Capillary electrophoresis with three-color fluorescence detection for the analysis of glycosphingolipid metabolism

De Novo Access to BODIPY C-Glycosides as Linker-Free Nonsymmetrical BODIPY-Carbohydrate Conjugates

Recent years have witnessed an increasing interest in the synthesis and study of BODIPY-glycoconjugates. Most of the described synthetic methods toward these derivatives involve postfunctional modifications of the BODIPY core followed by the covalent attachment of the fluorophore and the carbohydrate through a "connector". Conversely, few de novo synthetic approaches to linker-free carbohydrate-BODIPY hybrids have been described. We have developed a reliable modular, de novo, synthetic strategy to linker-free BODIPY-sugar derivatives using the condensation of pyrrole C-glycosides with a pyrrole-carbaldehyde derivative mediated by POCl3. This methodology allows labeling of carbohydrate biomolecules with fluorescent-enough BODIPYs within the biological window, stable in aqueous media, and able to display singlet oxygen generation.

Analytical Techniques for Single-Cell Biochemical Assays of Lipids

Lipids are essential cellular components forming membranes, serving as energy reserves, and acting as chemical messengers. Dysfunction in lipid metabolism and signaling is associated with a wide range of diseases including cancer and autoimmunity. Heterogeneity in cell behavior including lipid signaling is increasingly recognized as a driver of disease and drug resistance. This diversity in cellular responses as well as the roles of lipids in health and disease drive the need to quantify lipids within single cells. Single-cell lipid assays are challenging due to the small size of cells (∼1 pL) and the large numbers of lipid species present at concentrations spanning orders of magnitude. A growing number of methodologies enable assay of large numbers of lipid analytes, perform high-resolution spatial measurements, or permit highly sensitive lipid assays in single cells. Covered in this review are mass spectrometry, Raman imaging, and fluorescence-based assays including microscopy and microseparations.

Synthesis of carbohydrate–BODIPY hybrids

Owing to the relevance of fluorescently labeled carbohydrates in the study of biological processes, we have investigated several routes for the preparation of saccharides covalently linked to borondipyrromethene (BODIPY) fluorophores. We have shown that BODIPY dyes can be used as aglycons through synthetic saccharide protocols. In particular, a per-alkylated 8-(2-hydroxy-methylphenyl)-4,4′-dicyano-BODIPY derivative, which withstands glycosylation and protection/deprotection reaction conditions without decomposition, has been used in the stepwise synthesis of two fluorescently labeled trisaccharides. These saccharides displayed high water solubility and a low tendency to (H-)aggregation, a phenomenon that causes loss of photophysical efficiency in BODIPYs. Two additional synthetic strategies toward glyco-BODIPYs have also been described. The first method relies on a Ferrier-type C-glycosylation of the BODIPY core, leading to linker-free carbohydrate–BODIPY hybrids. Secondly, the application of the Nicholas propargylation reaction to 1,3,5,7-tetramethyl BODIPYs provides access to 2,6-dipropargylated BODIPYs that readily undergo CuAAC reactions with azido-containing sugars. From a photophysical standpoint, the BODIPY-labeled saccharides could be used as stable and fluorescent water-soluble chromophores, thereby addressing one of the current challenges in molecular imaging.

Photoactivatable Reporter to Perform Multiplexed and Temporally Controlled Measurements of Kinase and Protease Activity in Single Cells

Peptide bioreporters were developed to perform multiplexed measurements of the activation of epidermal growth factor receptor kinase (EGFR), Akt kinase (Akt/protein kinase B), and proteases/peptidases in single cells. The performance characteristics of the three reporters were assessed by measuring the reporter's proteolytic stability, kinetic constants for EGFR and Akt, and dephosphorylation rate. The reporter displaying optimal performance was composed of 6-carboxyfluorescein (6-FAM) on the peptide N-terminus, an Akt substrate sequence employing a threonine phosphorylation site for Akt, followed by a tri-D arginine linker, and finally an EGFR substrate sequence bearing a phosphatase-resistant 7-(S)-hydroxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid (L-htc) residue as the EGFR phosphorylation site. Importantly, use of a single electrophoretic condition separated the mono- and diphosphorylated products as well as proteolytic forms permitting the quantitation of multiple enzyme activities simultaneously using a single reporter. Because the Akt and EGFR substrates were linked, a known ratio (EGFR/Akt) of the reporter was loaded into cells. A photoactivatable version of the reporter was synthesized by adding two 4,5-dimethoxy-2-nitrobenzyl (DMNB) moieties to mask the EGFR and Akt phosphorylation sites. The DMNB moieties were readily photocleaved following exposure to 360 nm light, unmasking the phosphorylation sites on the reporter. The new photoactivatable reporter permitted multiplexed measurements of kinase signaling and proteolytic degradation in single cells in a temporally controlled manner. This work will facilitate the development of a new generation of multiplexed activity-based reporters capable of light-initiated measurement of enzymatic activity in single cells.

