A chemical approach to unraveling the biological function of the glycosylphosphatidylinositol anchor

Sensitive Method To Analyze Cell Surface GPI-Anchored Proteins Using DNA Hybridization Chain Reaction-Mediated Signal Amplification

GPI-anchored proteins (GPI-APs) are ubiquitous and essential but exist in low abundances on the cell surface, making their analysis and investigation especially challenging. To tackle the problem, a new method to detect and study GPI-APs based upon GPI metabolic engineering and DNA-facilitated fluorescence signal amplification was developed. In this context, cell surface GPI-APs were metabolically engineered using azido-inositol derivatives to introduce an azido group. This allowed GPI-AP coupling with alkyne-functionalized multifluorophore DNA assemblies generated by hybridization chain reaction (HCR). It was demonstrated that this approach could significantly improve the detection limit and sensitivity of GPI-APs, thereby enabling various biological studies, including the investigation of live cells. This new, enhanced GPI-AP detection method has been utilized to successfully explore GPI-AP engineering, analyze GPI-APs, and profile GPI-AP expression in different cells.

Bifunctional glycosphingolipid (GSL) probes to investigate GSL-interacting proteins in cell membranes

Glycosphingolipids (GSLs) are abundant glycolipids on cells and essential for cell recognition, adhesion, signal transduction, and so on. However, their lipid anchors are not long enough to cross the membrane bilayer. To transduce transmembrane signals, GSLs must interact with other membrane components, whereas such interactions are difficult to investigate. To overcome this difficulty, bifunctional derivatives of II3-β-N-acetyl-D-galactosamine-GA2 (GalNAc-GA2) and β-N-acetyl-D-glucosamine-ceramide (GlcNAc-Cer) were synthesized as probes to explore GSL-interacting membrane proteins in live cells. Both probes contain photoreactive diazirine in the lipid moiety, which can crosslink with proximal membrane proteins upon photoactivation, and clickable alkyne in the glycan to facilitate affinity tag addition for crosslinked protein pull-down and characterization. The synthesis is highlighted by the efficient assembly of simple glycolipid precursors followed by on-site lipid remodeling. These probes were employed to profile GSL-interacting membrane proteins in HEK293 cells. The GalNAc-GA2 probe revealed 312 distinct proteins, with GlcNAc-Cer probe-crosslinked proteins as controls, suggesting the potential influence of the glycan on GSL functions. Many of the proteins identified with the GalNAc-GA2 probe are associated with GSLs, and some have been validated as being specific to this probe. The versatile probe design and experimental protocols are anticipated to be widely applicable to GSL research.

Investigation of Glycosylphosphatidylinositol (GPI)-Plasma Membrane Interaction in Live Cells and the Influence of GPI Glycan Structure on the Interaction

Glycosylphosphatidylinositols (GPIs) need to interact with other components in the cell membrane to transduce transmembrane signals. A bifunctional GPI probe was employed for photoaffinity-based proximity labelling and identification of GPI-interacting proteins in the cell membrane. This probe contained the entire core structure of GPIs and was functionalized with photoreactive diazirine and clickable alkyne to facilitate its crosslinking with proteins and attachment of an affinity tag. It was disclosed that this probe was more selective than our previously reported probe containing only a part structure of the GPI core for cell membrane incorporation and an improved probe for studying GPI-cell membrane interaction. Eighty-eight unique membrane proteins, many of which are related to GPIs/GPI-anchored proteins, were identified utilizing this probe. The proteomics dataset is a valuable resource for further analyses and data mining to find new GPI-related proteins and signalling pathways. A comparison of these results with those of our previous probe provided direct evidence for the profound impact of GPI glycan structure on its interaction with the cell membrane.

Recent Advances in Chemical Synthesis of Amino Sugars

Amino sugars are a kind of carbohydrates with one or more hydroxyl groups replaced by an amino group. They play crucial roles in a broad range of biological activities. Over the past few decades, there have been continuing efforts on the stereoselective glycosylation of amino sugars. However, the introduction of glycoside bearing basic nitrogen is challenging using conventional Lewis acid-promoted pathways owing to competitive coordination of the amine to the Lewis acid promoter. Additionally, diastereomeric mixtures of O-glycoside are often produced if aminoglycoside lack a C2 substituent. This review focuses on the updated overview of the way to stereoselective synthesis of 1,2-cis-aminoglycoside. The scope, mechanism, and the applications in the synthesis of complex glycoconjugates for the representative methodologies were also included.

