A Mechanistic Model for the Development and Maintenance of Portocentral Gradients in Gene Expression in the Liver

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Abstract
In the liver, genes are expressed along a portocentral gradient. Based on their adaptive behavior, a gradient versus compartment type, and a dynamic versus stable type of gradient have been recognized. To understand at least in principle the development and maintenance of these gradients in gene expression in relation to the limited number of signal gradients, we propose a simple and testable model. The model uses portocentral gradients of signal molecules as input, while the output depends on two gene–specific variables, viz., the affinity of the gene for its regulatory factors and the degree of cooperativity that determines the response in the signal–transduction pathways. As a preliminary validity test for its performance, the model was tested on control and hormonally induced expression patterns of phosphoenolpyruvate carboxykinase (PCK), carbamoylphosphate synthetase I (CPS), and glutamine synthetase (GS). Affinity was found to determine the overall steepness of the gradient, whereas cooperativity causes these gradients to steepen locally, as is necessary for a compartment–like expression pattern. Interaction between two or more different signal gradients is necessary to ensure a stable expression pattern under different conditions. The diversity in sequence and arrangement of related DNA–response elements of genes appears to account for the gene–specific shape of the portocentral gradients in expression. The feasibility of testing the function of hepatocyte–specific DNA–response units in vivo is demonstrated by integrating such units into a ubiquitously active promoter/enhancer and analyzing the pattern of expression of these constructs in transgenic mice