Multiple trkA proteins in PC12 cells bind NGF with a slow association rate

Expression of Nerve Growth Factor and Its Receptor TrkA in the Reproductive System of Adult Zebrafish

Nerve growth factor (NGF), a member of the neurotrophin family, has emerged as an active mediator in different crucial events in the peripheral and central nervous system. At the same time, several studies showed that this neurotrophin can also play a role in non-neuronal tissues (e.g., among gonads). In spite of a large number of studies present in mammals, investigations devoted to NGF and its receptor TrkA in the reproductive system of other animal models, such as teleost fish, are scarce. To increase our knowledge of NGF and its receptor in a vertebrate gonads model, the present report describes the expression patterns of ngf and trka mRNA in the testis and ovary of adult zebrafish. By using chromogenic and fluorescence in situ hybridization, we demonstrate that in the testis of adult zebrafish, ngf and its receptor trka are mainly expressed in spermatogony B and spermatocytes. In the ovary of this fish, ngf and trka are expressed at different stages of oocyte development. Altogether, these results show that this neurotrophin and its receptor have an important role in the reproductive system that is conserved during vertebrate evolution.

Nerve growth factor (NGF) and NGF receptors in mesenchymal stem/stromal cells: Impact on potential therapies

Mesenchymal stem/stromal cells (MSCs) are promising for the treatment of degenerative diseases and traumatic injuries. However, MSC engraftment is not always successful and requires a strong comprehension of the cytokines and their receptors that mediate the biological behaviors of MSCs. The effects of nerve growth factor (NGF) and its two receptors, TrkA and p75NTR, on neural cells are well studied. Increasing evidence shows that NGF, TrkA, and p75NTR are also involved in various aspects of MSC function, including their survival, growth, differentiation, and angiogenesis. The regulatory effect of NGF on MSCs is thought to be achieved mainly through its binding to TrkA. p75NTR, another receptor of NGF, is regarded as a novel surface marker of MSCs. This review provides an overview of advances in understanding the roles of NGF and its receptors in MSCs as well as the effects of MSC‐derived NGF on other cell types, which will provide new insight for the optimization of MSC‐based therapy.

The evolution of nerve growth factor inhibition in clinical medicine

Nerve growth factor (NGF) is a neurotrophin that activates nociceptive neurons to transmit pain signals from the peripheral to the central nervous system and that exerts its effects on neurons by signalling through tyrosine kinase receptors. Antibodies that inhibit the function of NGF and small molecule inhibitors of NGF receptors have been developed and tested in clinical studies to evaluate the efficacy of NGF inhibition as a form of analgesia in chronic pain states including osteoarthritis and chronic low back pain. Clinical studies in individuals with painful knee and hip osteoarthritis have revealed that NGF inhibitors substantially reduce joint pain and improve function compared with NSAIDs for a duration of up to 8 weeks. However, the higher tested doses of NGF inhibitors also increased the risk of rapidly progressive osteoarthritis in a small percentage of those treated. This Review recaps the biology of NGF and the studies that have been performed to evaluate the efficacy of NGF inhibition for chronic musculoskeletal pain states. The adverse events associated with NGF inhibition and the current state of knowledge about the mechanisms involved in rapidly progressive osteoarthritis are also discussed and future studies proposed to improve understanding of this rare but serious adverse event.

Dental Pulp Stem Cells: Advances to Applications

Dental pulp stem cells (DPSCs) have a high capacity for differentiation and the ability to regenerate a dentin/pulp-like complex. Numerous studies have provided evidence of DPSCs’ differentiation capacity, such as in neurogenesis, adipogenesis, osteogenesis, chondrogenesis, angiogenesis, and dentinogenesis. The molecular mechanisms and functions of DPSCs’ differentiation process are affected by growth factors and scaffolds. For example, growth factors such as basic fibroblast growth factor (bFGF), transforming growth factor-β (TGF-β), nerve growth factor (NGF), platelet-derived growth factor (PDGF), and bone morphogenic proteins (BMPs) influence DPSC fate, including in differentiation, cell proliferation, and wound healing. In addition, several types of scaffolds, such as collagen, hydrogel, decellularized bioscaffold, and nanofibrous spongy microspheres, have been used to characterize DPSC cellular attachment, migration, proliferation, differentiation, and functions. An appropriate combination of growth factors and scaffolds can enhance the differentiation capacity of DPSCs, in terms of optimizing not only dental-related expression but also dental pulp morphology. For a cell-based clinical approach, focus has been placed on the tissue engineering triad [cells/bioactive molecules (growth factors)/scaffolds] to characterize DPSCs. It is clear that a deep understanding of the mechanisms of stem cells, including their aging, self-renewal, microenvironmental homeostasis, and differentiation correlated with cell activity, the energy for which is provided from mitochondria, should provide new approaches for DPSC research and therapeutics. Mitochondrial functions and dynamics are related to the direction of stem cell differentiation, including glycolysis, oxidative phosphorylation, mitochondrial metabolism, mitochondrial transcription factor A (TFAM), mitochondrial elongation, and mitochondrial fusion and fission proteins. This review summarizes the effects of major growth factors and scaffolds for regenerating dentin/pulp-like complexes, as well as elucidating mitochondrial properties of DPSCs for the development of advanced applications research.

