For embryos to develop properly, cells must communicate with one another over long ranges through diffusive chemical signals called morphogens. These signals act as instructive cues for the patterning of tissues, establishing the positions of different cell types according to the local morphogen concentration. Specifically, stem cells detect morphogens through receptors on their plasma membrane and choose their future cell fate accordingly – becoming muscle or bone, for example. Understanding how morphogen signaling is organized in the embryo may lead to future medical methods to repair or regrow damaged tissues. It could also provide insight into developmental defects resulting from gene mutations that disrupt morphogen signaling.
In recent years, it has become appreciated that morphogen receptors themselves, like morphogens, can also be spatially organized across tissues. For instance, several signaling receptors have been found to localize specifically on the top (“apical”) or bottom (“basolateral”) membrane of epithelial cells, altering where these cells can detect morphogens. Given this, we asked a simple question: does the localization of receptors determine where morphogen signaling occurs in the developing embryo? And if so, could the misregulation of receptor localization lead to developmental errors?
In our new paper, we investigated the role of receptor localization in regulating morphogen signaling within the early mouse embryo. By simulating the activity of a morphogen called BMP4 within the mouse embryo, we found that the location of BMP receptors crucially determined the profile of BMP signaling predicted by our simulations. Given these results, we hypothesized that any changes in BMP receptor localization within the mouse could have a striking effect on BMP signaling and patterning within the embryo. Immunofluorescence imaging of mouse embryos at the pre-gastrulation stage showed that BMP receptors indeed did have a specific localization. They localize exclusively at the basolateral membrane of the mouse epiblast, an epithelial tissue made up of embryonic stem cells. Furthermore, mislocalizing these receptors to the apical membrane leads to ectopic BMP signaling in the epiblast cells.
Why then are BMP receptors basolaterally localized? Our simulation results suggested one possibility: since the epiblast is held together by impermeable junctions, secreted BMP4 ligands must diffuse around the edge of the epiblast to reach the receptors underneath. As a consequence of this geometry, our model predicted that a robust BMP signaling gradient would form from the edges of the tissue inward across a wide range of morphogen concentrations. We tested this prediction in vivo by microinjecting BMP4 into the mouse embryo. We also created a BMP4 gradient in vitro using microfluidics. The test results support our prediction and demonstrate that receptor localization plays an essential role in regulating BMP signaling within the developing embryo.
We expect this regulatory mechanism to be broadly applicable in other developmental contexts. The sequence motif responsible for basolateral localization of BMP receptors is conserved throughout the vertebrate TGF-β receptor family.