Up-regulation of glial cell line-derived neurotrophic factor (GDNF) expression in regenerating muscle fibers in neuromuscular diseases

Pro-myogenic small molecules revealed by a chemical screen on primary muscle stem cells

Satellite cells are the canonical muscle stem cells that regenerate damaged skeletal muscle. Loss of function of these cells has been linked to reduced muscle repair capacity and compromised muscle health in acute muscle injury and congenital neuromuscular diseases. To identify new pathways that can prevent loss of skeletal muscle function or enhance regenerative potential, we established an imaging-based screen capable of identifying small molecules that promote the expansion of freshly isolated satellite cells. We found several classes of receptor tyrosine kinase (RTK) inhibitors that increased freshly isolated satellite cell numbers in vitro. Further exploration of one of these compounds, the RTK inhibitor CEP-701 (also known as lestaurtinib), revealed potent activity on mouse satellite cells both in vitro and in vivo. This expansion potential was not seen upon exposure of proliferating committed myoblasts or non-myogenic fibroblasts to CEP-701. When delivered subcutaneously to acutely injured animals, CEP-701 increased both the total number of satellite cells and the rate of muscle repair, as revealed by an increased cross-sectional area of regenerating fibers. Moreover, freshly isolated satellite cells expanded ex vivo in the presence of CEP-701 displayed enhanced muscle engraftment potential upon in vivo transplantation. We provide compelling evidence that certain RTKs, and in particular RET, regulate satellite cell expansion during muscle regeneration. This study demonstrates the power of small molecule screens of even rare adult stem cell populations for identifying stem cell-targeting compounds with therapeutic potential.

Muscle stem cell renewal suppressed by Gas1 can be reversed by GDNF in mice

Muscle undergoes progressive weakening and regenerative dysfunction with age due in part to the functional decline of skeletal muscle stem cells (MuSCs). MuSCs are heterogeneous but whether their gene expression changes with age and the implication of such changes are unclear. Here we show that in mice, Growth arrest-specific gene 1 (Gas1) is expressed in a small subset of young MuSCs with its expression progressively increasing in larger fractions of MuSCs later in life. Over-expression of Gas1 in young MuSCs and inactivation of Gas1 in aged MuSCs support that Gas1 reduces the quiescence and self-renewal capacity of MuSCs. Gas1 reduces Ret signaling, which is required for MuSC quiescence and self-renewal. Indeed, we show that the Ret ligand, Glial Cell-Derived Neurotrophic Factor (GDNF), can counteract Gas1 by stimulating Ret signaling and enhancing MuSC self-renewal and regeneration, thus improving muscle function. We propose that strategies aimed to target this pathway can be exploited to improve the regenerative decline of muscle stem cells.

Changing muscle function with sustained glial derived neurotrophic factor treatment of rabbit extraocular muscle

Recent microarray and RNAseq experiments provided evidence that glial derived neurotrophic factor (GDNF) levels were decreased in extraocular muscles from human strabismic subjects compared to age-matched controls. We assessed the effect of sustained GDNF treatment of the superior rectus muscles of rabbits on their physiological and morphological characteristics, and these were compared to naïve control muscles. Superior rectus muscles of rabbits were implanted with a sustained release pellet of GDNF to deliver 2μg/day, with the contralateral side receiving a placebo pellet. After one month, the muscles were assessed using in vitro physiological methods. The muscles were examined histologically for alteration in fiber size, myosin expression patterns, neuromuscular junction size, and stem cell numbers and compared to age-matched naïve control muscles. GDNF resulted in decreased force generation, which was also seen on the untreated contralateral superior rectus muscles. Muscle relaxation times were increased in the GDNF treated muscles. Myofiber mean cross-sectional areas were increased after the GDNF treatment, but there was a compensatory increase in expression of developmental, neonatal, and slow tonic myosin heavy chain isoforms. In addition, in the GDNF treated muscles there was a large increase in Pitx2-positive myogenic precursor cells. One month of GDNF resulted in significant extraocular muscle adaptation. These changes are interesting relative to the decreased levels of GDNF in the muscles from subjects with strabismus and preliminary data in infant non-human primates where sustained GDNF treatment produced a strabismus. These data support the view that GDNF has the potential for improving eye alignment in subjects with strabismus.

