Bone brittleness varies with genetic background in A/J and C57BL/6J inbred mice.

The contribution of genetic and environmental factors to variations in bone quality are understood poorly. We tested whether bone brittleness varies with genetic background using the A/J and C57BL/6J inbred mouse strains. Whole bone four-point bending tests revealed a 70% decrease in postyield deflection of A/J femurs compared with C57BL/6J, indicating that A/J femurs failed in a significantly more brittle manner. Cyclic loading studies indicated that A/J femurs accumulated damage differently than C57BL/6J femurs, consistent with their increased brittleness. Differences in matrix composition also were observed between the two mouse strains. A/J femurs had a 4.5% increase in ash content and an 11.8% decrease in collagen content. Interestingly, a reciprocal relationship was observed between femoral geometry and material stiffness; this relationship may have contributed to the brittle phenotype of A/J femurs. A/J femurs are more slender than those of C57BL/6J femurs; however, their 47% smaller moment of inertia appeared to be compensated by an increased tissue stiffness at the expense of altered tissue damageability. Importantly, these differences in whole bone mechanical properties between A/J and C57BL/6J femurs could not have been predicted from bone mass or density measures alone. The results indicated that bone brittleness is a genetically influenced trait and that it is associated with genetically determined differences in whole bone architecture, bone matrix composition, and mechanisms of cyclical damage accumulation.     

Quantitative trait loci that determine BMD in C57BL/6J and 129S1/SvImJ inbred mice.

BMD is highly heritable; however, little is known about the genes. To identify loci controlling BMD, we conducted a QTL analysis in a (B6 x 129) F2 population of mice. We report on additional QTLs and also narrow one QTL by combining the data from multiple crosses and through haplotype analysis. INTRODUCTION: Previous studies have identified quantitative trait loci (QTL) that determine BMD in mice; however, identification of genes underlying QTLs is impeded by the large size of QTL regions. MATERIALS AND METHODS: To identify loci controlling BMD, we performed a QTL analysis of 291 (B6 x 129) F2 females. Total body and vertebral areal BMD (aBMD) were determined by peripheral DXA when mice were 20 weeks old and had consumed a high-fat diet for 14 weeks. RESULTS AND CONCLUSIONS: Two QTLs were common for both total body and vertebral aBMD: Bmd20 on chromosome (Chr) 6 (total aBMD; peak cM 26, logarithm of odds [LOD] 3.8, and vertebral aBMD; cM 32, LOD 3.6) and Bmd22 on Chr 1 (total aBMD; cM 104, LOD 2.5, and vertebral aBMD; cM 98, LOD 2.6). A QTL on Chr 10 (Bmd21, cM 68, LOD 3.0) affected total body aBMD and a QTL on Chr 7 (Bmd9, cM 44, LOD 2.7) affected vertebral aBMD. A pairwise genome-wide search did not reveal significant gene-gene interactions. Collectively, the QTLs accounted for 21.6% of total aBMD and 17.3% of vertebral aBMD of the F(2) population variances. Bmd9 was previously identified in a cross between C57BL/6J and C3H/HeJ mice, and we narrowed this QTL from 34 to 22 cM by combining the data from these crosses. By examining the Bmd9 region for conservation of ancestral alleles among the low allele strains (129S1/SvImJ and C3H/HeJ) that differed from the high allele strain (C57BL/6J), we further narrowed the region to approximately 9.9 cM, where the low allele strains share a common haplotype. Identifying the genes for these QTLs will enhance our understanding of skeletal biology.     

Ultrafine mapping of SNPs from mouse strains C57BL/6J, DBA/2J, and C57BLKS/J for loci contributing to diabetes and atherosclerosis susceptibility

The inbred mouse strain C57BLKS/J (BKS) carrying a mutation of the leptin receptor lepr(-/-) (BKS-db) is a classic mouse model of type 2 diabetes. While BKS was originally presumed to be a substrain of C57BL/6J (B6), it has become apparent that its genome contains introgressed regions from a DBA/2 (DBA)-like strain and perhaps other unidentified sources. It has been hypothesized that the strikingly enhanced diabetes susceptibility of BKS-db compared with B6-db is conferred by this introgressed DNA. Using high-density single nucleotide polymorphisms, we have mapped the DBA and other contaminating DNA regions present in BKS. Thus, approximately 70% of its genome appears to derive from B6, with approximately 20% from DBA and another 9% from an unidentified donor. Comparison with 56 diverse inbred strains suggests that this donor may be a less common inbred strain or an outbred or wild strain. Using expression data from a B6 x DBA cross, we identified differentially regulated genes between these two strains. Those cis-regulated genes located on DBA-like blocks in BKS constitute primary candidates for genes contributing to diabetes susceptibility in the BKS-db strain. To further prioritize these candidates, we identified those cis-acting expression quantitative trait loci whose expression significantly correlates with diabetes-related phenotypes.     

