Carbohydrate–Neuroactive Hybrid Strategy for Metabolic Glycan Engineering of the Central Nervous System in Vivo

In Situ Glycan Analysis and Editing in Living Systems

Besides proteins and nucleic acids, carbohydrates are also ubiquitous building blocks of living systems. Approximately 70% of mammalian proteins are glycosylated. Glycans not only provide structural support for living systems but also act as crucial regulators of cellular functions. As a result, they are considered essential pieces of the life science puzzle. However, research on glycans has lagged far behind that on proteins and nucleic acids. The main reason is that glycans are not direct products of gene coding, and their synthesis is nontemplated. In addition, the diversity of monosaccharide species and their linkage patterns contribute to the complexity of the glycan structures, which is the molecular basis for their diverse functions. Research in glycobiology is extremely challenging, especially for the in situ elucidation of glycan structures and functions. There is an urgent need to develop highly specific glycan labeling tools and imaging methods and devise glycan editing strategies. This Perspective focuses on the challenges of in situ analysis of glycans in living systems at three spatial levels (i.e., cell, tissue, and in vivo) and highlights recent advances and directions in glycan labeling, imaging, and editing tools. We believe that examining the current development landscape and the existing bottlenecks can drive the evolution of in situ glycan analysis and intervention strategies and provide glycan-based insights for clinical diagnosis and therapeutics.

Selenium-based metabolic oligosaccharide engineering strategy for quantitative glycan detection

Metabolic oligosaccharide engineering (MOE) is a classical chemical approach to perturb, profile and perceive glycans in physiological systems, but probes upon bioorthogonal reaction require accessibility and the background signal readout makes it challenging to achieve glycan quantification. Here we develop SeMOE, a selenium-based metabolic oligosaccharide engineering strategy that concisely combines elemental analysis and MOE,enabling the mass spectrometric imaging of glycome. We also demonstrate that the new-to-nature SeMOE probes allow for detection, quantitative measurement and visualization of glycans in diverse biological contexts. We also show that chemical reporters on conventional MOE can be integrated into a bifunctional SeMOE probe to provide multimodality signal readouts. SeMOE thus provides a convenient and simplified method to explore the glyco-world.

In Vivo Applications of Bioorthogonal Reactions: Chemistry and Targeting Mechanisms

Bioorthogonal chemistry involves selective biocompatible reactions between functional groups that are not normally present in biology. It has been used to probe biomolecules in living systems, and has advanced biomedical strategies such as diagnostics and therapeutics. In this review, the challenges and opportunities encountered when translating in vitro bioorthogonal approaches to in vivo settings are presented, with a focus on methods to deliver the bioorthogonal reaction components. These methods include metabolic bioengineering, active targeting, passive targeting, and simultaneously used strategies. The suitability of bioorthogonal ligation reactions and bond cleavage reactions for in vivo applications is critically appraised, and practical considerations such as the optimum scheduling regimen in pretargeting approaches are discussed. Finally, we present our own perspectives for this area and identify what, in our view, are the key challenges that must be overcome to maximise the impact of these approaches.

A Ring Expansion─Stereoselective Cycloaddition of Carbohydrate-Derived Donor-Acceptor Cyclopropanes: Synthesis of Bridged Oxepanone-Indole Hybrids

An efficient method for the construction of sugar-derived chiral oxepanone-indole molecular hybrids is investigated. The reaction condition is optimized by monitoring the progress at various temperatures, with various solvents, and with different Lewis acid catalysts. Under optimized conditions, high stereoselectivity and efficiency are achieved in most of the formed cycloadducts. The accessibility of the strategy is evaluated by utilizing an array of carbohydrate-derived donor-acceptor cyclopropanes and variably substituted indole substrates. Additionally, quick access to the bridged indole-oxepanone framework is described by utilizing a diastereoselective (3+2) cycloaddition of aryl-substituted donor-acceptor cyclopropanes incorporated in a pyran ring.

