Mice lacking the LDL receptor associated protein (RAP) have a severe defect of thyroglobulin secretion into the colloid, associated with moderately increased serum TSH levels and histological features of early goiter. RAP is expressed also in renal proximal tubule cells, where it functions as a molecular chaperone for the endocytic receptor megalin, which is responsible for reabsorption of low molecular weight proteins from the glomerular filtrate. Here we investigated whether the thyroid phenotype in RAP knockout (KO) mice is associated with kidney alterations. By immunohistochemistry, we found that in RAP KO mice megalin expression on the apical membrane of renal proximal tubule cells was markedly reduced, with intracellular retention of the receptor. The reduced expression of megalin was associated with its impaired function. Thus, urinary protein concentrations and urinary protein excretion in 24 h were higher in RAP KO than in wild-type mice. Coomassie staining of urine samples revealed an increased intensity of low molecular mass bands in the urine of RAP KO mice, indicating that they had low molecular weight proteinuria. Therefore, we concluded that disruption of the RAP gene determines not only thyroid abnormalities, but also a severe defect of megalin expression and function in the kidney.
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