Mechanical Loading Synergistically Increases Trabecular Bone Volume and Improves Mechanical Properties in the Mouse when BMP Signaling Is Specifically Ablated in Osteoblasts

Preclinical Rodent Models for Human Bone Disease, Including a Focus on Cortical Bone

Preclinical models (typically ovariectomized rats and genetically altered mice) have underpinned much of what we know about skeletal biology. They have been pivotal for developing therapies for osteoporosis and monogenic skeletal conditions, including osteogenesis imperfecta, achondroplasia, hypophosphatasia, and craniodysplasias. Further therapeutic advances, particularly to improve cortical strength, require improved understanding and more rigorous use and reporting. We describe here how trabecular and cortical bone structure develop, are maintained, and degenerate with aging in mice, rats, and humans, and how cortical bone structure is changed in some preclinical models of endocrine conditions (eg, postmenopausal osteoporosis, chronic kidney disease, hyperparathyroidism, diabetes). We provide examples of preclinical models used to identify and test current therapies for osteoporosis, and discuss common concerns raised when comparing rodent preclinical models to the human skeleton. We focus especially on cortical bone, because it differs between small and larger mammals in its organizational structure. We discuss mechanisms common to mouse and human controlling cortical bone strength and structure, including recent examples revealing genetic contributors to cortical porosity and osteocyte network configurations during growth, maturity, and aging. We conclude with guidelines for clear reporting on mouse models with a goal for better consistency in the use and interpretation of these models.

Hyperthyroidism-driven bone loss depends on BMP receptor Bmpr1a expression in osteoblasts

Hyperthyroidism is a well-known trigger of high bone turnover that can lead to the development of secondary osteoporosis. Previously, we have shown that blocking bone morphogenetic protein (BMP) signaling systemically with BMPR1A-Fc can prevent bone loss in hyperthyroid mice. To distinguish between bone cell type-specific effects, conditional knockout mice lacking Bmpr1a in either osteoclast precursors (LysM-Cre) or osteoprogenitors (Osx-Cre) were rendered hyperthyroid and their bone microarchitecture, strength and turnover were analyzed. While hyperthyroidism in osteoclast precursor-specific Bmpr1a knockout mice accelerated bone resorption leading to bone loss just as in wildtype mice, osteoprogenitor-specific Bmpr1a deletion prevented an increase of bone resorption and thus osteoporosis with hyperthyroidism. In vitro, wildtype but not Bmpr1a-deficient osteoblasts responded to thyroid hormone (TH) treatment with increased differentiation and activity. Furthermore, we found an elevated Rankl/Opg ratio with TH excess in osteoblasts and bone tissue from wildtype mice, but not in Bmpr1a knockouts. In line, expression of osteoclast marker genes increased when osteoclasts were treated with supernatants from TH-stimulated wildtype osteoblasts, in contrast to Bmpr1a-deficient cells. In conclusion, we identified the osteoblastic BMP receptor BMPR1A as a main driver of osteoporosis in hyperthyroid mice promoting TH-induced osteoblast activity and potentially its coupling to high osteoclastic resorption.

Effects of Mechanical Stress Stimulation on Function and Expression Mechanism of Osteoblasts

Osteoclasts and osteoblasts play a major role in bone tissue homeostasis. The homeostasis and integrity of bone tissue are maintained by ensuring a balance between osteoclastic and osteogenic activities. The remodeling of bone tissue is a continuous ongoing process. Osteoclasts mainly play a role in bone resorption, whereas osteoblasts are mainly involved in bone remodeling processes, such as bone cell formation, mineralization, and secretion. These cell types balance and restrict each other to maintain bone tissue metabolism. Bone tissue is very sensitive to mechanical stress stimulation. Unloading and loading of mechanical stress are closely related to the differentiation and formation of osteoclasts and bone resorption function as well as the differentiation and formation of osteoblasts and bone formation function. Consequently, mechanical stress exerts an important influence on the bone microenvironment and bone metabolism. This review focuses on the effects of different forms of mechanical stress stimulation (including gravity, continuously compressive pressure, tensile strain, and fluid shear stress) on osteoclast and osteoblast function and expression mechanism. This article highlights the involvement of osteoclasts and osteoblasts in activating different mechanical transduction pathways and reports changings in their differentiation, formation, and functional mechanism induced by the application of different types of mechanical stress to bone tissue. This review could provide new ideas for further microscopic studies of bone health, disease, and tissue damage reconstruction.

