Na(+)-dependent high affinity uptake of L-glutamate in primary cultures of human fibroblasts isolated from three different types of tissue

Neuroinflammation in Primary Open-Angle Glaucoma

Primary open-angle glaucoma (POAG) is the second leading cause of irreversible blindness worldwide. Increasing evidence suggests oxidative damage and immune response defects are key factors contributing to glaucoma onset. Indeed, both the failure of the trabecular meshwork tissue in the conventional outflow pathway and the neuroinflammation process, which drives the neurodegeneration, seem to be linked to the age-related over-production of free radicals (i.e., mitochondrial dysfunction) and to oxidative stress-linked immunostimulatory signaling. Several previous studies have described a wide range of oxidative stress-related makers which are found in glaucomatous patients, including low levels of antioxidant defences, dysfunction/activation of glial cells, the activation of the NF-κB pathway and the up-regulation of pro-inflammatory cytokines, and so on. However, the intraocular pressure is still currently the only risk factor modifiable by medication or glaucoma surgery. This present review aims to summarize the multiple cellular processes, which promote different risk factors in glaucoma including aging, oxidative stress, trabecular meshwork defects, glial activation response, neurodegenerative insults, and the altered regulation of immune response.

Human Dermal Fibroblast: A Promising Cellular Model to Study Biological Mechanisms of Major Depression and Antidepressant Drug Response

BACKGROUND

Human dermal fibroblasts (HDF) can be used as a cellular model relatively easily and without genetic engineering. Therefore, HDF represent an interesting tool to study several human diseases including psychiatric disorders. Despite major depressive disorder (MDD) being the second cause of disability in the world, the efficacy of antidepressant drug (AD) treatment is not sufficient and the underlying mechanisms of MDD and the mechanisms of action of AD are poorly understood.

OBJECTIVE

The aim of this review is to highlight the potential of HDF in the study of cellular mechanisms involved in MDD pathophysiology and in the action of AD response.

METHODS

The first part is a systematic review following PRISMA guidelines on the use of HDF in MDD research. The second part reports the mechanisms and molecules both present in HDF and relevant regarding MDD pathophysiology and AD mechanisms of action.

RESULTS

HDFs from MDD patients have been investigated in a relatively small number of works and most of them focused on the adrenergic pathway and metabolism-related gene expression as compared to HDF from healthy controls. The second part listed an important number of papers demonstrating the presence of many molecular processes in HDF, involved in MDD and AD mechanisms of action.

CONCLUSION

The imbalance in the number of papers between the two parts highlights the great and still underused potential of HDF, which stands out as a very promising tool in our understanding of MDD and AD mechanisms of action.

Fibroblast glutamate transport in aging and in AD: correlations with disease severity

Altered glutamate transport and aberrant EAAT1 expression were shown in Alzheimer's disease (AD) brains. It is presently unknown whether these modifications are a consequence of neurodegeneration or play a pathogenetic role. However, recent findings of decreased glutamate uptake, EAAT1 protein and mRNA in AD platelets suggest that glutamate transporter modifications may be systemic and might explain the decreased glutamate uptake. We now used primary fibroblast cultures from 10 AD patients to further investigate the specific involvement of glutamate transporters in this disorder and in normal aging. Decreased glutamate uptake (p<0.001), EAAT1 expression (p<0.05) and mRNA (p<0.01) were observed in aged people, compared to younger controls. In AD fibroblasts, compared to age-matched controls, we observed further reductions of glutamate uptake (p<0.0005) and EAAT1 expression (p<0.005), while EAAT1 mRNA increase (p<0.001) was shown. EAAT1 parameters were mutually correlated (p<0.01) and correlations were shown with dementia severity (p<0.05 MMSE-expression, p<0.005 MMSE-mRNA). We suggest fibroblast cultures as possible ex vivo peripheral model to study the glutamate involvement and possible molecular and therapeutic targets in AD.

