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Age-related bone loss: lessons from the osteoporotic SAMP6 mouse model
International Congress Series 1260 (2004) 55 – 60
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Age-related bone loss: lessons from the osteoporotic SAMP6 mouse model Robert L. Jilka *, Robert J. Shmookler Reis, Stavros C. Manolagas Professor of Internal Medicine and VA Research Career Scientist, Center for Osteoporosis and Metabolic Bone Diseases, Central Arkansas Veterans Healthcare System, University of Arkansas for Medical Sciences, 4301 W. Markham, Slot 587 Little Rock, AR 72205 USA Received 19 June 2003; accepted 4 September 2003
Abstract. SAMP6 mice exhibit features of skeletal aging including reduced bone mineral density (BMD), diminished rate of bone formation, fewer mesenchymal stem cell (MSC) progenitors of osteoblasts, decreased osteoblasts and increased marrow adipocytes. A decrease in the self-renewal capacity of MSCs may account for the reduced osteoblast number and bone formation. Increased activation, or overexpression, of the transcription factor PPARg2 in SAMP6 mice may also contribute as features of the SAMP6 skeletal phenotype were reproduced by feeding the PPARg2 ligand rosiglitazone to normal mice. Genetic mapping studies utilizing F2 progeny of SAMP6 mice mated with either SAMR1 or the related AKR/J strain identified quantitative trait loci (QTLs) for vertebral BMD on chromosomes 2, 7, 11, 13, 16, 18, and X. Transfer of the AKR allele of the chromosome 2 QTL into SAMP6 mice by backcrossing caused a 5.0 – 5.4% increase in BMD, accounting for f50% of the BMD difference between SAMP6 and AKR/J. Studies in Scottish postmenopausal women revealed an association of the X chromosome locus with BMD, thus demonstrating the applicability of QTL mapping information derived from mice to humans. Future genetic and functional studies of SAMP6 mice should therefore provide clues as to why the production of osteoblasts is reduced during aging in mice and humans. D 2003 Elsevier B.V. All rights reserved. Keywords: SAMP6; Osteoporosis; Bone density; Osteoblast differentiation; Quantitative trait locus
1. Introduction Histologic studies have indicated that age-related bone loss is characterized by a deficit in the number of osteoblasts needed to replace the bone removed by osteoclasts during bone remodeling [1]. This contention is based on evidence that bone from elderly humans has decreased mean wall thickness—a histologic index of the amount of bone made by a team of
* Corresponding author. Tel.: +1-501-686-7896; fax: +501-686-8954. E-mail address: [email protected] (R.L. Jilka). 0531-5131/ D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0531-5131(03)01566-8
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Fig. 1. Determination of wall width. Photomicrograph of murine bone illustrating osteoblasts depositing osteoid on top of cement lines, which mark the extent of a previous episode of osteoclastic bone resorption. Wall width is an index of the amount of bone made by a team of osteoblasts during bone remodeling, and is determined by the distance between a cement line and a quiescent bone surface covered by lining cells.
osteoblasts (see Fig. 1). Osteoblast number is determined by the supply of new osteoblasts from local progenitors and the life span of the mature cell [2]. Based on these considerations, age-related bone loss must be due at least in part to changes in the mechanisms that govern osteoblast progenitor fate and/or the fate of the matrix-synthesizing osteoblast.
Fig. 2. Potential cell fates during osteoblast differentiation. Progenitors may (1) replicate, (2) die by apoptosis, or (3) differentiate into osteoblasts or adipocytes. Mature osteoblasts have one of the three fates; they may die, become embedded into the bone matrix as osteocytes, or become the lining cells covering the bone surface upon cessation of matrix synthesis.
