Biology subjects

Fallon, J. R.

Publications and source records attributed to Fallon, J. R..

5 recordsLinked to original sources

bioRxiv2023

The MuSK-BMP pathway regulates synaptic Nav1.4 localization and muscle excitability

The neuromuscular junction (NMJ) is the linchpin of nerve-evoked muscle contraction. Broadly considered, the function of the NMJ is to transduce a nerve action potential into a muscle fiber action potential (MFAP). Efficient information transfer requires both cholinergic signaling, responsible for the generation of endplate potentials (EPPs), and excitation, the activation of postsynaptic voltage-gated sodium channels (Nav1.4) to trigger MFAPs. In contrast to the cholinergic apparatus, the signaling pathways that organize Nav1.4 and muscle fiber excitability are poorly characterized. Muscle-specific kinase (MuSK), in addition to its Ig1 domain-dependent role as an agrin-LRP4 receptor, is also a BMP co-receptor that binds BMPs via its Ig3 domain and shapes BMP-induced signaling and transcriptional output. Here we probed the function of the MuSK-BMP pathway at the NMJ using mice lacking the MuSK Ig3 domain ( {Delta}Ig3-MuSK). Synapses formed normally in {Delta}Ig3-MuSK animals, but the postsynaptic apparatus was fragmented from the first weeks of life. Anatomical denervation was not observed at any age examined. Moreover, spontaneous and nerve-evoked acetylcholine release, AChR density, and endplate currents were comparable to WT. However, trains of nerve-evoked MFAPs in {Delta}Ig3-MuSK muscle were abnormal as revealed by increased jitter and blocking in single fiber electromyography. Further, nerve-evoked compound muscle action potentials (CMAPs), as well as twitch and tetanic muscle torque force production, were also diminished. Finally, Nav1.4 levels were reduced at {Delta}Ig3-MuSK synapses but not at the extrajunctional sarcolemma, indicating that the observed excitability defects are the result of impaired localization of this voltage-gated ion channel at the NMJ. We propose that MuSK plays two distinct roles at the NMJ: as an agrin-LRP4 receptor necessary for establishing and maintaining cholinergic signaling, and as a BMP co-receptor required for maintaining proper Nav1.4 density, nerve-evoked muscle excitability and force production. The MuSK-BMP pathway thus emerges as a target for modulating excitability and functional innervation, which are defective in conditions such as congenital myasthenic syndromes and aging. Significance StatementThe neuromuscular junction (NMJ) is required for nerve-evoked muscle contraction and movement, and its function is compromised during aging and disease. Although the mechanisms underlying neurotransmitter release and cholinergic response at this synapse have been studied extensively, the machinery necessary for nerve-evoked muscle excitation are incompletely characterized. We show that MuSK (Muscle-specific kinase), in its role as a BMP co-receptor, regulates NMJ structure as well as the localization of the voltage-gated sodium channels necessary for full nerve-evoked muscle fiber excitation and force production. This novel function of MuSK is structurally and mechanistically distinct from its role in organizing cholinergic machinery. The MuSK-BMP pathway thus presents a new opportunity to understand mechanisms that may preserve or enhance neuromuscular excitability in the face of aging and disease.

neuroscience↗

bioRxiv2023

MuSK-BMP signaling in adult muscle stem cells maintains quiescence and regulates myofiber size

A central question in adult stem cell biology is elucidating the signaling pathways regulating their dynamics and function in diverse physiological and age-related contexts. Muscle stem cells in adults (Satellite Cells; SCs) are generally quiescent but can activate and contribute to muscle repair and growth. Here we tested the role of the MuSK-BMP pathway in regulating adult SC quiescence by deletion of the BMP-binding MuSK Ig3 domain ( {Delta}Ig3-MuSK). At 3 months of age SC and myonuclei numbers and myofiber size were comparable to WT. However, at 5 months of age SC density was decreased while myofiber size, myonuclear number and grip strength were increased - indicating that SCs had activated and productively fused into the myofibers over this interval. Transcriptomic analysis showed that SCs from uninjured {Delta}Ig3-MuSK mice exhibit signatures of activation. Regeneration experiments showed that {Delta}Ig3-MuSK SCs maintain full stem cell function. Expression of {Delta}Ig3-MuSK in adult SCs was sufficient to break quiescence and increase myofiber size. We conclude that the MuSK-BMP pathway regulates SC quiescence and myofiber size in a cell autonomous, age-dependent manner. Targeting MuSK-BMP signaling in muscle stem cells thus emerges a therapeutic strategy for promoting muscle growth and function in the settings of injury, disease, and aging. HighlightsO_LIMuSK, in its role as a BMP co-receptor, regulates adult muscle stem cell quiescence C_LIO_LIThe MuSK-BMP pathway acts cell autonomously C_LIO_LIIncreased muscle size and function with preservation of myonuclear density and stemness in mice with attenuated MuSK-BMP signaling C_LI

cell biology↗

bioRxiv2023

Up-regulation of cholesterol synthesis pathways and limited neurodegeneration in a knock-in Sod1 mutant mouse model of ALS.