Profiling Glycosylphosphatidylinositol (GPI)-Interacting Proteins in the Cell Membrane Using a Bifunctional GPI Analogue as the Probe

Glycosylphosphatidylinositol (GPI) anchorage of cell surface proteins to the membrane is biologically important and ubiquitous in eukaryotes. However, GPIs do not contain long enough lipids to span the entire membrane bilayer. To transduce binding signals, GPIs must interact with other membrane components, but such interactions are difficult to define. Here, a new method was developed to explore GPI-interacting membrane proteins in live cell with a bifunctional analogue of the glucosaminylphosphatidylinositol motif conserved in all GPIs as a probe. This probe contained a diazirine functionality in the lipid and an alkynyl group on the glucosamine residue to respectively facilitate the cross-linkage of GPI-binding membrane proteins with the probe upon photoactivation and then the installation of biotin to the cross-linked proteins via a click reaction for affinity-based protein isolation and analysis. Profiling the proteins pulled down from the Hela cells revealed 94 unique and 18 overrepresented proteins compared to the control, and most of them are membrane proteins and many are GPI-related. The results have proved not only the concept of using the new bifunctional GPI probe to investigate GPI-binding membrane proteins but also the important role of inositol in the biological functions of GPI anchors and GPI-anchored proteins.

Polymer Chemistry in Living Cells

The polymerization of biomolecules is a central operation in biology that connects molecular signals with proliferative and information-rich events in cells. As molecules arrange precisely across 3-D space, they create new functional capabilities such as catalysis and transport highways and exhibit new phase separation phenomena that fuel nonequilibrium dynamics in cells. Hence, the observed polymer chemistry manifests itself as a molecular basis leading to cellular phenotypes, expressed as a multitude of hierarchical structures found in cell biology. Although many milestone discoveries had accompanied the rise of the synthetic polymer era, fundamental studies were realized within a closed, pristine environment and that their behavior in a complex multicomponent system remains challenging and thus unexplored. From this perspective, there is a rich trove of undiscovered knowledge that awaits the polymer science community that can revolutionize understanding in the interactive nanoscale world of the living cell.In this Account, we discuss the strategies that have enabled synthetic polymer chemistry to be conducted within the cells (membrane inclusive) and to establish monomer design principles that offer spatiotemporal control of the polymerization. As reaction considerations such as monomer concentration, polymer growth dynamics, and reactivities are intertwined with the subcellular environment and transport processes, we first provide a chemical narrative of each major cellular compartment. The conditions within each compartment will therefore set the boundaries on the type of polymer chemistry that can be conducted. Both covalent and supramolecular polymerization concepts are explored separately in the context of scaffold design, polymerization mechanism, and activation. To facilitate transport into a localized subcellular space, we show that monomers can be reversibly modified by targeting groups or stimulus-responsive motifs that react within the specific compartment. Upon polymerization, we discuss the characterization of the resultant polymeric structures and how these phase-separated structures would impact biological processes such as cell cycle, metabolism, and apoptosis. As we begin to integrate cellular biochemistry with in situ polymer science, we identify landmark challenges and technological hurdles that, when overcome, would lead to invaluable discoveries in macromolecular therapeutics and biology.

