NGF signals through TrkA to increase clathrin at the plasma membrane and enhance clathrin-mediated membrane trafficking

Nerve growth factor mediates activation of transient receptor potential vanilloid 1 in neurogenic pruritus of psoriasis

Pruritus is a common and painful symptom in psoriasis with profoundly negative impacts on quality of life. The underlying mechanisms of pruritus are complex and multifactorial, and accumulating evidence suggests that pruritus induced by neurogenic inflammation predominates in psoriasis. Nerve growth factor (NGF) -mediated transient receptor potential vanilloid receptor 1(TRPV1) pathway has emerged as a crucial node in the regulation of neurogenic pruritus. TRPV1 appears coupled to most pruritus-specific molecules via the neuro-immune axis. While the modes of regulation differ for each axis, TRPV1 is involved in substantial biochemical crosstalk-causing feedback loops with significant effects on neurogenic pruritus. Therefore, TRPV1 has emerged as a target molecular in drug development for pruritus in psoriasis. However, no significant clinical progress occurred in the development of systemic TRPV1 antagonists due to elevated core temperature. Thus, topical application of TRPV1 antagonists and interference with mediators linked to the TRPV1 activation pathway may be promising therapeutic options to ameliorate pruritus. This Review focuses on recent advances in complicated regulation of NGF-mediated TRPV1 pathway in psoriatic neurogenic pruritus, as well as the therapeutic options that arise from the dissection of the pathway.

Two-photon fluorescence imaging and specifically biosensing of norepinephrine on a 100-ms timescale

Norepinephrine (NE) is a key neurotransmitter in the central nervous system of organisms; however, specifically tracking the transient NE dynamics with high spatiotemporal resolution in living systems remains a great challenge. Herein, we develop a small molecular fluorescent probe that can precisely anchor on neuronal cytomembranes and specifically respond to NE on a 100-ms timescale. A unique dual acceleration mechanism of molecular-folding and water-bridging is disclosed, which boosts the reaction kinetics by ˃10⁵ and ˃10³ times, respectively. Benefiting from its excellent spatiotemporal resolution, the probe is applied to monitor NE dynamics at the single-neuron level, thereby, successfully snapshotting the fast fluctuation of NE levels at neuronal cytomembranes within 2 s. Moreover, two-photon fluorescence imaging of acute brain tissue slices reveals a close correlation between downregulated NE levels and Alzheimer's disease pathology as well as antioxidant therapy.

Growth cone macropinocytosis of neurotrophin receptor and neuritogenesis are regulated by neuron navigator 1

Neuron navigator 1 (Nav1) is a cytoskeleton-associated protein expressed during brain development that is necessary for proper neuritogenesis, but the underlying mechanisms are poorly understood. Here we show that Nav1 is present in elongating axon tracts during mouse brain embryogenesis. We found that depletion of Nav1 in cultured neurons disrupts growth cone morphology and neurotrophin-stimulated neuritogenesis. In addition to regulating both F-actin and microtubule properties, Nav1 promotes actin-rich membrane ruffles in the growth cone and promotes macropinocytosis at those membrane ruffles, including internalization of the TrkB receptor for the neurotrophin brain-derived neurotropic factor (BDNF). Growth cone macropinocytosis is important for downstream signaling, neurite targeting, and membrane recycling, implicating Nav1 in one or more of these processes. Depletion of Nav1 also induces transient membrane blebbing via disruption of signaling in the Rho GTPase signaling pathway, supporting the novel role of Nav1 in dynamic actin-based membrane regulation at the cell periphery. These data demonstrate that Nav1 works at the interface of microtubules, actin, and plasma membrane to organize the cell periphery and promote uptake of growth and guidance cues to facilitate neural morphogenesis during development.

Activation of neurotrophin signalling with light‑inducible receptor tyrosine kinases

Optogenetics combined with protein engineering based on natural light-sensitive dimerizing proteins has evolved as a powerful strategy to study cellular functions. The present study focused on tropomyosin kinase receptors (Trks) that have been engineered to be light-sensitive. Trk belongs to the superfamily of receptor tyrosine kinases (RTKs), which are single-pass transmembrane receptors that are activated by natural ligands and serve crucial roles in cellular growth, differentiation, metabolism and motility. However, functional variations exist among receptors fused with light-sensitive proteins. The present study proposed a signal transduction model for light-induced receptor activation. This model is based on analysis of previous light-induced Trk receptors reported to date and comparisons to the activation mechanism of natural receptors. In this model, quantitative differences on the dimerization induced from either top-to-bottom or bottom-to-up may lead to the varying amplitude of intracellular signals. We hypothesize that the top-to-bottom propagation is more favourable for activation and yields better results compared with the bottom-to-top direction. The careful delineation of the dimerization mechanisms fine-tuning activation will guide future design for an optimum cellular output with the precision of light.

Role of adaptin protein complexes in intracellular trafficking and their impact on diseases

Adaptin proteins (APs) play a crucial role in intracellular cell trafficking. The 'classical' role of APs is carried out by AP1‒3, which bind to clathrin, cargo, and accessory proteins. Accordingly, AP1-3 are crucial for both vesicle formation and sorting. All APs consist of four subunits that are indispensable for their functions. In fact, based on studies using cells, model organism knockdown/knock-out, and human variants, each subunit plays crucial roles and contributes to the specificity of each AP. These studies also revealed that the sorting and intracellular trafficking function of AP can exert varying effects on pathology by controlling features such as cell development, signal transduction related to the apoptosis and proliferation pathways in cancer cells, organelle integrity, receptor presentation, and viral infection. Although the roles and functions of AP1‒3 are relatively well studied, the functions of the less abundant and more recently identified APs, AP4 and AP5, are still to be investigated. Further studies on these APs may enable a better understanding and targeting of specific diseases.APs known or suggested locations and functions.

Brain-specific functions of the endocytic machinery

Endocytosis is an essential cellular process required for multiple physiological functions, including communication with the extracellular environment, nutrient uptake, and signaling by the cell surface receptors. In a broad sense, endocytosis is accomplished through either constitutive or ligand-induced invagination of the plasma membrane, which results in the formation of the plasma membrane-retrieved endocytic vesicles, which can either be sent for degradation to the lysosomes or recycled back to the PM. This additional function of endocytosis in membrane retrieval has been adopted by excitable cells, such as neurons, for membrane equilibrium maintenance at synapses. The last two decades were especially productive with respect to the identification of brain-specific functions of the endocytic machinery, which additionally include but not limited to regulation of neuronal differentiation and migration, maintenance of neuron morphology and synaptic plasticity, and prevention of neurotoxic aggregates spreading. In this review, we highlight the current knowledge of brain-specific functions of endocytic machinery with a specific focus on three brain cell types, neuronal progenitor cells, neurons, and glial cells.