Diet-induced type II diabetes in C57BL/6J mice

We investigated the effects of diet-induced obesity on glucose metabolism in two strains of mice, C57BL/6J and A/J. Twenty animals from each strain received ad libitum exposure to a high-fat high-simple-carbohydrate diet or standard Purina Rodent Chow for 6 mo. Exposure to the high-fat, high-simple-carbohydrate, low-fiber diet produced obesity in both A/J and C57BL/6J mice. Whereas obesity was associated with only moderate glucose intolerance and insulin resistance in A/J mice, obese C57BL/6J mice showed clear-cut diabetes with fasting blood glucose levels of greater than 240 mg/dl and blood insulin levels of greater than 150 microU/ml. C57BL/6J mice showed larger glycemic responses to stress and epinephrine in the lean state than AJ mice, and these responses were exaggerated by obesity. These data suggest that the C57BL/6J mouse carries a genetic predisposition to develop non-insulin-dependent (type II) diabetes. Furthermore, altered glycemic response to adrenergic stimulation may be a biologic marker for this genetic predisposition to develop type II diabetes.     

Identification of genetic loci that regulate bone adaptive response to mechanical loading in C57BL/6J and C3H/HeJ mice intercross

Strain-dependent differences in bone adaptive responses to loading among inbred mouse strains suggest that genetic background contributes significantly to adaptation to exercise. To explore the genetic regulation of response to loading, we performed a genome-wide search for linkage in a cross between two strains, a good responder, C57BL6/J (B6), and a poor responder, C3H/HeJ (C3H). Using a four-point bending model, the right tibia was loaded by applying 9 N force for 36 cycles for 12 days in 10-week-old female B6xC3H F2 mice. Changes in bone density (BMD) and bone size were evaluated in vivo by pQCT. Measurements from non-loaded left tibia were used as an internal control to calculate loading-induced percent increase in BMD and bone size, thus excluding the possibility of identifying background QTL(s) due to natural allelic variation in mapping strains. A genome-wide scan was performed using 111 microsatellite markers in DNA samples collected from 329 F2 mice. Heritability of bone adaptive response to loading was between 70 and 80%. The mean increase, expressed as percent of unloaded tibia, was 5% for BMD, 9% for periosteal circumference (PC), and 14% for cortical thickness in F2 mice (n = 329). All these phenotypes showed normal distributions. Absence of significant correlation between BMD response to four-point bending and body weight or bone size suggested that the bone adaptive response was independent of bone size. Interval mapping revealed that BMD response to four-point bending was influenced by three significant loci on Chrs 1 (log-of-odds ratio score (LOD) 3.4, 91.8 cM), 3 (LOD 3.6, 50.3 cM), and 8 (LOD 4.2, 60.1 cM) and one suggestive QTL on Chr 9 (LOD 2.5, 33.9 cM). Loading-induced increases in PC and Cth were influenced by four significant loci on Chrs 8 (LOD 3.0, 68.9 cM), 9 (LOD 3.0, 13.1 cM), 17 (LOD 3.0, 39.3 cM), and 18 (LOD 3.0, 0 cM) and two suggestive loci on Chr 9 (LOD 2.2, 24 cM) and 11 (LOD 2.1, 69.9 cM). Pairwise analysis showed the presence of several significant and suggestive interactions between loci on Chrs 1, 3, 8, and 13 for BMD trait. This is the first study that provides evidence for the presence of multiple genetic loci regulating bone anabolic responses to loading in the B6xC3H intercross. Knowledge of the genes underlying these loci could provide novel approaches to improve skeletal mass.     

Rejection of C57BL/6 skin grafts by (C57BL/6 X Mus musculus castaneus)F1 hybrids.

