Evidence for targeted vesicular glutamate exocytosis in osteoblasts

Cross-sectional study and bioinformatics analysis to reveal the correlations of osteoporosis in patients with Parkinson's disease

OBJECTIVES

Osteoporosis and Parkinson's disease (PD) are both aging-related diseases. PD patients with comorbid osteoporosis are vulnerable to the risk of fracture, which leads to a serious public health burden to the whole society. Therefore, this study sought to reveal the clinical and genetic correlations between PD and osteoporosis based on a cross-sectional study and bioinformatics analysis.

METHODS

A cross-sectional study of 95 PD patients and 99 healthy controls was conducted. Ordinal logistic regression analysis was utilized to investigate the clinical correlations between PD and osteoporosis. Two microarray datasets (GSE20292, GSE35958) including PD, osteoporosis and normal control samples were retrieved from the GEO database for GO analysis, KEGG pathway analysis and PPI network.

RESULTS

PD patients had lower 25(OH)VitD, FN BMD, BMD and T-score of the LS and TH, as well as poorer bone mass diagnosis, yet higher PINP compared to healthy controls. Both age and UPDRS II score of PD patients were adversely correlated with BMD of LS and TH. PD diagnosis acted as an independent risk factor of osteoporosis, and PD patients had approximately double risk for osteoporosis. Bioinformatics analysis further revealed that SNAP25, AQP4, SV2B, KCND3, and ABCA2 had important diagnostic value and risk prediction value for both PD and osteoporosis.

CONCLUSIONS

PD diagnosis can be used as an independent risk factor for osteoporosis. Moreover, SNAP25, AQP4, SV2B, KCND3 and ABCA2 as the top 5 hub genes have important diagnostic and risk predictive value for both PD and osteoporosis.

Osteoclast-derived extracellular vesicles are implicated in sensory neurons sprouting through the activation of epidermal growth factor signaling

BACKGROUND

Different pathologies, affecting the skeletal system, were reported to display altered bone and/or cartilage innervation profiles leading to the deregulation of the tissue homeostasis. The patterning of peripheral innervation is achieved through the tissue-specific expression of attractive or repulsive axonal guidance cues in specific space and time frames. During the last decade, emerging findings attributed to the extracellular vesicles (EV) trading a central role in peripheral tissue innervation. However, to date, the contribution of EV in controlling bone innervation is totally unknown.

RESULTS

Here we show that sensory neurons outgrowth induced by the bone resorbing cells-osteoclasts-is promoted by osteoclast-derived EV. The EV induced axonal growth is achieved by targeting epidermal growth factor receptor (EGFR)/ErbB2 signaling/protein kinase C phosphorylation in sensory neurons. In addition, our data also indicate that osteoclasts promote sensory neurons electrophysiological activity reflecting a possible pathway in nerve sensitization in the bone microenvironment, however this effect is EV independent.

CONCLUSIONS

Overall, these results identify a new mechanism of sensory bone innervation regulation and shed the light on the role of osteoclast-derived EV in shaping/guiding bone sensory innervation. These findings provide opportunities for exploitation of osteoclast-derived EV based strategies to prevent and/or mitigate pathological uncontrolled bone innervation.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Crosstalk of Brain and Bone-Clinical Observations and Their Molecular Bases

As brain and bone disorders represent major health issues worldwide, substantial clinical investigations demonstrated a bidirectional crosstalk on several levels, mechanistically linking both apparently unrelated organs. While multiple stress, mood and neurodegenerative brain disorders are associated with osteoporosis, rare genetic skeletal diseases display impaired brain development and function. Along with brain and bone pathologies, particularly trauma events highlight the strong interaction of both organs. This review summarizes clinical and experimental observations reported for the crosstalk of brain and bone, followed by a detailed overview of their molecular bases. While brain-derived molecules affecting bone include central regulators, transmitters of the sympathetic, parasympathetic and sensory nervous system, bone-derived mediators altering brain function are released from bone cells and the bone marrow. Although the main pathways of the brain-bone crosstalk remain ‘efferent’, signaling from brain to bone, this review emphasizes the emergence of bone as a crucial ‘afferent’ regulator of cerebral development, function and pathophysiology. Therefore, unraveling the physiological and pathological bases of brain-bone interactions revealed promising pharmacologic targets and novel treatment strategies promoting concurrent brain and bone recovery.

Molecular Targets for Combined Therapeutic Strategies to Limit Glioblastoma Cell Migration and Invasion

The highly invasive nature of glioblastoma imposes poor prospects for patient survival. Molecular evidence indicates glioblastoma cells undergo an intriguing expansion of phenotypic properties to include neuron-like signaling using excitable membrane ion channels and synaptic proteins, augmenting survival and motility. Neurotransmitter receptors, membrane signaling, excitatory receptors, and Ca2+ responses are important candidates for the design of customized treatments for cancers within the heterogeneous central nervous system. Relatively few published studies of glioblastoma multiforme (GBM) have evaluated pharmacological agents targeted to signaling pathways in limiting cancer cell motility. Transcriptomic analyses here identified classes of ion channels, ionotropic receptors, and synaptic proteins that are enriched in human glioblastoma biopsy samples. The pattern of GBM-enriched gene expression points to a major role for glutamate signaling. However, the predominant role of AMPA receptors in fast excitatory signaling throughout the central nervous system raises a challenge on how to target inhibitors selectively to cancer cells while maintaining tolerability. This review critically evaluates a panel of ligand- and voltage-gated ion channels and synaptic proteins upregulated in GBM, and the evidence for their potential roles in the pathological disease progress. Evidence suggests combinations of therapies could be more effective than single agents alone. Natural plant products used in traditional medicines for the treatment of glioblastoma contain flavonoids, terpenoids, polyphenols, epigallocatechin gallate, quinones, and saponins, which might serendipitously include agents that modulate some classes of signaling compounds highlighted in this review. New therapeutic strategies are likely to exploit evidence-based combinations of selected agents, each at a low dose, to create new cancer cell-specific therapeutics.

Synapsins are expressed at neuronal and non-neuronal locations in Octopus vulgaris

Synapsins are a family of phosphoproteins fundamental to the regulation of neurotransmitter release. They are typically neuron-specific, although recent evidence pointed to their expression in non-neuronal cells where they play a role in exocytosis and vesicle trafficking. In this work, we characterized synapsin transcripts in the invertebrate mollusk Octopus vulgaris and present evidence of their expression not only in the brain but also in male and female reproductive organs. We identified three synapsin isoforms phylogenetically correlated to that of other invertebrates and with a modular structure characteristic of mammalian synapsins with a central, highly conserved C domain, important for the protein functions, and less conserved A, B and E domains. Our molecular modeling analysis further provided a solid background for predicting synapsin functional binding to ATP, actin filaments and secretory vesicles. Interestingly, we found that synapsin expression in ovary and testis increased during sexual maturation in cells with a known secretory role, potentially matching the occurrence of a secretion process. This might indicate that its secretory role has evolved across animals according to cell activity in spite of cell identity. We believe that this study may yield insights into the convergent evolution of ubiquitously expressed proteins between vertebrates and invertebrates.

