Calcium and IP3 dynamics in cardiac myocytes: experimental and computational perspectives and approaches

Calcium plays a crucial role in excitation-contraction coupling (ECC), but it is also a pivotal second messenger activating Ca(2+)-dependent transcription factors in a process termed excitation-transcription coupling (ETC). Evidence accumulated over the past decade indicates a pivotal role of inositol 1,4,5-trisphosphate receptor (IP3R)-mediated Ca(2+) release in the regulation of cytosolic and nuclear Ca(2+) signals. IP3 is generated by stimulation of plasma membrane receptors that couple to phospholipase C (PLC), liberating IP3 from phosphatidylinositol 4,5-bisphosphate (PIP2). An intriguing aspect of IP3 signaling is the presence of the entire PIP2-PLC-IP3 signaling cascade as well as the presence of IP3Rs at the inner and outer membranes of the nuclear envelope (NE) which functions as a Ca(2+) store. The observation that the nucleus is surrounded by its own putative Ca(2+) store raises the possibility that nuclear IP3-dependent Ca(2+) release plays a critical role in ETC. This provides a potential mechanism of regulation that acts locally and autonomously from the global cytosolic Ca(2+) signal underlying ECC. Moreover, there is evidence that: (i) the sarcoplasmic reticulum (SR) and NE are a single contiguous Ca(2+) store; (ii) the nuclear pore complex is the major gateway for Ca(2+) and macromolecules to pass between the cytosol and the nucleoplasm; (iii) the inner membrane of the NE hosts key Ca(2+) handling proteins including the Na(+)/Ca(2+) exchanger (NCX)/GM1 complex, ryanodine receptors (RyRs), nicotinic acid adenine dinucleotide phosphate receptors (NAADPRs), Na(+)/K(+) ATPase, and Na(+)/H(+) exchanger. Thus, it appears that the nucleus represents a Ca(2+) signaling domain equipped with its own ion channels and transporters that allow for complex local Ca(2+) signals. Many experimental and modeling approaches have been used for the study of intracellular Ca(2+) signaling but the key to the understanding of the dual role of Ca(2+) mediating ECC and ECT lays in quantitative differences of local [Ca(2+)] in the nuclear and cytosolic compartment. In this review, we discuss the state of knowledge regarding the origin and the physiological implications of nuclear Ca(2+) transients in different cardiac cell types (adult atrial and ventricular myocytes) as well as experimental and mathematical approaches to study Ca(2+) and IP3 signaling in the cytosol and nucleus. In particular, we focus on the concept that highly localized Ca(2+) signals are required to translocate and activate Ca(2+)-dependent transcription factors (e.g., nuclear factor of activated T-cells, NFAT; histone deacetylase, HDAC) through phosphorylation/dephosphorylation processes.