Tracking TrkA’s Trafficking: NGF Receptor Trafficking Controls NGF Receptor Signaling

ZDHHC17 promotes axon outgrowth by regulating TrkA-tubulin complex formation

Correct axonal growth during nervous system development is critical for synaptic transduction and nervous system function. Proper axon outgrowth relies on a suitable growing environment and the expression of a series of endogenous neuronal factors. However, the mechanisms of these neuronal proteins involved in neuronal development remain unknown. ZDHHC17 is a member of the DHHC (Asp-His-His-Cys)-containing family, a family of highly homologous proteins. Here, we show that loss of function of ZDHHC17 in zebrafish leads to motor dysfunction in 3-day post-fertilization (dpf) larvae. We performed immunolabeling analysis to reveal that mobility dysfunction was due to a significant defect in the axonal outgrowth of spinal motor neurons (SMNs) without affecting neuron generation. In addition, we found a similar phenotype in zdhhc17 siRNA-treated neural stem cells (NSCs) and PC12 cells. Inhibition of zdhhc17 limited neurite outgrowth and branching in both NSCs and PC12. Furthermore, we discovered that the level of phosphorylation of extracellular-regulated kinase (ERK) 1/2, a major downstream effector of tyrosine kinase (TrkA), was largely upregulated in ZDHHC17 overexpressing PC12 cells by a mechanism independent on its palmitoyltransferase (PAT) activity. Specifically, ZDHHC17 is necessary for proper TrkA-tubulin module formation in PC12 cells. These results strongly indicate that ZDHHC17 is essential for correct axon outgrowth in vivo and in vitro. Our findings identify ZDHHC17 as an important upstream factor of ERK1/2 to regulate the interaction between TrkA and tubulin during neuronal development.

Membrane turnover and receptor trafficking in regenerating axons

Peripheral axonal regeneration requires surface-expanding membrane addition. The continuous incorporation of new membranes into the axolemma allows the pushing force of elongating microtubules to drive axonal growth cones forwards. Hence, a constant supply of membranes and cytoskeletal building blocks is required, often for many weeks. In human peripheral nerves, axonal tips may be more than 1 m away from the neuronal cell body. Therefore, in the initial phase of regeneration, membranes are derived from pre-existing vesicles or synthesised locally. Only later stages of axonal regeneration are supported by membranes and proteins synthesised in neuronal cell bodies, considering that the fastest anterograde transport mechanisms deliver cargo at 20 cm/day. Whereas endocytosis and exocytosis of membrane vesicles are balanced in intact axons, membrane incorporation exceeds membrane retrieval during regeneration to compensate for the loss of membranes distal to the lesion site. Physiological membrane turnover rates will not be established before the completion of target reinnervation. In this review, the current knowledge on membrane traffic in axonal outgrowth is summarised, with a focus on endosomal vesicles as the providers of membranes and carriers of growth factor receptors required for initiating signalling pathways to promote the elongation and branching of regenerating axons in lesioned peripheral nerves.

Overexpression of atypical protein kinase C in HeLa cells facilitates macropinocytosis via Src activation

Atypical protein kinase C (aPKC) is the first recognized kinase oncogene. However, the specific contribution of aPKC to cancer progression is unclear. The pseudosubstrate domain of aPKC is different from the other PKC family members, and therefore a synthetic peptide corresponding to the aPKC pseudosubstrate (aPKC-PS) sequence, which specifically blocks aPKC kinase activity, is a valuable tool to assess the role of aPKC in various cellular processes. Here, we learned that HeLa cells incubated with membrane permeable aPKC-PS peptide displayed dilated heterogeneous vesicles labeled with peptide that were subsequently identified as macropinosomes. A quantitative membrane binding assay revealed that aPKC-PS peptide stimulated aPKC recruitment to membranes and activated Src. Similarly, aPKC overexpression in transfected HeLa cells activated Src and induced macropinosome formation. Src-aPKC interaction was essential; substitution of the proline residues in aPKC that associate with the Src-SH3 binding domain rendered the mutant kinase unable to induce macropinocytosis in transfected cells. We propose that aPKC overexpression is a contributing factor to cell transformation by interacting with and consequently promoting Src activation and constitutive macropinocytosis, which increases uptake of extracellular factors, required for altered cell growth and accelerated cell migration.

