Glycosome biogenesis in trypanosomes and the de novo dilemma.

Trypanosomatid parasites, including Trypanosoma and Leishmania, are the causative agents of lethal diseases threatening millions of people around the world. These organisms compartmentalize glycolysis in essential, specialized peroxisomes called glycosomes. Peroxisome proliferation can occur through growth and division of existing organelles and de novo biogenesis from the endoplasmic reticulum. The level that each pathway contributes is debated. Current evidence supports the concerted contribution of both mechanisms in an equilibrium that can vary depending on environmental conditions and metabolic requirements of the cell. Homologs of a number of peroxins, the proteins involved in peroxisome biogenesis and matrix protein import, have been identified in T. brucei. Based on these findings, it is widely accepted that glycosomes proliferate through growth and division of existing organelles; however, to our knowledge, a de novo mechanism of biogenesis has not been directly demonstrated. Here, we review recent findings that provide support for the existence of an endoplasmic reticulum (ER)-derived de novo pathway of glycosome biogenesis in T. brucei. Two studies recently identified PEX13.1, a peroxin involved in matrix protein import, in the ER of procyclic form T. brucei. In other eukaryotes, peroxins including PEX13 have been found in the ER of cells undergoing de novo biogenesis of peroxisomes. In addition, PEX16 and PEX19 have been characterized in T. brucei, both of which are important for de novo biogenesis in other eukaryotes. Because glycosomes are rapidly remodeled via autophagy during life cycle differentiation, de novo biogenesis could provide a method of restoring glycosome populations following turnover. Together, the findings we summarize provide support for the hypothesis that glycosome proliferation occurs through growth and division of pre-existing organelles and de novo biogenesis of new organelles from the ER and that the level each mechanism contributes is influenced by glucose availability.