Sns and Kirre, theDrosophilaorthologs of Nephrin and Neph1,direct adhesion, fusion and formation of a slit diaphragm-like structure in insect nephrocytes

Abstract

The Immunoglobulin superfamily (IgSF) proteins Neph1 and Nephrin are co-expressed within podocytes in the kidney glomerulus, where they localize to the slit diaphragm (SD) and contribute to filtration between blood and urine. Herein, we demonstrate that their Drosophila orthologs Kirre (Duf) and Sns are co-expressed within binucleate garland cell nephrocytes (GCNs) that contribute to detoxification of the insect hemolymph by uptake of molecules through an SD-like nephrocyte diaphragm (ND) into labyrinthine channels that are active sites of endocytosis. The functions of Kirre and Sns in the embryonic musculature, to mediate adhesion and fusion between myoblasts to form multinucleate muscle fibers, have been conserved in the GCNs, where they contribute to adhesion of GCNs in the 'garland' and to their fusion into binucleate cells. Sns and Kirre proteins localize to the ND at the entry point into the labyrinthine channels and, like their vertebrate counterparts, are essential for its formation. Knockdown of Kirre or Sns drastically reduces the number of NDs at the cell surface. These defects are associated with a decrease in uptake of large proteins, suggesting that the ND distinguishes molecules of different sizes and controls access to the channels. Moreover, mutations in the Sns fibronectin-binding or immunoglobulin domains lead to morphologically abnormal NDs and to reduced passage of proteins into the labyrinthine channels for uptake by endocytosis, suggesting a crucial and direct role for Sns in ND formation and function. These data reveal significant similarities between the insect ND and the SD in mammalian podocytes at the level of structure and function.

An In Vivo Functional Analysis System for Renal Gene Discovery in Drosophila Pericardial Nephrocytes

“…11 The Drosophila nephrocyte has a highly specialized filtration structure called the nephrocyte diaphragm, which shares remarkable similarities to the slit diaphragm structure of the mammalian glomerular podocytes. 12,13 The nephrocyte foot processes created by extensive infolding of the plasma membrane generate ;30-nm slit pores in Drosophila nephrocytes compared with the 30-to 50-nm slit pores created by podocyte foot processes in mammalian glomerulus. These nephrocyte diaphragms and the basement membrane covering the entire nephrocyte together form a sizeand charge-selective filtration barrier to filter Drosophila hemolymph.…”

“…Genes encoding slit diaphragm components have been shown to be highly conserved from Drosophila to humans and required for nephrocyte function. 12,13 The molecular cores of the slit diaphragm structure are nephrin/Neph-1 in mammals, and their corresponding fly orthologs are Sticks and stones (Sns)/ Dumbfounded (Duf). Genetic defects in nephrin are responsible for the human congenital nephrotic syndrome of the Finnish type, leading to heavy proteinuria and kidney failure.…”

Involvement of nephrin in human placental trophoblast syncytialization

“…However, little is known about the potential roles of nephrin in extra-renal tissues. Sticks and stones (Sns), the Drosophila ortholog of nephrin, functions as a positive regulator of myoblast fusion and is also required for the adhesion and fusion of insect nephrocytes (Zhuang et al 2009). Nephrin also has a conserved function in the fusion of zebrafish and mouse myoblasts.…”

The Drosophila nephrocyte has a glomerular filtration system

“…This oversight is reasonable because the filtration system has only recently been discovered in Drosophila melanogaster and its existence has not yet been fully accepted in nephrology research. 2,3 The nephron is the basic functional and structural unit of the kidney; the glomerulus controls ultrafiltration and the renal tubule is responsible for reabsorption and secretion. 4,5 The Drosophila excretory system is composed of Malpighian tubules and nephrocytes.…”

“…2, 3 Our research group has developed a reliable functional readout for Drosophila nephrocytes in vivo and established an RNA interference system to identify genes that are required for nephrocyte function. 9 Using this system, we performed large-scale screens and showed that Drosophila nephrocytes share remarkable functional, structural and molecular similarities with mammalian renal proximal tubule cells.…”