Functional characterization of wild-type and mutant human sialin

Lack of significant ganglioside changes in Slc17a5 heterozygous mice: Relevance to FSASD and Parkinson's disease

Large population-based studies of Parkinson's disease (PD) have identified susceptibility genes, including SLC17A5. Biallelic mutations in SLC17A5, encoding the lysosomal sialic acid transporter sialin, cause the rare neurodegenerative disease, free sialic acid storage disorder (FSASD). To explore a potential biochemical link between FSASD and PD, we investigated ganglioside concentrations in a novel mouse model harboring the Slc17a5 p.Arg39Cys (p.R39C) variant. Our analysis revealed no significant alterations in ganglioside concentrations in heterozygous p.R39C mice, warranting further studies into other potential links between PD and sialin defects.

Comprehensive analysis of SLC17A5 variants in large European cohorts reveals no association with Parkinson's disease risk

BACKGROUND

Parkinson's disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss and α-synuclein aggregation. Aging is the primary risk factor, with both rare and common genetic variants playing a role. Previous studies have implicated lysosomal storage disorder (LSD)-related genes, including SLC17A5, in PD susceptibility.

OBJECTIVE

This study aimed to investigate the association of SLC17A5 variants, including rare and common variants and the FSASD-associated p.Arg39Cys missense variant, with PD risk in large European ancestry cohorts.

METHODS

Rare variant burden analyses were performed at minor allele frequency (MAF) thresholds of ≤1 % and ≤0.1 % in 7,184 PD cases and 51,650 controls using whole-genome and whole-exome sequencing data. Association testing of the p.Arg39Cys variant was conducted across five cohorts, encompassing both Finnish and non-Finnish Europeans. Common variant associations were examined using summary statistics from the largest European GWAS of PD.

RESULTS

No significant association was observed between rare SLC17A5 variants and PD at either MAF threshold. The p.Arg39Cys variant, though enriched in Finnish Europeans, showed no significant association with PD across several cohorts. Similarly, common SLC17A5 variants (MAF ≥1%) were not associated with PD risk.

CONCLUSION

Our findings do not support a role for SLC17A5 variants in PD susceptibility. While lysosomal dysfunction is central to PD pathogenesis, its contribution appears pathway-specific, with SLC17A5 unlikely to influence risk. Larger, multiethnic studies and functional analyses are needed to further investigate sialic acid metabolism in PD and related disorders.

Changes in glycosphingolipid levels in plasma and cerebrospinal fluid of individuals with Lysosomal Free Sialic Acid Storage Disorder

Lysosomal free sialic acid storage disorder (FSASD) is a rare, multisystem disease caused by biallelic pathogenic variants in SLC17A5, encoding the lysosomal transmembrane sialic acid exporter, sialin. Defective sialin function leads to sialic acid accumulation in lysosomes, contributing to neurodegeneration. While glycosphingolipid (GSL) metabolism is altered in other lysosomal storage disorders, its role in FSASD remains poorly understood, especially due to the restricted availability of biospecimens. This study investigated GSL levels in FSASD plasma and cerebrospinal fluid (CSF) using two normal-phase high-performance liquid chromatography assays. In plasma, GM1a was significantly elevated, while GM2 was decreased, with no significant alterations in other GSL species. In CSF, total GSLs, GM1a, GM3, GD3, GD1a, and GD1b were significantly elevated compared to comparison samples. These results reveal dysregulated GSL metabolism and suggest the potential of gangliosides as biomarkers. Further research is warranted to elucidate the biological implications of these alterations and their contributions to FSASD pathogenesis.

Atypical free sialic acid storage disorder associated with tissue specific mosaicism of SLC17A5

Free sialic acid storage disorder (FSASD) is a rare autosomal recessive lysosomal storage disease caused by pathogenic SLC17A5 variants with variable disease severity. We performed a multidisciplinary evaluation of an adolescent female with suspected lysosomal storage disease and conducted comprehensive studies to uncover the molecular etiology. The proband exhibited intellectual disability, a storage disease gestalt, and mildly elevated urine free sialic acid levels. Skin electron micrographs showed prominent cytoplasmic vacuolation. Clinical exome and genome sequencing identified a maternally-inherited SLC17A5 variant: c.533delC;p.Thr178Asnfs*34. RNASeq of proband skin fibroblasts revealed exon 3 skipping, which was not detected in RNA from proband blood or parental fibroblasts. Targeted deep sequencing of proband fibroblast DNA revealed a 184 bp deletion in ∼15 % of reads, encompassing the 3' end of exon 3. Illumina Complete Long Read sequencing confirmed the deletion was in the paternally-inherited allele and found in a mosaic state in proband fibroblasts and muscle but not in blood or buccal cells. Functional studies, including SLC17A5 knockout cells and transient transfections of mutated SLC17A5 demonstrated pathogenicity of the identified variants. We report an adolescent female with atypical FSASD with tissue-specific mosaicism for an intragenic deletion in SLC17A5, explaining the atypical clinical course, mild biochemical abnormalities, and long diagnostic process.

Transport mechanism of DgoT, a bacterial homolog of SLC17 organic anion transporters

The solute carrier 17 (SLC17) family contains anion transporters that accumulate neurotransmitters in secretory vesicles, remove carboxylated monosaccharides from lysosomes, or extrude organic anions from the kidneys and liver. We combined classical molecular dynamics simulations, Markov state modeling and hybrid first principles quantum mechanical/classical mechanical (QM/MM) simulations with experimental approaches to describe the transport mechanisms of a model bacterial protein, the D-galactonate transporter DgoT, at atomic resolution. We found that protonation of D46 and E133 precedes galactonate binding and that substrate binding induces closure of the extracellular gate, with the conserved R47 coupling substrate binding to transmembrane helix movement. After isomerization to an inward-facing conformation, deprotonation of E133 and subsequent proton transfer from D46 to E133 opens the intracellular gate and permits galactonate dissociation either in its unprotonated form or after proton transfer from E133. After release of the second proton, apo DgoT returns to the outward-facing conformation. Our results provide a framework to understand how various SLC17 transport functions with distinct transport stoichiometries can be attained through subtle variations in proton and substrate binding/unbinding.

GRPa-PRS: A risk stratification method to identify genetically-regulated pathways in polygenic diseases

Background

Polygenic risk scores (PRS) are tools used to evaluate an individual's susceptibility to polygenic diseases based on their genetic profile. A considerable proportion of people carry a high genetic risk but evade the disease. On the other hand, some individuals with a low risk of eventually developing the disease. We hypothesized that unknown counterfactors might be involved in reversing the PRS prediction, which might provide new insights into the pathogenesis, prevention, and early intervention of diseases.

Methods

We built a novel computational framework to identify genetically-regulated pathways (GRPas) using PRS-based stratification for each cohort. We curated two AD cohorts with genotyping data; the discovery (disc) and the replication (rep) datasets include 2722 and 2854 individuals, respectively. First, we calculated the optimized PRS model based on the three recent AD GWAS summary statistics for each cohort. Then, we stratified the individuals by their PRS and clinical diagnosis into six biologically meaningful PRS strata, such as AD cases with low/high risk and cognitively normal (CN) with low/high risk. Lastly, we imputed individual genetically-regulated expression (GReX) and identified differential GReX and GRPas between risk strata using gene-set enrichment and variational analyses in two models, with and without APOE effects. An orthogonality test was further conducted to verify those GRPas are independent of PRS risk. To verify the generalizability of other polygenic diseases, we further applied a default model of GRPa-PRS for schizophrenia (SCZ).

