| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
The Jackson Laboratory (K.M.D., K.L.S., L.G.H., L.R.D., C.A.-B., W.G.B., C.J.R.), Bar Harbor, Maine 04609; The University of Connecticut Medical Center (G.A.G., B.K.-J., J.L., B.E.K.), Farmington, Connecticut 06030; and The University of Texas Health Sciences Center at San Antonio (M.L.A.), San Antonio, Texas 78229
Address all correspondence and requests for reprints to: Clifford J. Rosen, M.D., The Jackson Laboratory, 600 Main Street, Bar Harbor, Maine 04609. E-mail: rofe{at}aol.com.
 |
Abstract |
|---|
 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
|
Introduction |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
To overcome the difficulty in finding genes that influence IGF-I, we turned to a quantitative trait locus (QTL) approach using two inbred strains of mice. As reported previously, C3H/HeJ (C3H) female mice have greater serum IGF-I concentrations as well as higher vBMD than C57BL/6J (B6) during and after development (2, 11, 12, 15). By crossing C3H and B6 and then intercrossing B6C3F1 mice, we identified several QTL in the female F2 progeny that independently segregated for serum IGF-I and femoral BMD (2, 12). Three QTL for IGF-I were found to contribute about 50% of the variance across the two strains and two of these (i.e. Igf1s1, Igf1s2) could be superimposed on femoral BMD QTL (2, 12). Previously we reported genomic and functional studies for one congenic strain, B6.C3H-6T (i.e. 6T, which carries the Igf1s1 QTL) (16, 17, 18, 19). In this study, we examined the IGF-I and skeletal phenotypes of a second congenic mouse strain, B6.C3H-10 (i.e.10T). This strain carries the Igf1s2 QTL on a B6 background with C3H alleles from the midportion of chromosome (Chr) 10. Based on previous F2 data for both the serum IGF-I phenotype and vertebral trabecular bone volume fraction (BV/TV), we predicted, a priori, that a congenic strain with this 30-cM region of the Chr 10 genome from C3H would show increased serum IGF-I and greater bone mass (12, 15). Findings from several nested congenic sublines indicated that allelic differences in this QTL profoundly affect both circulating IGF-I and skeletal remodeling.
|
Materials and Methods |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Phenotyping
Dual-energy x-ray absorptiometry (DXA) scanning by PIXImus.
We used PIXImus (GE-Lunar, Madison, WI) to assess areal whole-body BMD and body composition in 10T congenic and B6 control mice at 8 and 16 wk of age. This methodology has been validated in small animals. PIXImus scanning in mice for bone mineral content (BMC) and percent fat is both accurate and precise, although size must be considered when comparing strains. BMC by PIXImus is highly correlated with mineral content of hydroxyapatite standard of known density (r2 = 0.997).
Peripheral quantitative computed tomography (pQCT) for volumetric femoral BMD.
vBMD was measured on the entire left femur from groups of female and male B6 and congenic mice. Isolated femur length was measured with digital calipers (Stoelting, Wood Dale, IL), and then femurs were measured for density using the SA Plus densitometer (Orthometrics; Stratec SA Plus Research Unit, White Plains, NY). Calibration of the SA Plus instrument was done with hydroxyapatite standards of known density (50–1000 mg/mm3) with cylindrical diameters 2.4 mm and length 24 mm. Assessment of defined thickness aluminum foils indicated accurate measurement of the 0.25-mm-thick foil, whereas a 0.02-mm-thick foil could not be measured. The bone scans were analyzed with two different sets of thresholds to separate bone from soft tissue. Outer high density thresholds of 710 and 570 mg/cm3 were used to determine cortical bone areas and surfaces. This threshold was selected to yield area values consistent with histomorphometrically derived values. To determine mineral content, a second analysis was carried out with threshold settings of 220 and 400 mg/cm3. These lower thresholds were selected so that mineral from most partial voxels (0.07 mm) would be included in the analysis. Density values were calculated from the summed areas and associated mineral contents. Precision of the SA Plus for repeated measurement of a single femur was found to be 1.2%. Isolated femurs were scanned at seven locations at 2-mm intervals, beginning 0.8 mm from the distal ends of the epiphyseal condyles. Due to variation in femur lengths, the femoral head and necks could not be scanned at the same location for each bone and thus was not included in final data. Total vBMD values were calculated by dividing the total mineral content by the total bone volume (bone + marrow) and expressed as milligrams per cubic millimeter. Periosteal perimeters and cortical thickness data were obtained at the midshaft scan.
