A chimera carrying the functional domain of the orphan protein SLC7A14 in the backbone of SLC7A2 mediates trans-stimulated arginine transport

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.

Comprehensive review of amino acid transporters as therapeutic targets

The solute carrier (SLC) family, with more than 400 membrane-bound proteins, facilitates the transport of a wide array of substrates such as nutrients, ions, metabolites, and drugs across biological membranes. Amino acid transporters (AATs) are membrane transport proteins that mediate transfer of amino acids into and out of cells or cellular organelles. AATs participate in many important physiological functions including nutrient supply, metabolic transformation, energy homeostasis, redox regulation, and neurological regulation. Several AATs have been found to significantly impact the progression of human malignancies, and dysregulation of AATs results in metabolic reprogramming affecting tumor growth and progression. However, current clinical therapies that directly target AATs have not been developed. The purpose of this review is to highlight the structural and functional diversity of AATs, the molecular mechanisms in human diseases such as tumors, kidney diseases, and emerging therapeutic strategies for targeting AATs.

SLC7A14 imports GABA to lysosomes and impairs hepatic insulin sensitivity via inhibiting mTORC2

Lysosomal amino acid accumulation is implicated in several diseases, but its role in insulin resistance, the central mechanism to type 2 diabetes and many metabolic diseases, is unclear. In this study, we show the hepatic expression of lysosomal membrane protein solute carrier family 7 member 14 (SLC7A14) is increased in insulin-resistant mice. The promoting effect of SLC7A14 on insulin resistance is demonstrated by loss- and gain-of-function experiments. SLC7A14 is further demonstrated as a transporter resulting in the accumulation of lysosomal γ-aminobutyric acid (GABA), which induces insulin resistance via inhibiting mTOR complex 2 (mTORC2)'s activity. These results establish a causal link between lysosomal amino acids and insulin resistance and suggest that SLC7A14 inhibition may provide a therapeutic strategy in treating insulin resistance-related and GABA-related diseases and may provide insights into the upstream mechanisms for mTORC2, the master regulator in many important processes.

Mutation of SLC7A14 causes auditory neuropathy and retinitis pigmentosa mediated by lysosomal dysfunction

Lysosomes contribute to cellular homeostasis via processes including macromolecule degradation, nutrient sensing, and autophagy. Defective proteins related to lysosomal macromolecule catabolism are known to cause a range of lysosomal storage diseases; however, it is unclear whether mutations in proteins involved in homeostatic nutrient sensing mechanisms cause syndromic sensory disease. Here, we show that SLC7A14, a transporter protein mediating lysosomal uptake of cationic amino acids, is evolutionarily conserved in vertebrate mechanosensory hair cells and highly expressed in lysosomes of mammalian cochlear inner hair cells (IHCs) and retinal photoreceptors. Autosomal recessive mutation of SLC7A14 caused loss of IHCs and photoreceptors, leading to presynaptic auditory neuropathy and retinitis pigmentosa in mice and humans. Loss-of-function mutation altered protein trafficking and increased basal autophagy, leading to progressive cell degeneration. This study implicates autophagy-lysosomal dysfunction in syndromic hearing and vision loss in mice and humans.

Amino Acid Homeostasis in Mammalian Cells with a Focus on Amino Acid Transport

Amino acid homeostasis is maintained by import, export, oxidation, and synthesis of nonessential amino acids, and by the synthesis and breakdown of protein. These processes work in conjunction with regulatory elements that sense amino acids or their metabolites. During and after nutrient intake, amino acid homeostasis is dominated by autoregulatory processes such as transport and oxidation of excess amino acids. Amino acid deprivation triggers processes such as autophagy and the execution of broader transcriptional programs to maintain plasma amino acid concentrations. Amino acid transport plays a crucial role in the absorption of amino acids in the intestine, the distribution of amino acids across cells and organs, the recycling of amino acids in the kidney, and the recycling of amino acids after protein breakdown.

