Simultaneous analysis of enzyme structure and activity by kinetic capillary electrophoresis–MS

Conformational Modulation of Tissue Transglutaminase via Active Site Thiol Alkylating Agents: Size Does Not Matter

TG2 is a unique member of the transglutaminase family as it undergoes a dramatic conformational change, allowing its mutually exclusive function as either a cross-linking enzyme or a G-protein. The enzyme's dysregulated activity has been implicated in a variety of pathologies (e.g., celiac disease, fibrosis, cancer), leading to the development of a wide range of inhibitors. Our group has primarily focused on the development of peptidomimetic targeted covalent inhibitors, the nature and size of which were thought to be important features to abolish TG2's conformational dynamism and ultimately inhibit both its activities. However, we recently demonstrated that the enzyme was unable to bind guanosine triphosphate (GTP) when catalytically inactivated by small molecule inhibitors. In this study, we designed a library of models targeting covalent inhibitors of progressively smaller sizes (15 to 4 atoms in length). We evaluated their ability to inactivate TG2 by measuring their respective kinetic parameters kinact and KI. Their impact on the enzyme's ability to bind GTP was then evaluated and subsequently correlated to the conformational state of the enzyme, as determined via native PAGE and capillary electrophoresis. All irreversible inhibitors evaluated herein locked TG2 in its open conformation and precluded GTP binding. Therefore, we conclude that steric bulk and structural complexity are not necessary factors to consider when designing TG2 inhibitors to abolish G-protein activity.

Stabilizing transglutaminase 2 in the open conformation results in reactive astrocytes being more neurosupportive

Astrocytes play critical roles in supporting structural and metabolic homeostasis in the central nervous system (CNS). Inflammatory conditions bring about a range of poorly understood, heterogeneous, reactive phenotypes in astrocytes. Finding ways to manipulate the phenotype of reactive astrocytes, and leveraging a pro-recovery phenotype, holds promise in treating CNS injury. Previous studies have shown that the protein transglutaminase 2 (TG2) plays a significant role in determining the phenotype of reactive astrocytes. Recently it has been demonstrated that ablation of TG2 from astrocytes improves injury outcomes both in vitro and in vivo. Excitingly, in an in vivo mouse model, pharmacological inhibition of TG2 with the irreversible inhibitor VA4 phenocopies the neurosupportive effects of TG2 deletion in astrocytes. The focus of this study was to provide insights into the mechanisms by which TG2 deletion or inhibition of TG2 with VA4 result in a more neurosupportive astrocytic phenotype. Using a neuron-astrocyte co-culture model of neurite outgrowth, we show that VA4 treatment improves the ability of astrocytes to support neurite outgrowth on an injury-relevant matrix, further validating the ability of VA4 to phenocopy astrocytic TG2 deletion. VA4 treatment of neurons alone had no effect on neurite outgrowth. VA4 covalently binds to active site residues of TG2 that are exposed in its open conformation and are critical for its enzymatic function, and prevents TG2 from taking on a closed conformation, which interferes with its protein scaffolding function. To begin to understand how pharmacologically altering TG2's conformation affects its ability to regulate reactive astrocyte phenotypes, we assayed the impact of VA4 on TG2's interaction with Zbtb7a, a transcription factor that we have previously identified as a TG2 interactor, and whose functional outputs are significantly regulated by TG2. The results of these studies demonstrated that VA4 significantly decreases the interaction of TG2 and Zbtb7a. Further, previous findings indicate that TG2 may act as an epigenetic regulator, through its nuclear protein-protein interactions, to modulate gene expression. Since both TG2 and Zbtb7a interact with members of the Sin3a chromatin repressor complex, we assayed the effect of TG2 deletion and VA4 treatment on histone acetylation and found significantly greater acetylation with TG2 deletion or inhibition with VA4. Overall, this work points toward a possible epigenetic mechanism by which genetic deletion or acute inhibition of TG2 leads to enhanced astrocytic support of neurons.

Ion mobility in gas and liquid phases: How much orthogonality is obtained in capillary electrophoresis-ion mobility-mass spectrometry?