NGF Signaling in Endosomes

During the development of the nervous system, neurons respond to diffusible cues secreted by target cells. Because such target-derived factors regulate development, maturation, and maintenance of axons as well as somatodendritic compartments, signals initiated at distal axons must be retrogradely transmitted toward cell bodies. Neurotrophins, including the nerve growth factor (NGF), provide one of the best-known examples of target-derived growth factors. The cell biological processes of endocytosis and retrograde trafficking of their Trk receptors from growth cones to cell bodies are key mechanisms by which target-derived neurotrophins influence neurons. Evidence accumulated over the past several decades has begun to uncover the molecular mechanisms of formation, transport, and biological functions of these specialized endosomes called "signaling endosomes."

Challenges of BDNF-based therapies: From common to rare diseases

Neurotrophins are a well-known family of neurotrophic factors that play an important role both in the central and peripheral nervous systems, where they modulate neuronal survival, development, function and plasticity. Brain-derived neurotrophic factor (BDNF) possesses diverse biological functions which are mediated by the activation of two main classes of receptors, the tropomyosin-related kinase (Trk) B and the p75 neurotrophin receptor (p75^NTR). The therapeutic potential of BDNF has drawn attention since dysregulation of its signalling cascades has been suggested to underlie the pathogenesis of both common and rare diseases. Multiple strategies targeting this neurotrophin have been tested; most have found obstacles that ultimately hampered their effectiveness. This review focuses on the involvement of BDNF and its receptors in the pathophysiology of Alzheimer's disease (AD), Amyotrophic Lateral Sclerosis (ALS) and Rett Syndrome (RTT). We describe the known mechanisms leading to the impairment of BDNF/TrkB signalling in these disorders. Such mechanistic insight highlights how BDNF signalling compromise can take various shapes, nearly disease-specific. Therefore, BDNF-based therapeutic strategies must be specifically tailored and are more likely to succeed if a combination of resources is employed.

Mechanisms of neuronal survival safeguarded by endocytosis and autophagy

Multiple aspects of neuronal physiology crucially depend on two cellular pathways, autophagy and endocytosis. During endocytosis, extracellular components either unbound or recognized by membrane-localized receptors (termed "cargo") become internalized into plasma membrane-derived vesicles. These can serve to either recycle the material back to the plasma membrane or send it for degradation to lysosomes. Autophagy also uses lysosomes as a terminal degradation point, although instead of degrading the plasma membrane-derived cargo, autophagy eliminates detrimental cytosolic material and intracellular organelles, which are transported to lysosomes by means of double-membrane vesicles, referred to as autophagosomes. Neurons, like all non-neuronal cells, capitalize on autophagy and endocytosis to communicate with the environment and maintain protein and organelle homeostasis. Additionally, the highly polarized, post-mitotic nature of neurons made them adopt these two pathways for cell-specific functions. These include the maintenance of the synaptic vesicle pool in the pre-synaptic terminal and the long-distance transport of signaling molecules. Originally discovered independently from each other, it is now clear that autophagy and endocytosis are closely interconnected and share several common participating molecules. Considering the crucial role of autophagy and endocytosis in cell type-specific functions in neurons, it is not surprising that defects in both pathways have been linked to the pathology of numerous neurodegenerative diseases. In this review, we highlight the recent knowledge of the role of endocytosis and autophagy in neurons with a special focus on synaptic physiology and discuss how impairments in genes coding for autophagy and endocytosis proteins can cause neurodegeneration.

GDNF synthesis, signaling, and retrograde transport in motor neurons

Glial cell line–derived neurotrophic factor (GDNF) is a 134 amino acid protein belonging in the GDNF family ligands (GFLs). GDNF was originally isolated from rat glial cell lines and identified as a neurotrophic factor with the ability to promote dopamine uptake within midbrain dopaminergic neurons. Since its discovery, the potential neuroprotective effects of GDNF have been researched extensively, and the effect of GDNF on motor neurons will be discussed herein. Similar to other members of the TGF-β superfamily, GDNF is first synthesized as a precursor protein (pro-GDNF). After a series of protein cleavage and processing, the 211 amino acid pro-GDNF is finally converted into the active and mature form of GDNF. GDNF has the ability to trigger receptor tyrosine kinase RET phosphorylation, whose downstream effects have been found to promote neuronal health and survival. The binding of GDNF to its receptors triggers several intracellular signaling pathways which play roles in promoting the development, survival, and maintenance of neuron-neuron and neuron-target tissue interactions. The synthesis and regulation of GDNF have been shown to be altered in many diseases, aging, exercise, and addiction. The neuroprotective effects of GDNF may be used to develop treatments and therapies to ameliorate neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). In this review, we provide a detailed discussion of the general roles of GDNF and its production, delivery, secretion, and neuroprotective effects on motor neurons within the mammalian neuromuscular system.

Leucine zipper transcription factor-like 1 binds adaptor protein complex-1 and 2 and participates in trafficking of transferrin receptor 1

