Dynamics of ABC Transporter P-glycoprotein in Three Conformational States

The association between abcb1 gene polymorphism and clopidogrel response variability in stroke ischemic: a cross sectional study

BACKGROUND

Clopidogrel has been the primary choice of antiplatelet in ischemic stroke that inhibits adenosine diphosphate (ADP)-induced platelet aggregation. P-glycoprotein (P-gp) multidrug resistance-1 (MDR1) is a transmembrane efflux transporter in intestinal cells that plays a significant role in clopidogrel absorption, therefore may affect platelet aggregation. P-gp is encoded by the ABCB1 gene. This study aims to evaluate the effect of ABCB1 polymorphism on clopidogrel response variability in ischemic stroke patients and its genotype frequency.

METHODS

A cross-sectional study was conducted in ischemic stroke patients who received clopidogrel between 2020 and 2023 in RSUI/RSCM. All subjects were assessed for ABCB1 polymorphisms C3435T and C1236T. Platelet aggregation were measured using VerifyNow PRU. Clopidogrel response variability was classified into unresponsive (> 208 PRU), responsive (95-208 PRU), and bleeding risk (< 95 PRU).

RESULTS

124 subjects enrolled in this study, with 12,9% of subjects classified as non-responsive/resistant, 49,5% as responsive, and 41,9% as bleeding risk. ABCB1 C1236T homozygote wildtype (CC) was associated with 3,76 times higher bleeding risk than other variants (p = 0,008; 95%CI 1,41 - 10,07). Genotype frequency of ABCB1 C3435T homozygote wildtype, heterozygote, and homozygote variants were 35,9%, 43,5% and 16,9%, respectively; while the genotype frequency of ABCB1 C1236T were 17,8%, 39,5%, and 42,7%, respectively.

CONCLUSION

ABCB1 C1236T homozygote wildtype was associated with 3,76 times higher bleeding risk than other variants. The most common genotype frequency of ABCB1 C1236T was homozygote variant; while for ABCB1 C3435T was heterozygote.

Atomic force microscope kymograph analysis: A case study of two membrane proteins

Kymograph analysis is employed across the biological atomic force microscopy (AFM) community to boost temporal resolution. The method is well suited for revealing protein dynamics at the single molecule level in near-native conditions. Yet, kymograph analysis comes with limitations that depend on several factors including protein geometry and instrumental drift. This work focuses on conformational dynamics of difficult-to-study sparse distributions of membrane proteins. We compare and contrast AFM kymograph analysis for two proteins, one of which (SecDF) exhibits conformational dynamics primarily in the vertical direction (normal to the membrane surface) and the other (Pgp) exhibits a combination of lateral dynamics and vertical motion. Common experimental issues are analyzed including translational and rotational drift. Conformational transition detection is evaluated via kymograph simulations followed by state detection algorithms. We find that kymograph analysis is largely robust to lateral drift. Displacement of the AFM line scan trajectory away from the protein center of mass by a few nanometers, roughly half of the molecule diameter, does not significantly affect transition detection nor generate undue dwell time errors. On the other hand, for proteins like Pgp that exhibit significant azimuthal maximum height dependence, rotational drift can potentially produce artifactual transitions. Measuring the height of a membrane protein protrusion is generally superior to measurement of width, confirming intuition based on vertical resolution superiority. In low signal-to-noise scenarios, common state detection algorithms struggle with transition detection as opposed to infinite hidden Markov models. AFM kymography represents a valuable addition to the membrane biophysics toolkit; continued hardware and software improvements are poised to expand the method's impact in the field.

