APOL1-G1 in Nephrocytes Induces Hypertrophy and Accelerates Cell Death

Abstract

People of African ancestry carrying certain APOL1 mutant alleles are at elevated risk of developing renal diseases. However, the mechanisms underlying APOL1-associated renal diseases are unknown. Because the APOL1 gene is unique to humans and some primates, new animal models are needed to understand the function of APOL1 in vivo. We generated transgenic Drosophila fly lines expressing the human APOL1 wild type allele (G0) or the predominant APOL1 risk allele (G1) in different tissues. Ubiquitous expression of APOL1 G0 or G1 in Drosophila induced lethal phenotypes, and G1 was more toxic than was G0. Selective expression of the APOL1 G0 or G1 transgene in nephrocytes, fly cells homologous to mammalian podocytes, induced increased endocytic activity and accumulation of hemolymph proteins, dextran particles, and silver nitrate. As transgenic flies with either allele aged, nephrocyte function declined, cell size increased, and nephrocytes died prematurely. Compared with G0-expressing cells, however, G1-expressing cells showed more dramatic phenotypes, resembling those observed in cultured mammalian podocytes overexpressing APOL1-G1. Expressing the G0 or G1 APOL1 transgene in nephrocytes also impaired the acidification of organelles. We conclude that expression of an APOL1 transgene initially enhances nephrocyte function, causing hypertrophy and subsequent cell death. This new Drosophila model uncovers a novel mechanism by which upregulated expression of APOL1-G1 could precipitate renal disease in humans. Furthermore, this model may facilitate the identification of APOL1-interacting molecules that could serve as new drug targets to treat APOL1-associated renal diseases.

A tripartite complex of suPAR, APOL1 risk variants and αvβ3 integrin on podocytes mediates chronic kidney disease

“…The formation of such a triple complex results in the activation of multiple α v β 3 -integrin signaling pathways, which leads to the formation of autophagosomes, dysregulation of the actin cytoskeleton and, ultimately, cell detachment. Therefore, our results do not challenge a concurrent mechanism for cell injury emanating from podocyte-generated APOL1 proteins, as was previously suggested 27–30 . Rather, they suggest that, for the podocyte injury to occur, the secreted APOL1 risk-variant protein needs to be internalized by the cell in a urokinase receptor and β 3 -integrin-dependent manner (Supplementary Fig.…”

ApoL1 Overexpression Drives Variant-Independent Cytotoxicity

“…44 Two reports of ectopic expression of variant but not reference APOL1s in the Drosophila nephrocyte, a cell type with some similarities to the podocyte, increased endocytosis and loss of nephrocytes with aging. 45,46 Experiments in Saccharomyces cerevisiae also found a genetic interaction between APOL1 expression and deletion of components in the endocytic pathway. 46 Together, these results suggest that processes such as podocyte adhesion and endocytic trafficking, in addition to cytotoxicity, may also contribute to renal pathology associated with APOL1 risk variants.…”

APOL1 variants change C-terminal conformational dynamics and binding to SNARE protein VAMP8

“…Localization of VAMP8 with its cognate autophagosomal SNARE partner STX17 is diminished in presence of APOL1 variants (24). In addition, mice and Drosophila transgenic for human APOL1 kidney disease risk variants have been shown to have disordered endosomal trafficking and variant-dependent glomerular and nephrocyte injury (24,34,35). Our results suggest a model in which APOL1, in concert with VAMP8 or other SNARE proteins, identifies vesicles containing cargo capable of mediating cellular damage and their functional interaction mediates a cellular response capable of attenuating/mitigating the pathogenic potential of these cargos.…”