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

LM11a-31 Inhibits p75 Neurotrophin Receptor (p75 NTR ) Cleavage and is Neuroprotective in a Cell Culture Model of Parkinson's Disease

The p75 Neurotrophin Receptor (p75 NTR ) is a multifunctional transmembrane protein that mediates neuronal responses to pathological conditions in specific regions of the nervous system. In many biological contexts, p75 NTR signaling is initiated through sequential cleavage of the receptor by α- and γ-secretases, which releases receptor fragments for downstream signaling. Our previous work demonstrated that proteolytic processing of p75 NTR in this manner is stimulated by oxidative stress in Lund Human Mesencephalic (LUHMES) cells, a dopaminergic neuronal cell line derived from human mesencephalic tissue. Considering the vulnerability of dopaminergic neurons in the ventral mesencephalon to oxidative stress and neurodegeneration associated with Parkinson's disease (PD), we investigated the role of this signaling cascade in neurodegeneration and explored cellular processes that govern oxidative stress-induced p75 NTR signaling. In the present study, we provide evidence that oxidative stress induces cleavage of p75 NTR by promoting c-Jun N-terminal Kinase (JNK)-dependent internalization of p75 NTR from the cell surface. This activation of p75 NTR signaling is counteracted by tropomyosin-related kinase (Trk) receptor signaling; however, oxidative stress leads to Trk receptor downregulation, thereby enhancing p75 NTR processing. Importantly, we demonstrate that this pathway can be inhibited by LM11a-31, a small molecule modulator of p75 NTR , thereby conferring protection against neurodegeneration. Treatment with LM11a-31 significantly reduced p75 NTR cleavage and neuronal death associated with oxidative stress. These findings reveal novel mechanisms underlying activation of p75 NTR in response to oxidative stress, underscore a key role for p75 NTR in dopaminergic neurodegeneration, and highlight p75 NTR as a potential therapeutic target for reducing neurodegeneration in PD.

A Non-Canonical p75HER2 Signaling Pathway Underlying Trastuzumab Action and Resistance in Breast Cancer

Overexpression of HER2 occurs in 25% of breast cancer. Targeting HER2 has proven to be an effective therapeutic strategy for HER2-positive breast cancer. While trastuzumab is the most commonly used HER2 targeting agent, which has significantly improved outcomes, the overall response rate is low. To develop novel therapies to boost trastuzumab efficacy, it is critical to identify the mechanisms underlying trastuzumab action and resistance. We recently showed that the inhibition of breast cancer cell growth by trastuzumab is not through the inhibition of HER2 canonical signaling. Here we report the identification of a novel non-canonical HER2 signaling pathway and its interference by trastuzumab. We showed that HER2 signaled through a non-canonical pathway, regulated intramembrane proteolysis (RIP). In this pathway, HER2 is first cleaved by metalloprotease ADAM10 to produce an extracellular domain (ECD) that is released and the p95HER2 that contains the transmembrane domain (TM) and intracellular domain (ICD). p95HER2, if further cleaved by an intramembrane protease, γ-secretase, produced a soluble ICD p75HER2 with nuclear localization signal (NLS). p75HER2 is phosphorylated and translocated to the nucleus. Nuclear p75HER2 promotes cell proliferation. Trastuzumab targets this non-canonical HER2 pathway via inhibition of the proteolytic cleavage of HER2 by both ADAM10 and γ-secretase. However, p75HER2 pathway also confers resistance to trastuzumab once aberrantly activated. Combination of trastuzumab with ADAM10 and γ-secretase inhibitors completely blocks p75HER2 production in both BT474 and SKBR3 cells. We concluded that HER2 signals through the RIP signaling pathway that promotes cell proliferation and is targeted by trastuzumab. The aberrant HER2 RIP signaling confers resistance to trastuzumab that could be overcome by the application of inhibitors to ADAM10 and γ-secretase.

Cholinergic neurodegeneration and cholesterol metabolism dysregulation by constitutive p75NTR signaling in the p75exonIII-KO mice

Degeneration of basal forebrain cholinergic neurons (BFCNs) is a hallmark of Alzheimer's disease (AD). However, few mouse models of AD recapitulate the neurodegeneration of the cholinergic system. The p75 neurotrophin receptor, p75NTR, has been associated with the degeneration of BFCNs in AD. The senescence-accelerated mouse prone number 8 (SAMP8) is a well-accepted model of accelerated and pathological aging. To gain a better understanding of the role of p75NTR in the basal forebrain during aging, we generated a new mouse line, the SAMP8-p75exonIII-/-. Deletion of p75NTR in the SAMP8 background induces an increase in the number of BFCNs at birth, followed by a rapid decline during aging compared to the C57/BL6 background. This decrease in the number of BFCNs correlates with a worsening in the Y-maze memory test at 6 months in the SAMP8-p75exonIII-/-. We found that SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice expressed constitutively a short isoform of p75NTR that correlates with an upregulation of the protein levels of SREBP2 and its targets, HMGCR and LDLR, in the BF of both SAMP8-p75exonIII-/- and C57/BL6-p75exonIII-/- mice. As the neurodegeneration of the cholinergic system and the dysregulation of cholesterol metabolism are implicated in AD, we postulate that the generated SAMP8-p75exonIII-/- mouse strain might constitute a good model to study long-term cholinergic neurodegeneration in the CNS. In addition, our results support the role of p75NTR signaling in cholesterol biosynthesis regulation.

TrkA Co-Receptors: The Janus Face of TrkA?

Larotrectinib and Entrectinib are specific pan-Trk tyrosine kinase inhibitors (TKIs) approved by the Food and Drug Administration (FDA) in 2018 for cancers with an NTRK fusion. Despite initial enthusiasm for these compounds, the French agency (HAS) recently reported their lack of efficacy. In addition, primary and secondary resistance to these TKIs has been observed in the absence of other mutations in cancers with an NTRK fusion. Furthermore, when TrkA is overexpressed, it promotes ligand-independent activation, bypassing the TKI. All of these clinical and experimental observations show that genetics does not explain all therapeutic failures. It is therefore necessary to explore new hypotheses to explain these failures. This review summarizes the current status of therapeutic strategies with TrkA inhibitors, focusing on the mechanisms potentially involved in these failures and more specifically on the role of TrkA.

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.

The Rab11-regulated endocytic pathway and BDNF/TrkB signaling: Roles in plasticity changes and neurodegenerative diseases

Neurons are highly polarized cells that rely on the intracellular transport of organelles. This process is regulated by molecular motors such as dynein and kinesins and the Rab family of monomeric GTPases that together help move cargo along microtubules in dendrites, somas, and axons. Rab5-Rab11 GTPases regulate receptor trafficking along early-recycling endosomes, which is a process that determines the intracellular signaling output of different signaling pathways, including those triggered by BDNF binding to its tyrosine kinase receptor TrkB. BDNF is a well-recognized neurotrophic factor that regulates experience-dependent plasticity in different circuits in the brain. The internalization of the BDNF/TrkB complex results in signaling endosomes that allow local signaling in dendrites and presynaptic terminals, nuclear signaling in somas and dynein-mediated long-distance signaling from axons to cell bodies. In this review, we briefly discuss the organization of the endocytic pathway and how Rab11-recycling endosomes interact with other endomembrane systems. We further expand upon the roles of the Rab11-recycling pathway in neuronal plasticity. Then, we discuss the BDNF/TrkB signaling pathways and their functional relationships with the postendocytic trafficking of BDNF, including axonal transport, emphasizing the role of BDNF signaling endosomes, particularly Rab5-Rab11 endosomes, in neuronal plasticity. Finally, we discuss the evidence indicating that the dysfunction of the early-recycling pathway impairs BDNF signaling, contributing to several neurodegenerative diseases.

