Flagellar membrane fusion and protein exchange in trypanosomes; a new form of cell-cell communication?

Abstract

Diverse structures facilitate direct exchange of proteins between cells, including plasmadesmata in plants and tunnelling nanotubes in bacteria and higher eukaryotes. Here we describe a new mechanism of protein transfer, flagellar membrane fusion, in the unicellular parasite Trypanosoma brucei. When fluorescently tagged trypanosomes were co-cultured, a small proportion of double-positive cells were observed. The formation of double-positive cells was dependent on the presence of extracellular calcium and was enhanced by placing cells in medium supplemented with fresh bovine serum. Time-lapse microscopy revealed that double-positive cells arose by bidirectional protein exchange in the absence of nuclear transfer. Furthermore, super-resolution microscopy showed that this process occurred in ≤1 minute, the limit of temporal resolution in these experiments. Both cytoplasmic and membrane proteins could be transferred provided they gained access to the flagellum. Intriguingly, a component of the RNAi machinery (Argonaute) was able to move between cells, raising the possibility that small interfering RNAs are transported as cargo. Transmission electron microscopy showed that shared flagella contained two axonemes and two paraflagellar rods bounded by a single membrane. In some cases flagellar fusion was partial and interactions between cells were transient. In other cases fusion occurred along the entire length of the flagellum, was stable for several hours and might be irreversible. Fusion did not appear to be deleterious for cell function: paired cells were motile and could give rise to progeny while fused. The motile flagella of unicellular organisms are related to the sensory cilia of higher eukaryotes, raising the possibility that protein transfer between cells via cilia or flagella occurs more widely in nature.

KHARON Is an Essential Cytoskeletal Protein Involved in the Trafficking of Flagellar Membrane Proteins and Cell Division in African Trypanosomes

“…This observation further underscores the importance of flagellar membrane proteins in the biology and virulence of African trypanosomes. Furthermore, flagellar membranes of PF trypanosomes can fuse with each other and transfer both membrane-bound and cytosolic proteins between the two flagellarattached parasites (37), suggesting that flagella may be organelles involved in cell-cell communication. In addition, flagellar adenylate cyclases are involved in the process of social motility (8 -10), a coordinated motility process that is important for migration of parasites within the tsetse fly and for successful colonization of this insect vector (38).…”

Recent advances in trypanosomatid research: genome organization, expression, metabolism, taxonomy and evolution

“…Parasitology 9 exchange between two trypanosomes seems to occur by flagellar membrane exchange, and both short and long-term fusions have been observed in cultured trypanosomes (Imhof et al, 2016). The trypanosomatid flagellum, responsible for motility, contains the classical 9 + 2 axoneme (Ginger et al, 2008).…”

The languages of parasite communication

“…Filament formation was observed in the mosquito midgut as well as in culture, indicating that it is a natural process, most probably to facilitate contact between gametes and subsequent fertilisation [56]. An unusual form of direct communication has recently been described for procyclic form trypanosomes [57]. This involves flagellar membrane fusion and the rapid exchange of proteins between connected cells, without any transfer of DNA [57].…”

“…An unusual form of direct communication has recently been described for procyclic form trypanosomes [57]. This involves flagellar membrane fusion and the rapid exchange of proteins between connected cells, without any transfer of DNA [57]. The one prerequisite for exchange is that the proteins gain access to the flagellum.…”