Alvarez Jesus, Sohn Philip, Zeng Xin, Doetschman Thomas, Robbins David J, Serra Rosa
Department of Molecular and Cellular Physiology, University of Cincinnati College of Medicine, Cincinnati, OH 45267-0576, USA.
Development. 2002 Apr;129(8):1913-24. doi: 10.1242/dev.129.8.1913.
The development of endochondral bones requires the coordination of signals from several cell types within the cartilage rudiment. A signaling cascade involving Indian hedgehog (Ihh) and parathyroid hormone related peptide (PTHrP) has been described in which hypertrophic differentiation is limited by a signal secreted from chondrocytes as they become committed to hypertrophy. In this negative-feedback loop, Ihh inhibits hypertrophic differentiation by regulating the expression of Pthrp, which in turn acts directly on chondrocytes in the growth plate that express the PTH/PTHrP receptor. Previously, we have shown that PTHrP also acts downstream of transforming growth factor beta (TGFbeta) in a common signaling cascade to regulate hypertrophic differentiation in embryonic mouse metatarsal organ cultures. As members of the TGFbeta superfamily have been shown to mediate the effects of Hedgehog in several developmental systems, we proposed a model where TGFbeta acts downstream of Ihh and upstream of PTHrP in a cascade of signals that regulate hypertrophic differentiation in the growth plate. This report tests the hypothesis that TGFbeta signaling is required for the effects of Hedgehog on hypertrophic differentiation and expression of PTHRP: We show that Sonic hedgehog (Shh), a functional substitute for Ihh, stimulates expression of Tgfb2 and Tgfb3 mRNA in the perichondrium of embryonic mouse metatarsal bones grown in organ cultures and that TGFbeta signaling in the perichondrium is required for inhibition of differentiation and regulation of Pthrp expression by Shh. The effects of Shh are specifically dependent on TGFbeta2, as cultures from Tgfb3-null embryos respond to Shh but cultures from Tgfb2-null embryos do not. Taken together, these data suggest that TGFbeta2 acts as a signal relay between Ihh and PTHrP in the regulation of cartilage hypertrophic differentiation.
软骨内骨的发育需要软骨原基中几种细胞类型发出的信号相互协调。一种涉及印度刺猬因子（Ihh）和甲状旁腺激素相关肽（PTHrP）的信号级联反应已被描述，其中肥大分化受到软骨细胞在开始肥大时分泌的一种信号的限制。在这个负反馈回路中，Ihh通过调节Pthrp的表达来抑制肥大分化，而Pthrp反过来直接作用于生长板中表达PTH/PTHrP受体的软骨细胞。此前，我们已经表明，在一个共同的信号级联反应中，PTHrP也在转化生长因子β（TGFβ）下游发挥作用，以调节胚胎小鼠跖骨器官培养物中的肥大分化。由于TGFβ超家族的成员已被证明在几个发育系统中介导刺猬因子的作用，我们提出了一个模型，即TGFβ在调节生长板肥大分化的信号级联反应中，在Ihh下游和PTHrP上游发挥作用。本报告检验了这样一个假说，即TGFβ信号传导是刺猬因子对肥大分化和PTHRP表达产生影响所必需的：我们发现，作为Ihh功能替代物的音猬因子（Shh），刺激了器官培养中生长的胚胎小鼠跖骨骨膜中Tgfb2和Tgfb3 mRNA的表达，并且骨膜中的TGFβ信号传导是Shh抑制分化和调节Pthrp表达所必需的。Shh的作用特别依赖于TGFβ2，因为来自Tgfb³基因敲除胚胎的培养物对Shh有反应，而来自Tgfb²基因敲除胚胎的培养物则没有。综上所述，这些数据表明，在软骨肥大分化的调节中，TGFβ2充当了Ihh和PTHrP之间的信号中继。
