The open reading frame of the Na(+)-dependent glutamate transporter GLAST-1 is expressed in bone and a splice variant of this molecule is expressed in bone and brain

Glutamate transporters: Critical components of glutamatergic transmission

Glutamate is the major excitatory neurotransmitter in the vertebrate central nervous system. Once released, it binds to specific membrane receptors and transporters activating a wide variety of signal transduction cascades, as well as its removal from the synaptic cleft in order to avoid its extracellular accumulation and the overstimulation of extra-synaptic receptors that might result in neuronal death through a process known as excitotoxicity. Although neurodegenerative diseases are heterogenous in clinical phenotypes and genetic etiologies, a fundamental mechanism involved in neuronal degeneration is excitotoxicity. Glutamate homeostasis is critical for brain physiology and Glutamate transporters are key players in maintaining low extracellular Glutamate levels. Therefore, the characterization of Glutamate transporters has been an active area of glutamatergic research for the last 40 years. Transporter activity its regulated at different levels: transcriptional and translational control, transporter protein trafficking and membrane mobility, and through extensive post-translational modifications. The elucidation of these mechanisms has emerged as an important piece to shape our current understanding of glutamate actions in the nervous system.

Glutamate transporters, EAAT1 and EAAT2, are potentially important in the pathophysiology and treatment of schizophrenia and affective disorders

Glutamate is the predominant excitatory neurotransmitter in the human brain and it has been shown that prolonged activation of the glutamatergic system leads to nerve damage and cell death. Following release from the pre-synaptic neuron and synaptic transmission, glutamate is either taken up into the pre-synaptic neuron or neighbouring glia by transmembrane glutamate transporters. Excitatory amino acid transporter (EAAT) 1 and EAAT2 are Na⁺-dependant glutamate transporters expressed predominantly in glia cells of the central nervous system. As the most abundant glutamate transporters, their primary role is to modulate levels of glutamatergic excitability and prevent spill over of glutamate beyond the synapse. This role is facilitated through the binding and transportation of glutamate into astrocytes and microglia. The function of EAAT1 and EAAT2 is heavily regulated at the levels of gene expression, post-transcriptional splicing, glycosylation states and cell-surface trafficking of the protein. Both glutamatergic dysfunction and glial dysfunction have been proposed to be involved in psychiatric disorder. This review will present an overview of the roles that EAAT1 and EAAT2 play in modulating glutamatergic activity in the human brain, and mount an argument that these two transporters could be involved in the aetiologies of schizophrenia and affective disorders as well as represent potential drug targets for novel therapies for those disorders.

Down-Regulation of Excitatory Amino Acid Transporters EAAT1 and EAAT2 by the Kinases SPAK and OSR1

SPAK (SPS1-related proline/alanine-rich kinase) and OSR1 (oxidative stress-responsive kinase 1) are cell volume-sensitive kinases regulated by WNK (with-no-K[Lys]) kinases. SPAK/OSR1 regulate several channels and carriers. SPAK/OSR1 sensitive functions include neuronal excitability. Orchestration of neuronal excitation involves the excitatory glutamate transporters EAAT1 and EAAT2. Sensitivity of those carriers to SPAK/OSR1 has never been shown. The present study thus explored whether SPAK and/or OSR1 contribute to the regulation of EAAT1 and/or EAAT2. To this end, cRNA encoding EAAT1 or EAAT2 was injected into Xenopus oocytes without or with additional injection of cRNA encoding wild-type SPAK or wild-type OSR1, constitutively active (T233E)SPAK, WNK insensitive (T233A)SPAK, catalytically inactive (D212A)SPAK, constitutively active (T185E)OSR1, WNK insensitive (T185A)OSR1 or catalytically inactive (D164A)OSR1. The glutamate (2 mM)-induced inward current (I Glu) was taken as a measure of glutamate transport. As a result, I Glu was observed in EAAT1- and in EAAT2-expressing oocytes but not in water-injected oocytes, and was significantly decreased by coexpression of SPAK and OSR1. As shown for EAAT2, SPAK, and OSR1 decreased significantly the maximal transport rate but significantly enhanced the affinity of the carrier. The effect of wild-type SPAK/OSR1 on EAAT1 and EAAT2 was mimicked by (T233E)SPAK and (T185E)OSR1, but not by (T233A)SPAK, (D212A)SPAK, (T185A)OSR1, or (D164A)OSR1. Coexpression of either SPAK or OSR1 decreased the EAAT2 protein abundance in the cell membrane of EAAT2-expressing oocytes. In conclusion, SPAK and OSR1 are powerful negative regulators of the excitatory glutamate transporters EAAT1 and EAAT2.

