Regulation of nerve growth factor and its low-affinity receptor (p75NTR) during myogenic differentiation

Biological Aspects of Selected Myokines in Skeletal Muscle: Focus on Aging

In the last decade, clear evidence has emerged that the cellular components of skeletal muscle are important sites for the release of proteins and peptides called "myokines", suggesting that skeletal muscle plays the role of a secretory organ. After their secretion by muscles, these factors serve many biological functions, including the exertion of complex autocrine, paracrine and/or endocrine effects. In sum, myokines affect complex multi-organ processes, such as skeletal muscle trophism, metabolism, angiogenesis and immunological response to different physiological (physical activity, aging, etc.) or pathological states (cachexia, dysmetabolic conditions, chronic inflammation, etc.). The aim of this review is to describe in detail a number of myokines that are, to varying degrees, involved in skeletal muscle aging processes and belong to the group of proteins present in the functional environment surrounding the muscle cell known as the "Niche". The particular myokines described are those that, acting both from within the cell and in an autocrine manner, have a defined relationship with the modulation of oxidative stress in muscle cells (mature or stem) involved in the regulatory (metabolic or regenerative) processes of muscle aging. Myostatin, IGF-1, NGF, S100 and irisin are examples of specific myokines that have peculiar features in their mechanisms of action. In particular, the potential role of one of the most recently characterized myokines-irisin, directly linked to an active lifestyle-in reducing if not reversing senescence-induced oxidative damage is discussed in terms of its possible application as an agent able to counteract the deleterious effects of muscle aging.

p75NTR-/- mice exhibit an alveolar bone loss phenotype and inhibited PI3K/Akt/β-catenin pathway

OBJECTIVES

The aim of this study was to investigate the role of p75 neurotrophin receptor (p75NTR) in regulating the mouse alveolar bone development and the mineralization potential of murine ectomesenchymal stem cells (EMSCs). Moreover, we tried to explore the underlying mechanisms associated with the PI3K/Akt/β-catenin pathway.

MATERIALS AND METHODS

p75NTR knockout (p75NTR-/- ) mice and wild-type (WT) littermates were used. E12.5d p75NTR-/- and WT EMSCs were isolated in the same pregnant p75NTR-/+ mice from embryonic maxillofacial processes separately. Mouse alveolar bone mass was evaluated using micro-CT. Differential osteogenic differentiation pathways between p75NTR-/- and WT EMSCs were analysed by RNA-sequencing. The PI3K inhibitor LY294002 and PI3K agonist 740Y-P were used to regulate the PI3K/Akt pathway in EMSCs. p75NTR overexpression lentiviruses, p75NTR knock-down lentiviruses and recombined mouse NGF were used to transfect cells.

RESULTS

The alveolar bone mass was found reduced in the p75NTR knockout mouse comparing to the WT mouse. During mineralization induction, p75NTR-/- EMSCs displayed decreased osteogenic capacity and downregulated PI3K/Akt/β-catenin signalling. The PI3K/Akt/β-catenin pathway positively regulates the potential of differential mineralization in EMSCs. The promotive effect of p75NTR overexpression can be attenuated by LY294002, while the inhibitory effect of p75NTR knock-down on Runx2 and Col1 expression can be reversed by 740Y-P.

CONCLUSION

Deletion of p75NTR reduced alveolar bone mass in mice. P75NTR positively regulated the osteogenic differentiation of EMSCs via enhancing the PI3K/Akt/β-catenin pathway.

AAV1.NT-3 gene therapy increases muscle fiber diameter through activation of mTOR pathway and metabolic remodeling in a CMT mouse model

Neurotrophin 3 (NT-3) has well-recognized effects on peripheral nerve and Schwann cells, promoting axonal regeneration and associated myelination. In this study, we assessed the effects of AAV.NT-3 gene therapy on the oxidative state of the neurogenic muscle from the TremblerJ (Tr ^J ) mice at 16 weeks post-gene injection and found that the muscle fiber size increase was associated with a change in the oxidative state of muscle fibers towards normalization of the fiber type ratio seen in the wild type. NT-3-induced fiber size increase was most prominent for the fast twitch glycolytic fiber population. These changes in the Tr ^J muscle were accompanied by increased phosphorylation levels of 4E-BP1 and S6 proteins as evidence of mTORC1 activation. In parallel, the expression levels of the mitochondrial biogenesis regulator PGC1α, and the markers of glycolysis (HK1 and PK1) increased in the Tr^J muscle. In vitro studies showed that recombinant NT-3 can directly induce Akt/mTOR pathway activation in the TrkC expressing myotubes but not in myoblasts. In addition, myogenin expression levels were increased in myotubes while p75 ^NTR expression was downregulated compared to myoblasts, indicating that NT-3 induced myoblast differentiation is associated with mTORC1 activation. These studies for the first time have shown that NT-3 increases muscle fiber diameter in the neurogenic muscle through direct activation of mTOR pathway and that the fiber size increase is more prominent for fast twitch glycolytic fibers.

