Type I collagen synthesis in the context of osteogenesis imperfecta

Type I collagen — the major protein component of the extracellular matrix in bone, skin and tendon — is mainly secreted by osteoblasts, dermal fibroblasts and tenocytes. Despite the relatively simple structure of the collagen triple helix, the biosynthesis of type I procollagen is extremely complex, involving multiple steps and requiring an ensemble of proteins for post-translational modifications, folding, transport, secretion and quality control. The COL1A1 and COL1A2 transcripts are translated in the rough endoplasmic reticulum (rER), and the α(I)‑chains undergo a series of post-translational modifications. Helical prolines in position Y of the Gly-Xaa-Yaa repeat are hydroxylated in position C4 by prolyl 4‑hydroxylase 1 (P4H1), whereas specific prolines in the X positions are 3‑hydroxylated by P3H1 and P3H2. Some lysine residues are hydroxylated by lysyl hydroxylases (LH1 and LH2, encoded by PLOD1 and PLOD2 respectively), and glycosylation of hydroxylysines into galactosyl-hydroxylysine and glucosyl-galactosyl-hydroxylysine is catalysed by procollagen galactosyltransferase 1 and procollagen glucosyltransferase 1. After synthesis of the carboxy-terminal propeptide, it forms intra-chain disulfide bonds and remains attached to the rER membrane. Selection and association of the correct chains into a triple helix occur by diffusion of the C-propeptides attached to the rER membrane. A nucleus for triple helix formation is formed that staggers the chains in the correct order and initiates triple helix formation. Protein disulfide isomerase catalyses inter-chain disulfide bond formation, which stabilize the folding nucleus. Hydroxylation of proline residues and some lysine residues continues and triple helix formation proceeds from the C-terminal end towards the amino-terminal end. During this phase, 65 kDa FK506‑binding protein (FKBP65; encoded by FKBP10) and a complex formed by P3H1 — CRTAP– PPIase B (peptidyl-prolyl cis–trans isomerase B) — seem to play a crucial part. The complex is involved in the Hydroxylation of proline 986 of the collagen α1(I)-chain and α1(II)-chain and proline 707 of the α2(I)-chain, which are thought to be important for supramolecular assembly of collagen fibrils and to serve as binding sites for chaperones or small leucine-rich proteoglycans. Beyond its prolyl 3‑hydroxylase activity, the complex functions as a PPIase and chaperone for collagen folding. Indeed, the fast propagation of the triple helix requires the isomerization of cis peptide bonds that convert proline residues into trans configuration, mainly by PPIase B24. When most of the helix is folded, the N-propeptides associate and form the small triple helix within this domain. The newly formed triple helix is stabilized by serpin H1 (also known as HSP47; encoded by SERPINH1) and FKBP65. Further modifications occur during transport from the rER to the Golgi apparatus in special coat protein complex vesicles that contain melanoma inhibitory activity protein 3 (also known as TANGO1, encoded by MIA) and through the Golgi stack by cisternal maturation. Serpin H1 also has binding sites along the helical portion of the molecule and assists shuttling of folded collagen into the cis-Golgi. These biosynthetic steps depend on a proper rER environment (for example, optimal calcium levels and redox potential), and the quality-control mechanisms can lead to the activation of the unfolded protein response