Ligand-induced Structural Changes of the CD44 Hyaluronan-binding Domain Revealed by NMR

Bidirectional Allostery Mechanism in Catch-Bond Formation of CD44 Mediated Cell Adhesion

Catch-bonds, whereby noncovalent ligand-receptor interactions are counterintuitively reinforced by tensile forces, play a major role in cell adhesion under mechanical stress. A basic prerequisite for catch-bond formation, as implicated in classic catch-bond models, is that force-induced remodeling of the ligand binding interface occurs prior to bond rupture. However, what strategy receptor proteins utilize to meet such specific kinetic control remains elusive. Here we report a bidirectional allostery mechanism of catch-bond formation based on theoretical and molecular dynamics simulation studies. Binding of ligand allosterically reduces the threshold force for unlocking of otherwise stably folded force-sensing element (i.e., forward allostery), so that a much smaller tensile force can trigger the conformational switching of receptor protein to high binding-strength state via backward allosteric coupling before bond rupture. Such bidirectional allostery fulfills the specific kinetic control required by catch-bond formation and is likely to be commonly utilized in cell adhesion. The essential thermodynamic and kinetic features of receptor proteins essential for catch-bond formation were identified.

Material-specific binding peptides empower sustainable innovations in plant health, biocatalysis, medicine and microplastic quantification

Material-binding peptides (MBPs) have emerged as a diverse and innovation-enabling class of peptides in applications such as plant-/human health, immobilization of catalysts, bioactive coatings, accelerated polymer degradation and analytics for micro-/nanoplastics quantification. Progress has been fuelled by recent advancements in protein engineering methodologies and advances in computational and analytical methodologies, which allow the design of, for instance, material-specific MBPs with fine-tuned binding strength for numerous demands in material science applications. A genetic or chemical conjugation of second (biological, chemical or physical property-changing) functionality to MBPs empowers the design of advanced (hybrid) materials, bioactive coatings and analytical tools. In this review, we provide a comprehensive overview comprising naturally occurring MBPs and their function in nature, binding properties of short man-made MBPs (<20 amino acids) mainly obtained from phage-display libraries, and medium-sized binding peptides (20-100 amino acids) that have been reported to bind to metals, polymers or other industrially produced materials. The goal of this review is to provide an in-depth understanding of molecular interactions between materials and material-specific binding peptides, and thereby empower the use of MBPs in material science applications. Protein engineering methodologies and selected examples to tailor MBPs toward applications in agriculture with a focus on plant health, biocatalysis, medicine and environmental monitoring serve as examples of the transformative power of MBPs for various industrial applications. An emphasis will be given to MBPs' role in detecting and quantifying microplastics in high throughput, distinguishing microplastics from other environmental particles, and thereby assisting to close an analytical gap in food safety and monitoring of environmental plastic pollution. In essence, this review aims to provide an overview among researchers from diverse disciplines in respect to material-(specific) binding of MBPs, protein engineering methodologies to tailor their properties to application demands, re-engineering for material science applications using MBPs, and thereby inspire researchers to employ MBPs in their research.

Pan-cancer and single-cell analyses identify CD44 as an immunotherapy response predictor and regulating macrophage polarization and tumor progression in colorectal cancer

Introduction

Cluster of differentiation (CD) 44 is a non-kinase cell surface transmembrane glycoprotein critical for tumor maintenance and progression.

Methods

We conducted a systematic analysis of the expression profile and genomic alteration profile of CD44 in 33 types of cancer. The immune characteristics of CD44 were comprehensively explored by TIMER2.0 and CIBERSORT. In addition, the CD44 transcriptional landscape was examined at the single-cell level. Then, Pseudotime trajectory analysis of CD44 gene expression was performed using Monocle 2, and CellChat was utilized to compare the crosstalk differences between CD44+monocytes and CD44- monocytes. Tumor immune dysfunction and exclusion (TIDE) was used to evaluate the predictive ability of CD44 for immune checkpoint blockade (ICB) responses. The effects of CD44 on colorectal cancer (CRC) and macrophage polarization were investigated by knocking down the expression of CD44 in HCT-116 cell and macrophages in vitro.