Results

For each stratum, we conducted the same procedures in both the disc and rep datasets for comparison. In AD, we identified several well-known AD-related pathways, including amyloid-beta clearance, tau protein binding, and astrocyte response to oxidative stress. Additionally, we discovered resilience-related GRPs that are orthogonal to AD PRS, such as the calcium signaling pathway and divalent inorganic cation homeostasis. In SCZ, pathways related to mitochondrial function and muscle development were highlighted. Finally, our GRPa-PRS method identified more consistent differential pathways compared to another variant-based pathway PRS method.

Conclusions

We developed a framework, GRPa-PRS, to systematically explore the differential GReX and GRPas among individuals stratified by their estimated PRS. The GReX-level comparison among those strata unveiled new insights into the pathways associated with disease risk and resilience. Our framework is extendable to other polygenic complex diseases.

MFSD1 with its accessory subunit GLMP functions as a general dipeptide uniporter in lysosomes

The lysosomal degradation of macromolecules produces diverse small metabolites exported by specific transporters for reuse in biosynthetic pathways. Here we deorphanized the major facilitator superfamily domain containing 1 (MFSD1) protein, which forms a tight complex with the glycosylated lysosomal membrane protein (GLMP) in the lysosomal membrane. Untargeted metabolomics analysis of MFSD1-deficient mouse lysosomes revealed an increase in cationic dipeptides. Purified MFSD1 selectively bound diverse dipeptides, while electrophysiological, isotope tracer and fluorescence-based studies in Xenopus oocytes and proteoliposomes showed that MFSD1-GLMP acts as a uniporter for cationic, neutral and anionic dipeptides. Cryoelectron microscopy structure of the dipeptide-bound MFSD1-GLMP complex in outward-open conformation characterized the heterodimer interface and, in combination with molecular dynamics simulations, provided a structural basis for its selectivity towards diverse dipeptides. Together, our data identify MFSD1 as a general lysosomal dipeptide uniporter, providing an alternative route to recycle lysosomal proteolysis products when lysosomal amino acid exporters are overloaded.

Structure and inhibition of the human lysosomal transporter Sialin

Sialin, a member of the solute carrier 17 (SLC17) transporter family, is unique in its ability to transport not only sialic acid using a pH-driven mechanism, but also transport mono and diacidic neurotransmitters, such as glutamate and N-acetylaspartylglutamate (NAAG), into synaptic vesicles via a membrane potential-driven mechanism. While most transporters utilize one of these mechanisms, the structural basis of how Sialin transports substrates using both remains unclear. Here, we present the cryogenic electron-microscopy structures of human Sialin: apo cytosol-open, apo lumen-open, NAAG-bound, and inhibitor-bound. Our structures show that a positively charged cytosol-open vestibule accommodates either NAAG or the Sialin inhibitor Fmoc-Leu-OH, while its luminal cavity potentially binds sialic acid. Moreover, functional analyses along with molecular dynamics simulations identify key residues in binding sialic acid and NAAG. Thus, our findings uncover the essential conformational states in NAAG and sialic acid transport, demonstrating a working model of SLC17 transporters.

Inflammation-induced sialin mediates nitrate efflux in dysfunctional endothelium affecting NO bioavailability

AIM

The mechanism of NO bioavailability in endothelial dysfunction, the trigger for atherogenesis is still unclear as exogenous nitrate therapy fails to alleviate endothelial dysfunction. Recently, sialin, a nitrate transporter, has been linked to affect tissue nitrate/nitrite levels. Hence, we investigated the role of sialin in NO bioavailability in endothelial dysfunction.

METHODS

Serum-starved HUVECs were stimulated with either TNFα or AT-2 for 24 h either alone or in the presence of autophagy inducer or autophagy inhibitor alone. Nitric oxide, nitrite, and nitrate levels were measured in cell supernatant and cell lysate. Quantitative real-time PCR, Annexin V-PI, and monocyte adhesion assays were performed. Immunofluorescence staining for sialin, vWF, and LC3 was performed. STRING database was used to create protein interacting partners for sialin.

RESULTS

Sialin is strongly expressed in activated EC in vitro and atherosclerotic plaque as well as tumor neo-vessel ECs. Sialin mediates nitrate ion efflux and is negatively regulated by autophagy via mTOR pathway. Blocking sialin enhances NO bioavailability, autophagy, cell survival, and eNOS expression while decreasing monocyte adhesion. PPI shows LGALS8 to directly interact with sialin and regulate autophagy, cell-cell adhesion, and apoptosis.

CONCLUSION

Sialin is a potential novel therapeutic target for treating endothelial dysfunction in atherosclerosis and cancer.

Rare forms of hypomyelination and delayed myelination

Hypomyelination is defined by the evidence of an unchanged pattern of deficient myelination on two MRIs performed at least 6 months apart in a child older than 1 year. When the temporal criteria are not fulfilled, and the follow-up MRI shows a progression of the myelination even if still not adequate for age, hypomyelination is excluded and the pattern is instead consistent with delayed myelination. This can be mild and nonspecific in some cases, while in other cases there is a severe delay that in the first disease stages could be difficult to differentiate from hypomyelination. In hypomyelinating leukodystrophies, hypomyelination is due to a primary impairment of myelin deposition, such as in Pelizaeus Merzabcher disease. Conversely, myelin lack is secondary, often to primary neuronal disorders, in delayed myelination and some condition with hypomyelination. Overall, the group of inherited white matter disorders with abnormal myelination has expanded significantly during the past 20 years. Many of these disorders have only recently been described, for many of them only a few patients have been reported and this contributes to make challenging the diagnostic process and the interpretation of Next Generation Sequencing results. In this chapter, we review the clinical and radiologic features of rare and lesser known forms of hypomyelination and delayed myelination not mentioned in other chapters of this handbook.

Involvement of proton-coupled SLC49A4-mediated transport in the export of lysosomally trapped pyrilamine

Our recent study revealed that SLC49A4, known as disrupted in renal carcinoma 2, is a H+-coupled lysosomal exporter for pyridoxine (vitamin B6), a cationic compound, and involved in the regulation of its lysosomal and cellular levels. We here examined a possibility that this transporter might also transport cationic amphiphilic drugs (CADs) that are known to undergo lysosomal trapping, using pyrilamine, an H1-antagonist, as a model CAD and the COS-7 cell line as a model cell system for transient introduction of human SLC49A4 and a recombinant SLC49A4 protein (SLC49A4-AA), in which the N-terminal dileucine motif involved in lysosomal localization was removed by replacing with dialanine for redirected localization to the plasma membrane. The introduction of SLC49A4 into COS-7 cells induced a significant decrease in the accumulation of pyrilamine in the intracellular compartments in the cells treated with digitonin for permeabilization of plasma membranes, suggesting its operation for lysosomal pyrilamine export. Accordingly, functional analysis using the SLC49A4-AA mutant, which operates for cellular uptake at the plasma membrane, in transiently transfected COS-7 cells demonstrated its H+-coupled operation for pyrilamine transport, which was saturable with a Michaelis constant of 132 μM at pH 5.5. In addition, many CADs that may potentially undergo lysosomal trapping, which include imipramine, propranolol, verapamil, and some others, were found to inhibit SLC49A4-AA-mediated pyrilamine transport, suggesting their affinity for SLC49A4. These results suggest that SLC49A4 is involved in the lysosomal trapping of pyrilamine, operating for its exit. The CADs that inhibited SLC49A4-AA-mediated pyrilamine transport could also be SLC49A4 substrate candidates. Significance Statement SLC49A4 mediates the transport of pyrilamine in a H+-coupled manner at the lysosomal membrane. This could be a newly identified mechanism for lysosomal export involved in its lysosomal trapping.

