Differential endocytic sorting of p75NTR and TrkA in response to NGF: a role for late endosomes in TrkA trafficking

Optogenetic control of receptor-mediated growth cone dynamics in neurons

Development of neuronal connections is spatially and temporally controlled by extracellular cues which often activate their cognate cell surface receptors and elicit localized cellular responses. Here, we demonstrate the use of an optogenetic tool to activate receptor signaling locally to induce actin-mediated growth cone remodeling in neurons. Based on the light-induced interaction between Cryptochrome 2 (CRY2) and CIB1, we generated a bicistronic vector to co-expresses CRY2 fused to the intracellular domain of a guidance receptor and a membrane-anchored CIB1. When expressed in primary neurons, activation of the growth inhibitory PlexA4 receptor induced growth cone collapse, while activation of the growth stimulating TrkA receptor increased growth cone size. Moreover, local activation of either receptor not only elicited the predicted response in light-activated growth cones but also an opposite response in neighboring no-light-exposed growth cones of the same neuron. Finally, this tool was used to reorient growth cones toward or away from the site of light activation and to stimulate local actin polymerization for branch initiation along axonal shafts. These studies demonstrate the use of an optogenetic tool for precise spatial and temporal control of receptor signaling in neurons and support its future application in investigating cellular mechanisms of neuronal development and plasticity.

EFA6A, an Exchange Factor for Arf6, Regulates NGF-Dependent TrkA Recycling From Early Endosomes and Neurite Outgrowth in PC12 Cells

Endosomal trafficking of TrkA is a critical process for nerve growth factor (NGF)-dependent neuronal cell survival and differentiation. The small GTPase ADP-ribosylation factor 6 (Arf6) is implicated in NGF-dependent processes in PC12 cells through endosomal trafficking and actin cytoskeleton reorganization. However, the regulatory mechanism for Arf6 in NGF signaling is largely unknown. In this study, we demonstrated that EFA6A, an Arf6-specific guanine nucleotide exchange factor, was abundantly expressed in PC12 cells and that knockdown of EFA6A significantly inhibited NGF-dependent Arf6 activation, TrkA recycling from early endosomes to the cell surface, prolonged ERK1/2 phosphorylation, and neurite outgrowth. We also demonstrated that EFA6A forms a protein complex with TrkA through its N-terminal region, thereby enhancing its catalytic activity for Arf6. Similarly, we demonstrated that EFA6A forms a protein complex with TrkA in cultured dorsal root ganglion (DRG) neurons. Furthermore, cultured DRG neurons from EFA6A knockout mice exhibited disturbed NGF-dependent TrkA trafficking compared with wild-type neurons. These findings provide the first evidence for EFA6A as a key regulator of NGF-dependent TrkA trafficking and signaling.

HTT (huntingtin) and RAB7 co-migrate retrogradely on a signaling LAMP1-containing late endosome during axonal injury

ABBREVIATIONS

Atg5: Autophagy-related 5; Atg8a: Autophagy-related 8a; AL: autolysosome; AP: autophagosome; BAF1: bafilomycin A1; BDNF: brain derived neurotrophic factor; BMP: bone morphogenetic protein; Cyt-c-p: Cytochrome c proximal; CQ: chloroquine; DCTN1: dynactin 1; Dhc: dynein heavy chain; EE: early endosome; DYNC1I1: dynein cytoplasmic 1 intermediate chain 1; HD: Huntington disease; HIP1/Hip1: huntingtin interacting protein 1; HTT/htt: huntingtin; iNeuron: iPSC-derived human neurons; IP: immunoprecipitation; Khc: kinesin heavy chain; KIF5C: kinesin family member 5C; LAMP1/Lamp1: lysosomal associated membrane protein 1; LE: late endosome; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MAP3K12/DLK: mitogen-activated protein kinase kinase kinase 12; MAPK8/JNK/bsk: mitogen-activated protein kinase 8/basket; MAPK8IP3/JIP3: mitogen-activated protein kinase 8 interacting protein 3; NGF: nerve growth factor; NMJ: neuromuscular junction; NTRK1/TRKA: neurotrophic receptor tyrosine kinase 1; NRTK2/TRKB: neurotrophic receptor tyrosine kinase 2; nuf: nuclear fallout; PG: phagophore; PtdIns3P: phosphatidylinositol-3-phosphate; puc: puckered; ref(2)P: refractory to sigma P; Rilpl: Rab interacting lysosomal protein like; Rip11: Rab11 interacting protein; RTN1: reticulon 1; syd: sunday driver; SYP: synaptophysin; SYT1/Syt1: synaptotagmin 1; STX17/Syx17: syntaxin 17; tkv: thickveins; VF: vesicle fraction; wit: wishful thinking; wnd: wallenda.

Crystal structure of the PDZ4 domain of MAGI2 in complex with PBM of ARMS reveals a canonical PDZ recognition mode

Membrane-associated guanylate kinase, WW and PDZ domain-containing protein 2 (MAGI2) is a neuronal scaffold protein that plays critical roles at synaptic junctions by assembling neurotransmitter receptors and cell adhesion proteins through its multiple protein-protein interaction domains, including six PDZ domains, two phosphoserine-phosphothreonine binding WW domains, and a guanylate kinase GK domain. Previous studies showed that MAGI2 participates in formation of tetrameric complexes with PDZ-GEF1, TrkA receptor, and ankyrin repeat-rich membrane spanning (ARMS) protein at late endosomes and is crucial for neurite outgrowth. However, the molecular mechanism governing the assembly of these complexes remains unknown. Here, we characterize the direct interaction between MAGI2 and ARMS through multiple biochemical assays. Moreover, our solved crystal structure of the truncated PDZ4/PBM (PDZ binding motifs) complex of MAGI2 and ARMS proteins (MAGI2-PDZ4/ARMS-PBM) reveals that the binding interface lies between the αB/βB groove from the PDZ4 of MAGI2 and the C-terminal PBM from ARMS. The structure reveals high similarity to others in this protein family where canonical PDZ/PBM interactions are observed. However, the conserved "GLGF" motif in the PSD-95-PDZ3 changes to "GFGF" in the MAGI2-PDZ4/ARMS-PBM complex. We further validated our crystal structure through serial mutagenesis assays. Taken together, our study provides the biochemical details and binding mechanisms that underpin the stabilization of the MAGI2-PDZ4/ARMS-PBM complex, thereby offering a biochemical and structural basis for further understanding of the functional roles of MAGI2, ARMS, PDZ-GEF1, and TrkA in forming the tetrameric receptor complex in neuronal signaling.