LZTFL1 participates in immune synapse formation, ciliogenesis, and the localization of ciliary proteins, and knockout of LZTFL1 induces abnormal distribution of heterotetrameric adaptor protein complex-1 (AP-1) in the Lztfl1-knockout mouse photoreceptor cells, suggesting that LZTFL1 is involved in intracellular transport. Here, we demonstrate that in vitro LZTFL1 directly binds to AP-1 and AP-2 and coimmunoprecipitates AP-1 and AP-2 from cell lysates. DxxFxxLxxxR motif of LZTFL1 is essential for these bindings, suggesting LZTFL1 has roles in AP-1 and AP-2-mediated protein trafficking. Since AP-1 and AP-2 are known to be involved in transferrin receptor 1 (TfR1) trafficking, the effect of LZTFL1 on TfR1 recycling was analyzed. TfR1, AP-1 and LZTFL1 from cell lysates could be coimmunoprecipitated. However, pull-down results indicate there is no direct interaction between TfR1 and LZTFL1, suggesting that LZTFL1 interaction with TfR1 is indirect through AP-1. We report the colocalization of LZTFL1 and AP-1, AP-1 and TfR1 as well as LZTFL1 and TfR1 in the perinuclear region (PNR) and the cytoplasm, suggesting a potential complex between LZTFL1, AP-1 and TfR1. The results from the disruption of adaptin recruitment with brefeldin A treatment suggested ADP-ribosylation factor-dependent localization of LZFL1 and AP-1 in the PNR. Knockdown of AP-1 reduces the level of LZTFL1 in the PNR, suggesting that AP-1 plays a role in LZTFL1 trafficking. Knockout of LZTFL1 reduces the cell surface level and the rate of internalization of TfR1, leading to a decrease of transferrin uptake, efflux, and internalization. However, knockout of LZTFL1 did not affect the cell surface levels of epidermal growth factor receptor and cation-independent mannose 6-phosphate receptor, indicating that LZTFL1 specifically regulates the cell surface level of TfR1. These data support a novel role of LZTFL1 in regulating the cell surface TfR1 level by interacting with AP-1 and AP-2.

Pharmacologically diverse antidepressants facilitate TRKB receptor activation by disrupting its interaction with the endocytic adaptor complex AP-2

Several antidepressant drugs activate tropomyosin-related kinase B (TRKB) receptor, but it remains unclear whether these compounds employ a common mechanism for TRKB activation. Here, using MS, we found that a single intraperitoneal injection of fluoxetine disrupts the interaction of several proteins with TRKB in the hippocampus of mice. These proteins included members of adaptor protein complex-2 (AP-2) involved in vesicular endocytosis. The interaction of TRKB with the cargo-docking μ subunit of the AP-2 complex (AP2M) was confirmed to be disrupted by both acute and repeated fluoxetine treatments. Of note, fluoxetine disrupted the coupling between full-length TRKB and AP2M, but not the interaction between AP2M and the TRKB C-terminal region, indicating that the fluoxetine-binding site in TRKB lies outside the TRKB:AP2M interface. ELISA experiments revealed that in addition to fluoxetine, other chemically diverse antidepressants, such as imipramine, rolipram, phenelzine, ketamine, and its metabolite 2R,6R-hydroxynorketamine, also decreased the interaction between TRKB and AP2M in vitro Silencing the expression of AP2M in a TRKB-expressing mouse fibroblast cell line (MG87.TRKB) increased cell-surface expression of TRKB and facilitated its activation by brain-derived neurotrophic factor (BDNF), observed as levels of phosphorylated TRKB. Moreover, animals haploinsufficient for the Ap2m1 gene displayed increased levels of active TRKB, along with enhanced cell-surface expression of the receptor in cultured hippocampal neurons. Taken together, our results suggest that disruption of the TRKB:AP2M interaction is a common mechanism underlying TRKB activation by several chemically diverse antidepressants.

Molecular insight on the altered membrane trafficking of TrkA kinase dead mutants

We address the contribution of kinase domain structure and catalytic activity to membrane trafficking of TrkA receptor tyrosine kinase. We conduct a systematic comparison between TrkA-wt, an ATP-binding defective mutant (TrkA-K544N) and other mutants displaying separate functional impairments of phosphorylation, ubiquitination, or recruitment of intracellular partners. We find that only K544N mutation endows TrkA with restricted membrane mobility and a substantial increase of cell surface pool already in the absence of ligand stimulation. This mutation is predicted to drive a structural destabilization of the αC helix in the N-lobe by molecular dynamics simulations, and enhances interactions with elements of the actin cytoskeleton. On the other hand, a different TrkA membrane immobilization is selectively observed after NGF stimulation, requires both phosphorylation and ubiquitination to occur, and is most probably related to the signaling abilities displayed by the wt but not mutated receptors. In conclusion, our results allow to distinguish two different TrkA membrane immobilization modes and demonstrate that not all kinase-inactive mutants display identical membrane trafficking.

Below the Surface: IGF-1R Therapeutic Targeting and Its Endocytic Journey

Ligand-activated plasma membrane receptors follow pathways of endocytosis through the endosomal sorting apparatus. Receptors cluster in clathrin-coated pits that bud inwards and enter the cell as clathrin-coated vesicles. These vesicles travel through the acidic endosome whereby receptors and ligands are sorted to be either recycled or degraded. The traditional paradigm postulated that the endocytosis role lay in signal termination through the removal of the receptor from the cell surface. It is now becoming clear that the internalization process governs more than receptor signal cessation and instead reigns over the entire spatial and temporal wiring of receptor signaling. Governing the localization, the post-translational modifications, and the scaffolding of receptors and downstream signal components established the endosomal platform as the master regulator of receptor function. Confinement of components within or between distinct organelles means that the endosome instructs the cell on how to interpret and translate the signal emanating from any given receptor complex into biological effects. This review explores this emerging paradigm with respect to the cancer-relevant insulin-like growth factor type 1 receptor (IGF-1R) and discusses how this perspective could inform future targeting strategies.

Genome-wide association study in Finnish twins highlights the connection between nicotine addiction and neurotrophin signaling pathway

The heritability of nicotine dependence based on family studies is substantial. Nevertheless, knowledge of the underlying genetic architecture remains meager. Our aim was to identify novel genetic variants responsible for interindividual differences in smoking behavior. We performed a genome-wide association study on 1715 ever smokers ascertained from the population-based Finnish Twin Cohort enriched for heavy smoking. Data imputation used the 1000 Genomes Phase I reference panel together with a whole genome sequence-based Finnish reference panel. We analyzed three measures of nicotine addiction-smoking quantity, nicotine dependence and nicotine withdrawal. We annotated all genome-wide significant SNPs for their functional potential. First, we detected genome-wide significant association on 16p12 with smoking quantity (P = 8.5 × 10^-9 ), near CLEC19A. The lead-SNP stands 22 kb from a binding site for NF-κB transcription factors, which play a role in the neurotrophin signaling pathway. However, the signal was not replicated in an independent Finnish population-based sample, FINRISK (n = 6763). Second, nicotine withdrawal showed association on 2q21 in an intron of TMEM163 (P = 2.1 × 10^-9 ), and on 11p15 (P = 6.6 × 10^-8 ) in an intron of AP2A2, and P = 4.2 × 10^-7 for a missense variant in MUC6, both involved in the neurotrophin signaling pathway). Third, association was detected on 3p22.3 for maximum number of cigarettes smoked per day (P = 3.1 × 10^-8 ) near STAC. Associating CLEC19A and TMEM163 SNPs were annotated to influence gene expression or methylation. The neurotrophin signaling pathway has previously been associated with smoking behavior. Our findings further support the role in nicotine addiction.

