The role of N-glycosylation in the stability, trafficking and GABA-uptake of GABA-transporter 1. Terminal N-glycans facilitate efficient GABA-uptake activity of the GABA transporter

Common structural features in some of the sequentially distant neurotransmitter transporters N-termini

The N-terminal regions of SLC6 transporters are sequentially unrelated, and the majority of such transporters contain only relatively short peptide N-terminal extensions. Currently, it is not clear if a diversity of N-terminal sequences represents diverse functions among the transporters or if there are common functions hidden behind similar, as yet unidentified, structures. Using alignment of amino acid sequences with the hydropathy plot, disorder prediction, and calpain recognition sites, we show that common structural features among the N-termini of some transporters might exist.We previously showed that polymeric neurotransmitter transporter N-termini exhibit very similar profiles of dynamic, time-dependent 465-595-350-750 nm absorbance metachromasia in the Bradford assay. Here we report that under certain mild denaturing conditions, filamentous aggregation of glutathione S-transferase (GST) protein results in similar near-infrared metachromasia. This effect was eliminated by further GST protein denaturation and solubilization. The results suggest that aggregation of partially denatured GST stabilizes Coomassie dye docking sites, producing a near-infrared absorbance shift similar to that observed in the polymeric unstructured N-termini of transporters.

Aberrant glycosylation in schizophrenia: insights into pathophysiological mechanisms and therapeutic potentials

Schizophrenia (SCZ) is a severe neuropsychiatric disorder characterized by cognitive, affective, and social dysfunction, resulting in hallucinations, delusions, emotional blunting, and disordered thinking. In recent years, proteomics has been increasingly influential in SCZ research. Glycosylation, a key post-translational modification, can alter neuronal stability and normal signaling in the nervous system by affecting protein folding, stability, and cellular signaling. Recent research evidence suggests that abnormal glycosylation patterns exist in different brain regions in autopsy samples from SCZ patients, and that there are significant differences in various glycosylation modification types and glycosylation modifying enzymes. Therefore, this review explores the mechanisms of aberrant modifications of N-glycosylation, O-glycosylation, glycosyltransferases, and polysialic acid in the brains of SCZ patients, emphasizing their roles in neurotransmitter receptor function, synaptic plasticity, and neural adhesion. Additionally, the effects of antipsychotic drugs on glycosylation processes and the potential for glycosylation-targeted therapies are discussed. By integrating these findings, this review aims to provide a comprehensive perspective to further understand the role of aberrant glycosylation modifications in the pathophysiology of SCZ.

Haploinsufficiency underlies the neurodevelopmental consequences of SLC6A1 variants

Heterozygous variants in SLC6A1, encoding the GAT-1 GABA transporter, are associated with seizures, developmental delay, and autism. The majority of affected individuals carry missense variants, many of which are recurrent germline de novo mutations, raising the possibility of gain-of-function or dominant-negative effects. To understand the functional consequences, we performed an in vitro GABA uptake assay for 213 unique variants, including 24 control variants. De novo variants consistently resulted in a decrease in GABA uptake, in keeping with haploinsufficiency underlying all neurodevelopmental phenotypes. Where present, ClinVar pathogenicity reports correlated well with GABA uptake data; the functional data can inform future reports for the remaining 72% of unscored variants. Surface localization was assessed for 86 variants; two-thirds of loss-of-function missense variants prevented GAT-1 from being present on the membrane while GAT-1 was on the surface but with reduced activity for the remaining third. Surprisingly, recurrent de novo missense variants showed moderate loss-of-function effects that reduced GABA uptake with no evidence for dominant-negative or gain-of-function effects. Using linear regression across multiple missense severity scores to extrapolate the functional data to all potential SLC6A1 missense variants, we observe an abundance of GAT-1 residues that are sensitive to substitution. The extent of this missense vulnerability accounts for the clinically observed missense enrichment; overlap with hypermutable CpG sites accounts for the recurrent missense variants. Strategies to increase the expression of the wild-type SLC6A1 allele are likely to be beneficial across neurodevelopmental disorders, though the developmental stage and extent of required rescue remain unknown.

Molecular basis for substrate recognition and transport of human GABA transporter GAT1

γ-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in the central nervous system, is recycled through specific GABA transporters (GATs). GAT1, which is mainly expressed in the presynaptic terminals of axons, is a potential drug target of neurological disorders due to its essential role in GABA transport. Here we report four cryogenic electron microscopy structures of human GAT1, at resolutions of 2.2-3.2 Å. GAT1 in substrate-free form or in complex with the antiepileptic drug tiagabine exhibits an inward-open conformation. In the presence of GABA or nipecotic acid, inward-occluded structures are captured. The GABA-bound structure reveals an interaction network bridged by hydrogen bonds and ion coordination for GABA recognition. The substrate-free structure unwinds the last helical turn of transmembrane helix TM1a to release sodium ions and substrate. Complemented by structure-guided biochemical analyses, our studies reveal detailed mechanism of GABA recognition and transport, and elucidate mode of action of the inhibitors, nipecotic acid and tiagabine.

Experimental and Bioinformatic Insights into the Effects of Epileptogenic Variants on the Function and Trafficking of the GABA Transporter GAT-1

In this article, we identified a novel epileptogenic variant (G307R) of the gene SLC6A1, which encodes the GABA transporter GAT-1. Our main goal was to investigate the pathogenic mechanisms of this variant, located near the neurotransmitter permeation pathway, and compare it with other variants located either in the permeation pathway or close to the lipid bilayer. The mutants G307R and A334P, close to the gates of the transporter, could be glycosylated with variable efficiency and reached the membrane, albeit inactive. Mutants located in the center of the permeation pathway (G297R) or close to the lipid bilayer (A128V, G550R) were retained in the endoplasmic reticulum. Applying an Elastic Network Model, to these and to other previously characterized variants, we found that G307R and A334P significantly perturb the structure and dynamics of the intracellular gate, which can explain their reduced activity, while for A228V and G362R, the reduced translocation to the membrane quantitatively accounts for the reduced activity. The addition of a chemical chaperone (4-phenylbutyric acid, PBA), which improves protein folding, increased the activity of GAT-1WT, as well as most of the assayed variants, including G307R, suggesting that PBA might also assist the conformational changes occurring during the alternative access transport cycle.

Astrocytic GABA transporter 1 deficit in novel SLC6A1 variants mediated epilepsy: Connected from protein destabilization to seizures in mice and humans

OBJECTIVE

Mutations in γ-aminobutyric acid (GABA) transporter 1 (GAT-1)-encoding SLC6A1 have been associated with myoclonic atonic epilepsy and other phenotypes. We determined the patho-mechanisms of the mutant GAT-1, in order to identify treatment targets.

METHODS

We conducted whole-exome sequencing of patients with myoclonic atonic epilepsy (MAE) and characterized the seizure phenotypes and EEG patterns. We studied the protein stability and structural changes with homology modeling and machine learning tools. We characterized the function and trafficking of the mutant GAT-1 with 3H radioactive GABA uptake assay and confocal microscopy. We utilized different models including a knockin mouse and human astrocytes derived from induced pluripotent stem cells (iPSCs). We focused on astrocytes because of their direct impact of astrocytic GAT-1 in seizures.