Mechanisms of glutamate transport

L-Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and plays important roles in a wide variety of brain functions, but it is also a key player in the pathogenesis of many neurological disorders. The control of glutamate concentrations is critical to the normal functioning of the central nervous system, and in this review we discuss how glutamate transporters regulate glutamate concentrations to maintain dynamic signaling mechanisms between neurons. In 2004, the crystal structure of a prokaryotic homolog of the mammalian glutamate transporter family of proteins was crystallized and its structure determined. This has paved the way for a better understanding of the structural basis for glutamate transporter function. In this review we provide a broad perspective of this field of research, but focus primarily on the more recent studies with a particular emphasis on how our understanding of the structure of glutamate transporters has generated new insights.

Alternate splicing and expression of the glutamate transporter EAAT5 in the rat retina

Excitatory amino acid transporter 5 (EAAT5) is an unusual glutamate transporter that is expressed in the retina, where it is localised to two populations of glutamatergic neurons, namely the bipolar neurons and photoreceptors. EAAT5 exhibits two distinct properties, acting both as a slow glutamate transporter and as a glutamate-gated inhibitory receptor. The latter property is attributable to a co-associated chloride conductance. EAAT5 has previously been thought to exist only as a full-length form. We now demonstrate by PCR cloning and sequencing, the presence of five novel splice variant forms of EAAT5 which skip either partial or complete exons in the rat retina. Furthermore, we demonstrate that each of these variants is expressed at the protein level as assessed by Western blotting using splice-specific antibodies that we have generated. We conclude that EAAT5 exists in multiple spliced forms, and propose, based upon retention or absence of key structural features, that these variant forms may potentially exhibit distinct properties relative to the originally described form of EAAT5.

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 new splice variant of the glutamate-aspartate transporter: cloning and immunolocalization of GLAST1c in rat, pig and human brains

GLAST (EAAT1) is an abundant glial glutamate transporter in the mammalian brain. It plays important roles in terminating excitatory transmission in grey matter, as well as pathophysiological roles, including protecting white matter from excitotoxic injury. In normal brain, alternative splicing of GLAST has been described: GLAST1a and GLAST1b arise from the splicing out of exons 3 and 9, respectively. This study describes the isolation of a novel cDNA clone from neonatal hypoxic pig brain, referred to as GLAST1c, where exons 5 and 6 are skipped. GLAST1c encodes a protein of 430 amino acids. RT-PCR analysis showed that GLAST1c mRNA was readily detectable in control and hypoxic pig cortex, as well as in various brain regions of rat (cortex, mid, hind and cerebellum), and human cortex, retina and optic nerve. We have raised antibodies that selectively recognize GLAST1c and demonstrate expression of this novel splice variant in astrocytes and oligodendrocytes in rat brain, pig brain and human brain, including grey and white matter. Similarly expression of GLAST1c was observed in primary astrocyte cultures and in cultured oligodendrocytes. In unstimulated astrocytes GLAST1c exhibited an intracellular peri-nuclear distribution similar to that observed when GFP-tagged GLAST1c was transfected into COS 7 cells. In astrocytes this protein rapidly redistributed to the surface upon stimulation of protein kinase with phorbol esters. We conclude that GLAST1c may represent an astrocyte and oligodendrocyte glutamate transporter, though this could not be formally validated by D-aspartate uptake studies, due to the low transfection efficiency of constructs into COS 7 cells.

Expression of multiple glutamate transporter splice variants in the rodent testis

Glutamate is a regulated molecule in the mammalian testis. Extracellular regulation of glutamate in the body is determined largely by the expression of plasmalemmal glutamate transporters. We have examined by PCR, western blotting and immunocytochemistry the expression of a panel of sodium-dependent plasmalemmal glutamate transporters in the rat testis. Proteins examined included: glutamate aspartate transporter (GLAST), glutamate transporter 1 (GLT1), excitatory amino acid carrier 1 (EAAC1), excitatory amino acid transporter 4 (EAAT4) and EAAT5. We demonstrate that many of the glutamate transporters in the testis are alternately spliced. GLAST is present as exon-3- and exon-9-skipping forms. GLT1 was similarly present as the alternately spliced forms GLT1b and GLT1c, whereas the abundant brain form (GLT1a) was detectable only at the mRNA level. EAAT5 was also strongly expressed, whereas EAAC1 and EAAT4 were absent. These patterns of expression were compared with the patterns of endogenous glutamate localization and with patterns of d-aspartate accumulation, as assessed by immunocytochemistry. The presence of multiple glutamate transporters in the testis, including unusually spliced forms, suggests that glutamate homeostasis may be critical in this organ. The apparent presence of many of these transporters in the testis and sperm may indicate a need for glutamate transport by such cells.

