Kenneth Witwer, Johns Hopkins University School of Medicine, USA
In cross-kingdom RNA communication, the nematode Caenorhabditis elegans is the known champion, using ingested environmental RNA as a type of immune system (Jose, 2015). Such cross-kingdom communication has also been described or proposed in various host-pathogen interactions, e.g. in the relationship of retroviruses with their hosts (Klase et al, 2007; Wagschal et al, 2012). Applied to mammals, the so-called ‘dietary RNA hypothesis’ suggests that intact RNAs present in the food may enter the ingesting organism and exert gene expressional functions in its cells (Witwer and Hirschi, 2014). Studies of mammalian uptake have focused mostly on miRNAs, or ‘xenomiRs’ to denote their foreign origin. However, in the meantime, enthusiasm about the absorption and function of xenomiRs has been diminished by negative findings and evidence of contamination and experimental design flaws that account for apparently positive results (Mlotshwa et al, 2015). Nevertheless, despite scant and suspect evidence for the hypothesis, interest is likely to continue into the foreseeable future. Focusing on mammals exposed to plants and milk (Dickinson et al, 2013; Snow et al, 2013; Witwer et al, 2013; Baier et al, 2014; Tosar et al, 2015), experimental results and current understanding of RNA stability, transport, and function do not appear to be consistent with proposed forms of cross-kingdom communication. In many cases, presumably positive ncRNA findings were shown to result from, e.g. sample contaminations or artefacts caused by technical limitations of the applied technologies (Witwer, 2015).
Why may viruses enter cells and induce miRNA production, whereas plant miRNA apparently are unable to enter the body? – Viruses actively infect (invade) cells. When they come into contact with host cells, they actually interact with the cell membranes, for example, by means of fusogenic proteins that are generally not found on miRNA-containing lipid complexes (van Dongen et al, 2016). Furthermore, miRNAs produced from viral transcripts are made entirely by the host machinery and incorporated into host silencing complexes. It is not known how miRNAs from the diet, such as plant miRNAs, would enter cells, and a mechanism for transfer of miRNA from a plant AGO to a host AGO has not been proposed. miRNAs are not known to be released from AGO once bound (Martinez et al, 2002).
Have differences in the stability of miRNAs been recorded? – Generally, miRNAs are stable. A miRNA that is found in any body fluid must be protected. For instance, if it is present in the extracellular plasma of the blood, it must be AGO-bound. Even if the AGO is undetectable with current techniques, such as within extracellular vesicles, a protein binding partner must nevertheless be present. Therefore, the detection of an increase or a decrease of a miRNA in a certain matrix requires careful interpretation since its functionality may depend on the protein binding partner.