ALK2 functions as a BMP type I receptor and induces Indian hedgehog in chondrocytes during skeletal development.

UNLABELLED: Growth plate chondrocytes integrate multiple signals during normal development. The type I BMP receptor ALK2 is expressed in cartilage and expression of constitutively active (CA) ALK2 and other activated type I BMP receptors results in maturation-independent expression of Ihh in chondrocytes in vitro and in vivo. The findings suggest that BMP signaling modulates the Ihh/PTHrP signaling pathway that regulates the rate of chondrocyte differentiation.
INTRODUCTION: Bone morphogenetic proteins (BMPs) have an important role in vertebrate limb development. The expression of the BMP type I receptors BMPR-IA (ALK3) and BMPR-IB (ALK6) have been more completely characterized in skeletal development than ALK2.
METHODS: ALK2 expression was examined in vitro in isolated chick chondrocytes and osteoblasts and in vivo in the developing chick limb bud. The effect of overexpression of CA ALK2 and the other type I BMP receptors on the expression of genes involved in chondrocyte maturation was determined.
RESULTS: ALK2 was expressed in isolated chick osteoblasts and chondrocytes and specifically mediated BMP signaling. In the developing chick limb bud, ALK2 was highly expressed in mesenchymal soft tissues. In skeletal elements, expression was higher in less mature chondrocytes than in chondrocytes undergoing terminal differentiation. CA ALK2 misexpression in vitro enhanced chondrocyte maturation and induced Ihh. Surprisingly, although parathyroid hormone-related peptide (PTHrP) strongly inhibited CA ALK2 mediated chondrocyte differentiation, Ihh expression was minimally decreased. CA ALK2 viral infection in stage 19-23 limbs resulted in cartilage expansion with joint fusion. Enhanced periarticular expression of PTHrP and delayed maturation of the cartilage elements were observed. In the cartilage element, CA ALK2 misexpression precisely colocalized with the expression with Ihh. These findings were most evident in partially infected limbs where normal morphology was maintained. In contrast, BMP-6 had a normal pattern of differentiation-related expression. CA BMPR-IA and CA BMPR-IB overexpression similarly induced Ihh and PTHrP.
CONCLUSIONS: The findings show that BMP signaling induces Ihh. Although the colocalization of the activated type I receptors and Ihh suggests a direct BMP-mediated signaling event, other indirect mechanisms may also be involved. Thus, while BMPs act directly on chondrocytes to induce maturation, this effect is counterbalanced in vivo by induction of the Ihh/PTHrP signaling loop. The findings suggest that BMPs are integrated into the Ihh/PTHrP signaling loop and that a fine balance of BMP signaling is essential for normal chondrocyte maturation and skeletal development.