Carbohydrate based biomaterials for neural interface applications

Neuroprosthetic devices that record and modulate neural activities have demonstrated immense potential for bypassing or restoring lost neurological functions due to neural injuries and disorders. However, implantable electrical devices interfacing with brain tissue are susceptible to a series of inflammatory tissue responses along with mechanical or electrical failures which can affect the device performance over time. Several biomaterial strategies have been implemented to improve device-tissue integration for high quality and stable performance. Ranging from developing smaller, softer, and more flexible electrode designs to introducing bioactive coatings and drug-eluting layers on the electrode surface, such strategies have shown different degrees of success but with limitations. With their hydrophilic properties and specific bioactivities, carbohydrates offer a potential solution for addressing some of the limitations of the existing biomolecular approaches. In this review, we summarize the role of polysaccharides in the central nervous system, with a primary focus on glycoproteins and proteoglycans, to shed light on their untapped potential as biomaterials for neural implants. Utilization of glycosaminoglycans for neural interface and tissue regeneration applications is comprehensively reviewed to provide the current state of carbohydrate-based biomaterials for neural implants. Finally, we will discuss the challenges and opportunities of applying carbohydrate-based biomaterials for neural tissue interfaces.

An Augmented Method for Collecting PLGA Nanoparticles and the Fabrication of 1, 3, 4, 6-Tetra-O-acetyl-2-azido-2-deoxy-D-glucopyranose (Ac42AzGlc)-Loaded PLGA Nanoparticles for Efficient and Prospective in Vivo Metabolic Processing

We investigated two ways for fabricating 1, 3, 4, 6-tetra-O-acetyl-2-azido-2-deoxy-D-glucopyranose (Ac₄2AzGlc)-loaded poly (lactic-co-glycolic acid) PLGA nanoparticles in this article : 1) single emulsion solvent evaporation and 2) the nanoprecipitation method. Among the available methods of collecting nanoparticles using an ultra-high-speed centrifuge, we improvised a less-known method for collecting synthesized nanoparticles without a high-speed centrifuge, based on molecular weight (MW)-dependent centrifugal filters. These nanoparticles were collected in a tabletop centrifuge at a meager centrifugal force in the range of 200–300 xg whereas the conventional high-speed centrifuge method for nanoparticle recovery results in a hard nanoparticle pellet with poor resuspendability which hampers the yield and outcomes of the product. The Ac₄2AzGlc-loaded PLGA nanoparticles were spherical in shape with consistent and reliable nanometric particle size. The polydispersity indices were well within the acceptable limits. The preliminary studies in RAW 264.7 cell and C57BL/6 mice advocated efficient engineering in the former; however, the latter needs further confirmatory investigations. Preliminary in vivo studies with un-encapsulated Ac₄2AzGlc showed poor engineering of cardiac glycoproteins, opening up avenues for Ac₄2AzGlc-loaded nanoparticles for improved bioavailability and efficient metabolic engineering.

Mass Spectrometry Imaging for Glycome in the Brain

Glycans are diverse structured biomolecules that play crucial roles in various biological processes. Glycosylation, an enzymatic system through which various glycans are bound to proteins and lipids, is the most common and functionally crucial post-translational modification process. It is known to be associated with brain development, signal transduction, molecular trafficking, neurodegenerative disorders, psychopathologies, and brain cancers. Glycans in glycoproteins and glycolipids expressed in brain cells are involved in neuronal development, biological processes, and central nervous system maintenance. The composition and expression of glycans are known to change during those physiological processes. Therefore, imaging of glycans and the glycoconjugates in the brain regions has become a “hot” topic nowadays. Imaging techniques using lectins, antibodies, and chemical reporters are traditionally used for glycan detection. However, those techniques offer limited glycome detection. Mass spectrometry imaging (MSI) is an evolving field that combines mass spectrometry with histology allowing spatial and label-free visualization of molecules in the brain. In the last decades, several studies have employed MSI for glycome imaging in brain tissues. The current state of MSI uses on-tissue enzymatic digestion or chemical reaction to facilitate successful glycome imaging. Here, we reviewed the available literature that applied MSI techniques for glycome visualization and characterization in the brain. We also described the general methodologies for glycome MSI and discussed its potential use in the three-dimensional MSI in the brain.