MicroCT (micro-computed tomography) 40 femoral cross-sectional geometry and trabecular morphology.
Femurs from age-, genotype-, and gender-matched mice were scanned using a microCT40 microcomputed tomographic instrument (µCT40, Scanco Medical AG, Bassersdorf, Switzerland) to evaluate cross-sectional geometry at the femoral midshaft and trabecular bone volume fraction and microarchitecture in the secondary spongiosa of the distal femur (15). The femurs were scanned at low resolution, energy level of 55 KeV, and intensity of 145 µA. Eighteen slices were measured at the midpoint of each femur, with an isotropic pixel size of 12 µm and slice thickness of 12 µm, and used to compute the average total cross-sectional area (square millimeters), bone area (square millimeters), marrow area (square millimeters), and cortical thickness (micrometers). Images of the distal femur were acquired at the same parameters as the femoral midshaft. BV/TV and microarchitecture were evaluated in the secondary spongiosa, starting at approximately 0.6 mm proximal to the growth plate and extending proximally 1.5 mm.
IGF-I measurements
Serum IGF-I was measured by a RIA (ALPCO, Windham, NH) (16). IGF binding proteins (IGFBPs) were first separated from the IGF-I by an acid dissociation step. This was followed by the addition of a neutralization buffer containing excess recombinant human IGF-II, allowing the IGF-II to bind to the IGFBPs before immunoassay with a human anti-IGF-I polyclonal antibody. The sensitivity of the assay is 0.01 ng/ml IGF-I; the interassay coefficient of variation based on normal standards and pooled serum of C3H and B6 is approximately 6%. There is no cross-reactivity with IGF-II. Standards were included in each assay as well as normal pools from both progenitors. Because of the large numbers of samples run for IGF-I, all values were adjusted to the standard B6 mouse pools using a correction factor to account for interassay variation. In addition to serum measurements, IGF-I in conditioned media was measured using the same assay; the results were then corrected for milligrams of protein in the conditioned medium (i.e. nanograms per milligram of protein).
Quantitative PCR
Samples of liver and bone tissue from 8-wk 10T and B6 controls male mice were extracted for RNA as previously described (17). Briefly, total RNA was isolated from liver and bone using the Total RNA isolation system (Promega, Madison, WI) as per the manufacturer’s instructions. RNA was DNAase treated and subsequently purified further using RNA Easy spin columns (QIAGEN, Valencia, CA). RNA quality and quantity were assessed using an Agilent bioanalyzer (Caliper Technologies Corp., Hopkinton, MA). Five hundred nanograms of RNA was converted to cDNA in a reverse transcription reaction, using Superscript II (no. 18064-014; Invitrogen, Carlsbad, CA) and random hexamers as primers as per the manufacturer’s direction. The cDNA was then diluted 1:5 with water. For each PCR, 1 µl of diluted cDNA was added to 5 µl of 2 x iTaq SYBR Green Supermix with ROX (catalog no. 170-8851; Bio-Rad Laboratories, Hercules, CA) and 100 nM of each forward and reverse primer in a total reaction volume of 10 µl. Cycling conditions were 2 min hold at 50 C; 3 min hold at 95 C; 40 cycles of 95 C, 15 sec; 60 C, 1 min; and all reactions were run on the ABI 7900HT sequence detection system (Applied Biosystems, Warrington UK). Each of the 48 sets of gene-specific primers was analyzed by real-time PCR to assess transcript levels, with each primer pair run once per biological replicate. Four biological replicates were run per strain. Differential expression was assessed using a global pattern recognition algorithm (GPR) as previously described (20). In short, the global pattern recognition algorithm assesses each gene as a potential normalizer for all other genes on the run. It then calculates a score based on the fraction of normalizers against which a given gene was found to be significantly different. Fold change is then calculated as previously described (20, 21). In this manner the statistical significance of an observed expression level difference is assessed before fold change is calculated. With numerous normalizers used per examined gene, biases based on any one normalizer are eliminated.