Longitudinal Characterization of the Clinical Course of Intermediate-Severe Salla Disease

BACKGROUND

Biallelic pathogenic variants in SLC17A5 cause three forms of free sialic acid storage disease categorized based on severity from least to most severe: Salla disease, intermediate-severe Salla disease, and infantile free sialic acid storage disease. Intermediate-severe Salla disease is the most recently described form. Here, we report a longitudinal characterization of intermediate-severe Salla disease progression in two sisters carrying the following biallelic variants in SLC17A5: c.406A>G (p.Lys136Glu) and c.819+1G>A.

METHODS

A retrospective review of medical records was performed. A developmental questionnaire was completed to obtain further clinical information. For functional characterization of the predicted splice site variant, RNA was extracted from patient blood samples and sequenced.

RESULTS

Disease onset occurred within the first six months of life in both patients. Early childhood development was delayed with achievement of some milestones followed by a developmental plateau in late childhood. After this, both patients began a slow and progressive neurological regression in adolescence. Functional studies confirmed the pathogenicity of the c.819+1G>A variant, resulting in a frameshift and deletion of exon 6.

CONCLUSIONS

We present a detailed study describing the clinical course of intermediate-severe Salla disease with over 15 to 20 years of evolution and demonstrate the pathogenicity of the c.819+1G>A splice site variant.

Genetic differentiation and local adaptation of the Japanese honeybee, Apis cerana japonica

We examine the population genetic structure and divergence among the regional populations of the Japanese honeybee, Apis cerana japonica, by re-sequencing the genomes of 105 individuals from the three main Japanese islands with diverse climates. The genetic structure results indicated that these individuals are distinct from the mainland Chinese A. cerana samples. Furthermore, population structure analyses have identified three genetically distinct geographic regions in Japan: Northern (Tohoku-Kanto-Chubu districts), Central (Chugoku district), and Southern (Kyushu district). In some districts, "possible non-native" individuals, likely introduced from other regions in recent years, were discovered. Then, genome-wide scans were conducted to detect candidate genes for adaptation by two different approaches. We performed a population branch statistics (PBS) analysis to identify candidate genes for population-specific divergence. A latent factor mixed model (LFMM) was used to identify genes associated with climatic variables along a geographic gradient. The PBSmax analysis identified 25 candidate genes for population-specific divergence whereas the LFMM analysis identified 73 candidate genes for adaptation to climatic variables along a geographic gradient. However, no common genes were identified by both methods.

Substrate recognition and proton coupling by a bacterial member of solute carrier family 17

The solute carrier 17 family transports diverse organic anions using two distinct modes of coupling to a source of energy. Transporters that package glutamate and nucleotide into secretory vesicles for regulated release by exocytosis are driven by membrane potential but subject to allosteric regulation by H+ and Cl-. Other solute carrier 17 members including the lysosomal sialic acid exporter couple the flux of organic anion to cotransport of H+. To begin to understand how similar proteins can perform such different functions, we have studied Escherichia coli DgoT, a H+/galactonate cotransporter. A recent structure of DgoT showed many residues contacting D-galactonate, and we now find that they do not tolerate even conservative substitutions. In contrast, the closely related lysosomal H+/sialic acid cotransporter Sialin tolerates similar mutations, consistent with its recognition of diverse substrates with relatively low affinity. We also find that despite coupling to H+, DgoT transports more rapidly but with lower apparent affinity at high pH. Indeed, membrane potential can drive uptake, indicating electrogenic transport and suggesting a H+:galactonate stoichiometry >1. Located in a polar pocket of the N-terminal helical bundle, Asp46 and Glu133 are each required for net flux by DgoT, but the E133Q mutant exhibits robust exchange activity and rescues exchange by D46N, suggesting that these two residues operate in series to translocate protons. E133Q also shifts the pH sensitivity of exchange by DgoT, supporting a central role for the highly conserved TM4 glutamate in H+ coupling by DgoT.

Disrupted in renal carcinoma 2 (DIRC2/SLC49A4) is an H+-driven lysosomal pyridoxine exporter

Disrupted in renal carcinoma 2 (DIRC2) has gained interest because of its association with the development of renal cancer and cosegregation with a chromosomal translocation. It is a member of the SLC49 family (SLC49A4) and is considered to be an electrogenic lysosomal metabolite transporter; however, its molecular function has not been fully defined. To perform a detailed functional analysis of human DIRC2, we used a recombinant DIRC2 protein (DIRC2-AA), in which the N-terminal dileucine motif involved in its lysosomal localization was removed by replacing with dialanine for redirected localization to the plasma membrane, exposing intralysosomal segments to the extracellular space. The DIRC2-AA mutant induced the cellular uptake of pyridoxine (vitamin B6) under acidic conditions when expressed transiently in COS-7 cells. In addition, uptake was markedly inhibited by protonophores, indicating its function through an H+-coupled mechanism. In separate experiments, the transient overexpression of unmodified DIRC2 (tagged with HA) in human embryonic kidney 293 cells reduced cellular pyridoxine accumulation induced by transiently introduced human thiamine transporter 2/SLC19A3 (tagged with FLAG), a plasma membrane thiamine transporter that also transports pyridoxine. The cellular accumulation of pyridoxine in Caco-2 cells as a cell model was increased by the knockdown of endogenous DIRC2. Overall, the results indicate that DIRC2 is an H+-driven lysosomal pyridoxine exporter. Its overexpression leads to a reduction in cellular pyridoxine accumulation associated with reduced lysosomal accumulation and, conversely, its suppression results in an increase in lysosomal and cellular pyridoxine accumulation.

The molecular mechanism of sialic acid transport mediated by Sialin

Malfunction of the sialic acid transporter caused by various genetic mutations in the SLC17A5 gene encoding Sialin leads to a spectrum of neurodegenerative conditions called free sialic acid storage disorders. Unfortunately, how Sialin transports sialic acid/proton (H+) and how pathogenic mutations impair its function are poorly defined. Here, we present the structure of human Sialin in an inward-facing partially open conformation determined by cryo-electron microscopy, representing the first high-resolution structure of any human SLC17 member. Our analysis reveals two unique features in Sialin: (i) The H+ coupling/sensing requires two highly conserved Glu residues (E171 and E175) instead of one (E175) as implied in previous studies; and (ii) the normal function of Sialin requires the stabilization of a cytosolic helix, which has not been noticed in the literature. By mapping known pathogenic mutations, we provide mechanistic explanations for corresponding functional defects. We propose a structure-based mechanism for sialic acid transport mediated by Sialin.

Spns1 is a lysophospholipid transporter mediating lysosomal phospholipid salvage

The lysosome is central to the degradation of proteins, carbohydrates, and lipids and their salvage back to the cytosol for reutilization. Lysosomal transporters for amino acids, sugars, and cholesterol have been identified, and the metabolic fates of these molecules in the cytoplasm have been elucidated. Remarkably, it is not known whether lysosomal salvage exists for glycerophospholipids, the major constituents of cellular membranes. By using a transport assay screen against orphan lysosomal transporters, we identified the major facilitator superfamily protein Spns1 that is ubiquitously expressed in all tissues as a proton-dependent lysophosphatidylcholine (LPC) and lysophosphatidylethanolamine (LPE) transporter, with LPC and LPE being the lysosomal breakdown products of the most abundant eukaryotic phospholipids, phosphatidylcholine and phosphatidylethanolamine, respectively. Spns1 deficiency in cells, zebrafish embryos, and mouse liver resulted in lysosomal accumulation of LPC and LPE species with pathological consequences on lysosomal function. Flux analysis using stable isotope-labeled phospholipid apolipoprotein E nanodiscs targeted to lysosomes showed that LPC was transported out of lysosomes in an Spns1-dependent manner and re-esterified back into the cytoplasmic pools of phosphatidylcholine. Our findings identify a phospholipid salvage pathway from lysosomes to the cytosol that is dependent on Spns1 and critical for maintaining normal lysosomal function.

