High-throughput analysis of gene expression and alternative splicing in cardiac hypertrophy

Cardiac hypertrophy affects millions of Americans and occurs as a result of common disease states including high blood pressure and valvular disease. On the most basic level, cardiac hypertrophy is an enlargement of individual cardiomyocytes that predisposes the heart to myocardial infarction, arrhythmias, and heart failure. An intervention that could stop this remodeling process before significant impairment or pathology develops has been sought for decades [1,2]. Identifying novel mechanisms that regulate hypertrophy and would be amenable to drug targeting could potentially prevent costly and significant cardiovascular events. This proposal uses high-throughput techniques to explore two possible mechanisms underlying the process of cardiac hypertrophy.
Cardiomyocytes respond to a myriad of paracrine, hormonal, and mechanical signals. While many of the hormonal signaling pathways have been identified [2], a clear mechanotransduction pathway by which changes in cardiac workload produce changes in myocyte size and shape remains elusive [3]. Our previous studies have shown that the amount of shortening that individual cardiomyocytes experience may be an important mechanical cue in cardiac remodeling, particularly in determining myocyte shape [4,5]. Therefore, in aim 1, we will use an in vitro muscle culture system to control these mechanical cues with the goal of identifying genes that are responsive to changes in shortening. These genes may provide clues to how cardiomyocyte regulate cell shape during hypertrophy.
Over two decades ago, certain fetal genes involved in contraction and metabolism were identified as being re-expressed in cardiac hypertrophy. Collectively these genes were referred to as the ‘fetal gene program’. More recently, alternative splicing has been shown to be an important aspect of heart development [6–8]. Specifically, alternative mRNA isoforms undergo switches that are highly regulated throughout fetal and post-natal heart development. The second aim of this proposal will use whole-transcriptome analysis to characterize the similarities in alternative splicing between fetal and hypertrophied hearts in a rat model of hypertrophy.
In fetal hearts, these developmentally-regulated alternative splicing events are regulated by a handful of RNA-binding proteins (RBP), similar to the way transcription factors control gene expression. In the third aim, we will use similar computational approaches to identify which exons undergo alternative splice and which RBPs could potentially be mediating the re-expression of fetal splice variants in cardiac hypertrophy. Once candidate RBPs have been identified we will statistically test whether the presence of these motifs can explain the changes in exon inclusion we observed in both fetal and TAC hearts.
The overall goal of these studies is to provide new understanding of the mechanisms of myocyte shape regulation, alternative splicing, and cardiac hypertrophy.

Specific Aim 1: Identify genes responsive to changes in cardiomyocyte shortening as candidate regulators of cell shape. We hypothesize that only a few genes will be specifically regulated by changes in shortening. We will vary the amount of cardiomyocyte shortening in vitro, measure global gene expression, and identify shortening-responsive proteins and pathways as candidate regulators of cell shape. We will validate our experimental in vitro data with comparisons to genes that have similar patterns of expression in in vivo models of cardiac hypertrophy.

Specific Aim 2: Test the hypothesis that alternative splicing patterns present during fetal heart development are re-expressed during cardiac hypertrophy. We hypothesize that some alternative splicing isoforms that are expressed in fetal hearts will be expressed in cardiac hypertrophy, but not in normal adult hearts. We will measure alternative splicing using whole-transcriptome high-throughput sequencing (RNAseq) and compare mRNA isoform expression between cardiac hypertrophy, sham-operated adults, and fetal hearts to quantify amount of overlap of splicing patterns between hypertrophic and fetal hearts.

Specific Aim 3: Identify candidate RNA-binding proteins that mediate the common regulation of splicing in heart development and cardiac hypertrophy. We hypothesize that splicing changes seen in fetal and hypertrophied hearts are regulated by RNA-binding proteins (RBPs). We will identify alternative exons and search for short motifs that are over-represented in the regions flanking these exons to identify candidate RBPs that are responsible for alternative splicing in cardiac hypertrophy. To test the significance of the RBPs identified, we will specifically ask if the rates of exon inclusion for the exons that are flanked by a specific motif deviate from what would be expected if each exon was regulated independently.