Avian Purkinje neuronal cultures: extrinsic control of morphology by cell type and glutamate

An in vitro coculture system is described to study the avian Purkinje neuron and the interactions occurring with astrocytes and granule cells during development in the cerebellum. Astrocytes initially and granule cells later regulate Purkinje neuron morphology. The coculture system presented here provides an excellent system for investigating the morphological, immunocytochemical, and electrophysiological differentiation of Purkinje neurons under controlled conditions and for studying cell-cell interactions and extrinsic factors, e.g., glutamate in normal and neuropathological conditions.

Altered glutamate uptake in peripheral tissues from Down syndrome patients

Overexpression of APP and SOD induces beta-amyloid deposition and oxidative stress in Down syndrome (DS) patients. Both phenomena may impair glutamate transport and decreased glutamate uptake sites have been demonstrated in patient brains at autopsy. Since alterations of APP metabolism and oxidative damage are systemic, we investigated glutamate uptake in platelets and fibroblasts from DS patients to explore whether abnormalities in this process are inherent properties of DS cells and not secondary to neurodegeneration. Glutamate uptake was significantly decreased in platelets (P<0.005 vs. control) and fibroblasts (P<0.001 vs. control) from DS patients, particularly in those with free trisomy and with mitochondrial point mutations. Systemic impairment of glutamate uptake in DS is suggested, probably related to APP overexpression and mitochondrial dysfunction. Such mechanisms may contribute to neurodegeneration and dementia development in these patients.

Sustainable systemic delivery via a single injection of lentivirus into human skin tissue

The skin offers a tissue site accessible for delivery of gene-based therapeutics. To develop the capability for sustained systemic polypeptide delivery via cutaneous gene transfer, we generated and injected pseudotyped HIV-1 lentiviral vectors intradermally at a range of doses into human skin grafted on immune-deficient mice. Unlike Moloney murine leukemia virus (MLV)-based retrovectors, which failed to achieve detectable cutaneous gene transfer by this approach, lentivectors effectively targeted all major cell types within human skin tissue, including fibroblasts, endothelial cells, keratinocytes, and macrophages. After a single injection, lentivectors encoding human erythropoietin (EPO) produced dose-dependent increases in serum human EPO levels and hematocrit that increased rapidly within one month and remained stable subsequently. Delivered gene expression was confined locally at the injection site. Excision of engineered skin led to rapid and complete loss of human EPO in the bloodstream, confirming that systemic EPO delivery was entirely due to lentiviral targeting of cells within skin rather than via spread of the injected vector to visceral tissues. These findings indicate that the skin can sustain dosed systemic delivery of therapeutic polypeptides via direct lentivector injection and thus provide an accessible and reversible approach for gene-based delivery to the bloodstream.

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.

Neurochemistry of L-Glutamate Transport in the CNS: A Review of Thirty Years of Progress

The review highlights the landmark studies leading from the discovery and initial characterization of the Na+-dependent "high affinity" uptake in the mammalian brain to the cloning of individual transporters and the subsequent expansion of the field into the realm of molecular biology. When the data and hypotheses from 1970's are confronted with the recent developments in the field, we can conclude that the suggestions made nearly thirty years ago were essentially correct: the uptake, mediated by an active transport into neurons and glial cells, serves to control the extracellular concentrations of L-glutamate and prevents the neurotoxicity. The modern techniques of molecular biology may have provided additional data on the nature and location of the transporters but the classical neurochemical approach, using structural analogues of glutamate designed as specific inhibitors or substrates for glutamate transport, has been crucial for the investigations of particular roles that glutamate transport might play in health and disease. Analysis of recent structure/activity data presented in this review has yielded a novel insight into the pharmacological characteristics of L-glutamate transport, suggesting existence of additional heterogeneity in the system, beyond that so far discovered by molecular genetics. More compounds that specifically interact with individual glutamate transporters are urgently needed for more detailed investigations of neurochemical characteristics of glutamatergic transport and its integration into the glutamatergic synapses in the central nervous system. A review with 162 references.

