Optimizing the Substrate Uptake Rate of Solute Carriers

Effects of SCT genetic polymorphisms on methotrexate concentrations and toxicities in Chinese children with acute lymphoblastic leukemia

Solute carrier (SLC) transporters play a crucial role in facilitating the cellular uptake of various anticancer drugs, such as methotrexate (MTX). This study aimed to analyze the impact of nonsynonymous single nucleotide polymorphisms (SNPs) in SLC19A1, SLCO1B1, and SLCO1B3 on MTX exposure, toxicities, and prognosis in 148 patients with acute lymphoblastic leukemia (ALL). The SLCO1B3 rs7311358 polymorphism was significantly associated with the median dose-normalized MTX concentrations at 24 h (p < .05). There were significant differences in the proportions of patients with serum MTX levels >40 µmol/L at 24 h among SLC19A1 rs1051266 GG, GA, and AA genotype carriers (29.0, 24.7, and 6.2%, respectively, p < .05). The SLC19A1 rs1051266 G > A polymorphism also displayed significant associations with hematological (p < .05) and hepatic toxicities (p < .01). Our findings indicate that the analysis of SNPs in solute carrier transporters (SCTs) could offer valuable insights into the interpatient variability of MTX pharmacokinetics and toxicities in ALL children.

Electrochemical optimization of bioelectrochemically improved anaerobic digestion for agricultural digestates' valorisation to biomethane

Bioelectrochemically improved anaerobic digestion (AD-BES) represents an upgrading strategy for existing biogas plants, consisting of the integration of bioelectrodes within the AD reactor. For this study, a series of laboratory-scale AD-BES reactors were operated, valorising agricultural digestates through the production of biogas. The reactors were inoculated and started-up with three different digestates, leading to significant differences in the microbial community developed on the bioelectrodes. After the start-up was completed, the AD-BES were all fed with a unique digestate, to evaluate the stability of the bioelectrodes' biofilm performances against variations of the organic feedstock. In terms of methane (CH4) production rate, the presence of bioelectrodes allowed between 25 and 82% improvement, compared with control AD reactors. The application of an optimal voltage of 0.3 V resulted in an additional 40% improvement in CH4 production rate, but only when the biofilm was previously acclimated to the fed digestate. Comprehensive microbial characterization revealed that fed digestate significantly influences the composition and homogenization of microbial communities within AD-BES reactors, with applied voltage showing only a secondary effect. Even when reactors were transitioned to a uniform digestate feeding, resulting in closely similar microbial profiles, variations in CH4 production persisted, underscoring the lasting impact of initial microbial conditioning. A critical observation was the differentiation in archaeal colonization on bioelectrodes at 0.3 V, the voltage yielding the highest CH4 conversion. These insights suggest that while the microbial community structure depends on fed digestate, operational efficiency and methanogenic potential are intricately linked to both initial microbial establishment and the specific electrochemical conditions applied to AD-BES reactors.

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.

Allosteric modulators of solute carrier function: a theoretical framework

Large-scale drug screening is currently the basis for the identification of new chemical entities. This is a rather laborious approach, because a large number of compounds must be tested to cover the chemical space in an unbiased fashion. However, the structures of targetable proteins have become increasingly available. Thus, a new era has arguably been ushered in with the advent of methods, which allow for structure-based docking campaigns (i.e., virtual screens). Solute carriers (SLCs) are among the most promising drug targets. This claim is substantiated by the fact that a large fraction of the 400 solute carrier genes is associated with human diseases. The ability to dock large ligand libraries into selected structures of solute carriers has set the stage for rational drug design. In the present study, we show that these structure-based approaches can be refined by taking into account how solute carriers operate. We specifically address the feasibility of targeting solute carriers with allosteric modulators, because their actions differ fundamentally from those of ligands, which bind to the substrate binding site. For the pertinent analysis we used transition state theory in conjunction with the linear free energy relationship (LFER). These provide the theoretical framework to understand how allosteric modulators affect solute carrier function.

A mechanism of uncompetitive inhibition of the serotonin transporter

The serotonin transporter (SERT/SLC6A4) is arguably the most extensively studied solute carrier (SLC). During its eponymous action - that is, the retrieval of serotonin from the extracellular space - SERT undergoes a conformational cycle. Typical inhibitors (antidepressant drugs and cocaine), partial and full substrates (amphetamines and their derivatives), and atypical inhibitors (ibogaine analogues) bind preferentially to different states in this cycle. This results in competitive or non-competitive transport inhibition. Here, we explored the action of N-formyl-1,3-bis (3,4-methylenedioxyphenyl)-prop-2-yl-amine (ECSI#6) on SERT: inhibition of serotonin uptake by ECSI#6 was enhanced with increasing serotonin concentration. Conversely, the KM for serotonin was lowered by augmenting ECSI#6. ECSI#6 bound with low affinity to the outward-facing state of SERT but with increased affinity to a potassium-bound state. Electrophysiological recordings showed that ECSI#6 preferentially interacted with the inward-facing state. Kinetic modeling recapitulated the experimental data and verified that uncompetitive inhibition arose from preferential binding of ECSI#6 to the K+-bound, inward-facing conformation of SERT. This binding mode predicted a pharmacochaperoning action of ECSI#6, which was confirmed by examining its effect on the folding-deficient mutant SERT-PG601,602AA: preincubation of HEK293 cells with ECSI#6 restored export of SERT-PG601,602AA from the endoplasmic reticulum and substrate transport. Similarly, in transgenic flies, the administration of ECSI#6 promoted the delivery of SERT-PG601,602AA to the presynaptic specialization of serotonergic neurons. To the best of our knowledge, ECSI#6 is the first example of an uncompetitive SLC inhibitor. Pharmacochaperones endowed with the binding mode of ECSI#6 are attractive, because they can rescue misfolded transporters at concentrations, which cause modest transport inhibition.

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.