Positive regulation of Hedgehog signaling via phosphorylation of GLI2/GLI3 by DYRK2 kinase.

Hedgehog (Hh) signaling, an evolutionarily conserved pathway, plays an essential role in development and tumorigenesis, making it a promising drug target. Multiple negative regulators are known to govern Hh signaling; however, how activated Smoothened (SMO) participates in the activation of downstream GLI2 and GLI3 remains unclear. Herein, we identified the ciliary kinase DYRK2 as a positive regulator of the GLI2 and GLI3 transcription factors for Hh signaling. Transcriptome and interactome analyses demonstrated that DYRK2 phosphorylates GLI2 and GLI3 on evolutionarily conserved serine residues at the ciliary base, in response to activation of the Hh pathway. This phosphorylation induces the dissociation of GLI2/GLI3 from suppressor, SUFU, and their translocation into the nucleus. Loss of Dyrk2 in mice causes skeletal malformation, but neural tube development remains normal. Notably, DYRK2-mediated phosphorylation orchestrates limb development by controlling cell proliferation. Taken together, the ciliary kinase DYRK2 governs the activation of Hh signaling through the regulation of two processes: phosphorylation of GLI2 and GLI3 downstream of SMO and cilia formation. Thus, our findings of a unique regulatory mechanism of Hh signaling expand understanding of the control of Hh-associated diseases.

Insight into the mechanism of H+-coupled nucleobase transport.

Members of the nucleobase/ascorbic acid transporter (NAT) gene family are found in all kingdoms of life. In mammals, the concentrative uptake of ascorbic acid (vitamin C) by members of the NAT family is driven by the Na+ gradient, while the uptake of nucleobases in bacteria is powered by the H+ gradient. Here, we report the structure and function of PurTCp, a NAT family member from Colwellia psychrerythraea. The structure of PurTCp was determined to 2.80 Å resolution by X-ray crystallography. PurTCp forms a homodimer, and each protomer has 14 transmembrane segments folded into a transport domain (core domain) and a scaffold domain (gate domain). A purine base is present in the structure and defines the location of the substrate binding site. Functional studies reveal that PurTCp transports purines but not pyrimidines and that purine binding and transport is dependent on the pH. Mutation of a conserved aspartate residue close to the substrate binding site reveals the critical role of this residue in H+-dependent transport of purines. Comparison of the PurTCp structure with transporters of the same structural fold suggests that rigid-body motions of the substrate-binding domain are central for substrate translocation across the membrane.

Simple But Efficacious Enrichment of Integral Membrane Proteins and Their Interactions for In-Depth Membrane Proteomics.

Membrane proteins play essential roles in various cellular processes, such as nutrient transport, bioenergetic processes, cell adhesion, and signal transduction. Proteomics is one of the key approaches to exploring membrane proteins comprehensively. Bottom-up proteomics using LC-MS/MS has been widely used in membrane proteomics. However, the low abundance and hydrophobic features of membrane proteins, especially integral membrane proteins, make it difficult to handle the proteins and are the bottleneck for identification by LC-MS/MS. Herein, to improve the identification and quantification of membrane proteins, we have stepwisely evaluated methods of membrane enrichment for the sample preparation. The enrichment methods of membranes consisted of precipitation by ultracentrifugation and treatment by urea or alkaline solutions. The best enrichment method in the study, washing with urea after isolation of the membranes, resulted in the identification of almost twice as many membrane proteins compared with samples without the enrichment. Notably, the method significantly enhances the identified numbers of multispanning transmembrane proteins, such as solute carrier transporters, ABC transporters, and G-protein-coupled receptors, by almost sixfold. Using this method, we revealed the profiles of amino acid transport systems with the validation by functional assays and found more protein-protein interactions, including membrane protein complexes and clusters. Our protocol uses standard procedures in biochemistry, but the method was efficient for the in-depth analysis of membrane proteome in a wide range of samples.

Insight into the function of a unique voltage-sensor protein (TMEM266) and its short form in mouse cerebellum.

