The effects of transmembrane sequence and dimerization on cleavage of the p75 neurotrophin receptor by γ-secretase

Equilibrium between monomers and dimers of the death domain of the p75 neurotrophin receptor in solution

p75 neurotrophin receptor (p75NTR) contains a C-terminal globular protein module known as the death domain (DD), which plays a central role in apoptotic and inflammatory signaling through the formation of oligomeric protein complexes. A monomeric state of the p75NTR-DD also exists depending on its chemical environment in vitro. However, studies on the oligomeric states of the p75NTR-DD have produced conflicting findings and sparked great controversy. Here we present new evidence from biophysical and biochemical studies to demonstrate the coexistence of symmetric and asymmetric dimers of the p75NTR-DD, which may equilibrate with the monomeric form in solution and in the absence of any other protein. The reversible close-open solution behavior may be important for the p75NTR-DD to serve as an intracellular signaling hub. This result supports an intrinsic ability of the p75NTR-DD to self-associate, in congruence with the oligomerization properties of all members of the DD superfamily.

Gamma secretase activity modulates BMP-7-induced dendritic growth in primary rat sympathetic neurons

Autonomic dysfunction has been observed in Alzheimer's disease (AD); however, the effects of genes involved in AD on the peripheral nervous system are not well understood. Previous studies have shown that presenilin-1 (PSEN1), the catalytic subunit of the gamma secretase (γ-secretase) complex, mutations in which are associated with familial AD function, regulates dendritic growth in hippocampal neurons. In this study, we examined whether the γ-secretase pathway also influences dendritic growth in primary sympathetic neurons. Using immunoblotting and immunocytochemistry, molecules of the γ-secretase complex, PSEN1, PSEN2, PEN2, nicastrin and APH1a, were detected in sympathetic neurons dissociated from embryonic (E20/21) rat sympathetic ganglia. Addition of bone morphogenetic protein-7 (BMP-7), which induces dendrites in these neurons, did not alter expression or localization of γ-secretase complex proteins. BMP-7-induced dendritic growth was inhibited by siRNA knockdown of PSEN1 and by three γ-secretase inhibitors, γ-secretase inhibitor IX (DAPT), LY-411575 and BMS-299897. These effects were specific to dendrites and concentration-dependent and did not alter early downstream pathways of BMP signaling. In summary, our results indicate that γ-secretase activity enhances BMP-7 induced dendritic growth in sympathetic neurons. These findings provide insight into the normal cellular role of the γ-secretase complex in sympathetic neurons.

CD51 Intracellular Domain Promotes Cancer Cell Neurotropism through Interacting with Transcription Factor NR4A3 in Colorectal Cancer

The abundant nervous system in intestine provides the basis for perineural invasion (PNI) of colorectal cancer (CRC). PNI is defined as the invasion of the nerves by cancer cells. Although PNI is already known to be an independent prognostic factor in CRC, the molecular mechanism underlying PNI remains obscure. In this study, we first demonstrated that CD51 could promote the neurotropism of tumor cells through cleavage with γ-secretase to generate an intracellular domain (ICD). Mechanistically, ICD of CD51 could bind to the transcription factor NR4A3, and act as a coactivator to promote the expression of downstream effectors, such as NTRK1, NTRK3, and SEMA3E. Pharmacological inhibition of γ-secretase impedes PNI mediated by CD51 in CRC both in vitro and in vivo and may become a potential therapeutic target for PNI in CRC.

Helical stability of the GnTV transmembrane domain impacts on SPPL3 dependent cleavage

Signal-Peptide Peptidase Like-3 (SPPL3) is an intramembrane cleaving aspartyl protease that causes secretion of extracellular domains from type-II transmembrane proteins. Numerous Golgi-localized glycosidases and glucosyltransferases have been identified as physiological SPPL3 substrates. By SPPL3 dependent processing, glycan-transferring enzymes are deactivated inside the cell, as their active site-containing domain is cleaved and secreted. Thus, SPPL3 impacts on glycan patterns of many cellular and secreted proteins and can regulate protein glycosylation. However, the characteristics that make a substrate a favourable candidate for SPPL3-dependent cleavage remain unknown. To gain insights into substrate requirements, we investigated the function of a GxxxG motif located in the transmembrane domain of N-acetylglucosaminyltransferase V (GnTV), a well-known SPPL3 substrate. SPPL3-dependent secretion of the substrate's ectodomain was affected by mutations disrupting the GxxxG motif. Using deuterium/hydrogen exchange and NMR spectroscopy, we studied the effect of these mutations on the helix flexibility of the GnTV transmembrane domain and observed that increased flexibility facilitates SPPL3-dependent shedding and vice versa. This study provides first insights into the characteristics of SPPL3 substrates, combining molecular biology, biochemistry, and biophysical techniques and its results will provide the basis for better understanding the characteristics of SPPL3 substrates with implications for the substrates of other intramembrane proteases.

