Side branching and luminal lineage commitment by ID2 in developing mammary glands

Mammary glands develop through primary ductal elongation and side branching to maximize the spatial area. Although primary ducts are generated by bifurcation of terminal end buds, the mechanism through which side branching occurs is still largely unclear. Here, we show that inhibitor of DNA-binding 2 (ID2) drives side branch formation through differentiation of K6 + bipotent progenitor cells into CD61 + luminal progenitor cells. Id2 -null mice had side branching defects, along with developmental blockage of K6 + bipotent progenitor cells into CD61 + luminal progenitor cells. Notably, CD61 + luminal progenitor cells were found in budding and side branches, but not in terminal end buds. Hormone reconstitution studies using ovariectomized MMTV-NLS-Id2 transgenic mice revealed that ID2 is a key mediator of progesterone, which drives luminal lineage differentiation and side branching. Our results suggest that CD61 is a marker for side branches and that ID2 regulates side branch formation by inducing luminal lineage commitment from K6 + bipotent progenitor cells to CD61 + luminal progenitor cells.