Elucidating novel molecular mechanisms of irisin-mediated effects via integrin

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PROJECT SUMMARY: Exercise benefits the body in many ways. The functions of skeletal muscle, brain, liver, bone, adipose tissue and heart all gain from various types of physical activity and training. People suffering from disabilities, morbid obesity, or age-related diseases, are usually physically inactive, which exacerbates their symptoms and leads to development of other types of diseases, such as type 2 diabetes mellitus, cardiovascular diseases, and some forms of cancers. Scientific explorations of exercise have become more molecular, focusing on the pathways and molecules that mediate these benefits. Irisin has been identified as an exercise-induced hormone that embodies many adaptations to exercise in a variety of tissues/organs, including “browning” of subcutaneous adipose tissue, bone remodeling, improving cognitive deficits and neuropathology, and promoting myogenesis of skeletal muscles. In bone and fat, the effects of irisin are mediated via αv integrins, with αvβ5 identified as the major receptor. However, my biochemical and biophysical characterization of direct interaction between irisin and αvβ5 suggested extremely weak binding, while the concentrations of irisin that induce detectable amount of irisin-mediated effects in the body and in the cultured cells are really low, indicating a very high irisin/receptor binding affinity. This paradox could be explained by the existence of an additional factor that facilitates irisin/integrin interaction and irisin-mediated integrin activation. My preliminary data suggested that an exercise-induced circulating protein Hsp90α binds to integrin αvβ5, and functions as a cofactor to mediate the binding of irisin to integrin and irisin-induced integrin signaling. Irisin is different from many integrin ligands in that irisin is small, heavily glycosylated, and lacks the well-identified integrin binding motif, indicating a non-canonical ways of ligand binding to integrins. Biophysical and biochemical approaches will be used to characterize the complexes formed by irisin (WT and glycosylation mutants), αvβ5 and Hsp90α. The molecular model will be firstly tested in HEK293T cells ectopically expressing αv and β5, and muscle and fat cells, using molecular approaches and fluorescence microscopy. The effects of Hsp90α will be further evaluated in mice. Taken together, these studies will advance our understanding of irisin-mediated (or hormone-mediated, in general) integrin signaling, which will assist drug and antibody development to treat patients with obesity, aged-related diseases and neuro- or muscular degenerative disorders. The proposed project represents a great balance between biochemistry/biophysics as well as cell biology, in which I was trained during my Ph.D, and cell metabolism and animal physiology, which are the primary technologies employed by the Spiegelman lab. The resources provided by Bruce’s networks, DFCI and HMS, tremendously facilitated my research, and will support me to become an independent scientist.