Bringing Color to Sugars: The Chemical Assembly of Carbohydrates to BODIPY Dyes

The combination of carbohydrates with BODIPY fluorophores gives rise to a family of BODIPY-carbohydrate hybrids or glyco-BODIPYs, which mutually benefit from the encounter. Thus, from the carbohydrates standpoint, glyco-BODIPYs can be regarded as fluorescent glycoconjugate derivatives with application in imaging techniques, whereas from the fluorophore view the BODIPY-carbohydrate hybrids benefit from the biocompatibility, water-solubility, and reduced toxicity, among others, brought about by the sugar moiety. In this Account we have intended to present the collection of available methods for the synthesis of BODIPY-carbohydrate hybrids, with a focus on the chemical transformations on the BODIPY core.

A Concise Synthesis of a BODIPY-Labeled Tetrasaccharide Related to the Antitumor PI-88

A convergent synthetic route to a tetrasaccharide related to PI-88, which allows the incorporation of a fluorescent BODIPY-label at the reducing-end, has been developed. The strategy, which features the use of 1,2-methyl orthoesters (MeOEs) as glycosyl donors, illustrates the usefulness of suitably-designed BODIPY dyes as glycosyl labels in synthetic strategies towards fluorescently-tagged oligosaccharides.

Recent advances in the mass spectrometric analysis of glycosphingolipidome - A review

Aberrant expression of glycosphingolipids has been implicated in a myriad of diseases, but our understanding of the strucural diversity, spatial distribution, and biological function of this class of biomolecules remains limited. These challenges partly stem from a lack of sensitive tools that can detect, identify, and quantify glycosphingolipids at the molecular level. Mass spectrometry has emerged as a powerful tool poised to address most of these challenges. Here, we review the recent developments in analytical glycosphingolipidomics with an emphasis on sample preparation, mass spectrometry and tandem mass spectrometry-based structural characterization, label-free and labeling-based quantification. We also discuss the nomenclature of glycosphingolipids, and emerging technologies like ion mobility spectrometry in differentiation of glycosphingolipid isomers. The intrinsic advantages and shortcomings of each method are carefully critiqued in line with an individual's research goals. Finally, future perspectives on analytical sphingolipidomics are stated, including a need for novel and more sensive methods in isomer separation, low abundance species detection, and profiling the spatial distribution of glycosphingolipid molecular species in cells and tissues using imaging mass spectrometry.

BODIPYs as Chemically Stable Fluorescent Tags for Synthetic Glycosylation Strategies towards Fluorescently Labeled Saccharides

A series of fluorescent boron-dipyrromethene (BODIPY, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene) dyes have been designed to participate, as aglycons, in synthetic oligosaccharide protocols. As such, they served a dual purpose: first, by being incorporated at the beginning of the process (at the reducing-end of the growing saccharide moiety), they can function as fluorescent glycosyl tags, facilitating the detection and purification of the desired glycosidic intermediates, and secondly, the presence of these chromophores on the ensuing compounds grants access to fluorescently labeled saccharides. In this context, a sought-after feature of the fluorescent dyes has been their chemical robustness. Accordingly, some BODIPY derivatives described in this work can withstand the reaction conditions commonly employed in the chemical synthesis of saccharides; namely, glycosylation and protecting-group manipulations. Regarding their photophysical properties, the BODIPY-labeled saccharides obtained in this work display remarkable fluorescence efficiency in water, reaching quantum yield values up to 82 %, as well as notable lasing efficiencies and photostabilities.

Preserving Single Cells in Space and Time for Analytical Assays

Analytical assays performed within clinical laboratories influence roughly 70% of all medical decisions by facilitating disease detection, diagnosis, and management. Both in clinical and academic research laboratories, single-cell assays permit measurement of cell diversity and identification of rare cells, both of which are important in the understanding of disease pathogenesis. For clinically utility, the single-cell assays must be compatible with the clinical workflow steps of sample collection, sample transportation, pre-analysis processing, and single-cell assay; therefore, it is paramount to preserve cells in a state that resembles that in vivo rather than measuring signaling behaviors initiated in response to stressors such as sample collection and processing. To address these challenges, novel cell fixation (and more broadly, cell preservation) techniques incorporate programmable fixation times, reversible bond formation and cleavage, chemoselective reactions, and improved analyte recovery. These technologies will further the development of individualized, precision therapies for patients to yield improved clinical outcomes.