Quantitative autoradiography of Na+-dependent [3H]L-aspartate binding to L-glutamate transporters in rat brain: structure-activity studies using L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC) and 2-(carboxycyclopropyl)-glycine (CCG)

Sodium-dependent binding of [3H]L-aspartate was studied in thaw-mounted horizontal sections of fresh-frozen (i.e. not fixed) rat brain. After the incubation with [3H]L-aspartate, the sections were exposed against a 3H-sensitive film and the resulting autoradiograms were evaluated by quantitative densitometry. Effects of several inhibitors were examined and their potency expressed as IC50 and nH. Together with previously published data, the present study supports the view that [3H]L-aspartate binding to fresh-frozen sections of rat brain represents interaction of the radioligand with the substrate-binding sites on glutamate transporters. The most potent inhibitors were (2S,3S,4R)-2-(carboxycyclopropyl)-glycine (L-CCG III) and (2S,4R)-4-methylglutamate. In contrast, L-anti,endo-3,4-methanopyrrolidine dicarboxylate (L-a,e-MPDC) was about an order of magnitude less potent. Only subtle regional variations in the characteristics of inhibitors of [3H]L-aspartate binding were detected. It is not certain whether these differences reflect regional variations in the distribution of individual glutamate transporters or regional peculiarities in their pharmacological characteristics. In particular, (2S,4R)-4-methylglutamate, shown previously to differentiate between GLT-1 (principal glutamate transporter in the forebrain) and GLAST (expressed mainly in the cerebellum), did not strongly differentiate between the binding of [3H]L-aspartate in forebrain and cerebellum. Computer-assisted molecular modelling using selected glutamate analogues with restricted conformation (L-trans-pyrrolidine-2,4-dicarboxylate and four isomers of 2-(carboxycyclopropyl)-glycine: L- and D-CCG I, L-CCG III and L-CCG IV) identified at least one area of unfavourable steric interaction. We conclude that the quantitative autoradiographic studies using [3H]L-aspartate or other transporter-specific ligands, will be a useful tool to study the pharmacology of substrate binding sites on glutamate transporters in the mammalian brain in situ.

Glutamate transport in cultures from developing avian cerebellum: presence of GLT-1 immunoreactivity in Purkinje neurons

Immunocytochemical studies indicated that Purkinje cells cultured from chick embryonic cerebellum (embryonic day 8) strongly express a glutamate transporter EAAT2 cloned from human brain (GLT-1 in rat brain). At both 7 days and 14 days in culture, Purkinje neurons accumulated 1 microM [3H]L-glutamate via a potent "high-affinity" transport system that could be inhibited by D- and L-threo-3-hydroxyaspartate (D- and L-t-3OHA) and by L-trans-pyrrolidine-2,4-dicarboxylate (L-t-PDC). The order of potency of the three inhibitors was L-t-PDC approximately L-t-3OHA > D-t-30HA. Only the value of IC50 (concentration causing 50% inhibition) for D-t-3OHA significantly changed between 7 days (116 microM) and 14 days in culture (40 microM). All nH approximately 1, except in the case of the inhibition by D-t-3OHA at 14 days in culture (nH = 0.57), indicating the possible appearance of heterogeneity of the transport sites at later stages of culturing. Chronic inhibition of L-glutamate transport by L-t-PDC resulted in major changes in the morphology of Purkinje cells; particularly, the neurites almost completely regressed.