Mass Spectrometry Investigation of Some ATP-Binding Cassette (ABC) Proteins

Drug resistance remains one of the main causes of poor outcome in cancer therapy. It is also becoming evident that drug resistance to both chemotherapy and to antibiotics is driven by more than one mechanism. So far, there are at least eight recognized mechanisms behind such resistance. In this review, we choose to discuss one of these mechanisms, which is known to be partially driven by a class of transmembrane proteins known as ATP-binding cassette (ABC) transporters. In normal tissues, ABC transporters protect the cells from the toxic effects of xenobiotics, whereas in tumor cells, they reduce the intracellular concentrations of anticancer drugs, which ultimately leads to the emergence of multidrug resistance (MDR). A deeper understanding of the structures and the biology of these proteins is central to current efforts to circumvent resistance to both chemotherapy, targeted therapy, and antibiotics. Understanding the biology and the function of these proteins requires detailed structural and conformational information for this class of membrane proteins. For many years, such structural information has been mainly provided by X-ray crystallography and cryo-electron microscopy. More recently, mass spectrometry-based methods assumed an important role in the area of structural and conformational characterization of this class of proteins. The contribution of this technique to structural biology has been enhanced by its combination with liquid chromatography and ion mobility, as well as more refined labelling protocols and the use of more efficient fragmentation methods, which allow the detection and localization of labile post-translational modifications. In this review, we discuss the contribution of mass spectrometry to efforts to characterize some members of the ATP-binding cassette (ABC) proteins and why such a contribution is relevant to efforts to clarify the link between the overexpression of these proteins and the most widespread mechanism of chemoresistance.

Probing the allosteric NBD-TMD crosstalk in the ABC transporter MsbA by solid-state NMR

The ABC transporter MsbA plays a critical role in Gram-negative bacteria in the regulation of the outer membrane by translocating core-LPS across the inner membrane. Additionally, a broad substrate specificity for lipophilic drugs has been shown. The allosteric interplay between substrate binding in the transmembrane domains and ATP binding and turnover in the nucleotide-binding domains must be mediated via the NBD/TMD interface. Previous studies suggested the involvement of two intracellular loops called coupling helix 1 and 2 (CH1, CH2). Here, we demonstrate by solid-state NMR spectroscopy that substantial chemical shift changes within both CH1 and CH2 occur upon substrate binding, in the ATP hydrolysis transition state, and upon inhibitor binding. CH2 is domain-swapped within the MsbA structure, and it is noteworthy that substrate binding induces a larger response in CH2 compared to CH1. Our data demonstrate that CH1 and CH2 undergo structural changes as part of the TMD-NBD cross-talk.

The transport activity of the multidrug ABC transporter BmrA does not require a wide separation of the nucleotide-binding domains

ATP-binding cassette (ABC) transporters are ubiquitous membrane proteins responsible for the translocation of a wide diversity of substrates across biological membranes. Some of them confer multidrug or antimicrobial resistance to cancer cells and pathogenic microorganisms, respectively. Despite a wealth of structural data gained in the last two decades, the molecular mechanism of these multidrug efflux pumps remains elusive, including the extent of separation between the two nucleotide-binding domains (NBDs) during the transport cycle. Based on recent outward-facing structures of BmrA, a homodimeric multidrug ABC transporter from Bacillus subtilis, we introduced a cysteine mutation near the C-terminal end of the NBDs to analyze the impact of disulfide-bond formation on BmrA function. Interestingly, the presence of the disulfide bond between the NBDs did not prevent the ATPase, nor did it affect the transport of Hoechst 33342 and doxorubicin. Yet, the 7-amino-actinomycin D was less efficiently transported, suggesting that a further opening of the transporter might improve its ability to translocate this larger compound. We solved by cryo-EM the apo structures of the cross-linked mutant and the WT protein. Both structures are highly similar, showing an intermediate opening between their NBDs while their C-terminal extremities remain in close proximity. Distance measurements obtained by electron paramagnetic resonance spectroscopy support the intermediate opening found in these 3D structures. Overall, our data suggest that the NBDs of BmrA function with a tweezers-like mechanism distinct from the related lipid A exporter MsbA.