Optogenetic Delineation of Receptor Tyrosine Kinase Subcircuits in PC12 Cell Differentiation

Nerve growth factor elicits signaling outcomes by interacting with both its high-affinity receptor, TrkA, and its low-affinity receptor, p75NTR. Although these two receptors can regulate distinct cellular outcomes, they both activate the extracellular-signal-regulated kinase pathway upon nerve growth factor stimulation. To delineate TrkA subcircuits in PC12 cell differentiation, we developed an optogenetic system whereby light was used to specifically activate TrkA signaling in the absence of nerve growth factor. By using tyrosine mutants of the optogenetic TrkA in combination with pathway-specific pharmacological inhibition, we find that Y490 and Y785 each contributes to PC12 cell differentiation through the extracellular-signal-regulated kinase pathway in an additive manner. Optogenetic activation of TrkA eliminates the confounding effect of p75NTR and other potential off-target effects of the ligand. This approach can be generalized for the mechanistic study of other receptor-mediated signaling pathways.

The hallucinogen 2,5-dimethoxy-4-iodoamphetamine hydrochloride activates neurotrophin receptors in a neuronal cell line and promotes neurites extension

Decreased neurotrophic factors expression and neurotrophin receptors signalling have repeatedly been reported in association with stress, depression, and neurodegenerative disorders. We have previously identified the hallucinogen 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) as protective against trophic deprivation-induced cytotoxicity in human neuroblastoma SK-N-SH cells and established the dependence of this effect on the 5-HT_2A receptor, tyrosine kinases activity, and the extracellular signal-regulated kinase pathway. In the current study, we investigated the effect of DOI on tropomyosin-related kinase receptor A (TrkA) phosphorylation. Treatment with DOI increased TrkA tyrosine phosphorylation in SK-N-SH cells, determined by immunoprecipitation with TrkA antibody and immunoblotting with anti-phosphotyrosine- and TrkA-antibodies. Analysis of DOI's effect on individual TrkA residues in SK-N-SH cells showed that it increases TrkA Tyr490 phosphorylation (177 ± 23% after 5 μM DOI for 30 min compared to vehicle). Furthermore, DOI treatment increased the percentage of SK-N-SH cells extending neurites in a TrkA-dependent manner (17.2 ± 2.2% after 5 μM DOI treatment for 6 days compared to 5.6 ± 1.7% after vehicle). In a different cell model-lymphoblastoid cell lines-DOI treatment increased tropomyosin-related kinase receptor B (TrkB) phosphorylation, determined by immunoprecipitation with TrkB antibody and immunoblotting with anti-phosphotyrosine antibody and total Trk antibody. Our results identify the Trk receptors as a downstream target of the hallucinogen DOI. In light of the known involvement of Trk receptors in mental diseases, their participation in DOI-mediated effects warrants further investigation.

Mechanisms controlling neurite outgrowth in a pheochromocytoma cell line: the role of TRPC channels

