Prenatal flutamide enhances survival in a myogenic mouse model of spinal bulbar muscular atrophy

Myocytic androgen receptor overexpression does not affect sex differences in adaptation to chronic endurance exercise

Muscle-specific androgen receptor (AR) overexpression (HSAAR transgene) in sedentary male rats results in reduced adiposity, increased mitochondrial enzyme activity, and selective increase in Type 2b myofiber size. Here, we tested chronic endurance exercise interactions with this phenotype in both sexes. Across 9 weeks, rats ran 5×/week on motorized running wheels at increasing speeds and durations. Exercise reduced fat mass in all groups, but sex affected endurance exercise outcomes such that absolute lean mass increased only in females and total body mass decreased only in males. Expected sex differences were observed with males exhibiting greater total body and lean mass; absolute and relative fat mass; bone mineral density; extensor digitorum longus (EDL) myofiber size and glycolytic proportion; but lesser Type 2a and Type 1 myosin expression in tibialis anterior. Observed HSAAR outcomes were not altered by sex, with transgenic rats having greater lean mass, Type 2a myosin expression in soleus, and glycolytic myofiber size in EDL. Tibialis AR content was independently affected by sex, HSAAR, and exercise. No sex differences were observed in tibialis AR expression in wild-type rats, although HSAAR males had greater AR content than HSAAR females. We identified a moderate correlation between AR expression and glycolytic myofiber size, but not whole-body composition. Overall, results suggest myocytic AR overexpression and chronic exercise, despite sharing a similar phenotype to adaptation, are mediated by distinct mechanisms. Further, this study illustrates sex differences in adaptation to chronic endurance exercise, and suggests sex-similarity in the relationship between muscle AR and exercise response.

Adaptations in bone, lean, and total mass after forced endurance exercise are sex-dependent in rats.

Sex differences in muscle fiber-type size and proportion, lean body mass, and bone density are independent of exercise in rats.

Myocytic AR overexpression promotes lean body mass and glycolytic myofiber size in both sexes.

Skeletal muscle AR protein is elevated by chronic endurance exercise in rats, and these changes in AR content are correlated with improved glycolytic myofiber size.

Muscle BDNF improves synaptic and contractile muscle strength in Kennedy's disease mice in a muscle-type specific manner

KEY POINTS

Muscle-derived neurotrophic factors may offer therapeutic promise for treating neuromuscular diseases. We report that a muscle-derived neurotrophic factor, BDNF, rescues synaptic and muscle function in a muscle-type specific manner in mice modelling Kennedy's disease (KD). We also find that BDNF rescues select molecular mechanisms in slow and fast muscle that may underlie the improved cellular function. We also report for the first time that expression of BDNF, but not other members of the neurotrophin family, is perturbed in muscle from patients with KD. Given that muscle BDNF had divergent therapeutic effects that depended on muscle type, a combination of neurotrophic factors may optimally rescue neuromuscular function via effects on both pre- and postsynaptic function, in the face of disease.

ABSTRACT

Deficits in muscle brain-derived neurotrophic factor (BDNF) correlate with neuromuscular deficits in mouse models of Kennedy's disease (KD), suggesting that restoring muscle BDNF might restore function. To test this possibility, transgenic mice expressing human BDNF in skeletal muscle were crossed with '97Q' KD mice. We found that muscle BDNF slowed disease, doubling the time between symptom onset and endstage. BDNF also improved expression of genes in muscle known to play key roles in neuromuscular function, including counteracting the expression of neonatal isoforms induced by disease. Intriguingly, BDNF's ameliorative effects differed between muscle types: synaptic strength was rescued only in slow-twitch muscle, while contractile strength was improved only in fast-twitch muscle. In sum, muscle BDNF slows disease progression, rescuing select cellular and molecular mechanisms that depend on fibre type. Muscle BDNF expression was also affected in KD patients, reinforcing its translational and therapeutic potential for treating this disorder.