RESULTS

We identified four novel SLC6A1 variants associated with MAE and 2 to 4 Hz spike-wave discharges as a common EEG feature. Machine learning tools predicted that the variant proteins are destabilized. The variant protein had reduced expression and reduced GABA uptake due to endoplasmic reticular retention. The consistent observation was made in cortical and thalamic astrocytes from variant-knockin mice and human iPSC-derived astrocytes. The Slc6a+/A288V mouse, representative of MAE, had increased 5-7 Hz spike-wave discharges and absence seizures.

INTERPRETATION

SLC6A1 variants in various locations of the protein peptides can cause MAE with similar seizure phenotypes and EEG features. Reduced GABA uptake is due to decreased functional GAT-1, which, in thalamic astrocytes, could result in increased extracellular GABA accumulation and enhanced tonic inhibition, leading to seizures and abnormal EEGs.

Clinical, biochemical and genetic characteristics of MOGS-CDG: a rare congenital disorder of glycosylation

PURPOSE

To summarise the clinical, molecular and biochemical phenotype of mannosyl-oligosaccharide glucosidase-related congenital disorders of glycosylation (MOGS-CDG), which presents with variable clinical manifestations, and to analyse which clinical biochemical assay consistently supports diagnosis in individuals with bi-allelic variants in MOGS.

METHODS

Phenotypic characterisation was performed through an international and multicentre collaboration. Genetic testing was done by exome sequencing and targeted arrays. Biochemical assays on serum and urine were performed to delineate the biochemical signature of MOGS-CDG.

RESULTS

Clinical phenotyping revealed heterogeneity in MOGS-CDG, including neurological, immunological and skeletal phenotypes. Bi-allelic variants in MOGS were identified in 12 individuals from 11 families. The severity in each organ system was variable, without definite genotype correlation. Urine oligosaccharide analysis was consistently abnormal for all affected probands, whereas other biochemical analyses such as serum transferrin analysis was not consistently abnormal.

CONCLUSION

The clinical phenotype of MOGS-CDG includes multisystemic involvement with variable severity. Molecular analysis, combined with biochemical testing, is important for diagnosis. In MOGS-CDG, urine oligosaccharide analysis via matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry can be used as a reliable biochemical test for screening and confirmation of disease.

Roles of Endomembrane Alkali Cation/Proton Exchangers in Synaptic Function and Neurodevelopmental Disorders

Endomembrane alkali cation (Na⁺, K⁺)/proton (H⁺) exchangers (eNHEs) are increasingly associated with neurological disorders. These eNHEs play integral roles in regulating the luminal pH, processing, and trafficking of cargo along the secretory (Golgi and post-Golgi vesicles) and endocytic (early, recycling, and late endosomes) pathways, essential regulatory processes vital for neuronal development and plasticity. Given the complex morphology and compartmentalization of multipolar neurons, the contribution of eNHEs in maintaining optimal pH homeostasis and cargo trafficking is especially significant during periods of structural and functional development and remodeling. While the importance of eNHEs has been demonstrated in a variety of non-neuronal cell types, their involvement in neuronal function is less well understood. In this review, we will discuss their emerging roles in excitatory synaptic function, particularly as it pertains to cellular learning and remodeling. We will also explore their connections to neurodevelopmental conditions, including intellectual disability, autism, and attention deficit hyperactivity disorders.

Genetic mosaicism, intrafamilial phenotypic heterogeneity, and molecular defects of a novel missense SLC6A1 mutation associated with epilepsy and ADHD

BACKGROUND

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of neurodevelopmental disorders ranging from variable epilepsy syndromes, intellectual disability (ID), autism and others. To date, most identified mutations are de novo. We here report a pedigree of two siblings associated with myoclonic astatic epilepsy, attention deficit hyperactivity disorder (ADHD), and ID.

METHODS

Next-generation sequencing identified a missense mutation in the SLC6A1 gene (c.373G > A(p.Val125Met)) in the sisters but not in their shared mother who is also asymptomatic, suggesting gonadal mosaicism. We have thoroughly characterized the clinical phenotypes: EEG recordings identified features for absence seizures and prominent bursts of occipital intermittent rhythmic delta activity (OIRDA). The molecular pathophysiology underlying the clinical phenotypes was assessed using a multidisciplinary approach including machine learning, confocal microscopy, and high-throughput ³H radio-labeled GABA uptake assays in mouse astrocytes and neurons.

RESULTS

The GAT-1(Val125Met) mutation destabilizes the global protein conformation and reduces transporter protein expression at total and cell surface. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) in both HEK293T cells and astrocytes which may directly contribute to seizures in patients. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(Val125Met) to ~30% of the wildtype.

CONCLUSIONS

The seizure phenotypes, ADHD, and impaired cognition are likely caused by a partial loss-of-function of GAT-1 due to protein destabilization resulting from the mutation. Reduced GAT-1 function in astrocytes and neurons may consequently alter brain network activities such as increased seizures and reduced attention.

Aberrant glycosylation in schizophrenia: a review of 25 years of post-mortem brain studies

Glycosylation, the enzymatic attachment of carbohydrates to proteins and lipids, regulates nearly all cellular processes and is critical in the development and function of the nervous system. Axon pathfinding, neurite outgrowth, synaptogenesis, neurotransmission, and many other neuronal processes are regulated by glycans. Over the past 25 years, studies analyzing post-mortem brain samples have found evidence of aberrant glycosylation in individuals with schizophrenia. Proteins involved in both excitatory and inhibitory neurotransmission display altered glycans in the disease state, including AMPA and kainate receptor subunits, glutamate transporters EAAT1 and EAAT2, and the GABA_A receptor. Polysialylated NCAM (PSA-NCAM) and perineuronal nets, highly glycosylated molecules critical for axonal migration and synaptic stabilization, are both downregulated in multiple brain regions of individuals with schizophrenia. In addition, enzymes spanning several pathways of glycan synthesis show differential expression in brains of individuals with schizophrenia. These changes may be due to genetic predisposition, environmental perturbations, medication use, or a combination of these factors. However, the recent association of several enzymes of glycosylation with schizophrenia by genome-wide association studies underscores the importance of glycosylation in this disease. Understanding how glycosylation is dysregulated in the brain will further our understanding of how this pathway contributes to the development and pathophysiology of schizophrenia.

Endoplasmic reticulum retention and degradation of a mutation in SLC6A1 associated with epilepsy and autism

Mutations in SLC6A1, encoding γ-aminobutyric acid (GABA) transporter 1 (GAT-1), have been recently associated with a spectrum of epilepsy syndromes, intellectual disability and autism in clinic. However, the pathophysiology of the gene mutations is far from clear. Here we report a novel SLC6A1 missense mutation in a patient with epilepsy and autism spectrum disorder and characterized the molecular defects of the mutant GAT-1, from transporter protein trafficking to GABA uptake function in heterologous cells and neurons. The heterozygous missense mutation (c1081C to A (P361T)) in SLC6A1 was identified by exome sequencing. We have thoroughly characterized the molecular pathophysiology underlying the clinical phenotypes. We performed EEG recordings and autism diagnostic interview. The patient had neurodevelopmental delay, absence epilepsy, generalized epilepsy, and 2.5–3 Hz generalized spike and slow waves on EEG recordings. The impact of the mutation on GAT-1 function and trafficking was evaluated by ³H GABA uptake, structural simulation with machine learning tools, live cell confocal microscopy and protein expression in mouse neurons and nonneuronal cells. We demonstrated that the GAT-1(P361T) mutation destabilizes the global protein conformation and reduces total protein expression. The mutant transporter protein was localized intracellularly inside the endoplasmic reticulum (ER) with a pattern of expression very similar to the cells treated with tunicamycin, an ER stress inducer. Radioactive ³H-labeled GABA uptake assay indicated the mutation reduced the function of the mutant GAT-1(P361T), to a level that is similar to the cells treated with GAT-1 inhibitors. In summary, this mutation destabilizes the mutant transporter protein, which results in retention of the mutant protein inside cells and reduction of total transporter expression, likely via excessive endoplasmic reticulum associated degradation. This thus likely causes reduced functional transporter number on the cell surface, which then could cause the observed reduced GABA uptake function. Consequently, malfunctioning GABA signaling may cause altered neurodevelopment and neurotransmission, such as enhanced tonic inhibition and altered cell proliferation in vivo. The pathophysiology due to severely impaired GAT-1 function may give rise to a wide spectrum of neurodevelopmental phenotypes including autism and epilepsy.