Transient Receptor Potential Canonical (TRPC) channels are implicated in modulating neurite outgrowth. The expression pattern of TRPCs changes significantly during brain development, suggesting that fine-tuning TRPC expression may be important for orchestrating neuritogenesis. To study how alterations in the TRPC expression pattern affect neurite outgrowth, we used nerve growth factor (NGF)-differentiated rat pheochromocytoma 12 (PC12) cells, a model system for neuritogenesis. In PC12 cells, NGF markedly up-regulated TRPC1 and TRPC6 expression, but down-regulated TRPC5 expression while promoting neurite outgrowth. Overexpression of TRPC1 augmented, whereas TRPC5 overexpression decelerated NGF-induced neurite outgrowth. Conversely, shRNA-mediated knockdown of TRPC1 decreased, whereas shRNA-mediated knockdown of TRPC5 increased NGF-induced neurite extension. Endogenous TRPC1 attenuated the anti-neuritogenic effect of overexpressed TRPC5 in part by forming the heteromeric TRPC1-TRPC5 channels. Previous reports suggested that TRPC6 may facilitate neurite outgrowth. However, we found that TRPC6 overexpression slowed down neuritogenesis, whereas dominant negative TRPC6 (DN-TRPC6) facilitated neurite outgrowth in NGF-differentiated PC12 cells. Consistent with these findings, hyperforin, a neurite outgrowth promoting factor, decreased TRPC6 expression in NGF-differentiated PC12 cells. Using pharmacological and molecular biological approaches, we determined that NGF up-regulated TRPC1 and TRPC6 expression via a p75(NTR)-IKK(2)-dependent pathway that did not involve TrkA receptor signaling in PC12 cells. Similarly, NGF up-regulated TRPC1 and TRPC6 via an IKK(2) dependent pathway in primary cultured hippocampal neurons. Thus, our data suggest that a balance of TRPC1, TRPC5, and TRPC6 expression determines neurite extension rate in neural cells, with TRPC6 emerging as an NGF-dependent "molecular damper" maintaining a submaximal velocity of neurite extension.

Comparison of P2X and TRPV1 receptors in ganglia or primary culture of trigeminal neurons and their modulation by NGF or serotonin

BACKGROUND

Cultured sensory neurons are a common experimental model to elucidate the molecular mechanisms of pain transduction typically involving activation of ATP-sensitive P2X or capsaicin-sensitive TRPV1 receptors. This applies also to trigeminal ganglion neurons that convey pain inputs from head tissues. Little is, however, known about the plasticity of these receptors on trigeminal neurons in culture, grown without adding the neurotrophin NGF which per se is a powerful algogen. The characteristics of such receptors after short-term culture were compared with those of ganglia. Furthermore, their modulation by chronically-applied serotonin or NGF was investigated.

RESULTS

Rat or mouse neurons in culture mainly belonged to small and medium diameter neurons as observed in sections of trigeminal ganglia. Real time RT-PCR, Western blot analysis and immunocytochemistry showed upregulation of P2X(3) and TRPV1 receptors after 1-4 days in culture (together with their more frequent co-localization), while P2X(2) ones were unchanged. TRPV1 immunoreactivity was, however, lower in mouse ganglia and cultures. Intracellular Ca(2+) imaging and whole-cell patch clamping showed functional P2X and TRPV1 receptors. Neurons exhibited a range of responses to the P2X agonist alpha, beta-methylene-adenosine-5'-triphosphate indicating the presence of homomeric P2X(3) receptors (selectively antagonized by A-317491) and heteromeric P2X(2/3) receptors. The latter were observed in 16 % mouse neurons only. Despite upregulation of receptors in culture, neurons retained the potential for further enhancement of P2X(3) receptors by 24 h NGF treatment. At this time point TRPV1 receptors had lost the facilitation observed after acute NGF application. Conversely, chronically-applied serotonin selectively upregulated TRPV1 receptors rather than P2X(3) receptors.

CONCLUSION

Comparing ganglia and cultures offered the advantage of understanding early adaptive changes of nociception-transducing receptors of trigeminal neurons. Culturing did not prevent differential receptor upregulation by algogenic substances like NGF or serotonin, indicating that chronic application led to distinct plastic changes in the molecular mechanisms mediating pain on trigeminal nociceptors.

Plasticity in rat uterine sympathetic nerves: the role of TrkA and p75 nerve growth factor receptors

Uterine sympathetic innervation undergoes profound remodelling in response to physiological and experimental changes in the circulating levels of sex hormones. It is not known, however, whether this plasticity results from changes in the innervating neurons, the neuritogenic properties of the target tissue or both. Using densitometric immunohistochemistry, we analysed the effects of prepubertal chronic oestrogen treatment (three subcutaneous injections of 20 microg of beta-oestradiol 17-cypionate on days 25, 27 and 29 after birth), natural peripubertal transition and late pregnancy (19-20 days post coitum) on the levels of TrkA and p75 nerve growth factor receptors in uterine-projecting sympathetic neurons of the thoraco-lumbar paravertebral sympathetic chain (T7-L2) identified using the retrograde tracer Fluorogold. For comparative purposes, levels of TrkA and p75 were assessed in the superior cervical ganglion (SCG) following prepubertal chronic oestrogen treatment. These studies showed that the vast majority of uterine-projecting neurons expressed both TrkA and p75. Both prepubertal chronic oestrogen treatment and the peripubertal transition increased the ratio p75 to TrkA in uterine-projecting neurons, whereas pregnancy elicited the opposite effect. Prepubertal chronic oestrogen treatment had no effects on levels of TrkA or p75 in sympathetic neurons of the SCG. Taken together, our data suggest that neurotrophin receptor-mediated events may contribute to regulate sex hormone-induced plasticity in uterine sympathetic nerves, and are in line with the idea that, in vivo, plasticity in uterine nerves involves changes in both the target and the innervating neurons.