Structural selectivity and molecular nature of L-glutamate transport in cultured human fibroblasts

Uptake of L-[3H]glutamate by monolayers of fibroblasts cultured from human embryonic skin has been studied in the presence of several nonradioactive structural analogs of glutamate and aspartate. Results have suggested that the structural specificites of glutamate transporters in cultured human fibroblasts are similar to those of glutamate transporters in the mammalian brain. Only subtle differences have been detected: in the mammalian cerebral cortex, enantiomers of threo-3-hydroxyaspartate are almost equipotent as inhibitors of L-[3H]glutamate uptake while, in human fibroblasts, the D-isomer has been found to be an order of magnitude less potent than the corresponding L-isomer. Kinetic analysis of a model in which substrates are recognized by the glutamate transporter binding site(s) as both alpha- and beta-amino acids indicated that such a mechanism cannot explain the apparent negative cooperativity characterizing the effects of D- and L-aspartate. Molecular modeling has been used to estimate the optimum conformation of L-glutamate as it interacts with the transporter(s). Flow cytometry has indicated that all fibroblasts in culture express at least moderate levels of four glutamate transporters cloned from human brain. Small subpopulations (< 3%) of cells, however, were strongly labeled with antibodies against EAAT1 (GLAST) and EAAT2 (GLT-1) transporters. We conclude that these two transporters--known to be strongly expressed in brain tissue--can be principally responsible for the "high affinity" transport of glutamate also in nonneural cells.

Intracerebral microdialysis study of glutamate reuptake in awake, behaving rats

The central nervous system has high-affinity uptake systems for the clearance of amino acid transmitters. These systems are found in both neurons and astrocytes. Previous studies have shown that the uptake of amino acid transmitters by astrocytes in culture can be modulated by adrenergic agents. The objectives of this study were to develop a methodology that evaluates the brain's reuptake capacity for glutamate in awake, behaving animals and to determine whether glutamate reuptake is under alpha-adrenergic regulation in the intact central nervous system. Male Sprague-Dawley rats weighing 250-450 g were used in this study. The extraction fraction of L-[3H]glutamate with [14C]mannitol as a reference was measured. The cortical extraction fraction of L-[3H]glutamate corrected for [14C]mannitol (EL-glu) reaches steady state rapidly and is both stable and repeatable. EL-glu is a measure of L-glutamate reuptake and not metabolism. EL-glu is decreased in a dose-dependent manner by the addition of the glutamate reuptake blocker D,L-threo-beta-hydroxyaspartic acid or unlabeled L- glutamate. In addition, EL-glu is increased in a dose-dependent manner by the alpha1-adrenergic agonist phenylephrine, and this increase is blocked by the alpha-adrenergic antagonist phentolamine.

Transcriptional control in keratinocytes and fibroblasts using synthetic ligands

The skin is an attractive tissue for regulated target gene expression by virtue of its accessibility to topical regulating stimuli. We have used synthetic ligand-driven intracellular oligomerization to accomplish specific target gene regulation in human skin keratinocytes and fibroblasts. GAL4 DNA binding domains and VP16 transactivation domains, each linked to the FK506 binding protein, were expressed in normal human skin keratinocytes and fibroblasts. These hybrid proteins underwent heterodimerization via the novel intracellular dimerizing agent FK1012 to generate a heterodimeric activator of target gene expression in vitro. Dimeric FK1012, but not monomeric FK506M induced target gene expression in a dose-dependent fashion. FK1012 exerted no detectable nonspecific effects on expression of cutaneous genes and did not alter cellular proliferation kinetics. Controlled oligomerization of hybrid transcription activators offers a potential approach to target gene regulation in cells of normal human skin.

