Progressive Actomyosin Remodeling Defining a Structured Podocyte Injury Response in Nephrotic Syndrome
Nephrotic syndrome, the leading cause of chronic kidney disease worldwide, is associated with injury and eventual loss of podocytes, the specialized epithelial cells that form the final layer of the kidney's filtration apparatus. The progression from injury to loss is largely irreversible. However, some forms of nephrotic syndrome undergo remission, suggesting that injured podocytes may retain a capacity for repair, the structural basis of which remains unknown. Podocytes respond to injury by the stereotypical morphological change known as foot process effacement (FPE), which can be visualized using transmission electron microscopy. Using modified ultrastructure expansion microscopy (U-ExM) protocol combined with super-resolution microscopy, we discovered that injured podocytes exhibit a three-stage progression of FPE maturation, with kinetics determined by the degree of actomyosin contractility. Through longitudinal study of adriamycin-induced nephropathy in mice, we identified three sequential stages of actomyosin reorganization within the sarcomere-like structures (SLSs) that define the injury response: focal infiltration of myosin IIA into synaptopodin-rich foot processes (Stage 1), disordered intermingling of synaptopodin/myosin IIA (Stage 2), and the formation of highly organized periodic contractile arrays oriented perpendicular to the direction of capillary blood flow (Stage 3). Longitudinal quantification revealed predictable temporal dynamics, with mature Stage 3 predominating by day 7-9 post-injury. To determine whether this sequence is conserved in human disease, we analyzed kidney biopsies from patients with minimal change disease, focal segmental glomerulosclerosis, membranous nephropathy, and diabetic nephropathy. The same three stages were present across all four diseases, and their distribution tracked disease course: acute podocytopathies exhibited heterogeneous early-stage patterns, whereas chronic injuries converged on uniform, mature SLS organization. These findings demonstrate that podocyte injury is a graded, potentially reversible process rather than a binary switch. They identify structured actomyosin remodeling as a conserved wound response across mechanistically distinct kidney diseases and suggest that therapeutic strategies targeting specific stages of this progression could alter the trajectory of diverse glomerular diseases.