Circulating and urinary microRNAs in kidney disease.

microRNAs are small, noncoding RNAs that control gene/protein expression through target messengerRNA degradation and/or inhibition of protein synthesis. An array of experimental studies has shown the importance of microRNAs for disease initiation/progression. microRNAs are generally considered to act as intracellular modulators of gene expression. However, first studies in the cancer and cardiovascular field have elucidated that microRNAs are remarkably stable in the extracellular compartment (e.g., in blood or urine). The detection and quantification of circulating microRNAs may, thus, represent a novel noninvasive tool to detect and monitor disease activity. In addition, there might be a potential biologic relevance of circulating microRNAs for cell/cell communication. The aim of the present article is to give an outline of recent work on circulating and urinary microRNAs as well as their potential paracrine activity in kidney disease.