Low Intensity Pulsed Ultrasound for Bone Tissue Engineering

As explained by Wolff's law and the mechanostat hypothesis, mechanical stimulation can be used to promote bone formation. Low intensity pulsed ultrasound (LIPUS) is a source of mechanical stimulation that can activate the integrin/phosphatidylinositol 3-OH kinase/Akt pathway and upregulate osteogenic proteins through the production of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2). This paper analyzes the results of in vitro and in vivo studies that have evaluated the effects of LIPUS on cell behavior within three-dimensional (3D) titanium, ceramic, and hydrogel scaffolds. We focus specifically on cell morphology and attachment, cell proliferation and viability, osteogenic differentiation, mineralization, bone volume, and osseointegration. As shown by upregulated levels of alkaline phosphatase and osteocalcin, increased mineral deposition, improved cell ingrowth, greater scaffold pore occupancy by bone tissue, and superior vascularization, LIPUS generally has a positive effect and promotes bone formation within engineered scaffolds. Additionally, LIPUS can have synergistic effects by producing the piezoelectric effect and enhancing the benefits of 3D hydrogel encapsulation, growth factor delivery, and scaffold modification. Additional research should be conducted to optimize the ultrasound parameters and evaluate the effects of LIPUS with other types of scaffold materials and cell types.

Exercise improves bone formation by upregulating the Wnt3a/β-catenin signalling pathway in type 2 diabetic mice

BACKGROUND

The bone formation ability of type 2 diabetes is inhibited, and exercise can effectively improve the bone formation of T2DM. However, whether exercise can mediate the Wnt3a/β-catenin pathway to improve the mechanism of bone formation and metabolism still needs further research.

METHODS

A T2DM mouse model was established by a high-fat diet and STZ injection, and the mice were trained with swimming and downhill running exercise. Alizarin red staining is used to observe the changes of the left femoral trabecular bone; micro-CT is used to analyze the trabecular and cortical BMD, BV/TV, BS/BV, BS/TV, Tb.Th, Tb.Sp; the ALP staining of skull was used to observe the changes in ALP activity of bone tissues at the skull herringbone sutures; ALP staining was performed to observe the changes in the number of OBs and ALP activity produced by differentiation; Quantitative PCR was used to detect mRNA expression; Western blot was used to detect protein expression levels.

RESULTS

When the Wnt3a/β-catenin pathway in the bones of T2DM mice was inhibited, the bone formation ability of the mice was significantly reduced, resulting in the degradation of the bone tissue morphology and structure. Swimming caused the significant increase in body weight and Runx2 mRNA expression, while downhill running could significantly decrease the body weight of the mice, while the tibia length, wet weight, and the trabecular morphological structure of the distal femur and the indexes of bone histomorphology were significantly improved by activating the Wnt3a/β-catenin pathway.

CONCLUSIONS

Bone formation is inhibited in T2DM mice, leading to osteoporosis. Downhill running activates the Wnt3a/β-catenin pathway in the bones of T2DM mice, promotes OB differentiation and osteogenic capacity, enhances bone formation metabolism, and improves the bone morphological structure.

Skeletal muscle regeneration with robotic actuation-mediated clearance of neutrophils

[Figure: see text].