Diversity of function and mechanism in a family of organic anion transporters

Originally identified as transporters for inorganic phosphate, solute carrier 17 (SLC17) family proteins subserve diverse physiological roles. The vesicular glutamate transporters (VGLUTs) package the principal excitatory neurotransmitter glutamate into synaptic vesicles (SVs). In contrast, the closely related sialic acid transporter sialin mediates the flux of sialic acid in the opposite direction, from lysosomes to the cytoplasm. The two proteins couple in different ways to the H⁺ electrochemical gradient driving force, and high-resolution structures of the Escherichia coli homolog d-galactonate transporter (DgoT) and more recently rat VGLUT2 now begin to suggest the mechanisms involved as well as the basis for substrate specificity.

SLC46A3 is a lysosomal proton-coupled steroid conjugate and bile acid transporter involved in transport of active catabolites of T-DM1

Antibody-drug conjugates (ADCs) represent a new class of cancer therapeutics that enable targeted delivery of cytotoxic drugs to cancer cells. Although clinical efficacy has been demonstrated for ADC therapies, resistance to these conjugates may occur. Recently, SLC46A3, a lysosomal membrane protein, was revealed to regulate the efficacy of trastuzumab emtansine (T-DM1), a noncleavable ADC that has been widely used for treating breast cancer. However, the role of SLC46A3 in mediating T-DM1 cytotoxicity remains unclear. In this study, we discovered the function of SLC46A3 as a novel proton-coupled steroid conjugate and bile acid transporter. SLC46A3 preferentially recognized lipophilic steroid conjugates and bile acids as endogenous substrates. In addition, we found that SLC46A3 directly transports Lys-SMCC-DM1, a major catabolite of T-DM1, and potent SLC46A3 inhibitors attenuate the cytotoxic effects of T-DM1, suggesting a role in the escape of Lys-SMCC-DM1 from the lysosome into the cytoplasm. Our findings reveal the molecular mechanism by which T-DM1 kills cancer cells and may contribute to the rational development of ADCs that target SLC46A3.

Sialin mediates submandibular gland regeneration ability by affecting polysialic acid synthesis

OBJECTIVES

Sialin is a multifunctional molecule with a well-described role in physiological equilibrium regulation. The aim of this study was to elucidate the role of sialin in salivary glands regeneration.

MATERIALS AND METHODS

Submandibular gland duct ligation/deligation of rat was performed to develop a rat model of submandibular gland regeneration. Phenotype changes were investigated using western blotting and quantitative real-time polymerase chain reaction, as well as immunohistochemical staining. LV-slc17a5-RNAi vectors were injected into the submandibular glands via retroductal instillation to establish a stable sialin knockdown model.

RESULTS

Submandibular gland tissue structure could completely restore 28 days after duct deligation, when the duct had been ligated for 7 days. The expression of sialin, polysialic acid, and polysialyltransferase IV was significantly increased on day 0 after duct deligation, and it returned to the level of the control group at day 28. Moreover, sialin knockdown could weakened gland regeneration by reducing polysialic acid synthesis. Supplementing drinking water with polysialic acid precursors (ManNAc) in drinking water could partially rescue submandibular gland regeneration in sialin knockdown rats.

CONCLUSION

These data indicated that sialin was vital for submandibular gland regeneration which mediated the process of gland regeneration by affecting the polysialic acid synthesis.

Arginine-selective modulation of the lysosomal transporter PQLC2 through a gate-tuning mechanism

Lysosomes degrade and recycle cell components and integrate environmental and intracellular cues to regulate cell growth, metabolism, and autophagy. The lysosomal transporter PQLC2 exports cationic amino acids from lysosomes, and under amino acid starvation, it recruits to lysosomes a signaling complex implicated in neurological diseases. In this study, we show that PQLC2 transport activity is uncoupled from the lysosomal pH gradient and other ion gradients and that it is selectively modulated by arginine through a trans-inhibition mechanism. Kinetic modeling suggests that arginine accelerates the closing of its cytosolic gate. We propose a signaling model in which PQLC2 transduces the nutrient status to its cognate complex through opposing effects of lysosomal membrane potential and cytosolic arginine on its conformational state.

Lysosomes degrade excess or damaged cellular components and recycle their building blocks through membrane transporters. They also act as nutrient-sensing signaling hubs to coordinate cell responses. The membrane protein PQ-loop repeat-containing protein 2 (PQLC2; “picklock two”) is implicated in both functions, as it exports cationic amino acids from lysosomes and serves as a receptor and amino acid sensor to recruit the C9orf72/SMCR8/WDR41 complex to lysosomes upon nutrient starvation. Its transport activity is essential for drug treatment of the rare disease cystinosis. Here, we quantitatively studied PQLC2 transport activity using electrophysiological and biochemical methods. Charge/substrate ratio, intracellular pH, and reversal potential measurements showed that it operates in a uniporter mode. Thus, PQLC2 is uncoupled from the steep lysosomal proton gradient, unlike many lysosomal transporters, enabling bidirectional cationic amino acid transport across the organelle membrane. Surprisingly, the specific presence of arginine, but not other substrates (lysine, histidine), in the discharge (“trans”) compartment impaired PQLC2 transport. Kinetic modeling of the uniport cycle recapitulated the paradoxical substrate-yet-inhibitor behavior of arginine, assuming that bound arginine facilitates closing of the transporter’s cytosolic gate. Arginine binding may thus tune PQLC2 gating to control its conformation, suggesting a potential mechanism for nutrient signaling by PQLC2 to its interaction partners.

Clinical Development of Cell Therapies to Halt Lysosomal Storage Diseases: Results and Lessons Learned

Although cross-correction was discovered more than 50 years ago, and held the promise of drastically improving disease management, still no cure exists for lysosomal storage diseases (LSDs). Cell therapies hold the potential to halt disease progression: either a subset of autologous cells can be ex vivo/ in vivo transfected with the functional gene or allogenic wild type stem cells can be transplanted. However, majority of cell-based attempts have been ineffective, due to the difficulties in reversing neuronal symptomatology, in finding appropriate gene transfection approaches, in inducing immune tolerance, reducing the risk of graft versus host disease (GVHD) when allogenic cells are used and that of immune response when engineered viruses are administered, coupled with a limited secretion and uptake of some enzymes. In the last decade, due to advances in our understanding of lysosomal biology and mechanisms of cross-correction, coupled with progresses in gene therapy, ongoing pre-clinical and clinical investigations have remarkably increased. Even gene editing approaches are currently under clinical experimentation. This review proposes to critically discuss and compare trends and advances in cell-based and gene therapy for LSDs. Systemic gene delivery and transplantation of allogenic stem cells will be initially discussed, whereas proposed brain targeting methods will be then critically outlined.

Free sialic acid storage disorder: Progress and promise

Lysosomal free sialic acid storage disorder (FSASD) is an extremely rare, autosomal recessive, neurodegenerative, multisystemic disorder caused by defects in the lysosomal sialic acid membrane exporter SLC17A5 (sialin). SLC17A5 defects cause free sialic acid and some other acidic hexoses to accumulate in lysosomes, resulting in enlarged lysosomes in some cell types and 10-100-fold increased urinary excretion of free sialic acid. Clinical features of FSASD include coarse facial features, organomegaly, and progressive neurodegenerative symptoms with cognitive impairment, cerebellar ataxia and muscular hypotonia. Central hypomyelination with cerebellar atrophy and thinning of the corpus callosum are also prominent disease features. Around 200 FSASD cases are reported worldwide, with the clinical spectrum ranging from a severe infantile onset form, often lethal in early childhood, to a mild, less severe form with subjects living into adulthood, also called Salla disease. The pathobiology of FSASD remains poorly understood and FSASD is likely underdiagnosed. Known patients have experienced a diagnostic delay due to the rarity of the disorder, absence of routine urine sialic acid testing, and non-specific clinical symptoms, including developmental delay, ataxia and infantile hypomyelination. There is no approved therapy for FSASD. We initiated a multidisciplinary collaborative effort involving worldwide academic clinical and scientific FSASD experts, the National Institutes of Health (USA), and the FSASD patient advocacy group (Salla Treatment and Research [S.T.A.R.] Foundation) to overcome the scientific, clinical and financial challenges facing the development of new treatments for FSASD. We aim to collect data that incentivize industry to further develop, obtain approval for, and commercialize FSASD treatments. This review summarizes current aspects of FSASD diagnosis, prevalence, etiology, and disease models, as well as challenges on the path to therapeutic approaches for FSASD.

