Decoding the beat: physiological control mechanisms for robust heart development - SUMMARY A robust heartbeat is essential for vertebrate life, as it is critical for sustained transport of nutrients and waste throughout large body plans. Given this fact, the emergence and maturation of cardiac activity during embryogenesis must be extremely reliable and must be bidirectionally coordinated with development of the heart’s form. The calcium physiology of the heart exemplifies this idea - it is the fundamental driver of cardiac contraction by sarcomeric protein assemblies in each individual beat, but simultaneously triggers many signal transduction pathways and induces gene expression. Here we will explore the changing patterns of cardiac calcium signaling during embryonic development and investigate their implications for the heart’s maturation. Calcium signaling contributes both to normal developmental processes and to disease in the adult heart, suggesting that its tuning serves an important instructive role but must occur within a set of appropriate bounds. Identifying these bounds requires definition of the precise rules that map cellular calcium to downstream outputs during development. These rules have remained elusive because cellular calcium shows dynamics on the timescale of hundreds of milliseconds, and spatial scales ranging from subcellular structures to entire tissues. These patterns will evolve due to slower developmental processes, and their features that define reciprocal flows of information can only be parsed in intact, live samples. I have established methods to image cardiac physiological dynamics in tens of intact zebrafish embryos simultaneously, while administering non-destructive, spatiotemporally programmable optogenetic control of these processes. My approaches open the door to an unprecedented exploration of the high-dimensional signal encoding space traversed by calcium signaling in cardiac development. First, we will develop all-optical approaches to measure calcineurin and calmodulin/calmodulin kinase signaling in developing zebrafish hearts, record the dynamics of their behaviors over normal development, and dissect response mechanisms with optogenetic reconstruction of cardiac calcium physiology. Second, we will manipulate the physiology of developing zebrafish hearts and perform single cell functional genomics to define a comprehensive list of calcium-sensitive developmental gene expression programs. Finally, we will investigate the roles of cell type-specific calcium signaling mechanisms in cardiac chamber specification. Together, these studies will define new rules by which the heart perceives its emerging calcium physiology to make developmental decisions, tracing the basic biophysics of the cardiac cycle through signal transduction, gene expression, and to organ-scale phenotype. In doing so, we will gain broader insight into what delineates physiological versus pathological adaptation during cardiac development, and into fundamental information-encoding mechanisms of calcium signaling which have been of longstanding interest in disciplines as diverse as neuroscience, immunology, and regeneration.