NGF/TRKA Promotes ADAM17-Dependent Cleavage of P75 in Ovarian Cells: Elucidating a Pro-Tumoral Mechanism

Nerve growth factor (NGF) and its high-affinity receptor TRKA are overexpressed in epithelial ovarian cancer (EOC) displaying a crucial role in the disease progression. Otherwise, NGF interacts with its low-affinity receptor P75, activating pro-apoptotic pathways. In neurons, P75 could be cleaved by metalloproteinases (α and γ-secretases), leading to a decrease in P75 signaling. Therefore, this study aimed to evaluate whether the shedding of P75 occurs in EOC cells and whether NGF/TRKA could promote the cleavage of the P75 receptor. The immunodetection of the α-secretase, ADAM17, TRKA, P75, and P75 fragments was assessed by immunohisto/cytochemistry and Western blot in biopsies and ovarian cell lines. The TRKA and secretases' inhibition was performed using specific inhibitors. The results show that P75 immunodetection decreased during EOC progression and was negatively correlated with the presence of TRKA in EOC biopsies. NGF/TRKA increases ADAM17 levels and the fragments of P75 in ovarian cells. This effect is abolished when cells are previously treated with ADAM17, γ-secretase, and TRKA inhibitors. These results indicate that NGF/TRKA promotes the shedding of P75, involving the activation of secretases such as ADAM17. Since ADAM17 has been proposed as a screening marker for early detection of EOC, our results contribute to understanding better the role of ADAM17 and NGF/TRKA in EOC pathogenesis, which includes the NGF/TRKA-mediated cleavage of P75.

Brothers in arms: proBDNF/BDNF and sAPPα/Aβ-signaling and their common interplay with ADAM10, TrkB, p75NTR, sortilin, and sorLA in the progression of Alzheimer's disease

Brain-derived neurotrophic factor (BDNF) is an important modulator for a variety of functions in the central nervous system (CNS). A wealth of evidence, such as reduced mRNA and protein level in the brain, cerebrospinal fluid (CSF), and blood samples of Alzheimer's disease (AD) patients implicates a crucial role of BDNF in the progression of this disease. Especially, processing and subcellular localization of BDNF and its receptors TrkB and p75 are critical determinants for survival and death in neuronal cells. Similarly, the amyloid precursor protein (APP), a key player in Alzheimer's disease, and its cleavage fragments sAPPα and Aβ are known for their respective roles in neuroprotection and neuronal death. Common features of APP- and BDNF-signaling indicate a causal relationship in their mode of action. However, the interconnections of APP- and BDNF-signaling are not well understood. Therefore, we here discuss dimerization properties, localization, processing by α- and γ-secretase, relevance of the common interaction partners TrkB, p75, sorLA, and sortilin as well as shared signaling pathways of BDNF and sAPPα.

Long-distance regressive signaling in neural development and disease

Nervous system development proceeds via well-orchestrated processes involving a balance between progressive and regressive events including stabilization or elimination of axons, synapses, and even entire neurons. These progressive and regressive events are driven by functionally antagonistic signaling pathways with the dominant pathway eventually determining whether a neural element is retained or removed. Many of these developmental sculpting events are triggered by final target innervation necessitating a long-distance mode of communication. While long-distance progressive signaling has been well characterized, particularly for neurotrophic factors, there remains relatively little known about how regressive events are triggered from a distance. Here we discuss the emergent phenomenon of long-distance regressive signaling pathways. In particular, we will cover (a) progressive and regressive cues known to be employed after target innervation, (b) the mechanisms of long-distance signaling from an endosomal platform, (c) recent evidence that long-distance regressive cues emanate from platforms like death receptors or repulsive axon guidance receptors, and (d) evidence that these pathways are exploited in pathological scenarios. This article is categorized under: Nervous System Development > Vertebrates: General Principles Signaling Pathways > Global Signaling Mechanisms Establishment of Spatial and Temporal Patterns > Cytoplasmic Localization.

TrkA-mediated endocytosis of p75-CTF prevents cholinergic neuron death upon γ-secretase inhibition

The findings shed light into the adverse effects of GSIs observed in the Alzheimer’s field and explain, at least in part, the unexpected worsening in cognition observed in the semagacestat Phase 3 trial.

γ-secretase inhibitors (GSI) were developed to reduce the generation of Aβ peptide to find new Alzheimer’s disease treatments. Clinical trials on Alzheimer’s disease patients, however, showed several side effects that worsened the cognitive symptoms of the treated patients. The observed side effects were partially attributed to Notch signaling. However, the effect on other γ-secretase substrates, such as the p75 neurotrophin receptor (p75NTR) has not been studied in detail. p75NTR is highly expressed in the basal forebrain cholinergic neurons (BFCNs) during all life. Here, we show that GSI treatment induces the oligomerization of p75CTF leading to the cell death of BFCNs, and that this event is dependent on TrkA activity. The oligomerization of p75CTF requires an intact cholesterol recognition sequence (CRAC) and the constitutive binding of TRAF6, which activates the JNK and p38 pathways. Remarkably, TrkA rescues from cell death by a mechanism involving the endocytosis of p75CTF. These results suggest that the inhibition of γ-secretase activity in aged patients, where the expression of TrkA in the BFCNs is already reduced, could accelerate cholinergic dysfunction and promote neurodegeneration.

Nardilysin controls cardiac sympathetic innervation patterning through regulation of p75 neurotrophin receptor

Cardiac sympathetic innervation is critically involved in the regulation of circulatory dynamics. However, the molecular mechanism for the innervation patterning has remained elusive. Here, we demonstrate that nardilysin (NRDC, Nrdc), an enhancer of ectodomain shedding, regulates cardiac sympathetic innervation. Nardilysin-deficient (Nrdc^-/- ) mice show hypoplastic hearts, hypotension, bradycardia, and abnormal sympathetic innervation patterning. While the innervation of left ventricle (LV) of wild-type mice is denser in the subepicardium than in the subendocardium, Nrdc^-/- LV lacks such a polarity and is uniformly and more abundantly innervated. At the molecular level, the full-length form of p75 neurotrophin receptor (p75^NTR , Ngfr) is increased in Nrdc^-/- LV due to the reduced ectodomain shedding of p75^NTR . Importantly, the reduction of p75^NTR rescued the abnormal innervation phenotype of Nrdc^-/- mice. Moreover, sympathetic neuron-specific, but not cardiomyocyte-specific deletion of Nrdc recapitulated the abnormal innervation patterning of Nrdc^-/- mice. In conclusion, neuronal nardilysin critically regulates cardiac sympathetic innervation and circulatory dynamics via modulation of p75^NTR .

Endosomal dysfunction impacts extracellular vesicle release: Central role in Aβ pathology

Alzheimer's Disease (AD) is characterized by progressive loss of cognitive abilities; senile plaques represent the major histopathological findings. Amyloid precursor protein (APP) processing machinery, and its product amyloid-beta (Aβ) peptide, have been found in extracellular vesicles (EVs), specifically exosomes, which allows for Aβ peptide aggregation and subsequent senile plaques deposition. We review the APP processing imbalance in EVs, autophagic and endosomal pathways in AD. Increased intraluminal vesicle (ILV) production and exosome release appear to counteract the endosomal dysfunction of APP processing; however, this process results in elevated amyloidogenic processing of APP and augmented senile plaque deposition. Several players related to APP processing and dysfunctional endosomal-lysosomal-exosomal (and other EVs) pathway are described, and the interconnected systems are discussed. The components Arc, p75, Rab11 and retromer complex emerge as candidates for key convergent mechanisms that lead to increased EVs loaded with APP machinery and Aβ levels, in atrophy and damage of basal forebrain cholinergic neurons in AD.