Ion mobility-mass spectrometry (IM-MS) is an ever-evolving tool to separate ions in the gas phase according to electrophoretic mobility with subsequent mass determination. CE is rarely coupled to IM-MS, possibly due to similar separation mechanisms based on electrophoretic mobility. Here, we investigate the orthogonality of CE and ion mobility (IM) by analyzing a complex peptide mixture (tryptic digest of HeLa proteins) with trapped ion mobility mass spectrometry (TIMS-MS). Using the nanoCEasy interface, excellent sensitivity was achieved by identifying thousands of peptides and achieving a peak capacity of 7500 (CE: 203-323 in a 150 cm long capillary, IM: 27-31). Plotting CE versus mass and CE versus (inverse) mobility, a clear grouping in curved striped patterns is observed according to the charge-to-size and mass-to-charge ratios. The peptide charge in the acidic background electrolyte can be estimated from the number of basic amino acids, with a few exceptions where neighboring effects reduce the positive charge. A surprisingly high orthogonality of CE and IM is observed, which is obviously caused by solvation effects leading to different charges and sizes in the liquid phase compared to the gas phase. A high orthogonality of CE and ion mobility is expected to be observed for other peptide samples as well as other substance classes, making CE-IM-MS a promising tool for various applications.

Study of molecular interactions by nonequilibrium capillary electrophoresis of equilibrium mixtures: Originations, developments, and applications

Molecular interactions play a vital role in regulating various physiological and biochemical processes in vivo. Kinetic capillary electrophoresis (KCE) is an analytical platform that offers significant advantages in studying the thermodynamic and kinetic parameters of molecular interactions. It enables the simultaneous analysis of these parameters within an interaction pattern and facilitates the screening of binding ligands with predetermined kinetic parameters. Nonequilibrium capillary electrophoresis of equilibrium mixtures (NECEEM) was the first proposed KCE method, and it has found widespread use in studying molecular interactions involving proteins/aptamers, proteins/small molecules, and peptides/small molecules. The successful applications of NECEEM have demonstrated its promising potential for further development and broader application. However, there has been a dearth of recent reviews on NECEEM. To address this gap, our study provides a comprehensive description of NECEEM, encompassing its origins, development, and applications from 2015 to 2022. The primary focus of the applications section is on aptamer selection and screening of small-molecule ligands. Furthermore, we discuss important considerations in NECEEM experimental design, such as buffer suitability, detector selection, and protein adsorption. By offering this thorough review, we aim to contribute to the understanding, advancement, and wider utilization of NECEEM as a valuable tool for studying molecular interactions and facilitating the identification of potential ligands and targets.

Cell-Impermeable Inhibitors Confirm That Intracellular Human Transglutaminase 2 Is Responsible for the Transglutaminase-Associated Cancer Phenotype

Transglutaminase 2 (TG2) is a multifunctional enzyme primarily responsible for crosslinking proteins. Ubiquitously expressed in humans, TG2 can act either as a transamidase by crosslinking two substrates through formation of an Nε(ɣ-glutaminyl)lysine bond or as an intracellular G-protein. These discrete roles are tightly regulated by both allosteric and environmental stimuli and are associated with dramatic changes in the conformation of the enzyme. The pleiotropic nature of TG2 and multi-faceted activities have resulted in TG2 being implicated in numerous disease pathologies including celiac disease, fibrosis, and cancer. Targeted TG2 therapies have not been selective for subcellular localization, such that currently no tools exist to selectively target extracellular over intracellular TG2. Herein, we have designed novel TG2-selective inhibitors that are not only highly potent and irreversible, but also cell impermeable, targeting only extracellular TG2. We have also further derivatized the scaffold to develop probes that are intrinsically fluorescent or bear an alkyne handle, which target both intra- and extracellular TG2, in order to facilitate cellular labelling and pull-down assays. The fluorescent probes were internalized and imaged in cellulo, and provide the first implicit experimental evidence that by comparison with their cell-impermeable analogues, it is specifically intracellular TG2, and presumably its G-protein activity, that contributes to transglutaminase-associated cancer progression.

Proteomics Methodologies: The Search of Protein Biomarkers Using Microfluidic Systems Coupled to Mass Spectrometry

Protein biomarkers have been the subject of intensive studies as a target for disease diagnostics and monitoring. Indeed, biomarkers have been extensively used for personalized medicine. In biological samples, these biomarkers are most often present in low concentrations masked by a biologically complex proteome (e.g., blood) making their detection difficult. This complexity is further increased by the needs to detect proteoforms and proteome complexity such as the dynamic range of compound concentrations. The development of techniques that simultaneously pre-concentrate and identify low-abundance biomarkers in these proteomes constitutes an avant-garde approach to the early detection of pathologies. Chromatographic-based methods are widely used for protein separation, but these methods are not adapted for biomarker discovery, as they require complex sample handling due to the low biomarker concentration. Therefore, microfluidics devices have emerged as a technology to overcome these shortcomings. In terms of detection, mass spectrometry (MS) is the standard analytical tool given its high sensitivity and specificity. However, for MS, the biomarker must be introduced as pure as possible in order to avoid chemical noise and improve sensitivity. As a result, microfluidics coupled with MS has become increasingly popular in the field of biomarker discovery. This review will show the different approaches to protein enrichment using miniaturized devices and the importance of their coupling with MS.