γ-Aminobutyric acid transporters as relevant biological target: Their function, structure, inhibitors and role in the therapy of different diseases

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the nervous system. It plays a crucial role in many physiological processes. Upon release from the presynaptic element, it is removed from the synaptic cleft by reuptake due to the action of GABA transporters (GATs). GATs belong to a large SLC6 protein family whose characteristic feature is sodium-dependent relocation of neurotransmitters through the cell membrane. GABA transporters are characterized in many contexts, but their spatial structure is not fully known. They are divided into four types, which differ in occurrence and role. Herein, the special attention was paid to these transporting proteins. This comprehensive review presents the current knowledge about GABA transporters. Their distribution in the body, physiological functions and possible utilization in the therapy of different diseases were fully discussed. The important structural features were described based on published data, including sequence analysis, mutagenesis studies, and comparison with known SLC6 transporters for leucine (LeuT), dopamine (DAT) and serotonin (SERT). Moreover, the most important inhibitors of GABA transporters of various basic scaffolds, diverse selectivity and potency were presented.

A recurrent missense variant in SLC9A7 causes nonsyndromic X-linked intellectual disability with alteration of Golgi acidification and aberrant glycosylation

We report two unrelated families with multigenerational nonsyndromic intellectual disability (ID) segregating with a recurrent de novo missense variant (c.1543C>T:p.Leu515Phe) in the alkali cation/proton exchanger gene SLC9A7 (also commonly referred to as NHE7). SLC9A7 is located on human X chromosome at Xp11.3 and has not yet been associated with a human phenotype. The gene is widely transcribed, but especially abundant in brain, skeletal muscle and various secretory tissues. Within cells, SLC9A7 resides in the Golgi apparatus, with prominent enrichment in the trans-Golgi network (TGN) and post-Golgi vesicles. In transfected Chinese hamster ovary AP-1 cells, the Leu515Phe mutant protein was correctly targeted to the TGN/post-Golgi vesicles, but its N-linked oligosaccharide maturation as well as that of a co-transfected secretory membrane glycoprotein, vesicular stomatitis virus G (VSVG) glycoprotein, was reduced compared to cells co-expressing SLC9A7 wild-type and VSVG. This correlated with alkalinization of the TGN/post-Golgi compartments, suggestive of a gain-of-function. Membrane trafficking of glycosylation-deficient Leu515Phe and co-transfected VSVG to the cell surface, however, was relatively unaffected. Mass spectrometry analysis of patient sera also revealed an abnormal N-glycosylation profile for transferrin, a clinical diagnostic marker for congenital disorders of glycosylation. These data implicate a crucial role for SLC9A7 in the regulation of TGN/post-Golgi pH homeostasis and glycosylation of exported cargo, which may underlie the cellular pathophysiology and neurodevelopmental deficits associated with this particular nonsyndromic form of X-linked ID.

A missense mutation in SLC6A1 associated with Lennox-Gastaut syndrome impairs GABA transporter 1 protein trafficking and function

BACKGROUND

Mutations in SLC6A1 have been associated mainly with myoclonic atonic epilepsy (MAE) and intellectual disability. We identified a novel missense mutation in a patient with Lennox-Gastaut syndrome (LGS) characterized by severe seizures and developmental delay.

METHODS

Exome Sequencing was performed in an epilepsy patient cohort. The impact of the mutation was evaluated by ³H γ-aminobutyric acid (GABA) uptake, structural modeling, live cell microscopy, cell surface biotinylation and a high-throughput assay flow cytometry in both neurons and non neuronal cells.

RESULTS

We discovered a heterozygous missense mutation (c700G to A [pG234S) in the SLC6A1 encoding GABA transporter 1 (GAT-1). Structural modeling suggests the mutation destabilizes the global protein conformation. With transient expression of enhanced yellow fluorescence protein (YFP) tagged rat GAT-1 cDNAs, we demonstrated that the mutant GAT-1(G234S) transporter had reduced total protein expression in both rat cortical neurons and HEK 293 T cells. With a high-throughput flow cytometry assay and live cell surface biotinylation, we demonstrated that the mutant GAT-1(G234S) had reduced cell surface expression. ³H radioactive labeling GABA uptake assay in HeLa cells indicated a reduced function of the mutant GAT-1(G234S).

CONCLUSIONS

This mutation caused instability of the mutant transporter protein, which resulted in reduced cell surface and total protein levels. The mutation also caused reduced GABA uptake in addition to reduced protein expression, leading to reduced GABA clearance, and altered GABAergic signaling in the brain. The impaired trafficking and reduced GABA uptake function may explain the epilepsy phenotype in the patient.

Protein Quality Control in the Endoplasmic Reticulum

The site of protein folding and maturation for the majority of proteins that are secreted, localized to the plasma membrane or targeted to endomembrane compartments is the endoplasmic reticulum (ER). It is essential that proteins targeted to the ER are properly folded in order to carry out their function, as well as maintain protein homeostasis, as accumulation of misfolded proteins could lead to the formation of cytotoxic aggregates. Because protein folding is an error-prone process, the ER contains protein quality control networks that act to optimize proper folding and trafficking of client proteins. If a protein is unable to reach its native state, it is targeted for ER retention and subsequent degradation. The protein quality control networks of the ER that oversee this evaluation or interrogation process that decides the fate of maturing nascent chains is comprised of three general types of families: the classical chaperones, the carbohydrate-dependent system, and the thiol-dependent system. The cooperative action of these families promotes protein quality control and protein homeostasis in the ER. This review will describe the families of the ER protein quality control network and discuss the functions of individual members.

