Department of Immunology, Mayo Clinic College of Medicine, Rochester, Minnesota, United States of America.
PLoS Genet. 2009 Dec;5(12):e1000750. doi: 10.1371/journal.pgen.1000750. Epub 2009 Dec 4.
Osteogenesis Imperfecta (OI) is a human syndrome characterized by exquisitely fragile bones due to osteoporosis. The majority of autosomal dominant OI cases result from point or splice site mutations in the type I collagen genes, which are thought to lead to aberrant osteoid within developing bones. OI also occurs in humans with homozygous mutations in Prolyl-3-Hydroxylase-1 (LEPRE1). Although P3H1 is known to hydroxylate a single residue (pro-986) in type I collagen chains, it is unclear how this modification acts to facilitate collagen fibril formation. P3H1 exists in a complex with CRTAP and the peptidyl-prolyl isomerase cyclophilin B (CypB), encoded by the Ppib gene. Mutations in CRTAP cause OI in mice and humans, through an unknown mechanism, while the role of CypB in this complex has been a complete mystery. To study the role of mammalian CypB, we generated mice lacking this protein. Early in life, Ppib-/- mice developed kyphosis and severe osteoporosis. Collagen fibrils in Ppib-/- mice had abnormal morphology, further consistent with an OI phenotype. In vitro studies revealed that in CypB-deficient fibroblasts, procollagen did not localize properly to the golgi. We found that levels of P3H1 were substantially reduced in Ppib-/- cells, while CRTAP was unaffected by loss of CypB. Conversely, knockdown of either P3H1 or CRTAP did not affect cellular levels of CypB, but prevented its interaction with collagen in vitro. Furthermore, knockdown of CRTAP also caused depletion of cellular P3H1. Consistent with these changes, post translational prolyl-3-hydroxylation of type I collagen by P3H1 was essentially absent in CypB-deficient cells and tissues from CypB-knockout mice. These data provide significant new mechanistic insight into the pathophysiology of OI and reveal how the members of the P3H1/CRTAP/CypB complex interact to direct proper formation of collagen and bone.
成骨不全症（OI）是一种人类综合征，其特征是由于骨质疏松症导致骨骼极其脆弱。大多数常染色体显性 OI 病例是由 I 型胶原基因的点突变或剪接位点突变引起的，这些突变被认为导致发育中的骨骼内异常的类骨质。OI 也发生在脯氨酰-3-羟化酶-1（P3H1）纯合突变的人类中。尽管 P3H1 已知羟化 I 型胶原链中的单个残基（脯氨酸 986），但尚不清楚这种修饰如何促进胶原纤维的形成。P3H1 存在于与 CRTAP 和肽基脯氨酰顺反异构酶 cyclophilin B（CypB）的复合物中，后者由 Ppib 基因编码。CRTAP 中的突变导致小鼠和人类的 OI，但机制尚不清楚，而 CypB 在该复合物中的作用一直是个谜。为了研究哺乳动物 CypB 的作用，我们生成了缺乏这种蛋白的小鼠。在生命早期，Ppib-/- 小鼠出现脊柱后凸和严重的骨质疏松症。Ppib-/- 小鼠的胶原纤维形态异常，进一步符合 OI 表型。体外研究表明，在 CypB 缺陷型成纤维细胞中，前胶原不能正确定位到高尔基体。我们发现 Ppib-/- 细胞中的 P3H1 水平显著降低，而 CRTAP 不受 CypB 缺失的影响。相反，P3H1 或 CRTAP 的敲低都不会影响 CypB 的细胞水平，但会阻止其在体外与胶原的相互作用。此外，CRTAP 的敲低也导致细胞内 P3H1 的耗竭。与这些变化一致，P3H1 对 I 型胶原的翻译后脯氨酰-3-羟化作用在 CypB 缺陷型细胞和 CypB 敲除小鼠的组织中基本缺失。这些数据为 OI 的病理生理学提供了重要的新机制见解，并揭示了 P3H1/CRTAP/CypB 复合物的成员如何相互作用以指导胶原和骨骼的正确形成。
