Mechanical strain differentially regulates endothelial nitric-oxide synthase and receptor activator of nuclear kappa B ligand expression via ERK1/2 MAPK

The Role of Aryl Hydrocarbon Receptor in Bone Biology

Aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, is crucial in maintaining the skeletal system. Our study focuses on encapsulating the role of AhR in bone biology and identifying novel signaling pathways in musculoskeletal pathologies using the GEO dataset. The GEO2R analysis identified 8 genes (CYP1C1, SULT6B1, CYB5A, EDN1, CXCR4B, CTGFA, TIPARP, and CXXC5A) involved in the AhR pathway, which play a pivotal role in bone remodeling. The AhR knockout in hematopoietic stem cells showed alteration in several novel bone-related transcriptomes (eg, Defb14, ZNF 51, and Chrm5). Gene Ontology Enrichment Analysis demonstrated 54 different biological processes associated with bone homeostasis. Mainly, these processes include bone morphogenesis, bone development, bone trabeculae formation, bone resorption, bone maturation, bone mineralization, and bone marrow development. Employing Functional Annotation and Clustering through DAVID, we further uncovered the involvement of the xenobiotic metabolic process, p450 pathway, oxidation-reduction, and nitric oxide biosynthesis process in the AhR signaling pathway. The conflicting evidence of current research of AhR signaling on bone (positive and negative effects) homeostasis may be due to variations in ligand binding affinity, binding sites, half-life, chemical structure, and other unknown factors. In summary, our study provides a comprehensive understanding of the underlying mechanisms of the AhR pathway in bone biology.

Convergence of Calcium Channel Regulation and Mechanotransduction in Skeletal Regenerative Biomaterial Design

Cells are known to perceive their microenvironment through extracellular and intracellular mechanical signals. Upon sensing mechanical stimuli, cells can initiate various downstream signaling pathways that are vital to regulating proliferation, growth, and homeostasis. One such physiologic activity modulated by mechanical stimuli is osteogenic differentiation. The process of osteogenic mechanotransduction is regulated by numerous calcium ion channels-including channels coupled to cilia, mechanosensitive and voltage-sensitive channels, and channels associated with the endoplasmic reticulum. Evidence suggests these channels are implicated in osteogenic pathways such as the YAP/TAZ and canonical Wnt pathways. This review aims to describe the involvement of calcium channels in regulating osteogenic differentiation in response to mechanical loading and characterize the fashion in which those channels directly or indirectly mediate this process. The mechanotransduction pathway is a promising target for the development of regenerative materials for clinical applications due to its independence from exogenous growth factor supplementation. As such, also described are examples of osteogenic biomaterial strategies that involve the discussed calcium ion channels, calcium-dependent cellular structures, or calcium ion-regulating cellular features. Understanding the distinct ways calcium channels and signaling regulate these processes may uncover potential targets for advancing biomaterials with regenerative osteogenic capabilities.

The skeleton in a physical world

All organisms exist within a physical space and respond to physical forces as part of daily life. In higher organisms, the skeleton is critical for locomotion in the physical environment, providing a carapace upon which the animal can move to accomplish functions necessary for living. As such, the skeleton has responded evolutionarily, and does in real-time, to physical stresses placed on it to ensure that its structure supports its function in the sea, in the air, and on dry land. In this article, we consider how those cells responsible for remodeling skeletal structure respond to mechanical force including load magnitude, frequency, and cyclicity, and how force rearranges cellular structure in turn. The effects of these forces to balance the mesenchymal stem cell supply of bone-forming osteoblasts and energy storing adipocytes are addressed. That this phenotypic switching is achieved at the level of both gene transactivation and alteration of structural epigenetic controls of gene expression is considered. Finally, as clinicians, we consider this information as it applies to a prescriptive for intelligent exercise.

The effect of impact exercise on bone mineral density: A longitudinal study on non-athlete adolescents

PURPOSE

High impact exercise is known to induce osteogenic effects in the skeleton. However, less is known about the systemic effect of exercise practice in a potential adaptive mechanism of the skeletal accrual. This research aimed to assess the effect of impact exercise on bone mineral density (BMD) in the radius throughout adolescence.

METHODS

This study evaluated 1137 adolescents, at 13 and 17 years old, as part of the population-based cohort EPITeen. BMD (g/cm²) was measured at the ultradistal and proximal radius of the non-dominant forearm by dual-energy X-ray absorptiometry (DXA) using a Lunar® Peripheral Instantaneous X-ray Image device. The practice of (extra-curricular) exercise was categorized as: no exercise, exercise with high impact and exercise with low impact. Regression coefficients (β) and respective 95% confidence intervals (CI_95%) were used to estimate the association between exercise practice categories at 13 years old and BMD at 13 and 17 years old and BMD gain between evaluations.

RESULTS

In boys, at 13 years, BMD was similar between the ones not practicing exercise and those practicing exercise with low impact, and the gain of BMD was also similar in both groups. Still in boys, at 13 years, those who practiced exercise with high impact presented higher mean (standard-deviation) of BMD, comparing to the other two groups (no exercise and low impact exercise), and also significantly increased the BMD gain between 13 and 17 years (β = 0.013; CI_95%0.003;0.023). In girls, no statistically significant differences on BMD were found between the categories of exercise at 13 years and BMD at 17 years of age.

CONCLUSION

This research shows that the practice of high impact exercise could help to increase BMD more than low impact exercise even in a nonweight-bearing bone during adolescence.

Effects of Extracellular Osteoanabolic Agents on the Endogenous Response of Osteoblastic Cells

The complex multidimensional skeletal organization can adapt its structure in accordance with external contexts, demonstrating excellent self-renewal capacity. Thus, optimal extracellular environmental properties are critical for bone regeneration and inextricably linked to the mechanical and biological states of bone. It is interesting to note that the microstructure of bone depends not only on genetic determinants (which control the bone remodeling loop through autocrine and paracrine signals) but also, more importantly, on the continuous response of cells to external mechanical cues. In particular, bone cells sense mechanical signals such as shear, tensile, loading and vibration, and once activated, they react by regulating bone anabolism. Although several specific surrounding conditions needed for osteoblast cells to specifically augment bone formation have been empirically discovered, most of the underlying biomechanical cellular processes underneath remain largely unknown. Nevertheless, exogenous stimuli of endogenous osteogenesis can be applied to promote the mineral apposition rate, bone formation, bone mass and bone strength, as well as expediting fracture repair and bone regeneration. The following review summarizes the latest studies related to the proliferation and differentiation of osteoblastic cells, enhanced by mechanical forces or supplemental signaling factors (such as trace metals, nutraceuticals, vitamins and exosomes), providing a thorough overview of the exogenous osteogenic agents which can be exploited to modulate and influence the mechanically induced anabolism of bone. Furthermore, this review aims to discuss the emerging role of extracellular stimuli in skeletal metabolism as well as their potential roles and provide new perspectives for the treatment of bone disorders.