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.

Interaction between the transmembrane domains of neurotrophin receptors p75 and TrkA mediates their reciprocal activation

The neurotrophin receptors p75 and tyrosine protein kinase receptor A (TrkA) play important roles in the development and survival of the nervous system. Biochemical data suggest that p75 and TrkA reciprocally regulate the activities of each other. For instance, p75 is able to regulate the response of TrkA to lower concentrations of nerve growth factor (NGF), and TrkA promotes shedding of the extracellular domain of p75 by α-secretases in a ligand-dependent manner. The current model suggests that p75 and TrkA are regulated by means of a direct physical interaction; however, the nature of such interaction has been elusive thus far. Here, using NMR in micelles, multiscale molecular dynamics, FRET, and functional studies, we identified and characterized the direct interaction between TrkA and p75 through their respective transmembrane domains (TMDs). Molecular dynamics of p75-TMD mutants suggests that although the interaction between TrkA and p75 TMDs is maintained upon mutation, a specific protein interface is required to facilitate TrkA active homodimerization in the presence of NGF. The same mutations in the TMD protein interface of p75 reduced the activation of TrkA by NGF as well as reducing cell differentiation. In summary, we provide a structural model of the p75–TrkA receptor complex necessary for neuronal development stabilized by TMD interactions.

FLIM-FRET Analysis of Ras Nanoclustering and Membrane-Anchorage

On the plasma membrane, Ras is organized into laterally segregated proteo-lipid complexes called nanoclusters. The extent of Ras nanoclustering correlates with its signaling output, positioning nanocluster as dynamic signaling gain modulators. Recent evidence suggests that stacked dimers of Ras and Raf are elemental units at least of one type of Ras nanocluster. However, it is still incompletely understood, in which physiological contexts nanoclustering is regulated and which constituents are parts of nanocluster. Nonetheless, disruption of nanoclustering faithfully diminishes Ras activity in cells, suggesting Ras nanocluster as potential drug targets.While there are several methods available to study Ras nanocluster , fluorescence or Förster resonance energy transfer (FRET ) between fluorescently labeled, nanoclustered Ras proteins is a relatively simple readout. FRET measurements using fluorescence lifetime imaging microscopy (FLIM ) have proven to be robust and sensitive to determine Ras nanoclustering changes. Loss of FRET that emerges due to nanoclustering reports on all processes upstream of Ras nanoclustering, i.e., also on proper trafficking or lipid modification of Ras. Here we report our standard FLIM-FRET protocol to measure nanoclustering-dependent FRET of Ras in mammalian cells. Importantly, nanoclustering-dependent FRET is one of the few methods that can detect differences between the Ras isoforms.

c-Jun N-terminal Kinase Mediates Ligand-independent p75NTR Signaling in Mesencephalic Cells Subjected to Oxidative Stress

The p75 neurotrophin receptor (p75^NTR) is a multifunctional protein that regulates cellular responses to pathological conditions in specific regions of the nervous system. Activation of p75^NTR in certain neuronal populations induces proteolytic processing of the receptor, thereby generating p75^NTR fragments that facilitate downstream signaling. Expression of p75^NTR has been reported in neurons of the ventral midbrain, but p75^NTR signaling mechanisms in such cells are poorly understood. Here, we used Lund Human Mesencephalic cells, a population of neuronal cells derived from the ventral mesencephalon, to evaluate the effects of oxidative stress on p75^NTR signaling. Subjection of the cells to oxidative stress resulted in decreased cell-surface localization of p75^NTR and intracellular accumulation of p75^NTR fragments. Oxidative stress-induced p75^NTR processing was reduced by pharmacological inhibition of metalloproteases or γ-secretase, but was unaltered by blockade of the ligand-binding domain of p75^NTR. Furthermore, inhibition of c-Jun N-terminal Kinase (JNK) decreased p75^NTR cleavage induced by oxidative damage. Altogether, these results support a mechanism of p75^NTR activation in which oxidative stress stimulates JNK signaling, thereby facilitating p75^NTR processing via a ligand-independent mechanism involving induction of metalloprotease and γ-secretase activity. These findings reveal a novel role for JNK in ligand-independent p75^NTR signaling, and, considering the susceptibility of mesencephalic neurons to oxidative damage associated with Parkinson's disease (PD), merit further investigation into the effects of p75^NTR on PD-related neurodegeneration.

