Hybrid Microrna Control of Colon Cancer Stem Cell Asymmetric Division

Abstract

Colon cancer stem cells (CCSC) undergo both symmetric and asymmetric division, which balance differentiation versus self-renewal in the tumor cell population. This decision is determined by the microRNA miR-34a, whose spatial segregation generates a bimodal Notch response that determines cell fate outcomes. This bimodal response is caused by kinetic mutual sequestration between miR-34a and Notch mRNA. However, three questions have remained since we reported the above findings (Bu et al, Cell Stem Cell, 2013). First, does miR-34a generate bimodal responses from all target genes? Second, what is the relationship between miR-34a and the canonical cell fate determinant protein Numb, which also targets Notch to regulate cell fate symmetry? And third, do these mechanisms exist in normal stem cells? (1) A systematic study demonstrates that the kinetics of microRNA regulation is target-specific. Quantitative single-cell analysis revealed that miR-34a generates bimodal responses from a small subset of genes that are involved in cell fate determination, but regulating the majority of genes (e.g., metabolic and growth genes) in a non-bimodal manner. (2) We report that miR-34a and Numb directly interact to form an incoherent feedforward loop. This microRNA-protein hybrid circuitry synergistically enhances Notch and cell fate asymmetry by orders of magnitude and exhibits adaptive behavior to offset interference from other miR-34a target genes, hence buffering asymmetric cell fate outcomes from fluctuations in miR-34a levels. (3) We show that the hybrid control mechanism is active in the intestinal stem cell niche. Using innovative abdominal window and 3-D-printed intestinal insert, chronic in vivo multiphoton imaging further revealed the real-time spatiotemporal dynamics of the hybrid circuitry in live mice. This control is gradually subverted in late-stage cancer stem cells, contributing to their proliferation and malignancy.