Results

The expression of CD44 elevated in most cancers, predicting unfavorable prognosis. In addditon, CD44 was correlation with immune cell infiltration and key immune regulators. CD44+ monocytes had a higher information flow intensity than CD44- monocytes. CD44 had good predictive ability for immune checkpoint blockade responses. Knockdown of CD44 inhibited the proliferation, migration, and invasion of HCT-116 cell in vitro. Knockdown of CD44 inhibited M2 macrophage polarization.

Discussion

These findings suggest that CD44 is involved in regulating tumor development, macrophage polarization, and has certain predictive value for patient clinical prognosis and response to immunotherapy.

Exploiting the Features of Short Peptides to Recognize Specific Cell Surface Markers

Antibodies are the macromolecules of choice to ensure specific recognition of biomarkers in biological assays. However, they present a range of shortfalls including a relatively high production cost and limited tissue penetration. Peptides are relatively small molecules able to reproduce sequences of highly specific paratopes and, although they have less biospecificity than antibodies, they offer advantages like ease of synthesis, modifications of their amino acid sequences and tagging with fluorophores and other molecules required for detection. This work presents a strategy to design peptide sequences able to recognize the CD44 hyaluronic acid receptor present in the plasmalemma of a range of cells including human bone marrow stromal mesenchymal cells. The protocol of identification of the optimal amino acid sequence was based on the combination of rational design and in silico methodologies. This protocol led to the identification of two peptide sequences which were synthesized and tested on human bone marrow mesenchymal stromal cells (hBM-MSCs) for their ability to ensure specific binding to the CD44 receptor. Of the two peptides, one binds CD44 with sensitivity and selectivity, thus proving its potential to be used as a suitable alternative to this antibody in conventional immunostaining. In the context of regenerative medicine, the availability of this peptide could be harnessed to functionalize tissue engineering scaffolds to anchor stem cells as well as to be integrated into systems such as cell sorters to efficiently isolate MSCs from biological samples including various cell subpopulations. The data here reported can represent a model for developing peptide sequences able to recognize hBM-MSCs and other types of cells and for their integration in a range of biomedical applications.

Prediction of CD44 Structure by Deep Learning-Based Protein Modeling

CD44 is a cell surface glycoprotein transmembrane receptor that is involved in cell-cell and cell-matrix interactions. It crucially associates with several molecules composing the extracellular matrix, the main one of which is hyaluronic acid. It is ubiquitously expressed in various types of cells and is involved in the regulation of important signaling pathways, thus playing a key role in several physiological and pathological processes. Structural information about CD44 is, therefore, fundamental for understanding the mechanism of action of this receptor and developing effective treatments against its aberrant expression and dysregulation frequently associated with pathological conditions. To date, only the structure of the hyaluronan-binding domain (HABD) of CD44 has been experimentally determined. To elucidate the nature of CD44s, the most frequently expressed isoform, we employed the recently developed deep-learning-based tools D-I-TASSER, AlphaFold2, and RoseTTAFold for an initial structural prediction of the full-length receptor, accompanied by molecular dynamics simulations on the most promising model. All three approaches correctly predicted the HABD, with AlphaFold2 outperforming D-I-TASSER and RoseTTAFold in the structural comparison with the crystallographic HABD structure and confidence in predicting the transmembrane helix. Low confidence regions were also predicted, which largely corresponded to the disordered regions of CD44s. These regions allow the receptor to perform its unconventional activity.

