Activation of protein kinase C reduces GLAST in the plasma membrane of rat Müller cells in primary culture

Integrated serum pharmacochemistry and serum pharmacology to investigate the active components and mechanism of Bushen Huoxue Prescription in the treatment of diabetic retinopathy

This study aimed to investigate the active components of Bushen Huoxue Prescriptions (BHP), and further clarify the mechanism by the integration of serum pharmacochemistry and serum pharmacology based on spectrum-effect relationship in vivo. In this paper, the components absorbed into serum were analyzed by ultra-high performance liquid chromatography-quadrupole-Exactive Orbitrap-high resolution mass spectrometry (UPLC-Q-Exactive Orbitrap-HRMS). And Müller cells were chosen as target cells to further investigate the mechanism. After cell purification, the well-grown cells were identified by Hematoxylin-eosin staining (HE) staining and immunofluorescence assay, such as glial fibrillary acidic protein (GFAP) and glutamine synthetase (GS). The logarithmic phase cells were divided into normal group, model group and 12 BHP groups. The hyperglycemic and hypoxic model was induced by 50 mmol/L glucose and 1 mmol/L sodium disulfite. Enzyme-linked immunesorbnent assay (ELISA) was used to detect the expressions of five factors closely related to DR, named vascular endothelial growth factor (VEGF), hypoxia-inducible factor1-alpha (HIF-1α), protein kinase C-β (PKC-β), angiopoietin-2 (ANG-2) and transforming growth factor-β (TGF-β). Finally, the spectrum-effect relationship was investigated to screen the active components of BHP by partial least squares regression (PLSR). The results showed that 83 metabolic components, containing 30 prototypes and 53 metabolites were found in BHP serum. 12 characteristic common peaks were chosen to establish spectrum-effect relationship. Significantly, all the 12 BHP serum exhibited stronger inhibition on the expression of VEGF, PKC-β, and ANG-2, and the expression of VEGF, PKC-β, ANG-2 was chosen to establish the spectrum-effect relationship in vivo. The results of PLSR revealed that the content of methylation and sulfuration of caffeic acid, dehydroxylation and sulfation of Danshensu, daidzein, O-demethylangolanolin, cryptotanshinone, tanshinone IIA and protopanaxatriol were inversely correlated with VEGF expression of Müller cells; the areas of dihydrocaffeic acid, methylation and sulfuration of caffeic acid, dehydroxylation and sulfation of Danshensu, daidzein, cryptotanshinone, tanshinone IIA were negative correlation with the expression of PKC-β; while the coefficient of hydroxytyrosol sulfation, R-equol, O-demethylangolanolin, dihydrotanshinone IIA, hydrated cryptotanshinone, protopanaxatriol showed negative correlation with the expression of ANG-2. The above results indicated that cryptotanshinone, tanshinone IIA, daidzein and protopanaxatriol need be focused in our future research. In addition, this research idea provides feasible ways to investigate and determine pharmacodyamic material basis and screen the Q-markers of TCM and its formulas.

EAAT1-dependent slc1a3 Transcriptional Control depends on the Substrate Translocation Process

Glutamate, the major excitatory neurotransmitter in the vertebrate brain, is removed from the synaptic cleft by a family of sodium-dependent transporters expressed in neurons and glial cells. The bulk of glutamate uptake activity occurs in glial cells through the sodium-dependent glutamate/aspartate transporter (EAAT1/GLAST) and glutamate transporter 1 (EAAT2/GLT-1). EAAT1/GLAST is the predominant transporter within the cerebellum. It is highly enriched in Bergmann glial cells that span the cerebellar cortex and wrap the most abundant glutamatergic synapses in the central nervous system, the synapse formed by the parallel fibers and the Purkinje cells. In the past years, it has become evident that Bergmann glial cells are involved in glutamatergic transmission. Glutamate transporters are tightly regulated due to their essential role in tripartite synapses. Glutamate regulates EAAT1/GLAST function and gene expression in a receptor-dependent and receptor-independent manner. Through the use of the non-metabolizable EAAT1/GLAST ligand, D-Aspartate, and the well-established chick cerebellar Bergmann glia primary culture, in this contribution, we demonstrate that EAAT1/GLAST down-regulates its expression and function at the transcriptional level through the activation of a signaling pathway that includes the phosphatidyl inositol 3 kinase (PI3K), the Ca2+/diacylglycerol dependent protein kinase PKC and the nuclear factor κ-light-chain-enhancer of activated B cells (NF-κB). These results favor the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