Identification and mechanism evaluation of a novel osteogenesis promoting peptide from Tubulin Alpha-1C chain in Crassostrea gigas

Marine shellfish provides a series of biofunctionality account of its high-protein level. In this study, the osteogenic effect of a novel peptide, YRGDVVPK, from Crassostrea gigas protein hydrolysates on preosteoblast MC3T3-E1 proliferation was examined. Synthetic peptide with 100 nM significantly promoted the proliferation of MC3T3-E1 cells for a treatment of 72 h assayed by MTT method, and which was confirmed by the increase of alkaline phosphatase (ALP) activity. The peptide, YRGDVVPK, was docked with integrin α5β1 (PDB ID: 3VI4), which is a surface receptor of MC3T3-E1. The interaction of the peptide with integrin α5β1 (PDB ID: 3VI4) was analyzed by the molecular modeling algorithm of CDOCKER, which showed a more stable combination than the original ligand. The results suggested the novel peptide could promote the preosteoblast MC3T3-E1 proliferation probably by activating the signaling pathway of MAPK, which is induced through binding with peptide YRGDVVPK.

N-methyl-d-aspartate receptor mediated calcium influx supports in vitro differentiation of normal mouse megakaryocytes but proliferation of leukemic cell lines

Background

N-methyl-d-aspartate receptors (NMDARs) contribute calcium influx in megakaryocytic cells but their roles remain unclear; both pro- and anti-differentiating effects have been shown in different contexts.

Objectives

The aim of this study was to clarify NMDAR contribution to megakaryocytic differentiation in both normal and leukemic cells.

Methods

Meg-01, Set-2, and K-562 leukemic cell lines were differentiated using phorbol-12-myristate-13-acetate (PMA, 10 nmol L^-1) or valproic acid (VPA, 500 μmol L^-1). Normal megakaryocytes were grown from mouse marrow-derived hematopoietic progenitors (lineage-negative and CD41a-enriched) in the presence of thrombopoietin (30-40 nmol L^-1). Marrow explants were used to monitor proplatelet formation in the native bone marrow milieu. In all culture systems, NMDARs were inhibited using memantine and MK-801 (100 μmol L^-1); their effects compared against appropriate controls.

Results

The most striking observation from our studies was that NMDAR antagonists markedly inhibited proplatelet formation in all primary cultures employed. Proplatelets were either absent (in the presence of memantine) or short, broad and intertwined (with MK-801). Earlier steps of megakaryocytic differentiation (acquisition of CD41a and nuclear ploidy) were maintained, albeit reduced. In contrast, in leukemic Meg-01 cells, NMDAR antagonists inhibited differentiation in the presence of PMA and VPA but induced differentiation when applied by themselves.

Conclusions

NMDAR-mediated calcium influx is required for normal megakaryocytic differentiation, in particular proplatelet formation. However, in leukemic cells, the main NMDAR role is to inhibit differentiation, suggesting diversion of NMDAR activity to support leukemia growth. Further elucidation of the NMDAR and calcium pathways in megakaryocytic cells may suggest novel ways to modulate abnormal megakaryopoiesis.

Syntaxin 4a Regulates Matrix Vesicle-Mediated Bone Matrix Production by Osteoblasts

Osteoblasts secrete matrix vesicles and proteins to bone surfaces, but the molecular mechanisms of this secretion system remain unclear. The present findings reveal the roles of important genes in osteoblasts involved in regulation of extracellular matrix secretion. We especially focused on "soluble N-ethylmaleimide-sensitive factor (NSF) attachment protein receptor" (SNARE) genes and identified notable Syntaxin 4a (Stx4a) expression on the basolateral side of the plasma membrane of osteoblasts. Furthermore, Stx4a overexpression was found to increase mineralization by osteoblastic cells, whereas Stx4a knockdown reduced levels of mineralization. Also, BMP-4 and IGF-1 induced the localization of Stx4a to the basolateral side of the cells. To examine the function of Stx4a in osteoblasts, we generated osteoblast-specific Stx4a conditional knockout mice, which demonstrated an osteopenic phenotype due to reduced matrix secretion. Bone mineral density, shown by peripheral quantitative computed tomography (pQCT), was reduced in the femur metaphyseal and diaphyseal regions of Stx4a osteoblast-specific deficient mice, whereas bone parameters, shown by micro-computed tomography (μCT) and bone histomorphometric analysis, were also decreased in trabecular bone. In addition, primary calvarial cells from those mice showed decreased mineralization and lower secretion of matrix vesicles. Our findings indicate that Stx4a plays a critical role in bone matrix production by osteoblasts. © 2016 American Society for Bone and Mineral Research.

Structural Remodeling of Sympathetic Innervation in Atherosclerotic Blood Vessels: Role of Atherosclerotic Disease Progression and Chronic Social Stress

OBJECTIVE

The sympathetic nervous system (SNS) can undergo dramatic structural plasticity in response to behavioral factors and/or the presence of disease, leading to SNS hyperinnervation of peripheral tissues. The SNS has been proposed as an important mediator between stressful behavior and the progression of atherosclerosis in the vasculature. The present study examined whether structural remodeling of the SNS occurs in the vasculature in a genetically hyperlipidemic animal model of atherosclerosis, the Watanabe heritable hyperlipidemic rabbit (WHHL; relative to normolipidemic New Zealand white rabbits [NZW]), and whether SNS plasticity is driven by the progression of disease and/or by stressful social behavior.

METHODS

WHHL and NZW rabbits were assigned to an unstable or stable social environment for 4 months. Aortic atherosclerosis was assessed and SNS aortic innervation quantified using immunofluorescent microscopy.

RESULTS

Numerous SNS varicosities were observed throughout the aorta in WHHLs and NZWs, extending into the vascular media and intima, an innervation pattern not previously reported. WHHLs exhibited significantly greater innervation than NZWs (F(1,41) = 55.3, p < .001), with extensive innervation of the atherosclerotic neointima. The innervation density was highly correlated with the extent of disease in the WHHLs (r(21) = 0.855, p < .001). Social environment did not influence innervation in NZWs (aortic arch: p = .078, thoracic aorta: p = .34) or WHHLs (arch: p = .97, thoracic: p = .61).

CONCLUSIONS

The findings suggest that hyperinnervation is driven largely by the progression of disease rather than social environment. SNS innervation patterns observed in atherosclerotic human and mouse aortas were consistent with the rabbit, suggesting that SNS hyperinnervation of the diseased vessel wall is a general feature across mammalian species.

Alleviation of bone markers in rats induced nano-zinc oxide by qurecetin and α-lipolic acid

The purpose of this study was to evaluate the potential protective effect of qurecetin (Qur) and α-lipolic acid (ALA) to modulate the perturbation of bone turnover which is induced by nano-zinc oxide (n-ZnO). Rats were fasted overnight and randomly divided into two groups: G1, normal healthy animals and the other rats were administered zinc oxide nanoparticles orally by guava in a dose of 600 mg/kg body weight/d for 5 sequential days in Wistar albino male rats. N-ZnO-exposed animals were randomly sub-divided into three groups: G2, n-ZnO-exposed animals; G3, n-ZnO-exposed animals co-treated with Qur (200 mg/kg daily); and G4, n-ZnO-exposed animals co-treated with ALA (200 mg/kg). Qur and ALA were administered orally by guava daily for three sequential weeks from the beginning of the experiment. The results revealed a significant reduction of nitiric oxide (NO) and serum level and comet assay in n-ZnO exposure rats after treatment of Qur and ALA. It was found the alteration of pro-inflammatory markers (tumor necrosis factor alpha; TNF-α, interleukin-6; IL-6 and C-reactive protein; CRP), bone alkaline phosphatase (B-ALP, bone formation marker), and C-terminal peptide type I collagen (CTx, bone resorption marker) levels compared with the normal group. Co-administration of Qur and ALA in n-ZnO-exposed rats significantly alleviated the mentioned alterations of biochemical parameters. These results suggest that Qur and ALA as antioxidant agents may be a candidate for preventive and treatment applications of impaired bone markers induced bone loss caused by nano-particles of metal oxide.