Hybrid offspring from C57BL/6(B6) females mated to males of the subspecies Mus musculus castaneus received B6 skin grafts. No strong Y chromosome-linked histocompatibility genes were detected, although occasional rejection of parental grafts by both male and female hybrids was observed after long periods. Rejection was attributed to interaction of B6 and Castaneus-derived genes in the hybrids.     

Impact of chromosome 2 obesity loci on cardiovascular complications of insulin resistance in LDL receptor-deficient C57BL/6 mice

Previous characterization of mouse chromosome 2 identified genomic intervals that influence obesity, insulin resistance, and dyslipidemia. For this, resistant CAST/Ei (CAST) alleles were introgressed onto a susceptible C57BL/6J background to generate congenic strains with CAST alleles encompassing 67-162 Mb (multigenic obesity 6 [MOB6]) and 84-180 Mb (MOB5) from mouse chromosome 2. To examine the effects of each congenic locus on atherosclerosis and glucose disposal, we bred each strain onto a sensitizing LDL receptor-null (LDLR(-/-)) C57BL/6J background to predispose them to hypercholesterolemia and insulin resistance. LDLR(-/-) congenics and controls were characterized for measures of atherogenesis, insulin sensitivity, and obesity. We identified a genomic interval unique to the MOB6 congenic (72-84 Mb) that dramatically decreased atherosclerosis by approximately threefold and decreased insulin resistance. This region also reduced adiposity twofold. Conversely, the congenic region unique to MOB5 (162-180 Mb) increased insulin resistance but had little effect on atherosclerosis and adiposity. The MOB congenic intervals are concordant to human and rat quantitative trait loci influencing diabetes and atherosclerosis traits. Thus, our results define a strategy for studying the poorly understood interactions between diabetes and atherosclerosis and for identifying genes underlying the cardiovascular complications of insulin resistance.     

Bone development and age-related bone loss in male C57BL/6J mice

The objective of this study was to examine changes in the long bones of male C57BL/6J mice with growth and aging, and to consider the applicability of this animal for use in studying Type II osteoporosis. Male C57BL/6J mice were aged in our colony between 4 and 104 weeks (n=9-15/group). The right femur and humeri were measured for length and subjected to mechanical testing (3-point flexure) and compositional analysis. The left femurs were embedded and thick slices at the mid-diaphysis were assessed for morphology, formation indices, and bone structure. In young mice, rapid growth was marked by substantial increases in bone size, mineral mass, and mechanical properties. Maturity occurred between 12 and 42 weeks of age with the maintenance of bone mass and mechanical properties. From peak levels, mice aged for 104 weeks experienced decreased whole femur mass (12.1 and 18.6% for dry and ash mass, respectively), percentage mineralization (7.4%), diminished whole bone stiffness (29.2%), energy to fracture (51.8%), and decreased cortical thickness (20.1%). Indices of surface-based formation decreased rapidly from the onset of the study. However, the periosteal perimeter and, consequently, the cross-sectional moments of inertia continued to increase through 104 weeks, thus maintaining structural properties. This compensated for cortical thinning and increased brittleness due to decreased mineralization and stiffness. The shape of the mid-diaphysis became increasingly less elliptical in aged mice, and endocortical resorption and evidence of subsequent formation were present in 20-50% of femurs aged > or =78 weeks. This, combined with the appearance of excessive endocortical resorption after 52 weeks, indicated a shift in normal mechanisms regulating bone shape and location, and was suggestive of remodeling. The pattern of bone loss at the femoral mid-diaphysis in this study is markedly similar to that seen in cortical bone in the human femoral neck in Type II osteoporosis. This study has thus demonstrated that the male C57BL/6J mouse is a novel and appropriate model for use in studying endogenous, aging-related osteopenia and may be a useful model for the study of Type II osteoporosis.     

