Project Summary Heart failure (HF) is associated with a 5-year mortality of 50% and the incidence is still rising with most cases associated with ischemic cardiomyopathy. Identifying novel strategies to counteract cardiac pathological remodeling post myocardial infarction (MI) is of urgent clinical need. Long non-coding RNAs (lncRNA) have recently emerged as regulators of cardiac development and disease. However, the function of most lncRNAs remains unknown. We have characterized the first cardiac specific circadian lncRNA Circa, which affects cardiac remodeling post MI through regulating alternative splicing. To better understand the molecular mechanisms of Circa and its partner proteins in the heart, we propose the following specific aims: 1. Identify the RNA structures that are required for Circa nuclear localization and interaction with spliceosome. We will use the state-of-the-art chemical probing technique to define the secondary structure of Circa, then use structure mutants to identify its functional motifs. 2. Determine whether Circa acts in trans and whether its function is dependent on its oscillatory expression. We will unambiguously determine Circa function as a transcript by a series of “rescue” experiments in the novel knock out mice we created. Time restricted expression will be used to test the necessity of the oscillatory expression. 3. Define the molecular basis by which Circa affects splicing in the heart. Our preliminary data suggests Circa may suppress splicing regulator hnRNPA1. We will test the functional interaction between Circa and hnRNPA1 using a cardiac target gene we identified. We will further investigate the role of hnRNAPA1 in the post MI remodeling by identifying its splicing targets using eCLIPseq/RNAseq and test the function of hnRNPA1 in vitro and in vivo. Completion of this proposal will have significant impact in understanding splicing regulation in the heart during post MI pathological remodeling and potentially expand our therapeutic strategies for HF treatment.