Epac1 inhibition ameliorates pathological angiogenesis through coordinated activation of Notch and suppression of VEGF signaling

In this study, we investigated the roles of Epac1 in pathological angiogenesis and its potential as a novel therapeutic target for the treatment of vasoproliferative diseases. Genetic deletion of Epac1 ameliorated pathological angiogenesis in mouse models of oxygen-induced retinopathy (OIR) and carotid artery ligation. Moreover, genetic deletion or pharmacological inhibition of Epac1 suppressed microvessel sprouting from ex vivo aortic ring explants. Mechanistic studies revealed that Epac1 acted as a previously unidentified inhibitor of the γ-secretase/Notch signaling pathway via interacting with γ-secretase and regulating its intracellular trafficking while enhancing vascular endothelial growth factor signaling to promote pathological angiogenesis. Pharmacological administration of an Epac-specific inhibitor suppressed OIR-induced neovascularization in wild-type mice, recapitulating the phenotype of genetic Epac1 knockout. Our results demonstrate that Epac1 signaling is critical for the progression of pathological angiogenesis but not for physiological angiogenesis and that the newly developed Epac-specific inhibitors are effective in combating proliferative retinopathy.

Recent advances in understanding context-dependent mechanisms controlling neurotrophin signaling and function

Complex mechanisms control the signaling of neurotrophins through p75 NTR and Trk receptors, allowing cellular responses that are highly context dependent, particularly in the nervous system and particularly with regard to the neurotrophin brain-derived neurotrophic factor (BDNF). Recent reports describe a variety of sophisticated regulatory mechanisms that contribute to such functional flexibility. Mechanisms described include regulation of trafficking of alternative BDNF transcripts, regulation of post-translational processing and secretion of BDNF, engagement of co-receptors that influence localization and signaling of p75 NTR and Trk receptors, and control of trafficking of receptors in the endocytic pathway and during anterograde and retrograde axonal transport.

The p75NTR neurotrophin receptor is required to organize the mature neuromuscular synapse by regulating synaptic vesicle availability

The coordinated movement of organisms relies on efficient nerve-muscle communication at the neuromuscular junction. After peripheral nerve injury or neurodegeneration, motor neurons and Schwann cells increase the expression of the p75NTR pan-neurotrophin receptor. Even though p75NTR targeting has emerged as a promising therapeutic strategy to delay peripheral neuronal damage progression, the effects of long-term p75NTR inhibition at the mature neuromuscular junction have not been elucidated. We performed quantitative neuroanathomical analyses of the neuromuscular junction in p75NTR null mice by laser confocal and electron microscopy, which were complemented with electromyography, locomotor tests, and pharmacological intervention studies. Mature neuromuscular synapses of p75NTR null mice show impaired postsynaptic organization and ultrastructural complexity, which correlate with altered synaptic function at the levels of nerve activity-induced muscle responses, muscle fiber structure, force production, and locomotor performance. Our results on primary myotubes and denervated muscles indicate that muscle-derived p75NTR does not play a major role on postsynaptic organization. In turn, motor axon terminals of p75NTR null mice display a strong reduction in the number of synaptic vesicles and active zones. According to the observed pre and postsynaptic defects, pharmacological acetylcholinesterase inhibition rescued nerve-dependent muscle response and force production in p75NTR null mice. Our findings revealing that p75NTR is required to organize mature neuromuscular junctions contribute to a comprehensive view of the possible effects caused by therapeutic attempts to target p75NTR.

c-Jun N-terminal kinase (JNK)-dependent internalization and Rab5-dependent endocytic sorting mediate long-distance retrograde neuronal death induced by axonal BDNF-p75 signaling

During the development of the sympathetic nervous system, signals from tropomyosin-related kinase receptors (Trks) and p75 neurotrophin receptors (p75) compete to regulate survival and connectivity. During this process, nerve growth factor (NGF)- TrkA signaling in axons communicates NGF-mediated trophic responses in signaling endosomes. Whether axonal p75 signaling contributes to neuronal death and how signaling endosomes contribute to p75 signaling has not been established. Using compartmentalized sympathetic neuronal cultures (CSCGs) as a model, we observed that the addition of BDNF to axons increased the transport of p75 and induced death of sympathetic neurons in a dynein-dependent manner. In cell bodies, internalization of p75 required the activity of JNK, a downstream kinase mediating p75 death signaling in neurons. Additionally, the activity of Rab5, the key GTPase regulating early endosomes, was required for p75 death signaling. In axons, JNK and Rab5 were required for retrograde transport and death signaling mediated by axonal BDNF-p75 in CSCGs. JNK was also required for the proper axonal transport of p75-positive endosomes. Thus, our findings provide evidence that the activation of JNK by p75 in cell bodies and axons is required for internalization to a Rab5-positive signaling endosome and the further propagation of p75-dependent neuronal death signals.

The retromer complex safeguards against neural progenitor-derived tumorigenesis by regulating Notch receptor trafficking

The correct establishment and maintenance of unidirectional Notch signaling are critical for the homeostasis of various stem cell lineages. However, the molecular mechanisms that prevent cell-autonomous ectopic Notch signaling activation and deleterious cell fate decisions remain unclear. Here we show that the retromer complex directly and specifically regulates Notch receptor retrograde trafficking in Drosophila neuroblast lineages to ensure the unidirectional Notch signaling from neural progenitors to neuroblasts. Notch polyubiquitination mediated by E3 ubiquitin ligase Itch/Su(dx) is inherently inefficient within neural progenitors, relying on retromer-mediated trafficking to avoid aberrant endosomal accumulation of Notch and cell-autonomous signaling activation. Upon retromer dysfunction, hypo-ubiquitinated Notch accumulates in Rab7⁺ enlarged endosomes, where it is ectopically processed and activated in a ligand-dependent manner, causing progenitor-originated tumorigenesis. Our results therefore unveil a safeguard mechanism whereby retromer retrieves potentially harmful Notch receptors in a timely manner to prevent aberrant Notch activation-induced neural progenitor dedifferentiation and brain tumor formation.

Most cells in the animal body are tailored to perform particular tasks, but stem cells have not yet made their choice. Instead, they have unlimited capacity to divide and, with the right signals, they can start to specialize to become a given type of cells. In the brain, this process starts with a stem cell dividing. One of the daughters will remain a stem cell, while the other, the neural progenitor, will differentiate to form a mature cell such as a neuron. Keeping this tight balance is crucial for the health of the organ: if the progenitor reverts back to being a stem cell, there will be a surplus of undifferentiated cells that can lead to a tumor.

A one-way signal driven by the protein Notch partly controls the distinct fates of the two daughter cells. While the neural progenitor carries Notch at its surface, its neural stem cell sister has a Notch receptor on its membrane instead. This ensures that the Notch signaling goes in one direction, from the cell with Notch to the one sporting the receptor.

When a stem cell divides, one daughter gets more of a protein called Numb than the other. Numb pulls Notch receptors away from the external membrane and into internal capsules called endosomes. This guarantees that only one of the siblings will be carrying the receptors at its surface. Yet, sometimes the Notch receptors can get activated in the endosomes, which may make neural progenitors revert to being stem cells. It is still unclear what tools the cells have to stop this abnormal activation.

Here, Li et al. screened brain cells from fruit fly larvae to find out the genes that might play a role in suppressing the inappropriate Notch signaling. This highlighted a protein complex known as the retromer, which normally helps to transport proteins in the cell. Experiments showed that, in progenitors, the retromer physically interacts with Notch receptors and retrieves them from the endosomes back to the cell surface. If the retromer is inactive, the Notch receptors accumulate in the endosomes, where they can be switched on. It seems that, in fruit flies, the retromer acts as a bomb squad that recognizes and retrieves potentially harmful Notch receptors, thereby preventing brain tumor formation.

Several retromer components are less present in patients with various cancers, including glioblastoma, an aggressive form of brain cancer. The results by Li et al. may therefore shed light on the link between the protein complex and the emergence of the disease in humans.

