Direct visualization of specifically modified extracellular glycans in living animals

Heparan sulfates and heparan sulfate proteoglycans in hematopoiesis

ABSTRACT

From signaling mediators in stem cells to markers of differentiation and lineage commitment to facilitators for the entry of viruses, such as HIV-1, cell surface heparan sulfate (HS) glycans with distinct modification patterns play important roles in hematopoietic biology. In this review, we provide an overview of the importance of HS and the proteoglycans (HSPGs) to which they are attached within the major cellular subtypes of the hematopoietic system. We summarize the roles of HSPGs, HS, and HS modifications within each main hematopoietic cell lineage of both myeloid and lymphoid arms. Lastly, we discuss the biological advances in the detection of HS modifications and their potential to further discriminate cell types within hematopoietic tissue.

Extending the enzymatic toolbox for heparosan polymerization, depolymerization, and detection

Heparosan is an acidic polysaccharide expressed as a capsule polymer by pathogenic and commensal bacteria, e.g. by E. coli K5. As a precursor in the biosynthesis of heparan sulfate and heparin, heparosan has a high biocompatibility and is thus of interest for pharmaceutical applications. However, due to its low immunogenicity, developing antibodies against heparosan and detecting the polymer in biological samples has been challenging. In this study, we exploited the enzyme repertoire of E. coli K5 and the E. coli K5-specific bacteriophage ΦK5B for the controlled synthesis and depolymerization of heparosan. A fluorescently labeled heparosan nonamer was used as a priming acceptor to study the elongation mechanism of the E. coli K5 heparosan polymerases KfiA and KfiC. We could demonstrate that the enzymes act in a distributive manner, producing labeled heparosan of low dispersity. The enzymatically synthesized heparosan was a useful tool to identify the tailspike protein KflB of ΦK5B as heparosan lyase and to characterize its endolytic depolymerization mechanism. Most importantly, using site-directed mutagenesis and rational construct design, we generated an inactive version of KflB for the detection of heparosan in ELISA-based assays, on blots, and on bacterial and mammalian cells.

Synaptogenesis: unmasking molecular mechanisms using Caenorhabditis elegans

The nematode Caenorhabditis elegans is a research model organism particularly suited to the mechanistic understanding of synapse genesis in the nervous system. Armed with powerful genetics, knowledge of complete connectomics, and modern genomics, studies using C. elegans have unveiled multiple key regulators in the formation of a functional synapse. Importantly, many signaling networks display remarkable conservation throughout animals, underscoring the contributions of C. elegans research to advance the understanding of our brain. In this chapter, we will review up-to-date information of the contribution of C. elegans to the understanding of chemical synapses, from structure to molecules and to synaptic remodeling.

A glycan-based approach to cell characterization and isolation: Hematopoiesis as a paradigm

Cell surfaces display a wide array of molecules that confer identity. While flow cytometry and cluster of differentiation (CD) markers have revolutionized cell characterization and purification, functionally heterogeneous cellular subtypes remain unresolvable by the CD marker system alone. Using hematopoietic lineages as a paradigm, we leverage the extraordinary molecular diversity of heparan sulfate (HS) glycans to establish cellular "glycotypes" by utilizing a panel of anti-HS single-chain variable fragment antibodies (scFvs). Prospective sorting with anti-HS scFvs identifies functionally distinct glycotypes within heterogeneous pools of mouse and human hematopoietic progenitor cells and enables further stratification of immunophenotypically pure megakaryocyte-erythrocyte progenitors. This stratification correlates with expression of a heptad of HS-related genes that is reflective of the HS epitope recognized by specific anti-HS scFvs. While we show that HS glycotyping provides an orthogonal set of tools for resolution of hematopoietic lineages, we anticipate broad utility of this approach in defining and isolating novel, viable cell types across diverse tissues and species.