Expression of functional inhibitory neurotransmitter transporters GlyT1, GAT-1, and GAT-3 by astrocytes of inferior colliculus and hippocampus

Neuronal inhibition is mediated by glycine and/or GABA. Inferior colliculus (IC) neurons receive glycinergic and GABAergic inputs, whereas inhibition in hippocampus (HC) predominantly relies on GABA. Astrocytes heterogeneously express neurotransmitter transporters and are expected to adapt to the local requirements regarding neurotransmitter homeostasis. Here we analyzed the expression of inhibitory neurotransmitter transporters in IC and HC astrocytes using whole-cell patch-clamp and single-cell reverse transcription-PCR. We show that most astrocytes in both regions expressed functional glycine transporters (GlyTs). Activation of these transporters resulted in an inward current (I_Gly) that was sensitive to the competitive GlyT1 agonist sarcosine. Astrocytes exhibited transcripts for GlyT1 but not for GlyT2. Glycine did not alter the membrane resistance (R_M) arguing for the absence of functional glycine receptors (GlyRs). Thus, I_Gly was mainly mediated by GlyT1. Similarly, we found expression of functional GABA transporters (GATs) in all IC astrocytes and about half of the HC astrocytes. These transporters mediated an inward current (I_GABA) that was sensitive to the competitive GAT-1 and GAT-3 antagonists NO711 and SNAP5114, respectively. Accordingly, transcripts for GAT-1 and GAT-3 were found but not for GAT-2 and BGT-1. Only in hippocampal astrocytes, GABA transiently reduced R_M demonstrating the presence of GABA_A receptors (GABA_ARs). However, I_GABA was mainly not contaminated by GABA_AR-mediated currents as R_M changes vanished shortly after GABA application. In both regions, I_GABA was stronger than I_Gly. Furthermore, in HC the I_GABA/I_Gly ratio was larger compared to IC. Taken together, our results demonstrate that astrocytes are heterogeneous across and within distinct brain areas. Furthermore, we could show that the capacity for glycine and GABA uptake varies between both brain regions.

Expression, purification and structural analysis of functional GABA transporter 1 using the baculovirus expression system

The γ-aminobutyric acid (GABA) transporter 1 (GAT1) belongs to a family of Na⁺ and Cl⁻-coupled transport proteins and possesses 12 putative transmembrane domains. To perform structural analyses of the GAT1 protein, the GAT1/green fluorescent protein (GFP) fusion protein was functionally expressed in insect Sf9 cells by the BAC-TO-BAC^® baculovirus expression system. A two-step procedure to purify the GAT1/GFP fusion protein from insect Sf9 cells has been established and involves immunoaffinity chromatography using self-prepared anti-GFP antibodies and size-exclusion fast protein liquid chromatography (SE-FPLC). A yield of 200–300 μg of the GAT1/GFP protein could be purified from 400–600 mL of infected Sf9 cells. The purified protein was analyzed by transmission electron microscopy (TEM), which revealed that the GAT1/GFP fusion protein was isolated in its monomeric form.

Region specific contribution of ASIC2 to acidosis-and ischemia-induced neuronal injury

Acidosis in the brain plays a critical role in neuronal injury in neurological diseases, including brain ischemia. One key mediator of acidosis-induced neuronal injury is the acid-sensing ion channels (ASICs). Current literature has focused on ASIC1a when studying acid signaling. The importance of ASIC2, which is also widely expressed in the brain, has not been appreciated. We found here a region-specific effect of ASIC2 on acid-mediated responses. Deleting ASIC2 reduced acid-activated current in cortical and striatal neurons, but had no significant effect in cerebellar granule neurons. In addition, we demonstrated that ASIC2 was important for ASIC1a expression, and that ASIC2a but not 2b facilitated ASIC1a surface trafficking in the brain. Further, we showed that ASIC2 deletion attenuated acidosis/ischemia-induced neuronal injury in organotypic hippocampal slices but had no effect in organotypic cerebellar slices. Consistent with an injurious role of ASIC2, we showed that ASIC2 deletion significantly protected the mouse brain from ischemic damage in vivo. These data suggest a critical region-specific contribution of ASIC2 to neuronal injury and reveal an important functional difference between ASIC2a and 2b in the brain.

Abnormal subcellular localization of GABAA receptor subunits in schizophrenia brain

Inhibitory neurotransmission is primarily mediated by γ-aminobutyric acid (GABA) activating synaptic GABA type A receptors (GABA(A)R). In schizophrenia, presynaptic GABAergic signaling deficits are among the most replicated findings; however, postsynaptic GABAergic deficits are less well characterized. Our lab has previously demonstrated that although there is no difference in total protein expression of the α1-6, β1-3 or γ2 GABA(A)R subunits in the superior temporal gyrus (STG) in schizophrenia, the α1, β1 and β2 GABA(A)R subunits are abnormally N-glycosylated. N-glycosylation is a posttranslational modification that has important functional roles in protein folding, multimer assembly and forward trafficking. To investigate the impact that altered N-glycosylation has on the assembly and trafficking of GABA(A)Rs in schizophrenia, this study used western blot analysis to measure the expression of α1, α2, β1, β2 and γ2 GABA(A)R subunits in subcellular fractions enriched for endoplasmic reticulum (ER) and synapses (SYN) from STG of schizophrenia (N = 16) and comparison (N = 14) subjects and found evidence of abnormal localization of the β1 and β2 GABA(A)R subunits and subunit isoforms in schizophrenia. The β2 subunit is expressed as three isoforms at 52 kDa (β2(52 kDa)), 50 kDa (β2(50 kDa)) and 48 kDa (β2(48 kDa)). In the ER, we found increased total β2 GABA(A)R subunit (β2(ALL)) expression driven by increased β2(50 kDa), a decreased ratio of β(248 kDa):β2(ALL) and an increased ratio of β2(50 kDa):β2(48 kDa). Decreased ratios of β1:β2(ALL) and β1:β2(50 kDa) in both the ER and SYN fractions and an increased ratio of β2(52 kDa):β(248 kDa) at the synapse were also identified in schizophrenia. Taken together, these findings provide evidence that alterations of N-glycosylation may contribute to GABAergic signaling deficits in schizophrenia by disrupting the assembly and trafficking of GABA(A)Rs.

Role of N-glycosylation in renal betaine transport

The osmolyte and folding chaperone betaine is transported by the renal Na(+)-coupled GABA (γ-aminobutyric acid) symporter BGT-1 (betaine/GABA transporter 1), a member of the SLC6 (solute carrier 6) family. Under hypertonic conditions, the transcription, translation and plasma membrane (PM) insertion of BGT-1 in kidney cells are significantly increased, resulting in elevated betaine and GABA transport. Re-establishing isotonicity involves PM depletion of BGT-1. The molecular mechanism of the regulated PM insertion of BGT-1 during changes in osmotic stress is unknown. In the present study, we reveal a link between regulated PM insertion and N-glycosylation. Based on homology modelling, we identified two sites (Asn(171) and Asn(183)) in the extracellular loop 2 (EL2) of BGT-1, which were investigated with respect to trafficking, insertion and transport by immunogold-labelling, electron microscopy (EM), mutagenesis and two-electrode voltage clamp measurements in Xenopus laevis oocytes and uptake of radiolabelled substrate into MDCK (Madin-Darby canine kidney) and HEK293 (human embryonic kidney) cells. Trafficking and PM insertion of BGT-1 was clearly promoted by N-glycosylation in both oocytes and MDCK cells. Moreover, association with N-glycans at Asn(171) and Asn(183) contributed equally to protein activity and substrate affinity. Substitution of Asn(171) and Asn(183) by aspartate individually caused no loss of BGT-1 activity, whereas the double mutant was inactive, suggesting that N-glycosylation of at least one of the sites is required for function. Substitution by alanine or valine at either site caused a dramatic loss in transport activity. Furthermore, in MDCK cells PM insertion of N183D was no longer regulated by osmotic stress, highlighting the impact of N-glycosylation in regulation of this SLC6 transporter.