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The supply of new osteoblasts depends on the self-renewal, or replication, of mesenchymal stem cell (MSC) progenitors. This cell population can be studied in vitro by their ability to form a colony of osteoblastic cells that synthesize a mineralized matrix [3]. These progenitors are therefore called colony-forming unit osteoblast (CFU-OB). Fig. 2 presents a working model of those aspects of osteoblast differentiation that are likely to be of particular relevance for determining osteoblast number. The model is based on the importance of stem cells for tissue regeneration in adults [4]; the role of apoptosis as a means of governing progenitor and mature cell number in regenerating tissues, including the skeleton [5]; and the common ancestry of marrow adipocytes and osteoblasts [6]. Indeed, there is a reciprocal relationship between age-related bone loss and marrow adiposity [7]. Because the SAMP6 murine model of early senescence exhibits many of the features seen in the skeleton of aging animals and humans, we have utilized it to identify mechanisms that could reduce the production of osteoblasts during aging. 2. The osteoporotic phenotype of SAMP6 mice At 4– 6 months of age, SAMP6 mice exhibit reduced bone mineral density (BMD), fewer CFU-OBs, increased marrow adiposity, and decreased wall width [8,9]. We recently demonstrated that a significant proportion of CFU-OB in the murine femur undergo cell division in vivo [3]. Based on this, we reasoned that the reduction of CFU-OB in SAMP6 mice could be due to a decrease in their replicative capacity. In support of this contention, we have found that the rate of replication of CFU-OB in ex vivo cell cultures established from the marrow of SAMP6 mice was approximately 30% of that of cells from SAMR1 mice [10]. The increased marrow fat of SAMP6 mice, however, suggests that diversion of progenitors into the adipocyte lineage could also contribute to the reduction in osteoblast number. We have previously shown that activation of the nuclear hormone receptor PPARg2 stimulates the formation of adipocytes and inhibits the development of osteoblasts from MSCs [11,12]. Thus, increased activation, or overexpression, of PPARg2 in SAMP6 mice could contribute to decreased osteoblastogenesis. This notion is supported by the finding that addition of 30 nM of the PPARg2 activator rosiglitazone was sufficient to block osteoblastogenesis in bone marrow cultures from SAMP6 mice, whereas 3000 nM of the ligand was required to achieve a similar effect in cultures from control SAMR1 mice. The contention that activation of PPARg2 contributes to the osteoporotic phenotype was further supported by evidence that administration of rosiglitazone to normal C57Bl/6 mice caused bone loss that was associated with increased marrow adipocytes, decreased osteoblasts, decreased bone formation rate, and decreased wall width [13]. However, CFU-OB number was not affected and in vitro studies showed that rosiglitazone did not affect their replication. Thus, decreased CFU-OB replication and increased adipogenesis may independently contribute to age-related bone loss. 3. The genetic basis of the osteoporotic phenotype of SAMP6 mice We have sought genetic differences underlying the variation in BMD among SAM mice by identifying quantitative trait loci (QTLs) that associated with peak bone density. Inter-strain crosses were made between SAMP6 and two related strains: SAMR1 and
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AKR/J, each of which is identical to SAMP6 at f 60% of polymorphic markers, and which exhibit 9% and 18% higher vertebral BMD, respectively, than SAMP6. We identified five QTLs in F2 progeny of these crosses on chromosomes 2, 7, 11, 13 and 16 [14]. In addition, three QTLs were associated with postmaturity changes in BMD, defined as the ratio of 6-month to 4-month vertebral BMD. They were located on chromosomes 7, 18 and X [15]. To position these QTLs more precisely, recombinant-inbred lines for each QTL are being made by marker-directed backcrossing into either parental strain and then selecting mice heterozygous for the introgressed allelic markers for further breeding. Five such lines have been obtained for the chromosome 2 locus (Fig. 3). For each line, a variable portion of the AKR chromosome-2 QTL was isolated in SAMP6 genetic background. Introduction of this QTL increased spine BMD by 3 mg/mm2, most of which is attributable to a QTL near 37 cM. This QTL thus accounts for approximately 50% of the BMD difference between SAMP6 and AKR. We have also investigated the feasibility of transferring the QTL information obtained in our studies in SAM mice to humans. From the mouse QTL on the X chromosome, we located the corresponding (conserved-synteny) region of the human X chromosome by reference to the working-draft human genome sequence. DNA sequencing and single base extension techniques were developed to genotype at single nucleotide polymorphisms (SNPs) across this human genomic interval. In collaboration with Stuart H. Ralston at the University of Aberdeen, Aberdeen, UK, we then investigated the allele proportions for each SNP in a population of postmenopausal women from northeastern Scotland. Subjects were ordered by lumbar-spine BMD values adjusted for age, height and weight, to identify individuals in the highest and lowest bone density deciles of the normalized population distribution. Analysis of the SNP allele peak height ratios implied a significant difference between the high and low DNA pooled samples at one SNP, thus narrowing the candidate region to < 75 kilobase pairs.
