Osteoblastic glutamate receptor function regulates bone formation and resorption

Previous studies showed that a variety of bone cells express protein components necessary for neuronal-like glutamatergic signaling and implicated glutamate as having a role in mechanically induced bone remodeling. Initial functional studies concentrated on the role of glutamate signaling in bone resorption and provided compelling evidence to suggest that glutamate signaling through functional NMDA type ionotropic glutamate receptors (iGluRs) is a prerequisite for in vitro osteoclastogenesis. Originally, effects of iGluR antagonists seen in co-cultures were attributed to antagonists acting directly on osteoclast precursors. However, in the light of recent osteoblast studies it now seems likely that the observed effects on osteoclastogenesis are an indirect effect of modulating the function of pre-osteoblast present within these cultures. The presence of iGluRs in osteoblasts suggests a role for them in bone formation and this paper reviews and discusses the emerging data relating to the role of glutamate signaling in osteoblasts. A number of recently published studies have shown that osteoblasts not only express a wide number of 'pre-synaptic' glutamatergic proteins but also possess the ability to both regulate glutamate release and actively recycle extracellular glutamate. The functionality of osteoblastic 'post-synaptic' glutamatergic components has also been shown as both primary and clonal osteoblasts express electrophysiologically active iGluRs, metabotropic type glutamate receptors (mGluRs) along with a variety of glutamate receptor associated signaling proteins. There is, however, little published data regarding the actual role of glutamatergic signaling in osteoblastic bone formation. In vivo and in vitro studies performed provide evidence that glutamatergic signaling is a necessity for normal osteoblast function. In a number of different models of in vitro bone formation, the addition of non-competitive antagonists of iGluRs prevents the formation of mineralized bone, moreover antagonizing some sub-types of iGluR mediates the differentiation of pre-osteoblasts. iGluR antagonists modulate osteoblast function in a manner that correlates with the previously reported data regarding in vitro osteoclastogenesis. Interestingly iGluR mediated glutamate signaling appears to function differently in osteoblasts derived from flat and long bones. This implies the components of osteoblastic glutamatergic signaling may be adapted in vivo possibly to reflect the differential function of osteoblasts in those regions of the skeleton.     

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.     

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.     

The role of glutamate in the regulation of bone mass and architecture

Communication between the cells in bone underlies the way that the tissue functions physiologically, and in nearly all pathologies, the pathogenesis of skeletal diseases. The number of molecules involved in intercellular signalling in bone grows constantly and it is perhaps unsurprising that the list includes many with functions in other tissues. In recent years, evidence has accumulated to show that molecules involved in neurotransmission have paracrine roles in the skeleton. The focus of this review is the excitatory amino acid glutamate and its role in regulating bone formation and resorption. Specifically, this article will concentrate on the functional role of the system, and the reasons why mechanisms like synaptic transmission are relevant to what might appear to be a slow responding tissue, as the sites of expression of glutamate signalling components in bone have been reviewed already. While there is strong evidence for a regulatory role for glutamate in osteoblast and osteoclast differentiation and function in vitro, in vivo data is less advanced. Preliminary data from in vivo systems does however suggest that glutamate has a physiological function in the skeleton.     

Expression and function of P2 receptors in bone

ATP (adenosine 5'-triphosphate) is one of the most important extracellular regulatory molecules in the skeleton. Extracellular ATP and other nucleotides signal through P2 receptors, a diverse group of receptors that are widely expressed by bone cells. P2 receptors are divided into two subclasses; P2Y G-protein coupled receptors, and P2X ligand-gated ion channels, and there is functional and molecular evidence for the expression of these receptors on both osteoblasts and osteoclasts. In order to activate P2 receptors, nucleotides must be released into the bone microenvironment. ATP is present in mmol concentrations in cells and can be released by cell lysis, cell trauma or physiological mechanisms, possibly through ABC transporters. Following co-activation of P2Y and PTH1 receptors on osteoblasts, there are multiple levels of interaction in downstream signalling that eventually lead to synergistic expression of osteoblastic genes, providing a mechanism for integrating local and systemic regulatory signals in bone particularly with regard to the activation of bone remodelling. Activation of P2Y1 receptors on osteoblasts enhances expression of RANKL leading indirectly to an increase in osteoclast formation and resorption. Expression of P2X7 inducible pores on osteoclast precursor cell membranes allows fusion to form multinucleated osteoclasts and blockade of this receptor inhibits resorption. The capacity of extracellular nucleotides to provide a highly localized and transient signal coupled with the profound effects of P2 receptor activation on osteoblastic and osteoclastic cells and the synergistic interactions with systemic hormones, indicate that nucleotides have a strong influence over bone tissue growth and regeneration.     