Revising the mechanism of p75NTR activation: intrinsically monomeric state of death domains invokes the "helper" hypothesis

The neurotrophin receptor p75NTR plays crucial roles in neuron development and regulates important neuronal processes like degeneration, apoptosis and cell survival. At the same time the detailed mechanism of signal transduction is unclear. One of the main hypotheses known as the snail-tong mechanism assumes that in the inactive state, the death domains interact with each other and in response to ligand binding there is a conformational change leading to their exposure. Here, we show that neither rat nor human p75NTR death domains homodimerize in solution. Moreover, there is no interaction between the death domains in a more native context: the dimerization of transmembrane domains in liposomes and the presence of activating mutation in extracellular juxtamembrane region do not lead to intracellular domain interaction. These findings suggest that the activation mechanism of p75NTR should be revised. Thus, we propose a novel model of p75NTR functioning based on interaction with "helper" protein.

Fast-diffusing p75NTR monomers support apoptosis and growth cone collapse by neurotrophin ligands

Neurotrophins (NTs) are homodimeric growth factors displaying fundamental roles in the nervous system. Their activity stems from binding and activation of 3 different receptor types in nervous cell membranes. The p75 NT receptor (p75^NTR) was the first to be discovered in 1986; nevertheless, for the numerous structural and functional facets so far reported, its activation mechanisms have remained elusive. Here, we demonstrate that its pleiotropic functions are regulated by different redistributions of the receptors, which crucially depend on the available NT and on the involved subcellular compartment but are unrelated to its oligomerization state. Single-particle studies proved receptors to be monomers with a fast-diffusive behavior in the membrane with, at most, transient self-interactions on the millisecond time scale.

The p75 neurotrophin (NT) receptor (p75^NTR) plays a crucial role in balancing survival-versus-death decisions in the nervous system. Yet, despite 2 decades of structural and biochemical studies, a comprehensive, accepted model for p75^NTR activation by NT ligands is still missing. Here, we present a single-molecule study of membrane p75^NTR in living cells, demonstrating that the vast majority of receptors are monomers before and after NT activation. Interestingly, the stoichiometry and diffusion properties of the wild-type (wt) p75^NTR are almost identical to those of a receptor mutant lacking residues previously believed to induce oligomerization. The wt p75^NTR and mutated (mut) p75^NTR differ in their partitioning in cholesterol-rich membrane regions upon nerve growth factor (NGF) stimulation: We argue that this is the origin of the ability of wt p75^NTR , but not of mut p75^NTR, to mediate immature NT (proNT)-induced apoptosis. Both p75^NTR forms support proNT-induced growth cone retraction: We show that receptor surface accumulation is the driving force for cone collapse. Overall, our data unveil the multifaceted activity of the p75^NTR monomer and let us provide a coherent interpretative frame of existing conflicting data in the literature.

Death domain of p75 neurotrophin receptor: a structural perspective on an intracellular signalling hub

The death domain (DD) is a globular protein motif with a signature feature of an all-helical Greek-key motif. It is a primary mediator of a variety of biological activities, including apoptosis, cell survival and cytoskeletal changes, which are related to many neurodegenerative diseases, neurotrauma, and cancers. DDs exist in a wide range of signalling proteins including p75 neurotrophin receptor (p75^NTR ), a member of the tumour necrosis factor receptor superfamily. The specific signalling mediated by p75^NTR in a given cell depends on the type of ligand engaging the extracellular domain and the recruitment of cytosolic interactors to the intracellular domain, especially the DD, of the receptor. In solution, the p75^NTR -DDs mainly form a symmetric non-covalent homodimer. In response to extracellular signals, conformational changes in the p75^NTR extracellular domain (ECD) propagate to the p75^NTR -DD through the disulfide-bonded transmembrane domain (TMD) and destabilize the p75^NTR -DD homodimer, leading to protomer separation and exposure of binding sites on the DD surface. In this review, we focus on recent advances in the study of the structural mechanism of p75^NTR -DD signalling through recruitment of diverse intracellular interactors for the regulation and control of diverse functional outputs.

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.

Substrate processing in intramembrane proteolysis by γ-secretase - the role of protein dynamics

Intramembrane proteases comprise a number of different membrane proteins with different types of catalytic sites. Their common denominator is cleavage within the plane of the membrane, which usually results in peptide bond scission within the transmembrane helices of their substrates. Despite recent progress in the determination of high-resolution structures, as illustrated here for the γ-secretase complex and its substrate C99, it is still unknown how these enzymes function and how they distinguish between substrates and non-substrates. In principle, substrate/non-substrate discrimination could occur at the level of substrate binding and/or cleavage. Focusing on the γ-secretase/C99 pair, we will discuss recent observations suggesting that global motions within a substrate transmembrane helix may be much more important for defining a substrate than local unraveling at cleavage sites.