Imaging Glycosaminoglycan Modification Patterns In Vivo

Glycosaminoglycans (GAGs) such as heparan sulfates (HS) or chondroitin sulfates (CS) are long unbranched polymers of a disaccharide comprised of hexuronic acid and hexosamine. Attached to a protein backbone via a characteristic tetrasaccharide, the GAG chains are non-uniformly modified by sulfations, epimerizations, and deacetylations. The resultant glycan chains contain highly modified domains, separated by sections of sparse or no modifications. These GAG domains are central to the role of glycans in binding to proteins and mediating protein-protein interactions. Since HS and CS domains are not genetically encoded, they cannot be visualized and studied with conventional methods in vivo. We describe a transgenic approach using single chain variable fragment (scFv) antibodies that bind HS or CS. By transgenically expressing fluorescently tagged scFv antibodies, we can directly visualize both HS and CS domains in live Caenorhabditis elegans revealing unprecedented cellular specificity and evolutionary conservation (Attreed et al., Nat Methods 9(5): 477-479, 2012; Attreed et al., Glycobiology 26(8): 862-870, 2016) (unpublished). The approach allows concomitant co-labeling of multiple GAG domains, the study of GAG dynamics, and could lend itself to a genetic analysis of GAG domain biosynthesis or function.

The HSPG syndecan is a core organizer of cholinergic synapses

The extracellular matrix has emerged as an active component of chemical synapses regulating synaptic formation, maintenance, and homeostasis. The heparan sulfate proteoglycan (HSPG) syndecans are known to regulate cellular and axonal migration in the brain. They are also enriched at synapses, but their synaptic functions remain more elusive. Here, we show that SDN-1, the sole orthologue of syndecan in C. elegans, is absolutely required for the synaptic clustering of homomeric α7-like acetylcholine receptors (AChRs) and regulates the synaptic content of heteromeric AChRs. SDN-1 is concentrated at neuromuscular junctions (NMJs) by the neurally secreted synaptic organizer Ce-Punctin/MADD-4, which also activates the transmembrane netrin receptor DCC. Those cooperatively recruit the FARP and CASK orthologues that localize α7-like-AChRs at cholinergic NMJs through physical interactions. Therefore, SDN-1 stands at the core of the cholinergic synapse organization by bridging the extracellular synaptic determinants to the intracellular synaptic scaffold that controls the postsynaptic receptor content.

Specific heparan sulfate modifications stabilize the synaptic organizer MADD-4/Punctin at C. elegans neuromuscular junctions

Heparan sulfate proteoglycans contribute to the structural organization of various neurochemical synapses. Depending on the system, their role involves either the core protein or the glycosaminoglycan chains. These linear sugar chains are extensively modified by heparan sulfate modification enzymes, resulting in highly diverse molecules. Specific modifications of glycosaminoglycan chains may thus contribute to a sugar code involved in synapse specificity. Caenorhabditis elegans is particularly useful to address this question because of the low level of genomic redundancy of these enzymes, as opposed to mammals. Here, we systematically mutated the genes encoding heparan sulfate modification enzymes in C. elegans and analyzed their impact on excitatory and inhibitory neuromuscular junctions. Using single chain antibodies that recognize different heparan sulfate modification patterns, we show in vivo that these two heparan sulfate epitopes are carried by the SDN-1 core protein, the unique C. elegans syndecan orthologue, at neuromuscular junctions. Intriguingly, these antibodies differentially bind to excitatory and inhibitory synapses, implying unique heparan sulfate modification patterns at different neuromuscular junctions. Moreover, while most enzymes are individually dispensable for proper organization of neuromuscular junctions, we show that 3-O-sulfation of SDN-1 is required to maintain wild-type levels of the extracellular matrix protein MADD-4/Punctin, a central synaptic organizer that defines the identity of excitatory and inhibitory synaptic domains at the plasma membrane of muscle cells.

Roles of glycoconjugates in neural patterning in C. elegans

Establishment of neural circuits requires reproducible and precise interactions between growing axons, dendrites and their tissue environment. Cell adhesion molecules and guidance factors are involved in the process, but how specificity is achieved remains poorly understood. Glycans are the third major class of biopolymers besides nucleic acids and proteins, and are usually covalently linked to proteins to form glycoconjugates. Common to most glycans is an extraordinary level of molecular diversity, making them attractive candidates to contribute specificity during neural development. Indeed, many genes important for neural development encode glycoproteins, or enzymes involved in synthesizing or modifying glycans. Glycoconjugates are classified based on both the types of glycans and type of attachment that link them to proteins. Here I discuss progress in understanding the function of glycans, glycan modifications and glycoconjugates during neural development in Caenorhabditis elegans. I will also highlight relevance to human disease and known roles of glycoconjugates in regeneration.

