Insulin-Like Growth Factor-I Is Essential for Embryonic Bone Development (original) (raw)
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Endocrinology, 2007
IGF-I acts through endocrine and local, autocrine/paracrine routes. Disruption of both endocrine and local IGF-I action leads to neonatal lethality and impaired growth in various tissues including bone; however, the severity of growth and skeletal phenotype caused by disruption of endocrine IGF-I action is far less than with total IGF-I disruption. Based on these data and the fact that bone cells express IGF-I in high abundance, we and others predicted that locally produced IGF-I is also critical in regulating growth and bone accretion. To determine the role of local IGF-I, type 1␣2 collagen-Cre mice were crossed with IGF-I loxP mice to generate Cre؉ (conditional mutant) and Cre؊ (control) loxP homozygous mice. Surprisingly, approximately 40-50% of the conditional mutants died at birth, which is similar to total IGF-I disruption, but not observed in mice lacking circulating IGF-I. Expression of IGF-I in bone and muscle but not liver and brain was significantly decreased in the conditional mutant. Accordingly, circulating levels of serum IGF-I were also not affected. Disruption of local IGF-I dramatically reduced body weight 28-37%, femur areal bone mineral density 10-25%, and femur bone size 18-24% in growing mice. In addition, mineralization was reduced as early as during embryonic development. Consistently, histomorphometric analysis determined impaired osteoblast function as demonstrated by reduced mineral apposition rate (14-30%) and bone formation rate (35-57%). In conclusion, both local and endocrine IGF-I actions are involved in regulating growth of various tissues including bone, but they act via different mechanisms. (Endocrinology 148: 5706-5715, 2007
Journal of Bone and Mineral Research, 2011
Systemic derangements and perinatal death of generalized insulin-like growth factor 1 (IGF-1) and IGF-1 receptor (IGF-1R) knockout mice preclude definitive assessment of IGF-1R actions in growth-plate (GP) chondrocytes. We generated cartilage-specific Igf1r knockout (Cart Igf1r −/−) mice to investigate local control of chondrocyte differentiation in the GP by this receptor. These mice died shortly after birth and showed disorganized chondrocyte columns, delayed ossification and vascular invasion, decreased cell proliferation, increased apoptosis, and increased expression of parathyroid hormone-related protein (Pthrp) RNA and protein in their GPs. The increased Pthrp expression in the knockout GPs likely was due to an increase in gene transcription, as determined by the increased activity of a LacZ reporter that was inserted downstream of the endogenous PTHrP promoter and bred into the knockout mice. To circumvent the early death of Cart Igf1r −/− mice and investigate the role of IGF-1R during postnatal growth, we made tamoxifen (Tam)inducible, cartilage-specific Igf1r knockout (TamCart Igf1r −/−) mice. At 2 weeks of age and 7 to 8 days after Tam injection, the TamCart Igf1r −/− mice showed growth retardation with a disorganized GP, reduced chondrocyte proliferation, decreased type 2 collagen and Indian Hedgehog (Ihh) expression, but increased expression of PTHrP. Consistent with in vivo observations, in vitro knockout of the Igf1r gene by adenoviral expression of Cre recombinase suppressed cell proliferation, promoted apoptosis, and increased Pthrp expression. Our data indicate that the IGF-1R in chondrocytes controls cell growth, survival, and differentiation in embryonic and postnatal GPs in part by suppression of Pthrp expression.
Journal of bone metabolism, 2014
The osteocyte has long been considered to be the primary mechanosensory cell in the bone. Recent evidence has emerged that the osteocyte is also a key regulator of various bone and mineral metabolism and that its regulatory effects are in part mediated through locally produced osteocyte-derived factors, such as sclerostin, receptor activator of nuclear factor-kappa B ligand (RANKL), and fibroblast growth factor (FGF)-23. Osteocytes secrete large amounts of insulin-like growth factor (IGF)-I in bone. Although IGF-I produced locally by other bone cells, such as osteoblasts and chondrocytes, has been shown to play important regulatory roles in bone turnover and developmental bone growth, the functional role of osteocyte-derived IGF-I in the bone and mineral metabolism has not been investigated and remains unclear. However, results of recent studies in osteocyte Igf1 conditional knockout transgenic mice have suggested potential regulatory roles of osteocyte-derived IGF-I in various aspe...
