Two specific genetic variants of the apolipoprotein L1 (APOL1) gene are responsible for the high rate of kidney disease in people of recent African ancestry. Expression in cultured cells of these APOL1 risk variants, commonly referred to as G1 and G2, results in significant cytotoxicity. The underlying mechanism of this cytotoxicity is poorly understood. We hypothesized that this cytotoxicity is mediated by APOL1 risk variant-induced dysregulation of intracellular signaling relevant for cell survival. To test this hypothesis, we conditionally expressed WT human APOL1 (G0), the APOL1 G1 variant, or the APOL1 G2 variant in human embryonic kidney cells (T-REx-293) using a tetracycline-mediated (Tet-On) system. We found that expression of either G1 or G2 APOL1 variants increased apparent cell swelling and cell death compared with G0-expressing cells. These manifestations of cytotoxicity were preceded by G1 or G2 APOL1-induced net efflux of intracellular potassium as measured by X-ray fluorescence, resulting in the activation of stress-activated protein kinases (SAPKs), p38 MAPK, and JNK. Prevention of net K + efflux inhibited activation of these SAPKs by APOL1 G1 or G2. Furthermore, inhibition of SAPK signaling and inhibition of net K + efflux abrogated cytotoxicity associated with expression of APOL1 risk variants. These findings in cell culture raise the possibility that nephrotoxicity of APOL1 risk variants may be mediated by APOL1 risk variant-induced net loss of intracellular K + and subsequent induction of stress-activated protein kinase pathways. apolipoprotein L1 | kidney | African-American | genetics | protein kinase

Two coding variants in the apolipoprotein L1 (APOL1) gene (termed G1 and G2) are strongly associated with increased risk of nondiabetic kidney disease in people of recent African ancestry. The mechanisms by which the risk variants cause kidney damage, although not well-understood, are believed to involve injury to glomerular podocytes. The intracellular localization and function of APOL1 in podocytes remain unclear, with recent studies suggesting possible roles in the endoplasmic reticulum (ER), mitochondria, endosomes, lysosomes, and autophagosomes. Here, we demonstrate that APOL1 also localizes to intracellular lipid droplets (LDs). While a large fraction of risk variant APOL1 (G1 and G2) localizes to the ER, a significant proportion of wild-type APOL1 (G0) localizes to LDs. APOL1 transiently interacts with numerous organelles, including the ER, mitochondria, and endosomes. Treatment of cells that promote LD formation with oleic acid shifted the localization of G1 and G2 from the ER to LDs, with accompanying reduction of autophagic flux and cytotoxicity. Coexpression of G0 APOL1 with risk variant APOL1 enabled recruitment of G1 and G2 from the ER to LDs, accompanied by reduced cell death. The ability of G0 APOL1 to recruit risk variant APOL1 to LDs may help explain the recessive pattern of kidney disease inheritance. These studies establish APOL1 as a bona fide LD-associated protein, and reveal that recruitment of risk variant APOL1 to LDs reduces cell toxicity, autophagic flux, and cell death. Thus, interventions that divert APOL1 risk variants to LDs may serve as a novel therapeutic strategy to alleviate their cytotoxic effects.

People of recent African ancestry develop kidney disease at much higher rates than most other groups. Two specific coding variants in the Apolipoprotein-L1 gene termed G1 and G2 are the causal drivers of much of this difference in risk, following a recessive pattern of inheritance. However, most individuals with a high-risk genotype do not develop overt kidney disease, prompting interest in identifying those factors that interact with We performed an admixture mapping study to identify genetic modifiers of-associated kidney disease. Individuals with two risk alleles and focal segmental glomerulosclerosis (FSGS) have significantly increased African ancestry at the (also known as FAT10) locus. UBD is a ubiquitin-like protein modifier that targets proteins for proteasomal degradation. African ancestry at the locus correlates with lower levels of expression. In cell-based experiments, the disease-associated alleles (known as G1 and G2) lead to increased abundance of mRNA but to decreased levels of protein. gene expression inversely correlates with G1 and G2 -mediated cell toxicity, as well as with levels of G1 and G2 APOL1 protein in cells. These studies support a model whereby inflammatory stimuli up-regulate both and , which interact in a functionally important manner. UBD appears to mitigate-mediated toxicity by targeting it for destruction. Thus, genetically encoded differences in and expression appear to modify the -associated kidney phenotype.