Functional and Biochemical Consequences of Disease Variants in Neurotransmitter Transporters: A Special Emphasis on Folding and Trafficking Deficits

Neurotransmitters, such as γ-aminobutyric acid, glutamate, acetyl choline, glycine and the monoamines, facilitate the crosstalk within the central nervous system. The designated neurotransmitter transporters (NTTs) both release and take up neurotransmitters to and from the synaptic cleft. NTT dysfunction can lead to severe pathophysiological consequences, e.g. epilepsy, intellectual disability, or Parkinson’s disease. Genetic point mutations in NTTs have recently been associated with the onset of various neurological disorders. Some of these mutations trigger folding defects in the NTT proteins. Correct folding is a prerequisite for the export of NTTs from the endoplasmic reticulum (ER) and the subsequent trafficking to their pertinent site of action, typically at the plasma membrane. Recent studies have uncovered some of the key features in the molecular machinery responsible for transporter protein folding, e.g., the role of heat shock proteins in fine-tuning the ER quality control mechanisms in cells. The therapeutic significance of understanding these events is apparent from the rising number of reports, which directly link different pathological conditions to NTT misfolding. For instance, folding-deficient variants of the human transporters for dopamine or GABA lead to infantile parkinsonism/dystonia and epilepsy, respectively. From a therapeutic point of view, some folding-deficient NTTs are amenable to functional rescue by small molecules, known as chemical and pharmacological chaperones.

Inherited disorders of lysosomal membrane transporters

Disorders caused by defects in lysosomal membrane transporters form a distinct subgroup of lysosomal storage disorders (LSDs). To date, defects in only 10 lysosomal membrane transporters have been associated with inherited disorders. The clinical presentations of these diseases resemble the phenotypes of other LSDs; they are heterogeneous and often present in children with neurodegenerative manifestations. However, for pathomechanistic and therapeutic studies, lysosomal membrane transport defects should be distinguished from LSDs caused by defective hydrolytic enzymes. The involved proteins differ in function, localization, and lysosomal targeting, and the diseases themselves differ in their stored material and therapeutic approaches. We provide an overview of the small group of disorders of lysosomal membrane transporters, emphasizing discovery, pathomechanism, clinical features, diagnostic methods and therapeutic aspects. We discuss common aspects of lysosomal membrane transporter defects that can provide the basis for preclinical research into these disorders.

Amino Acids Bearing Aromatic or Heteroaromatic Substituents as a New Class of Ligands for the Lysosomal Sialic Acid Transporter Sialin

Sialin, encoded by the SLC17A5 gene, is a lysosomal sialic acid transporter defective in Salla disease, a rare inherited leukodystrophy. It also enables metabolic incorporation of exogenous sialic acids, leading to autoantibodies against N-glycolylneuraminic acid in humans. Here, we identified a novel class of human sialin ligands by virtual screening and structure-activity relationship studies. The ligand scaffold is characterized by an amino acid backbone with a free carboxylate, an N-linked aromatic or heteroaromatic substituent, and a hydrophobic side chain. The most potent compound, 45 (LSP12-3129), inhibited N-acetylneuraminic acid 1 (Neu5Ac) transport in a non-competitive manner with IC₅₀ ≈ 2.5 μM, a value 400-fold lower than the K_M for Neu5Ac. In vitro and molecular docking studies attributed the non-competitive character to selective inhibitor binding to the Neu5Ac site in a cytosol-facing conformation. Moreover, compound 45 rescued the trafficking defect of the pathogenic mutant (R39C) causing Salla disease. This new class of cell-permeant inhibitors provides tools to investigate the physiological roles of sialin and help develop pharmacological chaperones for Salla disease.

Ion transport and regulation in a synaptic vesicle glutamate transporter

Synaptic vesicles accumulate neurotransmitters, enabling the quantal release by exocytosis that underlies synaptic transmission. Specific neurotransmitter transporters are responsible for this activity and therefore are essential for brain function. The vesicular glutamate transporters (VGLUTs) concentrate the principal excitatory neurotransmitter glutamate into synaptic vesicles, driven by membrane potential. However, the mechanism by which they do so remains poorly understood owing to a lack of structural information. We report the cryo-electron microscopy structure of rat VGLUT2 at 3.8-angstrom resolution and propose structure-based mechanisms for substrate recognition and allosteric activation by low pH and chloride. A potential permeation pathway for chloride intersects with the glutamate binding site. These results demonstrate how the activity of VGLUTs can be coordinated with large shifts in proton and chloride concentrations during the synaptic vesicle cycle to ensure normal synaptic transmission.

Molecular, Structural, Functional, and Pharmacological Sites for Vesicular Glutamate Transporter Regulation

Vesicular glutamate transporters (VGLUTs) control quantal size of glutamatergic transmission and have been the center of numerous studies over the past two decades. VGLUTs contain two independent transport modes that facilitate glutamate packaging into synaptic vesicles and phosphate (Pi) ion transport into the synaptic terminal. While a transmembrane proton electrical gradient established by a vacuolar-type ATPase powers vesicular glutamate transport, recent studies indicate that binding sites and flux properties for chloride, potassium, and protons within VGLUTs themselves regulate VGLUT activity as well. These intrinsic ionic binding and flux properties of VGLUTs can therefore be modulated by neurophysiological conditions to affect levels of glutamate available for release from synapses. Despite their extraordinary importance, specific and high-affinity pharmacological compounds that interact with these sites and regulate VGLUT function, distinguish between the various modes of transport, and the different isoforms themselves, are lacking. In this review, we provide an overview of the physiologic sites for VGLUT regulation that could modulate glutamate release in an over-active synapse or in a disease state.

The mechanism and regulation of vesicular glutamate transport: Coordination with the synaptic vesicle cycle

The transport of classical neurotransmitters into synaptic vesicles generally relies on a H⁺ electrochemical gradient (∆μ_H+). Synaptic vesicle uptake of glutamate depends primarily on the electrical component ∆ψ as the driving force, rather than the chemical component ∆pH. However, the vesicular glutamate transporters (VGLUTs) belong to the solute carrier 17 (SLC17) family, which includes closely related members that function as H⁺ cotransporters. Recent work has also shown that the VGLUTs undergo allosteric regulation by H⁺ and Cl^-, and exhibit an associated Cl^- conductance. These properties appear to coordinate VGLUT activity with the large ionic shifts that accompany the rapid recycling of synaptic vesicles driven by neural activity. Recent structural information also suggests common mechanisms that underlie the apparently divergent function of SLC17 family members, and that confer allosteric regulation.