Innovative fluorescent probes for in vivo visualization of biomolecules in living Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) as a well-established multicellular model organism has been widely used in the biological field for half a century. Its numerous advantages including small body size, rapid life cycle, high-reproductive rate, well-defined anatomy, and conserved genome, has made C. elegans one of the most successful multicellular model organisms. Discoveries obtained from the C. elegans model have made great contributions to research fields such as development, aging, biophysics, immunology, and neuroscience. Because of its transparent body and giant cell size, C. elegans is also an ideal subject for high resolution and high-throughput optical imaging and analysis. During the past decade, great advances have been made to develop biomolecule-targeting techniques for noninvasive optical imaging. These novel technologies expanded the toolbox for qualitative and quantitative analysis of biomolecules in C. elegans. In this review, we summarize recently developed fluorescent probes or labeling techniques for visualizing biomolecules at the cellular, subcellular or molecular scale by using C. elegans as the major model organism or designed specifically for the applications in C. elegans. Combining the technological advantages of the C. elegans model with the novel fluorescent labeling techniques will provide new horizons for high-efficiency quantitative optical analysis in live organisms.

Enhanced visualization of cell surface glycans via a hybridization chain reaction

We apply a DNA hybridization chain reaction (HCR) to achieve sensitively amplified imaging of cell surface glycosylation.

RNA Aptamers with Specificity for Heparosan and Chondroitin Glycosaminoglycans

In this study, two respective groups of RNA aptamers have been selected against two main classes of glycosaminoglycans (GAGs), heparosan, and chondroitin, as they have proven difficult to specifically detect in biological samples. GAGs are linear, anionic, polydisperse polysaccharides found ubiquitously in nature, yet their detection remains problematic. GAGs comprised repeating disaccharide units, consisting of uronic acid and hexosamine residues that are often also sulfated at various positions. Monoclonal antibodies are frequently used in biology and medicine to recognize various biological analytes with high affinity and specificity. However, GAGs are conserved across the whole animal phylogenic tree and are nonimmunogenic in hosts traditionally used for natural antibody generation. Thus, it has been challenging to obtain high affinity, selective antibodies that recognize various GAGs. In the absence of anti-GAG antibodies, glycobiologists have relied on the use of specific enzymes to convert GAGs to oligosaccharides for analysis by mass spectrometry. Unfortunately, while these methods are sensitive, they can be labor-intensive and cannot be used for in situ detection of intact GAGs in cells and tissues. Aptamers are single-stranded oligonucleotide (DNA or RNA) ligands capable of high selectivity and high affinity detection of biological analytes. Aptamers can be developed in vitro by the systematic evolution of ligands by exponential enrichment (SELEX) to recognize nonimmunogenic targets, including neutral carbohydrates. This study utilizes the SELEX method to generate RNA aptamers, which specifically bind to the unmodified GAGs, heparosan, and chondroitin. Binding confirmation and cross-screening with other GAGs were performed using confocal microscopy to afford three specific GAGs to each target. Affinity constant of each RNA aptamer was obtained by fluorescent output after interaction with the respective GAG target immobilized on plates; the K D values were determined to be 0.71-1.0 μM for all aptamers. Upon the success of chemical modification (to stabilize RNA aptamers in actual biological systems) and fluorescent tagging (to only visualize RNA aptamers) of these aptamers, they would be able to serve as a specific detection reagent of these important GAGs in biological samples.

Enhanced Imaging of Specific Cell-Surface Glycosylation Based on Multi-FRET

Cell-surface glycosylation contains abundant biological information that reflects cell physiological state, and it is of great value to image cell-surface glycosylation to elucidate its functions. Here we present a hybridization chain reaction (HCR)-based multifluorescence resonance energy transfer (multi-FRET) method for specific imaging of cell-surface glycosylation. By installing donors through metabolic glycan labeling and acceptors through aptamer-tethered nanoassemblies on the same glycoconjugate, intramolecular multi-FRET occurs due to near donor-acceptor distance. Benefiting from amplified effect and spatial flexibility of the HCR nanoassemblies, enhanced multi-FRET imaging of specific cell-surface glycosylation can be obtained. With this HCR-based multi-FRET method, we achieved obvious contrast in imaging of protein-specific GalNAcylation on 7211 cell surfaces. In addition, we demonstrated the general applicability of this method by visualizing the protein-specific sialylation on CEM cell surfaces. Furthermore, the expression changes of CEM cell-surface protein-specific sialylation under drug treatment was accurately monitored. This developed imaging method may provide a powerful tool in researching glycosylation functions, discovering biomarkers, and screening drugs.