Journal of Bone and Mineral Research, 2015
To investigate the role of IGF-I signaling in osterix (OSX)-expressing cells in the skeleton, we generated IGF-I receptor (IGF-IR) knockout mice (OSX IGF-IRKO) (floxed-IGF-IR mice  OSX promoter-driven GFP-labeled cre-recombinase [ OSX GFPcre]), and monitored postnatal bone development. At day 2 after birth (P2), OSX GFP-cre was highly expressed in the osteoblasts in the bone surface of the metaphysis and in the prehypertrophic chondrocytes (PHCs) and inner layer of perichondral cells (IPCs). From P7, OSX GFP-cre was highly expressed in PHCs, IPCs, cartilage canals (CCs), and osteoblasts (OBs) in the epiphyseal secondary ossification center (SOC), but was only slightly expressed in the OBs in the metaphysis. Compared with the control mice, the IPC proliferation was decreased in the OSX IGF-IRKOs. In these mice, fewer IPCs invaded into the cartilage, resulting in delayed formation of the CC and SOC. Immunohistochemistry indicated a reduction of vessel number and lower expression of VEGF and ephrin B2 in the IPCs and SOC of OSX IGF-IRKOs. Quantitative real-time PCR revealed that the mRNA levels of the matrix degradation markers, MMP-9, 13 and 14, were decreased in the OSX IGF-IRKOs compared with the controls. The OSX IGF-IRKO also showed irregular morphology of the growth plate and less trabecular bone in the tibia and femur from P7 to 7 weeks, accompanied by decreased chondrocyte proliferation, altered chondrocyte differentiation, and decreased osteoblast differentiation. Our data indicate that during postnatal bone development, IGF-I signaling in OSX-expressing IPCs promotes IPC proliferation and cartilage matrix degradation and increases ephrin B2 production to stimulate vascular endothelial growth factor (VEGF) expression and vascularization. These processes are required for normal CC formation in the establishment of the SOC. Moreover, IGF-I signaling in the OSX-expressing PHC is required for growth plate maturation and osteoblast differentiation in the development of the metaphysis.
Physiological Genomics, 2007
It is well established that insulin-like growth factor (IGF)-I is critical for the regulation of peak bone mineral density (BMD) and bone width. However, the role of systemic vs. local IGF-I is not well understood. To determine the role local IGF-I plays in regulating BMD and bone width, we crossed IGF-I flox/flox mice with procollagen, typeIIαI-Cre mice to generate conditional mutants in which chondrocyte-derived IGF-I was disrupted. Bone parameters were measured by dual X-ray absorptiometry at 2, 4, 8, and 12 wk of age and peripheral quantitative computed tomography at 12 wk of age. Body length, areal BMD, and bone mineral content (BMC) were reduced ( P < 0.05) between 4 and 12 wk in the conditional mutant mice. Bone width was reduced 7% in the vertebrae and femur ( P < 0.05) of conditional mutant mice at 12 wk. Gains in body length and total body BMC and BMD were reduced by 27, 22, and 18%, respectively ( P < 0.05) in conditional mutant mice between 2 and 4 wk of age. Ex...