Disease Affects Bdnf Expression in Synaptic and Extrasynaptic Regions of Skeletal Muscle of Three SBMA Mouse Models

Spinal bulbar muscular atrophy (SBMA) is a slowly progressive, androgen-dependent neuromuscular disease in men that is characterized by both muscle and synaptic dysfunction. Because gene expression in muscle is heterogeneous, with synaptic myonuclei expressing genes that regulate synaptic function and extrasynaptic myonuclei expressing genes to regulate contractile function, we used quantitative PCR to compare gene expression in these two domains of muscle from three different mouse models of SBMA: the "97Q" model that ubiquitously expresses mutant human androgen receptor (AR), the 113Q knock-in (KI) model that expresses humanized mouse AR with an expanded glutamine tract, and the "myogenic" model that overexpresses wild-type rat AR only in skeletal muscle. We were particularly interested in neurotrophic factors because of their role in maintaining neuromuscular function via effects on both muscle and synaptic function, and their implicated role in SBMA. We confirmed previous reports of the enriched expression of select genes (e.g., the acetylcholine receptor) in the synaptic region of muscle, and are the first to report the synaptic enrichment of others (e.g., glial cell line-derived neurotrophic factor). Interestingly, all three models displayed comparably dysregulated expression of most genes examined in both the synaptic and extrasynaptic domains of muscle, with only modest differences between regions and models. These findings of comprehensive gene dysregulation in muscle support the emerging view that skeletal muscle may be a prime therapeutic target for restoring function of both muscles and motoneurons in SBMA.

Pre-clinical symptoms of SBMA may not be androgen-dependent: implications from two SBMA mouse models

A distinguishing aspect of spinal and bulbar muscular atrophy (SBMA) is its androgen-dependence, possibly explaining why only males are clinically affected. This disease, which impairs neuromuscular function, is linked to a polyglutamine expansion mutation in the androgen receptor (AR). In mouse models of SBMA, motor dysfunction is associated with pronounced defects in neuromuscular transmission, including defects in evoked transmitter release (quantal content, QC) and fiber membrane excitability (based on the resting membrane potential, RMP). However, whether such defects are androgen-dependent is unknown. Thus, we recorded synaptic potentials intracellularly from adult muscle fibers of transgenic (Tg) AR97Q male mice castrated pre-symptomatically. Although castration largely protects both QC and the RMP of fibers, correlating with the protective effect of castration on motor function, significant deficits in QC and RMP remained. Surprisingly, comparable defects in QC and RMP were also observed in pre-symptomatic AR97Q males, indicating that such defects emerge early and are pre-clinical. Exposing asymptomatic Tg females to androgens also induces both motor dysfunction and comparable defects in QC and RMP. Notably, asymptomatic Tg females also showed significant deficits in QC and RMP, albeit less severe, supporting their pre-clinical nature, but also raising questions about the androgen-dependence of pre-clinical symptoms. In summary, current evidence indicates that disease progression depends on androgens, but early pathogenic events may be triggered by the mutant AR allele independent of androgens. Such early, androgen-independent disease mechanisms may also be relevant to females carrying the SBMA allele.

Neural androgen receptors affect the number of surviving new neurones in the adult dentate gyrus of male mice

Adult hippocampal neurogenesis occurs in many mammalian species. In rats, the survival of new neurones within the hippocampus is modulated by the action of androgen via the androgen receptor (AR); however, it is not known whether this holds true in mice. Furthermore, the evidence is mixed regarding whether androgens act in neural tissue or via peripheral non-neural targets to promote new neurone survival in the hippocampus. We evaluated whether the action of androgen via AR underlies the survival of new neurones in mice, and investigated whether increasing AR selectively in neural tissue would increase new neurone survival in the hippocampus. We used the cre-loxP system to overexpress AR only in neural tissues (Nestin-AR). These males were compared with wild-type males, as well as control males with 1 of the 2 mutations required for overexpression. Mice were gonadectomised and injected with the DNA synthesis marker, bromodeoxyuridine (BrdU) and for 37 days (following BrdU injection), mice were treated with oil or dihydrotestosterone (DHT). Using immunohistochemistry, proliferation (Ki67) and survival (BrdU) of new neurones were both evaluated in the dorsal and ventral dentate gyrus. Dihydrotestosterone treatment increased the survival of new neurones in the entire hippocampus in wild-type mice and control mice that only have 1 of 2 necessary mutations for transgenic expression. However, DHT treatment did not increase the survival of new neurones in mice that overexpressed AR in neural tissue. Cell proliferation (Ki67) and cell death (pyknotic cells) were not affected by DHT treatment in wild-type or transgenic males. These results suggest that androgens act via neural AR to affect hippocampal neurogenesis by promoting cell survival; however, the relationship between androgen dose and new neurone survival is nonlinear.