Retrograde Degenerative Signaling Mediated by the p75 Neurotrophin Receptor Requires p150Glued Deacetylation by Axonal HDAC1

During development, neurons undergo apoptosis if they do not receive adequate trophic support from tissues they innervate or when detrimental factors activate the p75 neurotrophin receptor (p75NTR) at their axon ends. Trophic factor deprivation (TFD) or activation of p75NTR in distal axons results in a retrograde degenerative signal. However, the nature of this signal and the regulation of its transport are poorly understood. Here, we identify p75NTR intracellular domain (ICD) and histone deacetylase 1 (HDAC1) as part of a retrograde pro-apoptotic signal generated in response to TFD or ligand binding to p75NTR in sympathetic neurons. We report an unconventional function of HDAC1 in retrograde transport of a degenerative signal and its constitutive presence in sympathetic axons. HDAC1 deacetylates dynactin subunit p150Glued, which enhances its interaction with dynein. These findings define p75NTR ICD as a retrograde degenerative signal and reveal p150Glued deacetylation as a unique mechanism regulating axonal transport.

A Review of Notch Processing With New Insights Into Ligand-Independent Notch Signaling in T-Cells

The Notch receptor is an evolutionarily highly conserved transmembrane protein essential to a wide spectrum of cellular systems, and its deregulation has been linked to a vast number of developmental disorders and malignancies. Regulated Notch function is critical for the generation of T-cells, in which abnormal Notch signaling results in leukemia. Notch activation through trans-activation of the receptor by one of its ligands expressed on adjacent cells has been well defined. In this canonical ligand-dependent pathway, Notch receptor undergoes conformational changes upon ligand engagement, stimulated by a pulling-force on the extracellular fragment of Notch that results from endocytosis of the receptor-bound ligand into the ligand-expressing cell. These conformational changes in the receptor allow for two consecutive proteolytic cleavage events to occur, which release the intracellular region of the receptor into the cytoplasm. It can then travel to the nucleus, where it induces gene transcription. However, there is accumulating evidence that other pathways may induce Notch signaling. A ligand-independent mechanism of Notch activation has been described in which receptor processing is initiated via cell-internal signals. These signals result in the internalization of Notch into endosomal compartments, where chemical changes existing in this microenvironment result in the conformational modifications required for receptor processing. This review will present mechanisms underlying both canonical ligand-dependent and non-canonical ligand-independent Notch activation pathways and discuss the latter in the context of Notch signaling in T-cells.

Nogo-A interacts with TrkA to alter nerve growth factor signaling in Nogo-A-overexpressing PC12 cells

The Nogo-A protein, originally discovered as a potent myelin-associated inhibitor of neurite outgrowth, is also expressed by certain neurons, especially during development and after injury, but its role in neuronal function is not completely known. In this report, we overexpressed Nogo-A in PC12 cells to use as a model to identify potential neuronal signaling pathways affected by endogenously expressed Nogo-A. Unexpectedly, our results show that viability of Nogo-A-overexpressing cells was reduced progressively due to apoptotic cell death following NGF treatment, but only after 24 h. Inhibitors of neutral sphingomyelinase prevented this loss of viability, suggesting that NGF induced the activation of a ceramide-dependent cell death pathway. Nogo-A over-expression also changed NGF-induced phosphorylation of TrkA at tyrosines 490 and 674/675 from sustained to transient, and prevented the regulated intramembrane proteolysis of p75^NTR, indicating that Nogo-A was altering the function of the two neurotrophin receptors. Co-immunoprecipitation studies revealed that there was a physical association between TrkA and Nogo-A which appeared to be dependent on interactions in the Nogo-A-specific region of the protein. Taken together, our results indicate that Nogo-A influences NGF-mediated mechanisms involving the activation of TrkA and its interaction with p75^NTR.

Regulated intramembrane proteolysis: emergent role in cell signalling pathways

Receptor signalling events including those initiated following activation of cytokine and growth factor receptors and the well-characterised death receptors (tumour necrosis factor receptor, type 1, FasR and TRAIL-R1/2) are initiated at the cell surface through the recruitment and formation of intracellular multiprotein signalling complexes that activate divergent signalling pathways. Over the past decade, research studies reveal that many of these receptor-initiated signalling events involve the sequential proteolysis of specific receptors by membrane-bound proteases and the γ-secretase protease complexes. Proteolysis enables the liberation of soluble receptor ectodomains and the generation of intracellular receptor cytoplasmic domain fragments. The combined and sequential enzymatic activity has been defined as regulated intramembrane proteolysis and is now a fundamental signal transduction process involved in the termination or propagation of receptor signalling events. In this review, we discuss emerging evidence for a role of the γ-secretase protease complexes and regulated intramembrane proteolysis in cell- and immune-signalling pathways.

Retrograde apoptotic signaling by the p75 neurotrophin receptor

Neurotrophins are target-derived factors necessary for mammalian nervous system development and maintenance. They are typically produced by neuronal target tissues and interact with their receptors at axonal endings. Therefore, locally generated neurotrophin signals must be conveyed from the axon back to the cell soma. Retrograde survival signaling by neurotrophin binding to Trk receptors has been extensively studied. However, neurotrophins also bind to the p75 receptor, which can induce apoptosis in a variety of contexts. Selective activation of p75 at distal axon ends has been shown to generate a retrograde apoptotic signal, although the mechanisms involved are poorly understood. The present review summarizes the available evidence for retrograde proapoptotic signaling in general and the role of the p75 receptor in particular, with discussion of unanswered questions in the field. In-depth knowledge of the mechanisms of retrograde apoptotic signaling is essential for understanding the etiology of neurodegeneration in many diseases and injuries.

Nerve growth factor (NGF)-mediated regulation of p75(NTR) expression contributes to chemotherapeutic resistance in triple negative breast cancer cells

Triple negative breast cancer [TNBC] cells are reported to secrete the neurotrophin nerve growth factor [NGF] and express its receptors, p75 neurotrophin receptor [p75(NTR)] and TrkA, leading to NGF-activated pro-survival autocrine signaling. This provides a rationale for NGF as a potential therapeutic target for TNBC. Here we show that exposure of TNBC cells to NGF leads to increased levels of p75(NTR), which was diminished by NGF-neutralizing antibody or NGF inhibitors [Ro 08-2750 and Y1086]. NGF-mediated increase in p75(NTR) levels were partly due to increased transcription and partly due to inhibition of proteolytic processing of p75(NTR). In contrast, proNGF caused a decrease in p75(NTR) levels. Functionally, NGF-induced increase in p75(NTR) caused a decrease in the sensitivity of TNBC cells to apoptosis induction. In contrast, knock-down of p75(NTR) using shRNA or small molecule inhibition of NGF-p75(NTR) interaction [using Ro 08-2750] sensitized TNBC cells to drug-induced apoptosis. In patient samples, the expression of NGF and NGFR [the p75(NTR) gene] mRNA are positively correlated in several subtypes of breast cancer, including basal-like breast cancer. Together these data suggest a positive feedback loop through which NGF-mediated upregulation of p75(NTR) can contribute to the chemo-resistance of TNBC cells.

Recent advances in understanding neurotrophin signaling

The nerve growth factor family of growth factors, collectively known as neurotrophins, are evolutionarily ancient regulators with an enormous range of biological functions. Reflecting this long history and functional diversity, mechanisms for cellular responses to neurotrophins are exceptionally complex. Neurotrophins signal through p75 ^NTR, a member of the TNF receptor superfamily member, and through receptor tyrosine kinases (TrkA, TrkB, TrkC), often with opposite functional outcomes. The two classes of receptors are activated preferentially by proneurotrophins and mature processed neurotrophins, respectively. However, both receptor classes also possess neurotrophin-independent signaling functions. Signaling functions of p75 ^NTR and Trk receptors are each influenced by the other class of receptors. This review focuses on the mechanisms responsible for the functional interplay between the two neurotrophin receptor signaling systems.