Combining amperometry and mass spectrometry as a dual detection approach for capillary electrophoresis

Dual detection concepts (DDCs) are becoming more and more popular in analytical chemistry. In this work, we describe a novel DDC for capillary electrophoresis (CE) consisting of an amperometric detector (AD) and a mass spectrometer (MS). This detector combination has a good complementarity as the AD exhibits high sensitivity, whereas the MS provides excellent selectivity. Both detectors are based on a destructive detection principle, making a serial detector arrangement impossible. Thus, for the realization of the DDC, the CE flow was divided into two parts with a flow splitter. The DDC was characterized in a proof-of-concept study with ferrocene derivates and a nonaqueous background electrolyte. We could show that splitting the CE flow was a suitable method for the instrumental realization of the DDC consisting of two destructive detectors. By lowering the height of the AD compared to the MS, it was possible to synchronize the detector responses. Additionally, for the chosen model system, we confirmed that the AD was much more reproducible and had lower limits of detection (LODs) than the MS. The LODs were identical for the DDC and the single-detection arrangements, indicating no sensitivity decrease due to the CE flow splitting. The DDC was successfully applied to determine the drug and doping agent trimetazidine.

The Impact of Nε-Acryloyllysine Piperazides on the Conformational Dynamics of Transglutaminase 2

In addition to the classic functions of proteins, such as acting as a biocatalyst or binding partner, the conformational states of proteins and their remodeling upon stimulation need to be considered. A prominent example of a protein that undergoes comprehensive conformational remodeling is transglutaminase 2 (TGase 2), the distinct conformational states of which are closely related to particular functions. Its involvement in various pathophysiological processes, including fibrosis and cancer, motivates the development of theranostic agents, particularly based on inhibitors that are directed toward the transamidase activity. In this context, the ability of such inhibitors to control the conformational dynamics of TGase 2 emerges as an important parameter, and methods to assess this property are in great demand. Herein, we describe the application of the switchSENSE^® principle to detect conformational changes caused by three irreversibly binding N^ε-acryloyllysine piperazides, which are suitable radiotracer candidates of TGase 2. The switchSENSE^® technique is based on DNA levers actuated by alternating electric fields. These levers are immobilized on gold electrodes with one end, and at the other end of the lever, the TGase 2 is covalently bound. A novel computational method is introduced for describing the resulting lever motion to quantify the extent of stimulated conformational TGase 2 changes. Moreover, as a complementary biophysical method, native polyacrylamide gel electrophoresis was performed under similar conditions to validate the results. Both methods prove the occurrence of an irreversible shift in the conformational equilibrium of TGase 2, caused by the binding of the three studied N^ε-acryloyllysine piperazides.

Sulfated glycosaminoglycans inhibit transglutaminase 2 by stabilizing its closed conformation

Transglutaminases (TGs) catalyze the covalent crosslinking of proteins via isopeptide bonds. The most prominent isoform, TG2, is associated with physiological processes such as extracellular matrix (ECM) stabilization and plays a crucial role in the pathogenesis of e.g. fibrotic diseases, cancer and celiac disease. Therefore, TG2 represents a pharmacological target of increasing relevance. The glycosaminoglycans (GAG) heparin (HE) and heparan sulfate (HS) constitute high-affinity interaction partners of TG2 in the ECM. Chemically modified GAG are promising molecules for pharmacological applications as their composition and chemical functionalization may be used to tackle the function of ECM molecular systems, which has been recently described for hyaluronan (HA) and chondroitin sulfate (CS). Herein, we investigate the recognition of GAG derivatives by TG2 using an enzyme-crosslinking activity assay in combination with in silico molecular modeling and docking techniques. The study reveals that GAG represent potent inhibitors of TG2 crosslinking activity and offers atom-detailed mechanistic insights.

