTR) regulates death signaling and is required for subsequent cleavage by gamma-secretase

It has recently been shown that the p75 neurotrophin receptor (p75(NTR)), which is known to mediate neural cell death during development of the nervous system and in a range of adult neurodegenerative conditions, undergoes a regulated process of cell surface receptor cleavage, regulated intramembrane proteolysis (RIP). Here we show that neuronal death signaling occurs only following extracellular metalloprotease cleavage of p75(NTR) and palmitoylation of the resultant C-terminal fragment, causing its translocation to cholesterol-rich domains of the plasma membrane. Furthermore, death signaling is promoted by inhibition of intracellular gamma-secretase cleavage, a process which also occurs within the cholesterol-rich domains. In the presence of TrkA signaling, C-terminal fragment localization in these cholesterol-rich domains is prevented, thereby blocking neuronal death. Thus p75(NTR) activates neuronal death pathways in conditions where the balance of normal RIP is shifted toward extracellular domain cleavage due to increased metalloprotease activity, decreased TrkA activity or compromised gamma-secretase activity, all of which are features of neurodegenerative conditions such as Alzheimer's disease.

Metalloproteases and gamma-secretase: new membrane partners regulating p75 neurotrophin receptor signaling?

Signaling by the p75 neurotrophin receptor (p75) has been implicated in diverse neuronal responses, including the control of neuronal survival versus death and axonal regeneration and growth cone collapse, involving p75 in different neuropathological conditions. There are different levels of complexity regulating p75-mediated signaling. First, p75 can interact with different ligands and co-receptors in the plasma membrane, forming tripartite complexes, whose activation result in different cellular outcomes. Moreover, it was recently described that trafficking capacities of p75 in neurons are regulating, in addition to p75 downstream interactions, also the sequential cleavage of p75. The proteolytical processing of p75 involves, first, a shedding event that releases a membrane-bound carboxiterminal fragment (p75-CTF), followed by a gamma-secretase mediated cleavage, generating a soluble intracellular domain (p75-ICD) with signaling capabilities. The first shedding event, generating a p75-CTF, is the key step to regulating the production of p75-ICD, and although the generation of p75-ICD is important for both p75-mediated control of neuronal survival and the control of neurite outgrowth, little is known how both cleavage events are regulated. In this review, we argue that both sheddases and gamma-secretase are key membrane components regulating p75-mediated signaling transduction; therefore, further attention should be paid to their roles as p75 signaling regulators.

Modulation of semaphorin3A activity by p75 neurotrophin receptor influences peripheral axon patterning

The p75 neurotrophin receptor (p75(NTR)) interacts with multiple ligands and coreceptors. It is thought to mediate myelin growth inhibition as part of the Nogo receptor complex, in addition to its other roles. Paradoxically, however, peripheral axons of p75(ExonIII-/-) mutant embryos are severely stunted. This inhibition of axon growth may be a result of neurite elongation defects in p75(NTR) mutant neurons. Here, we show that p75(ExonIII-/-) DRG neurons are hypersensitive to the repellent molecule Semaphorin3A (Sema3A). NGF modulates Sema3A activity equally well in both the p75(NTR) mutant and wild-type neurons, indicating that the hypersensitivity of p75(NTR) mutant neurons is probably not related to their NGF receptor activity. Neuropilin1 and p75(NTR) partially colocalize in DRG growth cones. After Sema3A stimulation, the degree of colocalization is dramatically increased, particularly in clusters associated with Sema3A receptor complex activation. Coimmunoprecipitation studies show that p75(NTR) interacts directly with the Sema3A receptors Neuropilin1 and PlexinA4. When coexpressed with both Neuropilin1 and PlexinA4, p75(NTR) reduces the interaction between these two receptor components. Finally, p75(NTR)/Sema3A double-mutant embryos show growth similar to that observed in Sema3A-null mice. These data indicate that p75(NTR) is an important functional modulator of Sema3A activity and that, in the absence of p75(NTR), oversensitivity to Sema3A leads to severe reduction in sensory innervation. Our results also suggest that while inhibition of p75(NTR) in CNS injury may enhance nerve regeneration resulting from the inhibition of myelin-associated protein, it may also inhibit nerve regeneration through its modulation of Sema3A.

The p75 neurotrophin receptor

The pan neurotrophin receptor (p75(NTR)) is best known for mediating neural cell death during development as well as in the adult following injury, the latter making it a target for the treatment of neurodegenerative disease. Although p75(NTR) has been studied for over 30 years, a number of recent discoveries have changed our understanding of its regulation. Here we provide a brief overview of the p75(NTR) protein, its post-translational modifications, and the phenotype of p75(NTR)-deficient mice as a starting point for researchers unfamiliar with this complex receptor. The accepted mechanisms underlying the ability of p75(NTR) to regulate cell death as well as a number of other neural functions, most notably neuronal differentiation, neurite outgrowth and synaptic plasticity, are also summarised.