Voltage-sensing proteins generally consist of voltage-sensor domains and pore-gate domains, forming the voltage-gated ion channels. However, there are several unconventional voltage-sensor proteins that lack pore-gate domains, conferring them unique voltage-sensing machinery. TMEM266, which is expressed in cerebellum granule cells, is one of the interesting voltage-sensing proteins that has a putative intracellular coiled-coil and a functionally unidentified cytosolic region instead of a pore-gate domain. Here, we approached the molecular function of TMEM266 by performing co-immunoprecipitation experiments. We unexpectedly discovered that TMEM266 proteins natively interact with the novel short form splice variants that only have voltage-sensor domains and putative cytosolic coiled-coil region in cerebellum. The crystal structure of coiled-coil region of TMEM266 suggested that these coiled-coil regions play significant roles in forming homodimers. In vitro expression experiments supported the idea that short form TMEM266 (sTMEM266) or full length TMEM266 (fTMEM266) form homodimers. We also performed proximity labeling mass spectrometry analysis for fTMEM266 and sTMEM266 using Neuro-2A, neuroblastoma cells, and fTMEM266 showed more interacting molecules than sTMEM266, suggesting that the C-terminal cytosolic region in fTMEM266 binds to various targets. Finally, TMEM266-deficient animals showed the moderate abnormality in open-field test. The present study provides clues about the novel voltage-sensing mechanism mediated by TMEM266.

New roles for dopamine D2 and D3 receptors in pancreatic beta cell insulin secretion.

Although long-studied in the central nervous system, there is increasing evidence that dopamine (DA) has important roles in the periphery including in metabolic regulation. Insulin-secreting pancreatic ß-cells express the machinery for DA synthesis and catabolism, as well as all five DA receptors. In these cells, DA functions as a negative regulator of glucose-stimulated insulin secretion (GSIS), which is mediated by DA D2-like receptors including D2 (D2R) and D3 (D3R) receptors. However, the fundamental mechanisms of DA synthesis, storage, release, and signaling in pancreatic ß-cells and their functional relevance in vivo remain poorly understood. Here, we assessed the roles of the DA precursor L-DOPA in ß-cell DA synthesis and release in conjunction with the signaling mechanisms underlying DA's inhibition of GSIS. Our results show that the uptake of L-DOPA is essential for establishing intracellular DA stores in ß-cells. Glucose stimulation significantly enhances L-DOPA uptake, leading to increased DA release and GSIS reduction in an autocrine/paracrine manner. Furthermore, D2R and D3R act in combination to mediate dopaminergic inhibition of GSIS. Transgenic knockout mice in which ß-cell D2R or D3R expression is eliminated exhibit diminished DA secretion during glucose stimulation, suggesting a new mechanism where D2-like receptors modify DA release to modulate GSIS. Lastly, ß-cell-selective D2R knockout mice exhibit marked postprandial hyperinsulinemia in vivo. These results reveal that peripheral D2R and D3R receptors play important roles in metabolism through their inhibitory effects on GSIS. This opens the possibility that blockade of peripheral D2-like receptors by drugs including antipsychotic medications may significantly contribute to the metabolic disturbances observed clinically.

The LeuT-fold neurotransmitter:sodium symporter MhsT has two substrate sites.

Crystal structures of the neurotransmitter:sodium symporter MhsT revealed occluded inward-facing states with one substrate (Trp) bound in the primary substrate (S1) site and a collapsed extracellular vestibule, which in LeuT contains the second substrate (S2) site. In n-dodecyl-ß-d-maltoside, the detergent used to prepare MhsT for crystallization, the substrate-to-protein binding stoichiometry was determined by using scintillation proximity to be 1 Trp:MhsT. Here, using the same experimental approach, as well as equilibrium dialysis, we report that in n-decyl-ß-d-maltoside, or after reconstitution in lipid, MhsT, like LeuT, can simultaneously bind two Trp substrate molecules. Trp binding to the S2 site sterically blocks access to a substituted Cys at position 33 in the S2 site, as well as access to the deeper S1 site. Mutation of either the S1 or S2 site disrupts transport, consistent with previous studies in LeuT showing that substrate binding to the S2 site is an essential component of the transport mechanism.

Novel cystine transporter in renal proximal tubule identified as a missing partner of cystinuria-related plasma membrane protein rBAT/SLC3A1.

