Muscle expression of mutant androgen receptor accounts for systemic and motor neuron disease phenotypes in spinal and bulbar muscular atrophy

Therapeutic targeting of the polyglutamine androgen receptor in Spinal and Bulbar Muscular Atrophy

INTRODUCTION

Spinal and Bulbar Muscular Atrophy (SBMA) is a slowly progressive, X-linked, and sex-limited degenerative disorder affecting lower motor neurons and skeletal muscle which lacks disease-modifying therapies. This disease is caused by a CAG/polyglutamine (polyQ) tract expansion in the androgen receptor (AR) gene, and its pathogenesis is driven by toxic gain-of-function mechanisms. Affected men develop proximal limb and bulbar muscle weakness along with signs of partial androgen insensitivity.

AREAS COVERED

Toxicity of the polyQ AR is mediated by protein misfolding and nuclear translocation that follow ligand binding, resulting in the disruption of downstream homeostatic mechanisms. This review highlights what is known about disease pathogenesis and how this has been leveraged to test potential therapeutic approaches. The focus is on strategies that alleviate polyQ AR toxicity in SBMA, including those that alter AR function, diminish the expression of the encoding gene, or promote clearance of the misfolded, mutant protein.

EXPERT OPINION

We discuss emerging strategies to mitigate polyQ AR toxicity, including gene editing, RNA targeted therapies, and efforts to harness proteostatic mechanisms. These promising approaches are discussed in the context of challenges for drug discovery efforts that are faced when attempting to treat a rare and slowly progressive neurodegenerative disorder.

Activation of the muscle-to-brain axis ameliorates neurocognitive deficits in an Alzheimer's disease mouse model via enhancing neurotrophic and synaptic signaling

Skeletal muscle regulates central nervous system (CNS) function and health, activating the muscle-to-brain axis through the secretion of skeletal muscle-originating factors ("myokines") with neuroprotective properties. However, the precise mechanisms underlying these benefits in the context of Alzheimer's disease (AD) remain poorly understood. To investigate muscle-to-brain axis signaling in response to amyloid β (Aβ)-induced toxicity, we generated 5xFAD transgenic female mice with enhanced skeletal muscle function (5xFAD;cTFEB;HSACre) at prodromal (4-months old) and late (8-months old) symptomatic stages. Skeletal muscle TFEB overexpression reduced Aβ plaque accumulation in the cortex and hippocampus at both ages and rescued behavioral neurocognitive deficits in 8-month-old 5xFAD mice. These changes were associated with transcriptional and protein remodeling of neurotrophic signaling and synaptic integrity, partially due to the CNS-targeting myokine prosaposin (PSAP). Our findings implicate the muscle-to-brain axis as a novel neuroprotective pathway against amyloid pathogenesis in AD.

Msh3 and Pms1 Set Neuronal CAG-repeat Migration Rate to Drive Selective Striatal and Cortical Pathogenesis in HD Mice

Modifiers of Huntington's disease (HD) include mismatch repair (MMR) genes; however, their underlying disease-altering mechanisms remain unresolved. Knockout (KO) alleles for 9 HD GWAS modifiers/MMR genes were crossed to the Q140 Huntingtin (mHtt) knock-in mice to probe such mechanisms. Four KO mice strongly ( Msh3 and Pms1 ) or moderately ( Msh2 and Mlh1 ) rescue a triad of adult-onset, striatal medium-spiny-neuron (MSN)-selective phenotypes: somatic Htt DNA CAG-repeat expansion, transcriptionopathy, and mHtt protein aggregation. Comparatively, Q140 cortex also exhibits an analogous, but later-onset, pathogenic triad that is Msh3 -dependent. Remarkably, Q140/homozygous Msh3-KO lacks visible mHtt aggregates in the brain, even at advanced ages (20-months). Moreover, Msh3 -deficiency prevents striatal synaptic marker loss, astrogliosis, and locomotor impairment in HD mice. Purified Q140 MSN nuclei exhibit highly linear age-dependent mHtt DNA repeat expansion (i.e. repeat migration), with modal-CAG increasing at +8.8 repeats/month (R 2 =0.98). This linear rate is reduced to 2.3 and 0.3 repeats/month in Q140 with Msh3 heterozygous and homozygous alleles, respectively. Our study defines somatic Htt CAG-repeat thresholds below which there are no detectable mHtt nuclear or neuropil aggregates. Mild transcriptionopathy can still occur in Q140 mice with stabilized Htt 140-CAG repeats, but the majority of transcriptomic changes are due to somatic repeat expansion. Our analysis reveals 479 genes with expression levels highly correlated with modal-CAG length in MSNs. Thus, our study mechanistically connects HD GWAS genes to selective neuronal vulnerability in HD, in which Msh3 and Pms1 set the linear rate of neuronal mHtt CAG-repeat migration to drive repeat-length dependent pathogenesis; and provides a preclinical platform for targeting these genes for HD suppression across brain regions.

One Sentence Summary

Msh3 and Pms1 are genetic drivers of sequential striatal and cortical pathogenesis in Q140 mice by mediating selective CAG-repeat migration in HD vulnerable neurons.

Differentially disrupted spinal cord and muscle energy metabolism in spinal and bulbar muscular atrophy

Prior studies showed that polyglutamine-expanded androgen receptor (AR) is aberrantly acetylated and that deacetylation of the mutant AR by overexpression of nicotinamide adenine dinucleotide-dependent (NAD+-dependent) sirtuin 1 is protective in cell models of spinal and bulbar muscular atrophy (SBMA). Based on these observations and reduced NAD+ in muscles of SBMA mouse models, we tested the therapeutic potential of NAD+ restoration in vivo by treating postsymptomatic transgenic SBMA mice with the NAD+ precursor nicotinamide riboside (NR). NR supplementation failed to alter disease progression and had no effect on increasing NAD+ or ATP content in muscle, despite producing a modest increase of NAD+ in the spinal cords of SBMA mice. Metabolomic and proteomic profiles of SBMA quadriceps muscles indicated alterations in several important energy-related pathways that use NAD+, in addition to the NAD+ salvage pathway, which is critical for NAD+ regeneration for use in cellular energy production. We also observed decreased mRNA levels of nicotinamide riboside kinase 2 (Nmrk2), which encodes a key kinase responsible for NR phosphorylation, allowing its use by the NAD+ salvage pathway. Together, these data suggest a model in which NAD+ levels are significantly decreased in muscles of an SBMA mouse model and intransigent to NR supplementation because of decreased levels of Nmrk2.

Exercise attenuates polyglutamine-mediated neuromuscular degeneration in a mouse model of spinal and bulbar muscular atrophy

BACKGROUND

Spinal and bulbar muscular atrophy (SBMA) is a hereditary neuromuscular disorder caused by the expansion of trinucleotide cytosine-adenine-guanine (CAG) repeats, which encodes a polyglutamine (polyQ) tract in the androgen receptor (AR) gene. Recent evidence suggests that, in addition to motor neuron degeneration, defective skeletal muscles are also the primary contributors to the pathogenesis in SBMA. While benefits of physical exercise have been suggested in SBMA, underlying mechanism remains elusive.

