Mechanical loading modulates glutamate receptor subunit expression in bone

The emerging role of tranexamic acid and its principal target, plasminogen, in skeletal health

The worldwide burden of skeletal diseases such as osteoporosis, degenerative joint disease and impaired fracture healing is steadily increasing. Tranexamic acid (TXA), a plasminogen inhibitor and anti-fibrinolytic agent, is used to reduce bleeding with high effectiveness and safety in major surgical procedures. With its widespread clinical application, the effects of TXA beyond anti-fibrinolysis have been noticed and prompted renewed interest in its use. Some clinical trials have characterized the effects of TXA on reducing postoperative infection rates and regulating immune responses in patients undergoing surgery. Also, several animal studies suggest potential therapeutic effects of TXA on skeletal diseases such as osteoporosis and fracture healing. Although a direct effect of TXA on the differentiation and function of bone cells in vitro was shown, few mechanisms of action have been reported. Here, we summarize recent findings of the effects of TXA on skeletal diseases and discuss the underlying plasminogen-dependent and -independent mechanisms related to bone metabolism and the immune response. We furthermore discuss potential novel indications for TXA application as a treatment strategy for skeletal diseases.

Dementia, osteoporosis and fragility fractures: Intricate epidemiological relationships, plausible biological connections, and twisted clinical practices

Dementia, osteoporosis, and fragility fractures are chronic diseases, often co-existing in older adults. These conditions pose severe morbidity, long-term disability, and mortality, with relevant socioeconomic implications. While in the research arena, the discussion remains on whether dementia is the cause or the consequence of fragility fractures, healthcare professionals need a better understanding of the interplay between such conditions from epidemiological and physiological standpoints. With this review, we summarized the available literature surrounding the relationship between cognitive impairment, dementia, and both low bone mineral density (BMD) and fragility fractures. Given the strength of the bi-directional associations and their impact on the quality of life, we shed light on the biological connections between brain and bone systems, presenting the main mediators, including gut microbioma, and pathological pathways leading to the dysregulation of bone and brain metabolism. Ultimately, we synthesized the evidence about the impact of available pharmacological treatments for the prevention of fragility fractures on cognitive functions and individuals' outcomes when dementia coexists. Vice versa, the effects of symptomatic treatments for dementia on the risk of falls and fragility fractures are explored. Combining evidence alongside clinical practice, we discuss challenges and opportunities related to the management of older adults affected by cognitive impairment or dementia and at high risk for fragility fracture prevention, which leads to not only an improvement in patient health-related outcomes and survival but also a reduction in healthcare cost and socio-economic burden.

Unfavorable effects of memantine on the skeletal system in female rats

Memantine is an N-methyl-D-aspartate (NMDA) receptor antagonist used in the treatment of Alzheimer's disease (AD). NMDA receptors are expressed on bone cells. The aim of the present study was to investigate the effects of memantine on the rat musculoskeletal system. Taking into account that most of female AD patients are postmenopausal, the study was carried out on intact and ovariectomized (estrogen-deficient) rats. Mature Wistar rats were divided into following groups: non-ovariectomized (NOVX) control rats, NOVX rats treated with memantine, ovariectomized (OVX) control rats, and OVX rats treated with memantine. Memantine (2 mg/kg p.o.) was administered once daily for four weeks, starting one week after ovariectomy. The serum bone turnover marker and cytokine levels, bone density, mass, mineralization, mechanical properties, histomorphometric parameters of compact and cancellous bone, skeletal muscle mass and grip strength were determined. In NOVX rats, memantine slightly decreased the strength of compact bone of the femoral diaphysis (parameters in the yield point) and unfavorably affected histomorphometric parameters of cancellous bone (the femoral epiphysis and metaphysis). In OVX rats, in which estrogen deficiency induced osteoporotic changes, memantine increased the phosphorus content in the femoral bone mineral. No other effects on bone were observed in the memantine-treated OVX rats. In conclusion, the results of the present study indicated slight damaging skeletal effects of memantine in rats with normal estrogen levels.

Metabolic features of myeloma cells in the context of bone microenvironment: Implication for the pathophysiology and clinic of myeloma bone disease

Multiple myeloma (MM) is a hematological malignancy characterized by the accumulation of malignant plasma cells (PCs) into the bone marrow (BM). The complex interaction between the BM microenvironment and MM PCs can lead to severe impairment of bone remodeling. Indeed, the BM microenvironment exerts a critical role in the survival of malignant PCs. Growing evidence indicates that MM cells have several metabolic features including enhanced glycolysis and an increase in lactate production through the upregulation of glucose transporters and enzymes. More recently, it has been reported that MM cells arehighly glutamine addicted. Interestingly, these metabolic changes in MM cells may affect BM microenvironment cells by altering the differentiation process of osteoblasts from mesenchymal stromal cells. The identification of glutamine metabolism alterations in MM cells and bone microenvironment may provide a rationale to design new therapeutic approaches and diagnostic tools. The osteolytic lesions are the most frequent clinical features in MM patients, often characterized by pathological fractures and acute pain. The use of the newer imaging techniques such as Magnetic Resonance Imaging (MRI) and combined Positron Emission Tomography (PET) and Computerized Tomography (CT) has been introduced into clinical practice to better define the skeletal involvement. Currently, the PET/CT with ¹⁸F-fluorodeoxyglucose (FDG) is the diagnostic gold standard to detect active MM bone disease due to the high glycolytic activity of MM cells. However, new tracers are actively under investigation because a portion of MM patients remains negative at the skeletal level by ¹⁸F-FDG. In this review, we will summarize the existing knowledge on the metabolic alterations of MM cells considering their impact on the BM microenvironment cells and particularly in the subsequent formation of osteolytic bone lesions. Based on this, we will discuss the identification of possible new druggable targets and the use of novel metabolic targets for PET imaging in the detection of skeletal lesions, in the staging and treatment response of MM patients.

Glutamatergic system components as potential biomarkers and therapeutic targets in cancer in non-neural organs

Glutamate is one of the most abundant amino acids in the blood. Besides its role as a neurotransmitter in the brain, it is a key substrate in several metabolic pathways and a primary messenger that acts through its receptors outside the central nervous system (CNS). The two main types of glutamate receptors, ionotropic and metabotropic, are well characterized in CNS and have been recently analyzed for their roles in non-neural organs. Glutamate receptor expression may be particularly important for tumor growth in organs with high concentrations of glutamate and might also influence the propensity of such tumors to set metastases in glutamate-rich organs, such as the liver. The study of glutamate transporters has also acquired relevance in the physiology and pathologies outside the CNS, especially in the field of cancer research. In this review, we address the recent findings about the expression of glutamatergic system components, such as receptors and transporters, their role in the physiology and pathology of cancer in non-neural organs, and their possible use as biomarkers and therapeutic targets.