Inactive variants of death receptor p75NTR reduce Alzheimer's neuropathology by interfering with APP internalization

A prevalent model of Alzheimer's disease (AD) pathogenesis postulates the generation of neurotoxic fragments derived from the amyloid precursor protein (APP) after its internalization to endocytic compartments. The molecular pathways that regulate APP internalization and intracellular trafficking in neurons are incompletely understood. Here, we report that 5xFAD mice, an animal model of AD, expressing signaling-deficient variants of the p75 neurotrophin receptor (p75NTR ) show greater neuroprotection from AD neuropathology than animals lacking this receptor. p75NTR knock-in mice lacking the death domain or transmembrane Cys259 showed lower levels of Aβ species, amyloid plaque burden, gliosis, mitochondrial stress, and neurite dystrophy than global knock-outs. Strikingly, long-term synaptic plasticity and memory, which are completely disrupted in 5xFAD mice, were fully recovered in the knock-in mice. Mechanistically, we found that p75NTR interacts with APP at the plasma membrane and regulates its internalization and intracellular trafficking in hippocampal neurons. Inactive p75NTR variants internalized considerably slower than wild-type p75NTR and showed increased association with the recycling pathway, thereby reducing APP internalization and co-localization with BACE1, the critical protease for generation of neurotoxic APP fragments, favoring non-amyloidogenic APP cleavage. These results reveal a novel pathway that directly and specifically regulates APP internalization, amyloidogenic processing, and disease progression, and suggest that inhibitors targeting the p75NTR transmembrane domain may be an effective therapeutic strategy in AD.

Modulation of RAB7A Protein Expression Determines Resistance to Cisplatin through Late Endocytic Pathway Impairment and Extracellular Vesicular Secretion

BACKGROUND

Cisplatin (CDDP) is widely used in treatment of cancer, yet patients often develop resistance with consequent therapeutical failure. In CDDP-resistant cells alterations of endocytosis and lysosomal functionality have been revealed, although their causes and contribution to therapy response are unclear.

METHODS

We investigated the role of RAB7A, a key regulator of late endocytic trafficking, in CDDP-resistance by comparing resistant and sensitive cells using western blotting, confocal microscopy and real time PCR. Modulation of RAB7A expression was performed by transfection and RNA interference, while CDDP sensitivity and intracellular accumulation were evaluated by viability assays and chemical approaches, respectively. Also extracellular vesicles were purified and analyzed. Finally, correlations between RAB7A and chemotherapy response was investigated in human patient samples.

RESULTS

We demonstrated that down-regulation of RAB7A characterizes the chemoresistant phenotype, and that RAB7A depletion increases CDDP-resistance while RAB7A overexpression decreases it. In addition, increased production of extracellular vesicles is modulated by RAB7A expression levels and correlates with reduction of CDDP intracellular accumulation.

CONCLUSIONS

We demonstrated, for the first time, that RAB7A regulates CDDP resistance determining alterations in late endocytic trafficking and drug efflux through extracellular vesicles.

Differential effects of BDNF and neurotrophin 4 (NT4) on endocytic sorting of TrkB receptors

Neurotrophins are a family of growth factors playing key roles in the survival, development, and function of neurons. The neurotrophins brain-derived neurotrophic factor (BDNF) and NT4 both bind to and activate TrkB receptors, however, they mediate distinct neuronal functions. The molecular mechanism of how TrkB activation by BDNF and NT4 leads to diverse outcomes is unknown. Here, we report that BDNF and NT4 lead to differential endocytic sorting of TrkB receptors resulting in diverse biological functions in cultured cortical neurons. Fluorescent microscopy and surface biotinylation experiments showed that both neurotrophins stimulate internalization of TrkB with similar kinetics. Exposure to BDNF for 2-3 h reduced the surface pool of TrkB receptors to half, whereas a longer treatment (4-5 h) with NT4 was necessary to achieve a similar level of down-regulation. Although BDNF and NT4 induced TrkB phosphorylation with similar intensities, BDNF induced more rapid ubiquitination and degradation of TrkB than NT4. Interestingly, TrkB receptor ubiquitination by these ligands have substantially different pH sensitivities, resulting in varying degrees of receptor ubiquitination at lower pH levels. Consequently, NT4 was capable of maintaining longer sustained downstream signaling activation that correlated with reduced TrkB ubiquitination at endosomal pH. Thus, by leading to altered endocytic trafficking itineraries for TrkB receptors, BDNF and NT4 elicit differential TrkB signaling in terms of duration, intensity, and specificity, which may contribute to their functional differences in vivo. The neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-4 (NT4), both bind to and activate TrkB receptors, however, they mediate distinct neuronal functions. Here, we propose that BDNF and NT4 lead to differential endocytic sorting of TrkB receptors resulting in diverse biological functions. BDNF induces more rapid ubiquitination and degradation of TrkB than NT4. Consequently, NT4 is capable of maintaining more sustained signaling downstream of TrkB receptors.

GGA3 mediates TrkA endocytic recycling to promote sustained Akt phosphorylation and cell survival

GGA3 binds directly to the TrkA internal DXXLL motif and mediates TrkA endocytic recycling. This effect is dependent on the activation of Arf6. GGA3 is a key player in a novel DXXLL-mediated recycling machinery for TrkA, where it prolongs the activation of Akt signaling and survival responses.

Although TrkA postendocytic sorting significantly influences neuronal cell survival and differentiation, the molecular mechanism underlying TrkA receptor sorting in the recycling or degradation pathways remains poorly understood. Here we demonstrate that Golgi-localized, γ adaptin-ear–containing ADP ribosylation factor-binding protein 3 (GGA3) interacts directly with the TrkA cytoplasmic tail through an internal DXXLL motif and mediates the functional recycling of TrkA to the plasma membrane. We find that GGA3 depletion by siRNA delays TrkA recycling, accelerates TrkA degradation, attenuates sustained NGF-induced Akt activation, and reduces cell survival. We also show that GGA3’s effect on TrkA recycling is dependent on the activation of Arf6. This work identifies GGA3 as a key player in a novel DXXLL-mediated endosomal sorting machinery that targets TrkA to the plasma membrane, where it prolongs the activation of Akt signaling and survival responses.

The role of rab proteins in neuronal cells and in the trafficking of neurotrophin receptors

Neurotrophins are a family of proteins that are important for neuronal development, neuronal survival and neuronal functions. Neurotrophins exert their role by binding to their receptors, the Trk family of receptor tyrosine kinases (TrkA, TrkB, and TrkC) and p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily. Binding of neurotrophins to receptors triggers a complex series of signal transduction events, which are able to induce neuronal differentiation but are also responsible for neuronal maintenance and neuronal functions. Rab proteins are small GTPases localized to the cytosolic surface of specific intracellular compartments and are involved in controlling vesicular transport. Rab proteins, acting as master regulators of the membrane trafficking network, play a central role in both trafficking and signaling pathways of neurotrophin receptors. Axonal transport represents the Achilles' heel of neurons, due to the long-range distance that molecules, organelles and, in particular, neurotrophin-receptor complexes have to cover. Indeed, alterations of axonal transport and, specifically, of axonal trafficking of neurotrophin receptors are responsible for several human neurodegenerative diseases, such as Huntington's disease, Alzheimer's disease, amyotrophic lateral sclerosis and some forms of Charcot-Marie-Tooth disease. In this review, we will discuss the link between Rab proteins and neurotrophin receptor trafficking and their influence on downstream signaling pathways.

