Background: The signaling molecule stimulator of IFN genes (STING) was identified as a crucial regulator of the DNA-sensing cyclic GMP-AMP synthase (cGAS)-STING pathway, and this signaling pathway regulates inflammation and energy homeostasis under conditions of obesity, kidney fibrosis, and AKI. However, the role of STING in causing CKD, including diabetic kidney disease (DKD) and Alport syndrome, is unknown. Methods: To investigate whether STING activation contributes to the development and progression of glomerular diseases such as DKD and Alport syndrome, immortalized human and murine podocytes were differentiated for 14 days and treated with a STING-specific agonist. We used diabetic db/db mice, mice with experimental Alport syndrome, C57BL/6 mice, and STING knockout mice to assess the role of the STING signaling pathway in kidney failure. Results: In vitro, murine and human podocytes express all of the components of the cGAS-STING pathway. In vivo, activation of STING renders C57BL/6 mice susceptible to albuminuria and podocyte loss. STING is activated at baseline in mice with experimental DKD and Alport syndrome. STING activation occurs in the glomerular but not the tubulointerstitial compartment in association with autophagic podocyte death in Alport syndrome mice and with apoptotic podocyte death in DKD mouse models. Genetic or pharmacologic inhibition of STING protects from progression of kidney disease in mice with DKD and Alport syndrome and increases lifespan in Alport syndrome mice. Conclusion: The activation of the STING pathway acts as a mediator of disease progression in DKD and Alport syndrome. Targeting STING may offer a therapeutic option to treat glomerular diseases of metabolic and nonmetabolic origin or prevent their development, progression, or both.

Accumulating evidence indicates that inflammatory and immunologic processes play a significant role in the development and progression of glomerular diseases. Podocytes, the terminally differentiated epithelial cells, are crucial for maintaining the integrity of the glomerular filtration barrier. Once injured, podocytes cannot regenerate, leading to progressive proteinuric glomerular diseases. However, emerging evidence suggests that podocytes not only maintain the glomerular filtration barrier and are important targets of immune responses but also exhibit many features of immune-like cells, where they are involved in the modulation of the activity of innate and adaptive immunity. This dual role of podocytes may lead to the discovery and development of new therapeutic targets for treating glomerular diseases. This review aims to provide an overview of the innate immunity mechanisms involved in podocyte injury and the progression of proteinuric glomerular diseases.

Chronic kidney disease, including Alport Syndrome, is linked to collagen type IV mutations, lipid dysmetabolism, and altered sphingolipid pathways, with no targeted therapies currently available. Sphingomyelin phosphodiesterase acid-like 3b (SMPDL3b), a key regulator of sphingolipid metabolism and membrane receptor organization in podocytes, may drive disease via ceramide and sphingosine-1-phosphate pathways. This study tested whether altered SMPDL3b expression contributes to glomerular injury and renal decline in Alport Syndrome. Archived Alport Syndrome human biopsies were used for immunohistochemistry and NanoString re-analysis of SMPDL3b. Murine podocytes isolated from mouse models of Alport Syndrome were profiled using Illumina. Mouse models of Alport Syndrome and models with either podocyte-specific deletion or inducible overexpression of Smpdl3b were generated to assess renal function using liquid chromatography-mass spectrometry, matrix-assisted laser desorption ionization-mass spectrometry imaging and atomic force microscopy. We found a three-fold increase in SMPDL3b expression in glomeruli, tubules and murine podocytes isolated from Col4a3 knockout mice. Increased SMPDL3b expression occurred in association with alterations affecting kidney sphingolipid metabolism, increased glomerular but not tubular sphingosine-1-phosphate levels and reduced glomerular basement membrane and podocyte stiffness. Podocyte-specific Smpdl3b deletion in Col4a3 knockout mice was sufficient to restore sphingosine-1-phosphate levels, to reduce proteinuria, podocyte foot process effacement, and improve glomerular basement membrane and podocyte stiffness, but not sufficient to protect from kidney failure. Our study suggests that SMPDL3b may be a key modulator of proteinuria and podocyte integrity in Alport Syndrome, decoupling proteinuria from kidney failure, and suggests that improvement of glomerular structure and function may not always translate into protection from chronic kidney disease progression.