Heparan Sulfate Proteoglycans as Emerging Players in Synaptic Specificity

Neural circuits consist of distinct neuronal cell types connected in specific patterns. The specificity of these connections is achieved in a series of sequential developmental steps that involve the targeting of neurites, the identification of synaptic partners, and the formation of specific types of synapses. Cell-surface proteins play a critical role in each of these steps. The heparan sulfate proteoglycan (HSPG) family of cell-surface proteins is emerging as a key regulator of connectivity. HSPGs are expressed throughout brain development and play important roles in axon guidance, synapse development and synapse function. New insights indicate that neuronal cell types express unique combinations of HSPGs and HS-modifying enzymes. Furthermore, HSPGs interact with cell type-specific binding partners to mediate synapse development. This suggests that cell type-specific repertoires of HSPGs and specific patterns of HS modifications on the cell surface are required for the development of specific synaptic connections. Genome-wide association studies have linked these proteins to neurodevelopmental and neuropsychiatric diseases. Thus, HSPGs play an important role in the development of specific synaptic connectivity patterns important for neural circuit function, and their dysfunction may be involved in the development of brain disorders.

Applying DNA rolling circle amplification in fluorescence imaging of cell surface glycans labeled by a metabolic method

Glycans on the cell surfaces are essential for cellular communication. Metabolically labeling glycans can introduce unnatural sugars into cellular glycans, which can facilitate further labeling. We report herein imaging cell surface glycosylation by using click chemistry and DNA rolling circle amplification (RCA) to improve detection sensitivity. Through the RCA amplification, the image resolution of a cell was significantly improved and much fewer unnatural sugars were used than required previously. The advantage of this method is that it avoids introducing too much unnatural sugar, which can interfere with normal, physiological cell function. Simultaneously, the enhanced fluorescence intensity conveniently facilitates the detection of cells' own biosynthetic glycans by simply using a microplate reader. The results indicate that the metabolically labelling ability is different for different carbohydrates and different cells. Next, the RCA technique was adopted in a fluorescence resonance energy transfer (FRET)-based methodology that facilitated the glycan imaging of specific proteins on the cell surface. This method is broadly applicable to imaging the glycosylation of cellular proteins. Our results highlight the applications of RCA in metabolic glycan labeling, which can be used to monitor the glycosylation status on cells, and study the means by which glycosylation regulates cell function.

Conservation of anatomically restricted glycosaminoglycan structures in divergent nematode species

Heparan sulfates (HS) are glycosaminoglycans of the extracellular matrices and characterized by complex modification patterns owing to sulfations, epimerization, and acetylation. Distinct HS modification patterns have been shown to modulate protein-protein interactions during development in general and of the nervous system in particular. This has led to the heparan sulfate code hypothesis, which posits that specifically modified HS epitopes are distributed in a tissue and cell-specific fashion to orchestrate neural circuit formation. Whether an HS code exists in vivo, how specific or how evolutionarily conserved the anatomical distribution of an HS code may be has remained unknown. Here we conduct a systematic comparison of HS modification patterns in the nematode Caenorhabditis elegans using transgenic expression of 33 different HS-specific single chain variable fragment antibodies. We find that some HS modification patterns are widely distributed in the nervous system. In contrast, other HS modification patterns appear highly cell-specific in both non-neuronal and neuronal cells. Some patterns can be as restricted in their localization as to single neurites or synaptic connections between two neurons. This restricted anatomical localization of specific HS patterns can be evolutionarily conserved over a span of 80-100 million years in the divergent nematode species Caenorhabditis briggsae suggesting structural and, possibly functional conservation of glycosaminoglycan structures similar to proteins. These findings suggest a HS code with subcellularly localized, unique glycan identities in the nervous system.

