Functional and immunocytochemical characterization of the creatine transporter in rat hippocampal neurons

Probing binding and occlusion of substrate in the human creatine transporter-1 by computation and mutagenesis

In chordates, energy buffering is achieved in part through phosphocreatine, which requires cellular uptake of creatine by the membrane-embedded creatine transporter (CRT1/SLC6A8). Mutations in human slc6a8 lead to creatine transporter deficiency syndrome, for which there is only limited treatment. Here, we used a combined homology modeling, molecular dynamics, and experimental approach to generate a structural model of CRT1. Our observations support the following conclusions: contrary to previous proposals, C144, a key residue in the substrate binding site, is not present in a charged state. Similarly, the side chain D458 must be present in a protonated form to maintain the structural integrity of CRT1. Finally, we identified that the interaction chain Y148-creatine-Na+ is essential to the process of occlusion, which occurs via a "hold-and-pull" mechanism. The model should be useful to study the impact of disease-associated point mutations on the folding of CRT1 and identify approaches which correct folding-deficient mutants.

Taurine and Creatine Transporters as Potential Drug Targets in Cancer Therapy

Cancer cells are characterized by uncontrolled growth, proliferation, and impaired apoptosis. Tumour progression could be related to poor prognosis and due to this fact, researchers have been working on novel therapeutic strategies and antineoplastic agents. It is known that altered expression and function of solute carrier proteins from the SLC6 family could be associated with severe diseases, including cancers. These proteins were noticed to play important physiological roles through transferring nutrient amino acids, osmolytes, neurotransmitters, and ions, and many of them are necessary for survival of the cells. Herein, we present the potential role of taurine (SLC6A6) and creatine (SLC6A8) transporters in cancer development as well as therapeutic potential of their inhibitors. Experimental data indicate that overexpression of analyzed proteins could be connected with colon or breast cancers, which are the most common types of cancers. The pool of known inhibitors of these transporters is limited; however, one ligand of SLC6A8 protein is currently tested in the first phase of clinical trials. Therefore, we also highlight structural aspects useful for ligand development. In this review, we discuss SLC6A6 and SLC6A8 transporters as potential biological targets for anticancer agents.

Cooperative Binding of Substrate and Ions Drives Forward Cycling of the Human Creatine Transporter-1

Creatine serves as an ATP buffer and is thus an integral component of cellular energy metabolism. Most cells maintain their creatine levels via uptake by the creatine transporter (CRT-1, SLC6A8). The activity of CRT-1, therefore, is a major determinant of cytosolic creatine concentrations. We determined the kinetics of CRT-1 in real time by relying on electrophysiological recordings of transport-associated currents. Our analysis revealed that CRT-1 harvested the concentration gradient of NaCl and the membrane potential but not the potassium gradient to achieve a very high concentrative power. We investigated the mechanistic basis for the ability of CRT-1 to maintain the forward cycling mode in spite of high intracellular concentrations of creatine: this is achieved by cooperative binding of substrate and co-substrate ions, which, under physiological ion conditions, results in a very pronounced (i.e. about 500-fold) drop in the affinity of creatine to the inward-facing state of CRT-1. Kinetic estimates were integrated into a mathematical model of the transport cycle of CRT-1, which faithfully reproduced all experimental data. We interrogated the kinetic model to examine the most plausible mechanistic basis of cooperativity: based on this systematic exploration, we conclude that destabilization of binary rather than ternary complexes is necessary for CRT-1 to maintain the observed cytosolic creatine concentrations. Our model also provides a plausible explanation why neurons, heart and skeletal muscle cells must express a creatine releasing transporter to achieve rapid equilibration of the intracellular creatine pool.