Phosphorylation of NTRK1 at Y674/Y675 induced by TP53-dependent repression of PTPN6 expression: a potential novel prognostic marker for breast cancer

We have identified a ligand-independent mechanism whereby the tumor suppressor, TP53, induces nerve growth factor receptor, NTRK1, phosphorylation at Y674/Y675 (NTRK1-pY674/pY675), via the repression of the NTRK1-phosphatase, PTPN6. This results in suppression of breast cancer cell proliferation. In this investigation, we aimed to establish whether perturbation of the wild-type TP53-NTRK1-pY674/pY675-PTPN6 pathway has an impact on disease-free survival of breast cancer patients without neo-adjuvant treatment. A total of 308 tumor samples were stained for NTRK1, NTRK1-pY674/pY675, PTPN6, and TP53 expression. Association between expression levels and disease-free survival was determined by the univariate/multivariate and Kaplan-Meir methods of analysis. DNA from tumors was sequenced to identify mutant or wild-type TP53. Tumors expressing NTRK1-pY674/pY675 but with undetectable or low levels of PTPN6 and TP53 were associated with prolonged 5, 10, and 15 years' disease-free survival by 48%, 36%, and 37%, respectively, in the multivariate analysis (P<0.05). A similar result was observed in tumors expressing wild-type TP53, NTRK1-pY674/pY675, and low or undetectable levels of PTPN6. Given that estrogen receptor-positive breast cancers encode wild-type TP53, we analyzed this expression pattern in these tumors. Multivariate analysis showed that it was significantly and independently predictive of prolonged survival by 66%, 70%, and 84%, respectively, (P<0.05). The Kaplan-Meir method demonstrated that NTRK1-pY674/pY675 together with undetectable or low levels of PTPN6 correlated with 59% probability of disease-free survival (median survival 15 years), compared with 7% probability of disease-free survival (median survival 4.5 years) when absent. In luminal A tumors, the presence of this pattern was estimated to have a 61% probability of disease-free survival (median survival 15 years), compared with 6% probability of disease-free survival (median survival 3 years) when it was absent. These results strongly suggest that expression of NTRK1-pY674/pY675 together with wild-type TP53 and low levels of PTPN6 expression are predictors of improved disease-free survival and that they could be useful biomarkers to predict clinical outcome.

The microtubule-associated protein DCAMKL1 regulates osteoblast function via repression of Runx2

Deficiency of the microtubule-associated protein DCAMKL1 results in elevated bone mass via repression of osteoblast activation through Runx2 antagonization.

Osteoblasts are responsible for the formation and mineralization of the skeleton. To identify novel regulators of osteoblast differentiation, we conducted an unbiased forward genetic screen using a lentiviral-based shRNA library. This functional genomics analysis led to the identification of the microtubule-associated protein DCAMKL1 (Doublecortin-like and CAM kinase–like 1) as a novel regulator of osteogenesis. Mice with a targeted disruption of Dcamkl1 displayed elevated bone mass secondary to increased bone formation by osteoblasts. Molecular experiments demonstrated that DCAMKL1 represses osteoblast activation by antagonizing Runx2, the master transcription factor in osteoblasts. Key elements of the cleidocranial dysplasia phenotype observed in Runx2^+/− mice are reversed by the introduction of a Dcamkl1-null allele. Our results establish a genetic linkage between these two proteins in vivo and demonstrate that DCAMKL1 is a physiologically relevant regulator of anabolic bone formation.