Fas signal transduction triggers either proliferation or apoptosis in human fibroblasts

Although shown to be highly expressed by the epidermis in inflammatory skin disease, the ability of the Fas protein to trigger apoptosis in the distinct cell subpopulations of cutaneous tissue, particularly with regard to receptor density and the degree of crosslinking, has not been fully characterized. We therefore determined the effect of Fas cross-linking in primary human dermal fibroblasts at both high and low levels of Fas receptor expression. First, we examined the effects of the anti-Fas monoclonal antibody, CH-11, on fibroblasts expressing low basal levels of Fas. In these cells Fas aggregation stimulated proliferation by 160 +/- 10% over untreated controls. In contrast, the same concentration of CH-11 had an inhibitory effect on epidermal keratinocyte growth. Because Fas is upregulated in inflamed skin, we next examined the effects of Fas cross-linking on fibroblasts expressing augmented levels ofFas. Fibroblasts were either transfected with plasmids for overexpression of full length or bioengineered Fas receptors or were transduced with a retroviral Fas expression vector. In these cells Fas oligomerization triggered the morphologic changes indicative of apoptosis regardless of whether or not the Fas-signaling domain was tethered to the plasma membrane. These studies indicate that Fas oligomerization in dermal fibroblasts may initiate dual signaling programs, either proliferation or apoptosis, and that the chosen outcome may depend upon the magnitude of Fas aggregation.

The neurotransmitter candidature of sulphur-containing excitatory amino acids in the mammalian central nervous system

While L-glutamate (L-Glu) is considered to be the predominant excitatory amino acid transmitter in the mammalian CNS, other amino acids have come under scrutiny as possible rivals for such a role. These include four sulphur-containing analogues of L-Glu and L-aspartate known as the SAAs. The L-Glu analogues are L-homocysteic acid and L-homocysteine sulphinic acid, while the L-aspartate analogues are L-cysteic acid and L-cysteine sulphinic acid. They are mixed agonists of excitatory amino acid receptors on a variety of neurones and are reported to be present in and released from mammalian CNS tissue. This review serves to summarize the current state of research into the possibility that one or more of these compounds is indeed a transmitter within the mammalian CNS.

Differential actions of 3-(4-chlorophenyl) glutamic acid stereoisomers and L-trans-pyrrolidine-2,4-dicarboxylic acid upon L-homocysteic acid- and L-glutamic acid-induced responses from rat spinal motoneurones

The four recently synthesized stereoisomers of 3-(4-chlorophenyl) glutamic acid (chlorpheg) were individually examined for their abilities to potentiate depolarizations of neonatal rat motoneurones evoked by L-homocysteic acid (L-HCA, 10 microM). This property had previously been observed using the racemate and is believed to be mediated by uptake inhibition. Both the (2S,3S)- and (2S,3R)- isomers were selective potentiators of L-HCA- (vs L-Glu) induced depolarizations although the (2S,3S)- isomer was more effective. The (2R,3S)- isomer had a slight but significant depressant action which could be attributed to N-methyl-D-aspartate (NMDA) receptor antagonism. Comparison of the potentiating properties of (2S,3S)- and (2S,3R)-chlorpheg with those of L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC, a L-Glu uptake inhibitor) upon L-HCA- and L-Glu-evoked responses revealed that both chlorpheg isomers (500 microM each) selectively potentiated responses evoked by L-HCA (10 microM) but had no significant effect upon those evoked by L-Glu (50 microM). On the other hand, use of tPDC at the same concentration significantly enhanced the depolarizations evoked by both amino acids, although its action on L-Glu-evoked responses was greater. It is concluded that (i) the (2S,3S)- isomer and to a lesser extent, the (2S,3R)- isomer of chlorpheg are responsible for the potentiating actions seen with the chlorpheg racemate used in previous studies and (ii) (2R,3S)-chlorpheg is a weak NMDA antagonist. The apparently selective action of (2S,3S)- and (2S,3R)-chlorpheg upon L-HCA-relative to L-Glu-induced depolarizations supports the existence of multiple excitatory amino acid uptake sites, some of which may yet be unidentified.