The mechanosensory and mechanotransductive processes mediated by ion channels and the impact on bone metabolism: A systematic review

Mechanical environments were associated with alterations in bone metabolism. Ion channels present on bone cells are indispensable for bone metabolism and can be directly or indirectly activated by mechanical stimulation. This review aimed to discuss the literature reporting the mechanical regulatory effects of ion channels on bone cells and bone tissue. An electronic search was conducted in PubMed, Embase and Web of Science. Studies about mechanically induced alteration of bone cells and bone tissue by ion channels were included. Ion channels including TRP family channels, Ca²⁺ release-activated Ca²⁺ channels (CRACs), Piezo1/2 channels, purinergic receptors, NMDA receptors, voltage-sensitive calcium channels (VSCCs), TREK2 potassium channels, calcium- and voltage-dependent big conductance potassium (BK_Ca) channels, small conductance, calcium-activated potassium (SK_Ca) channels and epithelial sodium channels (ENaCs) present on bone cells and bone tissue participate in the mechanical regulation of bone development in addition to contributing to direct or indirect mechanotransduction such as altered membrane potential and ionic flux. Physiological (beneficial) mechanical stimulation could induce the anabolism of bone cells and bone tissue through ion channels, but abnormal (harmful) mechanical stimulation could also induce the catabolism of bone cells and bone tissue through ion channels. Functional expression of ion channels is vital for the mechanotransduction of bone cells. Mechanical activation (opening) of ion channels triggers ion influx and induces the activation of intracellular modulators that can influence bone metabolism. Therefore, mechanosensitive ion channels provide new insights into therapeutic targets for the treatment of bone-related diseases such as osteopenia and aseptic implant loosening.

Treadmill exercise influences the microRNA profiles in the bone tissues of mice

As an important regulator involved in cell activity, microRNAs (miRNAs) are important in the process of exercise influencing bone metabolism. The present study aimed to detect and select differentially expressed miRNAs in the bone tissues of mice trained on a treadmill, predict the target genes of these differentially expressed miRNAs and lay a foundation for exploring the effect of treadmill training on bone metabolism through miRNAs. In this experiment, after the mice were trained on a treadmill for 8 weeks, the mechanical properties of mouse femur bone were assessed, and the alkaline phosphatase (ALP) activity and osteocalcin (OCN) protein levels of the bone were assayed. miRNA microarray and reverse transcription-quantitative (RT-q)PCR were performed to select and validate differentially expressed miRNAs in the bone, and the target genes of these miRNAs were predicted with bioinformatics methods. In addition, the differentially expressed miRNAs in the bone tissues were compared with those in mechanically strained osteocytes in vitro. Treadmill training improved the mechanical properties of the femur bones of mice, and elevated the ALP activity and OCN protein level in the bone. In addition, 122 differentially expressed miRNAs were detected in the bone, of which nine were validated via RT-qPCR. Among the target genes of these differentially expressed miRNAs, certain candidates were involved in bone metabolism. A total of eight miRNAs were differentially expressed in both bone tissue and osteocytes, exhibiting the same expression trends, and various target genes of these eight miRNAs were also involved in bone metabolism. Treadmill training resulted in altered miRNA expression profiles in the bones of mice (mainly in osteocytes) and the differentially expressed miRNAs may serve important roles in regulating bone metabolism and osteogenic differentiation.

N-methyl-D-aspartate (NMDA) receptor expression and function is required for early chondrogenesis

Background

In vitro chondrogenesis depends on the concerted action of numerous signalling pathways, many of which are sensitive to the changes of intracellular Ca²⁺ concentration. N-methyl-D-aspartate (NMDA) glutamate receptor is a cation channel with high permeability for Ca²⁺. Whilst there is now accumulating evidence for the expression and function of NMDA receptors in non-neural tissues including mature cartilage and bone, the contribution of glutamate signalling to the regulation of chondrogenesis is yet to be elucidated.

Methods

We studied the role of glutamatergic signalling during the course of in vitro chondrogenesis in high density chondrifying cell cultures using single cell fluorescent calcium imaging, patch clamp, transient gene silencing, and western blotting.

Results

Here we show that key components of the glutamatergic signalling pathways are functional during in vitro chondrogenesis in a primary chicken chondrogenic model system. We also present the full glutamate receptor subunit mRNA and protein expression profile of these cultures. This is the first study to report that NMDA-mediated signalling may act as a key factor in embryonic limb bud-derived chondrogenic cultures as it evokes intracellular Ca²⁺ transients, which are abolished by the GluN2B subunit-specific inhibitor ifenprodil. The function of NMDARs is essential for chondrogenesis as their functional knock-down using either ifenprodil or GRIN1 siRNA temporarily blocks the differentiation of chondroprogenitor cells. Cartilage formation was fully restored with the re-expression of the GluN1 protein.

Conclusions

We propose a key role for NMDARs during the transition of chondroprogenitor cells to cartilage matrix-producing chondroblasts.

Bone Health in Rats With Temporal Lobe Epilepsy in the Absence of Anti-Epileptic Drugs

Rationale: Epilepsy patients often exhibit reduced bone mineral density and are at an increased risk of bone fracture. Whether these bone abnormalities are due to the use of anti-epileptic drugs (AED’s) or the disease itself is unknown. For example, although decreased bone health in epilepsy patients is generally attributed to the use of AED’s, seizures can also trigger a number of physiological processes that have the potential to affect bone. Therefore, to assess whether bone abnormalities occur in epilepsy in the absence of AED’s, the current study investigated mechanical characteristics and trabecular bone morphology in rats with chronic temporal lobe epilepsy.

Methods: Ten-week old male Wistar rats underwent kainic acid-induced status epilepticus (SE; n = 7) or a sham procedure (n = 9). Rats were implanted with EEG recording electrodes at nine weeks post-SE, and video-EEG was continuously recorded for one week at 10- and 22-weeks post-SE to confirm that SE rats had spontaneous seizures. Open-field testing to assess locomotion was conducted at 23-weeks post-SE. At 24-weeks post-SE, rats were euthanized and tibia were extracted to determine trabecular morphology by micro-computed tomography (µCT), while femurs were used to investigate mechanical properties via 3-point bending.

