Estrogen receptor-β regulates mechanical signaling in primary osteoblasts.

Mechanical loading is an important regulator in skeletal growth, maintenance, and aging. Estrogen receptors have a regulatory role in mechanically induced bone adaptation. Estrogen receptor-α (ERα) is known to enhance load-induced bone formation, whereas ERβ negatively regulates this process.

Regulatory mechanisms in bone following mechanical loading.

Bone responds with increased bone formation to mechanical loading, and the time course of bone formation after initiating mechanical loading is well characterized. However, the regulatory activities governing the loading-dependent changes in gene expression are not well understood. The goal of this study was to identify the time-dependent regulatory mechanisms that governed mechanical loading-induced gene expression in bone using a predictive bioinformatics algorithm.

Sost downregulation and local Wnt signaling are required for the osteogenic response to mechanical loading.

Sclerostin, the Wnt signaling antagonist encoded by the Sost gene, is secreted by osteocytes and inhibits bone formation by osteoblasts. Mechanical stimulation reduces sclerostin expression, suggesting that osteocytes might coordinate the osteogenic response to mechanical force by locally unleashing Wnt signaling. To investigate whether sclerostin downregulation is a pre-requisite for load-induced bone formation, we conducted experiments in transgenic mice (TG) engineered to maintain high levels of SOST expression during mechanical loading.

High-bone-mass-producing mutations in the Wnt signaling pathway result in distinct skeletal phenotypes.

Mutations among genes that participate in the canonical Wnt signaling pathway can lead to drastically different skeletal phenotypes, ranging from severe osteoporosis to severe osteosclerosis. Many high-bone-mass (HBM) causing mutations that occur in the LRP5 gene appear to impart the HBM phenotype, in part, by increasing resistance to soluble Wnt signaling inhibitors, including sclerostin. Sost loss-of-function mutant mice (Sost knock-out) and Lrp5 gain-of-function mutant mice (Lrp5 HBM knock-in) have high bone mass.

Anabolic and catabolic regimens of human parathyroid hormone 1-34 elicit bone- and envelope-specific attenuation of skeletal effects in Sost-deficient mice.

PTH is a potent calcium-regulating factor that has skeletal anabolic effects when administered intermittently or catabolic effects when maintained at consistently high levels. Bone cells express PTH receptors, but the cellular responses to PTH in bone are incompletely understood. Wnt signaling has recently been implicated in the osteo-anabolic response to the hormone.

Heterogeneous stock rat: a unique animal model for mapping genes influencing bone fragility.

Previously, we demonstrated that skeletal mass, structure and biomechanical properties vary considerably among 11 different inbred rat strains. Subsequently, we performed quantitative trait loci (QTL) analysis in four inbred rat strains (F344, LEW, COP and DA) for different bone phenotypes and identified several candidate genes influencing various bone traits. The standard approach to narrowing QTL intervals down to a few candidate genes typically employs the generation of congenic lines, which is time consuming and often not successful.

Non-overlapping functions for Pyk2 and FAK in osteoblasts during fluid shear stress-induced mechanotransduction.

Mechanotransduction, the process by which cells convert external mechanical stimuli such as fluid shear stress (FSS) into biochemical changes, plays a critical role in maintenance of the skeleton. We have proposed that mechanical stimulation by FSS across the surfaces of bone cells results in formation of unique signaling complexes called mechanosomes that are launched from sites of adhesion with the extracellular matrix and with other bone cells [1]. Deformation of adhesion complexes at the cell membrane ultimately results in alteration of target gene expression.

Alternative splicing in bone following mechanical loading.

It is estimated that more than 90% of human genes express multiple mRNA transcripts due to alternative splicing. Consequently, the proteins produced by different splice variants will likely have different functions and expression levels. Several genes with splice variants are known in bone, with functions that affect osteoblast function and bone formation.

A longitudinal study of the effect of genistein on bone in two different murine models of diminished estrogen-producing capacity.

This experiment was designed to assess the capacity of dietary genistein (GEN), to attenuate bone loss in ovariectomized (OVX) and ovary-intact VCD-treated mice. Pretreatment of mice with 4-vinylcyclohexene diepoxide (VCD) gradually and selectively destroys ovarian follicles whilst leaving ovarian androgen-producing cells largely intact. VCD induces a perimenopause-like condition prior to the onset of reproductive acyclicity.

Gene expression patterns in bone following mechanical loading.

The advent of high-throughput measurements of gene expression and bioinformatics analysis methods offers new ways to study gene expression patterns. The primary goal of this study was to determine the time sequence for gene expression in a bone subjected to mechanical loading during key periods of the bone-formation process, including expression of matrix-related genes, the appearance of active osteoblasts, and bone desensitization. A standard model for bone loading was employed in which the right forelimb was loaded axially for 3 minutes per day, whereas the left forearm served as a nonloaded contralateral control.

Identification of genes influencing skeletal phenotypes in congenic P/NP rats.

We previously showed that alcohol-preferring (P) rats have higher bone density than alcohol-nonpreferring (NP) rats. Genetic mapping in P and NP rats identified a major quantitative trait locus (QTL) between 4q22 and 4q34 for alcohol preference. At the same location, several QTLs linked to bone density and structure were detected in Fischer 344 (F344) and Lewis (LEW) rats, suggesting that bone mass and strength genes might cosegregate with genes that regulate alcohol preference.

Psychotropic drugs have contrasting skeletal effects that are independent of their effects on physical activity levels.