METHODS

We investigated the effect of running exercise in a transgenic mouse model of SBMA carrying human AR with 97 expanded CAGs (AR97Q). We assigned AR97Q mice to exercise and sedentary control groups, and mice in the exercise group received 1-h forced running wheel (5 m/min) 5 days a week for 4 weeks during the early stage of the disease. Motor function (grip strength and rotarod performance) and survival of each group were analysed, and histopathological and biological features in skeletal muscles and motor neurons were evaluated.

RESULTS

AR97Q mice in the exercise group showed improvement in motor function (~40% and ~50% increase in grip strength and rotarod performance, respectively, P < 0.05) and survival (median survival 23.6 vs. 16.7 weeks, P < 0.05) with amelioration of neuronal and muscular histopathology (~1.4-fold and ~2.8-fold increase in motor neuron and muscle fibre size, respectively, P < 0.001) compared to those in the sedentary group. Nuclear accumulation of polyQ-expanded AR in skeletal muscles and motor neurons was suppressed in the mice with exercise compared to the sedentary mice (~50% and ~30% reduction in 1C2-positive cells in skeletal muscles and motor neurons, respectively, P < 0.05). We found that the exercise activated 5'-adenosine monophosphate-activated protein kinase (AMPK) signalling and inhibited mammalian target of rapamycin pathway that regulates protein synthesis in skeletal muscles of SBMA mice. Pharmacological activation of AMPK inhibited protein synthesis and reduced polyQ-expanded AR proteins in C2C12 muscle cells.

CONCLUSIONS

Our findings suggest the therapeutic potential of exercise-induced effect via AMPK activation in SBMA.

A phenotypically robust model of spinal and bulbar muscular atrophy in Drosophila

Spinal and bulbar muscular atrophy (SBMA) is an X-linked disorder that affects males who inherit the androgen receptor (AR) gene with an abnormal CAG triplet repeat expansion. The resulting protein contains an elongated polyglutamine (polyQ) tract and causes motor neuron degeneration in an androgen-dependent manner. The precise molecular sequelae of SBMA are unclear. To assist with its investigation and the identification of therapeutic options, we report here a new model of SBMA in Drosophila melanogaster. We generated transgenic flies that express the full-length, human AR with a wild-type or pathogenic polyQ repeat. Each transgene is inserted into the same safe harbor site on the third chromosome of the fly as a single copy and in the same orientation. Expression of pathogenic AR, but not of its wild-type variant, in neurons or muscles leads to consistent, progressive defects in longevity and motility that are concomitant with polyQ-expanded AR protein aggregation and reduced complexity in neuromuscular junctions. Additional assays show adult fly eye abnormalities associated with the pathogenic AR species. The detrimental effects of pathogenic AR are accentuated by feeding flies the androgen, dihydrotestosterone. This new, robust SBMA model can be a valuable tool toward future investigations of this incurable disease.

Skeletal muscle TFEB signaling promotes central nervous system function and reduces neuroinflammation during aging and neurodegenerative disease

Skeletal muscle has recently arisen as a regulator of central nervous system (CNS) function and aging, secreting bioactive molecules known as myokines with metabolism-modifying functions in targeted tissues, including the CNS. Here, we report the generation of a transgenic mouse with enhanced skeletal muscle lysosomal and mitochondrial function via targeted overexpression of transcription factor E-B (TFEB). We discovered that the resulting geroprotective effects in skeletal muscle reduce neuroinflammation and the accumulation of tau-associated pathological hallmarks in a mouse model of tauopathy. Muscle-specific TFEB overexpression significantly ameliorates proteotoxicity, reduces neuroinflammation, and promotes transcriptional remodeling of the aged CNS, preserving cognition and memory in aged mice. Our results implicate the maintenance of skeletal muscle function throughout aging in direct regulation of CNS health and disease and suggest that skeletal muscle originating factors may act as therapeutic targets against age-associated neurodegenerative disorders.

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.

The value of serum creatinine as biomarker of disease progression in spinal and bulbar muscular atrophy (SBMA)

Serum creatinine has been indicated as a potential marker of motor function in SBMA and results form previous longitudinal studies pointed to its decline over time. This is a longitudinal retrospective study investigating creatinine changes over a 36-month-period in 73 patients with SBMA. Severity and progression of the disease was assessed according to serum creatine kinase (CK) values, manual muscle testing (MMT), SBMA functional rating scale (SBMAFRS) score, 6-min-walk test (6MWT) value, and spirometry (forced vital capacity, fVC%) obtained at the baseline and at each of the annual follow-up visits. Baseline serum creatinine concentrations positively correlated with 6MWT, the MMT megascore score of both the upper (ULM) and lower (LLM) limbs and SBMAFRS. No correlation was found with CK or fVC% values. Similar correlation results were achieved at all the subsequent time points. Longitudinal assessments conducted by the generalized estimating equations (GEE) method returned significant changes for SBMAFRS (- 1.41 points per year, p < 0.001), ULM and LLM (- 0.69, p = 0.01; and - 1.07, p < 0.001, respectively), 6MWT (- 47 m, p < 0.001) but not for creatinine (- 0.82, p > 0.05). We also observed that creatinine levels at baseline did not correlate with changes in the other measures from baseline at each annual visit. Our data do not support a role for serum creatinine as sensitive biomarker of disease progression, and possibily prognosis, in SBMA.

X-linked SBMA model mice display relevant non-neurological phenotypes and their expression of mutant androgen receptor protein in motor neurons is not required for neuromuscular disease

X-linked spinal and bulbar muscular atrophy (SBMA; Kennedy's disease) is a rare neuromuscular disorder characterized by adult-onset proximal muscle weakness and lower motor neuron degeneration. SBMA was the first human disease found to be caused by a repeat expansion mutation, as affected patients possess an expanded tract of CAG repeats, encoding polyglutamine, in the androgen receptor (AR) gene. We previously developed a conditional BAC fxAR121 transgenic mouse model of SBMA and used it to define a primary role for skeletal muscle expression of polyglutamine-expanded AR in causing the motor neuron degeneration. Here we sought to extend our understanding of SBMA disease pathophysiology and cellular basis by detailed examination and directed experimentation with the BAC fxAR121 mice. First, we evaluated BAC fxAR121 mice for non-neurological disease phenotypes recently described in human SBMA patients, and documented prominent non-alcoholic fatty liver disease, cardiomegaly, and ventricular heart wall thinning in aged male BAC fxAR121 mice. Our discovery of significant hepatic and cardiac abnormalities in SBMA mice underscores the need to evaluate human SBMA patients for signs of liver and heart disease. To directly examine the contribution of motor neuron-expressed polyQ-AR protein to SBMA neurodegeneration, we crossed BAC fxAR121 mice with two different lines of transgenic mice expressing Cre recombinase in motor neurons, and after updating characterization of SBMA phenotypes in our current BAC fxAR121 colony, we found that excision of mutant AR from motor neurons did not rescue neuromuscular or systemic disease. These findings further validate a primary role for skeletal muscle as the driver of SBMA motor neuronopathy and indicate that therapies being developed to treat patients should be delivered peripherally.