Results: All post-SE rats had spontaneous seizures at 10- and 22-weeks post-SE, while none of the sham rats had seizures. µCT trabecular analysis of tibia revealed no differences in total volume, bone volume, bone volume fraction, trabecular number, or trabecular separation between post-SE or sham rats, although post-SE rats did have increased trabecular thickness. There were also no group differences in total distance travelled in the open field suggesting that activity levels did not account for the increased trabecular thickness. In addition, no differences in mechanical properties of femurs were observed between the two groups.

Conclusion: There was a lack of overt bone abnormalities in rats with chronic temporal lobe epilepsy in the absence of AED treatment. Although further studies are still needed, these findings may have important implications towards understanding the source (e.g., AED treatments) of bone abnormalities in epilepsy patients.

The role of GPCRs in bone diseases and dysfunctions

The superfamily of G protein-coupled receptors (GPCRs) contains immense structural and functional diversity and mediates a myriad of biological processes upon activation by various extracellular signals. Critical roles of GPCRs have been established in bone development, remodeling, and disease. Multiple human GPCR mutations impair bone development or metabolism, resulting in osteopathologies. Here we summarize the disease phenotypes and dysfunctions caused by GPCR gene mutations in humans as well as by deletion in animals. To date, 92 receptors (5 glutamate family, 67 rhodopsin family, 5 adhesion, 4 frizzled/taste2 family, 5 secretin family, and 6 other 7TM receptors) have been associated with bone diseases and dysfunctions (36 in humans and 72 in animals). By analyzing data from these 92 GPCRs, we found that mutation or deletion of different individual GPCRs could induce similar bone diseases or dysfunctions, and the same individual GPCR mutation or deletion could induce different bone diseases or dysfunctions in different populations or animal models. Data from human diseases or dysfunctions identified 19 genes whose mutation was associated with human BMD: 9 genes each for human height and osteoporosis; 4 genes each for human osteoarthritis (OA) and fracture risk; and 2 genes each for adolescent idiopathic scoliosis (AIS), periodontitis, osteosarcoma growth, and tooth development. Reports from gene knockout animals found 40 GPCRs whose deficiency reduced bone mass, while deficiency of 22 GPCRs increased bone mass and BMD; deficiency of 8 GPCRs reduced body length, while 5 mice had reduced femur size upon GPCR deletion. Furthermore, deficiency in 6 GPCRs induced osteoporosis; 4 induced osteoarthritis; 3 delayed fracture healing; 3 reduced arthritis severity; and reduced bone strength, increased bone strength, and increased cortical thickness were each observed in 2 GPCR-deficiency models. The ever-expanding number of GPCR mutation-associated diseases warrants accelerated molecular analysis, population studies, and investigation of phenotype correlation with SNPs to elucidate GPCR function in human diseases.

Evaluation of the rivastigmine role against botulinum toxin-A-induced osteoporosis in albino rats: A biochemical, histological, and immunohistochemical study

The present study aimed to evaluate the role of rivastigmine against the effect of a single unilateral botulinum toxin-A (BTX-A) injection on the bone and bone marrow of adult albino rats 4 weeks after injection. Twenty-four Wistar albino rats were divided into four equal groups: group I, which received distilled water; group II, which received rivastigmine (0.3 mg/kg daily, intraperitoneally for 4 weeks); group III, which received BTX-A (4 IU in 0.2 mL physiological saline) single dose, intramuscularly; and group IV, which received BTX-A + rivastigmine. The results revealed that BTX-A induced a significant decrease in the calcium level with a significant increase in the phosphorus, alkaline phosphatase, C-reactive protein, and tumor necrosis factor α levels in serum. Furthermore, a significant increase in malondialdehyde with a significant decrease in reduced glutathione activities in both bone and bone marrow. Histologically, a distortion and thinning out of the compact bone and trabeculae of cancellous bone of the rat femur in the BTX-A group with an increase in adipocytes in adjacent bone marrow were detected. Immunohistochemically, Cluster of Differentiation 68 (CD68) showed a significant increase in both osteoclasts and bone marrow macrophage. Rivastigmine treatment could relieve the toxic effects induced by BTX-A. In conclusion, rivastigmine has a protective effect against the hazardous effects of BTX-A on bone and bone marrow.

Glutamate Receptor Agonists and Glutamate Transporter Antagonists Regulate Differentiation of Osteoblast Lineage Cells

Development and function of osteoblast lineage cells are regulated by a complex microenvironment consisting of the bone extracellular matrix, cells, systemic hormones and cytokines, autocrine and paracrine factors, and mechanical load. Apart from receptors that transduce extracellular signals into the cell, molecular transporters play a crucial role in the cellular response to the microenvironment. Transporter molecules are responsible for cellular uptake of nutritional components, elimination of metabolites, ion transport, and cell-cell communication. In this report, the expression of molecular transporters in osteoblast lineage cells was investigated to assess their roles in cell development and activity. Low-density arrays, covering membrane and vesicular transport molecules, were used to assess gene expression in osteoblasts representing early and late differentiation states. Receptors and transporters for the amino acid glutamate were found to be differentially expressed during osteoblast development. Glutamate is a neurotransmitter in the central nervous system, and the mechanisms of its release, signal transduction, and cellular reabsorption in the synaptic cleft are well understood. Less clear, however, is the control of equivalent processes in peripheral tissues. In primary osteoblasts, inhibition of glutamate transporters with nonselective inhibitors leads to an increase in the concentration of extracellular glutamate. This change was accompanied by a decrease in osteoblast proliferation, stimulation of alkaline phosphatase, and the expression of transcripts encoding osteocalcin. Enzymatic removal of extracellular glutamate abolished these pro-differentiation effects, as did the inhibition of PKC- and Erk1/2-signaling pathways. These findings demonstrate that glutamate signaling promotes differentiation and activation of osteoblast lineage cells. Consequently, the glutamate system may represent a putative therapeutic target to induce an anabolic response in the skeletal system. Known antagonists of glutamate transporters will serve as lead compounds in developing new and specific bioactive molecules.

