APOL1 kidney disease risk variants cause cytotoxicity by depleting cellular potassium and inducing stress-activated protein kinases

Abstract

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

APOL1-G1 in Nephrocytes Induces Hypertrophy and Accelerates Cell Death

“…Interestingly, these cytotoxic effects are remarkably similar to those previously reported in cultured renal epithelial cells overexpressing APOL1 risk variants. [25][26][27] Taken together, these data suggest that overexpression of G1 in Drosophila cells causes greater cytotoxicity than G0. It is important to note that Drosophila lacks not only endogenous APOL1 but likely also some (or perhaps all) genes encoding factors that in humans associate with APOL1 during transport, podocyte binding, endocytosis, intracellular trafficking, and metabolic processing.…”

“…27,44 Studies on cultured human embryonic kidney cells raise the possibility that nephrotoxic effects of APOL1 risk variants may be mediated by APOL1-induced net loss of intracellular K + and activation of stress-activated protein kinase pathways. 26 However, overexpression of APOL1 in cultured epithelial cells induced very rapid cell death. Culture conditions may not be sensitive enough to detect the early changes induced by APOL1 expression in vivo.…”

“…[21][22][23][24] Overexpression of APOL1 in cultured human kidney epithelial cells induces cytotoxic effects. [25][26][27] APOL1 is restricted to humans and some nonhuman primate species. Transgenic mice expressing APOL1-G2 developed preeclampsia and reduced podocyte cell density, but not renal disease.…”

Plasma Levels of Risk-Variant APOL1 Do Not Associate with Renal Disease in a Population-Based Cohort

“…The evidence against a loss-of-function model (in which APOL1 is a protective factor in the kidney) include: (1) APOL1 does not appear to be required for normal kidney function because a human homozygous null individual has been identified with apparently normal kidney function after several years of follow-up, 8,9 (2) the restricted species distribution of the APOL1 gene, 10,11 and (3) evidence for increased cytotoxic effects of G1 and G2 APOL1 when expressed in a variety of model systems. 7,[12][13][14] The bulk of APOL1 is found in the plasma, where it circulates as part of a specialized HDL particle consisting of APOA1, APOL1 and the hemoglobin binding protein HaptoglobinRelated Protein (HPR), which also serves as the receptor for uptake by trypanosomes. 15 A subset of APOL1-HDL has also been shown to bind IgM and forms the distinct lytic factor TLF-2.…”

APOL1 Nephrotoxicity: What Does Ion Transport Have to Do With It?

“…The specific molecular mechanisms by which renal risk variants APOL1 (G1 and G2) cause and accelerate kidney disease in human has been elusive. Expression of either G1 or G2 in cultured human cells elicits heightened cytotoxicity compared to G0 expression, and thus have been the model for research efforts at elucidating the mechanisms of APOL1 nephropathy by (4–7). I n vitro expression of G1 or G2 APOL1 in various cells of human origin has been reported to increase cell death by several mechanisms including necrosis, pyroptosis, apoptosis and autophagy (6–12).…”