Podoplanin is dispensable for mineralized tissue formation and maintenance in the Swiss outbred mouse background

Podoplanin, PDPN, is a mucin-type transmembrane glycoprotein widely expressed in many tissues, including lung, kidney, lymph nodes, and mineralized tissues. Its function is critical for lymphatic formation, differentiation of type I alveolar epithelial lung cells, and for bone response to biomechanical loading. It has previously been shown that Pdpn null mice die at birth due to respiratory failure emphasizing the importance of Pdpn in alveolar lung development. During the course of generation of Pdpn mutant mice, we found that most Pdpn null mice in the 129S6 and C57BL6/J mixed genetic background die at the perinatal stage, similar to previously published studies with Pdpn null mice, while all Pdpn null mice bred with Swiss outbred mice survived. Surviving mutant mice in the 129S6 and C57BL6/J mixed genetic background showed alterations in the osteocyte lacunocanalicular network, especially reduced osteocyte canaliculi in the tibial cortex with increased tibial trabecular bone. However, adult Pdpn null mice in the Swiss outbred background showed no overt differences in their osteocyte lacunocnalicular network, bone density, and no overt differences when challenged with exercise. Together, these data suggest that genetic variations present in the Swiss outbred mice compensate for the loss of function of PDPN in lung, kidney, and bone.

Axial mechanical loading to ex vivo mouse long bone regulates endochondral ossification and endosteal mineralization through activation of the BMP-Smad pathway during postnatal growth

Mechanical loading contributes to bone development, growth, and metabolism. However, the mechanisms underlying long bone mineralization via changes in loading during the growth period are unclear. The aim of the present study was to investigate the regulatory mechanisms underlying endochondral ossification and endosteal mineralization by developing an ex vivo organ culture model with cyclic axial mechanical loads. The metacarpal bones of 3-week-old C57BL/6 mice were exposed to mechanical loading (0, 7.8, and 78 mN) for 1 h/day for 4 days. Histomorphometry revealed that axial mechanical loading regulated the thickness of the calcified zone in the growth plate and endosteal mineralization in the diaphysis in a load-dependent manner. Mechanical loading also resulted in load-dependent upregulation of endochondral ossification and bone mineralization-related genes, including bone morphogenetic protein 2 (Bmp2). Recombinant human BMP-2 administration caused similar changes in tissue structures. Conversely, inhibition of the BMP-Smad pathway diminished the stimulatory effects of mechanical loading and BMP-2 administration, suggesting that the effects of mechanical loading may be exerted through activation of the BMP-Smad pathway with the results of gene ontology and pathway analyses. Mechanical loading increased alkaline phosphatase activity and decreased carbonic anhydrase IX (Car9) mRNA expression, resulting in a significant pH increase in the culture supernatant. We hypothesize that, through activation of the BMP-Smad pathway, mechanical loading downregulates Car9, which may alkalize the local milieu, thereby inducing bone formation and long bone mineralization. Our results showed that cyclic axial mechanical loading increased endochondral ossification and endosteal mineralization in developing mouse long bones, which may have resulted from changes in the pH, ALP activity, and Pi/PPi of the extracellular environment. These findings advance our understanding of the regulation of mineralization mechanisms by mechanical loading mediated through activation of the BMP-Smad pathway.

Treadmill exercise facilitated rotator cuff healing is coupled with regulating periphery neuropeptides expression in a murine model