Brain-derived neurotrophic factor: a newly described mediator of angiogenesis

Recent studies indicate that, in addition to its neuropoietic actions, brain derived neurotrophic factor (BDNF) promotes endothelial cell survival and induces neoangiogenesis in ischemic tissues. Unlike many vascular growth factors that act on many vascular beds, BDNF activity is relatively restricted to central arteries, vessels of cardiac and skeletal muscle, and skin. Studies of newly described biologic mediators that act on large-vessel and microvascular beds in these organs will help us to better understand organ-specific vascular development, as well as to develop novel therapeutic strategies to improve the condition of patients with cardiac and peripheral vascular disease. In this review, we summarize dual proangiogenic actions of BDNF, which, through local activation of TrkB receptor, expressed on a subpopulation of endothelial cells and, in addition, by recruitment of bone marrow-derived cells, contribute to neoangiogenesis.

Familial Alzheimer's disease presenilin 1 mutation M146V increases gamma secretase cutting of p75NTR in vitro

The cholinergic neurons of the basal forebrain are amongst the first to degenerate in Alzheimer's disease. These neurons are unique in the brain, expressing the tyrosine kinase receptor TrkA, together with the common neurotrophin receptor p75NTR; both of which bind nerve growth factor. Activation of the TrkA receptor is important in the maintenance of cell viability, whereas the p75NTR receptor has been implicated in apoptosis. Mutations in the gene for presenilin 1, a multi-transmembrane aspartyl protease, are known to cause familial Alzheimer's disease. This is thought to be due to their effect on gamma-secretase-dependent processing of amyloid precursor protein and subsequent formation of amyloid. Since p75NTR was recently shown to undergo gamma-secretase regulated intramembrane proteolysis, this study examines the effect of familial Alzheimer mutations on processing of p75NTR. PC12 cells were stably transfected with familial mutations M146V, A246E and deltaE9 and wild-type presenilin 1 and were examined here for gamma-secretase-dependent proteolysis of p75NTR. Overexpression of wild-type presenilin 1 did not increase gamma-secretase-mediated cleavage of p75NTR. However, by contrast, the presence of the M146V mutation was shown to significantly increase cleavage of p75NTR compared with the other mutations. Survival of cholinergic neurons will depend on the balance between the receptors TrkA and p75NTR, and their respective signalling pathways. Thus alterations in p75NTR proteolysis may influence this equilibrium. The novel finding that a mutation may increase processing of p75NTR may have implications for the pathogenic outcome in Alzheimer's disease.

Hypo-osmolar stress induces p75NTR expression by activating Sp1-dependent transcription

Injury-induced expression of the p75 neurotrophin receptor (p75NTR) in the CNS facilitates neuronal apoptosis and prevents neuronal regrowth, but the mechanisms regulating p75NTR expression are poorly characterized. In this study, we showed that hypo-osmolarity induces p75NTR expression in primary neurons, and, using a comparative genomics approach, we identified conserved elements in the 25 kb upstream sequences of the rat, mouse, and human p75NTR genes. We found that only one of these, a proximal region rich in Sp1 sites, responds to changes in hypo-osmolarity. We then showed that Sp1 DNA binding activity is increased in cells exposed to hypo-osmolarity, established that hypo-osmolarity enhanced Sp1 binding to the endogenous p75NTR promoter, and showed that Sp1 is required for p75NTR expression induced by hypo-osmolarity. We examined how Sp1 is regulated to effect these changes and established that Sp1 turnover is strongly inhibited by hypo-osmolarity. We propose that stress-induced Sp1 accumulation that results from reductions in Sp1 turnover rate contributes to injury-induced gene expression.

The dark side of the NGF family: neurotrophins in neoplasias

Although neurotrophins of the nerve growth factor (NGF) family are best known for their neurite outgrowth-inducing and survival-promoting effects on neuronal cells, these are actually pleiotropic growth factors acting physiologically on many different cell types of our body. As for many other growth factors, dysregulation of neurotrophin signal transduction is found in a number of tumors where they can accompany or contribute to malignant transformation. Interestingly, tropomyosin-related kinase (Trk) receptor activation can either support or suppress tumor growth, depending on the tumor type. These same divergent responses have been observed with neurotrophins binding to the p75NTR neurotrophin receptor on tumor cells. This article summarizes the current knowledge on the role of neurotrophins and their receptors in malignancies, with special focus on tumors of neuropathological interest.

