Zhang Donghui, Schwarz Edward M, Rosier Randy N, Zuscik Michael J, Puzas J Edward, O'Keefe Regis J
Center for Musculoskeletal Research, University of Rochester, Rochester, New York 14642, USA.
J Bone Miner Res. 2003 Sep;18(9):1593-604. doi: 10.1359/jbmr.2003.18.9.1593.
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.
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.
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.
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.
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.
生长板软骨细胞在正常发育过程中整合多种信号。I型骨形态发生蛋白受体ALK2在软骨中表达，组成型激活（CA）ALK2和其他激活的I型骨形态发生蛋白受体的表达导致体外和体内软骨细胞中Ihh的成熟非依赖性表达。这些发现表明骨形态发生蛋白信号传导调节Ihh/PTHrP信号通路，该通路调节软骨细胞分化速率。
骨形态发生蛋白（BMPs）在脊椎动物肢体发育中起重要作用。与ALK2相比，I型BMP受体BMPR-IA（ALK3）和BMPR-IB（ALK6）的表达在骨骼发育中得到了更全面的表征。
在体外分离的鸡软骨细胞和成骨细胞中以及在发育中的鸡肢芽体内检测ALK2的表达。确定CA ALK2和其他I型BMP受体过表达对软骨细胞成熟相关基因表达的影响。
ALK2在分离的鸡成骨细胞和软骨细胞中表达，并特异性介导BMP信号传导。在发育中的鸡肢芽中，ALK2在间充质软组织中高度表达。在骨骼成分中，未成熟软骨细胞中的表达高于正在进行终末分化的软骨细胞。体外CA ALK2表达错误增强了软骨细胞成熟并诱导了Ihh。令人惊讶的是，尽管甲状旁腺激素相关肽（PTHrP）强烈抑制CA ALK2介导的软骨细胞分化，但Ihh表达仅略有下降。在19-23期肢体中进行CA ALK2病毒感染导致软骨扩张并伴有关节融合。观察到PTHrP的关节周围表达增强和软骨成分的成熟延迟。在软骨成分中，CA ALK2表达错误与Ihh的表达精确共定位。这些发现在保持正常形态的部分感染肢体中最为明显。相比之下，BMP-6具有正常的分化相关表达模式。CA BMPR-IA和CA BMPR-IB过表达同样诱导Ihh和PTHrP。
这些发现表明BMP信号传导诱导Ihh。尽管激活的I型受体和Ihh的共定位表明存在直接的BMP介导的信号事件，但也可能涉及其他间接机制。因此，虽然BMP直接作用于软骨细胞以诱导成熟，但这种作用在体内通过诱导Ihh/PTHrP信号环而被抵消。这些发现表明BMP被整合到Ihh/PTHrP信号环中，并且BMP信号的精细平衡对于正常软骨细胞成熟和骨骼发育至关重要。