Effect of helper and/or cytotoxic T-lymphocyte depletion on low-dose streptozocin-induced diabetes in C57BL/6J mice

The low-dose streptozocin (STZ) model of diabetes has been reported to involve direct STZ beta-cytotoxicity and/or immunologically mediated beta-cell destruction. Because the T-lymphocyte dependency of such a model is controversial, we further assessed the role of T-lymphocytes by determining the occurrence and magnitude of hyperglycemia as well as the pancreatic insulin contents in both STZ-injected nude C57BL/6J male mice and STZ-injected euthymic C57BL/6J male mice selectively depleted in helper and/or cytotoxic T-lymphocytes with monoclonal antibodies (MoAbs). The effectiveness of MoAb treatment was assessed in lymph node cells by flow-microfluorometry analysis and in spleen cells by concanavalin A stimulation, allospecific cytotoxic T-lymphocyte activity, and T-lymphocyte lymphokine production. Sixteen days after the first STZ injection, hyperglycemia (plasma glucose greater than 200 mg/dl) occurred in significantly fewer helper T-lymphocyte-depleted mice (P less than .005) or helper and cytotoxic T-lymphocyte-depleted mice (P less than .001) than in non-MoAb-treated mice. However, a progressive increase in the number of mice with hyperglycemia ensued in all MoAb-treated groups, and 2 mo after STZ was administered, the prevalence of hyperglycemia, mean plasma glucose levels, and pancreatic insulin contents did not differ significantly from the values obtained in the non-MoAb-treated animals. Similarly, STZ-injected C57BL/6J male nude mice developed hyperglycemia that was associated with a marked decrease in pancreatic insulin contents on a time course comparable with that of STZ-injected euthymic C57BL/6J male mice depleted in helper or in helper and cytotoxic T-lymphocytes by MoAbs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-related changes in trabecular architecture differ in female and male C57BL/6J mice.

We used microCT and histomorphometry to assess age-related changes in bone architecture in male and female C57BL/6J mice. Deterioration in vertebral and femoral trabecular microarchitecture begins early, continues throughout life, is more pronounced at the femoral metaphysis than in the vertebrae, and is greater in females than males. INTRODUCTION: Despite widespread use of mice in the study of musculoskeletal disease, the age-related changes in murine bone structure and the relationship to whole body BMD changes are not well characterized. Thus, we assessed age-related changes in body composition, whole body BMD, and trabecular and cortical microarchitecture at axial and appendicular sites in mice. MATERIALS AND METHODS: Peripheral DXA was used to assess body composition and whole body BMD in vivo, and microCT and histomorphometry were used to measure trabecular and cortical architecture in excised femora, tibia, and vertebrae in male and female C57BL/6J mice at eight time-points between 1 and 20 mo of age (n = 6-9/group). RESULTS: Body weight and total body BMD increased with age in male and female, with a marked increase in body fat between 6 and 12 mo of age. In contrast, trabecular bone volume (BV/TV) was greatest at 6-8 wk of age and declined steadily thereafter, particularly in the metaphyseal region of long bones. Age-related declines in BV/TV were greater in female than male. Trabecular bone loss was characterized by a rapid decrease in trabecular number between 2 and 6 mo of age, and a more gradual decline thereafter, whereas trabecular thickness increased slowly over life. Cortical thickness increased markedly from 1 to 3 mo of age and was maintained or slightly decreased thereafter. CONCLUSIONS: In C57BL/6J mice, despite increasing body weight and total body BMD, age-related declines in vertebral and distal femoral trabecular bone volume occur early and continue throughout life and are more pronounced in females than males. Awareness of these age-related changed in bone morphology are critical for interpreting the skeletal response to pharmacologic interventions or genetic manipulation in mice.     

Quantitative trait loci for femoral and lumbar vertebral bone mineral density in C57BL/6J and C3H/HeJ inbred strains of mice.