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.

Impact of alcohol abuse on protein expression of midkine and excitatory amino acid transporter 1 in the human prefrontal cortex

BACKGROUND

Alcoholism is associated with shrinkage of brain tissue and reduction in the number of neurons and dendritic arbors particularly in the prefrontal cortex. These changes correlate with the cognitive defects common in alcoholics. A recent study investigated the mRNA expression of selected genes in the prefrontal cortex and found that the levels of mRNA encoding the neurotrophic factor, midkine (MDK), and the excitatory amino acid transporter 1 (EAAT1) were significantly higher in alcoholics compared with nonalcoholic controls. This study aimed to investigate, whether the transcriptional changes observed result in alterations to protein expression. Additionally, the study aimed to expand our understanding of MDK and EAAT1 action by localizing their expression within morphologically and functionally distinct layers of this brain region.

METHODS

Quantitative changes in protein levels of MDK and EAAT1 were investigated in alcoholic and control cases using Western blots. Immunohistochemistry was utilized to localize proteins expression in formalin-fixed sagittal sections of the prefrontal cortex.

RESULTS

A marked increase was revealed in protein expression of both genes in the prefrontal cortex of chronic alcoholics. MDK-like immunofluorescence in alcoholic and control cases was present in nuclei throughout the prefrontal cortex and was particularly apparent in cell bodies of astrocytes in cortical layer II. Immunolabeling of the EAAT1 was densest in cortical layer II in control cases and induced in deeper layers in alcoholic cases.

CONCLUSION

Midkine promotes neuronal outgrowth and survival. The up-regulation of MDK protein expression may indicate the induction of reparative processes. The amino acid transporter is vital for the removal of glutamate from the synaptic cleft. At alcohol withdrawal, extracellular glutamate is thought to reach excitotoxic concentrations. Up-regulation of EAAT1 throughout the cortical layers may indicate an attempt to combat elevated glutamate concentrations. The predominant expression of the two proteins in layer II of the cortex implies a region-specific role of astrocytes.

Lipids in the assembly of membrane proteins and organization of protein supercomplexes: implications for lipid-linked disorders

Lipids play important roles in cellular dysfunction leading to disease. Although a major role for phospholipids is in defining the membrane permeability barrier, phospholipids play a central role in a diverse range of cellular processes and therefore are important factors in cellular dysfunction and disease. This review is focused on the role of phospholipids in normal assembly and organization of the membrane proteins, multimeric protein complexes, and higher order supercomplexes. Since lipids have no catalytic activity, it is difficult to determine their function at the molecular level. Lipid function has generally been defined by affects on protein function or cellular processes. Molecular details derived from genetic, biochemical, and structural approaches are presented for involvement of phosphatidylethanolamine and cardiolipin in protein organization. Experimental evidence is presented that changes in phosphatidylethanolamine levels results in misfolding and topological misorientation of membrane proteins leading to dysfunctional proteins. Examples are presented for diseases in which proper protein folding or topological organization is not attained due to either demonstrated or proposed involvement of a lipid. Similar changes in cardiolipin levels affects the structure and function of individual components of the mitochondrial electron transport chain and their organization into supercomplexes resulting in reduced mitochondrial oxidative phosphorylation efficiency and apoptosis. Diseases in which mitochondrial dysfunction has been linked to reduced cardiolipin levels are described. Therefore, understanding the principles governing lipid-dependent assembly and organization of membrane proteins and protein complexes will be useful in developing novel therapeutic approaches for disorders in which lipids play an important role.

Existence of an endogenous glutamate and aspartate transporter in Chinese hamster ovary cells

Chinese hamster ovary cells show endogenous high-affinity Na(+)-dependent glutamate transport activity. This transport activity is kinetically similar to a glutamate transporter family strategically expressed in the central nervous system and is pharmacologically unlike glutamate transporter-1 or excitatory amino acid carrier 1. The cDNA of a glutamate/aspartate transporter (GLAST)-like transporter was obtained and analyzed. The deduced amino acid sequence showed high similarity to human, mouse, and rat GLAST. We concluded that a GLAST-like glutamate transporter exists in Chinese hamster ovary cells that might confer the endogenous high-affinity Na(+)-dependent glutamate transport activity evident in these cells.