Structure and photochromic properties of a novel diarylethene containing a 9-fluorenone hydrazone Schiff base moiety

A novel diarylethene containing a 9-fluorenone hydrazone Schiff base moiety is designed and synthesized. The structure is characterized by single-crystal diffraction analysis and it shows excellent photochromism in the single crystalline phase. Moreover, the diarylethene product shows relatively high fluorescence modulation efficiency and good fatigue resistance.

"Fix and assay": separating in-cellulo sphingolipid reactions from analytical assay in time and space using an aldehyde-based fixative

Chemical cytometry using capillary electrophoresis (CE) is a powerful tool for measuring single-cell enzyme activity. However, these measurements are often confounding as dynamic processes within cells rapidly change depending on environment, meaning that cell handling, transport, and storage can affect signaling pathways and alter results. To meet these challenges, we describe a method utilizing aldehyde fixation to simultaneously terminate cellular reactions across a population, freezing reaction results in time prior to analytical analysis. Fluorescent sphingosine was loaded into cells of different lineages (leukemia and lymphoma cell lines and primary leukemia cells) and allowed to react before fixing. The remaining sphingosine and any products formed were then quantified with chemical cytometry utilizing CE. When cells were loaded with sphingosine followed by glyoxal fixation and immediate analysis, 55 ± 5% of lipid was recoverable compared to an unfixed control. Storage of fixed cells for 24 h showed no statistical differences in total amount of recoverable sphingolipid compared to samples analyzed immediately after fixation-though there was a difference in recovery of low-abundance products. Sphingosine kinase activity decreased in response to inhibitor treatment compared to treatment with a DMSO vehicle (21 ± 3% product formed in inhibitor-treated cells vs. 57 ± 2% in control cells), which was mirrored in single-cell measurements. This "fix and assay" strategy enables measurement of sphingosine kinase activity in single cells followed by subsequent analytical assay separated in space and time from reaction initiation, enabling greater temporal control over intracellular reactions and improving future compatibility with clinical workflow.

Large-scale Top-down Proteomics Using Capillary Zone Electrophoresis Tandem Mass Spectrometry

Capillary zone electrophoresis-electrospray ionization-tandem mass spectrometry (CZE-ESI-MS/MS) has been recognized as a useful tool for top-down proteomics that aims to characterize proteoforms in complex proteomes. However, the application of CZE-MS/MS for large-scale top-down proteomics has been impeded by the low sample-loading capacity and narrow separation window of CZE. Here, a protocol is described using CZE-MS/MS with a microliter-scale sample-loading volume and a 90-min separation window for large-scale top-down proteomics. The CZE-MS/MS platform is based on a linear polyacrylamide (LPA)-coated separation capillary with extremely low electroosmotic flow, a dynamic pH-junction-based online sample concentration method with a high efficiency for protein stacking, an electro-kinetically pumped sheath flow CE-MS interface with extremely high sensitivity, and an ion trap mass spectrometer with high mass resolution and scan speed. The platform can be used for the high-resolution characterization of simple intact protein samples and the large-scale characterization of proteoforms in various complex proteomes. As an example, a highly efficient separation of a standard protein mixture and a highly sensitive detection of many impurities using the platform is demonstrated. As another example, this platform can produce over 500 proteoform and 190 protein identifications from an Escherichia coli proteome in a single CZE-MS/MS run.

Design and Application of Sensors for Chemical Cytometry

The bulk cell population response to a stimulus, be it a growth factor or a cytotoxic agent, neglects the cell-to-cell variability that can serve as a friend or as a foe in human biology. Biochemical variations among closely related cells furnish the basis for the adaptability of the immune system but also act as the root cause of resistance to chemotherapy by tumors. Consequently, the ability to probe for the presence of key biochemical variables at the single-cell level is now recognized to be of significant biological and biomedical impact. Chemical cytometry has emerged as an ultrasensitive single-cell platform with the flexibility to measure an array of cellular components, ranging from metabolite concentrations to enzyme activities. We briefly review the various chemical cytometry strategies, including recent advances in reporter design, probe and metabolite separation, and detection instrumentation. We also describe strategies for improving intracellular delivery, biochemical specificity, metabolic stability, and detection sensitivity of probes. Recent applications of these strategies to small molecules, lipids, proteins, and other analytes are discussed. Finally, we assess the current scope and limitations of chemical cytometry and discuss areas for future development to meet the needs of single-cell research.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2013-2014

This review is the eighth update of the original article published in 1999 on the application of Matrix-assisted laser desorption/ionization mass spectrometry (MALDI) mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2014. Topics covered in the first part of the review include general aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, fragmentation, and arrays. The second part of the review is devoted to applications to various structural types such as oligo- and poly- saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. The third part of the review covers medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2018 Wiley Periodicals, Inc. Mass Spec Rev 37:353-491, 2018.