Intramuscular delivery of p75NTR ectodomain by an AAV vector attenuates cognitive deficits and Alzheimer's disease-like pathologies in APP/PS1 transgenic mice

The neurotrophin receptor p75 (p75NTR) is a receptor for amyloid-beta (Aβ) and mediates Aβ-induced neurodegenerative signals. The ectodomain of p75NTR (p75ECD) is a physiological protective factor against Aβ in Alzheimer's disease (AD). We have previously demonstrated that the shedding of p75ECD from the cell surface is down-regulated in AD brains and restoration of the p75ECD level in the brain, through intracranial administration of p75ECD by adeno-associated virus vectors, attenuates AD-like pathologies in an AD mouse model. In this study, we further investigated the feasibility and efficacy of peripheral administration of AAV-p75ECD on brain amyloid burden and associated pathogenesis. We found that intramuscular delivery of AAV-p75ECD increased the level of p75ECD in the blood, significantly improved the behavioral phenotype of amyloid precursor protein/PS1 transgenic mice, and reduced brain amyloid burden, attenuated Tau hyperphosphorylation, and neuroinflammation. Furthermore, intramuscular delivery of AAV-p75ECD was well tolerated. Our results indicate that peripheral delivery of p75ECD represents a safe and effective therapeutic strategy for AD. The ectodomain of p75NTR (p75ECD) is a physiological protective factor against amyloid-beta (Aβ) in Alzheimer's disease (AD). Intramuscular delivery of AAV-p75ECD increased the p75ECD levels in the blood, reduced brain amyloid burden through a 'peripheral sink' mechanism and alleviates AD-type pathologies. Peripheral delivery of p75ECD represents a promising therapeutic strategy for AD.

Presenilin 1 and Presenilin 2 Target γ-Secretase Complexes to Distinct Cellular Compartments

γ-Secretase complexes achieve the production of amyloid peptides playing a key role in Alzheimer disease. These proteases have many substrates involved in important physiological functions. They are composed of two constant subunits, nicastrin and PEN2, and two variable ones, presenilin (PS1 or PS2) and APH1 (APH1aL, APH1aS, or APH1b). Whether the composition of a given γ-secretase complex determines a specific cellular targeting remains unsolved. Here we combined a bidirectional inducible promoter and 2A peptide technology to generate constructs for the temporary, stoichiometric co-expression of six different combinations of the four γ-secretase subunits including EGFP-tagged nicastrin. These plasmids allow for the formation of functional γ-secretase complexes displaying specific activities and maturations. We show that PS1-containing γ-secretase complexes were targeted to the plasma membrane, whereas PS2-containing ones were addressed to the trans-Golgi network, to recycling endosomes, and, depending on the APH1-variant, to late endocytic compartments. Overall, these novel constructs unravel a presenilin-dependent subcellular targeting of γ-secretase complexes. These tools should prove useful to determine whether the cellular distribution of γ-secretase complexes contributes to substrate selectivity and to delineate regulations of their trafficking.

γ-Secretase Activity Is Required for Regulated Intramembrane Proteolysis of Tumor Necrosis Factor (TNF) Receptor 1 and TNF-mediated Pro-apoptotic Signaling

The γ-secretase protease and associated regulated intramembrane proteolysis play an important role in controlling receptor-mediated intracellular signaling events, which have a central role in Alzheimer disease, cancer progression, and immune surveillance. An increasing number of γ-secretase substrates have a role in cytokine signaling, including the IL-6 receptor, IL-1 receptor type I, and IL-1 receptor type II. In this study, we show that following TNF-converting enzyme-mediated ectodomain shedding of TNF type I receptor (TNFR1), the membrane-bound TNFR1 C-terminal fragment is subsequently cleaved by γ-secretase to generate a cytosolic TNFR1 intracellular domain. We also show that clathrin-mediated internalization of TNFR1 C-terminal fragment is a prerequisite for efficient γ-secretase cleavage of TNFR1. Furthermore, using in vitro and in vivo model systems, we show that in the absence of presenilin expression and γ-secretase activity, TNF-mediated JNK activation was prevented, assembly of the TNFR1 pro-apoptotic complex II was reduced, and TNF-induced apoptosis was inhibited. These observations demonstrate that TNFR1 is a γ-secretase substrate and suggest that γ-secretase cleavage of TNFR1 represents a new layer of regulation that links the presenilins and the γ-secretase protease to pro-inflammatory cytokine signaling.

p75NTR ectodomain is a physiological neuroprotective molecule against amyloid-beta toxicity in the brain of Alzheimer's disease

In Alzheimer's disease (AD), neurodegenerative signals such as amyloid-beta (Aβ) and the precursors of neurotrophins, outbalance neurotrophic signals, causing synaptic dysfunction and neurodegeneration. The neurotrophin receptor p75 (p75NTR) is a receptor of Aβ and mediates Aβ-induced neurodegenerative signals. The shedding of its ectodomain from the cell surface is physiologically regulated; however, the function of the diffusible p75NTR ectodomain (p75ECD) after shedding remains largely not known. Here, we show that p75ECD levels in cerebrospinal fluid and in the brains of Alzheimer's patients and amyloid-beta precursor protein (APP)/PS1 transgenic mice were significantly reduced, due to inhibition of the sheddase-tumor necrosis factor-alpha-converting enzyme by Aβ. Restoration of p75ECD to the normal level by brain delivery of the gene encoding human p75ECD before or after Aβ deposition in the brain of APP/PS1 mice reversed the behavioral deficits and AD-type pathologies, such as Aβ deposit, apoptotic events, neuroinflammation, Tau phosphorylation and loss of dendritic spine, neuronal structures and synaptic proteins. Furthermore, p75ECD can also reduce amyloidogenesis by suppressing β-secretase expression and activities. Our data demonstrate that p75ECD is a physiologically neuroprotective molecule against Aβ toxicity and would be a novel therapeutic target and biomarker for AD.

Neuron-type-specific signaling by the p75NTR death receptor is regulated by differential proteolytic cleavage

Signaling by the p75 neurotrophin receptor (p75(NTR), also known as NGFR) is often referred to as cell-context dependent, but neuron-type-specific signaling by p75(NTR) has not been systematically investigated. Here, we report that p75(NTR) signals very differently in hippocampal neurons (HCNs) and cerebellar granule neurons (CGNs), and we present evidence indicating that this is partly controlled by differential proteolytic cleavage. Nerve growth factor (NGF) induced caspase-3 activity and cell death in HCNs but not in CGNs, whereas it stimulated NFκB activity in CGNs but not in HCNs. HCNs and CGNs displayed different patterns of p75(NTR) proteolytic cleavage. Whereas the p75(NTR) carboxy terminal fragment (CTF) was more abundant than the intracellular domain (ICD) in HCNs, CGNs exhibited fully processed ICD with very little CTF. Pharmacological or genetic blockade of p75(NTR) cleavage by γ-secretase abolished NGF-induced upregulation of NFκB activity and enabled induction of CGN death, phenocopying the functional profile of HCNs. Thus, the activities of multifunctional receptors, such as p75(NTR), can be tuned into narrower activity profiles by cell-type-specific differences in intracellular processes, such as proteolytic cleavage, leading to very different biological outcomes.

