|Ph.D Student||Fleming Britannia|
|Subject||Signaling Pathways Regulating the Formation of the|
|Department||Department of Medicine||Supervisor||Professor Thomas Schultheiss|
|Full Thesis text|
The intermediate mesoderm (IM) is the embryonic source of all kidney tissue in vertebrates. The factors that regulate the formation of the IM are not yet well understood. Through investigations in the chick embryo, the current study identifies and characterizes Vg1/Nodal signaling (“Nodal-like signaling”) and Mitogen-Activated Protein Kinase (MAPK) signaling as novel regulators of IM formation.
Excess Nodal-like signaling at gastrulation stages resulted in expansion of the IM at the expense of the adjacent paraxial mesoderm, whereas inhibition of Nodal-like signaling caused repression of IM gene expression. IM formation was found to be sensitive to levels of the Nodal-like pathway coreceptor Cripto, with elevation of Cripto leading to expansion of the IM and inhibition of Cripto resulting in repression of IM genes, indicating that the observed effects are specific to the Nodal-like signaling. Supporting this conclusion, it was also found that IM formation was inhibited by the Nodal inhibitors Lefty and a truncated form of the secreted molecule Cerberus. The IM-promoting effects of Nodal-like signaling were distinct from the known effects of this pathway on mesoderm formation and left-right patterning, a finding that can be attributed to specific time windows for the activities of these Nodal-like functions.
Our research has also identified a link between Nodal-like and Bone Morphogenetic Protein (BMP) signaling in the induction of IM. Activation of IM genes by Nodal-like signaling required an active BMP signaling pathway, and Nodal-like signals induced phosphorylation of Smad1/5/8, which is normally associated with activation of BMP signaling. We postulate that Nodal-like signaling regulates IM formation by modulating the IM-inducing effects of BMP signaling.
Finally, the results of these studies identify MAPK signaling as an additional pathway playing a role in IM formation. Activation of this pathway induced significant broadening of the IM, whereas its inhibition resulted in reduced expression of early kidney genes. Interestingly, the alterations in IM gene expression were not always cell autonomous, suggesting a paracrine mode of signaling.
Understanding the combination of signals that generates IM in vivo is an important prerequisite for attempts to generate kidney tissue in the laboratory. This research adds both Nodal-like signaling and MAPK signaling as regulators of IM formation and also emphasizes that their IM-inducing effects are very specific with respect to time and location in the embryo. Therefore, the studies described in this thesis could ultimately help generate kidney tissue from stem cells for potential therapies in the future.