The primary goal of this study was to determine whether the IGF1R in mature osteoblasts and osteocytes was required for the catabolic actions of continuous parathyroid hormone (cPTH). Igf1r was deleted from male and female FVN/B mice by breeding with mice expressing cre recombinase under control of the osteocalcin promoter ((0CN) Igfr1(-/-) ). Littermates lacking the cre recombinase served as controls. PTH, 60 μg/kg/d, was administered continuously by Alzet minipumps for 4 weeks. Blood was obtained for indices of calcium metabolism. The femurs were examined by micro-computed tomography for structure, immunohistochemistry for IGF1R expression, histomorphometry for bone formation rates (BFR), mRNA levels by qPCR, and bone marrow stromal cell cultures (BMSC) for alkaline phosphatase activity (ALP(+) ), mineralization, and osteoblast-induced osteoclastogenesis. Whereas cPTH led to a reduction in trabecular bone volume/tissue volume (BV/TV) and cortical thickness in the control females, no change was found in the control males. Although trabecular BV/TV and cortical thickness were reduced in the (0CN) Igfr1(-/-) mice of both sexes, no further reduction after cPTH was found in the females, unlike the reduction in males. BFR was stimulated by cPTH in the controls but blocked by Igf1r deletion in the females. The (0CN) Igfr1(-/-) male mice showed a partial response. ALP(+) and mineralized colony formation were higher in BMSC from control males than from control females. These markers were increased by cPTH in both sexes, but BMSC from male (0CN) Igfr1(-/-) also were increased by cPTH, unlike those from female (0CN) Igfr1(-/-) . cPTH stimulated receptor activator of NF-κB ligand (RANKL) and decreased osteoprotegerin and alkaline phosphatase expression more in control female bone than in control male bone. Deletion of Igf1r blocked these effects of cPTH in the female but not in the male. However, PTH stimulation of osteoblast-driven osteoclastogenesis was blocked by deleting Igfr1 in both sexes. We conclude that cPTH is catabolic in female but not male mice. Moreover, IGF1 signaling plays a greater role in the skeletal actions of cPTH in the female mouse than in the male mouse, which may underlie the sex differences in the response to cPTH.

Skeletal loading and unloading has a pronounced impact on bone remodeling, a process also regulated by insulin-like growth factor 1 (IGF-1) signaling. Skeletal unloading leads to resistance to the anabolic effect of IGF-1, while reloading after unloading restores responsiveness to IGF-1. However, a direct study of the importance of IGF-1 signaling in the skeletal response to mechanical loading remains to be tested. In this study, we assessed the skeletal response of osteoblast-specific Igf-1 receptor deficient (Igf-1r^-/- ) mice to unloading and reloading. The mice were hindlimb unloaded for 14 days and then reloaded for 16 days. Igf-1r^-/- mice displayed smaller cortical bone and diminished periosteal and endosteal bone formation at baseline. Periosteal and endosteal bone formation decreased with unloading in Igf-1r^+/+ mice. However, the recovery of periosteal bone formation with reloading was completely inhibited in Igf-1r^-/- mice, although reloading-induced endosteal bone formation was not hampered. These changes in bone formation resulted in the abolishment of the expected increase in total cross-sectional area with reloading in Igf-1r^-/- mice compared to the control mice. These results suggest that the Igf-1r in mature osteoblasts has a critical role in periosteal bone formation in the skeletal response to mechanical loading.