The importance of side branches of glycosylphosphatidylinositol anchors: a molecular dynamics perspective

Many proteins are anchored to the cell surface of eukaryotes using a unique family of glycolipids called glycosylphosphatidylinositol (GPI) anchors. These glycolipids also exist without a covalently bound protein, in particular on the cell surfaces of protozoan parasites where they are densely populated. GPIs and GPI-anchored proteins participate in multiple cellular processes such as signal transduction, cell adhesion, protein trafficking and pathogenesis of Malaria, Toxoplasmosis, Trypanosomiasis and prion diseases, among others. All GPIs share a common conserved glycan core modified in a cell-dependent manner with additional side glycans or phosphoethanolamine residues. Here, we use atomistic molecular dynamic simulations and perform a systematic study to evaluate the structural properties of GPIs with different side chains inserted in lipid bilayers. Our results show a flop-down orientation of GPIs with respect to the membrane surface and the presentation of the side chain residues to the solvent. This finding agrees well with experiments showing the role of the side residues as active epitopes for recognition of GPIs by macrophages and induction of GPI-glycan-specific immune responses. Protein-GPI interactions were investigated by attaching parasitic GPIs to Green Fluorescent Protein. GPIs are observed to recline on the membrane surface and pull down the attached protein close to the membrane facilitating mutual contacts between protein, GPI and the lipid bilayer. This model is efficient in evaluating the interaction of GPIs and GPI-anchored proteins with membranes and can be extended to study other parasitic GPIs and proteins and develop GPI-based immunoprophylaxis to treat infectious diseases.

Chemische Synthese und Semisynthese von lipidierten Proteinen

Lipidierung ist eine ubiquitäre Modifikation von Peptiden und Proteinen, die entweder co‐ oder posttranslational auftreten kann. Für die Vielzahl von Lipidklassen wurde gezeigt, dass diese viele entscheidende biologische Aktivitäten, z. B. die Regulierung der Signalweiterleitung, Zell‐Zell‐Adhäsion sowie die Anlagerung von Proteinen an Lipid‐Rafts und Phospholipidmembranen, beeinflussen. Während die Natur Enzyme nutzt, um Lipidmodifikationen in Proteine einzubringen, ist ihre Nutzung für die chemoenzymatische Herstellung von lipidierten Proteinen häufig ineffizient. Eine Alternative ist die Kombination moderner synthetischer und semisynthetischer Techniken, um lipidierte Proteine in reiner und homogen modifizierter Form zu erhalten. Dieser Aufsatz erörtert Fortschritte in der Entwicklung der Lipidierungs‐ und Ligationschemie und deren Anwendung in der Synthese und Semisynthese homogen lipidierter Proteine, die es ermöglichen, den Einfluss dieser Modifikationen auf die Proteinstruktur und ‐funktion zu untersuchen.

Chemical Synthesis and Semisynthesis of Lipidated Proteins

Lipidation is a ubiquitous modification of peptides and proteins that can occur either co‐ or post‐translationally. An array of different lipid classes can adorn proteins and has been shown to influence a number of crucial biological activities, including the regulation of signaling, cell–cell adhesion events, and the anchoring of proteins to lipid rafts and phospholipid membranes. Whereas nature employs a range of enzymes to install lipid modifications onto proteins, the use of these for the chemoenzymatic generation of lipidated proteins is often inefficient or impractical. An alternative is to harness the power of modern synthetic and semisynthetic technologies to access lipid‐modified proteins in a pure and homogeneously modified form. This Review aims to highlight significant advances in the development of lipidation and ligation chemistry and their implementation in the synthesis and semisynthesis of homogeneous lipidated proteins that have enabled the influence of these modifications on protein structure and function to be uncovered.

Sortase A: A chemoenzymatic approach for the labeling of cell surfaces

Sortase A, a transpeptidase enzyme is present in many Gram-positive bacteria and helps in the recruitment of the cell surface proteins. Over the last two decades, Sortase A has become an attractive tool for performing in vivo and in vitro ligations. Sortase A-mediated ligation has continuously been used for its specificity, robustness, and highly efficient nature. These properties make it a popular choice among protein engineers as well as researchers from different fields. In this review, we give an overview of Sortase A-mediated ligation of various molecules on the cell surfaces, which can have diverse applications in interdisciplinary fields.