The transmembrane domain of the p75 neurotrophin receptor stimulates phosphorylation of the TrkB tyrosine kinase receptor

The function of protein products generated from intramembraneous cleavage by the γ-secretase complex is not well defined. The γ-secretase complex is responsible for the cleavage of several transmembrane proteins, most notably the amyloid precursor protein that results in Aβ, a transmembrane (TM) peptide. Another protein that undergoes very similar γ-secretase cleavage is the p75 neurotrophin receptor. However, the fate of the cleaved p75 TM domain is unknown. p75 neurotrophin receptor is highly expressed during early neuronal development and regulates survival and process formation of neurons. Here, we report that the p75 TM can stimulate the phosphorylation of TrkB (tyrosine kinase receptor B). In vitro phosphorylation experiments indicated that a peptide representing p75 TM increases TrkB phosphorylation in a dose- and time-dependent manner. Moreover, mutagenesis analyses revealed that a valine residue at position 264 in the rat p75 neurotrophin receptor is necessary for the ability of p75 TM to induce TrkB phosphorylation. Because this residue is just before the γ-secretase cleavage site, we then investigated whether the p75(αγ) peptide, which is a product of both α- and γ-cleavage events, could also induce TrkB phosphorylation. Experiments using TM domains from other receptors, EGFR and FGFR1, failed to stimulate TrkB phosphorylation. Co-immunoprecipitation and biochemical fractionation data suggested that p75 TM stimulates TrkB phosphorylation at the cell membrane. Altogether, our results suggest that TrkB activation by p75(αγ) peptide may be enhanced in situations where the levels of the p75 receptor are increased, such as during brain injury, Alzheimer's disease, and epilepsy.

Transmembrane Interactions of Full-length Mammalian Bitopic Cytochrome-P450-Cytochrome-b5 Complex in Lipid Bilayers Revealed by Sensitivity-Enhanced Dynamic Nuclear Polarization Solid-state NMR Spectroscopy

The dynamic protein-protein and protein-ligand interactions of integral bitopic membrane proteins with a single membrane-spanning helix play a plethora of vital roles in the cellular processes associated with human health and diseases, including signaling and enzymatic catalysis. While an increasing number of high-resolution structural studies of membrane proteins have successfully manifested an in-depth understanding of their biological functions, intact membrane-bound bitopic protein-protein complexes pose tremendous challenges for structural studies by crystallography or solution NMR spectroscopy. Therefore, there is a growing interest in developing approaches to investigate the functional interactions of bitopic membrane proteins embedded in lipid bilayers at atomic-level. Here we demonstrate the feasibility of dynamic nuclear polarization (DNP) magic-angle-spinning NMR techniques, along with a judiciously designed stable isotope labeling scheme, to measure atomistic-resolution transmembrane-transmembrane interactions of full-length mammalian ~72-kDa cytochrome P450-cytochrome b5 complex in lipid bilayers. Additionally, the DNP sensitivity-enhanced two-dimensional 13C/13C chemical shift correlations via proton driven spin diffusion provided distance constraints to characterize protein-lipid interactions and revealed the transmembrane topology of cytochrome b5. The results reported in this study would pave ways for high-resolution structural and topological investigations of membrane-bound full-length bitopic protein complexes under physiological conditions.

Piecing it together: Unraveling the elusive structure-function relationship in single-pass membrane receptors

The challenge of crystallizing single-pass plasma membrane receptors has remained an obstacle to understanding the structural mechanisms that connect extracellular ligand binding to cytosolic activation. For example, the complex interplay between receptor oligomerization and conformational dynamics has been, historically, only inferred from static structures of isolated receptor domains. A fundamental challenge in the field of membrane receptor biology, then, has been to integrate experimentally observable dynamics of full-length receptors (e.g. diffusion and conformational flexibility) into static structural models of the disparate domains. In certain receptor families, e.g. the ErbB receptors, structures have led somewhat linearly to a putative model of activation. In other families, e.g. the tumor necrosis factor (TNF) receptors, structures have produced divergent hypothetical mechanisms of activation and transduction. Here, we discuss in detail these and other related receptors, with the goal of illuminating the current challenges and opportunities in building comprehensive models of single-pass receptor activation. The deepening understanding of these receptors has recently been accelerated by new experimental and computational tools that offer orthogonal perspectives on both structure and dynamics. As such, this review aims to contextualize those technological developments as we highlight the elegant and complex conformational communication between receptor domains. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.

