The blood–brain barrier (BBB) ensures central nervous system (CNS) homeostasis by strictly controlling the passage of molecules and solutes from the bloodstream into the CNS. Complex and continuous tight junctions (TJs) between brain endothelial cells block uncontrolled paracellular diffusion of molecules across the BBB, with claudin-5 being its dominant TJs protein. However, claudin-5 deficient mice still display ultrastructurally normal TJs, suggesting the contribution of other claudins or tight-junction associated proteins in establishing BBB junctional complexes. Expression of claudin-12 at the BBB has been reported, however the exact function and subcellular localization of this atypical claudin remains unknown. We created claudin-12-lacZ-knock-in C57BL/6J mice to explore expression of claudin-12 and its role in establishing BBB TJs function during health and neuroinflammation. We furthermore performed a broad standardized phenotypic check-up of the mouse mutant. Making use of the lacZ reporter allele, we found claudin-12 to be broadly expressed in numerous organs. In the CNS, expression of claudin-12 was detected in many cell types with very low expression in brain endothelium. Claudin-12lacZ/lacZ C57BL/6J mice lacking claudin-12 expression displayed an intact BBB and did not show any signs of BBB dysfunction or aggravated neuroinflammation in an animal model for multiple sclerosis. Determining the precise localization of claudin-12 at the BBB was prohibited by the fact that available anti-claudin-12 antibodies showed comparable detection and staining patterns in tissues from wild-type and claudin-12lacZ/lacZ C57BL/6J mice. Our present study thus shows that claudin-12 is not essential in establishing or maintaining BBB TJs integrity. Claudin-12 is rather expressed in cells that typically lack TJs suggesting that claudin-12 plays a role other than forming classical TJs. At the same time, in depth phenotypic screening of clinically relevant organ functions of claudin-12lacZ/lacZ C57BL/6J mice suggested the involvement of claudin-12 in some neurological but, more prominently, in cardiovascular functions.

中文翻译：

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血脑屏障紧密连接功能不需要 Claudin-12

血脑屏障 (BBB) 通过严格控制分子和溶质从血流进入 CNS 来确保中枢神经系统 (CNS) 稳态。脑内皮细胞之间复杂且连续的紧密连接 (TJ) 阻止分子不受控制地跨 BBB 的细胞旁扩散，其中 claudin-5 是其主要的 TJ 蛋白。然而，claudin-5 缺陷小鼠仍显示超微结构正常的 TJ，表明其他 claudin 或紧密连接相关蛋白在建立 BBB 连接复合物方面的贡献。已经报道了 BBB 上 claudin-12 的表达，但是这种非典型 claudin 的确切功能和亚细胞定位仍然未知。我们创建了 claudin-12-lacZ-knock-in C57BL/6J 小鼠，以探索 claudin-12 的表达及其在健康和神经炎症期间建立 BBB TJ 功能的作用。我们还对小鼠突变体进行了广泛的标准化表型检查。利用 lacZ 报告基因等位基因，我们发现 claudin-12 在许多器官中广泛表达。在 CNS 中，在许多细胞类型中检测到 claudin-12 的表达，在脑内皮中的表达非常低。缺乏 claudin-12 表达的 Claudin-12lacZ/lacZ C57BL/6J 小鼠显示出完整的 BBB，并且在多发性硬化症动物模型中未显示任何 BBB 功能障碍或加重神经炎症的迹象。由于可用的抗 claudin-12 抗体在野生型和 claudin-12lacZ/lacZ C57BL/6J 小鼠的组织中显示出类似的检测和染色模式，因此无法确定 claudin-12 在 BBB 的精确定位。因此，我们目前的研究表明，claudin-12 在建立或维持 BBB TJ 完整性方面不是必不可少的。Claudin-12 在通常缺乏 TJs 的细胞中表达，这表明 claudin-12 除了形成经典 TJs 之外还发挥着其他作用。同时，对 claudin-12lacZ/lacZ C57BL/6J 小鼠临床相关器官功能的深入表型筛选表明，claudin-12 参与一些神经功能，但更重要的是参与心血管功能。因此，我们目前的研究表明，claudin-12 在建立或维持 BBB TJ 完整性方面不是必不可少的。Claudin-12 在通常缺乏 TJs 的细胞中表达，这表明 claudin-12 除了形成经典 TJs 之外还发挥着其他作用。同时，对 claudin-12lacZ/lacZ C57BL/6J 小鼠临床相关器官功能的深入表型筛选表明，claudin-12 参与一些神经功能，但更突出的是参与心血管功能。因此，我们目前的研究表明，claudin-12 在建立或维持 BBB TJ 完整性方面不是必不可少的。Claudin-12 在通常缺乏 TJs 的细胞中表达，这表明 claudin-12 除了形成经典 TJs 之外还发挥着其他作用。同时，对 claudin-12lacZ/lacZ C57BL/6J 小鼠临床相关器官功能的深入表型筛选表明，claudin-12 参与一些神经功能，但更重要的是参与心血管功能。