Fig. 3. Vertebral BMD of recombinant inbred lines. Each backcross line contains a variable portion of the AKR/J chromosome 2 allele (hatched bars) on the SAMP6 background (solid bars). The boundaries of the introgressed regions were determined by genotyping at microsatellite markers indicated by vertical lines, with genetic positions (in cM) indicated above.
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4. Conclusions The occurrence of an osteoporotic phenotype early in the adult life of SAMP6 mice has permitted the study of age-related bone loss in a fashion that allows integration of cellular, molecular, and genetic information. Identification of the genes that control BMD in SAMP6 mice, and elucidation of the control of osteoblast progenitor fate, should provide new clues as to why the production of osteoblasts is reduced during aging in mice and humans. Acknowledgements The authors thank T. Bellido, S. Kousteni, C.A. O’Brien, A.M. Parfitt, and R.S. Weinstein for their advice and help in the development of some of the concepts presented in this manuscript. This work was supported by the NIH (P01 AG13918 to S.C.M.) and the Department of Veterans Affairs. References [1] A.M. Parfitt, Bone-forming cells in clinical conditions, in: B.K. Hall (Ed.), Bone, vol. 1, The Osteoblast and Osteocyte. Telford Press and CRC Press, Boca Raton, FL, 1990, pp. 351 – 429. [2] S.C. Manolagas, Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis, Endocr. Rev. 21 (2) (2000) 115 – 137. [3] G.B. Di Gregorio, M. Yamamoto, A.A. Ali, E. Abe, P. Roberson, S.C. Manolagas, et al., Attenuation of the self-renewal of transit-amplifying osteoblast progenitors in the murine bone marrow by 17 beta-estradiol, J. Clin. Invest. 107 (7) (2001) 803 – 812. [4] M. Loeffler, C.S. Potten, Stem cells and cellular pedigrees—a conceptual introduction, in: C.S. Potten (Ed.), Stem Cells, Academic Press, San Diego, CA, 1997, pp. 1 – 27. [5] B.F. Boyce, L. Xing, R.L. Jilka, T. Bellido, R.S. Weinstein, A.M. Parfitt, et al., Apoptosis in Bone Cells, in: J.P. Bilezikian, L.G. Raisz, G. Rodan (Eds.), Principles of Bone Biology, 2nd ed., Academic Press, San Diego, 2002, pp. 151 – 168. [6] J.E. Aubin, J.T. Triffitt, Mesenchymal stem cells and osteoblast differentiation, in: J.P. Bilezikian, L.G. Raisz, G. Rodan (Eds.), Principles of Bone Biology, 2nd ed., Academic Press, San Diego, 2002, pp. 59 – 81. [7] S.G. Moore, K.L. Dawson, Red and yellow marrow in the femur: age-related changes in appearance at MR imaging, Radiology 175 (1990) 219 – 223. [8] O. Kajkenova, B. Lecka-Czernik, I. Gubrij, S.P. Hauser, K. Takahashi, A.M. Parfitt, et al., Increased adipogenesis and myelopoiesis in the bone marrow of SAMP6, a murine model of defective osteoblastogenesis and low turnover osteopenia, J. Bone Miner. Res. 12 (11) (1997) 1772 – 1779. [9] R.L. Jilka, R.S. Weinstein, K. Takahashi, A.M. Parfitt, S.C. Manolagas, Linkage of decreased bone mass with impaired osteoblastogenesis in a murine model of accelerated senescence, J. Clin. Invest. 97 (7) (1996) 1732 – 1740. [10] G.B. DiGregorio-Taguchi, I. Gubrij, C. Smith, A.M. Parfitt, R.L. Jilka, The transit amplifying mesenchymal progenitor cell compartment, not the stem cell compartment, is the principal site of control of osteoblast production in normal and pathologic bone remodeling, J. Bone Miner. Res. 15 (2000) S376. [11] B. Lecka-Czernik, E.J. Moerman, D.F. Grant, J.M. Lehmann, S.C. Manolagas, R.L. Jilka, Divergent effects of selective peroxisome proliferator-activated receptor-g2 ligands on adipocyte versus osteoblast differentiation, Endocrinology 143 (6) (2002) 2376 – 2384. [12] B. Lecka-Czernik, I. Gubrij, E.J. Moerman, O. Kajkenova, D.A. Lipschitz, S.C. Manolagas, et al., Inhibition of Osf2/Cbfa1 expression and terminal osteoblast differentiation by PPARgamma2, J. Cell. Biochem. 74 (3) (1999) 357 – 371. [13] R.L. Jilka, B. Lecka-Czernik, A.A. Ali, C.E. O’Brien, R.S. Weinstein, S.C. Manolagas, Activation of PPARg2 by rosiglitazone causes bone loss associated with increased marrow adiposity and decreased osteoblast number in mice, J. Bone Miner. Res. 16 (2001) S319.