Summary statement

EAAT1/GLAST down-regulates its expression and function at the transcriptional level by activating a signaling pathway that includes PI3K, PKC and NF-κB, favoring the notion of an activity-dependent fine-tuning of glutamate recycling and its synaptic transactions through glial cells.

Rapid Regulation of Glutamate Transport: Where Do We Go from Here?

Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system (CNS). A family of five Na⁺-dependent transporters maintain low levels of extracellular glutamate and shape excitatory signaling. Shortly after the research group of the person being honored in this special issue (Dr. Baruch Kanner) cloned one of these transporters, his group and several others showed that their activity can be acutely (within minutes to hours) regulated. Since this time, several different signals and post-translational modifications have been implicated in the regulation of these transporters. In this review, we will provide a brief introduction to the distribution and function of this family of glutamate transporters. This will be followed by a discussion of the signals that rapidly control the activity and/or localization of these transporters, including protein kinase C, ubiquitination, glutamate transporter substrates, nitrosylation, and palmitoylation. We also include the results of our attempts to define the role of palmitoylation in the regulation of GLT-1 in crude synaptosomes. In some cases, the mechanisms have been fairly well-defined, but in others, the mechanisms are not understood. In several cases, contradictory phenomena have been observed by more than one group; we describe these studies with the goal of identifying the opportunities for advancing the field. Abnormal glutamatergic signaling has been implicated in a wide variety of psychiatric and neurologic disorders. Although recent studies have begun to link regulation of glutamate transporters to the pathogenesis of these disorders, it will be difficult to determine how regulation influences signaling or pathophysiology of glutamate without a better understanding of the mechanisms involved.

Regulation of Glutamate, GABA and Dopamine Transporter Uptake, Surface Mobility and Expression

Neurotransmitter transporters limit spillover between synapses and maintain the extracellular neurotransmitter concentration at low yet physiologically meaningful levels. They also exert a key role in providing precursors for neurotransmitter biosynthesis. In many cases, neurons and astrocytes contain a large intracellular pool of transporters that can be redistributed and stabilized in the plasma membrane following activation of different signaling pathways. This means that the uptake capacity of the brain neuropil for different neurotransmitters can be dynamically regulated over the course of minutes, as an indirect consequence of changes in neuronal activity, blood flow, cell-to-cell interactions, etc. Here we discuss recent advances in the mechanisms that control the cell membrane trafficking and biophysical properties of transporters for the excitatory, inhibitory and modulatory neurotransmitters glutamate, GABA, and dopamine.

Calcineurin Controls Expression of EAAT1/GLAST in Mouse and Human Cultured Astrocytes through Dynamic Regulation of Protein Synthesis and Degradation

Alterations in the expression of glutamate/aspartate transporter (GLAST) have been associated with several neuropathological conditions including Alzheimer's disease and epilepsy. However, the mechanisms by which GLAST expression is altered are poorly understood. Here we used a combination of pharmacological and genetic approaches coupled with quantitative PCR and Western blot to investigate the mechanism of the regulation of GLAST expression by a Ca²⁺/calmodulin-activated phosphatase calcineurin (CaN). We show that treatment of cultured hippocampal mouse and fetal human astrocytes with a CaN inhibitor FK506 resulted in a dynamic modulation of GLAST protein expression, being downregulated after 24-48 h, but upregulated after 7 days of continuous FK506 (200 nM) treatment. Protein synthesis, as assessed by puromycin incorporation in neo-synthesized polypeptides, was inhibited already after 1 h of FK506 treatment, while the use of a proteasome inhibitor MG132 (1 μM) shows that GLAST protein degradation was only suppressed after 7 days of FK506 treatment. In astrocytes with constitutive genetic ablation of CaN both protein synthesis and degradation were significantly inhibited. Taken together, our data suggest that, in cultured astrocytes, CaN controls GLAST expression at a posttranscriptional level through regulation of GLAST protein synthesis and degradation.

Neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease

Neurodegenerative disorders possess common pathological mechanisms, such as protein aggregation, inflammation, oxidative stress (OS) and excitotoxicity, raising the possibility of shared therapeutic targets. As a result of the selective cellular and regional expression of group III metabotropic glutamate (mGlu) receptors, drugs targeting such receptors have demonstrated both neuroprotective properties and symptomatic improvements in several models of neurodegeneration. In recent years, the discovery and development of subtype-selective ligands for the group III mGlu receptors has gained pace, allowing further research into the functions of these receptors and revealing their roles in health and disease. Activation of this class of receptors results in neuroprotection, with a variety of underlying mechanisms implicated. Group III mGlu receptor stimulation prevents excitotoxicity by inhibiting glutamate release from neurons and microglia and increasing glutamate uptake by astrocytes. It also attenuates the neuroinflammatory response by reducing glial reactivity and encourages neurotrophic phenotypes. This article will review the current literature with regard to the neuroprotective and symptomatic effects of group III mGlu receptor activation and discuss their promise as therapeutic targets in neurodegenerative disease. We review the neuroprotective and symptomatic effects of targeting group III mGlu receptors in neurodegenerative disease: Excess extracellular glutamate causes overactivation of NMDA receptors resulting in excitotoxicity. Externalization of phosphatidylserine stimulates phagocytosis of neurons by activated microglia, which contribute to damage through glutamate and pro-inflammatory factor release. Reactive astrocytes produce cytotoxic factors enhancing neuronal cell death. Activation of group III mGlu receptors by glutamate and/or mGlu receptor ligands results in inhibition of glutamate release from presynaptic terminals and microglia, reducing excitotoxicity. Astrocytic glutamate uptake is increased and microglia produce neurotrophic factors.

Molecular and biochemical characterization of folate transport proteins in retinal Müller cells

PURPOSE

To analyze the mechanisms of folate uptake in retinal Müller cells.

METHODS

RT-PCR and Western blot analysis were performed in freshly isolated neural retina and RPE/eyecup, primary mouse Müller cells, and rMC-1 cells for the three known folate transport proteins folate receptor alpha (FRalpha), proton-coupled folate transporter (PCFT), and reduced folate carrier (RFC). Laser scanning confocal microscopy (LSCM) and immunoelectron microscopy were used to determine the subcellular location of FRalpha and PCFT in primary Müller cells. The pH dependence of the uptake of [(3)H]-methyltetrahydrofolate ([(3)H]-MTF) was assayed in Müller cells in the presence/absence of thiamine pyrophosphate, an inhibitor of RFC.

RESULTS

FRalpha and PCFT are expressed abundantly in the retina in several cell layers, including the inner nuclear layer; they are present in primary mouse Müller cells and rMC-1 cells. LSCM localized these proteins to the plasma membrane, nuclear membrane, and perinuclear region. Immunoelectron microscopic studies revealed the colocalization of FRalpha and PCFT on the plasma membrane and nuclear membrane and within endosomal structures. Müller cell uptake of [(3)H]-MTF was robust at pH 5.0 to 6.0, consistent with PCFT activity, but also at neutral pH, reflecting RFC function. RFC was expressed in mouse Müller cells that had been allowed to proliferate in culture, but not in freshly isolated primary cells.