Ndr2 Kinase Controls Neurite Outgrowth and Dendritic Branching Through α1 Integrin Expression

The serine/threonine kinase Ndr2 has been shown to control the inside-out activation of the β₁subunit of integrins and the formation of neurites in both primary neurons and neurally differentiated pheochromacytoma (PC12) cells. In this study, we demonstrate that Ndr2 kinase furthermore determines the substrate specificity of neurite extension in PC12 cells via expression of α₁β₁ integrins. We show that stable overexpression of Ndr2 in PC12 cells increases neurite growth and extension on poly-D-lysine substrate, likely involving an increased expression of active β₁ integrin in the growth tips of these cells. By contrast, the Ndr2 overexpressing cells do not show the α₁β₁ integrin-mediated enhancement of neurite growth on collagen IV substrate that can be seen in control cells. Moreover, they entirely fail to increase in response to activation of α₁β₁ integrins via a soluble KTS ligand and show a diminished accumulation of α₁ integrin in neurite tips, although the expression of this subunit is induced during differentiation to comparable levels as in control cells. Finally, we demonstrate that Ndr2 overexpression similarly inhibits the α₁β₁ integrin-dependent dendritic growth of primary hippocampal neurons on laminin 111 substrate. By contrast, lack of Ndr2 impairs the dendritic growth regardless of the substrate. Together, these results suggest that Ndr2 regulates α₁ integrin trafficking in addition to β₁ integrin subunit activation and thereby controls the neurite growth on different extracellular matrix (ECM) substrates.

TrkB neurotrophic activities are blocked by α-synuclein, triggering dopaminergic cell death in Parkinson's disease

BDNF/TrkB neurotrophic signaling is essential for dopaminergic neuronal survival, and the activities are reduced in the substantial nigra (SN) of Parkinson's disease (PD). However, whether α-Syn (alpha-synuclein) aggregation, a hallmark in the remaining SN neurons in PD, accounts for the neurotrophic inhibition remains elusive. Here we show that α-Syn selectively interacts with TrkB receptors and inhibits BDNF/TrkB signaling, leading to dopaminergic neuronal death. α-Syn binds to the kinase domain on TrkB, which is negatively regulated by BDNF or Fyn tyrosine kinase. Interestingly, α-Syn represses TrkB lipid raft distribution, decreases its internalization, and reduces its axonal trafficking. Moreover, α-Syn also reduces TrkB protein levels via up-regulation of TrkB ubiquitination. Remarkably, dopamine's metabolite 3,4-Dihydroxyphenylacetaldehyde (DOPAL) stimulates the interaction between α-Syn and TrkB. Accordingly, MAO-B inhibitor rasagiline disrupts α-Syn/TrkB complex and rescues TrkB neurotrophic signaling, preventing α-Syn-induced dopaminergic neuronal death and restoring motor functions. Hence, our findings demonstrate a noble pathological role of α-Syn in antagonizing neurotrophic signaling, providing a molecular mechanism that accounts for its neurotoxicity in PD.

The neurotrophin receptor signaling endosome: Where trafficking meets signaling

Neurons are the largest cells in the body and form subcellular compartments such as axons and dendrites. During both development and adulthood building blocks must be continually trafficked long distances to maintain the different regions of the neuron. Beyond building blocks, signaling complexes are also transported, allowing for example, axons to communicate with the soma. The critical roles of signaling via ligand-receptor complexes is perhaps best illustrated in the context of development, where they are known to regulate polarization, survival, axon outgrowth, dendrite development, and synapse formation. However, knowing 'when' and 'how much' signaling is occurring does not provide the complete story. The location of signaling has a significant impact on the functional outcomes. There are therefore complex and functionally important trafficking mechanisms in place to control the precise spatial and temporal aspects of many signal transduction events. In turn, many of these signaling events affect trafficking mechanisms, setting up an intricate connection between trafficking and signaling. In this review we will use neurotrophin receptors, specifically TrkA and TrkB, to illustrate the cell biology underlying the links between trafficking and signaling. Briefly, we will discuss the concepts of how trafficking and signaling are intimately linked for functional and diverse signaling outputs, and how the same protein can play different roles for the same receptor depending on its localization. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.

Retrolinkin recruits the WAVE1 protein complex to facilitate BDNF-induced TrkB endocytosis and dendrite outgrowth

Retrolinkin, a neuronal membrane protein, coordinates with endophilin A1 and mediates early endocytic trafficking and signal transduction of the ligand-receptor complex formed between brain-derived neurotrophic factor (BDNF) and its receptor, tropomyosin-related kinase B (TrkB), in dendrites of CNS neurons. Here we report that retrolinkin interacts with the CYFIP1/2 subunit of the WAVE1 complex, a member of the WASP/WAVE family of nucleation-promoting factors that binds and activates the Arp2/3 complex to promote branched actin polymerization. WAVE1, not N-WASP, is required for BDNF-induced TrkB endocytosis and dendrite outgrowth. Disruption of the interaction between retrolinkin and CYFIP1/2 impairs recruitment of WAVE1 to neuronal plasma membrane upon BDNF addition and blocks internalization of activated TrkB. We also show that WAVE1-mediated endocytosis of BDNF-activated TrkB is actin dependent and clathrin independent. These results not only reveal the mechanistic role of retrolinkin in BDNF-TrkB endocytosis, but also indicate that WASP/WAVE-dependent actin polymerization during endocytosis is regulated by cell type-specific and cargo-specific modulators.