The Role of KV7.3 in Regulating Osteoblast Maturation and Mineralization

KCNQ (KV7) channels are voltage-gated potassium (KV) channels, and the function of KV7 channels in muscles, neurons, and sensory cells is well established. We confirmed that overall blockade of KV channels with tetraethylammonium augmented the mineralization of bone-marrow-derived human mesenchymal stem cells during osteogenic differentiation, and we determined that KV7.3 was expressed in MG-63 and Saos-2 cells at the mRNA and protein levels. In addition, functional KV7 currents were detected in MG-63 cells. Inhibition of KV7.3 by linopirdine or XE991 increased the matrix mineralization during osteoblast differentiation. This was confirmed by alkaline phosphatase, osteocalcin, and osterix in MG-63 cells, whereas the expression of Runx2 showed no significant change. The extracellular glutamate secreted by osteoblasts was also measured to investigate its effect on MG-63 osteoblast differentiation. Blockade of KV7.3 promoted the release of glutamate via the phosphorylation of extracellular signal-regulated kinase 1/2-mediated upregulation of synapsin, and induced the deposition of type 1 collagen. However, activation of KV7.3 by flupirtine did not produce notable changes in matrix mineralization during osteoblast differentiation. These results suggest that KV7.3 could be a novel regulator in osteoblast differentiation.

[The cancer paradigm in pulmonary arterial hypertension: towards anti-remodeling therapies targeting metabolic dysfunction?]

Pulmonary arterial hypertension (PAH) is a rare, complex and multifactorial disease in which pulmonary vascular remodeling plays a major role ending in right heart failure and death. Current specific therapies of PAH that mainly target the vasoconstriction/vasodilatation imbalance are not curative. Bi-pulmonary transplantation remains the only option in patients resistant to current therapies. It is thus crucial to identify novel vascular anti-remodeling therapeutic targets. This remodeling displays several properties of cancer cells, especially overproliferation and apoptosis resistance of pulmonary vascular cells, hallmarks of cancer related to the metabolic shift known as the "Warburg effect". The latter is characterized by a shift of ATP production, from oxidative phosphorylation to low rate aerobic glycolysis. In compensation, the cancer cells exhibit exacerbated glutaminolysis thus resulting in glutamine addiction, necessary to their overproliferation. Glutamine intake results in glutamate production, a molecule at the crossroads of energy metabolism and cancer cell communication, thus contributing to cell proliferation. Accordingly, therapeutic strategies targeting glutamate production, its release into the extracellular space and its membrane receptors have been suggested to treat different types of cancers, not only in the central nervous system but also in the periphery. We propose that similar strategies targeting glutamatergic signaling may be considered in PAH, especially as they could affect not only the vascular remodeling but also the right heart hypertrophy known to involve the glutaminolysis pathway. Ongoing studies aim to characterize the involvement of the glutamate pathway and its receptors in vascular remodeling, and the therapeutic potential of specific molecules targeting this pathway.

Membrane depolarization regulates intracellular RANKL transport in non-excitable osteoblasts

Parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (VD3) are important factors in Ca(2+) homeostasis, and promote osteoclastogenesis by modulating receptor activator of nuclear factor kappa-B ligand (RANKL) mRNA expression. However, their contribution to RANKL intracellular transport (RANKLiT), including the trigger for RANKL lysosomal vesicle (RANKL-lv) fusion to the cell membrane, is unclear. In neurons, depolarization of membrane potential increases the intracellular Ca(2+) level ([Ca(2+)]i) and promotes neurotransmitter release via fusion of the synaptic vesicles to the cell membrane. To determine whether membrane depolarization also regulates cellular processes such as RANKLiT in MC3T3-E1 osteoblasts (OBs), we generated a light-sensitive OB cell line and developed a system for altering their membrane potential via delivery of a blue light stimulus. In the membrane fraction of RANKL-overexpressing OBs, PTH and VD3 increased the membrane-bound RANKL (mbRANKL) level at 10 min after application without affecting the mRNA expression level, and depolarized the cell membrane while transiently increasing [Ca(2+)]i. In our novel OB line stably expressing the channelrhodopsin-wide receiver, blue light-induced depolarization increased the mbRANKL level, which was reversed by treatment of blockers for L-type voltage-gated Ca(2+) channels and Ca(2+) release from the endoplasmic reticulum. In co-cultures of osteoclast precursor-like RAW264.7 cells and light-sensitive OBs overexpressing RANKL, light stimulation induced an increase in tartrate-resistant acid phosphatase activity and promoted osteoclast differentiation. These results indicate that depolarization of the cell membrane is a trigger for RANKL-lv fusion to the membrane and that membrane potential contributes to the function of OBs. In addition, the non-genomic action of VD3-induced RANKL-lv fusion included the membrane-bound VD3 receptor (1,25D3-MARRS receptor). Elucidating the mechanism of RANKLiT regulation by PTH and VD3 will be useful for the development of drugs to prevent bone loss in osteoporosis and other bone diseases.

Functional distribution of synapsin I in human sperm

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Synapsin I was localized in the human sperm equatorial segment.

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Presence of synapsin I was confirmed by dot and Western blotting techniques.

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Treatment of sperm with anti-synapsin antibodies significantly decreased motility.

Proteins known to function during cell–cell communication and exocytosis in neurons and other secretory cells have recently been reported in human sperm. Synapsins are a group of proteins that have been very well characterized in neurons, but little is known about synapsin function in other cell types. Based upon previous findings and the known function of synapsin, we tested the hypothesis that synapsin I was present in human sperm. Washed, capacitated, and acrosome induced sperm preparations were used to evaluate the functional distribution of synapsin I using immunocytochemistry. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were used for protein blotting techniques. Immunolocalization revealed synapsin I was enriched in the sperm equatorial segment. Protein extracts from mouse brain, mouse testis/epididymis, and human semen were positive for synapsin I using several different antibodies, and dot blot results were confirmed by Western blot analyses. Finally, treatment of capacitated and acrosome reaction induced samples with anti-synapsin antibodies significantly reduced sperm motility. Localization of synapsin I in human sperm is a novel finding. The association of synapsin I with the sperm equatorial segment and effects on motility are suggestive of a role associated with capacitation and/or acrosome reaction, processes that render sperm capable of fertilizing an oocyte.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Endocrinology of bone/brain crosstalk

Bone metabolism is regulated by the action of two skeletal cells: osteoblasts and osteoclasts. This process is controlled by many genetic, hormonal and lifestyle factors, but today more and more studies have allowed us to identify a neuronal regulation system termed 'bone-brain crosstalk', which highlights a direct relationship between bone tissue and the nervous system. The first documentation of an anatomic relationship between nerves and bone was made via a wood cut by Charles Estienne in Paris in 1545. His diagram demonstrated nerves entering and leaving the bones of a skeleton. Later, several studies were conducted on bone innervation and, as of today, many observations on the regulation of bone remodeling by neurons and neuropeptides that reside in the CNS have created a new research field, that is, neuroskeletal research.