CONCLUSIONS

FRalpha and PCFT are expressed in retinal Müller cells and colocalize in the endosomal compartment, suggesting that the two proteins may work coordinately to mediate folate uptake. The unexpected finding of RFC expression and activity in cultured Müller cells may reflect the upregulation of this protein under proliferative conditions.

Neural progenitor potential in cultured Müller glia: effects of passaging and exogenous growth factor exposure

The Müller radial glial cell is the principal support cell of the adult mammalian retina. Recent reports suggest that these cells retain the capacity to proliferate, express phenotypes reminiscent of retinal progenitor cells (RPC) and generate neuron-like progeny. We isolated rodent Müller cells and generated cultures that could be passaged under conditions used in neural stem/progenitor cell colonies. We demonstrate that during the early period of primary culture, Müller glia proliferate into sphere colonies and express a select regimen of phenotypes normally seen in RPCs. This effect correlates temporally with the loss of retinal neurons post-dissection. When chronically maintained in vitro, Müller cells can be repeatedly passaged, and up-regulate early RPC phenotypes that are suggestive of cellular de-differentiation. Furthermore, exposure of Müller glial cultures to differentiating conditions containing growth factors stimulates Müller glia to up-regulate phenotypes associated with retinal neurons. These data provide further evidence that isolated, adult Müller glia retain functional and phenotypic features of RPCs.

Acute regulation of sodium-dependent glutamate transporters: a focus on constitutive and regulated trafficking

The acidic amino acid glutamate activates a family of ligand-gated ion channels to mediate depolarization that can be as short-lived as a few milliseconds and activates a family of G protein-coupled receptors that couple to both ion channels and other second messenger pathways. Glutamate is the predominant excitatory neurotransmitter in the mammalian central nervous system and is required for essentially all motor, sensory, and cognitive functions. In addition, glutamate-mediated signaling is required for development and the synaptic plasticity thought to underlie memory formation and retrieval. The levels of glutamate in brain approach 10 mmol/kg and most cells in the CNS express at least one of the receptor subtypes. Unlike acetylcholine that mediates "rapid" excitatory neurotransmission at the neuromuscular junction, there is no evidence for extracellular inactivation of glutamate. Instead, glutamate is cleared by a family of Na(+)-dependent transport systems that are found on glial processes that sheath the synapse and found on the pre- and postsynaptic elements of neurons. These transporters ensure crisp excitatory transmission by maintaining synaptic concentrations below those required for tonic activation of glutamate receptors under baseline conditions (approximately 1 microM) and serve to limit activation of glutamate receptors after release. During the past few years, it has become clear that like many of the other neurotransmitter transporters discussed in this volume of Handbook of Experimental Pharmacology, the activity of these transporters can be rapidly regulated by a variety of effectors. In this chapter, a broad overview of excitatory signaling will be followed by a brief introduction to the family of Na(+)-dependent glutamate transporters and a detailed discussion of our current understanding of the mechanisms that control transporter activity. The focus will be on our current understanding of the mechanisms that could regulate transporter activity within minutes, implying that this regulation is independent of transcriptional or translational control mechanisms. The glutamate transporters found in forebrain are regulated by redistributing the proteins to or from the plasma membrane; the signals involved and the net effects on transporter activity are being defined. In addition, there is evidence to suggest that the intrinsic activity of these transporters is also regulated by mechanisms that are independent of transporter redistribution; less is known about these events. As this field progresses, it should be possible to determine how this regulation affects physiologic and pathologic events in the CNS.