LINGO-1 protein interacts with the p75 neurotrophin receptor in intracellular membrane compartments

Axon outgrowth inhibition in response to trauma is thought to be mediated via the binding of myelin-associated inhibitory factors (e.g. Nogo-66, myelin-associated glycoprotein, oligodendrocyte myelin glycoprotein, and myelin basic protein) to a putative tripartite LINGO-1·p75(NTR)·Nogo-66 receptor (NgR) complex at the cell surface. We found that endogenous LINGO-1 expression in neurons in the cortex and cerebellum is intracellular. Mutation or truncation of the highly conserved LINGO-1 C terminus altered this intracellular localization, causing poor intracellular retention and increased plasma membrane expression. p75(NTR) associated predominantly with natively expressed LINGO-1 containing immature N-glycans, characteristic of protein that has not completed trans-Golgi-mediated processing, whereas mutant forms of LINGO-1 with enhanced plasma membrane expression did not associate with p75(NTR). Co-immunoprecipitation experiments demonstrated that LINGO-1 and NgR competed for binding to p75(NTR) in a manner that is difficult to reconcile with the existence of a LINGO-1·p75(NTR)·NgR ternary complex. These findings contradict models postulating functional LINGO-1·p75(NTR)·NgR complexes in the plasma membrane.

The conundrum of the high-affinity NGF binding site formation unveiled?

The homodimer NGF (nerve growth factor) exerts its neuronal activity upon binding to either or both distinct transmembrane receptors TrkA and p75(NTR). Functionally relevant interactions between NGF and these receptors have been proposed, on the basis of binding and signaling experiments. Namely, a ternary TrkA/NGF/p75(NTR) complex is assumed to be crucial for the formation of the so-called high-affinity NGF binding sites. However, the existence, on the cell surface, of direct extracellular interactions is still a matter of controversy. Here, supported by a small-angle x-ray scattering solution study of human NGF, we propose that it is the oligomerization state of the secreted NGF that may drive the formation of the ternary heterocomplex. Our data demonstrate the occurrence in solution of a concentration-dependent distribution of dimers and dimer of dimers. A head-to-head molecular assembly configuration of the NGF dimer of dimers has been validated. Overall, these findings prompted us to suggest a new, to our knowledge, model for the transient ternary heterocomplex, i.e., a TrkA/NGF/p75(NTR) ligand/receptors molecular assembly with a (2:4:2) stoichiometry. This model would neatly solve the problem posed by the unconventional orientation of p75(NTR) with respect to TrkA, as being found in the crystal structures of the TrkA/NGF and p75(NTR)/NGF complexes.

Neurotrophins regulate ApoER2 proteolysis through activation of the Trk signaling pathway

Background

ApoER2 and the neurotrophin receptors Trk and p75^NTR are expressed in the CNS and regulate key functional aspects of neurons, including development, survival, and neuronal function. It is known that both ApoER2 and p75^NTR are processed by metalloproteinases, followed by regulated intramembrane proteolysis. TrkA activation by nerve growth factor (NGF) increases the proteolytic processing of p75^NTR mediated by ADAM17. Reelin induces the sheeding of ApoER2 ectodomain depending on metalloproteinase activity. However, it is not known if there is a common regulation mechanism for processing these receptors.

Results

We found that TrkA activation by NGF in PC12 cells induced ApoER2 processing, which was dependent on TrkA activation and metalloproteinases. NGF-induced ApoER2 proteolysis was independent of mitogen activated protein kinase activity and of phosphatidylinositol-3 kinase activity. In contrast, the basal proteolysis of ApoER2 increased when both kinases were pharmacologically inhibited. The ApoER2 ligand reelin regulated the proteolytic processing of its own receptor but not of p75^NTR. Finally, in primary cortical neurons, which express both ApoER2 and TrkB, we found that the proteolysis of ApoER2 was also regulated by brain-derived growth factor (BDNF).

Conclusions

Our results highlight a novel relationship between neurotrophins and the reelin-ApoER2 system, suggesting that these two pathways might be linked to regulate brain development, neuronal survival, and some pathological conditions.

Role of p75 neurotrophin receptor in stem cell biology: more than just a marker

p75(NTR), the common receptor for both neurotrophins and proneurotrophins, has been widely studied because of its role in many tissues, including the nervous system. More recently, a close relationship between p75(NTR) expression and pluripotency has been described. p75(NTR) was shown to be expressed in various types of stem cells and has been used to prospectively isolate stem cells with different degrees of potency. Here, we give an overview of the current knowledge on p75(NTR) in stem cells, ranging from embryonic to adult stem cells, and cancer stem cells. In an attempt to address its potential role in the control of stem cell biology, the molecular mechanisms underlying p75(NTR) signaling in different models are also highlighted. p75(NTR)-mediated functions include survival, apoptosis, migration, and differentiation, and depend on cell type, (pro)neurotrophin binding, interacting transmembrane co-receptors expression, intracellular adaptor molecule availability, and post-translational modifications, such as regulated proteolytic processing. It is therefore conceivable that p75(NTR) can modulate cell-fate decisions through its highly ramified signaling pathways. Thus, elucidating the potential implications of p75(NTR) activity as well as the underlying molecular mechanisms of p75(NTR) will shed new light on the biology of both normal and cancer stem cells.

The p75 neurotrophin receptor evades the endolysosomal route in neuronal cells, favouring multivesicular bodies specialised for exosomal release

The p75 neurotrophin receptor (p75, also known as NGFR) is a multifaceted signalling receptor that regulates neuronal physiology, including neurite outgrowth, and survival and death decisions. A key cellular aspect regulating neurotrophin signalling is the intracellular trafficking of their receptors; however, the post-endocytic trafficking of p75 is poorly defined. We used sympathetic neurons and rat PC12 cells to study the mechanism of internalisation and post-endocytic trafficking of p75. We found that p75 internalisation depended on the clathrin adaptor protein AP2 and on dynamin. More surprisingly, p75 evaded the lysosomal route at the level of the early endosome, instead accumulating in two different types of endosomes, Rab11-positive endosomes and multivesicular bodies (MVBs) positive for CD63, a marker of the exosomal pathway. Consistently, depolarisation by KCl induced the liberation of previously endocytosed full-length p75 into the extracellular medium in exosomes. Thus, p75 defines a subpopulation of MVBs that does not mature to lysosomes and is available for exosomal release by neuronal cells.

p75 neurotrophin receptor cleavage by α- and γ-secretases is required for neurotrophin-mediated proliferation of brain tumor-initiating cells

Malignant gliomas are highly invasive, proliferative, and resistant to treatment. Previously, we have shown that p75 neurotrophin receptor (p75NTR) is a novel mediator of invasion of human glioma cells. However, the role of p75NTR in glioma proliferation is unknown. Here we used brain tumor-initiating cells (BTICs) and show that BTICs express neurotrophin receptors (p75NTR, TrkA, TrkB, and TrkC) and their ligands (NGF, brain-derived neurotrophic factor, and neurotrophin 3) and secrete NGF. Down-regulation of p75NTR significantly decreased proliferation of BTICs. Conversely, exogenouous NGF stimulated BTIC proliferation through α- and γ-secretase-mediated p75NTR cleavage and release of its intracellular domain (ICD). In contrast, overexpression of the p75NTR ICD induced proliferation. Interestingly, inhibition of Trk signaling blocked NGF-stimulated BTIC proliferation and p75NTR cleavage, indicating a role of Trk in p75NTR signaling. Further, blocking p75NTR cleavage attenuated Akt activation in BTICs, suggesting role of Akt in p75NTR-mediated proliferation. We also found that p75NTR, α-secretases, and the four subunits of the γ-secretase enzyme were elevated in glioblastoma multiformes patients. Importantly, the ICD of p75NTR was commonly found in malignant glioma patient specimens, suggesting that the receptor is activated and cleaved in patient tumors. These results suggest that p75NTR proteolysis is required for BTIC proliferation and is a novel potential clinical target.