Implications of enigmatic transglutaminase 2 (TG2) in cardiac diseases and therapeutic developments

Cardiac diseases are the leading cause of mortality and morbidity worldwide. Mounting evidence suggests that transglutaminases (TGs), tissue TG (TG2) in particular, are involved in numerous molecular responses underlying the pathogenesis of cardiac diseases. The TG family has several intra- and extracellular functions in the human body, including collagen cross-linking, angiogenesis, cell growth, differentiation, migration, adhesion as well as survival. TGs are thiol- and calcium-dependent acyl transferases that catalyze the formation of a covalent bond between the γ-carboxamide group of a glutamine residue and an amine group, thus increasing the stability, rigidity, and stiffness of the myocardial extracellular matrix (ECM). Excessive accumulation of cross-linked collagen leads to increase myocardial stiffness and fibrosis. Beyond TG2 extracellular protein cross-linking action, mounting evidence suggests that this pleiotropic TG isozyme may also promote fibrotic diseases through cell survival and profibrotic pathway activation at the signaling, transcriptional and translational levels. Due to its multiple functions and localizations, TG2 fulfils critical yet incompletely understood roles in myocardial fibrosis and associated heart diseases, such as cardiac hypertrophy, heart failure, and age-related myocardial stiffness under several conditions. This review summarizes current knowledge and existing gaps regarding the ECM-dependent and ECM-independent roles of TG2 and highlights the therapeutic prospects of targeting TG2 to treat cardiac diseases.

Structure-activity relationships of hydrophobic alkyl acrylamides as tissue transglutaminase inhibitors

Tissue transglutaminase (TG2) is a multifunctional protein that plays biological roles based on its ability to catalyse protein cross-linking and to function as a non-canonical G-protein known as Ghα. The non-regulated activity of TG2 has been implicated in fibrosis, celiac disease and the survival of cancer stem cells, underpinning the therapeutic potential of cell permeable small molecule inhibitors of TG2. In the current study, we designed a small library of inhibitors to explore the importance of a terminal hydrophobic moiety, as well as the length of the tether to the irreversible acrylamide warhead. Subsequent kinetic evaluation using an in vitro activity assay provided values for the k _inact and K _I parameters for each of these irreversible inhibitors. The resulting structure-activity relationship (SAR) clearly indicated the affinity conferred by dansyl and adamantyl moieties, as well as the efficiency provided by the shortest warhead tether. We also provide the first direct evidence of the capability of these inhibitors to suppress the GTP binding ability of TG2, at least partially. However, it is intriguing to note that the SAR trends observed herein are opposite to those predicted by molecular modelling - namely that longer tether groups should improve binding affinity by allowing for deeper insertion of the hydrophobic moiety into a hydrophobic pocket on the enzyme. This discrepancy leads us to question whether the existing crystallographic structures of TG2 are appropriate for docking non-peptidic inhibitors. In the absence of a more relevant crystallographic structure, the data from rigorous kinetic studies, such as those provided herein, are critically important for the development of future small molecule TG2 inhibitors.

Studying protein structure and function by native separation–mass spectrometry

Alterations in protein structure may have profound effects on biological function. Analytical techniques that permit characterization of proteins while maintaining their conformational and functional state are crucial for studying changes in the higher order structure of proteins and for establishing structure-function relationships. Coupling of native protein separations with mass spectrometry is emerging rapidly as a powerful approach to study these aspects in a reliable, fast and straightforward way. This Review presents the available native separation modes for proteins, covers practical considerations on the hyphenation of these separations with mass spectrometry and highlights the involvement of affinity-based separations to simultaneously obtain structural and functional information of proteins. The impact of these approaches is emphasized by selected applications addressing biomedical and biopharmaceutical research questions.

Faster, better, and cheaper: harnessing microfluidics and mass spectrometry for biotechnology

High-throughput screening technologies are widely used for elucidating biological activities. These typically require trade-offs in assay specificity and sensitivity to achieve higher throughput. Microfluidic approaches enable rapid manipulation of small volumes and have found a wide range of applications in biotechnology providing improved control of reaction conditions, faster assays, and reduced reagent consumption. The integration of mass spectrometry with microfluidics has the potential to create high-throughput, sensitivity, and specificity assays. This review introduces the widely-used mass spectrometry ionization techniques that have been successfully integrated with microfluidics approaches such as continuous-flow system, microchip electrophoresis, droplet microfluidics, digital microfluidics, centrifugal microfluidics, and paper microfluidics. In addition, we discuss recent applications of microfluidics integrated with mass spectrometry in single-cell analysis, compound screening, and the study of microorganisms. Lastly, we provide future outlooks towards online coupling, improving the sensitivity and integration of multi-omics into a single platform.

Development of an 18F-Labeled Irreversible Inhibitor of Transglutaminase 2 as Radiometric Tool for Quantitative Expression Profiling in Cells and Tissues

The transamidase activity of transglutaminase 2 (TGase 2) is considered to be important for several pathophysiological processes including fibrotic and neoplastic tissue growth, whereas in healthy cells this enzymatic function is predominantly latent. Methods that enable the highly sensitive detection of TGase 2, such as application of radiolabeled activity-based probes, will support the exploration of the enzyme's function in various diseases. In this context, the radiosynthesis and detailed in vitro radiopharmacological evaluation of an ¹⁸F-labeled N^ε-acryloyllysine piperazide are reported. Robust and facile detection of the radiotracer-TGase 2 complex by autoradiography of thin layer plates and polyacrylamide gels after chromatographic and electrophoretic separation owing to irreversible covalent bond formation was demonstrated for the isolated protein, cell lysates, and living cells. By use of this radiotracer, quantitative data on the expression profile of activatable TGase 2 in mouse organs and selected tumors were obtained for the first time by autoradiography of tissue sections.