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[14] H. Benes, R.S. Weinstein, W. Zheng, J.J. Thaden, R.L. Jilka, S.C. Manolagas, et al., Chromosomal mapping of osteopenia-associated quantitative trait loci using closely related mouse strains, J. Bone Miner. Res. 15 (4) (2000) 626 – 633. [15] R.J. Shmookler Reis, H. Benes, R. McClure, P. Kang, R.S. Weinstein, R.S. Shelton, et al., The effect of sex on genetic determinants of pre- and post-maturity bone accrual in mice, J. Bone Miner. Res. 16 (2001) S351.
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Age-related bone loss: lessons from the osteoporotic SAMP6 mouse model Robert L. Jilka *, Robert J. Shmookler Reis, Stavros C. Manolagas Professor of Internal Medicine and VA Research Career Scientist, Center for Osteoporosis and Metabolic Bone Diseases, Central Arkansas Veterans Healthcare System, University of Arkansas for Medical Sciences, 4301 W. Markham, Slot 587 Little Rock, AR 72205 USA Received 19 June 2003; accepted 4 September 2003
Abstract. SAMP6 mice exhibit features of skeletal aging including reduced bone mineral density (BMD), diminished rate of bone formation, fewer mesenchymal stem cell (MSC) progenitors of osteoblasts, decreased osteoblasts and increased marrow adipocytes. A decrease in the self-renewal capacity of MSCs may account for the reduced osteoblast number and bone formation. Increased activation, or overexpression, of the transcription factor PPARg2 in SAMP6 mice may also contribute as features of the SAMP6 skeletal phenotype were reproduced by feeding the PPARg2 ligand rosiglitazone to normal mice. Genetic mapping studies utilizing F2 progeny of SAMP6 mice mated with either SAMR1 or the related AKR/J strain identified quantitative trait loci (QTLs) for vertebral BMD on chromosomes 2, 7, 11, 13, 16, 18, and X. Transfer of the AKR allele of the chromosome 2 QTL into SAMP6 mice by backcrossing caused a 5.0 – 5.4% increase in BMD, accounting for f50% of the BMD difference between SAMP6 and AKR/J. Studies in Scottish postmenopausal women revealed an association of the X chromosome locus with BMD, thus demonstrating the applicability of QTL mapping information derived from mice to humans. Future genetic and functional studies of SAMP6 mice should therefore provide clues as to why the production of osteoblasts is reduced during aging in mice and humans. D 2003 Elsevier B.V. All rights reserved. Keywords: SAMP6; Osteoporosis; Bone density; Osteoblast differentiation; Quantitative trait locus
1. Introduction Histologic studies have indicated that age-related bone loss is characterized by a deficit in the number of osteoblasts needed to replace the bone removed by osteoclasts during bone remodeling [1]. This contention is based on evidence that bone from elderly humans has decreased mean wall thickness—a histologic index of the amount of bone made by a team of
* Corresponding author. Tel.: +1-501-686-7896; fax: +501-686-8954. E-mail address: [email protected] (R.L. Jilka). 0531-5131/ D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0531-5131(03)01566-8
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Fig. 1. Determination of wall width. Photomicrograph of murine bone illustrating osteoblasts depositing osteoid on top of cement lines, which mark the extent of a previous episode of osteoclastic bone resorption. Wall width is an index of the amount of bone made by a team of osteoblasts during bone remodeling, and is determined by the distance between a cement line and a quiescent bone surface covered by lining cells.