Deorphaning a solute carrier 22 family member, SLC22A15, through functional genomic studies

The human solute carrier 22A (SLC22A) family consists of 23 members, representing one of the largest families in the human SLC superfamily. Despite their pharmacological and physiological importance in the absorption and disposition of a range of solutes, eight SLC22A family members remain classified as orphans. In this study, we used a multifaceted approach to identify ligands of orphan SLC22A15. Ligands of SLC22A15 were proposed based on phylogenetic analysis and comparative modeling. The putative ligands were then confirmed by metabolomic screening and uptake assays in SLC22A15 transfected HEK293 cells. Metabolomic studies and transporter assays revealed that SLC22A15 prefers zwitterionic compounds over cations and anions. We identified eight zwitterions, including ergothioneine, carnitine, carnosine, gabapentin, as well as four cations, including MPP⁺ , thiamine, and cimetidine, as substrates of SLC22A15. Carnosine was a specific substrate of SLC22A15 among the transporters in the SLC22A family. SLC22A15 transport of several substrates was sodium-dependent and exhibited a higher Km for ergothioneine, carnitine, and carnosine compared to previously identified transporters for these ligands. This is the first study to characterize the function of SLC22A15. Our studies demonstrate that SLC22A15 may play an important role in determining the systemic and tissue levels of ergothioneine, carnosine, and other zwitterions.

The Creatine Transporter Unfolded: A Knotty Premise in the Cerebral Creatine Deficiency Syndrome

Creatine provides cells with high-energy phosphates for the rapid reconstitution of hydrolyzed adenosine triphosphate. The eponymous creatine transporter (CRT1/SLC6A8) belongs to a family of solute carrier 6 (SLC6) proteins. The key role of CRT1 is to translocate creatine across tissue barriers and into target cells, such as neurons and myocytes. Individuals harboring mutations in the coding sequence of the human CRT1 gene develop creatine transporter deficiency (CTD), one of the pivotal underlying causes of cerebral creatine deficiency syndrome. CTD encompasses an array of clinical manifestations, including severe intellectual disability, epilepsy, autism, development delay, and motor dysfunction. CTD is characterized by the absence of cerebral creatine, which implies an indispensable role for CRT1 in supplying the brain cells with creatine. CTD-associated variants dramatically reduce or abolish creatine transport activity by CRT1. Many of these are point mutations that are known to trigger folding defects, leading to the retention of encoded CRT1 proteins in the endoplasmic reticulum and precluding their delivery to the cell surface. Misfolding of several related SLC6 transporters also gives rise to detrimental pathologic conditions in people; e.g., mutations in the dopamine transporter induce infantile parkinsonism/dystonia, while mutations in the GABA transporter 1 cause treatment-resistant epilepsy. In some cases, folding defects are amenable to rescue by small molecules, known as pharmacological and chemical chaperones, which restore the cell surface expression and transport activity of the previously non-functional proteins. Insights from the recent molecular, animal and human case studies of CTD add toward our understanding of this complex disorder and reveal the wide-ranging effects elicited upon CRT1 dysfunction. This grants novel therapeutic prospects for the treatment of patients afflicted with CTD, e.g., modifying the creatine molecule to facilitate CRT1-independent entry into brain cells, or correcting folding-deficient and loss-of-function CTD variants using pharmacochaperones and/or allosteric modulators. The latter justifies a search for additional compounds with a capacity to correct mutation-specific defects.

Studies of structural determinants of substrate binding in the Creatine Transporter (CreaT, SLC6A8) using molecular models

Creatine is a crucial metabolite that plays a fundamental role in ATP homeostasis in tissues with high-energy demands. The creatine transporter (CreaT, SLC6A8) belongs to the solute carrier 6 (SLC6) transporters family, and more particularly to the GABA transporters (GATs) subfamily. Understanding the molecular determinants of specificity within the SLC6 transporters in general, and the GATs in particular is very challenging due to the high similarity of these proteins. In the study presented here, our efforts focused on finding key structural features involved in binding selectivity for CreaT using structure-based computational methods. Due to the lack of three-dimensional structures of SLC6A8, our approach was based on the realization of two reliable homology models of CreaT using the structures of two templates, i.e. the human serotonin transporter (hSERT) and the prokaryotic leucine transporter (LeuT). Our models reveal that an optimal complementarity between the shape of the binding site and the size of the ligands is necessary for transport. These findings provide a framework for a deeper understanding of substrate selectivity of the SLC6 family and other LeuT fold transporters.