Defective axonal transport of Rab7 GTPase results in dysregulated trophic signaling

Retrograde trophic signaling of nerve growth factor (NGF) supports neuronal survival and differentiation. Dysregulated trophic signaling could lead to various neurological disorders. Charcot-Marie-Tooth type 2B (CMT2B) is one of the most common inherited peripheral neuropathies characterized by severe terminal axonal loss. Genetic analysis of human CMT2B patients has revealed four missense point mutations in Rab7, a small GTPase that regulates late endosomal/lysosomal pathways, but the exact pathological mechanism remains poorly understood. Here, we show that these Rab7 mutants dysregulated axonal transport and diminished the retrograde signaling of NGF and its TrkA receptor. We found that all CMT2B Rab7 mutants were transported significantly faster than Rab7(wt) in the anterograde direction, accompanied with an increased percentile of anterograde Rab7-vesicles within axons of rat E15.5 dorsal root ganglion (DRG) neurons. In PC12M cells, the CMT2B Rab7 mutants drastically reduced the level of surface TrkA and NGF binding, presumably by premature degradation of TrkA. On the other hand, siRNA knock-down of endogenous Rab7 led to the appearance of large TrkA puncta in enlarged Rab5-early endosomes within the cytoplasm, suggesting delayed TrkA degradation. We also show that CMT2B Rab7 mutants markedly impaired NGF-induced Erk1/2 activation and differentiation in PC12M cells. Further analysis revealed that CMT2B Rab7 mutants caused axonal degeneration in rat E15.5 DRG neurons. We propose that Rab7 mutants induce premature degradation of retrograde NGF-TrkA trophic signaling, which may potentially contribute to the CMT2B disease.

Intracellular interaction of newly synthesized nerve growth factor and its receptors

In autocrine cells, both a ligand and its receptors are synthesized in the same cell, but their intracellular interaction is not well known. We examined it using PC84 cells, a mutant PC12 cell line expressing nerve growth factor (NGF). We have already reported that the intracellular precursor of TrkA was phosphorylated and that MAP kinase was phosphorylated in PC84 cells. In this paper we found that the NGF receptors, TrkA and p75NTR, existed mainly as precursors, and most p75NTR localized inside PC84 cells. The phosphorylation of MAP kinase was also observed even when PC84 cells were incubated with anti-NGF antibody to block the extracellular interaction. These results suggest the possibility that newly synthesized NGF could interact intracellularly with the receptors in PC84 cells.

Tracing the endocytic pathways and trafficking kinetics of cell signaling receptors using single QD nanoparticles

Cellular signaling is the fundamental process through which cells communicate with each other and respond to their environment. Regulation of this cellular signaling is crucial for healthy cellular function. Malfunctions in signaling are the cause for many diseases and disorders and therefore are under heavy investigation. The molecular mechanisms that underlie cellular signaling rely upon complex and dynamic processes of receptor intracellular trafficking. The specific endosomal pathways and kinetics through which receptors are intracellularly transported regulate the strength and duration of cellular signaling. In even more subtle and complex aspects, the cell orchestrates the individual motions of many receptors, through multiple different pathways, simultaneously. Despite the fundamental role of endosomal trafficking in signal regulation, it has been technically challenging to study since intracellular trafficking is complex and dynamic, with millions of individual receptors simultaneously undergoing trafficking in different endocytic stages. Here, we describe the use of single nanoparticle quantum dot (QD) probes to quantitatively investigate the endocytic trafficking pathways that receptors undergo following ligand activation. This new capability to directly visualize and quantitate cellular signaling at the level of individual receptors inside the cell has broad and important value for understanding fundamental cell signaling processes and the action and effect of therapeutics upon signaling.

The role of TRKA signaling in IL-10 production by apoptotic tumor cell-activated macrophages