Nerve Growth Factor and Estrogen Receptor mRNA Expression in Paravertebral Muscles of Patients With Adolescent Idiopathic Scoliosis: A Preliminary Study

STUDY DESIGN

Comparison of nerve growth factor (NGF) and estrogen receptor (ER)α messenger ribonucleic acid (mRNA) expression in bilateral paravertebral muscles in adolescent idiopathic scoliosis (AIS). This expression in AIS was compared with that of normal control subjects.

OBJECTIVES

To investigate NGF and ERα mRNA expression in bilateral paravertebral muscles in AIS and control subjects to clarify its association with the development and progression of spinal curvature.

SUMMARY OF BACKGROUND DATA

Paravertebral muscle abnormalities in AIS patients have been investigated through various methods. Despite the roles of NGF and ER in human skeletal muscles, the association with idiopathic scoliosis is still unclear.

METHODS

A total of 14 AIS patients (average age, 15.9 ± 2.2 years; average Cobb angle, 48.2° ± 8.9°) and 8 controls (average age, 27.3 ± 9.3 years) were included. Muscle samples were harvested from bilateral paravertebral muscles at the apical vertebral level. Nerve growth factor and ERα mRNA expression was evaluated by the real-time polymerase chain reaction. The researchers compared expression levels in bilateral paravertebral muscles in each group. The expression ratio, the expression at the convex side relative to the concave side, was compared between groups and the correlation between Cobb angle and expression ratio was analyzed.

RESULTS

Nerve growth factor and ERα mRNA expression on the convex side was higher than on the concave side in the AIS group (p = .024 and .007, respectively) and the expression ratio of NGF and ERα in the AIS group was higher than that of control subjects (p = .004 and .017, respectively). The expression ratio of NGF and the Cobb angle were significantly correlated (r = -0.5728; p = .0323).

CONCLUSIONS

In the AIS group, both NGF and ERα mRNA expression was asymmetric. The AIS group had higher expression ratios than control group and the NGF expression ratio was positively correlated to the Cobb angle.

Nerve growth factor stimulation of ERK1/2 phosphorylation requires both p75NTR and α9β1 integrin and confers myoprotection towards ischemia in C2C12 skeletal muscle cell model

The functions of nerve growth factor (NGF) in skeletal muscles physiology and pathology are not clear and call for an updated investigation. To achieve this goal we sought to investigate NGF-induced ERK1/2 phosphorylation and its role in the C2C12 skeletal muscle myoblasts and myotubes. RT-PCR and western blotting experiments demonstrated expression of p75(NTR), α9β1 integrin, and its regulator ADAM12, but not trkA in the cells, as also found in gastrocnemius and quadriceps mice muscles. Both proNGF and βNGF induced ERK1/2 phosphorylation, a process blocked by (a) the specific MEK inhibitor, PD98059; (b) VLO5, a MLD-disintegrin with relative selectivity towards α9β1 integrin; and (c) p75(NTR) antagonists Thx-B and LM-24, but not the inactive control molecule backbone Thx. Upon treatment for 4 days with either anti-NGF antibody or VLO5 or Thx-B, the proliferation of myoblasts was decreased by 60-70%, 85-90% and 60-80% respectively, indicative of trophic effect of NGF which was autocrinically released by the cells. Exposure of myotubes to ischemic insult in the presence of βNGF, added either 1h before oxygen-glucose-deprivation or concomitant with reoxygenation insults, resulted with about 20% and 33% myoprotection, an effect antagonized by VLO5 and Thx-B, further supporting the trophic role of NGF in C2C12 cells. Cumulatively, the present findings propose that proNGF and βNGF-induced ERK1/2 phosphorylation in C2C12 cells by functional cooperation between p75(NTR) and α9β1 integrin, which are involved in myoprotective effects of autocrine released NGF. Furthermore, the present study establishes an important trophic role of α9β1 in NGF-induced signaling in skeletal muscle model, resembling the role of trkA in neurons. Future molecular characterization of the interactions between NGF receptors in the skeletal muscle will contribute to the understanding of NGF mechanism of action and may provide novel therapeutic targets.