Spatiotemporal intracellular dynamics of neurotrophin and its receptors. Implications for neurotrophin signaling and neuronal function

Neurons possess a polarized morphology specialized to contribute to neuronal networks, and this morphology imposes an important challenge for neuronal signaling and communication. The physiology of the network is regulated by neurotrophic factors that are secreted in an activity-dependent manner modulating neuronal connectivity. Neurotrophins are a well-known family of neurotrophic factors that, together with their cognate receptors, the Trks and the p75 neurotrophin receptor, regulate neuronal plasticity and survival and determine the neuronal phenotype in healthy and regenerating neurons. Is it now becoming clear that neurotrophin signaling and vesicular transport are coordinated to modify neuronal function because disturbances of vesicular transport mechanisms lead to disturbed neurotrophin signaling and to diseases of the nervous system. This chapter summarizes our current understanding of how the regulated secretion of neurotrophin, the distribution of neurotrophin receptors in different locations of neurons, and the intracellular transport of neurotrophin-induced signaling in distal processes are achieved to allow coordinated neurotrophin signaling in the cell body and axons.

BDNF transcripts, proBDNF and proNGF, in the cortex and hippocampus throughout the life span of the rat

Neurotrophins are established molecular mediators of neuronal plasticity in the adult brain. We analyzed the impact of aging on brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) protein isoforms, their receptors, and on the expression patterns of multiple 5' exon-specific BDNF transcripts in the rat cortex and hippocampus throughout the life span of the rat (6, 12, 18, and 24 months of age). ProNGF was increased during aging in both structures. Mature NGF gradually decreased in the cortex, and, in 24-month-old animals, it was 30% lower than that in adult 6-month-old rats. The BDNF expression did not change during aging, while proBDNF accumulated in the hippocampus of aged rats. Hippocampal total BDNF mRNA was lower in 12-month-old animals, mostly as a result of a decrease of BDNF transcripts 1 and 2. In contrast to the region-specific regulation of specific exon-containing BDNF mRNAs in adult animals, the same BDNF RNA isoforms (containing exons III, IV, or VI) were present in both brain structures of aged animals. Deficits in neurotrophin signaling were supported by the observed decrease in Trk receptor expression which was accompanied by lower levels of the two main downstream effector kinases, pAkt and protein kinase C. The proteolytic processing of p75NTR observed in 12-month-old rats points to an additional regulatory mechanism in early aging. The changes described herein could contribute to reduced brain plasticity underlying the age-dependent decline in cognitive function.

Small-molecule modulation of neurotrophin receptors: a strategy for the treatment of neurological disease

Neurotrophins and their receptors modulate multiple signalling pathways to regulate neuronal survival and to maintain axonal and dendritic networks and synaptic plasticity. Neurotrophins have potential for the treatment of neurological diseases. However, their therapeutic application has been limited owing to their poor plasma stability, restricted nervous system penetration and, importantly, the pleiotropic actions that derive from their concomitant binding to multiple receptors. One strategy to overcome these limitations is to target individual neurotrophin receptors — such as tropomyosin receptor kinase A (TRKA), TRKB, TRKC, the p75 neurotrophin receptor or sortilin — with small-molecule ligands. Such small molecules might also modulate various aspects of these signalling pathways in ways that are distinct from the programmes triggered by native neurotrophins. By departing from conventional neurotrophin signalling, these ligands might provide novel therapeutic options for a broad range of neurological indications.

An intracellular domain fragment of the p75 neurotrophin receptor (p75(NTR)) enhances tropomyosin receptor kinase A (TrkA) receptor function

Facilitation of nerve growth factor (NGF) signaling by the p75 neurotrophin receptor (p75(NTR)) is critical for neuronal survival and differentiation. However, the interaction between p75(NTR) and TrkA receptors required for this activity is not understood. Here, we report that a specific 29-amino acid peptide derived from the intracellular domain fragment of p75(NTR) interacts with and potentiates binding of NGF to TrkA-expressing cells, leading to increased neurite outgrowth in sympathetic neurons as a result of enhanced Erk1/2 and Akt signaling. An endogenous intracellular domain fragment of p75(NTR) (p75(ICD)) containing these 29 amino acids is produced by regulated proteolysis of the full-length receptor. We demonstrate that generation of this fragment is a requirement for p75(NTR) to facilitate TrkA signaling in neurons and propose that the juxtamembrane region of p75(ICD) acts to cause a conformational change within the extracellular domain of TrkA. This finding provides new insight into the mechanism by which p75(NTR) and TrkA interact to enhance neurotrophic signaling.

Mapping of the interaction site between sortilin and the p75 neurotrophin receptor reveals a regulatory role for the sortilin intracellular domain in p75 neurotrophin receptor shedding and apoptosis

Neurotrophins comprise a group of neuronal growth factors that are essential for the development and maintenance of the nervous system. However, the immature pro-neurotrophins promote apoptosis by engaging in a complex with sortilin and the p75 neurotrophin receptor (p75(NTR)). To identify the interaction site between sortilin and p75(NTR), we analyzed binding between chimeric receptor constructs and truncated p75(NTR) variants by co-immunoprecipitation experiments, surface plasmon resonance analysis, and FRET. We found that complex formation between sortilin and p75(NTR) relies on contact points in the extracellular domains of the receptors. We also determined that the interaction critically depends on an extracellular juxtamembrane 23-amino acid sequence of p75(NTR). Functional studies further revealed an important regulatory function of the sortilin intracellular domain in p75(NTR)-regulated intramembrane proteolysis and apoptosis. Thus, although the intracellular domain of sortilin does not contribute to p75(NTR) binding, it does regulate the rates of p75(NTR) cleavage, which is required to mediate pro-neurotrophin-stimulated cell death.

p75NTR is mainly responsible for Aβ toxicity but not for its internalization: a primary study

Accumulating evidence indicates that the intraneuronal accumulation of beta-amyloid peptide (Aβ) is earlier than the formation of extraneuronal amyloid plaque but the mechanism of the accumulation remains unclear. p75NTR is a receptor for Aβ and interacts with Aβ in vitro and in vivo but whether p75NTR mediates Aβ internalization and intraneuronal accumulation is not known. In this study, we aim to determine if p75NTR mediates Aβ internalization, which might provide new insights into Aβ metabolism and toxicity. FRET analysis in PC12 cells showed that internalized Aβ was close to p75NTR. Aβ1-42 could be internalized in PC12 cells in a concentration-dependent manner but the antibody to the p75NTR extracellular domain did not prevent its internalization. Aβ1-42 could also be internalized in mouse neonatal cortical neurons and the deletion of p75NTR in these neurons did not prevent its internalization but prevented Aβ neurotoxicity. Cholesterol at 10 μM significantly increased Aβ1-42 internalization in PC12 cells. Internalized Aβ1-42 is mainly co-localized with Beclin-1 (a biomarker of autophagosomes) but not with endosomal and lysomal markers. p75NTR may not play a main role in Aβ internalization at the concentrations tested but is responsible for Aβ induced toxicity in primary neurons. Internalized Aβ is mainly sorted to autophagosomes for metabolism.

The many substrates of presenilin/γ-secretase

The Alzheimer's disease (AD)-associated amyloid-β protein precursor (AβPP) is cleaved by α-, β-, and presenilin (PS)/γ-secretases through sequential regulated proteolysis. These proteolytic events control the generation of the pathogenic amyloid-β (Aβ) peptide, which excessively accumulates in the brains of individuals afflicted by AD. A growing number of additional proteins cleaved by PS/γ-secretase continue to be discovered. Similarly to AβPP, most of these proteins are type-I transmembrane proteins involved in vital signaling functions regulating cell fate, adhesion, migration, neurite outgrowth, or synaptogenesis. All the identified proteins share common structural features, which are typical for their proteolysis. The consequences of the PS/γ-secretase-mediated cleavage on the function of many of these proteins are largely unknown. Here, we review the current literature on the proteolytic processing mediated by the versatile PS/γ-secretase complex. We begin by discussing the steps of AβPP processing and PS/γ-secretase complex composition and localization, which give clues to how and where the processing of other PS/γ-secretase substrates may take place. Then we summarize the typical features of PS/γ-secretase-mediated protein processing. Finally, we recapitulate the current knowledge on the possible physiological function of PS/γ-secretase-mediated cleavage of specific substrate proteins.