Heterodimeric amino acid transporters play crucial roles in epithelial transport, as well as in cellular nutrition. Among them, the heterodimer of a membrane protein b(0,+)AT/SLC7A9 and its auxiliary subunit rBAT/SLC3A1 is responsible for cystine reabsorption in renal proximal tubules. The mutations in either subunit cause cystinuria, an inherited amino aciduria with impaired renal reabsorption of cystine and dibasic amino acids. However, an unsolved paradox is that rBAT is highly expressed in the S3 segment, the late proximal tubules, whereas b(0,+)AT expression is highest in the S1 segment, the early proximal tubules, so that the presence of an unknown partner of rBAT in the S3 segment has been proposed. In this study, by means of coimmunoprecipitation followed by mass spectrometry, we have found that a membrane protein AGT1/SLC7A13 is the second partner of rBAT. AGT1 is localized in the apical membrane of the S3 segment, where it forms a heterodimer with rBAT. Depletion of rBAT in mice eliminates the expression of AGT1 in the renal apical membrane. We have reconstituted the purified AGT1-rBAT heterodimer into proteoliposomes and showed that AGT1 transports cystine, aspartate, and glutamate. In the apical membrane of the S3 segment, AGT1 is suggested to locate itself in close proximity to sodium-dependent acidic amino acid transporter EAAC1 for efficient functional coupling. EAAC1 is proposed to take up aspartate and glutamate released into luminal fluid by AGT1 due to its countertransport so that preventing the urinary loss of aspartate and glutamate. Taken all together, AGT1 is the long-postulated second cystine transporter in the S3 segment of proximal tubules and a possible candidate to be involved in isolated cystinuria.

18F-FBPA as a tumor-specific probe of L-type amino acid transporter 1 (LAT1): a comparison study with 18F-FDG and 11C-Methionine PET.

PURPOSE: The purpose of this study was to evaluate the usefulness of L-4-borono-2-18F-fluoro-phenylalanine (18F-FBPA) as a tumor-specific probe, in comparison to 18F-FDG and 11C-methionine (Met), focusing on its transport selectivity by L-type amino acid transporter 1 (LAT1), which is highly upregulated in cancers. METHODS: Cellular analyses of FBPA were performed to evaluate the transportablity and Km value. PET studies were performed in rat xenograft models of C6 glioma (n = 12) and in rat models of turpentine oil-induced subcutaneous inflammation (n = 9). The kinetic parameters and uptake values on static PET images were compared using the one-tissue compartment model (K1, k2) and maximum standardized uptake value (SUVmax). RESULTS: The cellular analyses showed that FBPA had a lower affinity to a normal cell-type transporter LAT2 and induced less efflux through LAT2 among FBPA, Met, and BPA, while the efflux through LAT1 induced by FBPA was similar among the three compounds. The Km value of 18F-FBPA for LAT1 (196.8 ± 11.4 µM) was dramatically lower than that for LAT2 (2813.8 ± 574.5 µM), suggesting the higher selectivity of 18F-FBPA for LAT1. K1 and k2 values were significantly smaller in 18F-FBPA PET (K1 = 0.04 ± 0.01 ml/ccm/min and k2 = 0.07 ± 0.01 /min) as compared to 11C-Met PET (0.22 ± 0.09 and 0.52 ± 0.10, respectively) in inflammatory lesions. Static PET analysis based on the SUVmax showed significantly higher accumulation of 18F-FDG in the tumor and inflammatory lesions (7.2 ± 2.1 and 4.6 ± 0.63, respectively) as compared to both 18F-FBPA (3.2 ± 0.40 and 1.9 ± 0.19) and 11C-Met (3.4 ± 0.43 and 1.6 ± 0.11). No significant difference was observed between 18F-FBPA and 11C-Met in the static PET images. CONCLUSION: This study shows the utility of 18F-FBPA as a tumor-specific probe of LAT1 with low accumulation in the inflammatory lesions.

Structure-activity relationship of a novel series of inhibitors for cancer type transporter L-type amino acid transporter 1 (LAT1).

