2020 Research Updates
Priorities in Managing KD Symptoms
Luca Zampedri, University College London, presents results of a KD patient survey "Priorities in Managing KD Symptoms" that he conducted prior to the KDA/KD-UK joint conference in 2020. To view the video, click here.
Deubiquitinase USP7 contributes to the pathogenicity of spinal and bulbar muscular atrophy
We found that the deubiquitinase USP7 preferentially interacts with polyQ-expanded AR, and that lowering USP7 levels reduced mutant AR aggregation and cytotoxicity in cell models of SBMA. Moreover, USP7 knockdown suppressed disease phenotypes in SBMA and spinocerebellar ataxia type 3 (SCA3) fly models, and monoallelic knockout of Usp7 ameliorated several motor deficiencies in transgenic SBMA mice. USP7 overexpression resulted in reduced AR ubiquitination, indicating the direct action of USP7 on AR. To learn more, click here.
Split hand and motor axonal hyperexcitability in spinal and bulbar muscular atrophy
Split hand is not specific to ALS and can be caused by the peripheral mechanism alone in SBMA, whereas the effect of upper motor neuron lesion cannot be excluded in ALS. Our results also suggest that SBMA and ALS share common axonal excitability changes; increased nodal persistent sodium and reduced potassium currents that may accelerate motor neuronal death and differently affect axons-innervating different muscles. Ion channel modulators could be a therapeutic option for both SBMA and ALS. To learn more, click here.
Disease mechanism, biomarker and therapeutics for spinal and bulbar muscular atrophy (SBMA)
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 fasttwitch 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. To learn more, click here.
Androgen Receptor Genetic Variant Predicts COVID-19 Disease Severity: A Prospective Longitudinal Study of Hospitalized COVID-19 Male Patients
Our data suggest that longer AR CAG score is associated with more severe COVID-19 disease. In some androgen mediated disease, short CAG has been associate with worse prognosis, e.g., in prostate cancer.6 However, in skeletal muscle, a long CAG repeat length produces higher androgen mediated activity.7 We believe this discrepancy can be explained by the tissue dependent expression of co-factors important for activation of the androgen response element (ARE).8 For example, protein arginine methyltransferase 6 has been shown to be highly expressed in lung and has been shown to be a specific co-activator of the androgen receptor.9 The results of this study suggest that the AR CAG repeat length could potentially be used as a biomarker to identify male COVIID-19 patients at risk for ICU admissions. To learn more, click here.
ClC-2-like Chloride Current Alterations in a Cell Model of Spinal and Bulbar Muscular Atrophy, a Polyglutamine Disease
Here, we identified and characterized chloride currents most likely belonging to the chloride channel-2 (ClC-2) subfamily, which showed significantly increased amplitudes in the SBMA cells. The treatment with the pituitary adenylyl cyclase-activating polypeptide (PACAP), a neuropeptide with a proven protective effect in a mouse model of SBMA, recovered chloride channel current alterations in SBMA cells. These observations suggest that the CIC-2 currents are affected in SBMA, an alteration that may contribute and potentially determine the pathophysiology of the disease. Learn more by clicking here.
Kennedy’s disease: an under‑recognized motor neuron disorder
Kennedy’s disease or spinal bulbar muscular atrophy is a rare, inherited and slowly progressive multisystem disease mostly manifesting with a motor neuron disease phenotype leading to disability. The slow progression, partial androgen insensitivity, electrophysiological evidence of sensory neuronopathy, and relatively spared central nervous system pathways help differentiate it from amyotrophic lateral sclerosis. To date, there is no treatment or cure with clinical care mainly focused on accurate diagnosis, symptom management, patient education, and genetic counselling. To learn more, click here.
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). To learn more, click here.
Linking epigenetic dysregulation, mitochondrial impairment, and metabolic dysfunction in SBMA motor neurons
Overall, our study indicates a molecular link between mitochondrial function, metabolism and epigenetic regulation that may be important for motor neuron survival. Our analysis suggests that SBMA motor neurons are sensitive to insufficient metabolic adaptation and its exacerbation of mitochondrial dysfunction, which then leads to neurodegeneration. We propose that epigenetic dysregulation of the metabolic genes contributes to impairment of mitochondrial ATP production through perturbation of compensatory metabolic pathways and insufficient substrate production in response to disrupted cellular energy homeostasis.
The reversibility of this process provides opportunities for therapeutic intervention through metabolic pathways that impact mitochondrial function in SBMA and perhaps other neurodegenerative diseases. To read more, click here.
Myoglobin: A new biomarker for spinal and bulbar muscular atrophy?
This study showed that the serum Myo levels increased in all patients with SBMA, but serum Myo levels in all patients with ALS were within the normal range.
Our study also showed that there was a correlation between the Myo level and the course of disease in patients with SBMA, which seems to suggest that Myo can be more sensitive in reflecting the extent of damage of muscle fibers in patients with SBMA than CK.
We believe that serum Myo levels can be used in the differential diagnosis and for prognosis in patients with SBMA, as well as to evaluate efficacy of therapeutic interventions. To read more, click here.
Wide range of reduced penetrance alleles in spinal and bulbar muscular atrophy: a model based approach
Our analysis reveals an unexpectedly high frequency of expanded SBMA-associated alleles, with (CAG)n ≥35 present in 107/100,000 and (CAG)n ≥38 present in 27/100,000 of the general population. We argue against the cut-off model for the penetrance of SBMA and suggest that penetrance gradually increases from 35 to approximately 46 repeats, above which it reaches a plateau approaching maximum value. Therefore, asymptomatic men of the general population with no/ unknown SBMA family history are free of risk when carrying (CAG)n ≤34, are at intermediate but increasing risk for developing SBMA (with greater risk for longer alleles) when carrying (CAG)n ≈35–46 and have close to 100% risk of developing the disease when carrying (CAG)n ≥47. To read the article, click here.