Rejuvenation of the aged neuromuscular junction by exercise

Age-dependent declines in muscle function are observed across species. The loss of mobility resulting from the decline in muscle function represents an important health issue and a key determinant of quality of life for the elderly. It is believed that changes in the structure and function of the neuromuscular junction are important contributors to the observed declines in motor function with increased age. Numerous studies indicate that the aging muscle is an important contributor to the deterioration of the neuromuscular junction but the cellular and molecular mechanisms driving the degeneration of the synapse remain incompletely described. Importantly, growing data from both animal models and humans indicate that exercise can rejuvenate the neuromuscular junction and improve motor function. In this review we will focus on the role of muscle-derived neurotrophin signaling in the rejuvenation of the aged neuromuscular junction in response to exercise.

The GDNF Family: A Role in Cancer?

The glial cell line–derived neurotrophic factor (GDNF) family of ligands (GFLs) comprising of GDNF, neurturin, artemin, and persephin plays an important role in the development and maintenance of the central and peripheral nervous system, renal morphogenesis, and spermatogenesis. Here we review our current understanding of GFL biology, and supported by recent progress in the area, we examine their emerging role in endocrine-related and other non–hormone-dependent solid neoplasms. The ability of GFLs to elicit actions that resemble those perturbed in an oncogenic phenotype, alongside mounting evidence of GFL involvement in tumor progression, presents novel opportunities for therapeutic intervention.

Effects of acetylcholine and electrical stimulation on glial cell line-derived neurotrophic factor production in skeletal muscle cells

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic factor required for survival of neurons in the central and peripheral nervous system. Specifically, GDNF has been characterized as a survival factor for spinal motor neurons. GDNF is synthesized and secreted by neuronal target tissues, including skeletal muscle in the peripheral nervous system; however, the mechanisms by which GDNF is synthesized and released by skeletal muscle are not fully understood. Previous results suggested that cholinergic neurons regulate secretion of GDNF by skeletal muscle. In the current study, GDNF production by skeletal muscle myotubes following treatment with acetylcholine was examined. Acetylcholine receptors on myotubes were identified with labeled alpha-bungarotoxin and were blocked using unlabeled alpha-bungarotoxin. The question of whether electrical stimulation has a similar effect to that of acetylcholine was also investigated. Cells were stimulated with voltage pulses; at 1 and 5 Hz frequencies for times ranging from 30 min to 48 h. GDNF content in myotubes and GDNF in conditioned culture medium were quantified by enzyme-linked immunosorbant assay. Results suggest that acetylcholine and short-term electrical stimulation reduce GDNF secretion, while treatment with carbachol or long-term electrical stimulation enhances GDNF production by skeletal muscle.

Sustained delivery of VEGF maintains innervation and promotes reperfusion in ischemic skeletal muscles via NGF/GDNF signaling

Tissue reinnervation following trauma, disease, or transplantation often presents a significant challenge. Here, we show that the delivery of vascular endothelial growth factor (VEGF) from alginate hydrogels ameliorates loss of skeletal muscle innervation after ischemic injury by promoting both maintenance and regrowth of damaged axons in mice. Nerve growth factor (NGF) and glial-derived neurotrophic factor (GDNF) mediated VEGF-induced axonal regeneration, and the expression of both is induced by VEGF presentation. Using both in vitro and in vivo modeling approaches, we demonstrate that the activity of NGF and GDNF regulates VEGF-driven angiogenesis, controlling endothelial cell sprouting and blood vessel maturation. Altogether, these studies produce evidence of new mechanisms of VEGF action, further broaden the understanding of the roles of NGF and GDNF in angiogenesis and axonal regeneration, and suggest approaches to improve axonal and ischemic tissue repair therapies.

Transplantated mesenchymal stem cells derived from embryonic stem cells promote muscle regeneration and accelerate functional recovery of injured skeletal muscle

We previously established that mesenchymal stem cells originating from mouse embryonic stem (ES) cells (E-MSCs) showed markedly higher potential for differentiation into skeletal muscles in vitro than common mesenchymal stem cells (MSCs). Further, the E-MSCs exhibited a low risk for teratoma formation. Here we evaluate the potential of E-MSCs for differentiation into skeletal muscles in vivo and reveal the regeneration and functional recovery of injured muscle by transplantation. E-MSCs were transplanted into the tibialis anterior (TA) muscle 24 h following direct clamping. After transplantation, the myogenic differentiation of E-MSCs, TA muscle regeneration, and re-innervation were morphologically analyzed. In addition, footprints and gaits of each leg under spontaneous walking were measured by CatWalk XT, and motor functions of injured TA muscles were precisely analyzed. Results indicate that >60% of transplanted E-MSCs differentiated into skeletal muscles. The cross-sectional area of the injured TA muscles of E-MSC–transplanted animals increased earlier than that of control animals. E-MSCs also promotes re-innervation of the peripheral nerves of injured muscles. Concerning function of the TA muscles, we reveal that transplantation of E-MSCs promotes the recovery of muscles. This is the first report to demonstrate by analysis of spontaneous walking that transplanted cells can accelerate the functional recovery of injured muscles. Taken together, the results show that E-MSCs have a high potential for differentiation into skeletal muscles in vivo as well as in vitro. The transplantation of E-MSCs facilitated the functional recovery of injured muscles. Therefore, E-MSCs are an efficient cell source in transplantation.