Monitoring signaling by the p75(NTR) receptor utilizing a caspase-3 activation assay amenable to small-molecule screening

p75(NTR) is a neurotrophin receptor that can mediate either survival or death of neurons depending on the cell context. Modulation of p75(NTR) is a promising strategy to promote neuronal survival for treatment of cognitive disorders such as Alzheimer's disease. Despite years of investigation into the signaling mechanisms of p75(NTR), no p75(NTR) signaling assay has yet been developed that is compatible with efficient screening of small-molecule modulators. In this work, we developed a homogeneous cell-based assay for screening p75(NTR) modulators and studying p75(NTR) function. Stimulation of p75(NTR)-transfected cells using either nerve growth factor (NGF) or Pro-NGF resulted in an enhanced caspase-3 activity as assessed by cleavage of a fluorescent caspase-3 substrate. Optimization of the assay with respect to time, cell density, NGF and Pro-NGF concentration, and other factors provided a twofold increase in the caspase-3 activity compared to background. Withdrawal of serum during the NGF or Pro-NGF treatment period was found to be essential for p75(NTR)-dependent caspase-3 activation. We validated the method by demonstrating that a signaling-incompetent p75(NTR) mutant could not substitute for wild-type p75(NTR) in mediating caspase-3 activation. A focused library screen identified new inhibitors of p75(NTR) signaling. This method will be useful for identifying small-molecule modulators of p75(NTR) as well as further characterizing downstream signaling events.

The low-affinity receptor for neurotrophins p75NTR plays a key role for satellite cell function in muscle repair acting via RhoA

Regeneration of muscle fibers, lost during pathological muscle degeneration or after injuries, is mediated by the production of new myofibres. This process, sustained by the resident stem cells of the muscle, the satellite cells, is finely regulated by local cues, in particular by cytokines and growth factors. Evidence in the literature suggests that nerve growth factor (NGF) is involved in muscle fiber regeneration; however, its role and mechanism of action were unclear. We have investigated this issue in in vivo mouse models of muscle regeneration and in primary myogenic cells. Our results demonstrate that NGF acts through its low-affinity receptor p75(NTR) in a developmentally regulated signaling pathway necessary to myogenic differentiation and muscle repair in vivo. We also demonstrate that this action of NGF is mediated by the down-regulation of RhoA-GTP signaling in myogenic cells.

FGF2 stimulation of the pyrophosphate-generating enzyme, PC-1, in pre-osteoblast cells is mediated by RUNX2

Pyrophosphate is an established inhibitor of hydroxyapatite deposition and crystal growth, yet when hydrolyzed into phosphate, it becomes a substrate for hydroxyapatite deposition. Pyrophosphate-generating enzyme (PC-1), Ank, and tissue nonspecific alkaline phosphatase (Tnap) are three factors that regulate extracellular pyrophosphate levels through its generation, transport, and hydrolysis. We previously showed that fibroblast growth factor 2 (FGF2) induces PC-1 and Ank while inhibiting Tnap expression and mineralization in MC3T3E1(C4) calvarial pre-osteoblast cells. In this study, we showed similar FGF2 regulation of these genes in primary pre-osteoblast cultures. In contrast to Ank and Tnap that are regulated by FGF2 in multiple cell types, we found regulation of PC-1 to be selective to pre-osteoblastic cells and to require the osteoblast-related transcription factor, Runx2. Specifically, FGF2 was unable to induce PC-1 expression in Runx2-negative nonbone cells or in calvarial cells from Runx2-deficient mice. Transfection of these cells with a Runx2 expression vector restored FGF2 responsiveness. FGF2 was also shown to stimulate recruitment of Runx2 to the endogenous PC-1 promoter in MC3T3E1(C4) cells, as measured by chromatin immunoprecipitation. Taken together, our results establish that FGF2 is a specific inducer of PC-1 in pre-osteoblast cells and that FGF2 induces PC-1 expression through a mechanism involving Runx2.

Functional role of sortilin in myogenesis and development of insulin-responsive glucose transport system in C2C12 myocytes