GLAST1b, the exon-9 skipping form of the glutamate-aspartate transporter EAAT1 is a sensitive marker of neuronal dysfunction in the hypoxic brain

In normal brain, we previously demonstrated that the exon-9 skipping form of glutamate-aspartate transporter (GLAST; which we refer to as GLAST1b) is expressed by small populations of neurons that appear to be sick or dying and suggested that these cells were subject to inappropriate local glutamate-mediated excitation. To test this hypothesis we examined the expression of GLAST1b in the hypoxic pig brain. In this model glial glutamate transporters such as GLAST and glutamate transporter 1 (GLT-1) are down-regulated in susceptible regions, leading to regional loss of glutamate homeostasis and thus to brain damage. We demonstrate by immunohistochemistry that in those brain regions where astroglial glutamate transporters are lost, GLAST1b expression is induced in populations of neurons and to a lesser extent in some astrocytes. These neurons were also immunolabeled by antibodies against the carboxyl-terminal region of GLAST but did not label with antibodies directed against the amino-terminal region. Our Western blotting data indicate that GLAST1b expressed by neurons lacks the normal GLAST amino-terminal region and may be further cleaved to a smaller approximately 30-kDa fragment. We propose that GLAST1b represents a novel and sensitive marker for the detection of neurons at risk of dying in response to hypoxic and other excitotoxic insults and may have wider applicability in experimental and clinical contexts.

Central nervous system expression of the exon 9 skipping form of the glutamate transporter GLAST

We have raised antibodies that selectively recognize an exon 9 skipping form of GLAST. We demonstrate expression of this protein in brains of rats, cats, monkeys and humans. Immunolabelling was present in scattered populations of neurons, particularly in cerebral cortex and colliculi. Neurons were often present in small clusters and exhibited a range of morphologies, from apparently normal to highly degenerate. GLAST1b was also expressed by some glial cells. Cortical neurons expressing the exon 9 skipping form of GLAST also labelled with antibodies against the C- or N-terminal regions of GLAST. We suggest that alternate splicing of GLAST by subpopulations of neurons may indicate some dysfunction in these cells, and may be an indicator of inappropriate local excitation.

Expression of the exon 3 skipping form of GLAST, GLAST1a, in brain and retina

GLAST is a glial glutamate transporter; mRNA for a splice variant, GLAST1a, which lacks exon 3, has previously been identified. To detect GLAST1a protein, we generated antibodies against a peptide sequence encompassing the splice site. We demonstrate by Western blotting and immunocytochemistry the expression of GLAST1a in brains and retinae. Robust immunolabelling was present in the cerebellar Bergmann glia, and weaker labelling was evident in the retinal Müller cells. GLAST1a is differentially targeted to some cellular compartments such as the end feet of the Müller cells. As GLAST1a protein may interfere with the transport of glutamate by normally spliced GLAST, differentially targeted expression of GLAST1a may represent a mechanism for selectively regulating GLAST function in the mammalian nervous system.

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.

Transporters for L-glutamate: an update on their molecular pharmacology and pathological involvement

L-Glutamate (Glu) is the major excitatory neurotransmitter in the mammalian CNS and five types of high-affinity Glu transporters (EAAT1-5) have been identified. The transporters EAAT1 and EAAT2 in glial cells are responsible for the majority of Glu uptake while neuronal EAATs appear to have specialized roles at particular types of synapses. Dysfunction of EAATs is specifically implicated in the pathology of neurodegenerative conditions such as amyotrophic lateral sclerosis, epilepsy, Huntington's disease, Alzheimer's disease and ischemic stroke injury, and thus treatments that can modulate EAAT function may prove beneficial in these conditions. Recent advances have been made in our understanding of the regulation of EAATs, including their trafficking, splicing and post-translational modification. This article summarises some recent developments that improve our understanding of the roles and regulation of EAATs.

Possible expression of a particular gamma-aminobutyric acid transporter isoform responsive to upregulation by hyperosmolarity in rat calvarial osteoblasts