Astrocyte and muscle-derived secreted factors differentially regulate motoneuron survival

During development, motoneurons (MNs) undergo a highly stereotyped, temporally and spatially defined period of programmed cell death (PCD), the result of which is the loss of 40-50% of the original neuronal population. Those MNs that survive are thought to reflect the successful acquisition of limiting amounts of trophic factors from the target. In contrast, maturation of MNs limits the need for target-derived trophic factors, because axotomy of these neurons in adulthood results in minimal neuronal loss. It is unclear whether MNs lose their need for trophic factors altogether or whether, instead, they come to rely on other cell types for nourishment. Astrocytes are known to supply trophic factors to a variety of neuronal populations and thus may nourish MNs in the absence of target-derived factors. We investigated the survival-promoting activities of muscle- and astrocyte-derived secreted factors and found that astrocyte-conditioned media (ACM) was able to save substantially more motoneurons in vitro than muscle-conditioned media (MCM). Our results indicate that both ACM and MCM are significant sources of MN trophic support in vitro and in ovo, but only ACM can rescue MNs after unilateral limb bud removal. Furthermore, we provide evidence suggesting that MCM facilitates the death of a subpopulation of MNs in a p75(NTR) - and caspase-dependent manner; however, maturation in ACM results in MN trophic independence and reduced vulnerability to this negative, pro-apoptotic influence from the target.

TrkA receptor activation by nerve growth factor induces shedding of the p75 neurotrophin receptor followed by endosomal gamma-secretase-mediated release of the p75 intracellular domain

Neurotrophins are trophic factors that regulate important neuronal functions. They bind two unrelated receptors, the Trk family of receptor-tyrosine kinases and the p75 neurotrophin receptor (p75). p75 was recently identified as a new substrate for gamma-secretase-mediated intramembrane proteolysis, generating a p75-derived intracellular domain (p75-ICD) with signaling capabilities. Using PC12 cells as a model, we studied how neurotrophins activate p75 processing and where these events occur in the cell. We demonstrate that activation of the TrkA receptor upon binding of nerve growth factor (NGF) regulates the metalloprotease-mediated shedding of p75 leaving a membrane-bound p75 C-terminal fragment (p75-CTF). Using subcellular fractionation to isolate a highly purified endosomal fraction, we demonstrate that p75-CTF ends up in endosomes where gamma-secretase-mediated p75-CTF cleavage occurs, resulting in the release of a p75-ICD. Moreover, we show similar structural requirements for gamma-secretase processing of p75 and amyloid precursor protein-derived CTFs. Thus, NGF-induced endocytosis regulates both signaling and proteolytic processing of p75.

Neurotrophin-regulated signalling pathways

Neurotrophins are a family of closely related proteins that were identified initially as survival factors for sensory and sympathetic neurons, and have since been shown to control many aspects of survival, development and function of neurons in both the peripheral and the central nervous systems. Each of the four mammalian neurotrophins has been shown to activate one or more of the three members of the tropomyosin-related kinase (Trk) family of receptor tyrosine kinases (TrkA, TrkB and TrkC). In addition, each neurotrophin activates p75 neurotrophin receptor (p75NTR), a member of the tumour necrosis factor receptor superfamily. Through Trk receptors, neurotrophins activate Ras, phosphatidyl inositol-3 (PI3)-kinase, phospholipase C-gamma1 and signalling pathways controlled through these proteins, such as the MAP kinases. Activation of p75NTR results in activation of the nuclear factor-kappaB (NF-kappaB) and Jun kinase as well as other signalling pathways. Limiting quantities of neurotrophins during development control the number of surviving neurons to ensure a match between neurons and the requirement for a suitable density of target innervation. The neurotrophins also regulate cell fate decisions, axon growth, dendrite growth and pruning and the expression of proteins, such as ion channels, transmitter biosynthetic enzymes and neuropeptide transmitters that are essential for normal neuronal function. Continued presence of the neurotrophins is required in the adult nervous system, where they control synaptic function and plasticity, and sustain neuronal survival, morphology and differentiation. They also have additional, subtler roles outside the nervous system. In recent years, three rare human genetic disorders, which result in deleterious effects on sensory perception, cognition and a variety of behaviours, have been shown to be attributable to mutations in brain-derived neurotrophic factor and two of the Trk receptors.

Does the p75 neurotrophin receptor mediate Abeta-induced toxicity in Alzheimer's disease?