CD44 receptor targeted nanoparticles augment immunity against tuberculosis in mice

We describe a role of CD44-mediated signaling during host-defense against tuberculosis (TB) using a mouse model of TB and studies in M. tuberculosis (Mtb) infected human macrophage (MФ). Liposomes targeting CD44 using thioaptamers (CD44TA-LIP) were designed and tested as new vaccines to boost host immunity in TB. CD44TA-LIP enhanced killing of Mtb in human MФ, which correlated with an increased production of pro-inflammatory cytokines IL-1β, TNF-α and IL-12. CD44TA-LIP activated MФ showed an enhanced MHC-II dependent antigen presentation to CD4 T-cells. Inhibition of cellular proliferation and cytoskeleton rearrangement pathways downstream of CD44 signaling abrogated CD44TA-LIP-induced antimicrobial effects. Blockade of inflammatory pathways also reduced antigen presentation by MФ and activation of CD4 T cells. Mtb infected MФ treated with CD44TA-LIP exhibited increased nitric oxide and HβD2 defensin peptide production. Among Mtb infected mice with increased lung and spleen loads of organisms, intranasal administration of CD44TA-LIP led to a ten-fold reduction of colony forming units of Mtb and elevated IFN-γ + CD4, effector, central and resident memory T cells. Biodistribution studies demonstrated that CD44TA-LIP preferentially accumulated in the lungs and were associated with CD11b + cells. CD44TA-LIP treated mice showed no weight loss or increased liver LDH levels. This study highlights the importance of CD44-mediated signaling in host-defense during TB and the therapeutic potential of CD44TA-LIP.

OSCILLATORS IN THE MICROVASCULATURE - GLYCOCALYX AND BEYOND

The growing recognition of abundance of oscillating functions in biological systems has motivated this brief overview which narrows down on the microvasculature. Specifically, it encompasses self-sustained oscillations of blood flow, hematocrit and viscosity at bifurcations; their effects on the oscillations of endothelial glycocalyx, mechanotransduction and its termination to prime endothelial cells for the subsequent mechanical signaling event; oscillating affinity of hyaluronan-CD44 binding domain; spontaneous contractility of actomyosin complexes in the cortical actin web, its effects on the tension of the plasma membrane; reversible effects of sirtuin-1 on endothelial glycocalyx; and effects of plasma membrane tension on endo-and exocytosis. Some potential interactions between those oscillators - their coupling - are discussed together with their transition into chaotic movements. Future in-depth understanding of the oscillatory activities in the microvasculature could serve as a guide to its chronotherapy under pathological conditions.

CD44 In Sarcomas: A Comprehensive Review and Future Perspectives

It is widely accepted that the tumor microenvironment, particularly the extracellular matrix, plays an essential role in the development of tumors through the interaction with specific protein-membrane receptors. One of the most relevant proteins in this context is the transmembrane protein CD44. The role of CD44 in tumor progression, invasion, and metastasis has been well established in many cancers, although a comprehensive review concerning its role in sarcomas has not been published. CD44 is overexpressed in most sarcomas and several in vitro and in vivo experiments have shown a direct effect on tumor progression, dissemination, and drug resistance. Moreover, CD44 has been revealed as a useful marker for prognostic and diagnostic (CD44v6 isoform) in osteosarcoma. Besides, some innovative treatments such as HA-functionalized liposomes therapy have become an excellent CD44-mediated intracellular delivery system for osteosarcoma. Unfortunately, the reduced number of studies deciphering the prognostic/diagnostic value of CD44 in other sarcoma subgroups, neither than osteosarcoma, in addition to the low number of patients involved in those studies, have produced inconclusive results. In this review, we have gone through the information available on the role of CD44 in the development, maintenance, and progression of sarcomas, analyzing their implications at the prognostic, therapeutic, and mechanistic levels. Moreover, we illustrate how research involving the specific role of CD44 in the different sarcoma subgroups could suppose a chance to advance towards a more innovative perspective for novel therapies and future clinical trials.