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.

Trk-dependent ADAM17 activation facilitates neurotrophin survival signaling

Signaling by TrkA and TrkB receptor tyrosine kinase is required for peripheral neuron survival. TrkA and TrkB signaling is facilitated by the p75 neurotrophin receptor (p75NTR), a member of the tumor necrosis factor (TNF) receptor superfamily, through mechanisms that remain obscure. Here, we demonstrate that TrkA and TrkB induces MEK-dependent phosphorylation of the transmembrane cysteine protease ADAM17 (a disintegrin and metalloprotease 17) at the intracellular residue threonine 735. Phosphorylation at this site activates ADAM17 and causes cleavage of p75NTR and production of the receptors' intracellular domain (p75NTR(ICD)) in PC12 cells and in primary cerebellar granule neurons. We show that Trk-induced ADAM17 phosphorylation and generation of the p75NTR(ICD) is required for neurotrophin-induced Erk and Akt activation and for neurotrophin-dependent survival signaling. Survival of PC12 cells maintained in 10 ng/ml nerve growth factor drops by 47% in cells depleted of ADAM17; this survival deficit is resolved if the p75NTR(ICD) is overexpressed in the ADAM17 depleted cells. These studies identify a novel signaling circuit in which Trk activates ADAM17-dependent p75NTR(ICD) production to feedback to sustain Trk signaling and Trk-dependent survival.

Proteolytic processing of protocadherin proteins requires endocytosis

The α-, β-, and γ-protocadherins (Pcdhα, Pcdhβ, and Pcdhγ) comprise a large family of single-pass transmembrane proteins predominantly expressed in the nervous system. These proteins contain six cadherin-like extracellular domains, and proteolysis of Pcdhα and Pcdhγ by the γ-secretase complex releases their intracellular domains into the cytoplasm where they may function locally and/or enter the nucleus and affect gene expression. Thus, cleavage of Pcdhs may function to link intercellular contacts and intracellular signaling. Here we report that shedding of the Pcdhα extracellular domain and subsequent processing by γ-secretase require endocytosis and that Pcdhs interact with the regulator of vesicular sorting ESCRT-0 in undifferentiated cells. We also find that the accumulation of Pcdh cleavage products is regulated during development. Differentiation leads to an increase in the interactions between Pcdh proteins and a decrease in the accumulation of cleavage products. We conclude that Pcdh processing requires endocytosis and that the level of cleavage products is regulated during neuronal differentiation.

Sequential and gamma-secretase-dependent processing of the betacellulin precursor generates a palmitoylated intracellular-domain fragment that inhibits cell growth

Betacellulin (BTC) belongs to the family of epidermal growth factor (EGF)-like growth factors that are expressed as transmembrane precursors and undergo proteolytic ectodomain shedding to release soluble mature ligands. BTC is a dual-specificity ligand for ErbB1 and ErbB4 receptors, and can activate unique signal-transduction pathways that are beneficial for the function, survival and regeneration of pancreatic beta-cells. We have previously shown that BTC precursor (proBTC) is cleaved by ADAM10 to generate soluble ligand and a stable, transmembrane remnant (BTC-CTF). In this study, we analyzed the fate of the BTC-CTF in greater detail. We demonstrated that proBTC is cleaved by ADAM10 to produce BTC-CTF, which then undergoes intramembrane processing by presenilin-1- and/or presenilin-2-dependent gamma-secretase to generate an intracellular-domain fragment (BTC-ICD). We found that the proBTC cytoplasmic domain is palmitoylated and that palmitoylation is not required for ADAM10-dependent cleavage but is necessary for the stability and gamma-secretase-dependent processing of BTC-CTF to generate BTC-ICD. Additionally, palmitoylation is required for nuclear-membrane localization of BTC-ICD, as demonstrated by the redistribution of non-palmitoylated BTC-ICD mutant to the nucleoplasm. Importantly, a novel receptor-independent role for BTC-ICD signaling is suggested by the ability of BTC-ICD to inhibit cell growth in vitro.

p75 neurotrophin receptor-mediated apoptosis in sympathetic neurons involves a biphasic activation of JNK and up-regulation of tumor necrosis factor-alpha-converting enzyme/ADAM17

During the development of the sympathetic nervous system, the p75 neurotrophin receptor (p75NTR) has a dual function: promoting survival together with TrkA in response to NGF, but inducing cell death upon binding pro or mature brain-derived neurotrophic factor (BDNF). Apoptotic signaling through p75NTR requires activation of the stress kinase, JNK. However, the receptor also undergoes regulated proteolysis, first by a metalloprotease, and then by gamma-secretase, in response to pro-apoptotic ligands and this is necessary for receptor mediated neuronal death (Kenchappa, R. S., Zampieri, N., Chao, M. V., Barker, P. A., Teng, H. K., Hempstead, B. L., and Carter, B. D. (2006) Neuron 50, 219-232). Hence, the relationship between JNK activation and receptor proteolysis remains to be defined. Here, we report that JNK3 activation is necessary for p75NTR cleavage; however, following release of the intracellular domain, there is a secondary activation of JNK3 that is cleavage dependent. Receptor proteolysis and apoptosis were prevented in sympathetic neurons from jnk3(-/-) mice, while activation of JNK by ectopic expression of MEKK1 induced p75NTR cleavage and cell death. Proteolysis of the receptor was not detected until 6 h after BDNF treatment, suggesting that JNK3 promotes cleavage through a transcriptional mechanism. In support of this hypothesis, BDNF up-regulated tumor necrosis factor-alpha-converting enzyme (TACE)/ADAM17 mRNA and protein in wild-type, but not jnk3(-/-) sympathetic neurons. Down-regulation of TACE by RNA interference blocked BDNF-induced p75NTR cleavage and apoptosis, indicating that this metalloprotease is responsible for the initial processing of the receptor. Together, these results demonstrate that p75NTR-mediated activation of JNK3 is required for up-regulation of TACE, which promotes receptor proteolysis, leading to prolonged activation of JNK3 and subsequent apoptosis in sympathetic neurons.

Nerve growth factor as a paradigm of neurotrophins related to Alzheimer's disease

Converging lines of evidence on the possible connection between NGF signaling and Alzheimer's diseases (AD) are unraveling new facets which could depict this neurotrophin (NTF) in a more central role. AD animal models have provided evidence that a shortage of NGF supply may induce an AD-like syndrome. In vitro experiments, moreover, are delineating a possible temporal and causal link between APP amiloydogenic processing and altered post-translational tau modifications. After NGF signaling interruption, the pivotal upstream players of the amyloid cascade (APP, beta-secretase, and active form of gamma-secretase) are up-regulated, leading to an increased production of amyloid beta peptide (Abeta) and to its intracellular aggregation in molecular species of different sizes. Contextually, the Abeta released pool generates an autocrine toxic loop in the same healthy neurons. At the same time tau protein undergoes anomalous, GSKbeta-mediated, phosphorylation at specific pathogenetic sites (Ser262 and Thr 231), caspase(s) and calpain- I- mediated truncation, detachment from microtubules with consequent cytoskeleton collapse and axonal transport impairment. All these events are inhibited when the amyloidogenic processing is reduced by beta and gamma secretase inhibitors or anti-Abeta antibodies and appear to be causally correlated to TrkA, p75CTF, Abeta, and PS1 molecular association in an Abeta-mediated fashion. In this scenario, the so-called trophic action exerted by NGF (and possibly also by other neurotrophins) in these targets neurons is actually the result of an anti-amyloidogenic activity.