Defects in Neuromuscular Transmission May Underlie Motor Dysfunction in Spinal and Bulbar Muscular Atrophy

Spinal and bulbar muscular atrophy (SBMA) in men is an androgen-dependent neuromuscular disease caused by expanded CAG repeats in the androgen receptor (AR). Whether muscle or motor neuron dysfunction or both underlies motor impairment in SBMA is unknown. Muscles of SBMA mice show significant contractile dysfunction, implicating them as a likely source of motor dysfunction, but whether disease also impairs neuromuscular transmission is an open question. Thus, we examined synaptic function in three well-studied SBMA mouse models-the AR97Q, knock-in (KI), and myogenic141 models-by recording in vitro miniature and evoked end-plate potentials (MEPPs and EPPs, respectively) intracellularly from adult muscle fibers. We found striking defects in neuromuscular transmission suggesting that toxic AR in SBMA impairs both presynaptic and postsynaptic mechanisms. Notably, SBMA causes neuromuscular synapses to become weak and muscles to become hyperexcitable in all three models. Presynaptic defects included deficits in quantal content, reduced size of the readily releasable pool, and impaired short-term facilitation. Postsynaptic defects included prolonged decay times for both MEPPs and EPPs, marked resistance to μ-conotoxin (a sodium channel blocker), and enhanced membrane excitability. Quantitative PCR revealed robust upregulation of mRNAs encoding neonatal isoforms of the AChR (γ-subunit) and the voltage-gated sodium channel (NaV1.5) in diseased adult muscles of all three models, consistent with the observed slowing of synaptic potentials and resistance to μ-conotoxin. These findings suggest that muscles of SBMA patients regress to an immature state that impairs neuromuscular function.

SIGNIFICANCE STATEMENT

We have discovered that SBMA is accompanied by marked defects in neuromuscular synaptic transmission involving both presynaptic and postsynaptic mechanisms. For three different mouse models, we find that diseased synapses are weak, having reduced quantal content due to reductions in the size of the readily releasable pool and/or probability of release. Synaptic potentials in diseased adult fibers are slowed, explained by an aberrant upregulation of the neonatal isoform of the acetylcholine receptor. Diseased fibers also show marked resistance to μ-conotoxin, explained by an aberrant upregulation in the neonatal isoform of the sodium channel, and are hyperexcitable, reminiscent of myotonic dystrophy, showing anode-break action potentials. This work identifies several new molecular targets for recovering function in SBMA.

Non-Cell-Autonomous Regulation of Retrograde Motoneuronal Axonal Transport in an SBMA Mouse Model

Defects in axonal transport are seen in motoneuronal diseases, but how that impairment comes about is not well understood. In spinal bulbar muscular atrophy (SBMA), a disorder linked to a CAG/polyglutamine repeat expansion in the androgen receptor (AR) gene, the disease-causing AR disrupts axonal transport by acting in both a cell-autonomous fashion in the motoneurons themselves, and in a non-cell-autonomous fashion in muscle. The non-cell-autonomous mechanism is suggested by data from a unique “myogenic” transgenic (TG) mouse model in which an AR transgene expressed exclusively in skeletal muscle fibers triggers an androgen-dependent SBMA phenotype, including defects in retrograde transport. However, motoneurons in this TG model retain the endogenous AR gene, leaving open the possibility that impairments in transport in this model also depend on ARs in the motoneurons themselves. To test whether non-cell-autonomous mechanisms alone can perturb retrograde transport, we generated male TG mice in which the endogenous AR allele has the testicular feminization mutation (Tfm) and, consequently, is nonfunctional. Males carrying the Tfm allele alone show no deficits in motor function or axonal transport, with or without testosterone treatment. However, when Tfm males carrying the myogenic transgene (Tfm/TG) are treated with testosterone, they develop impaired motor function and defects in retrograde transport, having fewer retrogradely labeled motoneurons and deficits in endosomal flux based on time-lapse video microscopy of living axons. These findings demonstrate that non-cell-autonomous disease mechanisms originating in muscle are sufficient to induce defects in retrograde transport in motoneurons.