Structural Characterization of the p75 Neurotrophin Receptor: A Stranger in the TNFR Superfamily

Although p75 neurotrophin receptor (p75^NTR) was the founding member of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF), it is an atypical TNFRSF protein. p75^NTR like TNF-R1 and Fas-R contain an extracellular domain with four cysteine-rich domains (CRD) and a death domain (DD) in the intracellular region. While TNFRSF proteins are activated by trimeric TNFSF ligands, p75^NTR forms dimers activated by dimeric neurotrophins that are structurally unrelated to TNFSF proteins. In addition, although p75^NTR shares with other members the interaction with the TNF receptor-associated factors to activate the NF-κB and cell death pathways, p75^NTR does not interact with the DD-containing proteins FADD, TRADD, or MyD88. By contrast, the DD of p75^NTR is able to recruit several protein interactors via a full catalog of DD interactions not described before in the TNFRSF. p75-DD forms homotypic symmetrical DD-DD complexes with itself and with the related p45-DD; forms heterotypic DD-CARD interactions with the RIP2-CARD domain, and forms a new interaction between a DD and RhoGDI. All these features, in addition to its promiscuous interactions with several ligands and coreceptors, its processing by α- and γ-secretases, the dimeric nature of its transmembrane domain and its "special" juxtamembrane region, make p75^NTR a truly stranger in the TNFR superfamily. In this chapter, I will summarize the known structural aspects of p75^NTR and I will analyze from a structural point of view, the similitudes and differences between p75^NTR and the other members of the TNFRSF.

Cancer stem cell drugs target K-ras signaling in a stemness context

Cancer stem cells (CSCs) are considered to be responsible for treatment relapse and have therefore become a major target in cancer research. Salinomycin is the most established CSC inhibitor. However, its primary mechanistic target is still unclear, impeding the discovery of compounds with similar anti-CSC activity. Here, we show that salinomycin very specifically interferes with the activity of K-ras4B, but not H-ras, by disrupting its nanoscale membrane organization. We found that caveolae negatively regulate the sensitivity to this drug. On the basis of this novel mechanistic insight, we defined a K-ras-associated and stem cell-derived gene expression signature that predicts the drug response of cancer cells to salinomycin. Consistent with therapy resistance of CSC, 8% of tumor samples in the TCGA-database displayed our signature and were associated with a significantly higher mortality. Using our K-ras-specific screening platform, we identified several new candidate CSC drugs. Two of these, ophiobolin A and conglobatin A, possessed a similar or higher potency than salinomycin. Finally, we established that the most potent compound, ophiobolin A, exerts its K-ras4B-specific activity through inactivation of calmodulin. Our data suggest that specific interference with the K-ras4B/calmodulin interaction selectively inhibits CSC.

SPRED1 Interferes with K-ras but Not H-ras Membrane Anchorage and Signaling

The Ras/mitogen-activated protein kinase (MAPK) signaling pathway is tightly controlled by negative feedback regulators, such as the tumor suppressor SPRED1. The SPRED1 gene also carries loss-of-function mutations in the RASopathy Legius syndrome. Growth factor stimulation translocates SPRED1 to the plasma membrane, triggering its inhibitory activity. However, it remains unclear whether SPRED1 there acts at the level of Ras or Raf. We show that pharmacological or galectin-1 (Gal-1)-mediated induction of B- and C-Raf-containing dimers translocates SPRED1 to the plasma membrane. This is facilitated in particular by SPRED1 interaction with B-Raf and, via its N terminus, with Gal-1. The physiological significance of these novel interactions is supported by two Legius syndrome-associated mutations that show diminished binding to both Gal-1 and B-Raf. On the plasma membrane, SPRED1 becomes enriched in acidic membrane domains to specifically perturb membrane organization and extracellular signal-regulated kinase (ERK) signaling of active K-ras4B (here, K-ras) but not H-ras. However, SPRED1 also blocks on the nanoscale the positive effects of Gal-1 on H-ras. Therefore, a combinatorial expression of SPRED1 and Gal-1 potentially regulates specific patterns of K-ras- and H-ras-dependent signaling output. More broadly, our results open up the possibility that related SPRED and Sprouty proteins act in a similar Ras and Raf isoform-specific manner.

Juxtamembrane contribution to transmembrane signaling

Signaling across the cell membrane mediated by transmembrane receptors plays an important role in diverse biological processes. Recent studies have indicated that, in a number of single-span transmembrane receptors, the intracellular juxtamembrane (JM) sequence linking the transmembrane helix with the rest of the cytoplasmic domain participates directly in the signaling process via several novel mechanisms. This review briefly highlights several modes of JM dynamics in the context of signal transduction that are shared by different types of transmembrane receptors.

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.

