Acanthamoeba castellanii is the etiological agent of amoebic
keratitis and is present in the environment in trophozoite or
cyst forms. Both forms can infect the vertebrate host and colonize different tissues. The high resistance of
cysts to standard drugs used in clinics contributes to the lack of effective treatments. Therefore, in this context, studies have emerged to understand
cyst physiology and metabolism.
Phosphate transporters are
proteins responsible for the uptake of extracellular
inorganic phosphate and transport to the cytosol. This work aims to verify the relationship between Pi transport and energetic metabolism in
cysts of A. castellanii. The
phosphate uptake ratio was higher in
cysts compared with trophozoites. Recently, three sequences related to
phosphate transporters have been identified in the A. castellanii genome (AcPHS1, AcPHS2, and AcPHS3); the
messenger RNA expression levels of which differ depending on the amoeba life form. Pi uptake in
cysts displayed peak activity at alkaline pH, whereas Pi transport in trophozoites was not affected in the same pH ranges.
Cysts harbor a low-affinity Pi transport system (K0,5 and Vmax values of 1.76 ± 0.26 mM and 104.6 ± 6.3 nmol Pi × h-1 × 106 cells) compared to the trophozoite
phosphate transport system. Pi transport seems important for anaerobic
adenosine triphosphate synthesis in
cysts, which initially occurs through the glycolytic pathway and subsequently through the
pyruvate ferredoxin oxidoreductase pathway. Altogether, these results suggest that contrary to that previously postulated,
cysts are active metabolic forms, and, as noted in trophozoites,
phosphate uptake is important for energetic metabolism.