Alzheimer's disease is characterized by the over-production and accumulation of amyloidogenic Abeta peptide, which can induce cell death in vitro. It has been suggested that the death signal could be transduced by the pan neurotrophin receptor (p75NTR). p75NTR is well known for its ability to mediate neuronal death in neurodegenerative conditions and is inextricably linked with changes that occur in Alzheimer's disease. Moreover, Abeta binds to p75NTR, activating signalling cascades. However, the complexity of p75NTR-mediated signalling, which does not always promote cell death, leaves open the possibly of Abeta promoting death via an alternative signalling pathway or the regulation of other p75NTR-mediated actions. This review focuses on the interactions between Abeta and p75NTR in the context of the broader p75NTR signalling field, and offers alternative explanations for how p75NTR might contribute to the aetiology of Alzheimer's disease.

Functional monoclonal antibodies to p75 neurotrophin receptor raised in knockout mice

In this study, p75NTREXONIII knockout mice were used as immune-naive hosts to produce functional antibodies to human p75NTR. Three monoclonal antibodies were produced and named MLR1, MLR2 and MLR3, and isotyped as IgG1, IgG2a and IgG2a, respectively. MLR1 and MLR2 bound to human p75NTR with higher affinity than the well-characterized ME20.4 in ELISA and also recognized p75NTR present on neurons in both rat and mouse. MLR1 and MLR2 bound to nerves known to express p75NTR following injection into Balb/C mice but not p75NTREXONIII knockout mice, indicating the antibodies are directed against the ligand binding extracellular region absent in knockout mice. Both MLR1 and MLR2 partially blocked NGF induced cell death in a mouse cell-line that expresses p75NTR but not TrKA. Importantly, intracerebroventricular injections indicated MLR2 was internalized within the cell bodies of mouse basal forebrain neurons, further demonstrating that this antibody is biologically active.

Intracellular sphingosine 1-phosphate mediates the increased excitability produced by nerve growth factor in rat sensory neurons

Our previous studies found that nerve growth factor (NGF), via ceramide, enhanced the number of action potentials (APs) evoked by a ramp of depolarizing current in capsaicin-sensitive sensory neurons. Ceramide can be metabolized by ceramidase to sphingosine (Sph), and Sph to sphingosine 1-phosphate (S1P) by sphingosine kinase. It is well established that each of these products of sphingomyelin metabolism can act as intracellular signalling molecules. This raises the question as to whether the enhanced excitability produced by NGF was mediated directly by ceramide or required additional metabolism to Sph and/or S1P. Sph applied externally did not affect the neuronal excitability, whereas internally perfused Sph augmented the number of APs evoked by the depolarizing ramp. Furthermore, internally perfused S1P enhanced the number of evoked APs. This sensitizing action of NGF, ceramide and internally perfused Sph was abolished by dimethylsphingosine (DMS), an inhibitor of sphingosine kinase. In contrast, internally perfused S1P enhanced the number of evoked APs in the presence of DMS. These observations support the idea that the metabolism of ceramide/Sph to S1P is critical for the sphingolipid-induced modulation of excitability. Both internally perfused Sph and S1P inhibited the outward K+ current by 25-35% for the step to +60 mV. The Sph- and S1P-sensitive currents had very similar current-voltage relations, suggesting that they were likely to be the same. In addition, the Sph-induced suppression of the K+ current was blocked by pretreatment with DMS. These findings demonstrate that intracellular S1P derived from ceramide acts as an internal second messenger to regulate membrane excitability; however, the effector system whereby S1P modulates excitability remains undetermined.

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.

The p75 neurotrophin receptor negatively modulates dendrite complexity and spine density in hippocampal neurons

The correlation between functional and structural neuronal plasticity is by now well documented. However, the molecular mechanisms translating patterns of neuronal activity into specific changes in the structure of neurons remain unclear. Neurotrophins can be released in an activity-dependent manner, and they are capable of controlling both neuronal morphology and functional synaptic changes. They are thus attractive molecules to be studied in the context of synaptic plasticity. In the CNS, most of the work so far has focused on the role of BDNF and of its tyrosine kinase B receptor (TrkB), but relatively little is known about the function of the pan-neurotrophin receptor p75NTR. In this study, we show in loss-of-function experiments that postnatal hippocampal pyramidal cells in two mutant lines of p75NTR have a higher spine density and greater dendritic complexity than wild-type (WT) mice. Conversely, in a gain-of-function approach, p75NTR overexpression in WT neurons significantly reduces dendritic complexity, as well as spine density in all dendritic compartments. These results show that p75NTR negatively modulates dendritic morphology in adult hippocampal pyramidal neurons and documents a new case of functional antagonism between Trk and p75NTR signaling.