L-type amino acid transporter 1 (LAT1) is known as a cancer-type amino acid transporter. In cancer cells, LAT1 is responsible for the cellular uptake of many essential amino acids including leucine that activates mechanistic/mammalian target of rapamycin (mTOR), regulating cancer cell growth. In this study, we designed a novel series of LAT1 inhibitors, SKN101-105, based on the structure of triiodothyronine (T3), a known LAT1 blocker. The compounds consist of core structure of 2-amino-3-[3,5-dichloro-4-(naphthalene-1-methoxy)-phenyl]-propanoic acid and different modifications on the naphthalene. Among them, the compounds including SKN103 with a modified phenyl group at C-7 position of naphthalene inhibited LAT1-mediated leucine transport, whereas SKN102 with a phenyl group at C-6 position did not, indicating the importance of the position of substituents on the naphthalene for the interaction with LAT1. SKN103 was suggested to be a non-transportable blocker rather than a substrate of LAT1 and inhibited LAT1 in a competitive manner with the Ki value of 2.1 µM. SKN103 suppressed mTOR activity and the growth of cancer cells. Moreover, SKN103 in combination with cisplatin additively enhanced the growth inhibition in cancer cells. This study provides an additional insight into the structure-activity relationship of LAT1 ligands, which could lead to designing desirable LAT1 inhibitors.

Interaction of α-Lipoic Acid with the Human Na+/Multivitamin Transporter (hSMVT).

The human Na(+)/multivitamin transporter (hSMVT) has been suggested to transport α-lipoic acid (LA), a potent antioxidant and anti-inflammatory agent used in therapeutic applications, e.g. in the treatment of diabetic neuropathy and Alzheimer disease. However, the molecular basis of the cellular delivery of LA and in particular the stereospecificity of the transport process are not well understood. Here, we expressed recombinant hSMVT in Pichia pastoris and used affinity chromatography to purify the detergent-solubilized protein followed by reconstitution of hSMVT in lipid bilayers. Using a combined approach encompassing radiolabeled LA transport and equilibrium binding studies in conjunction with the stabilized R-(+)- and S-(-)-enantiomers and the R,S-(+/-) racemic mixture of LA or lipoamide, we identified the biologically active form of LA, R-LA, to be the physiological substrate of hSMVT. Interaction of R-LA with hSMVT is strictly dependent on Na(+). Under equilibrium conditions, hSMVT can simultaneously bind ~2 molecules of R-LA in a biphasic binding isotherm with dissociation constants (Kd) of 0.9 and 7.4 µm. Transport of R-LA in the oocyte and reconstituted system is exclusively dependent on Na(+) and exhibits an affinity of ~3 µm. Measuring transport with known amounts of protein in proteoliposomes containing hSMVT in outside-out orientation yielded a catalytic turnover number (kcat) of about 1 s(-1), a value that is well in agreement with other Na(+)-coupled transporters. Our data suggest that hSMVT-mediated transport is highly specific for R-LA at our tested concentration range, a finding with wide ramifications for the use of LA in therapeutic applications.

Specific transport of 3-fluoro-l-α-methyl-tyrosine by LAT1 explains its specificity to malignant tumors in imaging.

3-(18)F-l-α-methyl-tyrosine ([18F]FAMT), a PET probe for tumor imaging, has advantages of high cancer-specificity and lower physiologic background. FAMT-PET has been proved useful in clinical studies for the prediction of prognosis, the assessment of therapy response and the differentiation of malignant tumors from inflammation and benign lesions. The tumor uptake of [18F]FAMT in PET is strongly correlated with the expression of L-type amino acid transporter 1 (LAT1), an isoform of system L upregulated in cancers. In this study, to assess the transporter-mediated mechanisms in FAMT uptake by tumors, we examined amino acid transporters for FAMT transport. We synthesized [14C]FAMT and measured its transport by human amino acid transporters expressed in Xenopus oocytes. The transport of FAMT was compared with that of l-methionine, a well-studied amino acid PET probe. The significance of LAT1 in FAMT uptake by tumor cells was confirmed by siRNA knockdown. Among amino acid transporters, [14C]FAMT was specifically transported by LAT1, whereas l-[14C]methionine was taken up by most of the transporters. Km of LAT1-mediated [14C]FAMT transport was 72.7 µM, similar to that for endogenous substrates. Knockdown of LAT1 resulted in the marked reduction of [14C]FAMT transport in HeLa S3 cells, confirming the contribution of LAT1 in FAMT uptake by tumor cells. FAMT is highly specific to cancer-type amino acid transporter LAT1, which explains the cancer-specific accumulation of [18F]FAMT in PET. This, vice versa, further supports the cancer-specific expression of LAT1. This study has established FAMT as a LAT1-specific molecular probe to monitor the expression of a potential tumor biomarker LAT1.