Death Domain Signaling by Disulfide-Linked Dimers of the p75 Neurotrophin Receptor Mediates Neuronal Death in the CNS

The p75 neurotrophin receptor (p75(NTR)) mediates neuronal death in response to neural insults by activating a caspase apoptotic pathway. The oligomeric state and activation mechanism that enable p75(NTR) to mediate these effects have recently been called into question. Here, we have investigated mutant mice lacking the p75(NTR) death domain (DD) or a highly conserved transmembrane (TM) cysteine residue (Cys(259)) implicated in receptor dimerization and activation. Neuronal death induced by proneurotrophins or epileptic seizures was assessed and compared with responses in p75(NTR) knock-out mice and wild-type animals. Proneurotrophins induced apoptosis of cultured hippocampal and cortical neurons from wild-type mice, but mutant neurons lacking p75(NTR), only the p75(NTR) DD, or just Cys(259) were all equally resistant to proneurotrophin-induced neuronal death. Homo-FRET anisotropy experiments demonstrated that both NGF and proNGF induce conformational changes in p75(NTR) that are dependent on the TM cysteine. In vivo, neuronal death induced by pilocarpine-mediated seizures was significantly reduced in the hippocampus and somatosensory, piriform, and entorhinal cortices of all three strains of p75(NTR) mutant mice. Interestingly, the levels of protection observed in mice lacking the DD or only Cys(259) were identical to those of p75(NTR) knock-out mice even though the Cys(259) mutant differed from the wild-type receptor in only one amino acid residue. We conclude that, both in vitro and in vivo, neuronal death induced by p75(NTR) requires the DD and TM Cys(259), supporting the physiological relevance of DD signaling by disulfide-linked dimers of p75(NTR) in the CNS.

SIGNIFICANCE STATEMENT

A detailed understanding of the physiological significance of distinct structural determinants in the p75 neurotrophin receptor (p75(NTR)) is crucial for the identification of suitable drug targets in this receptor. We have tested the relevance of the p75(NTR) death domain (DD) and the highly conserved transmembrane residue Cys(259) for the ability of p75(NTR) to induce apoptosis in neurons of the CNS using gene-targeted mutant mice. The physiological importance of these determinants had been contested in some recent in vitro studies. Our results indicate a requirement for DD signaling by disulfide-linked dimers of p75(NTR) for neuronal death induced by proneurotrophins and epileptic seizures. These new mouse models will be useful for clarifying different aspects of p75(NTR) physiology.

Structural Basis of p75 Transmembrane Domain Dimerization

Dimerization of single span transmembrane receptors underlies their mechanism of activation. p75 neurotrophin receptor plays an important role in the nervous system, but the understanding of p75 activation mechanism is still incomplete. The transmembrane (TM) domain of p75 stabilizes the receptor dimers through a disulfide bond, essential for the NGF signaling. Here we solved by NMR the three-dimensional structure of the p75-TM-WT and the functionally inactive p75-TM-C257A dimers. Upon reconstitution in lipid micelles, p75-TM-WT forms the disulfide-linked dimers spontaneously. Under reducing conditions, p75-TM-WT is in a monomer-dimer equilibrium with the Cys(257) residue located on the dimer interface. In contrast, p75-TM-C257A forms dimers through the AXXXG motif on the opposite face of the α-helix. Biochemical and cross-linking experiments indicate that AXXXG motif is not on the dimer interface of p75-TM-WT, suggesting that the conformation of p75-TM-C257A may be not functionally relevant. However, rather than mediating p75 homodimerization, mutagenesis of the AXXXG motif reveals its functional role in the regulated intramembrane proteolysis of p75 catalyzed by the γ-secretase complex. Our structural data provide an insight into the key role of the Cys(257) in stabilization of the weak transmembrane dimer in a conformation required for the NGF signaling.

Detection of p75NTR Trimers: Implications for Receptor Stoichiometry and Activation

The p75 neurotrophin receptor (p75(NTR)) is a multifunctional receptor that participates in many critical processes in the nervous system, ranging from apoptosis to synaptic plasticity and morphological events. It is a member of the tumor necrosis factor receptor (TNFR) superfamily, whose members undergo trimeric oligomerization. Interestingly, p75(NTR) interacts with dimeric ligands (i.e., proneurotrophins or mature neurotrophins), but several of the intracellular adaptors that mediate p75(NTR) signaling are trimeric (i.e., TNFR-associated factor 6 or TRAF6). Consequently, the active receptor signaling unit remains uncertain. To identify the functional receptor complex, we evaluated its oligomerization in vitro and in mice brain tissues using a combination of biochemical techniques. We found that the most abundant homotypic arrangement for p75(NTR) is a trimer and that monomers and trimers coexist at the cell surface. Interestingly, trimers are not required for ligand-independent or ligand-dependent p75(NTR) activation in a growth cone retraction functional assay. However, monomers are capable of inducing acute morphological effects in neurons. We propose that p75(NTR) activation is regulated by its oligomerization status and its levels of expression. These results indicate that the oligomeric state of p75(NTR) confers differential responses and offers an explanation for the diverse and contradictory actions of this receptor in the nervous system.

