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.     

The p75 neurotrophin receptor

The low-affinity p75 molecule and trk tyrosine kinases serve as receptors for target-derived neurotrophins. While the mechanism by which receptor tyrosine kinases impart intracellular signaling has become well understood, the precise roles of the p75 receptor are not fully defined. The p75 neurotrophin receptor belongs to a family of transmembrane molecules which also serve as receptors for the tumor necrosis factor family of cytokines. Each receptor shares a common extracellular structure highlighted by conserved cysteine-rich repeats. Because NGF, BDNF, NT-3, and NT-4/5 bind to p75 with similar affinity, p75 may either act as a common subunit in a neurotrophin receptor complex with trk family members, or act by independent mechanisms to mediate biological actions of each neurotrophin. 1994 John Wiley & Sons, Inc.     

Antisense peptide nucleic acid-mediated knockdown of the p75 neurotrophin receptor delays motor neuron disease in mutant SOD1 transgenic mice

Re-expression of the death-signalling p75 neurotrophin receptor (p75NTR) is associated with injury and neurodegeneration in the adult nervous system. The induction of p75NTR expression in mature degenerating spinal motor neurons of humans and transgenic mice with amyotrophic lateral sclerosis (ALS) suggests a role of p75NTR in the progression of motor neuron disease (MND). In this study, we designed, synthesized and evaluated novel antisense peptide nucleic acid (PNA) constructs targeting p75NTR as a potential gene knockdown therapeutic strategy for ALS. An 11-mer antisense PNA directed at the initiation codon, but not downstream gene sequences, dose-dependently inhibited p75NTR expression and death-signalling by nerve growth factor (NGF) in Schwann cell cultures. Antisense phosphorothioate oligonucleotide (PS-ODN) sequences used for comparison failed to confer such inhibitory activity. Systemic intraperitoneal administration of this antisense PNA to mutant superoxide dismutase 1 (SOD1G93A) transgenic mice significantly delayed locomotor impairment and mortality compared with mice injected with nonsense or scrambled PNA sequences. Reductions in p75NTR expression and subsequent caspase-3 activation in spinal cords were consistent with increased survival in antisense PNA-treated mice. The uptake of fluorescent-labelled antisense PNA in the nervous system of transgenic mice was also confirmed. This study suggests that p75NTR may be a promising antisense target in the treatment of ALS.     

p75受体信号转导途径的研究进展

神经生长因子 (ｎｅｒｖｅｇｒｏｗｔｈｆａｃｔｏｒ,ＮＧＦ)通过与两种受体结合而发挥作用。一种是高亲和性受体ＴｒｋＡ受体 ,它是由原癌基因Ｔｒｋ编码的蛋白质酪氨酸激酶 ;另一种是低亲和性受体ｐ75,它以相同的亲和力与神经营养素 (ｎｅｕｒｏｔｒｏｐｈｉｎ ,ＮＴ)家族的各成员结合 ,因此被称为神经营养素受体ｐ75(ｐ75ＮＴＲ)。ＮＧＦ与ＴｒｋＡ受体结合后发挥其促进神经细胞生长和存活的作用 ,而近年来发现ｐ75受体在特定条件下却能够诱导某些神经细胞和胶质细胞的凋亡 ,由于ｐ75ＮＴＲ的这一作用 ,它受到越来越多的关注。1 .ｐ75Ｎ…     

p75 neurotrophin receptor regulates agonist-induced pulmonary vasoconstriction

A member of the TNF receptor family, the p75 neurotrophin receptor (p75(NTR)) has been previously shown to play a role in the regulation of fibrin deposition in the lung. However, the role of p75(NTR) in the regulation of pulmonary vascular tone in the lung is unknown. In the present study, we evaluated the expression of p75(NTR) in mouse pulmonary arteries and the putative role of p75(NTR) in modulating pulmonary vascular tone and agonist responsiveness using wild-type (WT) and p75(NTR) knockout (p75(-/-)) mice. Our data indicated that p75(NTR) is expressed in both smooth muscle and endothelial cells within the pulmonary vascular wall in WT mice. Pulmonary artery rings from p75(-/-) mice exhibited significantly elevated active tension due to endothelin-1-mediated Ca(2+) influx. Furthermore, the contraction due to capacitative Ca(2+) entry (CCE) in response to phenylephrine-mediated active depletion of intracellular Ca(2+) stores was significantly enhanced compared with WT rings. The contraction due to CCE induced by passive store depletion, however, was comparable between WT and p75(-/-) rings. Active tension induced by serotonin, U-46619 (a thromboxane A(2) analog), thrombin, 4-aminopyridine (a K(+) channel blocker), and high extracellular K(+) in p75(-/-) rings was similar to that in WT rings. Deletion of p75(NTR) did not alter pulmonary vasodilation to sodium nitroprusside (a nitric oxide donor). These data suggest that intact p75(NTR) signaling may play a role in modulating pulmonary vasoconstriction induced by endothelin-1 and by active store depletion.     