Study Models of Drug-Drug Interactions Involving P-Glycoprotein: The Potential Benefit of P-Glycoprotein Modulation at the Kidney and Intestinal Levels

P-glycoprotein (P-gp) is a crucial membrane transporter situated on the cell's apical surface, being responsible for eliminating xenobiotics and endobiotics. P-gp modulators are compounds that can directly or indirectly affect this protein, leading to changes in its expression and function. These modulators can act as inhibitors, inducers, or activators, potentially causing drug-drug interactions (DDIs). This comprehensive review explores diverse models and techniques used to assess drug-induced P-gp modulation. We cover several approaches, including in silico, in vitro, ex vivo, and in vivo methods, with their respective strengths and limitations. Additionally, we explore the therapeutic implications of DDIs involving P-gp, with a special focus on the renal and intestinal elimination of P-gp substrates. This involves enhancing the removal of toxic substances from proximal tubular epithelial cells into the urine or increasing the transport of compounds from enterocytes into the intestinal lumen, thereby facilitating their excretion in the feces. A better understanding of these interactions, and of the distinct techniques applied for their study, will be of utmost importance for optimizing drug therapy, consequently minimizing drug-induced adverse and toxic effects.

P-glycoprotein mechanical functional analysis using in silico molecular modeling: Pharmacokinetic variability according to ABCB1 c.2677G > T/A genetic polymorphisms

P-glycoprotein (P-gp) is a widely membrane-expressed multi-drug transporter. It is unclear whether the pharmacokinetic diversity of P-gp substrates is highly dependent on ABCB1 polymorphisms encoding P-gp. The purpose of this study is to analyze the mechanistic function of P-gp through in silico molecular modeling and to approach the resolution of controversy over pharmacokinetic differences according to ABCB1 polymorphisms. P-gp conformations of apo, ligand-docked, and outward-facing states can be modeled based on structural information of human P-gp. And polymorphic P-gp structures were constructed through homology modeling. ABCB1 c.2677G > T/A (Ala893Ser/Thr), did not correspond to P-gp's nucleotide-binding-domain (NBD) or drug-binding-pocket (DBP) or involve mechanical conformational changes. Although amino acid substitution by ABCB1 c.2677G > T/A caused a 30 % increased strain in an α-helix hinge between the NBD and DBP in P-gp's internal tunnel, there were no overall structural changes compared to wild-type. ABCB1 c.2677G > T/A may increase torsional energy, impacting conformational change rate, but this does not significantly affect P-gp's general functioning. Fexofenadine docking into P-gp's DBP explained the substrate interaction, but no effect by ABCB1 c.2677G > T/A was confirmed. Our findings provide additional insights useful in resolving the debate about the influence of ABCB1 polymorphisms on the interindividual pharmacokinetic variability of P-gp substrates.

Isolation, cloning, and tissue distribution and functional analysis of ShP-glycoprotein in the freshwater crab Sinopotamon henanense exposed to Cd and Cd-QDs

P-glycoprotein (Pgp), a member of ATP binding cassette (ABC) transporter family, can extrude toxic substances out of cells by mediating multi-xenobiotic resistance (MXR) in aquatic organisms, however, its regulation and association with MXR are still unclear. In this work, the genetic information of Pgp in freshwater crab Sinopotamon henanense (ShPgp) was revealed for the first time. ShPgp with a total of 4488 bp was cloned and analyzed, which includes 4044 bp open reading frame, 353 bp 3' untranslated region, and 91 bp 5' untranslated region. The recombinant ShPGP were expressed in Saccharomyces cerevisiae and taken for SDS-PAGE and western blot analysis. ShPGP was widely expressed in the midgut, hepatopancreas, testis, ovary, gill, hemocytes, accessory gonad and myocardium of the crabs studied. The images of immunohistochemistry indicated that ShPgp was mainly distributed in the cytoplasm and cell membrane. When the crabs were exposed to cadmium or cadmium containing quantum dots (Cd-QDs), not only the relative expression of ShPgp mRNA and the protein produced were enhanced, but also the MXR activity and ATP contents. The relative expression of target genes related to energy metabolism, detoxification and apoptosis was also determined in the carbs exposed to Cd or Cd-QDs. The results showed that bcl-2 was significantly down-regulated, while other genes were up-regulated except PPAR (not affected). However, when the Shpgp in treated crabs was interfering by knockdown technique, their apoptosis and the expression of proteolytic enzyme genes and transcription factors MTF1 and HSF1 were also elevated, while the expression of apoptosis inhibiting and fat metabolism genes were compromised. Based on the observation, we concluded that MTF1 and HSF1 were involved in gene transcription regulation of mt and MXR, respectively, while PPAR had limited regulatory effect on those genes in S. henanense. NF-κB may play a negligible role in the process of apoptosis in testes induced by cadmium or Cd-QDs. However, the detail information regarding Pgp involvement in SOD or MT, and its association with apoptosis during xenobiotics insults remain to be explored.

