Nucleophosmin (NPM) Promotes Regulated Cell Death in Podocyte Injury

The mechanism of regulated podocyte death during glomerulopathy is unknown, limiting therapeutic interventions to protect this essential kidney cell. We hypothesize that cytosolic nucleophosmin (NPM) accumulation promotes Bax-mediated podocyte injury during experimental and clinical glomerulopathy. Cytosolic and total NPM were quantified in differentiated human podocytes subjected to mechanistically diverse stressors in vitro, in kidney tissue harvested from conditional, podocyte-specific integrin-linked kinase conditional knockout (ILK-cKO) mice, and in kidney tissue from patients with focal segmental glomerulosclerosis (FSGS), membranous nephropathy, or diabetic nephropathy using histologic and immunologic techniques. The effect of stress on NPM/Bax complex formation, mitochondrial Bax accumulation, and podocyte survival was assessed using an organelle-specific stain, NPM/Bax co-immunoprecipitation, and a colorimetric cell survival assay, respectively. To establish a potential role for NPM in regulated podocyte injury, NPM content was reduced and increased using molecular techniques, and the therapeutic effect of peptides designed to competitively inhibit NPM/Bax interaction was examined. Cytosolic NPM accumulation increased after transient exposure to Adriamycin or hydrogen peroxide. Hydrogen peroxide increased cytosolic NPM/Bax complex formation and increased mitochondrial Bax accumulation, early hallmarks of regulated cell death. siRNA-mediated NPM suppression significantly increased human podocyte survival, whereas NPM over-expression significantly reduced podocyte survival after acute stress. A novel TP10-fused peptide reduced NPM/Bax interaction and significantly increased podocyte survival after stress. In contrast to 2-day-old pups, increased NPM expression and cytosolic NPM accumulation were detected in podocytes of 4-week-old ILK-KO mice, an early FSGS model, as well as in the glomeruli of kidney tissue harvested from patients with diverse forms of clinical glomerulopathy. Cytosolic NPM translocation accompanies experimental and clinical podocyte injury, promotes regulated podocyte death after stress, and is a promising target for protecting podocytes against glomerular injury.