p75 and TrkA receptors are both required for uptake of NGF in adult sympathetic neurons: use of a novel fluorescent NGF conjugate

We have developed and tested the biological activity and specificity of a novel fluorescent dextran-Texas Red-nerve growth factor (DTR-NGF) conjugate. DTR-NGF was found to promote survival and neurite outgrowth in cultured dissociated sympathetic neurons similarly to native NGF. The conjugate was taken up and transported retrogradely by terminal sympathetic nerves innervating the iris to neurons in the ipsilateral superior cervical ganglion (SCG) of young adult rats. Uptake and transport was assessed by counting numbers of labelled neurons and by measuring intensity of neuronal labelling using confocal microscopy and image analysis. DTR-NGF labelling in SCG neurons was shown to be dose-dependent with an EC(50) of 75 ng. Similar concentrations of unconjugated DTR resulted in no neuronal labelling. DTR-NGF uptake was competed off using a 50-fold excess of native NGF, resulting in a 73% reduction in numbers of labelled neurons. Pretreatment of nerve terminals with function-blocking antibodies against the low (p75) and high (TrkA) affinity NGF receptors resulted in a large (85-93%) reduction in numbers of DTR-NGF labelled neurons. Anti-p75 and anti-TrkA antibodies had comparable effects which were concentration-dependent. These findings indicate that both receptors are required for uptake of NGF in adult rat sympathetic neurons. In particular, the results provide strong evidence that the p75 receptor plays a more active role in transducing the NGF signal than has been proposed.

Retinoic acid combined with neurotrophin-3 enhances the survival and neurite outgrowth of embryonic sympathetic neurons

Both nerve growth factor (NGF) and neurotrophin-3 (NT-3) are necessary for the survival of embryonic sympathetic neurons in vivo. All-trans retinoic acid (atRA) has been shown to promote neurite outgrowth and long-term survival of chick embryonic sympathetic neurons cultured in the presence of NGF. The present study shows that atRA can also potentiate the survival and neurite outgrowth-promoting activities of NT-3. This was accomplished by enhancing the survival of existing neurons, as cell proliferation was unaffected by exposure to atRA. atRA also enhanced neurite outgrowth of the NT-3-treated cells; however, the neurites appeared thicker and less branched than cells treated with atRA in combination with NGF. Using a quantitative PCR assay, trkA and p75(NTR) mRNAs, but not trkC mRNA, were increased ( approximately 1.5- to 2-fold) after 72 and 48 hr of exposure of the cultures to atRA, respectively. The atRA-induced increase in trkA mRNA may play a role in the enhanced survival of neurons cultured in the presence of either NGF or NT-3, as both neurotrophins have been shown to signal through this receptor. The time course of these mRNA changes would indicate that atRA does not regulate the neurotrophin receptor mRNA directly, rather, intervening gene transcription is required. Thus, during development, atRA may play a role in fine-tuning embryonic responsiveness to both NT-3 and NGF.

Ganglioside GM1 potentiates NGF action on axotomised medial septal cholinergic neurons

Transection of the fimbria fornix leads to retrograde degeneration of axotomised septal cholinergic neurons as manifested by loss of choline acetyltransferase and p75NGFR immunoreactivity. Intracerebroventricularly administered nerve growth factor initiated at the time of axotomy can prevent these changes. We have shown that concurrent intraperitoneal administration of GM1 with a low and otherwise unprotective intracerebroventricular dose of nerve growth factor, can also prevent the loss of these fimbria fornix axotomised cholinergic neurons, where GM1 alone does not have this effect. This study further confirms the neuroprotective actions of GM1 and suggests that it may interact to potentiate the effect of nerve growth factor on these axotomised septal cholinergic neurons.

