Fatty Acid Resource Allocation and the Survival of African Trypanosomes in the Host Bloodstream
Authors: Paul Kimberly S , McKnight Ciara A. , Vigueira Patrick A. 
Published: November 2023

Trypanosoma brucei, a lethal unicellular parasite transmitted by the tsetse fly, infects the bloodstream and cerebrospinal fluid of humans, causing African sleeping sickness.

In the blood, T. brucei evades the adaptive immune system by varying its major surface coat antigen, the Variant Surface Glycoprotein (VSG). VSG is anchored to the plasma membrane by a glycosylphosphatidylinositol (GPI)‐anchor whose fatty acid moiety is exclusively myristate, a saturated 14‐carbon fatty acid. VSG GPI‐myristoylation creates a high demand for myristate by the parasite, yet myristate is rare in blood. The parasite meets its myristate demand through an ER‐associated fatty acid synthesis pathway specialized for synthesizing myristate. T. brucei also has a mechanism to evade the innate immune system that involves up‐regulation of VSG endocytosis/recycling with concomitant degradation of surface‐bound antibodies and serum components. This up‐regulation of endocytosis enables the parasite to “scrub” its surface coat, enabling it to escape complement‐mediated lysis. The vesicle trafficking events governing endocytosis and exocytosis are dependent upon myristoylated proteins (e.g. ARF, ARL), and thus represent a separate sink for myristate in addition to VSG GPI‐myristoylation. We placed the cellular myristate supply under stress by using RNA interference (RNAi) to reduce expression of the first enzyme in the fatty acid synthesis pathway, Acetyl‐CoA Carboxylase (ACC). Previous work showed that a bloodstream form (BF) ACC RNAi strain exhibited reduced virulence in mice, but normal growth in culture, suggesting that major cellular functions were not disrupted. As the phenotype in mice was observed in a time frame before a specific antibody response could develop, we hypothesized that ACC was required for immune evasion via endocytosis upregulation. To test our hypothesis, we examined endocytosis in BF ACC RNAi cells using fluorescent dextran and transferrin as markers for fluid‐phase and receptor‐mediated endocytosis, respectively. After 9 days of ACC RNAi, we observed a significant reduction in fluid phase (88%) and receptor‐mediated endocytosis (87%). Next, we tested the clearance of surface bound antibodies and found that ACC RNAi caused a significant 26% delay in surface antibody clearance. Lastly, we tested avoidance of complement‐mediated lysis. ACC RNAi resulted in a significant 42% increase in complement‐mediated lysis. We propose that ACC RNAi caused a myristate shortage in the cells that was insufficient to meet the demands of both the GPI‐myristoylation and protein myristoylation pathways. As GPI‐myristoylation occurs in the same organelle as fatty acid synthesis, we are currently testing the hypothesis that GPI‐myristoylation consumed the lion's share of the myristate in the ACC RNAi cells, leaving little left over for protein myristoylation, thereby leading to under‐myristoylated trafficking factors and reduced endocytic function.

Support or Funding Information

Supported by funds from the NIH R15 AI081207 and Clemson University Creative Inquiry Program.