Determining the impact of mitochondrial dysfunction on stem cell dynamics and proliferation within the colon

Abstract

Clonally expanded mitochondrial DNA (mtDNA) point mutations have been shown to cause mitochondrial dysfunction in the form of cytochrome c oxidase deficiency (COX deficiency) within ageing human colonic epithelium. Currently, there are a lack of robust stem cell markers within human colonic epithelium, however the detection of mitochondrial dysfunction has been shown to be a useful stem cell lineage marker, enabling the investigation of stem cell dynamics within colonic crypts. Using the most robust data set of COX deficiency frequency and mtDNA mutation number, a computational model that simulates stem cell dynamics within human colonic crypts was constructed. In silico stem cell modelling suggests: there are approximately 5 stem cells within human colonic crypts, stem cell divisions occur asymmetrically the majority of the time, and that infrequent symmetric stem cell division permits niche succession of individual stem cell clones. However, the in silico data was unable to match the biological data when a model simulating neutral drift stem cell dynamics was used, suggesting a change in stem cell biology when mitochondrial dysfunction was present. In order to assess the impact of mitochondrial dysfunction within colonic stem cells in vivo, a mouse model of mitochondrial dysfunction was crossed with a mouse model enabling the visualisation of cells expressing Lgr5 (a well-accepted stem cell marker). Double thymidine analogue labelling was used to identify cells traversing through the cell cycle, together with a marker of proliferation. This data suggests that stem cells with mitochondrial dysfunction have a small but significant increase in cell cycle progression rate compared to normal stem cells. When these data were included in the model, a better fit to the biological data was achieved. These findings suggest that mitochondrial dysfunction does significantly impact on stem cell homeostasis. As stem cells with mitochondrial dysfunction are more likely to out-compete normal stem cells over time, this may have potential implications for an increased risk of cancer propagation within the colon.