Neuromuscular junctions are pathological but not denervated in two mouse models of spinal bulbar muscular atrophy

Spinal bulbar muscular atrophy (SBMA) is a progressive, late onset neuromuscular disease causing motor dysfunction in men. While the morphology of the neuromuscular junction (NMJ) is typically affected by neuromuscular disease, whether NMJs in SBMA are similarly affected by disease is not known. Such information will shed light on whether defective NMJs might contribute to the loss of motor function and represent a potential therapeutic target for treating symptoms of SBMA. To address this gap in information, the morphology of NMJs was examined in two mouse models of SBMA, a myogenic model that overexpresses wildtype androgen receptor (AR) exclusively in muscle fibres and a knockin (KI) model expressing a humanized mutant AR gene. The tripartite motor synapse consisting of motor nerve terminal, terminal Schwann cells (tSCs) and postsynaptic specialization were visualized and analysed using confocal microscopy. Counter to expectation, we found no evidence of denervation in either model, but junctions in both models show pathological fragmentation and an abnormal synaptophysin distribution consistent with functionally weak synapses. Neurofilament accumulations were observed only in the myogenic model, even though axonal transport dysfunction is characteristic of both models. The ultrastructure of NMJs revealed additional pathology, including deficits in docked vesicles presynaptically, wider synaptic clefts, and simpler secondary folds postsynaptically. The observed pathology of NMJs in diseased SBMA mice is likely the morphological correlates of defects in synaptic function which may underlie motor impairments associated with SBMA.

Androgen-dependent loss of muscle BDNF mRNA in two mouse models of SBMA

Transgenic expression of neurotrophic factors in skeletal muscle has been found to protect mice from neuromuscular disease, including spinal bulbar muscular atrophy (SBMA), triggering renewed interest in neurotrophic factors as therapeutic agents for treating neuromuscular disease. Because SBMA is an androgen-dependent disease, and brain-derived neurotrophic factor (BDNF) mediates effects of androgens on neuromuscular systems, we asked whether BDNF expression is impaired in two different transgenic (Tg) mouse models of SBMA, the so called "97Q" and "myogenic" SBMA models. The 97Q model globally overexpresses a full length human AR with 97 glutamine repeats whereas the myogenic model of SBMA overexpresses a wild-type rat androgen receptor (AR) only in skeletal muscle fibers. Using quantitative PCR, we find that muscle BDNF mRNA declines in an androgen-dependent manner in both models, paralleling changes in motor function, with robust deficits (6-8 fold) in both fast and slow twitch muscles of impaired Tg males. Castration rescues or reverses disease-related deficits in muscle BDNF mRNA in both models, paralleling its effect on motor function. Moreover, when disease is acutely induced in Tg females, both motor function and muscle BDNF mRNA expression plummet, with the deficit in muscle BDNF emerging before overt motor dysfunction. That androgen-dependent motor dysfunction is tightly associated with a robust and early down-regulation of muscle BDNF mRNA suggests that BDNF delivered to skeletal muscle may have therapeutic value for SBMA.

Contractile dysfunction in muscle may underlie androgen-dependent motor dysfunction in spinal bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is characterized by progressive muscle weakness linked to a polyglutamine expansion in the androgen receptor (AR). Current evidence indicates that mutant AR causes SBMA by acting in muscle to perturb its function. However, information about how muscle function is impaired is scant. One fundamental question is whether the intrinsic strength of muscles, an attribute of muscle independent of its mass, is affected. In the current study, we assess the contractile properties of hindlimb muscles in vitro from chronically diseased males of three different SBMA mouse models: a transgenic (Tg) model that broadly expresses a full-length human AR with 97 CAGs (97Q), a knock-in (KI) model that expresses a humanized AR containing a CAG expansion in the first exon, and a Tg myogenic model that overexpresses wild-type AR only in skeletal muscle fibers. We found that hindlimb muscles in the two Tg models (97Q and myogenic) showed marked losses in their intrinsic strength and resistance to fatigue, but were minimally affected in KI males. However, diseased muscles of all three models showed symptoms consistent with myotonic dystrophy type 1, namely, reduced resting membrane potential and deficits in chloride channel mRNA. These data indicate that muscle dysfunction is a core feature of SBMA caused by at least some of the same pathogenic mechanisms as myotonic dystrophy. Thus mechanisms controlling muscle function per se independent of mass are prime targets for SBMA therapeutics.

