STING-Dependent Podocyte Injury Is Associated with Impaired Mitophagy in Diabetic Kidney Disease (DKD)

Background: Inflammation plays a significant role in the pathogenesis of diabetic kidney disease (DKD), the most prevalent cause of end stage kidney disease in the USA. We have demonstrated increased glomerular and podocyte expression of the stimulator of interferon genes (STING) pathway in DKD, suggesting that modulated innate immune responses may contribute to glomerular injury. Release of mitochondrial DNA (mtDNA) into the cytoplasm leads to STING activation but the exact mechanism is unknown. Cytosolic mtDNA leakage in DKD is associated with loss of mitochondrial transcription factor A (TFAM) and altered PTEN-induced putative kinase 1 (PINK1)-mediated mitophagy. We hypothesize that TFAM deficiency causes mtDNA escape and STING-mediated podocyte injury via defective PINK1-mediated mitophagy. Methods: In vitro, human podocytes with knockdown of STING (shSTING), TFAM (shTFAM) and scrambled control (shCTRL) were used to evaluate mitophagy and cytosolic mtDNA. STING and PINK1 expression in shTFAM was assessed by Western blot. In vivo, 16-week-old control (db/+) and diabetic (db/db) mice were used to evaluate TFAM and PINK1 expression by Western blot, mitochondrial morphology by TEM and renal cortical cytosolic mtDNA content by qRT-PCR. Control and STING knockout (STING KO) mice were treated with mtDNA (5mg/kg, 24h) and mice with podocyte specific Tfam deletion (pTFAMfl/fl) were injected with streptozotocin (STZ, 40 mg/kg) to induce diabetes followed by urinary albumin-to-creatinine ratio (ACR), histological and serum analyses. Two-tailed t-test or One-Way ANOVA followed by Tukey’s post-test were used to detect statistical changes. Results: Increased levels of renal cytosolic mtDNA in db/db mice were associated with decreased glomerular TFAM expression and increased PINK1 expression. shTFAM podocytes had increased PINK1 expression and less efficient mitophagosome formation. STING deficiency protected from mtDNA-induced injury in vitro and in vivo. While pTFAMfl/fl mice had no baseline renal injury, STZ treatment resulted in worse renal injury in pTFAMfl/fl mice compared to wildtype. Conclusion: Our data suggest that podocyte STING activation is driven by TFAM deficiency -mediated mitophagy in association with increased PINK1. Targeting the TFAM/PINK1/STING pathway may lead to new treatment options for DKD.