Bone fracture healing: perspectives according to molecular basis

Fractures have a great impact on health all around the world and with fracture healing optimization; this problem could be resolved partially. To make a practical contribution to this issue, the knowledge of bone tissue, cellularity, and metabolism is essential, especially cytoskeletal architecture and its transformations according to external pressures. Special physical and chemical characteristics of the extracellular matrix (ECM) allow the transmission of mechanical stimuli from outside the cell to the plasmatic membrane. The osteocyte cytoskeleton is conformed by a complex network of actin and microtubules combined with crosslinker proteins like vinculin and fimbrin, connecting and transmitting outside stimuli through EMC to cytoplasm. Herein, critical signaling pathways like Cx43-depending ones, MAPK/ERK, Wnt, YAP/TAZ, Rho-ROCK, and others are activated due to mechanical stimuli, resulting in osteocyte cytoskeletal changes and ECM remodeling, altering the tissue and, therefore, the bone. In recent years, the osteocyte has gained more interest and value in relation to bone homeostasis as a great coordinator of other cell populations, thanks to its unique functions. By integrating the latest advances in relation to intracellular signaling pathways, mechanotransmission system of the osteocyte and bone tissue engineering, there are promising experimental strategies, while some are ready for clinical trials. This work aims to show clearly and precisely the integration between cytoskeleton and main molecular pathways in relation to mechanotransmission mechanism in osteocytes, and the use of this theoretical knowledge in therapeutic tools for bone fracture healing.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

Constitutive activation of ERK1/2 signaling protects against myocardial ischemia via inhibition of mitochondrial fragmentation in the aging heart

Background

Studies have shown that the ability of the myocardium to tolerate ischemia becomes significantly compromised with age. During ischemia, several endogenous protective signals are activated to protect the heart from injury, among which extracellular-signal regulated kinase (ERK) 1/2 signaling has been established as playing a pivotal role. However, in aging hearts, the activation of ERK1/2 is compromised. Mitogen-activated protein kinase/ERK kinase (MEK) is a major regulator of ERK1/2 signaling. In the present study, we investigated whether transduction of CaMEK, a constitutively activated MEK, using adeno-associated virus serotype 9 (AAV9) could protect the aging heart against ischemia.

Methods

Myocardial ischemia models were established in aging mice and senescent cardiomyocytes, and AAV9-mediated delivery of CaMEK was applied. Echocardiography, fluorescent staining, transmission electron microscopy, flow cytometry, and immunoblotting were used to explore the effects of CaMEK and their underlying mechanism.

Results

AAV9-CaMEK activated ERK1/2 signaling and exerted cardioprotective effects against ischemia in aging hearts. Specifically, CaMEK transduction decreased dynamin-related protein-1 (Drp1) expression and phosphorylation at serine 616, resulting in improved mitochondrial morphology and function in aging ischemic hearts. Furthermore, CaMEK transduction exerted similar protective effects in senescent cardiomyocytes under hypoxia. Meanwhile, with the inhibition of ERK1/2 signaling in senescent cardiomyocytes under hypoxia, the opposite effects were observed, including an increase in mitochondrial fragmentation and aggravation of mitochondrial dysfunction and cell apoptosis.

Conclusions

Our results suggested that AAV9-CaMEK alleviated ischemia-induced myocardium injury in the aging heart, at least in part, through inhibition of mitochondrial fragmentation. Therefore, AAV9-CaMEK is a potential intervention for prevention of ischemia-induced injury of the aging myocardium.

Mechanical strain-mediated reduction in RANKL expression is associated with RUNX2 and BRD2

Mechanical loading-related strains trigger bone formation by osteoblasts while suppressing resorption by osteoclasts, uncoupling the processes of formation and resorption. Osteocytes may orchestrate this process in part by secreting sclerostin (SOST), which inhibits osteoblasts, and expressing receptor activator of nuclear factor-κB ligand (RANKL/TNFSF11) which recruits osteoclasts. Both SOST and RANKL are targets of the master osteoblastic transcription factor RUNX2. Subjecting human osteoblastic Saos-2 cells to strain by four point bending down-regulates their expression of SOST and RANKL without altering RUNX2 expression. RUNX2 knockdown increases basal SOST expression, but does not alter SOST down-regulation following strain. Conversely, RUNX2 knockdown does not alter basal RANKL expression, but prevents its down-regulation by strain. Chromatin immunoprecipitation revealed RUNX2 occupies a region of the RANKL promoter containing a consensus RUNX2 binding site and its occupancy of this site decreases following strain. The expression of epigenetic acetyl and methyl writers and readers was quantified by RT-qPCR to investigate potential epigenetic bases for this change. Strain and RUNX2 knockdown both down-regulate expression of the bromodomain acetyl reader BRD2. BRD2 and RUNX2 co-immunoprecipitate, suggesting interaction within regulatory complexes, and BRD2 was confirmed to interact with the RUNX2 promoter. BRD2 also occupies the RANKL promoter and its occupancy was reduced following exposure to strain. Thus, RUNX2 may contribute to bone remodeling by suppressing basal SOST expression, while facilitating the acute strain-induced down-regulation of RANKL through a mechanosensitive epigenetic loop involving BRD2.

Combating osteoporosis and obesity with exercise: leveraging cell mechanosensitivity

Osteoporosis, a condition of skeletal decline that undermines quality of life, is treated with pharmacological interventions that are associated with poor adherence and adverse effects. Complicating efforts to improve clinical outcomes, the incidence of obesity is increasing, predisposing the population to a range of musculoskeletal complications and metabolic disorders. Pharmacological management of obesity has yet to deliver notable reductions in weight and debilitating complications are rarely avoided. By contrast, exercise shows promise as a non-invasive and non-pharmacological method of regulating both osteoporosis and obesity. The principal components of exercise - mechanical signals - promote bone and muscle anabolism while limiting formation and expansion of fat mass. Mechanical regulation of bone and marrow fat might be achieved by regulating functions of differentiated cells in the skeletal tissue while biasing lineage selection of their common progenitors - mesenchymal stem cells. An inverse relationship between adipocyte versus osteoblast fate selection from stem cells is implicated in clinical conditions such as childhood obesity and increased marrow adiposity in type 2 diabetes mellitus, as well as contributing to skeletal frailty. Understanding how exercise-induced mechanical signals can be used to improve bone quality while decreasing fat mass and metabolic dysfunction should lead to new strategies to treat chronic diseases such as osteoporosis and obesity.