IGF-I RNase protection assay
A 386-bp DNA fragment consisting of the 3' end of intron 1, the entire 72 bp of exon 2, the contiguous 241 bp containing exon 3, and the 5' end of exon 4 was obtained from a mouse IGF-I clone and was subcloned into pGEM 2 (13). The plasmid was linearized with EcoR1, and T7 RNA polymerase was used to synthesize a 32P-labled antisense RNA using 
-32P-UTP (DuPont NEN Life Science Products, Boston, MA) and reagents from Ambion, Inc. (Austin, TX). The probe was hybridized with equivalent amounts of total RNA. Hybridization, subsequent RNase digestion of unhybridized RNA and collection, and electrophoresis of protected probe fragments were carried out as described by Adamo et al. (13). Hybridization of the probe to IGF-I mRNAs yields a protected band of 241 nt, reflecting exon 1 transcripts, and a series of bands ranging from approximately 290 to approximately 310 bp, reflecting IGF-I mRNAs initiated from the clusters of exon 2 start sites. RNase protection assays (RPA) for mouse actin used a construct, also supplied by Ambion.
Histomorphometry
Femurs were dissected free of tissue and fixed in 4% paraformaldehyde at the time the animals were killed. The undecalcified femurs were then dehydrated in increasing concentrations of ethanol, cleared in xylene, and embedded in methyl methacrylate. Five-micrometer-thick longitudinal serial sections were cut on a Reichert-Jung Polycut S microtome (Reichert-Jung, Heidelberg, Germany). Sections were taken from the middle of the femur, in which a central vein is located. Some sections were stained with modified Masson trichrome stain for static measurements (22). Histomorphometric measurements were made in a blinded, nonbiased manner using the OsteoMeasure computerized image analysis system (OsteoMetrics, Inc., Atlanta, GA) interfaced with an Optiphot Nikon microscope (Nikon Inc., Melville, NY) at a magnification of x20. The terminology and units used are those recommended by the Histomorphometry Nomenclature Committee of the American Society for Bone and Mineral Research (23). All measurements were confined to the secondary spongiosa and restricted to an area between 400 and 2000 m distal to the growth plate-metaphyseal junction of the distal femur. Cortical measurements were made 4000 m distal to the same growth plate.
Adult bone marrow stromal cell cultures for IGF-I
Bone marrow was removed from 10T and B6 8-wk-male mice and immediately plated in dishes. Adherent cells were removed after 2 d, and these cells were plated in six-well dish plates and cultured for 7 d in DMEM and 10% fetal calf serum with medium change at d 3. After 7 d, medium was replaced with MEM, glycerophosphate, and phosphoascorbate. After 24 h, the conditioned medium is removed and assayed for both IGF-I and protein content.
Statistics
Data are expressed as mean ± SEM in tables and figures. Statistical evaluation was performed using the JMP (SAS, Cary, NC) ANOVA software program for ANOVA and for linear model regression analyses. The pQCT data were adjusted for effects of body weight and femur length, whereas the DXA (PIXImus) data were adjusted for effects of body weight. Differences between adjusted least squares means for B6 and each congenic subline were tested by Student’s t test, with significance declared when P < 0.05 was observed. Differences between 8 and 16 wk in serum IGF-I for B6 and the congenic sublines were also tested by ANOVA.