Postoperative exercise has been found able to accelerate bone-tendon (B-T) healing. In this study, we systematically compared tendon-to-bone healing in mice subjected to postoperative treadmill exercise and free cage recovery in a murine rotator cuff repair model. Specifically, C57BL/6 mice underwent unilateral supraspinatus tendon (SST) detachment and repair were randomly allocated into treadmill group and control group. Treadmill group received daily treadmill running initiated from postoperative day 7 while the control group was allowed free cage activity. Mice were euthanized at postoperative 4 and 8 weeks for synchrotron radiation micro-computed tomography (SR-μCT), histology and biomechanical tests to investigate the effect of treadmill running on B-T healing. The results indicated that treadmill running initiated at day 7 postoperatively was able to accelerate B-T healing, as evidenced by better tendon-to-bone maturation and increased mechanical property. Recent studies show that peripheral neuropeptides are closely associated with musculoskeletal tissue repair. We furtherly conducted quantitative reverse transcription-polymerase chain reaction and immunofluorescence staining to investigate the temporal-spatial expression of calcitonin gene-related peptide (CGRP), substance P (SP), and peripheral neuropeptide Y (NPY) to verify whether they are related to rotator cuff healing. Our results show increased expression of CGRP, SP, and NPY at the healing site under the effect of mechanical stimulation. In conclusion, delayed postoperative exercise with moderate strength appears to accelerate the early phase of B-T healing, a process that may prove to be linked to increased expression of periphery neuropeptides known to play a role in tissue healing.

Medium-Intensity Treadmill Exercise Exerts Beneficial Effects on Bone Modeling Through Bone Marrow Mesenchymal Stromal Cells

As a type of multipotential cells, bone marrow mesenchymal stromal cells (BMMSCs) can differentiate into chondrocytes, osteoblasts, and adipocytes under different loading condition or specific microenvironment. Previous studies have shown that BMMSCs and their lineage-differentiated progeny (for example, osteoblasts), and osteocytes are mechanosensitive in bone. The appropriate physical activity and exercise could help attenuate bone loss, effectively stimulate bone formation, increase bone mineral density (BMD), prevent the progression of osteoporosis, and reduce the risk of bone fractures. Bone morphogenetic protein (BMP) is originally discovered as a protein with heterotopic bone-inducing activity in the bone matrix that exerts a critical role in multiple stages of bone metabolism. In the present study, the medium-intensity treadmill exercise enhanced bone formation and increased osteocalcin (OCN) and osteopontin (OPN) mRNA expression as well as activation of the BMP-Smad signaling pathway in vivo. In order to investigate the effect of a BMP-Smad signaling pathway, we injected mice with activated enzyme inhibitors (LDN-193189HCL) and subjected the mice to treadmill exercise intervention. LDN-193189HCL attenuated the BMD and bone mass mediated by medium-intensity exercise and BMP-Smad signaling pathway.

Four-Point Bending Testing for Mechanical Assessment of Mouse Bone Structural Properties

One of the primary functions of bone is to support the skeleton by withstanding load. In the diseased state, bone's ability to perform this function is altered. Quantification of the features of bone that support its functional behavior, and how they may change with disease, is accomplished through mechanical testing. As such, mechanical testing is a useful tool for scientists studying orthopedic-related diseases. Furthermore, a common animal model used to investigate disease and its treatment is the mouse. Therefore, in this chapter we (1) describe central concepts of mechanical testing, (2) describe factors that influence the mechanical behavior of bone, and (3) describe the application of a widely used mechanical testing technique, four-point bending, to the mouse bone for characterization of its structural properties.

Controversy of physiological vs. pharmacological effects of BMP signaling: Constitutive activation of BMP type IA receptor-dependent signaling in osteoblast lineage enhances bone formation and resorption, not affecting net bone mass