The role of transient receptor potential polycystin channels in bone diseases

Transient receptor potential (TRP) channels are cation channels which act as molecular sensors that enable cells to detect and respond to a plethora of mechanical and environmental cues. TRPs are involved in various physiological processes, such as mechanosensation, non-inception and thermosensation, while mutations in genes encoding them can lead to pathological conditions, called "channelopathies". The subfamily of transient receptor potential polycystins (TRPPs), Polycystin 1 (PC1, TRPP1) and Polycystin 2 (PC2, TRPP2), act as mechanoreceptors, sensing external mechanical forces, including strain, stretch and fluid shear stress, triggering a cascade of signaling pathways involved in osteoblastogenesis and ultimately bone formation. Both in vitro studies and research on animal models have already identified their implications in bone homeostasis. However, uncertainty veiling the role of polycystins (PCs) in bone disease urges studies to elucidate further their role in this field. Mutations in TRPPs have been related to autosomal polycystic kidney disease (ADKPD) and research groups try to identify their role beyond their well-established contribution in kidney disease. Such an elucidation would be beneficial for identifying signaling pathways where polycystins are involved in bone diseases related to exertion of mechanical forces such as osteoporosis, osteopenia and craniosynostosis. A better understanding of the implications of TRPPs in bone diseases would possibly lay the cornerstone for effective therapeutic schemes.

The role of aryl hydrocarbon receptor in bone remodeling

Bone remodeling is a persistent process for maintaining skeletal system homeostasis, and it depends on the dynamic equilibrium between bone-forming osteoblasts and bone-resorbing osteoclasts. Aryl hydrocarbon receptor (Ahr), a ligand-activated transcription factor, plays a pivotal role in regulating skeletal system. In order to better understand the role of Ahr in bone remodeling, we focused on bone remodeling characteristic, and the effects of Ahr on bone formation and differentiation, which suggest that Ahr is a critical control factor in the process of bone remodeling. Moreover, we discussed the impacts of Ahr on several signaling pathways related to bone remodeling, hoping to provide a theoretical basis to improve bone remodeling.

Osteocytes regulate osteoblast differentiation and osteoclast activity through Interleukin-6 under mechanical loading

Osteocytes are the major mechanosensors that respond to mechanical strain and regulate bone formation and resorption.

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.

Mechanical loading and the synthesis of 1,25(OH)2D in primary human osteoblasts

The metabolite 1,25-dihydroxyvitamin D (1,25(OH)2D) is synthesized from its precursor 25-hydroxyvitamin D (25(OH)D) by human osteoblasts leading to stimulation of osteoblast differentiation in an autocrine or paracrine way. Osteoblast differentiation is also stimulated by mechanical loading through activation of various responses in bone cells such as nitric oxide signaling. Whether mechanical loading affects osteoblast differentiation through an enhanced synthesis of 1,25(OH)2D by human osteoblasts is still unknown. We hypothesized that mechanical loading stimulates the synthesis of 1,25(OH)2D from 25(OH)D in primary human osteoblasts. Since the responsiveness of bone to mechanical stimuli can be altered by various endocrine factors, we also investigated whether 1,25(OH)2D or 25(OH)D affect the response of primary human osteoblasts to mechanical loading. Primary human osteoblasts were pre-incubated in medium with/without 25(OH)D3 (400 nM) or 1,25(OH)2D3 (100 nM) for 24h and subjected to mechanical loading by pulsatile fluid flow (PFF). The response of osteoblasts to PFF was quantified by measuring nitric oxide, and by PCR analysis. The effect of PFF on the synthesis of 1,25(OH)2D3 was determined by subjecting osteoblasts to PFF followed by 24h post-incubation in medium with/without 25(OH)D3 (400 nM). We showed that 1,25(OH)2D3 reduced the PFF-induced NO response in primary human osteoblasts. 25(OH)D3 did not significantly alter the NO response of primary human osteoblasts to PFF, but 25(OH)D3 increased osteocalcin and RANKL mRNA levels, similar to 1,25(OH)2D3. PFF did not increase 1,25(OH)2D3 amounts in our model, even though PFF did increase CYP27B1 mRNA levels and reduced VDR mRNA levels. CYP24 mRNA levels were not affected by PFF, but were strongly increased by both 25(OH)D3 and 1,25(OH)2D3. In conclusion, 1,25(OH)2D3 may affect the response of primary human osteoblasts to mechanical stimuli, at least with respect to NO production. Mechanical stimuli may affect local vitamin D metabolism in primary human osteoblasts. Our results suggest that 1,25(OH)2D3 and mechanical loading, both stimuli of the differentiation of osteoblasts, interact at the cellular level.

Planar cell polarity aligns osteoblast division in response to substrate strain

Exposure of bone to dynamic strain increases the rate of division of osteoblasts and also influences the directional organization of the cellular and molecular structure of the bone tissue that they produce. Here, we report that brief exposure to dynamic substrate strain (sufficient to rapidly stimulate cell division) influences the orientation of osteoblastic cell division. The initial proliferative response to strain involves canonical Wnt signaling and can be blocked by sclerostin. However, the strain-related orientation of cell division is independently influenced through the noncanonical Wnt/planar cell polarity (PCP) pathway. Blockade of Rho-associated coiled kinase (ROCK), a component of the PCP pathway, prevents strain-related orientation of division in osteoblast-like Saos-2 cells. Heterozygous loop-tail mutation of the core PCP component van Gogh-like 2 (Vangl2) in mouse osteoblasts impairs the orientation of division in response to strain. Examination of bones from Vangl2 loop-tail heterozygous mice by µCT and scanning electron microscopy reveals altered bone architecture and disorganized bone-forming surfaces. Hence, in addition to the well-accepted role of PCP involvement in response to developmental cues during skeletal morphogenesis, our data reveal that this pathway also acts postnatally, in parallel with canonical Wnt signaling, to transduce biomechanical cues into skeletal adaptive responses. The simultaneous and independent actions of these two pathways appear to influence both the rate and orientation of osteoblast division, thus fine-tuning bone architecture to meet the structural demands of functional loading.

Strain energy density gradients in bone marrow predict osteoblast and osteoclast activity: a finite element study

Huiskes et al. hypothesized that mechanical strains sensed by osteocytes residing in trabecular bone dictate the magnitude of load-induced bone formation. More recently, the mechanical environment in bone marrow has also been implicated in bone׳s response to mechanical stimulation. In this study, we hypothesize that trabecular load-induced bone formation can be predicted by mechanical signals derived from an integrative µFE model, incorporating a description of both the bone and marrow phase. Using the mouse tail loading model in combination with in vivo micro-computed tomography (µCT) we tracked load induced changes in the sixth caudal vertebrae of C57BL/6 mice to quantify the amount of newly mineralized and eroded bone volumes. To identify the mechanical signals responsible for adaptation, local morphometric changes were compared to micro-finite element (µFE) models of vertebrae prior to loading. The mechanical parameters calculated were strain energy density (SED) on trabeculae at bone forming and resorbing surfaces, SED in the marrow at the boundary between bone forming and resorbing surfaces, along with SED in the trabecular bone and marrow volumes. The gradients of each parameter were also calculated. Simple regression analysis showed mean SED gradients in the trabecular bone matrix to significantly correlate with newly mineralized and eroded bone volumes R(2)=0.57 and 0.41, respectively, p<0.001). Nevertheless, SED gradients in the marrow were shown to be the best predictor of osteoblastic and osteoclastic activity (R(2)=0.83 and 0.60, respectively, p<0.001). These data suggest that the mechanical environment of the bone marrow plays a significant role in determining osteoblast and osteoclast activity.

