Testosterone treatment fails to accelerate disease in a transgenic mouse model of spinal and bulbar muscular atrophy

Skeletal muscle in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), the major adult-onset motor neuron disease, has been viewed almost exclusively as a disease of upper and lower motor neurons, with muscle changes interpreted as a consequence of the progressive loss of motor neurons and neuromuscular junctions. This has led to the prevailing view that the involvement of muscle in ALS is only secondary to motor neuron loss. Skeletal muscle and motor neurons reciprocally influence their respective development and constitute a single functional unit. In ALS, multiple studies indicate that skeletal muscle dysfunction might contribute to progressive muscle weakness, as well as to the final demise of neuromuscular junctions and motor neurons. Furthermore, skeletal muscle has been shown to participate in disease pathogenesis of several monogenic diseases closely related to ALS. Here, we move the narrative towards a better appreciation of muscle as a contributor of disease in ALS. We review the various potential roles of skeletal muscle cells in ALS, from passive bystanders to active players in ALS pathophysiology. We also compare ALS to other motor neuron diseases and draw perspectives for future research and treatment.

Bicalutamide and Trehalose Ameliorate Spinal and Bulbar Muscular Atrophy Pathology in Mice

Spinal and bulbar muscular atrophy (SBMA) is characterized by motor neuron (MN) degeneration that leads to slowly progressive muscle weakness. It is considered a neuromuscular disease since muscle has a primary role in disease onset and progression. SBMA is caused by a CAG triplet repeat expansion in the androgen receptor (AR) gene. The translated poly-glutamine (polyQ) tract confers a toxic gain of function to the mutant AR altering its folding, causing its aggregation into intracellular inclusions, and impairing the autophagic flux. In an in vitro SBMA neuronal model, we previously showed that the antiandrogen bicalutamide and trehalose, a natural disaccharide stimulating autophagy, block ARpolyQ activation, reduce its nuclear translocation and toxicity and facilitate the autophagic degradation of cytoplasmic AR aggregates. Here, in a knock-in SBMA mouse model (KI AR113Q), we show that bicalutamide and trehalose ameliorated SBMA pathology. Bicalutamide reversed the formation of the AR insoluble forms in KI AR113Q muscle, preventing autophagic flux blockage. We demonstrated that apoptosis is activated in KI AR113Q muscle, and that both compounds prevented its activation. We detected a decrease of mtDNA and an increase of OXPHOS enzymes, already at early symptomatic stages; these alterations were reverted by trehalose. Overall, bicalutamide and/or trehalose led to a partial recovery of muscle morphology and function, and improved SBMA mouse motor behavior, inducing an extension of their survival. Thus, bicalutamide and trehalose, by counteracting ARpolyQ toxicity in skeletal muscle, are valuable candidates for future clinical trials in SBMA patients.

Neuromuscular junction pathology is correlated with differential motor unit vulnerability in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an X-linked, neuromuscular neurodegenerative disease for which there is no cure. The disease is characterized by a selective decrease in fast-muscle power (e.g., tongue pressure, grip strength) accompanied by a selective loss of fast-twitch muscle fibers. However, the relationship between neuromuscular junction (NMJ) pathology and fast-twitch motor unit vulnerability has yet to be explored. In this study, we used a cross-model comparison of two mouse models of SBMA to evaluate neuromuscular junction pathology, glycolytic-to-oxidative fiber-type switching, and cytoskeletal alterations in pre- and postsynaptic termini of tibialis anterior (TA), gastrocnemius, and soleus hindlimb muscles. We observed significantly increased NMJ and myofiber pathology in fast-twitch, glycolytic motor units of the TA and gastrocnemius compared to slow-twitch, oxidative motor units of the soleus, as seen by decreased pre- and post-synaptic membrane area, decreased pre- and post-synaptic membrane colocalization, increased acetylcholine receptor compactness, a decrease in endplate area and complexity, and deficits in neurofilament heavy chain. Our data also show evidence for metabolic dysregulation and myofiber atrophy that correlate with severity of NMJ pathology. We propose a model in which the dynamic communicative relationship between the motor neuron and muscle, along with the developmental subtype of the muscle, promotes motor unit subtype specific vulnerability, metabolic alterations, and NMJ pathology.

