Limited capacity of proximal tubular proteolysis in mice with proteinuria.

Albuminuria is associated with the additional loss in the urine of small molecular weight proteins normally degraded by the proximal convoluted tubule (PCT), and competition for binding to the megalin/cubilin reuptake system has been considered the likely cause. We have previously reported that deficiency of the intrinsic lysosomal protein Limp-2 causes tubular proteinuria due to reduced fusion of endosomes with lysosomes in the PCT leading to inadequate proteolysis. To determine whether this mechanism also contributes to the tubular proteinuria induced by albumin overload in normal mice, wild-type (WT) mice received daily BSA injections intraperitoneally for 10 days, using untreated Limp-2(-/-) mice as positive controls for inadequate proteolysis. BSA overload induced significant urinary loss of megalin and cubilin ligands in WT mice. Tubular uptake of Alexa-conjugated BSA, administered by intravenous injection, was not reduced in the PCT of mice receiving intraperitoneal BSA. Expression of the tubular protein receptor megalin was also unchanged. There was a delay in proteolysis of reabsorbed proteins in WT mice receiving BSA, evidenced by an increased quantity of retinol-binding protein (RBP) in the kidney cortex, increased basal distribution of endocytosed RBP in cells of the PCT, and persistence of exogenous Alexa-conjugated BSA and RBP after injection. Upregulation of cathepsin L and normal fusion of lysosomes with endosomes were apparently not sufficient to maintain normal clearance of endocytosed proteins. The data suggest that in the presence of competition from albumin overload, reabsorption of filtered proteins is limited by the capacity of lysosomal degradation rather than receptor-mediated endocytosis.