A comparative synthesis of transcriptomic analyses reveals major differences between WSSV-susceptible Litopenaeus vannamei and WSSV-refractory Macrobrachium rosenbergii

Since the 1990s White Spot Syndrome Virus (WSSV) has severely affected shrimp aquaculture worldwide causing a global pandemic of White Spot Disease (WSD) in penaeid culture. However, not all decapod species that can be infected by WSSV show the same susceptibility to the virus, thus raising interesting questions regarding the potential genetic traits that might confer resistance to WSSV. In order to shed light into the genetic markers of WSSV resistance, we employed a dual approach: i) we initially analysed the transcriptomes derived from the hepatopancreas of two species, the susceptible white shrimp Litopenaeus vannamei and the refractory fresh water prawn Macrobrachium rosenbergii, both infected with WSSV. We found a large number of differentially expressed genes (DEGs) belonging to the immune system (mostly anti-microbial peptides and haemolymph clotting components) that were generally up-regulated in M. rosenbergii and down-regulated in L. vannamei. Further, in both species we identified many up-regulated DEGs that were related to metabolism (suggesting a metabolic shift during the infection) and, interestingly, in L. vannamei only, we found several DEGs that were related to moult and suggested an inhibition of the moult cycle in this species following WSSV infection. ii) we then identified a limited number of genetic markers putatively linked with WSD tolerance by employing an ecological genomics approach in which we compared published reports with our own RNA-seq datasets for different decapod species infected with WSSV. Using this second comparative approach, we found nine candidate genes which are consistently down-regulated in susceptible species and up-regulated in refractory species and which have a role in immune response. Together our data offer novel insights into gene expression differences that can be found in susceptible and refractory decapod species infected with WSSV and provide a valuable resource towards our understanding of the potential genetic basis of tolerance to WSSV.

Structures suggest a mechanism for energy coupling by a family of organic anion transporters

Members of the solute carrier 17 (SLC17) family use divergent mechanisms to concentrate organic anions. Membrane potential drives uptake of the principal excitatory neurotransmitter glutamate into synaptic vesicles, whereas closely related proteins use proton cotransport to drive efflux from the lysosome. To delineate the divergent features of ionic coupling by the SLC17 family, we determined the structure of Escherichia coli D-galactonate/H⁺ symporter D-galactonate transporter (DgoT) in 2 states: one open to the cytoplasmic side and the other open to the periplasmic side with substrate bound. The structures suggest a mechanism that couples H⁺ flux to substrate recognition. A transition in the role of H⁺ from flux coupling to allostery may confer regulation by trafficking to and from the plasma membrane.

The first structures of a family of organic anion transporters reveal an interaction with protons that is conserved from bacterial transporters to the mammalian proteins that transport glutamate into synaptic vesicles.

Chemical glycomics enrichment: imaging the recycling of sialic acid in living cells

The development of metabolic oligosaccharide engineering (MOE) over the past two decades enabled the bioimaging studies of glycosylation processes in physio-pathological contexts. Herein, we successfully applied the chemical reporter strategy to image the fate of sialylated glycoconjugates in healthy and sialin-deficient patient fibroblasts. This chemical glycomics enrichment is a powerful tool for tracking sialylated glycoconjugates and probing lysosomal recycling capacities. Thus, such strategies appear fundamental for the characterization of lysosomal storage diseases.

Biochemical and molecular analyses of infantile sialic acid storage disease in a patient with nonimmune hydrops fetalis

Nonimmune hydrops fetalis is the most severe clinical manifestation of lysosomal storage diseases (LSDs). Around 14 different LSDs have been accounted for as 1-15% of the cause of nonimmune hydrops fetalis. We report a Korean infant affected by an extremely rare but severe form of sialic acid storage disease. The patient presented with nonimmune hydrops fetalis, dysmorphic facial features, hepatosplenomegaly, and dysostosis multiplex and died at 39 days of age due to persistent pulmonary hypertension. LSD was suspected based on the presence of diffuse vacuolation of syncytiotrophoblast, villous stromal cells, and intermediate trophoblast in placental biopsy. Increased excretion of urinary free sialic acid was detected by liquid chromatography-tandem mass spectrometry. The patient was compound heterozygous of the c.908G>A (p.Trp303Ter) and the splicing mutation c.1259+5G>T (IVS9+5 G>T) in the SLC17A5 gene.

SNAT7 is the primary lysosomal glutamine exporter required for extracellular protein-dependent growth of cancer cells

Lysosomes degrade cellular components sequestered by autophagy or extracellular material internalized by endocytosis and phagocytosis. The macromolecule building blocks released by lysosomal hydrolysis are then exported to the cytosol by lysosomal transporters, which remain undercharacterized. In this study, we designed an in situ assay of lysosomal amino acid export based on the transcription factor EB (TFEB), a master regulator of lysosomal biogenesis that detects lysosomal storage. This assay was used to screen candidate lysosomal transporters, leading to the identification of sodium-coupled neutral amino acid transporter 7 (SNAT7), encoded by the SLC38A7 gene, as a lysosomal transporter highly selective for glutamine and asparagine. Cell fractionation confirmed the lysosomal localization of SNAT7, and flux measurements confirmed its substrate selectivity and showed a strong activation by the lysosomal pH gradient. Interestingly, gene silencing or editing experiments revealed that SNAT7 is the primary permeation pathway for glutamine across the lysosomal membrane and it is required for growth of cancer cells in a low free-glutamine environment, when macropinocytosis and lysosomal degradation of extracellular proteins are used as an alternative source of amino acids. SNAT7 may, thus, represent a novel target for glutamine-related anticancer therapies.

Identification of a large intronic transposal insertion in SLC17A5 causing sialic acid storage disease

BACKGROUND

Sialic acid storage diseases are neurodegenerative disorders characterized by accumulation of sialic acid in the lysosome. These disorders are caused by mutations in SLC17A5, the gene encoding sialin, a sialic acid transporter located in the lysosomal membrane. The most common form of sialic acid storage disease is the slowly progressive Salla disease, presenting with hypotonia, ataxia, epilepsy, nystagmus and findings of cerebral and cerebellar atrophy. Hypomyelination and corpus callosum hypoplasia are typical as well. We report a 16 year-old boy with an atypically mild clinical phenotype of sialic acid storage disease characterized by psychomotor retardation and a mixture of spasticity and rigidity but no ataxia, and only weak features of hypomyelination and thinning of corpus callosum on MRI of the brain.

RESULTS

The thiobarbituric acid method showed elevated levels of free sialic acid in urine and fibroblasts, indicating sialic acid storage disease. Initial Sanger sequencing of SLC17A5 coding regions did not show any pathogenic variants, although exon 9 could not be sequenced. Whole exome sequencing followed by RNA and genomic DNA analysis identified a homozygous 6040 bp insertion in intron 9 of SLC17A5 corresponding to a long interspersed element-1 retrotransposon (KF425758.1). This insertion adds two splice sites, both resulting in a frameshift which in turn creates a premature stop codon 4 bp into intron 9.

CONCLUSIONS

This study describes a novel pathogenic variant in SLC17A5, namely an intronic transposal insertion, in a patient with mild biochemical and clinical phenotypes. The presence of a small fraction of normal transcript may explain the mild phenotype. This case illustrates the importance of including lysosomal sialic acid storage disease in the differential diagnosis of developmental delay with postnatal onset and hypomyelination, as well as intronic regions in the genetic investigation of inborn errors of metabolism.

Protons Regulate Vesicular Glutamate Transporters through an Allosteric Mechanism

The quantal nature of synaptic transmission requires a mechanism to transport neurotransmitter into synaptic vesicles without promoting non-vesicular efflux across the plasma membrane. Indeed, the vesicular transport of most classical transmitters involves a mechanism of H(+) exchange, which restricts flux to acidic membranes such as synaptic vesicles. However, vesicular transport of the principal excitatory transmitter glutamate depends primarily on membrane potential, which would drive non-vesicular efflux, and the role of protons is unclear. Adapting electrophysiology to record currents associated with the vesicular glutamate transporters (VGLUTs), we characterize a chloride conductance that is gated by lumenal protons and chloride and supports glutamate uptake. Rather than coupling stoichiometrically to glutamate flux, lumenal protons and chloride allosterically activate vesicular glutamate transport. Gating by protons serves to inhibit what would otherwise be substantial non-vesicular glutamate efflux at the plasma membrane, thereby restricting VGLUT activity to synaptic vesicles.

