Student Theses and Dissertations

Date of Award

2021

Document Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Abstract

Using self-organizing human models of gastrulation, the Brivanlou/Siggia labs previously showed that (i) BMP4 initiates the cascade of events leading to gastrulation; (ii) BMP4 signal-reception is restricted to the basolateral domain; and (iii) in a human-specific manner, BMP4 directly induces the expression of NOGGIN. In this work, I interrogate the interplay of the morphogen/inhibitor pair BMP4/NOGGIN within our model of the polarized human embryonic disc. Here, I report the surprising discovery that in human epiblasts, NOGGIN and BMP4 were selectively secreted into opposite sides of the polarized epithelium, effectively segregating the ligand and its inhibitor into distinct extracellular spaces. Moreover, utilizing a microfluidic flow chamber, I unambiguously demonstrated the long-distance diffusion of NOGGIN through the extracellular medium at the apical side as well as the short-distance diffusion of BMP4 over few direct neighbors. Surprisingly, I further demonstrate that apically-applied NOGGIN can inhibit basally-applied BMP4 across a tight hESC epithelium, which points to mechanisms of trans-epithelial transport. And finally, I trace the transcytosis route of apical NOGGIN through the endosomal system. Overall, I highlight a complex mechanism where apically-secreted NOGGIN diffuses over long distance in the apical compartment; NOGGIN is then internalized, transcytosed, and trafficked to the basal-lateral surface close to the subcellular locus where BMP receptors are located. This apical-to-basal transcytosis was indispensable for NOGGIN inhibition. Taken together, the segregation of activator/inhibitor into distinct extracellular spaces challenges classical views of morphogen movement. This discovery challenges current dogma which assumes that the morphogen activator and inhibitor diffuse in the same compartment. I propose that the transport of morphogen inhibitors regulates the spatial availability of morphogens during embryogenesis. My study shed light on an important and unexpected level of regulation for the transport of morphogen inhibitors whose role in spatially restricting the spread of morphogen signaling ultimately shapes the embryonic body plan.

Comments

A Thesis Presented to the Faculty of The Rockefeller University in Partial Fulfillment of the Requirements for the degree of Doctor of Philosophy

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