Dedifferentiation and aberrations of the endolysosomal compartment characterize the early stage of nephropathic cystinosis

Abstract

Nephropathic cystinosis, a lysosomal storage disease causedbymutations intheCTNSgeneencoding the lysosomal cystine transporter cystinosin, is characterized by generalized proximal tubule (PT) dysfunction that progresses, if untreated, to end-stage renal disease. The pathogenesis of defective PT cellular transport in nephropathic cystinosis remains unclear. We characterized a recently generated line of C57BL/6 Ctns mice and analyzed endocytic uptake, lysosome function, and dedifferentiation and proliferation markers using primary cultures of PT epithelial cells derived from Ctns-/- and Ctns+/+ littermates. Metabolic studies revealed that Ctns-/- mice show a progressive PT dysfunction characterized by low-molecular-weight (LMW) proteinuria, glucosuria and phosphaturia, before structural damage and in the absence of renal failure. These changes are related to decreased expression of the multi-ligand receptors megalin and cubilin and to increased dedifferentiation (ZONABtranscription factor) and proliferation (PCNAand Cyclin D1) rates. Studies on PT cells derived from Ctns-/- kidneys confirmedcystine overload, with accumulationof enlarged, dysfunctional lysosomes and reduced expression of endocytic receptors reflected by decreased uptake of specific ligands. These changes were related to a loss of integrity of tight junctions with a nuclear translocation of ZONAB and increased proliferation, as observed in Ctns-/- kidneys. These data reveal that the absence of cystinosin in PT cells triggers aberrations of the endolysosomal compartment, transport defects and an abnormal transcription program in the early stage of nephropathic cystinosis. Insights into the earlymanifestations of cystinosismayoffer newtargets for intervention, before irreversible renal damage. © The Author 2013. Published by Oxford University Press. All rights reserved.