Bone, 2004
Targeted gene studies have demonstrated the importance of insulin-like growth factor-I (IGF-I) for osteoblast (OB) differentiation and the acquisition of peak bone mineral density (BMD). The skeletal response to allelic differences in IGF-I expression can also be measured in vivo, using congenic mice. We created a congenic strain with reduced (approximately 20%) circulating IGF-I (C3H.B6-6T [6T]) by backcrossing a small genomic region (30 cM) of Chromosome 6 (Chr6) from C3H/HeJ (C3H) onto a C57Bl/6J (B6) background. 6T female mice have lower serum IGF-I (P b 0.001 vs. B6) but similar growth hormone (GH) and serum IGF binding protein (IGFBP) concentrations as B6. At 16 weeks of age, congenics have greater body fat (P b 0.02 vs. B6) despite less total body weight, and exhibit smaller femoral cross-sectional size (P = 0.001), reduced cortical thickness (P b 0.001) and lower trabecular BV/TV (P b 0.05) than B6. 6T mice also have suppressed serum leptin (P b 0.01), but compared to B6 have similar markers of bone resorption (i.e., urine CTx and serum TRAP 5B). At 8 weeks of age, skeletal IGF-I mRNA from long bones was reduced by 40% (P b 0.05) as were liver mRNA transcripts (i.e., 50%, P b 0.01). Osteoblast progenitors from the bone marrow of 6T mice formed less colony forming unit fibroblasts by crystal violet staining than B6 (P b 0.007) and had significantly reduced alkaline phosphatase-positive colonies than B6(P b 0.0001). In addition, staining of bone marrow with oil red O revealed greater numbers of adipocytes in 6T than B6. Several candidate genes in the Chr6 QTL were excluded by lack of strain-related expression differences in bone, but genes positively regulating adipocyte differentiation including Alox 5 and PPAR-g require further study as either bpathwayQ or candidate genes. In summary, allelic differences in a QTL on Chr6 result in altered IGF-I gene expression, changes in OB lineage allocation, and reduced peak bone mass. Congenic mice are useful models not only for mapping genes related to bone mass but also for elucidating the biology underlying various skeletal phenotypes associated with more subtle manipulation of the mouse genome. D
Bone, 1997
Recent work has demonstrated differences in femoral bone mineral density between two common inbred strains of mice, C3H/HeJ (C3H) and C57BL/6J (B6), across a wide age range. To investigate one possible mechanism that could affect acquisition and maintenance of bone mass in mice, we studied circulatory and skeletal insulin-like growth factor-I (IGF-I) and femoral bone mineral density (F-BMD) by pQCT in C3H and ]36 progenitor strains, as well as serum IGF-I obtained from matings between these two strains and mice bred from subsequent F I intercrosses (F2). Serum IGF-I measured by radioimmunoassay was more than 35% higher in virgin progenitor C3H than virgin B6 at I, 4, 8, and I0 months of age, and iin 8-month-old C3H compared with B6 retired breeders (p < 0.001). In the progenitors, there was also a strong correlation between serum IGF-I and serum alkaline phosphatase (r-0.51,p = 0.001). In the 4 month F 1 females IGF-I levels and F-BMD were intermediate between C3H and B6 progenitors. In contrast, groups of F 2 mice with the highest or lowest BMD also had the highest or lowest serum IGF-I (p = 0.0001). IGF-I accounted for >35% of the variance in F-BMD among the F 2 mice. Conditioned media from newborn C3H calvarial cultures had higher concentrations of IGF-I than media from B6 cultures, and cell layer extracts from C3H calvariae exhibited greater alkaline phosphatase activity than cultures from B6 calvarial cells (p < 0.0001). The skeletal content of IGF-I in C3H tibiae, femorae, and calvariae (6-14 weeks of age) was also significantly higher than IGF-I content in the same bones of the B6 mice (p < 0.05). These data suggest that a possible mechanism for the difference in acquisition and maintenance of bone mass between these two inbred strains is related to systemic and skeletal IGF-I synthesis.
IGF-1 and bone: New discoveries from mouse models
Journal of Bone and Mineral Research, 2010
Insulin-like growth factor-1 (IGF-1) plays a central role in cellular growth, differentiation, survival, and cell cycle progression. It is expressed early during development and its effects are mediated through binding to a tyrosine kinase receptor, the insulin-like growth factor-1 receptor (IGF-1R). In the circulation, the IGFs bind to IGF-binding proteins (IGFBPs), which determine their bioavailability and regulate the interaction between the IGFs and IGF-1R. Studies in animal models and in humans have established critical roles for IGFs in skeletal growth and development. In this review we present new and old findings from mouse models of the IGF system and discuss their clinical relevance to normal and pathological skeletal physiology. ß