SIGNIFICANCE STATEMENT

The p75 neurotrophin receptor (p75(NTR)) regulates a wide range of cellular functions, including apoptosis, neuronal processes remodeling, and synaptic plasticity. The goal of our work was to inquire whether oligomers of the receptor are required for function. Here we report that p75(NTR) predominantly assembles as a trimer, similar to other tumor necrosis factor receptors. Interestingly, monomers and trimers coexist at the cell surface, but trimers are not required for p75(NTR) activation in a functional assay. However, monomers are capable of inducing acute morphological effects in neurons. Identification of the oligomerization state of p75(NTR) begins to provide insights to the mechanisms of signal initiation of this noncatalytic receptor, as well as to develop therapeutic interventions to diminish its activity.

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.

Ligand-induced dynamics of neurotrophin receptors investigated by single-molecule imaging approaches

Neurotrophins are secreted proteins that regulate neuronal development and survival, as well as maintenance and plasticity of the adult nervous system. The biological activity of neurotrophins stems from their binding to two membrane receptor types, the tropomyosin receptor kinase and the p75 neurotrophin receptors (NRs). The intracellular signalling cascades thereby activated have been extensively investigated. Nevertheless, a comprehensive description of the ligand-induced nanoscale details of NRs dynamics and interactions spanning from the initial lateral movements triggered at the plasma membrane to the internalization and transport processes is still missing. Recent advances in high spatio-temporal resolution imaging techniques have yielded new insight on the dynamics of NRs upon ligand binding. Here we discuss requirements, potential and practical implementation of these novel approaches for the study of neurotrophin trafficking and signalling, in the framework of current knowledge available also for other ligand-receptor systems. We shall especially highlight the correlation between the receptor dynamics activated by different neurotrophins and the respective signalling outcome, as recently revealed by single-molecule tracking of NRs in living neuronal cells.

Heterodimerization of p45-p75 modulates p75 signaling: structural basis and mechanism of action

The formation of a p45-p75 heterodimer overrides p75’s inhibition of nerve regeneration by stopping p75 homodimers from forming and creating a complex with the Nogo receptor.

The p75 neurotrophin receptor, a member of the tumor necrosis factor receptor superfamily, is required as a co-receptor for the Nogo receptor (NgR) to mediate the activity of myelin-associated inhibitors such as Nogo, MAG, and OMgp. p45/NRH2/PLAIDD is a p75 homologue and contains a death domain (DD). Here we report that p45 markedly interferes with the function of p75 as a co-receptor for NgR. P45 forms heterodimers with p75 and thereby blocks RhoA activation and inhibition of neurite outgrowth induced by myelin-associated inhibitors. p45 binds p75 through both its transmembrane (TM) domain and DD. To understand the underlying mechanisms, we have determined the three-dimensional NMR solution structure of the intracellular domain of p45 and characterized its interaction with p75. We have identified the residues involved in such interaction by NMR and co-immunoprecipitation. The DD of p45 binds the DD of p75 by electrostatic interactions. In addition, previous reports suggested that Cys257 in the p75 TM domain is required for signaling. We found that the interaction of the cysteine 58 of p45 with the cysteine 257 of p75 within the TM domain is necessary for p45–p75 heterodimerization. These results suggest a mechanism involving both the TM domain and the DD of p45 to regulate p75-mediated signaling.

Injuries to the brain and spinal cord often result in paralysis due to the fact that the injured nerves cannot regrow to reach their normal targets and carry out their functions. At the injury sites, there are proteins released from the damaged myelin that bind the Nogo receptor (NgR) on the nerve and inhibit its regeneration. The NgR needs to form a complex with the p75 neurotrophin receptor in order to mediate this inhibitory signal. Here we found that p45, a homologue of p75, can also bind to p75 and block its inhibitory activity when overexpressed. To perform its function, p75 needs to dimerize through both its transmembrane and intracellular domains, facilitating the recruitment of several proteins. Our structural and functional studies show that p45 binds specifically to conserved regions in the p75 transmembrane and the intracellular domain and that this blocks p75 dimerization along with its downstream signaling. Thus, this study demonstrates that altering the oligomerization of p75 is a good strategy to override p75's inhibitory effects on nerve regeneration, and it opens the door for the design of specific p75 inhibitors for therapeutic applications.