Significant differences in vertebral (9%) and femoral (50%) adult bone mineral density (BMD) between the C57BL/6J (B6) and C3H/HeJ (C3H) inbred strains of mice have been subjected to genetic analyses for quantitative trait loci (QTL). Nine hundred eighty-six B6C3F2 females were analyzed to gain insight into the number of genes that regulate peak BMD and their locations. Femurs and lumbar vertebrae were isolated from 4-month-old B6C3F2 females at skeletal maturity and then BMD was determined by peripheral quantitative computed tomography (pQCT). Estimates of BMD heritability were 83% for femurs and 72% for vertebrae. Genomic DNA from F2 progeny was screened for 107 polymerase chain reaction (PCR)-based markers discriminating B6 and C3H alleles on all 19 autosomes. The regression analyses of markers on BMD revealed ten chromosomes (1, 2, 4, 6, 11, 12, 13, 14, 16, and 18) carrying QTLs for femurs and seven chromosomes (1, 4, 7, 9, 11, 14, and 18) carrying QTLs for vertebrae, each with log10 of the odds ratio (LOD) scores of 2.8 or better. The QTLs on chromosomes (Chrs) 2, 6, 12, 13, and 16 were unique to femurs, whereas the QTLs on Chrs 7 and 9 were unique to vertebrae. When the two bone sites had a QTL on the same chromosome, the same marker had the highest, although different, LOD score. A pairwise comparison by analysis of variance (ANOVA) did not reveal significant gene x gene interactions between QTLs for either bone site. BMD variance accounted for by individual QTLs ranged from 1% to 10%. Collectively, the BMD QTLs for femurs accounted for 35.1% and for vertebrae accounted for 23.7 % of the F2 population variances in these bones. When mice were homozygous c3/c3 in the QTL region, 8 of the 10 QTLs increased, while the remaining two QTLs on Chrs 6 and 12 decreased, femoral BMD. Similarly, when mice were homozygous c3/c3 in the QTL region for the vertebrae, five of the seven QTLs increased, while two QTLs on Chrs 7 and 9 decreased, BMD. These findings show the genetic complexity of BMD with multiple genes participating in its regulation. Although 5 of the 12 QTLs are considered to be skeleton-wide loci and commonly affect both femurs and vertebrae, each of the bone sites also exhibited unique QTLs. Thus, the BMD phenotype can be partitioned into its genetic components and the effects of these loci on normal bone biology can be determined. Importantly, the BMD QTLs that we have identified are in regions of the mouse genome that have known human homology, and the QTLs will become useful experimental tools for mechanistic and therapeutic analyses of bone regulatory genes.     

Increased glycogen synthase kinase-3 activity in diabetes- and obesity-prone C57BL/6J mice.

Although the precise mechanisms contributing to insulin resistance and type 2 diabetes are unknown, it is believed that defects in downstream components of the insulin signaling pathway may be involved. In this work, we hypothesize that a serine/threonine kinase, glycogen synthase kinase-3 (GSK-3), may be pertinent in this regard. To test this hypothesis, we examined GSK-3 activity in two inbred mouse strains known to be susceptible (C57BL/6J) or resistant (A/J) to diet-induced obesity and diabetes. Examination of GSK-3 in fat, liver, and muscle tissues of C57BL/6J mice revealed that GSK-3 activity increased twofold in the epididymal fat tissue and remained unchanged in muscle and liver of mice fed a high-fat diet, compared with their low-fat diet-fed counterparts. In contrast, GSK-3 activity did not change in the epididymal fat tissue of A/J mice, regardless of the type of diet they were fed. In addition, both basal and diet-induced GSK-3 activity was higher (2.3- and 3.2-fold, respectively) in the adipose tissue of C57BL/6J mice compared with that in A/J mice. Taken together, our studies suggest an unsuspected link between increased GSK-3 activity and development of insulin resistance and type 2 diabetes in fat tissue of C57BL/6J mice, and implicate GSK-3 as a potential factor contributing to susceptibility of C57BL/6J mice to diet-induced diabetes.     

Control of expression of insulin resistance and hyperglycemia by different genetic factors in diabetic C57BL/6J mice

The inheritance of the tendency to develop diet-induced non-insulin-dependent (type II) diabetes was analyzed in crosses between diabetes-prone C57BL/6J (BL/6) mice and diabetes-resistant A/J mice. The effects of a diabetogenic diet on blood glucose and insulin levels, insulin sensitivity, and weight were evaluated in F1 and both (BL/6 X A/J) F1 X BL/6 and (BL/6 X A/J) F1 X A/J backcross mice. These results suggest that diet-induced hyperglycemia is largely determined by a recessive gene and diet-induced insulin resistance by a dominant gene. Analyses of both backcrosses indicated that insulin sensitivity and blood glucose levels were unrelated, suggesting that they are controlled by different genetic factors. This conclusion was supported by data from nine recombinant inbred BXA strains in which no correlation was observed between these variables. Furthermore, insulin sensitivity and body weight correlated differently in the two backcross groups, suggesting that insulin resistance is not simply a function of obesity. The number of genes that predominantly influence diabetic traits was estimated by comparing the variance observed in (BL/6 X A/J) F1 X BL/6 backcross mice with that observed in parental mice. The data suggest that relatively few genes predominantly affect the diabetic phenotype in this murine model.     