Neurotrophin signaling endosomes: biogenesis, regulation, and functions

In the nervous system, communication between neurons and their post-synaptic target cells is critical for the formation, refinement and maintenance of functional neuronal connections. Diffusible signals secreted by target tissues, exemplified by the family of neurotrophins, impinge on nerve terminals to influence diverse developmental events including neuronal survival and axonal growth. Key mechanisms of action of target-derived neurotrophins include the cell biological processes of endocytosis and retrograde trafficking of their Trk receptors from growth cones to cell bodies. In this review, we summarize the molecular mechanisms underlying this endosome-mediated signaling, focusing on the instructive role of neurotrophin signaling itself in directing its own trafficking. Recent studies have linked impaired neurotrophin trafficking to neurodevelopmental disorders, highlighting the relevance of neurotrophin endosomes in human health.

Transmembrane prostatic acid phosphatase (TMPAP) delays cells in G1 phase of the cell cycle

BACKGROUND

Prostate adenocarcinoma is the most common form of prostate cancer. We have previously shown in a murine model that prostatic acid phosphatase (PAP) deficiency leads to increased cell proliferation and development of prostate adenocarcinoma. The association between PAP and prostate cancer has been reported. Indeed, high PAP enzymatic activity is detected in the serum of patients with metastatic disease while its expression is reduced in prostate cancer tissue. However, the molecular mechanisms behind the onset of the disease remains poorly understood. We previously identified a novel transmembrane prostatic acid phosphatase (TMPAP) isoform, which interacts with snapin. TMPAP is expressed on the plasma membrane, as well as endosomal/lysosomal and exosomal membrane vesicles by means of a tyrosine-based lysosomal targeting motif (Yxxϕ).

METHODS

We used stable overexpression of the secreted isoform (SPAP) and TMPAP in LNCaP cells, live cell imaging, microarray and qRT-PCR analyses, and fluid phase uptake of HRP and transferrin.

RESULTS

Our results indicate that the stable overexpression of TMPAP, but not SPAP in LNCaP cells reduces cell growth while increasing endo/exocytosis and cell size. Specifically, cells overexpressing TMPAP accumulate in the G1 phase of the cell cycle, and show altered gene expression profile.

CONCLUSIONS

Our data suggests that TMPAP may function as a non-canonical tumor suppressor by delaying cell growth in G1 phase of the cell cycle.

7,8-dihydroxyflavone, a small molecular TrkB agonist, is useful for treating various BDNF-implicated human disorders

Brain-derived neurotrophic factor (BDNF) regulates a variety of biological processes predominantly via binding to the transmembrane receptor tyrosine kinase TrkB. It is a potential therapeutic target in numerous neurological, mental and metabolic disorders. However, the lack of efficient means to deliver BDNF into the body imposes an insurmountable hurdle to its clinical application. To address this challenge, we initiated a cell-based drug screening to search for small molecules that act as the TrkB agonist. 7,8-Dihydroxyflavone (7,8-DHF) is our first reported small molecular TrkB agonist, which has now been extensively validated in various biochemical and cellular systems. Though binding to the extracellular domain of TrkB, 7,8-DHF triggers TrkB dimerization to induce the downstream signaling. Notably, 7,8-DHF is orally bioactive that can penetrate the brain blood barrier (BBB) to exert its neurotrophic activities in the central nervous system. Numerous reports suggest 7,8-DHF processes promising therapeutic efficacy in various animal disease models that are related to deficient BDNF signaling. In this review, we summarize our current knowledge on the binding activity and specificity, structure-activity relationship, pharmacokinetic and metabolism, and the pre-clinical efficacy of 7,8-DHF against some human diseases.

IL-7 induces clathrin-mediated endocytosis of CD127 and subsequent degradation by the proteasome in primary human CD8 T cells

Interleukin-7 (IL-7), a key immunoregulatory cytokine, plays an essential role in peripheral T-cell homeostasis and function. Signaling via the IL-7 receptor is tightly regulated and we and others have shown IL-7 provides negative feedback on its own signaling by downregulating expression of the IL-7 receptor alpha-chain (CD127) through both suppression of CD127 gene transcription and by internalization of existing CD127 proteins from the cell membrane. We show here for the first time in primary human CD8 T cells that upon stimulation with IL-7, CD127 is internalized through clathrin-coated pits, a process dependent on both lipid-raft formation and the activity of dynamin. As visualized by confocal microscopy, CD127 shows increased co-localization with clathrin within 5 min of IL-7 stimulation and within 15-30 min is seen in multiple intracellular punctae co-localizing with the early endosomal marker EEA1. By 2 h after addition of IL-7, CD127 staining associates with the late endosomal marker RAB7 and with the proteasomal 20S subunit. By inducing receptor internalization and translocation from early endosomes to the proteasome, IL-7 directly influences its receptor density on the cell surface and thus regulates the intensity of its own signaling cascades. Given the important role IL-7 plays in T-cell development, homeostasis and function, deciphering how expression of its receptor is controlled on the cell surface is essential in understanding how T-cell activity can be regulated in different microenvironments and in response to different pathogens.

Blocking GSK3β-mediated dynamin1 phosphorylation enhances BDNF-dependent TrkB endocytosis and the protective effects of BDNF in neuronal and mouse models of Alzheimer's disease

Endocytosis of tropomyosin related kinase B (TrkB) receptors has critical roles in brain-derived neurotrophic factor (BDNF) mediated signal transduction and biological function, however the mechanism that is governing TrkB endocytosis is still not completely understood. In this study, we showed that GSK3β, a key kinase in neuronal development and survival, could regulate TrkB endocytosis through phosphorylating dynamin1 (Dyn1) but not dynamin2 (Dyn2). Moreover, we found that beta-amyloid (Aβ) oligomer exposure could impair BDNF-dependent TrkB endocytosis and Akt activation through enhancing GSK3β activity in cultured hippocampal neurons, which suggested that BDNF-induced TrkB endocytosis and the subsequent signaling were impaired in neuronal model of Alzheimer's disease (AD). Notably, we found that inhibiting GSK3β phosphorylating Dyn1 by using TAT-Dyn1SpS could rescue the impaired TrkB endocytosis and Akt activation upon BDNF stimuli under Aβ exposure. Finally, TAT-Dyn1SpS could facilitate BDNF-mediated neuronal survival and cognitive enhancement in mouse models of AD. These results clarified a role of GSK3β in BDNF-dependent TrkB endocytosis and the subsequent signaling, and provided a potential new strategy by inhibiting GSK3β-induced Dyn1 phosphorylation for AD treatment.

