The role of cholesterol recognition (CARC/CRAC) mirror codes in the allosterism of the human organic cation transporter 2 (OCT2, SLC22A2)

Effect of 7-ketocholesterol incorporation on substrate binding affinity and turnover rate of the organic cation transporter 2 (OCT2)

The organic cation transporter 2 (OCT2) is pivotal in the renal elimination of several positively charged molecules. OCT2 mode of transport is profoundly influenced by the level of membrane cholesterol. The aim of this study was to investigate the effect of oxidized cholesterol on OCT2 transport activity in human embryonic kidney 293 cells stably transfected with OCT2 (OCT2-HEK293) and in primary renal proximal tubular epithelial cells (RPTEC). Cholesterol was exchanged with 7-ketocholesterol, the main product of cholesterol auto-oxidation, by exposing cells to sterol-saturated methyl-β-cyclodextrin (mβcd). After a 30 min-exposure, approximately 50% of the endogenous cholesterol was replaced by 7-ketocholesterol without significant changes in total sterol level. In the presence of 7-ketocholesterol, [3H]1-methyl-4-phenylpyridinium (MPP+) uptake was significantly reduced in both cell lines. 7-ketocholesterol incorporation did not affect lipid raft integrity, nor OCT2 surface expression and spatial organization. The inhibitory effect of 7-ketocholesterol on MPP+ uptake was abolished by the presence of MPP+ in the trans-compartment. In the presence of 7-ketocholesterol, both Kt and Vmax of MPP+ influx decreased. Molecular docking using OCT2 structure in outward occluded conformation showed overlapping poses and similar binding energies between cholesterol and 7-ketocholesterol. The thermal stability of OCT2 was not changed when cholesterol was replaced with 7-ketocholesterol. We conclude that 7-ketocholesterol confers a higher rigidity to the carrier by reducing its conformational entropy, arguably as a result of changes in plasma membrane physical properties, thereby facilitating the achievement of a higher affinity state at the expense of the mobility and overall cycling rate of the transporter.

Characterization of ligand-induced thermal stability of the human organic cation transporter 2 (OCT2)

Introduction: The human organic cation transporter 2 (OCT2) is involved in the transport of endogenous quaternary amines and positively charged drugs across the basolateral membrane of proximal tubular cells. In the absence of a structure, the progress in unraveling the molecular basis of OCT2 substrate specificity is hampered by the unique complexity of OCT2 binding pocket, which seemingly contains multiple allosteric binding sites for different substrates. Here, we used the thermal shift assay (TSA) to better understand the thermodynamics governing OCT2 binding to different ligands.

Methods: Molecular modelling and in silico docking of different ligands revealed two distinct binding sites at OCT2 outer part of the cleft. The predicted interactions were assessed by cis-inhibition assay using [3H]1-methyl-4-phenylpyridinium ([3H]MPP+) as a model substrate, or by measuring the uptake of radiolabeled ligands in intact cells. Crude membranes from HEK293 cells harboring human OCT2 (OCT2-HEK293) were solubilized in n-Dodecyl-β-D-Maltopyranoside (DDM), incubated with the ligand, heated over a temperature gradient, and then pelleted to remove heat-induced aggregates. The OCT2 in the supernatant was detected by western blot.

Results: Among the compounds tested, cis-inhibition and TSA assays showed partly overlapping results. Gentamicin and methotrexate (MTX) did not inhibit [3H]MPP+ uptake but significantly increased the thermal stabilization of OCT2. Conversely, amiloride completely inhibited [3H]MPP+ uptake but did not affect OCT2 thermal stabilization. [3H]MTX intracellular level was significantly higher in OCT2-HEK293 cells than in wild type cells. The magnitude of the thermal shift (ΔTm) did not provide information on the binding. Ligands with similar affinity showed markedly different ΔTm, indicating different enthalpic and entropic contributions for similar binding affinities. The ΔTm positively correlated with ligand molecular weight/chemical complexity, which typically has high entropic costs, suggesting that large ΔTm reflect a larger displacement of bound water molecules.

Discussion: In conclusion, TSA might represent a viable approach to expand our knowledge on OCT2 binding descriptors.

Cholesterol in Class C GPCRs: Role, Relevance, and Localization

G-protein coupled receptors (GPCRs), one of the largest superfamilies of cell-surface receptors, are heptahelical integral membrane proteins that play critical roles in virtually every organ system. G-protein-coupled receptors operate in membranes rich in cholesterol, with an imbalance in cholesterol level within the vicinity of GPCR transmembrane domains affecting the structure and/or function of many GPCRs, a phenomenon that has been linked to several diseases. These effects of cholesterol could result in indirect changes by altering the mechanical properties of the lipid environment or direct changes by binding to specific sites on the protein. There are a number of studies and reviews on how cholesterol modulates class A GPCRs; however, this area of study is yet to be explored for class C GPCRs, which are characterized by a large extracellular region and often form constitutive dimers. This review highlights specific sites of interaction, functions, and structural dynamics involved in the cholesterol recognition of the class C GPCRs. We summarize recent data from some typical family members to explain the effects of membrane cholesterol on the structural features and functions of class C GPCRs and speculate on their corresponding therapeutic potential.

OCTN1: A Widely Studied but Still Enigmatic Organic Cation Transporter Linked to Human Pathology and Drug Interactions

The Novel Organic Cation Transporter, OCTN1, is the first member of the OCTN subfamily; it belongs to the wider Solute Carrier family SLC22, which counts many members including cation and anion organic transporters. The tertiary structure has not been resolved for any cation organic transporter. The functional role of OCNT1 is still not well assessed despite the many functional studies so far conducted. The lack of a definitive identification of OCTN1 function can be attributed to the different experimental systems and methodologies adopted for studying each of the proposed ligands. Apart from the contradictory data, the international scientific community agrees on a role of OCTN1 in protecting cells and tissues from oxidative and/or inflammatory damage. Moreover, the involvement of this transporter in drug interactions and delivery has been well clarified, even though the exact profile of the transported/interacting molecules is still somehow confusing. Therefore, OCTN1 continues to be a hot topic in terms of its functional role and structure. This review focuses on the most recent advances on OCTN1 in terms of functional aspects, physiological roles, substrate specificity, drug interactions, tissue expression, and relationships with pathology.