Background: Lysosomal enzymuria is usually considered to be a non-specific marker of renal injury, but little is known about lysosomal enzyme excretion in renal proximal tubular cell disorders such as the renal Fanconi syndrome (FS). We examined excretion of two lysosomal enzymes and the cation-independent mannose-6-phosphate receptor (CI-MPR) in patients with inherited FS.
Methods: The lysosomal enzyme cathepsin D was measured by ELISA and isolated by pepstatin-agarose affinity chromatography; N-acetyl-beta-d-glucosaminidase (NAG) was assayed colorimetrically, as was the cytosolic enzyme lactate dehydrogenase (LDH). Cathepsin D, procathepsin D and CI-MPR were also detected by western blotting. No patient had a serum creatinine concentration >170 micromol/L. Soluble CI-MPR, isolated from fetal calf serum and bound to agarose, was used to probe cathepsin D for mannose-6-phosphate (M6P).
Results: Increased excretion of cathepsin D (mean = 44-fold) and NAG (mean = 12-fold) was found in FS patients: Dent's disease (n = 5), cystinosis (n = 4), Lowe syndrome (n = 3) and 'autosomal dominant idiopathic FS' (ADIF) (n = 2). Increased cathepsin D excretion was confirmed by western blotting; excretion of procathepsin D and LDH was not increased. When compared with control subjects, CI-MPR excretion was also increased in FS (n = 6). Thus, significantly increased excretion of lysosomal enzymes and CI-MPR was found in all cases of FS examined. Cathepsin D binding to CI-MPR-agarose was inhibited by M6P.
Conclusions: We conclude that underlying gene defects in FS may disrupt normal membrane trafficking of CI-MPR, leading to mistrafficking of lysosomal enzymes via a default pathway from the Golgi to the apical surface of proximal tubule cells rather than to lysosomes. Lysosomal enzymes are then secreted into the tubular fluid and excreted in the urine. This contrasts with the widely held view that cell necrosis is the cause of lysosomal enzymuria in renal disease. Moreover, cathepsin D in FS urine is M6P-tagged.