The Drosophila nephrocyte: back on stage.

Drosophila nephrocytes have been an object of study for more than a century. They were named by Bruntz, based on observations of cells around the heart, the central nervous system, and in the sternal area in scorpions. Kowalevsky suggested that they function as a storage kidney, because they can absorb ammonia carmine from the hemolymph. Despite decades of study, however, the precise function of the nephrocyte has remained elusive. Two papers in the current issue of JASN now extend their function, making these remarkable cells still more surprising. The adult Drosophila nephrocytes are categorized as thoracic nephrocytes or abdominal nephrocytes. They derive from larval nephrocytes, among the largest cells in the larval body, which are classified as garland cells or pericardial cells based on their location. Garland cells lie close to the esophagus or between the salivary glands. Pericardial nephrocytes develop from the cardiogenicmesodermby the late embryo/early larval stage; they arrange into two rows of 20–25 flanking each side of the heart from the first to the sixth segment. Using transmission electronmicroscopy of adultDrosophila nephrocytes, Mills and King found that the plasmamembrane of the pericardial nephrocyte is invaginated to form elaborate sheets and tubules along with organelles that resemble lysosomes. Functional lysosomes were confirmed by histochemicalmethods andwere presumed to act as a garbage disposal system. By studying the third-instar larva ofCalliphora, Crossley documented the ultrastructure of pericardial cells and demonstrated a desmosome-like structure, very similar to the podocyte slit diaphragm that is required for proper filtration within the vertebrate kidney. Although Schwinck observed that pericardial cells of Panorpa synthesize proteins and export them to hemolymph, more attention has been paid to the role of nephrocytes in taking up materials from the hemolymph. Pericardial nephrocytes were demonstrated to absorb foreignmaterials from the hemolymph, providing selection by size and charge. Based on dye injections, Hollande proposed that the Lepidopteran pericardial cell absorbs and stores toxic compounds from the hemolymph, hydrolyzing them to soluble, nontoxic molecules that are released into the hemolymph, perhaps to be excreted by the Malpighian tubules. This hypothesis was later supported by Wigglesworth’s observation in bloodsucking insects that hemoglobin protein constituents consumed in a blood meal are broken down into biliverdin and stored in the nephrocytes, while the iron is released to the hemolymph. Recently, Drosophila nephrocytes have returned to center stage. Weavers et al. and Zhuang et al. independently showed that the molecular structure of the nephrocyte diaphragm is similar to that of the podocyte slit diaphragm. Reducing key components of the nephrocyte diaphragm, the Nephrin or Neph1 orthologs SNS and Kirre/Duf, respectively, led to structural defects. This strongly suggests that nephrocytes act in amanner analogous to our podocytes, thefly nephrocyte diaphragm presumably functioning similar to the mammalian slit diaphragm to regulate filtration. Weavers et al. therefore proposed that the nephrocyte diaphragm functions as a filter to exclude large hemolymph constituents from the labyrinthine channels, and that this filtration depends on the proper functioning of SNS/Nephrin and Kirre/Neph1. This led to the attractive hypothesis that the nephrocyte acts primarily as afly podocyte (filtration), perhaps without the need for an associated renal proximal tubule (reabsorption). This work broadened interest in nephrocytes as a useful model for understanding kidney filtration. The charge and size-selectivity of nephrocytes is reminiscent of podocytes, a crucial component of the mammalian glomerular filtration barrier. Genetic mutations that affect constituent proteins within the slit diaphragm lead to severe proteinuria and kidney failure in humans. Proteinuria can also be caused by defects in the re-absorptionmachinery in the proximal tubule, involving the major proteins in the endocytic receptor complex (e.g., megalin, cubilin, and amnionless). In a pair of articles in the current issue of JASN, Zhang et al. use genetic approaches to further explore the functions of the Drosophila nephrocytes. Establishing a stable transgenic fly line producing a secreted fluorescently tagged protein (ANFRFP) that accumulates in nephrocytes—reminiscent of the secreted GFP system described by Ferrandon et al.,—they assessed the ability of loci to regulate protein uptake. For example, they demonstrated that mutating Sns/Nephrin, Kirre/Neph1, orDrosophila podocin abolished secreted protein accumulation in nephrocytes. Using an unbiased screen, they then identified .70 genes required for nephrocyte function. These provide an important resource for exploring nephrocyte function. Published online ahead of print. Publication date available at www.jasn.org.