Positionally-dependent chondrogenesis induced by BMP4 is co-regulated by Sox9 and Msx2

Cranial neural crest cells emigrate from the posterior midbrain and anterio r hindbrain to populate the first branchial arch and eventually differentia te into multiple cell lineages in the maxilla and mandible during craniofac ial morphogenesis. In the developing mouse mandibular process, the expressi on profiles of BMP4, Msx2, Sox9, and type II collagen demonstrate temporall y and spatially restrictive localization patterns suggestive of their funct ions in the patterning and differentiation of cartilage, Under serumless cu lture conditions, beads soaked in BMP4 and implanted into embryonic day 10 (E10) mouse mandibular explants induced ectopic cartilage formation in the proximal position of the explant. However, BMP4-soaked beads implanted at t he rostral position did not have an inductive effect. Ectopic chondrogenesi s was associated with the up-regulation of Sox9 and Msx2 expression in the immediate vicinity of the BMP4 beads 24 hours after implantation. Control b eads had no effect on cartilage induction or Msx2 and Sox9 expression. SoxS was induced at all sites of BMP4 bead implantation In contrast, Msx2 expre ssion was induced more intensely at the rostral position when compared with the proximal position, and suggested that Msx2 expression was inhibitory t o chondrogenesis. To test the hypothesis that over-expression of Msx2 inhib its chondrogenesis, we ectopically expressed Msx2 in the mandibular process organ culture system using adenovirus gene delivery strategy. Microinjecti on of the Msx2-adenovirus to the proximal position inhibited BMP4-induced c hondrogenesis, Over-expression of Msx2 also resulted in the abrogation of e ndogenous cartilage and the down-regulation of type II collagen expression, Taken together, these results suggest that BMP4 induces chondrogenesis, th e pattern of which is positively regulated by SoxS and negatively by Msx2, Chondrogenesis only occurs at sites where SoxS expression is high relative to that of Msx2. The combinatorial action of these transcription factors ap pear to establish a threshold for SoxS function and thereby restricts the p osition of chondrogenesis, Published 2000 Wiley-Liss, Inc.(dagger).