Processing of CXCL12 by Different Osteoblast-Secreted Cathepsins

Abstract

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.

Identification and Characterization of Circulating Variants of CXCL12 from Human Plasma: Effects on Chemotaxis and Mobilization of Hematopoietic Stem and Progenitor Cells

“…This proteolytic activity is most likely not related to proteases that are known to process Lys 89 from CXCL12. These proteases include carboxypeptidase N [44] and M [45], and Cathepsin X [46,47]. These exoproteases are not present in thrombocytes.…”

“…Carboxypeptidases N and M exclusively process Lys 89 [44], whereas Cathepsin X is secreted by human osteoblasts and reveals a C-terminal exopeptidase activity for CXCL12 , which cleaves 15 amino acids up to proline P 74 [47]. A putative carboxypeptidase for C-terminal processing of CXCL12 represents a thrombin-activatable fibrinolysis inhibitor that is expressed in thrombocytes and is able to process C-terminal Lys residues [48].…”

Granulocyte-Colony-Stimulating Factor Stimulation of Bone Marrow Mesenchymal Stromal Cells Promotes CD34+ Cell Migration Via a Matrix Metalloproteinase-2-Dependent Mechanism

“…Interestingly, a recent report indicates that G-CSF treatment of human CD34 + cells induces expression and activation of MT1-MMP and mediates pro-MMP-2 activation and CD34 + cell migration across a reconstituted basement membrane [27]. Another very recent study suggests a contribution of BM osteoblasts in the mobilizing process of HSPCs by degrading CXCL12 through the secretion of B, K, and L cathepsins [37].…”

CD133+ and Nestin+ Glioma Stem-Like Cells Reside Around CD31+ Arterioles in Niches that Express SDF-1α, CXCR4, Osteopontin and Cathepsin K

“…The high expression of CTSK in high-grade glioma was validated by RT-qPCR, western blotting and immunohistochemistry (Verbovšek et al 2014). CTSK can cleave and degrade SDF-1α in human bone marrow (Staudt et al 2012). SDF-1α is a chemoattractant that is produced by stromal cells and is involved in homing of CXCR4 + stem cells/progenitor cells and leukocytes (Ehninger and Trumpp 2011;Teicher and Fricker 2010).…”