|
Results |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
The differences in serum IGF-I were mirrored by expression changes in the liver as determined by RPAs. Eight-week 10T male mice (n = 6 per strain) showed significantly increased exon 2 transcripts of the Igf1 gene, compared with B6 (n = 4) (P < 0.05) (Fig. 1
), a pattern identical with what has been reported for progenitor C3H hepatic transcripts of Igf1 (13). RT-PCR of 8-wk male liver from 10T and B6 confirmed a 1.8 ± 0.6-fold greater expression of Igf1 mRNA in 10T vs. B6 (P < 0.05). Similarly, in three determinations of 8-wk whole femorae (i.e. cortical and trabecular bone plus marrow contents), skeletal Igf1 mRNA by RT-PCR was 1.33 ± 0.08-fold greater in 10T than B6 (P < 0.05). Expression differences in IGF-I in whole bone were also reflected by strain differences in secreted IGF-I from conditioned media of adult bone marrow stromal cells (i.e. 10T IGF-I secreted: 579 ± 41 ng/mg protein vs. B6 IGF-I secreted: 350 ± 20 ng/mg protein, P < 0.01). Finally, two important skeletal factors also showed differential strain expression by quantitative PCR of long bones. In 10T mice, Runx2, a bone-specific transcription factor necessary for osteoblast differentiation, exhibited 1.63-fold greater mRNA expression relative to B6 (P < 0.05), whereas message for IGFBP-5, an IGFBP regulated by IGF-I, was 2.12-fold higher in 10T than B6 femorae (P < 0.05). Thus, serum IGF-I as well as hepatic and skeletal expression of IGF-I was greater in 10T than B6, a pattern also seen in C3H mice (13). Moreover, these strain differences were reflected in secreted IGF-I from adult marrow cells and the expression in bone of two downstream targets of IGF-1 activity, IGFBP-5 and Runx2.
|
). Volumetric BMD of the total femur and midshaft was measured by pQCT. 10T females had nearly identical femoral size as B6 females, with slightly but not significantly greater cortical thickness; there were no differences in cortical bone density or periosteal circumference between strains. Male 10Ts were not different from male B6 for any cortical parameter (data not shown).
|
). For male 10T mice, BV/TV was slightly but not significantly higher vs. B6, but like female 10T, trabecular number was markedly increased (P < 0.005) and there was a trend toward lower structure model index (SMI) (Table 2). We next performed dynamic bone histomorphometry from the distal femur of 16-wk congenic and B6 males and females (n = 11/strain). Once again, we found that female 10T mice had higher distal femoral BV/TV than B6 (P < 0.05). Also, the number of osteoclasts per bone perimeter and per bone surface was markedly increased in 10T females, compared with B6 females (P < 0.01). On the other hand, for males, there were strain differences, compared with B6, that paralleled female 10T changes, but these were not statistically significant (see Table 3).
|
|
). We chose to study female nested congenics for two reasons: 1) we mapped the Igf1s2 QTL in females only (12); and 2) differences in trabecular bone mass by microCT and histomorphometry were greatest in females. Fixed nested congenics as determined by comprehensive genotyping in the Chr 10 locus (see Fig. 2), were then tested for serum IGF-I at both 8 and 16 wk using a minimum of 10 mice per subline per age. We found that serum IGF-I at 8 wk was significantly greater in female 10-4, 10-7, 10-10, and 10-13 than B6 (P < 0.05 by ANOVA), and at 16 wk, sublines 10-4, 10-7, 10-10, and 10-13 were markedly greater than B6 (i.e. > 20% higher; P < 0.0003 by ANOVA; Fig. 3).
|
|
Skeletal phenotyping of the 10-4 subline
To determine whether the skeletal phenotype was carried in the first set of nested sublines with the highest IGF-I levels, we measured femoral vBMD and BV/TV at 16 wk in sublines with a minimum of 10 mice/strain; i.e.:10-1, 10-2, 10-3, 10-4, and B6 controls. Only subline 10-4 showed statistically significant differences in measurements of trabecular bone by microCT and cortical bone by pQCT from B6. Distal femoral bone volume, total femoral mineral, midcortical density, trabecular number, connectivity density, and SMI were all significantly different in 10-4, compared with B6 (P < 0.02 vs. B6 for all parameters, see Table 4). Other nested sublines showed no differences in trabecular or cortical parameters when compared with B6 (data not shown). Hence, greater circulating IGF-I in at least one nested subline was associated with enhanced bone mineral and mass.
