Cathepsin X is secreted by human osteoblasts, digests CXCL-12 and impairs adhesion of hematopoietic stem and progenitor cells to osteoblasts.
BACKGROUND
Hematopoietic stem cells are retained within discrete bone marrow niches through the effects of cell adhesion molecules and chemokine gradients. However, a small proportion of hematopoietic stem cells can also be found trafficking in the peripheral blood. During induced stem cell mobilization a proteolytic microenvironment is generated, but whether proteases are also involved in physiological trafficking of hematopoietic stem cells is not known. In the present study we examined the expression, secretion and function of the cysteine protease cathepsin X by cells of the human bone marrow.


METHODS
Human osteoblasts, bone marrow stromal cells and hematopoietic stem and progenitor cells were analyzed for the secretion of cathepsin X by western blotting, active site labeling, immunofluorescence staining and activity assays. A possible involvement of cathepsin X in cell adhesion and CXCL-12-mediated cell migration was studied in functional assays. Matrix-assisted laser desorption and ionization time-of-flight (MALDI-TOF) analysis revealed the digestion mechanism of CXCL-12 by cathepsin X.


RESULTS
Osteoblasts and stromal cells secrete cathepsin X, whereas hematopoietic stem and progenitor cells do not. Using a cathepsin X-selective substrate, we detected the catalytic activity of cathepsin X in cell culture supernatants of osteoblasts. Activated cathepsin X is able to reduce cellular adhesive interactions between CD34(+) hematopoietic stem and progenitor cells and adherent osteoblasts. The chemokine CXCL-12, a highly potent chemoattractant for hematopoietic stem cells secreted by osteoblasts, is readily digested by cathepsin X.


CONCLUSIONS
The exo-peptidase cathepsin X has been identified as a new member of the group of CXCL-12-degrading enzymes secreted by non-hematopoietic bone marrow cells. Functional data indicate that cathepsin X can influence hematopoietic stem and progenitor cell trafficking in the bone marrow.
Staudt
Nicole D
ND
Center for Medical Research, Section for Transplantation Immunology and Immunohematology, Waldhörnlestrasse 22, 72072 Tübingen, Germany.
Aicher
Wilhelm K
WK
Kalbacher
Hubert
H
Stevanovic
Stefan
S
Carmona
Adriana K
AK
Bogyo
Matthew
M
Klein
Gerd
G
D001853:Bone Marrow; D056663:Cathepsin Z / Q000378:metabolism*; D002448:Cell Adhesion; D002465:Cell Movement; D002478:Cells, Cultured; D054377:Chemokine CXCL12 / Q000378:metabolism*; D006412:Hematopoietic Stem Cells / Q000166:cytology* / Q000378:metabolism; D006801:Humans; D010006:Osteoblasts / Q000166:cytology* / Q000378:metabolism
C515255:CXCL12 protein, human; D054377:Chemokine CXCL12; D056663:Cathepsin Z