MicroRNAs: one of the sexiest topics in the science community these days! Pick up a copy of Science, Nature, or Cell and you can usually find something to do with microRNAs within. What do these have to do with the kidney? Well, it's still a little premature, but there are certainly microRNAs present in the kidney, and recent studies have demonstrated that specific microRNAs are upregulated in conditions such as polycystic kidney disease and renal cell carcinomas, for instance.
How microRNAs work: microRNAs are single-stranded RNA molecules of between 21-23 nucleotides in length which are partially complementary to regions in multiple mRNAs. Once they bind to these mRNAs, the microRNAs either inhibit translation or completely degrade their target RNAs. A specific enzymatic machinery--comprised of the proteins Dicer and the RISC complex--is responsible for inhibiting mRNAs via microRNAs.
In essence, the power of microRNAs are that a single microRNA can regulate the expression of multiple genes working in parallel to achieve a similar biologic effect. This technology is of particular use to the pharmaceutical industry: one can envision targeting a particular microRNA which inhibit several pathways to prevent a disease process, such as atherosclerosis, renal fibrosis, or cyst formation to think of a few possibilities. The field is still very new. I wouldn't be surprised if a future Nobel Prize came out of this work. Three scientists (Drs. Ruvkun, Baulcombe, and Ambros) working on microRNAs recently won the 2008 Lasker Prize--considered by many to be the "precursor" prize to the Nobel.