Sortilin has been implicated in the formation of insulin-responsive GLUT4 storage vesicles in adipocytes by regulating sorting events between the trans-Golgi-network and endosomes. We herein show that sortilin serves as a potent myogenic differentiation stimulator for C2C12 myocytes by cooperatively functioning with p75NTR, which subsequently further contributes to development of the insulin-responsive glucose transport system in C2C12 myotubes. Sortilin expression was up-regulated upon C2C12 differentiation, and overexpression of sortilin in C2C12 cells significantly stimulated myogenic differentiation, a response that was completely abolished by either anti-p75NTR- or anti-nerve growth factor (NGF)-neutralizing antibodies. Importantly, small interference RNA-mediated suppression of endogenous sortilin significantly inhibited C2C12 differentiation, indicating the physiological significance of sortilin expression in the process of myogenesis. Although sortilin overexpression in C2C12 myotubes improved insulin-induced 2-deoxyglucose uptake, as previously reported, this effect apparently resulted from a decrease in the cellular content of GLUT1 and an increase in GLUT4 via differentiation-dependent alterations at both the gene transcriptional and the post-translational level. In addition, cellular contents of Ubc9 and SUMO-modified proteins appeared to be increased by sortilin overexpression. Taken together, these data demonstrate that sortilin is involved not only in development of the insulin-responsive glucose transport system in myocytes, but is also directly involved in muscle differentiation via modulation of proNGF-p75NTR.

The nerve growth factor and its receptors in airway inflammatory diseases

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

Bioengineered nerve regeneration and muscle reinnervation

The peripheral nervous system has the intrinsic capacity to regenerate but the reinnervation of muscles is often suboptimal and results in limited recovery of function. Injuries to nerves that innervate complex organs such as the larynx are particularly difficult to treat. The many functions of the larynx have evolved through the intricate neural regulation of highly specialized laryngeal muscles. In this review, we examine the responses of nerves and muscles to injury, focusing on changes in the expression of neurotrophic factors, and highlight differences between the skeletal limb and laryngeal muscle systems. We also describe how artificial nerve conduits have become a useful tool for delivery of neurotrophic factors as therapeutic agents to promote peripheral nerve repair and might eventually be useful in the treatment of laryngeal nerve injury.

A novel function of differentiation revealed by cDNA microarray profiling of p75NTR-regulated gene expression

The expression of the p75 neurotrophin receptor (p75NTR) is diminished in epithelial cells during progression of prostate cancer in vivo and in vitro. Previous studies have demonstrated a role for p75NTR as a tumor suppressor in prostate growth. To better understand the molecular mechanism of p75(NTR) on tumor suppression, we utilized a complementary deoxyribonucleic acid microarray composed of approximately 6,000 human cancer-related genes to determine the gene expression pattern altered by re-introduction of p75NTR into PC-3 prostate tumor cells. Comparison of the transcripts in the neo and p75NTR-transfected cells revealed 52 differentially expressed genes, of which 21 were up-regulated and 31 were down-regulated in the presence of p75NTR. Based on the known biological functions of the p75NTR-regulated genes, we observed that p75NTR modulated the expression of genes that are critically involved in the regulation of differentiation as well as cell adhesion, signal transduction, apoptosis, tumor cell invasion, and metastasis. Several differentially expressed genes identified by microarray were selected for confirmation using quantitative real-time polymerase chain reaction. Immunoblot analysis further confirmed increased cellular retinoic acid-binding protein I (CRABPI) and IGFBP5 protein levels and decreased level of PLAUR protein with increasing p75NTR protein expression. As CRABPI was elevated far more than any other genes, we observed that the retinoids, all-trans retinoic acid and 9-cis retinoic acid, that bind CRABPI, promoted nitroblue tetrazolium-associated functional cell differentiation in p75NTR PC-3 cells, but not in neo control PC-3 cells. Subsequent examination of the retinoic acid receptors (RARs) expression levels demonstrated an absence of RAR-beta in the neo control cells and re-expression in the p75NTR expressing cells, consistent with previous findings where RAR-beta is believed to play a critical role as a tumor suppressor gene that is lost during de-differentiation of prostate epithelial cells. Whereas the RAR-alpha and -gamma protein levels remained unchanged, retinoid X receptor (RXR)-alpha and -beta also exhibited increasing protein levels with re-expression of the p75NTR protein. Moreover, the ability of p75NTR siRNA to knockdown levels of RAR-beta, RXR-alpha, and RXR-beta supports the specificity of the functional involvement of p75NTR in differentiation. Hence, re-expression of the p75NTR appears to partially reverse de-differentiation of prostate cancer cells by up-regulating the expression of CRABPI for localized sequestration of retinoids that are available to newly up-regulated RAR-beta, RXR-alpha, and RXR-beta.