Metabolic Labeling and Imaging of N-Linked Glycans in Arabidopsis Thaliana

Molecular imaging of glycans has been actively pursued in animal systems for the past decades. However, visualization of plant glycans remains underdeveloped, despite that glycosylation is essential for the life cycle of plants. Metabolic glycan labeling in Arabidopsis thaliana by using N-azidoacetylglucosamine (GlcNAz) as the chemical reporter is reported. GlcNAz is metabolized through the salvage pathway of N-acetylglucosamine (GlcNAc) and incorporated into N-linked glycans, and possibly intracellular O-GlcNAc. Click-labeling with fluorescent probes enables visualization of newly synthesized N-linked glycans. N-glycosylation in the root tissue was discovered to possess distinct distribution patterns in different developmental zones, suggesting that N-glycosylation is regulated in a developmental stage-dependent manner. This work shows the utility of metabolic glycan labeling in elucidating the function of N-linked glycosylation in plants.

Directional Trans-Synaptic Labeling of Specific Neuronal Connections in Live Animals

Understanding animal behavior and development requires visualization and analysis of their synaptic connectivity, but existing methods are laborious or may not depend on trans-synaptic interactions. Here we describe a transgenic approach for in vivo labeling of specific connections in Caenorhabditis elegans, which we term iBLINC. The method is based on BLINC (Biotin Labeling of INtercellular Contacts) and involves trans-synaptic enzymatic transfer of biotin by the Escherichia coli biotin ligase BirA onto an acceptor peptide. A BirA fusion with the presynaptic cell adhesion molecule NRX-1/neurexin is expressed presynaptically, whereas a fusion between the acceptor peptide and the postsynaptic protein NLG-1/neuroligin is expressed postsynaptically. The biotinylated acceptor peptide::NLG-1/neuroligin fusion is detected by a monomeric streptavidin::fluorescent protein fusion transgenically secreted into the extracellular space. Physical contact between neurons is insufficient to create a fluorescent signal, suggesting that synapse formation is required. The labeling approach appears to capture the directionality of synaptic connections, and quantitative analyses of synapse patterns display excellent concordance with electron micrograph reconstructions. Experiments using photoconvertible fluorescent proteins suggest that the method can be utilized for studies of protein dynamics at the synapse. Applying this technique, we find connectivity patterns of defined connections to vary across a population of wild-type animals. In aging animals, specific segments of synaptic connections are more susceptible to decline than others, consistent with dedicated mechanisms of synaptic maintenance. Collectively, we have developed an enzyme-based, trans-synaptic labeling method that allows high-resolution analyses of synaptic connectivity as well as protein dynamics at specific synapses of live animals.

A transgenic approach to live imaging of heparan sulfate modification patterns

Heparan sulfate (HS) glycosaminoglycan chains contain highly modified HS domains that are separated by sections of sparse or no modification. HS domains are central to the role of HS in protein binding and mediating protein-protein interactions in the extracellular matrix. Since HS domains are not genetically encoded, they are impossible to visualize and study with conventional methods in vivo. Here we describe a transgenic approach using previously described single chain variable fragment (scFv) antibodies that bind HS in vitro and on tissue sections with different specificities. By engineering a secretion signal and a fluorescent protein to the scFvs and transgenically expressing these fluorescently tagged antibodies in Caenorhabditis elegans, we are able to directly visualize specific HS domains in live animals (Attreed et al. Nat Methods 9(5):477-479, 2012). The approach allows concomitant colabeling of multiple epitopes, the study of HS dynamics and, could lend itself to a genetic analysis of HS domain biosynthesis or to visualize other nongenetically encoded or posttranslational modifications.

The cellular and molecular mechanisms of vertebrate lens development

The ocular lens is a model system for understanding important aspects of embryonic development, such as cell specification and the spatiotemporally controlled formation of a three-dimensional structure. The lens, which is characterized by transparency, refraction and elasticity, is composed of a bulk mass of fiber cells attached to a sheet of lens epithelium. Although lens induction has been studied for over 100 years, recent findings have revealed a myriad of extracellular signaling pathways and gene regulatory networks, integrated and executed by the transcription factor Pax6, that are required for lens formation in vertebrates. This Review summarizes recent progress in the field, emphasizing the interplay between the diverse regulatory mechanisms employed to form lens progenitor and precursor cells and highlighting novel opportunities to fill gaps in our understanding of lens tissue morphogenesis.