Integrative Analysis of Genomics and Transcriptome Data to Identify Potential Functional Genes of BMDs in Females

Osteoporosis is known to be highly heritable. However, to date, the findings from more than 20 genome-wide association studies (GWASs) have explained less than 6% of genetic risks. Studies suggest that the missing heritability data may be because of joint effects among genes. To identify novel heritability for osteoporosis, we performed a system-level study on bone mineral density (BMD) by weighted gene coexpression network analysis (WGCNA), using the largest GWAS data set for BMD in the field, Genetic Factors for Osteoporosis Consortium (GEFOS-2), and a transcriptomic gene expression data set generated from transiliac bone biopsies in women. A weighted gene coexpression network was generated for 1574 genes with GWAS nominal evidence of association (p ≤ 0.05) based on dissimilarity measurement on the expression data. Twelve distinct gene modules were identified, and four modules showed nominally significant associations with BMD (p ≤ 0.05), but only one module, the yellow module, demonstrated a good correlation between module membership (MM) and gene significance (GS), suggesting that the yellow module serves an important biological role in bone regulation. Interestingly, through characterization of module content and topology, the yellow module was found to be significantly enriched with contractile fiber part (GO:044449), which is widely recognized as having a close relationship between muscle and bone. Furthermore, detailed submodule analyses of important candidate genes (HOMER1, SPTBN1) by all edges within the yellow module implied significant enrichment of functional connections between bone and cytoskeletal protein binding. Our study yielded novel information from system genetics analyses of GWAS data jointly with transcriptomic data. The findings highlighted a module and several genes in the model as playing important roles in the regulation of bone mass in females, which may yield novel insights into the genetic basis of osteoporosis. © 2016 American Society for Bone and Mineral Research.

Excitatory amino acid glutamate: role in peripheral nociceptive transduction and inflammation in experimental and clinical osteoarthritis

Although a large proportion of patients with osteoarthritis (OA) show inflammation in their affected joints, the pathological role of inflammation in the development and progression of OA has yet to be clarified. Glutamate is considered an excitatory amino acid (EAA) neurotransmitter in the mammalian central nervous system (CNS). There are cellular membrane glutamate receptors and transporters for signal input modulation and termination as well as vesicular glutamate transporters (VGLUTs) for signal output through exocytotic release. Glutamate been shown to mediate intercellular communications in bone cells in a manner similar to synaptic transmission within the CNS. Glutamate-mediated events may also contribute to the pathogenesis and ongoing processes of peripheral nociceptive transduction and inflammation of experimental arthritis models as well as human arthritic conditions. This review will discuss the differential roles of glutamate signaling and blockade in peripheral neuronal and non-neuronal joint tissues, including bone remodeling systems and their potentials to impact OA-related inflammation and progression. This will serve to identify several potential targets to direct novel therapies for OA. Future studies will further elucidate the role of glutamate in the development and progression of OA, as well as its association with the clinical features of the disease.

MicroRNA-218, microRNA-191*, microRNA-3070a and microRNA-33 are responsive to mechanical strain exerted on osteoblastic cells

MicroRNA (miRNA) is an important regulator of cell differentiation and function. Mechanical strain is important in the growth and differentiation of osteoblasts. Therefore, mechanresponsive miRNA may be important in the response of osteoblasts to mechanical strain. The purpose of the present study was to select and identify the mechanoresponsive miRNAs of osteoblasts. Mouse osteoblastic MC3T3-E1 cells were cultured in cell culture dishes and stimulated with a mechanical tensile strain of 2,50 με at 0.5 Hz, and the activity of alkaline phosphatase (ALP), mRNA levels of ALP, osteocalcin (OCN), and collagen type I (Col I), and protein levels of bone morphogenetic proteins (BMPs) in the cell culture medium were assayed. Following miRNA microarray and reverse transcription-quantitative polymerase chain reaction analyses, differentially expressed miRNAs in the mechanically strained cells and unstrained cells were selected and identified. Using bioinformatics analysis, the target genes of the miRNAs were then predicted. The results revealed that the mechanical strain of 2,500 με increased the activity of ALP, the mRNA levels of ALP, OCN and Col I, and the protein levels of bone morphogenetic protein(BMP)-2 and BMP-4 Continuous mechanical stimulation for 8 h had the most marked stimulant effects. miR-218, miR-191*, miR-3070a and miR-33 were identified as differentially expressed miRNAs in the mechanically strained MC3T3-E1 cells. Certain target genes of these four miRNAs were involved in osteoblastic differentiation. These findings indicated that a mechanical strain of 2,500 με, particularly for a period of 8 h, promoted osteoblastic differentiation, and the four mechanoresponsive miRNAs identified may be a potential regulator of osteoblastic differentiation and their response to mechanical strain.

Shear stress-induced Ca(2+) elevation is mediated by autocrine-acting glutamate in osteoblastic MC3T3-E1 cells

Mechanical loading is an important regulatory factor in bone homeostasis. Neurotransmitters, such as glutamate and ATP, are known to be released from osteoblasts, but their roles have been less studied. In this study, we investigated the role of transmitter release in mechanotransduction. To identify from where transmitters were released, focal fluid flow was applied to a single cell of MC3T3-E1, mouse calvaria-derived osteoblastic cell line, by using a glass micropipette. Intracellular Ca(2+) elevation induced by the focal shear stress was eliminated by either GdCl3, a mechanosensing channel inhibitor, or removal of extracellular Ca(2+). On the other hand, the focal shear stress-induced Ca(2+) elevation was also significantly suppressed by inositol triphosphate receptor antagonist or vesicular release inhibitors. These results suggest that not only mechanosensitive channel-mediated Ca(2+) influx but also some autocrine transmitters are involved in mechanotransduction. Additionally, glutamate receptor antagonists, but not ATP receptor antagonist, suppressed most of the focal shear stress-induced Ca(2+) elevation. Therefore, it is suggested that glutamate is released from osteoblasts following the activation of mechanosensitive Ca(2+) channels and acts in an autocrine manner. The glutamate release may have a significant role in the initial event of mechanotransduction in bone tissue.

The potential of targeting NMDA receptors outside the CNS

INTRODUCTION

NMDA receptor (NMDAR) is an ionotropic glutamate receptor with a high permeability to calcium and a unique feature of controlling numerous calcium-dependent processes. Apart from being widely distributed in the CNS, the presence of NMDAR and its potential significance in a variety of non-neuronal cells and tissues has become an interesting research topic.