Transglutaminase 2 Inhibitor LDN 27219 Age-Dependently Lowers Blood Pressure and Improves Endothelium-Dependent Vasodilation in Resistance Arteries

Transglutaminase 2 (TG2) is an enzyme which in the open conformation exerts transamidase activity, leading to protein cross-linking and fibrosis. In the closed conformation, TG2 participates in transmembrane signaling as a G protein. The unspecific transglutaminase inhibitor cystamine causes vasorelaxation in rat resistance arteries. However, the role of TG2 conformation in vascular function is unknown. We investigated the vascular effects of selective TG2 inhibitors by myography in isolated rat mesenteric and human subcutaneous resistance arteries, patch-clamp studies on vascular smooth muscle cells, and blood pressure measurements in rats and mice. LDN 27219 promoted the closed TG2 conformation and inhibited transamidase activity in mesenteric arteries. In contrast to TG2 inhibitors promoting the open conformation (Z-DON, VA5), LDN 27219 concentration-dependently relaxed rat and resistance human arteries by a mechanism dependent on nitric oxide, large-conductance calcium-activated and voltage-gated potassium channels 7, lowering blood pressure. LDN 27219 also potentiated acetylcholine-induced relaxation by opening potassium channels in the smooth muscle; these effects were abolished by membrane-permeable TG2 inhibitors promoting the open conformation. In isolated arteries from 35- to 40-week-old rats, transamidase activity was increased, and LDN 27219 improved acetylcholine-induced relaxation more than in younger rats. Infusion of LDN 27219 decreased blood pressure more effectively in 35- to 40-week than 12- to 14-week-old anesthetized rats. In summary, pharmacological modulation of TG2 to the closed conformation age-dependently lowers blood pressure and, by opening potassium channels, potentiates endothelium-dependent vasorelaxation. Our findings suggest that promoting the closed conformation of TG2 is a potential strategy to treat age-related vascular dysfunction and lowers blood pressure.

Evaluation of ion mobility in capillary electrophoresis coupled to mass spectrometry for the identification in metabolomics

Currently, feature annotation remains one of the main challenges in untargeted metabolomics. In this context, the information provided by high-resolution mass spectrometry (HRMS) in addition to accurate mass can improve the quality of metabolite annotation, and MS/MS fragmentation patterns are widely used. Accurate mass and a separation index, such as retention time or effective mobility (μ_eff ), in chromatographic and electrophoretic approaches, respectively, must be used for unequivocal metabolite identification. The possibility of measuring collision cross-section (CCS) values by using ion mobility (IM) is becoming increasingly popular in metabolomic studies thanks to the new generation of IM mass spectrometers. Based on their similar separation mechanisms involving electric field and the size of the compounds, the complementarity of ^DT CCS_N2 and μ_eff needs to be evaluated. In this study, a comparison of ^DT CCS_N2 and μ_eff was achieved in the context of feature identification ability in untargeted metabolomics by capillary zone electrophoresis (CZE) coupled with HRMS. This study confirms the high correlation of ^DT CCS_N2 with the mass of the studied metabolites as well as the orthogonality between accurate mass and μ_eff , making this combination particularly interesting for the identification of several endogenous metabolites. The use of IM-MS remains of great interest for facilitating the annotation of neutral metabolites present in the electroosmotic flow (EOF) that are poorly or not separated by CZE.

THE USE OF MASS SPECTROMETRY TO STUDY ZN-METALLOPROTEASE-SUBSTRATE INTERACTIONS

Zinc metalloproteases (ZnMPs) participate in diverse biological reactions, encompassing the synthesis and degradation of all the major metabolites in living organisms. In particular, ZnMPs have been recognized to play a very important role in controlling the concentration level of several peptides and/or proteins whose homeostasis has to be finely regulated for the correct physiology of cells. Dyshomeostasis of aggregation-prone proteins causes pathological conditions and the development of several different diseases. For this reason, in recent years, many analytical approaches have been applied for studying the interaction between ZnMPs and their substrates and how environmental factors can affect enzyme activities. In this scenario, mass spectrometric methods occupy a very important role in elucidating different aspects of ZnMPs-substrates interaction. These range from identification of cleavage sites to quantitation of kinetic parameters. In this work, an overview of all the main achievements regarding the application of mass spectrometric methods to investigating ZnMPs-substrates interactions is presented. A general experimental protocol is also described which may prove useful to the study of similar interactions. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Integration of High-Resolution Radiation Detector for Hybrid Microchip Electrophoresis