Increased Transduction of Skeletal Muscle Cells by Fibroblast Growth Factor-Modified Adenoviral Vectors

Gene therapy for Duchenne muscular dystrophy will likely require that the corrective dystrophin gene be delivered to a high fraction of muscle fibers in vivo. Because of the large size of the dystrophin cDNA, adenoviral (Ad) vectors have been developed for this application. However, Ad vectors transduce mature muscle inefficiently in part due to downregulation of Ad receptors on these cells. To circumvent this problem, we have tested fibroblast growth factor-2 (FGF) and insulin-like growth factor (IGF) as ligands for their ability to enhance Ad transduction of muscle cells. In this work, we demonstrate that covalent conjugation of FGF, but not IGF, to Ad5 vectors mediates substantial increases in transduction of skeletal muscle cells in vitro and dystrophic in vivo. Ad5 vectors expressing reporter genes were cross-linked to the ligands, using bifunctional polyethylene glycol (PEG) molecules. Ad-PEG-FGF mediated 1000- and 200-fold increases in transduction on C2C12 myoblasts and myotubes in vitro when compared with Ad5, Ad-PEG, or Ad-PEG-IGF. When tested in vivo in mdx mice, Ad-PEG-FGF mediated 6-fold higher transduction in skeletal muscle than unmodified Ad5. Similar results were seen when using lacZ as a reporter gene to observe transduction qualitatively. These data suggest that FGF may be a useful cell-binding ligand to enhance gene delivery by Ad and other vectors into skeletal muscle for the gene therapy of Duchenne muscular dystrophy and other muscle-related diseases.

The role of neurotrophins in muscle under physiological and pathological conditions

This review summarizes the various effects of neurotrophins in skeletal muscle and how these proteins act as potential regulators of development, maintenance, function, and regeneration of skeletal muscle fibers. Increasing evidence suggests that this family of neurotrophic factors not only modulates survival and function of innervating motoneurons and proprioceptive neurons but also development and differentiation of myoblasts and muscle fibers. Neurotrophins and neurotrophin receptors play a role in the coordination of muscle innervation and functional differentiation of neuromuscular junctions. However, neurotrophin receptors are also expressed in differentiating muscle cells, in particular at early developmental stages in myoblasts before they fuse. In adults with pathological conditions such as human degenerative and inflammatory muscle disorders, variations of neurotrophin expression are found, but the role of neurotrophins under such conditions is still not clear. The goal of this review is to provide a basis for a better understanding and future studies on the role of these factors under such pathological conditions and for treatment of human muscle diseases.

p75NTR-mediated signaling promotes the survival of myoblasts and influences muscle strength

During muscle development, the p75(NTR) is expressed transiently on myoblasts. The temporal expression pattern of the receptor raises the possibility that the receptor is influencing muscle development. To test this hypothesis, p75(NTR)-deficient mutant mice were tested for muscle strength by using a standard wire gripe strength test and were found to have significantly decreased strength relative to that of normal mice. When normal mybolasts were examined in vivo for expression of NGF receptors, p75(NTR) was detected on myoblasts but the high affinity NGF receptor, trk A, was not co-expressed with p75(NTR). In vitro, proliferating C2C12 and primary myoblasts co-expressed the p75(NTR) and MyoD, but immunofluorescent analysis of primary myoblasts and RT-PCR analysis of C2C12 mRNA revealed that myoblasts were devoid of trk A. In contrast to the cell death functions that characterize the p75(NTR) in neurons, p75(NTR)-positive primary and C2C12 myoblasts did not differentiate or undergo apoptosis in response to neurotrophins. Rather, myoblasts survived and even proliferated when grown at subconfluent densities in the presence of the neurotrophins. Furthermore, when myoblasts treated with NGF were lysed and immunoprecipitated with antibodies against phosphorylated I-kappaB and AKT, the cells contained increased levels of both phospho-proteins, both of which promote cell survival. By contrast, neurotrophin-treated myoblasts did not induce phosphorylation of Map Kinase p42/44 or p38, indicating the survival was not mediated by the trk A receptor. Taken together, the data indicate that the p75(NTR) mediates survival of myoblasts prior to differentiation and that the activity of this receptor during myogenesis is important for developing muscle.

Amphoterin stimulates myogenesis and counteracts the antimyogenic factors basic fibroblast growth factor and S100B via RAGE binding

