Section for Transplantation Immunology and Immunohematology, Center for Medical Research, University Medical Clinic, University of Tübingen, Tübingen, Germany.
Stem Cells Dev. 2012 Jul 20;21(11):1924-35. doi: 10.1089/scd.2011.0307. Epub 2012 Jan 4.
Hematopoietic stem and progenitor cells (HSPCs) are known to reside in specialized niches at the endosteum in the trabecular bone. Osteoblasts are the major cell type of the endosteal niche. It is well established that secreted proteases are involved in cytokine-induced mobilization processes that release stem cell from their niches. However, migratory processes such as the regular trafficking of HSPCs between their niches and the periphery are not fully understood. In the current study we analyzed whether osteoblast-secreted cysteine cathepsins are able to reduce the direct interaction of HSPCs with bone-forming osteoblasts. Isolated human osteoblasts were shown to secrete proteolytically active cysteine cathepsins, such as cathepsins B, K, L, and X. All of these cathepsins were able to digest, although with different efficacy, the chemokine CXCL12, which is known to be important for retaining HSPCs in their niches. Of the 4 identified cathepsins, only cathepsin X was able to reduce binding of HSPCs to osteoblasts. Interestingly, nonactivated pro-cathepsin X and mature cathepsin X did not interfere with HSPC-osteoblast interactions. Only pro-cathepsin X treated with dithiothreitol, which unfolds but does not lead to full maturation of cathepsin X, significantly reduced HSPC adhesion to osteoblasts. These observations argue for a role of the accessible cathepsin X prodomain in diminishing cell binding. Our findings strongly suggest that the cysteine cathepsins B, K, and L constitutively secreted by osteoblasts are part of the fine-tuned regulation of CXCL12 in the bone marrow, whereas pro-cathepsin X with its prodomain can affect HSPC trafficking in the niche.
造血干细胞和祖细胞 (HSPCs) 已知存在于骨小梁的骨内膜的特殊龛位中。成骨细胞是骨内膜龛位的主要细胞类型。已证实，分泌的蛋白酶参与细胞因子诱导的动员过程，使干细胞从其龛位中释放出来。然而，迁移过程，如 HSPCs 在其龛位和外周之间的常规迁移，尚未完全了解。在本研究中，我们分析了成骨细胞分泌的半胱氨酸组织蛋白酶是否能够减少 HSPCs 与形成骨的成骨细胞的直接相互作用。分离的人成骨细胞被证明分泌具有蛋白水解活性的半胱氨酸组织蛋白酶，如组织蛋白酶 B、K、L 和 X。所有这些组织蛋白酶都能够消化，尽管效力不同，但已知对保留 HSPCs 在其龛位中很重要的趋化因子 CXCL12。在鉴定出的 4 种组织蛋白酶中，只有组织蛋白酶 X 能够减少 HSPCs 与成骨细胞的结合。有趣的是，非激活的原组织蛋白酶 X 和成熟的组织蛋白酶 X 不会干扰 HSPC-成骨细胞相互作用。只有用二硫苏糖醇处理的原组织蛋白酶 X 才能显著降低 HSPC 与成骨细胞的黏附。这些观察结果表明，可及的组织蛋白酶 X 前肽域在减少细胞结合中起作用。我们的研究结果强烈表明，成骨细胞持续分泌的半胱氨酸组织蛋白酶 B、K 和 L 是骨髓中 CXCL12 精细调节的一部分，而具有前肽域的原组织蛋白酶 X 可以影响龛位中的 HSPC 迁移。
