| Bmps are signaling molecules critical for the development of multiple organs. Bmp expression and a Bmp activity gradient is readily detectable in the mouse telencephalon, the embryonic structure that eventually yields the choroid plexus and cerebral cortex. However, the precise roles of this Bmp gradient in the developing telencephalon have been difficult to clarify due to the redundancy of Bmps in development, the embryonic lethality of early Bmp abrogation, the complex interactions that regulate target genes, and a lack of well-studied Bmp readouts that have confounded previous studies.;In this dissertation, I address the roles of Bmps in the telencephalon first by studying how cells respond to a Bmp gradient by characterizing readouts, and secondly by studying how Bmp readouts are regulated to generate patterning. My studies using an in vitro model of a Bmp concentration gradient demonstrated that 1) an extracellular Bmp concentration gradient is translated to an all-or-none, or ultrasensitive expression of endogenous Bmp target genes despite the finding that 2) the extracellular Bmp concentration gradient is translated to an intracellular gradient of the activated downstream signaling molecule. To address the mechanism of ultrasensitivity, I generated a novel transgenic mouse line called BRE-gal that would report Bmp signaling activity based upon a construct that, to our knowledge, only contained Bmp response elements (BRE). Using BRE-gal, I found evidence that cortical hem fate can be specified over a range of Bmp signaling levels, and that Lhx2 does not negatively regulate Bmp signaling levels in suppressing hem fate. I then established that the mechanism underlying the Bmp ultrasensitivity could be reduced to interactions between the Bmp signaling pathway and the growth factors Fgf2 and Egf. Finally, I provide evidence suggesting that the Fgf2/Egf-mediated ultrasensitivity phenomenon observed in culture plays a role in the refinement of Bmp target genes in tissue. Taken together, my studies have established a new tool with which to dissect the regulation of Bmp signaling, shed insights into how discrete changes in cellular behavior emerge from a signaling gradient, further developed the model for cortical hem development, and contributed a novel model for border formation. |