Complex cooperative functions of heparan sulfate proteoglycans shape nervous system development in Caenorhabditis elegans

The development of the nervous system is a complex process requiring the integration of numerous molecular cues to form functional circuits. Many cues are regulated by heparan sulfates, a class of linear glycosaminoglycan polysaccharides. These sugars contain distinct modification patterns that regulate protein-protein interactions. Misexpressing the homolog of KAL-1/anosmin-1, a neural cell adhesion molecule mutant in Kallmann syndrome, in Caenorhabditis elegans causes a highly penetrant, heparan sulfate-dependent axonal branching phenotype in AIY interneurons. In an extended forward genetic screen for modifiers of this phenotype, we identified alleles in new as well as previously identified genes involved in HS biosynthesis and modification, namely the xylosyltransferase sqv-6, the HS-6-O-sulfotransferase hst-6, and the HS-3-O-sulfotransferase hst-3.2. Cell-specific rescue experiments showed that different HS biosynthetic and modification enzymes can be provided cell-nonautonomously by different tissues to allow kal-1-dependent branching of AIY. In addition, we show that heparan sulfate proteoglycan core proteins that carry the heparan sulfate chains act genetically in a highly redundant fashion to mediate kal-1-dependent branching in AIY neurons. Specifically, lon-2/glypican and unc-52/perlecan act in parallel genetic pathways and display synergistic interactions with sdn-1/syndecan to mediate kal-1 function. Because all of these heparan sulfate core proteins have been shown to act in different tissues, these studies indicate that KAL-1/anosmin-1 requires heparan sulfate with distinct modification patterns of different cellular origin for function. Our results support a model in which a three-dimensional scaffold of heparan sulfate mediates KAL-1/anosmin-1 and intercellular communication through complex and cooperative interactions. In addition, the genes we have identified could contribute to the etiology of Kallmann syndrome in humans.

Syndecan promotes axon regeneration by stabilizing growth cone migration

Growth cones facilitate the repair of nervous system damage by providing the driving force for axon regeneration. Using single-neuron laser axotomy and in vivo time-lapse imaging, we show that syndecan, a heparan sulfate (HS) proteoglycan, is required for growth cone function during axon regeneration in C. elegans. In the absence of syndecan, regenerating growth cones form but are unstable and collapse, decreasing the effective growth rate and impeding regrowth to target cells. We provide evidence that syndecan has two distinct functions during axon regeneration: (1) a canonical function in axon guidance that requires expression outside the nervous system and depends on HS chains and (2) an intrinsic function in growth cone stabilization that is mediated by the syndecan core protein, independently of HS. Thus, syndecan is a regulator of a critical choke point in nervous system repair.

Genetically encoded system to track histone modification in vivo

Post-translational histone modifications play key roles in gene regulation, development, and differentiation, but their dynamics in living organisms remain almost completely unknown. To address this problem, we developed a genetically encoded system for tracking histone modifications by generating fluorescent modification-specific intracellular antibodies (mintbodies) that can be expressed in vivo. To demonstrate, an H3 lysine 9 acetylation specific mintbody (H3K9ac-mintbody) was engineered and stably expressed in human cells. In good agreement with the localization of its target acetylation, H3K9ac-mintbody was enriched in euchromatin, and its kinetics measurably changed upon treatment with a histone deacetylase inhibitor. We also generated transgenic fruit fly and zebrafish stably expressing H3K9ac-mintbody for in vivo tracking. Dramatic changes in H3K9ac-mintbody localization during Drosophila embryogenesis could highlight enhanced acetylation at the start of zygotic transcription around mitotic cycle 7. Together, this work demonstrates the broad potential of mintbody and lays the foundation for epigenetic analysis in vivo.

