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Open Access Publications from the University of California

Scholarly Works (7 results)

  • Thesis
  • Peer Reviewed
UC San Diego Electronic Theses and Dissertations (2022)

Spinal and Bulbar Muscular Atrophy (SBMA) is a rare neuromuscular disorder caused by a CAG repeat expansion mutation in first exon of the Androgen Receptor (AR) gene. The mutant protein is thus referred to as polyQ-AR. AR’s natural ligand, testosterone, is required for polyQ-AR to cause neuromuscular decline, thus only men are clinically affected. Historically, SBMA has been considered a motor neuron disease, but recent works from our group and others implicate skeletal muscle as a primary site of pathogenicity caused by polyQ-AR. Given that AR is a potent facilitator of skeletal muscle hypertrophy, but polyQ-AR causes progressive muscle atrophy starting in the third of fourth decade of life despite normal male development, the differences between AR and polyQ-AR molecular activity in skeletal muscle are particularly intriguing and remain elusive. Moreover, we have previously shown that polyQ-AR expression in skeletal muscle of SBMA mice is required for motor neuron pathology, which suggests a skeletal muscle-driven mechanism for motor neuron degeneration in SBMA. The neuromuscular junction (NMJ), which is a large and highly specialized synapse between motor neurons and skeletal muscle, is likely the main site of pathogenicity driving this non-cell autonomous motor neuron demise. This dissertation first provides an overview of SBMA and a published review on what is currently known about NMJ involvement in motor neuron disease. Next, I present findings in aged muscle rescued SBMA mice that develop latent phenotypes that are highly relevant to SBMA patients but have not previously been explored in animal models. In parallel, I show that motor neuron rescued SBMA mice do not experience any notable attenuation of neuromuscular decline, definitively highlighting the central role of polyQ-AR expression in skeletal muscle in driving neuromuscular decline in SBMA. Lastly, transcriptome analysis and metabolic profiling of pre-symptomatic SBMA mice provides important clues as to how polyQ-AR could initiate muscle pathology at the molecular level. Taken together, this dissertation reinforces the timely and important paradigm shift in the field toward investigating skeletal muscle, not motor neurons, as therapeutic targets in SBMA and prompts the reclassification of this repeat expansion disorder as a neuromuscular disease or even a primary myopathy

    • Article
    • Peer Reviewed
    UC San Diego Previously Published Works (2017)
    Motor neuron (MN) diseases are progressive disorders resulting from degeneration of neuromuscular junctions (NMJs), which form the connection between MNs and muscle fibers. NMJ-in-a-dish models have been developed to examine human MN-associated dysfunction with disease; however such coculture models have randomly oriented myotubes with immature synapses that contract asynchronously. Mechanically patterned (MP) extracellular matrix with alternating soft and stiff stripes improves current NMJ-in-a-dish models by inducing both mouse and human myoblast durotaxis to stripes where they aligned, differentiated, and fused into patterned myotubes. Compared to conventional culture on rigid substrates or unpatterned hydrogels, MP substrates supported increased differentiation and fusion, significantly larger acetylcholine (ACh) receptor clusters, and increased expression of MuSK and Lrp4, two cell surface receptors required for NMJ formation. Robust contractions were observed when mouse myotubes were stimulated by ACh, with twitch duration and frequency most closely resembling those for mature muscle on MP substrates. Fused myotubes, when cocultured with MNs, were able to form even larger NMJs. Thus MP matrices produce more functionally active NMJs-in-a-dish, which could be used to elucidate disease pathology and facilitate drug discovery.
      Cover page: Mechanically patterned neuromuscular junctions-in-a-dish have improved functional maturation
      • Article
      • Peer Reviewed
      Muscle stem cells (MuSCs) exhibit distinct behavior during successive phases of developmental myogenesis. However, how their transition to adulthood is regulated is poorly understood. Here, we show that fetal MuSCs resist progenitor specification and exhibit altered division dynamics, intrinsic features that are progressively lost postnatally. After transplantation, fetal MuSCs expand more efficiently and contribute to muscle repair. Conversely, niche colonization efficiency increases in adulthood, indicating a balance between muscle growth and stem cell pool repopulation. Gene expression profiling identified several extracellular matrix (ECM) molecules preferentially expressed in fetal MuSCs, including tenascin-C, fibronectin, and collagen VI. Loss-of-function experiments confirmed their essential and stage-specific role in regulating MuSC function. Finally, fetal-derived paracrine factors were able to enhance adult MuSC regenerative potential. Together, these findings demonstrate that MuSCs change the way in which they remodel their microenvironment to direct stem cell behavior and support the unique demands of muscle development or repair.
        Cover page: Autonomous Extracellular Matrix Remodeling Controls a Progressive Adaptation in Muscle Stem Cell Regenerative Capacity during Development
        • Article
        • Peer Reviewed
        UC Irvine Previously Published Works (2023)
        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.
          Cover page: 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.
          • Article
          • Peer Reviewed
          UC Irvine Previously Published Works (2022)
          Amyotrophic lateral sclerosis (ALS) is a heterogenous neurodegenerative disorder that affects motor neurons and voluntary muscle control1. ALS heterogeneity includes the age of manifestation, the rate of progression and the anatomical sites of symptom onset. Disease-causing mutations in specific genes have been identified and define different subtypes of ALS1. Although several ALS-associated genes have been shown to affect immune functions2, whether specific immune features account for ALS heterogeneity is poorly understood. Amyotrophic lateral sclerosis-4 (ALS4) is characterized by juvenile onset and slow progression3. Patients with ALS4 show motor difficulties by the time that they are in their thirties, and most of them require devices to assist with walking by their fifties. ALS4 is caused by mutations in the senataxin gene (SETX). Here, using Setx knock-in mice that carry the ALS4-causative L389S mutation, we describe an immunological signature that consists of clonally expanded, terminally differentiated effector memory (TEMRA) CD8 T cells in the central nervous system and the blood of knock-in mice. Increased frequencies of antigen-specific CD8 T cells in knock-in mice mirror the progression of motor neuron disease and correlate with anti-glioma immunity. Furthermore, bone marrow transplantation experiments indicate that the immune system has a key role in ALS4 neurodegeneration. In patients with ALS4, clonally expanded TEMRA CD8 T cells circulate in the peripheral blood. Our results provide evidence of an antigen-specific CD8 T cell response in ALS4, which could be used to unravel disease mechanisms and as a potential biomarker of disease state.
            Cover page: Clonally expanded CD8 T cells characterize amyotrophic lateral sclerosis-4
            • Article
            • Peer Reviewed
            UC Irvine Previously Published Works (2023)
            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.
              Cover page: Skeletal muscle TFEB signaling promotes central nervous system function and reduces neuroinflammation during aging and neurodegenerative disease.
              • Article
              • Peer Reviewed
              UC Irvine Previously Published Works (2022)
                Cover page: Author Correction: Clonally expanded CD8 T cells characterize amyotrophic lateral sclerosis-4Creative Commons 'BY' version 4.0 license
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