AREAS COVERED

The current review summarizes prevailing knowledge on the role of NMDARs in the kidney, bone and parathyroid gland, three main organs responsible for calcium homeostasis, as well as in the heart, an organ whose function is highly dependable on balanced intracellular calcium concentrations. The review also examines studies that have advanced our understanding of the therapeutic potential of NMDAR agonists and antagonists in renal, cardiovascular and bone pathologies.

EXPERT OPINION

NMDARs have a preeminent role in many physiological and pathological processes outside the CNS. In certain organs and/or disease conditions, activating the NMDAR leads to beneficial effects for the target organ, whereas in other diseases cell signaling downstream of NMDAR activation can exacerbate their pathology. Therefore, targeting NMDARs therapeutically is rather intricate work, and surely requires more extensive investigation in order to properly tune up the diverse NMDAR's actions translating them into beneficial cellular responses.

Aging diminishes lamellar and woven bone formation induced by tibial compression in adult C57BL/6

Aging purportedly diminishes the ability of the skeleton to respond to mechanical loading, but recent data show that old age did not impair loading-induced accrual of bone in BALB/c mice. Here, we hypothesized that aging limits the response of the tibia to axial compression over a range of adult ages in the commonly used C57BL/6. We subjected the right tibia of old (22 month), middle-aged (12 month) and young-adult (5 month) female C57BL/6 mice to peak periosteal strains (measured near the mid-diaphysis) of -2200 με and -3000 με (n=12-15/age/strain) via axial tibial compression (4 Hz, 1200 cycles/day, 5 days/week, 2 weeks). The left tibia served as a non-loaded, contralateral control. In mice of every age, tibial compression that engendered a peak strain of -2200 με did not alter cortical bone volume but loading to a peak strain of -3000 με increased cortical bone volume due in part to woven bone formation. Both loading magnitudes increased total volume, medullary volume and periosteal bone formation parameters (MS/BS, BFR/BS) near the cortical midshaft. Compared to the increase in total volume and bone formation parameters of 5-month mice, increases were less in 12- and 22-month mice by 45-63%. Moreover, woven bone incidence was greatest in 5-month mice. Similarly, tibial loading at -3000 με increased trabecular BV/TV of 5-month mice by 18% (from 0.085 mm3/mm3), but trabecular BV/TV did not change in 12- or 22-month mice, perhaps due to low initial BV/TV (0.032 and 0.038 mm3/mm3, respectively). In conclusion, these data show that while young-adult C57BL/6 mice had greater periosteal bone formation following loading than middle-aged or old mice, aging did not eliminate the ability of the tibia to accrue cortical bone.

NMDARs in neurological diseases: a potential therapeutic target

N-methyl-D-aspartate ionotropic glutamate receptor (NMDARs) is a ligand-gated ion channel that plays a critical role in excitatory neurotransmission, brain development, synaptic plasticity associated with memory formation, central sensitization during persistent pain, excitotoxicity and neurodegenerative diseases in the central nervous system (CNS). Within iGluRs, NMDA receptors have been the most actively investigated for their role in neurological diseases, especially neurodegenerative pathologies such as Alzheimer's and Parkinson's diseases. It has been demonstrated that excessive activation of NMDA receptors (NMDARs) plays a key role in mediating some aspects of synaptic dysfunction in several CNS disorders, so extensive research has been directed on the discovery of compounds that are able to reduce NMDARs activity. This review discusses the role of NMDARs on neurological pathologies and the possible therapeutic use of agents that target this receptor. Additionally, we delve into the role of NMDARs in Alzheimer's and Parkinson's diseases and the receptor antagonists that have been tested on in vivo models of these pathologies. Finally, we put into consideration the importance of antioxidants to counteract oxidative capacity of the signaling cascade in which NMDARs are involved.

AMPA/kainate glutamate receptors contribute to inflammation, degeneration and pain related behaviour in inflammatory stages of arthritis

Objectives

Synovial fluid glutamate concentrations increase in arthritis. Activation of kainate (KA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptors (GluRs) increase interleukin-6 (IL-6) release and cause arthritic pain, respectively. We hypothesised that AMPA and KA GluRs are expressed in human arthritis, and that intra-articular NBQX (AMPA/KA GluR antagonist) prevents pain and pathology in antigen-induced arthritis (AIA).

Methods

GluR immunohistochemistry was related to synovial inflammation and degradation in osteoarthritis (OA) and rheumatoid arthritis (RA). A single intra-articular NBQX injection was given at induction, and knee swelling and gait of AIA and AIA+NBQX rats compared over 21 days, before imaging, RT-qPCR, histology and immunohistochemistry of joints. Effects of NBQX on human primary osteoblast (HOB) activity were determined.

Results

AMPAR2 and KA1 immunolocalised to remodelling bone, cartilage and synovial cells in human OA and RA, and rat AIA. All arthritic tissues showed degradation and synovial inflammation. NBQX reduced GluR abundance, knee swelling (p<0.001, days 1–21), gait abnormalities (days 1–2), end-stage joint destruction (p<0.001), synovial inflammation (p<0.001), and messenger RNA expression of meniscal IL-6 (p<0.05) and whole joint cathepsin K (p<0.01). X-ray and MRI revealed fewer cartilage and bone erosions, and less inflammation after NBQX treatment. NBQX reduced HOB number and prevented mineralisation.

Conclusions

AMPA/KA GluRs are expressed in human OA and RA, and in AIA, where a single intra-articular injection of NBQX reduced swelling by 33%, and inflammation and degeneration scores by 34% and 27%, respectively, exceeding the efficacy of approved drugs in the same model. AMPA/KA GluR antagonists represent a potential treatment for arthritis.

Glutamate signaling in healthy and diseased bone

Bone relies on multiple extracellular signaling systems to maintain homeostasis of its normal structure and functions. The amino acid glutamate is a fundamental extracellular messenger molecule in many tissues, and is used in bone for both neural and non-neural signaling. This review focuses on the non-neural interactions, and examines the evolutionarily ancient glutamate signaling system in the context of its application to normal bone functioning and discusses recent findings on the role of glutamate signaling as they pertain to maintaining healthy bone structure. The underlying mechanisms of glutamate signaling and the many roles glutamate plays in modulating bone physiology are featured, including those involved in osteoclast and osteoblast differentiation and mature cell functions. Moreover, the relevance of glutamate signaling systems in diseases that affect bone, such as cancer and rheumatoid arthritis, is discussed, and will highlight how the glutamate system may be exploited as a viable therapeutic target. We will identify novel areas of research where knowledge of glutamate communication mechanisms may aid in our understanding of the complex nature of bone homeostasis. By uncovering the contributions of glutamate in maintaining healthy bone, the reader will discover how this complex molecular signaling system may advance our capacity to treat bone pathologies.