Mouse models suggest that undercarboxylated osteocalcin (ucOC), produced by the skeleton, protects against type 2 diabetes development, whereas human studies have been inconclusive. We aimed to determine if ucOC or total OC is associated with incident type 2 diabetes or changes in fasting glucose, insulin resistance (HOMA-IR), or beta-cell function (HOMA-Beta). A subcohort (n = 338; 50% women; 36% black) was identified from participants without diabetes at baseline in the Health, Aging, and Body Composition Study. Cases of incident type 2 diabetes (n = 137) were defined as self-report at an annual follow-up visit, use of diabetes medication, or elevated fasting glucose during 8 years of follow-up. ucOC and total OC were measured in baseline serum. Using a case-cohort design, the association between biomarkers and incident type 2 diabetes was assessed using robust weighted Cox regression. In the subcohort, linear regression models analyzed the associations between biomarkers and changes in fasting glucose, HOMA-IR, and HOMA-Beta over 9 years. Higher levels of ucOC were not statistically associated with increased risk of incident type 2 diabetes (adjusted hazard ratio = 1.06 [95% confidence interval, 0.84-1.34] per 1 standard deviation [SD] increase in ucOC). Results for %ucOC and total OC were similar. Adjusted associations of ucOC, %ucOC, and total OC with changes in fasting glucose, HOMA-IR, and HOMA-Beta were modest and not statistically significant. We did not find evidence of an association of baseline undercarboxylated or total osteocalcin with risk of incident type 2 diabetes or with changes in glucose metabolism in older adults. © 2022 American Society for Bone and Mineral Research (ASBMR).

Insulin-like growth factors (IGFs) are important local regulators during fracture healing. Although IGF1 deficiency is known to increase the risk of delayed union or non-union fractures in the elderly population, the underlying mechanisms that contribute to this defect remains unclear. In this study, IGF1 signaling during fracture healing was investigated in an osteoblast-specific IGF1 receptor (IGF1R) conditional knockout (KO) mouse model. A closed tibial fracture was induced in IGF1R(flox/flox) /2.3-kb α1(1)-collagen-Cre (KO) and IGF1R(flox/flox) (control) mice aged 12 weeks. Fracture callus samples and nonfractured tibial diaphysis were collected and analyzed by μCT, histology, immunohistochemistry, histomorphometry, and gene expression analysis at 10, 15, 21, and 28 days after fracture. A smaller size callus, lower bone volume accompanied by a defect in mineralization, bone microarchitectural abnormalities, and a higher cartilage volume were observed in the callus of these KO mice. The levels of osteoblast differentiation markers (osteocalcin, alkaline phosphatase, collagen 1α1) were significantly reduced, but the early osteoblast transcription factor runx2, as well as chondrocyte differentiation markers (collagen 2α1 and collagen 10α1) were significantly increased in the KO callus. Moreover, increased numbers of osteoclasts and impaired angiogenesis were observed during the first 15 days of fracture repair, but decreased numbers of osteoclasts were found in the later stages of fracture repair in the KO mice. Although baseline nonfractured tibias of KO mice had decreased trabecular and cortical bone compared to control mice, subsequent studies with mice expressing the 2.3-kb α1(1)-collagen-Cre ERT2 construct and given tamoxifen at the time of fracture and so starting with comparable bone levels showed similar impairment in fracture repair at least initially. Our data indicate that not only is the IGF1R in osteoblasts involved in osteoblast differentiation during fracture repair, but it plays an important role in coordinating chondrocyte, osteoclast, and endothelial responses that all contribute to the endochondral bone formation required for normal fracture repair.

In lactating mothers, the high calcium (Ca²⁺) demand for milk production triggers significant bone loss¹. Although oestrogen normally counteracts excessive bone resorption by promoting bone formation, this sex steroid drops precipitously during this postpartum period. Here we report that brain-derived cellular communication network factor 3 (CCN3) secreted from KISS1 neurons of the arcuate nucleus (ARC^KISS1) fills this void and functions as a potent osteoanabolic factor to build bone in lactating females. We began by showing that our previously reported female-specific, dense bone phenotype² originates from a humoral factor that promotes bone mass and acts on skeletal stem cells to increase their frequency and osteochondrogenic potential. This circulatory factor was then identified as CCN3, a brain-derived hormone from ARC^KISS1 neurons that is able to stimulate mouse and human skeletal stem cell activity, increase bone remodelling and accelerate fracture repair in young and old mice of both sexes. The role of CCN3 in normal female physiology was revealed after detecting a burst of CCN3 expression in ARC^KISS1 neurons coincident with lactation. After reducing CCN3 in ARC^KISS1 neurons, lactating mothers lost bone and failed to sustain their progeny when challenged with a low-calcium diet. Our findings establish CCN3 as a potentially new therapeutic osteoanabolic hormone for both sexes and define a new maternal brain hormone for ensuring species survival in mammals.