Amyotrophic lateral sclerosis (ALS) is a severe neurodegenerative disorder affecting brain and spinal cord motor neurons. Mutations in the copper/zinc superoxide dismutase gene (SOD1) are associated with [~]20% of inherited and 1-2% of sporadic ALS cases. Much has been learned from mice expressing transgenic copies of mutant SOD1, which typically involve high-level transgene expression, thereby differing from ALS patients expressing one mutant gene copy. To generate a model that more closely represents patient gene expression, we created a knock-in point mutation (G85R, a human ALS-causing mutation) in the endogenous mouse Sod1 gene, leading to mutant SOD1G85R protein expression. Heterozygous Sod1G85R mutant mice resemble wild type, whereas homozygous mutants have reduced body weight and lifespan, a mild neurodegenerative phenotype, and express very low mutant SOD1 protein levels with no detectable SOD1 activity. Homozygous mutants exhibit partial neuromuscular junction denervation at 3-4 months of age. Spinal cord motor neuron transcriptome analyses of homozygous Sod1G85R mice revealed up-regulation of cholesterol synthesis pathway genes compared to wild type. Transcriptome and phenotypic features of these mice are similar to Sod1 knock-out mice, suggesting the Sod1G85R phenotype is largely driven by loss of SOD1 function. By contrast, cholesterol synthesis genes are down-regulated in severely affected human TgSOD1G93A transgenic mice at 4 months. Our analyses implicate dysregulation of cholesterol or related lipid pathway genes in ALS pathogenesis. The Sod1G85R knock-in mouse is a useful ALS model to examine the importance of SOD1 activity in control of cholesterol homeostasis and motor neuron survival. SIGNIFICANCE STATEMENTAmyotrophic lateral sclerosis is a devastating disease involving the progressive loss of motor neurons and motor function for which there is currently no cure. Understanding biological mechanisms leading to motor neuron death is critical for developing new treatments. Using a new knock-in mutant mouse model carrying a Sod1 mutation that causes ALS in patients, and in the mouse, causes a limited neurodegenerative phenotype similar to Sod1 loss-of-function, we show that cholesterol synthesis pathway genes are up-regulated in mutant motor neurons, whereas the same genes are down-regulated in transgenic SOD1 mice with a severe phenotype. Our data implicate dysregulation of cholesterol or other related lipid genes in ALS pathogenesis and provide new insights that could contribute to strategies for disease intervention.

neuroscience↗

bioRxiv2022

The MuSK-BMP pathway maintains myofiber size in slow muscle through regulation of Akt-mTOR signaling

Myofiber size regulation is critical in health, disease, and aging. MuSK (muscle-specific kinase) is a BMP (bone morphogenetic protein) co-receptor that promotes and shapes BMP signaling. MuSK is expressed at all neuromuscular junctions and is also present extrasynaptically in the slow soleus muscle. To investigate the role of the MuSK-BMP pathway in vivo we generated mice lacking the BMP-binding MuSK Ig3 domain. These {Delta}Ig3-MuSK mice are viable and fertile with innervation levels comparable to wild type. In 3-month-old mice myofibers are smaller in the slow soleus, but not in the fast tibialis anterior (TA). Transcriptomic analysis revealed soleus-selective decreases in RNA metabolism and protein synthesis pathways as well as dysregulation of IGF1-Akt-mTOR pathway components. Biochemical analysis showed that Akt-mTOR signaling is reduced in soleus but not TA. We propose that the MuSK-BMP pathway acts extrasynaptically to maintain myofiber size in slow muscle by promoting protein synthetic pathways including IGF1-Akt-mTOR signaling. These results reveal a novel mechanism for regulating myofiber size in slow muscle and introduce the MuSK-BMP pathway as a target for promoting muscle growth and combatting atrophy.

cell biology↗

bioRxiv2018

The FXR2P low complexity domain drives assembly of multiple fibril types with differing ribosome association in neurons

RNA binding proteins (RBPs) typically function in higher order assemblages to regulate RNA localization and translation. The Fragile X homolog FXR2P is an RBP essential for formation of Fragile X granules, which associate with axonal mRNA and ribosomes in the intact brain. Here we performed an unbiased EGFP insertional mutagenesis screen to probe for FXR2P domains important for assembly into higher order structural states in neurons. Fifteen of the 18 unique in-frame FXR2PEGFP fusions tested formed cytosolic granules. However, EGFP insertion within a 23 amino acid region of the low complexity (LC) domain induced formation of distinct FXR2PEGFP fibrils (A and B) that were found in isolation or assembled into highly ordered bundles. Type A and B complexes exhibited different developmental timelines, ultrastructure and ribosome association with ribosomes absent from bundled Type B fibrils. The formation of both fibril types was dependent on an intact RNA binding domain. We conclude that formation of these higher order FXR2P assemblages with alternative structural and compositional states in neurons requires collaboration between the LC and RNA binding domains.\n\nSummary StatementThe Fragile X protein FXR2P forms multiple types of fibrillar assemblages with differential ribosome associations in neurons through cooperation between its RNA binding and LC domains.

neuroscience↗