Membrane trafficking in osteoblasts and osteoclasts: new avenues for understanding and treating skeletal diseases

The endocytic and exocytic/secretory pathways are two major intracellular membrane trafficking routes that regulate numerous cellular functions in a variety of cell types. Osteoblasts and osteoclasts, two major bone cells responsible for bone remodeling and homeostasis, are no exceptions. During the past few years, emerging evidence has pinpointed a critical role for endocytic and secretory pathways in osteoblast and osteoclast differentiation and function. The endosomal membrane provides a platform to integrate bone tropic signals of hormones and growth factors in osteoblasts. In osteoclasts, endocytosis, followed by transcytosis, of degraded bone matrix promotes bone resorption. Secretory pathways, especially lysosome secretion, not only participate in bone matrix deposition by osteoblasts and degradation of mineralized bone matrix by osteoclasts; they may also be involved in the coupling of bone resorption and bone formation during bone remodeling. More importantly, mutations in genes encoding regulatory factors within the endocytic and secretory pathways have been identified as causes for bone diseases. Identification of the molecular mechanisms of these genes in bone cells may provide new therapeutic targets for skeletal disorders.

Differential effects of short term feeding of a soy protein isolate diet and estrogen treatment on bone in the pre-pubertal rat

Background

Previous reports suggest that beneficial effects of soy on bone quality are due to the estrogenic actions of isoflavone phytochemicals associated with the protein. However, mechanistic studies comparing the effects of soy diet and estrogens on bone, particularly in rapidly growing animals are lacking.

Methodology and Principal Findings

We studied the effects of short term feeding of soy protein isolate (SPI) on bone in comparison to the effects of 17β-estradiol (E2) in pre-pubertal rats. Female rats were weaned to one of 4 treatments: 1) a control casein-based diet (CAS); 2) CAS with subcutaneous E2 (10 µg/kg/d) (CAS+E2); 3) a SPI-containing diet (SPI); or 4) SPI with subcutaneous E2 (SPI) or SPI with 10 µg/kg/d E2 (SPI+E2) for 14 days beginning on postnatal day 20. SPI increased while E2 decreased bone turnover compared to CAS. In contrast, both treatments decreased serum sclerostin levels. Microarray analysis of RNA isolated from bone revealed 652 genes regulated by SPI, 491 genes regulated by E2, and 266 genes regulated by both SPI diet and E2 compared to CAS. The expression of caveolin-1, a protein localized in the cell membrane, was down-regulated (p<0.05) in rats fed SPI, but not by E2 compared to rats fed casein. Down-regulation of caveolin-1 by SPI was associated with increased BMP2, Smad and Runx2 expression in bone and osteoblasts (p<0.05).

Conclusions/Significance

These results suggest SPI and E2 have different effects on bone turnover prior to puberty. Approximately half of the genes are regulated in the same direction by E2 or SPI, but in combination, SPI blocks the estrogen effects and returns the profile towards control levels. In addition, there are E2 specific and SPI-specific gene changes related to regulation of bone formation.

Synapsin II is involved in the molecular pathway of lithium treatment in bipolar disorder

Bipolar disorder (BD) is a debilitating psychiatric condition with a prevalence of 1–2% in the general population that is characterized by severe episodic shifts in mood ranging from depressive to manic episodes. One of the most common treatments is lithium (Li), with successful response in 30–60% of patients. Synapsin II (SYN2) is a neuronal phosphoprotein that we have previously identified as a possible candidate gene for the etiology of BD and/or response to Li treatment in a genome-wide linkage study focusing on BD patients characterized for excellent response to Li prophylaxis. In the present study we investigated the role of this gene in BD, particularly as it pertains to Li treatment. We investigated the effect of lithium treatment on the expression of SYN2 in lymphoblastoid cell lines from patients characterized as excellent Li-responders, non-responders, as well as non-psychiatric controls. Finally, we sought to determine if Li has a cell-type-specific effect on gene expression in neuronal-derived cell lines. In both in vitro models, we found SYN2 to be modulated by the presence of Li. By focusing on Li-responsive BD we have identified a potential mechanism for Li response in some patients.

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.

Muscle changes can account for bone loss after botulinum toxin injection

Studies to date have assumed that botulinum toxin type A (BTX) affects bone indirectly, through its action on muscle. We hypothesized that BTX has no discernable effect on bone morphometry, independent of its effect on muscle. Therefore, we investigated whether BTX had an additional effect on bone when combined with tenotomy compared to tenotomy in isolation. Female BALB/c mice (n = 73) underwent one of the following procedures in the left leg: BTX injection and Achilles tenotomy (BTX-TEN), BTX injection and sham surgery (BTX-sham), Achilles tenotomy (TEN), or sham surgery (sham). BTX groups were injected with 20 μL of BTX (1 U/100 g) in the posterior lower hindlimb. At 4 weeks, muscle cross-sectional area (MCSA) and tibial bone morphometry were assessed using micro-CT. Each treatment, other than sham, resulted in significant muscle and bone loss (P < 0.05). BTX-TEN experienced the greatest muscle loss (23-45% lower than other groups) and bone loss (20-30% lower bone volume fraction than other groups). BTX-sham had significantly lower MCSA and bone volume fraction than TEN and sham. After adjusting for differences in MCSA, there were no significant between-group differences in bone properties. We found that BTX injection resulted in more adverse muscle and bone effects than tenotomy and that effects were amplified when the procedures were combined. However, between-group differences in bone could be accounted for by MCSA. We conclude that any independent effect of BTX on bone morphometry is likely small or negligible compared with the effect on muscle.

Development of an oligo DNA microarray for the European sea bass and its application to expression profiling of jaw deformity

Background

The European sea bass (Dicentrarchus labrax) is a marine fish of great importance for fisheries and aquaculture. Functional genomics offers the possibility to discover the molecular mechanisms underlying productive traits in farmed fish, and a step towards the application of marker assisted selection methods in this species. To this end, we report here on the development of an oligo DNA microarray for D. labrax.

Results

A database consisting of 19,048 unique transcripts was constructed, of which 12,008 (63%) could be annotated by similarity and 4,692 received a GO functional annotation. Two non-overlapping 60mer probes were designed for each unique transcript and in-situ synthesized on glass slides using Agilent SurePrint™ technology. Probe design was positively completed for 19,035 target clusters; the oligo microarray was then applied to profile gene expression in mandibles and whole-heads of fish affected by prognathism, a skeletal malformation that strongly affects sea bass production. Statistical analysis identified 242 transcripts that are significantly down-regulated in deformed individuals compared to normal fish, with a significant enrichment in genes related to nervous system development and functioning. A set of genes spanning a wide dynamic range in gene expression level were selected for quantitative RT-PCR validation. Fold change correlation between microarray and qPCR data was always significant.

Conclusions

The microarray platform developed for the European sea bass has a high level of flexibility, reliability, and reproducibility. Despite the well known limitations in achieving a proper functional annotation in non-model species, sufficient information was obtained to identify biological processes that are significantly enriched among differentially expressed genes. New insights were obtained on putative mechanisms involved on mandibular prognathism, suggesting that bone/nervous system development might play a role in this phenomenon.