Tumor-associated macrophages (TAMs) are a major supportive component within neoplasms. Mechanisms of macrophage (MΦ) attraction and differentiation to a tumor-promoting phenotype, which is characterized by pronounced interleukin (IL)-10 production, are under investigation. We report that supernatants of dying cancer cells induced substantial IL-10 release from primary human MΦs, dependent on signaling through tyrosine kinase receptor A (TRKA or neurotrophic tyrosine kinase receptor type 1 (NTRK1)). Mechanistically, sphingosine-1-phosphate (S1P) release from apoptotic cancer cells triggered src-dependent shuttling of cytosolic TRKA to the plasma membrane via S1P receptor signaling. Plasma membrane-associated TRKA, which was activated by constitutively autocrine secreted nerve growth factor, used phosphatidylinositol 3-kinase (PI3K)/AKT and p38 mitogen-activated protein kinase (MAPK) signaling to induce IL-10. Interestingly, TRKA-dependent signaling was required for cytokine production by TAMs isolated from primary murine breast cancer tissue. Besides IL-10, this pathway initiated secretion of IL-6, tumor necrosis factor-α (TNF-α) and monocyte chemotactic protein-1 (MCP-1), indicating relevance in cancer-associated inflammation. Our findings highlight a fine-tuned regulatory system including S1P-dependent TRKA trafficking for executing TAM-like cell function in vitro as well as in vivo.

CMT2B-associated Rab7 mutants inhibit neurite outgrowth

Charco-Marie-Tooth type 2B (CMT2B) neuropathy is a rare autosomal-dominant axonal disorder characterized by distal weakness, muscle atrophy, and prominent sensory loss often complicated by foot ulcerations. CMT2B is associated with mutations of the Rab7 protein, a small GTPase controlling late endocytic traffic. Currently, it is still unknown how these mutations cause the neuropathy. Indeed, CMT2B selectively affects neuronal processes, despite the ubiquitous expression of Rab7. Therefore, this study focused on whether these disorder-associated mutations exert an effect on neurite outgrowth. We observed a marked inhibition of neurite outgrowth upon expression of all the CMT2B-associated mutants in the PC12 and Neuro2A cell lines. Thus, our data strongly support previous genetic data which proposed that these Rab7 mutations are indeed causally related to CMT2B. Inhibition of neurite outgrowth by these CMT2B-associated Rab7 mutants was confirmed biochemically by impaired up-regulation of growth-associated protein 43 (GAP43) in PC12 cells and of the nuclear neuronal differentiation marker NeuN in Neuro2A cells. Expression of a constitutively active Rab7 mutant had a similar effect to the expression of the CMT2B-associated Rab7 mutants. The active behavior of these CMT2B-associated mutants is in line with their previously demonstrated increased GTP loading, thus confirming that active Rab7 mutants are responsible for CMT2B. Our findings provide an explanation for the ability of CMT2B-associated Rab7 mutants to override the activity of wild-type Rab7 in heterozygous patients. Thus, our data suggest that lowering the activity of Rab7 in neurons could be a targeted therapy for CMT2B.

The use of immobilized neurotrophins to support neuron survival and guide nerve fiber growth in compartmentalized chambers

We answered two major questions: (1) does retrograde signaling involve retrograde transport of nerve growth factor (NGF); and (2) is a gradient of immobilized NGF sufficient to promote and guide local axonal growth? To answer these questions, we developed a technique that resulted in stably immobilized NGF and combined this with compartmented chambers. NGF was photochemically-immobilized on a chitosan surface either in the cell body (CB) compartment, distal axon (DA) compartment, or both. Neuron survival and axon outgrowth were found to be insignificantly different from positive controls where soluble NGF was present. When NGF was immobilized on chitosan surfaces in the DA compartment, and in the absence of soluble NGF, neuron survival was observed, likely due to the retrograde signal of the activated TrkA receptor and NGF-induced signals, but not the retrograde signal of NGF itself. Axons were guided towards the higher end of the step concentration gradient of NGF that was photoimmobilized on the chitosan surface in the DA compartment by laser confocal patterning, demonstrating axonal guidance. These studies provide better insight into NGF signaling mechanisms which are important to both understanding developmental disorders and degenerative diseases of the nervous system, as well as improving design strategies to promote nerve regeneration after injury.

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.

Signaling by neuronal tyrosine kinase receptors: relevance for development and regeneration

Receptor tyrosine kinase activation by binding of neurotrophic factors determines neuronal morphology and identity, migration of neurons to appropriate destinations, and integration into functional neural circuits as well as synapse formation with appropriate targets at the right time and at the right place. This review summarizes the most important aspects of intraneuronal signaling mechanisms and induced gene expression changes that underlie morphological and neurochemical consequences of receptor tyrosine kinase activation in central and peripheral neurons.