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.

Chronic BMAA exposure combined with TDP-43 mutation elicits motor neuron dysfunction phenotypes in mice

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease with an average age-of-onset of ∼60 years and is usually fatal within 2-5 years of diagnosis. Mouse models based upon single gene mutations do not recapitulate all ALS pathological features. Environmental insults may also contribute to ALS, and β-N-methylamino-L-alanine (BMAA) is an environmental toxin linked with an increased risk of developing ALS. BMAA, along with cycasin, are hypothesized to be the cause of the Guam-ALS epicenter of the 1950s. We developed a multihit model based on low expression of a dominant familial ALS TDP-43 mutation (Q331K) and chronic low-dose BMAA exposure. Our two-hit mouse model displayed a motor phenotype absent from either lesion alone. By LC/MS analysis, free BMAA was confirmed at trace levels in brain, and were as high as 405 ng/mL (free) and 208 ng/mL (protein-bound) in liver. Elevated BMAA levels in liver were associated with dysregulation of the unfolded protein response (UPR) pathway. Our data represent initial steps towards an ALS mouse model resulting from combined genetic and environmental insult.

Defective excitation-contraction coupling and mitochondrial respiration precede mitochondrial Ca2+ accumulation in spinobulbar muscular atrophy skeletal muscle

Polyglutamine expansion in the androgen receptor (AR) causes spinobulbar muscular atrophy (SBMA). Skeletal muscle is a primary site of toxicity; however, the current understanding of the early pathological processes that occur and how they unfold during disease progression remains limited. Using transgenic and knock-in mice and patient-derived muscle biopsies, we show that SBMA mice in the presymptomatic stage develop a respiratory defect matching defective expression of genes involved in excitation-contraction coupling (ECC), altered contraction dynamics, and increased fatigue. These processes are followed by stimulus-dependent accumulation of calcium into mitochondria and structural disorganization of the muscle triads. Deregulation of expression of ECC genes is concomitant with sexual maturity and androgen raise in the serum. Consistent with the androgen-dependent nature of these alterations, surgical castration and AR silencing alleviate the early and late pathological processes. These observations show that ECC deregulation and defective mitochondrial respiration are early but reversible events followed by altered muscle force, calcium dyshomeostasis, and dismantling of triad structure.

Epigenetic Modifications by Estrogen and Androgen in Alzheimer's Disease

For the development and maintenance of neuron networks in the brain, epigenetic mechanisms are necessary, as indicated by recent findings. This includes some of the high-order brain processes, such as behavior and cognitive functions. Epigenetic mechanisms could influence the pathophysiology or etiology of some neuronal diseases, altering disease susceptibility and therapy responses. Recent studies support epigenetic dysfunctions in neurodegenerative and psychiatric conditions, such as Alzheimer's disease (AD). These dysfunctions in epigenetic mechanisms also play crucial roles in the transgenerational effects of the environment on the brain and subsequently in the inheritance of pathologies. The possible role of gonadal steroids in the etiology and progression of neurodegenerative diseases, including Alzheimer's disease, has become the subject of a growing body of research over the last 20 years. Recent scientific findings suggest that epigenetic changes, driven by estrogen and androgens, play a vital role in brain functioning. Therefore, exploring the role of estrogen and androgen-based epigenetic changes in the brain is critical for the deeper understanding of AD. This review highlights the epigenetic modifications caused by these two gonadal steroids and the possible therapeutic strategies for AD.

Clinical Pharmacokinetics of Approved RNA Therapeutics

RNA-mediated drugs are a rapidly growing class of therapeutics. Over the last five years, the list of FDA-approved RNA therapeutics has expanded owing to their unique targets and prolonged pharmacological effects. Their absorption, distribution, metabolism, and excretion (ADME) have important clinical im-plications, but their pharmacokinetic properties have not been fully understood. Most RNA therapeutics have structural modifications to prevent rapid elimination from the plasma and are administered intravenously or subcutaneously, with some exceptions, for effective distribution to target organs. Distribution of drugs into tissues depends on the addition of a moiety that can be transported to the target and RNA therapeutics show a low volume of distribution because of their molecular size and negatively-charged backbone. Nucleases metabolize RNA therapeutics to a shortened chain, but their metabolic ratio is relatively low. Therefore, most RNA therapeutics are excreted in their intact form. This review covers not only ADME features but also clinical pharmacology data of the RNA therapeutics such as drug-drug interaction or population pharmacokinetic analyses. As the market of RNA therapeutics is expected to rapidly expand, comprehensive knowledge will contribute to interpreting and evaluating the pharmacological properties.

The role of ubiquitination in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neurodegenerative and neuromuscular genetic disease caused by the expansion of a polyglutamine-encoding CAG tract in the androgen receptor (AR) gene. The AR is an important transcriptional regulator of the nuclear hormone receptor superfamily; its levels are regulated in many ways including by ubiquitin-dependent degradation. Ubiquitination is a post-translational modification (PTM) which plays a key role in both AR transcriptional activity and its degradation. Moreover, the ubiquitin-proteasome system (UPS) is a fundamental component of cellular functioning and has been implicated in diseases of protein misfolding and aggregation, including polyglutamine (polyQ) repeat expansion diseases such as Huntington's disease and SBMA. In this review, we discuss the details of the UPS system, its functions and regulation, and the role of AR ubiquitination and UPS components in SBMA. We also discuss aspects of the UPS that may be manipulated for therapeutic effect in SBMA.

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.

Skeletal Muscle Pathogenesis in Polyglutamine Diseases

Polyglutamine diseases are characterized by selective dysfunction and degeneration of specific types of neurons in the central nervous system. In addition, nonneuronal cells can also be affected as a consequence of primary degeneration or due to neuronal dysfunction. Skeletal muscle is a primary site of toxicity of polyglutamine-expanded androgen receptor, but it is also affected in other polyglutamine diseases, more likely due to neuronal dysfunction and death. Nonetheless, pathological processes occurring in skeletal muscle atrophy impact the entire body metabolism, thus actively contributing to the inexorable progression towards the late and final stages of disease. Skeletal muscle atrophy is well recapitulated in animal models of polyglutamine disease. In this review, we discuss the impact and relevance of skeletal muscle in patients affected by polyglutamine diseases and we review evidence obtained in animal models and patient-derived cells modeling skeletal muscle.