osteoblasts (see Fig. 1). Osteoblast number is determined by the supply of new osteoblasts from local progenitors and the life span of the mature cell [2]. Based on these considerations, age-related bone loss must be due at least in part to changes in the mechanisms that govern osteoblast progenitor fate and/or the fate of the matrix-synthesizing osteoblast.
Fig. 2. Potential cell fates during osteoblast differentiation. Progenitors may (1) replicate, (2) die by apoptosis, or (3) differentiate into osteoblasts or adipocytes. Mature osteoblasts have one of the three fates; they may die, become embedded into the bone matrix as osteocytes, or become the lining cells covering the bone surface upon cessation of matrix synthesis.
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The supply of new osteoblasts depends on the self-renewal, or replication, of mesenchymal stem cell (MSC) progenitors. This cell population can be studied in vitro by their ability to form a colony of osteoblastic cells that synthesize a mineralized matrix [3]. These progenitors are therefore called colony-forming unit osteoblast (CFU-OB). Fig. 2 presents a working model of those aspects of osteoblast differentiation that are likely to be of particular relevance for determining osteoblast number. The model is based on the importance of stem cells for tissue regeneration in adults [4]; the role of apoptosis as a means of governing progenitor and mature cell number in regenerating tissues, including the skeleton [5]; and the common ancestry of marrow adipocytes and osteoblasts [6]. Indeed, there is a reciprocal relationship between age-related bone loss and marrow adiposity [7]. Because the SAMP6 murine model of early senescence exhibits many of the features seen in the skeleton of aging animals and humans, we have utilized it to identify mechanisms that could reduce the production of osteoblasts during aging. 2. The osteoporotic phenotype of SAMP6 mice At 4– 6 months of age, SAMP6 mice exhibit reduced bone mineral density (BMD), fewer CFU-OBs, increased marrow adiposity, and decreased wall width [8,9]. We recently demonstrated that a significant proportion of CFU-OB in the murine femur undergo cell division in vivo [3]. Based on this, we reasoned that the reduction of CFU-OB in SAMP6 mice could be due to a decrease in their replicative capacity. In support of this contention, we have found that the rate of replication of CFU-OB in ex vivo cell cultures established from the marrow of SAMP6 mice was approximately 30% of that of cells from SAMR1 mice [10]. The increased marrow fat of SAMP6 mice, however, suggests that diversion of progenitors into the adipocyte lineage could also contribute to the reduction in osteoblast number. We have previously shown that activation of the nuclear hormone receptor PPARg2 stimulates the formation of adipocytes and inhibits the development of osteoblasts from MSCs [11,12]. Thus, increased activation, or overexpression, of PPARg2 in SAMP6 mice could contribute to decreased osteoblastogenesis. This notion is supported by the finding that addition of 30 nM of the PPARg2 activator rosiglitazone was sufficient to block osteoblastogenesis in bone marrow cultures from SAMP6 mice, whereas 3000 nM of the ligand was required to achieve a similar effect in cultures from control SAMR1 mice. The contention that activation of PPARg2 