There have now been a number of reports demonstrating the surprisingly stable presence of microRNAs in extracellular compartments such as blood and urine. This area, however, has long struggled to gain widespread acceptance in the RNAi community because of concerns that they could just represent microRNAs associated with cellular debris. On the other hand, their occurrence in increasingly better defined 30-100nm lipid nanoparticles that are generated by intracellular membrane budding and then exocytosis outside of cells strongly argue for a physiological role for these ‘exosomes’ with implications for small RNA diagnostics and therapeutics.
Speculation that exosomes could function as intercellular transport vehicles for RNAs, especially microRNAs, is relatively recent. Before this, they were largely thought to play a role in immunomodulation (both stimulation and induction of tolerance), pathogen spread, and for cancer cells to manipulate their microenvironment for example by setting free matrix metalloproteinases that would facilitate their metastatic spread (for a review, see Schorey and Bhatnagar). The recent focus on microRNAs is justified, however, because small RNAs are clearly enriched in intact form in these particles. Moreover, evidence for actual cellular RNA transfer has been presented by showing that mRNAs in exosomes from one cell type can be translated in a second recipient cell (Valadi et al, 2007), and that small silencing RNAs (microRNAs, siRNAs) can be similarly transferred and mediate modest silencing (Kosaka et al, 2010).
My initial interest in exosomes was founded on the prospect for microRNA diagnostics based on fluids instead of sometimes difficult-to-obtain biopsies. A physiological role for exosomes would therefore add to the confidence that such microRNAs would be of sufficient abundance in such fluids and that their content may be subject to less variability than might be expected for degradation products. Overall, despite Rosetta Genomics’ failed attempt to develop a blood-based screening test for colon cancer, the theoretical foundation for further pursuing such microRNA Dx is quite strong now.
The natural spread of silencing from one cell to another could also be relevant to RNAi Therapeutics since delivery of a large amount of small RNAs to one cell may facilitate the silencing in a neighboring cell through the exosomal pathway. This may be particularly useful for DNA-directed RNAi approaches where vector transduction of a target tissue may not be complete, but once transduced, a cell is able to generate more than sufficient amounts of siRNAs. I can vaguely remember now-defunct ddRNAi company Nucleonics making such claims which, ironically, actually may have hurt their credibility. Although I would susupect that such an effect would not be terribly strong, it could certainly contribute to more homogeneous silencing especially with highly potent siRNAs. This spread would be analogous to a process referred to a systemic silencing well known for organisms like plants and worms, but for which a homologous process has not been believed to exist in humans.
Finally, the study of ‘physiological liposomes’ may allow us to learn a trick or two for the design of RNAi Therapeutics delivery technologies, especially those based on liposomes. Ceramide for example has recently been shown to be important for the small RNA transfer function of exosomes in a study apparently co-founded by Japanese company Dainippon Sumitomo which has also recently expanded their early collaborative efforts on RNAi Therapeutics with Silence Therapeutics, including Silence’s lipoplexes and liposomes. In addition to studying the lipid composition of exosomes, other areas of interest might be the mechanism of cell uptake and membrane fusion. Maybe after all, RNAi delivery may not be as ‘unnatural’ and therefore daunting as once thought.
For a link to a dedicated website on exosomes click here.