SNARE-dependent glutamate release in megakaryocytes

Objective

The identification of signaling pathways involved in megakaryocytopoiesis is essential for development of novel therapeutics to treat hematological disorders. Following our previous findings that megakaryocytes express functional channel-forming N-methyl-D-aspartate-type glutamate receptors, here we aimed to determine the glutamate release capacity in undifferentiated and differentiated megakaryocytes and the role of soluble N-ethyl maleimide-sensitive factor attachment protein receptor (SNARE) proteins that are known to be associated with vesicular exocytosis.

Materials and Methods

Using the megakaryocytic cell line MEG-01, primary megakaryocytes, and tissue sections of bone marrow, reverse transcription polymerase chain reaction, Western blot analysis, and immunolocalization were employed to detect factors required for vesicular glutamate release. Vesicle recycling was monitored by acridine orange and FM1-43 staining and glutamate release activity was assessed by an enzyme-linked fluorimetric assay. Genetically modified MEG-01 cells, with deletion or overexpression of SNARE and vesicular proteins, were also examined for glutamate release activity.

Results

We demonstrated that megakaryocytes express numerous proteins required for vesicular glutamate release, including core SNARE proteins, vesicle-associated membrane protein, soluble N-ethyl maleimide-sensitive factor attachment protein−23, and syntaxin, as well as specific glutamate-loading vesicle proteins, VGLUT1 and VGLUT2. Moreover, active vesicle recycling and differentiation-dependent glutamate release were observed in megakaryocytes. Vesicle-associated membrane protein−deficient MEG-01 cells, which are impaired in vesicle recycling, showed a 30% decrease in released glutamate, whereas overexpression of VGLUT1 exhibited up to a 2.2-fold increase in glutamate release.

Conclusion

These data show that glutamate release from megakaryocytes occurs in a SNARE-dependent, exocytotic manner and is increased during differentiation, suggesting that manipulation of glutamate signaling could influence megakaryocytopoiesis and, therefore, offer a suitable target for the treatment of thrombosis and other hematological disorders.

DOC2B, C2 domains, and calcium: A tale of intricate interactions

Ca(+2)-dependent exocytosis involves vesicle docking, priming, fusion, and recycling. This process is performed and regulated by a vast number of synaptic proteins and depends on proper protein-protein and protein-lipid interactions. Double C2 domain (DOC2) is a protein family of three isoforms found while screening DNA libraries with a C2 probe. DOC2 has three domains: the Munc13-interacting domain and tandem C2s (designated C2A and C2B) connected by a short polar linker. The C2 domain binds phospholipids in a Ca(2+)-dependent manner. This review focuses on the ubiquitously expressed isoform DOC2B. Sequence alignment of the tandem C2 protein family in mouse revealed high homology (81%) between rabphilin-3A and DOC2B proteins. We created a structural model of DOC2B's C2A based on the crystal structure of rabphilin-3A with and without calcium and found that the calcium-binding loops of DOC2B move upon calcium binding, enabling efficient plasma membrane penetration of its C2A. Here, we discuss the potential relation between the DOC2B bioinformatical model and its function and suggest a possible working model for its interaction with other proteins of the exocytotic machinery, including Munc13, Munc18, and syntaxin.

1alpha,25(OH)(2) vitamin D(3) induction of ATP secretion in osteoblasts

In the absence of mechanical stimulation, brief exposure of osteoblasts to 1alpha,25(OH)(2)vitamin D(3) (1,25D) triggers plasma membrane electrical responses that couple to exocytosis. Here we describe for the first time 1,25D induction of exocytotic ATP release in static ROS 17/2.8 and SAOS-2 cells and primary calvarial osteoblasts expressing a vitamin D receptor (VDR). We found that 10 nM 1,25D optimally induced 45 +/- 1% and 40 +/- 1% of partial and complete exocytotic events, respectively, from a 1,25D-sensitive pool of ATP-containing secretory vesicles within 60 s. We measured a dose-dependent 1,25D induction of ATP secretion, with maximal response of approximately 6.2-fold (16.93 +/- 1.82 nM for SAOS-2) and 3.1-fold (18.89 +/- 1.39 nM for ROS 17/2.8) obtained with 10 nM 1,25D compared with basal ATP levels (2.75 +/- 0.39 nM, SAOS-2; 6.09 +/- 0.58 nM, ROS 17/2.8 cells). The natural metabolite 25(OH)vitamin D(3) (25D, 10 nM) induced a significant 3.6-fold increase of ATP release in ROS 17/2.8 cells, but there was no induction with the antagonist 1beta,25(OH)(2)vitamin D(3) (1beta,25D, 10 nM) or the steroid 17beta-estradiol (10 nM). 1,25D-induced ATP secretion was abolished when cells were preincubated with inhibitors of vesicular exocytosis. siRNA VDR silencing prevented 1,25D stimulation of ATP exocytosis in ROS 17/2.8 and SAOS-2 cells. Similarly, 1,25D failed to stimulate ATP exocytosis in primary osteoblasts from a VDR knockout mouse. ATP secretion coupled to 1,25D induction of cytosolic calcium and chloride channel potentiation. Rapid 1,25D stimulation of ATP secretion involving nontranscriptional VDR functions in osteoblasts may help explain 1,25D bone anabolic properties.

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.

Increase in excitatory amino acid concentration and transporters expression in osteoarthritic knees of anterior cruciate ligament transected rabbits

OBJECTIVE

The present study aimed to determine the role of excitatory amino acids (EAAs) and EAA transporters (EAATs) in an osteoarthritis (OA) model of rabbit knees.

METHODS

OA was induced in New Zealand white male rabbits by anterior cruciate ligament transection (ACLT) in one knee of one hind limb; the other knee left unoperated. Rabbits that received ACLT of knee were assigned to the ACLT group (n=6), while a sham-operated group (n=6) underwent arthrotomy with no ACLT. Six naïve rabbits that received no surgery were used as normal control. The width of the knee joint was measured to determine the severity of joint inflammation. Before operation and at 10, 20, and 30 weeks after operation, knee joint dialysates were collected by microdialysis and assayed for EAAs by high-performance liquid chromatography. Gross morphology and histopathology and EAATs glutamate/aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1) expression in the articular cartilage of the knees were evaluated by immunohistochemistry and western blot analysis.

RESULTS

In the ACLT knees, a significant increase in the joint width was observed (5.3+/-0.9 mm, P<0.05) at 30 weeks after operation, while the sham-operated and naïve knees showed no difference as compared with the basal values. The concentrations (microM) of aspartate and glutamate in knee dialysates at 30 weeks after ACLT in naïve, sham, and ACLT were 0.36+/-0.07 and 4.5+/-1.10; 0.38+/-0.09 and 4.61+/-1.11; 0.67+/-0.18 and 9.71+/-2.89, respectively. Levels of glutamate and aspartate in the dialysates obtained from the ACLT knees increased by 213.3+/-29.6% and 187.5+/-33.8% (P<0.05) when compared to those in the sham-operated knees. Both naïve and ACLT chondrocytes were positively stained by antibodies against GLAST and GLT-1. GLAST and GLT-1 protein expressions were significantly increased in the ACLT knees (P<0.05).

CONCLUSION

Our findings indicate an involvement of EAAs and EAATs in the pathogenesis of OA in ACLT rabbits.

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.