contributes to the osteoporotic phenotype was further supported by evidence that administration of rosiglitazone to normal C57Bl/6 mice caused bone loss that was associated with increased marrow adipocytes, decreased osteoblasts, decreased bone formation rate, and decreased wall width [13]. However, CFU-OB number was not affected and in vitro studies showed that rosiglitazone did not affect their replication. Thus, decreased CFU-OB replication and increased adipogenesis may independently contribute to age-related bone loss. 3. The genetic basis of the osteoporotic phenotype of SAMP6 mice We have sought genetic differences underlying the variation in BMD among SAM mice by identifying quantitative trait loci (QTLs) that associated with peak bone density. Inter-strain crosses were made between SAMP6 and two related strains: SAMR1 and
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AKR/J, each of which is identical to SAMP6 at f 60% of polymorphic markers, and which exhibit 9% and 18% higher vertebral BMD, respectively, than SAMP6. We identified five QTLs in F2 progeny of these crosses on chromosomes 2, 7, 11, 13 and 16 [14]. In addition, three QTLs were associated with postmaturity changes in BMD, defined as the ratio of 6-month to 4-month vertebral BMD. They were located on chromosomes 7, 18 and X [15]. To position these QTLs more precisely, recombinant-inbred lines for each QTL are being made by marker-directed backcrossing into either parental strain and then selecting mice heterozygous for the introgressed allelic markers for further breeding. Five such lines have been obtained for the chromosome 2 locus (Fig. 3). For each line, a variable portion of the AKR chromosome-2 QTL was isolated in SAMP6 genetic background. Introduction of this QTL increased spine BMD by 3 mg/mm2, most of which is attributable to a QTL near 37 cM. This QTL thus accounts for approximately 50% of the BMD difference between SAMP6 and AKR. We have also investigated the feasibility of transferring the QTL information obtained in our studies in SAM mice to humans. From the mouse QTL on the X chromosome, we located the corresponding (conserved-synteny) region of the human X chromosome by reference to the working-draft human genome sequence. DNA sequencing and single base extension techniques were developed to genotype at single nucleotide polymorphisms (SNPs) across this human genomic interval. In collaboration with Stuart H. Ralston at the University of Aberdeen, Aberdeen, UK, we then investigated the allele proportions for each SNP in a population of postmenopausal women from northeastern Scotland. Subjects were ordered by lumbar-spine BMD values adjusted for age, height and weight, to identify individuals in the highest and lowest bone density deciles of the normalized population distribution. Analysis of the SNP allele peak height ratios implied a significant difference between the high and low DNA pooled samples at one SNP, thus narrowing the candidate region to < 75 kilobase pairs.
Fig. 3. Vertebral BMD of recombinant inbred lines. Each backcross line contains a variable portion of the AKR/J chromosome 2 allele (hatched bars) on the SAMP6 background (solid bars). The boundaries of the introgressed regions were determined by genotyping at microsatellite markers indicated by vertical lines, with genetic positions (in cM) indicated above.