Cyclocreatine Transport by SLC6A8, the Creatine Transporter, in HEK293 Cells, a Human Blood-Brain Barrier Model Cell, and CCDSs Patient-Derived Fibroblasts

PURPOSE

Cyclocreatine, a creatine analog, is a candidate drug for treating patients with cerebral creatine deficiency syndromes (CCDSs) caused by creatine transporter (CRT, SLC6A8) deficiency, which reduces brain creatine level. The purpose of this study was to clarify the characteristics of cyclocreatine transport in HEK293 cells, which highly express endogenous CRT, in hCMEC/D3 cells, a human blood-brain barrier (BBB) model, and in CCDSs patient-derived fibroblasts with CRT mutations.

METHODS

Cells were incubated at 37°C with [¹⁴C]cyclocreatine (9 μM) and [¹⁴C]creatine (9 μM) for specified periods of times in the presence or absence of inhibitors, while the siRNAs were transfected by lipofection. Protein expression and mRNA expression were quantified using targeted proteomics and quantitative PCR, respectively.

RESULTS

[¹⁴C]Cyclocreatine was taken up by HEK293 cells in a time-dependent manner, while exhibiting saturable kinetics. The inhibition and siRNA knockdown studies demonstrated that the uptake of [¹⁴C]cyclocreatine by both HEK293 and hCMEC/D3 cells was mediated predominantly by CRT as well as [¹⁴C]creatine. In addition, uptake of [¹⁴C]cyclocreatine and [¹⁴C]creatine by the CCDSs patient-derived fibroblasts was found to be largely reduced.

CONCLUSION

The present study suggests that cyclocreatine is a CRT substrate, where CRT is the predominant contributor to influx of cyclocreatine into the brain at the BBB. Our findings provide vital insights for the purposes of treating CCDSs patients using cyclocreatine.

Rescue by 4-phenylbutyrate of several misfolded creatine transporter-1 variants linked to the creatine transporter deficiency syndrome

Diseases arising from misfolding of SLC6 transporters have been reported over recent years, e.g. folding-deficient mutants of the dopamine transporter and of the glycine transporter-2 cause infantile/juvenile Parkinsonism dystonia and hyperekplexia, respectively. Mutations in the coding sequence of the human creatine transporter-1 (hCRT-1/SLC6A8) gene result in a creatine transporter deficiency syndrome, which varies in its clinical manifestation from epilepsy, mental retardation, autism, development delay and motor dysfunction to gastrointestinal symptoms. Some of the mutations in hCRT-1 occur at residues, which are highly conserved across the SLC6 family. Here, we examined 16 clinically relevant hCRT-1 variants to verify the conjecture that they were misfolded and that this folding defect was amenable to correction. Confocal microscopy imaging revealed that the heterologously expressed YFP-tagged mutant CRTs were trapped in the endoplasmic reticulum (ER), co-localised with the ER-resident chaperone calnexin. In contrast, the wild type hCRT-1 reached the plasma membrane. Preincubation of transiently transfected HEK293 cells with the chemical chaperone 4-phenylbutyrate (4-PBA) restored ER export and surface expression of as well as substrate uptake by several folding-deficient CRT-1 mutants. A representative mutant (hCRT-1-P544L) was expressed in rat primary hippocampal neurons to verify pharmacochaperoning in a target cell: 4-PBA promoted the delivery of hCRT-1-P544L to the neurite extensions. These observations show that several folding-deficient hCRT-1 mutants can be rescued. This proof-of-principle justifies the search for additional pharmacochaperones to restore folding of 4PBA-unresponsive hCRT-1 mutants. Finally, 4-PBA is an approved drug in paediatric use: this provides a rationale for translating the current insights into clinical trials. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Pharmacological Characterization of [3H]ATPCA as a Substrate for Studying the Functional Role of the Betaine/GABA Transporter 1 and the Creatine Transporter