Bone morphogenetic proteins (BMPs) were first described over 50 years ago as potent inducers of ectopic bone formation when administrated subcutaneously. Preclinical studies have extensively examined the osteoinductive properties of BMPs in vitro and new bone formation in vivo. BMPs (BMP-2, BMP-7) have been used in orthopedics over 15 years. While osteogenic function of BMPs has been widely accepted, our previous studies demonstrated that loss-of-function of BMP receptor type IA (BMPR1A), a potent receptor for BMP-2, increased net bone mass by significantly inhibiting bone resorption in mice, indicating a positive role of BMP signaling in bone resorption. The physiological role of BMPs (i.e. osteogenic vs. osteoclastogenic) is still largely unknown. The purpose of this study was to investigate the physiological role of BMP signaling in endogenous long bones during adult stages. For this purpose, we conditionally and constitutively activated the Smad-dependent canonical BMP signaling thorough BMPR1A in osteoblast lineage cells using the mutant mice (Col1CreER™:caBmpr1a). Because trabecular bones were largely increased in the loss-of-function mouse study for BMPR1A, we hypothesized that the augmented BMP signaling would affect endogenous trabecular bones. In the mutant bones, the Smad phosphorylation was enhanced within physiological level three-fold while the resulting gross morphology, bodyweights, bone mass/shape/length, serum calcium/phosphorus levels, collagen cross-link patterns, and healing capability were all unchanged. Interestingly, we found; 1) increased expressions of both bone formation and resorption markers in femoral bones, 2) increased osteoblast and osteoclast numbers together with dynamic bone formation parameters by trabecular bone histomorphometry, 3) modest bone architectural phenotype with reduced bone quality (i.e. reduced trabecular bone connectivity, larger diametric size but reduced cortical bone thickness, and reduced bone mechanical strength), and 4) increased expression of SOST, a downstream target of the Smad-dependent BMPR1A signaling, in the mutant bones. This study is clinically insightful because gain-of-function of BMP signaling within a physiological window does not increase bone mass while it alters molecular and cellular aspects of osteoblast and osteoclast functions as predicted. These findings help explain the high-doses of BMPs (i.e. pharmacological level) in clinical settings required to substantially induce a bone formation, concurrent with potential unexpected side effects (i.e. bone resorption, inflammation) presumably due to a broader population of cell-types exposed to the high-dose BMPs rather than osteoblastic lineage cells.

Loss of BMP signaling mediated by BMPR1A in osteoblasts leads to differential bone phenotypes in mice depending on anatomical location of the bones

Bone morphogenetic protein (BMP) signaling in osteoblasts plays critical roles in skeletal development and bone homeostasis. Our previous studies showed loss of function of BMPR1A, one of the type 1 receptors for BMPs, in osteoblasts results in increased trabecular bone mass in long bones due to an imbalance between bone formation and bone resorption. Decreased bone resorption was associated with an increased mature-to-immature collagen cross-link ratio and mineral-matrix ratios in the trabecular compartments, and increased tissue-level biomechanical properties. Here, we investigated the bone mass, bone composition and biomechanical properties of ribs and spines in the same genetically altered mouse line to compare outcomes by loss of BMPR1A functions in bones from different anatomic sites and developmental origins. Bone mass was significantly increased in both cortical and trabecular compartments of ribs with minimal to modest changes in compositions. While tissue-levels of biomechanical properties were not changed between control and mutant animals, whole bone levels of biomechanical properties were significantly increased in association with increased bone mass in the mutant ribs. For spines, mutant bones showed increased bone mass in both cortical and trabecular compartments with an increase of mineral content. These results emphasize the differential role of BMP signaling in osteoblasts in bones depending on their anatomical locations, functional loading requirements and developmental origin.

Dynamic finite element analysis of implants for femoral neck fractures simulating walking

BACKGROUND

To examine postoperative complications for osteosynthesizing femoral neck fractures (Pauwels III), biomechanical analysis should be conducted under dynamic conditions simulating for walking, not static conditions. Among the two main aims of this study, one is to pioneer the technique of dynamic finite element (FE) analysis, and the other is to compare stress distribution between two implants during walking.

MATERIALS AND METHODS

First, we performed an inverse dynamic analysis with optimization method using a musculoskeletal model to calculate the inter-segmental and muscular forces during walking. Second, three FE models were prepared: (I) intact hip joint, (II) fractures treated with two Hansson pins (HP), and (III) fractures with Dual SC Screws (DSCS) maintaining an angular stability. The direction and magnitude of the loadings varied continuously. Stress distribution during the walking was evaluated by using a dynamic explicit method. We examined the time-dependent von Mises stresses at two representative spots: medial cortex at the femoral neck fracture site and lateral pin (presumed) insertion holes.