|
, 10-7, 10-10, and 10-13 have higher serum IGF-I than B6 at both 8 and 16 wk, whereas 10-1, 10-2, and 10-5 do not. The most obvious candidate gene carried within the sublines that show higher serum IGF-I is Igf1 itself (Fig. 4). However, three other genes are of interest: Socs2, which is an inhibitor of growth hormone signaling; Elk-3 (Ets domain protein), which is an activator of transcription in the IGF-I signaling pathway; and Kitl (Kit ligand), which can stimulate osteoclastogenesis. Finally, it is worth noting that QTL for two other phenotypes that may be regulated by IGF-I, vertebral BV/TV and whole-body growth, have also been mapped to this region (15, 24 ; and http://www.informatics.jax.org) (see Fig. 4). Thus, using sublines as mapping tools, we can narrow the list of candidate genes and subsequently determine by differential expression and sequence analysis the genetic determinants of IGF-I within this locus.
|
|
Discussion |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
Our data provide the first in vivo evidence of a functional relationship between allelic differences in IGF-I and bone acquisition. Unlike knockout or transgenic models, congenic mice exhibit only modest differences in circulating and skeletal IGF-I expression. Yet in many ways, the phenotypic feature of these mice parallel changes seen with targeted genomic strategies. For example, we recently showed that targeted overexpression of Igf1 to bone using a Col1A1 promoter led to greater bone remodeling and increased osteoclastogenesis in adult mice, a finding remarkably similar to the 10T and 10-4 sublines (6). Thus, congenics provide proof of concept that a QTL in the midregion of mouse Chr 10 has a significant effect on both IGF-I expression and bone acquisition.
Because congenics carry allelic differences in a single locus, these mice also reflect the genetic heterogeneity inherent in the human population, thus providing a platform for understanding how heritable differences in IGF-I impact the skeleton (26). For example, the syntenic chromosomal region for mouse Chr 10 in humans is Chr12 q 21–24, which has been reported to have a major femoral BMD QTL in at least three human cohorts (1, 3, 4, 27). And single-nucleotide polymorphisms in the Igf1 gene have recently been associated with both femoral BMD and circulating IGF-I in a large Chinese sibling cohort (28). Moreover, data from the current study are consistent with recent work identifying a QTL for BV/TV in B6C3F2 progeny in the same region as the Igf1s2 QTL (15). Two other groups have found BMD QTL in the midregion of Chr 10 in different inbred strain crosses, and Klein et al. identified a femoral moment of inertia QTL between B6 and DBA/2 in the same region (29, 30, 31). Hence, there appears to be a conserved locus on mouse Chr 10 in mice and humans that affects both IGF-I and skeletal acquisition.
The phenotypic features of our congenics raise several important and provocative questions. First, there was some degree of gender specificity for the skeletal phenotypes of the sublines. For example, serum IGF-I was increased in both males and female 10T, but differences in BV/TV and osteoclast number, compared with B6, were more pronounced in 10T females than 10T males. A similar pattern was noted in the nested congenic subline 10-4, i.e. females exhibited marked increases in trabecular number and connectivity density with increased serum IGF-I, compared with B6, whereas male 10-4 mice showed slightly greater trabecular bone mass but significantly increased serum IGF-I in relation to B6.
Gender specificity for skeletal phenotypes has been described previously (24). Both Klein et al. and our group (17, 31) have reported gender differences for peak bone mass and serum IGF-I among congenic strains. The mechanisms surrounding this feature of QTL analysis are not clear yet are probably more frequent than reported. It should be noted that we mapped QTL for BMD and IGF-I in the original B6C3F2 progeny by phenotyping female mice only (12). Hence, we do not know whether male F2 mice would have a QTL for serum IGF-I and BMD in the same mid region of Chr 10. But QTL studies in progress, particularly in lit/lit mice on a C3H and B6 background, may help define gender specificity and its relationship to the GH/IGF-I axis (32).