Slc7a14 Is Indispensable in Zebrafish Retinas

Previous study has identified SLC7A14 as a new causative gene of retinitis pigmentosa (RP). However, the role of SLC7A14 has not been fully characterized. The goal of this study was to investigate the biological features of slc7a14 in zebrafish. To determine the expression of slc7a14 in developing zebrafish, we performed in situ hybridization (ISH) and quantitative real-time PCR. Morpholino knockdown and overexpression experiments were performed to study the role of slc7a14 in zebrafish retinas. Immunostaining was carried out to observe structural changes. Visual motor responses (VMR) and optokinetic responses (OKR) were analyzed to assess visual behaviors. Terminal deoxynucleotidyl transferase (dUTP) nick-end labeling (TUNEL) staining was performed to survey apoptotic retinal cells. We found that slc7a14 was highly expressed in neuronal tissues, including the brain, spinal cord and retina, and that the expression levels increased during early embryogenesis. Consistently, ISH showed a similar expression pattern. Knockdown of slc7a14 led to dose-dependent microphthalmia that was reversed by overexpression. The immunostaining results revealed that the rod-specific protein zpr-3 and the retinal pigment epithelium-specific protein zpr-2 (decreased to 44.48%) were significantly suppressed in the slc7a14-silenced morphants. Notably, visual behaviors (the VMR and the OKR) were severely impaired in the slc7a14-deficient morphant, especially the VMR OFF response. In addition, apoptotic cells were observed in the retina at 3 days post fertilization (dpf) and 5 dpf by TUNEL assay. Our results demonstrated that slc7a14 is essential for visually mediated behaviors in zebrafish. Temporary silencing of slc7a14 in larvae led to severe visual impairments, consistent with the manifestations observed in RP patients. Our findings provide further insights into the genetic mechanisms of RP predisposition caused by SLC7A14 mutations.

Phenotypic variability of SLC7A14 mutations in patients with inherited retinal dystrophy

BACKGROUND

Inherited retinal dystrophy (IRD) is a group of retinal disorders that are both clinically and genetically diverse, typically with loss of photoreceptor function. Herein, we aimed to identify the underlying genetic defect in IRD patients with mutations in the SLC7A14 gene.

METHODS

A targeted exome capture panel was applied for mutational screening of SLC7A14. Targeted exome sequencing (TES) was performed on 200 non-syndromic and unrelated autosomal recessive or sporadic IRD families. Candidate variants were validated by direct sequencing and further examined using bioinformatics analyses for determination of their potential effect.

RESULTS

We identified compound heterozygous missense mutations (c.988G>A, p.G330R; c.1970G>A, p.R657Q) in an autosomal recessive retinitis pigmentosa (RP) case and a homozygous mutation (c.988G>A, p.G330R) in a simplex case with Leber congenital amaurosis (LCA) in the SLC7A14 gene. Both G330R and R657Q were deleterious based on in silico predictive tools. Our proposed topological model of the SLC7A14 polypeptide suggested that both G330R and R657Q affected evolutionarily highly conserved amino acid residues in SLC7A14 that occurred in transmembrane helixes. Structural modeling revealed a broken arginine and aspartic acid connection between residues 657 and 406.

CONCLUSIONS

We applied TES to the molecular diagnosis of patients with IRD and for the first time identified SLC7A14 mutations in two unrelated families with RP and LCA separately. Our findings uniquely add the knowledge of the phenotypic variability of SLC7A14 mutations.

Screening for SLC7A14 gene mutations in patients with autosomal recessive or sporadic retinitis pigmentosa

PURPOSE

In this study, we aimed to detect mutations in the SLC7A14 cationic transporter gene, which has recently been reported as a causative gene for retinitis pigmentosa (RP), in Japanese patients with autosomal recessive (AR) or sporadic RP.

