Early in development, the midgut must rotate so that its ventral margin shifts to the left;failure to do so results in a malrotation and can lead to catastrophic midgut volvulus. It has long been assumed that gut rotation is intrinsic to the tube itself;however, my research has demonstrated that rotation is instead determined by asymmetric cellular changes within the dorsal mesentery that suspends the gut. This mesentery has four juxtaposed yet distinct cellular compartments distributed along its left-right axis, and changes in each are required for correct gut rotation. Combined with the unique accessibility of the chicken egg, this cellular architecture has established the dorsal mesentery as a powerful model system to define, in vivo, the fundamental genetic and cellular mechanisms through which organs acquire their spatial organization, which is a prerequisite for normal functioning. The genesis of gut rotation traces its origins to the early left-right symmetry-breaking transcription factor Pitx2. In mice and birds, Pitx2 is necessary and sufficient to produce the leftward tilt, and this rotation is randomized in embryos deficient for Pitx2 activity. However, th mechanisms by which this transcription factor directs downstream cellular changes necessary to cause gut rotation remain unknown. To identify cellular targets of Pitx2 in each of the four compartments, we employed laser capture microdissection to isolate then catalog the genes expressed in each cellular compartment at the time of the leftward tilt. Using these data, the firs aim pursues cascades involving subsets of genes that are critical for signaling, for recognizing extracellular cues, and for remodeling cytoskeletal architecture. The roles of key players will be assessed by introducing gain- or reduction- of function gene constructs into each compartment. In our second aim, we address previously unknown asymmetries in the formation of intra-mesenteric arteries that bring blood to the gut, using experimental approaches similar to the first aim but assaying for positive and negative regulators of vasculogenesis. In our third aim, we expand our studies using mouse models of asymmetric organ development and use chromatin immunoprecipitations in vivo to identify bona fide Pitx2 transcriptional targets. Lessons learned from these experiments will impact the study of other regions of the gut, and of tubular organs in general, some of which share strikingly similar features of morphogenesis and genetic patterning with the vertebrate midgut.
Gut malrotation is a birth defect of abnormal intestinal rotation that occurs once in approximately 500 live births. While malrotation predisposes affected babies to volvulus, a catastrophic strangulation of the intestine and its blood supply, th origin of this anomaly remains entirely unknown. We have developed the genetic tools to answer the fundamental questions surrounding proper gut rotation making use of the accessibility of the chicken egg and genetic mouse models to ultimately improve diagnosis in neonates.
|Welsh, Ian C; Kwak, Hojoong; Chen, Frances L et al. (2015) Chromatin Architecture of the Pitx2 Locus Requires CTCF- and Pitx2-Dependent Asymmetry that Mirrors Embryonic Gut Laterality. Cell Rep 13:337-49|
|Mahadevan, Aparna; Welsh, Ian C; Sivakumar, Aravind et al. (2014) The left-right Pitx2 pathway drives organ-specific arterial and lymphatic development in the intestine. Dev Cell 31:690-706|
|Welsh, Ian C; Thomsen, Michael; Gludish, David W et al. (2013) Integration of left-right Pitx2 transcription and Wnt signaling drives asymmetric gut morphogenesis via Daam2. Dev Cell 26:629-44|