Biogenesis and function of the NGF/TrkA signaling endosome

Target-derived neurotrophin nerve growth factor (NGF) and its receptor TrkA are well known for retrograde signaling to promote survival and innervation of sympathetic and sensory neurons. In recent years, the signaling endosome model has been used to describe the sustained NGF/TrkA retrograde signaling as a process of endocytosis and retrograde transport of NGF/TrkA-containing endosomes from the axon terminal to the cell body for activation of NGF-inducible gene expression responsible for neuronal survival and development. Here, we review the biogenesis and function of NGF, TrkA, and the signaling endosome and discuss possible roles of Rab GTPases in the biogenesis and trafficking of signaling endosomes.

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.

Biochemical and biophysical investigation of the brain-derived neurotrophic factor mimetic 7,8-dihydroxyflavone in the binding and activation of the TrkB receptor

7,8-dihydroxyflavone (7,8-DHF), a newly identified small molecular TrkB receptor agonist, rapidly activates TrkB in both primary neurons and the rodent brain and mimics the physiological functions of the cognate ligand BDNF. Accumulating evidence supports that 7,8-DHF exerts neurotrophic effects in a TrkB-dependent manner. Nonetheless, the differences between 7,8-DHF and BDNF in activating TrkB remain incompletely understood. Here we show that 7,8-DHF and BDNF exhibit different TrkB activation kinetics in which TrkB maturation may be implicated. Employing two independent biophysical approaches, we confirm that 7,8-DHF interacts robustly with the TrkB extracellular domain, with a Kd of ∼10 nm. Although BDNF transiently activates TrkB, leading to receptor internalization and ubiquitination/degradation, in contrast, 7,8-DHF-triggered TrkB phosphorylation lasts for hours, and the internalized receptors are not degraded. Notably, primary neuronal maturation may be required for 7,8-DHF but not for BDNF to elicit the full spectrum of TrkB signaling cascades. Hence, 7,8-DHF interacts robustly with the TrkB receptor, and its agonistic effect may be mediated by neuronal development and maturation.

The adaptor protein GULP promotes Jedi-1-mediated phagocytosis through a clathrin-dependent mechanism

During the development of the peripheral nervous system, the large number of apoptotic neurons generated are phagocytosed by glial precursor cells. This clearance is mediated, in part, through the mammalian engulfment receptor Jedi-1. However, the mechanisms by which Jedi-1 mediates phagocytosis are poorly understood. Here we demonstrate that Jedi-1 associates with GULP, the mammalian homologue of CED-6, an adaptor protein required for phagocytosis mediated by the nematode engulfment receptor CED-1. Silencing GULP or mutating the NPXY motif in Jedi-1, which is required for GULP binding, prevents Jedi-1-mediated phagocytosis. How GULP promotes engulfment is not known. Of interest, we find that Jedi-1-induced phagocytosis requires GULP binding to clathrin heavy chain (CHC). During engulfment, CHC is tyrosine phosphorylated, which is required for Jedi-mediated engulfment. Both phosphoclathrin and actin accumulate around engulfed microspheres. Furthermore, knockdown of CHC in HeLa cells prevents Jedi-1-mediated engulfment of microspheres, and knockdown in glial precursors prevents the engulfment of apoptotic neurons. Taken together, these results reveal that Jedi-1 signals through recruitment of GULP, which promotes phagocytosis through a noncanonical phosphoclathrin-dependent mechanism.

Local versus long-range neurotrophin receptor signalling: endosomes are not just carriers for axonal transport

Neurotrophins play a critical role in neuronal development and survival, as well as maintenance of the adult nervous system. Neurotrophins can mediate their effects by signalling locally at the nerve terminal, or signalling retrogradely from the axonal terminal to the cell soma to regulate gene expression. Given that the axon terminals of many nerve cells can be up to a metre away from their soma, neurons have evolved specialized long-range signalling platforms that depend on a highly regulated network of intracellular membrane compartments termed "signalling endosomes". Endosomal trafficking of activated receptors controls not only the axonal retrograde signals but also local receptor recycling and degradation. Endosomal trafficking involving the sorting and compartmentalizing of different signals, which are subsequently distributed to the appropriate cellular destination, can at least partially explain how neurotrophins generate a diverse array of signalling outcomes. Although signalling endosomes provide a useful model for understanding how different cell surface receptor-mediated signals are generated and transported, the precise role, identity and functional definition of a signalling endosome remains unclear. In this review we will discuss the regulation of local versus long-range neurotrophin signalling, with a specific focus on recent developments in the role of endosomes in regulating the fate of Trk receptors.

Review series: From uncertain beginnings: initiation mechanisms of clathrin-mediated endocytosis

Clathrin-mediated endocytosis is a central and well-studied trafficking process in eukaryotic cells. How this process is initiated is likely to be a critical point in regulating endocytic activity spatially and temporally, but the underlying mechanisms are poorly understood. During the early stages of endocytosis three components—adaptor and accessory proteins, cargo, and lipids—come together at the plasma membrane to begin the formation of clathrin-coated vesicles. Although different models have been proposed, there is still no clear picture of how these three components cooperate to initiate endocytosis, which may indicate that there is some flexibility underlying this important event.

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.

The non-canonical roles of clathrin and actin in pathogen internalization, egress and spread

The role of clathrin in pathogen entry has received much attention and has highlighted the adaptability of clathrin during internalization. Recent studies have now uncovered additional roles for clathrin and have put the spotlight on its role in pathogen spread. Here, we discuss the manipulation of clathrin by pathogens, with specific attention to the processes that occur at the plasma membrane. In the majority of cases, both clathrin and the actin cytoskeleton are hijacked, so we also examine the interplay between these two systems and their role during pathogen internalization, egress and spread.

