Our research is focused on understanding how hormone producing cells in the small intestine, pancreas, stomach, and colon differentiate from uncommitted precursor cells to adopt an endocrine cell fate. My research group relies heavily on transgenic mice as a model to examine developmental biology in the GI tract. The major areas currently under investigation in the Leiter laboratory include: 1) Identification of precursor cells that give rise to endocrine precursor cells in the GI tract, 2) Elucidation of the role of the Wnt, Rb, and Notch signaling pathways in regulating cell fate specification in the GI tract, 3) Characterization of the cells from which human carcinoid tumors originate, and 4) Analysis of the mechanism that the basic helix loop helix transcription factor, NeuroD1 activates gut hormone expression in cells as they terminally differentiate.
Importance of GI endocrine cells in health and disease
Major targets of GI hormones include other digestive organs, insulin-expressing pancreatic ß cells, and the brain, including regions that control food intake. The ability to change gut hormone function/development may therefore lead to potential new approaches for treating obesity and diabetes mellitus , two major health problems in the US,
The GI tract is an attractive system for studying differentiation and represents the largest reservoir of stem cells. The intestinal epithelium continuously self-renews itself and continues to differentiate throughout life unlike many organs where differentiation ceases after birth.
Identification of precursor cells of GI endocrine cells
Studies from knockout mice suggest that differentiation of intestinal and pancreatic endocr ine cells depends on the expression of the basic helix loop helix transcription factors Neurogenin3 (Ngn3) and NeuroD. We have used recombination based cell lineage analysis to trace the cell fate of Ngn3+ cells and show that most intestinal and pancreatic endocrine cells arise from Ngn3-expressing precursor cells. Surprisingly, our studies showed that some Ngn3-expressing cells that express low levels of Ngn3 can adopt nonendocrine cell fates. In contrast, cell lineage analysis of NeuroD expressing cells revealed that once a cell expresses this protein, it is irreversibly committed to become an endocrine cell.
We believe that recombination based lineage analysis is an important complement to knock out mice because it does not rely on loss of function as a read out. One surprising outcome was that most endocrine cells in the body of the stomach do not arise from Ngn3+ cells. This observation illustrates the value of recombination based methods since Ngn3 knockout mice die shortly after birth, prior to development of endocrine cells in the stomach. Our lab is currently focusing on identifying the precursors of gastric enteroendocrine cells.
Signaling pathways in GI endocrine cell differentiation
Both the notch and Wnt signaling pathways are conserved throughout metazoan development. Both pathways have been implicated in the control of differentiation within the GI tract although their role in endocrine cell specification is less clear. Abnormal Wnt signaling is a major factor in the development of colorectal cancer. Wnt signaling is clearly required for maintenance of intestinal stem cells but its role in specification of enteroendocrine cells is unclear. Using several transgenic mouse models, we have shown that enteroendocrine cells do not require the Wnt pathway. We have also found that early precursors to enteroendocrine cells respond to abnormal wnt signaling and develop small intestinal polyps and eventually carcinoma. As immature cells further differentiate, they fail to respond to abnormal Wnt signals. We are currently investigating the mechanism of this loss of responsive during differentiation.
Notch signaling plays an important in role in the selection of alternate cell fates by controlling the activity of basic helix loop helix transcription factors like Ngn3. Disruption of notch signaling leads to precocious endocrine differentiation in the pancreas and intestine, suggesting that Notch signals select or suppress endocrine differentiation. We are currently using several mouse models to study the role of Notch and Notch associated proteins for their potential role in determining endocrine cell fate through out the GI tract.
Origin of endocrine cancers in the gastrointestinal tract
We have generated transgenic mice that develop small intestinal adenomas and carcinoma with neuroendocrine features. These mice may represent a model for human GI cancers with mixed neuroendocrine and nonendocrine features. The tumors that develop in our mice also express the hormone serotonin and thus share some similarities to midgut serotonin-expressing carcinoid tumors in humans. From cell lineage tracing studies, we have found that some serotonin cells arise independently of Ngn3 and NeuroD. We believe this population of cells are candidates for becoming carcinoid tumors and will examine their potential to develop carcinoid tumors following activation of the Wnt pathway.
Function of the basic helix loop helix protein BETA2/NeuroD1 in activation of GI hormone transcription
Our laboratory has studied how the basic helix loop helix transcription factor, NeuroD/BETA2, activates expression of the gene encoding the hormone secretin. NeuroD acts as a master regulator of terminal differentiation of secretin by both activating secretin gene expression and inducing cell cycle arrest. We are currently focused on interactions between NeuroD and other transcription factors the bind to the secretin enhancer. NeuroD is a relatively weak activator of transcription. However, its activity is enhanced by interactions with the widely expressed proteins, Sp1 and FinB which bind to nearby sites on the enhancer. It appears that NeuroD stabilizes Sp1 binding to the enhancer. In addition, the effects of NeuroD appear to be regulated by several other proteins including the corepressor, CtBP and possibly by p52.