To View Your Biomolecule, Click inside the Cell

The surging development of bioorthogonal chemistry has profoundly transformed chemical biology over the last two decades. Involving chemical partners that specifically react together in highly complex biological fluids, this branch of chemistry now allows researchers to probe biomolecules in their natural habitat through metabolic labelling technologies. Chemical reporter strategies include metabolic glycan labelling, site-specific incorporation of unnatural amino acids in proteins, and post-synthetic labelling of nucleic acids. While a majority of literature reports mark cell-surface exposed targets, implementing bioorthogonal ligations in the interior of cells constitutes a more challenging task. Owing to limiting factors such as membrane permeability of reagents, fluorescence background due to hydrophobic interactions and off-target covalent binding, and suboptimal balance between reactivity and stability of the designed molecular reporters and probes, these strategies need mindful planning to achieve success. In this review, we discuss the hurdles encountered when targeting biomolecules localized in cell organelles and give an easily accessible summary of the strategies at hand for imaging intracellular targets.

Mass Spectrometric Method for the Unambiguous Profiling of Cellular Dynamic Glycosylation

Various biological processes at the cellular level are regulated by glycosylation which is a highly microheterogeneous post-translational modification (PTM) on proteins and lipids. The dynamic nature of glycosylation can be studied through metabolic incorporation of non-natural sugars into glycan epitopes and their detection using bio-orthogonal probes. However, this approach possesses a significant drawback due to nonspecific background reactions and ambiguity of non-natural sugar metabolism. Here, we report a probe-free strategy for their direct detection by glycoproteomics and glycomics using mass spectrometry (MS). The method dramatically simplifies the detection of non-natural functional group bearing monosaccharides installed through promiscuous sialic acid, N-acetyl-d-galactosamine (GalNAc) and N-acetyl-d-glucosamine (GlcNAc) biosynthetic pathways. Multistage enrichment of glycoproteins by cellular fractionation, subsequent ZIC-HILIC (zwitterionic-hydrophilic interaction chromatography) based glycopeptide enrichment, and a spectral enrichment algorithm for the MS data processing enabled direct detection of non-natural monosaccharides that are incorporated at low abundance on the N/O-glycopeptides along with their natural counterparts. Our approach allowed the detection of both natural and non-natural sugar bearing glycopeptides, N- and O-glycopeptides, differentiation of non-natural monosaccharide types on the glycans and also their incorporation efficiency through quantitation. Through this, we could deduce interconversion of monosaccharides during their processing through glycan salvage pathway and subsequent incorporation into glycan chains. The study of glycosylation dynamics through this method can be conducted in high throughput, as few sample processing steps are involved, enabling understanding of glycosylation dynamics under various external stimuli and thereby could bolster the use of metabolic glycan engineering in glycosylation functional studies.

Sialic acid glycoengineering using N-acetylmannosamine and sialic acid analogs

Sialic acids cap the glycans of cell surface glycoproteins and glycolipids. They are involved in a multitude of biological processes and aberrant sialic acid expression is associated with several pathologies. Sialic acids modulate the characteristics and functions of glycoproteins and regulate cell-cell as well as cell-extracellular matrix interactions. Pathogens such as influenza virus use sialic acids to infect host cells and cancer cells exploit sialic acids to escape from the host's immune system. The introduction of unnatural sialic acids with different functionalities into surface glycans enables the study of the broad biological functions of these sugars and presents a therapeutic option to intervene with pathological processes involving sialic acids. Multiple chemically modified sialic acid analogs can be directly utilized by cells for sialoglycan synthesis. Alternatively, analogs of the natural sialic acid precursor sugar N-Acetylmannosamine (ManNAc) can be introduced into the sialic acid biosynthesis pathway resulting in the intracellular conversion into the corresponding sialic acid analog. Both, ManNAc and sialic acid analogs, have been employed successfully for a large variety of glycoengineering applications such as glycan imaging, targeting toxins to tumor cells, inhibiting pathogen binding, or altering immune cell activity. However, there are significant differences between ManNAc and sialic acid analogs with respect to their chemical modification potential and cellular metabolism that should be considered in sialic acid glycoengineering experiments.