Exploring the Role of Posttranslational Modifications in Spinal and Bulbar Muscular Atrophy

Spinal and Bulbar Muscular Atrophy (SBMA) is an X-linked adult-onset progressive neuromuscular disease that affects the spinal and bulbar motor neurons and skeletal muscles. SBMA is caused by expansion of polymorphic CAG trinucleotide repeats in the Androgen Receptor (AR) gene, resulting in expanded glutamine tract in the AR protein. Polyglutamine (polyQ) expansion renders the mutant AR protein toxic, resulting in the formation of mutant protein aggregates and cell death. This classifies SBMA as one of the nine known polyQ diseases. Like other polyQ disorders, the expansion of the polyQ tract in the AR protein is the main genetic cause of the disease; however, multiple other mechanisms besides the polyQ tract expansion also contribute to the SBMA disease pathophysiology. Posttranslational modifications (PTMs), including phosphorylation, acetylation, methylation, ubiquitination, and SUMOylation are a category of mechanisms by which the functionality of AR has been found to be significantly modulated and can alter the neurotoxicity of SBMA. This review summarizes the different PTMs and their effects in regulating the AR function and discusses their pathogenic or protective roles in context of SBMA. This review also includes the therapeutic approaches that target the PTMs of AR in an effort to reduce the mutant AR-mediated toxicity in SBMA.

The protective role of exercise against age-related neurodegeneration

Endurance exercise is a widely accessible, low-cost intervention with a variety of benefits to multiple organ systems. Exercise improves multiple indices of physical performance and stimulates pronounced health benefits reducing a range of pathologies including metabolic, cardiovascular, and neurodegenerative disorders. Endurance exercise delays brain aging, preserves memory and cognition, and improves symptoms of neurodegenerative pathologies like Amyotrophic Lateral Sclerosis, Alzheimer's disease, Parkinson's disease, Huntington's disease, and various ataxias. Potential mechanisms underlying the beneficial effects of exercise include neuronal survival and plasticity, neurogenesis, epigenetic modifications, angiogenesis, autophagy, and the synthesis and release of neurotrophins and cytokines. In this review, we discuss shared benefits and molecular pathways driving the protective effects of endurance exercise on various neurodegenerative diseases in animal models and in humans.

Nucleic Acid-Based Therapeutic Approach for Spinal and Bulbar Muscular Atrophy and Related Neurological Disorders

The recent advances in nucleic acid therapeutics demonstrate the potential to treat hereditary neurological disorders by targeting their causative genes. Spinal and bulbar muscular atrophy (SBMA) is an X-linked and adult-onset neurodegenerative disorder caused by the expansion of trinucleotide cytosine-adenine-guanine repeats, which encodes a polyglutamine tract in the androgen receptor gene. SBMA belongs to the family of polyglutamine diseases, in which the use of nucleic acids for silencing a disease-causing gene, such as antisense oligonucleotides and small interfering RNAs, has been intensively studied in animal models and clinical trials. A unique feature of SBMA is that both motor neuron and skeletal muscle pathology contribute to disease manifestations, including progressive muscle weakness and atrophy. As both motor neurons and skeletal muscles can be therapeutic targets in SBMA, nucleic acid-based approaches for other motor neuron diseases and myopathies may further lead to the development of a treatment for SBMA. Here, we review studies of nucleic acid-based therapeutic approaches in SBMA and related neurological disorders and discuss current limitations and perspectives to apply these approaches to patients with SBMA.

Genetic architecture of motor neuron diseases

Motor neuron diseases (MNDs) are rare and frequently fatal neurological disorders in which motor neurons within the brainstem and spinal cord regions slowly die. MNDs are primarily caused by genetic mutations, and > 100 different mutant genes in humans have been discovered thus far. Given the fact that many more MND-related genes have yet to be discovered, the growing body of genetic evidence has offered new insights into the diverse cellular and molecular mechanisms involved in the aetiology and pathogenesis of MNDs. This search may aid in the selection of potential candidate genes for future investigation and, eventually, may open the door to novel interventions to slow down disease progression. In this review paper, we have summarized detailed existing research findings of different MNDs, such as amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), spinal bulbar muscle atrophy (SBMA) and hereditary spastic paraplegia (HSP) in relation to their complex genetic architecture.

Harmony Lost: Cell-Cell Communication at the Neuromuscular Junction in Motor Neuron Disease

The neuromuscular junction (NMJ) is a specialized synapse that is the point of connection between motor neurons and skeletal muscle. Although developmental studies have established the importance of cell-cell communication at the NMJ for the integrity and full functionality of this synapse, the contribution of this structure as a primary driver in motor neuron disease pathogenesis remains uncertain. Here, we consider the biology of the NMJ and review emerging lines of investigation that are highlighting the importance of cell-cell interaction at the NMJ in spinal muscular atrophy (SMA), X-linked spinal and bulbar muscular atrophy (SBMA), and amyotrophic lateral sclerosis (ALS). Ongoing research may reveal NMJ targets and pathways whose therapeutic modulation will help slow the progression of motor neuron disease, offering a novel treatment paradigm for ALS, SBMA, SMA, and related disorders.

Gene therapy with AR isoform 2 rescues spinal and bulbar muscular atrophy phenotype by modulating AR transcriptional activity

Spinal and bulbar muscular atrophy (SBMA) is an X-linked, adult-onset neuromuscular condition caused by an abnormal polyglutamine (polyQ) tract expansion in androgen receptor (AR) protein. SBMA is a disease with high unmet clinical need. Recent studies have shown that mutant AR-altered transcriptional activity is key to disease pathogenesis. Restoring the transcriptional dysregulation without affecting other AR critical functions holds great promise for the treatment of SBMA and other AR-related conditions; however, how this targeted approach can be achieved and translated into a clinical application remains to be understood. Here, we characterized the role of AR isoform 2, a naturally occurring variant encoding a truncated AR lacking the polyQ-harboring domain, as a regulatory switch of AR genomic functions in androgen-responsive tissues. Delivery of this isoform using a recombinant adeno-associated virus vector type 9 resulted in amelioration of the disease phenotype in SBMA mice by restoring polyQ AR-dysregulated transcriptional activity.

Forgot to Exercise? Exercise Derived Circulating Myokines in Alzheimer's Disease: A Perspective

Regular exercise plays an essential role in maintaining healthy neurocognitive function and central nervous system (CNS) immuno-metabolism in the aging CNS. Physical activity decreases the risk of developing Alzheimer's Disease (AD), is associated with better AD prognosis, and positively affects cognitive function in AD patients. Skeletal muscle is an important secretory organ, communicating proteotoxic and metabolic stress to distant tissues, including the CNS, through the secretion of bioactive molecules collectively known as myokines. Skeletal muscle undergoes significant physical and metabolic remodeling during exercise, including alterations in myokine expression profiles. This suggests that changes in myokine and myometabolite secretion may underlie the well-documented benefits of exercise in AD. However, to date, very few studies have focused on specific alterations in skeletal muscle-originating secreted factors and their potential neuroprotective effects in AD. In this review, we discuss exercise therapy for AD prevention and intervention, and propose the use of circulating myokines as novel therapeutic tools for modifying AD progression.