The receptor for advanced glycation end products (RAGE), a multiligand receptor of the immunoglobulin superfamily, has been implicated in the inflammatory response, diabetic angiopathy and neuropathy, neurodegeneration, cell migration, tumor growth, neuroprotection, and neuronal differentiation. We show here that (i) RAGE is expressed in skeletal muscle tissue and its expression is developmentally regulated and (ii) RAGE engagement by amphoterin (HMGB1), a RAGE ligand, in rat L6 myoblasts results in stimulation of myogenic differentiation via activation of p38 mitogen-activated protein kinase (MAPK), up-regulation of myogenin and myosin heavy chain expression, and induction of muscle creatine kinase. No such effects were detected in myoblasts transfected with a RAGE mutant lacking the transducing domain or myoblasts transfected with a constitutively inactive form of the p38 MAPK upstream kinase, MAPK kinase 6, Cdc42, or Rac-1. Moreover, amphoterin counteracted the antimyogenic activity of the Ca(2+)-modulated protein S100B, which was reported to inhibit myogenic differentiation via inactivation of p38 MAPK, and basic fibroblast growth factor (bFGF), a known inhibitor of myogenic differentiation, in a manner that was inversely related to the S100B or bFGF concentration and directly related to the extent of RAGE expression. These data suggest that RAGE and amphoterin might play an important role in myogenesis, accelerating myogenic differentiation via Cdc42-Rac-1-MAPK kinase 6-p38 MAPK.

Epithelial growth control by neurotrophins: leads and lessons from the hair follicle

Neurotrophins (NTs) exert many growth-regulatory functions beyond the nervous system. For example, murine hair follicles (HF) show developmentally and spatio-temporally stringently controlled expression of NTs, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and NT-4, and their cognate receptors, tyrosine kinase A-C (TrkA-C) and p75 neurotrophin receptor (p75NTR). Follicular NT and NT receptor expression exhibit significant, hair cycle-dependent fluctuations on the gene and protein level, which are mirrored by changes in nerve fiber density and neurotransmitter/neuropeptide content in the perifollicular neural networks. NT-3/TrkC and NGF/TrkA signaling stimulate HF development, while NT-3, NT-4 and BDNF inhibit the growth (anagen) of mature HF by the induction of apoptosis-driven HF regression (catagen). p75NTR stimulation inhibits HF development and stimulates catagen. Since the HF is thus both a prominent target and key peripheral source of NT, dissecting the role of NTs in the control of HF morphogenesis and cyclic remodeling provides a uniquely accessible, and easily manipulated, clinically relevant experimental model, which has many lessons to teach. Given that our most recent data also implicate NTs in human hair growth control, selective NT receptor agonists and antagonists may become innovative therapeutic tools for the management of hair growth disorders (alopecia, effluvium, hirsutism). Since, however, the same NT receptor agonists that inhibit hair growth (e.g., BDNF, NT-4) can actually stimulate epidermal keratinocyte proliferation, NT may exert differential effects on defined keratinocyte subpopulations. The studies reviewed here provide new clues to understanding the complex roles of NT in epithelial tissue biology and remodeling in vivo, and invite new applications for synthetic NT receptor ligands for the treatment of epithelial growth disorders, exploiting the HF as a lead model.

An anti-apoptotic role for NGF receptors in human rhabdomyosarcoma

The expression and biological function of Nerve Growth Factor (NGF) receptors was studied in a panel of rhabdomyosarcoma cell lines derived from embryonal and alveolar histotype. All the cell lines expressed both the high affinity receptor TrkA and the low affinity receptor p75(NTR). Treatment with exogenous NGF did not considerably alter rhabdomyosarcoma cell growth or differentiation, but significantly inhibited spontaneous apoptosis as well as apoptosis, and induced by serum starvation or apoptosis induced by treatment with cycloheximide (CHX). Rhabdomyosarcoma cell lines expressed NGF and other neurotrophins and trace amounts of NGF protein were found in the supernatants of rhabdomyosarcoma cell cultures. Blocking the putative autocrine loop with an anti-NGF antibody resulted in an increase in apoptosis compared with control cultures. These data suggest that the simultaneous presence of both high and low affinity NGF receptors engaged by endogenous or exogenous NGF might contribute to the escape from apoptosis exhibited by the rhabdomyosarcoma cells.

Nerve growth factor (NGF) influences differentiation and proliferation of myogenic cells in vitro via TrKA