For decades, there has been immense progress in miniaturizing analytical methods based on electrophoresis to improve sensitivity and to reduce sample volumes, separation times, and/or equipment cost and space requirements, in applications ranging from analysis of biological samples to environmental analysis to forensics. In the field of radiochemistry, where radiation-shielded laboratory space is limited, there has been great interest in harnessing the compactness, high efficiency, and speed of microfluidics to synthesize short-lived radiolabeled compounds. We recently proposed that analysis of these compounds could also benefit from miniaturization and have been investigating capillary electrophoresis (CE) and hybrid microchip electrophoresis (hybrid-MCE) as alternatives to the typically used high-performance liquid chromatography (HPLC). We previously showed separation of the positron-emission tomography (PET) imaging tracer 3'-deoxy-3'-fluorothymidine (FLT) from its impurities in a hybrid-MCE device with UV detection, with similar separation performance to HPLC, but with improved speed and lower sample volumes. In this paper, we have developed an integrated radiation detector to enable measurement of the emitted radiation from radiolabeled compounds. Though conventional radiation detectors have been incorporated into CE systems in the past, these approaches cannot be readily integrated into a compact hybrid-MCE device. We instead employed a solid-state avalanche photodiode (APD)-based detector for real-time, high-sensitivity β particle detection. The integrated system can reliably separate [18F]FLT from its impurities and perform chemical identification via coinjection with nonradioactive reference standard. This system can quantitate samples with radioactivity concentrations as low as 114 MBq/mL (3.1 mCi/mL), which is sufficient for analysis of radiochemical purity of radiopharmaceuticals.

Straight nano-electrospray ionization and its coupling of mobility capillary electrophoresis to mass spectrometry

Mobility capillary electrophoresis (MCE) was developed previously in our group, which has the capabilities of ion separation and biomolecule hydrodynamic radius analysis. The coupling of MCE with mass spectrometry (MS) would greatly improve complex sample identification capability as well as system detection sensitivity. In the present study, a simple and robust ionization source, named as straight nano-electrospray ionization (nanoESI) source was developed, which was applied to couple MCE with MS. A stainless-steel needle attached directly at the end of an MCE capillary was used as the nanoESI emitter, and the connection between this emitter to the liquid flow in the MCE separation channel was established through a liquid bridge. After optimization, this straight nanoESI source enhanced the ion signal intensity by ~10 times when compared with a commercial nanoESI source. The MCE-straight nanoESI-MS system was also characterized in terms of mixture separation and peptide hydrodynamic radius measurements. Compared to our previous work when a UV detector was used in a commercial Lumex CE system (model Capel 105 M, St. Petersburg, Russia), peptides with much lower concentrations could be analyzed (from ~1 mg/mL to ~20 μg/mL) in terms of radius measurement.

Mobility Capillary Electrophoresis-Restrained Modeling Method for Protein Structure Analysis in Mixtures

Protein stereostructure analysis in mixtures still remains challenging, especially large-scale analysis such as in proteomics. With the capability of measuring the hydrodynamic radius of ions in the liquid phase, mobility capillary electrophoresis (MCE) has been applied to study the structure of peptides. In this study, MCE was extended for protein mixture separation and their corresponding hydrodynamic radius analyses. After ellipsoid approximation, the results obtained by MCE experiments were then used as a restraint in molecular dynamics simulations to predict the most probable structure of each protein. Besides a three-protein mixture, a mixture of disulfide bond reduced insulin was also studied by this MCE-restrained modeling method. The results obtained by this method agree with literature studies, and mass spectrometry experiments were also carried out to confirm our findings.

Selectivity enhancement in capillary electrophoresis by means of two-dimensional separation or dual detection concepts

For the identification and quantification of analytes in complex samples, highly selective analytical strategies are required. The selectivity of single separation techniques such as gas chromatography (GC), liquid chromatography (LC), or capillary electrophoresis (CE) with common detection principles can be enhanced by hyphenating orthogonal separation techniques but also by using complementary detection systems. In this review, two-dimensional systems containing CE in at least one dimension are reviewed, namely LC-CE or 2D CE systems. Particular attention is paid to the aspect of selectivity enhancement due to the orthogonality of the different separation mechanisms. As an alternative concept, dual detection approaches are reviewed using the common detectors of CE such as UV/VIS, laser-induced fluorescence, capacitively coupled contactless conductivity (C⁴D), electrochemical detection, and mass spectrometry. Special emphasis is given to dual detection systems implementing the highly flexible C⁴D as one detection component. Selectivity enhancement can be achieved in case of complementarity of the different detection techniques.