IB4(+) nociceptors mediate persistent muscle pain induced by GDNF

Skeletal muscle is a well-known source of glial cell line-derived neurotrophic factor (GDNF), which can produce mechanical hyperalgesia. Since some neuromuscular diseases are associated with both increased release of GDNF and intense muscle pain, we explored the role of GDNF as an endogenous mediator in muscle pain. Intramuscularly injected GDNF induced a dose-dependent (0.1-10 ng/20 μl) persistent (up to 3 wk) mechanical hyperalgesia in the rat. Once hyperalgesia subsided, injection of prostaglandin E(2) at the site induced a prolonged mechanical hyperalgesia (>72 h) compared with naïve rats (<4 h; hyperalgesic priming). Selective neurotoxic destruction of IB4(+) nociceptors attenuated both GDNF hyperalgesia and hyperalgesic priming. Ergonomic muscular injury induced by eccentric exercise or mechanical vibration increased muscle GDNF levels at 24 h, a time point where rats also exhibited marked muscle hyperalgesia. Intrathecal antisense oligodeoxynucleotides to mRNA encoding GFRα1, the canonical binding receptor for GDNF, reversibly inhibited eccentric exercise- and mechanical vibration-induced muscle hyperalgesia. Finally, electrophysiological recordings from nociceptors innervating the gastrocnemius muscle in anesthetized rats, revealed significant increase in response to sustained mechanical stimulation after local GDNF injection. In conclusion, these data indicate that GDNF plays a role as an endogenous mediator in acute and induction of chronic muscle pain, an effect likely to be produced by GDNF action at GFRα1 receptors located in IB4(+) nociceptors.

GDNF induces mechanical hyperalgesia in muscle by reducing I(BK) in isolectin B4-positive nociceptors

We have assessed the mechanism underlying glial cell-derived neurotrophic factor (GDNF)-induced mechanical hyperalgesia in the gastrocnemius muscle, using patch clamp electrophysiology, in vivo electrophysiology and behavioral studies. Cultured isolectin B4-positive (IB4+) dorsal root ganglion neurons that innervated this muscle were held under current clamp; the majority developed an increase in action potential duration (a factor of increase of 2.29±0.24, compared to 1.13±0.17 in control, P<0.01) in response to GDNF (200 ng/ml) by 15 min after application. They also demonstrated a depolarization of resting membrane potential, but without significant changes in rheobase, action potential peak, or after-hyperpolarization. Large-conductance voltage- and calcium-activated potassium (BK) channels, which have recently been shown to play a role in the repolarization of IB4+ nociceptors, were inhibited under voltage clamp, as indicated by a significant reduction in the iberiotoxin-sensitive current. In vivo single-fiber recording from muscle afferents revealed that injection of iberiotoxin into their peripheral nociceptive field caused an increase in nociceptor firing in response to a 60s suprathreshold stimulus (an increase from 392.2±119.8 spikes to 596.1±170.8 spikes, P<0.05). This was observed in the absence of changes in the mechanical threshold. Finally, injection of iberiotoxin into the gastrocnemius muscle produced dose-dependent mechanical hyperalgesia. These data support the suggestion that GDNF induces nociceptor sensitization and mechanical hyperalgesia, at least in part, by inhibiting BK current in IB4+ nociceptors.

