Spns1 controls megalin dependent endocytosis via iron misregulation

Proximal tubule endocytosis is essential for protecting the kidney from potentially damaging proteinuria and for mediating metabolic pathways, such as the activation of Vitamin D. We have identified the lysosomal membrane protein, Spns1, as a novel iron conductance that is indispensable for the normal endocytic function of the proximal tubule. Conditional knockout of Spns1 with a Cre-lox system in megalin-expressing cells in mice leads to arrest of both pinocytosis and receptor-mediated endocytosis in the proximal tubule. This endocytic defect is associated with iron overload in proximal tubules as measured by immunofluorescence of ferritin abundance and by a reduction in nascent TfRc mRNA transcripts. We have recapitulated previous findings in drosophila and zebrafish that Spns1 knockout causes endolysosomal dysfunction including abnormally enlarged lysosomes. The endocytic defect resulting from conditional Spns1 knockout can be rescued nutritionally with an iron deficient diet or genetically through double knockout of both Spns1 and the Fe2+ transporter, DMT1. Surprisingly, the endocytic defect in mice with Spns1 conditional knockout occurs despite normal megalin expression at the proximal tubule apical membrane. This data raises the possibilty that post-translational regulation of megalin activation may be nutrient-responsive.