The role of glutamate transporters in bone cell signalling

The amino acid L-glutamate mediates signals at excitatory synapses in the CNS where its effects are controlled by co-ordinated activities of various types of glutamate receptor and transporter. This signalling mechanism has proved to be far more ubiquitous with many different cell types responding to glutamate. The glutamate transporter GLAST-1 was the first component of this pathway identified in bone where its expression was found to be mechanoresponsive in osteocytes. There is now a wealth of evidence supporting a role for this signalling mechanism in bone. Osteoblasts can release glutamate in a regulated manner and express functional glutamate receptors that influence their differentiation and osteogenic activity. Likewise, osteoclasts express functional glutamate receptors that influence their bone resorbing capacity. This article considers the various functions of glutamate transporters in this signalling pathway, and the evidence supporting an important role of glutamate signalling in regulating bone cell activities.     

Building a synapse

L-glutamate is the primary neurotransmitter at excitatory synapses in the vertebrate CNS and at arthropod neuromuscular junctions (NMJs). However, the molecular mechanisms that trigger the recruitment of glutamate receptors at the onset of synaptogenesis and promote their stabilization at postsynaptic densities remain poorly understood. We have reported the discovery of a novel, evolutionary conserved molecule, Neto, essential for clustering of ionotropic glutamate receptors (iGluRs) at Drosophila NMJ. Neto is the first auxiliary subunit described in Drosophila and is the only non-channel subunit absolutely required for functional iGluRs. Here we review the role of Drosophila Neto in synapse assembly, its similarities with other Neto proteins and a new perspective on how glutamatergic synapses are physically assembled and stabilized.     

Emerging evidence for a similar role of glutamate receptors in the nervous and immune systems

The role of glutamate receptors in synaptic transmission and excitotoxicity in the nervous system is well established. Recent evidence has emerged that glutamatergic mechanisms also exist in a wide variety of non-neuronal cells. In the case of thymocytes and lymphocytes, several types of glutamate receptor are expressed which can induce functional changes. This review focuses on the cellular function of NMDA-activated ionotropic and groups I and III metabotropic glutamate receptors in lymphocytes. Levels of exogenous and endogenous circulatory agonists and antagonists for lymphocyte glutamate receptors, notably homocysteine metabolites, are markedly increased in certain disease states and may be involved in disorders of the immune system. In addition to glutamate and aspartate, these compounds are active at glutamate receptors and increase the excitotoxic effects of glutamate in both neurons and lymphocytes. Increased levels of compounds acting at glutamate receptors may be risk factors for organ damage, for example in both heart and kidney disease. We conclude that glutamate is involved in signaling in immunocompetent cells and that the expression of both ionotropic and metabotropic glutamate receptors may have regulatory functions in immunocompetent cells, as well as in the nervous system. In addition, glutamate may serve as a signaling agent between the immune and nervous systems.     

Modulation of excitatory neurotransmission by neuronal/glial signalling molecules: interplay between purinergic and glutamatergic systems

Glutamate is the main excitatory neurotransmitter of the central nervous system (CNS), released both from neurons and glial cells. Acting via ionotropic (NMDA, AMPA, kainate) and metabotropic glutamate receptors, it is critically involved in essential regulatory functions. Disturbances of glutamatergic neurotransmission can be detected in cognitive and neurodegenerative disorders. This paper summarizes the present knowledge on the modulation of glutamate-mediated responses in the CNS. Emphasis will be put on NMDA receptor channels, which are essential executive and integrative elements of the glutamatergic system. This receptor is crucial for proper functioning of neuronal circuits; its hypofunction or overactivation can result in neuronal disturbances and neurotoxicity. Somewhat surprisingly, NMDA receptors are not widely targeted by pharmacotherapy in clinics; their robust activation or inhibition seems to be desirable only in exceptional cases. However, their fine-tuning might provide a promising manipulation to optimize the activity of the glutamatergic system and to restore proper CNS function. This orchestration utilizes several neuromodulators. Besides the classical ones such as dopamine, novel candidates emerged in the last two decades. The purinergic system is a promising possibility to optimize the activity of the glutamatergic system. It exerts not only direct and indirect influences on NMDA receptors but, by modulating glutamatergic transmission, also plays an important role in glia-neuron communication. These purinergic functions will be illustrated mostly by depicting the modulatory role of the purinergic system on glutamatergic transmission in the prefrontal cortex, a CNS area important for attention, memory and learning.     