Structure-activity relations of leucine derivatives reveal critical moieties for cellular uptake and activation of mTORC1-mediated signaling.

Among amino acids, leucine is a potential signaling molecule to regulate cell growth and metabolism by activating mechanistic target of rapamycin complex 1 (mTORC1). To reveal the critical structures of leucine molecule to activate mTORC1, we examined the structure-activity relationships of leucine derivatives in HeLa S3 cells for cellular uptake and for the induction of phosphorylation of p70 ribosomal S6 kinase 1 (p70S6K), a downstream effector of mTORC1. The activation of mTORC1 by leucine and its derivatives was the consequence of two successive events: the cellular uptake by L-type amino acid transporter 1 (LAT1) responsible for leucine uptake in HeLa S3 cells and the activation of mTORC1 following the transport. The structural requirement for the recognition by LAT1 was to have carbonyl oxygen, alkoxy oxygen of carboxyl group, amino group and hydrophobic side chain. In contrast, the requirement for mTORC1 activation was more rigorous. It additionally required fixed distance between carbonyl oxygen and alkoxy oxygen of carboxyl group, and amino group positioned at α-carbon. L-Configuration in chirality and appropriate length of side chain with a terminal isopropyl group were also important. This confirmed that LAT1 itself is not a leucine sensor. Some specialized leucine sensing mechanism with rigorous requirement for agonistic structures should exist inside the cells because leucine derivatives not transported by LAT1 did not activate mTORC1. Because LAT1-mTOR axis is involved in the regulation of cell growth and cancer progression, the results from this study may provide a new insight into therapeutics targeting both LAT1 and leucine sensor.

Transport of 3-fluoro-L-α-methyl-tyrosine (FAMT) by organic ion transporters explains renal background in [(18)F]FAMT positron emission tomography.

A PET tracer for tumor imaging, 3-(18)F-l-α-methyl-tyrosine ([(18)F]FAMT), has advantages of high cancer-specificity and low physiological background. In clinical studies, FAMT-PET has been proved useful for the detection of malignant tumors and their differentiation from inflammation and benign lesions. The tumor specific uptake of FAMT is due to its high-selectivity to cancer-type amino acid transporter LAT1 among amino acid transporters. In [(18)F]FAMT PET, kidney is the only organ that shows high physiological background. To reveal transporters involved in renal accumulation of FAMT, we have examined [(14)C]FAMT uptake on the organic ion transporters responsible for the uptake into tubular epithelial cells. We have found that OAT1, OAT10 and OCTN2 transport [(14)C]FAMT. The [(14)C]FAMT uptake was inhibited by probenecid, furosemide and ethacrynic acid, consistent with the properties of the transporters. The amino acid uptake inhibitor, 2-amino-2-norbornanecarboxylic acid (BCH), also inhibited the [(14)C]FAMT uptake, whereas OCTN2-mediated [(14)C]FAMT uptake was Na(+)-dependent. We propose that FAMT uptake by OAT1, OAT10 and OCTN2 into tubular epithelial cells could contribute to the renal accumulation of FAMT. The results from this study would provide clues to the treatments to reduce renal background and enhance tumor uptake as well as to designing PET tracers with less renal accumulation.

Boronophenylalanine, a boron delivery agent for boron neutron capture therapy, is transported by ATB0,+, LAT1 and LAT2.