Classic studies have established that muscle cells exert trophic actions on neurons of the developing peripheral nervous system through the production of neurotrophins. For this reason neurotrophins are also known as 'target-derived factors'. During differentiation, muscle cells also express some neurotrophin receptors, such as the low-affinity p75 neurotrophin receptor, which binds all neurotrophins, and the high affinity tyrosine kinase receptor TrKA, nerve growth factor (NGF) transducing receptor. The functional roles of these receptors in muscle cells are still unclear and only fragmentary and controversial data are available regarding the responsiveness of muscle cells to NGF. The aim of the present study is to investigate the effects of NGF on cells of myogenic lineage. The rat myogenic cell line L6, primary cultures of adult human myoblasts, and the human rhabdomyosarcoma cell line TE-671 were used in this study. As expected, all the three cell types expressed NGF, p75 and TrKA. NGF was expressed by L6 and primary myoblasts following differentiation, but it was constitutively expressed at high levels in the TE-671 rhabdomyosarcoma cells. In L6 myoblasts, p75 receptor was expressed in myoblasts but not in myotubes early after plating; while some primary human myoblasts expressed it at all the time-points tested. Some fusiform cells of the TE-671 rhabdomyosarcoma cell line also expressed p75. TrKA was constitutively immunodetected in all the three cell lines, suggesting that these cells may respond to NGF. Addition of exogenous NGF increased the fusion rate of both primary and L6 myoblasts, as well as the proliferation of the slowly dividing primary myoblasts. Consistently, blocking the action of endogenously produced NGF with a specific neutralizing antibody decreased the percentage of fusion in both primary and L6 myoblasts. On the contrary, blocking the binding of NGF to p75 did not affect the percentage of fusion. Furthermore, neither exogenous NGF nor NGF- or p75-neutralizing antibodies appeared to affect the rhabdomyosarcoma cells, which have a high proliferation rate and do not fuse. Pharmacological inhibition of TrKA signal transduction with K252a (in the nM range) and tyrphostin AG879 (in the low microM range) resulted in a dramatic dose-dependent decrease in proliferation of all of the myogenic cell lines tested. Interestingly, this was especially evident in the rapidly dividing rhabdomyosarcoma cell line. The TrKA inhibitors also blocked fusion of L6 and primary myoblasts and induced morphological changes characterized by the flattening of the cells and a 'spider-like' rearrangement of the intermediate filaments in all three cell lines with some minor differences. A transfection study showed that p75-overexpressing L6 cells do not fuse and present changes in their morphology similar to the TrKA-inhibitors treated L6 cells. These data support the notion that NGF expression in skeletal muscle is not only associated with a classical target-derived neurotrophic function for peripheral nervous system neurons, but also with an autocrine action which affects the proliferation, fusion into myotubes, and cell morphology of developing myoblasts. The present data also suggest that these effects of NGF are mediated by TrKA receptors and that a sustained presence of NGF is needed for increase fusion into myotubes. Lastly, the dramatic anti-proliferative effect of TrKA inhibitors on myogenic cells, and especially on the TE-671 rhabdomyosarcoma cell line, suggests that pharmacological interference with NGF signal transduction could be effective in the control of these malignancies.

Muscular dystrophy in adult and aged anti-NGF transgenic mice resembles an inclusion body myopathy

The role of nerve growth factor (NGF) and its receptors in the physiology of skeletal muscles has not been extensively studied in animal models. We describe the production of transgenic lines of mice expressing a neutralizing antibody against NGF (alphaD11) and the morphological and histochemical analysis of skeletal muscles from adult and aged anti-NGF mice. This study reveals that the chronic deprivation of NGF results in a decreased size of myofibers of dorsal and hindlimb muscles in adult but not in postnatal day (P)2 mice. In myofibers from adult anti-NGF mice, the presence of central nuclei, vacuolization of the cytoplasm, and inflammatory cell infiltration was observed. The immunohistochemical analysis of these muscular fibers revealed an upregulation of p75 expression, a decrease in adenosine triphosphatase (ATP)ase activity, and a subsarcolemmal Congo Red-positive staining. Immunostaining with an antibody against amyloid precursor protein showed an increased labeling of the cytoplasm of myofibers from adult and aged anti-NGF mice. These features are reminiscent of human myopathies, such as inclusion body myositis. We conclude that NGF deficits might be relevant for a class of human myopathies.

A role for p75 neurotrophin receptor in the control of hair follicle morphogenesis

During hair follicle (HF) morphogenesis, p75 neurotrophin receptor (p75NTR) reportedly is the first growth factor receptor found to be expressed by those fibroblasts that later develop into the dermal papilla (DP) of the HF. However, the functional role of p75NTR in HF morphogenesis is still unknown. Studying HF development in fetal and neonatal C57BL/6 murine back skin, we show that p75NTR-immunoreactivity (IR) is prominently expressed by DP fibroblasts as well as by skin nerves during the early steps of HF development. In contrast, p75NTR-IR disappears from the DP in the fully developed HF and it is expressed only in the epithelial outer root sheath of the HF. Compared to age-matched wild-type animals, p75NTR knockout (-/-) mice show significant acceleration of HF morphogenesis, and DP fibroblasts of p75NTR knockout mice show reduced proliferative activity in situ, indicating alterations in their transition from proliferation to differentiation. Although no significant differences in the expression of adhesion molecules (NCAM), selected morphogens (TGFbeta-2, HGF/SF, FGF-2, KGF), or their receptors (TGFbetaR-II, m-met, FGFR-1) were seen between DP of p75NTR knockout and wild-type mice, p75NTR mutants showed a prominent upregulation of FGFR-2, a high-affinity receptor for KGF, in both follicular DP and epithelium. Furthermore, the administration of anti-KGF neutralizing antibody significantly inhibited acceleration of HF morphogenesis in p75NTR knockout mice in vivo. These observations suggest that p75NTR plays an important role during HF morphogenesis, functioning as a receptor that negatively controls HF development, most likely via alterations in DP fibroblast proliferation/differentiation and via downregulation of KGF/FGFR-2 signaling in the HF.