The effect of tunicamycin on the glucose uptake, growth, and cellular adhesion in the protozoan parasite Crithidia fasciculata

Crithidia fasciculata represents a very interesting model organism to study biochemical, cellular, and genetic processes unique to members of the family of the Trypanosomatidae. Thus, C. fasciculata parasitizes several species of insects and has been widely used to test new therapeutic strategies against parasitic infections. By using tunicamycin, a potent inhibitor of glycosylation in asparaginyl residues of glycoproteins (N-glycosylation), we demonstrate that N-glycosylation in C. fasciculata cells is involved in modulating glucose uptake, dramatically impacting growth, and cell adhesion. C. fasciculata treated with tunicamycin was severely affected in their ability to replicate and to adhere to polystyrene substrates and losing their ability to aggregate into small and large groups. Moreover, under tunicamycin treatment, the parasites were considerably shorter and rounder and displayed alterations in cytoplasmic vesicles formation. Furthermore, glucose uptake was significantly impaired in a tunicamycin dose-dependent manner; however, no cytotoxic effect was observed. Interestingly, this effect was reversible. Thus, when tunicamycin was removed from the culture media, the parasites recovered its growth rate, cell adhesion properties, and glucose uptake. Collectively, these results suggest that changes in the tunicamycin-dependent glycosylation levels can influence glucose uptake, cell growth, and adhesion in the protozoan parasite C. fasciculata.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Glycosylation of closely spaced acceptor sites in human glycoproteins

Asparagine-linked glycosylation of proteins by the oligosaccharyltransferase (OST) occurs when acceptor sites or sequons (N-x≠P-T/S) on nascent polypeptides enter the lumen of the rough endoplasmic reticulum. Metazoan organisms assemble two isoforms of the OST that have different catalytic subunits (STT3A or STT3B) and partially non-overlapping cellular roles. Potential glycosylation sites move past the STT3A complex, which is associated with the translocation channel, at the protein synthesis elongation rate. Here, we investigated whether close spacing between acceptor sites in a nascent protein promotes site skipping by the STT3A complex. Biosynthetic analysis of four human glycoproteins revealed that closely spaced sites are efficiently glycosylated by an STT3B-independent process unless the sequons contain non-optimal sequence features, including extreme close spacing between sequons (e.g. NxTNxT) or the presence of paired NxS sequons (e.g. NxSANxS). Many, but not all, glycosylation sites that are skipped by the STT3A complex can be glycosylated by the STT3B complex. Analysis of a murine glycoprotein database revealed that closely spaced sequons are surprisingly common, and are enriched for paired NxT sites when the gap between sequons is less than three residues.

Structural and functional conservation of CLEC-2 with the species-specific regulation of transcript expression in evolution

CLEC-2 was first identified by sequence similarity to C-type lectin-like molecules with immune functions and has been reported as a receptor for the platelet-aggregating snake venom toxin rhodocytin and the endogenous sialoglycoprotein podoplanin. Recent researches indicate that CLEC-2-deficient mice were lethal at the embryonic stage associated with disorganized and blood-filled lymphatic vessels and severe edema. In view of a necessary role of CLEC-2 in the individual development, it is of interest to investigate its phylogenetic homology and highly conserved functional regions. In this work, we reported that CLEC-2 from different species holds with an extraordinary conservation by sequence alignment and phylogenetic tree analysis. The functional structures including N-linked oligosaccharide sites and ligand-binding domain implement a structural and functional conservation in a variety of species. The glycosylation sites (N120 and N134) are necessary for the surface expression CLEC-2. CLEC-2 from different species possesses the binding activity of mouse podoplanin. Nevertheless, the expression of CLEC-2 is regulated with a species-specific manner. The alternative splicing of pre-mRNA, a regulatory mechanism of gene expression, and the binding sites on promoter for several key transcription factors vary between different species. Therefore, CLEC-2 shares high sequence homology and functional identity. However the transcript expression might be tightly regulated by different mechanisms in evolution.

N-glycosylation of acid-sensing ion channel 1a regulates its trafficking and acidosis-induced spine remodeling

Acid-sensing ion channel-1a (ASIC1a) is a potential therapeutic target for multiple neurological diseases. We studied here ASIC1a glycosylation and trafficking, two poorly understood processes pivotal in determining the functional outcome of an ion channel. We found that most ASIC1a in the mouse brain was fully glycosylated. Inhibiting glycosylation with tunicamycin reduced ASIC1a surface trafficking, dendritic targeting, and acid-activated current density. N-glycosylation of the two glycosylation sites, Asn393 and Asn366, has differential effects on ASIC1a biogenesis. Maturation of Asn393 increased ASIC1a surface and dendritic trafficking, pH sensitivity, and current density. In contrast, glycosylation of Asn366 was dispensable for ASIC1a function and may be a rate-limiting step in ASIC1a biogenesis. In addition, we revealed that acidosis reduced the density and length of dendritic spines in a time- and ASIC1a-dependent manner. ASIC1a N366Q, which showed increased glycosylation and dendritic targeting, potentiated acidosis-induced spine loss. Conversely, ASIC1a N393Q, which had diminished dendritic targeting and inhibited ASIC1a current dominant-negatively, had the opposite effect. These data tie N-glycosylation of ASIC1a with its trafficking. More importantly, by revealing a site-specific effect of acidosis on dendritic spines, our findings suggest that these processes have an important role in regulating synaptic plasticity and determining long-term consequences in diseases that generate acidosis.

Alternative glycosylation modulates function of IgG and other proteins - implications on evolution and disease

BACKGROUND

Nearly all membrane and secreted proteins, as well as numerous intracellular proteins are glycosylated. However, contrary to proteins which are defined by their individual genetic templates, glycans are encoded in a complex dynamic network of hundreds of genes which participate in the complex biosynthetic pathway of protein glycosylation.

SCOPE OF REVIEW

This review summarizes present knowledge about the importance of alternative glycosylation of IgG and other proteins.

MAJOR CONCLUSIONS

Numerous proteins depend on correct glycosylation for proper function. Very good example for this is the alternative glycosylation of IgG whose effector functions can be completely changed by the addition or removal of a single monosaccharide residue from its glycans.

GENERAL SIGNIFICANCE

The change in the structure of a protein requires mutations in DNA and subsequent selection in the next generation, while even slight alterations in activity or intracellular localization of one or more biosynthetic enzymes are sufficient for the creation of novel glycan structures, which can then perform new functions. Glycome composition varies significantly between individuals, which makes them slightly or even significantly different in their ability to execute specific molecular pathways with numerous implications for development and progression of various diseases. This article is part of a Special Issue entitled Glycoproteomics.

SLC6 neurotransmitter transporters: structure, function, and regulation

The neurotransmitter transporters (NTTs) belonging to the solute carrier 6 (SLC6) gene family (also referred to as the neurotransmitter-sodium-symporter family or Na(+)/Cl(-)-dependent transporters) comprise a group of nine sodium- and chloride-dependent plasma membrane transporters for the monoamine neurotransmitters serotonin (5-hydroxytryptamine), dopamine, and norepinephrine, and the amino acid neurotransmitters GABA and glycine. The SLC6 NTTs are widely expressed in the mammalian brain and play an essential role in regulating neurotransmitter signaling and homeostasis by mediating uptake of released neurotransmitters from the extracellular space into neurons and glial cells. The transporters are targets for a wide range of therapeutic drugs used in treatment of psychiatric diseases, including major depression, anxiety disorders, attention deficit hyperactivity disorder and epilepsy. Furthermore, psychostimulants such as cocaine and amphetamines have the SLC6 NTTs as primary targets. Beginning with the determination of a high-resolution structure of a prokaryotic homolog of the mammalian SLC6 transporters in 2005, the understanding of the molecular structure, function, and pharmacology of these proteins has advanced rapidly. Furthermore, intensive efforts have been directed toward understanding the molecular and cellular mechanisms involved in regulation of the activity of this important class of transporters, leading to new methodological developments and important insights. This review provides an update of these advances and their implications for the current understanding of the SLC6 NTTs.