Blocking nerve growth factor binding to the p75 neurotrophin receptor on sympathetic neurons transiently reduces trkA activation but does not affect neuronal survival

Nerve growth factor interacts with the trkA tyrosine kinase receptor and with the p75 neurotrophin receptor. It is clear that trkA mediates most, if not all, of the stereotypical responses of sympathetic neurons to nerve growth factor but the role of the p75 neurotrophin receptor is unclear. In this study, we have asked whether a functional interaction between p75 neurotrophin receptor and trkA exists in primary sympathetic neurons by disrupting nerve growth factor binding to p75 neurotrophin receptor. Acute assays reveal that blocking antibodies directed against p75 neurotrophin receptor reduce nerve growth factor-mediated trkA tyrosine phosphorylation and reduce the amount of nerve growth factor which binds the trkA receptor. This reduction in trkA activity is relatively short-lived in vitro and blocking antibodies to p75 neurotrophin receptor do not inhibit long-term survival of nerve growth factor-dependent primary neurons. Together, these data indicate that p75 neurotrophin receptor and trkA interact within primary neurons to enhance nerve growth factor binding to the trkA receptor under conditions of acute but not chronic nerve growth factor exposure.

A potential interaction of p75 and trkA NGF receptors revealed by affinity crosslinking and immunoprecipitation

Nerve growth factor binds independently to two transmembrane receptors, the p75 neurotrophin receptor and the p140trk (trkA) tyrosine kinase receptor, which are both co-expressed in the majority of neuronal cells that respond to NGF. Previous findings have suggested that appropriate co-expression of the two receptors gives rise to high affinity NGF binding sites and increased neurotrophin responsiveness; however, evidence demonstrating a direct interaction between the two receptors in cell lines has been lacking. Here we have utilized affinity crosslinking agents with 125I-NGF to detect an association of trkA and p75 receptors in embryonic spinal cord and brain tissues enriched in the two receptors. Although multimeric complexes of trkA and p75 were not detected by affinity crosslinking, immunoprecipitation of cross-linked NGF-receptor complexes with trk-specific antibodies resulted in selective immunoprecipitation of crosslinked p75. Our results indicate that the trkA and p75 receptors can potentially interact, and that such an association may be responsible for the generation of high affinity NGF binding sites.

p75 nerve growth factor receptor modulates p140trkA kinase activity, but not ligand internalization, in PC12 cells

The biological activity of nerve growth factor (NGF) has been shown to be mediated by the p140trkA receptor tyrosine kinase, while the role of the p75 NGF receptor (p75NGFR) is still unresolved. Here we have investigated the relative contribution of p140trkA and p75NGFR to early consequences of NGF binding: ligand internalization, p140trkA autophosphorylation, and tyrosine phosphorylation of Shc, phospholipase C gamma-1 (PLC gamma-1), and extracellular signal-regulated kinases (ERKs). It was found that NGF internalization was neither prevented by blocking p140trkA activity using the protein kinase inhibitors methylthioadenosine, staurosporine, and K-252a, nor by inhibiting NGF binding to p75NGFR with antibodies. However, when NGF binding to p140trkA was reduced by the use of a synthetic peptide corresponding to amino acids 36-53 of human p140trkA, internalization of NGF was decreased. Thus, at least in PC12 cells, internalization appears to require binding of NGF to p140trkA, but occurs irrespective of p140trkA kinase activity and ligand occupancy of p75NGFR. The NGF triple mutant Lys-32/Lys-34/Glu-35 to Ala, which has been demonstrated to bind to p140trkA, but not to p75NGFR, induced tyrosine phosphorylation more rapidly than wild-type NGF. Likewise, NGF-induced tyrosine phosphorylation was accelerated when NGF binding to p75NGFR was prevented with REX-IgG. These findings indicate that NGF bindign by p75NGFR may modulate NGF-induced p140trkA kinase activity.

Disruption of NGF binding to the low affinity neurotrophin receptor p75LNTR reduces NGF binding to TrkA on PC12 cells

The role of the low affinity neurotrophin receptor, p75LNTR, in NGF-mediated signal transduction has been examined. Our results show that treatment of PC12 cells with MC192, a monoclonal antibody directed against p75LNTR, results in reduced NGF binding to TrkA and attenuated TrkA activation. Use of mutant NGF that binds TrkA but not p75LNTR shows that the MC192 effect requires that NGF bind the p75LNTR receptor. To explore the possibility that MC192 disrupts some normal functional role of p75LNTR, BDNF was used to block binding of NGF to p75LNTR on PC12 cells. By preventing NGF binding to p75LNTR, NGF binding to TrkA and NGF-mediated signal transduction were reduced. We propose that p75LNTR normally acts to increase binding of NGF to TrkA, possibly by increasing the local NGF concentration in the microenvironment surrounding the cell surface TrkA receptor.