Distinct Spatial and Temporal Expression Patterns of Two Type I Receptors for Bone Morphogenetic Proteins During Mouse Embryogenesis

Bone morphogenetic proteins (BMPs) are multifunctional proteins structurally related to transforming growth factor-beta (TGF beta) and activin that can induce cartilage and bone growth in vivo. Members of the TGF beta superfamily exert their biological effects via heteromeric serine/threonine kinase complexes of type I and type II receptors. We previously obtained six different type I receptors, termed activin receptor-like kinase-1 (ALK-1) to -6. ALK-5 is a TGF beta type I receptor, ALK-2 and ALK-4 are activin type I receptors, and ALK-3 and ALK-6 are type I receptors for osteogenic protein-1 (OP-1)/bone morphogenetic protein-7 (BMP-7) and BMP-4. Here we report the complementary DNA cloning of the mouse homolog of ALK-3, which is highly conserved between mouse and man. ALK-3 messenger RNA (mRNA) is ubiquitously expressed in various adult mouse tissues, whereas ALK-6 mRNA is only found in brain and lung. The distribution of ALK-3 and ALK-6 mRNA in the postimplantation mouse embryo [6.5-15.5 days postcoitum (pc)] was studied by in situ hybridization. ALK-3 was nearly ubiquitously expressed throughout these stages of development, but was notably absent in the liver. In contrast, ALK-6 showed a more restricted expression pattern. ALK-6 mRNA was absent in early postimplantation embryos, was detected first in 9.5 days pc embryos, and persisted until 15.5 days pc. In midgestation embryos, ALK-6 transcripts were detected in mesenchymal precartilage condensations, premuscle masses, blood vessels, central nervous system, parts of the developing ear and eye, and epithelium. The expression in sites of developing cartilage and bone supports the idea that ALK-3 and -6 are receptors for BMPs in vivo. In addition, the expression of these genes in many soft tissues suggests broader functions for BMPs in embryogenesis.