Distinct binding kinetics of E-, P- and L-selectins to CD44

Molecular-level selectin-cluster of differentiation 44 (CD44) interactions are far from clear because of the complexity and diversity of CD44 glycosylation and isoforms expressed on various types of cells. By combining experimental measurements and simulation predictions, the binding kinetics of three selectin members to the recombinant CD44 were quantified and the corresponding microstructural mechanisms were explored, respectively. Experimental results showed that the E-selectin-CD44 interactions mainly mediated the firm adhesion of microbeads under shear flow with the strongest rupture force. P- and L-selectins had similar interaction strength but different association and dissociation rates by mediating stable rolling and transient adhesions of microbeads, respectively. Molecular docking and molecular dynamics (MD) simulations predicted that the binding epitopes of CD44 to selectins are all located at the side face of each selectin, although the interfaces denoted as the hinge region are between lectin and epidermal growth factor domains of E-selectin, Lectin domain side of P-selectin and epidermal growth factor domain side of L-selectin, respectively. The lowest binding free energy, the largest rupture force and the longest lifetime for E-selectin, as well as the comparable values for P- and L-selectins, demonstrated in both equilibration and steered MD simulations, supported the above experimental results. These results offer basic data for understanding the functional differences of selectin-CD44 interactions.

Molecular Dynamics Simulation Study on Allosteric Regulation of CD44-Hyaluronan Binding as a Force Sensing Mechanism

CD44 protein exists on surfaces of a variety of human cells, acts as a receptor for the hyaluronan (HA) molecule, and mediates cell adhesion via the HA binding in leukocyte trafficking, cell rolling, and so on. The molecular structures of both CD44 and HA are well known, and the previous work shows that the external-mechanical force induces the partially disordered (PD) conformation from the ordered (O) conformation of CD44. The PD conformation has the higher HA affinity compared to the O conformation. However, the details of force-sensing mechanics have remained unclear. This study provides new insights into allosteric regulation of HA binding by conformational shift from the O to the PD conformation of the CD44 HA binding domain by using the classical molecular dynamics simulations. The O conformation was more favorable than the PD conformation under the equilibrium state, and the O conformation showed weak HA-binding affinity. Our simulation suggests that the PD conformation induced by the external force can refold to a compact structure similar to the O conformation keeping the bound HA. This new conformation showed a higher affinity than the O and PD conformations. Our results show that the unfolding of a remote disordered region from the ligand binding site by the external force allosterically regulates the HA affinity. This study promotes understanding not only the mechanism of CD44-mediated cell rolling but also the allosteric regulation induced by the external mechanical force.

NMR Characterization of the Interactions Between Glycosaminoglycans and Proteins

Glycosaminoglycans (GAGs) constitute a considerable fraction of the glycoconjugates found on cellular membranes and in the extracellular matrix of virtually all mammalian tissues. The essential role of GAG-protein interactions in the regulation of physiological processes has been recognized for decades. However, the underlying molecular basis of these interactions has only emerged since 1990s. The binding specificity of GAGs is encoded in their primary structures, but ultimately depends on how their functional groups are presented to a protein in the three-dimensional space. This review focuses on the application of NMR spectroscopy on the characterization of the GAG-protein interactions. Examples of interpretation of the complex mechanism and characterization of structural motifs involved in the GAG-protein interactions are given. Selected families of GAG-binding proteins investigated using NMR are also described.

Proteolytic Processing of CD44 and Its Implications in Cancer

CD44 is a transmembrane glycoprotein expressed in several healthy and tumor tissues. Modifications in its structure contribute differently to the activity of this molecule. One modification that has provoked interest is the consecutive cleavage of the CD44 extracellular ectodomain by enzymes that belong mainly to the family of metalloproteases. This process releases biologically active substrates, via alternative splice forms of CD44, that generate CD44v3 or v6 isoforms which participate in the transcriptional regulation of genes and proteins associated to signaling pathways involved in the development of cancer. These include the protooncogene tyrosine-protein kinase Src (c-Src)/signal transducer and activator of transcription 3 (STAT3), the epithelial growth factor receptor, the estrogen receptor, Wnt/βcatenin, or Hippo signaling pathways all of which are associated to cell proliferation, differentiation, or cancer progression. Whereas CD44 still remains as a very useful prognostic cell marker in different pathologies, the main topic is that the generation of CD44 intracellular fragments assists the regulation of transcriptional proteins involved in the cell cycle, cell metabolism, and most importantly, the regulation of some stem cell-associated markers.

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.