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in the brain, but widely distributed in different peripheral organs. We have previously shown the functional expression of GABA(B) receptors required for GABAergic signal input by cultured rat calvarial osteoblasts. This study focused on the possible functional expression of the machinery required for GABAergic signal termination such as GABA transporters. In rat calvarial osteoblasts cultured for 7 days, [(3)H]GABA accumulation was observed in a temperature-, sodium- and chloride-dependent manner, consisting of a single component with a K(m) value of 789.6+/-9.0 microM and a V(max) value of 4.4+/-0.1 nmol/min/mg protein, respectively. Both nipecotic and L-2,4-diaminobutyric acids significantly inhibited [(3)H]GABA accumulation in a concentration-dependent manner. Constitutive expression was seen with mRNA for the betaine/GABA transporter-1 (BGT-1) and taurine transporter (TauT), while hyperosmotic cultivation led to significant increases in both [(3)H]GABA accumulation and BGT-1 mRNA expression without affecting TauT mRNA expression. Highly immunoreactive cells were detected for the BGT-1 isoform at the surface of trabecular bone of neonatal rat tibias. Sustained exposure to GABA significantly inhibited alkaline phosphatase (ALP) activity, but not cellular viability, at concentrations above 0.1 mM in osteoblasts cultured for 3 to 28 days. Nipecotic acid not only decreased ALP activity alone, but also further decreased ALP activity in osteoblasts cultured in the presence of GABA. These results suggest that the BGT-1 isoform may be functionally expressed by rat calvarial osteoblasts to play a hitherto unidentified role in mechanisms underlying hyperosmotic regulation of osteoblastogenesis.

Excitatory amino acid transporters expressed by synovial fibroblasts in rats with collagen-induced arthritis

Although previous studies have demonstrated increased levels of the brain neurotransmitter glutamate (Glu) in the synovial fluid from patients with arthritis, not much attention has been paid to the possible role of Glu in joint synovial tissues to date. Constitutive expression of mRNA was for the first time shown with glutamate aspartate transporter, glutamate transporter-1 and excitatory amino acid carrier-1 (EAAC1), in addition to with particular ionotropic and metabotropic Glu receptors, in cultured synovial fibroblasts prepared from knee joints of male Lewis rats. Immunohistochemical analysis revealed high localization of immunoreactive EAAC1 at synovial tissues. The accumulation of [3H]Glu occurred in a temperature- and sodium-dependent manner in cultured synovial fibroblasts, with a Km of 23.1+/-1.1 microM and a Vmax of 237.1+/-31.1 pmol/(mg protein min), respectively. In rats with arthritis induced by immunization to type-II collagen, marked increases were seen in hind paw volume, cytokine mRNA expression and Glu levels in synovial tissues, in addition to histological erosion. In cultured synovial fibroblasts prepared from these arthritic rats, [3H]Glu accumulation was drastically increased with biochemical and pharmacological profiles similar to those seen in normal synovial fibroblasts. The exposure to Glu at 500 microM doubled the incorporation of 5-bromo-2'-deoxyuridine in cultured synovial fibroblasts of arthritic but not normal rats, without significantly affecting mRNA expression of different cytokines in both synovial fibroblasts. These results suggest that Glu may at least in part play a role in mechanisms associated with cellular proliferation through particular transporters functionally expressed by synovium in rheumatoid arthritis.

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.

A novel alternative splicing form of excitatory amino acid transporter 1 is a negative regulator of glutamate uptake

Abstract EAAT1 is a major glutamate transporter in the CNS and is required for normal neurotransmission and neuroprotection from excitotoxicity. In the present study, we have identified a novel form of the human EAAT1, named here as EAAT1ex9skip, which lacks the entire exon 9. Quantitative PCR analysis indicates that this variant is expressed throughout the CNS, both in grey matter and axonal tracts, at levels ranging between 10% and 20% of the full-length EAAT1 form. When expressed in HEK293 cells, EAAT1ex9skip mRNA is translated into a truncated protein localized in the endoplasmic reticulum. EAAT1ex9skip has no functional glutamate uptake activity but instead, exerts a dominant negative effect over full-length EAAT1 function. In turn, co-expression of full-length EAAT1 and EAAT1ex9skip variants reduces the insertion of the former into the plasma membrane. Together, these results indicate that the EAAT1ex9skip splice variant is a negative regulator of full-length EAAT1 function in the human brain.

Functional expression of particular isoforms of excitatory amino acid transporters by rodent cartilage

In the present study, we have attempted to demonstrate functional expression by the rodent cartilage of particular isoforms of excitatory amino acid transporters (EAATs) essentially required for central glutamatergic signal termination. Constitutive expression of mRNA was shown for the first time with the neuronal EAAT subtype excitatory amino acid carrier-1 (EAAC1), in addition to glial subtypes such as glutamate aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1), in rat costal chondrocytes cultured for 7-21 days on reverse transcription polymerase chain reaction (RT-PCR). Western blotting analysis confirmed the expression of corresponding proteins for both GLAST and GLT-1 in cultured chondrocytes. The accumulation of [(3)H]glutamate (Glu) occurred in a temperature- and sodium-dependent manner with biochemical and pharmacological profiles similar to those seen for brain EAATs in chondrocytes cultured for 7 days, while [(3)H]Glu accumulation consisted of a single component with a K(m) of 39.1+/-2.3 microM and a V(max) of 1320+/-120pmol/mg protein/min, respectively. In organotypic cultured metatarsals isolated before vascularization from embryonic mice, where cells underwent maturational development from resting to proliferating, prehypertrophic, hypertrophic and calcified chondrocytes in a progressive order of cellular differentiation, moreover, mRNA expression was seen for GLAST, GLT-1 and EAAT4 but not for EAAC1 subtypes. Immunohistochemical analysis revealed distribution profiles different from each other with GLAST, GLT-1 and EAAT4 isoforms in sections of cultured metatarsals and isolated tibiae. These results suggest that extracellular Glu could be cleared up into intracellular locations through particular glial and/or neuronal EAAT isoforms functionally expressed by the rodent cartilage.