Gambogic amide selectively upregulates TrkA expression and triggers its activation

BACKGROUND

Gambogic amide is the first identified small molecular agonist for TrkA receptor. It mimics NGF functions by selectively activating TrkA receptor and preventing neuron death. However, its function different from that of NGF remains unknown.

METHODS

In the current study, we detect the effect of gambogic amide on TrkA expression using TrkA-expressing cell lines in vitro and hippocampi from mice treated with gambogic amide.

RESULTS

We have confirmed that gambogic amide displays robust neurotrophic activities in provoking neurite outgrowth in vitro. However, gambiogic amide displays a different kinetics from NGF in activating TrkA signals. NGF swiftly provokes TrkA activation and quickly induces TrkA degradation, while gambogic amid selectively upregulates TrkA protein and mRNA levels in a time-dependent manner. Administration of this compound in mice also activates TrkA receptor in hippocampus and promotes TrkA transcription and expression.

CONCLUSION

This study provides a novel mechanism of how gambogic amide regulates TrkA receptor, other than mimicking NGF in triggering TrkA activation.

Syntaxin 8 modulates the post-synthetic trafficking of the TrkA receptor and inflammatory pain transmission

Nerve growth factor (NGF) promotes the survival, maintenance, and neurite outgrowth of sensory and sympathetic neurons, and the effects are mediated by TrkA receptor signaling. Thus, the cell surface location of the TrkA receptor is crucial for NGF-mediated functions. However, the regulatory mechanism underlying TrkA cell surface levels remains incompletely understood. In this study, we identified syntaxin 8 (STX8), a Q-SNARE protein, as a novel TrkA-binding protein. Overexpression and knockdown studies showed that STX8 facilitates TrkA transport from the Golgi to the plasma membrane and regulates the surface levels of TrkA but not TrkB receptors. Furthermore, STX8 modulates downstream NGF-induced TrkA signaling and, consequently, the survival of NGF-dependent dorsal root ganglia neurons. Finally, knockdown of STX8 in rat dorsal root ganglia by recombinant adeno-associated virus serotype 6-mediated RNA interference led to analgesic effects on formalin-induced inflammatory pain. These findings demonstrate that STX8 is a modulator of TrkA cell surface levels and biological functions.

LAMTOR2-mediated modulation of NGF/MAPK activation kinetics during differentiation of PC12 cells

LAMTOR2 (p14), a part of the larger LAMTOR/Ragulator complex, plays a crucial role in EGF-dependent activation of p42/44 mitogen-activated protein kinases (MAPK, ERK1/2). In this study, we investigated the role of LAMTOR2 in nerve growth factor (NGF)-mediated neuronal differentiation. Stimulation of PC12 (rat adrenal pheochromocytoma) cells with NGF is known to activate the MAPK. Pharmacological inhibition of MEK1 as well as siRNA–mediated knockdown of both p42 and p44 MAPK resulted in inhibition of neurite outgrowth. Contrary to expectations, siRNA–mediated knockdown of LAMTOR2 effectively augmented neurite formation and neurite length of PC12 cells. Ectopic expression of a siRNA-resistant LAMTOR2 ortholog reversed this phenotype back to wildtype levels, ruling out nonspecific off-target effects of this LAMTOR2 siRNA approach. Mechanistically, LAMTOR2 siRNA treatment significantly enhanced NGF-dependent MAPK activity, and this effect again was reversed upon expression of the siRNA-resistant LAMTOR2 ortholog. Studies of intracellular trafficking of the NGF receptor TrkA revealed a rapid colocalization with early endosomes, which was modulated by LAMTOR2 siRNA. Inhibition of LAMTOR2 and concomitant destabilization of the remaining members of the LAMTOR complex apparently leads to a faster release of the TrkA/MAPK signaling module and nuclear increase of activated MAPK. These results suggest a modulatory role of the MEK1 adapter protein LAMTOR2 in NGF-mediated MAPK activation required for induction of neurite outgrowth in PC12 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.

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-dependent recycling facilitates TrkB translocation to postsynaptic density during LTP via a Rab11-dependent pathway

Brain-derived neurotrophic factor (BDNF) plays an important role in the activity-dependent regulation of synaptic structure and function via tropomyosin related kinase B (TrkB) receptor activation. However, whether BDNF could regulate TrkB levels at synapse during long-term potentiation (LTP) is still unknown. We show in cultured rat hippocampal neurons that chemical LTP (cLTP) stimuli selectively promote endocytic recycling of BDNF-dependent full-length TrkB (TrkB-FL) receptors, but not isoform T1 (TrkB.T1) receptors, via a Rab11-dependent pathway. Moreover, neuronal-activity-enhanced TrkB-FL recycling could facilitate receptor translocation to postsynaptic density and enhance BDNF-induced extracellular signal-regulated kinase and phosphatidylinositol 3-kinase activation and rat hippocampal neuron survival. Finally, we found that cLTP could stimulate the switch of Rab11 from an inactive to an active form and that GTP-bound Rab11 could enhance the interaction between TrkB-FL and PSD-95. Therefore, the recycling endosome could serve as a reserve pool to supply TrkB-FL receptors for LTP maintenance. These findings provide a mechanistic link between Rab11-dependent endocytic recycling and TrkB modulation of synaptic plasticity.

NGF causes TrkA to specifically attract microtubules to lipid rafts

Membrane protein sorting is mediated by interactions between proteins and lipids. One mechanism that contributes to sorting involves patches of lipids, termed lipid rafts, which are different from their surroundings in lipid and protein composition. Although the nerve growth factor (NGF) receptors, TrkA and p75(NTR) collaborate with each other at the plasma membrane to bind NGF, these two receptors are endocytosed separately and activate different cellular responses. We hypothesized that receptor localization in membrane rafts may play a role in endocytic sorting. TrkA and p75(NTR) both reside in detergent-resistant membranes (DRMs), yet they responded differently to a variety of conditions. The ganglioside, GM1, caused increased association of NGF, TrkA, and microtubules with DRMs, but a decrease in p75(NTR). When microtubules were induced to polymerize and attach to DRMs by in vitro reactions, TrkA, but not p75(NTR), was bound to microtubules in DRMs and in a detergent-resistant endosomal fraction. NGF enhanced the interaction between TrkA and microtubules in DRMs, yet tyrosine phosphorylated TrkA was entirely absent in DRMs under conditions where activated TrkA was detected in detergent-sensitive membranes and endosomes. These data indicate that TrkA and p75(NTR) partition into membrane rafts by different mechanisms, and that the fraction of TrkA that associates with DRMs is internalized but does not directly form signaling endosomes. Rather, by attracting microtubules to lipid rafts, TrkA may mediate other processes such as axon guidance.