Advanced Bottom-Up Engineering of Living Architectures

Bottom-up tissue engineering is a promising approach for designing modular biomimetic structures that aim to recapitulate the intricate hierarchy and biofunctionality of native human tissues. In recent years, this field has seen exciting progress driven by an increasing knowledge of biological systems and their rational deconstruction into key core components. Relevant advances in the bottom-up assembly of unitary living blocks toward the creation of higher order bioarchitectures based on multicellular-rich structures or multicomponent cell-biomaterial synergies are described. An up-to-date critical overview of long-term existing and rapidly emerging technologies for integrative bottom-up tissue engineering is provided, including discussion of their practical challenges and required advances. It is envisioned that a combination of cell-biomaterial constructs with bioadaptable features and biospecific 3D designs will contribute to the development of more robust and functional humanized tissues for therapies and disease models, as well as tools for fundamental biological studies.

In vivo selective imaging of metabolic glycosylation with a tetrazine-modified upconversion nanoprobe

A novel nanoprobe (UCNP-T) for the specific labeling and real-time imaging of glycans on the cell membrane via ratiometric UCL imaging was developed.

A bioorthogonal time-resolved luminogenic probe for metabolic labelling and imaging of glycans

A terbium complex Tb-1 was demonstrated to undergo bioorthogonal ligation with engineered cell-surface glycans, which results in a much less efficient LRET and a 5-fold increase in long-lived terbium emission with low toxicity.

A multifunctional lipid that forms contrast-agent liposomes with dual-control release capabilities for precise MRI-guided drug delivery

Monitoring of nanoparticle-based therapy in vivo and controlled drug release are urgently needed for the precise treatment of disease. We have synthesized a multifunctional Gd-DTPA-ONB (GDO) lipid by introducing the Gd-DTPA contrast agent moiety into an o-nitro-benzyl ester lipid. By design, liposomes formed from the GDO lipid combine MRI tracking ability and dual-trigger release capabilities with maximum sensitivity (because all lipids bear the cleavable moiety) without reducing the drug encapsulation rate. We first confirmed that both photo-treatment and pH-triggered hydrolysis are able to cleave the GDO lipid and lyse GDO liposomes. We then investigated the efficiency of drug release via the combined release processes for GDO liposomes loaded with doxorubicin (DOX). Relative to neutral pH, the release efficiency in acidic environment increased by 10.4% (at pH = 6.5) and 13.3% (at pH = 4.2). This pH-dependent release response is conducive to distinguishing pathological tissue such as tumors and endolysosomal compartments. The photo-induced release efficiency increases with illumination time as well as with distance of the pH from neutral. Photolysis increased the release efficiency by 13.8% at pH = 4.2, which is remarkable considering the already increased amount of drug release in the acidic environment. In addition, the relaxation time of GDO liposomes was 4.1 times that of clinical Gd-DTPA, with brighter T₁-weighted imaging in vitro and in vivo. Real-time MRI imaging and in vivo fluorescence experiments demonstrated tumor targeting and MRI guided release. Furthermore, significant tumor growth inhibition in a treatment experiment using DOX-loaded GDO liposomes clearly demonstrated the benefit of photo-treatment for efficacy: the tumor size in the photo-treatment group was 3.7 times smaller than in the control group. The present study thus highlights the benefit of the design idea of combining efficient imaging/guiding, targeting, and triggerable release functions in one lipid molecule for drug delivery applications.

Potentiometric analysis of sialic acid with a flexible carbon cloth based on boronate affinity and molecularly imprinted polymers

A potentiometric sensor for sialic acid detection was designed based on a boronic acid-containing MIP modified carbon cloth electrode.

