The tricellular junctional protein Ildr2 helps maintain glomerular podocyte architecture

Abstract

Podocytes are highly specialized epithelial cells of the glomerular filtration barrier essential for maintaining proper kidney function. Their intricate cellular projections, called foot processes, wrap around the capillary endothelium, interdigitate, and connect to one another via slit diaphragm intercellular junctions to form a sieve-like barrier. Compromise to this unique podocyte architecture can lead to altered glomerular function and eventual kidney disease. We previously identified Immunoglobulin-like domain containing receptor 2 (Ildr2) as a component of the podocyte foot process through proteomic analyses. Ildr2 is a tricellular tight junction constituent and localizes to distinct puncta in podocytes that likely represent these specialized junctions. However, the significance of tricellular tight junctions and Ildr2 to podocyte integrity is unknown. To this end, we generated a conditional knockout mouse with podocyte-specific deletion of Ildr2 (Podocin-Cretg/+Ildr2fl/fl or Ildr2PodKO). Kidneys from Ildr2PodKO adult animals show disruptions to podocyte foot process architecture including effacement and the unique formation of electron dense strands connecting subsets of processes. Additionally, the glomerular basement membrane was significantly thicker in conditional knockouts and histological analyses revealed glomerular collagen and glycoprotein accumulation. Surprisingly, Ildr2PodKO animals displayed no significant albuminuria. When Ildr2PodKO mice were challenged with a stress to kidney function, immunohistochemistry revealed podocyte loss from a subset of glomeruli with concomitant detection of podocytes in urine, although function was still preserved. Collectively, our data provide novel insights into the function of Ildr2 and suggest tricellular junctions help preserve podocyte architecture but are likely not necessary to maintain proper filtration in no or low stress physiological states.