Charcot-Marie-Tooth disease and intracellular traffic

Mutations of genes whose primary function is the regulation of membrane traffic are increasingly being identified as the underlying causes of various important human disorders. Intriguingly, mutations in ubiquitously expressed membrane traffic genes often lead to cell type- or organ-specific disorders. This is particularly true for neuronal diseases, identifying the nervous system as the most sensitive tissue to alterations of membrane traffic. Charcot-Marie-Tooth (CMT) disease is one of the most common inherited peripheral neuropathies. It is also known as hereditary motor and sensory neuropathy (HMSN), which comprises a group of disorders specifically affecting peripheral nerves. This peripheral neuropathy, highly heterogeneous both clinically and genetically, is characterized by a slowly progressive degeneration of the muscle of the foot, lower leg, hand and forearm, accompanied by sensory loss in the toes, fingers and limbs. More than 30 genes have been identified as targets of mutations that cause CMT neuropathy. A number of these genes encode proteins directly or indirectly involved in the regulation of intracellular traffic. Indeed, the list of genes linked to CMT disease includes genes important for vesicle formation, phosphoinositide metabolism, lysosomal degradation, mitochondrial fission and fusion, and also genes encoding endosomal and cytoskeletal proteins. This review focuses on the link between intracellular transport and CMT disease, highlighting the molecular mechanisms that underlie the different forms of this peripheral neuropathy and discussing the pathophysiological impact of membrane transport genetic defects as well as possible future ways to counteract these defects.

Neurotrophin signaling via long-distance axonal transport

Neurotrophins are a family of target-derived growth factors that support survival, development, and maintenance of innervating neurons. Owing to the unique architecture of neurons, neurotrophins that act locally on the axonal terminals must convey their signals across the entire axon for subsequent regulation of gene transcription in the cell nucleus. This long-distance retrograde signaling, a motor-driven process that can take hours or days, has been a subject of intense interest. In the last decade, live-cell imaging with high sensitivity has significantly increased our capability to track the transport of neurotrophins, their receptors, and subsequent signals in real time. This review summarizes recent research progress in understanding neurotrophin-receptor interactions at the axonal terminal and their transport dynamics along the axon. We emphasize high-resolution studies at the single-molecule level and also discuss recent technical advances in the field.

Beyond amyloid: the future of therapeutics for Alzheimer's disease

Currently, the field is awaiting the results of several pivotal Phase III clinical Alzheimer's disease (AD) trials that target amyloid-β (Aβ). In light of the recent biomarker studies that indicate Aβ levels are at their most dynamic 5-10 years before the onset of clinical symptoms, it is becoming uncertain whether direct approaches to target Aβ will achieve desired clinical efficacy. AD is a complex neurodegenerative disease caused by dysregulation of numerous neurobiological networks and cellular functions, resulting in synaptic loss, neuronal loss, and ultimately impaired memory. While it is clear that Aβ plays a key role in the pathogenesis of AD, it may be a challenging and inefficient target for mid-to-late stage AD intervention. Throughout the course of AD, multiple pathways become perturbed, presenting a multitude of possible therapeutic avenues for design of AD intervention and prophylactic therapies. In this chapter, we sought to first provide an overview of Aβ-directed strategies that are currently in development, and the pivotal Aβ-targeted trials that are currently underway. Next, we delve into the biology and therapeutic designs associated with other key areas of research in the field including tau, protein trafficking and degradation pathways, ApoE, synaptic function, neurotrophic/neuroprotective strategies, and inflammation and energy utilization. For each area we have provided a comprehensive and balanced overview of the therapeutic strategies currently in preclinical and clinical development, which will shape the future therapeutic landscape of AD.

The STAT3 beacon: IL-6 recurrently activates STAT 3 from endosomal structures

Endocytic trafficking plays an important role in signal transduction. Signal transducer and activator of transcription 3 (STAT3) and mitogen-activate protein kinase (MAPK) have both been localized to endosomal structures and are dependent upon endocytosis for downstream function. While the dependence of MAPK signaling upon endosomes has been well characterized, the involvement of endosomes in regulating STAT3 signaling has not been defined. Consequently, this study evaluated the role of endosomes in the initiation, modulation, amplification and persistence of interleukin-6(IL-6)-induced STAT3 signal transduction and transcription, and utilized IL-6-induced MAPK signaling as a comparator. Using pharmacologic treatment and temperature control of endocytic trafficking, pulse-chase treatments and in vitro kinase assays, STAT3 was found to interact with endosomes in a markedly different fashion than MAPK. STAT3 was activated by direct interaction with internal structures upstream of the late endosome following IL-6 exposure and persistent STAT3 signaling depended upon recurrent activation from endocytic structures. Further, STAT3 subcellular localization was not dependent upon endocytic trafficking. Instead, STAT3 transiently interacted with endosomes and relocated to the nucleus by an endosome-independent mechanism. Finally, endocytic trafficking played a central role in regulating STAT3 serine 727 phosphorylation through crosstalk with the MAPK signaling system. Together, these data reveal endosomes as central to the genesis, course and outcome of STAT3 signal transduction and transcription.

Regulation of trafficking of activated TrkA is critical for NGF-mediated functions

Upon activation by nerve growth factor (NGF), TrkA is internalized, trafficked and sorted through different endosomal compartments. Proper TrkA trafficking and sorting are crucial events as alteration of these processes hinders NGF-mediated functions. However, it is not fully known which proteins are involved in the trafficking and sorting of TrkA. Here we report that Nedd4-2 regulates the trafficking of TrkA and NGF functions in sensory neurons. Depletion of Nedd4-2 disrupts the correct sorting of activated TrkA at the early and late endosome stages, resulting in an accumulation of TrkA in these compartments and, as a result of the reduced trafficking to the degradative pathway, TrkA is either reverted to the cell surface through the recycling pathway or retrogradely transported to the cell body. In addition, Nedd4-2 depletion enhances TrkA signaling and the survival of NGF-dependent dorsal root ganglion neurons, but not those of brain-derived neurotrophic factor-dependent neurons. Furthermore, neurons from a knock-in mouse expressing a TrkA mutant that does not bind Nedd4-2 protein exhibit increased NGF-mediated signaling and cell survival. Our data indicate that TrkA trafficking and sorting are regulated by Nedd4-2 protein.

