Membranes and vesicles in RNA interference and intercellular RNA transport
RNA interference (RNAi), RNA-induced sequence-specific degradation of mRNA, has emerged as a major mechanism of gene regulation in most eukaryotes and has important implications in biomedical research and drug development. Extensive research has led to a relatively detailed understanding of this process in the short time since its discovery. However, a much less explored aspect of RNAi is the uptake and transport of RNAi silencing signals between cells in animals. Intriguingly, recent studies suggest that this represents a new means of cell-cell communication. Increased knowledge about spreading of RNAi is also of importance for the development of RNA-based drugs in order to efficiently deliver the drug to the location of the target mRNA. In some organisms, such as the nematode Caenorhabditis elegans, RNAi uptake and spreading occurs with high efficiency. By taking advantage of the many molecular and genetic tools available for C. elegans, a number of proteins required for RNA transport has been discovered.
From our previous studies, we know that the SID-5 protein is required for cell-cell transport of RNAi silencing signals in C. elegans and that it localizes to late endosomes/multivesicular bodies (MVBs). Studies in Drosophila and mammalian cells have shown that MVBs are required for efficient cell autonomous RNAi whereas intact lysosomes seem to limit the RNAi efficiency. The co-localization of SID-5 and MVBs indicates that the RNAi transport pathway could also rely on endocytosis. However, the function of SID-5 in RNAi uptake and spreading is still unclear. We therefore performed a membrane yeast-two-hybrid screen, which identified several putative SID-5 interacting proteins. Current research is focused on further analysis of these proteins and their potential interactions with SID-5. We are also investigating whether SID-5 and its associated proteins affect microRNA-mediated silencing. Together, these experiments will give us clues to how endolysosomal compartments regulate small RNA pathways and transport of RNA between cells in animals.