Abstract
Kidney proximal tubular cells (PTCs) are highly specialized for ultrafiltrate reabsorption and serve as paradigm of apical epithelial differentiation. Vps34/PI3-kinase type III (PI3KC3) regulates endosomal dynamics, macroautophagy and lysosomal function. However, its in vivo role in PTCs has not been evaluated. Conditional deletion of Vps34/PI3KC3 in PTCs by Pax8-Cre resulted in early (P7) PTC dysfunction, manifested by Fanconi-like syndrome, followed by kidney failure (P14) and death. By confocal microscopy, Vps34∆/∆ PTCs showed preserved apico-basal specification (brush border, NHERF-1 versus Na+/K+-ATPase, ankyrin-G) but basal redistribution of late-endosomes/lysosomes (LAMP-1) and mis-localization to lysosomes of apical recycling endocytic receptors (megalin, cubilin) and apical non-recycling solute carriers (NaPi-IIa, SGLT-2). Defective endocytosis was confirmed by Texas-red-ovalbumin tracing and reduced albumin content. Disruption of Rab-11 and perinuclear galectin-3 compartments suggested mechanistic clues for defective receptor recycling and apical biosynthetic trafficking. p62-dependent autophagy was triggered yet abortive (p62 co-localization with LC3 but not LAMP-1) and PTCs became vacuolated. Impaired lysosomal positioning and blocked autophagy are known causes of cell stress. Thus, early trafficking defects show that Vps34 is a key in vivo component of molecular machineries governing apical vesicular trafficking, thus absorptive function in PTCs. Functional defects underline the essential role of Vps34 for PTC homeostasis and kidney survival.
Highlights
Kidney proximal tubular cells (PTCs) are highly specialized for ultrafiltrate reabsorption and serve as paradigm of apical epithelial differentiation
For conditional deletion in kidney PTCs, Vps34fl mice were crossed with Wnt4-Cre mice or Pax8-Cre mice
We tested the role of the Vps[34] isoform of PI3K on the differentiation and maintenance of PTCs in vivo, by conditional deletion using Cre driven by two different promoters expressed at different time points during embryonic development
Summary
Kidney proximal tubular cells (PTCs) are highly specialized for ultrafiltrate reabsorption and serve as paradigm of apical epithelial differentiation. Efficient reabsorption of water and solutes from the ultrafiltrate is ensured by abundance of channels and transporters (NaPi-IIa for phosphate, SGLT-2 for glucose) at the highly developed apical brush border membrane. These transporters are anchored by ezrin and by multi-PDZ scaffold proteins including Na+/H+-exchanger regulatory factor-1 (NHERF-1)[2,3,4,5]. In PTCs, impaired apical trafficking due to defective recycling causes a generalized reabsorption deficit, known as kidney Fanconi syndrome, in Dent’s disease[16,17], Lowe disease[18] and possibly cystinosis[19]. PI3P can be generated at endosomes by dephosphorylation of diphosphoinositides by PdIns(3,5P)[2] phosphatases[24]
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