Hyaluronan synthase assembles hyaluronan on a [GlcNAc(β1,4)]n-GlcNAc(α1→)UDP primer and hyaluronan retains this residual chitin oligomer as a cap at the nonreducing end

Systemic Glycosaminoglycan Clearance by HARE/Stabilin-2 Activates Intracellular Signaling

Scavenger receptors perform essential functions, critical to maintaining mammalian physiologic homeostasis by continuously clearing vast numbers of biomolecules from blood, interstitial fluid and lymph. Stabilin-2 (Stab2) and the Hyaluronic Acid Receptor for Endocytosis (HARE), a proteolytic isoform of Stab2, are important scavenger receptors responsible for the specific binding and internalization (leading to degradation) of 22 discrete molecules, macromolecular complexes and cell types. One-third of these ligands are glycosaminoglycans (GAGs). Full-length Stab2, but not HARE, mediates efficient phagocytosis of apoptotic cells and bacteria via binding to target surface ligands. HARE, the C-terminal half of Stab2, mediates endocytosis of all the known soluble ligands. HA was the first ligand identified, in 1981, prior to receptor purification or cloning. Seven other GAG ligands were subsequently identified: heparin, dermatan sulfate, chondroitin and chondroitin sulfates A, C, D and E. Synthetic dextran sulfate is also a GAG mimic and ligand. HARE signaling during HA endocytosis was first discovered in 2008, and we now know that activation of HARE/Stab2 signaling is stimulated by receptor-mediated endocytosis or phagocytosis of many, but not all, of its ligands. This review focuses on the HARE-mediated GAG activation of intracellular signaling, particularly the Extracellular Signal-Regulated Kinase 1/2 pathway.

Chitinases and Chitinase-Like Proteins in Obstructive Lung Diseases - Current Concepts and Potential Applications

Chitinases, enzymes that cleave chitin’s chain to low molecular weight chitooligomers, are widely distributed in nature. Mammalian chitinases belong to the 18-glycosyl-hydrolase family and can be divided into two groups: true chitinases with enzymatic activity (AMCase and chitotriosidase) and chitinase-like proteins (CLPs) molecules which can bind to chitin or chitooligosaccharides but lack enzymatic activity (eg, YKL-40). Chitinases are thought to be part of an innate immunity against chitin-containing parasites and fungal infections. Both groups of these hydrolases are lately evaluated also as chemical mediators or biomarkers involved in airway inflammation and fibrosis. The aim of this article is to present the current knowledge on the potential role of human chitinases and CLPs in the pathogenesis, diagnosis, and course of obstructive lung diseases. We also assessed the potential role of chitinase and CLPs inhibitors as therapeutic targets in chronic obstructive pulmonary disease and asthma.

The journey of hyaluronan research in the Journal of Biological Chemistry

Hyaluronan has a very simple structure. It is a linear glycosaminoglycan composed of disaccharide units of GlcNAc and d-glucuronic acid with alternating β-1,4 and β-1,3 glycosidic bonds that can be repeated 20,000 or more times, a molecular mass >8 million Da, and a length >20 μm. However, it has a very complex biology. It is a major, ubiquitous component of extracellular matrices involved in everything from fertilization, development, inflammations, to cancer. This JBC Review highlights some of these processes that were initiated through publications in the Journal of Biological Chemistry.

A two-phase model for the non-processive biosynthesis of homogalacturonan polysaccharides by the GAUT1:GAUT7 complex

Homogalacturonan (HG) is a pectic glycan in the plant cell wall that contributes to plant growth and development and cell wall structure and function, and interacts with other glycans and proteoglycans in the wall. HG is synthesized by the galacturonosyltransferase (GAUT) gene family. Two members of this family, GAUT1 and GAUT7, form a heteromeric enzyme complex in Arabidopsis thaliana Here, we established a heterologous GAUT expression system in HEK293 cells and show that co-expression of recombinant GAUT1 with GAUT7 results in the production of a soluble GAUT1:GAUT7 complex that catalyzes elongation of HG products in vitro The reaction rates, progress curves, and product distributions exhibited major differences dependent upon small changes in the degree of polymerization (DP) of the oligosaccharide acceptor. GAUT1:GAUT7 displayed >45-fold increased catalytic efficiency with DP11 acceptors relative to DP7 acceptors. Although GAUT1:GAUT7 synthesized high-molecular-weight polymeric HG (>100 kDa) in a substrate concentration-dependent manner typical of distributive (nonprocessive) glycosyltransferases with DP11 acceptors, reactions primed with short-chain acceptors resulted in a bimodal product distribution of glycan products that has previously been reported as evidence for a processive model of GT elongation. As an alternative to the processive glycosyltransfer model, a two-phase distributive elongation model is proposed in which a slow phase, which includes the de novo initiation of HG and elongation of short-chain acceptors, is distinguished from a phase of rapid elongation of intermediate- and long-chain acceptors. Upon reaching a critical chain length of DP11, GAUT1:GAUT7 elongates HG to high-molecular-weight products.