Quantitative trait loci analysis of structural and material skeletal phenotypes in C57BL/6J and DBA/2 second-generation and recombinant inbred mice.

QTL analyses identified several chromosomal regions influencing skeletal phenotypes of the femur and tibia in BXD F2 and BXD RI populations of mice. QTLs for skeletal traits co-located with each other and with correlated traits such as body weight and length, adipose mass, and serum alkaline phosphatase. INTRODUCTION: Past research has shown substantial genetic influence on bone quality, and the impact of reduced bone mass on our aging population has heightened the interest in skeletal genetic research. MATERIALS AND METHODS: Quantitative trait loci (QTL) analyses were performed on morphologic measures and structural and material properties of the femur and tibia in 200-day-old C57BL/6J x DBA/2 (BXD) F2 (second filial generation; n = 400) and BXD recombinant inbred (RI; n = 23 strains) populations of mice. Body weight, body length, adipose mass, and serum alkaline phosphatase were correlated phenotypes included in the analyses. RESULTS: Skeletal QTLs for morphologic bone measures such as length, width, cortical thickness, and cross-sectional area mapped to nearly every chromosome. QTLs for both structural properties (ultimate load, yield load, or stiffness) and material properties (stress and straincharacteristics and elastic modulus) mapped to chromosomes 4, 6, 9, 12, 13, 15, and 18. QTLs that were specific to structural properties were identified on chromosomes 1, 2, 3, 7, 8, and 17, and QTLs that were specific to skeletal material properties were identified on chromosomes 5, 11, 16, and 19. QTLs for body size (body weight, body length, and adipose mass) often mapped to the same chromosomal regions as those identified for skeletal traits, suggesting that several QTLs identified as influencing bone could be mediated through body size. CONCLUSION: New QTLs, not previously reported in the literature, were identified for structural and material properties and morphological measures of the mouse femur and tibia. Body weight and length, adipose mass, and serum alkaline phosphatase were correlated phenotypes that mapped in close proximity of skeletal chromosomal loci. The more specific measures of bone quality included in this investigation enhance our understanding of the functional significance of previously identified QTLs.     

Strain differences in bone density and calcium metabolism between C3H/HeJ and C57BL/6J mice.

Previous reports indicate that peak bone density is significantly higher in C3H/HeJ (C3H) than in C57BL/6J (C57BL) mice, making these two inbred strains useful models for studying the genetic basis for peak bone density. The following study was undertaken to examine whether strain differences in the bone density of C3H and C57BL mice are associated with differences in intestinal calcium (Ca) absorption. Calcium absorption was measured by the balance technique and animals received two injections of fluorochromes 5 days apart before killing. Subsequently, the femurs were removed and, following measurement of volumetric density, the left femur was divided into three equal parts and the middle third served as the femoral cortical diaphysis. Femur diaphyseal volumetric bone density, ash, and Ca content were 10%, 29%, and 29% higher in C3H than in C57BL mice (p < 0.001), respectively. Bone length, periosteal mineral apposition rate, and periosteal bone formation rate of femoral diaphyseal cortical bone were not significantly different between the two strains of mice, but the marrow area of C57BL mice was almost twofold that of C3H mice (p < 0.0001). Intestinal Ca absorption and 1,25-dihydroxyvitamin D [1,25(OH)2D]-stimulated Ca2+ uptake by intestinal mucosal cells were 38% and 51% higher in C3H than in C57BL mice p < 0.001), respectively. Serum Ca and 1,25(OH)2D levels were 6% and 32% higher in C3H than in C57BL mice (p < 0.001), respectively, and the number of intestinal-occupied vitamin D receptors was 51% higher in C3H than in C57BL mice (p < 0.01). In a second experiment, three groups of C3H mice and three groups of C57BL mice were fed diets that contained 0.4%, 0.1%, or 0.02% Ca, and serum Ca, 1,25(OH)2D, parathyroid hormone (PTH), and intestinal Ca absorption measured. At all dietary Ca levels, C3H mice maintained positive Ca absorption and absorbed significantly more Ca than C57BL mice. In contrast, at low dietary Ca levels (0.1% and 0.02% Ca), C57BL mice maintained negative Ca absorption. Low dietary Ca increased serum PTH significantly in C57BL but not in C3H mice, and decreased serum 1,25(OH)2D and Ca levels in both strains of mice. Our findings indicate that the C57BL mice relied more on the mobilization of Ca from bone to maintain extracellular Ca homeostasis than the C3H mice. We conclude that strain differences in bone mass and density between C3H and C57BL mice is expressed, in part, through the vitamin D and PTH endocrine systems and their effects on the maintenance of extracellular Ca homeostasis.     