RESULTS

In general, stress values are always changing during walking cycle. Regarding medial femoral neck cortex at the fracture line, intact model showed almost consistent value. Both HP model and DSCS model amounted the highest around 30 MPa. At lateral holes, highest values were 18.8, 104.0, and 63.1 MPa of intact, HP, and DSCS models, respectively.

CONCLUSION

Thus, our analysis simulating the real walking will be useful in evaluating time-varying stress distribution to assess postoperative complication.

CLINICAL RELEVANCE

DSCS is expected to be paramount for treatment of unstable femoral neck fractures.

The Effects of Light and Moderate Intensity Exercise on the Femoral Bone and Cerebellum of d-Galactose-Exposed Rats

Aging causes the degeneration of organs of the locomotor system, including the cerebellum and bones. Exercise may reverse this deterioration. d-galactose has been frequently used in rodents to accelerate aging. The present study aimed at investigating the effects of exercise on cerebellar and serum growth factors, motor activity, and the number of bone cells of the femoral head of d-galactose-treated rats. Twenty-four male Wistar rats were divided randomly into four groups, that is, three treated groups injected with 300 mg/(mL·kg) body weight (bw) d-galactose solution daily for 4 weeks, and a control group injected with normal saline. Following the 4-week administration of d-galactose solution, two of the treated groups performed light- (45% VO₂max) and moderate- (55% VO₂max) intensity exercise, by running on a treadmill 4 × a week for 4 weeks. Locomotor activity was examined in rotarod and open field tests. The cerebellar and serum Insulin-like Growth Factor 1 (IGF-1) and Brain-Derived Neurotrophic Factor (BDNF) levels were measured using enzyme-linked immunosorbent assay (ELISA). The number of osteoblasts and osteoclasts of femoral head was estimated using unbiased stereological methods. It was found that the number of osteoclasts was higher in the d-galactose-treated group than the normal control and moderate-intensity exercise groups. No significant difference between groups was found in the rotarod and open field test performance, IGF-1 and BDNF levels, as well as number of osteoblasts. In conclusion, a 4-week administration of high-dosed-galactose caused the increase of the number of osteoclasts. A subsequent 4-week moderate-intensity exercise reversed this increase to the normal level.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

Bone morphogenetic protein signaling through ACVR1 and BMPR1A negatively regulates bone mass along with alterations in bone composition

Bone quantity and bone quality are important factors in determining the properties and the mechanical functions of bone. This study examined the effects of disrupting bone morphogenetic protein (BMP) signaling through BMP receptors on bone quantity and bone quality. More specifically, we disrupted two BMP receptors, Acvr1 and Bmpr1a, respectively, in Osterix-expressing osteogenic progenitor cells in mice. We examined the structural changes to the femora from 3-month old male and female conditional knockout (cKO) mice using micro-computed tomography (micro-CT) and histology, as well as compositional changes to both cortical and trabecular compartments of bone using Raman spectroscopy. We found that the deletion of Acvr1 and Bmpr1a, respectively, in an osteoblast-specific manner resulted in higher bone mass in the trabecular compartment. Disruption of Bmpr1a resulted in a more significantly increased bone mass in the trabecular compartment. We also found that these cKO mice showed lower mineral-to-matrix ratio, while tissue mineral density was lower in the cortical compartment. Collagen crosslink ratio was higher in both cortical and trabecular compartments of male cKO mice. Our study suggested that BMP signaling in osteoblast mediated by BMP receptors, namely ACVR1 and BMPR1A, is critical in regulating bone quantity and bone quality.

Cyclic mechanical stretch enhances BMP9-induced osteogenic differentiation of mesenchymal stem cells

PURPOSE

The purpose of this study was to investigate whether mechanical stretch can enhance the bone morphogenetic protein 9 (BMP9)-induced osteogenic differentiation in MSCs.

