Role of ldb1 in adult intestinal homeostasis.

Ldb1 is an essential co-regulator of transcription in embryonic development. It acts in conjunction with nuclear LIM-homeodomain and LIM-only proteins to control key events of organogenesis as precursor cells enter lineage specification. Here we ask whether Ldb1 exerts control over stem cell activation and differentiation throughout the life of the organism as required for tissue homeostasis. To help answer this question, we have generated conditional Ldb1 mouse mutants with an Ldb1 floxed/floxed;ROSA26CreER genotype. Tamoxifen treatment of 60 day-old mutant animals results in near-ubiquitous Cre-mediated Ldb1 inactivation. As a consequence, the stem cell microenvironment of intestinal crypts is drastically affected. Cells that normally express Ldb1 together with markers that identify them as lineage progenitors cease to retain bromodeoxyuridine and are gradually lost. Ldb1 inactivation in intestinal crypts and/or in neighboring mesenchymal cells also triggers activation of Wnt signaling in the stem cell niches of the small intestine. Cell proliferation is markedly increased in the epithelia of the small intestine, and Lgr5-expressing stem cells disappear from the base of the crypts. This perturbation of the normal process of tissue homeostasis causes apoptosis, and the animals do not survive. We conclude that Ldb1-mediated transcriptional regulation plays a major role in adult intestinal homeostasis.