Blood vessels pattern heparan sulfate gradients between their apical and basolateral aspects

A hallmark of immune cell trafficking is directional guidance via gradients of soluble or surface bound chemokines. Vascular endothelial cells produce, transport and deposit either their own chemokines or chemokines produced by the underlying stroma. Endothelial heparan sulfate (HS) was suggested to be a critical scaffold for these chemokine pools, but it is unclear how steep chemokine gradients are sustained between the lumenal and ablumenal aspects of blood vessels. Addressing this question by semi-quantitative immunostaining of HS moieties around blood vessels with a pan anti-HS IgM mAb, we found a striking HS enrichment in the basal lamina of resting and inflamed post capillary skin venules, as well as in high endothelial venules (HEVs) of lymph nodes. Staining of skin vessels with a glycocalyx probe further suggested that their lumenal glycocalyx contains much lower HS density than their basolateral extracellular matrix (ECM). This polarized HS pattern was observed also in isolated resting and inflamed microvascular dermal cells. Notably, progressive skin inflammation resulted in massive ECM deposition and in further HS enrichment around skin post capillary venules and their associated pericytes. Inflammation-dependent HS enrichment was not compromised in mice deficient in the main HS degrading enzyme, heparanase. Our results suggest that the blood vasculature patterns steep gradients of HS scaffolds between their lumenal and basolateral endothelial aspects, and that inflammatory processes can further enrich the HS content nearby inflamed vessels. We propose that chemokine gradients between the lumenal and ablumenal sides of vessels could be favored by these sharp HS scaffold gradients.

Heparan sulfate 3-O-sulfation: a rare modification in search of a function

Many protein ligands bind to heparan sulfate, which results in their presentation, protection, oligomerization or conformational activation. Binding depends on the pattern of sulfation and arrangement of uronic acid epimers along the chains. Sulfation at the C3 position of glucosamine is a relatively rare, yet biologically significant modification, initially described as a key determinant for binding and activation of antithrombin and later for infection by type I herpes simplex virus. In mammals, a family of seven heparan sulfate 3-O-sulfotransferases installs sulfate groups at this position and constitutes the largest group of sulfotransferases involved in heparan sulfate formation. However, to date very few proteins or biological systems have been described that are influenced by 3-O-sulfation. This review describes our current understanding of the prevalence and structure of 3-O-sulfation sites, expression and substrate specificity of the 3-O-sulfotransferase family and the emerging roles of 3-O-sulfation in biology.

Distinct 3-O-sulfated heparan sulfate modification patterns are required for kal-1-dependent neurite branching in a context-dependent manner in Caenorhabditis elegans

Heparan sulfate (HS) is an unbranched glycosaminoglycan exhibiting substantial molecular diversity due to multiple, nonuniformly introduced modifications, including sulfations, epimerization, and acetylation. HS modifications serve specific and instructive roles in neuronal development, leading to the hypothesis of a HS code that regulates nervous system patterning. Although the in vivo roles of many of the HS modifications have been investigated, very little is known about the function of HS 3-O-sulfation in vivo. By examining patterning of the Caenorhabditis elegans nervous system in loss of function mutants of the two 3-O-sulfotransferases, hst-3.1 and hst-3.2, we found HS 3-O-sulfation to be largely dispensable for overall neural development. However, generation of stereotypical neurite branches in hermaphroditic-specific neurons required hst-3.1, hst-3.2, as well as an extracellular cell adhesion molecule encoded by kal-1, the homolog of Kallmann Syndrome associated gene 1/anosmin-1. In contrast, kal-1−dependent neurite branching in AIY neurons required catalytic activity of hst-3.2 but not hst-3.1. The context-dependent requirement for hst-3.2 and hst-3.1 indicates that both enzymes generate distinct types of HS modification patterns in different cell types, which regulate kal-1 to promote neurite branching. We conclude that HS 3-O-sulfation does not play a general role in establishing the HS code in C. elegans but rather plays a specialized role in a context-dependent manner to establish defined aspects of neuronal circuits.

Chemical probing of glycans in cells and organisms

Among the four major building blocks of life, glycans play essential roles in numerous physiological and pathological processes. Due to their non-templated biosynthesis, advances towards elucidating the molecular details of glycan functions are relatively slow compared with the pace of protein and nucleic acid research. Over the past 30 years, chemical tools have emerged as powerful allies to genetics and molecular biology in the study of glycans in their native environment. This tutorial review will provide an overview of the recent technological developments in the field, as well as the progress in the application of these techniques to probe glycans in cells and organisms.