Intracellular functions of APOL1 in the kidney

ABSTRACT
Chronic kidney disease (CKD) in African Americans is one of the largest racial health disparities in the United
States. The cause for the increased risk has been attributed to recessive inheritance of allelic variants in the
gene for apolipoprotein L1 (APOL1). These APOL1 variants, known as G1 and G2, do not cause CKD on their
own, but CKD is caused by a combination of the inherited genetic risk plus exposure to a triggering environmental
stressor (a gene-environment interaction). Despite the association of CKD risk with APOL1 variants more than
ten years ago, the biological function of APOL1 in the kidney and the mechanism of pathogenesis in the setting
of a disease stressor remain unclear. Our long-term goal is to understand the genetics and biochemical
mechanism of CKD in African Americans caused by these APOL1 polymorphisms. To accomplish this goal, we
are studying HIV-associated nephropathy (HIVAN), the CKD most strongly associated APOL1 variants, and the
only CKD where the environmental stressor is known (HIV infection). HIVAN is an ideal disease model to dissect
biochemical pathways and cellular events intersected by APOL1 function, viral infection, and CKD. Our recent
studies have demonstrated, for the first time, a function for the common APOL1 allele, known as G0, in providing
protection against podocyte losses in HIVAN. Since APOL1 risk is a recessively inherited trait, this suggests
CKD may be caused, in part, by a loss-of-function process (i.e. absence of G0). In new preliminary data, G0
appears to associate with Toll-like receptors (TLRs) in intracellular vesicles containing HIV and facilitate signaling
events initiated by interferon regulatory factor (IRF)-3, and these processes were absent with the APOL1 risk
variants. Methods will use established in vitro HIV infection of human podocyte cell lines, in vivo models of HIVAN
and BAC-APOL1 transgenic mice that replicate endogenous human APOL1 expression. Aims will examine both
initial response to HIV infection in podocytes, and long-term in vivo studies of intercrosses between the HIVAN
and BAC-APOL1 transgenic mice evaluating effects on renal function and pathology. These studies also will
establish temporal and magnitude of stressor-induced APOL1 expression, the mechanism of the altered innate
immune activation through TLRs to IRF-3/7 signaling, and effect on podocyte phenotype (survival and cell
adhesion). The dominance of the G0 protective effect over the risk variant dysfunction will be tested in podocytes
and mouse models co-expressing G0 and the risk variants. These studies should advance our understanding of
gain- versus loss-of-function mechanism associated with the recessive inheritance of APOL1 risk alleles, and
the necessity of induced APOL1 expression to drive stress responses. Determining the contribution of G0
function versus risk variant dysfunction will have important clinical impact on further therapy design, as it will
establish whether replacement of G0 or suppression of the risk variants would be the more effective strategy.

Grant Number: 5R01DK127638-05
NIH Institute/Center: NIH
Principal Investigator: Leslie Bruggeman