Glutamate signaling in bone

Mechanical loading plays a key role in the physiology of bone, allowing bone to functionally adapt to its environment, however characterization of the signaling events linking load to bone formation is incomplete. A screen for genes associated with mechanical load-induced bone formation identified the glutamate transporter GLAST, implicating the excitatory amino acid, glutamate, in the mechanoresponse. When an osteogenic load (10 N, 10 Hz) was externally applied to the rat ulna, GLAST (EAAT1) mRNA, was significantly down-regulated in osteocytes in the loaded limb. Functional components from each stage of the glutamate signaling pathway have since been identified within bone, including proteins necessary for calcium-mediated glutamate exocytosis, receptors, transporters, and signal propagation. Activation of ionotropic glutamate receptors has been shown to regulate the phenotype of osteoblasts and osteoclasts in vitro and bone mass in vivo. Furthermore, glutamatergic nerves have been identified in the vicinity of bone cells expressing glutamate receptors in vivo. However, it is not yet known how a glutamate signaling event is initiated in bone or its physiological significance. This review will examine the role of the glutamate signaling pathway in bone, with emphasis on the functions of glutamate transporters in osteoblasts.

A by-product of glutathione production in cancer cells may cause disruption in bone metabolic processes

Bone is a frequent site for metastasis of breast and prostate cancers, often resulting in pathologic changes in bone metabolism and severe pain. The mechanisms involved are not well understood, but tumour cells may release factors that interfere with bone homeostasis. Several observations have led us to hypothesize that the functional disruptions in bone metastasis are the result of a biological process common to many cell types. The high metabolic activity characteristic of cancer cells often upregulates oxidative stress protection mechanisms such as the antioxidant molecule glutathione. In maintaining redox balance, this normal metabolic response may result in unintended pathologic effects in certain sensitive organ sites. Malignant glioma cells kill surrounding neurons in the brain specifically by secreting the amino acid glutamate, an obligatory waste product of glutathione synthesis. We suggest that glutamate release is a plausible mechanism that may account for the pathologic changes in bone metastasis, since bone, like brain, is also highly sensitive to glutamatergic disruption. This report reviews the available evidence to draw a mechanistic connection between tumour cell oxidative stress and the pathology seen in patients with bone metastasis.

Coupling of the NMDA receptor to neuroprotective and neurodestructive events

NMDA (N-methyl-D-aspartate) receptors are a subtype of ionotropic glutamate receptor with an important role in the physiology and pathophysiology of central neurons. Inappropriate levels of Ca(2+) influx through the NMDA receptor can contribute to neuronal loss in acute trauma such as ischaemia and traumatic brain injury, as well as certain neurodegenerative diseases such as Huntington's disease. However, normal physiological patterns of NMDA receptor activity can promote neuroprotection against both apoptotic and excitotoxic insults. As a result, NMDA receptor blockade can promote neuronal death outright or render neurons vulnerable to secondary trauma. Thus responses to NMDA receptor activity follow a classical hormetic dose-response curve: both too much and too little can be harmful. There is a growing knowledge of the molecular mechanisms underlying both the neuroprotective and neurodestructive effects of NMDA receptor activity, as well as the factors that determine whether an episode of NMDA receptor activity is harmful or beneficial. It is becoming apparent that oxidative stress plays a role in promoting neuronal death in response to both hyper- and hypo-activity of the NMDA receptor. Increased understanding in this field is leading to the discovery of new therapeutic targets and strategies for excitotoxic disorders, as well as a growing appreciation of the harmful consequences of NMDA receptor blockade.

Signalling cascades in mechanotransduction: cell-matrix interactions and mechanical loading

Mechanical loading of articular cartilage stimulates the metabolism of resident chondrocytes and induces the synthesis of molecules to maintain the integrity of the cartilage. Mechanical signals modulate biochemical activity and changes in cell behavior through mechanotransduction. Compression of cartilage results in complex changes within the tissue including matrix and cell deformation, hydrostatic and osmotic pressure, fluid flow, altered matrix water content, ion concentration and fixed charge density. These changes are detected by mechanoreceptors on the cell surface, which include mechanosensitive ion channels and integrins that on activation initiate intracellular signalling cascades leading to tissue remodelling. Excessive mechanical loading also influences chondrocyte metabolism but unlike physiological stimulation leads to a quantitative imbalance between anabolic and catabolic activity resulting in depletion of matrix components. In this article we focus on the role of mechanical signalling in the maintenance of articular cartilage, and discuss how alterations in normal signalling can lead to pathology.

Cancer cell lines release glutamate into the extracellular environment

Bone is one of the most frequent sites for metastasis of breast and prostate cancers. Bone metastases are associated with pathologic changes in bone turnover and severe pain. The mechanisms that trigger these effects are not well understood, but it is postulated that tumour cells release factors which interfere with signalling processes critical to bone homeostasis. We have identified that several cancer cell lines known to cause bone disruption in animal models of bone metastasis appear to secrete glutamate into their extracellular environment in vitro. Although these cells also express specific glutamate receptors, the implications of this potentially disruptive chemical signal are discussed in relation to normal glutamate-dependent communication processes in bone and a possible mechanistic connection is made between tumour cell glutamate release and the development of pathological changes in bone turnover.

Effects of artificial gravity during bed rest on bone metabolism in humans

We report results from a study designed to explore the utility of artificial gravity (AG) as a countermeasure to bone loss induced by microgravity simulation. After baseline testing, 15 male subjects underwent 21 days of 6 degrees head-down bed rest to simulate the deconditioning associated with spaceflight. Eight of the subjects underwent 1 h of centrifugation (AG; 1 G(z) at the heart, 2.5 G(z) at the feet) each day for 21 days, whereas seven of the subjects served as untreated controls (Con). Blood and urine were collected before, during, and after bed rest for bone marker determinations. Bone mineral density (BMD) and bone mineral content (BMC) were determined by dual-energy X-ray absorptiometry and peripheral quantitative computerized tomography before and after bed rest. Urinary excretion of bone resorption markers increased during bed rest, but the AG and Con groups did not differ significantly. The same was true for serum C-telopeptide. During bed rest, bone alkaline phosphatase (ALP) and total ALP tended to be lower in the AG group (P = 0.08, P = 0.09). Neither BMC nor BMD changed significantly from the pre-bed rest period in AG or Con groups, and the two groups were not significantly different. However, when AG and Con data were combined, there was a significant (P < 0.05) effect of time for whole body total BMC and total hip and trochanter BMD. These data failed to demonstrate efficacy of this AG prescription to prevent the changes in bone metabolism observed during 3 wk of bed rest.