IB4-saporin attenuates acute and eliminates chronic muscle pain in the rat

The function of populations of nociceptors in muscle pain syndromes remain poorly understood. We compared the contribution of two major classes, isolectin B4-positive (IB4(+)) and IB4-negative (IB4(-)) nociceptors, in acute and chronic inflammatory and ergonomic muscle pain. Baseline mechanical nociceptive threshold was assessed in the gastrocnemius muscle of rats treated with IB4-saporin, which selectively destroys IB4(+) nociceptors. Rats were then submitted to models of acute inflammatory (intramuscular carrageenan)- or ergonomic intervention (eccentric exercise or vibration)-induced muscle pain, and each of the three models also evaluated for the transition from acute to chronic pain, manifest as prolongation of prostaglandin E2 (PGE(2))-induced hyperalgesia, after recovery from the hyperalgesia induced by acute inflammation or ergonomic interventions. IB4-saporin treatment did not affect baseline mechanical nociceptive threshold. However, compared to controls, IB4-saporin treated rats exhibited shorter duration mechanical hyperalgesia in all three models and attenuated peak hyperalgesia in the ergonomic pain models. And, IB4-saporin treatment completely prevented prolongation of PGE(2)-induced mechanical hyperalgesia. Thus, IB4(+) and IB4(-) neurons contribute to acute muscle hyperalgesia induced by diverse insults. However, only IB4+ nociceptors participate in the long term consequence of acute hyperalgesia.

Cyclosporine A in Ullrich congenital muscular dystrophy: long-term results

Six individuals with Ullrich congenital muscular dystrophy (UCMD) and mutations in the genes-encoding collagen VI, aging 5–9, received 3–5 mg/kg of cyclosporine A (CsA) daily for 1 to 3.2 years. The primary outcome measure was the muscle strength evaluated with a myometer and expressed as megalimbs. The megalimbs score showed significant improvement (P = 0.01) in 5 of the 6 patients. Motor function did not change. Respiratory function deteriorated in all. CsA treatment corrected mitochondrial dysfunction, increased muscle regeneration, and decreased the number of apoptotic nuclei. Results from this study demonstrate that long-term treatment with CsA ameliorates performance in the limbs, but not in the respiratory muscles of UCMD patients, and that it is well tolerated. These results suggest considering a trial of CsA or nonimmunosuppressive cyclosporins, that retains the PTP-desensitizing properties of CsA, as early as possible in UCMD patients when diaphragm is less compromised.

Cholinergic neurons regulate secretion of glial cell line-derived neurotrophic factor by skeletal muscle cells in culture

Glial cell line-derived neurotrophic factor (GDNF) has been identified as a potent survival factor for both central and peripheral neurons. GDNF has been shown to be a potent survival factor for motor neurons during programmed cell death and continuous treatment with GDNF maintains hyperinnervation of skeletal muscle in adulthood. However, little is known about factors regulating normal production of endogenous GDNF in skeletal muscle. This study aimed to examine the role that motor neurons play in regulating GDNF secretion by skeletal muscle. A co-culture of skeletal muscle cells (C2C12) and cholinergic neurons, glioma×neuroblastoma hybrid cells (NG108-15) were used to create nerve-muscle interactions in vitro. Acetylcholine receptors (AChRs) on nerve-myotube co-cultures were blocked with alpha-bungarotoxin (α-BTX). GDNF protein content in cells and in culture medium was analyzed by enzyme-linked immunosorbant assay (ELISA) and western blotting. GDNF localization was examined by immunocytochemistry. The nerve-muscle co-culture study indicated that the addition of motor neurons to skeletal muscle cells reduced the secretion of GDNF by skeletal muscle. The results also showed that blocking AChRs with α-BTX reversed the action of neural cells on GDNF secretion by skeletal muscle. Although ELISA results showed no GDNF in differentiated NG108-15 cells grown alone, immunocytochemical analysis showed that GDNF was localized in NG108-15 cells co-cultured with C2C12 myotubes. These results suggest that motor neurons may be regulating their own supply of GDNF secreted by skeletal muscle and that activation of AChRs may be involved in this process.

Increased expression of B-lymphocyte chemoattractant, but not pro-inflammatory cytokines, in muscle tissue in rhesus chronic Lyme borreliosis

Inflammation in skeletal muscle is a consistent feature of Lyme borreliosis, both in the human disease and experimental models. This study had two goals: to evaluate the expression of selected pro-inflammatory and chemokine genes in skeletal muscle in the Rhesus model of Lyme disease, and to identify unexpected cytokine genes involved in Lyme myositis. Two different techniques for measuring cytokine messenger RNA (mRNA) levels were used to achieve these goals: gene expression microarrays and. real-time RT-PCR (Taqman). Muscle from necropsies and biopsies were used, and were obtained from both infected and uninfected non-human primates (NHPs). Although many cytokines were found expressed in muscle tissue, pro-inflammatory cytokines commonly associated with inflammation were not consistently upregulated in infected muscles relative to uninfected muscles. However, B-lymphocyte chemoattractant (BLC), a chemokine implicated in the trafficking of B-cells into tissue, was increased in expression. This study is the first to extensively characterize cytokine gene expression in chronically inflamed tissue in Lyme borreliosis.