CXCR7 protein expression in human adult brain and differentiated neurons

BACKGROUND

CXCR7 and CXCR4 are receptors for the chemokine CXCL12, which is involved in essential functions of the immune and nervous systems. Although CXCR7 transcripts are widely expressed throughout the central nervous system, little is known about its protein distribution and function in the adult brain. To evaluate its potential involvement in CXCL12/CXCR4 signaling in differentiated neurons, we studied CXCR7 protein expression in human brain and cultured neurons.

METHODOLOGY/PRINCIPAL FINDINGS

Immunohistochemistry and RT-PCR analyses of cortex and hippocampus from control and HIV-positive subjects provided the first evidence of CXCR7 protein expression in human adult neurons, under normal and pathological conditions. Furthermore, confocal microscopy and binding assays in cultured neurons show that CXCR7 protein is mainly located into cytoplasm, while little to no protein expression is found on neuronal plasma membrane. Interestingly, specific CXCR7 ligands that inhibit CXCL12 binding to CXCR7 do not alter CXCR4-activated survival signaling (pERK/pAkt) in rat cortical neurons. Neuronal CXCR7 co-localizes to some extent with the endoplasmic reticulum marker ERp29, but not with early/late endosome markers. Additionally, large areas of overlap are detected in the intracellular pattern of CXCR7 and CXCR4 expression.

CONCLUSIONS/SIGNIFICANCE

Overall, these results implicate CXCR4 as the main CXCL12 signaling receptor on the surface of differentiated neurons and suggest that CXCR7 may interact with CXCR4 at the intracellular level, possibly affecting CXCR4 trafficking and/or coupling to other proteins.

Alpha1,6-fucosyltransferase-deficient mice exhibit multiple behavioral abnormalities associated with a schizophrenia-like phenotype: importance of the balance between the dopamine and serotonin systems

Previously, we reported that α1,6-fucosyltransferase (Fut8)-deficient (Fut8(-/-)) mice exhibit emphysema-like changes in the lung and severe growth retardation due to dysregulation of TGF-β1 and EGF receptors and to abnormal integrin activation, respectively. To study the role of α1,6-fucosylation in brain tissue where Fut8 is highly expressed, we examined Fut8(-/-) mice using a combination of neurological and behavioral tests. Fut8(-/-) mice exhibited multiple behavioral abnormalities consistent with a schizophrenia-like phenotype. Fut8(-/-) mice displayed increased locomotion compared with wild-type (Fut8(+/+)) and heterozygous (Fut8(+/-)) mice. In particular, Fut8(-/-) mice showed strenuous hopping behavior in a novel environment. Working memory performance was impaired in Fut8(-/-) mice as evidenced by the Y-maze tests. Furthermore, Fut8(-/-) mice showed prepulse inhibition (PPI) deficiency. Intriguingly, although there was no significant difference between Fut8(+/+) and Fut8(+/-) mice in the PPI test under normal conditions, Fut8(+/-) mice showed impaired PPI after exposure to a restraint stress. This result suggests that reduced expression of Fut8 is a plausible cause of schizophrenia and related disorders. The levels of serotonin metabolites were significantly decreased in both the striatum and nucleus accumbens of the Fut8(-/-) mice. Likewise, treatment with haloperidol, which is an antipsychotic drug that antagonizes dopaminergic and serotonergic receptors, significantly reduced hopping behaviors. The present study is the first to clearly demonstrate that α1,6-fucosylation plays an important role in the brain, and that it might be related to schizophrenia-like behaviors. Thus, the results of the present study provide new insights into the underlying mechanisms responsible for schizophrenia and related disorders.

The life cycle of human equilibrative nucleoside transporter 1: from ER export to degradation

Nucleoside transporters (NTs) play an essential role in the transport of nucleosides across cellular membranes. Equilibrative NTs (ENTs) allow facilitated diffusion of nucleosides and the prototypic ENT, hENT1, is primarily localized to the plasma membrane (PM). hENT1 is responsible for the uptake of nucleoside analog drugs used in treating viral infections and cancer, but despite its clinical importance, virtually nothing is known about the dynamics of the hENT1 life cycle including trafficking to the PM, endocytosis and degradation. Therefore, we followed the life cycle of tagged hENT1 (GFP- or FLAG-) transiently transfected into mammalian cells to gain insight into the sequence of events, timing and underlying mechanisms regulating the hENT1 life cycle. Protein translocation to the PM was examined using fixed and live cell confocal microscopy while endocytosis and degradation were analyzed by cell surface biotinylation and [(35)S] pulse chase analysis respectively. We determined that tagged hENT1 is trafficked to the PM in association with microtubules and incorporated in the plasma membrane where it subsequently undergoes clathrin-mediated endocytosis and recycling. Finally, internalized protein is degraded via the lysosomal pathway and observations suggest the complete life cycle of tagged hENT1 within these cells is approximately 14 hours.

Involvement of sialic acid in the regulation of γ--aminobutyric acid uptake activity of γ-aminobutyric acid transporter 1

The γ-aminobutyric acid (GABA) transporters (GATs) have long been recognized for their key role in the uptake of neurotransmitters. The GAT1 belongs to the family of Na(+)- and Cl(-)-coupled transport proteins, which possess 12 putative transmembrane (TM) domains and three N-glycosylation sites on the extracellular loop between TM domains 3 and 4. Previously, we demonstrated that terminal trimming of N-glycans is important for the GABA uptake activity of GAT1. In this work, we examined the effect of deficiency, removal or oxidation of surface sialic acid residues on GABA uptake activity to investigate their role in the GABA uptake of GAT1. We found that the reduced concentration of sialic acid on N-glycans was paralleled by a decreased GABA uptake activity of GAT1 in Chinese hamster ovary (CHO) Lec3 cells (mutant defective in sialic acid biosynthesis) in comparison to CHO cells. Likewise, either enzymatic removal or chemical oxidation of terminal sialic acids using sialidase or sodium periodate, respectively, resulted in a strong reduction in GAT1 activity. Kinetic analysis revealed that deficiency, removal or oxidation of terminal sialic acids did not affect the K(m) GABA values. However, deficiency and removal of terminal sialic acids of GAT1 reduced the V(max) GABA values with a reduced apparent affinity for extracellular Na(+). Oxidation of cell surface sialic acids also strongly reduced V(max) without affecting both affinities of GAT1 for GABA and Na(+), respectively. These results demonstrated for the first time that the terminal sialic acid of N-linked oligosaccharides of GAT1 plays a crucial role in the GABA transport process.

The effect of individual N-glycans on enzyme activity

In a series of investigations, N-glycosylation has proven to be a key determinant of enzyme secretion, activity, binding affinity and substrate specificity, enabling a protein to fine-tune its activity. In the majority of cases elimination of all putative N-glycosylation sites of an enzyme results in significantly reduced protein secretion levels, while removal of individual N-glycosylation sites often leads to the expression of active enzymes showing markedly reduced catalytic activity, with the decreased activity often commensurate with the number of glycosylation sites available, and the fully deglycosylated enzymes showing only minimal activity relative to their glycosylated counterparts. On the other hand, several cases have also recently emerged where deglycosylation of an enzyme results in significantly increased catalytic activity, binding affinity and altered substrate specificity, highlighting the very unique and diverse roles that individual N-glycans play in regulating enzyme function.

