Branching morphogenesis in the lung serves as a model for the complex patterning that is reiterated in multiple organs throughout development. Beta-catenin and Wnt signaling mediate critical functions in cell fate specification and differentiation, but specific functions during branching morphogenesis have remained unclear. Here, we show that Wnt/beta-catenin signaling regulates proximal-distal differentiation of airway epithelium. Inhibition of Wnt/beta-catenin signaling, either by expression of Dkk1 or by tissue-specific deletion of beta-catenin, results in disruption of distal airway development and expansion of proximal airways. Wnt/beta-catenin functions upstream of BMP4, FGF signaling, and N-myc. Moreover, we show that beta-catenin and LEF/TCF activate the promoters of BMP4 and N-myc. Thus, Wnt/beta-catenin signaling is a critical upstream regulator of proximal-distal patterning in the lung, in part, through regulation of N-myc, BMP4, and FGF signaling.

The anterior heart field (AHF), which contributes to the outflow tract and right ventricle of the heart, is defined in part by expression of the LIM homeobox transcription factor Isl-1. The importance of Isl-1-positive cells in cardiac development and homeostasis is underscored by the finding that these cells are required for cardiac development and act as cardiac stem/progenitor cells within the postnatal heart. However, the molecular pathways regulating these cells' expansion and differentiation are poorly understood. We show that Isl-1-positive AHF progenitor cells in mice were responsive to Wnt/β-catenin signaling, and these responsive cells

Wnt7b is a Wnt ligand that has been demonstrated to play critical roles in several developmental processes, including lung airway and vascular development and chorion-allantois fusion during placental development. Wnt signaling involves the binding of Wnt ligands to cell surface receptors of the frizzled family and coreceptors of the LRP5/6 family. However, little is known of the ligand-receptor specificity exhibited by different Wnts, Fzds, and LRPs in Wnt signaling. Expression analysis of Fzds and LRP5/6 in the developing lung and vasculature showed that Fzd1, -4, -7, and -10 and LRP5/6 are expressed in tissue-specific patterns during lung development. Fzd1, -4, and -7 are expressed primarily in the developing lung mesenchyme, and Fzd10 is expressed in airway epithelium. LRP5 and LRP6 are expressed in airway epithelium during lung development, whereas LRP5 but not LRP6 expression is observed in the muscular component of large blood vessels, including the aorta. Cell transfection studies demonstrate that Wnt7b can activate the canonical Wnt pathway but not the noncanonical Wnt pathway in a cell-specific manner. Biochemical analysis demonstrates that Wnt7b can bind to Fzd1 and -10 on the cell surface and cooperatively activate canonical Wnt signaling with these receptors in the presence of LRP5. Together, these data demonstrate that Wnt7b signals through Fzd1 and -10 and LRP5 and implicate these Wnt coreceptors in the regulation of lung airway and vascular development.

Although NH2-terminal mutant huntingtin (htt) fragments cause neurological disorders in Huntington's disease (HD), it is unclear how toxic htt fragments are generated and contribute to the disease process. Here, we report that complex NH2-terminal mutant htt fragments smaller than the first 508 amino acids were generated in htt-transfected cells and HD knockin mouse brains. These fragments constituted neuronal nuclear inclusions and appeared before neurological symptoms. The accumulation and aggregation of these htt fragments were associated with an age-dependent decrease in proteasome activity and were promoted by inhibition of proteasome activity. These results suggest that decreased proteasome activity contributes to late onset htt toxicity and that restoring the ability to remove NH2-terminal fragments will provide a more effective therapy for HD than inhibiting their production.

Arsenic is a well-recognized human carcinogen, yet the mechanism by which it causes human cancer has not been elucidated. MicroRNAs (miRNAs) are a big family of small noncoding RNAs and negatively regulate the expression of a large number of protein-coding genes. We investigated the role of miRNAs in arsenic-induced human bronchial epithelial cell malignant transformation and tumor formation. We found that prolonged exposure of immortalized p53-knocked down human bronchial epithelial cells (p53(low)HBECs) to low levels of arsenite (NaAsO₂, 2.5 μM) caused malignant transformation that was accompanied by epithelial to mesenchymal transition (EMT) and reduction in the levels of miR-200 family members. Stably reexpressing miR-200b in arsenite-transformed cells (As-p53(low)HBECs) completely reversed their transformed phenotypes, as evidenced by inhibition of colony formation in soft agar and prevention of xenograft tumor formation in nude mice. Moreover, stably expressing miR-200b alone in parental nontransformed p53(low)HBECs was sufficient to completely prevent arsenite exposure from inducing EMT and malignant transformation. Further mechanistic studies showed that depletion of miR-200 in arsenite-transformed cells involved induction of the EMT-inducing transcription factors zinc-finger E-box-binding homeobox factor 1 (ZEB1) and ZEB2 and increased methylation of miR-200 promoters. Stably expressing ZEB1 alone in parental nontransformed p53(low)HBECs was sufficient to deplete miR-200, induce EMT and cause cell transformation, phenocopying the oncogenic effect of 16-week arsenite exposure. These findings establish for the first time a causal role for depletion of miR-200b expression in human cell malignant transformation and tumor formation resulting from arsenic exposure.