Sphingolipids are a conserved family of lipids built upon a sphingoid base backbone. Sphingolipids serve structural membrane functions, whereas their metabolism produces signaling molecules involved in regulating mammalian development, immune function, inflammation and cellular stress responses. Studies supported by this grant have explored the role of sphingolipids in the model organism Drosophila melanogaster. These studies have resulted in the chemical characterization of Drosophila sphingoid bases and the identification of novel endogenous Drosophila sphingolipids with potent growth-inhibitory activity. Further, we have shown that Sply mutants lacking expression of Drosophila sphingosine-1-phosphate (S1P) lyase (SPL), which is responsible for the final step of sphingolipid degradation, accumulate sphingolipid intermediates and develop a progressive myopathy in the thoracic muscles needed to power flight. The Sply myopathy is corrected by reducing sphingolipid production, indicating sphingolipid intermediates play a causative role. We have observed that mutants in key membrane proteins such as dynamin and dystroglycan phenocopy the Sply myopathy. We have also conducted cell-based investigations that suggest that Sply/SPL is required for normal AKT signaling, protein translation, myoblast fusion, myoblast gene expression and control of autophagy. Many of these interactions have been corroborated in murine C2C12 cells, indicating that SPL plays a conserved role in myoblast survival, fusion and differentiation. We have also found that sphingosine kinase and S1P lyase are dynamically upregulated during murine muscle regeneration, leading to a transient peak in S1P levels in regenerating muscle. These collective observations have led us to propose our central hypothesis, which states that SPL plays a critical role in muscle biology, development and homeostasis.
The specific aims of our proposal are, thus: 1) To define the role of SPL in muscle cell biology;2) To dissect the role of SPL in muscle development;and 3) To establish the role of SPL in muscle atrophy and regeneration. Our long-term goals are to exploit Drosophila to elucidate the role of sphingolipids in biology, membrane function, and tissue homeostasis and to define the relevance of these findings to human disease. Due to species-specific structural differences in sphingolipids and the lack of S1P receptors in lower eukaryotes, we do not expect the two systems to be equivalent. Thus, our goals are to compare the biochemical and molecular events associated with SPL loss in Drosophila and murine cell and animal models, with the intent of developing a comprehensive understanding of how SPL functions in the context of muscle tissue and how its function may have been modified throughout evolution. These studies should be readily achieved by our team, which has extensive experience in sphingolipid biochemistry, Drosophila genetics, and the genetics and pathology of MD.
The studies proposed in this application will elucidate mechanisms by which sphingolipids influence key processes involved in muscle development and regeneration, including myogenic gene expression, membrane fusion, satellite cell activation, muscle membrane structure and extracellular matrix processing. Our research plan includes studies to specifically address the translational relevance of S1P metabolism in mammalian muscle injury, atrophy and regeneration using preclinical rodent models. The information gained from this work is therefore relevant to muscle-wasting syndromes, age-related frailty, inherited muscular dystrophies and related musculoskeletal disorders arising from defects of extracellular matrix remodeling. Skeletal muscle is also a prime target organ for gene therapy, since engineered myoblasts can be made to fuse with mature muscle, generating a stable hybrid organ within the adult. Thus, our studies of myoblast fusion may be relevant to an array of disorders potentially treatable by genetic therapy. Our proposed studies will also characterize how sphingolipids interact with the PI3K/AKT pathway, a key membrane-localized signaling hub that regulates cell survival, protein synthesis, and autophagy and gene expression. Our main focus is on how sphingolipids influence this pathway in muscle. However, understanding endogenous regulators of the AKT pathway is relevant to the vastly important medical problem of insulin- resistance and metabolic syndrome, whereas devising new pharmacological agents to inhibit PI3K/AKT signaling is a major objective in the treatment of cancer. Excessive autophagy underlies many human disease states including neurodegenerative diseases, cardiomyopathies, cancer, programmed cell death, and bacterial and viral infections. Thus, our research plan has potentially broad translational significance.
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