Gene Expression of Bone Matrix Proteins and Endothelin Receptors in Endothelin‐1–Deficient Mice Revealed by In Situ Hybridization

Endothelin‐1 (ET‐1) was first found as a vasoconstrictor protein excreted by vascular endothelial cells, but recently ET‐1 has been considered to have widespread functions that include regulation of osteochondrogenic metabolism. We analyzed sections of head regions in ET‐1 knockout mice that are known to have abnormalities in pharyngeal arch–derived tissues and found that there was severe hypoplasia in facial bones. The hypoplasia suggests that the matrix mineralization system of facial bones is disrupted in ET‐1−/− homozygous mice. To elucidate whether osteogenic cells in facial bones are the targets for ET‐1 and whether expression of bone matrix genes are modulated by ET‐1, we examined gene expression of ET‐1 receptors, ETA and ETB, and that of the bone matrix proteins, osteonectin (ON) and osteopontin (OP), both in the head regions of ET‐1+/− heterozygous and ET‐1−/− homozygous mice by means of in situ hybridization. Different patterns of expression between ETA and ETB mRNAs were observed in both groups. In 18.5 days post coitus fetuses, ETA mRNA was most strongly expressed in osteogenic cells along craniofacial bones, but ETB mRNA was most strongly expressed in trunks of trigeminal nerve. This finding suggests that ET‐1 may modulate osteogenic cells through ETA receptor but not through ETB receptor. The expression patterns of ETA, OP, and ON mRNAs were distinct between the two groups. In the lower jaw of ET‐1+/− heterozygous mice, the ETA, ON, and OP mRNA positive cells were scattered in the inner and outer regions of the thick bone matrix, but in ET‐1−/− homozygous mice, cells containing those mRNAs were located close to each other at the surface of thin bone matrix. However, cellular expression of ON and OP mRNAs in osteogenic cells of ET‐1−/− homozygous mice was not suppressed as compared with ET‐1+/− heterozygous mice. We conclude that ET‐1 may regulate proliferation and migration of osteogenic cells in the maxillofacial region, rather than modulating the expression level of ON and OP mRNAs.