MATERIALS AND METHODS

We included 146 unrelated Japanese patients with AR or sporadic RP who lacked mutations in genes known to be associated with RP despite next-generation sequencing-based screening. We sequenced the seven SLC7A14 coding exons along with their flanking intronic DNA using the Sanger method. The detected polymorphisms were assessed for their pathogenicity with in silico prediction tools. For those who had heterozygous, nonsynonymous variants, we performed multiplex ligation-dependent probe amplification (MLPA) to search for additional deletion/duplication.

RESULTS

We detected four distinct SLC7A14 polymorphisms excluding synonymous polymorphisms. Two of these polymorphisms were assessed as detrimental by in silico prediction tools. However, all of the mutations were heterozygous. Neither homozygous polymorphisms nor compound heterozygous polymorphisms, which are considered detrimental variants, were detected. Neither deletion nor duplication was found with MLPA in patients with heterozygous variants.

CONCLUSIONS

The four SLC7A14 mutations detected herein were unlikely to be pathogenic in this Japanese cohort. The frequency and pathogenicity of SLC7A14 mutations may vary depending on ethnicity, and these mutations may be rare in Japanese patients.

Plasma arginine/ADMA ratio as a sensitive risk marker for atherosclerosis: Shimane CoHRE study

BACKGROUND

Asymmetric dimethylarginine (ADMA), which acts an endogenous inhibitor of nitric oxide synthase (NOS), is involved in the pathogenesis of cardiovascular disease. Arginine (Arg) may regulate vascular endothelial function, since Arg is the substrate of NO competing with ADMA. In our previous study, low Arg/ADMA ratio is an independent risk for microangiopathy-related cerebral damage.

PURPOSE

Here, we performed a cross-sectional study to evaluate the association between the Arg/ADMA ratio and the maximal intima-media thickness (IMT) in the carotid artery.

SUBJECTS AND METHODS

Participants were 785 community-dwelling Japanese people without any severe disorders. Plasma concentration of Arg and ADMA in fasting blood sample was determined using HPLC. IMT was measured in the bilateral carotid artery by ultrasonography.

RESULTS

Among quartiles stratified by the Arg/ADMA ratio, ANOVA showed a significant difference in IMT and the IMT in Q1 (the lowest quartile) was significantly higher than that in Q4 (the highest quartile). In multiple linear regression analysis, age, the male gender, lower BMI, the presence of hypertension and lower Arg/ADMA ratio were independently correlated with IMT, while IMT was not correlated with Arg or ADMA alone. In addition, the Arg/ADMA ratio was associated with IMT independent of age, sex, BMI and the presence of hypertension with odds ratio 0.21 (95%CI: 0.05-0.88) in multiple logistic regression analysis for IMT 1.5 mm or more.

CONCLUSION

Imbalance of Arg and ADMA is independently involved in the progression of atherosclerosis, and the Arg/ADMA ratio may be a sensitive marker for atherosclerosis.

SLC7A14 linked to autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) is characterized by degeneration of the retinal photoreceptors and is the leading cause of inherited blindness worldwide. Although few genes are known to cause autosomal recessive RP (arRP), a large proportion of disease-causing genes remain to be revealed. Here we report the identification of SLC7A14, a potential cationic transporter, as a novel gene linked to arRP. Using exome sequencing and direct screening of 248 unrelated patients with arRP, we find that mutations in the SLC7A14 gene account for 2% of cases of arRP. We further demonstrate that SLC7A14 is specifically expressed in the photoreceptor layer of the mammalian retina and its expression increases during postnatal retinal development. In zebrafish, downregulation of slc7a14 expression leads to an abnormal eye phenotype and defective light-induced locomotor response. Furthermore, targeted knockout of Slc7a14 in mice results in retinal degeneration with abnormal ERG response. This suggests that SLC7A14 has an important role in retinal development and visual function.

Retinitis pigmentosa is the leading cause of inherited blindness worldwide. Here, the authors use exome sequencing to identify mutations in SLC7A14 that may be linked to the disease, and provide functional support for the role of this gene in retinal development and visual function in mice and zebrafish.