Age-Related Changes in Bone Morphology Are Accelerated in Group VIA Phospholipase A 2 (iPLA 2 (cid:1) )-Null Mice

Phospholipases A 2 (PLA 2 ) hydrolyze the sn (cid:1) 2 fatty acid substituent, such as arachidonic acid, from phospholipids, and arachidonate metabolites are recognized mediators of bone modeling. We have previously generated knockout (KO) mice lacking the group VIA PLA 2 (iPLA 2 (cid:2) ), which participates in a variety of signaling events; iPLA 2 (cid:2) mRNA is expressed in bones of wild-type (WT) but not KO mice. Cortical bone size, trabecular bone volume, bone mineralizing surfaces, and bone strength are similar in WT and KO mice at 3 months and decline with age in both groups, but the decreases are more pronounced in KO mice. The lower bone mass phenotype observed in KO mice is not associated with an increase in osteoclast abundance/activity or a decrease in osteoblast density, but is accompanied by an increase in bone marrow fat. Relative to WT mice, undifferentiated bone marrow stromal cells (BMSCs) from KO mice express higher levels of PPAR- (cid:3) and lower levels of Runx2 mRNA, and this correlates with increased adipogenesis and decreased osteogenesis in BMSCs from these mice. pholipid remodeling, signal transduction, cell proliferation, and apoptosis. 12 We have generated iPLA 2 (cid:1) -null mice by homologous recombination, and these mice exhibit defective spermatozoa motility, 13 pancreatic islet insulin secretion, 14 and signaling in macrophages. 15 iPLA 2 (cid:1) mRNA is expressed in a wide variety of wild-type (WT) tissues 13 including bone marrow and spinal cord. 16 Here, we used 1) dual-energy X-ray absorptiometry (DEXA) scanning to determine bone mineral density (BMD), 2) microcomputed tomography ( (cid:2) CT) analyses to assess cortical and trabecular bone morphology, 3) whole-bone mechanical testing to examine bone strength, 4) histomorphometry to examine osteoclasts and osteoblasts, 5) calcein labeling to assess dynamic bone formation, 6) real-time polymer-ase chain reaction (PCR) to determine expression of adipogenic and osteogenic transcriptional factors, and 7) ex vivo differentiation of bone marrow progenitor cells into bone-forming cells or adipocytes in WT and iPLA 2 (cid:1) -null female mice. Our findings indicate that iPLA 2 (cid:1) is expressed in normal bone and that iPLA 2 (cid:1) -null mice exhibit accelerated aging-related bone loss. Red-mond, we determined cortical bone area, moments of inertia about the x - and y -axes (where the x axis is aligned with the medial-lateral direction) and average cortical thickness. Results were averaged over the 12 slices. Trabecular bone analyses were done throughout the entire length of the L4 vertebra. The vertebral body starting immediately adjacent to the caudal endplate region was selected for analyses using contours inside the cortical shell on each two-dimensional image. Trabecular (Tb) bone volume relative to total bone volume (TbBV/TBV), number (Tb.N (cid:2) ), thickness (Tb.Th (cid:2) ), and spacing (Tb.Sp (cid:2) ) were determined using the manufacturer’s three-dimensional analysis tools ( (cid:2) denotes the direct method of calculation, not based on stereological models).