The intracellular protein phosphatase magnesium-dependent 1A negatively regulates osteoclast commitment.

Increased protein phosphatase magnesium-dependent 1A (PPM1A) levels in patients with ankylosing spondylitis regulate osteoblast differentiation in bony ankylosis; however, the potential mechanisms that regulate osteoclast (OC) differentiation in relation to abnormal bone formation remain unclear. Therefore, we generated conditional gene knockout (PPM1A^fl/fl ;LysM-Cre) mice and evaluated their bone phenotype.

The bone phenotypes of LysM-Cre (n=6) and PPM1A^fl/fl ;LysM-Cre mice (n=6) were assessed via micro-computed tomography. OC differentiation was induced by culturing bone marrow-derived macrophages in the presence of the RANKL and M-CSF, and was evaluated by counting tartrate-resistant acid phosphatase-positive multinucleated cells. The mRNA expressions of PPM1A, RANK and OC-specific genes were examined by quantitative real-time PCR, and protein levels were determined using Western blotting. Surface RANK expression was analysed by fluorescent flow cytometry.

The PPM1A^fl/fl ;LysM-Cre mice displayed reduced bone mass (p<0.001) and increased OC differentiation (p<0.001) and OC-specific gene expression (p<0.05) compared with their LysM-Cre littermates. Mechanistically, reduced PPM1A function in OC precursors in PPM1A^fl/fl ;LysM-Cre mice induced OC lineage commitment by up-regulating RANK expression (p<0.01) via p38 MAPK activation in response to M-CSF. PPM1A expression in macrophages was decreased by TLR4 activation (p<0.05). The ankylosing spondylitis disease activity score was negatively correlated with the expression of PPM1A in peripheral blood mononuclear cells from Ax SpA patients (γ=-0.7072, p<0.0001).

The loss of PPM1A function in OC precursors driven by inflammatory signals contributes to OC lineage commitment and differentiation by elevating RANK expression, reflecting a potential role of PPM1A in dynamic bone metabolism in Ax SpA.