Effects of L-trans-pyrrolidine-2,4-dicarboxylate and L-threo-3-hydroxyaspartate on the binding of [3H]L-aspartate, [3H]alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), [3H]DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoate (CGP 39653), [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and [3H]kainate studied by autoradiography in rat forebrain

L-trans-Pyrrolidine-2,4-dicarboxylate (L-t-PDC) and L-threo-3- hydroxyaspartate (L-t-3OHA), compounds known to interact strongly with the Na(+)-dependent high affinity uptake of excitatory amino acids in central nervous tissue, were tested as potential inhibitors of binding to glutamate receptors and transport sites in frozen sections of rat brain. [3H] alpha-amino-3-hydroxy- 5-methyl-4-isoxazolepropionate (AMPA), [3H]6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), and [3H] kainate were used as ligands for the binding sites on the "non-NMDA" classes of glutamate receptors and [3H]DL-(E)-2-amino-4-propyl-5-phosphono-3-pentenoate (CGP 39653) was used to label NMDA receptor binding sites. The Na(+)-dependent glutamate-uptake site was marked by [3H]L-aspartate. The autoradiograms, obtained by exposing 3H-sensitive film to sections of rat forebrain preincubated with 3H-labelled ligands, were scanned by laser beam and quantified. Distribution patterns of the receptor and transporter sites visualized by the 3H-labelled ligands were compatible with previously published results. [3H]CNQX binding, however, was found to be significantly decreased by Na+.L-t-3OHA was about an order of magnitude stronger than L-t-PDC as an inhibitor of [3H]L-aspartate binding. Neither of the compounds had any important effect at the "non-NMDA" receptor binding sites but L-t-3OHA was a weak inhibitor of [3H]CGP 39653 binding (< 40% at 100 microM). The results suggest that, at low nanomolar concentrations, both compounds are likely to be selective for Na(+)-dependent high affinity glutamate transporter sites. Moreover, L-t-3OHA seems to have a sufficiently high affinity for the site to be almost certainly useful, if available in a 3H-labelled form, as a ligand in autoradiographic studies.

Glutamate decarboxylase solubilized from the rat cerebral cortex by two different concentrations of Triton X-100: effects of glutamate analogues and analysis by SDS-PAGE/western blotting using GAD6 and K2 antibodies

Analysis of two preparations (containing 0.1% and 0.5% Triton X-100) of glutamate decarboxylase (GAD) by Western blotting using GAD6 and K2 antibodies specifically recognizing two GAD isoenzymes, GAD65 and GAD67, respectively, indicated that the higher concentration of Triton X-100 at best only moderately favoured solubilization of GAD67. Several glutamate analogues were found to be either equally potent or equally inactive as inhibitors of glutamate decarboxylase activities in the two preparations. Among typical ligands for glutamate receptors and transporters, only quinolinic and L-cysteine sulphinic acids were weak inhibitors of GAD. Kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (AMPA), 3-((RS)-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), L-threo-3-hydroxy-aspartate, L-trans-pyrrolidine-2,4-dicarboxylate, dihydrokainate, kynurenic acid and N-methyl-D-aspartate were inactive. Even though the activity of glutamate decarboxylase in homogenates of rat cerebral cortex is higher at 0.5% than at 0.1% Triton X-100, structural requirements of the enzyme active site appear to be independent of Triton X-100 concentration. Furthermore, since the less soluble component of the enzyme activity contains about the same ratio of GAD65 to GAD67 as the more soluble one, it does not seem that the fractionation with Triton X-100 can be easily used to separate the two isoenzymes from each other.

The regulation of sodium-dependent transport of anionic amino acids in cultured human fibroblasts

In cultured human fibroblasts the transport of anionic amino acids through the sodium-dependent system X-AG is stimulated rapidly and transiently by phorbol 12,13-dibutyrate. Transport stimulation is consistent with an effect due to the activation of protein kinase C. Bradykinin (1 microM) and PDGF-AA (100 ng/ml) also stimulate the activity of system X-AG. The bradykinin effect appears to be fully dependent upon PKC activation whereas the stimulation of aspartate transport by PDGF-AA is also due to PKC-independent mechanisms.