p75 neurotrophin receptor functions as a survival receptor in brain-metastatic melanoma cells

The p75 neurotrophin receptor (p75(NTR)), a common receptor for members of the neurotrophins (NT) family, was previously identified as a molecular determinant of brain metastasis. We have also reported that NT treatment of murine and human brain-metastatic melanoma cells affects their invasive capacities and increases the production of heparanase, an important and unique extracellular matrix (ECM) degradative enzyme. Neurotrophism can be a survival-support mechanism for brain-metastatic cells and a survival assay was devised to mimic the growth limiting conditions of rapidly expanding metastatic tumors prior to neoangiogenesis. We report that p75(NTR) promoted the survival of brain-metastatic melanoma cells but not melanocytes in stress cultures conditions. Secondly, melanoma cells fluorescently sorted for high p75(NTR) expression (p75(NTR-H) cells) had an up to a 15-fold greater survival than those sorted for low p75(NTR) expression (p75(NTR-L) cells). Thirdly, cells overexpressing p75(NTR) associated with the growth fraction and provided these cells with an inherent growth advantage. Finally, we observed an increased survival of sorted p75(NTR-L) cells, dependent upon treatment of NT members whose functional receptors are present on these cells. Together, these results delineate that p75(NTR)-mediated trophic support profoundly affects competitive melanoma-cell survival when the tumor cell microenvironment becomes growth limiting.     

Polyubiquitination of the neurotrophin receptor p75 directs neuronal cell survival

Specific binding of nerve growth factor (NGF) to p75 neurotrophin receptor (p75(NTR)) leads to p75(NTR) polyubiquitination and its subsequent interaction with TRAF6 resulting in neuronal cell survival. However, when the binding of NGF to p75(NTR) was blocked with p75 antiserum, p75(NTR) polyubiquitination and neuronal cell survival were impaired. Results showed that tyrosine phosphorylation of p75(NTR) increased the polyubiquitination of p75(NTR) and contributed to the observed apparent neuroprotective effects. Similar to p75(NTR) polyubiquitination, interaction of TRAF6 with p75(NTR) was NGF/tyrosine phosphorylation dependent suggesting that TRAF6 might function as an E3 ubiquitin ligase. In sum, the results show that specific binding of NGF to p75(NTR) mediates neuronal cell survival.     

The p75 neurotrophin receptor is a central regulator of glioma invasion

The invasive nature of cancers in general, and malignant gliomas in particular, is a major clinical problem rendering tumors incurable by conventional therapies. Using a novel invasive glioma mouse model established by serial in vivo selection, we identified the p75 neurotrophin receptor (p75^NTR) as a critical regulator of glioma invasion. Through a series of functional, biochemical, and clinical studies, we found that p75^NTR dramatically enhanced migration and invasion of genetically distinct glioma and frequently exhibited robust expression in highly invasive glioblastoma patient specimens. Moreover, we found that p75^NTR-mediated invasion was neurotrophin dependent, resulting in the activation of downstream pathways and producing striking cytoskeletal changes of the invading cells. These results provide the first evidence for p75^NTR as a major contributor to the highly invasive nature of malignant gliomas and identify a novel therapeutic target.     