METHODS

Recombinant adenoviruses were used to overexpress the BMP9 in C3H10T1/2 MSCs. Cells were seeded onto six-well BioFlex collagen I-coated plates and subjected to cyclic mechanical stretch [6% elongation at 60 cycles/minute (1 Hz)] in a Flexercell FX-4000 strain unit for up to 12 hours. Immunostaining and confocal microscope were used to detect cytoskeleton organization. Cell cycle progression was checked by flow cytometry. Alkaline phosphatase activity was measured with a Chemiluminescence Assay Kit and was quantified with a histochemical staining assay. Matrix mineralization was examined by Alizarin Red S Staining.

RESULTS

Mechanical stretch induces cytoskeleton reorganization and inhibits cell proliferation by preventing cells entry into S phase of the cell cycle. Although mechanical stretch alone does not induce the osteogenic differentiation of C3H10T1/2 MSCs, co-stimulation with mechanical stretch and BMP9 enhances alkaline phosphatase activity. The expression of key lineage-specific regulators (e.g., osteocalcin (OCN), SRY-related HMG-box 9, and runt-related transcription factor 2) is also increased after the co-stimulation, compared to the mechanical stretch stimulation along. Furthermore, mechanical stretch augments the BMP9-mediated bone matrix mineralization of C3H10T1/2 MSCs.

CONCLUSIONS

Our results suggest that mechanical stretch enhances BMP9-induced osteoblastic lineage specification in C3H10T1/2 MSCs.

The Relationship of Collagen Structural and Compositional Heterogeneity to Tissue Mechanical Properties: A Chemical Perspective

Collagen is the primary protein component in mammalian connective tissues. Over the last 20 years, evidence has mounted that collagen matrices exhibit substantial heterogeneity in their hierarchical structures and that this heterogeneity plays important roles in both structure and function. Herein, an overview of studies addressing the nanoscale compositional and structural heterogeneity is provided and connected to work exploring the mechanical implications for a number of tissues.

Parathyroid hormone's enhancement of bones' osteogenic response to loading is affected by ageing in a dose- and time-dependent manner

Decreased effectiveness of bones' adaptive response to mechanical loading contributes to age-related bone loss. In young mice, intermittent administration of parathyroid hormone (iPTH) at 20-80μg/kg/day interacts synergistically with artificially applied loading to increase bone mass. Here we report investigations on the effect of different doses and duration of iPTH treatment on mice whose osteogenic response to artificial loading is impaired by age. One group of aged, 19-month-old female C57BL/6 mice was given 0, 25, 50 or 100μg/kg/day iPTH for 4weeks. Histological and μCT analysis of their tibiae revealed potent iPTH dose-related increases in periosteally-enclosed area, cortical area and porosity with decreased cortical thickness. There was practically no effect on trabecular bone. Another group was given a submaximal dose of 50μg/kg/day iPTH or vehicle for 2 or 6weeks with loading of their right tibia three times per week for the final 2weeks. In the trabecular bone of these mice the loading-related increase in BV/TV was abrogated by iPTH primarily by reduction of the increase in trabecular number. In their cortical bone, iPTH treatment time-dependently increased cortical porosity. Loading partially reduced this effect. The osteogenic effects of iPTH and loading on periosteally-enclosed area and cortical area were additive but not synergistic. Thus in aged, unlike young mice, iPTH and loading appear to have separate effects. iPTH alone causes a marked increase in cortical porosity which loading reduces. Both iPTH and loading have positive effects on cortical periosteal bone formation but these are additive rather than synergistic.

