Stem cells are self-renewing cells constitutively or conditionally activated during tissue homeostasis and regeneration. Understanding the biology of stem cells can provide valuable insights into therapeutic applications in regenerative medicine. Despite the recent advance in stem cell research, the lack of the strong consensus on stem cell markers is the current limitation. Moreover, due to the fine interaction between intrinsic and extrinsic factors in modulating stem cells, utilizing animal models is imperative to understand stem cell behavior.
To overcome such technical limitation, we developed the genetically engineered mouse models that enable us to visualize, quantify, and perform the cellular and genetic manipulation of stem cells in vivo. Employing these mouse models, we are addressing the following questions: the molecular and cellular mechanism of stem cell quiescence exit, expansion, plasticity, and heterogeneity. Similarly, we are also studying tumor-initiating cells in regards to intra-/inter-tumor heterogeneity, plasticity, therapeutic resistance, and metastasis.
Intestinal stem cells
Proliferative Intestinal stem cells (ISCs) continuously divide into the differentiated cells during intestinal homeostasis, whereas relatively quiescent ISCs are conditionally activated for tissue regeneration.
Mechanism of stem cell expansion in regeneration and cancer
Expansion of PAF expressing (PAF+) stem cells in the regenerating intestine Dpi (days post ionizing radiation injury). Arrows indicate the PAF+ cells. We found that PAF-Myc axis is essential for the expansion of stem cells during intestinal regeneration and tumorigenesis (Kim et al., Developmental Cell accepted).
Mechanism of quiescence exit of Tert+ stem cells
Upon tissue injury, tissue stem cells are activated and expanded for tissue regeneration. In the small intestine, Lgr5+ ISCs (intestinal stem cells) (proliferative) and Tert+ ISCs (quiescent) coexist. Upon radiation injury, Lgr5+ ISCs are depleted (cell death), while Tert+ ISCs undergo quiescence exit and expanded for rebuilding the intestinal epithelium. Recently, we unveiled how quiescence exit of Tert+ ISCs is initiated (Suh et al., 2017).
Tert mouse models - visualize and manipulate self-renewing cells
Visualizing tissue stem cells using a functional stem cell marker
Despite many stem cell markers, there is no strong consensus on those. TERT, a catalytic subunit of telomerase, is specifically expressed in the self-renewing cells including stem cells, germ cells, regenerating cells, and cancer cells. Nonetheless, detecting TERT+ cells in vivo was still challenging. To overcome this current technical limitation, we established TERT knock-in mice by inserting tdTomato (red fluorescent protein)-CreERT2 (tamoxifen-inducible genetic recombination), a TCE cassette, into the TERT allele, using gene targeting of embryonic stem cells (ESC) (a). TERT-TCE is highly expressed in ESC colony (b) and ESCs (c). In the small intestine, TCE expression indicating TERT-expressing cells, tissue stem cells, is detected in position +3~5 (d and e) (by Sohee Jun).
Jun S et al., Nature Communications 2016b (PDF)
Suh HN et al., Cell Reports 2017 (PDF)