Understanding cellular glycan surfaces in the central nervous system

Glycosylation, the enzymatic process by which glycans are attached to proteins and lipids, is the most abundant and functionally important type of post-translational modification associated with brain development, neurodegenerative disorders, psychopathologies and brain cancers. Glycan structures are diverse and complex; however, they have been detected and targeted in the central nervous system (CNS) by various immunohistochemical detection methods using glycan-binding proteins such as anti-glycan antibodies or lectins and/or characterized with analytical techniques such as chromatography and mass spectrometry. The glycan structures on glycoproteins and glycolipids expressed in neural stem cells play key roles in neural development, biological processes and CNS maintenance, such as cell adhesion, signal transduction, molecular trafficking and differentiation. This brief review will highlight some of the important findings on differential glycan expression across stages of CNS cell differentiation and in pathological disorders and diseases such as Alzheimer's disease, Parkinson's disease, Huntington's disease, multiple sclerosis, amyotrophic lateral sclerosis, schizophrenia and brain cancer.

Asking more from metabolic oligosaccharide engineering

Metabolic Oligosaccharide Engineering (MOE) is a groundbreaking strategy which has been largely used in the last decades, as a powerful strategy for glycans understanding. The present review aims to highlight recent studies that are pushing the boundaries of MOE applications.

Glycans form one of the four classes of biomolecules, are found in every living system and present a huge structural and functional diversity. As an illustration of this diversity, it has been reported that more than 50% of the human proteome is glycosylated and that 2% of the human genome is dedicated to glycosylation processes. Glycans are involved in many biological processes such as signalization, cell–cell or host pathogen interactions, immunity, etc. However, fundamental processes associated with glycans are not yet fully understood and the development of glycobiology is relatively recent compared to the study of genes or proteins. Approximately 25 years ago, the studies of Bertozzi's and Reutter's groups paved the way for metabolic oligosaccharide engineering (MOE), a strategy which consists in the use of modified sugar analogs which are taken up into the cells, metabolized, incorporated into glycoconjugates, and finally detected in a specific manner. This groundbreaking strategy has been widely used during the last few decades and the concomitant development of new bioorthogonal ligation reactions has allowed many advances in the field. Typically, MOE has been used to either visualize glycans or identify different classes of glycoproteins. The present review aims to highlight recent studies that lie somewhat outside of these more traditional approaches and that are pushing the boundaries of MOE applications.

Chemical and biological methods for probing the structure and functions of polysialic acids

Owing to its poly-anionic charge and large hydrodynamic volume, polysialic acid (polySia) attached to neural cell adhesion molecule regulates axon–axon and axon–substratum interactions and signalling, particularly, in the development of the central nervous system (CNS). Expression of polySia is spatiotemporally regulated by the action of two polysialyl transferases, namely ST8SiaII and ST8SiaIV. PolySia expression peaks during late embryonic and early post-natal period and maintained at a steady state in adulthood in neurogenic niche of the brain. Aberrant polySia expression is associated with neurological disorders and brain tumours. Investigations on the structure and functions, over the past four decades, have shed light on the physiology of polySia. This review focuses on the biological, biochemical, and chemical tools available for polySia engineering. Genetic knockouts, endo-neuraminidases that cleave polySia, antibodies, exogenous expression, and neuroblastoma cells have provided deep insights into the ability of polySia to guide migration of neuronal precursors in neonatal brain development, neuronal clustering, axonal pathway guidance, and axonal targeting. Advent of metabolic sialic acid engineering using ManNAc analogues has enabled reversible and dose-dependent modulation polySia in vitro and ex vivo. In vivo, ManNAc analogues readily engineer the sialoglycans in peripheral tissues, but show no effect in the brain. A recently developed carbohydrate-neuroactive hybrid strategy enables a non-invasive access to the brain in living animals across the blood–brain barrier. A combination of recent advances in CNS drugs and imaging with ManNAc analogues for polySia modulation would pave novel avenues for understanding intricacies of brain development and tackling the challenges of neurological disorders.