Dysregulation of Muscle-Specific MicroRNAs as Common Pathogenic Feature Associated with Muscle Atrophy in ALS, SMA and SBMA: Evidence from Animal Models and Human Patients

Motor neuron diseases (MNDs) are neurodegenerative disorders characterized by upper and/or lower MN loss. MNDs include amyotrophic lateral sclerosis (ALS), spinal muscular atrophy (SMA), and spinal and bulbar muscular atrophy (SBMA). Despite variability in onset, progression, and genetics, they share a common skeletal muscle involvement, suggesting that it could be a primary site for MND pathogenesis. Due to the key role of muscle-specific microRNAs (myomiRs) in skeletal muscle development, by real-time PCR we investigated the expression of miR-206, miR-133a, miR-133b, and miR-1, and their target genes, in G93A-SOD1 ALS, Δ7SMA, and KI-SBMA mouse muscle during disease progression. Further, we analyzed their expression in serum of SOD1-mutated ALS, SMA, and SBMA patients, to demonstrate myomiR role as noninvasive biomarkers. Our data showed a dysregulation of myomiRs and their targets, in ALS, SMA, and SBMA mice, revealing a common pathogenic feature associated with muscle impairment. A similar myomiR signature was observed in patients’ sera. In particular, an up-regulation of miR-206 was identified in both mouse muscle and serum of human patients. Our overall findings highlight the role of myomiRs as promising biomarkers in ALS, SMA, and SBMA. Further investigations are needed to explore the potential of myomiRs as therapeutic targets for MND treatment.

Selective suppression of polyglutamine-expanded protein by lipid nanoparticle-delivered siRNA targeting CAG expansions in the mouse CNS

Polyglutamine (polyQ) diseases are inherited neurodegenerative disorders caused by expansion of cytosine-adenine-guanine (CAG)-trinucleotide repeats in causative genes. These diseases include spinal and bulbar muscular atrophy (SBMA), Huntington’s disease, dentatorubral-pallidoluysian atrophy, and spinocerebellar ataxias. Targeting expanded CAG repeats is a common therapeutic approach to polyQ diseases, but concomitant silencing of genes with normal CAG repeats may lead to toxicity. Previous studies have shown that CAG repeat-targeting small interfering RNA duplexes (CAG-siRNAs) have the potential to selectively suppress mutant proteins in in vitro cell models of polyQ diseases. However, in vivo application of these siRNAs has not yet been investigated. In this study, we demonstrate that an unlocked nucleic acid (UNA)-modified CAG-siRNA shows high selectivity for polyQ-expanded androgen receptor (AR) inhibition in in vitro cell models and that lipid nanoparticle (LNP)-mediated delivery of the CAG-siRNA selectively suppresses mutant AR in the central nervous system of an SBMA mouse model. In addition, a subcutaneous injection of the LNP-delivered CAG-siRNA efficiently suppresses mutant AR in the skeletal muscle of the SBMA mouse model. These results support the therapeutic potential of LNP-delivered UNA-modified CAG-siRNAs for selective suppression of mutant proteins in SBMA and other polyQ diseases.

Disease mechanism, biomarker and therapeutics for spinal and bulbar muscular atrophy (SBMA)

Spinal and bulbar muscular atrophy (SBMA) is a hereditary neuromuscular disorder caused by CAG trinucleotide expansion in the gene encoding the androgen receptor (AR). In the central nervous system, lower motor neurons are selectively affected, whereas pathology of patients and animal models also indicates involvement of skeletal muscle including loss of fast-twitch type 2 fibres and increased slow-twitch type 1 fibres, together with a glycolytic-to-oxidative metabolic switch. Evaluation of muscle and fat using MRI, in addition to biochemical indices such as serum creatinine level, are promising biomarkers to track the disease progression. The serum level of creatinine starts to decrease before the onset of muscle weakness, followed by the emergence of hand tremor, a prodromal sign of the disease. Androgen-dependent nuclear accumulation of the polyglutamine-expanded AR is an essential step in the pathogenesis, providing therapeutic opportunities via hormonal manipulation and gene silencing with antisense oligonucleotides. Animal studies also suggest that hyperactivation of Src, alteration of autophagy and a mitochondrial deficit underlie the neuromuscular degeneration in SBMA and provide alternative therapeutic targets.

Myoglobin: a new biomarker for spinal and bulbar muscular atrophy?

OBJECTIVES

There is a primary muscular affection in spinal and bulbar muscular atrophy (SBMA). Myoglobin (Myo) is mainly distributed in the myocardium and skeletal muscle. The purpose of the study was to explore the significance of serum Myo detection in the diagnosis and clinical evaluation of SBMA.

MATERIALS AND METHODS

In this study, serum creatine kinase (CK), Myo, and Troponin T (cTNT) levels were assessed in 80 patients with SBMA and were compared with those of 60 patients with amyotrophic lateral sclerosis (ALS). All measurement data were analyzed using the t-test and enumeration data using the χ2-test.

RESULTS

The rate of abnormal Myo levels in the SBMA group was 100%, however, none of the patients with ALS had an abnormal Myo level. There was no overlap between the two groups. The Myo levels in patients with SBMA were correlated with the course of the disease. Further, their CK level was significantly elevated compared with that in patients with ALS, however, there was an overlap between the two groups. The serum cTNT level in patients with SBMA was not significantly different from that in patients with ALS.

CONCLUSION

Myo, as a simple, inexpensive, and readily available biochemical indicator, is likely to be used for the differentiation between SBMA and ALS, and used as a new biomarker for the clinical evaluation of SBMA.

A Crucial Role for the Protein Quality Control System in Motor Neuron Diseases

Motor neuron diseases (MNDs) are fatal diseases characterized by loss of motor neurons in the brain cortex, in the bulbar region, and/or in the anterior horns of the spinal cord. While generally sporadic, inherited forms linked to mutant genes encoding altered RNA/protein products have also been described. Several different mechanisms have been found altered or dysfunctional in MNDs, like the protein quality control (PQC) system. In this review, we will discuss how the PQC system is affected in two MNDs—spinal and bulbar muscular atrophy (SBMA) and amyotrophic lateral sclerosis (ALS)—and how this affects the clearance of aberrantly folded proteins, which accumulate in motor neurons, inducing dysfunctions and their death. In addition, we will discuss how the PQC system can be targeted to restore proper cell function, enhancing the survival of affected cells in MNDs.

Metabolic alterations in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a rare, X-linked neuromuscular disease characterised by lower motor neurons degeneration, slowly progressive myopathy and multisystem involvement. SBMA is caused by trinucleotide repeat expansion in the first exon of the androgen receptor (AR) gene on chromosome X that encodes a polyglutamine (polyQ) tract in the AR protein. Disease onset occurs between 30-60 years of age with easy fatigability, muscle cramps, and weakness in the limbs. In addition to neuromuscular involvement, in SBMA phenotype, many non-neural manifestations are present. Recently, some studies have reported a high prevalence of metabolic and liver disorders in patients with SBMA. Particularly, fatty liver and insulin resistance (IR) have been found in many SBMA patients. The alteration of AR function and the androgen insensitivity can be involved in both fatty liver and IR. In turn, IR and liver alterations can influence neuromuscular damage through different mechanisms. These data lead to consider SBMA as a metabolic as well as a neuromuscular disease. The mechanism of metabolic alterations, their link with the neuromuscular damage, the effects on the course of disease and their treatment will have to be yet fully clarified.