Point mutations in dimerization motifs of the transmembrane domain stabilize active or inactive state of the EphA2 receptor tyrosine kinase

The EphA2 receptor tyrosine kinase plays a central role in the regulation of cell adhesion and guidance in many human tissues. The activation of EphA2 occurs after proper dimerization/oligomerization in the plasma membrane, which occurs with the participation of extracellular and cytoplasmic domains. Our study revealed that the isolated transmembrane domain (TMD) of EphA2 embedded into the lipid bicelle dimerized via the heptad repeat motif L(535)X3G(539)X2A(542)X3V(546)X2L(549) rather than through the alternative glycine zipper motif A(536)X3G(540)X3G(544) (typical for TMD dimerization in many proteins). To evaluate the significance of TMD interactions for full-length EphA2, we substituted key residues in the heptad repeat motif (HR variant: G539I, A542I, G553I) or in the glycine zipper motif (GZ variant: G540I, G544I) and expressed YFP-tagged EphA2 (WT, HR, and GZ variants) in HEK293T cells. Confocal microscopy revealed a similar distribution of all EphA2-YFP variants in cells. The expression of EphA2-YFP variants and their kinase activity (phosphorylation of Tyr(588) and/or Tyr(594)) and ephrin-A3 binding were analyzed with flow cytometry on a single cell basis. Activation of any EphA2 variant is found to occur even without ephrin stimulation when the EphA2 content in cells is sufficiently high. Ephrin-A3 binding is not affected in mutant variants. Mutations in the TMD have a significant effect on EphA2 activity. Both ligand-dependent and ligand-independent activities are enhanced for the HR variant and reduced for the GZ variant compared with the WT. These findings allow us to suggest TMD dimerization switching between the heptad repeat and glycine zipper motifs, corresponding to inactive and active receptor states, respectively, as a mechanism underlying EphA2 signal transduction.

FRET-reporter nanoparticles to monitor redox-induced intracellular delivery of active compounds

FRET-reporter particles for redox-induced release of active compounds in cells were developed. This particle system allowed following the intracellular cleavage of delivered compounds after particle internalization.

Cellular FRET-Biosensors to Detect Membrane Targeting Inhibitors of N-Myristoylated Proteins

Hundreds of eukaryotic signaling proteins require myristoylation to functionally associate with intracellular membranes. N-myristoyl transferases (NMT) responsible for this modification are established drug targets in cancer and infectious diseases. Here we describe NANOMS (NANOclustering and Myristoylation Sensors), biosensors that exploit the FRET resulting from plasma membrane nanoclustering of myristoylated membrane targeting sequences of Gα_i2, Yes- or Src-kinases fused to fluorescent proteins. When expressed in mammalian cells, NANOMS report on loss of membrane anchorage due to chemical or genetic inhibition of myristoylation e.g. by blocking NMT and methionine-aminopeptidase (Met-AP). We used Yes-NANOMS to assess inhibitors of NMT and a cherry-picked compound library of putative Met-AP inhibitors. Thus we successfully confirmed the activity of DDD85646 and fumagillin in our cellular assay. The developed assay is unique in its ability to identify modulators of signaling protein nanoclustering, and is amenable to high throughput screening for chemical or genetic inhibitors of functional membrane anchorage of myristoylated proteins in mammalian cells.

Multiple motifs regulate apical sorting of p75 via a mechanism that involves dimerization and higher-order oligomerization

The sorting signals that direct proteins to the apical surface of polarized epithelial cells are complex and can include posttranslational modifications, such as N- and O-linked glycosylation. Efficient apical sorting of the neurotrophin receptor p75 is dependent on its O-glycosylated membrane proximal stalk, but how this domain mediates targeting is unknown. Protein oligomerization or clustering has been suggested as a common step in the segregation of all apical proteins. Like many apical proteins, p75 forms dimers, and we hypothesized that formation of higher-order clusters mediated by p75 dimerization and interactions of the stalk facilitate its apical sorting. Using fluorescence fluctuation techniques (photon-counting histogram and number and brightness analyses) to study p75 oligomerization status in vivo, we found that wild-type p75-green fluorescent protein forms clusters in the trans-Golgi network (TGN) but not at the plasma membrane. Disruption of either the dimerization motif or the stalk domain impaired both clustering and polarized delivery. Manipulation of O-glycan processing or depletion of multiple galectins expressed in Madin-Darby canine kidney cells had no effect on p75 sorting, suggesting that the stalk domain functions as a structural prop to position other determinants in the lumenal domain of p75 for oligomerization. Additionally, a p75 mutant with intact dimerization and stalk motifs but with a dominant basolateral sorting determinant (Δ250 mutant) did not form oligomers, consistent with a requirement for clustering in apical sorting. Artificially enhancing dimerization restored clustering to the Δ250 mutant but was insufficient to reroute this mutant to the apical surface. Together these studies demonstrate that clustering in the TGN is required for normal biosynthetic apical sorting of p75 but is not by itself sufficient to reroute a protein to the apical surface in the presence of a strong basolateral sorting determinant. Our studies shed new light on the hierarchy of polarized sorting signals and on the mechanisms by which newly synthesized proteins are segregated in the TGN for eventual apical delivery.

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.