Just a few years ago miRNAs were viewed as pretty solitary transcripts, exhibiting their regulatory muti-tasking skills primarily within their micro-environments. Recent evidence however has shown that miRNAs can spring from their cellular confines when secreted in exosomes, our cells’ version of public transit. A few groups have hypothesized that these exosome-associated miRNAs might play a role in intercellular communication, but some key bits of mechanistic evidence have been elusive. This prompted several groups, including a crew of talented of folks from the Netherlands and US, to take a closer look at what seems to be extroverted behavior from miRNAs.
In data published in PNAS last week, the team looked at how Epstein Barr Virus (EBV)-transformed cells such as, lymphoblastoid B cells (LCL), can communicate with primary immature monocyte-derived dendritic cells (aka MoDC). The team used a co-culturing method where a top chamber of MoDC cells was separated from stained LCL cells by a 1 uM membrane. This way only the stained exosomes could move across to mingle with the MoDC cells. Here’s what unfolded next:
- The MoDC cells took on the fluorescence indicating that they were internalizing the exosomes from the LCL cells.
- EBV miRNAs from the LCL cells were present in the MoDC cells supporting that miRNAs were packaged in the exosomes.
Now things get really interesting. It’s been shown that viruses might use miRNAs to hijack host miRNA machinery to keep immune responses in the dark. To verify this and to check if the exosome-associated miRNAs were functional, the team shifted to HeLa cells, where they expressed a miRNA reporter vector containing the 3’ UTR of CXCL11, an immunostimulatory gene that has been shown to be repressed by an EBV miRNA, BHRF1-3.
- After incubating the cells with purified BHRF1-3 -positive exosomes, they saw an 80% reduction in reporter activity showing that the miRNAs could repress as usual.
- To validated the repression, they used another reporter with a mutation in the BHRF1-3 binding site which significantly reduced the level of repression
Check out PNAS, March 2010 and get dialed into the miRNA social scene.