Cholesterol or linoleic acid are essential exogenous lipids for tick cell proliferation

Sterol molecules play indispensable roles in biology of cells and organisms. Ticks, pathogen-transmitting blood-feeding arthropods, lack the genes for the sterol biosynthetic pathway, yet cholesterol is their integral molecular constituent. Here, we demonstrate that the absence of de novo sterol biosynthesis in ticks is compensated by specific acquisition mechanisms of exogenous cholesterol. In embryo-derived Ixodes ricinus tick cell line, cholesterol, but not other sterols (ergosterol, sitosterol), sustains cellular proliferation in lipid-depleted media. In addition to cholesterol, linoleic acid (18:2n-6) also promotes proliferation. Unlike vertebrate cells, tick cells allocate only a minor fraction of cholesterol to the plasma membrane, with the majority of it being trafficked to and stored in intracellular compartments, including lysosomes and lipid droplets. Using RNA-seq, we further show that levels of mRNA transcripts that encode several components of cholesterol exporting machinery, which is localised in lysosomes, are upregulated in response to exogenous lipids. Together, these findings identify cholesterol and linoleic acid as critical determinants of tick cell lipid homeostasis and proliferation.