Sam68 Deficiency Protects Mice from Age-Related Bone Loss
Src null animals are known to have bone metabolism defects [48], and since Sam68 is a substrate of Src, we decided to analyze the Sam68 mice for skeletal abnormalities. Cohorts of Sam68+/+ and Sam68−/− mice were euthanized at 4 and 12 months of age for skeletal phenotyping. To minimize differences in the bone phenotype that might arise secondary to gender or weight differences, we selected age-matched female mice for these analyses. The female mice demonstrated similar increases in body weight, although 4-month-old Sam68−/− mice weighed less than Sam68+/+ mice, and similar changes in bone lengths in the axial and appendicular skeleton between 4 and 12 months of age (Table 2). Faxitron radiography (Faxitron X-ray Corporation, Wheeling, Illinois, United States) (Figure 3A) and micro–computed tomography (CT) (Figure 3B) revealed significant cortical thinning (arrow) and a reduction in metaphyseal bone (asterisk) in the distal femora of 12-month-old Sam68+/+ mice compared with 4-month-old mice of either genotype and 12-month-old Sam68−/− mice. Similar reductions in trabecular bone were shown by Faxitron and micro-CT in the fifth lumbar vertebrae of the 12-month-old Sam68+/+ mice but not in the Sam68−/− mice (unpublished data). Total body bone mineral content (BMC), quantified with a Lunar PIXImus mouse densitometer (GE-Lunar, Madison, Wisconsin, United States), increased in both Sam68+/+ and Sam68−/− mice between 4 months and 12 months of age, but only reached significance in the Sam68−/− mice (Table 2). Similarly, a greater increase in BMC was seen in the femur and vertebra in the Sam68−/− mice. Bone mineral density (BMD) remained constant or decreased in the wild type mice but increased significantly in the total body and in the femoral and vertebral regions of interest in the Sam68−/− (Table 2). These data showed that Sam68−/− mice continued to thrive and accrue bone in the axial and appendicular skeleton for longer than 12 months. This was in contrast to the situation in age-matched, littermate controls in which a significant amount of bone was lost over the same timeframe.
Table 2  Morphology and Bone Mineral Density in 4- and 12-Month-Old Female Mice
Figure 3  Radiologic Assessment of the Femur of Young and Old Sam68+/+ and Sam68−/− Mice
(A) Mice were given a lethal dose of anesthetic at the indicated times, and contact radiographs of the distal femora were obtained on a Faxitron MX20 equipped with an FPX-2 Imaging system. Representative radiographs of the distal femur of Sam68+/+ (+/+) and Sam68−/− (−/−) mice revealed comparable radiopacity at 4 months (left). At 12 months (right), cortical thinning (arrow) and radiolucency (asterisk) were apparent in the distal femur of +/+ mice but not −/− mice.
(B) Bones were dissected free of soft tissue and fixed overnight in 4% paraformaldehyde before scanning on a Skyscan 1072 static instrument equipped with 3D Creator analytical software. Representative three-dimensional re-constructions and two-dimensional cross-sectional scans demonstrated similar architecture in the distal femur of Sam68+/+ (+/+) and Sam68−/− (−/−) mice. In keeping with the results from Faxitron x-ray, trabecular bone (asterisk) and cortical thickness (arrow) were reduced in the femur of 12-month-old +/+ mice compared with all other groups. The images are representative of those from five to seven animals in each group.