bioRxiv2018
Intestinal crypts are responsible for the total cell renewal of the lining of the intestines, occurring every 4-7 days in humans. This rapid turnover is governed by the complex interplay between signalling pathways and the cell cycle within individual cells in the crypts. The role of Wnt signalling in governing cell proliferation and differentiation in the intestinal crypt has been extensively studied, with increased signalling found towards the lower regions of the crypt. Recent studies have shown that the Wnt signalling gradient found within the crypt may arise as a result of division-based spreading from a Wnt reservoir at the crypt base. The discovery of the Hippo pathways involvement in maintaining crypt homeostasis is more recent; a mechanistic understanding of Hippo pathway dynamics, and its possible cross-talk with the Wnt pathway, remains lacking. To explore how the interplay between these pathways may control crypt homeostasis and dysplasia we developed and analysed a mathematical model coupling an ordinary differential equation description of Wnt and Hippo signalling with a cell-based description of cell movement, proliferation and contact inhibition. Furthermore, we compared an imposed Wnt gradient with a division-based Wnt gradient model. Our results suggest that Hippo signalling affects the Wnt pathway by reducing the presence of free cytoplasmic {beta}-catenin, causing cell cycle arrest. This process slows the rate of crypt turnover, previously attributed only to mutations in Wnt pathway components commonly observed in colorectal cancers. We also show that a division-based spreading of Wnt can form a Wnt gradient, resulting in proliferative dynamics comparable to imposed-gradient models. This suggests that imposed-gradient models are a reasonable approximation of experimentally observed Wnt gradients, however they provide little insight into the source of Wnt. Finally, a simulated APC double mutant, with misregulated Wnt and Hippo signalling activity, is predicted to be capable of completing a more rapid monoclonal conversion of the crypt than a Wnt-only mutant.\n\nAuthor summaryColorectal cancer is the fourth leading cause of cancer death worldwide. It is generally established that colorectal cancer occurs upon disruption of the dynamics of crypts, tubular indentations lining the walls of the gastrointestinal tract that are responsible for renewal of the intestinal epithelium. Thus, understanding the mechanisms that regulate intestinal crypt dynamics is instrumental to understand how colorectal cancer can emerge. We used mathematical modelling to explore how crypt dynamics emerge from the interactions across several biological scales, including mutations that cause an abnormal cell response to changes at the cell or tissue level, cell proliferation and motility within the crypt. Our model describes, in each cell within the crypt, levels and dynamics of genes involved in two signalling pathways - the canonical Wnt and Hippo pathways - relevant for cell proliferation and adhesion. In simulations, we predict how the crosstalk between these pathways affects crypt dynamics in both healthy and dysplastic cases. Furthermore, our simulations match recent biological evidence of a novel mechanism, based on cell division, causing heterogeneous and graded localisation of different cell types (dividing or not) within the crypt. Our results suggest the possibility of combined disease treatment, targeting both the Wnt and Hippo pathways.