Insect Stage-Specific Adenylate Cyclases Regulate Social Motility in African Trypanosomes

Abstract

b Sophisticated systems for cell-cell communication enable unicellular microbes to act as multicellular entities capable of grouplevel behaviors that are not evident in individuals. These group behaviors influence microbe physiology, and the underlying signaling pathways are considered potential drug targets in microbial pathogens. Trypanosoma brucei is a protozoan parasite that causes substantial human suffering and economic hardship in some of the most impoverished regions of the world. T. brucei lives on host tissue surfaces during transmission through its tsetse fly vector, and cultivation on surfaces causes the parasites to assemble into multicellular communities in which individual cells coordinate their movements in response to external signals. This behavior is termed "social motility," based on its similarities with surface-induced social motility in bacteria, and it demonstrates that trypanosomes are capable of group-level behavior. Mechanisms governing T. brucei social motility are unknown. Here we report that a subset of receptor-type adenylate cyclases (ACs) in the trypanosome flagellum regulate social motility. RNA interference-mediated knockdown of adenylate cyclase 6 (AC6), or dual knockdown of AC1 and AC2, causes a hypersocial phenotype but has no discernible effect on individual cells in suspension culture. Mutation of the AC6 catalytic domain phenocopies AC6 knockdown, demonstrating that loss of adenylate cyclase activity is responsible for the phenotype. Notably, knockdown of other ACs did not affect social motility, indicating segregation of AC functions. These studies reveal interesting parallels in systems that control social behavior in trypanosomes and bacteria and provide insight into a feature of parasite biology that may be exploited for novel intervention strategies.

The Social Life of African Trypanosomes

“…The same phenotype was observed after dual knockdown of ACP1&2 [38]; these two ACs are closely related, but diverge at their C-termini and are differentially localised to the tip and entire flagellum, respectively [37]. At first sight it might seem puzzling that the stock used by Lopez et al [38] produced fewer than half the number of radial projections seen by Imhof et al [28], although both were derived from T. b. were 5-fold more abundant in late procyclic forms [26]. Since the 3' untranslated regions (UTRs) of the two transcripts are unique, however, this allows them to be distinguished by qRT-PCR [38] and RNA-Seq.…”

“…Intriguingly, knockdown of ACP6 by RNA interference resulted in a hypersocial phenotype with the production of more radial projections than the parental stock, suggesting that decreased levels of cAMP favour SoMo [38]. The same phenotype was observed after dual knockdown of ACP1&2 [38]; these two ACs are closely related, but diverge at their C-termini and are differentially localised to the tip and entire flagellum, respectively [37].…”

“…After a hiatus of several years following the first report on SoMo [7], four new publications have appeared within a few months of each other [26,28,38,44].…”

“…At first sight it might seem puzzling that the stock used by Lopez et al [38] produced fewer than half the number of radial projections seen by Imhof et al [28], although both were derived from T. b. were 5-fold more abundant in late procyclic forms [26]. Since the 3' untranslated regions (UTRs) of the two transcripts are unique, however, this allows them to be distinguished by qRT-PCR [38] and RNA-Seq. Based on reads mapping to the 3' UTRs, ACP1 mRNA is 5 times more abundant in late procyclic forms, whereas ACP2 is reduced almost 2-fold (A. Naguleswaran and IR, unpublished data).…”

The languages of parasite communication

“…Mixing PDEB1 knockdown mutants with wild-type cells overcomes the defect, indicating that the downstream signalling cascade is still intact [19]. Negative regulators of SoMo have also been identified: knocking down the procyclic form-specific adenylate cyclases ACP1/2 and ACP6 result in a hypersocial phenotype, with the production of more radial projections than in communities expressing wild-type levels [21]. Interestingly, ACP1 and ACP6 both localise to the flagellar tip [22].…”