MEF2 impairment underlies skeletal muscle atrophy in polyglutamine disease

Polyglutamine (polyQ) tract expansion leads to proteotoxic misfolding and drives a family of nine diseases. We study spinal and bulbar muscular atrophy (SBMA), a progressive degenerative disorder of the neuromuscular system caused by the polyQ androgen receptor (AR). Using a knock-in mouse model of SBMA, AR113Q mice, we show that E3 ubiquitin ligases which are a hallmark of the canonical muscle atrophy machinery are not induced in AR113Q muscle. Similarly, we find no evidence to suggest dysfunction of signaling pathways that trigger muscle hypertrophy or impairment of the muscle stem cell niche. Instead, we find that skeletal muscle atrophy is characterized by diminished function of the transcriptional regulator Myocyte Enhancer Factor 2 (MEF2), a regulator of myofiber homeostasis. Decreased expression of MEF2 target genes is age- and glutamine tract length-dependent, occurs due to polyQ AR proteotoxicity, and is associated with sequestration of MEF2 into intranuclear inclusions in muscle. Skeletal muscle from R6/2 mice, a model of Huntington disease which develops progressive atrophy, also sequesters MEF2 into inclusions and displays age-dependent loss of MEF2 target genes. Similarly, SBMA patient muscle shows loss of MEF2 target gene expression, and restoring MEF2 activity in AR113Q muscle rescues fiber size and MEF2-regulated gene expression. This work establishes MEF2 impairment as a novel mechanism of skeletal muscle atrophy downstream of toxic polyglutamine proteins and as a therapeutic target for muscle atrophy in these disorders.

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.

Deterioration of muscle force and contractile characteristics are early pathological events in spinal and bulbar muscular atrophy mice

Spinal and bulbar muscular atrophy (SBMA), also known as Kennedy's Disease, is a late-onset X-linked progressive neuromuscular disease, which predominantly affects males. The pathological hallmarks of the disease are selective loss of spinal and bulbar motor neurons, accompanied by weakness, atrophy and fasciculations of bulbar and limb muscles. SBMA is caused by a CAG repeat expansion in the gene that encodes the androgen receptor (AR) protein. Disease manifestation is androgen dependent and results principally from a toxic gain of AR function. There are currently no effective treatments for this debilitating disease. It is important to understand the course of the disease in order to target therapeutics to key pathological stages. This is especially relevant in disorders such as SBMA, for which disease can be identified before symptom onset, through family history and genetic testing. To fully characterise the role of muscle in SBMA, we undertook a longitudinal physiological and histological characterisation of disease progression in the AR100 mouse model of SBMA. Our results show that the disease first manifests in skeletal muscle, before any motor neuron degeneration, which only occurs in late-stage disease. These findings reveal that alterations in muscle function, including reduced muscle force and changes in contractile characteristics, are early pathological events in SBMA mice and suggest that muscle-targeted therapeutics may be effective in SBMA.This article has an associated First Person interview with the first author of the paper.

The Role of Sex and Sex Hormones in Neurodegenerative Diseases

Neurodegenerative diseases (NDs) are a wide class of disorders of the central nervous system (CNS) with unknown etiology. Several factors were hypothesized to be involved in the pathogenesis of these diseases, including genetic and environmental factors. Many of these diseases show a sex prevalence and sex steroids were shown to have a role in the progression of specific forms of neurodegeneration. Estrogens were reported to be neuroprotective through their action on cognate nuclear and membrane receptors, while adverse effects of male hormones have been described on neuronal cells, although some data also suggest neuroprotective activities. The response of the CNS to sex steroids is a complex and integrated process that depends on (i) the type and amount of the cognate steroid receptor and (ii) the target cell type-either neurons, glia, or microglia. Moreover, the levels of sex steroids in the CNS fluctuate due to gonadal activities and to local metabolism and synthesis. Importantly, biochemical processes involved in the pathogenesis of NDs are increasingly being recognized as different between the two sexes and as influenced by sex steroids. The aim of this review is to present current state-of-the-art understanding on the potential role of sex steroids and their receptors on the onset and progression of major neurodegenerative disorders, namely, Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, and the peculiar motoneuron disease spinal and bulbar muscular atrophy, in which hormonal therapy is potentially useful as disease modifier.

Genetic approaches to the treatment of inherited neuromuscular diseases

Inherited neuromuscular diseases are a heterogeneous group of developmental and degenerative disorders that affect motor unit function. Major challenges toward developing therapies for these diseases include heterogeneity with respect to clinical severity, age of onset and the primary cell type that is affected (e.g. motor neurons, skeletal muscle and Schwann cells). Here, we review recent progress toward the establishment of genetic therapies to treat inherited neuromuscular disorders that affect both children and adults with a focus on spinal muscular atrophy, Charcot-Marie-Tooth disease and spinal and bulbar muscular atrophy. We discuss clinical features, causative mutations and emerging approaches that are undergoing testing in preclinical models and in patients or that have received recent approval for clinical use. Many of these efforts employ antisense oligonucleotides to alter pre-mRNA splicing or diminish target gene expression and use viral vectors to replace expression of mutant genes. Finally, we discuss remaining challenges for optimizing the delivery and effectiveness of these approaches. In sum, therapeutic strategies for neuromuscular diseases have shown encouraging results, raising hope that recent strides will translate into significant clinical benefits for patients with these disorders.

Unveiling synapse pathology in spinal bulbar muscular atrophy by genome-wide transcriptome analysis of purified motor neurons derived from disease specific iPSCs

Spinal bulbar muscular atrophy (SBMA) is an adult-onset, slowly progressive motor neuron disease caused by abnormal CAG repeat expansion in the androgen receptor (AR) gene. Although ligand (testosterone)-dependent mutant AR aggregation has been shown to play important roles in motor neuronal degeneration by the analyses of transgenic mice models and in vitro cell culture models, the underlying disease mechanisms remain to be fully elucidated because of the discrepancy between model mice and SBMA patients. Thus, novel human disease models that recapitulate SBMA patients’ pathology more accurately are required for more precise pathophysiological analysis and the development of novel therapeutics. Here, we established disease specific iPSCs from four SBMA patients, and differentiated them into spinal motor neurons. To investigate motor neuron specific pathology, we purified iPSC-derived motor neurons using flow cytometry and cell sorting based on the motor neuron specific reporter, HB9^e438::Venus, and proceeded to the genome-wide transcriptome analysis by RNA sequences. The results revealed the involvement of the pathology associated with synapses, epigenetics, and endoplasmic reticulum (ER) in SBMA. Notably, we demonstrated the involvement of the neuromuscular synapse via significant upregulation of Synaptotagmin, R-Spondin2 (RSPO2), and WNT ligands in motor neurons derived from SBMA patients, which are known to be associated with neuromuscular junction (NMJ) formation and acetylcholine receptor (AChR) clustering. These aberrant gene expression in neuromuscular synapses might represent a novel therapeutic target for SBMA.

