Adult stem cells in mice : visualization and characterization using genetic mouse models

The onset of each living organism starts with pluripotent stem cells that have the ability to differentiate into all the different cell types of an organism. However, during the earliest stages of development, the pluripotent stem cells will stepwise lose their developmental potential. The cells that can only give rise to a limited, yet related set of cell types, are called multipotent stem cells. Relatively few multipotent stem cells remain in an adult body, known as adult stem cells. Here, we describe the generation of multiple knockin- and knockout mice for the identification and characterization of adult stem cell populations in the mouse. We describe the expression pattern of Prominin-1/CD133 on intestinal stem cells and early progenitors in the mouse small intestine. In adult skin, Lgr6 turned out to mark an uncharacterized cell population above the hair follicle bulge. During development, Lgr6 cells establish all three major lineages of the skin, i.e. the hair follicle, sebaceous glands and interfollicular epidermis. Under normal homeostasis during adulthood, Lgr6 stem cells continuously fuel sebaceous glands and interfollicular epidermis, while the contribution toward the hair lineage diminished with age. During regenerative responses, Lgr6 stem cells generate persistent progeny for the long-term repair of full-thickness wounds. Using R26R-Confetti we have been able to mark multiple intestinal stem cells in the same niche with distinguishable fluorescent proteins. Using short-term and long-term fate mapping of each crypt stem cell, we demonstrate that during normal homeostasis most of the stem cell divisions are symmetric in fate. As a consequence, equal stem cells compete for residency at the crypt base, leading the crypt to drift towards clonality over time. Last, we used the R26R-Confetti system to study two stem cell phenomena in intestinal cancer biology. First, we studied “field cancerization” by activating the oncogene K-ras in Lgr5 intestinal stem cells. The transformed stem cell becomes a super-competitor because of its shortened cell cycle length and starts outcompeting the neighbouring wild-type stem cells for residency at the crypt base. In the second scenario we show preliminary lineage tracing data of Lgr5+ adenoma cells that behave as their wild-type counterparts, suggesting that Lgr5 marks intestinal cancer stem cells.