Multivesicular bodies in neurons: distribution, protein content, and trafficking functions

Multivesicular bodies (MVBs) are intracellular endosomal organelles characterized by multiple internal vesicles that are enclosed within a single outer membrane. MVBs were initially regarded as purely prelysosomal structures along the degradative endosomal pathway of internalized proteins. MVBs are now known to be involved in numerous endocytic and trafficking functions, including protein sorting, recycling, transport, storage, and release. This review of neuronal MVBs summarizes their research history, morphology, distribution, accumulation of cargo and constitutive proteins, transport, and theories of functions of MVBs in neurons and glia. Due to their complex morphologies, neurons have expanded trafficking and signaling needs, beyond those of "geometrically simpler" cells, but it is not known whether neuronal MVBs perform additional transport and signaling functions. This review examines the concept of compartment-specific MVB functions in endosomal protein trafficking and signaling within synapses, axons, dendrites and cell bodies. We critically evaluate reports of the accumulation of neuronal MVBs based on evidence of stress-induced MVB formation. Furthermore, we discuss potential functions of neuronal and glial MVBs in development, in dystrophic neuritic syndromes, injury, disease, and aging. MVBs may play a role in Alzheimer's, Huntington's, and Niemann-Pick diseases, some types of frontotemporal dementia, prion and virus trafficking, as well as in adaptive responses of neurons to trauma and toxin or drug exposure. Functions of MVBs in neurons have been much neglected, and major gaps in knowledge currently exist. Developing truly MVB-specific markers would help to elucidate the roles of neuronal MVBs in intra- and intercellular signaling of normal and diseased neurons.

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

Control of TrkA-induced cell death by JNK activation and differential expression of TrkA upon DNA damage

TrkA, a receptor for nerve growth factor, plays a crucial role in neuronal cell growth and differentiation. However, overactivation of TrkA signaling leads to cell death in various cell types. TrkA-mediated cell death shows some similarities to DNA damage-induced cell death. In this study, we examined how TrkA-induced cell death is regulated upon DNA damage. Cytoplasmic expression of TrkA protein was differentially modulated during the camptothecin-induced DNA damage response in TrkA-expressing U2OS cells. TrkA-induced cell death was synergistically increased by DNA damage, but it was blocked in the presence of the JNK inhibitor SP600125. Overexpression of a 54-kDa JNK isoform (JNK1alpha2) aggravated TrkA-induced cell death and was associated with TrkA functional activation. These results suggest that TrkA shares a functional connection with other mediators in the DNA damage response via JNK signaling.

Differential effects of myopathy-associated caveolin-3 mutants on growth factor signaling

Caveolin-3 is an important scaffold protein of cholesterol-rich caveolae. Mutations of caveolin-3 cause hereditary myopathies that comprise remarkably different pathologies. Growth factor signaling plays an important role in muscle physiology; it is influenced by caveolins and cholesterol-rich rafts and might thus be affected by caveolin-3 dysfunction. Prompted by the observation of a marked chronic peripheral neuropathy in a patient suffering from rippling muscle disease due to the R26Q caveolin-3 mutation and because TrkA is expressed by neuronal cells and skeletal muscle fibers, we performed a detailed comparative study on the effect of pathogenic caveolin-3 mutants on the signaling and trafficking of the TrkA nerve growth factor receptor and, for comparison, of the epidermal growth factor receptor. We found that the R26Q mutant slightly and the P28L strongly reduced nerve growth factor signaling in TrkA-transfected cells. Surface biotinylation experiments revealed that the R26Q caveolin-3 mutation markedly reduced the internalization of TrkA, whereas the P28L did not. Moreover, P28L expression led to increased, whereas R26Q expression decreased, epidermal growth factor signaling. Taken together, we found differential effects of the R26Q and P28L caveolin-3 mutants on growth factor signaling. Our findings are of clinical interest because they might help explain the remarkable differences in the degree of muscle lesions caused by caveolin-3 mutations and also the co-occurrence of peripheral neuropathy in the R26Q caveolinopathy case presented.

Axonal targeting of Trk receptors via transcytosis regulates sensitivity to neurotrophin responses

Axonal targeting of trophic receptors is critical for neuronal responses to extracellular developmental cues, yet the underlying trafficking mechanisms remain unclear. Here, we report that tropomyosin-related kinase (Trk) receptors for target-derived neurotrophins are anterogradely trafficked to axons via transcytosis in sympathetic neurons. Using compartmentalized cultures, we show that mature receptors on neuronal soma surfaces are endocytosed and remobilized via Rab11-positive recycling endosomes into axons. Inhibition of dynamin-dependent endocytosis disrupted anterograde transport and localization of TrkA receptors in axons. Anterograde TrkA delivery and exocytosis into axon growth cones is enhanced by nerve growth factor (NGF), acting locally on distal axons. Perturbing endocytic recycling attenuated NGF-dependent signaling and axon growth while enhancing recycling conferred increased neuronal sensitivity to NGF. Our results reveal regulated transcytosis as an unexpected mode of Trk trafficking that serves to rapidly mobilize ready-synthesized receptors to growth cones, thus providing a positive feedback mechanism by which limiting concentrations of target-derived neurotrophins enhance neuronal sensitivity.

Fates of neurotrophins after retrograde axonal transport: phosphorylation of p75NTR is a sorting signal for delayed degradation

Neurotrophins can mediate survival or death of neurons. Opposing functions of neurotrophins are based on binding of these ligands to two distinct types of receptors: trk receptors and p75NTR. Previous work showed that target-derived NGF induces cell death, whereas BDNF and NT-3 enhance survival of neurons in the isthmo-optic nucleus of avian embryos. To determine the fate of retrogradely transported neurotrophins and test whether their sorting differs between neurotrophins mediating survival- or death-signaling pathways, we traced receptor-binding, sorting, and degradation kinetics of target-applied radiolabeled neurotrophins that bind in this system to trk receptors (BDNF, NT-3) or only to p75NTR (NGF). At the ultrastructural level, the p75NTR-bound NGF accumulates with a significant delay in multivesicular bodies and organelles of the degradation pathway on arrival in the cell body when compared with trk-bound BDNF or NT-3. This delayed lysosomal accumulation was restricted to target-derived NGF, but was not seen when NGF was supplied to the soma in vitro. The kinase inhibitors K252a and Gö6976 alter the kinetics of organelle accumulation: phosphorylation of p75NTR is a sorting signal for delayed sequestering of p75NTR-bound NGF in multivesicular bodies and delayed degradation in lysosomes when compared with trk-bound neurotrophins. Mutagenesis and mass spectrometry studies indicate that p75NTR is phosphorylated by conventional protein kinase C on serine 266. We conclude that, in addition to the known phosphorylation of trks, the phosphorylation of p75NTR can also significantly affect neuronal survival in vivo by changing the intracellular sorting and degradation kinetics of its ligands and thus signaling duration.