Potential of Cell Surface Engineering with Biocompatible Polymers for Biomedical Applications

The regulation of the cellular surface with biomaterials can contribute to the progress of biomedical applications. In particular, the cell surface is exposed to immunological surveillance and reactions in transplantation therapy, and modulation of cell surface properties might improve transplantation outcomes. The transplantation of therapeutic cells, tissue, and organs is an effective and fundamental treatment and has contributed to saving lives and improving quality of life. Because of shortages, donor cells, tissues, and organs are carefully transplanted with the goal of retaining activity and viability. However, some issues remain to be resolved in terms of reducing side effects, improving graft survival, managing innate and adaptive immune responses, and improving transplant storage and procedures. Given that the transplantation process involves multiple steps and is technically complicated, an engineering approach together with medical approaches to resolving these issues could enhance success. In particular, cell surface engineering with biocompatible polymers looks promising for improving transplantation therapy and has potential for other biomedical applications. Here we review the significance of polymer-based surface modification of cells and organs for biomedical applications, focusing on the following three topics: Cell protection: cellular protection through local immune regulation using cell surface modification with biocompatible polymers. This protection could extend to preventing attack by the host immune system, freeing recipients from taking immunosuppressive drugs, and avoiding a second transplantation. Cell attachment: cell manipulation, which is an important technique for delivery of therapeutic cells and their alignment for recellularization of decellularized tissues and organs in regenerative therapy. Cell fusion: fusion of different cells, which can lead to the formation of new functional cells that could be useful for generating, e.g., immunologically competent or metabolically active cells.

Identification of a homology-independent linchpin domain controlling mouse and bank vole prion protein conversion

Prions are unorthodox pathogens that cause fatal neurodegenerative diseases in humans and other mammals. Prion propagation occurs through the self-templating of the pathogenic conformer PrP^Sc, onto the cell-expressed conformer, PrP^C. Here we study the conversion of PrP^C to PrP^Sc using a recombinant mouse PrP^Sc conformer (mouse protein-only recPrP^Sc) as a unique tool that can convert bank vole but not mouse PrP^C substrates in vitro. Thus, its templating ability is not dependent on sequence homology with the substrate. In the present study, we used chimeric bank vole/mouse PrP^C substrates to systematically determine the domain that allows for conversion by Mo protein-only recPrP^Sc. Our results show that that either the presence of the bank vole amino acid residues E227 and S230 or the absence of the second N-linked glycan are sufficient to allow PrP^C substrates to be converted by Mo protein-only recPrP^Sc and several native infectious prion strains. We propose that residues 227 and 230 and the second glycan are part of a C-terminal domain that acts as a linchpin for bank vole and mouse prion conversion.

Prions are unconventional infectious agents that lack nucleic acids such as DNA and RNA, and the mechanism by which prions replicate is not fully understood. It has been established that a central feature of the replication mechanism involves the misfolding of a host protein (PrP^C) into an infectious shape termed PrP^Sc, but it is unclear how this misfolding occurs. Interestingly, it has been observed that a particular animal species, the European bank vole, is unusually susceptible to prion infection and that this near-universal susceptibility is caused by the specific PrP^C sequence of this protein. Here we use a powerful and unique biochemical system to determine the specific region of bank vole PrP^C that is primarily responsible for its propensity to misfold into PrP^Sc. This critical region, which is located at the extreme C-terminal end of the protein, appears to act as a linchpin domain that normally stabilizes the shape of PrP^C and thereby regulates its misfolding into PrP^Sc.

Coarse-Grained Molecular Model for the Glycosylphosphatidylinositol Anchor with and without Protein

Glycosylphosphatidylinositol (GPI) anchors are a unique class of complex glycolipids that anchor a great variety of proteins to the extracellular leaflet of plasma membranes of eukaryotic cells. These anchors can exist either with or without an attached protein called GPI-anchored protein (GPI-AP) both in vitro and in vivo. Although GPIs are known to participate in a broad range of cellular functions, it is to a large extent unknown how these are related to GPI structure and composition. Their conformational flexibility and microheterogeneity make it difficult to study them experimentally. Simplified atomistic models are amenable to all-atom computer simulations in small lipid bilayer patches but not suitable for studying their partitioning and trafficking in complex and heterogeneous membranes. Here, we present a coarse-grained model of the GPI anchor constructed with a modified version of the MARTINI force field that is suited for modeling carbohydrates, proteins, and lipids in an aqueous environment using MARTINI’s polarizable water. The nonbonded interactions for sugars were reparametrized by calculating their partitioning free energies between polar and apolar phases. In addition, sugar–sugar interactions were optimized by adjusting the second virial coefficients of osmotic pressures for solutions of glucose, sucrose, and trehalose to match with experimental data. With respect to the conformational dynamics of GPI-anchored green fluorescent protein, the accessible time scales are now at least an order of magnitude larger than for the all-atom system. This is particularly important for fine-tuning the mutual interactions of lipids, carbohydrates, and amino acids when comparing to experimental results. We discuss the prospective use of the coarse-grained GPI model for studying protein-sorting and trafficking in membrane models.