Polyglutamine-Expanded Androgen Receptor Alteration of Skeletal Muscle Homeostasis and Myonuclear Aggregation Are Affected by Sex, Age and Muscle Metabolism

Polyglutamine (polyQ) expansions in the androgen receptor (AR) gene cause spinal and bulbar muscular atrophy (SBMA), a neuromuscular disease characterized by lower motor neuron (MN) loss and skeletal muscle atrophy, with an unknown mechanism. We generated new mouse models of SBMA for constitutive and inducible expression of mutant AR and performed biochemical, histological and functional analyses of phenotype. We show that polyQ-expanded AR causes motor dysfunction, premature death, IIb-to-IIa/IIx fiber-type change, glycolytic-to-oxidative fiber-type switching, upregulation of atrogenes and autophagy genes and mitochondrial dysfunction in skeletal muscle, together with signs of muscle denervation at late stage of disease. PolyQ expansions in the AR resulted in nuclear enrichment. Within the nucleus, mutant AR formed 2% sodium dodecyl sulfate (SDS)-resistant aggregates and inclusion bodies in myofibers, but not spinal cord and brainstem, in a process exacerbated by age and sex. Finally, we found that two-week induction of expression of polyQ-expanded AR in adult mice was sufficient to cause premature death, body weight loss and muscle atrophy, but not aggregation, metabolic alterations, motor coordination and fiber-type switch, indicating that expression of the disease protein in the adulthood is sufficient to recapitulate several, but not all SBMA manifestations in mice. These results imply that chronic expression of polyQ-expanded AR, i.e. during development and prepuberty, is key to induce the full SBMA muscle pathology observed in patients. Our data support a model whereby chronic expression of polyQ-expanded AR triggers muscle atrophy through toxic (neomorphic) gain of function mechanisms distinct from normal (hypermorphic) gain of function mechanisms.

Role of Mutant TBP in Regulation of Myogenesis on Muscle Satellite Cells

In polyglutamine (PolyQ) diseases, mutant proteins cause not only neurological problems but also peripheral tissue abnormalities. Among all systemic damages, skeletal muscle dystrophy is the severest. Previously by studying knock-in (KI) mouse models of spinal cerebellar ataxia 17 (SCA17), it was found that mutant TATA box binding protein (TBP) decreases its interaction with myogenic differentiation antigen, thus reducing the expression of skeletal muscle structural proteins and resulting in muscle degeneration. In this paper, the role of mutant TBP in myogenesis was investigated. Single myofibers were isolated from tibialis anterior muscles of wild type (WT) and SCA17KI mice. The 1TBP18 staining confirmed the expression of mutant TBP in muscle satellite cells in SCA17KI mice. In the BaCl₂-induced TA muscle injury, H&E cross-section staining showed no significant change in myofibril size before and after BaCl₂ treatment, and there was no significant difference in centralized nuclei between WT and SCA17KI mice, suggesting that mutant TBP had no significant effect on muscle regeneration. In the cultured primary myoblasts from WT and SCA17KI mice in vitro, representative BrdU immunostaining showed no significant difference in proliferation of muscle satellite cells. The primary myoblasts were then induced to differentiate and immunostained for eMyHC, and the staining showed there was no significant difference in differentiation of primary myoblasts between WT and SCA1KI mice. Our findings confirmed that mutant TBP had no significant effect on myogenesis.

Molecular Mechanisms and Therapeutics for SBMA/Kennedy's Disease

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by a polyglutamine (polyQ) expansion in the androgen receptor (AR). Despite the fact that the monogenic cause of SBMA has been known for nearly 3 decades, there is no effective treatment for this disease, underscoring the complexity of the pathogenic mechanisms that lead to a loss of motor neurons and muscle in SBMA patients. In the current review, we provide an overview of the system-wide clinical features of SBMA, summarize the structure and function of the AR, discuss both gain-of-function and loss-of-function mechanisms of toxicity caused by polyQ-expanded AR, and describe the cell and animal models utilized in the study of SBMA. Additionally, we summarize previously conducted clinical trials which, despite being based on positive results from preclinical studies, proved to be largely ineffective in the treatment of SBMA; nonetheless, these studies provide important insights as researchers develop the next generation of therapies.

Molecular Mechanisms and Therapeutics for SCA17

Spinocerebellar ataxia type 17 (SCA17) is caused by polyglutamine (polyQ) expansion in the TATA box-binding protein (TBP), which functions as a general transcription factor. Like other polyQ expansion-mediated diseases, SCA17 is characterized by late-onset and selective neurodegeneration, despite the disease protein being ubiquitously expressed in the body. To date, the pathogenesis of polyQ diseases is not fully understood, and there are no effective treatments for these devastating disorders. The well-characterized function of TBP and typical neurodegeneration in SCA17 give us opportunities to understand how polyQ expansion causes selective neurodegeneration and to develop effective therapeutics. In this review, we discuss the molecular mechanisms behind SCA17, focusing on transcriptional dysregulation as its major cause. Mounting evidence suggests that reversing transcriptional alterations induced by mutant TBP and reducing the expression of mutant TBP are promising strategies to treat SCA17.

Src inhibition attenuates polyglutamine-mediated neuromuscular degeneration in spinal and bulbar muscular atrophy

Spinal and bulbar muscular atrophy (SBMA) is a neuromuscular disease caused by an expanded CAG repeat in the androgen receptor (AR) gene. Here, we perform a comprehensive analysis of signaling pathways in a mouse model of SBMA (AR-97Q mice) utilizing a phosphoprotein assay. We measure the levels of 17 phosphorylated proteins in spinal cord and skeletal muscle of AR-97Q mice at three stages. The level of phosphorylated Src (p-Src) is markedly increased in the spinal cords and skeletal muscles of AR-97Q mice prior to the onset. Intraperitoneal administration of a Src kinase inhibitor improves the behavioral and histopathological phenotypes of the transgenic mice. We identify p130Cas as an effector molecule of Src and show that the phosphorylated p130Cas is elevated in murine and cellular models of SBMA. These results suggest that Src kinase inhibition is a potential therapy for SBMA.

Muscle pathology of hereditary motor and sensory neuropathy with proximal dominant involvement with TFG mutation

BACKGROUND

Hereditary motor and sensory neuropathy with proximal dominant involvement (HMSN-P) is characterized by adult onset, a slowly progressive course and autosomal dominant inheritance. It remains unclear whether myopathic changes occur histopathologically.