Proteasomal inhibition alters the trafficking of the neurotrophin receptor TrkA

Neurotrophin receptors of the Trk family promote neuronal survival. The signal transduction of Trk receptors is regulated by endosomal trafficking. Monoubiquitination of receptor tyrosine kinases is an established signal for sorting of internalized receptors to late endosomes. The NGF receptor TrkA is sorted to late endosomes and undergoes ubiquitination, indicating a so far undefined regulatory role of proteasomal activity in the trafficking of TrkA. Surprisingly, we found that proteasomal inhibition alters the trafficking of TrkA from the late endosomal sorting pathway to the recycling pathway. Many neurodegenerative diseases are associated with impaired proteasomal activity. Thus, our study suggests that missorting of neurotrophic receptors might contribute to neuronal death in those neurodegenerative diseases that are known to be associated with impaired proteasomal function.

Essential role of Hrs in endocytic recycling of full-length TrkB receptor but not its isoform TrkB.T1

Brain-derived neurotrophic factor (BDNF) signaling through its receptor, TrkB, modulates survival, differentiation, and synaptic activity of neurons. Both full-length TrkB (TrkB-FL) and its isoform T1 (TrkB.T1) receptors are expressed in neurons; however, whether they follow the same endocytic pathway after BDNF treatment is not known. In this study we report that TrkB-FL and TrkB.T1 receptors traverse divergent endocytic pathways after binding to BDNF. We provide evidence that in neurons TrkB.T1 receptors predominantly recycle back to the cell surface by a "default" mechanism. However, endocytosed TrkB-FL receptors recycle to a lesser extent in a hepatocyte growth factor-regulated tyrosine kinase substrate (Hrs)-dependent manner which relies on its tyrosine kinase activity. The distinct role of Hrs in promoting recycling of internalized TrkB-FL receptors is independent of its ubiquitin-interacting motif. Moreover, Hrs-sensitive TrkB-FL recycling plays a role in BDNF-induced prolonged mitogen-activated protein kinase (MAPK) activation. These observations provide evidence for differential postendocytic sorting of TrkB-FL and TrkB.T1 receptors to alternate intracellular pathways.

High-resolution fractionation of signaling endosomes containing different receptors

Receptor endocytosis is regulated by ligand binding, and receptors may signal after endocytosis in signaling endosomes. We hypothesized that signaling endosomes containing different types of receptors may be distinct from one another and have different physical characteristics. To test this hypothesis, we developed a high-resolution organelle fractionation method based on mass and density, optimized to resolve endosomes from other organelles. Three different types of receptors undergoing ligand-induced endocytosis were localized predominately in endosomes that were resolved from one another using this method. Endosomes containing activated receptor tyrosine kinases (RTKs), TrkA and EGFR, were similar to one another. Endosomes containing p75(NTR) (in the tumor necrosis receptor superfamily) and PAC1 (a G-protein-coupled receptor) were distinct from each other and from RTK endosomes. Receptor-specific endosomes may direct the intracellular location and duration of signal transduction pathways to dictate response to signals and determine cell fate.

The protein phosphatase 2A regulatory subunits B'beta and B'delta mediate sustained TrkA neurotrophin receptor autophosphorylation and neuronal differentiation

Nerve growth factor (NGF) is critical for the differentiation and maintenance of neurons in the peripheral and central nervous system. Sustained autophosphorylation of the TrkA receptor tyrosine kinase and long-lasting activation of downstream kinase cascades are hallmarks of NGF signaling, yet our knowledge of the molecular mechanisms underlying prolonged TrkA activity is incomplete. Protein phosphatase 2A (PP2A) is a heterotrimeric Ser/Thr phosphatase composed of a scaffolding, catalytic, and regulatory subunit (B, B', and B" gene families). Here, we employ a combination of pharmacological inhibitors, regulatory subunit overexpression, PP2A scaffold subunit exchange, and RNA interference to show that PP2A containing B' family regulatory subunits participates in sustained NGF signaling in PC12 cells. Specifically, two neuron-enriched regulatory subunits, B'beta and B'delta, recruit PP2A into a complex with TrkA to dephosphorylate the NGF receptor on Ser/Thr residues and to potentiate its intrinsic Tyr kinase activity. Acting at the receptor level, PP2A/ B'beta and B'delta enhance NGF (but not epidermal growth factor or fibroblast growth factor) signaling through the Akt and Ras-mitogen-activated protein kinase cascades and promote neuritogenesis and differentiation of PC12 cells. Thus, select PP2A heterotrimers oppose desensitization of the TrkA receptor tyrosine kinase, perhaps through dephosphorylation of inhibitory Ser/Thr phosphorylation sites on the receptor itself, to maintain neurotrophin-mediated developmental and survival signaling.

Paradigm shifts in the cell biology of STAT signaling

In recent years several of the key tenets of the original cytokine-STAT-signaling paradigm had to be revised. First, the notion that nonphosphorylated "inactive" STATs are present in the cytoplasm as free monomers which dimerized only subsequent to Tyr-phosphorylation has been replaced by the understanding that nonphosphorylated STATs in the cytoplasm exist largely as dimers and high molecular mass "statosome" complexes. Second, the notion that phosphorylation, either of Tyr or Ser residues or both, in STAT species is required for transcriptional activation has been replaced by the realization that nonphosphorylated STATs can be transcriptionally active albeit with respect to sets of target genes distinct from phosphorylated STATs. Third, the notion that it is the activation by phosphorylation of STATs at the plasma membrane that then leads to their import into the nucleus has been replaced by the recognition that even nonphosphorylated STATs shuttle between the cytoplasm and nucleus at all times in a constitutive manner. Fourth, the notion that the trans-cytoplasmic transit of STATs from the plasma membrane to the nuclear import machinery takes place exclusively as a free cytosolic process has been replaced by the understanding that at least a portion of this trans-cytoplasmic transit is mediated via membrane-associated caveolar and endocytic trafficking (the "signaling endosome" hypothesis). Fifth, the targeting and sequestration of activated STAT3 to long-lived endosomes in the cytoplasm requires consideration of STAT3-mediated "signal transduction" from the plasma membrane to cytoplasmic membrane destinations potentially for function(s) in the cytoplasm. Indeed, in tissue sections many discrete histologic cell types display PY-STAT3 almost exclusively in the cytoplasm with little, if any, in the nucleus. New challenges include determining the structural bases for the recruitment of nonphosphorylated dimeric STAT species to the cytosolic face of membranes including at the cytoplasmic tails of respective receptor complexes, the conformational changes subsequent to phosphorylation and the structural bases for the targeting and functions of STAT proteins within the cytoplasm per se.