The betaine/γ-aminobutyric acid (GABA) transporter 1 (BGT1) is one of the four GABA transporters (GATs) involved in the termination of GABAergic neurotransmission. Although suggested to be implicated in seizure management, the exact functional importance of BGT1 in the brain is still elusive. This is partly owing to the lack of potent and selective pharmacological tool compounds that can be used to probe its function. We previously reported the identification of 2-amino-1,4,5,6-tetrahydropyrimidine-5-carboxylic acid (ATPCA), a selective substrate for BGT1 over GAT1/GAT3, but also an agonist for GABA_A receptors. With the aim of providing new functional insight into BGT1, we here present the synthesis and pharmacological characterization of the tritiated analogue, [³H]ATPCA. Using traditional uptake assays at recombinant transporters expressed in cell lines, [³H]ATPCA displayed a striking selectivity for BGT1 among the four GATs ( K_m and V_max values of 21 μM and 3.6 nmol ATPCA/(min × mg protein), respectively), but was also found to be a substrate for the creatine transporter (CreaT). In experiments with mouse cortical cell cultures, we observed a Na⁺-dependent [³H]ATPCA uptake in neurons, but not in astrocytes. The neuronal uptake could be inhibited by GABA, ATPCA, and a noncompetitive BGT1-selective inhibitor, indicating functional BGT1 in neurons. In conclusion, we report [³H]ATPCA as a novel radioactive substrate for both BGT1 and CreaT. The dual activity of the radioligand makes it most suitable for use in recombinant studies.

Abnormal N-Glycosylation of a Novel Missense Creatine Transporter Mutant, G561R, Associated with Cerebral Creatine Deficiency Syndromes Alters Transporter Activity and Localization

Cerebral creatine deficiency syndromes (CCDSs) are caused by loss-of-function mutations in creatine transporter (CRT, SLC6A8), which transports creatine at the blood-brain barrier and into neurons of the central nervous system (CNS). This results in low cerebral creatine levels, and patients exhibit mental retardation, poor language skills and epilepsy. We identified a novel human CRT gene missense mutation (c.1681 G>C, G561R) in Japanese CCDSs patients. The purpose of the present study was to evaluate the reduction of creatine transport in G561R-mutant CRT-expressing 293 cells, and to clarify the mechanism of its functional attenuation. G561R-mutant CRT exhibited greatly reduced creatine transport activity compared to wild-type CRT (WT-CRT) when expressed in 293 cells. Also, the mutant protein is localized mainly in intracellular membrane fraction, while WT-CRT is localized in plasma membrane. Western blot analysis revealed a 68 kDa band of WT-CRT protein in plasma membrane fraction, while G561R-mutant CRT protein predominantly showed bands at 55, 110 and 165 kDa in crude membrane fraction. The bands of both WT-CRT and G561R-mutant CRT were shifted to 50 kDa by N-glycosidase treatment. Our results suggest that the functional impairment of G561R-mutant CRT was probably caused by incomplete N-linked glycosylation due to misfolding during protein maturation, leading to oligomer formation and changes of cellular localization.

Novel, primate-specific PDE10A isoform highlights gene expression complexity in human striatum with implications on the molecular pathology of bipolar disorder

Bipolar disorder is a highly heritable neuropsychiatric disorder affecting nearly 2.5% of the population. Prior genetic studies identified a panel of common and rare single-nucleotide polymorphisms associated with the disease that map to the first intron of the PDE10A gene. RNA sequencing of striatal brain tissue from bipolar and healthy control subjects identified a novel transcript of PDE10A, named PDE10A19, that codes for a PDE10A isoform with a unique N terminus. Genomic sequences that can encode the novel N terminus were conserved in other primates but not rodents. The RNA transcript was expressed at equal or greater levels in the human striatum compared with the two annotated transcripts, PDE10A1 and PDE10A2. The PDE10A19 transcript was detected in polysomal fractions; western blotting experiments confirmed that the RNA transcript is translated into protein. Immunocytochemistry studies using transfected mouse striatal and cortical neurons demonstrated that the PDE10A19 protein distributes to the cytosol, like PDE10A1, and unlike PDE10A2, which is associated with plasma membranes. Immunoprecipitation and immunocytochemical experiments revealed that the PDE10A19 isoform interacts physically with PDE10A2 and, when expressed at elevated levels, interferes with the plasma membrane localization of PDE10A2. These studies illustrate the complexity of PDE10A gene expression in the human brain and highlight the need to unravel the gene's complex and complete coding capabilities along with its transcriptional and translational regulation to guide the development of therapeutic agents that target the protein for the treatment of neuropsychiatric illness.