Synaptotagmin VII regulates bone remodeling by modulating osteoclast and osteoblast secretion

Maintenance of bone mass and integrity requires a tight balance between resorption by osteoclasts and formation by osteoblasts. Exocytosis of functional proteins is a prerequisite for the activity of both cells. In the present study, we show that synaptotagmin VII, a calcium sensor protein that regulates exocytosis, is associated with lysosomes in osteoclasts and bone matrix protein-containing vesicles in osteoblasts. Absence of synaptotagmin VII inhibits cathepsin K secretion and formation of the ruffled border in osteoclasts and bone matrix protein deposition in osteoblasts, without affecting the differentiation of either cell. Reflecting these in vitro findings, synaptotagmin VII-deficient mice are osteopenic due to impaired bone resorption and formation. Therefore, synaptotagmin VII plays an important role in bone remodeling and homeostasis by modulating secretory pathways functionally important in osteoclasts and osteoblasts.

Responses of differentiated MC3T3-E1 osteoblast-like cells to reactive oxygen species

MC3T3-E1 osteoblast-like cells represent a suitable model for studying osteogenic development in vitro. The current investigation extends our previous work on the response of these cells to hydrogen peroxide by considering the effects of reactive oxygen species from other sources, and by determining whether differentiation alters sensitivity to oxidative damage. Aspects of hydrogen peroxide-mediated apoptotic and necrotic death were also examined. Cell viability was determined using the Alamar Blue assay; and accompanying morphological changes monitored by phase-contrast microscopy. Sensitivity to hydrogen peroxide increased significantly in cultures which had been induced to differentiate. Hydrogen peroxide and copper (II) ions, when combined, produced greater damage than hydrogen peroxide alone, whilst the hydroxyl radical scavengers mannitol or dimethylsulphoxide had no effect. Cyclosporin A and nicotinamide afforded partial protection. The tryptophan metabolite, 3-hydroxykynurenine significantly reduced viability, although 3-hydroxyanthranilic acid did not. The xanthine/xanthine oxidase system also reduced cell viability, an effect prevented by catalase but potentiated by superoxide dismutase. S-nitroso-N-acetylpenicillamine did not impair viability at the concentrations tested. Cultures were resistant to mitochondrial poisoning by potassium cyanide, but succumbed to 24-h exposures to 3-nitropropionic acid (1 mM). The results reveal a differential sensitivity of MC3T3-E1 cells to hydrogen peroxide-induced oxidative stress, an enhancement of sensitivity by cellular differentiation, and a potential preference for the glycolytic pathway by MC3T3-E1 cells. This study gives new insight into how bone cells may succumb to the toxic effects of oxidative stress generated by different stimuli and has relevance to conditions such as osteoporosis.

Proteomic and bioinformatic analysis of epithelial tight junction reveals an unexpected cluster of synaptic molecules

BACKGROUND

Zonula occludens, also known as the tight junction, is a specialized cell-cell interaction characterized by membrane "kisses" between epithelial cells. A cytoplasmic plaque of approximately 100 nm corresponding to a meshwork of densely packed proteins underlies the tight junction membrane domain. Due to its enormous size and difficulties in obtaining a biochemically pure fraction, the molecular composition of the tight junction remains largely unknown.

RESULTS

A novel biochemical purification protocol has been developed to isolate tight junction protein complexes from cultured human epithelial cells. After identification of proteins by mass spectroscopy and fingerprint analysis, candidate proteins are scored and assessed individually. A simple algorithm has been devised to incorporate transmembrane domains and protein modification sites for scoring membrane proteins. Using this new scoring system, a total of 912 proteins have been identified. These 912 hits are analyzed using a bioinformatics approach to bin the hits in 4 categories: configuration, molecular function, cellular function, and specialized process. Prominent clusters of proteins related to the cytoskeleton, cell adhesion, and vesicular traffic have been identified. Weaker clusters of proteins associated with cell growth, cell migration, translation, and transcription are also found. However, the strongest clusters belong to synaptic proteins and signaling molecules. Localization studies of key components of synaptic transmission have confirmed the presence of both presynaptic and postsynaptic proteins at the tight junction domain. To correlate proteomics data with structure, the tight junction has been examined using electron microscopy. This has revealed many novel structures including end-on cytoskeletal attachments, vesicles fusing/budding at the tight junction membrane domain, secreted substances encased between the tight junction kisses, endocytosis of tight junction double membranes, satellite Golgi apparatus and associated vesicular structures. A working model of the tight junction consisting of multiple functions and sub-domains has been generated using the proteomics and structural data.

CONCLUSION

This study provides an unbiased proteomics and bioinformatics approach to elucidate novel functions of the tight junction. The approach has revealed an unexpected cluster associating with synaptic function. This surprising finding suggests that the tight junction may be a novel epithelial synapse for cell-cell communication.

REVIEWERS

This article was reviewed by Gáspár Jékely, Etienne Joly and Neil Smalheiser.

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.

Secretion of L-glutamate from osteoclasts through transcytosis

Osteoclasts are involved in the catabolism of the bone matrix and eliminate the resulting degradation products through transcytosis, but the molecular mechanism and regulation of transcytosis remain poorly understood. Upon differentiation, osteoclasts express vesicular glutamate transporter 1 (VGLUT1), which is essential for vesicular storage and subsequent exocytosis of glutamate in neurons. VGLUT1 is localized in transcytotic vesicles and accumulates L-glutamate. Osteoclasts secrete L-glutamate and the bone degradation products upon stimulation with KCl or ATP in a Ca2+-dependent manner. KCl- and ATP-dependent secretion of L-glutamate was absent in osteoclasts prepared from VGLUT1-/- knockout mice. Osteoclasts express mGluR8, a class III metabotropic glutamate receptor. Its stimulation by a specific agonist inhibits secretion of L-glutamate and bone degradation products, whereas its suppression by a specific antagonist stimulates bone resorption. Finally, it was found that VGLUT1-/- mice develop osteoporosis. Thus, in bone-resorbing osteoclasts, L-glutamate and bone degradation products are secreted through transcytosis and the released L-glutamate is involved in autoregulation of transcytosis. Glutamate signaling may play an important role in the bone homeostasis.

Hydrogen peroxide-induced oxidative stress in MC3T3-E1 cells: The effects of glutamate and protection by purines

Glutamate has toxic effects on a number of tissues, partly by inducing toxic (e.g., oxidative) stress, whereas adenosine can be protective. Since there is evidence that glutamate and adenosine receptors are present in bone, we set out to study whether oxidative stress, induced by hydrogen peroxide (H2O2), affected viability in the MC3T3-E1 osteoblast-like cell line and whether treatment with adenosine receptor ligands attenuated this. Hydrogen peroxide (100 microM to 5 mM) reduced the viability of the MC3T3-E1 cells, while catalase reversed this cell loss and itself had some mitogenic effect. Superoxide dismutase (SOD) increased the number of viable cells alone but failed to modify significantly the effect of H2O2 treatments. Glutamate (100 microM, 1 mM) and NMDA (10 microM), applied alone for up to 1 h, had a mitogenic effect (P < 0.05). Adenosine A1 and A2A receptor agonists and antagonists at low and high concentrations showed some mitogenic effects when added singly, but only high concentrations of the agonists showed significant protection against cell death resulting from H2O2 treatments. Contributions from both apoptotic and necrotic pathways were implicated in the H2O2-induced cell loss as was demonstrated by the use of the caspase-3 inhibitor (Z-DEVD-fmk) and the PARP-1 inhibitor (DPQ). The results demonstrate that hydrogen peroxide was toxic to MC3T3-E1 cells, whereas glutamate was not and may even have a trophic influence. Adenosine and its receptors afforded some protection to osteoblasts against cellular death mediated partly by apoptosis and partly by necrosis.