Overexpression of functional TrkA receptors after internalisation in human airway smooth muscle cells

Trafficking of the TrkA receptor after stimulation by NGF is of emerging importance in structural cells in the context of airway inflammatory diseases. We have recently reported the expression of functional TrkA receptors in human airway smooth muscle cells (HASMC). We have here studied the TrkA trafficking mechanisms in these cells. TrkA disappearance from the cell membrane was induced within 5 min of NGF (3pM) stimulation. Co-immunoprecipitation of clathrin-TrkA was revealed, and TrkA internalisation inhibited either by clathrin inhibitors or by siRNA inducing downregulation of endogenous clathrin. TrkA internalised receptors were totally degraded in lysosomes, with no recycling phenomenon. Newly synthesized TrkA receptors were thereafter re-expressed at the cell membrane within 10 h. TrkA re-synthesis was inhibited by blockade of clathrin-dependent internalisation, but not of TrkA receptors lysosomal degradation. Finally, we observed that NGF multiple stimulations progressively increased TrkA expression in HASMC, which was associated with an increase in NGF/TrkA-dependent proliferation. In conclusion, we show here the occurrence of clathrin-dependent TrkA internalisation and lysosomal degradation in the airway smooth muscle, followed by upregulated re-synthesis of functional TrkA receptors and increased proliferative effect in the human airway smooth muscle. This may have pathophysiological consequences in airway inflammatory diseases.

Ligand-bound quantum dot probes for studying the molecular scale dynamics of receptor endocytic trafficking in live cells

Endocytic receptor trafficking is a complex, dynamic process underlying fundamental cell function. An integrated understanding of endocytosis at the level of single or small numbers of ligand bound-receptor complexes inside live cells is currently hampered by technical limitations. Here, we develop and test ligand nerve growth factor-bound quantum dot (NGF-QD) bioconjugates for imaging discrete receptor endocytic events inside live NGF-responsive PC12 cells. Using single particle tracking, QD hybrid gel coimmunoprecipitation, and immuno-colocalization, we illustrate and validate the use of QD-receptor complexes for imaging receptor trafficking at synchronized time points after QD-ligand-receptor binding and internalization (t = 15-150 min). The unique value of these probes is illustrated by new dynamic observations: (1) that endocytosis proceeds at strikingly regulated fashion, and (2) that diffusive and active forms of transport inside cells are rapid and efficient. QDs are powerful intracellular probes that can provide biologists with new capabilities and fresh insight for studying endocytic receptor signaling events, in real time, and at the resolution of single or small numbers of receptors in live cells.

Inhibition of Abl tyrosine kinase enhances nerve growth factor-mediated signaling in Bcr-Abl transformed cells via the alteration of signaling complex and the receptor turnover

Receptor tyrosine kinase-mediated signaling is tightly regulated by a number of cytoplasmic signaling molecules. In this report, we show that Bcr-Abl transformed chronic myelogenous leukemia (CML) cell lines, K562 and Meg-01, express the receptor for nerve growth factor (NGF), TrkA, on the cell surface; however, the NGF-mediated signal is not particularly strong. Treatment with imatinib, a potent inhibitor of Bcr-Abl tyrosine kinase, downmodulates phosphorylation of downstream molecules. Upon stimulation with NGF, Erk and Akt are phosphorylated to a much greater degree in imatinib-treated cells than in untreated cells. Knockdown of expression of Bcr-Abl using small interfering RNA technique also enhanced NGF-mediated Akt phosphorylation, indicating that Bcr-Abl kinase modifies NGF signaling directly. Imatinib treatment also enhanced NGF signaling in rat adrenal pheochromocytoma cell line PC12 that expresses TrkA and c-Abl, suggesting that it is not only restoration of responsiveness to NGF after blocking oncoprotein activity, but also c-Abl tyrosine kinase per se may be a negative regulator of growth factor signaling. Furthermore, inhibition of Abl tyrosine kinase enhanced clearance of surface TrkA after NGF treatment and simultaneously enhanced NGF-mediated signaling, suggesting that as in neuronal cells 'signaling endosomes' are formed in hematopoietic cells. To examine the role of TrkA in CML cells, we studied cell growth or colony formation in the presence or absence of imatinib with or without NGF. We found that NGF treatment induces cell survival in imatinib-treated CML cell lines, as well as colony formation of primary CD34+ CML cells, strongly suggesting that NGF/TrkA signaling contributes to aberrant signaling in CML.

TrkA receptor endolysosomal degradation is both ubiquitin and proteasome dependent

Gaps in our knowledge exist regarding the degradation of the tropomyosin-regulated kinase A (TrkA) receptor after addition of neurotrophin, nerve growth factor (NGF). TrkA is rapidly and transiently ubiquitinated upon addition of NGF. Here, we demonstrate that the polyubiquitin tag plays a definitive role in receptor sorting. Treatment of PC12 cells with lactacystin prevented NGF-dependent deubiquitination and degradation of TrkA. However, treatment with methylamine, bafilomycin or leupeptin, did not prevent NGF-dependent deubiquitination but blocked the degradation of TrkA. Employing co-immunoprecipitation, biochemical fractionation and confocal microscopy, the kinetics of receptor trafficking post-internalization was observed to occur as a sequel from endosome/multivesicular body, proteasomes, culminating with degradation in the lysosomes. The trafficking of the polyubiquitin-deficient TrkA receptor mutant K485R was impaired and likewise failed to degrade revealing that the receptor escapes degradation. The interaction of TrkA with proteasomes was confirmed by purification and co-immunoprecipitation. We provide evidence that proteasomal deubiquitinating enzymes trim K63-ubiquitin chains from the TrkA receptor prior to its delivery to lysosomes for degradation. Taken together, our results reveal the existence of a novel proteasome-dependent step in receptor degradation.

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

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

Tyr-701 is a new regulatory site for neurotrophin receptor TrkA trafficking and function

Tropomyosin-related kinase A (TrkA) receptor mediates the effects exerted by nerve growth factor on several subpopulations of neuronal cells. Ligand binding to TrkA induces receptor autophosphorylation on several tyrosine residues and the activation of signaling cascades. In this study, we describe a new site relevant for TrkA regulation, the tyrosine 701 (Y701), which is important for receptor trafficking and activation. Y701 replacement by aspartate or phenylalanine reduces receptor internalization rate and decreases the colocalization and association of TrkA with clathrin heavy chain, demonstrating that Y701 constitutes a YxxPhi (YRKF701-704) trafficking motif relevant for the regulation of receptor endocytosis. In accordance with this hypothesis, the colocalization of Y701 mutant receptors with a lysosomal marker is also reduced giving support to the involvement of the YRKF701-704 motif in the lysosomal targeting of TrkA receptors. Contrary to what was expected, substitution of Y701 for an Asp in order to mimic phosphorylation, impairs TrkA ability to mediate nerve growth factor-induced differentiation, although the mutant receptor retains its in vitro kinase activity. This is the first evidence that a Tyr residue can simultaneously regulate TrkA receptor trafficking and activity.