NMDA receptor expression and activity in osteoarthritic human articular chondrocytes

OBJECTIVE

Classical neuronal signalling molecules such as substance P and glutamate have been identified in cartilage and have roles in regulation of chondrocyte function. This study looks at expression and activity of the ionotropic glutamate NMDA (N-methyl-D-aspartic acid) receptor (NMDAR) in human osteoarthritic (OA) chondrocytes.

METHOD

Chondrocytes were obtained from human knee joint arthroplasty specimens. NMDAR subunits and PSD-95 (postsynaptic density protein 95) expression were analysed by reverse transcription-polymerase chain reaction and Western blotting. Activity of NMDAR was assayed by radioactive calcium(45) uptake and changes in membrane potential in the presence and absence of NMDA and NMDAR antagonists and blockade of cell membrane ion channels.

RESULTS

NMDAR 1, 2A, 2B and PSD-95 were detected in human OA chondrocytes whereas NR2B was absent from normal chondrocytes. NMDA induced calcium flux into OA chondrocytes and cell membrane depolarisation. These responses were blocked by NMDAR antagonists, removal of extracellular calcium, inhibition of nNOS (neuronal nitric oxide synthase) activity and uncoupling of NMDAR from PSD-95. Blockade of sodium channels by tetrodotoxin resulted in NMDA-induced membrane hyperpolarisation which was, in turn inhibited by apamin, a blocker of SK channels. NMDA-induced changes in cell membrane potential were not affected by l-type and stretch activated calcium channel inhibitors.

CONCLUSIONS

Human OA and normal articular chondrocytes differ in the expression of NMDAR subunits. In OA chondrocytes NMDAR signalling requires extracellular calcium, association with PSD-95, and nNOS activity. Downstream signalling results in activation of tetrodotoxin sensitive sodium channels and SK channels, a response that differs from that of normal chondrocytes suggesting altered activity of NMDAR in OA.

Wnt and steroid pathways control glutamate signalling by regulating glutamine synthetase activity in osteoblastic cells

Glutamate signalling has recently been found functional also outside the central nervous system, especially in bone. Glutamate is converted to glutamine by glutamine synthetase (GS), which is therefore able to regulate intracellular concentrations of glutamate. We previously characterized the induction of GS expression by glucocorticoids (GCs) in human osteoblast-like cells. Besides this observation, the mechanisms controlling GS in bone are unknown. Therefore, the aim of our present study was to investigate further the regulation of GS in osteoblastic cells. We observed that vitamin D inhibited basal and, even more efficiently, GC-stimulated GS activity by affecting both the mRNA and protein levels of the enzyme in human MG-63 osteoblast-like cells. In osteoblasts derived from rat bone marrow stem cells (rMSCs), GS activity was induced accordingly by the osteogenic culture conditions including GCs. Also in these primary cells, vitamin D clearly inhibited GS activity. In addition, the canonical Wnt signalling pathway was characterized as a negative regulator of GS activity. All these changes in GS activity were reflected on the intracellular glutamate concentration. Our results provide novel evidence that GS activity and expression are regulated by several different signalling pathways in osteoblastic cells. Therefore, GS is a strategic enzyme in controlling glutamate concentration in bone environment: GCs decreased the amount of this signalling molecule while vitamin D and Wnt signalling pathway increased it. Interestingly, GS activity and expression declined rapidly when the rMSC derived osteoblasts began to mineralize. Due to its downregulation during osteoblast mineralization, GS could be held as a marker for osteoblast development. Further supporting this, GS activity was stimulated and intracellular glutamate concentration maintained by the N-methyl-d-aspartate (NMDA) type glutamate receptor antagonist MK801, which inhibited osteogenic differentiation of the rMSCs. GS, a novel target for both steroidal and Wnt pathways in bone, might be a central player in the regulation of osteoblastogenesis and/or intercellular signal transmission. Therefore, the proper understanding of the interplay of these three signalling cascades, i.e., steroidal, Wnt, and glutamate signalling, gives vital information on how bone cells communicate together aiming to keep bone healthy.

Regulation of the maturation of osteoblasts and osteoclastogenesis by glutamate

Glutamate, an important central excitatory neurotransmitter, is also secreted by osteoblasts and may be involved in the regulation of bone metabolism. Glutamate receptors for N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) are demonstrated in bone cells. Here we investigated the in vivo effects of glutamate by local injection of AMPA, NMDA, and their antagonists into tibia as well as their in vitro effects on the maturation of osteoblasts and formation of osteoclasts. AMPA receptor antagonist CNQX and NMDA receptor antagonist MK-801 significantly inhibited the maturation and mineralization of osteoblasts in high-glutamate alpha-MEM. On the other hand, AMPA and NMDA up-regulated the mineralized deposition and osteocalcin mRNA expression of primary osteoblasts cultured in glutamate-free DMEM. AMPA and NMDA induced the phosphorylation of extracellular signal-related kinases (ERK) in osteoblasts within 15 min. In addition, NMDA but not AMPA up-regulated the number of osteoclasts while MK801 antagonized this potentiating effect. To explore the action of glutamate agonists on bone formation in animal model, AMPA was locally injected into tibia and it was found that the bone volume in secondary spongiosa significantly increased and co-treatment of CNQX antagonized the enhancing effect of AMPA. These results suggest that glutamate may play a physiological role in regulating the maturation of osteoblasts and osteoclastogenesis. Activation of both AMPA and NMDA receptors regulates the maturation of osteoblasts. NMDA but not AMPA affects receptor for activation of NF-kappaB ligand (RANKL)-induced osteoclastogenesis.