Sonic Hedgehog-Gli1 Signaling and Cellular Retinoic Acid Binding Protein 1 Gene Regulation in Motor Neuron Differentiation and Diseases
We employed the same strategy to engineer healthy vs. degenerated MN1 cells to model SBMA neurons . MN1 cells containing AR-24Q (control) remain healthy, whereas MN1 cells containing the diseased version, AR-65Q, become degenerated in cultures As shown in Figure 2b, the diseased SBMA/MN1 neurons, AR-65Q, also have a dramatically reduced CRABP1 level, as compared to the healthy control, AR-24Q. These results, in two motor neuron disease models, consistently show that Crabp1 gene activity is positively correlated with a healthy state in motor neurons, whereas down regulation of the Crabp1 gene is correlated with degeneration in motor neurons, such as those in ALS and SBMA. To read the article, click here.
Arginine is a disease modifier for polyQ disease models that stabilizes polyQ protein conformation
In this study, we identified arginine as a potent polyQ aggregation inhibitor that acts by inhibiting the formation of misfolded and oligomeric toxic protein species before the formation of insoluble aggregates. We also confirmed its therapeutic effects on neurological symptoms and protein aggregation pathology using two different animal models of polyQ diseases. In addition, we showed that arginine may exert a therapeutic effect on the dendritic arborization of the Purkinje cell in the cerebellum. To read the article, click here.
Muscle BDNF: A Potential Therapeutic Target for Kennedy’s Disease
Novel to this study, the authors developed a 97Q mouse with Cre-dependent overexpression of BDNF (97Q/BDNF) specifically in muscle. This resulted in significant increases in muscle-specific BDNF expression by 150-fold in the fast-twitch tibialis anterior (TA) and 45-fold in the slow-twitch soleus. The hang test, used to determine disease progression, demonstrates neuromuscular impairment and motor coordination in mouse models by measuring how long a mouse can hold their bodyweight using an overhanging bar. In the current study, disease onset was defined as a hang time < 120 seconds for two consecutive days and a hang-time < 30 seconds represented disease end-stage. Overexpression of BDNF was found to significantly increase time to disease onset, endstage and further, doubled survival time. To read the article, click here.
Cell-Clearing Systems Bridging Repeat Expansion Proteotoxicity and Neuromuscular Junction Alterations in ALS and SBMA
The coordinated activities of autophagy and the ubiquitin proteasome system (UPS) are key to preventing the aggregation and toxicity of misfold-prone proteins which manifest in a number of neurodegenerative disorders. These include proteins which are encoded by genes containing nucleotide repeat expansions. In the present review we focus on the overlapping role of autophagy and the UPS in repeat expansion proteotoxicity associated with chromosome 9 open reading frame 72 (C9ORF72) and androgen receptor (AR) genes, which are implicated in two motor neuron disorders, amyotrophic lateral sclerosis (ALS) and spinal-bulbar muscular atrophy (SBMA), respectively. At baseline, both C9ORF72 and AR regulate autophagy, while their aberrantly-expanded isoforms may lead to a failure in both autophagy and the UPS, further promoting protein aggregation and toxicity within motor neurons and skeletal muscles. Besides proteotoxicity, autophagy and UPS alterations are also implicated in neuromuscular junction (NMJ) alterations, which occur early in both ALS and SBMA. In fact, autophagy and the UPS intermingle with endocytic/secretory pathways to regulate axonal homeostasis and neurotransmission by interacting with key proteins which operate at the NMJ, such as agrin, acetylcholine receptors (AChRs), and adrenergic beta2 receptors (B2-ARs). Thus, alterations of autophagy and the UPS configure as a common hallmark in both ALS and SBMA disease progression. To read the article, click here.
Muscle BDNF improves synaptic and contractile muscle strength in Kennedy's disease mice in a muscle‐type specific manner.
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. To read the article, click here.
MEF2 impairment underlies skeletal muscle atrophy in polyglutamine disease
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. To read the article, click here.
Enhanced Clearance of Neurotoxic Misfolded Proteins by the Natural Compound Berberine and Its Derivatives
We tested the natural alkaloid berberine (BBR) and some derivatives for their capability to enhance misfolded protein clearance in cell models of NDs, evaluating which degradative pathway mediates their action. Results: We found that both BBR and its semisynthetic derivatives promote degradation of mutant androgen receptor (ARpolyQ) causative of spinal and bulbar muscular atrophy, acting mainly via proteasome and preventing ARpolyQ aggregation. To read the article, click here.
Deterioration of muscle force and contractile characteristics are early pathological events in spinal and bulbar muscular atrophy mice
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, prior to any motor neuron degeneration, which only occurs in late stage disease. These findings reveal 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. To read the article. Click here.
Unveiling synapse pathology in spinal bulbar muscular atrophy by genome-wide transcriptome analysis of purified motor neurons derived from disease specific iPSCs
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. ...we established disease specific iPSCs from four SBMA patients, and differentiated them into spinal motor neurons. The results revealed the involvement of the pathology associated with synapses, epigenetics, and endoplasmic reticulum (ER) in SBMA. To read the article. Click here.
Polyglutamine-Expanded Androgen Receptor Alteration of Skeletal Muscle Homeostasis and Myonuclear Aggregation Are Affected by Sex, Age and Muscle Metabolism
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.
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. To read the article. Click here.