Electrogenic glutamate transporters in the CNS: molecular mechanism, pre-steady-state kinetics, and their impact on synaptic signaling

Glutamate is the major excitatory neurotransmitter in the mammalian CNS. The spatiotemporal profile of the glutamate concentration in the synapse is critical for excitatory synaptic signalling. The control of this spatiotemporal concentration profile requires the presence of large numbers of synaptically localized glutamate transporters that remove pre-synaptically released glutamate by uptake into neurons and adjacent glia cells. These glutamate transporters are electrogenic and utilize energy stored in the transmembrane potential and the Na+/K+-ion concentration gradients to accumulate glutamate in the cell. This review focuses on the kinetic and electrogenic properties of glutamate transporters, as well as on the molecular mechanism of transport. Recent results are discussed that demonstrate the multistep nature of the transporter reaction cycle. Results from pre-steady-state kinetic experiments suggest that at least four of the individual transporter reaction steps are electrogenic, including reactions associated with the glutamate-dependent transporter halfcycle. Furthermore, the kinetic similarities and differences between some of the glutamate transporter subtypes and splice variants are discussed. A molecular mechanism of glutamate transport is presented that accounts for most of the available kinetic data. Finally, we discuss how synaptic glutamate transporters impact on glutamate receptor activity and how transporters may shape excitatory synaptic transmission.

Transmembrane protein topology mapping by the substituted cysteine accessibility method (SCAM(TM)): application to lipid-specific membrane protein topogenesis

We provide an overview of lipid-dependent polytopic membrane protein topogenesis, with particular emphasis on Escherichia coli strains genetically altered in their lipid composition and strategies for experimentally determining the transmembrane organization of proteins. A variety of reagents and experimental strategies are described including the use of lipid mutants and thiol-specific chemical reagents to study lipid-dependent and host-specific membrane protein topogenesis by substituted cysteine site-directed chemical labeling. Employing strains in which lipid composition can be controlled temporally during membrane protein synthesis and assembly provides a means to observe dynamic changes in protein topology as a function of membrane lipid composition.

Glutamate signaling system in bone

L-glutamate (Glu) has been thought to be an excitatory amino acid neurotransmitter in the mammalian central nervous system (CNS). The hypothesis is supported by successful cloning of a number of genes encoding different signaling molecules, such as Glu receptors for signal input, Glu transporters for signal termination, and vesicular Glu transporters for signal output through exocytotic release. Limited information is available in the literature with regard to an extracellular transmitter role of Glu in peripheral neuronal and non-neuronal tissues, whereas recent molecular biological analyses including ours give rise to a novel function for Glu as an autocrine and/or paracrine factor in bone comprised of osteoblasts, osteoclasts, and osteocytes, in addition to other peripheral tissues including pancreas, adrenal, and pituitary glands. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying maintenance of cellular homeostasis as an excitatory neurotransmitter in the CNS and as an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, therefore, we summarized the possible signaling by Glu as an extracellular signal mediator in mechanisms underlying maintenance of cellular homeostasis with a focus on bone tissues.

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.

Glutamate signaling in peripheral tissues

The hypothesis that l-glutamate (Glu) is an excitatory amino acid neurotransmitter in the mammalian central nervous system is now gaining more support after the successful cloning of a number of genes coding for the signaling machinery required for this neurocrine at synapses in the brain. These include Glu receptors (signal detection), Glu transporters (signal termination) and vesicular Glu transporters (signal output through exocytotic release). Relatively little attention has been paid to the functional expression of these molecules required for Glu signaling in peripheral neuronal and non-neuronal tissues; however, recent molecular biological analyses show a novel function for Glu as an extracellular signal mediator in the autocrine and/or paracrine system. Emerging evidence suggests that Glu could play a dual role in mechanisms underlying the maintenance of cellular homeostasis - as an excitatory neurotransmitter in the central neurocrine system and an extracellular signal mediator in peripheral autocrine and/or paracrine tissues. In this review, the possible Glu signaling methods are outlined in specific peripheral tissues including bone, testis, pancreas, and the adrenal, pituitary and pineal glands.