Abundant production of brain-derived neurotrophic factor by adult visceral epithelia. Implications for paracrine and target-derived Neurotrophic functions

Brain-derived neurotrophic factor (BDNF) plays a crucial role for the survival of visceral sensory neurons during development. However, the physiological sources and the function of BDNF in the adult viscera are poorly described. We have investigated the cellular sources and the potential role of BDNF in adult murine viscera. We found markedly different amounts of BDNF protein in different organs. Surprisingly, BDNF levels in the urinary bladder, lung, and colon were higher than those found in the brain or skin. In situ hybridization experiments revealed that BDNF mRNA was made by visceral epithelial cells, several types of smooth muscle, and neurons of the myenteric plexus. Epithelia that expressed BDNF lacked both the high- and low-affinity receptors for BDNF, trkB and p75(NTR). In contrast, both receptors were present on neurons of the peripheral nervous system. Studies with BDNF-/-mice demonstrated that epithelial and smooth muscle cells developed normally in the absence of BDNF. These data provide evidence that visceral epithelia are a major source, but not a target, of BDNF in the adult viscera. The abundance of BDNF protein in certain internal organs suggests that this neurotrophin may regulate the function of adult visceral sensory and motor neurons.

Evidence for the participation of nerve growth factor and its low-affinity receptor (p75NTR) in the regulation of the myogenic program

We have studied expression and function of neurotrophins and their receptors during myogenic differentiation of C2C12 cells, a clonal cell line derived from mouse muscle that is capable of in vitro differentiation. The genes coding for nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and their common low-affinity receptor p75(neurotrophin receptor) (p75NTR) were shown to be expressed in C2C12 myoblasts and downregulated during myogenic differentiation and fusion into myotubes. Cocultures with dorsal root ganglia from day 8 chick embryos revealed neurite-promoting activities of C2C12 cells that ceased with myogenic differentiation. These data suggest a temporal and developmental window for the effect of myogenic cell-derived neurotrophins on neuronal as well as on myogenic cell populations. NGF was shown to increase DNA synthesis and cell growth of C2C12 myoblasts and to enhance myogenic differentiation in this cell line. We present evidence that NGF-mediated processes take place at stages preceding myogenic differentiation. Enhanced muscle differentiation was also seen in p75NTR-overexpressing C2C12 myoblasts which maintained high levels of receptors but ceased to produce NGF during differentiation. In contrast, when exogenous NGF was present at the onset of myogenic differentiation of receptor-overexpressing cells, muscle cell development was strongly repressed. This indicates that downregulation of p75NTR is necessary for guiding myogenic cells towards terminal differentiation. Since none of the trk high-affinity neurotrophin receptors could be demonstrated in C2C12 cells, we conclude that NGF mediates its nonneurotrophic effect via its low-affinity receptor in an autocrine fashion.

MyoD and MEF2A mediate activation and repression of the p75NGFR gene during muscle development

In an effort to clarify transient expression of the NGF low-affinity receptor p75NGFR during muscle development we have focused on the molecular mechanisms involved in the initiation and cessation of p75NGFR gene expression. Using quiescent C3H10T1/2 fibroblast as a tool, we observed that induction of differentiation competence in MyoD-transfected 10T1/2 fibroblasts was accompanied by the initiation of p75NGFR expression. Moreover, we could show that the bHLH transcription factor MyoD itself is a powerful candidate for transcriptional activation of the p75NGFR gene in muscle precursor cells. By means of MyoD-mutants we have found that both the amino terminus of the MyoD molecule as well as the bHLH-region are essential for transcriptional activity on the p75NGFR promoter. The fact that myocyte enhancer factor MEF2A inactivated MyoD-induced p75NGFR promoter activity strongly suggests that cell-specific regulation of the p75NGFR gene might be strictly dependent on the intracellular composition and balance of the appropriate bHLH-transcription factors and their modulators.