Advantages and potential limitations of applying AFM kymograph analysis to pharmaceutically relevant membrane proteins in lipid bilayers

Membrane proteins play critical roles in disease and in the disposition of many pharmaceuticals. A prime example is P-glycoprotein (Pgp) which moves a diverse range of drugs across membranes and out of the cell before a therapeutic payload can be delivered. Conventional structural biology methods have provided a valuable framework for comprehending the complex conformational changes underlying Pgp function, which also includes ATPase activity, but the lack of real-time information hinders understanding. Atomic force microscopy (AFM) is a single-molecule technique that is well-suited for studying active membrane proteins in bilayers and is poised to advance the field beyond static snapshots. After verifying Pgp activity in surface-support bilayers, we used kymograph analysis in conjunction with AFM imaging and simulations to study structural transitions at the 100 ms timescale. Though kymographs are frequently employed to boost temporal resolution, the limitations of the method have not been well characterized, especially for sparse non-crystalline distributions of pharmaceutically relevant membrane proteins like Pgp. Common experimental challenges are analyzed, including protein orientation, instrument noise, and drift. Surprisingly, a lateral drift of 75% of the protein dimension leads to only a 12% probability of erroneous state transition detection; average dwell time error achieves a maximum value of 6%. Rotational drift of proteins like Pgp, with azimuthally-dependent maximum heights, can lead to artifactual transitions. Torsional constraints can alleviate this potential pitfall. Confidence in detected transitions can be increased by adding conformation-altering ligands such as non-hydrolysable analogs. Overall, the data indicate that AFM kymographs are a viable method to access conformational dynamics for Pgp, but generalizations of the method should be made with caution.

Cross-Linking Mass Spectrometry on P-Glycoprotein

The ABC transporter P-glycoprotein (Pgp) has been found to be involved in multidrug resistance in tumor cells. Lipids and cholesterol have a pivotal role in Pgp's conformations; however, it is often difficult to investigate it with conventional structural biology techniques. Here, we applied robust approaches coupled with cross-linking mass spectrometry (XL-MS), where the natural lipid environment remains quasi-intact. Two experimental approaches were carried out using different cross-linkers (i) on living cells, followed by membrane preparation and immunoprecipitation enrichment of Pgp, and (ii) on-bead, subsequent to membrane preparation and immunoprecipitation. Pgp-containing complexes were enriched employing extracellular monoclonal anti-Pgp antibodies on magnetic beads, followed by on-bead enzymatic digestion. The LC-MS/MS results revealed mono-links on Pgp's solvent-accessible residues, while intraprotein cross-links confirmed a complex interplay between extracellular, transmembrane, and intracellular segments of the protein, of which several have been reported to be connected to cholesterol. Harnessing the MS results and those of molecular docking, we suggest an epitope for the 15D3 cholesterol-dependent mouse monoclonal antibody. Additionally, enriched neighbors of Pgp prove the strong connection of Pgp to the cytoskeleton and other cholesterol-regulated proteins. These findings suggest that XL-MS may be utilized for protein structure and network analyses in such convoluted systems as membrane proteins.