The efficacy of boron neutron capture therapy relies on the selective delivery of boron carriers to malignant cells. p-Boronophenylalanine (BPA), a boron delivery agent, has been proposed to be localized to cells through transporter-mediated mechanisms. In this study, we screened aromatic amino acid transporters to identify BPA transporters. Human aromatic amino acid transporters were functionally expressed in Xenopus oocytes and examined for BPA uptake and kinetic parameters. The roles of the transporters in BPA uptake were characterized in cancer cell lines. For the quantitative assessment of BPA uptake, HPLC was used throughout the study. Among aromatic amino acid transporters, ATB(0,+), LAT1 and LAT2 were found to transport BPA with Km values of 137.4 ± 11.7, 20.3 ± 0.8 and 88.3 ± 5.6 µM, respectively. Uptake experiments in cancer cell lines revealed that the LAT1 protein amount was the major determinant of BPA uptake at 100 µM, whereas the contribution of ATB(0,+) became significant at 1000 µM, accounting for 20-25% of the total BPA uptake in MCF-7 breast cancer cells. ATB(0,+), LAT1 and LAT2 transport BPA at affinities comparable with their endogenous substrates, suggesting that they could mediate effective BPA uptake in vivo. The high and low affinities of LAT1 and ATB(0,+), respectively, differentiate their roles in BPA uptake. ATB(0,+), as well as LAT1, could contribute significantly to the tumor accumulation of BPA at clinical dose.

Nicotine induces dendritic spine remodeling in cultured hippocampal neurons.

Cholinergic neurons in the CNS are involved in synaptic plasticity and cognition. Both muscarinic and nicotinic acetylcholine receptors (nAChRs) influence plasticity and cognitive function. The mechanism underlying nAChR-induced plasticity, however, has remained elusive. Here, we demonstrate morphological changes in dendritic spines following activation of α4ß2* nAChRs, which are expressed on glutamatergic pre-synaptic termini of cultured hippocampal neurons. Exposure of the neurons to nicotine resulted in a lateral enlargement of spine heads. This was abolished by dihydro-ß-erythroidine, an antagonist of α4ß2* nAChRs, but not by α-bungarotoxin, an antagonist of α7 nAChRs. Tetanus toxin or a mixture of 2-amino-5-phosphonovaleric acid and 6-cyano-7-nitroquinoxaline-2,3-dione, antagonists of NMDA- and AMPA-type glutamate receptors, blocked the nicotine-induced spine remodeling. In addition, nicotine exerted full spine-enlarging response in the post-synaptic neuron whose ß2 nAChR expression was knocked down. Finally, pre-treatment with nicotine enhanced the Ca(2+)-response of the neurons to glutamate. These data suggest that nicotine influences the activity of glutamatergic neurotransmission through the activation of pre-synaptic α4ß2 nAChRs, resulting in the modulation of spinal architecture and responsiveness. The present findings may represent one of the cellular mechanisms underlying cholinergic tuning of brain function. Activation of nicotinic acetylcholine receptors (nAChRs) in brain influences plasticity and cognition. Here, activation of α4ß2* nAChRs, which are expressed on glutamatergic presynaptic termini, results in the enlargement of dendritic spines through the modulation of the glutamatergic neurotransmission. The remodeled spinal architecture might be responsible for the change in responsiveness of neural circuitry, leading to cholinergic tuning of brain function.

Identification of three distinct cell populations for urate excretion in human kidneys.

In humans, uric acid is an end-product of purine metabolism. Urate excretion from the human kidney is tightly regulated by reabsorption and secretion. At least eleven genes have been identified as human renal urate transporters. However, it remains unclear whether all renal tubular cells express the same set of urate transporters. Here, we show renal tubular cells are divided into three distinct cell populations for urate handling. Analysis of healthy human kidneys at single-cell resolution revealed that not all tubular cells expressed the same set of urate transporters. Only 32% of tubular cells were related to both reabsorption and secretion, while the remaining tubular cells were related to either reabsorption or secretion at 5% and 63%, respectively. These results provide physiological insight into the molecular function of the transporters and renal urate handling on single-cell units. Our findings suggest that three different cell populations cooperate to regulate urate excretion from the human kidney, and our proposed framework is a step forward in broadening the view from the molecular to the cellular level of transport capacity.

A multi-hierarchical approach reveals d-serine as a hidden substrate of sodium-coupled monocarboxylate transporters.