METHODS

We encountered 2 patients in a family with a heterozygous p.P285L mutation in TRK-fused gene (TFG), which is known to cause HMSN-P. The affected individuals developed proximal-dominant muscle weakness in their 40s, which slowly progressed to a motor neuron disease-like phenotype.

RESULTS

Muscle biopsy showed myopathic pathology including fiber size variability, increased internal nuclei, fiber splitting, and core-like structures, associated with neurogenic changes: large groups of atrophic fibers and fiber type-grouping. Immunohistochemistry revealed sarcoplasmic aggregates of TFG, TDP-43, and p62 without congophilic material.

CONCLUSIONS

The present study demonstrates myopathic changes in HMSN-P. Although the mechanisms underlying the skeletal muscle involvement remain to be elucidated, immunohistochemistry suggests that abnormal protein aggregation may be involved in the myopathic pathology.

The Regulation of the Small Heat Shock Protein B8 in Misfolding Protein Diseases Causing Motoneuronal and Muscle Cell Death

Misfolding protein diseases are a wide class of disorders in which the aberrantly folded protein aggregates accumulate in affected cells. In the brain and in the skeletal muscle, misfolded protein accumulation induces a variety of cell dysfunctions that frequently lead to cell death. In motoneuron diseases (MNDs), misfolded proteins accumulate primarily in motoneurons, glial cells and/or skeletal muscle cells, altering motor function. The deleterious effects of misfolded proteins can be counteracted by the activity of the protein quality control (PQC) system, composed of chaperone proteins and degradative systems. Here, we focus on a PQC system component: heat shock protein family B (small) member 8 (HSPB8), a chaperone induced by harmful stressful events, including proteotoxicity. In motoneuron and muscle cells, misfolded proteins activate HSPB8 transcription and enhance HSPB8 levels, which contributes to prevent aggregate formation and their harmful effects. HSPB8 acts not only as a chaperone, but also facilitates the autophagy process, to enable the efficient clearance of the misfolded proteins. HSPB8 acts as a dimer bound to the HSP70 co-chaperone BAG3, a scaffold protein that is also capable of binding to HSP70 (associated with the E3-ligase CHIP) and dynein. When this complex is formed, it is transported by dynein to the microtubule organization center (MTOC), where aggresomes are formed. Here, misfolded proteins are engulfed into nascent autophagosomes to be degraded via the chaperone-assisted selective autophagy (CASA). When CASA is insufficient or impaired, HSP70 and CHIP associate with an alternative co-chaperone, BAG1, which routes misfolded proteins to the proteasome for degradation. The finely tuned equilibrium between proteasome and CASA activity is thought to be crucial for maintaining the functional cell homeostasis during proteotoxic stresses, which in turn is essential for cell survival. This fine equilibrium seems to be altered in MNDs, like Amyotrophic lateral sclerosis (ALS) and spinal and bulbar muscular atrophy (SBMA), contributing to the onset and the progression of disease. Here, we will review how misfolded proteins may affect the PQC system and how the proper activity of this system can be restored by boosting or regulating HSPB8 activity, with the aim to ameliorate disease progression in these two fatal MNDs.

Low-Cost Gait Analysis for Behavioral Phenotyping of Mouse Models of Neuromuscular Disease

Measurement of animal locomotion is a common behavioral tool used to describe the phenotype of a given disease, injury, or drug model. The low-cost method of gait analysis demonstrated here is a simple but effective measure of gait abnormalities in murine models. Footprints are analyzed by painting a mouse's feet with non-toxic washable paint and allowing the subject to walk through a tunnel on a sheet of paper. The design of the testing tunnel takes advantage of natural mouse behavior and their affinity for small dark places. The stride length, stride width, and toe spread of each mouse is easily measured using a ruler and a pencil. This is a well-established and reliable method, and it generates several metrics that are analogous to digital systems. This approach is sensitive enough to detect changes in stride early in phenotype presentation, and due to its non-invasive approach, it allows for testing of groups across life-span or phenotypic presentation.

Androgen receptor antagonism accelerates disease onset in the SOD1G93A mouse model of amyotrophic lateral sclerosis

BACKGROUND AND PURPOSE

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease typically more common in males, implicating androgens in progression of both patients and mouse models. Androgen effects are mediated by androgen receptor which is highly expressed in spinal motor neurons and skeletal muscles. To clarify the role of androgen receptors in ALS, we therefore examined the effect of androgen receptor antagonism in the SOD1G93A mouse model.

EXPERIMENTAL APPROACH

The androgen receptor antagonist, flutamide, was administered to presymptomatic SOD1G93A mice as a slow-release subcutaneous implant (5 mg·day-1 ). Testosterone, flutamide, and metabolite levels were measured in blood and spinal cord tissue by LC-MS-MS. Effects on disease onset and progression were assessed using motor function tests, survival, muscle, and neuropathological analyses.

KEY RESULTS

Flutamide was metabolised to 2-hydroxyflutamide achieving steady-state plasma levels across the study duration and reached the spinal cord at pharmacologically active concentrations. Flutamide treatment accelerated disease onset and locomotor dysfunction in male SOD1G93A mice, but not female mice, without affecting survival. Analysis of hindlimb muscles revealed exacerbation of myofibre atrophy in male SOD1G93A mice treated with flutamide, although motor neuron pathology was not affected.

CONCLUSION AND IMPLICATIONS

The androgen receptor antagonist accelerated disease onset in male SOD1G93A mice, leading to exacerbated muscle pathology, consistent with a role of androgens in modulating disease severity, sexual dimorphism, and peripheral pathology in ALS. These results also demonstrate a key contribution of skeletal muscle pathology to disease onset, but not outcome, in this mouse model of ALS.

DNA methylation inhibitor attenuates polyglutamine-induced neurodegeneration by regulating Hes5

Spinal and bulbar muscular atrophy (SBMA) is a polyglutamine-mediated neuromuscular disease caused by a CAG repeat expansion in the androgen receptor (AR) gene. While transcriptional dysregulation is known to play a critical role in the pathogenesis of SBMA, the underlying molecular pathomechanisms remain unclear. DNA methylation is a fundamental epigenetic modification that silences the transcription of various genes that have a CpG-rich promoter. Here, we showed that DNA methyltransferase 1 (Dnmt1) is highly expressed in the spinal motor neurons of an SBMA mouse model and in patients with SBMA. Both genetic Dnmt1 depletion and treatment with RG108, a DNA methylation inhibitor, ameliorated the viability of SBMA model cells. Furthermore, a continuous intracerebroventricular injection of RG108 mitigated the phenotype of SBMA mice. DNA methylation array analysis identified hairy and enhancer of split 5 (Hes5) as having a CpG island with hyper-methylation in the promoter region, and the Hes5 expression was strongly silenced in SBMA. Moreover, Hes5 over-expression rescued the SBMA cells possibly by inducing Smad2 phosphorylation. Our findings suggest DNA hyper-methylation underlies the neurodegeneration in SBMA.