Bone mechanobiology, gravity and tissue engineering: effects and insights

Bone homeostasis strongly depends on fine tuned mechanosensitive regulation signals from environmental forces into biochemical responses. Similar to the ageing process, during spaceflights an altered mechanotransduction occurs as a result of the effects of bone unloading, eventually leading to loss of functional tissue. Although spaceflights represent the best environment to investigate near-zero gravity effects, there are major limitations for setting up experimental analysis. A more feasible approach to analyse the effects of reduced mechanostimulation on the bone is represented by the 'simulated microgravity' experiments based on: (1) in vitro studies, involving cell cultures studies and the use of bioreactors with tissue engineering approaches; (2) in vivo studies, based on animal models; and (3) direct analysis on human beings, as in the case of the bed rest tests. At present, advanced tissue engineering methods allow investigators to recreate bone microenvironment in vitro for mechanobiology studies. This group and others have generated tissue 'organoids' to mimic in vitro the in vivo bone environment and to study the alteration cells can go through when subjected to unloading. Understanding the molecular mechanisms underlying the bone tissue response to mechanostimuli will help developing new strategies to prevent loss of tissue caused by altered mechanotransduction, as well as identifying new approaches for the treatment of diseases via drug testing. This review focuses on the effects of reduced gravity on bone mechanobiology by providing the up-to-date and state of the art on the available data by drawing a parallel with the suitable tissue engineering systems.

Endocrine regulation of bone and energy metabolism in hibernating mammals

Precise coordination among organs is required to maintain homeostasis throughout hibernation. This is particularly true in balancing bone remodeling processes (bone formation and resorption) in hibernators experiencing nutritional deprivation and extreme physical inactivity, two factors normally leading to pronounced bone loss in non-hibernating mammals. In recent years, important relationships between bone, fat, reproductive, and brain tissues have come to light. These systems share interconnected regulatory mechanisms of energy metabolism that potentially protect the skeleton during hibernation. This review focuses on the endocrine and neuroendocrine regulation of bone/fat/energy metabolism in hibernators. Hibernators appear to have unique mechanisms that protect musculoskeletal tissues while catabolizing their abundant stores of fat. Furthermore, the bone remodeling processes that normally cause disuse-induced bone loss in non-hibernators are compared to bone remodeling processes in hibernators, and possible adaptations of the bone signaling pathways that protect the skeleton during hibernation are discussed. Understanding the biological mechanisms that allow hibernators to survive the prolonged disuse and fasting associated with extreme environmental challenges will provide critical information regarding the limit of convergence in mammalian systems and of skeletal plasticity, and may contribute valuable insight into the etiology and treatment of human diseases.

Defining a stimuli-response relationship in compensatory lung growth following major resection

Major lung resection is a robust model that mimics the consequences of loss-of-functioning lung units. We previously observed in adult canines, following 42% and 58% lung resection, a critical threshold of stimuli intensity for the initiation of compensatory lung growth. To define the range and limits of this stimuli-response relationship, we performed morphometric analysis on the remaining lobes of adult dogs, 2-3 years after surgical removal of ∼ 70% of lung units in the presence or absence of mediastinal shift. Results were expressed as ratios to that in corresponding control lobes. Lobar expansion and extravascular tissue growth (∼ 3.8- and ∼ 2.0-fold of normal, respectively) were heterogeneous; the lobes remaining next to the diaphragm exhibited a greater response. Tissue growth and capillary formation, indexed by double-capillary profiles, increased, regardless of mediastinal shift. Septal collagen fibers increased up to 2.7-fold, suggesting a greater need for structural support. Compared with previous cohorts following less-extensive resection, tissue volume and gas-exchange surface areas increased significantly only in the infracardiac lobe following 42% resection, exceeded two- to threefold in all lobes following 58% resection, and then exhibited diminished gains following ∼ 70% resection. In contrast, alveolar-capillary formation increased with incremental resection without reaching an upper limit. Overall structural regrowth was most vigorous and uniform following 58% resection. The diminishment of gains in tissue growth, following ∼ 70% resection, could reflect excessive or maldistributed mechanical stress that threatens septal integrity. Results also suggest additional independent stimuli of alveolar-capillary formation, possibly related to the postresection augmentation of regional perfusion.

Shaping development through mechanical strain: the transcriptional basis of diet-induced phenotypic plasticity in a cichlid fish

Adaptive phenotypic plasticity, the ability of an organism to change its phenotype to match local environments, is increasingly recognized for its contribution to evolution. However, few empirical studies have explored the molecular basis of plastic traits. The East African cichlid fish Astatoreochromis alluaudi displays adaptive phenotypic plasticity in its pharyngeal jaw apparatus, a structure that is widely seen as an evolutionary key innovation that has contributed to the remarkable diversity of cichlid fishes. It has previously been shown that in response to different diets, the pharyngeal jaws change their size, shape and dentition: hard diets induce an adaptive robust molariform tooth phenotype with short jaws and strong internal bone structures, while soft diets induce a gracile papilliform tooth phenotype with elongated jaws and slender internal bone structures. To gain insight into the molecular underpinnings of these adaptations and enable future investigations of the role that phenotypic plasticity plays during the formation of adaptive radiations, the transcriptomes of the two divergent jaw phenotypes were examined. Our study identified a total of 187 genes whose expression differs in response to hard and soft diets, including immediate early genes, extracellular matrix genes and inflammatory factors. Transcriptome results are interpreted in light of expression of candidate genes-markers for tooth size and shape, bone cells and mechanically sensitive pathways. This study opens up new avenues of research at new levels of biological organization into the roles of phenotypic plasticity during speciation and radiation of cichlid fishes.

p38-MAPK signaling pathway is not involved in osteogenic differentiation during early response of mesenchymal stem cells to continuous mechanical strain

Mechanical stimuli play a significant role in the regulation of bone remodeling during orthodontic tooth movement. However, the correlation between mechanical strain and bone remodeling is still poorly understood. In this study, we used a model of continuous mechanical strain (CMS) on bone mesenchymal stem cells (BMSCs) to investigate the proliferation and osteogenic differentiation of BMSCs and the mechanism of mechano-transduction. A CMS of 10 % at 1 Hz suppressed the proliferation of BMSCs and induced early osteogenic differentiation within 48 h by activating Runx2 and increasing alkaline phosphatase (ALP) activity and mRNA expression of osteogenesis-related genes (ALP, collagen type I, and osteopontin). Regarding mitogen-activated protein kinase (MAPK) activation, CMS induced phased phosphorylation of p38 consisting of a rapid induction of p38 MAPK at 10 min and a rapid decay after 1 h. Furthermore, the potent p38 inhibitor SB203580 blocked the induction of p38 MAPK signaling, but had little effect on subsequent osteogenic events. These results demonstrate that mechanical strain may act as a stimulator to induce the differentiation of BMSCs into osteoblasts, which is a vital function for bone formation in orthodontic tooth movement. However, activation of the p38 signaling pathway may not be involved in this process.