Transporter research primarily relies on the canonical substrates of well-established transporters. This approach has limitations when studying transporters for the low-abundant micromolecules, such as micronutrients, and may not reveal physiological functions of the transporters. While d-serine, a trace enantiomer of serine in the circulation, was discovered as an emerging biomarker of kidney function, its transport mechanisms in the periphery remain unknown. Here, using a multi-hierarchical approach from body fluids to molecules, combining multi-omics, cell-free synthetic biochemistry, and ex vivo transport analyses, we have identified two types of renal d-serine transport systems. We revealed that the small amino acid transporter ASCT2 serves as a d-serine transporter previously uncharacterized in the kidney and discovered d-serine as a non-canonical substrate of the sodium-coupled monocarboxylate transporters (SMCTs). These two systems are physiologically complementary, but ASCT2 dominates the role in the pathological condition. Our findings not only shed light on renal d-serine transport, but also clarify the importance of non-canonical substrate transport. This study provides a framework for investigating multiple transport systems of various trace micromolecules under physiological conditions and in multifactorial diseases.

Parkinson's disease-associated ATP13A2/PARK9 functions as a lysosomal H+,K+-ATPase.

Mutations in the human ATP13A2 (PARK9), a lysosomal ATPase, cause Kufor-Rakeb Syndrome, an early-onset form of Parkinson's disease (PD). Here, we demonstrate that ATP13A2 functions as a lysosomal H+,K+-ATPase. The K+-dependent ATPase activity and the lysosomal K+-transport activity of ATP13A2 are inhibited by an inhibitor of sarco/endoplasmic reticulum Ca2+-ATPase, thapsigargin, and K+-competitive inhibitors of gastric H+,K+-ATPase, such as vonoprazan and SCH28080. Interestingly, these H+,K+-ATPase inhibitors cause lysosomal alkalinization and α-synuclein accumulation, which are pathological hallmarks of PD. Furthermore, PD-associated mutants of ATP13A2 show abnormal expression and function. Our results suggest that the H+/K+-transporting function of ATP13A2 contributes to acidification and α-synuclein degradation in lysosomes.

Establishment of stable cell lines with high expression of heterodimers of human 4F2hc and human amino acid transporter LAT1 or LAT2 and delineation of their differential interaction with α-alkyl moieties.

System L is a major transport system for cellular uptake of neutral amino acids. Among system L transporters, L-type amino acid transporter 1 (LAT1) is responsible for the nutrient uptake in cancer cells, whereas L-type amino acid transporter 2 (LAT2) is a transporter for non-cancer cells. In this study, we have established HEK293 cell lines stably expressing high levels of human LAT1 and LAT2 forming heterodimers with native human 4F2hc of the cells. We have found that L-[(14)C]alanine is an appropriate substrate to examine the function of LAT2, whereas L-[(14)C]leucine is used for LAT1. By using L-[(14)C]alanine on LAT2, we have for the first time directly evaluated the function of human LAT2 expressed in mammalian cells and obtained its reliable kinetics. Using α-alkyl amino acids including α-methyl-alanine and α-ethyl-L-alanine, we have demonstrated that α-alkyl groups interfere with the interaction with LAT2. These cell lines with higher practical advantages would be useful for screening and analyzing compounds to develop LAT1-specific drugs that can be used for cancer diagnosis and therapeutics. The strategy that we took to establish the cell lines would also be applicable to the other heterodimeric transporters with important therapeutic implications.

Ca2+-mediated higher-order assembly of heterodimers in amino acid transport system b0,+ biogenesis and cystinuria.

Cystinuria is a genetic disorder characterized by overexcretion of dibasic amino acids and cystine, causing recurrent kidney stones and kidney failure. Mutations of the regulatory glycoprotein rBAT and the amino acid transporter b0,+AT, which constitute system b0,+, are linked to type I and non-type I cystinuria respectively and they exhibit distinct phenotypes due to protein trafficking defects or catalytic inactivation. Here, using electron cryo-microscopy and biochemistry, we discover that Ca2+ mediates higher-order assembly of system b0,+. Ca2+ stabilizes the interface between two rBAT molecules, leading to super-dimerization of b0,+AT-rBAT, which in turn facilitates N-glycan maturation and protein trafficking. A cystinuria mutant T216M and mutations of the Ca2+ site of rBAT cause the loss of higher-order assemblies, resulting in protein trapping at the ER and the loss of function. These results provide the molecular basis of system b0,+ biogenesis and type I cystinuria and serve as a guide to develop new therapeutic strategies against it. More broadly, our findings reveal an unprecedented link between transporter oligomeric assembly and protein-trafficking diseases.