When:  Tuesday, March 19th  @ 10:30 am
Where: Colburn Lab Room 366
Committee Chair:  Dr. Babatunde Ogunnaike and Dr. Rajanikanth Vadigepalli
Committee:  Dr. Ryan Zurakowski, Dr. Abhyudai Singh, Dr. April Kloxin

Title:

MODELING CALCIUM SIGNALING DYNAMICS IN THE LIVER: FROM SINGLE CELL TO MULTI-SCALE

Abstract: 

Free Ca2+ in the intracellular domain regulates a wide range of liver functions. In hepatocytes, the primary cell type that makes up 80% of the liver volume, important intracellular functions such as glycogenolysis, bile secretion, gluconeogenesis, etc. can be modulated by free cytosolic Ca2+. Consequently, aberrant intracellular Ca2+ dynamics can affect liver function adversely and lead to diseases such as NAFLD and cholestasis.

Ca2+ dynamics in hepatocytes are tightly regulated by a combination of a complex intracellular network and extracellular stimuli. Circulating hormonal stimuli elicit cytosolic Ca2+ spikes in hepatocytes that exhibit interesting spatio-temporal dynamics at the single cell and the tissue scale. At the single cell level, intracellular Ca2+ spiking frequency depends upon the strength of extracellular stimulus. This frequency encoded response to extracellular stimuli alters protein activity, energy metabolism and even gene expression in cells. At higher spatial dimensions, circulating Ca2+ mobilizing stimuli lead to a wave-like propagation of Ca2+ signal through liver lobules. Although Ca2+ spiking and its relevance has been investigated at different spatial scales in isolation, little is known about how spatio-temporal Ca2+ dynamics influence liver function in a multi-scale context.

In this work, we adopted a computational modeling and high-dimensional time series analysis approach to model Ca2+ spiking in single hepatocytes, its spatial organization in liver lobules, and its importance in whole-body function. Starting with an ODE-based computational model of Ca2+ spiking in single hepatocytes, we identified the key regulators of intracellular Ca2+ spiking frequency. We extended our single cell model to the lobular context by allowing gap junction-mediated molecular exchange between adjacent hepatocytes. We used the lobular scale model to predict that spatial gradients of intracellular Ca2+ signaling components and gap junction coupling are required for wave-like propagation of Ca2+ signals thorough liver lobules observed experimentally.

We then acquired a high dimensional time series of vasopressin induced Ca2+ response across 1300 hepatocytes. We characterized causal networks within liver lobules using transfer entropy – an information theoretic measure of directed information transfer.  Our network analysis revealed that hepatocytes are causally linked with multiple other hepatocytes. However, even though Ca2+ signal propagates through liver lobules in a wave-like fashion, causal influences are not aligned unidirectionally.

Next, we integrated our lobular scale model of Ca2+ signaling with a whole-body model of glucose metabolism. Using this multi-organ, multi-scale model, we investigated the importance of spatio-temporal organization of Ca2+ signal propagation in liver lobules in maintaining systemic glucose homeostasis under periods of high glycemic demand. Our model simulations revealed that synchronized intracellular Ca2+ spiking in hepatocytes across liver lobules increases liver glucose output. In combination, this work presents a high-resolution quantitative accounting of how molecular signals organize tissue-scale cellular response and regulate organ-scale metabolic function in the liver.

As a counterview of overriding cellular heterogeneity at the single cell level in protein function and signaling cascades, we also analyzed single cell mRNA expression heterogeneity in single hepatocytes to identify how perturbations – both acute (partial hepatectomy) and chronic (chronic alcohol adaptation), can affect liver function and regeneration. Using a high dimensional single cell transcriptomic dataset and confocal microscopy, we identified the shifts in single cell molecular states that could underlie aberrant liver regeneration due to chronic alcohol adaptation observed experimentally.

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