Fine Reading: Trans-kingdom Cross Talk
Small RNAs on the Move
by Elio
We have lived with the discovery of small RNAs as regulatory molecules for nearly 30 years, so for most readers of this blog this is old hat. But some of us oldsters are still reeling from the novelty and importance of the findings. It seems odd that a subject of this significance, one that has so many ramifications, should have been relatively slow in coming. The analytical tools needed for its discovery were available long before the 1980's. So why the delay? One reason is that the idea of gene silencing by small RNAs did not fit neatly into the Zeitgeist, the prevailing scheme of things at the time. It took a lot of work to unravel the mechanism, which goes to show you what you already knew, that what counts is being open to new thoughts.
A Botrytis cinerea can transfer Bc-siRNA to its host. This process has been shown to be dependent on AGO1 in the host, Arabidopsis thaliana and on both Dcl1 and 2 in B. cinerea. B Human miRNAs can be translocated to the malaria-parasite, P. falciparum, where they interfere with translation. C The nematode C. elegans has been shown to take up E. coli-produced ncRNAs that subsequently influence their foraging behavior. This is dependent on the C. elegans protein RDE‑2, that is essential for RNAi. D The Chagas disease-causing parasite, T cruzi, produces tRNA-derived sRNAs (tsRNAs) that are exported from the cell in vesicles. These vesicles are shown to increase infectability of host cells, suggesting this might be caused by the tsRNAs. This has not been shown directly though. E The expression of sRNA-generating constructs to silence genes in pathogens, or other closely associated species, has now been demonstrated for many species combinations. This process is suggested to be dependent on Dcl1, since Dcl2, 3, and 4 seem to be dispensable to induce silencing by an Arabidopsis-expressed hairpin in the insect, Helicoverpa armigera. Source
The story is now well established and parts of it fairly well understood. Yet, there are surprises left. One of them is the extent to which small RNAs are being teleported across huge phylogenetic chasms. Here, national borders don't seem to count for much and molecules made by members of one kingdom are active on those of another. Not surprisingly, such interactions are known mainly between organisms that interact with one another, such as symbionts, and parasites and their hosts. This topic is handsomely treated in a clearly written and highly stimulating review by Knip and collaborators. Examples treated include cross-talk between bacteria, fungi, plants, nematodes, insects, and vertebrates in a large number of pairwise combinations. Some of this communication has positive effects, some negative. Notice that such molecular intercellular communication requires that they be secreted by their makers and transported into the recipients, where their presence must be duly noticed. These events are not always understood. In fact, much is left to be discovered. Reading this review will give you an opening into of what is left to be understood.
Reference
Knip M, Constantin ME, Thordal-Christensen H (2014). Trans-kingdom Cross-Talk: Small RNAs on the Move. PLoS genetics, 10 (9). PMID 25188222
