Differential compartmentalization of BMP4/NOGGIN requires NOGGIN trans-epithelial transport

Using self-organizing human models of gastrulation, we previously showed that (1) BMP4 initiates the cascade of events leading to gastrulation, (2) BMP4 signal reception is restricted to the basolateral domain, and (3) in a human-specific manner, BMP4 directly induces the expression of NOGGIN. Here, we report the surprising discovery that in human epiblasts, NOGGIN and BMP4 were secreted into opposite extracellular spaces. Interestingly, apically presented NOGGIN could inhibit basally delivered BMP4. Apically imposed microfluidic flow demonstrated that NOGGIN traveled in the apical extracellular space. Our co-localization analysis detailed the endocytotic route that trafficked NOGGIN from the apical space to the basolateral intercellular space where BMP4 receptors were located. This apical-basal transcytosis was indispensable for NOGGIN inhibition. Taken together, the segregation of activator/inhibitor into distinct extracellular spaces challenges classical views of morphogen movement. We propose that the transport of morphogen inhibitors regulates the spatial availability of morphogens during embryogenesis. [Display omitted] •NOGGIN and BMP4 are secreted to separate, opposite extracellular spaces•Apically applied NOGGIN can inhibit basally applied BMP4•NOGGIN undergoes apical-basal transcytosis•The endocytotic route involved in NOGGIN trafficking is characterized Phan-Everson et al. demonstrates that in a model human epiblast, the morphogen BMP4 and its inhibitor NOGGIN are secreted to opposite extracellular spaces. NOGGIN is then trafficked across the polarized hESC epithelium. Tissue polarity can influence morphogen signaling by restricting the spaces morphogens are secreted to and mediating their transport.