Changes in the expression and current of the Na+/K+ pump in the snail nervous system after exposure to a static magnetic field

Compelling evidence supports the use of a moderate static magnetic field (SMF) for therapeutic purposes. In order to provide insight into the mechanisms underlying SMF treatment, it is essential to examine the cellular responses elicited by therapeutically applied SMF, especially in the nervous system. The Na(+)/K(+) pump, by creating and maintaining the gradient of Na(+) and K(+) ions across the plasma membrane, regulates the physiological properties of neurons. In this study, we examined the expression of the Na(+)/K(+) pump in the isolated brain-subesophageal ganglion complex of the garden snail Helix pomatia, along with the immunoreactivity and current of the Na(+)/K(+) pump in isolated snail neurons after 15 min exposure to a moderate (10 mT) SMF. Western blot and immunofluorescence analysis revealed that 10 mT SMF did not significantly change the expression of the Na(+)/K(+) pump α-subunit in the snail brain and the neuronal cell body. However, our immunofluorescence data showed that SMF treatment induced a significant increase in the Na(+)/K(+) pump α-subunit expression in the neuronal plasma membrane area. This change in Na(+)/K(+) pump expression was reflected in pump activity as demonstrated by the pump current measurements. Whole-cell patch-clamp recordings from isolated snail neurons revealed that Na(+)/K(+) pump current density was significantly increased after the 10 mT SMF treatment. The SMF-induced increase was different in the two groups of control snail neurons, as defined by the pump current level. The results obtained could represent a physiologically important response of neurons to 10 mT SMF comparable in strength to therapeutic applications.

Diversity of clathrin function: new tricks for an old protein

Clathrin is considered the prototype vesicle coat protein whose self-assembly mediates sorting of membrane cargo and recruitment of lipid modifiers. Detailed knowledge of clathrin biochemistry, structure, and interacting proteins has accumulated since the first observation, almost 50 years ago, of its role in receptor-mediated endocytosis of yolk protein. This review summarizes that knowledge, and focuses on properties of the clathrin heavy and light chain subunits and interaction of the latter with Hip proteins, to address the diversity of clathrin function beyond conventional receptor-mediated endocytosis. The distinct functions of the two human clathrin isoforms (CHC17 and CHC22) are discussed, highlighting CHC22's specialized involvement in traffic of the GLUT4 glucose transporter and consequent role in human glucose metabolism. Analysis of clathrin light chain function and interaction with the actin-binding Hip proteins during bacterial infection defines a novel actin-organizing function for CHC17 clathrin. By considering these diverse clathrin functions, along with intracellular sorting roles and influences on mitosis, further relevance of clathrin function to human health and disease is established.

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.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Endocytic trafficking of neurotrophins in neural development

During the formation of neuronal circuits, neurons respond to diffusible cues secreted by target tissues. Often, target-derived signals act on nerve terminals to influence local growth events; in other cases, they are transported long distances back to neuronal cell bodies to effect transcriptional changes necessary for neuronal survival and differentiation. Neurotrophins provide one of the best examples of target-derived cues that elicit an astonishingly diverse array of neuronal responses. Endocytic trafficking of neurotrophins and their receptors is a fundamental feature of neurotrophin signaling, allowing neurotrophins to control neuronal survival by retrograde transport of signaling endosomes containing ligand-receptor complexes. In this review we summarize recent findings that provide new insight into the interplay between neurotrophin signaling and trafficking.

Genomic analysis of individual differences in ethanol drinking: evidence for non-genetic factors in C57BL/6 mice

Genetic analysis of factors affecting risk to develop excessive ethanol drinking has been extensively studied in humans and animal models for over 20 years. However, little progress has been made in determining molecular mechanisms underlying environmental or non-genetic events contributing to variation in ethanol drinking. Here, we identify persistent and substantial variation in ethanol drinking behavior within an inbred mouse strain and utilize this model to identify gene networks influencing such “non-genetic” variation in ethanol intake. C57BL/6NCrl mice showed persistent inter-individual variation of ethanol intake in a two-bottle choice paradigm over a three-week period, ranging from less than 1 g/kg to over 14 g/kg ethanol in an 18 h interval. Differences in sweet or bitter taste susceptibility or litter effects did not appreciably correlate with ethanol intake variation. Whole genome microarray expression analysis in nucleus accumbens, prefrontal cortex and ventral midbrain region of individual animals identified gene expression patterns correlated with ethanol intake. Results included several gene networks previously implicated in ethanol behaviors, such as glutamate signaling, BDNF and genes involved in synaptic vesicle function. Additionally, genes functioning in epigenetic chromatin or DNA modifications such as acetylation and/or methylation also had expression patterns correlated with ethanol intake. In verification for the significance of the expression findings, we found that a histone deacetylase inhibitor, trichostatin A, caused an increase in 2-bottle ethanol intake. Our results thus implicate specific brain regional gene networks, including chromatin modification factors, as potentially important mechanisms underlying individual variation in ethanol intake.

Integrated genomics and proteomics of the Torpedo californica electric organ: concordance with the mammalian neuromuscular junction

Background

During development, the branchial mesoderm of Torpedo californica transdifferentiates into an electric organ capable of generating high voltage discharges to stun fish. The organ contains a high density of cholinergic synapses and has served as a biochemical model for the membrane specialization of myofibers, the neuromuscular junction (NMJ). We studied the genome and proteome of the electric organ to gain insight into its composition, to determine if there is concordance with skeletal muscle and the NMJ, and to identify novel synaptic proteins.

Results

Of 435 proteins identified, 300 mapped to Torpedo cDNA sequences with ≥2 peptides. We identified 14 uncharacterized proteins in the electric organ that are known to play a role in acetylcholine receptor clustering or signal transduction. In addition, two human open reading frames, C1orf123 and C6orf130, showed high sequence similarity to electric organ proteins. Our profile lists several proteins that are highly expressed in skeletal muscle or are muscle specific. Synaptic proteins such as acetylcholinesterase, acetylcholine receptor subunits, and rapsyn were present in the electric organ proteome but absent in the skeletal muscle proteome.

Conclusions

Our integrated genomic and proteomic analysis supports research describing a muscle-like profile of the organ. We show that it is a repository of NMJ proteins but we present limitations on its use as a comprehensive model of the NMJ. Finally, we identified several proteins that may become candidates for signaling proteins not previously characterized as components of the NMJ.