Membrane topology and post-translational modification of the Saccharomyces cerevisiae essential protein Rot1

ROT1 is an essential gene that has been related to cell wall biosynthesis, the actin cytoskeleton and protein folding. In order to help to understand its molecular function, we carried out a characterization of the Rot1 protein. It is primarily located at the endoplasmic reticulum-nuclear membrane facing the lumen. Rot1 migrates more slowly than expected, which might suggest post-translational modification. Our results indicate that Rot1 is a protein that is neither GPI-anchored nor O-glycosylated. In contrast, it is N-glycosylated. By a directed mutagenesis of several Asn residues, we identified that the protein is simultaneously glycosylated at N103, N107 and N139. Although the mutation of these three N sites is not lethal, cellular growth is impaired. Sequence analysis predicts a transmembrane domain at the C-terminus. This fragment affects neither the targeting of the Rot1 protein to the ER nor its N-glycosylation, although it is important for the anchoring of the protein to the membrane and for its functionality. The existence of a signal sequence at the N-terminus has been suggested. However, deletion of this fragment impedes neither translocation to the ER nor N-glycosylation, but it is required for cell viability. Finally, we found that Rot1 is translocated to the ER by an SRP-independent post-translational mechanism which depends on Sec62.

Functional role of N-glycosylation from ADAM10 in processing, localization and activity of the enzyme

A disintegrin and metalloprotease 10 (ADAM10) is a type I transmembrane glycoprotein with four potential N-glycosylation sites (N267, N278, N439 and N551), that cleaves several plasma membrane proteins. In this work, ADAM10 was found to contain high-mannose and complex-type glycans. Individual N-glycosylation site mutants S269A, T280A, S441A, T553A were constructed, and results indicated that all sites were occupied. T280A was found to accumulate in the endoplasmic reticulum as the non-processed precursor of the enzyme. Furthermore, it exhibited only residual levels of metalloprotease activity in vivo towards the L1 cell adhesion molecule, as well as in vitro, using a ProTNF-alpha peptide as substrate. S441A showed increased ADAM10 susceptibility to proteolysis. Mutation of N267, N439 and N551 did not completely abolish enzyme activity, however, reduced levels were found. ADAM10 is sorted into secretory vesicles, the exosomes. Here, a fraction of ADAM10 from exosomes was found to contain more processed N-linked glycans than the cellular enzyme. In conclusion, N-glycosylation is crucial for ADAM10 processing and resistance to proteolysis, and results suggest that it is required for full-enzyme activity.

Glycine transporter dimers: evidence for occurrence in the plasma membrane

Different Na(+)/Cl(-)-dependent neurotransmitter transporters of the SLC6a family have been shown to form dimers or oligomers in both intracellular compartments and at the cell surface. In contrast, the glycine transporters (GlyTs) GlyT1 and -2 have been reported to exist as monomers in the plasma membrane based on hydrodynamic and native gel electrophoretic studies. Here, we used cysteine substitution and oxidative cross-linking to show that of GlyT1 and GlyT2 also form dimeric complexes within the plasma membrane. GlyT oligomerization at the cell surface was confirmed for both GlyT1 and GlyT2 by fluorescence resonance energy transfer microscopy. Endoglycosidase treatment and surface biotinylation further revealed that complex-glycosylated GlyTs form dimers located at the cell surface. Furthermore, substitution of tryptophan 469 of GlyT2 by an arginine generated a transporter deficient in dimerization that was retained intracellulary. Based on these results and GlyT structures modeled by using the crystal structure of the bacterial homolog LeuT(Aa), as a template, residues located within the extracellular loop 3 and at the beginning of transmembrane domain 6 are proposed to contribute to the dimerization interface of GlyTs.

Beta N-acetylglucosaminyltransferase V (Mgat5) deficiency reduces the depression-like phenotype in mice

The central nervous system (CNS) is rich in glycoconjugates, located on cell surface and in extracellular matrix. The products of Golgi UDP-GlcNAc:N-acetylglucosaminyltransferases (encoded by Mgat1, Mgat2, Mgat4 and Mgat5) act sequentially to generate the GlcNAc-branched complex-type N-glycans on glycoprotein receptors. While elimination of all the branched N-glycans in Mgat1(-/-) mouse embryos is lethal at neural tube fold stage, decreased branching is associated with late developmental defects similar to type 2 of congenital disorders of glycosylation, with developmental and psychomotor abnormalities. To study the role of complex-type N-glycans in brain function, we tested Mgat5(-/-) mice in a battery of neurological and behavioral tests. Despite the absence of tri- and tetra-antennary products, Mgat5(-/-) mice were not different from their wild-type littermates in physical and neurological assessments, anxiety level, startle reactivity and sensorimotor gating. However, they displayed a robust decrease in the immobility time in the forced swim test and the tail suspension test independent of locomotor activity, interpreted as a change in depression-like behavior. This effect was accentuated after chronic mild stress. Comparable increase in plasma corticosterone of Mgat5(+/+) and Mgat5(-/-) mice in response to acute stress shows an intact function of the hypothalamus-pituitary-adrenal axis. A change in social interactions was also observed. Our results indicate that Mgat5 modification of complex-type N-glycans on CNS glycoproteins is involved in the regulation of depression-like behavior.

Acute regulation of mouse AE2 anion exchanger requires isoform-specific amino acid residues from most of the transmembrane domain

The widely expressed anion exchanger polypeptide AE2/SLC4A2 is acutely inhibited by acidic intracellular (pH(i)), by acidic extracellular pH (pH(o)), and by the calmodulin inhibitor, calmidazolium, whereas it is acutely activated by NH(4)(+). The homologous erythroid/kidney AE1/SLC4A1 polypeptide is insensitive to these regulators. Each of these AE2 regulatory responses requires the presence of AE2's C-terminal transmembrane domain (TMD). We have now measured (36)Cl(-) efflux from Xenopus oocytes expressing bi- or tripartite AE2-AE1 chimeras to define TMD subregions in which AE2-specific sequences contribute to acute regulation. The chimeric AE polypeptides were all functional at pH(o) 7.4, with the sole exception of AE2((1-920))/AE1((613-811))/AE2((1120-1237)). Reciprocal exchanges of the large third extracellular loops were without effect. AE2 regulation by pH(i), pH(o) and NH(4)(+) was retained after substitution of C-terminal AE2 amino acids 1120-1237 (including the putative second re-entrant loop, two TM spans and the cytoplasmic tail) with the corresponding AE1 sequence. In contrast, the presence of this AE2 C-terminal sequence was both necessary and sufficient for inhibition by calmidazolium. All other tested TMD substitutions abolished AE2 pH(i) sensitivity, abolished or severely attenuated sensitivity to pH(o) and removed sensitivity to NH(4)(+). Loss of AE2 pH(i) sensitivity was not rescued by co-expression of a complementary AE2 sequence within separate full-length chimeras or AE2 subdomains. Thus, normal regulation of AE2 by pH and other ligands requires AE2-specific sequence from most regions of the AE2 TMD, with the exceptions of the third extracellular loop and a short C-terminal sequence. We conclude that the individual TMD amino acid residues previously identified as influencing acute regulation of AE2 exert that influence within a regulatory structure requiring essential contributions from multiple regions of the AE2 TMD.