We still to need clarify the relationship between genes controlling circulating IGF-I and those influencing bone acquisition. The Chr 10 QTL region represents approximately 57 Mb of genomic DNA with hundreds of genes that could influence both IGF-I and bone mass. Conversely, the QTL may possess two sets of genes for these phenotypes: one that regulates IGF-I expression and one that independently contributes to bone acquisition. Only through extensive skeletal phenotyping of the congenic sublines (e.g. 10-7 or 10-10), which carry a much narrower genomic region than 10-4, will we be able to determine how many genes may be present in this region and which ones regulate circulating and/or skeletal IGF regulatory pathways. Those studies are currently ongoing.
Another surprising finding was the developmental differences in serum IGF-I in the nested congenic sublines, compared with B6 progenitors. Several years ago we noted that in B6 females, between 8 and 16 wk, there was a modest but statistically significant decline in circulating IGF-I, not seen in female C3H mice (11). As we developed congenic sublines including 10T, we noted that differences in serum IGF-I in these strains were considerably greater at 16 than 8 wk of age, compared with age-matched B6 controls (i.e. > 20%). This was due to a decline in serum IGF-I for B6 during the late phases of pubertal growth, whereas serum IGF-I in the sublines either did not change or increased. Similarly, the bone phenotype in the congenics was much more pronounced at 16 vs. 8 wk of age. These developmental differences resemble changes in female C3H mice during this period and strongly suggest that the genomic region of interest in mouse Chr 10 is temporally regulated. Whether this is due to enhanced growth hormone sensitivity, changes in sex steroids, or other factors remains to be determined.
With respect to identifying where the alleles are positioned that result in the IGF-I and skeletal phenotypes of the congenics, there are several plausible candidate genes. The 10T, 10-4, 10-7, 10-10, and 10-13 congenic mice all carry Igf1 alleles from C3H, making it conceivable that a cis-acting polymorphism within that gene, or an enhancer adjacent to the Igf1 gene, could affect transcriptional activity. We previously found a polymorphic sequence in the P1 promoter B6 in the Igf1 gene that differed between C3H and that may be responsible for increased transcription (14). We recently also identified a single nucleotide polymorphism in the 3'-untranslated region of the Igf1 gene between B6 and C3H that might affect message stability and is currently being investigated. Interestingly, two other groups (33, 34) have also found QTL in the same Chr 10 region for Igfbp-5 and Igfbp-3, two IGFBPs that are regulated by IGF-I. Moreover, association studies in humans have reported polymorphisms in the IGF-I gene and serum levels of the protein (1).
Another candidate gene in this QTL is Socs-2, suppressor of cytokine signaling. It inhibits GH-mediated induction of IGF-I and knockout of this gene results in gigantism with increased body weight and greater femoral length (35). However, several lines of evidence suggest that Socs-2 is not a candidate gene. First, before narrowing the sublines, we measured hepatic Socs-2 expression by RPA in 10T and found slightly higher levels in 10T vs. B6 (Adamo, M. L., personal communication). This would not be consistent with either the skeletal or IGF-I phenotype of 10T or 10-4. Second, the 10-7 subline does not carry the Socs-2 gene yet has higher serum IGF-I than B6. Thus, we suspect Socs-2 is not the gene driving the skeletal or IGF-I phenotypes in the 10 sublines. But the proximity of this gene to Igf1 demonstrates how structural aspects of the mammalian genome align nicely with functional components.
We can eliminate another candidate, Kitl, a protein that can stimulate osteoclast recruitment because it is outside the locus identified from our new sublines (see Fig. 2). On the other hand, Elk-3, a transcription factor, is located closer to the Igf1 gene and currently cannot be excluded. There is one report (36) of a targeted mutation in Elk-3 that is associated with early lethality due to vascular defects and chylothorax. Although this gene is an unlikely candidate, it is conceivable that the Igf1s2 QTL could carry a gene such as Elk-3, which targets expression of early genes in development, and thereby affects peak bone mass by up-regulating IGF-I transcription (36). Indeed, our group recently reported a strong QTL for areal BMD in the same region on Chr 10(i.e. 
60 cM) in F2 female mice from a B6 x 129 cross (29). This would imply conservation across inbred strains of a gene or genes that affect peak bone acquisition.