Cyclic Compressive Stress Regulates Apoptosis in Rat Osteoblasts: Involvement of PI3K/Akt and JNK MAPK Signaling Pathways

It is widely accepted that physiological mechanical stimulation suppresses apoptosis and induces synthesis of extracellular matrix by osteoblasts; however, the effect of stress overloading on osteoblasts has not been fully illustrated. In the present study, we investigated the effect of cyclic compressive stress on rat osteoblasts apoptosis, using a novel liquid drop method to generate mechanical stress on osteoblast monolayers. After treatment with different levels of mechanical stress, apoptosis of osteoblasts and activations of mitogen-activated protein kinases (MAPKs) and PI3-kinase (PI3K)/Akt signaling pathways were investigated. Osteoblasts apoptosis was observed after treated with specific inhibitors prior to mechanical stimulation. Protein levels of Bax/Bcl-2/caspase-3 signaling were determined using western blot with or without inhibitors of PI3K/Akt and phosphorylation of c-jun N-terminal kinase (JNK) MAPK. Results showed that mechanical stimulation led to osteoblasts apoptosis in a dose-dependent manner and a remarkable activation of MAPKs and PI3K/Akt signaling pathways. Activation of PI3K/Akt protected against apoptosis, whereas JNK MAPK increased apoptosis via regulation of Bax/Bcl-2/caspase-3 activation. In summary, the PI3K/Akt and JNK MAPK signaling pathways played opposing roles in osteoblasts apoptosis, resulting in inhibition of apoptosis upon small-magnitude stress and increased apoptosis upon large-magnitude stress.

Overexpression of Gα11 in Osteoblast Lineage Cells Suppresses the Osteoanabolic Response to Intermittent PTH and Exercise

Intermittent parathyroid hormone (iPTH) treatment and mechanical loading are osteoanabolic stimuli that are partially mediated through actions on G protein-coupled receptors (GPCRs). GPCR signaling can be altered by heterotrimeric G protein Gα subunits levels, which can therefore lead to altered responses to such stimuli. Previous studies have suggested that enhanced signaling through the Gαq/11 pathway inhibits the osteoanabolic actions of PTH. The influence of Gαq/11 signaling on mechanotransduction, however, has not been reported in vivo. Using transgenic mice that specifically overexpress Gα11 in osteoblast lineage cells (G11-Tg mice), we investigated the skeletal effects of elevated Gα11 levels on iPTH and mechanical loading by treadmill exercise. Both regimens increased trabecular and cortical bone in Wild-Type (WT) mice as a result of increased bone formation. In G11-Tg mice, there was no change in trabecular or cortical bone and no increase in bone formation in response to iPTH or exercise. While exercise reduced osteoclast parameters in WT mice, these changes were diminished in G11-Tg mice as expression of M-csf and Trap remained increased. Collectively, our results suggest that osteoblastic upregulation of Gα11 is inhibitory to osteoanabolic actions of both PTH and exercise, and that its suppression may be a promising target for treating bone loss.

Common mechanisms in development and disease: BMP signaling in craniofacial development

BMP signaling is one of the key pathways regulating craniofacial development. It is involved in the early patterning of the head, the development of cranial neural crest cells, and facial patterning. It regulates development of its mineralized structures, such as cranial bones, maxilla, mandible, palate, and teeth. Targeted mutations in the mouse have been instrumental to delineate the functional involvement of this signaling network in different aspects of craniofacial development. Gene polymorphisms and mutations in BMP pathway genes have been associated with various non-syndromic and syndromic human craniofacial malformations. The identification of intricate cellular interactions and underlying molecular pathways illustrate the importance of local fine-regulation of Bmp signaling to control proliferation, apoptosis, epithelial-mesenchymal interactions, and stem/progenitor differentiation during craniofacial development. Thus, BMP signaling contributes both to shape and functionality of our facial features. BMP signaling also regulates postnatal craniofacial growth and is associated with dental structures life-long. A more detailed understanding of BMP function in growth, homeostasis, and repair of postnatal craniofacial tissues will contribute to our ability to rationally manipulate this signaling network in the context of tissue engineering.