The transduction of forces such as blood pressure and muscle stretch into biological signals is central to cardiovascular function, with altered states leading to heart failure, hypertension, and arrhythmias. Within development, alteration of flow can produce congenital defects, such as the severe cardiomyocyte hypo- proliferation seen in hypoplastic left heart syndrome. Yet, the identity of key mechanosensors remains unknown. One potential means of mechanotransduction is the production of stretch-sensitive ionic currents. These have been observed electrophysiologically in neonatal/adult cardiovascular tissue, and perhaps play a role in pressure sensation. Nevertheless, study of this mechanism has been hindered by our ignorance of the identity of the channels creating them, because electrophysiology is difficult to adapt for cloning. Moreover, though stretch-sensitive currents are found in neonatal tissue, their presence in the embryonic heart remains unexplored. Identifying these channels will likely advance our knowledge and provide novel therapeutic targets for cardiovascular disease. For example, inhibitors of these currents are effective against atrial fibrillation in animal models, but designing high-affinty drugs will depend on isolating the channels themselves. Similarly, studying the role these channels play in cardiomyocyte proliferation may prove critical to field of regenerative medicine, as the maturation of the fetal heart is known to depend on the forces created by a heartbeat. Thus, the aims of this proposal are to examine whether stretch-sensitive currents play a role in the proliferation of embryonic cardiac tissue, and to clone a stretch-activated calcium channel. To identify these currents in early cardiogenesis, embryonic tissue will be freshly dissected and examined electrophysiologically at several points in early development. We will culture these cells under conditions of stretch, while blocking currents pharmacologically, assaying for subsequent cardiomyocyte proliferation. For the aim of cloning these channels, we will perform independent genomic and proteomic screens, given the absence of suitable high- affinity channel blockers. The genomic approach will use a Drosophila-RNAi screen for clones that inhibit Ca2+ entry in response to stretch. This approach has had robust success for cloning novel channels and Ca2+ signaling molecules. We will then identify mammalian orthologs computationally. The proteomic approach will take advantage of the finding that stretch-activated channel are upregulated when cells are cultured under flowing, as opposed to static, media. Thus, surface proteins will be labeled and purified under static and flow conditions, and the identity of those whose surface expression increases with flow will be determined by mass spectroscopic methods. Putative stretch-activated channels will be isolated from within this subset. Successful completion of this project will provide novel therapeutic targets for cardiac disease, identify molecules involved in the response to force, and allow the creation of new tools to study the role of force in organ development.
The ability of the cardiovascular system to sense blood pressure and flow is central to its normal function, becoming altered in diseases such as hypertension, heart failure, cardiac arrhythmias, and certain congenital heart defects. By identifying key molecules that sense blood pressure, we hope to gain new insight into how these diseases develop and establish these sensors as targets for the design of novel drugs.
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