ABC Transporters in Bacterial Nanomachineries

Members of the superfamily of ABC transporters are found in all domains of life. Most of these primary active transporters act as isolated entities and export or import their substrates in an ATP-dependent manner across biological membranes. However, some ABC transporters are also part of larger protein complexes, so-called nanomachineries that catalyze the vectorial transport of their substrates. Here, we will focus on four bacterial examples of such nanomachineries: the Mac system providing drug resistance, the Lpt system catalyzing vectorial LPS transport, the Mla system responsible for phospholipid transport, and the Lol system, which is required for lipoprotein transport to the outer membrane of Gram-negative bacteria. For all four systems, we tried to summarize the existing data and provide a structure-function analysis highlighting the mechanistical aspect of the coupling of ATP hydrolysis to substrate translocation.

Hydrogen-deuterium exchange coupled to mass spectrometry: A multifaceted tool to decipher the molecular mechanism of transporters

Transporters regulate trafficking through the biological membrane of living cells and organelles. Therefore, these proteins play an important role in key cellular processes. Obtaining a molecular-level description of the mechanism of transporters is highly desirable to understand and modulate such processes. Different challenges currently complicate this effort, mostly due to transporters' intrinsic properties. They are dynamic and often averse to in vitro characterization. The crossing of the membrane via a transporter depends on both global and local structural changes that will enable substrate binding from one side of the membrane and release on the other. Dedicated approaches are required to monitor these dynamic changes, ideally within the complex membrane environment. Hydrogen-deuterium exchange coupled to mass spectrometry (HDX-MS) has recently emerged as a powerful biophysical tool to understand transporters' mechanism. This mini-review aims to offer to the reader an overview of the field of HDX-MS applied to transporters. It first summarizes the current workflow for HDX-MS measurements on transporters. It then provides illustrative examples on the molecular insights that are accessible thanks to the technique; following conformational transitions between different states, observing structural changes upon ligand binding and finally understanding the role of lipid-protein interactions.

Structural mass spectrometry approaches to understand multidrug efflux systems

Multidrug efflux pumps are ubiquitous across both eukaryotes and prokaryotes, and have major implications in antimicrobial and multidrug resistance. They reside within cellular membranes and have proven difficult to study owing to their hydrophobic character and relationship with their compositionally complex lipid environment. Advances in structural mass spectrometry (MS) techniques have made it possible to study these systems to elucidate critical information on their structure-function relationships. For example, MS techniques can report on protein structural dynamics, stoichiometry, connectivity, solvent accessibility, and binding interactions with ligands, lipids, and other proteins. This information proving powerful when used in conjunction with complementary structural biology methods and molecular dynamics (MD) simulations. In the present review, aimed at those not experts in MS techniques, we report on the current uses of MS in studying multidrug efflux systems, practical considerations to consider, and the future direction of the field. In the first section, we highlight the importance of studying multidrug efflux proteins, and introduce a range of different MS techniques and explain what information they yield. In the second section, we review recent studies that have utilised MS techniques to study and characterise a range of different multidrug efflux systems.

Long Range Communication between the Drug-Binding Sites and Nucleotide Binding Domains of the Efflux Transporter ABCB1

The ABC efflux pump P-glycoprotein (P-gp) transports a wide variety of drugs and is inhibited by others. Some drugs stimulate ATP hydrolysis at the nucleotide binding domains (NBDs) and are transported, others uncouple ATP hydrolysis and transport, and others inhibit ATP hydrolysis. The molecular basis for the different behavior of these drugs is not well understood despite the availability of several structural models of P-gp complexes with ligands bound. Hypothetically, ligands differentially alter the conformational dynamics of peptide segments that mediate the coupling between the drug binding sites and the NBDs. Here, we explore by hydrogen-deuterium exchange mass spectrometry the dynamic consequences of a classic substrate and inhibitor, vinblastine and zosuquidar, binding to mouse P-gp (mdr1a) in lipid nanodiscs. The dynamics of P-gp in nucleotide-free, pre-hydrolysis, and post-hydrolysis states in the presence of each drug reveal distinct mechanisms of ATPase stimulation and implications for transport. For both drugs, there are common regions affected in a similar manner, suggesting that particular networks are the key to stimulating ATP hydrolysis. However, drug binding effects diverge in the post-hydrolysis state, particularly in the intracellular helices (ICHs 3 and 4) and neighboring transmembrane helices. The local dynamics and conformational equilibria in this region are critical for the coupling of drug binding and ATP hydrolysis and are differentially modulated in the catalytic cycle.