Local delivery of nitric oxide: targeted delivery of therapeutics to bone and connective tissues

Non-invasive treatment of injuries and disorders affecting bone and connective tissue remains a significant challenge facing the medical community. A treatment route that has recently been proposed is nitric oxide (NO) therapy. Nitric oxide plays several important roles in physiology with many conditions lacking adequate levels of NO. As NO is a radical, localized delivery via NO donors is essential to promoting biological activity. Herein, we review current literature related to therapeutic NO delivery in the treatment of bone, skin and tendon repair.

Mechanical regulation of signaling pathways in bone

A wide range of cell types depend on mechanically induced signals to enable appropriate physiological responses. The skeleton is particularly dependent on mechanical information to guide the resident cell population towards adaptation, maintenance and repair. Research at the organ, tissue, cell and molecular levels has improved our understanding of how the skeleton can recognize the functional environment, and how these challenges are translated into cellular information that can site-specifically alter phenotype. This review first considers those cells within the skeleton that are responsive to mechanical signals, including osteoblasts, osteoclasts, osteocytes and osteoprogenitors. This is discussed in light of a range of experimental approaches that can vary parameters such as strain, fluid shear stress, and pressure. The identity of mechanoreceptor candidates is approached, with consideration of integrins, pericellular tethers, focal adhesions, ion channels, cadherins, connexins, and the plasma membrane including caveolar and non-caveolar lipid rafts and their influence on integral signaling protein interactions. Several mechanically regulated intracellular signaling cascades are detailed including activation of kinases (Akt, MAPK, FAK), β-catenin, GTPases, and calcium signaling events. While the interaction of bone cells with their mechanical environment is complex, an understanding of mechanical regulation of bone signaling is crucial to understanding bone physiology, the etiology of diseases such as osteoporosis, and to the development of interventions to improve bone strength.

Induction of endothelial nitric oxide synthase expression by IL-17 in human vascular endothelial cells: implications for vascular remodeling in transplant vasculopathy

IL-17 is a signature cytokine of Th17 cells, a recently described subset of effector CD4 T cells implicated in the development of several pathologies. We have examined the role of IL-17 in regulating endothelial NO synthase (eNOS) expression in human vascular endothelial cells (ECs) because of the key role of eNOS in determining the pathological outcome of immune-mediated vascular diseases. In cultured ECs, IL-17 increased expression of eNOS, eNOS phosphorylation at Ser(1177), and NO production. The induction of eNOS expression by IL-17 was prevented by the pharmacological inhibition of NF-κB, MEK, and JNK, as well as by small interfering RNA-mediated gene silencing of these signaling pathways. The expression of IL-17 was then examined by immunohistochemistry in human arteries affected by transplant vasculopathy (TV), a vascular condition that is a leading reflection of chronic heart transplant rejection. IL-17 was expressed by infiltrating leukocytes in the intima of arteries with TV, and the majority of IL-17-positive cells were T cells. The number of IL-17-positive cells was not correlated with the intima/media ratio, but was negatively correlated with the amount of luminal occlusion. There was also a significant positive correlation between the number of IL-17-positive cells and the density of eNOS-expressing luminal ECs in arteries with TV. Altogether, these findings show that IL-17 induces the expression of eNOS in human ECs and that this may facilitate outward expansion of arteries afflicted with TV.

Análise da influência do ultrassom de baixa intensidade na região de reparo ósseo em ratos sob ausência de carga

Há evidências de que o ultrassom (US) de baixa intensidade pode acelerar a regeneração óssea. Este trabalho objetivou verificar a ação do US no defeito ósseo, criado experimentalmente em tíbias de ratos sob ausência de carga. Vinte Rattus novergicus albinus, Wistar adultos, divididos em: G1 (n=10), grupo experimental de 15 dias sem suspensão, e G2 (n=10), grupo experimental de 15 dias suspenso pela cauda, foram submetidos à osteotomia em ambas as tíbias e à aplicação do US, frequência de 1,5 MHz, ciclo de trabalho 1:4, 30 mW/cm², nas tíbias direitas por 12 sessões de 20 minutos. Após o sacrifício, as tíbias foram submetidas à análise da Densidade Mineral Óssea (DMO). Os resultados demonstraram DMO de 0,139±0,018 g/cm² para tíbia tratada; 0,131±0,009 g/cm² para tíbia controle no G1; e no G2 registrou-se 0,120±0,009 g/cm² para tíbia tratada e 0,106±0,017 g/cm² para tíbia controle. Houve diferença significante entre os grupos nos quais o G2 apresentou menor DMO, o que demonstra que a suspensão prejudica a manutenção das propriedades ósseas, e entre as tíbias tratadas e controles do G2, demonstrando que o US acelerou o processo de reparo, concluindo que a impossibilidade do estímulo mecânico causada pela não deambulação em um processo de reparo ósseo pode ser minimizada pela ação do US. No G1, a aplicação do US não teve influência significante no aumento da DMO, talvez pelo fato dos animais já terem estímulo mecânico suficiente à formação óssea.

Effects of membrane cholesterol depletion and GPI-anchored protein reduction on osteoblastic mechanotransduction

We previously demonstrated that oscillatory fluid flow activates MC3T3-E1 osteoblastic cell calcium signaling pathways via a mechanism involving ATP releases and P2Y(2) puringeric receptors. However, the molecular mechanisms by which fluid flow initiates cellular responses are still unclear. Accumulating evidence suggests that lipid rafts, one of the important membrane structural components, may play an important role in transducing extracellular fluid shear stress to intracellular responses. Due to the limitations of current techniques, there is no direct approach to study the role of lipid rafts in transmitting fluid shear stress. In this study, we targeted two important membrane components associated with lipid rafts, cholesterol, and glycosylphosphatidylinositol-anchored proteins (GPI-anchored proteins), to disrupt the integrity of cell membrane structures. We first demonstrated that membrane cholesterol depletion with the treatment of methyl-β-cyclodextrin inhibits oscillatory fluid flow induced intracellular calcium mobilization and ERK1/2 phosphorylation in MC3T3-E1 osteoblastic cells. Secondly, we used a novel approach to decrease the levels of GPI-anchored proteins on cell membranes by overexpressing glycosylphosphatidylinositol-specific phospholipase D in MC3T3-E1 osteoblastic cells. This resulted in significant inhibition of intracellular calcium mobilization and ERK1/2 phosphorylation in response to oscillatory fluid flow. Finally, we demonstrated that cholesterol depletion inhibited oscillatory fluid flow induced ATP releases, which were responsible for the activation of calcium signaling pathways in MC3T3-E1 osteoblastic cells. Our findings suggest that cholesterol and GPI-anchored proteins, two membrane structural components related to lipid rafts, may play an important role in osteoblastic cell mechanotransduction.

