Renal proximal tubule receptors Cubilin and Amnionless mediate protein reabsorption in Drosophila nephrocytes. Fujian Zhang¹, Ying Zhao¹, Yufang Chao¹, Katherine Muir¹, Zhe Han^1,2. 1) Department of Internal Medicine, University of Michigan, Ann Arbor, MI; 2) Department of Cell and Developmental Biology.

   Filtration and reabsorption are two fundamental roles of the renal system. Remarkable similarities have been found between insect nephrocyte and the mammalian glomerular podocyte for filtration, but it remains unclear whether there is an organ or cell to perform protein reabsorption in flies. Here we show that the Drosophila nephrocyte has remarkable molecular, structural and functional similarities to the renal proximal tubule cell. From a genetic screen for genes required for nephrocyte function, we identified two novel Drosophila genes encoding orthologues of mammalian Cubilin and Amnionless (AMN), two major receptors for protein reabsorption in the renal proximal tubule. Mutations in Cubilin or AMN lead to Imerslund-Gräsbeck syndrome (IGS), a genetic disease associated with persisting proteinuria. We found that dCubilin and dAMN are specifically expressed in the Drosophila nephrocytes and function as co-receptors for protein uptake, suggesting that nephrocytes may carry out the similar function as renal proximal tubules. Targeted expression of human AMN in Drosophila nephrocytes is sufficient to rescue the protein uptake defect caused by dAMN RNAi knockdown, suggesting that functions of the Cubilin/AMN co-receptors are evolutionarily conserved from flies to humans. Electron microscopy analysis and toxin stress assay demonstrated that Cubilin/AMN-mediated protein reabsorption is not only required for maintaining nephrocyte ultrastructure, but also important for survivability of flies in toxic stress condition. Our data suggests that the insect nephrocyte combines filtration with protein reabsorption using evolutionarily conserved genes and subcellular structures, and can serve as a simplified model for both podocytes and renal proximal tubules.