A time course of bone response to jump exercise in C57BL/6J mice

 Exercise, by way of mechanical loading, provides a physiological stimulus to which bone tissue adapts by increased bone formation. The mechanical stimulus due to physical activity depends on both the magnitude and the duration of the exercise. Earlier studies have demonstrated that jump training for 4 weeks produces a significant bone formation response in C57BL/6J mice. An early time point with significant increase in bone formation response would be helpful in: (1) designing genetic quantitative trait loci (QTL) studies to investigate genes regulating the bone adaptive response to mechanical stimulus; and (2) mechanistic studies to investigate early stimulus to bone tissue. Consequently, we investigated the bone structural response after 2, 3, and 4 weeks of exercise with a loading cycle of ten jumps a day. We used biochemical markers and peripheral quantitative computed tomography (pQCT) of excised femur to measure bone density, bone mineral content (BMC), and area. Four-week-old mice were separated into control (n= 6) and jump groups (n= 6), and the latter groups of mice were subjected to jump exercise of 2-week, 3-week, and 4-week duration. Data (pQCT) from a mid-diaphyseal slice were used to compare bone formation parameters between exercise and control groups, and between different time points. There was no statistically significant change in bone response after 2 weeks of jump exercise as compared with the age-matched controls. After 3 weeks of jump exercise, the periosteal circumference, which is the most efficient means of measuring adaptation to exercise, was increased by 3% (P < 0.05), and total and cortical area were increased by 6% (P < 0.05) and 11% (P < 0.01), respectively. Total bone mineral density (BMD) increased by 11% (P < 0.01). The biggest changes were observed in cortical and total BMC, with the increase in total BMC being 12% (P < 0.01). Interestingly, the increase in BMC was observed throughout the length of the femur and was not confined to the mid-diaphysis. Consistent with earlier studies, mid-femur bone mass and area remained significantly elevated in the 4-week exercise group when compared with the control group of mice. The levels of the biochemical markers osteocalcin, skeletal alkaline phosphatase, and C-telopeptide were not significantly different between the exercise and control groups, indicating the absence of any systemic response due to the exercise. We conclude that a shorter exercise regimen, of 3 weeks, induced a bone response that was greater than or equal to that of 4 weeks of jump exercise reported earlier. Received: October 1, 2001 / Accepted: January 18, 2002     

Changes in bone structure and mass with advancing age in the male C57BL/6J mouse.

To determine whether the mouse loses bone with aging and whether the changes mimic those observed in human aging, we examined the changes in the tibial metaphysis and diaphysis in the male C57BL/6J mouse over its life span using microcomputed tomography (microCT). Cancellous bone volume fraction (BV/TV) decreased 60% between 6 weeks and 24 months of age. Loss was characterized by decreased trabecular number (Tb.N), increased trabecular spacing (Tb.Sp), and decreased connectivity. Anisotropy decreased while the structure model index increased with age. Cortical bone thickness increased between 6 weeks and 6 months of age and then decreased continuously to 24 months (-12%). Cortical bone area (Ct.Ar) remained constant between 6 and 24 months. Fat-free weight reached a peak at 12 months and gradually declined to 24 months. Total mass lost between 12 and 24 months reached 10%. Overall, the age-related changes in skeletal mass and architecture in the mouse were remarkably similar to those seen in human aging. Furthermore, the rapid early loss of cancellous bone suggests that bone loss is not just associated with old age in the mouse but rather occurs as a continuum from early growth. We conclude that the C57BL/6J male mouse maybe a useful model to study at least some aspects of age-related bone loss in humans.