Remodeling of Cellular Surfaces via Fast Disulfide-Thiol Exchange To Regulate Cell Behaviors

Remodeling of cellular surfaces is shown highly effective in the manipulation and control of cell behaviors via nonbiological means. By 5-thio-2-nitrobenzoate-mediated, fast, and reversible disulfide-thiol exchange, a sequential layer by layer assembly process was developed to grow albumin protein shells on cellular surfaces fixed by a disulfide-linked network, in a cytocompatible manner. The artificial shells, accomplished by a double-assembly process, were sustainable up to >1 day, and thereafter gradually bioabsorbed with unaffected cell viability. The surface engineering process enabled dynamic remodeling of cellular surfaces that effectively controlled cell behaviors including regulated cell proliferation, enhanced uptake efficiency of dextran-fluorescein isothiocyanate that is known for cell-impermeability, and targeted imaging. This unique approach was well-validated on tumor cells (B16), immune cells (DC2.4), and neutrophils, showing its potential universality for most of the cells that are rich in thiols. The new strategy will show promise in cell manipulation and targeted imaging.

Introducing Oxo-Phenylacetyl (OPAc) as a Protecting Group for Carbohydrates

A series of oxo-phenylacetyl (OPAc)-protected saccharides, with divergent base sensitivity profiles against benzoyl (Bz) and acetyl (Ac) were synthesized, and KHSO₅/AcCl in methanol was identified as an easy, mild, selective, and efficient deprotecting reagent for their removal in the perspective of carbohydrate synthesis. Timely monitoring of AcCl reagent was supportive in both sequential and simultaneous deprotecting of OPAc, Bz, and Ac. The salient feature of our method is the orthogonal stability against different groups, its ease to generate different valuable acceptors using designed monosaccharides, and use of OPAc as a glycosyl donar.

Cell surface engineering and application in cell delivery to heart diseases

Cell-based therapy has expanded its influence in cancer immunotherapy, regenerative medicine, and tissue engineering. Due to their secretory functions, differentiation capabilities, specific homing effects through chemotaxis, distinctive therapeutic potentials, and ex vivo expandability, cells have become an attractive reagent for advanced therapeutic strategies. Therefore, the ability to modify cells and manipulate their functions according to intended therapeutic designs has been the central scientific interest in the field of biomedical research. Many innovative methods have been developed with genetic modification of cells being the most advanced cell surface engineering technique. Although genetic modification is a powerful tool, it has a limited applicability due to the permanent modifications made on cells. Alternatively, many endeavors have been made to develop surface engineering techniques that can circumvent the limitations of genetic modification. In this review, current methods of non-genetic cell surface modification, including chemical conjugations, polymeric encapsulation, hydrophobic insertion, enzymatic and metabolic addition, will be introduced. Moreover, cell surface engineering plausible for cardiac remodeling and the future prospective will be discussed at the end.

Synthesis of biotin-labelled core glycans of GPI anchors and their application in the study of GPI interaction with pore-forming bacterial toxins

A convergent strategy was developed for the first-time synthesis of biotin-labeled GPI core glycans. These GPI conjugates are useful for various biological studies showcased by their application in the scrutiny of pore-forming bacterial toxin-GPI interaction, revealing that the phosphate group at the GPI inositol 1-O-position had a significant impact on GPI-toxin binding.

A molecular dynamics model for glycosylphosphatidyl-inositol anchors: “flop down” or “lollipop”?

Computational model for GPI anchors tested in DMPC and POPC bilayers. The free anchor rarely occurs as an erected “lollipop-like” conformation, it rather “flops down” onto the bilayer surface. Yet an attached protein (here green fluorescent protein) exhibits extensive orientational flexibility due to the phospho-ethanolamine linker.

