Mechanisms of Bronchial Myogenesis
Bronchial myogenesis involves the differentiation of local embryonic mesenchymal cells into smooth muscle (SM) cells. The mechanisms that determine this process are largely unknown. Our recent studies suggested that laminin-2 induces SM differentiation by promoting mesenchymal cell spreading/elongation ( 1-3). In unrelated studies we found that heterogeneous ribonucleoprotein-H (hnRNP-H), an RNA-binding protein involved in alternative splicing, inhibits SM differentiation and bronchial myogenesis. Based on our preliminary data, we hypothesize that bronchial myogenesis involves activation of a laminin-integrin mediated pathway that results in downregulation of hnRNP-H. This in turn causes a switch in serum response factor (SRF) pre-mRNA alternative splicing that favors SM gene expression. We also hypothesize that altered hnRNP-H levels contribute to the myofibroblast phenotype seen in lung disease. The studies proposed in this application will address these hypotheses. Aim #1 will elucidate whether stimulation of SM differentiation by laminin-2 involves regulation of hnRNP-H and what is the signaling pathway connecting them. Based on preliminary studies we will focus on integrins alpha1beta1 and alpha2beta1, RhoA and MAPK 38. Aim #2 will establish the role of hnRNP-H in SRF pre-mRNA alternative splicing and how each SRF isoform impacts on hnRNP-H-mediated SM differentiation. It has been recently shown that SRF pre-mRNA splices into dominant positive and negative isoforms and our preliminary data suggest that these are involved in bronchial myogenesis. Aim #3 will determine whether modulation of hnRNP-H levels/activity affects the phenotype of mature SM cells, fibroblasts and lung myofibroblasts. The myofibroblasts will be studied using a bleomycin model of lung fibrosis. Our pilot data showed a decrease in hnRNP-H levels in this model. In summary, this project will define a novel molecular pathway controlling bronchial myogenesis. This pathway may be shared by many different myogenic stimuli. We anticipate that these studies will provide novel knowledge that will advance considerably our understanding of lung development and diseases caused by, or resulting from abnormal SM quantity, distribution and function.