Chemical fixation to arrest phospholipid signaling for chemical cytometry

Chemical cytometry is a powerful tool for measuring biological processes such as enzymatic signaling at the single cell level. Among these technologies, single-cell capillary zone electrophoresis (CZE) has emerged as a powerful tool to assay a wide range of cellular metabolites. However, analysis of dynamic processes within cells remains challenging as signaling pathways are rapidly altered in response to changes in the cellular environment, including cell manipulation and storage. To address these limitations, we describe a method for chemical fixation of cells to stop the cellular reactions to preserve the integrity of key signaling molecules or reporters within the cell and to enable the cell to act as a storage reservoir for the reporter and its metabolites prior to assay by single-cell CZE. Fluorescent phosphatidylinositol 4,5-bisphosphate reporters were loaded into cells and the cells were chemically fixed and stored prior to analysis. The reporter and its metabolites were electrophoretically separated by single-cell CZE. Chemical fixation parameters such as fixative, fixation time, storage solution, storage duration, and extraction solution were optimized. When cells were loaded with a fluorescent C6- or C16-PIP₂ followed by glutaraldehyde fixation and immediate analysis, 24±2% and 139±12% of the lipid was recoverable, respectively, when compared to an unfixed control. Storage of the cells for 24h yielded recoverable lipid of 61±3% (C6-PIP₂) and 55±5% (C16-PIP₂) when compared to cells analyzed immediately after fixation. The metabolites observed with and without fixation were identical. Measurement of phospholipase C activity in single leukemic cells in response to an agonist demonstrated the capability of chemical fixation coupled to single-cell CZE to yield an accurate snapshot of cellular reactions with the probe. This methodology enables cell assay with the reporter to be separated in space and time from reporter metabolite quantification while preserving assay integrity.

Get to Understand More from Single-Cells: Current Studies of Microfluidic-Based Techniques for Single-Cell Analysis

This review describes the microfluidic techniques developed for the analysis of a single cell. The characteristics of microfluidic (e.g., little sample amount required, high-throughput performance) make this tool suitable to answer and to solve biological questions of interest about a single cell. This review aims to introduce microfluidic related techniques for the isolation, trapping and manipulation of a single cell. The major approaches for detection in single-cell analysis are introduced; the applications of single-cell analysis are then summarized. The review concludes with discussions of the future directions and opportunities of microfluidic systems applied in analysis of a single cell.

Syntheses of lactosyl ceramide analogues carrying novel bifunctional BODIPY dyes directed towards the differential analysis of multiplexed glycosphingolipids by MS/MS using iTRAQ

Lactosyl ceramide analogues carrying novel bifunctional BODIPY-based fluorescent tags were designed and synthesised for live cell imaging. Addition of azide functionality on the fluorophore facilitated isobaric tagging for quantitative multiplexed analysis of biomolecules based on tandem mass spectrometry.

Single cell metabolic profiling of tumor mimics

Chemical cytometry employs modern analytical methods to study the differences in composition between single cells to better understand development, cellular differentiation, and disease. Metabolic cytometry is a form of chemical cytometry wherein cells are incubated with and allowed to metabolize fluorescently labeled small molecules. Capillary electrophoresis with laser-induced fluorescence detection is then used to characterize the extent of metabolism at the single cell level. To date, all metabolic cytometry experiments have used conventional two-dimensional cell cultures. HCT 116 spheroids are a three-dimensional cell culture system, morphologically and phenotypically similar to tumors. Here, intact HCT 116 multicellular spheroids were simultaneously incubated with three fluorescently labeled glycosphingolipid substrates, GM3-BODIPY-FL, GM1-BODIPY-TMR, and lactosylceramide-BODIPY-650/665. These substrates are spectrally distinct, and their use allows the simultaneous probing of metabolism at three different points in the glycolipid metabolic cascade. Beginning with intact spheroids, a serial trypsinization and trituration procedure was used to isolate single cells from spatially distinct regions of the spheroid. Cells from the distinct regions showed unique metabolic patterns. Treatment with the lysosomal inhibitor and potential chemotherapeutic chloroquine consistently decreased catabolism for all substrates. Nearly 200 cells were taken for analysis. Principal component analysis with a multivariate measure of precision was used to quantify cell-to-cell variability in glycosphingolipid metabolism as a function of cellular localization and chloroquine treatment. While cells from different regions exhibited differences in metabolism, the heterogeneity in metabolism did not differ significantly across the experimental conditions.

