Glomerular disease such as diabetic kidney disease (DKD) and focal segmental glomerulosclerosis (FSGS) are the leading causes of end-stage renal disease. The treatment options of both diseased are very limited. Through proteomic analysis of glomeruli in diabetic rats, we identified protein S (PS) as a highly regulated protein in early DKD. PS is known to bind to TAM receptors (Tyro3, Axl, and Mer), which belong to a family of receptor tyrosine kinases that mediates regulation of inflammation and cell survival. Individual TAM receptors appear to mediate different functions. During last funding period, we found that PS protects podocytes from injury in early DKD and FSGS through the effects mediated by Tyro3. These findings are included in two recent papers [Zhong F JASN 2018] [Zhong F JCI Insights 2018] and the key findings from the paper include: 1) In human kidney, we found that Tyro3 is expressed mostly in podocytes. Glomerular Tyro3 expression is reduced in DKD and FSGS and positively correlates with eGFR in DKD patients. In addition, low glomerular Tyro3 mRNA levels predicts the progression of primary glomerular disease in the NUPTUNE study. These findings highlight the importance of Tyro3 in human glomerular disease. 2) In cultured human podocytes, PS has anti-inflammatory effects through inhibition of NF-KB activation and anti-apoptotic effects through induction of AKT phosphorylation. Tyro3, but not Axl and Mer, mediates these protective effects of PS in podocytes. 3) Knockout of PS in podocytes aggravates DKD while overexpression of PS attenuated it. 4) Morfolino-mediated knockdown of tyro3 led to altered glomerular filtration barrier development in zebrafish larvae. 5) Tyro3 knockout mice developed more DKD and Adriamycin-induced nephropathy (ADRN) while induction of Tyro3 expression in podocytes attenuated DKD, ADRN, and HIV-associated nephropathy (HIVAN). Together, these findings suggest that PS-Tyro3 is a key renal protective pathway against podocyte injury in both DKD and FSGS. Since PS could not be used as a drug due to its effect on coagulation, we propose here to develop new agonists of Tyro3 as potential drugs to treat DKD and FSGS. To address this, we propose in the aim 1 to determine the specificity of Tyro3 in comparison to Axl and Mer on the ligand-receptor interaction and the activation of downstream pathways in podocytes. These studies will help us to design specific agonists for Tyro3. Since Tyro3 could be cleaved into soluble Tyro3 (sTyro3) which serves as a decoy receptor and the levels of sTyro3 increase in DKD patients, we will determine the mechanism of Tyro3 cleavage and the role of sTyro3 in regulation of inflammation and podocyte apoptosis in diabetic condition. These studies will help us to determine whether inhibition of Tyro3 cleavage could be another approach to enhance Tyro3-mediated renal protective pathway in glomerular disease. In the aim 2, we propose to develop Tyro3 agonists as drugs for treatment of DKD and FSGS. By using structure activity relationship (SAR) studies of Tyro3 we have designed and synthesized 10 compounds as Tyro3 agonists. Based on their biological activity we selected two potential hits (compound 8 and 10) to further determine the binding activity and selectivity to Tyro3 and the biological activity in cultured human podocytes and the renal-protective effects in mice with DKD and ADRN. Finally, we will refine the structure of hit compounds to develop the lead compound to increase potency, solubility, bioavailability, and decrease toxicity for treatment of DKD and FSGS.
Diabetic kidney disease (DKD) and focal segmental glomerulosclerosis (FSGS) are common cause of end- stage renal disease in the US and in our veterans. However current regimen provides only partial therapeutic effects and the incidence and prevalence of these diseases remains high, suggesting that the key pathogenic mechanisms driving DKD progression are not adequately inhibited by current treatments. Therefore, better understanding of mechanisms that drive the disease progression is urgently required and therefore we could develop novel therapeutic measures for DKD and FSGS patients. The proposed studies may lead to the discovery of a new class of drugs to treat these two diseases.
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