FIG. 11.

Relationship between proline, trypanosomes, and the insect host. (A) Trypanosomes exhibit a complex life cycle, cycling between a vertebrate host and an insect vector. On infection of a vertebrate host, metacyclic trypomastigotes will differentiate into the intracellular amastigote form, a form characterized by no visible flagella. The amastigotes will then reproduce within host cells before differentiating into intracellular epimastigotes, followed by differentiation into trypomastigotes and subsequent bursting of the host cell. The trypomastigotes are then free to infect new cells or be passed into the insect vector. In the insect vector, trypomastigotes differentiate into procyclic trypomastigotes capable of reproduction. The procyclic trypomastigotes will then differentiate into epimastigotes, followed by differentiation into the infective metacyclic trypomastigote form. The trypomastigote and epimastigote forms are primarily distinguished by the location of the kinetoplast (solid, black circle) relative to the nucleus (dashed circle). Proline and its catabolism have been implicated in the differentiation of trypanosomes from the epimastigote to the metacyclic trypomastigote forms and from the amastigote to the intracellular epimastigote forms. (B) In the tsetse fly, proline is utilized as an energy source by flight muscles or during lactation. Alanine generated from proline is then transported to fat bodies for proline regeneration. This conversion of alanine back to proline relies on pyridoxal 5′-phosphate (vitamin B₆) generated by the symbiont Wigglesworthia. Trypanosomes actively uptake proline from the insect host for utilization in numerous processes, including energy production, cellular differentiation, and osmotic stress protection. To see this illustration in color, the reader is referred to the web version of this article at www.liebertpub.com/ars