Microbial extracellular RNAs and their roles in human diseases

Extracellular RNAs (exRNAs) are released by extracellular vesicles, small membranous nanoparticles secreted by all cell types. When transported into cells, exRNAs can modulate gene expression or cellular responses in the target cells since many small RNAs have regulatory functions. Indeed, it is widely acknowledged that endogenous exRNAs in the human body are related to various diseases. However, microbial exRNAs have been less studied, and their connection to host diseases has just begun to be explored. In this review, I will discuss analytical methods for exRNAs and the potential use of exRNAs as disease biomarkers. I also consider current progress in understanding the regulation of host mechanisms by microbial exRNAs as inter-kingdom communication, efforts to utilize extracellular vesicles as therapeutic vehicles loaded with engineered RNA cargos, and a putative connection between microbial exRNA-based regulation of host responses and human diseases such as Alzheimer’s. This overview aims to present novel insights into pathogenesis with regard to the function of microbial exRNAs as “disease-relevant travelers.” Impact statement: The number of commensal bacteria in the body surpasses the number of actual human cells. Thus, various interactions between microbes and human cells constitute an inevitable phenomenon. Recent evidence has led to bacterial extracellular RNAs (exRNAs) being proposed as good candidates for microbe–host inter-kingdom communication tools as they can modulate the expression of host genes. However, research findings on the relevance of interactions between extracellular RNA and human diseases are still in their infancy. Nevertheless, substantial data suggest that microbial exRNAs are implicated in various human diseases both at local and distant sites. By exploring various scenarios for the involvement of microbial exRNAs in human diseases, we may better understand the role of exRNAs as “communication signals” for diseases and thereby develop novel therapeutic strategies by using them and their carrier extracellular vesicles.