using the ER-associated degradation pathway or the autophagy-mediated lysosomal degradation system to eliminate molecules that were not properly folded. Once secreted, the propeptides of procollagen are cleaved by a disintegrin and metalloproteinase with thrombospondin motifs 2 (ADAMTS2) and bone morphogenetic protein 1 (BMP1) into mature type I collagen. This initiates collagen fibre formation and these fibrils are stabilized by crosslink formation, in which certain lysine and hydroxylysine residues in the triple helix and the telopeptides are oxidized by lysyl oxidases and converted into allysine and hydroxyallysine. These residues then initially form divalent crosslinks that convert into mature trivalent pyridinoline and pyrrole crosslinks to stabilize the fibril structure in tissues. Bone formation consists of the secretion of bone extracellular matrix components (mainly type I collagen) by osteoblasts. The unmineralized bone matrix (osteoid) subsequently becomes mineralized. In addition, osteoblasts and osteocytes release many cytokines, including receptor activator of nuclear factor-κB ligand (RANKL; also known as TNFSF11) and osteoprotegerin (OPG, encoded by TNFRSF11B), which regulate bone resorption by osteoclasts. RANKL acts on osteoclast precursor cells by binding to receptor activator of nuclear factor-κB (RANK; also known as TNFRSF11A) on their surface, thereby favouring their differentiation to osteoclasts. OPG, by interacting with RANKL, prevents the binding of RANKL to RANK. Some osteoblasts become embedded in the mineralized bone matrix and differentiate to osteocytes, which produce, among other factors, sclerostin, an inhibitor of the WNT pathway that is known to stimulate bone formation by stimulating osteoblast activity. Linked with a dotted arrow to the GeneProduct nodes are diseases caused by mutation in the respective gene. Adapted from [5]

I型胶原——骨骼、皮肤和肌腱细胞外基质的主要蛋白质成分——主要由成骨细胞、真皮成纤维细胞和肌腱细胞分泌。尽管胶原三螺旋结构相对简单，但I型前胶原的生物合成过程极为复杂，涉及多个步骤，并需要一系列蛋白质进行翻译后修饰、折叠、运输、分泌和质量控制。COL1A1和COL1A2转录本在粗糙内质网（rER）中翻译，α(I)链经历一系列翻译后修饰。Gly-Xaa-Yaa重复序列中的Y位螺旋丙氨酸在丙氨酸4-羟化酶1（P4H1）的作用下在C4位置发生羟基化，而X位特定丙氨酸则由P3H1和P3H2进行3-羟基化。某些赖氨酸残基由赖氨酸羟化酶（LH1和LH2，分别由PLOD1和PLOD2编码）进行羟基化，并且通过前胶原半乳糖基转移酶1和前胶原葡萄糖基转移酶1催化将羟基赖氨酸糖基化成半乳糖基羟基赖氨酸和葡萄糖基半乳糖基羟基赖氨酸。在合成羧基端前肽后，它形成链内二硫键并附着于rER膜上。通过rER膜上附着C前肽的扩散来选择和将正确的链组合成三螺旋。形成三螺旋核，使链以正确的顺序交错排列并启动三螺旋形成。蛋白质二硫键异构酶催化链间二硫键的形成，从而稳定折叠核。丙氨酸残基和某些赖氨酸残基的羟基化继续进行，三螺旋形成从羧基端向氨基端进行。在此阶段，65 kDa的FK506结合蛋白（FKBP65；由FKBP10编码）和由P3H1—CRTAP–PPIase B（肽基丙氨酸异构酶B）组成的复合体似乎起着至关重要的作用。该复合体参与胶原α1(I)-链和α1(II)-链中丙氨酸986位以及α2(I)-链中丙氨酸707位的羟基化，这些位置被认为对于胶原纤维的超分子组装至关重要，并作为伴侣蛋白或小富亮氨酸富含蛋白聚糖的结合位点。除了其丙氨酸3-羟化酶活性外，该复合体还作为PPIase和胶原折叠的伴侣蛋白。事实上，三螺旋的快速传播需要将顺式肽键异构化为反式配置的异构化，主要是由PPIase B24完成的。当大部分螺旋折叠后，N前肽结合并形成该域内的微小三螺旋。新形成的三螺旋由丝肽H1（也称为HSP47；由SERPINH1编码）和FKBP65稳定。进一步的修饰发生在从rER到高尔基体的运输过程中，在包含黑色素瘤抑制蛋白3（也称为TANGO1，由MIA编码）的特殊囊泡复合物中，并通过高尔基体堆栈中的囊腔成熟。丝肽H1在其分子的螺旋部分沿具有结合位点，并协助折叠的胶原进入顺式高尔基体。这些生物合成步骤依赖于适当的rER环境（例如，最佳的钙水平和氧化还原电位），并且质量控制机制可能导致通过ER相关降解途径或自噬介导的溶酶体降解系统激活未折叠蛋白反应，以消除未正确折叠的分子。一旦分泌，前胶原的前肽由具有血栓烷生成基序的金属蛋白酶2（ADAMTS2）和骨形态发生蛋白1（BMP1）切割成成熟的I型胶原。这启动了胶原纤维的形成，这些纤维通过交联形成得到稳定，其中三螺旋和末端肽中的某些赖氨酸和羟基赖氨酸残基被赖氨酸氧化酶氧化成乙赖氨酸和羟基乙赖氨酸。这些残基最初形成二价交联，进而转化为成熟的三角吡啶醇和吡咯交联，以稳定组织中的纤维结构。骨形成包括成骨细胞分泌骨细胞外基质成分（主要是I型胶原）。随后，未矿化的骨基质（骨样组织）发生矿化。此外，成骨细胞和破骨细胞释放许多细胞因子，包括核因子-κB受体激活剂配体（RANKL；也称为TNFSF11）和骨保护素（OPG，由TNFRSF11B编码），它们通过调节破骨细胞介导的骨吸收来发挥作用。RANKL通过与破骨细胞前体细胞表面的核因子-κB受体激活剂（RANK；也称为TNFRSF11A）结合，从而促进其分化为破骨细胞。通过相互作用，OPG阻止RANKL与RANK的结合。一些成骨细胞嵌入到矿化的骨基质中并分化为破骨细胞，其中产生许多其他因素，包括硬骨素，硬骨素是WNT途径的抑制剂，已知通过刺激成骨细胞活性来刺激骨形成。与基因产物节点相连的虚线箭头表示由相应基因突变引起的疾病。改编自[5]