Cell-to-Cell Heterogeneity in Trypanosomes.

Recent developments in single-cell and single-molecule techniques have revealed surprising levels of heterogeneity among isogenic cells. These advances have transformed the study of cell-to-cell heterogeneity into a major area of biomedical research, revealing that it can confer essential advantages, such as priming populations of unicellular organisms for future environmental stresses. Protozoan parasites, such as trypanosomes, face multiple and often hostile environments, and to survive, they undergo multiple changes, including changes in morphology, gene expression, and metabolism. But why does only a subset of proliferative cells differentiate to the next life cycle stage? Why do only some bloodstream parasites undergo antigenic switching while others stably express one variant surface glycoprotein? And why do some parasites invade an organ while others remain in the bloodstream? Building on extensive research performed in bacteria, here we suggest that biological noise can contribute to the fitness of eukaryotic pathogens and discuss the importance of cell-to-cell heterogeneity in trypanosome infections.

Spatial integration of transcription and splicing in a dedicated compartment sustains monogenic antigen expression in African trypanosomes.

Highly selective gene expression is a key requirement for antigenic variation in several pathogens, allowing evasion of host immune responses and maintenance of persistent infections1. African trypanosomes-parasites that cause lethal diseases in humans and livestock-employ an antigenic variation mechanism that involves monogenic antigen expression from a pool of >2,600 antigen-coding genes2. In other eukaryotes, the expression of individual genes can be enhanced by mechanisms involving the juxtaposition of otherwise distal chromosomal loci in the three-dimensional nuclear space3-5. However, trypanosomes lack classical enhancer sequences or regulated transcription initiation6,7. In this context, it has remained unclear how genome architecture contributes to monogenic transcription elongation and transcript processing. Here, we show that the single expressed antigen-coding gene displays a specific inter-chromosomal interaction with a major messenger RNA splicing locus. Chromosome conformation capture (Hi-C) revealed a dynamic reconfiguration of this inter-chromosomal interaction upon activation of another antigen. Super-resolution microscopy showed the interaction to be heritable and splicing dependent. We found a specific association of the two genomic loci with the antigen exclusion complex, whereby VSG exclusion 1 (VEX1) occupied the splicing locus and VEX2 occupied the antigen-coding locus. Following VEX2 depletion, loss of monogenic antigen expression was accompanied by increased interactions between previously silent antigen genes and the splicing locus. Our results reveal a mechanism to ensure monogenic expression, where antigen transcription and messenger RNA splicing occur in a specific nuclear compartment. These findings suggest a new means of post-transcriptional gene regulation.