Background: We have very recently shown that the deficiency of Apolipoprotein A-I (apoA-I), a key-element in HDL biogenesis, results in osteoporosis in mice, under yet unidentified mechanisms. CLCX12 is a chemoattractant cytokine, which is expressed by bone marrow cells of osteoblastic lineage and binds to its receptor CXCR4. Annexin-2 (ANXA2) is an adhesion molecule that directly binds CLCX12 facilitating CXCL12-dependent haematopoietic progenitor cell migration. Herein, we aimed at investigating whether aberrations in ANXA2/CLCX12 interaction are involved in impaired osteoblastic function in apoA-I deficient osteoporotic mice.
Methods: Whole bone marrow cells (WBMCs) were isolated, from the femora of apoA-I-/- (n=6) and wild-type (n=6) C57BL/6 mice and assessed for the expression of the mesenchymal stem cell markers PTPRC/CD45 and SCA1/Ly6, the osteoblast-specific markers Runx2, Osterix, Col1a1, RANKL and the osteoblastic bone marrow niche regulators ANXA2, CLCX12 and its receptor CXCR4, with qRT-PCR.
Results: The expression of the two the mesenchymal stem cell markers examined did not reveal significant differences between the knock-out and the WT group. WBMCs from ApoA-I-/- mice displayed strongly decreased mRNA levels of the osteoblastic regulators Runx2, Osterix, Col1a1 and RANKL, compared with their WT counterparts. Moreover, the expression of ANXA2 and CLCX12 was significantly reduced, while the expression or the receptor CXCR4 was greatly augmented (possibly via feedback cell reaction-mechanism) in the WBMC of the ApoA-I-/- compared to the WT mice.
Conclusions: ApoA-I deficiency, results in reduced osteoblastogenic capacity of WBMCs; however, it has no effect on the mesenchymal stem cell pool. This finding implies that apoA-I defficiency affects late stages of MSC progression reducing the entry into the level of bone formation. Additionally, our data provide novel mechanistic evidence that apoA-I and thus HDL may control bone synthesis via the ANXA2-CLCX12 interaction.
Disclosure: This work was supported by the European Communitys Seventh Framework Programme [FP7/2009-20-14] grant agreement PIRG02-GA-2009-256402 and the University of Patras Karatheodoris Research Grant (D155/2010-2013) (both awarded to D.J. Papachristou) and The University of Patras Karatheodoris research grant (E073) (awarded to S. A. Syggelos). This work is part of the activities of the intramural research network OsteoNet of the University of Patras.