Distribution of the creatine transporter throughout the human brain reveals a spectrum of creatine transporter immunoreactivity

Creatine is a molecule that supports energy metabolism in cells. It is carried across the plasma membrane by the creatine transporter. There has been recent interest in creatine for its neuroprotective effects in neurodegenerative diseases and its potential as a therapeutic agent. This study represents the first systematic investigation of the distribution of the creatine transporter in the human brain. We have used immunohistochemical techniques to map out its location and the intensity of staining. The transporter was found to be strongly expressed, especially in the large projection neurons of the brain and spinal cord. These include the pyramidal neurons in the cerebral cortex, Purkinje cells in the cerebellar cortex, and motor neurons of the somatic motor and visceromotor cranial nerve nuclei and the ventral horn of the spinal cord. Many other neurons in the brain also had some degree of creatine transporter immunoreactivity. By contrast, the medium spiny neurons of the striatum and the catecholaminergic neurons of the substantia nigra and locus coeruleus, which are implicated in neurodegenerative diseases, showed a very low to almost absent level of immunoreactivity for the transporter. We propose that the distribution may reflect the energy consumption by different cell types and that the extent of creatine transporter expression is proportional to the cell's energy requirements. Furthermore, the distribution indicates that supplemented creatine would be widely taken up by brain cells, although possibly less by those cells that degenerate in Huntington's and Parkinson's diseases.

X-linked creatine transporter deficiency: clinical aspects and pathophysiology

Creatine transporter deficiency was discovered in 2001 as an X-linked cause of intellectual disability characterized by cerebral creatine deficiency. This review describes the current knowledge regarding creatine metabolism, the creatine transporter and the clinical aspects of creatine transporter deficiency. The condition mainly affects the brain while other creatine requiring organs, such as the muscles, are relatively spared. Recent studies have provided strong evidence that creatine synthesis also occurs in the brain, leading to the intriguing question of why cerebral creatine is deficient in creatine transporter deficiency. The possible mechanisms explaining the cerebral creatine deficiency are discussed. The creatine transporter knockout mouse provides a good model to study the disease. Over the past years several treatment options have been explored but no treatment has been proven effective. Understanding the pathogenesis of creatine transporter deficiency is of paramount importance in the development of an effective treatment.

Creatine pretreatment protects cortical axons from energy depletion in vitro

Creatine is a natural nitrogenous guanidino compound involved in bioenergy metabolism. Although creatine has been shown to protect neurons of the central nervous system (CNS) from experimental hypoxia/ischemia, it remains unclear if creatine may also protect CNS axons, and if the potential axonal protection depends on glial cells. To evaluate the direct impact of creatine on CNS axons, cortical axons were cultured in a separate compartment from their somas and proximal neurites using a modified two-compartment culture device. Axons in the axon compartment were subjected to acute energy depletion, an in vitro model of white matter ischemia, by exposure to 6mM sodium azide for 30 min in the absence of glucose and pyruvate. Energy depletion reduced axonal ATP by 65%, depolarized axonal resting potential, and damaged 75% of axons. Application of creatine (10 mM) to both compartments of the culture at 24h prior to energy depletion significantly reduced axonal damage by 50%. In line with the role of creatine in the bioenergy metabolism, this application also alleviated the axonal ATP loss and depolarization. Inhibition of axonal depolarization by blocking sodium influx with tetrodotoxin also effectively reduced the axonal damage caused by energy depletion. Further study revealed that the creatine effect was independent of glial cells, as axonal protection was sustained even when creatine was applied only to the axon compartment (free from somas and glial cells) for as little as 2h. In contrast, application of creatine after energy depletion did not protect axons. The data provide the first evidence that creatine pretreatment may directly protect CNS axons from energy deficiency.