RNA binding proteins in Idiopathic Pulmonary Fibrosis

Abstract

RNA binding proteins (RBPs) associate with target ribonucleic acids (RNAs) through conventional or unconventional RNA-binding domains to regulate messenger RNA (mRNA) biogenesis and metabolism. Under certain settings like cellular stress, membrane-free cytoplasmic aggregates, termed stress granules (SGs), are formed, which enclose RBPs and untranslated or stalled RNA transcripts. In organs with a high and complex RNA metabolism like the brain or the lungs, the task of RBPs and SGs becomes even more complex. The complex role of RBPs has been elegantly studied in the field of neurodegeneration, but their role in lung diseases remains poorly understood. Idiopathic pulmonary fibrosis (IPF) is a life-threatening, chronic lung disease that is characterized by alveolar epithelial cell injury and severe fibroblast proliferation, thus resulting in disturbed epithelial-mesenchymal crosstalk and, ultimately, fibrosis. The goal of this study was to better understand the role of RBPs and SGs in lung fibrosis. For this, explanted lungs and interstitial fibroblasts isolated from IPF patients or healthy donors, mouse model of fibrosis as well as an in vitro amiodarone induced lung fibrosis model were employed. In general, a differential regulation of several RBPs including transactive response DNA binding protein 43 kDa (TDP43), fused in sarcoma/translocated in liposarcoma (FUS/TLS), serum response factor (SRF), muscleblind-like splicing regulator 1 (MBNL1) and polyadenylate-binding protein 1 (PABPC1) was encountered in IPF and in the amiodarone model. Of interest, some RBPs, namely FUS, TDP43 and PABPC1, were observed to be particularly increased in the interstitial fibroblasts of IPF patients. In addition to this, overexpression of FUS in healthy primary fibroblasts resulted in an increase in the cell proliferation marker proliferating cell nuclear antigen (PCNA), indicating that the increase in FUS is sufffcient to trigger their proliferation. Further, decreased staining for both FUS and PABPC1, was observed in IPF precision cut lung slices (PCLS) which were treated ex vivo with the anti-fibrotic drug pirfenidone. This indicated that pirfenidone may exert its anti-fibrotic effects, at least in part by targeting RBPs like FUS and PABPC1. Taken together, our study, for the first time, documents strict cell specific and altered expression and localization of RBPs and SG proteins in IPF. Our results also imply, that patient lungs and therapeutic targeting of RBPs may prove beneficial in IPF.