p75 neurotrophin receptor inhibits invasion and metastasis of gastric cancer

The p75 neurotrophin receptor (p75NTR) is a focus for study at present. However, its function in gastric cancer was not elucidated. Here, we investigated its relation with metastasis of gastric cancer. By immunohistochemistry, we found that the positive rate of p75NTR expression in metastatic gastric cancer was 15.09% (16 of 106), which was lower compared with nonmetastatic gastric cancer (64.15%; 68 of 106). The average staining score in nonmetastatic gastric cancer was significantly higher than in metastatic gastric cancer (1.21 +/- 0.35 versus 0.23 +/- 0.18; P<0.01). p75NTR protein level was also lowly expressed in the highly liver-metastatic gastric cancer cell line XGC9811-L compared with other gastric cancer cell lines by Western blotting. It could also significantly inhibit the in vitro adhesive, invasive, and migratory and in vivo metastatic abilities of gastric cancer cell lines SGC7901 and MKN45 by reducing urokinase-type plasminogen activator (uPA) and matrix metalloproteinase (MMP)-9 proteins and by increasing tissue inhibitor of matrix metalloproteinase (TIMP)-1 protein. Further studies showed that p75NTR could suppress the nuclear factor-kappaB (NF-kappaB) signal. SN50, a specific inhibitor of NF-kappaB, which could inhibit in vitro invasive and migratory abilities of gastric cancer cells, reduced expression of uPA and MMP9 proteins and increased expression of TIMP1 protein. Taken together, p75NTR had the function of inhibiting the invasive and metastatic abilities of gastric cancer cells, which was mediated, at least partially, by down-regulation of uPA and MMP9 proteins and up-regulation of TIMP1 protein via the NF-kappaB signal transduction pathway. Our studies suggested that p75NTR may be used as a new potential therapeutic target in metastatic gastric cancer.     

High-affinity homologous peptide nucleic acid probes for targeting a quadruplex-forming sequence from a MYC promoter element

Guanine-rich DNA and RNA sequences are known to fold into secondary structures known as G-quadruplexes. Recent biochemical evidence along with the discovery of an increasing number of sequences in functionally important regions of the genome capable of forming G-quadruplexes strongly indicates important biological roles for these structures. Thus, molecular probes that can selectively target quadruplex-forming sequences (QFSs) are envisioned as tools to delineate biological functions of quadruplexes as well as potential therapeutic agents. Guanine-rich peptide nucleic acids have been previously shown to hybridize to homologous DNA or RNA sequences forming PNA-DNA (or RNA) quadruplexes. For this paper we studied the hybridization of an eight-mer G-rich PNA to a quadruplex-forming sequence derived from the promoter region of the MYC proto-oncogene. UV melting analysis, fluorescence assays, and surface plasmon resonance experiments reveal that this PNA binds to the MYC QFS in a 2:1 stoichiometry and with an average binding constant Ka = (2.0 +/- 0.2) x 10(8) M(-1) or Kd = 5.0 nM. In addition, experiments carried out with short DNA targets revealed a dependence of the affinity on the sequence of bases in the loop region of the DNA. A structural model for the hybrid quadruplex is proposed, and implications for gene targeting by G-rich PNAs are discussed.     

The p75 neurotrophin receptor interacts with multiple MAGE proteins

The p75 neurotrophin receptor has been implicated in diverse aspects of neurotrophin signaling, but the mechanisms by which its effects are mediated are not well understood. Here we identify two MAGE proteins, necdin and MAGE-H1, as interactors for the intracellular domain of p75 and show that the interaction is enhanced by ligand stimulation. PC12 cells transfected with necdin or MAGE-H1 exhibit accelerated differentiation in response to nerve growth factor. Expression of these two MAGE proteins is predominantly cytoplasmic in PC12 cells, and necdin was found to be capable of homodimerization, suggesting that it may act as a cytoplasmic adaptor to recruit a signaling complex to p75. These findings indicate that diverse MAGE family members can interact with the p75 receptor and highlight type II MAGE proteins as a potential family of interactors for signaling proteins containing type II death domains.     

A comparison of the cytoplasmic domains of the Fas receptor and the p75 neurotrophin receptor

The p75 neurotrophic receptor (p75) shares structural features with the Fas receptor (FasR). Both receptors contain extracellular cysteine-rich repeats, a single transmembrane domain, and intracellular death domains. However, it has not been clearly established whether their death domains are equivalent in their ability to mediate apoptosis. To understand better the role of p75 during apoptosis, we constructed chimeric receptors that contained the extracellular portion of the FasR and the intracellular portion of p75. These chimeric receptors, one containing the p75 transmembrane domain and the other containing the FasR transmembrane portion, as well as wild-type p75 and Fas receptors, were transiently transfected into human U373 glioma cells and human embryonic kidney 293 cells (293 cells), which are both responsive to Fas-mediated apoptosis. Whereas expression of FasR was sufficient to induce apoptosis in U373 and 293 cells, expression of p75 and the chimeric receptors induced only minimal levels of cell death compared to FasR. The results indicate that the magnitudes of FasR- and p75-induced killing are different and suggest that the death domain of p75 does not function in the same manner as the FasR death domain.     

NMR structure of the death domain of the p75 neurotrophin receptor.

