Collagen XI is a fibril-forming collagen that regulates collagen fibrillogenesis. Collagen XI is normally associated with collagen II-containing tissues such as cartilage, but it also is expressed broadly during development in collagen I-containing tissues, including tendons. The goals of this study are to define the roles of collagen XI in regulation of tendon fibrillar structure and the relationship to function. A conditional Col11a1-null mouse model was created to permit the spatial and temporal manipulation of Col11a1 expression. We hypothesize that collagen XI functions to regulate fibril assembly, organization and, therefore, tendon function. Previous work using cho mice with ablated Col11a1 alleles supported roles for collagen XI in tendon fibril assembly. Homozygous cho/cho mice have a perinatal lethal phenotype that limited the studies. To circumvent this, a conditional Col11a1^flox/flox mouse model was created where exon 3 was flanked with loxP sites. Breeding with Scleraxis-Cre (Scx-Cre) mice yielded a tendon-specific Col11a1-null mouse line, Col11a1^Δten/Δten. Col11a1^flox/flox mice had no phenotype compared to wild type C57BL/6 mice and other control mice, e.g., Col11a1^flox/flox and Scx-Cre. Col11a1^flox/flox mice expressed Col11a1 mRNA at levels comparable to wild type and Scx-Cre mice. In contrast, in Col11a1^Δten/Δten mice, Col11a1 mRNA expression decreased to baseline in flexor digitorum longus tendons (FDL). Collagen XI protein expression was absent in Col11a1^Δten/Δten FDLs, and at ~50% in Col11a1^+/Δten compared to controls. Phenotypically, Col11a1^Δten/Δten mice had significantly decreased body weights (p < 0.001), grip strengths (p < 0.001), and with age developed gait impairment becoming hypomobile. In the absence of Col11a1, the tendon collagen fibrillar matrix was abnormal when analyzed using transmission electron microscopy. Reducing Col11a1 and, therefore collagen XI content, resulted in abnormal fibril structure, loss of normal fibril diameter control with a significant shift to small diameters and disrupted parallel alignment of fibrils. These alterations in matrix structure were observed in developing (day 4), maturing (day 30) and mature (day 60) mice. Altering the time of knockdown using inducible I-Col11a1^−/− mice indicated that the primary regulatory foci for collagen XI was in development. In mature Col11a1^Δten/Δten FDLs a significant decrease in the biomechanical properties was observed. The decrease in maximum stress and modulus suggest that fundamental differences in the material properties in the absence of Col11a1 expression underlie the mechanical deficiencies. These data demonstrate an essential role for collagen XI in regulation of tendon fibril assembly and organization occurring primarily during development.

胶原蛋白 XI 是一种形成纤维的胶原蛋白，可调节胶原纤维的形成。胶原蛋白 XI 通常与含胶原蛋白 II 的组织（如软骨）相关，但它也在含胶原蛋白 I 的组织（包括肌腱）的发育过程中广泛表达。本研究的目的是确定胶原蛋白 XI 在调节肌腱纤维结构中的作用以及与功能的关系。创建了条件Col11a1 -null 小鼠模型以允许对Col11a1表达进行空间和时间操作。我们假设胶原蛋白 XI 的功能是调节纤维组装、组织，从而调节肌腱功能。先前使用带有消融Col11a1 的cho小鼠进行的工作等位基因支持胶原蛋白 XI 在肌腱原纤维组装中的作用。纯合cho/cho小鼠的围产期致死表型限制了研究。为了避免这种情况，创建了一个条件Col11a1 ^flox/flox小鼠模型，其中外显子 3 两侧是loxP位点。用 Scleraxis-Cre ( Scx-Cre ) 小鼠繁殖产生了一个肌腱特异性Col11a1 - null 小鼠品系，Col11a1 ^Δten/Δten。与野生型 C57BL/6 小鼠和其他对照小鼠（例如Col11a1 ^flox/flox和Scx-Cre）相比，Col11a1 ^flox/flox小鼠没有表型。Col11a1 ^flox/flox小鼠表达Col11a1 mRNA 的水平与野生型和Scx-Cre小鼠相当。相比之下，在Col11a1 ^Δten/Δten小鼠中，指长屈肌腱 (FDL) 的Col11a1 mRNA 表达降低至基线。与对照相比，Col11a1 ^Δten/Δten FDL 中不存在胶原蛋白 XI 蛋白表达，并且在Col11a1 ^+/Δten中表达约 50% 。从表型上看，Col11a1 ^Δten/Δten小鼠的体重（p < 0.001）、握力（p < 0.001）显着降低，并且随着年龄的增长步态障碍变得行动不便。在没有Col11a1的情况下，当使用透射电子显微镜分析时，肌腱胶原纤维基质异常。减少Col11a1并因此减少胶原蛋白 XI 含量，导致异常的原纤维结构、正常原纤维直径控制的丧失，显着转移到小直径并破坏原纤维的平行排列。在发育（第 4 天）、成熟（第 30 天）和成熟（第 60 天）小鼠中观察到基质结构的这些改变。使用诱导型 I- Col11a1 ^-/-小鼠改变敲低时间表明胶原蛋白 XI 的主要调节灶正在发育中。在成熟的Col11a1 ^Δten/Δten观察到 FDL 的生物力学性能显着降低。最大应力和模量的降低表明，在没有Col11a1表达的情况下，材料特性的根本差异是机械缺陷的基础。这些数据证明了胶原蛋白 XI 在调节主要发生在发育过程中的肌腱原纤维组装和组织方面的重要作用。