An isoimmune response to human sperm clathrin in an infertile woman with systemic lupus erythematosus

We have employed a proteomic approach to study the immune response to human sperm in an infertile female patient suffering from systemic lupus erythematosus (SLE). Human sperm antigenic extracts were resolved by means of two-dimensional electrophoresis and electroblotted onto nitrocellulose membranes. The membranes were incubated with serum from the SLE patient. Sperm antigens that were reactive to polyclonal antibodies were next visualized on X-ray film, using the enhanced chemiluminescence (ECL). Three spots corresponding to the positions of sperm immunoreactive antigens on a nitrocellulose membrane were localized in a silver stained gel and subjected to mass spectrometry. A database search of the sequences recognized by the analyzed SLE serum revealed its homology to the clathrin heavy chain (CHC). Further analysis revealed that anti-CHC antibody reacted with multiple sperm antigenic determinants, resolved by either one- or two-dimensional electrophoresis. When studied by immunofluorescence, we demonstrated anti-CHC antibody reactivity with the sperm tail tip (corresponding to the sperm agglutination pattern), also with the principal piece and with cytoplasmic droplets around the sperm midpiece. Live sperm clearly exhibited reactivity with the midpiece. This study demonstrates clathrin heavy chain on human sperm using serum of an infertile individual with a concomitant autoimmune disease.

Shiga toxin increases formation of clathrin-coated pits through Syk kinase

Clathrin-dependent endocytosis is a main entry mechanism for the glycolipid-binding Shiga toxin (Stx), although clathrin-independent pathways are also involved. Binding of Stx to its receptor Gb3 not only is essential for Stx retrograde transport to the endoplasmic reticulum and toxicity but also activates signaling through the tyrosine kinase Syk. We previously described that Syk activity is important for Stx entry, but it remained unclear how this kinase modulates endocytosis of Stx. Here we characterized the effects of Stx and Syk on clathrin-coated pit formation. We found that acute treatment with Stx results in an increase in the number of clathrin-coated profiles as determined by electron microscopy and on the number of structures containing the endocytic AP-2 adaptor at the plasma membrane determined by live-cell spinning disk confocal imaging. These responses to Stx require functional Syk activity. We propose that a signaling pathway mediated by Syk and modulated by Stx leads to an increased number of endocytic clathrin-coated structures, thus providing a possible mechanism by which Stx enhances its own endocytosis.

Src-family kinase signaling, actin-mediated membrane trafficking and organellar dynamics in the control of cell fate: lessons to be learned from the adenovirus E4orf4 death factor

Evidence has accumulated that there are different modes of regulated cell death, which share overlapping signaling pathways. Cytoskeletal-dependent inter-organellar communication as a result of protein and lipid trafficking in and out of organelles has emerged as a common, key issue in the regulation of cell death modalities. The movement of proteins and lipids between cell compartments is believed to relay death signals in part through modifications of organelles dynamics. Little is known, however, regarding how trafficking is integrated within stress signaling pathways directing organelle-specific remodeling events. In this review, we discuss emerging evidence supporting a role for regulated changes in actin dynamics and intracellular membrane flow. Based on recent findings using the adenovirus E4orf4 death factor as a probing tool to tackle the mechanistic underpinnings that control alternative modes of cell death, we propose the existence of multifunctional platforms at the endosome-Golgi interface regulated by SFK-signaling. These endosomal platforms could be mobilized during cell activation processes to reorganize cellular membranes and promote inter-organelle signaling.

Investigation of neuromuscular abnormalities in neurotrophin-3-deficient mice

Neurotrophin-3 (NT-3) is a trophic factor that is essential for the normal development and maintenance of proprioceptive sensory neurons and is widely implicated as an important modulator of synaptic function and development. We have previously found that animals lacking NT-3 have a number of structural abnormalities in peripheral nerves and skeletal muscles. Here we investigated whether haploinsufficiency-induced reduction in NT-3 resulted in impaired neuromuscular performance and synaptic function. Motor nerve terminal function was tested by monitoring the uptake/release of the fluorescent membrane dye FM1-43 by the electrophysiological examination of synaptic transmission and electron microscopic determination of synaptic vesicle density at the presynaptic active zone. We investigated skeletal muscle form and function by measuring force in response to both nerve-mediated and direct muscle stimulation and by quantification of fiber number and area from transverse sections. Synaptic transmission was not markedly different between the two groups, although the uptake and release of FM1-43 were impaired in mature NT-3-deficient mice but not in immature mice. The electron microscopic examination of mature nerve terminals showed no genotype-dependent variation in the number of synaptic vesicles near the active zone. NT-3(+/-) mice had normal soleus muscle fiber numbers but their fibers had smaller cross-sectional areas and were more densely-packed than wild-type littermates. Moreover, the muscles of adult NT-3-deficient animals were weaker than those of wild-type animals to both nerve and direct muscle stimulation. The results indicate that a reduction in NT-3 availability during development impairs motor nerve terminal maturation and synaptic vesicle recycling and leads to a reduction in muscle fiber diameter.

Endocytosis and signalling: intertwining molecular networks

Cell signalling and endocytic membrane trafficking have traditionally been viewed as distinct processes. Although our present understanding is incomplete and there are still great controversies, it is now recognized that these processes are intimately and bidirectionally linked in animal cells. Indeed, many recent examples illustrate how endocytosis regulates receptor signalling (including signalling from receptor tyrosine kinases and G protein-coupled receptors) and, conversely, how signalling regulates the endocytic pathway. The mechanistic and functional principles that underlie the relationship between signalling and endocytosis in cell biology are becoming increasingly evident across many systems.

Rab GTPase regulation of VEGFR2 trafficking and signaling in endothelial cells

OBJECTIVE

Vascular endothelial growth factor receptor 2 (VEGFR2) is a receptor tyrosine kinase that regulates vascular physiology. However, mechanism(s) by which VEGFR2 signaling and trafficking is coordinated are not clear. Here, we have tested endocytic Rab GTPases for regulation of VEGFR2 trafficking and signaling linked to endothelial cell migration.

METHODS AND RESULTS

Quiescent VEGFR2 displays endosomal localization and colocalization with the Rab5a GTPase, an early endosome fusion regulator. Expression of GTP or GDP-bound Rab5a mutants block activated VEGFR2 trafficking and degradation. Manipulation of Rab7a GTPase activity associated with late endosomes using overexpression of wild-type or mutant proteins blocks activated VEGFR2 trafficking and degradation. Depletion of Rab7a decreased VEGFR2 Y1175 phosphorylation but increased p42/44 (pERK1/2) MAPK phosphorylation. Endothelial cell migration was increased by Rab5a depletion but decreased by Rab7a depletion.

CONCLUSIONS

Rab5a and Rab7a regulate VEGFR2 trafficking toward early and late endosomes. Our data suggest that VEGFR2-mediated regulation of endothelial function is dependent on different but specific Rab-mediated GTP hydrolysis activity required for endosomal trafficking.