Chemical Synthesis of GPI Glycan-Peptide Conjugates by Traceless Staudinger Ligation

A new strategy has been developed for GPI glycan-peptide conjugate synthesis based upon a traceless Staudinger reaction between a peptide phosphinothioester and a GPI glycan azide. The strategy was first studied and optimized with simple peptides and GPI glycans, which offered excellent yields of the desired conjugates in both organic and aqueous solvents. It was then used to successfully synthesize an analogue of the human CD52 antigen containing the whole CD52 peptide sequence and the conserved trimannose motif of all GPI anchors.

Hybrid Congregation of Islet Single Cells and Curcumin-Loaded Polymeric Microspheres as an Interventional Strategy to Overcome Apoptosis Associated with Pancreatic Islets Transplantation

Hypoxic or near-anoxic conditions that occur in the core of transplanted islets induce necrosis and apoptosis during the early stages after transplantation, primarily due to loss of vascularization during the isolation process. Moreover, secretion of various cytokines from pancreatic islets is detrimental to the viability of islet cells in vitro. In this study, we aimed to protect pancreatic islet cells against apoptosis by establishing a method for in situ delivery of curcumin to the pancreatic islets. Self-assembled heterospheroids composed of pancreatic islet cells and curcumin-loaded polymeric microspheres were prepared by the three-dimensional cell culture technique. Release of curcumin in the microenvironment of pancreatic islets promoted survival of the islets. In hypoxic culture conditions, which mimic the in vivo conditions after transplantation, viability of the islets was significantly improved, as indicated by a decreased expression of pro-apoptotic protein and an increased expression of anti-apoptotic protein. Additionally, oxidative stress-induced cell death was suppressed. Thus, unlike co-transplantation of pancreatic islets and free microspheres, which provided a wide distribution of microspheres throughout the transplanted area, the heterospheroid transplantation resulted in colocalization of pancreatic islet cells and microspheres, thereby exerting beneficial effects on the cells.

Progress and challenges in macroencapsulation approaches for type 1 diabetes (T1D) treatment: Cells, biomaterials, and devices

Macroencapsulation technology has been an attractive topic in the field of treatment for Type 1 diabetes due to mechanical stability, versatility, and retrievability of the macro-capsule design. Macro-capsules can be categorized into extravascular and intravascular devices, in which solute transport relies either on diffusion or convection, respectively. Failure of macroencapsulation strategies can be due to limited regenerative capacity of the encased insulin-producing cells, sub-optimal performance of encapsulation biomaterials, insufficient immunoisolation, excessive blood thrombosis for vascular perfusion devices, and inadequate modes of mass transfer to support cell viability and function. However, significant technical advancements have been achieved in macroencapsulation technology, namely reducing diffusion distance for oxygen and nutrients, using pro-angiogenic factors to increase vascularization for islet engraftment, and optimizing membrane permeability and selectivity to prevent immune attacks from host's body. This review presents an overview of existing macroencapsulation devices and discusses the advances based on tissue-engineering approaches that will stimulate future research and development of macroencapsulation technology. Biotechnol. Bioeng. 2016;113: 1381-1402. © 2015 Wiley Periodicals, Inc.

QM/MM studies of contemporary and novel membrane raft fluorescent probes

We have studied a number of contemporary and novel membrane probes, selected for their structural similarity to membrane raft components, in order to properly anchor themselves within a sphingolipid/cholesterol rich region. A QM/MM approach was adopted in order to understand the structural and electrostatic influences of fluorescence emission shifts of the probes in different lipid and solvation environments. The proposed modifications to the membrane probes have shown encouraging data relating not only to emission shifts within the membrane, but also their ability to anchor within a membrane raft domain and the stability to internalization within a membrane system.