VGluT2 expression in painful Achilles and patellar tendinosis: evidence of local glutamate release by tenocytes

The pathogenesis of chronic tendinopathy is unclear. We have previously measured high intratendinous levels of glutamate in patients with tendinosis, suggesting potential roles of glutamate in the modulation of pain, vascular function, and degenerative changes including apoptosis of tenocytes. However, the origin of free glutamate found in tendon tissue is completely unknown. Surgical biopsies of pain-free normal tendons and tendinosis tendons (Achilles and patellar) were examined immunohistochemically using antibodies against vesicular glutamate transporters (VGluT1 and VGluT2), as indirect markers of glutamate release. In situ hybridization for VGluT2 mRNA was also conducted. Specific immunoreactions for VGluT2, but not VGluT1, could be consistently detected in tenocytes. However, there were interindividual variations in the levels of immunoreactivity. The level of immunoreaction for VGluT2 was higher in tendinosis tendons compared to normal tendons (p < 0.05). In situ hybridization of VGluT2 demonstrated that mRNA was localized in a similar pattern as the protein, with marked expression by certain tenocytes, particularly those showing abnormal appearances. Reactivity for VGluT1 and -2 was absent from nerves and vessel structures in both normal and painful tendons. The current data demonstrate that tenocytes may be involved in the regulation of extracellular glutamate levels in tendons. Specifically, the observations suggest that free glutamate may be locally produced and released by tenocytes, rather than by peripheral neurons. Excessive free glutamate is expected to impact a variety of autocrine and paracrine functions important in the development of tendinosis, including tenocyte proliferation and apoptosis, extracellular matrix metabolism, nociception, and blood flow.

The dichotomy of NMDA receptor signaling

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors plays a Jekyll and Hyde role in the mammalian central nervous system. In pathological scenarios such as ischemia, Ca2+ influx through the NMDA receptor is a key mediator of cell death. However, physiological levels of NMDA-receptor activity can promote neuronal survival and resistance to trauma and play important roles in synaptic plasticity and transmission. This dichotomy may explain the poor tolerance and efficacy of NMDA-receptor antagonists in clinical trials for excitotoxic trauma. There is a growing understanding of the signaling events that mediate the opposing effects of NMDA-receptor activity and the factors that determine whether an episode of NMDA-receptor activity will promote survival or death. This knowledge may lead to therapeutic strategies that enable the selective blockade of prodeath signaling cassettes while sparing physiological signaling to survival and plasticity.

Ontogenetic structural and material variations in ovine calcanei: a model for interpreting bone adaptation

Experimental models are needed for resolving relative influences of genetic, epigenetic, and nonheritable functionally induced (extragenetic) factors in the emergence of developmental adaptations in limb bones of larger mammals. We examined regional/ontogenetic morphologic variations in sheep calcanei, which exhibit marked heterogeneity in structural and material organization by skeletal maturity. Cross-sections and lateral radiographs of an ontogenetic series of domesticated sheep calcanei (fetal to adult) were examined for variations in biomechanically important structural (cortical thickness and trabecular architecture) and material (percent ash and predominant collagen fiber orientation) characteristics. Results showed delayed development of variations in cortical thickness and collagen fiber orientation, which correlate with extragenetic factors, including compression/tension strains of habitual bending in respective dorsal/plantar cortices and load-related thresholds for modeling/remodeling activities. In contrast, the appearance of trabecular arches in utero suggests strong genetic/epigenetic influences. These stark spatial/temporal variations in sheep calcanei provide a compelling model for investigating causal mechanisms that mediate this construction. In view of these findings, it is also suggested that the conventional distinction between genetic and epigenetic factors in limb bone development be expanded into three categories: genetic, epigenetic, and extragenetic factors.

Current perspectives on NMDA-type glutamate signalling in bone

Bone is a complex, evolving tissue, architecturally defined by the activities of osteoclasts and osteoblasts that continually resorb and replace the mineralised matrix. Numerous regulatory mechanisms exist to control bone remodelling and the maintenance of bone mass. The consequences of inappropriate or uncoupled bone resorption and formation are significant and invariably lead to different disease states, the most prevalent being osteoporosis. In recent years, much attention has focused on unravelling the systemic and local signalling interactions that influence the differentiation and function of bone cells with a view to developing our understanding of bone biology and identifying potential new targets for therapeutic intervention. Several lines of evidence indicate that neurotransmitters and neuromodulators have influential roles to play in the regulation of bone remodelling and much of this research has involved analysis of the excitatory amino acid glutamate. This review will summarise current understanding of glutamate signalling in bone cells, addressing specifically the function of N-methyl-D-aspartate (NMDA)-type glutamate receptor signalling mechanisms, and will address the functional significance and future prospects for this area of research.

Osteocyte Lacuna Population Densities in Sheep, Elk and Horse Calcanei

Osteocytes, the most prevalent cell type in bone, appear to communicate via gap junctions. In limb-bone diaphyses, it has been hypothesized that these cellular networks have the capacity to monitor habitual strains, which can differ significantly between cortical locations of the same bone. Regional differences in microdamage associated with prevalent/predominant strain mode (tension, compression, or shear) and/or magnitude may represent an important "variable" detected by this network. This hypothesis was indirectly addressed by examining bones subjected to habitual bending for correlations of osteocyte lacuna population densities (n/mm(2) bone area, Ot.Lc.N/B.Ar) with locations experiencing high and low strain, and/or prevalent/predominant tension, compression, and shear. We examined dorsal ("compression"), plantar ("tension"), and medial/lateral ("shear" or neutral axis) cortices of mid-diaphyseal sections of calcanei of adult sheep, elk, and horses. Ot.Lc.N/B.Ar data, quantified in backscattered electron images, were also evaluated in a context of various additional structural and material variables (e.g. % ash, cortical thickness, porosity, and secondary osteon population). Results showed significant differences in dorsal versus plantar comparisons with the highest Ot.Lc.N/B.Ar in dorsal cortices of sheep and elk (p < 0.0001); but this was a statistical trend in the equine calcanei (p = 0.14). There were no consistent transcortical (pericortical to endocortical) differences, and Ot.Lc.N/B.Ar in neutral axes was not consistently different from dorsal/plantar cortices. Correlations of Ot.Lc.N/B.Ar with structural and material parameters were also poor and/or inconsistent within or between species. These results provide little or no evidence that the number of osteocyte lacunae has a functional role in mechanotransduction pathways that are typically considered in bone adaptation. Although dorsal/plantar differences may be adaptations for prevalent/predominant strain modes and/or associated microdamage, it is also plausible that they are strongly influenced by differences in the bone formation rates that produced the tissue in these locations.