(568b) Simulating Ca2+ Signal Propagation in Exact 3D Reconstructions of Hepatic Lobules

Ca²⁺ is a critical regulator of a wide range of cellular functions in the liver, including bile transport, proliferation, apoptosis, and intermediary metabolism. Studies focused on imaging Ca²⁺ at tissue-scale have revealed remarkable tissue-scale spatial organization in cellular response to extracellular Ca²⁺ agonists, suggesting a mechanism for synchronizing cellular functions across a liver lobule. In the presence of circulating stimuli such as hormones and growth factors, hepatocytes exhibit rapid Ca²⁺ release from intracellular stores (“spikes”) that appear to propagate in a wave-like fashion across a liver lobule. Available experimental evidence suggests that gap junction mediated IP₃ transfer between adjacent hepatocytes is necessary for generating the spatial waves of intracellular Ca²⁺ release. However, it is not clear if gap junction mediated processes are sufficient to lead to a tissue-scale synchronization of Ca²⁺ response, and whether such cell-cell interactions can override the effect of extensive variability in the expression of hormonal signaling pathways. We pursued a computational modeling approach to examine how spatial patterns of expression of signaling pathways, together with gap junction mediated cell-cell interactions, lead to tissue-scale synchronization of Ca²⁺ dynamics.

We performed an integrated in vivo imaging, data-driven causality analysis, and computational model-based analysis of Ca²⁺ signal propagation in mouse liver lobules. Simulations of ODE-based models in one- and two-dimensional representations of liver tissue suggested that lobular scale Ca²⁺ waves require spatially organized gradients in the expression of hormonal signaling components as well as IP₃ exchange between adjacent hepatocytes^1,2. Based on the experimentally observed wave-like propagation, we hypothesized that the cell-cell causal influences unidirectionally radiate outward in a sequential manner from pericentral vein to periportal vein in a liver lobule. However, a data-driven, causal network modeling of potential pair-wise influences between 1300 individual hepatocytes suggested that spatially co-localized subsets of cells formed causally connected “islands”. Within these islands, hepatocytes were causally connected to up to six neighbors. The predicted information flow between adjacent hepatocytes within these islands was not consistently aligned in a unidirectional fashion from central vein towards portal triads.

We extended the ODE-based modeling to incorporate three-dimensional topology and cell-cell contacts in hepatic lobules, and examined the dynamics of Ca²⁺ wave propagation in a three-dimensionally organized liver tissue. We acquired high resolution confocal images of sections of hepatic lobules up to 100 μm in thickness. We segmented hepatocytes, and hepatic vasculature in the acquired z-stack of images to generate an exact reconstruction of the imaged tissue volume. We then simulated an ODE-based computational model of intra- and inter-cellular signaling using the reconstructed 3D tissue architecture to investigate the contribution of lobular topology to Ca²⁺ signal propagation. In the simulations, the expression of two hormonal signaling components, represented by two cell-specific model parameters, were initialized based on published single cell transcriptomic data³.

Our simulations revealed that with model initialization based on single cell transcriptomic data in [3], flow of Ca²⁺ signal from the central vein towards the portal triads requires gap junction mediated IP₃ exchange between adjacent hepatocytes. Additionally, information flow between adjacent hepatocytes within a 2D slice may not be aligned in a unidirectional fashion, as observed in our data-driven causality analysis in [2], likely because of cell-cell contacts in 3D lobular topology.

References:

1. Verma, Aalap, Hirenkumar Makadia, Jan B. Hoek, Babatunde A. Ogunnaike, and Rajanikanth Vadigepalli. "Computational Modeling of Spatiotemporal Ca²⁺ Signal Propagation Along Hepatocyte Cords." IEEE Transactions on Biomedical Engineering 63, no. 10 (2016): 2047-2055.
2. Verma, Aalap, Anil Noronha Antony, Babatunde A. Ogunnaike, Jan B. Hoek and Rajanikanth Vadigepalli. “Causality Analysis and Cell Network Modeling of Spatial Calcium Signaling Patterns in Liver Lobules.” Frontiers in Physiology, under review
3. Halpern, Keren Bahar, Rom Shenhav, Orit Matcovitch-Natan, Beáta Tóth, Doron Lemze, Matan Golan, Efi E. Massasa et al. "Single-cell spatial reconstruction reveals global division of labour in the mammalian liver." Nature 542, no. 7641 (2017): 352.