B7_Calcium Signalling

Bioquant (University of Heidelberg)
Prof. Dr. Thomas Höfer (PL), Dipl.-Phys. Anneke Brümmer (PhD student)
Prof. Dr. Ursula Kummer (PL), Ralph Gauges (PhD student)

Life Imaging Center (University of Freiburg)
Dr. Roland Nitschke (PL)

Project summary:

Calcium ions serve as second messenger in a multitude of signaling pathways that regulate cell metabolism, proliferation and apoptosis. The imaging of calcium in single cells has uncovered complex dynamics of hormone- and growth-factor evoked calcium oscillations and waves. In the last decade, research on hepatocytes and other cell types has linked these dynamics with the complex gating properties of the calcium-release channel in the endoplasmic reticulum, the InsP3 receptor. However, with recent progress in imaging and molecular biology, the calcium oscillator is now emerging as a highly connected cellular module that involves ion channel regulation, lipid signaling, and protein phosphorylation. In this integrated project we aim to identify the feedback mechanisms that drive hormone-evoked calcium oscillations in primary mouse hepatocytes and study how calcium oscillations cross-talk with other signaling processes. In the first phase of the project and previous work, we have developed new models for calcium signaling in hepatocytes with external experimental collaborators (Kummer et al., 2000; Höfer et al., 2002; Larsen et al., 2004; Politi et al., 2006; Gaspers et al., 2006). These models show that Ca2+ feedbacks into hormone receptor and IP3 dynamics are crucial for agonist-specific spike patterns and the efficient frequency encoding of agonist dose. We will extend these models to include phospho-inositide metabolism, parametrize them, and use systematic perturbations of Ca2+ oscillations with caged compounds and pharmacological agents for experimental testing. This will establish whether rat and mouse cells use conserved or divergent oscillator mechanisms. Complex spatio-temporal dynamics in the form of intracellular and intercellular Ca2+ waves are a distinguishing feature of Ca2+ signaling. Starting from our models of the local dynamics, we will develop experimentally-based reaction-diffusion models for Ca2+ and IP3 waves. Gene expression data from the Gebhard group (Platform Cell Biology) will be used in conjunction with these models to elucidate the molecular basis of pacemakers regions that drive synchronized Ca2+ waves in liver lobules. Because Ca2+ regulates a many signaling proteins (such as PKC, calcineurin, CaM kinase) and phosphorylation affects the Ca2+ signaling machinery (receptors, PLC-Gamma, IP3 receptors), cross-talk with other signaling pathways are investigated in collaboration with Projects B3, B5 and B6. We will test whether Ca2+ oscillation patterns can be used as a sensitive indicator of cross-talk. The modeling of the 'non-canonical' Wnt pathway will be used to study the effect of Ca2+ on protein phosphorylation and serve as a starting point for the integration of signal-transduction models.