Schwann cell-derived extracellular vesicles maintain peripheral nerve function under homeostatic conditions and promote damage in type 2 diabetes, independent of myelination - ABSTRACT Type 2 diabetes (T2D) affects approximately 537 million people worldwide. Peripheral neuropathy (PN) is the most common complication of diabetes, affecting 60% of individuals with T2D. Glycemic control is the only PN treatment, but it often fails to restore nerve function in T2D. Dyslipidemia is an independent risk factor for PN, but there is a gap in our understanding of how it leads to nerve damage. Thus, there is a critical need to understand mechanisms by which dyslipidemia promotes nerve injury in T2D to develop targeted, mechanism- based therapies. The American Diabetes Association recommends diet and exercise to correct lipid profiles and prevent PN in T2D. In addition to clinical data on exercise benefits in PN, we discovered that a dietary reversal (DR) paradigm restores the nerve lipidome and nerve function in an established nongenetic model of T2D and PN. These animals are fed a high fat diet (HFD) and treated with low dose streptozotocin (STZ) to mirror the human condition. What remains unknown is the cellular and molecular mechanisms underlying the restoration of nerve function. One mechanism by which diet and/or exercise mediate their beneficial effects in metabolically active tissues is through the release of extracellular vesicles (EVs). EVs mediate intercellular communication by transferring cargo containing proteins, lipids, and miRNA that support ATP production in recipient tissues during bioenergetic stress. With respect to the peripheral nervous system, Schwann cells (SCs) maintain nerve health through orchestrated metabolic communication via homeostatic EV transfer from SCs to axons. We recently discovered that SC dysfunction promotes secretion of pathologic EVs carrying miR- 15b, a regulator of lipid metabolism, ATP levels, and neurodegeneration, leading to axonal damage and PN. Our objective is to uncover the mechanisms underlying DR- and exercise- mediated improvements in PN, focusing on SC-derived EVs, SC single-cell transcriptomics, and the overall effect on nerve lipid metabolism, bioenergetics, and function. We hypothesize that following diet and/or exercise, SCs transfer homeostatic EVs to axons and improve PN by normalizing nerve lipid metabolism and bioenergetics. To test this hypothesis, we will 1) assess the effects of DR, voluntary exercise in a running wheel (Ex), and DR/Ex on T2D-induced SC injury and nerve function to determine how T2D and/or the intervention(s) impact the SC-specific transcriptome, SC EV cargo, and SC-axon crosstalk; 2) interrogate the effect of SC-derived EVs on neuronal health in an in vitro PN model to identify mechanisms of EV-mediated SC-axon crosstalk through miRNA changes, uncovering the contribution of SC-axon coupling to nerve damage and PN, and 3) determine how SC-derived EVs contribute to PN in T2D mice to determine the roles of EVs and miR-15b in SC-axon crosstalk and PN and confirm whether EVs and miR-15b are suitable for therapeutic targeting in PN. These studies will have a significant impact by advancing our understanding of nerve injury mechanisms in T2D and identifying SC-specific therapies for PN.
