Abstract 28: Formation of Podocyte-Derived Microparticles in Diabetic Glomerular Injury

Abstract

Hypertension is a significant cause of progressive kidney disease, particularly in the presence of diabetes. Under such conditions, increased glomerular capillary pressure subjects podocytes, specialized glomerular epithelial cells critical to filtration, to mechanical stress resulting in podocyte injury/dysfunction. Microparticles (MPs) are small (0.1-1.0 μm), membranous vesicles shed from the cell surface following injury. However, whether podocyte MP formation reflects glomerular injury is unknown. We examined MP formation by podocytes in vitro and in vivo. Conditionally immortalized human podocytes were exposed to 10% equibiaxial cyclic stretch (a mimic of increased intraglomerular pressure), high glucose (HG, 25 mM), mannitol (osmotic control), angiotensin II (Ang II, 500 nM) or transforming growth factor beta (TGF-β, 5 ng/mL). Additionally, urinary podocyte MPs were quantified in two mouse models of diabetic kidney disease: streptozotocin (STZ) and OVE26. MPs were characterized by nanoparticle tracking analysis and quantified by Annexin V (total MPs) or podocalyxin (podocyte MPs) labeling and flow cytometry. Podocyte-derived vesicles were identifiable in both media and urine samples with a mean size of 236 nm by nanoparticle tracking analysis. In vitro, cyclic stretch was associated with a 3-fold increase in MP release after 24 hours (P<0.01, n=6). HG increased MP release 5-fold after 24 hours (P<0.05, n=6). Mannitol had no effect on MP formation by either normal or stretched podocytes and neither Ang II, nor TGF-β altered podocyte MP formation over 24 hours. In vivo, both models of diabetes displayed typical hallmarks of renal injury (proteinuria, mesangial expansion). In OVE26 mice urinary podocyte MPs were elevated compared with their wild-type littermates (17479±8329 vs. 7 ±7, P<0.05, n=5-7). Similarly, STZ-treated mice displayed increased urinary podocyte MPs as compared with untreated (18035±3813 vs. 43±34, P<0.001, n=9-18) and urinary MPs levels were positively correlated with albuminuria (r2=0.74, P<0.01). Our results suggest that podocytes produce MPs which are released into urine and are indicative of glomerular injury. Such processes may be mediated by intraglomerular capillary pressure and hyperglycemia.