A sequential bioorthogonal dual strategy: ManNAl and SiaNAl as distinct tools to unravel sialic acid metabolic pathways

A new sequential orthogonal dual strategy to unravel the intracellular trafficking and cellular uptake mechanism of sialic acid.

Structure, Function, and Drug Interactions of Neurotransmitter Transporters in the Postgenomic Era

Vesicular neurotransmitter transporters are responsible for the accumulation of neurotransmitters in secretory vesicles and play essential roles in chemical transmission. The SLC17 family contributes to sequestration of anionic neurotransmitters such as glutamate, aspartate, and nucleotides. Identification and subsequent cellular and molecular biological studies of SLC17 transporters unveiled the principles underlying the actions of these transporters. Recent progress in reconstitution methods in combination with postgenomic approaches has advanced studies on neurotransmitter transporters. This review summarizes the molecular properties of SLC17-type transporters and recent findings regarding the novel SLC18 transporter.

Autophagy regulates the stability of sialin, a lysosomal sialic acid transporter

Macroautophagy plays a critical role in catabolizing cytosolic components via lysosomal degradation. Recent findings from our studies indicate that basal autophagy is required for the efficient lysosomal catabolism of sialyloligosaccharides, and that the downregulation of sialin, a lysosomal transporter of sialic acids can cause a significant delay in the cytosolic accumulation of such glycans. The findings reported herein show that the sialin protein level was increased when the autophagy process was inhibited. This effect appears to be specific to sialin, since the amount of LAMP1, another lysosomal membrane protein, remains constant under the same conditions. Our results suggest that autophagy may regulate the stability of sialin, and it could lead to the cytosolic accumulation of sialyloligosaccharides in autophagy-defective cells.

Vesicular neurotransmitter transporters: an approach for studying transporters with purified proteins

Vesicular storage and subsequent release of neurotransmitters are the key processes of chemical signal transmission. In this process, vesicular neurotransmitter transporters are responsible for loading the signaling molecules. The use of a "clean biochemical" approach with purified, recombinant transporters has helped in the identification of novel vesicular neurotransmitter transporters and in the analysis of the control of signal transmission.

Gene disruption of Mfsd8 in mice provides the first animal model for CLN7 disease

Mutations in the major facilitator superfamily domain containing 8 (MFSD8) gene coding for the lysosomal CLN7 membrane protein result in CLN7 disease, a lysosomal storage disease of childhood. CLN7 disease belongs to a group of inherited disorders, called neuronal ceroid lipofuscinoses (NCL), which are characterized by the accumulation of autofluorescent ceroid lipopigments, neuroinflammation, photoreceptor- and neurodegeneration. We have disrupted the Mfsd8 gene by insertion of a lacZ gene-trap cassette between exons 1 and 2 in mice and have analyzed the impact of Cln7 depletion on neuronal and visceral tissues. Analysis of lacZ reporter gene activity in heterozygous Mfsd8((wt/tm1a)) mice showed strong Mfsd8 mRNA expression in the cerebral cortex, in the hippocampus and in the kidney. Homozygous Mfsd8((tm1a/tm1a)) mice were viable and fertile and resembled biochemically the NCL-phenotype of human CLN7 patients including the accumulation of autofluorescent material in the brain and peripheral tissues and of subunit c of mitochondrial ATP synthase in the cerebellum and nuclei of distinct brain regions, and the degeneration of photoreceptor cells in the retina. Lysosomal storage was found in large neurons of the medulla, the hippocampus and in Purkinje cells of the cerebellum in mutant mice. The ultrastructure of the storage material revealed dense lamellar bodies with irregular forms within cerebellar and hippocampal neurons. In the brain loss of Cln7 was accompanied by mild reactive microgliosis and subtle astrogliosis by 10months of age, respectively. In summary we have generated a mouse model which is partly valuable as some but not all neuropathological features of human CLN7 disease are recapitulated thus representing an animal model to study CLN7-specific disease mechanisms.

Vesicular neurotransmitter transporters: mechanistic aspects

Secondary transporters driven by a V-type H⁺-ATPase accumulate nonpeptide neurotransmitters into synaptic vesicles. Distinct transporter families are involved depending on the neurotransmitter. Monoamines and acetylcholine on the one hand, and glutamate and ATP on the other hand, are accumulated by SLC18 and SLC17 transporters, respectively, which belong to the major facilitator superfamily (MFS). GABA and glycine accumulate through a common SLC32 transporter from the amino acid/polyamine/organocation (APC) superfamily. Although crystallographic structures are not yet available for any vesicular transporter, homology modeling studies of MFS-type vesicular transporters based on distantly related bacterial structures recently provided significant advances, such as the characterization of substrate-binding pockets or the identification of spatial clusters acting as hinge points during the alternating-access cycle. However, several basic issues, such as the ion stoichiometry of vesicular amino acid transporters, remain unsettled.

Synapse location during growth depends on glia location

Synaptic contacts are largely established during embryogenesis and are then maintained during growth. To identify molecules involved in this process, we conducted a forward genetic screen in C. elegans and identified cima-1. In cima-1 mutants, synaptic contacts are correctly established during embryogenesis, but ectopic synapses emerge during postdevelopmental growth. cima-1 encodes a solute carrier in the SLC17 family of transporters that includes sialin, a protein that when mutated in humans results in neurological disorders. cima-1 does not function in neurons but rather functions in the nearby epidermal cells to correctly position glia during postlarval growth. Our findings indicate that CIMA-1 antagonizes the FGF receptor (FGFR), and does so most likely by inhibiting FGFR's role in epidermal-glia adhesion rather than signaling. Our data suggest that epidermal-glia crosstalk, in this case mediated by a transporter and the FGF receptor, is vital to preserve embryonically derived circuit architecture during postdevelopmental growth.

Vesicular uptake of N-acetylaspartylglutamate is catalysed by sialin (SLC17A5)

NAAG (N-acetylaspartylglutamate) is an abundant neuropeptide in the vertebrate nervous system. It is released from synaptic terminals in a calcium-dependent manner and has been shown to act as an agonist at the type II metabotropic glutamate receptor mGluR3. It has been proposed that NAAG may also be released from axons. So far, however, it has remained unclear how NAAG is transported into synaptic or other vesicles before it is secreted. In the present study, we demonstrate that uptake of NAAG and the related peptide NAAG2 (N-acetylaspartylglutamylglutamate) into vesicles depends on the sialic acid transporter sialin (SLC17A5). This was demonstrated using cell lines expressing a cell surface variant of sialin and by functional reconstitution of sialin in liposomes. NAAG uptake into sialin-containing proteoliposomes was detectable in the presence of an active H+-ATPase or valinomycin, indicating that transport is driven by membrane potential rather than H+ gradient. We also show that sialin is most probably the major and possibly only vesicular transporter for NAAG and NAAG2, because ATP-dependent transport of both peptides was not detectable in vesicles isolated from sialin-deficient mice.

SLC17: a functionally diverse family of organic anion transporters

Molecular studies have determined that the SLC17 transporters, a family of nine proteins initially implicated in phosphate transport, mediate the transport of organic anions. While their role in phosphate transport remains uncertain, it is now clear that the transport of organic anions facilitated by this family of proteins is involved in diverse processes ranging from the vesicular storage of the neurotransmitters, to urate metabolism, to the degradation and metabolism of glycoproteins.