Synthesis of hyaluronic acid oligosaccharides with a GlcNAc-GlcA repeating pattern and their binding affinity with CD44

Hyaluronic acid (HA) is a ubiquitous glycosaminoglycan in the extracellular matrix and a ligand of CD44, a transmembrane glycoprotein that is important in cell migration. Crystal and NMR studies found a hexasaccharide of the pattern (GlcA-GlcNAc)3 as the shortest HA that could bind to CD44, but molecular dynamics simulations indicated that a tetrasaccharide of the pattern (GlcNAc-GlcA)2 is the key structure interacting with CD44. Access to oligomers with such a repeat pattern is crucial in binding studies with CD44. Here we developed a synthetic procedure to afford the HA oligosaccharides with the GlcNAc-GlcA repeating unit and measured the binding interaction between these sugars and human CD44 by isothermal titration calorimetry (ITC). During the chemical synthesis, we successfully generated the β-glycosidic bond in the absence of neighbouring group participation and overcome the issues in the oxidation step. In addition, ammonia-free dissolving metal reduction for debenzylation and azido reduction has been applied in carbohydrate synthesis for the first time. ITC analysis revealed that the HA tetrasaccharide (GlcNAc-GlcA)2 could indeed interact and bind to the human CD44.

3D cultures for modeling nanomaterial-based photothermal therapy

Photothermal therapy (PTT) is one of the most promising techniques for cancer tumor ablation. Nanoparticles are increasingly being investigated for use with PTT and can serve as theranostic agents. Based on the ability of near-infrared nano-photo-absorbers to generate heat under laser irradiation, PTT could prove advantageous in certain situations over more classical cancer therapies. To analyze the efficacy of nanoparticle-based PTT, preclinical in vitro studies typically use 2D cultures, but this method cannot completely mimic the complex tumor organization, bioactivity, and physiology that all control the complex penetration depth, biodistribution, and tissue diffusion parameters of nanomaterials in vivo. To fill this knowledge gap, 3D culture systems have been explored for PTT analysis. These models provide more realistic microenvironments that allow spatiotemporal oxygen gradients and cancer cell adaptations to be considered. This review highlights the work that has been done to advance 3D models for cancer microenvironment modeling, specifically in the context of advanced, functionalized nanoparticle-directed PTT.

Structural Characterization of the CD44 Stem Region for Standard and Cancer-Associated Isoforms

CD44 is widely expressed in most vertebrate cells, whereas the expression of CD44v6 is restricted to only a few tissues and has been considered to be associated with tumor progression and metastasis. Thus, CD44v6 has been recognized as a promising prognostic biomarker and therapeutic target for various cancers for more than a decade. However, despite many experimental studies, the structural dynamics and differences between CD44s and CD44v6, particularly in their stem region, still remain elusive. Here, a computational study was conducted to address these problems. We found that the stem of CD44s adopted predominantly two conformations, one featuring antiparallel β-sheets and the other featuring parallel β-sheets, whereas the stem of CD44v6 adopted mainly one conformation with relatively highly suppressed β-sheet contents. Moreover, Phe215 was found to be essential in the β-sheets of both CD44s and CD44v6. We finally found intramolecular Phe215-Trp224 hydrogen-bonding interactions and hydrophobic interactions with Phe215 that cooperatively drove conformational differences upon the addition of the v6 region to CD44. Our study elucidated the structural differences between the stem regions of CD44s and CD44v6 and thus can offer useful structural information for drug design to specifically target CD44v6 in promising clinical applications.

Molecular weight impact on the mechanical forces between hyaluronan and its receptor

Hyaluronan (HA) possesses manifold mechanical and signaling properties in the body. Most of these activities are largely regulated by its molecular weight, which often triggers opposing functions. However the molecular basis for such function distinction between HA size categories remains unclear. Using a combination of biophysical techniques, we measured the physical forces between HA ligand and its specific receptor CD44 in both normal and lateral directions, at different HA molecular weights and bound states. It was found that the impact of HA multivalency is more than just the sum of separate monovalent bindings. The HA-CD44 specific interaction enhances with HA molecular weight and the maximum binding occurs at ∼1000 kD, possibly due to the balance between multivalent HA zipping effect and conformational entropy. High friction patches, probably from CD44 protein clustering, was observed in friction force microscopy (FFM) upon HA shearing, which is also dependent on HA molecular weight. These results could help to understand the biophysical mechanism of HA in regulating CD44-induced physiological activities and thus facilitate the new design of HA-based material in fine tuning the receptor responses.