Microarray analysis of the tendinopathic rat supraspinatus tendon: glutamate signaling and its potential role in tendon degeneration

Degenerative tendon injury or "tendinopathy" is one of the most common disorders of the musculoskeletal system. We used a rat model (Soslowsky LJ, Thomopoulos S, Tun S, Flanagan CL, Keefer CC, Mastaw J, and Carpenter JE. J Shoulder Elbow Surg 9: 79-84, 2000) to identify novel gene expression in the exercised-induced degenerated supraspinatus tendon by microarray and real-time PCR analyses. We identified several novel groups of differentially expressed genes, including those involved in apoptosis and related stress responses, and also genes that appear to be involved in glutamate signaling in tendon tissue, similar to recent findings by us in a microarray study of healing in the transected Achilles tendon of the rat (24). Until recently this kind of cellular communication was thought only to exist in cells of the central nervous system (CNS), where it is vital for CNS function. We further show that glutamate appears to induce a proapoptotic response in cultured tendon cells, similar to the "excitotoxic" response of cells in the CNS that become overstimulated. This may prove to be at least a partial cause of degeneration in overused tendon tissue and allow the development of treatments or "prehibilitation" regimens for tendinopathy based on currently used non-toxic glutamate antagonists.

Microarray analysis of healing rat Achilles tendon: evidence for glutamate signaling mechanisms and embryonic gene expression in healing tendon tissue

Tendon healing is a complex process consisting of a large number of intricate pathways roughly divided into the phases of inflammation, proliferation, and remodeling. Although these processes have been extensively studied at a variety of levels in recent years, there is still much that remains unknown. This study used microarray analyses to investigate the process at a genetic level in healing rat Achilles tendon at 1, 7, and 21 days postinjury, roughly representing the inflammation, proliferation, and remodeling phases. An interesting temporal expression profile was demonstrated, identifying both known and novel genes and pathways involved in the progression of tendon healing. Both inflammatory response and pro-proliferative genes were shown to be significantly upregulated from 24 h postinjury through to 21 days. Day 7 showed the largest increase in genetic activity, particularly with the expression of collagens and other extracellular matrix genes. Interestingly, there was also evidence of central nervous system-like glutamate-based signaling machinery present in tendon cells, as has recently been shown in bone. This type of signaling mechanism has not previously been shown to exist in tendon. Another novel finding from these analyses is that there appears to be several genes upregulated during healing which have exclusively or primarily been characterized as key modulators of proliferation and patterning during embryonic development. This may suggest that similar pathways are employed in wound healing as in the tightly regulated progression of growth and development in the embryo. These results could be of use in designing novel gene-based therapies to increase the efficacy and efficiency of tendon healing.

The glutamatergic system outside the CNS and in cancer biology

Glutamate is a major excitatory neurotransmitter in the CNS. The signalling machinery consists of: glutamate receptors, which are responsible for signal input; plasma glutamate transporters, which are responsible for signal termination; and vesicular glutamate transporters for signal output through exocytic release. Recently, data have suggested that the glutamatergic system plays an important role in non-neuronal tissues. In addition, the expression of glutamatergic system has been implicated in tumour biology. This review outlines the evidence, which suggests that the glutamatergic system may have an important role in cancer biology.

Group III metabotropic glutamate receptor activation inhibits Ca2+ influx and nitric oxide synthase activity in bone marrow stromal cells

Nitric oxide (NO) is pivotal to bone physiology. In the central nervous system constitutive, Ca(2+)-calmodulin regulated NO synthase activity and glutamate signalling are intimately linked. Since L-glutamate signalling occurs in bone and is implicated in bone regulation, we have investigated the effect of L-glutamate on NO synthase in bone-derived cells. Treatment of marrow stromal cells with L-glutamate reduced basal NO synthase activity by 40%. Imaging showed that L-glutamate caused a rapid, usually localised and slowly-reversible fall in [Ca(2+)](i). This effect was resistant to disruption of intracellular Ca(2+) stores but sensitive to extracellular La(3+) or omission of extracellular Ca(2+), demonstrating that glutamate acts by inhibition of membrane Ca(2+) influx. The only previous description of such an effect of L-glutamate is via activation of the group III receptor, mGluR6, in the retina. Using Western blotting and RT-PCR we detected mGluR6 protein and transcripts in marrow stromal cells. The effects of L-glutamate on NOS activity and [Ca(2+)](i) in marrow stromal cells were abolished by a group III mGluR inhibitor, (S)-2-amino-2-methyl-4-phosphonobutyric acid. Recording of membrane potential showed that, similarly to the effects of retinal mGluR6 activation, L-glutamate induced membrane hyperpolarisation (-16 +/- 2 mV), which was also sensitive to group III mGluR inhibition. L-glutamate had no effect on cAMP levels. We conclude that activation of a group III mGluR in bone marrow stromal cells inhibits a Ca(2+)-permeable plasma membrane channel, reducing [Ca(2+)](i) and suppressing generation of NO. These observations directly link bone L-glutamate signalling to processes central to bone growth and regulation.

Autocrine/paracrine stimulation of purinergic receptors in osteoblasts: contribution of vesicular ATP release

Extracellular nucleotides such as ATP and UTP are released in response to mechanical stimulation in different cell systems. It is becoming increasingly evident that ATP release plays a role in autocrine and paracrine stimulation of osteoblasts. Mechanical stimulation, as shear stress, membrane stretch or hypo-osmotic swelling, as well as oscillatory fluid flow, stimulates ATP release from different osteoblastic cell lines. Human osteoblast-like initial transfectant (HOBIT) cells release ATP in response to mechanical stimulation. In the present study, we show that HOBIT cells are activated by nanomolar levels of extracellular ATP, concentrations that can be detected under resting conditions and increase following hypotonic shock. Cell activation by hypotonic medium induced intracellular Ca2+ oscillations, and Egr-1 synthesis and DNA-binding activity. Quinacrine staining of living, resting cells revealed a granular fluorescence, typical of ATP-storing vesicles. Monensin prevented quinacrine staining and considerably inhibited hypotonic-induced ATP release. Finally, elevated levels of cytosolic Ca2+ activated massive ATP release and a dose-dependent loss of quinacrine granules. The contribution of a vesicular mechanism for ATP release is proposed to sustain paracrine osteoblast activation.

Rab3D regulates a novel vesicular trafficking pathway that is required for osteoclastic bone resorption

Rab3 proteins are a subfamily of GTPases, known to mediate membrane transport in eukaryotic cells and play a role in exocytosis. Our data indicate that Rab3D is the major Rab3 species expressed in osteoclasts. To investigate the role of Rab3D in osteoclast physiology we examined the skeletal architecture of Rab3D-deficient mice and found an osteosclerotic phenotype. Although basal osteoclast number in null animals is normal the total eroded surface is significantly reduced, suggesting that the resorptive defect is due to attenuated osteoclast activity. Consistent with this hypothesis, ultrastructural analysis reveals that Rab3D(-/-) osteoclasts exhibit irregular ruffled borders. Furthermore, while overexpression of wild-type, constitutively active, or prenylation-deficient Rab3D has no significant effects, overexpression of GTP-binding-deficient Rab3D impairs bone resorption in vitro. Finally, subcellular localization studies reveal that, unlike wild-type or constitutively active Rab3D, which associate with a nonendosomal/lysosomal subset of post-trans-Golgi network (TGN) vesicles, inactive Rab3D localizes to the TGN and inhibits biogenesis of Rab3D-bearing vesicles. Collectively, our data suggest that Rab3D modulates a post-TGN trafficking step that is required for osteoclastic bone resorption.