Chemical Approach to a Whole Body Imaging of Sialo-N-Linked Glycans

PET and noninvasive fluorescence imaging of the sialo-N-linked glycan derivatives are described. To establish the efficient labeling protocol for N-glycans and/or glycoconjugates, new labeling probes of fluorescence and ⁶⁸Ga-DOTA, as the positron emission nucleus for PET, through rapid 6π-azaelectrocyclization were designed and synthesized, (E)-ester aldehydes. The high reactivity of these probes enabled the labeling of lysine residues in peptides, proteins, and even amino groups on the cell surfaces at very low concentrations of the target molecules (~10⁻⁸ M) within a short reaction time (~5 min) to result in "selective" and "non-destructive" labeling of the more accessible amines. The first MicroPET of glycoproteins, ⁶⁸Ga-DOTA-orosomucoid and asialoorosomucoid, successfully visualized the differences in the circulatory residence of glycoproteins, in the presence or absence of sialic acids. In vivo dynamics of the new N-glycoclusters, prepared by the "self-activating" Huisgen cycloaddition reaction, could also be affected significantly by their partial structures at the non-reducing end, i.e., the presence or absence of sialic acids, and/or sialoside linkages to galactose. Azaelectrocyclization chemistry is also applicable to the engineering of the proteins and/or the cell surfaces by the oligosaccharides; lymphocytes chemically engineered by sialo-N-glycan successfully target the tumor implanted in BALB/C nude mice, detected by noninvasive fluorescence imaging.

Preparation of semisynthetic lipoproteins with fluorescent cholesterol anchor and their introduction to the cell membrane with minimal disruption of the membrane

The exogenous introduction of fluorescent lipoproteins into cell membranes is a method for visualizing the cellular traffic of membrane associated proteins, and also for altering the cell surface in a controlled manner. In order to achieve the cell membrane anchoring of proteins and their subsequent fluorescence based detection, a cholesterol derivative was designed. The headgroup of the novel cholesterol anchor contains a fluorescent reporter and a thiol reactive maleimide for protein conjugation. Protein conjugation was demonstrated by the addition of a green fluorescent maleimido anchor to the C-terminus of a Cys extended red fluorescent protein, mCherry. The resulting dual fluorescent cholesteryl lipoprotein was successfully separated from the micellar associates of the surplus fluorescent lipid anchor without denaturing the protein, and the lipoprotein containing only the covalently linked, stoichiometric fluorescent lipid was efficiently delivered to the plasma membrane of live cells. It was demonstrated that the membrane fluorescence could be directly assigned to the protein-anchor conjugate, because no excess of fluorescent lipid species were present during the imaging experiment and the protein and anchor fluorescence colocalized in the cell membrane. Molecular dynamics simulations and subsequent trajectory analysis suggest also the spontaneous and stable membrane association of the cholesterol anchor. Thus, the method could be beneficially applied for studying membrane associated proteins and for preparing mimetics of glycosylphosphatidylinositol (GPI)-anchored proteins to target cholesterol-rich membrane microdomains.

Recent progress in synthetic and biological studies of GPI anchors and GPI-anchored proteins

Covalent attachment of glycosylphosphatidylinositols (GPIs) to the protein C-terminus is one of the most common posttranslational modifications in eukaryotic cells. In addition to anchoring surface proteins to the cell membrane, GPIs also have many other important biological functions, determined by their unique structure and property. This account has reviewed the recent progress made in disclosing GPI and GPI-anchored protein biosynthesis, in the chemical and chemoenzymatic synthesis of GPIs and GPI-anchored proteins, and in understanding the conformation, organization, and distribution of GPIs in the lipid membrane.

Stereoselective α-glycosylation of C(6)-hydroxyl myo-inositols via nickel catalysis-application to the synthesis of GPI anchor pseudo-oligosaccharides

Glycosylphosphatidyl inositol (GPI) anchors play a key role in many eukaryotic biological pathways. Stereoselective synthesis of GPI anchor analogues have proven to be critical for probing the biosynthesis, structure, and biological properties of these compounds. Challenges that have emerged from these efforts include the preparation of the selectively protected myo-inositol building blocks and the stereoselective construction of glucosamine α-linked myo-inositol containing pseudodisaccharide units. Herein, we describe the effectiveness of the cationic nickel(II) catalyst, Ni(4-F-PhCN)4(OTf)2, at promoting selective formation of 1,2-cis-2-amino glycosidic bonds between the C(2)-N-substituted benzylideneamino trihaloacetimidate donors and C(6)-hydroxyl myo-inositol acceptors. This catalytic coupling process allows rapid access to pseudosaccharides of GPI anchors in good yields and with excellent levels of α-selectivity (α:β=10:1-20:1). In stark contrast, activation of trichloroacetimidate donors containing the C(2)-N-substituted benzylidene group with TMSOTf and BF3(.)OEt2 provided the desired pseudodisaccharides as a 1:1 mixture of α- and β-isomers.

