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The cataract and glucosuria associated monocarboxylate transporter MCT12 is a new creatine transporter.

ABSTRACT: Creatine transport has been assigned to creatine transporter 1 (CRT1), encoded by mental retardation associated SLC6A8. Here, we identified a second creatine transporter (CRT2) known as monocarboxylate transporter 12 (MCT12), encoded by the cataract and glucosuria associated gene SLC16A12. A non-synonymous alteration in MCT12 (p.G407S) found in a patient with age-related cataract (ARC) leads to a significant reduction of creatine transport. Furthermore, Slc16a12 knockout (KO) rats have elevated creatine levels in urine. Transport activity and expression characteristics of the two creatine transporters are distinct. CRT2 (MCT12)-mediated uptake of creatine was not sensitive to sodium and chloride ions or creatine biosynthesis precursors, breakdown product creatinine or creatine phosphate. Increasing pH correlated with increased creatine uptake. Michaelis-Menten kinetics yielded a Vmax of 838.8 pmol/h/oocyte and a Km of 567.4 µm. Relative expression in various human tissues supports the distinct mutation-associated phenotypes of the two transporters. SLC6A8 was predominantly found in brain, heart and muscle, while SLC16A12 was more abundant in kidney and retina. In the lens, the two transcripts were found at comparable levels. We discuss the distinct, but possibly synergistic functions of the two creatine transporters. Our findings infer potential preventive power of creatine supplementation against the most prominent age-related vision impaired condition.

SUBMITTER: Abplanalp J

PROVIDER: S-EPMC3723308 | biostudies-literature | 2013 Aug

REPOSITORIES: biostudies-literature

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The cataract and glucosuria associated monocarboxylate transporter MCT12 is a new creatine transporter.

Abplanalp Jeannette J Laczko Endre E Philp Nancy J NJ Neidhardt John J Zuercher Jurian J Braun Philipp P Schorderet Daniel F DF Munier Francis L FL Verrey François F Berger Wolfgang W Camargo Simone M R SM Kloeckener-Gruissem Barbara B

Human molecular genetics 20130410 16

Creatine transport has been assigned to creatine transporter 1 (CRT1), encoded by mental retardation associated SLC6A8. Here, we identified a second creatine transporter (CRT2) known as monocarboxylate transporter 12 (MCT12), encoded by the cataract and glucosuria associated gene SLC16A12. A non-synonymous alteration in MCT12 (p.G407S) found in a patient with age-related cataract (ARC) leads to a significant reduction of creatine transport. Furthermore, Slc16a12 knockout (KO) rats have elevated ...[more]

PMID: 23578822

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Project description:BackgroundGuanidinoacetate (GAA) induces epileptogenesis and neurotoxicity in the brain. As epileptic animal models have been reported to show elevated cerebral GAA levels, the processing mechanism of GAA in the brain is important for maintaining brain homeostasis. We have revealed that GAA in the cerebrospinal fluid (CSF) is removed by incorporation into the choroid plexus epithelial cells (CPxEpic), which form the blood-CSF barrier (BCSFB). However, the processing mechanism of GAA incorporated into CPxEpic remains unknown. We have reported that monocarboxylate transporter 12 (MCT12) functions as an efflux transporter of GAA and creatine, a metabolite of GAA, in the kidneys and liver. Therefore, we aimed to clarify the role of MCT12 in GAA dynamics in CPxEpic.MethodsProtein expression and localization in CPxEpic were evaluated using immunohistochemistry. Metabolic analysis was performed using high-performance liquid chromatography (HPLC) 24 h after the addition of [14C]GAA to TR-CSFB3 cells, which are conditionally immortalized rat CPxEpic. The efflux transport of [14C]creatine was evaluated in TR-CSFB3 cells after transfection with MCT12 small interfering RNA (siRNA). The CSF-to-brain parenchyma transfer of creatine was measured after intracerebroventricular injection in rats.ResultsImmunohistochemical staining revealed that MCT12-derived signals merged with those of the marker protein at the apical membrane of CPxEpic, suggesting that MCT12 is localized on the apical membrane of CPxEpic. The expression levels of guanidinoacetate N-methyltransferase (GAMT), which catalyzes the conversion of GAA to creatine, in TR-CSFB3 cells was also indicated, and GAA was considered to be metabolized to creatine after influx transport into CPxEpic, after which creatine was released into the CSF. Creatine release from TR-CSFB3 cells decreased following MCT12 knockdown. The contribution ratio of MCT12 to the release of creatine was more than 50%. The clearance of CSF-to-brain parenchyma transfer of creatine was 4.65 µL/(min·g brain), suggesting that biosynthesized creatine in CPxEpic is released into the CSF and supplied to the brain parenchyma.ConclusionsIn CPxEpic, GAA is metabolized to creatine via GAMT. Biosynthesized creatine is then released into the CSF via MCT12 and supplied to the brain parenchyma.