Finally, with respect to the skeletal phenotype, several lines of evidence suggest there may be increased osteoclastic activity in the 10T and 10-4 congenic female mice. We found that 16-wk 10T females had higher BV/TV than B6; this was associated with greater numbers of osteoclasts and more trabeculae. The 10-4 subline also had more trabeculae, greater bone volume, lower SMI, and increased trabecular connectivity, compared with B6. These findings would be consistent with a high bone turnover state, particularly by 16 wk of age, a time when there are major differences in circulating IGF-I, compared with the progenitors. Whether these findings are related to increased skeletal IGF-I expression in all the major sublines is not clear yet. Nevertheless, IGF-I can stimulate bone remodeling and particularly bone resorption when administered exogenously to mice and rats or when overexpressed in bone (37). The mechanism for IGF-I induction of osteoclastogenesis is not well delineated, but IGF-I can stimulate receptor activator of nuclear factor-
B ligand expression in ST-2 cells (25). Further studies will more clearly define the skeletal phenotype in the nested congenic sublines and the role of IGF-I in bone remodeling.
In summary, we generated nested congenic mice that have increased circulating IGF-I and a pronounced skeletal phenotype. These sublines are important model systems not only for dissecting the relationship between allelic differences in local/systemic IGF-I and bone acquisition but also for identifying heritable determinants of IGF-I expression.
|
Footnotes |
|---|
Disclosure: K.M.D., K.L.S., G.A.G., B.K.-J., M.L.A., L.R.D., C.A.-B., W.G.B., C.J.R., and L.G.H. have nothing to declare. J.L. has received consulting fees of less than $10,000 and has equity ownership/stock options of less than $10,000; and B.E.K. has disclosed receipt of lecture fees of less than $10,000.
First Published Online May 4, 2006
Abbreviations: BMC, Bone mineral content; BMD, bone mineral density; BV/TV, trabecular bone volume fraction; Chr, chromosome; DXA, dual-energy x-ray absorptiometry; IGFBP, IGF binding protein; microCT, micro-computed tomography; pQCT, peripheral quantitative computed tomography; QTL, quantitative trait loci; RPA, RNase protection assay; SMI, structure model index; vBMD, volumetric femoral BMD.
Received March 3, 2006.
Accepted for publication April 24, 2006.
|
References |
|---|
|
TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
|---|
B ligand in vitro and OPG in vivo. J Clin Endocrinol Metab 87:4273–4279This article has been cited by other articles:
 |
|
 |
|
  V. E. DeMambro, D. R. Clemmons, L. G. Horton, M. L. Bouxsein, T. L. Wood, W. G. Beamer, E. Canalis, and C. J. Rosen Gender-Specific Changes in Bone Turnover and Skeletal Architecture in Igfbp-2-Null Mice Endocrinology, May 1, 2008; 149(5): 2051 - 2061. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 C. K. Haston, W. Li, A. Li, M. Lafleur, and J. E. Henderson Persistent Osteopenia in Adult Cystic Fibrosis Transmembrane Conductance Regulator-deficient Mice Am. J. Respir. Crit. Care Med., February 1, 2008; 177(3): 309 - 315. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 J. Leger, I. Mercat, C. Alberti, D. Chevenne, P. Armoogum, J. Tichet, and P. Czernichow The relationship between the GH/IGF-I axis and serum markers of bone turnover metabolism in healthy children Eur. J. Endocrinol., November 1, 2007; 157(5): 685 - 692. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
B. Lecka-Czernik, C. Ackert-Bicknell, M. L. Adamo, V. Marmolejos, G. A. Churchill, K. R. Shockley, I. R. Reid, A. Grey, and C. J. Rosen Activation of Peroxisome Proliferator-Activated Receptor {gamma} (PPAR{gamma}) by Rosiglitazone Suppresses Components of the Insulin-Like Growth Factor Regulatory System in Vitro and in Vivo Endocrinology, February 1, 2007; 148(2): 903 - 911. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| Endocrinology | Endocrine Reviews | J. Clin. End. & Metab. |
| Molecular Endocrinology | Recent Prog. Horm. Res. | All Endocrine Journals |