Are vesicular neurotransmitter transporters potential treatment targets for temporal lobe epilepsy?

The vesicular neurotransmitter transporters (VNTs) are small proteins responsible for packing synaptic vesicles with neurotransmitters thereby determining the amount of neurotransmitter released per vesicle through fusion in both neurons and glial cells. Each transporter subtype was classically seen as a specific neuronal marker of the respective nerve cells containing that particular neurotransmitter or structurally related neurotransmitters. More recently, however, it has become apparent that common neurotransmitters can also act as co-transmitters, adding complexity to neurotransmitter release and suggesting intriguing roles for VNTs therein. We will first describe the current knowledge on vesicular glutamate transporters (VGLUT1/2/3), the vesicular excitatory amino acid transporter (VEAT), the vesicular nucleotide transporter (VNUT), vesicular monoamine transporters (VMAT1/2), the vesicular acetylcholine transporter (VAChT) and the vesicular γ-aminobutyric acid (GABA) transporter (VGAT) in the brain. We will focus on evidence regarding transgenic mice with disruptions in VNTs in different models of seizures and epilepsy. We will also describe the known alterations and reorganizations in the expression levels of these VNTs in rodent models for temporal lobe epilepsy (TLE) and in human tissue resected for epilepsy surgery. Finally, we will discuss perspectives on opportunities and challenges for VNTs as targets for possible future epilepsy therapies.

Infantile Sialic Acid Storage Disease: Two Unrelated Inuit Cases Homozygous for a Common Novel SLC17A5 Mutation

Infantile sialic acid storage disease (ISSD) is a lysosomal storage disease characterized by accumulation of covalently unlinked (free) sialic acid in multiple tissues. ISSD and Salla disease (a predominantly neurological disorder) are allelic disorders caused by recessive mutations of a lysosomal anionic monosaccharide transporter, SLC17A5. While Salla disease is common in Finland due to a founder-effect mutation (p.Arg39Cys), ISSD is comparatively rare in all populations studied.Here, we describe the clinical and molecular features of two unrelated Canadian Inuit neonates with a virtually identical presentation of ISSD. Both individuals presented antenatally with fetal hydrops, dying shortly following delivery. Urinary free sialic acid excretion was markedly increased in the one case in which urine could be obtained for testing; postmortem examination showed a picture of widespread lysosomal storage in both. Both children were homozygous for a novel splice site mutation (NM_012434:c.526-2A>G) resulting in skipping of exon 4 and an ensuing frameshift. Analysis of a further 129 pan-Arctic Inuit controls demonstrated a heterozygous carrier rate of 1/129 (~0.4 %) in our sample. Interestingly, lysosomal enzyme studies showed an unexplained ninefold increase in neuraminidase activity, with lesser elevations in the activities of several other lysosomal enzymes. Our results raise the possibility of a common founder mutation presenting as hydrops in this population. Furthermore, if confirmed in subsequent cases, the marked induction of neuraminidase activity seen here may prove useful in the clinical diagnosis of ISSD.

In vitro correction of disorders of lysosomal transport by microvesicles derived from baculovirus-infected Spodoptera cells

Infection of Spodoptera frugiperda (Sf9) cells by baculovirus (BV) is well established for transgene expression of soluble proteins, but few correctly folded transmembrane proteins have been so produced. We here report the use of the BV/Sf9 (BVES) method for the expression and transfer, via microvesicles, of the exclusive lysosomal exporters for cystine and sialic acid, human cystinosin and sialin. These proteins and their mRNA are released into the culture medium as very low-density microvesicles (~1.05 g/ml), which do not label for lysobisphosphatidic acid. The presence of the human transgene proteins in the vesicles was confirmed by western blotting and confirmed and quantified by mass spectrometry. Addition of vesicles to cultures of human fibroblast lines deficient in either cystinosin or sialin produced a progressive depletion of stored lysosomal cystine or sialic acid, respectively. The depletion effect was slow (T1/2 ~48 h), saturable (down to ~40% of initial after 4 days) and stable (>one week). Surprisingly, BV infection of Spodoptera appeared to induce expression and release into microvesicles of the insect orthologue of cystinosin, but not of sialin. We conclude that BVES is an effective method to express and transfer functional transmembrane proteins so as to study their properties in mammalian cells, and has a generic potential for transport protein replacement therapy.

Heptahelical protein PQLC2 is a lysosomal cationic amino acid exporter underlying the action of cysteamine in cystinosis therapy

Cystinosin, the lysosomal cystine exporter defective in cystinosis, is the founding member of a family of heptahelical membrane proteins related to bacteriorhodopsin and characterized by a duplicated motif termed the PQ loop. PQ-loop proteins are more frequent in eukaryotes than in prokaryotes; except for cystinosin, their molecular function remains elusive. In this study, we report that three yeast PQ-loop proteins of unknown function, Ypq1, Ypq2, and Ypq3, localize to the vacuolar membrane and are involved in homeostasis of cationic amino acids (CAAs). We also show that PQLC2, a mammalian PQ-loop protein closely related to yeast Ypq proteins, localizes to lysosomes and catalyzes a robust, electrogenic transport that is selective for CAAs and strongly activated at low extracytosolic pH. Heterologous expression of PQLC2 at the yeast vacuole rescues the resistance phenotype of an ypq2 mutant to canavanine, a toxic analog of arginine efficiently transported by PQLC2. Finally, PQLC2 transports a lysine-like mixed disulfide that serves as a chemical intermediate in cysteamine therapy of cystinosis, and PQLC2 gene silencing trapped this intermediate in cystinotic cells. We conclude that PQLC2 and Ypq1-3 proteins are lysosomal/vacuolar exporters of CAAs and suggest that small-molecule transport is a conserved feature of the PQ-loop protein family, in agreement with its distant similarity to SWEET sugar transporters and to the mitochondrial pyruvate carrier. The elucidation of PQLC2 function may help improve cysteamine therapy. It may also clarify the origin of CAA abnormalities in Batten disease.

Sialin (SLC17A5) functions as a nitrate transporter in the plasma membrane

In vivo recycling of nitrate (NO(3)(-)) and nitrite (NO(2)(-)) is an important alternative pathway for the generation of nitric oxide (NO) and maintenance of systemic nitrate-nitrite-NO balance. More than 25% of the circulating NO(3)(-) is actively removed and secreted by salivary glands. Oral commensal bacteria convert salivary NO(3)(-) to NO(2)(-), which enters circulation and leads to NO generation. The transporters for NO(3)(-) in salivary glands have not yet been identified. Here we report that sialin (SLC17A5), mutations in which cause Salla disease and infantile sialic acid storage disorder (ISSD), functions as an electrogenic 2NO(3)(-)/H(+) cotransporter in the plasma membrane of salivary gland acinar cells. We have identified an extracellular pH-dependent anion current that is carried by NO(3)(-) or sialic acid (SA), but not by Br(-), and is accompanied by intracellular acidification. Both responses were reduced by knockdown of sialin expression and increased by the plasma membrane-targeted sialin mutant (L22A-L23A). Fibroblasts from patients with ISSD displayed reduced SA- and NO(3)(-)-induced currents compared with healthy controls. Furthermore, expression of disease-associated sialin mutants in fibroblasts and salivary gland cells suppressed the H(+)-dependent NO(3)(-) conductance. Importantly, adenovirus-dependent expression of the sialinH183R mutant in vivo in pig salivary glands decreased NO(3)(-) secretion in saliva after intake of a NO(3)(-)-rich diet. Taken together, these data demonstrate that sialin mediates nitrate influx into salivary gland and other cell types. We suggest that the 2NO(3)(-)/H(+) transport function of sialin in salivary glands can contribute significantly to clearance of serum nitrate, as well as nitrate recycling and physiological nitrite-NO homeostasis.