Differentiation of a culture-adapted mutant bloodstream form of Trypanosoma brucei into the procyclic form results in growth arrest of the cells

Abstract

The bloodstream forms of Trypanosoma brucei brucei monomorphic strain 427 serially passaged in rats can differentiate in vitro equally well in HMI-9, HMI-10, SDM-79 or Cunningham's medium into the insect (procyclic) forms by a simple temperature shift from 37 to 26°C in the presence of citrate and cis-aconitate. The procyclic forms thus generated can also continue to multiply at 26°C without replacing the culture medium. The same strain of T. brucei pre-adapted to grow as bloodstream forms in HMI-10 medium at 37°C is also capable of differentiating showing a similar rate of variant surface glycoprotein (VSG) disappearance and appearance of phospho enolpyruvate carboxykinase (PEPCK) under the same experimental conditions. However, appearance of both procyclin mRNA and procyclin protein is much delayed and the resulting procyclic forms cannot multiply. The culture-adapted bloodstream forms are capable of infecting rats, and the cells thus harvested from the rats can differentiate but cannot multiply in the same manner as the original culture-adapted bloodstream forms. Apparently, a certain variant has been selected during the adaptation of T. brucei bloodstream forms from rat blood to the culture medium. This variant could be a useful tool for identifying the genes involved in differentiation of T. brucei and multiplication of the procyclic forms. Comparison of the protein profiles between the wild-type and the variant during differentiation showed that a major protein band of about 70 kDa remained in the non-dividing variant procyclic forms but vanished in the rapidly dividing wild type procyclic forms. N-terminal determinations indicated that the 70-kDa protein band consists of bovine serum albumin and fetuin. Presumably these two serum proteins are actively taken up by the bloodstream forms via endocytosis. Since the procyclic forms are incapable of endocytosis, the serum proteins may be rapidly diluted in the growing wild type procyclic cells but remain unchanged in the non-dividing procyclic cells of the variant. Further studies are underway in trying to identify the key distinctions between these two lines of cells at the molecular level.