Rap1-PDZ-GEF1 interacts with a neurotrophin receptor at late endosomes, leading to sustained activation of Rap1 and ERK and neurite outgrowth

Neurotrophins, such as NGF and BDNF, induce sustained activation of Rap1 small G protein and ERK, which are essential for neurite outgrowth. We show involvement of a GDP/GTP exchange factor (GEF) for Rap1, PDZ-GEF1, in these processes. PDZ-GEF1 is activated by GTP-Rap1 via a positive feedback mechanism. Upon NGF binding, the TrkA neurotrophin receptor is internalized from the cell surface, passes through early endosomes, and arrives in late endosomes. A tetrameric complex forms between PDZ-GEF1, synaptic scaffolding molecule and ankyrin repeat-rich membrane spanning protein which interacts directly with the TrkA receptor. At late endosomes, the complex induces sustained activation of Rap1 and ERK, resulting in neurite outgrowth. In cultured rat hippocampal neurons, PDZ-GEF1 is recruited to late endosomes in a BDNF-dependent manner involved in BDNF-induced neurite outgrowth. Thus, the interaction of PDZ-GEF1 with an internalized neurotrophin receptor transported to late endosomes induces sustained activation of both Rap1 and ERK and neurite outgrowth.

Endosomal transport of neurotrophins: roles in signaling and neurodegenerative diseases

The internalization and retrograde axonal transport of neurotrophin receptors is important for their retrograde signal transduction supporting neuronal differentiation, plasticity, and survival. To influence transcription, neurotrophin signals initiated at synapses have to be conveyed retrogradely to the cell body. Signaling endosomes containing neurotrophin receptor signaling complexes mediate retrograde neurotrophin signaling from synapses to the nucleus. Interestingly, many neurodegenerative diseases, including Alzheimer's disease, Niemann Pick disease Type C, and Charcot-Marie-Tooth neuropathies, show alterations of vesicular transport, suggesting that traffic jams within neuronal processes may cause neurodegeneration. Although most of these diseases are complex and may be modulated by diverse pathways contributing to neuronal death, altered neurotrophin transport is emerging as a strong candidate influence on neurodegeneration. In this article, we review the mechanisms of internalization and endocytic trafficking of neurotrophin receptors, and discuss the potential roles of perturbations in neurotrophin trafficking in a number of neurodegenerative diseases.

Neurotrophins Redirect p75NTRfrom a Clathrin‐Independent to a Clathrin‐Dependent Endocytic Pathway Coupled to Axonal Transport

The p75 neurotrophin receptor (p75(NTR)) plays multiple roles in neuronal physiology through interactions with many ligands and coreceptors. However, its intracellular neuronal trafficking prior to and after neurotrophin activation is still poorly characterized. We have previously shown that in response to nerve growth factor (NGF), p75(NTR) is retrogradely transported along the axons of motor neurons (MNs) in carriers shared with NGF, brain-derived neurotrophic factor and the tyrosine kinase receptor TrkB. Here, we report that NGF does not enhance the internalization or degradation of p75(NTR), which undergoes a rapid dynamin-dependent and clathrin-independent recycling process in MNs. Instead, incubation of cells with NGF leads to the redirection of a pool of plasma membrane p75(NTR) into clathrin-coated pits. The subsequent internalization of p75(NTR) via clathrin-mediated endocytosis, as well as the activity of Rab5, are essential for the sorting of the p75(NTR)-containing endosomes to the axonal retrograde transport pathway and for the delivery of p75(NTR) to the soma. Our findings suggest that the spatial regulation of p75(NTR) signalling is controlled by these ligand-driven routes of endocytosis.

The nerve growth factor and its receptors in airway inflammatory diseases

The nerve growth factor (NGF) belongs to the neurotrophin family and induces its effects through activation of 2 distinct receptor types: the tropomyosin-related kinase A (TrkA) receptor, carrying an intrinsic tyrosine kinase activity in its intracellular domain, and the receptor p75 for neurotrophins (p75NTR), belonging to the death receptor family. Through activation of its TrkA receptor, NGF activates signalling pathways, including phospholipase Cgamma (PLCgamma), phosphatidyl-inositol 3-kinase (PI3K), the small G protein Ras, and mitogen-activated protein kinases (MAPK). Through its p75NTR receptor, NGF activates proapoptotic signalling pathways including the MAPK c-Jun N-terminal kinase (JNK), ceramides, and the small G protein Rac, but also activates pathways promoting cell survival through the transcription factor nuclear factor-kappaB (NF-kappaB). NGF was first described by Rita Levi-Montalcini and collaborators as an important factor involved in nerve differentiation and survival. Another role for NGF has since been established in inflammation, in particular of the airways, with increased NGF levels in chronic inflammatory diseases. In this review, we will first describe NGF structure and synthesis and NGF receptors and their signalling pathways. We will then provide information about NGF in the airways, describing its expression and regulation, as well as pointing out its potential role in inflammation, hyperresponsiveness, and remodelling process observed in airway inflammatory diseases, in particular in asthma.

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

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

Rab5 and Rab7 control endocytic sorting along the axonal retrograde transport pathway

Vesicular pathways coupling the neuromuscular junction with the motor neuron soma are essential for neuronal function and survival. To characterize the organelles responsible for this long-distance crosstalk, we developed a purification strategy based on a fragment of tetanus neurotoxin (TeNT H(C)) conjugated to paramagnetic beads. This approach enabled us to identify, among other factors, the small GTPase Rab7 as a functional marker of a specific pool of axonal retrograde carriers, which transport neurotrophins and their receptors. Furthermore, Rab5 is essential for an early step in TeNT H(C) sorting but is absent from axonally transported vesicles. Our data demonstrate that TeNT H(C) uses a retrograde transport pathway shared with p75(NTR), TrkB, and BDNF, which is strictly dependent on the activities of both Rab5 and Rab7. Therefore, Rab7 plays an essential role in axonal retrograde transport by controlling a vesicular compartment implicated in neurotrophin traffic.

Attractive axon guidance involves asymmetric membrane transport and exocytosis in the growth cone

Asymmetric elevation of the Ca(2+) concentration in the growth cone can mediate both attractive and repulsive axon guidance. Ca(2+) signals that are accompanied by Ca(2+)-induced Ca(2+) release (CICR) trigger attraction, whereas Ca(2+) signals that are not accompanied by CICR trigger repulsion. The molecular machinery downstream of Ca(2+) signals, however, remains largely unknown. Here we report that asymmetric membrane trafficking mediates growth cone attraction. Local photolysis of caged Ca(2+), together with CICR, on one side of the growth cone of a chick dorsal root ganglion neuron facilitated the microtubule-dependent centrifugal transport of vesicles towards the leading edge and their subsequent vesicle-associated membrane-protein 2 (VAMP2)-mediated exocytosis on the side with an elevated Ca(2+) concentration. In contrast, Ca(2+) signals without CICR had no effect on the vesicle transport. Furthermore, pharmacological inhibition of VAMP2-mediated exocytosis prevented growth cone attraction, but not repulsion. These results strongly suggest that growth cone attraction and repulsion are driven by distinct mechanisms, rather than using the same molecular machinery with opposing polarities.