Recent advances in capillary electrophoresis-mass spectrometry: Instrumentation, methodology and applications

Capillary electrophoresis (CE) offers fast and high-resolution separation of charged analytes from small injection volumes. Coupled to mass spectrometry (MS), it represents a powerful analytical technique providing (exact) mass information and enables molecular characterization based on fragmentation. Although hyphenation of CE and MS is not straightforward, much emphasis has been placed on enabling efficient ionization and user-friendly coupling. Though several interfaces are now commercially available, research on more efficient and robust interfacing with nano-electrospray ionization (ESI), matrix-assisted laser desorption/ionization (MALDI) and inductively coupled plasma mass spectrometry (ICP) continues with considerable results. At the same time, CE-MS has been used in many fields, predominantly for the analysis of proteins, peptides and metabolites. This review belongs to a series of regularly published articles, summarizing 248 articles covering the time between June 2016 and May 2018. Latest developments on hyphenation of CE with MS as well as instrumental developments such as two-dimensional separation systems with MS detection are mentioned. Furthermore, applications of various CE-modes including capillary zone electrophoresis (CZE), nonaqueous capillary electrophoresis (NACE), capillary gel electrophoresis (CGE) and capillary isoelectric focusing (CIEF) coupled to MS in biological, pharmaceutical and environmental research are summarized.

Magnifying ion mobility spectrometry-mass spectrometry measurements for biomolecular structure studies

Ion mobility spectrometry-mass spectrometry (IMS-MS) provides information about the structures of gas-phase ions in the form of a collision cross section (CCS) with a neutral buffer gas. Indicating relative ion size, a CCS value alone is of limited utility. Although such information can be used to propose different conformer types, finer details of structure are not captured. The increased accessibility of IMS-MS measurements with commercial instrumentation in recent years has ballooned its usage in combination with separate measurements to provide enhanced data from which greater structural inferences can be drawn. This short review presents recent outstanding developments in scientific research that employs complementary measurements that when combined with IMS-MS data are used to characterize the structures of a wide range of compounds.

The application of ion-mobility mass spectrometry for structure/function investigation of protein complexes

Ion-mobility mass spectrometry (IM-MS) is an approach that can provide information on the stoichiometry, composition, protein contacts and topology of protein complexes. The power of this approach lies not only in its sensitivity and speed of analysis, but also in the fact that it is a technique that can capture the repertoire of conformational states adopted by protein assemblies. Here, we describe the array of available IM-MS based tools, and demonstrate their application to the structural characterization of various protein complexes, including challenging systems as amyloid aggregates and membrane proteins. We also discuss recent studies in which IM-MS was applied towards investigations of conformational transitions and stabilization effects induced by protein interactions.

Recent Progress toward Microfluidic Quality Control Testing of Radiopharmaceuticals

Radiopharmaceuticals labeled with short-lived positron-emitting or gamma-emitting isotopes are injected into patients just prior to performing positron emission tomography (PET) or single photon emission tomography (SPECT) scans, respectively. These imaging modalities are widely used in clinical care, as well as in the development and evaluation of new therapies in clinical research. Prior to injection, these radiopharmaceuticals (tracers) must undergo quality control (QC) testing to ensure product purity, identity, and safety for human use. Quality tests can be broadly categorized as (i) pharmaceutical tests, needed to ensure molecular identity, physiological compatibility and that no microbiological, pyrogenic, chemical, or particulate contamination is present in the final preparation; and (ii) radioactive tests, needed to ensure proper dosing and that there are no radiochemical and radionuclidic impurities that could interfere with the biodistribution or imaging. Performing the required QC tests is cumbersome and time-consuming, and requires an array of expensive analytical chemistry equipment and significant dedicated lab space. Calibrations, day of use tests, and documentation create an additional burden. Furthermore, in contrast to ordinary pharmaceuticals, each batch of short-lived radiopharmaceuticals must be manufactured and tested within a short period of time to avoid significant losses due to radioactive decay. To meet these challenges, several efforts are underway to develop integrated QC testing instruments that automatically perform and document all of the required tests. More recently, microfluidic quality control systems have been gaining increasing attention due to vastly reduced sample and reagent consumption, shorter analysis times, higher detection sensitivity, increased multiplexing, and reduced instrumentation size. In this review, we describe each of the required QC tests and conventional testing methods, followed by a discussion of efforts to directly miniaturize the test or examples in the literature that could be implemented for miniaturized QC testing.