Approaches to minimize the effects of P-glycoprotein in drug transport: A review

P-glycoprotein (P-gp) is a transporter protein that is come under the ATP binding cassette family of proteins. It is situated on the surface of the intestine epithelium, where P-gp substrate binds to the transporter and is pumped into the intestine lumen by the ATP-driven energy-dependent process. In this review, we summarize the role of the P-gp efflux transporter situated on the intestine, the clinical importance of P-gp related drug interactions, and approaches to minimize the effect of P-gp in drug transport. This review also focuses on the impact of P-gp on the bioavailability of the orally administered drug. Many drug's oral bioavailabilities can improve by concomitant use of P-gp inhibitors. Multidrug resistance are reduced by using some naturally occurring compounds obtained from plants and several synthetic P-gp inhibitors. Formulation strategies, one of the most important approaches to mimic the P-gp transporter's action, finally enhancing the oral bioavailability of the drug by inhibiting its P-gp efflux. Vitamin E TPGS, Gelucire 44/14 and other pharmaceutical/formulation excipients inhibit the P-gp efflux. A prodrug approach might be a useful strategy to overcome drug resistance. Prodrug helps to enhance the solubility or alter the pharmacokinetic properties but does not diminish the pharmacological action.

Advances in Hydrogen/Deuterium Exchange Mass Spectrometry and the Pursuit of Challenging Biological Systems

Solution-phase hydrogen/deuterium exchange (HDX) coupled to mass spectrometry (MS) is a widespread tool for structural analysis across academia and the biopharmaceutical industry. By monitoring the exchangeability of backbone amide protons, HDX-MS can reveal information about higher-order structure and dynamics throughout a protein, can track protein folding pathways, map interaction sites, and assess conformational states of protein samples. The combination of the versatility of the hydrogen/deuterium exchange reaction with the sensitivity of mass spectrometry has enabled the study of extremely challenging protein systems, some of which cannot be suitably studied using other techniques. Improvements over the past three decades have continually increased throughput, robustness, and expanded the limits of what is feasible for HDX-MS investigations. To provide an overview for researchers seeking to utilize and derive the most from HDX-MS for protein structural analysis, we summarize the fundamental principles, basic methodology, strengths and weaknesses, and the established applications of HDX-MS while highlighting new developments and applications.

Hydrogen/Deuterium Exchange and Nuclear Magnetic Resonance Spectroscopy Reveal Dynamic Allostery on Multiple Time Scales in the Serine Protease Thrombin

A deeper understanding of how hydrogen/deuterium exchange mass spectrometry (HDX-MS) reveals allostery is important because HDX-MS can reveal allostery in systems that are not amenable to nuclear magnetic resonance (NMR) spectroscopy. We were able to study thrombin and its complex with thrombomodulin, an allosteric regulator, by both HDX-MS and NMR. In this Perspective, we compare and contrast the results from both experiments and from molecular dynamics simulations. NMR detects changes in the chemical environment around the protein backbone N-H bond vectors, providing residue-level information about the conformational exchange between distinct states. HDX-MS detects changes in amide proton solvent accessibility and H-bonding. Taking advantage of NMR relaxation dispersion measurements of the time scale of motions, we draw conclusions about the motions reflected in HDX-MS experiments. Both experiments detect allostery, but they reveal different components of the allosteric transition. The insights gained from integrating NMR and HDX-MS into thrombin dynamics enable a clearer interpretation of the evidence for allostery revealed by HDX-MS in larger protein complexes and assemblies that are not amenable to NMR.

