Regulation of Endothelial-to-Mesenchymal Transition by MicroRNAs in Chronic Allograft Dysfunction

Abstract

Fibrosis is a universal finding in chronic allograft dysfunction, and it is characterized by an accumulation of extracellular matrix. The precise source of the myofibroblasts responsible for matrix deposition is not understood, and pharmacological strategies for prevention or treatment of fibrosis remain limited. One source of myofibroblasts in fibrosis is an endothelial-to-mesenchymal transition (EndMT), a process first described in heart development and involving endothelial cells undergoing a phenotypic change to become more like mesenchymal cells. Recently, lineage tracing of endothelial cells in mouse models allowed studies of EndMT in vivo and reported 27% to 35% of myofibroblasts involved in cardiac fibrosis and 16% of isolated fibroblasts in bleomycin-induced pulmonary fibrosis to be of endothelial origin. Over the past decade, mature microRNAs (miRNAs) have increasingly been described as key regulators of biological processes through repression or degradation of targeted mRNA. The stability and abundance of miRNAs in body fluids make them attractive as potential biomarkers, and progress is being made in developing miRNA targeted therapeutics. In this review, we will discuss the evidence of miRNA regulation of EndMT from in vitro and in vivo studies and the potential relevance of this to heart, lung, and kidney allograft dysfunction.

Figure 1

Allograft injury results in release of cytokines, chemokines and increase in inflammatory and immune cells in the allograft which induce differentiation of different cells, including tissue-resident fibroblasts, bone marrow progenitor cells, endothelial cells, epithelial cells and pericytes, into myofibroblasts. Myofibroblasts produce collagen and collagen deposition results in fibrosis associated with chronic allograft failure.

Figure 2

Summary of main signalling pathways involved in changing gene expression in EndMT and how microRNAs that are upregulated (orange) or downregulated (purple) interact with these pathways. Red arrows indicate inhibition. Green arrows indicate activation.

Figure 3

In the nucleus, the miRNA gene is transcribed by RNA polymerase II into the double stranded pri-miRNA. The enzyme Drosha RNase III endonuclease works with cofactor DiGeorge syndrome critical region 8 (DGCR8) to cleave both strands of pri-miRNA near the base of the primary stem loop to produce a shorter 60-70 nucleotide precursor known as pre-miRNA. Pre-miRNA is then actively exported into the cytoplasm via Exportin5 channel. Here, a second RNase III endonuclease, Dicer, cuts both strands of pre-miRNA near the base of the stem loop to leave a duplex of the mature miRNA and a similar sized complementary fragment of the opposing arm (miRNA*). The duplex is separated by incorporation of miRNA strand into RISC by binding to argonaute proteins.