Osteoblast-specific Knockout of the Insulin-like Growth Factor (IGF) Receptor Gene Reveals an Essential Role of IGF Signaling in Bone Matrix Mineralization

Abstract

To examine the local actions of IGF signaling in skeletal tissue in a physiological context, we have used Cremediated recombination to disrupt selectively in mouse osteoblasts the gene encoding the type 1 IGF receptor (Igf1r). Mice carrying this bone-specific mutation were of normal size and weight but, in comparison with normal siblings, demonstrated a striking decrease in cancellous bone volume, connectivity, and trabecular number, and an increase in trabecular spacing. These abnormalities correlated with a striking decrease in the rate of mineralization of osteoid that occurred despite an unexpected osteoblast and osteoclast hyperactivity, detected from the significant increments in both osteoblast and erosion surfaces. Our findings indicate that IGF1 is essential for coupling matrix biosynthesis to sustained mineralization. This action is likely to be particularly important during the pubertal growth spurt when rapid bone formation and consolidation are required.Body size and linear bone growth in mammals is affected by cellular signaling pathways controlled by growth factors and hormones (1). In this regard, a major growth-promoting signaling system consisting of the insulin-like growth factors (IGF, 1 IGF1 and IGF2) and the type 1 IGF receptor (IGF1R) regulates embryonic growth, as shown by gene knockout experiments in mice (1). IGF1 acting through IGF1R also plays central roles in postnatal growth either independently or by mediating growth hormone functions (2). Signaling through the IGF1R tyrosine kinase receptor not only promotes cell proliferation, but also mediates anti-apoptotic actions (3, 4). The IGF system includes a second receptor (IGF2R) devoid of signaling properties, but serving IGF2 turnover, and at least six IGF-binding proteins (IGFBPs) of obscure functional significance (single and also some double mouse mutations ablating IGFBPs have not revealed as yet significant consequences in growth impairment). 2 The IGFs are produced locally in various tissues, including bones, and exert autocrine/paracrine functions, but they are also present in serum, mostly associated with IGFBPs. Whether the circulating IGFs act systemically as hormones is currently controversial (5, 6).A number of in vitro and in vivo studies are progressively unraveling the significance of the IGF system for skeletal development and metabolic control (for a review see Ref. 7). IGF1, by stimulating the proliferation of chondrocytes in the growth plate, plays an essential role in longitudinal bone growth (2) and is also involved in the formation of trabecular bone. In fact, chondrocytes and bone cells produce IGFs and express IGF1R (see for example Refs. 8 and 9). Studies using osteoblast culture systems have shown that IGF1 stimulates osteoblast proliferation, accelerates their differentiation, and enhances bone matrix production (10, 11). In addition, IGF1 is being recognized as a critical factor for bone cell survival (12)(13)(14). Finally, IGF1 also appears to regulate bone resorption, either directly or through its actio...

The p38α MAPK positively regulates osteoblast function and postnatal bone acquisition

“…Another transcription factor, Atf4 regulates bone matrix deposition by mature osteoblasts [25]. Furthermore, several signaling pathways have been shown to regulate postnatal osteoblast function including Ihh [26], Wnt/Lrp5 [27], TGFb-BMP/Smads [28][29][30], BMP/MEKK2/MEK5-7/Jnk [31], b-adrenergic [32], IGF-1 [16], insulin [33] and PTH signaling [34]. The p38 MAPK pathway has been recently shown to be an essential regulator of osteoblastogenesis during skeletogenesis [12,13].…”

“…1a) [15] with mice expressing the Cre recombinase under the control of the osteocalcin promoter (Ocn-Cre) [16]. Ocn-Cre mice, in which Cre expression exclusively occurs in osteoblasts at birth [16], represent a unique tool to address postnatal osteoblast function. Mice homozygous for floxed p38a alleles (p38a f/f ) were crossed with Ocn-Cre mice to generate Ocn-Cre;p38a f/?…”

Promotion of Osteoblast Differentiation in Mesenchymal Cells Through Cbl-Mediated Control of STAT5 Activity

“…The actions of GH in osteoblasts are in part mediated by STAT5 activation which upregulates IGF-1 expression [51,59]. IGF-1 signaling in turn activates RUNX2 activity and promotes osteoblast differentiation in MSC [60][61][62]. Our finding that c-Cbl silencing increased IGF-1 expression in vitro and in vivo suggests that IGF-1 may functionally contribute to the observed induction of osteoblast differentiation.…”

Inactivation of the androgen receptor in bone-forming cells leads to trabecular bone loss in adult female mice

“…It is unlikely that the phenotypic differences can be attributed to the toxicity of the Cre itself, 35 as no bone phenotype has been demonstrated in this particular parental osteocalcin-Cre mouse used by us 22 or others. 36 No AR activity has been shown after the deletion of the exon 2 29 used in our mice, whereas the residual activity of the AR after exon 3 deletion complicates the analysis of other mice. The recombination efficacy measured as AR mRNA expression in the bone-forming cells of our mouse model was good (79% as determined in male mice), which is in line with the previous reports utilizing the osteocalcin-Cre construct.…”

“…The recombination efficacy measured as AR mRNA expression in the bone-forming cells of our mouse model was good (79% as determined in male mice), which is in line with the previous reports utilizing the osteocalcin-Cre construct. 36 The remaining In mouse models, the role of the AR in the regulation of cortical bone thickness and perimeter has been somewhat controversial. The expression of the AR has been reported to be higher in cortical bone osteocytes compared with those in the trabecular bone.…”