Drug delivery and targeting to brain tumors: considerations for crossing the blood-brain barrier

Introduction: The blood-brain barrier (BBB) selectively impedes the transportation of drug molecules into the brain, which makes the drug delivery and targeting of brain tumors very challenging.Areas covered: Having surveyed the recent literature, comprehensive insights are given into the impacts of the BBB on the advanced drug delivery and targeting modalities for brain tumors.Expert opinion: Brain capillary endothelial cells form the BBB in association with astrocytes, pericytes, neurons, and extracellular matrix. Coop of these forms the complex setting of neurovascular unite. The BBB maintains the brain homeostasis by restrictive controlling of the blood circulating nutrients/substances trafficking. Despite substantial progress on therapy of brain tumors, there is no impeccable strategy to safely deliver chemotherapeutics into the brain. Various strategies have been applied to deliver chemotherapeutics into the brain (e.g. BBB opening, direct delivery by infusion, injection, microdialysis, and implants, and smart nanosystems), which hold different pros and cons. Of note, smart nanoscale multifunctional nanomedicines can serve as targeting, imaging, and treatment modality for brain tumors. Given that aggressive brain tumors (e.g. gliomas) are often unresponsive to any treatments, an in-depth understanding of the molecular/cellular complexity of brain tumors might help the development of smart and effective treatment modalities.

Does the ATP-bound EQ mutant reflect the pre- or post-ATP hydrolysis state in the catalytic cycle of human P-glycoprotein (ABCB1)?

P-glycoprotein (P-gp, ABCB1) is an ABC transporter associated with the development of multidrug resistance to chemotherapy. During its catalytic cycle, P-gp undergoes significant conformational changes. Recently, atomic structures of some of these conformations have been resolved using cryo-electron microscopy. The ATP hydrolysis-defective mutant of the catalytic glutamate residue of the Walker B motif (E556Q/E1201Q) has been used to determine the structure of the ATP-bound inward-closed conformation of P-gp. Here, we show that this mutant does not appear to undergo the same steps as wild-type P-gp. We discuss conformational differences in the EQ mutant that may lead to a better understanding of the catalytic cycle of P-gp and propose that additional structural studies with wild-type P-gp are required.

Cholesterol Asymmetrically Modulates the Conformational Ensemble of the Nucleotide-Binding Domains of P-Glycoprotein in Lipid Nanodiscs

P-Glycoprotein (P-gp) is an ATP-dependent efflux pump that clears a wide variety of drugs and toxins from cells. P-gp undergoes large-scale structural changes and demonstrates conformational heterogeneity even within a single catalytic or drug-bound state, although the role of heterogeneity remains unclear. P-gp is found in a variety of cell types that vary in lipid composition, which modulates its activity. An understanding of structural or dynamic changes due to the lipid environment is lacking. We aimed to determine the effects of cholesterol in a membrane on the conformational behavior of P-gp in lipid nanodiscs. The presence of cholesterol stimulates ATP hydrolysis and alters lipid order and fluidity. Hydrogen/deuterium exchange mass spectrometry demonstrates that cholesterol in the membrane induces asymmetric, long-range changes in the distributions and exchange kinetics of conformations of the nucleotide-binding domains, correlating the effects of lipid composition on activity with specific changes in the P-gp conformational landscape.

Design, synthesis, and biological evaluation of hederagenin derivatives with improved aqueous solubility and tumor resistance reversal activity

