Bone morphogenetic protein signaling pathway plays multiple roles during gastrointestinal tract development.

The bone morphogenetic protein (BMP) signaling pathway plays an essential role during gastrointestinal (GI) tract development in vertebrates. In the present study, we use an antibody that recognizes the phosphorylated and activated form of Smad1, 5, and 8 to examine (by immunohistochemistry) the endogenous patterns of BMP signaling pathway activation in the developing GI tract. We show that the endogenous BMP signaling pathway is activated in the mesoderm, the endoderm, and the enteric nervous system (ENS) of the developing chick GI tract and is more widespread than BMP ligand expression patterns. Using an avian-specific retroviral misexpression technique to activate or inhibit BMP signaling pathway activity in the mesoderm of the gut, we show that BMP activity is required for the pattern, the development, and the differentiation of all three tissue types of the gut: mesoderm (that forms the visceral smooth muscle), endoderm (that forms the epithelium), and ectoderm (that forms the ENS). These results demonstrate that BMP signaling is activated in all the tissue layers of the GI tract during the development and plays a role during interactions and reciprocal communications of these tissue layers.

Hh pathway expression in human gut tissues and in inflammatory gut diseases.

Sonic hedgehog (Shh) directs early gut patterning via epithelial-mesenchymal signaling and remains expressed in endoderm-derived tissues into the adult period. In human adult gut epithelium SHH/SHH expression is strongest in basal layers, which suggests that SHH may function in the maintenance of gut epithelial stem or progenitor cells. Recent publications suggest a role for aberrant SHH/SHH expression in gut epithelial neoplasias. We hypothesized that the regenerating gut epithelium in inflammatory gut disorders would show an upregulation of SHH/SHH signaling and this abnormal signal may explain the increased incidence of neoplasia in these diseases. Archived healthy gut and inflammatory gut diseased tissues were analyzed by RNA in situ hybridization and immunohistochemistry to describe location and levels of SHH signaling. We show that SHH/SHH and its receptor PTCH1/PTCH1 expression is restricted to the glandular epithelium of the gut, in an antiluminal pattern (strongest in basal layers and weak to absent in luminal epithelium). Inflammatory diseases of the gut show dramatic increases in epithelial SHH signaling. Expression increases in inflamed glandular epithelium (including metaplastic glandular epithelium), losing its radial (crypt-villous) polarity, and expression appears upregulated and present in all epithelial cells. We also describe strong SHH/SHH and PTCH1/PTCH1 expression in intraepithelial and mucosal inflammatory cells. We suggest that SHH signaling in inflammatory diseases of the gut acts to ensure stem cell restitution of damaged mucosal epithelium. However, such signaling may also present a risk for neoplastic transformation.