Cardiac myocytes are striated muscle cells with myofilaments running along their length and Z-discs in a transverse direction. These cells contain structures that are sensitive to mechanical stretch and transduce this signal into a biochemical response. By altering the major axis of biaxial, mechanical stretch, I hypothesized that different signaling pathways and transcription regulators would be activated and lead to divergent gene expression profiles in cardiac myocytes.
To investigate the effect of stretch axis, cardiac myocytes were cultured on a micropatterned substrate, and the primary stretch axis was applied either parallel or transverse to the myofibril direction. RNA sequencing (RNA-Seq) was conducted to study whole genomic expression changes after acute cardiac myocyte stretch. After 30 minutes of stretch, 53 and 168 genes were considered differentially expressed (DE) from transverse and longitudinal stretch, respectively. After 4 hours, the number of DE genes increased to 795 in longitudinal stretch while it decreased to 35 in transverse stretch. Gene ontology (GO) term analysis indicated enrichment of transcription factor (TF) activity and protein kinase activity by both stretch axes; whereas longitudinal but not transverse stretch induced expression of genes involved in sarcomere organization and cytoskeletal protein binding.
Although researchers have identified sensors, pathways, and TFs involved in cardiac mechanotransduction, I, for the first time, integrated this information into a logic-based network model of cardiac myocyte stretch signaling. This model was validated against independent data and correctly predicted the effect of stretch or effect of inhibitors on stretch response in 54 of 61 experiments.
This network model was used in conjunction with the RNA-Seq data to identify the mechanisms regulating gene expression changes due to longitudinal and transverse stretch. Analysis of this network identified serum response factor (SRF) and myocyte enhancer factor-2 (MEF2) as critical TFs in regulating longitudinal stretch-induced gene changes whose activity is modulated by protein kinase C (PKC). In addition, cyclic adenosine monophosphate response element-binding protein (CREB) was found to be activated by both longitudinal and transverse stretch. The network model provides evidence that cardiac myocytes engage different transcriptional regulators in response to different principal orientations of biaxial stretch.