Structure-Activity Relationships of Potent, Targeted Covalent Inhibitors That Abolish Both the Transamidation and GTP Binding Activities of Human Tissue Transglutaminase

Human tissue transglutaminase (hTG2) is a multifunctional enzyme. It is primarily known for its calcium-dependent transamidation activity that leads to formation of an isopeptide bond between glutamine and lysine residues found on the surface of proteins, but it is also a GTP binding protein. Overexpression and unregulated hTG2 activity have been associated with numerous human diseases, including cancer stem cell survival and metastatic phenotype. Herein, we present a series of targeted covalent inhibitors (TCIs) based on our previously reported Cbz-Lys scaffold. From this structure-activity relationship (SAR) study, novel irreversible inhibitors were identified that block the transamidation activity of hTG2 and allosterically abolish its GTP binding ability with a high degree of selectivity and efficiency (k_inact/K_I > 10⁵ M^-1 min^-1). One optimized inhibitor (VA4) was also shown to inhibit epidermal cancer stem cell invasion with an EC₅₀ of 3.9 μM, representing a significant improvement over our previously reported "hit" NC9.

Novel volumetric method for highly repeatable injection in microchip electrophoresis

A novel injector for microchip electrophoresis (MCE) has been designed and evaluated that achieves very high repeatability of injection volume suitable for quantitative analysis. It eliminates the injection biases in electrokinetic injection and the dependence on pressure and sample properties in hydrodynamic injection. The microfluidic injector, made of poly(dimethylsiloxane) (PDMS), operates similarly to an HPLC injection valve. It contains a channel segment (chamber) with a well-defined volume that serves as an "injection loop". Using on-chip microvalves, the chamber can be connected to the sample source during the "loading" step, and to the CE separation channel during the "injection" step. Once the valves are opened in the second state, electrophoretic potential is applied to separate the sample. For evaluation and demonstration purposes, the microinjector was connected to a 75 μm ID capillary and UV absorbance detector. For single compounds, a relative standard deviation (RSD) of peak area as low as 1.04% (n = 11) was obtained, and for compound mixtures, RSD as low as 0.40% (n = 4) was observed. Using the same microchip, the performance of this new injection technique was compared to hydrodynamic injection and found to have improved repeatability and less dependence on sample viscosity. Furthermore, a non-radioactive version of the positron-emission tomography (PET) imaging probe, FLT, was successfully separated from its known 3 structurally-similar byproducts with baseline resolution, demonstrating the potential for rapid, quantitative analysis of impurities to ensure the safety of batches of short-lived radiotracers. Both the separation efficiency and injection repeatability were found to be substantially higher when using the novel volumetric injection approach compared to electrokinetic injection (performed in the same chip). This novel microinjector provides a straightforward way to improve the performance of hydrodynamic injection and enables extremely repeatable sample volume injection in MCE. It could be used in any MCE application where volume repeatability is needed, including the quantitation of impurities in pharmaceutical or radiopharmaceutical samples.

Inhibition or ablation of transglutaminase 2 impairs astrocyte migration

Astrocytes play numerous complex roles that support and facilitate the function of neurons. Further, when there is an injury to the central nervous system (CNS) they can both facilitate or ameliorate functional recovery depending on the location and severity of the injury. When a CNS injury is relatively severe a glial scar is formed, which is primarily composed of astrocytes. The glial scar can be both beneficial, by limiting inflammation, and detrimental, by preventing neuronal projections, to functional recovery. Thus, understanding the processes and proteins that regulate astrocyte migration in response to injury is still of fundamental importance. One protein that is likely involved in astrocyte migration is transglutaminase 2 (TG2); a multifunctional protein expressed ubiquitously throughout the brain. Its functions include transamidation and GTPase activity, among others, and previous studies have implicated TG2 as a regulator of migration. Therefore, we examined the role of TG2 in primary astrocyte migration subsequent to injury. Using wild type or TG2^-/- astrocytes, we manipulated the different functions and conformation of TG2 with novel irreversible inhibitors or mutant versions of the protein. Results showed that both inhibition and ablation of TG2 in primary astrocytes significantly inhibit migration. Additionally, we show that the deficiency in migration caused by deletion of TG2 can only be rescued with the native protein and not with mutants. Finally, the addition of TGFβ rescued the migration deficiency independent of TG2. Taken together, our study shows that transamidation and GTP/GDP-binding are necessary for inhibiting astrocyte migration and it is TGFβ independent.