Metabotropic glutamate receptors in mechanisms of plasticity of the central nervous system in the honeybee <Emphasis Type="Italic">Apis mellifera</Emphasis>

Localization of metabotropic glutamate receptors (MGR) in head ganglion of honeybee Apis mellifera, and mechanisms of participation of activated MGR in CNS plasticity are investigated by means of complex approach using immunochemical, electrophysiological and behavioral methods. Influence of MGR activation on GABAergic system and inotropic glutamate receptors (IGR) of AMPA- and NMDA-subtypes is studied. MGR are revealed in lateral and medial calices of mushroom bodies. The inhibiting influence of MGR on AMPA- and NMDA receptors is shown using method of conditioned reflex. Previous activation of MGR neutralizes the inhibiting effect of GABA. Modulating role of heterogeneous MGR population in mechanisms of CNS plasticity on the level of glutamatergic synapse, and at interaction with GABAergic system is discussed.     

ATP: a ubiquitous gliotransmitter integrating neuron-glial networks

Astrocytes are ideally situated to integrate glial and neuronal functions and neurovascular coupling by way of their multiple contacts with neurons, glia and blood vessels. There is a high degree of specialisation of astroglial membranes at the different sites of contact, including the expression of neurotransmitter receptors, ion channels, transporters and gap junctional proteins. An apparently universal property of astrocytes throughout the CNS is their responsiveness to ATP acting via metabotropic P2Y receptors, with a prominent role for the P2Y1 receptor subtype. Activation of astroglial P2Y receptors triggers a rise in intracellular calcium, which is the substrate for astroglial excitability and intercellular communication. In addition, astrocytes have a number of mechanisms for the release of ATP, which can be considered a 'gliotransmitter'. Astrocytes may be the most widespread source of ATP release in the CNS, and astroglial ATP and its metabolite adenosine activate purine receptors on neurons, microglia, oligodendrocytes and blood vessels. There is compelling evidence that astroglial ATP and adenosine regulate neuronal synaptic strength, although the physiological significance of this astrocyte-to-neuron signalling is questioned. A less appreciated aspect of astrocyte signalling is that they also release neurotransmitters onto other glia. Notably, both ATP and adenosine control microglial behaviour and regulate oligodendrocyte differentiation and myelination. P2 receptors also mediate injury responses in all glial cell types, with a prominent role for the P2X7 receptor subtype. In addition, ATP is a potent vasoconstrictor and astrocytes provide a route for coupling blood flow to neuronal activity by way of their synaptic and perivascular connections. Thus, astrocytes are the fulcrum of neuron-glial-vascular networks and purinergic signalling is the primary mechanism by which astrocytes can integrate the functions of these diverse elements.     

Group III Metabotropic Glutamate Receptors: Pharmacology,Physiology and Therapeutic Potential

Glutamate, the primary excitatory neurotransmitter in the central nervous system (CNS), exerts neuromodulatory actions via the activation of metabotropic glutamate (mGlu) receptors. There are eight known mGlu receptor subtypes (mGlu1-8), which are widely expressed throughout the brain, and are divided into three groups (I–III), based on signalling pathways and pharmacological profiles. Group III mGlu receptors (mGlu4/6/7/8) are primarily, although not exclusively, localised on presynaptic terminals, where they act as both auto- and hetero-receptors, inhibiting the release of neurotransmitter. Until recently, our understanding of the role of individual group III mGlu receptor subtypes was hindered by a lack of subtype-selective pharmacological tools. Recent advances in the development of both orthosteric and allosteric group III-targeting compounds, however, have prompted detailed investigations into the possible functional role of these receptors within the CNS, and revealed their involvement in a number of pathological conditions, such as epilepsy, anxiety and Parkinson’s disease. The heterogeneous expression of group III mGlu receptor subtypes throughout the brain, as well as their distinct distribution at glutamatergic and GABAergic synapses, makes them ideal targets for therapeutic intervention. This review summarises the advances in subtype-selective pharmacology, and discusses the individual roles of group III mGlu receptors in physiology, and their potential involvement in disease.     

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.     

The diversity of mechanisms of the ion channels blockade as a way towards designing new pharmacological agents

Dysfunctions of glutamatergic synaptic neurotransmission often accompany various CNS disorders. Action of excessive glutamate, which causes excitotoxic effects by neuron depolarization and massive calcium influx can lead to cell death. Despite obvious importance of development of anti-glutamic neuroprotectors, among great number of known antagonists of ionotropic glutamate receptors only memantine is used in medicinal practice. One of the sources of numerous side effects caused by glutamate receptor antagonists is that the drugs usually inhibit receptors, which mediate both normal and pathological CNS processes. A possible approach to overcoming the problem is to develop the drugs whose action is enhanced in potentially pathological conditions such as high-frequency activation, high glutamate concentration, depolarized membrane, etc. Action of many classes of antagonists depends on pattern of receptor activation and on membrane voltage. In the present work, we discuss several peculiarities of channel blocking mechanisms from the viewpoint of neuroprotector development. In particular, we compare channel blockers which demonstrate different types of interaction with the channel gating machinery, we consider different types of voltage dependence and consider action of channel blockers, which can permeate through the channel. We conclude that meticulous analysis of the mechanism of action of the glutamate receptor channel antagonists could help to approach predicting of in vivo action using in vitro data.     