Distinct reaction mechanisms for hyaluronan biosynthesis in different kingdoms of life

Hyaluronan (HA) is an acidic high molecular weight cell surface polysaccharide ubiquitously expressed by vertebrates, some pathogenic bacteria and even viruses. HA modulates many essential physiological processes and is implicated in numerous pathological conditions ranging from autoimmune diseases to cancer. In various pathogens, HA functions as a non-immunogenic surface polymer that reduces host immune responses. It is a linear polymer of strictly alternating glucuronic acid and N-acetylglucosamine units synthesized by HA synthase (HAS), a membrane-embedded family-2 glycosyltransferase. The enzyme synthesizes HA and secretes the polymer through a channel formed by its own membrane-integrated domain. To reveal how HAS achieves these tasks, we determined the biologically functional units of bacterial and viral HAS in a lipid bilayer environment by co-immunoprecipitation, single molecule fluorescence photobleaching, and site-specific cross-linking analyses. Our results demonstrate that bacterial HAS functions as an obligate homo-dimer with two functional HAS copies required for catalytic activity. In contrast, the viral enzyme, closely related to vertebrate HAS, functions as a monomer. Using site-specific cross-linking, we identify the dimer interface of bacterial HAS and show that the enzyme uses a reaction mechanism distinct from viral HAS that necessitates a dimeric assembly.

Planning, evaluating and vetting receptor signaling studies to assess hyaluronan size-dependence and specificity

Exciting discoveries in many diverse fields of hyaluronan (HA) biology over the last 40 years have centered around the ability of HA to bind cell surface HA receptors (e.g., CD44, Layilin, LYVE-1, HARE/Stab2 and RHAMM) and sometimes also to activate intracellular signal transduction pathways, frequently involving ERK1/2. Although perplexing, a major characteristic of HA-mediated signal pathway activation for some receptors has been a dependence on the size of the bound HA. Receptors that directly interact with HA, which may not include TLR2/4, bind very well to any HA molecule >8-20 sugars, depending on the receptor. Despite their ability to bind virtually any size HA, only HA chains of a particular mass range can activate receptor-mediated cell signaling. Many studies have demonstrated parts of this emerging story by utilizing different: HA receptors, cell types, animal models, HA sources, HA sizes, assays to assess HA mass and varying controls to verify HA specificity or HA size-dependence. Recent reports have highlighted issues with potential endotoxin contamination of HA fragments, especially those generated by hyaluronidase digestion. Also, researchers unfamiliar with HA polydispersity must adjust to working with, and interpreting data for, preparations without a unique molecular mass (molecular weight). The confusion, uncertainty and skepticism generated by these and other factors has hindered the development of a general consensus about HA-specific and HA-size dependent receptor activation. An overview of issues, suggested strategies and validating controls is presented to aid those planning an HA-mediated receptor signaling study or those trying to evaluate the literature.

Activation of Matrix Hyaluronan-Mediated CD44 Signaling, Epigenetic Regulation and Chemoresistance in Head and Neck Cancer Stem Cells

Head and neck squamous cell carcinoma (HNSCC) is a solid tumor composed by a genotypically and phenotypically heterogeneous population of neoplastic cells types. High recurrence rate and regional metastases lead to major morbidity and mortality. Recently, many studies have focused on cellular and molecular mechanisms of tumor progression that can help to predict prognosis and to choose the best therapeutic approach for HNSCC patients. Hyaluronan (HA), an important glycosaminoglycan component of the extracellular matrix (ECM), and its major cell surface receptor, CD44, have been suggested to be important cellular mediators influencing tumor progression and treatment resistance in head and neck cancer. HNSCC contains a small subpopulation of cells that exhibit a hallmark of CD44-expressing cancer stem cell (CSC) properties with self-renewal, multipotency, and a unique potential for tumor initiation. HA has been shown to stimulate a variety of CSC functions including self-renewal, clone formation and differentiation. This review article will present current evidence for the existence of a unique small population of CD44v3^highALDH^high-expressing CSCs in HNSCC. A special focus will be placed on the role of HA/CD44-induced oncogenic signaling and histone methyltransferase, DOT1L activities in regulating histone modifications (via epigenetic changes) and miRNA activation. Many of these events are essential for the CSC properties such as Nanog/Oct4/Sox2 expression, spheroid/clone formation, self-renewal, tumor cell migration/invasion, survival and chemotherapeutic drug resistance in HA-activated head and neck cancer. These newly-discovered HA/CD44-mediated oncogenic signaling pathways delineate unique tumor dynamics with implications for defining the drivers of HNSCC progression processes. Most importantly, the important knowledge obtained from HA/CD44-regulated CSC signaling and functional activation could provide new information regarding the design of novel drug targets to overcome current therapeutic drug resistance which will have significant treatment implications for head and neck cancer patients.