Antiandrogen flutamide protects male mice from androgen-dependent toxicity in three models of spinal bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a late-onset, progressive neurodegenerative disease linked to a polyglutamine (polyQ) expansion in the androgen receptor (AR). Men affected by SBMA show marked muscle weakness and atrophy, typically emerging midlife. Given the androgen-dependent nature of this disease, one might expect AR antagonists to have therapeutic value for treating SBMA. However, current work from animal models suggests otherwise, raising questions about whether polyQ-expanded AR exerts androgen-dependent toxicity through mechanisms distinct from normal AR function. In this study, we asked whether the nonsteroidal AR antagonist flutamide, delivered via a time-release pellet, could reverse or prevent androgen-dependent AR toxicity in three different mouse models of SBMA: the AR97Q transgenic (Tg) model, a knock-in (KI) model, and a myogenic Tg model. We find that flutamide protects mice from androgen-dependent AR toxicity in all three SBMA models, preventing or reversing motor dysfunction in the Tg models and significantly extending the life span in KI males. Given that flutamide effectively protects against androgen-dependent disease in three different mouse models of SBMA, our data are proof of principle that AR antagonists have therapeutic potential for treating SBMA in humans and support the notion that toxicity caused by polyQ-expanded AR uses at least some of the same mechanisms as normal AR before diverging to produce disease and muscle atrophy.

Mechanisms mediating spinal and bulbar muscular atrophy: investigations into polyglutamine-expanded androgen receptor function and dysfunction

Spinal and bulbar muscular atrophy (SBMA, Kennedy’s disease), a late-onset neuromuscular disorder, is caused by expansion of the polymorphic polyglutamine tract in the androgen receptor (AR). The AR is a ligand-activated transcription factor, but plays roles in other cellular pathways. In SBMA, selective motor neuron degeneration occurs in the brainstem and spinal cord, thus the causes of neuronal dysfunction have been studied. However, pathogenic pathways in muscles may also be involved. Cultured cells, fly and mouse models are used to study the molecular mechanisms leading to SBMA. Both the structure of the polyglutamine-expanded AR (polyQ AR) and its interactions with other proteins are altered relative to the normal AR. The ligand-dependent translocation of the polyQ AR to the nucleus appears to be critical, as are interdomain interactions. The polyQ AR, or fragments thereof, can form nuclear inclusions, but their pathogenic or protective nature is unclear. Other data suggests soluble polyQ AR oligomers can be harmful. Post-translational modifications such as phosphorylation, acetylation, and ubiquitination influence AR function and modulate the deleterious effects of the polyQ AR. Transcriptional dysregulation is highly likely to be a factor in SBMA; deregulation of non-genomic AR signaling may also be involved. Studies on polyQ AR-protein degradation suggest inhibition of the ubiquitin proteasome system and changes to autophagic pathways may be relevant. Mitochondrial function and axonal transport may also be affected by the polyQ AR. Androgens, acting through the AR, can be neurotrophic and are important in muscle development; hence both loss of normal AR functions and gain of novel harmful functions by the polyQ AR can contribute to neurodegeneration and muscular atrophy. Thus investigations into polyQ AR function have shown that multiple complex mechanisms lead to the initiation and progression of SBMA.

Androgen receptors in muscle fibers induce rapid loss of force but not mass: implications for spinal bulbar muscular atrophy

INTRODUCTION

Testosterone (T) induces motor dysfunction in transgenic (Tg) mice that overexpress wild-type androgen receptor (AR) in skeletal muscles. Because many genes implicated in motor neuron disease are expressed in skeletal muscle, mutant proteins may act in muscle to cause dysfunction in motor neuron disease.

METHODS

We examined contractile properties of the extensor digitorum longus (EDL) and soleus (SOL) muscles in vitro after 5 and 3 days of T treatment in motor-impaired Tg female mice.