A retrograde neuronal survival response: target-derived neurotrophins regulate MEF2D and bcl-w

Survival and maturation of dorsal root ganglia sensory neurons during development depend on target-derived neurotrophins. These target-derived signals must be transmitted across long distances to alter gene expression. Here, we address the possibility that long-range retrograde signals initiated by target-derived neurotrophins activate a specialized transcriptional program. The transcription factor MEF2D is expressed in sensory neurons; we show that expression of this factor is induced in response to target-derived neurotrophins that stimulate the distal axons. We demonstrate that MEF2D regulates expression of an anti-apoptotic bcl-2 family member, bcl-w. Expression of mef2d and bcl-w is stimulated in response to activation of a Trk-dependent ERK5/MEF2 pathway, and our data indicate that this pathway promotes sensory neuron survival. We find that mef2d and bcl-w are members of a larger set of retrograde response genes, which are preferentially induced by neurotrophin stimulation of distal axons. Thus, activation of an ERK5/MEF2D transcriptional program establishes and maintains the cellular constituents of functional sensory circuits.

Emerging roles for Rab family GTPases in human cancer

Member of the Ras-associated binding (Rab) family of small GTPases function as molecular switches regulating vesicular transport in eukaryotes cells. Their pathophysiological roles in human malignancies are less well-known compared to members of Ras and Rho families. Several members of the Rab family have, however, been shown to be aberrantly expressed in various cancer tissues. Recent findings have also revealed , in particular, Rab25 as a determinant of tumor progression and aggressiveness of epithelial cancers. Rab25 associates with alpha5beta1 integrin, and enhances tumor cell invasion by directing the localization of integrin-containing vesicles to the leading edge of matrix invading pseudopodia. We summarized here recent integrin on Rab25 and other Rabs implicated to be involved in a variety of human cancers, and discussed plausible mechanisms of how dysregulation of Rab expression could be tumorigenic or tumor suppressive.

Trophic activity derived from bone marrow mononuclear cells increases peripheral nerve regeneration by acting on both neuronal and glial cell populations

A rat model of complete sciatic nerve transection was used to evaluate the effect of bone marrow mononuclear cells (BMMC) transplanted to the injury site immediately after lesion. Rats treated with BMMC had both sensory and motor axons reaching the distal stump earlier compared to untreated animals. In addition, BMMC transplantation reduced cell death in dorsal root ganglia (DRG) compared to control animals. Transplanted BMMC remained in the lesion site for several days but there is no evidence of BMMC differentiation into Schwann cells. However, an increase in the number of Schwann cells, satellite cells and astrocytes was observed in the treated group. Moreover, neutralizing antibodies for nerve growth factor (NGF) (but not for brain-derived neurotrophic factor and ciliary-derived neurotrophic factor) added to the BMMC-conditioned medium reduced neurite growth of sensory and sympathetic neurons in vitro, suggesting that BMMC release NGF, improve regeneration of the sciatic nerve in the adult rat and stimulate Schwann and satellite cell proliferation or a combination of both.

Characterization of the Rab7K157N mutant protein associated with Charcot-Marie-Tooth type 2B

Four missense mutations, that target highly conserved amino acid residues in the small GTPase Rab7, have been associated with the Charcot-Marie-Tooth (CMT) type 2B phenotype. CMT2B peripheral axonal neuropathies are characterized by severe sensory loss, often complicated by infections, arthropathy, and amputations. Here, we have investigated the biochemical and functional properties of the Rab7 K157N mutated protein. Interestingly, Rab7 K157N showed altered nucleotide exchange rate and GTP hydrolysis compared to the wild type protein. Consistently, the majority of the expressed protein in HeLa cells was bound to GTP. In addition, Rab7 K157N was able to restore EGF degradation, previously inhibited by Rab7 silencing. Altogether these data indicate that Rab7 K157N, similarly to the other three mutated proteins causative of CMT2B, is predominantly in the GTP-bound form and behaves as an active mutant. Therefore, activated forms of Rab7 protein cause the CMT2B disease.