Kinetic And Biologic Properties of Recombinant ChIFN-γExpressed via CELO-Virus Vector

In this study, a replicative fowl adenovirus serotype 1 (CELO) recombinant expressing chicken interferon-gamma (ChIFN-gamma) was constructed. In the engineered recombinant, the ChIFN-gamma gene was placed under the control of cytomegalovirus (CMV) promoter. The ChIFN-gamma expression cassette was inserted in the right end of the CELO genome (D fragment), which was able to carry the largest insertion of foreign DNA without affecting the replication functions of the vector. The recombinant ChIFN-gamma (rChIFN-gamma) produced in the CELO-virus expression system was characterized by comparing its biologic activities with that of rChIFN-gamma produced via the baculovirus expression system (Bac-ChIFN-gamma). CELO-ChIFN-gamma inhibited the replication of cytolytic virus in chicken embryo fibroblasts (CEFs) and activated macrophages in a better manner than did Bac-ChIFN-gamma . Moreover, the in vitro and in vivo stability of the CELO-derived rChIFN-gamma was considerably higher than that of the Bac-ChIFN-gamma. The CELO-ChIFN-gamma recombinant vector was able to replicate in vitro in the loghorn male hepatoma (LMH) hepatocyte cell line and to produce detectable levels of recombinant cytokine in supernatant as early as 90 min post-infection. Therefore, the CELO-virus expression system is an appropriate system for high-level expression of biologically active and stable ChIFN-gamma.

Mice with genetically altered GABA transporter subtype I (GAT1) expression show altered behavioral responses to ethanol

It is widely accepted that the GABAergic system plays an important role in the action of ethanol in vivo. GABA transporter subtype 1 (GAT1) constructs high affinity reuptake sites in the CNS and regulates GABAergic transmissions. In this study, mice lacking the GAT1 were developed by homologous recombination. Both hetero- and homozygous GAT1 mutant mice were tested for ethanol, saccharin or quinine consumption, ethanol-conditioned place preference, ethanol-conditioned taste aversion, ethanol-simulated motor activity, and ethanol-induced sedation/hypnosis. The GAT1(-/-) mice showed decreased ethanol aversion and ethanol reward, and insensitivity to both the sedative/hypnotic and the motor stimulant effects of ethanol, along with increased avoidance of quinine preference and consumption. GAT1(+/-) mice showed significantly increased consumption of ethanol and saccharin, however, enhanced the rewarding and preference effect of ethanol, increased avoidance of quinine, and higher sensitivity to the motor stimulant effect of ethanol. These results demonstrate that GAT1, perhaps in a bi-directional way, modulates some behavioral effects of ethanol. The GAT1 mutant mice provided us a very useful model to investigate the mechanisms of ethanol action in vivo.

Posttranscriptional regulation of Git1p, the glycerophosphoinositol/glycerophosphocholine transporter of Saccharomyces cerevisiae

Glycerophosphoinositol (GroPIns) and glycerophosphocholine (GroPCho) are the products of phospholipase-B mediated deacylation of phosphatidylinositol and phosphatidylcholine, respectively. GroPIns and GroPCho are transported across the Saccharomyces cerevisiae plasma membrane into the cell via the transporter encoded by GIT1. Previous studies have shown that GIT1 expression is regulated by inorganic phosphate (P(i)) availability through the transcription factors Pho2p and Pho4p. We now report that posttranscriptional mechanisms also regulate Git1p activity in response to P(i) availability. Mutations that inhibit endocytosis and vacuolar proteolysis inhibit Git1p degradation, indicating that Git1p downregulation involves internalization and subsequent degradation in the vacuole. Similar to the effect seen with P(i), provision of cells with high levels of the Git1p substrates, GroPIns and GroPCho, posttranscriptionally downregulates Git1p activity. Unlike P(i), high levels of GroPCho and GroPIns do not repress GIT1 promoter-driven reporter gene activity. These results indicate that Git1p transport activity is regulated at multiple levels by P(i) availability. In addition, the results indicate that the Git1p substrates (and alternate phosphate sources) GroPIns and GroPCho behave distinctly from P(i) in their ability to affect GIT1 expression.

SNAP-25/syntaxin 1A complex functionally modulates neurotransmitter gamma-aminobutyric acid reuptake

Neurotransmitter gamma-aminobutyric acid (GABA) release to the synaptic clefts is mediated by the formation of a soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, which includes two target SNAREs syntaxin 1A and SNAP-25 and one vesicle SNARE VAMP-2. The target SNAREs syntaxin 1A and SNAP-25 form a heterodimer, the putative intermediate of the SNARE complex. Neurotransmitter GABA clearance from synaptic clefts is carried out by the reuptake function of its transporters to terminate the postsynaptic signaling. Syntaxin 1A directly binds to the neuronal GABA transporter GAT-1 and inhibits its reuptake function. However, whether other SNARE proteins or SNARE complex regulates GABA reuptake remains unknown. Here we demonstrate that SNAP-25 efficiently inhibits GAT-1 reuptake function in the presence of syntaxin 1A. This inhibition depends on SNAP-25/syntaxin 1A complex formation. The H3 domain of syntaxin 1A is identified as the binding sites for both SNAP-25 and GAT-1. SNAP-25 binding to syntaxin 1A greatly potentiates the physical interaction of syntaxin 1A with GAT-1 and significantly enhances the syntaxin 1A-mediated inhibition of GAT-1 reuptake function. Furthermore, nitric oxide, which promotes SNAP-25 binding to syntaxin 1A to form the SNARE complex, also potentiates the interaction of syntaxin 1A with GAT-1 and suppresses GABA reuptake by GAT-1. Thus our findings delineate a further molecular mechanism for the regulation of GABA reuptake by a target SNARE complex and suggest a direct coordination between GABA release and reuptake.

Functional influence ofN-glycosylation in OCT2-mediated tetraethylammonium transport

OCT2, an organic cation transporter critical for removal of many drugs and toxins from the body, contains consensus sites for N-glycosylation at amino acid position 71, 96, and 112. However, the extent to which these sites are glycosylated by the cell, and the influence glycosylation has on OCT2 function, remains unknown. To address these issues, the acquisition of N-glycosylation was disrupted by mutating the amino acid asparagine (N) to glutamine (Q) at these sites in the rabbit ortholog of OCT2, which was expressed in Chinese hamster ovary cells. Disruption of N-glycosylation followed by Western blotting indicated that each site is indeed glycosylated and that OCT2 contains no other sites of N-glycosylation. Plasma membrane expression (determined by surface biotinylation) of the N112Q mutant, but not N71Q or N96Q mutants, was fourfold lower than that of wild-type OCT2, and unglycosylated OCT2 (N71Q/N96Q/N112Q) was sequestered in an unidentified intracellular compartment. The N71Q, N96Q, and N112Q mutants had a higher affinity (∼2-fold) for tetraethylammonium (TEA). Maximum transport rate was reduced in the N96Q (3-fold) and N112Q (5-fold) mutants, but not the N71Q mutant, and unglycosylated OCT2 failed to transport TEA (associated with its absence in the plasma membrane). Whereas the reduction in maximum transport rate of the N112Q mutant is consistent with its reduced plasma membrane expression, the lower rate of the N96Q mutant, which appeared to traffic properly, suggests that glycosylation at N96 increases the transporter turnover number.