Project description:Creatine kinase (CK) is an energy storage enzyme that plays an important role in energy metabolism. CK/phosphocreatine functions as an energy buffer and links ATP production sites with ATP utilization sites. Several key mutations in the ?A-crystallin (cryaa) and ?B-crystallin (cryab) genes have been linked with autosomal-dominant, hereditary human cataracts. The cryaa-R49C mutation was identified in a four-generation Caucasian family. We previously identified an increase in the quantity of CK complexed with ?-crystallin in the lenses of knock-in mice expressing the cryaa-R49C mutation using proteomic analyses. Increased levels of CK in postnatal cataractous lenses may indicate increased ATP requirements during early cataract development. To gain a further understanding of the relationship between CK and ?-crystallin, we investigated whether ?-crystallin interacts with and forms complexes with CK, in vitro. Isothermal titration calorimetry (ITC) showed that each CK dimer bound to 28 ?-crystallin subunits, with a Kd of 3.3 × 10-7 M, and that the interaction between ?-crystallin and CK was endothermic, thermodynamically favorable, and entropy-driven. High-salt concentrations did not affect the interaction between CK and ?-crystallin, suggesting that the interaction between CK and ?-crystallin is primarily hydrophobic. Gel permeation chromatography (GPC) detected water-soluble ?-crystallin and CK complexes, as determined by increased light scattering after complex formation. In addition, CK and ?-crystallin formed partially-water-insoluble, high-molecular-mass complexes. Enzyme-linked immunosorbent assay (ELISA)-based enzymatic activity analyses of lens homogenates showed a 17-fold increase in CK activity in the postnatal lenses of cryaa-R49C knock-in mice. These studies indicate that the interaction between ?-crystallin and CK is functionally important and that increased CK levels may be necessary to meet the increased ATP demands of ATP-dependent functions in cataractous lenses.

Project description:Monocarboxylate transporter 8 (MCT8) deficiency or the Allan-Herndon-Dudley Syndrome (AHDS) is an X-linked psychomotor disability syndrome with around 320 clinical cases described worldwide. SLC16A2 gene mutations, encoding the thyroid hormone (TH) transporter MCT8, result in intellectual disability due to impaired TH uptake in the developing brain. MCT8 deficiency is a multi-organ affecting disease with a predominant neuronal cell-based pathology, with the glial component inadequately investigated. However, deficiency in myelin, a key component of white matter (WM) enabling fast nerve conduction, is a TH-dependent hallmark of the disease. Nevertheless, analysis of the myelin status in AHDS patients has led to conflicting interpretations. The majority of individual case studies reported delayed myelination, that was restored later in life. In contrast, post-mortem studies and high-resolution MRIs detected WM (micro-) abnormalities throughout adolescence, suggesting permanent hypomyelination. Thus, interpretations vary depending on methodology to investigate WM microstructure. Further, it is unknown whether the mutation within the MCT8 is linked to the severity of the myelin deficiency. Consequently, terminology is inconsistent among reports, and AHDS is occasionally misdiagnosed as another WM disorder. The evolutionary conserved TH signaling pathway that promotes the generation of myelinating oligodendrocytes enabled deciphering how the lack of MCT8 might affect myelinogenesis. Linking patient findings on myelination to those obtained from models of MCT8 deficiency revealed underlying pathophysiological mechanisms, but knowledge gaps remain, notably how myelination progresses both spatially and temporally in MCT8 deficiency. This limits predicting how myelin integrity might benefit therapeutically, and when to initiate. A recurrent observation in clinical trials is the absence of neurological improvement. Testing MCT8-independent thyromimetics in models, and evaluating treatments used in other demyelinating diseases, despite different etiologies, is crucial to propose new therapeutic strategies combatting this devastating disease.

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The cataract and glucosuria associated monocarboxylate transporter MCT12 is a new creatine transporter.

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The cataract and glucosuria associated monocarboxylate transporter MCT12 is a new creatine transporter.

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