Zona pellucida-binding protein 2 (ZPBP2) and several proteins containing BX7B motifs in human sperm may have hyaluronic acid binding or recognition properties

STUDY QUESTION

Are there novel hyaladherins in human sperm?

SUMMARY ANSWER

Zona pellucida-binding protein 2 (ZPBP2), containing a Link-like hyaluronic acid (HA)-binding domain, and several other proteins containing BX7B motifs, such as ADAM32 and Midkine, may be novel hyaladherins with HA-binding properties.

WHAT IS KNOWN ALREADY

HA-binding proteins (hyaladherins), which can bind HA surrounding the cumulus-oophorus complex, are distinct from hyases such as PH 20 (SPAM1) and are expressed by mature spermatozoa. Although HABP1 and CD44 are reasonably well characterized hyaladherins and the former has been implicated in sperm-oocyte interactions, the overall significance of sperm hyaladherins for male fertility is still poorly understood.

STUDY DESIGN, SIZE, DURATION

This was a laboratory-based investigation into human sperm hyaladherins undertaken as part of a three year PhD programme sponsored by the EU Marie Curie Training network, Reprotrain.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Protein homogenates of sperm obtained from young men of unknown fertility (N = 4) were partitioned into HA-binding and non-binding fractions by a protein affinity 'panning' method; their subsequent characterization was by liquid chromatography-tandem mass spectrometry (LC-MS-MS) and partitioning behaviour was confirmed by western blotting. Sequences of proteins from both fractions were submitted to PDBsum to look for orthologous entries (PDB codes) and all returned codes were queried against the matching protein using SAS (Sequences Annotated by Structure) looking for structural similarities between them. A systematic search for other common features of hyaladherins was also undertaken.

MAIN RESULTS AND THE ROLE OF CHANCE

The presence of BX7B sequence motifs found in several well-described hyaladherins including RHAMM was used to assess efficacy of potential hyaladherin partitioning by the HA substrate. The data showed that 50% (14/28) and 34.5% (28/81) of proteins in the bound and unbound fractions, respectively, contained these motifs (one-tailed Z-score = 1.45; P = 0.074), indicating weak discrimination by the substrate. Querying PDBsum with sequences for all bound proteins returned several PDB codes matching ZPBP2 with the HA-binding Link domain of the hyaladherin, CD44. Western blot analysis confirmed the affinity partitioning of proteins indicated by the LC-MS/MS results, with ADAM32 (containing two BX7B motifs) and ZPBP2 (containing a Link-like HA-binding domain) present only in the binding fraction. There remains the possibility that the putative hyaladherins uncovered by this study were coincidentally enriched by HA-binding.

LARGE SCALE DATA

The full proteomics data set is available on request.

LIMITATIONS REASONS FOR CAUTION

The protein extraction methods or the HA substrate used to pan them in this study were probably not ideal, as hyaladherins expected to be present in sperm homogenates (such as CD44 and RHAMM) were not detected.

WIDER IMPLICATIONS OF THE FINDINGS

The results provide evidence that ZPBP2, found only in the bound fraction, may have hyaladherin-like properties, which could reflect the evolutionary background context of contemporary sperm-oocyte interaction mechanisms.

STUDY FUNDING AND COMPETING INTEREST(S)

An EU Marie Curie Sklodowska Initial Training Network Scholarship, supporting Ms Torabi, is gratefully acknowledged. This project was also supported and funded by the Efficacy and Mechanism Evaluation Programme, a UK MRC and NIHR partnership (Grant No 11/14/ 34). There is no conflict of interest in relation to this work.