Simultaneous monitoring of insulin and islet amyloid polypeptide secretion from islets of Langerhans on a microfluidic device

A method was developed that allowed simultaneous monitoring of the acute secretory dynamics of insulin and islet amyloid polypeptide (IAPP) from islets of Langerhans using a microfluidic system with two-color detection. A flow-switching feature enabled changes in the perfusion media within 5 s, allowing rapid exchange of the glucose concentrations delivered to groups of islets. The perfusate was continuously sampled by electroosmotic flow and mixed online with Cy5-labeled insulin, fluorescein isothiocyanate (FITC)-labeled IAPP, anti-insulin, and anti-IAPP antibodies in an 8.15 cm mixing channel maintained at 37 °C. The immunoassay mixture was injected for 0.3 s onto a 1.5 cm separation channel at 11.75 s intervals and immunoassay reagents detected using 488 and 635 nm lasers with two independent photomultiplier tubes for detection of the FITC and Cy5 signal. RSD of the bound-to-free immunoassay ratios ranged from 2 to 7% with LODs of 20 nM for insulin and 1 nM for IAPP. Simultaneous secretion profiles of the two peptides were monitored from groups of 4-10 islets during multiple step changes in glucose concentration. Insulin and IAPP were secreted in an approximately 10:1 ratio and displayed similar responses to step changes from 3 to 11 or 20 mM glucose. The ability to monitor the secretory dynamics of multiple peptides from islets of Langerhans in a highly automated fashion is expected to be a useful tool for investigating hormonal regulation of glucose homeostasis.

Measurement of protein tyrosine phosphatase activity in single cells by capillary electrophoresis

A fluorescent peptide substrate was used to measure dephosphorylation by protein tyrosine phosphatases (PTP) in cell lysates and single cells and to investigate the effect of environmental toxins on PTP activity in these systems. Dephosphorylation of the substrate by PTPN1 and PTPN2 obeyed Michaelis-Menten kinetics, with KM values of 770 ± 250 and 290 ± 54 nM, respectively. Dose-response curves and IC50 values were determined for the inhibition of these two enzymes by the environmental toxins Zn(2+) and 1,2-naphthoquinone, as well as pervanadate. In A431 cell lysates, the reporter was a poor substrate for peptidases (degradation rate of 100 ± 8.2 fmol min(-1) mg(-1)) but an excellent substrate for phosphatases (dephosphorylation rate of 1.4 ± 0.3 nmol min(-1) mg(-1)). Zn(2+), 1,2-naphthoquinone, and pervanadate inhibited dephosphorylation of the reporter in cell lysates with IC50 values of 470 nM, 35 μM, and 100 nM, respectively. Dephosphorylation of the reporter, following loading into living single cells, occurred at rates of at least 2 pmol min(-1) mg(-1). When single cells were exposed to 1,2-naphthoquinone (50 μM), Zn(2+) (100 μM), and pervandate (1 mM), dephosphorylation was inhibited with median values and first and third quartile values of 41 (Q1 = 0%, Q3 = 96%), 50 (Q1 = 46%, Q3 = 74%), and 53% (Q1 = 36%, Q3 = 77%), respectively, demonstrating both the impact of these toxic exposures on cell signaling and the heterogeneity of response between cells. This approach will provide a valuable tool for the study of PTP dynamics, particularly in small, heterogeneous populations such as human biopsy specimens.

Manipulating ionic strength to improve single cell electrophoretic separations

A capillary electrophoresis system with ultrasensitive two-color laser-induced fluorescence detection was used to probe the effect of ionic strength on single cell separations of glycosphingolipids. Differentiated PC12 cells were incubated with two ganglioside substrates tagged with different fluorophores within the BODIPY family such that two distinct metabolic patterns could be simultaneously monitored. Aspiration of single differentiated PC12 cells suspended in a phosphate-buffered saline solution showed excessive peak dispersion, poor resolution, and peak efficiencies below 100,000 theoretical plates. Aspiration of single differentiated PC12 cells suspended in deionized water corrected peak dispersion. Average peak efficiencies ranged between 400,000 and 600,000 theoretical plates. Improved performance was due to the dilution of the high salt concentrations inside of single neuronal-like cells to produce field amplified sample stacking. Single cell separations showed the highest resolution when aspiration of single differentiated PC12 cells suspended in deionized water were separated using a running buffer of high ionic strength. The improvement in resolution allowed for the identification of analytes not previously detected in single cell metabolism studies.