The challenging world of extracellular RNAs of helminth parasites

Abstract

In the last years, cell free or extracellular RNAs (ex-RNAs) have emerged as novel intercellular messengers between animal cells, including pathogens. In infectious diseases, ex-RNAs represent novel players in the host-pathogen and pathogen-pathogen interplays and have been described in parasitic helminths from the three major taxonomic groups: nematodes, trematodes and cestodes. Altogether, it is estimated that approximately 30 percent of the world's population is infected with helminths, which cause debilitating diseases and syndromes.Ex-RNAs are protected from degradation by encapsulation in extracellular vesicles (EV), or association to proteins or lipoproteins, and have been detected in the excretion/secretion products of helminth parasites, with EV as the preferred extracellular compartment under study. EV is the generic term used to describe a heterogenous group of subcellular membrane-bound particles, with varying sizes, biogenesis, density and composition. However, recent data suggests that this is not the only means used by helminth parasites to secrete RNAs since ex-RNAs can also be found in EV-depleted samples. Furthermore, the use of pathogen ex-RNAs as biomarkers promise the advent of new diagnostic tools though this field is still in early stages of exploration.In this review, we summarize current knowledge of vesicular and non-vesicular ex-RNAs secretion in helminth parasites, their potential as biomarkers and the evidence of their role in parasite and host reciprocal communication, together with unanswered questions in the field.