ABSTRACT Proteinuria, commonly caused by podocyte dysfunction or injury, is an invariable finding in patients with glomerular disease and is by itself a risk factor for long-term prognosis. Evidence suggests that the multi- tasking melanocortin hormone system plays a protective role in stress response in multiple organ systems, including the kidney. The clinical effectiveness of melanocortin therapy with adrenocorticotropin in inducing remission of steroid-resistant nephrotic syndrome points to a steroidogenic-independent anti-proteinuric activity of the melanocortinergic pathway. By harnessing the naturally occurring loss-of-function mutations in melanocortin 1 receptor (MC1R) in both humans and mice, our latest study demonstrated that MC1R is likely dispensable for the proteinuria-reducing and podocyte protective effect of melanocortin therapy. Rather, we found that MC5R is predominantly expressed in glomerular podocytes in vivo and in vitro, and MC5R agonists seem to confer a podocyte protective effect. Building on our previous work, this study will examine the role of the MC5R-mediated melanocortinergic pathway in regulating podocyte injury, explore the sites and modes of action and test an entirely novel strategy for treating proteinuric glomerulopathies based on targeting this pathway. Aim 1 will define and validate the role of MC5R versus MC1R in mediating the anti-proteinuric and podocyte protective effect by comparing the therapeutic efficacy of diverse melanocortins in wild-type, MC1R- null and MC5R knockout (MC5R-/-) mice with Adriamycin nephropathy or nephrotoxic serum nephritis. The contribution of hematopoietic MC5R to the beneficial effect of melanocortin therapy will be assessed in MC5R-/- mice receiving adoptive transfer of syngeneic wild-type bone marrow-derived cells. Furthermore, the podocyte autonomous effect of MC5R will be examined in MC5R-/- mice with podocyte specific MC5R reconstitution. Aim 2 will explore the mechanisms underlying the podocyte protective effect of MC5R signaling. How melanocortin treatment affects podocyte death, cytoskeleton disorganization and expression of NFB-dependent podocytopathic mediators will be assessed in conditionally immortalized and primary podocytes. The essential role of MC5R will be defined using primary MC5R-/- podocytes and those with MC5R reconstitution. Studies will further examine if the podocyte protective effect of MC5R signaling is conveyed by inhibitory phosphorylation of GSK3β, a key transducer of MC5R signaling and a point of convergence for multiple pathways involved in podocyte injury. Aim 3 will test the rescue effect of a novel and highly selective MC5R agonist on established podocyte injury elicited by Adriamycin or nephrotoxic serum in mice and by puromycin aminonucleoside in rats. The mechanisms responsible for the podocyte protective effect of melanocortinergic signaling will be validated in vivo. Collectively, these studies will provide a mechanistic view of the ro...