Membrane Permeabilization by Trypanosome Lytic Factor, a Cytolytic Human High Density Lipoprotein

Abstract

Trypanosome lytic factor (TLF) is a subclass of human high density lipoprotein (HDL) that mediates an innate immune killing of certain mammalian trypanosomes, most notably Trypanosoma brucei brucei, the causative agent of a wasting disease in cattle. Mechanistically, killing is initiated in the lysosome of the target trypanosome where the acidic pH facilitates a membranedisrupting activity by TLF. Here we utilize a model liposome system to characterize the membrane binding and permeabilizing activity of TLF and its protein constituents, haptoglobinrelated protein (Hpr), apolipoprotein L-1 (apoL-1), and apolipoprotein A-1 (apoA-1). We show that TLF efficiently binds and permeabilizes unilamellar liposomes at lysosomal pH, whereas non-lytic human HDL exhibits inefficient permeabilizing activity. Purified, delipidated Hpr and apoL-1 both efficiently permeabilize lipid bilayers at low pH. Trypanosome lytic factor, apoL-1, and apoA-1 exhibit specificity for anionic membranes, whereas Hpr permeabilizes both anionic and zwitterionic membranes. Analysis of the relative particle sizes of susceptible liposomes reveals distinctly different membrane-active behavior for native TLF and the delipidated protein components. We propose that lysosomal membrane damage in TLF-susceptible trypanosomes is initiated by the stable association of the TLF particle with the lysosomal membrane and that this is a property unique to this subclass of human HDL.

APOL1 Localization in Normal Kidney and Nondiabetic Kidney Disease

“…As a constituent of HDL 3 particles, it has been assumed, but not demonstrated, that APOL1 participates in cholesterol metabolism. In addition, APOL1 forms ion channels in lipid bilayers 20,21 and may disrupt membranes, 22 but these studies, while informative about APOL1's trypanolytic activity, do not illuminate function within normal cells. Currently, the function of APOL1 in any of the resident cells of the kidney is entirely speculative.…”

“…5,6 Using 1000 Genomes Project sequence data, 7 two groups demonstrated that coding variants within the neighboring APOL1 gene, 8,9 which is centromeric to MYH9 on 22q12.3, accounted for the association signal. APOL1 is one of six closely related apolipoprotein L family member genes clustered on chromosome 22. APOL1 is restricted to the genomes of humans and some nonhuman primates.…”

The Plasma Membrane of Bloodstream-form African Trypanosomes Confers Susceptibility and Specificity to Killing by Hydrophobic Peptides

“…Calcein Release Assays-Permeabilization of unilamellar liposomes was assayed as described previously (21). Briefly, liposomes were diluted 1:1000 into 50 mM Tris (pH 7.0), and calcein fluorescence was monitored at 513 nm when excited at 484 nm.…”

The Changing Face of HDL and the Best Way to Measure It

“…Trypanosome lytic factor (TLF), an HDL subfraction containing apoA-I, apoL-1, and haptoglobin-related protein (HRP), is responsible for the innate killing of susceptible trypanosome parasites (127)(128)(129)(130). The HRP component of TLF binds with high affinity to the cell-surface receptor of the parasite, promoting endocytosis and trafficking of the TLF-parasite particle complex to the lysosome (131)(132)(133)(134). Once contained within the acidified lysosome of the trypanosome, the apoL-1 component of TLF mediates lysosomal rupture and spilling of lysosomal contents into the cytoplasm, effectively killing the trypanosome.…”