RESULTS

Both muscles showed deficits in tetanic force after 5 days of T treatment, without losses in muscle mass, protein content, or fiber number. After 3 days of T treatment, only SOL showed a deficit in tetanic force comparable to that of 5 days of treatment. In both treatments, EDL showed slowed twitch kinetics, whereas SOL showed deficits in the twitch/tetanus ratio.

CONCLUSIONS

These results suggest calcium-handling mechanisms in muscle fibers are defective in motor-impaired mice.

New routes to therapy for spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy's disease, is a genetically inherited neuromuscular disorder characterized by loss of lower motor neurons in the brainstem and spinal cord and skeletal muscle fasciculation, weakness, and atrophy. SBMA is caused by expansion of a polyglutamine (polyQ) tract in the gene coding for the androgen receptor (AR). PolyQ expansions cause at least eight other neurological disorders, which are collectively known as polyQ diseases. SBMA is unique in the family of polyQ diseases in that the disease manifests fully in male individuals only. The sex specificity of SBMA is the result of the interaction between mutant AR and its natural ligand, testosterone. Here, we will discuss emerging therapeutic perspectives for SBMA in light of recent findings regarding disease pathogenesis.

The anabolic/androgenic steroid nandrolone exacerbates gene expression modifications induced by mutant SOD1 in muscles of mice models of amyotrophic lateral sclerosis

Anabolic/androgenic steroids (AAS) are drugs that enhance muscle mass, and are often illegally utilized in athletes to improve their performances. Recent data suggest that the increased risk for amyotrophic lateral sclerosis (ALS) in male soccer and football players could be linked to AAS abuse. ALS is a motor neuron disease mainly occurring in sporadic (sALS) forms, but some familial forms (fALS) exist and have been linked to mutations in different genes. Some of these, in their wild type (wt) form, have been proposed as risk factors for sALS, i.e. superoxide dismutase 1 (SOD1) gene, whose mutations are causative of about 20% of fALS. Notably, SOD1 toxicity might occur both in motor neurons and in muscle cells. Using gastrocnemius muscles of mice overexpressing human mutant SOD1 (mutSOD1) at different disease stages, we found that the expression of a selected set of genes associated to muscle atrophy, MyoD, myogenin, atrogin-1, and transforming growth factor (TGF)β1, is up-regulated already at the presymptomatic stage. Atrogin-1 gene expression was increased also in mice overexpressing human wtSOD1. Similar alterations were found in axotomized mouse muscles and in cultured ALS myoblast models. In these ALS models, we then evaluated the pharmacological effects of the synthetic AAS nandrolone on the expression of the genes modified in ALS muscle. Nandrolone administration had no effects on MyoD, myogenin, and atrogin-1 expression, but it significantly increased TGFβ1 expression at disease onset. Altogether, these data suggest that, in fALS, muscle gene expression is altered at early stages, and AAS may exacerbate some of the alterations induced by SOD1 possibly acting as a contributing factor also in sALS.

Impaired motoneuronal retrograde transport in two models of SBMA implicates two sites of androgen action

Spinal and bulbar muscular atrophy (SBMA) impairs motor function in men and is linked to a CAG repeat mutation in the androgen receptor (AR) gene. Defects in motoneuronal retrograde axonal transport may critically mediate motor dysfunction in SBMA, but the site(s) where AR disrupts transport is unknown. We find deficits in retrograde labeling of spinal motoneurons in both a knock-in (KI) and a myogenic transgenic (TG) mouse model of SBMA. Likewise, live imaging of endosomal trafficking in sciatic nerve axons reveals disease-induced deficits in the flux and run length of retrogradely transported endosomes in both KI and TG males, demonstrating that disease triggered in muscle can impair retrograde transport of cargo in motoneuron axons, possibly via defective retrograde signaling. Supporting the idea of impaired retrograde signaling, we find that vascular endothelial growth factor treatment of diseased muscles reverses the transport/trafficking deficit. Transport velocity is also affected in KI males, suggesting a neurogenic component. These results demonstrate that androgens could act via both cell autonomous and non-cell autonomous mechanisms to disrupt axonal transport in motoneurons affected by SBMA.