Popular psychotropic drugs, like the antidepressant selective serotonin reuptake inhibitors (SSRIs) and tricyclic antidepressants (TCAs), and the mood stabilizer lithium, may have skeletal effects. In particular, preclinical observations suggest a direct negative effect of SSRIs on the skeleton. A potential caveat in studies of the skeletal effects of psychotropic drugs is the hypoactive (skeletal unloading) phenotype they induce.

Numerical modeling of long bone adaptation due to mechanical loading: correlation with experiments.

The process of external bone adaptation in cortical bone is modeled mathematically using finite element (FE) stress analysis coupled with an evolution model, in which adaptation response is triggered by mechanical stimulus represented by strain energy density. The model is applied to experiments in which a rat ulna is subjected to cyclic loading, and the results demonstrate the ability of the model to predict the bone adaptation response. The FE mesh is generated from micro-computed tomography (microCT) images of the rat ulna, and the stress analysis is carried out using boundary and loading conditions on the rat ulna obtained from the experiments [Robling, A.

Bone loss or lost bone: rationale and recommendations for the diagnosis and treatment of early postmenopausal bone loss.

Recent reports suggest that bone loss begins during late perimenopause at a dramatic rate, even before estrogen levels plummet. During the ensuing 5 years, there is evidence of the beginnings of microarchitectural deterioration, which impacts bone strength and ultimately enhances its propensity to fracture. The diagnosis of osteoporosis based on T-scores alone, or through stratification for a high fracture risk by FRAX, excludes these women who are rapidly losing bone.

Lengthening of mouse hindlimbs with joint loading.

For devising clinical approaches to treating limb length discrepancies, strategies that will generate differential longitudinal growth need to be improved. This report addresses the following question: does knee loading increase bone length of the loaded hindlimb? Knee loading has been shown to induce anabolic responses on the periosteal and endosteal surfaces, but its effects on longitudinal bone growth have not yet been examined. In the present studies, loads were applied to the left hindlimb (5-min bouts at 0.

Genes influencing spinal bone mineral density in inbred F344, LEW, COP, and DA rats.

Previously, we identified the regions of chromosomes 10q12-q31 and 15p16-q21 harbor quantitative trait loci (QTLs) for lumbar volumetric bone mineral density (vBMD) in female F2 rats derived from Fischer 344 (F344) x Lewis (LEW) and Copenhagen 2331 (COP) x Dark Agouti (DA) crosses. The purpose of this study is to identify the candidate genes within these QTL regions contributing to the variation in lumbar vBMD. RNA was extracted from bone tissue of F344, LEW, COP, and DA rats.

Mechanical signaling for bone modeling and remodeling.

Proper development of the skeleton in utero and during growth requires mechanical stimulation. Loading results in adaptive changes in bone that strengthen bone structure. Bone's adaptive response is regulated by the ability of resident bone cells to perceive and translate mechanical energy into a cascade of structural and biochemical changes within the cells a process known as mechanotransduction.

The emerging role of serotonin (5-hydroxytryptamine) in the skeleton and its mediation of the skeletal effects of low-density lipoprotein receptor-related protein 5 (LRP5).

Novel molecular pathways obligatory for bone health are being rapidly identified. One pathway recently revealed involves gut-derived 5-hydroxytryptamine (5-HT) mediation of the complete skeletal effects of low-density lipoprotein receptor-related protein 5 (LRP5). Mounting evidence supports 5-HT as an important regulatory compound in bone with previous evidence demonstrating that bone cells possess functional pathways for responding to 5-HT.

Differentially expressed genes strongly correlated with femur strength in rats.

The region of chromosome 1q33-q54 harbors quantitative trait loci (QTL) for femur strength in COPxDA and F344xLEW F2 rats. The purpose of this study is to identify the genes within this QTL region that contribute to the variation in femur strength. Microarray analysis was performed using RNA extracted from femurs of COP, DA, F344 and LEW rats.

Joint loading-driven bone formation and signaling pathways predicted from genome-wide expression profiles.

Joint loading is a recently developed loading modality that induces anabolic responses by lateral loads applied to a synovial joint such as an elbow and a knee. The present study extended this loading modality to an ankle and addressed a question: does ankle loading promote bone formation in the tibia? If so, what signaling pathways are involved in the anabolic responses? Using C57BL/6 female mice as a model system, lateral loads of 0.5 N were applied to the ankle at 5 Hz for 3 min/day for 3 consecutive days and load-driven bone formation was evaluated at three tibial cross-sections (the proximal, middle, and distal diaphysis).

Mechanobiology of the skeleton.

Mechanical loading of the skeleton is essential for the development, growth, and maintenance of strong, weight-bearing bones. Bone strength is plastic and can be modulated in adults, as illustrated by the increased bone mass in the playing arms of athletes as compared with their nonplaying arms. Our studies have shown that mechanical loading improves bone strength by inducing bone formation in regions of high strain energy.

P2X7 nucleotide receptor plays an important role in callus remodeling during fracture repair.

The P2X7 nucleotide receptor (P2X7R) is an ATP-gated ion channel expressed in bone cells. Homozygous null P2X7R (P2X7R(-/-)) mice have reduced bone formation, so we hypothesized that P2X7R(-/-) mice have impaired fracture healing compared to P2X7R(+/+) control mice. To test the hypothesis, adult P2X7R(-/-) mice and P2X7R(+/+) mice were studied.

Epistasis between QTLs for bone density variation in Copenhagen x dark agouti F2 rats.

The variation in several of the risk factors for osteoporotic fracture, including bone mineral density (BMD), has been shown to be strongly influenced by genetic differences. However, the genetic architecture of BMD is complex in both humans and in model organisms. We previously reported quantitative trait locus (QTL) results for BMD from a genome screen of 828 F2 progeny of Copenhagen and dark agouti rats.