Mechanical loading modulates glutamate receptor subunit expression in bone

The cellular mechanisms coupling mechanical loading with bone remodeling remain unclear. In the CNS, the excitatory amino acid glutamate (Glu) serves as a potent neurotransmitter exerting its effects via various membrane Glu receptors (GluR). Nerves containing Glu exist close to bone cells expressing functional GluRs. Demonstration of a mechanically sensitive glutamate/aspartate transporter protein and the ability of glutamate to stimulate bone resorption in vitro suggest a role for glutamate linking mechanical load and bone remodeling. We used immunohistochemical techniques to identify the expression of N-methyl-d-aspartate acid (NMDA) and non-NMDA (AMPA or kainate) ionotropic GluR subunits on bone cells in vivo. In bone sections from young adult rats, osteoclasts expressed numerous GluR subunits including AMPA (GluR2/3 and GluR4), kainic acid (GluR567) and NMDA (NMDAR2A, NMDAR2B and NMDAR2C) receptor subtypes. Bone lining cells demonstrated immunoexpression for NMDAR2A, NMDAR2B, NMDAR2C, GluR567, GluR23, GluR2 and GluR4 subunits. Immunoexpression was not evident on osteocytes, chondrocytes or vascular channels. To investigate the effects of mechanical loading on GluR expression, we used a Materials Testing System (MTS) to apply 10 N sinusoidal axial compressive loads percutaneously to the right limbs (radius/ulna, tibia/fibula) of rats. Each limb underwent 300-load cycles/day (cycle rate, 1 Hz) for 4 consecutive days. Contralateral, non-loaded limbs served as controls. Mechanically loaded limbs revealed a load-induced loss of immunoexpression for GluR2/3, GluR4, GluR567 and NMDAR2A on osteoclasts and NMDAR2A, NMDAR2B, GluR2/3 and GluR4 on bone lining cells. Both neonatal rabbit and rat osteoclasts were cultured on bone slices to investigate the effect of the NMDA receptor antagonist, MK801, and the AMPA/kainic acid receptor antagonist, NBQX, on osteoclast resorptive activity in vitro. The inhibition of resorptive function seen suggested that both NMDAR and kainic acid receptor function are required for normal osteoclast function. While the exact role of ionotropic GluRs in skeletal tissue remains unclear, the modulation of GluR subunit expression by mechanical loading lends further support for participation of Glu as a mechanical loading effector. These ionotropic receptors appear to be functionally relevant to normal osteoclast resorptive activity.

Dramatic decrease of innervation density in bone after ovariectomy

Recent studies have demonstrated that bone is highly innervated and contains neuromediators that have functional receptors on bone cells. However, no data exist concerning the quantitative changes of innervation during bone loss associated with estrogen withdrawal. To study the involvement of nerve fibers in the regulation of bone remodeling, we have evaluated the modifications of innervation in a classical in vivo model of osteopenia in rats, ovariectomy (OVX). Skeletal innervation was studied by immunocytochemistry using antibodies directed against specific neuronal markers, neurofilament 200 and synaptophysin, and the neuromediator glutamate. Sciatic neurectomy, another model of bone loss due to limb denervation and paralysis, was used to validate our quantitative image analysis technique of immunostaining for nerve markers. Female Wistar rats at 12 wk of age were sham-operated (SHAM) or ovariectomized (OVX). Bone mineral density measurement and bone histomorphometry analysis of tibiae 14 d after surgery demonstrated a significant bone loss in OVX compared with SHAM. We observed an important reduction of nerve profile density in tibiae of OVX animals compared with SHAM animals, whereas innervation density in skin and muscles was similar for OVX and control rats. Quantitative image analysis of immunostainings demonstrated a significant decrease of the percentage of immunolabeling per total bone volume of neurofilament 200, synaptophysin, and glutamate in both the primary and secondary spongiosa of OVX rats compared with SHAM. These data indicate for the first time that OVX-induced bone loss in rat tibiae is associated with a reduction in nerve profile density, suggesting a functional link between the nervous system and the bone loss after ovariectomy.

Synapsin-like immunoreactivity is present in hair cells and efferent terminals of the toadfish crista ampullaris

The synapsins are presynaptic membrane-associated proteins involved in neurotransmitter release. They are differentially expressed in tissues and cells of the central and peripheral nervous system. In vestibular end organs of mammals, synapsin I-like immunoreactivity has been reported in efferent and afferent terminals and in afferent nerve calyces surrounding type I hair cells. In addition, synapsin I has recently been described in several non-neural cell lines. The present study was conducted to locate synapsin-like immunoreactivity in the neuronal and non-neuronal cells of the fish crista ampullaris, to examine the possibility that the non-neuronal sensory receptor cells express synapsins in vivo. Synapsin-like immunostaining was visualized by immunofluorescence detection in wholemounts of the toadfish crista ampullaris using multiphoton laser scanning microscopy and by electron microscopic visualization of post-embedding immunogold labeling. The results demonstrate that synapsin-like immunoreactivity is present in vestibular hair cells and efferent boutons of the toadfish crista ampullaris. Afferent endings are not labeled. Staining in hair cells is not associated with the synaptic ribbons, suggesting that there is an additional, non-synaptic role for the synapsins in some non-neuronal cells of vertebrates. Moreover, while the cristae of amniote and anamniote species share many functional attributes, differences in their synaptic vesicle-associated protein profiles appear to reflect their disparate hair cell populations.

Accumulation of [3H] glutamate in cultured rat calvarial osteoblasts

In the present study, we have attempted to demonstrate constitutive and functional expression in bone of particular glutamate transporters (GluTs) required for signal termination in glutamatergic signaling process. Reverse transcription polymerase chain reaction revealed constitutive expression of mRNA for the neuronal GluT subtype excitatory amino acid carrier-1, in addition to glial subtypes such as glutamate aspartate transporter and glutamate transporter-1, in rat calvarial osteoblasts cultured for 7-21 days in vitro (DIV). The accumulation of [3H]glutamate (Glu) occurred in a temperature- and sodium-dependent manner with pharmacological profiles similar to those for brain GluTs in osteoblasts cultured for 7 DIV, while three different agonists at ionotropic Glu receptors significantly inhibited the accumulation of [3H]Glu in osteoblasts. Although [3H]Glu accumulation consisted of a single component with a K(m) value of 26.0 +/- 5.8 microM and a V(max) value of 960 +/- 122 nmol/(min mg protein), respectively, in osteoblasts cultured for 7 DIV, in vitro maturation led to a significant decrease in V(max) value to 290 +/- 33 nmol/(min mg protein) without significantly affecting K(m) values on 21 DIV. These results suggest that Glu could be incorporated into intracellular locations through glial and/or neuronal GluT subtypes expressed in cultured rat calvarial osteoblasts.