Effect of the distribution and clustering of the type I A BMP receptor (ALK3) with the type II BMP receptor on the activation of signalling pathways.

Bone morphogenetic proteins (BMPs) play an important role during embryonic development, especially in chondrogenesis, osteogenesis, neurogenesis and hematopoiesis. There are over 19 BMPs known in mammalians, but only three BMP-type-I receptors and three BMP-type-II receptors are known so far to mediate these responses. Previous reports provide evidence to support that oligomerisation of BMP receptors influences the activation of the downstream BMP signalling pathways, the Smad or the p38 MAPK pathway. To further explore the importance of BMP receptor clustering in signalling, image correlation spectroscopy has been used to investigate the clustering and distribution of BMP receptors at the surface of the cell membrane. Here we demonstrate that the co-expression of the BMP-type-II receptor (BRII) influences the aggregation and the distribution of the BMP-type-Ia receptor (BRIa) in COS7 cells and in A431 cells. We also demonstrate that BMP-2 stimulation of the cells leads to a rearrangement of receptor complexes at the cell surface. Using A431 cells and limb bud-derived mesenchymal cells, we show that co-expression of the BRII and a constitutive active BRIa-ca is necessary for the activation of the Smad pathway. Importantly using a kinase-inactive BRII the rearrangement of BRIa is blocked. Together, these findings suggest that rearrangement of the receptors at the cell surface prior to forming preformed ligand independent complexes plays a critical role in activation of the Smad pathway. It also suggests further that the kinase activity of BRII is needed for signalling beyond the activation of BRIa at the GS domain.