Chemical biology of glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositols (GPIs) are complex glycolipids that are covalently linked to the C-terminus of proteins as a posttranslational modification. They anchor the attached protein to the cell membrane and are essential for normal functioning of eukaryotic cells. GPI-anchored proteins are structurally and functionally diverse. Many GPIs have been structurally characterized but comprehension of their biological functions, beyond the simple physical anchoring, remains largely speculative. Work on functional elucidation at a molecular level is still limited. This Review focuses on the roles of GPI unraveled by using synthetic molecules and summarizes the structural diversity of GPIs, as well as their biological and chemical syntheses.

Ultrathin polymeric coatings based on hydrogen-bonded polyphenol for protection of pancreatic islet cells

Though transplantation of pancreatic islet cells has emerged as a promising treatment for Type 1 diabetes its clinical application remains limited due to a number of limitations including both pathogenic innate and adaptive immune responses. We report here on a novel type of multifunctional cytoprotective material applied to coat living pancreatic islets. The coating utilizes hydrogen-bonded interactions of a natural polyphenol (tannic acid) with poly(N-vinylpyrrolidone) deposited on the islet surface via non-ionic layer-by-layer assembly. We demonstrate that the coating is conformal over the surface of mammalian islets including those derived from rat, non-human primate (NHP), and human. In contrast to unmodified controls, the coated islets maintain their viability and β-cell functionality for at least 96 hours in vitro. We also determine that the coating demonstrates immunomodulatory cytoprotective properties suppressing pro-inflammatory cytokine synthesis in stimulated bone marrow-derived macrophages and diabetogenic BDC-2.5 T cells. The coating material combines high chemical stability under physiologically relevant conditions with capability of suppressing cytokine synthesis, crucial parameters for prolonged islet integrity, viability, and function in vivo. Our study offers new opportunities in the area of advanced multifunctional materials to be used for a cell-based transplantation therapy.

Cell surface biotinylation by azaelectrocyclization: easy-handling and versatile approach for living cell labeling

Versatile method for living cell labeling has been established. Cell surfaces are initially biotinylated by azaelectrocyclization, and then treated with the fluorescence-labeled avidin or the anti-biotin antibody.

Target-selective fluorescent "switch-on" protein labeling by 6π-azaelectrocyclization

Application of azaelectrocyclization and FRET techniques to lysine groups enabled the selective and sensitive detection of a target protein from a mixture, with high fluorescence contrast.

Chemical synthesis and functionalization of clickable glycosylphosphatidylinositol anchors

Glycosylphosphatidylinositol (GPI) anchorage is a common posttranslational modification of eukaryotic proteins. Chemical synthesis of structurally defined GPIs and GPI derivatives is a necessary step toward understanding the properties and functions of these molecules in biological systems. In this work, the synthesis of several functionalized GPI anchors was accomplished using the para-methoxybenzyl (PMB) group for permanent hydroxyl protection, which allowed the incorporation of functionalities that are incompatible with permanent protecting groups traditionally used in carbohydrate synthesis. A flexible convergent-divergent assembly strategy enabled efficient access to a diverse set of target structures, including "clickable" Alkynyl-GPIs 1 and 2 and Azido-GPI 3. For global deprotection, a one-pot reaction was employed to afford the target GPIs in excellent yields (85-97%). Fully deprotected clickable GPIs 2 and 3 were readily conjugated to imaging and affinity probes via Cu(I)-catalyzed and Cu-free strain-promoted [3+2] cycloaddition, respectively, resulting in GPI-Fluor 4 and GPI-Biotin 5.