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4. Conclusions The occurrence of an osteoporotic phenotype early in the adult life of SAMP6 mice has permitted the study of age-related bone loss in a fashion that allows integration of cellular, molecular, and genetic information. Identification of the genes that control BMD in SAMP6 mice, and elucidation of the control of osteoblast progenitor fate, should provide new clues as to why the production of osteoblasts is reduced during aging in mice and humans. Acknowledgements The authors thank T. Bellido, S. Kousteni, C.A. O’Brien, A.M. Parfitt, and R.S. Weinstein for their advice and help in the development of some of the concepts presented in this manuscript. This work was supported by the NIH (P01 AG13918 to S.C.M.) and the Department of Veterans Affairs. References [1] A.M. Parfitt, Bone-forming cells in clinical conditions, in: B.K. Hall (Ed.), Bone, vol. 1, The Osteoblast and Osteocyte. Telford Press and CRC Press, Boca Raton, FL, 1990, pp. 351 – 429. [2] S.C. Manolagas, Birth and death of bone cells: basic regulatory mechanisms and implications for the pathogenesis and treatment of osteoporosis, Endocr. Rev. 21 (2) (2000) 115 – 137. [3] G.B. Di Gregorio, M. Yamamoto, A.A. Ali, E. Abe, P. Roberson, S.C. Manolagas, et al., Attenuation of the self-renewal of transit-amplifying osteoblast progenitors in the murine bone marrow by 17 beta-estradiol, J. Clin. Invest. 107 (7) (2001) 803 – 812. [4] M. Loeffler, C.S. Potten, Stem cells and cellular pedigrees—a conceptual introduction, in: C.S. Potten (Ed.), Stem Cells, Academic Press, San Diego, CA, 1997, pp. 1 – 27. [5] B.F. Boyce, L. Xing, R.L. Jilka, T. Bellido, R.S. Weinstein, A.M. Parfitt, et al., Apoptosis in Bone Cells, in: J.P. Bilezikian, L.G. Raisz, G. Rodan (Eds.), Principles of Bone Biology, 2nd ed., Academic Press, San Diego, 2002, pp. 151 – 168. [6] J.E. Aubin, J.T. Triffitt, Mesenchymal stem cells and osteoblast differentiation, in: J.P. Bilezikian, L.G. Raisz, G. Rodan (Eds.), Principles of Bone Biology, 2nd ed., Academic Press, San Diego, 2002, pp. 59 – 81. [7] S.G. Moore, K.L. Dawson, Red and yellow marrow in the femur: age-related changes in appearance at MR imaging, Radiology 175 (1990) 219 – 223. [8] O. Kajkenova, B. Lecka-Czernik, I. Gubrij, S.P. Hauser, K. Takahashi, A.M. Parfitt, et al., Increased adipogenesis and myelopoiesis in the bone marrow of SAMP6, a murine model of defective osteoblastogenesis and low turnover osteopenia, J. Bone Miner. Res. 12 (11) (1997) 1772 – 1779. [9] R.L. Jilka, R.S. Weinstein, K. Takahashi, A.M. Parfitt, S.C. Manolagas, Linkage of decreased bone mass with impaired osteoblastogenesis in a murine model of accelerated senescence, J. Clin. Invest. 97 (7) (1996) 1732 – 1740. [10] G.B. DiGregorio-Taguchi, I. Gubrij, C. Smith, A.M. Parfitt, R.L. Jilka, The transit amplifying mesenchymal progenitor cell compartment, not the stem cell compartment, is the principal site of control of osteoblast production in normal and pathologic bone remodeling, J. Bone Miner. Res. 15 (2000) S376. [11] B. Lecka-Czernik, E.J. Moerman, D.F. Grant, J.M. Lehmann, S.C. Manolagas, R.L. Jilka, Divergent effects of selective peroxisome proliferator-activated receptor-g2 ligands on adipocyte versus osteoblast differentiation, Endocrinology 143 (6) (2002) 2376 – 2384. [12] B. Lecka-Czernik, I. Gubrij, E.J. Moerman, O. Kajkenova, D.A. Lipschitz, S.C. Manolagas, et al., Inhibition of Osf2/Cbfa1 expression and terminal osteoblast differentiation by PPARgamma2, J. Cell. Biochem. 74 (3) (1999) 357 – 371. [13] R.L. Jilka, B. Lecka-Czernik, A.A. Ali, C.E. O’Brien, R.S. Weinstein, S.C. Manolagas, Activation of PPARg2 by rosiglitazone causes bone loss associated with increased marrow adiposity and decreased osteoblast number in mice, J. Bone Miner. Res. 16 (2001) S319.
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[14] H. Benes, R.S. Weinstein, W. Zheng, J.J. Thaden, R.L. Jilka, S.C. Manolagas, et al., Chromosomal mapping of osteopenia-associated quantitative trait loci using closely related mouse strains, J. Bone Miner. Res. 15 (4) (2000) 626 – 633. [15] R.J. Shmookler Reis, H. Benes, R. McClure, P. Kang, R.S. Weinstein, R.S. Shelton, et al., The effect of sex on genetic determinants of pre- and post-maturity bone accrual in mice, J. Bone Miner. Res. 16 (2001) S351.