Effect of low-magnitude, high-frequency vibration on osteogenic differentiation of rat mesenchymal stromal cells

Whole body vibration (WBV), consisting of a low-magnitude, high-frequency (LMHF) signal, is anabolic to bone in vivo and may act through alteration of the lineage commitment of mesenchymal stromal cells (MSC). We investigated the effect of LMHF vibration on rat bone marrow-derived MSCs (rMSCs) in an in vitro system. We subjected rMSCs to repeated (six) bouts of 1-h vibration at 0.3g and 60 Hz in the presence of osteogenic (OS) induction medium. The OS differentiation of rMSCs under the loaded and non-loaded conditions was assessed by examining cell proliferation, alkaline phosphatase (ALP) activity, mRNA expression of various osteoblast-associated markers [ALP, Runx2, osterix (Osx), collagen type I alpha 1 (COL1A1), bone sialoprotein (BSP), osteopontin (OPN), and osteocalcin (OCN)], and matrix mineralization. LMHF vibration did not enhance the OS differentiation of rMSCs. Surprisingly, the mRNA level of Osx, a transcription factor necessary for osteoblast formation, was decreased, and matrix mineralization was inhibited. Our findings suggest that LMHF vibration may exert its anabolic effects in vivo via mechanosensing of a cell type different from MSCs.

Comparison of the biological response of osteoblasts after tension and compression

The aim of this study was to investigate the difference in the biological response of osteoblasts when stretched and compressed. A cellular cyclic tension and compression apparatus (CCTCA) was designed to stretch and compress cells under the same conditions. After stretching or compressing MC3T3-E1 with continuously increased strain for 5 hours, cellular cytoskeletal modulation was detected by immunohistochemical assay with actin antibody. Real-time polymerase chain reaction was performed at 1, 3, and 5 hours to detect local factors related to bone remodelling. Statistical analysis was undertaken with analysis of variance and the Kruskal-Wallis. Following stretching or compression for 5 hours, MC3T3-E1 attached to the culture dishes grew well. Compared with the control, the microfilaments orientated parallel with each other and were clearly observed by laser scanning confocal microscope after 5 hours of stretching. The morphology of MC3T3-E1 cells was thinner and longer than the control. However, microfilaments presented a disordered arrangement after 5 hours of compression, and the MC3T3-E1 cells decreased in size. Gene expression of Wnt10b and Lrp5 increased during tension but more in the compression groups at 1, 3, and 5 hours. The ratio of osteoprotegerin to receptor activator for nuclear factor kappa B ligand increased in the tension group compared with the control but decreased in the compression group at 5 hours.

Effects of resistance and aerobic exercise on physical function, bone mineral density, OPG and RANKL in older women

This study compared the effects of a resistance training protocol and a moderate-impact aerobic training protocol on bone mineral density (BMD), physical ability, serum osteoprotegerin (OPG), and receptor activator of nuclear factor kappa B ligand (RANKL) levels. Seventy-one older women were randomly assigned to resistance exercise (RE), aerobic exercise (AE) or a control group (CON). Both interventions were conducted 3 times per week for 8 months. Outcome measures included proximal femur BMD, muscle strength, balance, body composition, serum OPG, and RANKL levels. Potential confounding variables included dietary intake, accelerometer-based physical activity (PA), and molecularly defined lactase nonpersistence. After 8 months, only RE group exhibited increases in BMD at the trochanter (2.9%) and total hip (1.5%), and improved body composition. Both RE and AE groups improved balance. No significant changes were observed in OPG and RANKL levels, and OPG/RANKL ratio. Lactase nonpersistence was not associated with BMD changes. No group differences were observed in baseline values or change in dietary intakes and daily PA. Data suggest that 8 months of RE may be more effective than AE for inducing favourable changes in BMD and muscle strength, whilst both interventions demonstrate to protect against the functional balance control that is strongly related to fall risk.

Human dental pulp cells exhibit bone cell-like responsiveness to fluid shear stress

BACKGROUND AIMS

For engineering bone tissue to restore, for example, maxillofacial defects, mechanosensitive cells are needed that are able to conduct bone cell-specific functions, such as bone remodelling. Mechanical loading affects local bone mass and architecture in vivo by initiating a cellular response via loading-induced flow of interstitial fluid. After surgical removal of ectopically impacted third molars, human dental pulp tissue is an easily accessible and interesting source of cells for mineralized tissue engineering. The aim of this study was to determine whether human dental pulp-derived cells (DPC) are responsive to mechanical loading by pulsating fluid flow (PFF) upon stimulation of mineralization in vitro.

METHODS

Human DPC were incubated with or without mineralization medium containing differentiation factors for 3 weeks. Cells were subjected to 1-h PFF (0.7 ± 0.3 Pa, 5 Hz) and the response was quantified by measuring nitric oxide (NO) and prostaglandin E₂ (PGE₂) production, and gene expression of cyclooxygenase (COX)-1 and COX-2.

RESULTS

We found that DPC are intrinsically mechanosensitive and, like osteogenic cells, respond to PFF-induced fluid shear stress. PFF stimulated NO and PGE₂ production, and up-regulated COX-2 but not COX-1 gene expression. In DPC cultured under mineralizing conditions, the PFF-induced NO, but not PGE₂, production was significantly enhanced.

CONCLUSIONS

These data suggest that human DPC, like osteogenic cells, acquire responsiveness to pulsating fluid shear stress in mineralizing conditions. Thus DPC might be able to perform bone-like functions during mineralized tissue remodeling in vivo, and therefore provide a promising new tool for mineralized tissue engineering to restore, for example, maxillofacial defects.

Morphological and proteomic analysis of early stage of osteoblast differentiation in osteoblastic progenitor cells

Bone remodeling relies on a dynamic balance between bone formation and resorption, mediated by osteoblasts and osteoclasts, respectively. Under certain stimuli, osteoprogenitor cells may differentiate into premature osteoblasts and further into mature osteoblasts. This process is marked by increased alkaline phosphatase (ALP) activity and mineralized nodule formation. In this study, we induced osteoblast differentiation in mouse osteoprogenitor MC3T3-E1 cells and divided the process into three stages. In the first stage (day 3), the MC3T3-E1 cell under osteoblast differentiation did not express ALP or deposit a mineralized nodule. In the second stage, the MC3T3-E1 cell expressed ALP but did not form a mineralized nodule. In the third stage, the MC3T3-E1 cell had ALP activity and formed mineralized nodules. In the present study, we focused on morphological and proteomic changes of MC3T3-E1 cells in the early stage of osteoblast differentiation - a period when premature osteoblasts transform into mature osteoblasts. We found that mean cell area and mean stress fiber density were increased in this stage due to enhanced cell spreading and decreased cell proliferation. We further analyzed the proteins in the signaling pathway of regulation of the cytoskeleton using a proteomic approach and found upregulation of IQGAP1, gelsolin, moesin, radixin, and Cfl1. After analyzing the focal adhesion signaling pathway, we found the upregulation of FLNA, LAMA1, LAMA5, COL1A1, COL3A1, COL4A6, and COL5A2 as well as the downregulation of COL4A1, COL4A2, and COL4A4. In conclusion, the signaling pathway of regulation of the cytoskeleton and focal adhesion play critical roles in regulating cell spreading and actin skeleton formation in the early stage of osteoblast differentiation.

