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

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.

Silencing GnT-V reduces oxaliplatin chemosensitivity in human colorectal cancer cells through N-glycan alteration of organic cation transporter member 2

Organic cation transporter member 2 (OCT2) is an N-glycosylated transporter that has been shown to be closely associated with the transport of antitumor drugs. Oxaliplatin, a platinum-based drug, is used for the chemotherapy of colorectal cancer (CRC). However, oxaliplatin resistance is a major challenge in the treatment of advanced CRC. The aim of the present study was to better understand the mechanism underlying the chemosensitivity of CRC cells to oxaliplatin. The present study describes a potential novel strategy for enhancing oxaliplatin sensitivity involving the glycosylation of this drug transporter, specifically the modification of β-1,6-N-acetylglucosamine (GlcNAc) residues by N-acetylglucosaminyltransferase V (GnT-V). The results revealed that the downregulation of GnT-V inhibited the oxaliplatin chemosensitivity of CW-2 cells. Furthermore, the knockdown of GnT-V caused a marked reduction in the presence of β-1,6-GlcNAc structures on OCT2 and decreased the localization of OCT2 in the cytomembrane, which were associated with a reduced uptake of oxaliplatin in wild-type and oxaliplatin-resistant CW-2 cells. Overall, the study provides novel insights into the molecular mechanism by which GnT-V regulates the chemosensitivity to oxaliplatin, which involves the modulation of the drug transporter OCT2 by N-glycosylation in CRC cells.

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.

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.

[Elucidation of Disease Mechanisms Based on Transport Function at Tissue Barriers and Challenges in Drug Development]

Tissue barriers contribute to the maintenance of homeostasis in the body, and tissue barrier dysfunction presents a risk factor for a variety of diseases. The blood-brain barrier (BBB) is a major tissue barrier acting as a static barrier and dynamic interface that plays an important role in the maintenance of central nervous system homeostasis. We show the functional characterization of the brain-to-blood efflux transport system of amyloid-β peptide (Aβ) across the BBB. We found that activated vitamin D3 may be a candidate agent for modulating the Aβ clearance across the BBB. Cerebral creatine deficiency syndromes are caused by loss-of-function mutations in the creatine transporter (CRT; SLC6A8), which transports creatine at the BBB. We found that functional impairment of CRT due to a G561R mutation resulted in incomplete N-linked glycosylation due to misfolding during protein maturation, leading to impaired creatine transport activity at the BBB. To develop a delivery system for biomedicine across the tissue barrier, we established a screening system to identify cell-penetrating peptides by a combination of in vitro cell permeability screening assays and phage display technology. Using this system, we identified cyclic hepta-peptides that are able to facilitate intestinal absorption of phages in vitro and in vivo, which are promising candidates as a carrier for macromolecular biomedicines. In conclusion, these studies focusing on the dynamic interface of tissue barriers will contribute to knowledge on disease pathogenesis as well as the development of a targeted biomedicine delivery system.

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'.

Relax, Cool Down and Scaffold: How to Restore Surface Expression of Folding-Deficient Mutant GPCRs and SLC6 Transporters

Many diseases arise from mutations, which impair protein folding. The study of folding-deficient variants of G protein-coupled receptors and solute carrier 6 (SLC6) transporters has shed light on the folding trajectory, how it is monitored and how misfolding can be remedied. Reducing the temperature lowers the energy barrier between folding intermediates and thereby eliminates stalling along the folding trajectory. For obvious reasons, cooling down is not a therapeutic option. One approach to rescue misfolded variants is to use membrane-permeable orthosteric ligands. Antagonists of GPCRs are—in many instances—effective pharmacochaperones: they restore cell surface expression provided that they enter cells and bind to folding intermediates. Pharmacochaperoning of SLC6 transporters is less readily achieved because the ionic conditions in the endoplasmic reticulum (ER) are not conducive to binding of typical inhibitors. The second approach is to target the heat-shock protein (HSP) relay, which monitors the folding trajectory on the cytosolic side. Importantly, orthosteric ligands and HSP-inhibitors are not mutually exclusive. In fact, pharmacochaperones and HSP-inhibitors can act in an additive or synergistic manner. This was exemplified by rescuing disease-causing, folding-deficient variants of the human dopamine transporters with the HSP70 inhibitor pifithrin-μ and the pharmacochaperone noribogaine in Drosophila melanogaster.

SLC6 Transporter Folding Diseases and Pharmacochaperoning

The human genome encodes 19 genes of the solute carrier 6 (SLC6) family; non-synonymous changes in the coding sequence give rise to mutated transporters, which are misfolded and thus cause diseases in the affected individuals. Prominent examples include mutations in the transporters for dopamine (DAT, SLC6A3), for creatine (CT1, SLC6A8), and for glycine (GlyT2, SLC6A5), which result in infantile dystonia, mental retardation, and hyperekplexia, respectively. Thus, there is an obvious unmet medical need to identify compounds, which can remedy the folding deficit. The pharmacological correction of folding defects was originally explored in mutants of the serotonin transporter (SERT, SLC6A4), which were created to study the COPII-dependent export from the endoplasmic reticulum. This led to the serendipitous discovery of the pharmacochaperoning action of ibogaine. Ibogaine and its metabolite noribogaine also rescue several disease-relevant mutants of DAT. Because the pharmacology of DAT and SERT is exceptionally rich, it is not surprising that additional compounds have been identified, which rescue folding-deficient mutants. These compounds are not only of interest for restoring DAT function in the affected children. They are also likely to serve as useful tools to interrogate the folding trajectory of the transporter. This is likely to initiate a virtuous cycle: if the principles underlying folding of SLC6 transporters are understood, the design of pharmacochaperones ought to be facilitated.