Multidrug resistance (MDR) has become a major obstacle to malignancies treatment by chemotherapeutic drugs, therefore, it is important to develop MDR reversal agents with high activity. We have previously found that the hederagenin (HD) derivative HBQ showed good tumor MDR reversal activity in vitro and in vivo but had poor solubility. In this study, to enhance the aqueous solubility and tumor MDR reversal activity of HBQ, three series of HD derivatives were designed and synthesized. Nitrogen-containing heterocyclic-substituted, PEGylated, and ring-A substituted derivatives significantly reversed the MDR phenotype of KBV (multidrug-resistant oral epidermoid carcinoma) cells toward paclitaxel at a concentration of 10 μM in MTT assays. The PEGylated derivatives 10c-10e had increased aqueous solubility compared with HBQ by 18-657 fold, while maintaining tumor MDR reversal activity. The most in vitro active compound 10c possessed good chemical stability to an esterase over 24 h and enhanced the sensitivity of KBV cells to paclitaxel and vincristine with IC₅₀ values of 4.58 and 0.79 nM, respectively. Mechanism studies indicated that compound 10c increased the accumulation of P-glycoprotein (P-gp) substrates rhodamine 123 and Flutax1 in KBV cells and MCF-7T (paclitaxel-resistant breast carcinoma) cells, that is to say, compound 10c exerted the reversal effect of tumor MDR by inhibiting the efflux function of P-gp. Finally, the structure-activity relationships were further investigated by analyzing the relationship between structure and tumor MDR reversal activity of HD derivatives. This study highlights the potential of PEGylated HD derivatives such as compound 10c for the development of tumor MDR reversal agents and provides information for the further improvement of the aqueous solubility and tumor MDR reversal activity of HD derivatives in the future.

EPO and EPO-Receptor System as Potential Actionable Mechanism for the Protection of Brain and Heart in Refractory Epilepsy and SUDEP

The most important activity of erythropoietin (EPO) is the regulation of erythrocyte production by activation of the erythropoietin receptor (EPO-R), which triggers the activation of anti-apoptotic and proliferative responses of erythroid progenitor cells. Additionally, to erythropoietic EPO activity, an antiapoptotic effect has been described in a wide spectrum of tissues. EPO low levels are found in the central nervous system (CNS), while EPO-R is expressed in most CNS cell types. In spite of EPO-R high levels expressed during the hypoxicischemic brain, insufficient production of endogenous cerebral EPO could be the cause of determined circuit alterations that lead to the loss of specific neuronal populations. In the heart, high EPO-R expression in cardiac progenitor cells appears to contribute to myocardial regeneration under EPO stimulation. Several lines of evidence have linked EPO to an antiapoptotic role in CNS and in heart tissue. In this review, an antiapoptotic role of EPO/EPO-R system in both brain and heart under hypoxic conditions, such as epilepsy and sudden death (SUDEP) has been resumed. Additionally, their protective effects could be a new field of research and a novel therapeutic strategy for the early treatment of these conditions and avoid SUDEP.

Complementarity of Hydrogen/Deuterium Exchange Mass Spectrometry and Cryo-Electron Microscopy

Methodological improvements in both single particle cryo-electron microscopy (cryo-EM) and hydrogen/deuterium exchange mass spectrometry (HDX-MS) mean that the two methods are being more frequently used together to tackle complex problems in structural biology. There are many benefits to this combination, including for the analysis of low-resolution density, for structural validation, in the analysis of individual proteins versus the same proteins in large complexes, studies of allostery, protein quality control during cryo-EM construct optimization, and in the study of protein movements/dynamics during function. As will be highlighted in this review, through careful considerations of potential sample and conformational heterogeneity, many joint studies have recently been demonstrated, and many future studies using this combination are anticipated.

A glimpse into the molecular mechanism of integral membrane proteins through hydrogen-deuterium exchange mass spectrometry

Integral membrane proteins (IMPs) control countless fundamental biological processes and constitute the majority of drug targets. For this reason, uncovering their molecular mechanism of action has long been an intense field of research. They are, however, notoriously difficult to work with, mainly due to their localization within the heterogeneous of environment of the biological membrane and the instability once extracted from the lipid bilayer. High‐resolution structures have unveiled many mechanistic aspects of IMPs but also revealed that the elucidation of static pictures has limitations. Hydrogen–deuterium exchange coupled to mass spectrometry (HDX‐MS) has recently emerged as a powerful biophysical tool for interrogating the conformational dynamics of proteins and their interactions with ligands. Its versatility has proven particularly useful to reveal mechanistic aspects of challenging classes of proteins such as IMPs. This review recapitulates the accomplishments of HDX‐MS as it has matured into an essential tool for membrane protein structural biologists.