Regulation of the glutamate transporter EAAT1 by the ubiquitin ligase Nedd4-2 and the serum and glucocorticoid-inducible kinase isoforms SGK1/3 and protein kinase B

Surface expression of the glial glutamate transporter EAAT1 is stimulated by insulin-like growth factor 1 through activation of phosphatidylinositol-3-kinase. Downstream targets include serum and glucocorticoid-sensitive kinase isoforms SGK1, SGK2 and SGK3, and protein kinase B. SGK1 regulates Nedd4-2, a ubiquitin ligase that prepares cell membrane proteins for degradation. To test whether Nedd4-2, SGK1, SGK3 and protein kinase B regulate EAAT1, cRNA encoding EAAT1 was injected into Xenopus oocytes with or without additional injection of wild-type Nedd4-2, constitutively active S422DSGK1, inactive K127NSGK1, wild-type SGK3 and/or constitutively active T308D,S473DPKB. Glutamate induces a current in Xenopus oocytes expressing EAAT1, but not in water-injected oocytes, which is decreased by co-expression of Nedd4-2, an effect reversed by additional co-expression of S422DSGK1, SGK3 and T308D,S473DPKB, but not K127NSGK1. Site-directed mutagenesis of the SGK1 phosphorylation sites in the Nedd4-2 protein (S382A,S468ANedd4-2) and in the EAAT1 protein (T482AEAAT1, T482DEAAT1) significantly blunts the effect of S422DSGK1. Moreover, the current is significantly larger in T482DEAAT1- than in T482AEAAT1-expressing oocytes, indicating that a negative charge mimicking phosphorylation at T482 increases transport. The experiments reveal a powerful novel mechanism that regulates the activity of EAAT1. This mechanism might participate in the regulation of neuronal excitability and glutamate transport in other tissues.

Modulation of cellular differentiation by N-methyl-D-aspartate receptors in osteoblasts

N-methyl-D-aspartate (NMDA) receptors for the central neurotransmitter l-glutamate (Glu) have been shown to be present in both osteoblasts and osteoclasts. Sustained exposure to the NMDA channel antagonist dizocilpine (MK-801) significantly prevented increases in both alkaline phosphatase activity and Ca2+ accumulation in a concentration-dependent manner in osteoblasts cultured for 7-28 days in vitro (DIV), without significantly affecting cell survivability. Osteocalcin expression was markedly reduced in the presence of MK-801 in osteoblasts cultured for 28 DIV. Both an NMDA domain antagonist and a glycine domain antagonist similarly prevented Ca2+ accumulation in osteoblasts exposed for 28 consecutive DIV. MK-801 was effective in significantly inhibiting Ca2+ accumulation determined at 28 DIV in osteoblasts exposed before 7 DIV but was ineffective in cells exposed after 11-21 DIV. Sustained exposure to MK-801 significantly inhibited DNA binding activity and expression of core binding factor alpha-1 (CBFA1) in osteoblasts exposed after 7 DIV up to 28 DIV, but not in those exposed before 7 DIV. These results suggest that heteromeric NMDA receptor channels may be functionally expressed to regulate mechanisms underlying cellular differentiation rather than proliferation and/or maturation through modulation of expression of CBFA1 in cultured rat calvarial osteoblasts.

Facilitation of glutamate release by ionotropic glutamate receptors in osteoblasts

Constitutive expression of mRNA was seen for the vesicular glutamate transporter brain-specific Na(+)-dependent inorganic phosphate cotransporter (BNPI), but not differentiation-associated Na(+)-dependent inorganic phosphate cotransporter, in rat calvarial osteoblasts cultured for 7 and 21 days in vitro (DIV). Three different agonists for ionotropic glutamate receptors (iGluR) at 1mM, as well as 50mM KCl, significantly increased the release of endogenous L-glutamate from osteoblasts cultured for 7DIV when determined 5 min after the addition by using a high performance liquid chromatograph. The inhibitor of desensitization of DL-alpha-amino-3-hydroxy-5-methylisoxasole-4-propionate (AMPA) receptors cyclothiazide significantly potentiated and prolonged the release of endogenous L-glutamate evoked by AMPA in a dose-dependent manner. The release evoked by AMPA was significantly prevented by the addition of an AMPA receptor antagonist as well as by the removal of Ca(2+) ions. These results suggest that endogenous L-glutamate could be released from intracellular vesicular constituents associated with BNPI through activation of particular iGluR subtypes expressed in cultured rat calvarial osteoblasts.