Homeostatic regulation of germinal stem cell proliferation by the GDNF/FSH pathway

Stem cell regulatory mechanisms are difficult to study because self-renewal and production of differentiated progeny, which are both strictly controlled, occur simultaneously in these cells. To focus on the self-renewal mechanism alone, we investigated the behavior of germinal stem cells (GSCs) in progeny-deficient testes with defective GSC differentiation. In these testes, we found that the proliferation of undifferentiated spermatogonia, some of which are GSCs, was accelerated by high concentrations of glial cell line-derived neurotrophic factor (GDNF). Furthermore, we found that follicle-stimulating hormone (FSH) stimulation via homeostatic control was one of the major regulators of GDNF concentration. These results suggest that in mammalian testes, GSC proliferation and population size are regulated homeostatically by the GDNF/FSH pathway.

GDNF gene delivery to injured adult CNS motor neurons promotes axonal growth, expression of the trophic neuropeptide CGRP, and cellular protection

Glial-cell-line--derived neurotrophic factor (GDNF) has been identified as a potent survival and differentiation factor for several neuronal populations in the central nervous system (CNS), but to date, distinct effects of GDNF on motor axon growth and regeneration in the adult have not been demonstrated. In the present study, ex vivo gene delivery was used to directly examine whether GDNF can influence axonal growth, expression of neuronal regeneration-related genes, and sustain the motor neuronal phenotype after adult CNS injury. Adult Fischer 344 rats underwent unilateral transections of the hypoglossal nerve, followed by intramedullary grafts of fibroblasts genetically modified to secrete GDNF. Control animals received lesions and grafts of cells expressing a reporter gene. Two weeks later, GDNF gene delivery (1) robustly promoted the growth of lesioned hypoglossal motor axons, (2) altered the expression and intracellular trafficking of the growth-related protein calcitonin gene-related peptide (CGRP), and (3) significantly sustained the cholinergic phenotype in 84 +/- 6% of hypoglossal neurons compared with 39 +/- 6% in control animals (P < 0.001). This is the first neurotrophic factor identified to increase the in vivo expression of the trophic peptide CGRP and the first report that GDNF promotes motor axonal growth in vivo in the adult CNS. Taken together with previous in vitro studies, these findings serve as the foundation for a model wherein GDNF and CGRP interact in a paracrine manner to regulate neuromuscular development and regeneration.

Leukemia inhibitory factor, glial cell line-derived neurotrophic factor, and their receptor expressions following muscle crush injury

Using in situ hybridization histochemistry, we characterized the spatiotemporal gene expression patterns of leukemia inhibitory factor (LIF) and glial cell line-derived neurotrophic factor (GDNF), and their receptor components (LIFR, GFR-alpha1, RET) induced in muscle cells, intramuscular nerves, and motoneurons in the regeneration processes of both muscle cells and nerves following muscle contusion. Muscle contusion induced upregulation of GDNF and GFR-alpha1 mRNAs in Schwann cell-like cells in the intramuscular nerves and of LIFR mRNA in damaged muscle cells. LIFR, GFR-alpha1, and RET mRNA expressions in motoneurons were upregulated following muscle contusion. Muscle contusion also induced more rapid, prominent transactivations of GFR-alpha1 and RET genes in motoneurons than did sciatic nerve axotomy. These findings suggest that rapid and prominent upregulation of the receptor components for LIF and GDNF in motoneurons is important for the regeneration of intramuscular motor nerves damaged by muscle contusion.

Expression of human GFR alpha-1 (GDNF receptor) at the neuromuscular junction and myelinated nerves

Motor neurons have been known to require a wide variety of neurotrophic factors for their survival. As one of the target-derived trophic factors, glial cell line-derived neurotrophic factor (GDNF) has been shown to exert its effects on motor neurons via a receptor complex including GDNF receptor alpha 1 (GFR alpha-1). Immunoreactivity of GFR alpha-1 was observed at myelinated peripheral nerves and neuromuscular junction (NMJ) of human skeletal muscles. Reverse transcriptase polymerase chain reaction (RT-PCR) analyses showed that mRNA of GFR alpha-1 existed in the ventral horn of human spinal cord, but not in the skeletal muscles. The results suggested that GFR alpha-1 might play a key role for uptake and internalization of GDNF at the human NMJ.