Characterization of circulating APOL1 protein complexes in African Americans

Abstract

This article is available online at http://www.jlr.org G1 and G2 renal-risk variants on chromosome 22q13.1 ( 1-3 ). Although APOL1 mRNA and APOL1 protein are present in human kidney ( 4, 5 ), the major APOL1 reservoir appears to be circulating protein ( 5, 6 ). APOL1 was initially discovered as a minor apolipoprotein of plasma HDLs ( 6 ); however, its distribution among HDL subfractions has not been well-defi ned. APOL1 nephropathy variants associate with HDL subfraction concentrations ( 7 ) and CVD risk, although controversial results have been reported with CVD ( 8-11 ). Trypanosome lytic factors (TLF1 and TLF2) contain APOL1 protein ( 12 ) and are minor HDL subfractions in humans that contribute to innate immunity via protection from infection, including from African trypanosomes ( 13 ).Association was not observed between plasma APOL1 concentrations and APOL1 genotype in African Americans with treated HIV infection; plasma APOL1 levels also did not associate with the risk of HIV-associated nephropathy or chronic kidney disease ( 14 ). Whether serum APOL1 protein levels and their distribution among HDL particles are associated with APOL1 genotypes in healthy individuals is unknown. Because free APOL1 protein may be taken up by podocytes in vitro ( 5 ), it remains critical to determine whether serum APOL1 concentrations or the structure/composition of variant APOL1 proteins and their associated complexes are specifi c to the G1 and G2 renalrisk variants, relative to nonrisk G0.To address these issues, serum APOL1 protein levels and size distribution were examined in age-and gender-matched African Americans without kidney disease based on APOL1 genotype using fast protein LC (FPLC) and immunoblot analysis. Proteomics analysis was performed to compare the composition of APOL1 protein-containing complexes. These Abstract APOL1 gene renal-risk variants are associated with nephropathy and CVD in African Americans; however, little is known about the circulating APOL1 variant proteins which reportedly bind to HDL. We examined whether APOL1 G1 and G2 renal-risk variant serum concentrations or lipoprotein distributions differed from nonrisk G0 APOL1 in African Americans without nephropathy. Serum APOL1 protein concentrations were similar regardless of APOL1 genotype. In addition, serum APOL1 protein was bound to protein complexes in two nonoverlapping peaks, herein referred to as APOL1 complex A (12.2 nm diameter) and complex B (20.0 nm diameter). Neither of these protein complexes associated with HDL or LDL. Proteomic analysis revealed that complex A was composed of APOA1, haptoglobin-related protein (HPR), and complement C3, whereas complex B contained APOA1, HPR, IgM, and fibronectin. Serum HPR was less abundant on complex B in individuals with G1 and G2 renal-risk variant genotypes, relative to G0 ( P = 0.0002-0.037). These circulating complexes may play roles in HDL metabolism and susceptibility to CVD. A major breakthrough in human molecular genetics was identifi cation of the powerful contribution of APOL1 gene ...

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

“…1). Given that circulating APOL1 is a component of a lipid-rich trypanolytic complex that has an electrophoretic mobility of high-density lipoprotein (HDL) particles 18 , we assessed the functionality of recombinant APOL1 proteins by measuring the binding affinity of APOL1 to HDL. All three variant APOL1 proteins (G0, G1 and G2) exhibited similarly high affinities for HDL ( K D , ~150 nM; Supplementary Fig.…”

“…The binding experiments were run with 10-mM HEPES buffered saline (10 mM HEPES, 150-mM NaCl, 1-mM MgCl 2 , 0.5-mM MnCl 2 , 0.05% N -octyl-β- d -glucose, pH 7.1 for the study of α v β 3 integrin, α 3 β 1 integrin and HDL) or (10-mM HEPES, 150-mM NaCl, 3-mM EDTA, 0.05% Tween 20, pH 7.5 for a few sets with suPAR). HDL was used as a positive control 18 . Data were referenced with blank (ethanolamine) response units.…”

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

“…These results are comparable with those reported recently by Weckerle et al, although in that study, an RRG associated decrease in HPR association was noted specifically in the IgM containing APOL1 complex. 25 …”

The Apolipoprotein L1 Gene and Cardiovascular Disease

“…Circulating APOL1 protein concentration does not appear to be dependent on APOL1 renal-risk variant genotypes. 7,38 APOL1 messenger RNA and protein have been detected in vascular cells, which supports the potential for a local vascular effect as with nephropathy risk. 39,40 If APOL1 renal-risk alleles exhibit opposing effects on CVD and nephropathy risk, an effect supported by AA-DHS, treatments that improve renal outcomes might have the potential to aggravate CVD.…”

“…This is especially relevant because APOL1 protein travels in the circulation bound to a subset of trypanosome lytic factors 1 and 2 lipoprotein particles; it does not appear to be bound to typical plasma high-density lipoprotein (HDL) particles as has often been reported. 7,8 To date, there is no evidence of association between APOL1 genotypes and circulating APOL1 protein concentrations or components of the standard lipid profile, such as low-density lipoprotein (LDL) cholesterol, HDL cholesterol, or triglycerides. [9][10][11][12][13] A report of more than 2,000 African American participants in the REGARDS study detected positive association between APOL1 renal-risk variants and small HDL cholesterol particle concentrations.…”