Evidence for pleiotropic factors in genetics of the musculoskeletal system

There are both theoretical and empirical underpinnings that provide evidence that the musculoskeletal system develops, functions, and ages as a whole. Thus, the risk of osteoporotic fracture can be viewed as a function of loading conditions and the ability of the bone to withstand the load. Both bone loss (osteoporosis) and muscle wasting (sarcopenia) are the two sides of the same coin, an involution of the musculoskeletal system. Skeletal loads are dominated by muscle action; both bone and muscle share environmental, endocrine and paracrine influences. Muscle also has an endocrine function by producing bioactive molecules, which can contribute to homeostatic regulation of both bone and muscle. It also becomes clear that bone and muscle share genetic determinants; therefore the consideration of pleiotropy is an important aspect in the study of the genetics of osteoporosis and sarcopenia. The aim of this review is to provide an additional evidence for existence of the tight genetic co-regulation of muscles and bones, starting early in development and still evident in aging. Recently, important papers were published, including those dealing with the cellular mechanisms and anatomic substrate of bone mechanosensitivity. Further evidence has emerged suggesting that the relationship between skeletal muscle and bone parameters extends beyond the general paradigm of bone responses to mechanical loading. We provide insights into several pathways and single genes, which apparently have a biologically plausible pleiotropic effect on both bones and muscles; the list is continuing to grow. Understanding the crosstalk between muscles and bones will translate into a conceptual framework aimed at studying the pleiotropic genetic relationships in the etiology of complex musculoskeletal disease. We believe that further progress in understanding the common genetic etiology of osteoporosis and sarcopenia will provide valuable insight into important biological underpinnings for both musculoskeletal conditions. This may translate into new approaches to reduce the burden of both conditions, which are prevalent in the elderly population.

Stretch-induced inhibition of Wnt/beta-catenin signaling in mineralizing osteoblasts

Wnt signaling is important for bone formation and osteoblastic differentiation. Recent findings indicate a stimulating role of Wnt signaling in bone mechanotransduction. However, negative effects of Wnt signaling on osteoblast differentiation and mineralization have been described as well. We conducted in vitro stretch experiments using human pre-osteoblasts to study short- and long-term effects of mechanical loading on Wnt/beta-catenin signaling. As the extracellular regulated kinase (ERK) pathway is known to be involved in mechanotransduction in osteoblasts, we also evaluated its role in Wnt/beta-catenin signaling. Stretch experiments up to 21 days (using stretch episodes of 15 min, alternated with 90 min rest) resulted in higher mineralization compared to static control cultures. We found that 15 min of stretch initially increased nuclear beta-catenin, but ultimately resulted in significant decrease at 12 and 40 h after stretch. Downregulation of Wnt-responsive element activity 16 h after stretch, using a luciferase construct, further supported these findings. The presence of the ERK inhibitor U0126 did not alter the stretch-induced decrease of beta-catenin levels. Our data indicate a biphasic effect of mechanical loading on beta-catenin in mineralizing human differentiating osteoblasts, which is independent of the ERK pathway. The osteogenic potential of our loading regime was confirmed by an increase in osteogenic differentiation markers such as alkaline phosphatase activity and calcium deposition after 3 weeks of culture. We conjecture that the biphasic aspect of Wnt/beta-catenin signaling with a strong decrease up to 40 h after the stretch induction, is important for the anabolic effects of mechanical stretch on bone.

Concerted stimuli regulating osteo-chondral differentiation from stem cells: phenotype acquisition regulated by microRNAs

Bone and cartilage are being generated de novo through concerted actions of a plethora of signals. These act on stem cells (SCs) recruited for lineage-specific differentiation, with cellular phenotypes representing various functions throughout their life span. The signals are rendered by hormones and growth factors (GFs) and mechanical forces ensuring proper modelling and remodelling of bone and cartilage, due to indigenous and programmed metabolism in SCs, osteoblasts, chondrocytes, as well as osteoclasts and other cell types (eg T helper cells).This review focuses on the concerted action of such signals, as well as the regulatory and/or stabilizing control circuits rendered by a class of small RNAs, designated microRNAs. The impact on cell functions evoked by transcription factors (TFs) via various signalling molecules, also encompassing mechanical stimulation, will be discussed featuring microRNAs as important members of an integrative system. The present approach to cell differentiation in vitro may vastly influence cell engineering for in vivo tissue repair.

Signaling networks and transcription factors regulating mechanotransduction in bone

Mechanical stimulation has a critical role in the development and maintenance of the skeleton. This function requires the perception of extracellular stimuli as well as their conversion into intracellular biochemical responses. This process is called mechanotransduction and is mediated by a plethora of molecular events that regulate bone metabolism. Indeed, mechanoreceptors, such as integrins, G protein-coupled receptors, receptor protein tyrosine kinases, and stretch-activated Ca(2+) channels, together with their downstream effectors coordinate the transmission of load-induced signals to the nucleus and the expression of bone-related genes. During the past decade, scientists have gained increasing insight into the molecular networks implicated in bone mechanotransduction. In the present paper, we consider the major signaling cascades and transcription factors that control bone and cartilage mechanobiology and discuss the influence of the mechanical microenvironment on the determination of skeletal morphology.

Effects of high frequency loading on RANKL and OPG mRNA expression in ST-2 murine stromal cells

Background

Oscillatory fluid flow (OFF)-induced shear stress leads to positive bone remodeling through pro-formative and anti-resorptive effects on bone cells. In this study, the effects of high frequency OFF on expression of receptor activator of NF-κB ligand (RANKL) and osteoprotegerin (OPG), two important regulators of osteoclast differentiation, were investigated.

Methods

Cells were exposed to 1 Pa peak shear stress using three loading frequencies (1, 10, and 20 Hz) widely employed in cell, animal, and clinical studies of bone remodeling. Two separate experiments were performed where either the total number of cycles (3600 cycles) or the total loading time (60 min) was kept constant. Real-time RT-PCR was used to quantify mRNA levels of RANKL, OPG.

Results

3600 cycles of OFF at 1 Hz and 10 Hz loading decreased RANKL/OPG ratio. Interestingly, these results were due to different mechanisms where at 1 Hz the decrease was due to an increase in OPG mRNA, whereas at 10 Hz the decrease was due to a decrease in RANKL mRNA.

Conclusion

Although high frequency OFF does not appear to further enhance the decrease in the RANKL/OPG ratio, these results suggest a potential to differentially control the change in either RANKL or OPG mRNA expression by applying different loading frequencies.