Kennedy disease (X-linked recessive bulbospinal neuronopathy): A comprehensive review from pathophysiology to therapy

Kennedy's disease, also known as spinal and bulbar muscular atrophy (SBMA), is a rare, adult-onset, X-linked recessive neuromuscular disease caused by expansion of a CAG repeat sequence in exon 1 of the androgen receptor gene (AR) encoding a polyglutamine (polyQ) tract. The polyQ-expanded AR accumulates in nuclei, and initiates degeneration and loss of motor neurons and dorsal root ganglia. While the disease has long been considered a pure lower motor neuron disease, recently, the presence of major hyper-creatine-kinase (CK)-emia and myopathic alterations on muscle biopsy has suggested the presence of a primary myopathy underlying a wide range of clinical manifestations. The disease, which affects male adults, is characterized by muscle weakness and atrophy localized proximally in the limbs, and bulbar involvement. Sensory disturbances are associated with the motor phenotype, but may be subclinical. The most frequent systemic symptom is gynecomastia related to androgen insensitivity, but other abnormalities, such as heart rhythm and urinary disturbances, have also been reported. The course of the disease is slowly progressive with normal life expectancy. The diagnosis of SBMA is based on genetic testing, with 38 CAG repeats taken as pathogenic. Despite several therapeutic attempts made in mouse models, no effective disease-modifying therapy is yet available, although symptomatic therapy is beneficial for the management of the weakness, fatigue and bulbar symptoms.

Beyond aggression: Androgen-receptor blockade modulates social interaction in wild meerkats

In male vertebrates, androgens are inextricably linked to reproduction, social dominance, and aggression, often at the cost of paternal investment or prosociality. Testosterone is invoked to explain rank-related reproductive differences, but its role within a status class, particularly among subordinates, is underappreciated. Recent evidence, especially for monogamous and cooperatively breeding species, suggests broader androgenic mediation of adult social interaction. We explored the actions of androgens in subordinate, male members of a cooperatively breeding species, the meerkat (Suricata suricatta). Although male meerkats show no rank-related testosterone differences, subordinate helpers rarely reproduce. We blocked androgen receptors, in the field, by treating subordinate males with the antiandrogen, flutamide. We monitored androgen concentrations (via baseline serum and time-sequential fecal sampling) and recorded behavior within their groups (via focal observation). Relative to controls, flutamide-treated animals initiated less and received more high-intensity aggression (biting, threatening, feeding competition), engaged in more prosocial behavior (social sniffing, grooming, huddling), and less frequently initiated play or assumed a 'dominant' role during play, revealing significant androgenic effects across a broad range of social behavior. By contrast, guarding or vigilance and measures of olfactory and vocal communication in subordinate males appeared unaffected by flutamide treatment. Thus, androgens in male meerkat helpers are aligned with the traditional trade-off between promoting reproductive and aggressive behavior at a cost to affiliation. Our findings, based on rare endocrine manipulation in wild mammals, show a more pervasive role for androgens in adult social behavior than is often recognized, with possible relevance for understanding tradeoffs in cooperative systems.

Trinucleotide repeats and protein folding and disease: the perspective from studies with the androgen receptor

The androgen receptor (AR), a ligand activated transcription factor plays a number of roles in reproduction, homeostasis and pathogenesis of disease. It has two major polymorphic sequences; a polyglutamine and a polyglycine repeat that determine the length of the protein and influence receptor folding, structure and function. Here, we review the role the folding of the AR plays in the pathogenesis of spinal-bulbar muscular atrophy (SBMA), a neuromuscular degenerative disease arising from expansion of the polyglutamine repeat. We discuss current management for SBMA patients and how research on AR structure function may lead to future drug treatments.

Synergic prodegradative activity of Bicalutamide and trehalose on the mutant androgen receptor responsible for spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is an X-linked motoneuron disease due to a CAG triplet-repeat expansion in the androgen receptor (AR) gene, which is translated into an elongated polyglutamine (polyQ) tract in AR protein (ARpolyQ). ARpolyQ toxicity is activated by the AR ligand testosterone (or dihydrotestosterone), and the polyQ triggers ARpolyQ misfolding and aggregation in spinal cord motoneurons and muscle cells. In motoneurons, testosterone triggers nuclear toxicity by inducing AR nuclear translocation. Thus, (i) prevention of ARpolyQ nuclear localization, combined with (ii) an increased ARpolyQ cytoplasmic clearance, should reduce its detrimental activity. Using the antiandrogen Bicalutamide (Casodex^®), which slows down AR activation and nuclear translocation, and the disaccharide trehalose, an autophagy activator, we found that, in motoneurons, the two compounds together reduced ARpolyQ insoluble forms with higher efficiency than that obtained with single treatments. The ARpolyQ clearance was mediated by trehalose-induced autophagy combined with the longer cytoplasmic retention of ARpolyQ bound to Bicalutamide. This allows an increased recognition of misfolded species by the autophagic system prior to their migration into the nucleus. Interestingly, the combinatory use of trehalose and Bicalutamide was also efficient in the removal of insoluble species of AR with a very long polyQ (Q112) tract, which typically aggregates into the cell nuclei. Collectively, these data suggest that the combinatory use of Bicalutamide and trehalose is a novel approach to facilitate ARpolyQ clearance that has to be tested in other cell types target of SBMA (i.e. muscle cells) and in vivo in animal models of SBMA.

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.

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.