Atomistic fingerprint of hyaluronan-CD44 binding

Hyaluronan is a polyanionic, megadalton-scale polysaccharide, which initiates cell signaling by interacting with several receptor proteins including CD44 involved in cell-cell interactions and cell adhesion. Previous studies of the CD44 hyaluronan binding domain have identified multiple widespread residues to be responsible for its recognition capacity. In contrast, the X-ray structural characterization of CD44 has revealed a single binding mode associated with interactions that involve just a fraction of these residues. In this study, we show through atomistic molecular dynamics simulations that hyaluronan can bind CD44 with three topographically different binding modes that in unison define an interaction fingerprint, thus providing a plausible explanation for the disagreement between the earlier studies. Our results confirm that the known crystallographic mode is the strongest of the three binding modes. The other two modes represent metastable configurations that are readily available in the initial stages of the binding, and they are also the most frequently observed modes in our unbiased simulations. We further discuss how CD44, fostered by the weaker binding modes, diffuses along HA when attached. This 1D diffusion combined with the constrained relative orientation of the diffusing proteins is likely to influence the aggregation kinetics of CD44. Importantly, CD44 aggregation has been suggested to be a possible mechanism in CD44-mediated signaling.

Hyaluronan is a natural sugar polymer in our bodies. Besides acting as a space-filling agent for example in multiple connective tissues, it also functions as a cellular cue in cancer and inflammation. Our tissues sense hyaluronan through receptors—proteins that sit at the surface of cells and grab the molecules they are expected to recognize. Although the knowledge associated with hyaluronan and its receptors is constantly accumulating, the molecular-level insight is largely missing or incomplete due to the lack of techniques able to probe the dynamics of protein–carbohydrate interactions with sufficiently high resolution. In this work, we characterize the binding of hyaluronan to its receptor CD44 with atomistic precision. We achieve this level of precision by employing atomistic molecular dynamics simulations. This computational technique allows one to follow the movement of atoms of a virtual system at scales beyond the resolution of any experimental technique. Our work specifically focuses on the different stages of hyaluronan–CD44 binding, and we observe the process to involve three different binding modes, making it more versatile than previously thought. Our insights, therefore, promote the understanding of the interplay between hyaluronan and HA, thereby fostering development of new drugs or inhibitors to malignancies, such as cancer metastasis.

The Cell Surface Receptor CD44: NMR-Based Characterization of Putative Ligands

The cell surface receptor CD44 is a glycoprotein belonging to the hyaluronan-binding proteins, termed hyaladherins. CD44 is expressed in a wide variety of isoforms in many cells and, in particular, is present on the surface of malignant cells where it is involved in the onset and progression of cancer. In a first attempt to identify novel CD44-binding agents, we first characterized, with NMR spectroscopic techniques, several agents that were reported to bind to human CD44 (hCD44). To our surprise, however, none of these putative CD44-binding agents, including a peptide that is in phase 2 clinical trials (A6 peptide) and a recently reported fragment hit, were found to interact significantly with recombinant hCD44(21-178). Nonetheless, we further report that a fragment-screening campaign, with solution NMR spectroscopy as the detection method, identified a viable fragment hit that bound in a potentially functional pocket on the surface of CD44, opposite to the hyaluronic acid binding site. We hypothesize that this pocket could be indirectly associated with the cellular and in vivo activity of the A6 peptide, which would provide a novel framework for the possible development of therapeutically viable CD44 antagonists.

A user-friendly web portal for analyzing conformational changes in structures of Mycobacterium tuberculosis

Initiation of the Tuberculosis Structural Consortium has resulted in the expansion of the Mycobacterium tuberculosis (MTB) protein structural database. Currently, 969 experimentally solved structures are available for 354 MTB proteins. This includes multiple crystal structures for a given protein under different functional conditions, such as the presence of different ligands or mutations. In depth analysis of the multiple structures reveal that subtle differences exist in conformations of a given protein under varied conditions. Therefore, it is immensely important to understand the conformational differences between the multiple structures of a given protein in order to select the most suitable structure for molecular docking and structure-based drug designing. Here, we introduce a web portal ( http://bmi.icmr.org.in/mtbsd/torsion.php ) that we developed to provide comparative data on the ensemble of available structures of MTB proteins, such as Cα root means square deviation (RMSD), sequence identity, presence of mutations and torsion angles. Additionally, torsion angles were used to perform principal component analysis (PCA) to identify the conformational differences between the structures. Additionally, we present a few case studies to demonstrate this database. Graphical Abstract Conformational changes seen in the structures of the enoyl-ACP reductase protein encoded by the Mycobacterial gene inhA.