The intracellular domain of the p75 neurotrophin receptor (p75ICD) lacks catalytic activity but contains a motif similar to death domains found in the cytoplasmic regions of members of the tumor necrosis factor receptor family and their downstream targets. Although some aspects of the signaling pathways downstream of p75 have been elucidated recently, mechanisms of receptor activation and proximal signaling events are unknown. Here we report the nuclear magnetic resonance (NMR) structure of the 145 residue long p75ICD. The death domain of p75ICD consists of two perpendicular sets of three helices packed into a globular structure. The polypeptide segment connecting the transmembrane and death domains as well as the serine/threonine-rich C-terminal end are highly flexible in p75ICD. Unlike the death domains involved in signaling by the TNF receptor and Fas, p75ICD does not self-associate in solution. A surface area devoid of charged residues in the p75ICD death domain may indicate a potential site of interaction with downstream targets.     

Cleavage of p75 neurotrophin receptor by alpha-secretase and gamma-secretase requires specific receptor domains

The p75 neurotrophin receptor (p75(NTR)), a member of the tumor necrosis factor superfamily of receptors, undergoes multiple proteolytic cleavage events. These events are initiated by an alpha-secretase-mediated release of the extracellular domain followed by a gamma-secretase-mediated intramembrane cleavage. However, the specific determinants of p75(NTR) cleavage events are unknown. Many other substrates of gamma-secretase cleavage have been identified, including Notch, amyloid precursor protein, and ErbB4, indicating there is broad substrate recognition by gamma-secretase. Using a series of deletion mutations and chimeric receptors of p75(NTR) and the related Fas receptor, we have identified domains that are essential for p75(NTR) proteolysis. The initial alpha-secretase cleavage was extracellular to the transmembrane domain. Unfortunately, deletion mutants were not capable of defining the requirements of ectodomain shedding. Although this cleavage is promiscuous with respect to amino acid sequence, its position with respect to the transmembrane domain is invariant. The generation of chimeric receptors exchanging different domains of noncleavable Fas receptor with p75(NTR), however, revealed that a discrete domain above the membrane is sufficient for efficient cleavage of p75(NTR). Mass spectrometric analysis confirmed the cleavage can occur with a truncated p75(NTR) displaying only 15 extracellular amino acids in the stalk region.     

Bex1, a novel interactor of the p75 neurotrophin receptor, links neurotrophin signaling to the cell cycle

A screening for intracellular interactors of the p75 neurotrophin receptor (p75NTR) identified brain-expressed X-linked 1 (Bex1), a small adaptor-like protein of unknown function. Bex1 levels oscillated during the cell cycle, and preventing the normal cycling and downregulation of Bex1 in PC12 cells sustained cell proliferation under conditions of growth arrest, and inhibited neuronal differentiation in response to nerve growth factor (NGF). Neuronal differentiation of precursors isolated from the brain subventricular zone was also reduced by ectopic Bex1. In PC12 cells, Bex1 overexpression inhibited the induction of NF-kappaB activity by NGF without affecting activation of Erk1/2 and AKT, while Bex1 knockdown accelerated neuronal differentiation and potentiated NF-kappaB activity in response to NGF. Bex1 competed with RIP2 for binding to the p75NTR intracellular domain, and elevating RIP2 levels restored the ability of cells overexpressing Bex1 to differentiate in response to NGF. Together, these data establish Bex1 as a novel link between neurotrophin signaling, the cell cycle, and neuronal differentiation, and suggest that Bex1 may function by coordinating internal cellular states with the ability of cells to respond to external signals.     

High-affinity TrkA and p75 neurotrophin receptor complexes: A twisted affair

Neurotrophin signaling is essential for normal nervous system development and adult function. Neurotrophins are secreted proteins that signal via interacting with two neurotrophin receptor types: the multifaceted p75 neurotrophin receptor and the tropomyosin receptor kinase receptors. In vivo, neurons compete for the limited quantities of neurotrophins, a process that underpins neural plasticity, axonal targeting, and ultimately survival of the neuron. Thirty years ago, it was discovered that p75 neurotrophin receptor and tropomyosin receptor kinase A form a complex and mediate high-affinity ligand binding and survival signaling; however, despite decades of functional and structural research, the mechanism of modulation that yields this high-affinity complex remains unclear. Understanding the structure and mechanism of high-affinity receptor generation will allow development of pharmaceuticals to modulate this function for treatment of the many nervous system disorders in which altered neurotrophin expression or signaling plays a causative or contributory role. Here we re-examine the key older literature and integrate it with more recent studies on the topic of how these two receptors interact. We also identify key outstanding questions and propose a model of inside-out allosteric modulation to assist in resolving the elusive high-affinity mechanism and complex.