Fractal analysis and ionic dependence of endocytotic membrane activity of human breast cancer cells

The endocytic membrane activities of two human breast cancer cell lines (MDA-MB-231 and MCF-7) of strong and weak metastatic potential, respectively, were studied in a comparative approach. Uptake of horseradish peroxidase was used to follow endocytosis. Dependence on ionic conditions and voltage-gated sodium channel (VGSC) activity were characterized. Fractal methods were used to analyze quantitative differences in vesicular patterning. Digital quantification showed that MDA-MB-231 cells took up more tracer (i.e., were more endocytic) than MCF-7 cells. For the former, uptake was totally dependent on extracellular Na(+) and partially dependent on extracellular and intracellular Ca(2+) and protein kinase activity. Analyzing the generalized fractal dimension (D(q )) and its Legendre transform f(alpha) revealed that under control conditions, all multifractal parameters determined had values greater for MDA-MB-231 compared with MCF-7 cells, consistent with endocytic/vesicular activity being more developed in the strongly metastatic cells. All fractal parameters studied were sensitive to the VGSC blocker tetrodotoxin (TTX). Some of the parameters had a "simple" dependence on VGSC activity, if present, whereby pretreatment with TTX reduced the values for the MDA-MB-231 cells and eliminated the differences between the two cell lines. For other parameters, however, there was a "complex" dependence on VGSC activity. The possible physical/physiological meaning of the mathematical parameters studied and the nature of involvement of VGSC activity in control of endocytosis/secretion are discussed.

Mechanisms of Strain-Mediated Mesenchymal Stem Cell Apoptosis

Mechanical conditioning of mesenchymal stem cells (MSCs) has been adopted widely as a biophysical signal to aid tissue engineering applications. The replication of in vivo mechanical signaling has been used in in vitro environments to regulate cell differentiation, and extracellular matrix synthesis, so that both the chemical and mechanical properties of the tissue-engineered construct are compatible with the implant site. While research in these areas contributes to tissue engineering, the effects of mechanical strain on MSC apoptosis remain poorly defined. To evaluate the effects of uniaxial cyclic tensile strain on MSC apoptosis and to investigate mechanotransduction associated with strain-mediated cell death, MSCs seeded on a 2D silicone membrane were stimulated by a range of strain magnitudes for 3days. Mechanotransduction was investigated using the stretch-activated cation channel blocker gadolinium chloride, the L-type voltage-activated calcium channel blocker nicardipine, the c-jun NH2-terminal kinase (JNK) blocker D-JNK inhibitor 1, and the calpain inhibitor MDL 28170. Apoptosis was assessed through DNA fragmentation using the terminal deoxynucleotidyl transferase mediated-UTP-end nick labeling method. Results demonstrated that tensile strains of 7.5% or greater induce apoptosis in MSCs. L-type voltage-activated calcium channels coupled mechanical stress to activation of calpain and JNK, which lead to apoptosis through DNA fragmentation. The definition of the in vitro boundary conditions for tensile strain and MSCs along with a proposed mechanism for apoptosis induced by mechanical events positively contributes to the development of MSC biology, bioreactor design for tissue engineering, and development of computational methods for mechanobiology.

NMDA enhances stretching-induced differentiation of osteoblasts through the ERK1/2 signaling pathway

Activation of the excitatory neurotransmitter N-methyl-d-aspartate (NMDA) and stretching both increase Ca(2+) influx in osteoblastic cells. We postulated that NMDA would enhance the osteoblastic cell's response to stretching. The goal of this study was to investigate, in the presence of the neurotransmitter NMDA, the effect of mechanical loading on osteoblast's stage of differentiation and the mitogen-activated protein kinase (MAPK) signaling pathway associated with it. Rat primary osteoblastic cells were subjected to cyclic, equibiaxial stretch for 48 h in the presence or absence of NMDA. Pretreatment with 0.5 mM NMDA significantly enhanced the stretching magnitude-dependent increase in osteogenesis markers. MK801, an antagonist of NMDA receptors, abolished those responses. To further study the mechanism of this response, osteoblastic cells were stretched for 5, 15, or 60 min in the absence of NMDA. Cyclic stretch induced a rapid increase in extracellular signal-regulated kinase ERK1/2 phosphorylation with the peak at 15 min, but no changes were noted in p38 and JNK pathway signaling. NMDA could enhance ERK1/2 phosphorylation stimulated by stretching. U0126, an inhibitor of ERK1/2, blocked the increase in osteogenesis markers. In conclusion, the current study demonstrates that there is a synergistic effect between mechanical stimulation and NMDA in osteoblasts. ERK1/2 signaling may be the common pathway in the increased response to stretching in the presence of NMDA in osteoblastic cells.

Beta-catenin levels influence rapid mechanical responses in osteoblasts

Mechanical loading of bone initiates an anabolic, anticatabolic pattern of response, yet the molecular events involved in mechanical signal transduction are not well understood. Wnt/beta-catenin signaling has been recognized in promoting bone anabolism, and application of strain has been shown to induce beta-catenin activation. In this work, we have used a preosteoblastic cell line to study the effects of dynamic mechanical strain on beta-catenin signaling. We found that mechanical strain caused a rapid, transient accumulation of active beta-catenin in the cytoplasm and its translocation to the nucleus. This was followed by up-regulation of the Wnt/beta-catenin target genes Wisp1 and Cox2, with peak responses at 4 and 1 h of strain, respectively. The increase of beta-catenin was temporally related to the activation of Akt and subsequent inactivation of GSK3beta, and caveolin-1 was not required for these molecular events. Application of Dkk-1, which disrupts canonical Wnt/LRP5 signaling, did not block strain-induced nuclear translocation of beta-catenin or up-regulation of Wisp1 and Cox2 expression. Conditions that increased basal beta-catenin levels, such as lithium chloride treatment or repression of caveolin-1 expression, were shown to enhance the effects of strain. In summary, mechanical strain activates Akt and inactivates GSK3beta to allow beta-catenin translocation, and Wnt signaling through LRP5 is not required for these strain-mediated responses. Thus, beta-catenin serves as both a modulator and effector of mechanical signals in bone cells.

Metformin enhances the differentiation and mineralization of osteoblastic MC3T3-E1 cells via AMP kinase activation as well as eNOS and BMP-2 expression

It is unclear whether metformin, one of the anti-hyperglycemic agents commonly used for type 2 diabetes, could affect bone formation through activation of AMP-activated protein kinase (AMPK). In order to clarify this issue, we investigated the effects of metformin on the differentiation and mineralization of osteoblastic MC3T3-E1 cells as well as intracellular signal transduction. Metformin (50 microM) significantly increased collagen-I and osteocalcin mRNA expression, stimulated alkaline phosphatase activity, and enhanced cell mineralization. Moreover, metformin significantly activated AMPK in dose- and time-dependent manners, and induced endothelial nitric oxide synthase (eNOS) and bone morphogenetic protein-2 (BMP-2) expressions. Supplementation of Ara-A (0.1mM), a specific AMPK inhibitor, significantly reversed the metformin-induced eNOS and BMP-2 expressions. Our findings suggest that metformin can induce the differentiation and mineralization of osteoblasts via activation of the AMPK signaling pathway, and that this drug might be beneficial for not only diabetes but also osteoporosis by promoting bone formation.