Regulation and dysregulation of glutamate transporters

Glutamate is the primary excitatory neurotransmitter in the central nervous system. During synaptic activity, glutamate is released into the synaptic cleft and binds to glutamate receptors on the pre- and postsynaptic membrane as well as on neighboring astrocytes in order to start a number of intracellular signaling cascades. To allow for an efficient signaling to occur, glutamate levels in the synaptic cleft have to be maintained at very low levels. This process is regulated by glutamate transporters, which remove excess extracellular glutamate via a sodium-potassium coupled uptake mechanism. When extracellular glutamate levels rise to about normal, glutamate overactivates glutamate receptors, triggering a multitude of intracellular events in the postsynaptic neuron, which ultimately results in neuronal cell death. This phenomenon is known as excitotoxicity and is the underlying mechanisms of a number of neurodegenerative diseases. A dysfunction of the glutamate transporter is thought to contribute to cell death during excitotoxicity. Therefore, efforts have been made to understand the regulation of glutamate transporter function. Transporter activity can be regulated in different ways, including through gene expression, transporter protein targeting and trafficking and through posttranslational modifications of the transporter protein. The identification of these mechanisms has helped to understand the role of glutamate transporters during pathology and will aid in the development of therapeutic strategies with the transporter as a desirable target.

Activation of Group II/III metabotropic glutamate receptors attenuates LPS-induced astroglial neurotoxicity via promoting glutamate uptake

Altered glial function that leads to oxidative stress and excitotoxicity may contribute to the initiation or progression of neuronal death in neurodegenerative diseases. We report the pivotal role of astroglial Group II and III metabotropic glutamate receptors (mGluR) against neurotoxicity. Activation of Group II or III mGluR on astrocytes with selective agonists DCG-IV or L-AP4 respectively inhibited astroglial lipopolysaccharide (LPS)-conditioned medium induced apoptosis of primary cultured mesencephalic neurons. Specific Group II or III mGluR antagonists APICA or MSOP completely abolished the neuroprotective effects of DCG-IV and L-AP4. Morphologic analysis showed that DCG-IV or L-AP4 could also attenuate the astroglial neurotoxicity to dopaminergic neurons. Measurement of extracellular glutamate concentration and [(3)H]-glutamate uptake showed that the restoration of glutamate uptake capability in LPS-treated astrocytes might be involved in the neuroprotective effects of activating astroglial Group II or III mGluR. Furthermore, we found that the repression of astroglial uptake function could be revived by GSH, and both Group II and III mGluR agonists could recover the endogenous reduced glutathione (GSH) level in LPS-treated astrocytes. These results suggested that the possible mechanisms of neuroprotection by either Type II or Type III mGluR activation may involve restoration of endogenous GSH, in turn affording recovery of astroglial capability to take up glutamate.

Differential regulation of GLAST immunoreactivity and activity by protein kinase C: evidence for modification of amino and carboxyl termini

Many neurotransmitter transporters, including the GLT-1 and EAAC1 subtypes of the glutamate transporter, are regulated by protein kinase C (PKC) and these effects are associated with changes in cell surface expression. In the present study, the effects of PKC activation on the glutamate aspartate transporter (GLAST) subtype of glutamate transporter were examined in primary astrocyte cultures. Acute (30 min) exposure to the phorbol 12-myristate 13-acetate (PMA) increased (approximately 20%) transport activity but had the opposite effect on both total and cell surface immunoreactivity. Chronic treatment (6 or 24 h) with PMA had no effect on transport activity but caused an even larger decrease in total and cell surface immunoreactivity. This loss of immunoreactivity was observed using antibodies directed against three different cytoplasmic epitopes, and was blocked by the PKC antagonist, bisindolylmaleimide II. We provide biochemical and pharmacological evidence that the activity observed after treatment with PMA is mediated by GLAST. Two different flag-tagged variants of the human homolog of GLAST were introduced into astrocytes using lentiviral vectors. Although treatment with PMA caused a loss of transporter immunoreactivity, flag immunoreactivity did not change in amount or size. Together, these studies suggest that activation of PKC acutely up-regulates GLAST activity, but also results in modification of several different intracellular epitopes so that they are no longer recognized by anti-GLAST antibodies. We found that exposure of primary cultures of neurons/astrocytes to transient hypoxia/glucose deprivation also caused a loss of GLAST immunoreactivity that was attenuated by the PKC antagonist, bisindolylmaleimide II, suggesting that some acute insults previously thought to cause a loss of GLAST protein may mimic the phenomenon observed in the present study.