Activated leukocyte cell adhesion molecule (ALCAM or CD166) modulates bone phenotype and hematopoiesis

Activated Leukocyte Cell Adhesion Molecule (ALCAM/CD166), is expressed on osteoblasts (OB) and hematopoietic stem cells (HSC) residing in the hematopoietic niche, and may have important regulatory roles in bone formation. Because HSC numbers are reduced 77% in CD166^-/- mice, we hypothesized that changes in bone phenotype and consequently the endosteal niche may partially be responsible for this alteration. Therefore, we investigated bone phenotype and OB function in CD166^-/- mice. Although osteoclastic measures were not affected by loss of CD¹⁶⁶, CD166^-/- mice exhibited a modest increase in trabecular bone fraction (42%), and increases in osteoid deposition (72%), OB number (60%), and bone formation rate (152%). Cortical bone geometry was altered in CD166^-/- mice resulting in up to 81% and 49% increases in stiffness and ultimate force, respectively. CD166^-/- OB displayed elevated alkaline phosphatase (ALP) activity and mineralization, and increased mRNA expression of Fra 1, ALP, and osteocalcin. Overall, CD166^-/- mice displayed modestly elevated trabecular bone volume fraction with increased OB numbers and deposition of osteoid, and increased OB differentiation in vitro, possibly suggesting more mature OB are secreting more osteoid. This may explain the decline in HSC number in vivo because immature OB are mainly responsible for hematopoiesis enhancing activity.