Neuro-Compatible Metabolic Glycan Labeling of Primary Hippocampal Neurons in Noncontact, Sandwich-Type Neuron-Astrocyte Coculture

Glycans are intimately involved in several facets of neuronal development and neuropathology. However, the metabolic labeling of surface glycans in primary neurons is a difficult task because of the neurotoxicity of unnatural monosaccharides that are used as a metabolic precursor, hindering the progress of metabolic engineering in neuron-related fields. Therefore, in this paper, we report a neurosupportive, neuron-astrocyte coculture system that neutralizes the neurotoxic effects of unnatural monosaccharides, allowing for the long-term observation and characterization of glycans in primary neurons in vitro. Polysialic acids in neurons are selectively imaged, via the metabolic labeling of sialoglycans with peracetylated N-azidoacetyl-d-mannosamine (Ac₄ManNAz), for up to 21 DIV. Two-color labeling shows that neuronal activities, such as neurite outgrowth and recycling of membrane components, are highly dynamic and change over time during development. In addition, the insertion sites of membrane components are suggested to not be random, but be predominantly localized in developing neurites. This work provides a new research platform and also suggests advanced 3D systems for metabolic-labeling studies of glycans in primary neurons.

Caspase-3/-7-Specific Metabolic Precursor for Bioorthogonal Tracking of Tumor Apoptosis

Apoptosis is one of the most important intracellular events in living cell, which is a programmed cell death interrelated with caspase enzyme activity for maintaining homeostasis in multicellular organisms. Therefore, direct apoptosis imaging of living cells can provide enormous advantages for diagnosis, drug discovery, and therapeutic monitoring in various diseases. However, a method of direct apoptosis imaging has not been fully validated, especially for live cells in in vitro and in vivo. Herein, we developed a new apoptosis imaging technology via a direct visualization of active caspase-3/-7 activity in living cells. For this, we synthesized a caspase-3/-7-specific cleavable peptide (KGDEVD) conjugated triacetylated N-azidoacetyl-D-mannosamine (Apo-S-Ac₃ManNAz), wherein the Apo-S-Ac₃ManNAz can be cleaved by the active caspase-3/-7 in live apoptotic cells and the cleaved Ac₃ManNAz molecules can further generate targetable azido groups (N₃) on the living cell surface. Importantly, the azido groups on the apoptotic tumor cells could be visualized with Cy5.5-conjugated dibenzylcyclooctyne (DBCO-Cy5.5) via bioorthogonal click chemistry in vitro cell culture condition and in vivo tumor-bearing mice. Therefore, our Apo-S-Ac₃ManNAz can be utilized for the further applications in tumor therapy as a monitoring tool for anticancer efficacy and optimization of anticancer new drugs in cell culture system and in tumor-bearing mice.

Molecular imaging based on metabolic glycoengineering and bioorthogonal click chemistry

Metabolic glycoengineering is a powerful technique that can introduce various chemical groups to cellular glycan by treatment of unnatural monosaccharide. Particularly, this technique has enabled many challenging trials for molecular imaging in combination with click chemistry, which provides fast and specific chemical conjugation reaction of imaging probes to metabolically-modified live cells. This review introduces recent progress in molecular imaging based on the combination of these two cutting-edge techniques. First, these techniques showed promising results in specific tumor cell imaging for cancer diagnosis and therapy. The related researches showed the surface of tumor cells could be labeled with bioorthogonal chemical groups by metabolic glycoengineering, which can be further conjugated with fluorescence dyes or nanoparticles with imaging probes by click chemistry, in vitro and in vivo. This method can be applied to heterogeneous tumor cells regardless of genetic properties of different tumor cells. Furthermore, the amount of targeting moieties on tumor cells can be freely controlled externally by treatment of unnatural monosaccharide. Second, this sequential use of metabolic glycoengineering and click chemistry is also useful in cell tracking to monitor the localization of the inoculated therapeutic cells including chondrocytes and stem cells. This therapeutic cell-labeling technique provided excellent viability of chondrocytes and stem cells during the whole process in vitro and in vivo. It can provide long-term and safe therapeutic cell imaging compared to traditional methods. These overall studies demonstrate the great potential of metabolic glycoengineering and click chemistry in live cell imaging.