Demonstration of expression of mRNA for particular AMPA and kainate receptor subunits in immature and mature cultured rat calvarial osteoblasts

Reverse transcription polymerase chain reaction revealed expression of mRNA for particular subunits of ionotropic glutamate receptors (iGluR) in primary cultures of rat calvarial osteoblastic cells under immature to mature states. These included GluR3, KA1 and KA2 subunits, in addition to NR1 and NR2D subunits. These results suggest that glutamate may play an unidentified role in mechanisms associated with cellular development through particular subunits of iGluR in rat calvarial osteoblasts.

Functional GABA(B) receptors expressed in cultured calvarial osteoblasts

In immature and mature primary cultured rat calvarial osteoblasts, both mRNA and corresponding proteins were constitutively expressed for 2 splice variants of GABA(B) receptor (GABA(B)R) subunits but not for any known GABA(A) and GABA(C) receptor subunits. The agonist for GABA(B)R baclofen significantly inhibited cAMP formation induced by forskolin in a manner sensitive to the antagonist 2-hydroxysaclofen. Similar expression was seen with mRNA for GABA(B)R-1a and -1b splice variants in the murine calvarial osteoblast cell line MC3TC-E1 cells cultured for 7-21 days in vitro (DIV). In these MC3T3-E1 cells, baclofen not only inhibited the activity of alkaline phosphatase, but also exacerbated Ca2+ accumulation, throughout the culture period up to 28 DIV. These results suggest that GABA may play an unidentified role in mechanisms associated with cellular proliferation, differentiation, and/or development through functional GABA(B)R constitutively expressed in cultured osteoblasts.

Glutamate uptake

Brain tissue has a remarkable ability to accumulate glutamate. This ability is due to glutamate transporter proteins present in the plasma membranes of both glial cells and neurons. The transporter proteins represent the only (significant) mechanism for removal of glutamate from the extracellular fluid and their importance for the long-term maintenance of low and non-toxic concentrations of glutamate is now well documented. In addition to this simple, but essential glutamate removal role, the glutamate transporters appear to have more sophisticated functions in the modulation of neurotransmission. They may modify the time course of synaptic events, the extent and pattern of activation and desensitization of receptors outside the synaptic cleft and at neighboring synapses (intersynaptic cross-talk). Further, the glutamate transporters provide glutamate for synthesis of e.g. GABA, glutathione and protein, and for energy production. They also play roles in peripheral organs and tissues (e.g. bone, heart, intestine, kidneys, pancreas and placenta). Glutamate uptake appears to be modulated on virtually all possible levels, i.e. DNA transcription, mRNA splicing and degradation, protein synthesis and targeting, and actual amino acid transport activity and associated ion channel activities. A variety of soluble compounds (e.g. glutamate, cytokines and growth factors) influence glutamate transporter expression and activities. Neither the normal functioning of glutamatergic synapses nor the pathogenesis of major neurological diseases (e.g. cerebral ischemia, hypoglycemia, amyotrophic lateral sclerosis, Alzheimer's disease, traumatic brain injury, epilepsy and schizophrenia) as well as non-neurological diseases (e.g. osteoporosis) can be properly understood unless more is learned about these transporter proteins. Like glutamate itself, glutamate transporters are somehow involved in almost all aspects of normal and abnormal brain activity.

Group III metabotropic glutamate receptors in rat cultured calvarial osteoblasts

Reverse transcription polymerase chain reaction revealed expression of mRNAs for particular receptors for the central neurotransmitter l-glutamic acid (Glu) in primary cultures of rat calvarial osteoblastic cells under premature to mature states according to the duration of days in vitro. These included metabotropic Glu receptors (mGluR) such as mGluR4 and mGluR8, in addition to several ionotropic Glu receptor subunits including NR1 and NR2D. Expression of mRNAs was not detected with other mGluR and NR2A-C subunits irrespective of the maturity of cultured cells. The agonist for group III mGluR L-(+)-2-amino-4-phosphonobutyric acid significantly inhibited the forskolin-induced accumulation of cAMP in premature osteoblasts, which occurred in a manner sensitive to prevention by the group III mGluR antagonist (RS)-alpha-cyclopropyl-4-phosphonophenylglycine. These results suggest that Glu may at least in part play a role in mechanisms associated with cellular proliferation and/or differentiation through group III mGluR functionally expressed in rat calvarial osteoblastic cells.