The role of glutamate transporters in neurodegenerative diseases and potential opportunities for intervention

Extracellular concentrations of the predominant excitatory neurotransmitter, glutamate, and related excitatory amino acids are maintained at relatively low levels to ensure an appropriate signal-to-noise ratio and to prevent excessive activation of glutamate receptors that can result in cell death. The latter phenomenon is known as 'excitotoxicity' and has been associated with a wide range of acute and chronic neurodegenerative disorders, as well as disorders that result in the loss of non-neural cells such as oligodendroglia in multiple sclerosis. Unfortunately clinical trials with glutamate receptor antagonists that would logically seem to prevent the effects of excessive receptor activation have been associated with untoward side effects or little clinical benefit. In the mammalian CNS, the extracellular concentrations of glutamate are controlled by two types of transporters; these include a family of Na(+)-dependent transporters and a cystine-glutamate exchange process, referred to as system X(c)(-). In this review, we will focus primarily on the Na(+)-dependent transporters. A brief introduction to glutamate as a neurotransmitter will be followed by an overview of the properties of these transporters, including a summary of the presumed physiologic mechanisms that regulate these transporters. Many studies have provided compelling evidence that impairing the function of these transporters can increase the sensitivity of tissue to deleterious effects of aberrant activation of glutamate receptors. Over the last decade, it has become clear that many neurodegenerative disorders are associated with a change in localization and/or expression of some of the subtypes of these transporters. This would suggest that therapies directed toward enhancing transporter expression might be beneficial. However, there is also evidence that glutamate transporters might increase the susceptibility of tissue to the consequences of insults that result in a collapse of the electrochemical gradients required for normal function such as stroke. In spite of the potential adverse effects of upregulation of glutamate transporters, there is recent evidence that upregulation of one of the glutamate transporters, GLT-1 (also called EAAT2), with beta-lactam antibiotics attenuates the damage observed in models of both acute and chronic neurodegenerative disorders. While it seems somewhat unlikely that antibiotics specifically target GLT-1 expression, these studies identify a potential strategy to limit excitotoxicity. If successful, this type of approach could have widespread utility given the large number of neurodegenerative diseases associated with decreases in transporter expression and excitotoxicity. However, given the massive effort directed at developing glutamate receptor agents during the 1990s and the relatively modest advances to date, one wonders if we will maintain the patience needed to carefully understand the glutamatergic system so that it will be successfully targeted in the future.     

谷氨酸信号通路及其在骨力学信号转导通路中潜在的功能

自在中枢神经系统中发现谷氨酸发挥功能以来,谷氨酸受体及其在突触内膜偶联的信号通路,就成为神经系统研究的重要内容。近年的研究显示,谷氨酸受体及其胞内信号通路在包括骨在内的非神经组织中表达和发挥功能,在骨细胞中有表达谷氨酸受体、转运子的证据,因而有假说认为谷氨酸成了骨中力学信号潜在的转导子,但还缺乏有利的证据支持。简要综述了谷氨酸信号通路及其在骨中的功能,并就其在骨力学信号转导中潜在的功能和作用机制进行了探讨。     

Regulatory mechanisms of AMPA receptors in synaptic plasticity

Activity-dependent changes in the strength of excitatory synapses are a cellular mechanism for the plasticity of neuronal networks that is widely recognized to underlie cognitive functions such as learning and memory. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors (AMPARs) are the main transducers of rapid excitatory transmission in the mammalian CNS, and recent discoveries indicate that the mechanisms which regulate AMPARs are more complex than previously thought. This review focuses on recent evidence that alterations to AMPAR functional properties are coupled to their trafficking, cytoskeletal dynamics and local protein synthesis. These relationships offer new insights into the regulation of AMPARs and synaptic strength by cellular signalling.     

Beyond the role of glutamate as a neurotransmitter

Glutamate is the principal excitatory neurotransmitter of the central nervous system, but many studies have expanded its functional repertoire by showing that glutamate receptors are present in a variety of non-excitable cells. How does glutamate receptor activation modulate their activity? Do non-excitable cells release glutamate, and, if so, how? These questions remain enigmatic. Here, we review the current knowledge on glutamatergic signalling in non-neuronal cells, with a special emphasis on astrocytes.