Revealing the Mechanisms of Protein Disorder and N-Glycosylation in CD44-Hyaluronan Binding Using Molecular Simulation

The extracellular N-terminal hyaluronan binding domain (HABD) of CD44 is a small globular domain that confers hyaluronan (HA) binding functionality to this large transmembrane glycoprotein. When recombinantly expressed by itself, HABD exists as a globular water-soluble protein that retains the capacity to bind HA. This has enabled atomic-resolution structural biology experiments that have revealed the structure of HABD and its binding mode with oligomeric HA. Such experiments have also pointed to an order-to-disorder transition in HABD that is associated with HA binding. However, it had remained unclear how this structural transition was involved in binding since it occurs in a region of HABD distant from the HA-binding site. Furthermore, HABD is known to be N-glycosylated, and such glycosylation can diminish HA binding when the associated N-glycans are capped with sialic acid residues. The intrinsic flexibility of disordered proteins and of N-glycans makes it difficult to apply experimental structural biology approaches to probe the molecular mechanisms of how the order-to-disorder transition and N-glycosylation can modulate HA binding by HABD. We review recent results from molecular dynamics simulations that provide atomic-resolution mechanistic understanding of such modulation to help bridge gaps between existing experimental binding and structural biology data. Findings from these simulations include: Tyr42 may function as a molecular switch that converts the HA-binding site from a low affinity to a high affinity state; in the partially disordered form of HABD, basic amino acids in the C-terminal region can gain sufficient mobility to form direct contacts with bound HA to further stabilize binding; and terminal sialic acids on covalently attached N-glycans can form charge-paired hydrogen bonding interactions with basic amino acids that could otherwise bind to HA, thereby blocking HA binding to glycosylated CD44 HABD.

Molecular mechanism for the action of the anti-CD44 monoclonal antibody MEM-85

The hyaluronate receptor CD44 plays role in cell adhesion and migration and is involved in tumor metastasis. The extracellular domain of CD44 comprises the hyaluronate-binding domain (HABD) and the membrane-proximal stem region; the short intracellular portion interacts with adaptor proteins and triggers signaling pathways. Binding of hyaluronate to CD44 HABD induces an allosteric conformational change, which results in CD44 shedding. A poorly characterized epitope in human CD44 HABD is recognized by the murine monoclonal antibody MEM-85, which cross-blocks hyaluronate binding to CD44 and also induces CD44 shedding. MEM-85 is of therapeutic interest, as it inhibits growth of lung cancer cells in murine models. In this work, we employed a combination of biophysical methods to determine the MEM-85 binding epitope in CD44 HABD and to provide detailed insight into the mechanism of MEM-85 action. In particular, we constructed a single-chain variable fragment (scFv) of MEM-85 as a tool for detailed characterization of the CD44 HABD-antibody complex and identified residues within CD44 HABD involved in the interaction with scFv MEM-85 by NMR spectroscopy and mutational analysis. In addition, we built a rigid body model of the CD44 HABD-scFv MEM-85 complex using a low-resolution structure obtained by small-angle X-ray scattering. The MEM-85 epitope is situated in the C-terminal part of CD44 HABD, rather than the hyaluronate-binding groove, and the binding of MEM-85 induces a structural reorganization similar to that induced by hyaluronate. Therefore, the mechanism of MEM-85 cross-blocking of hyaluronate binding is likely of an allosteric, relay-like nature.

Mechanical force effect on the two-state equilibrium of the hyaluronan-binding domain of CD44 in cell rolling

CD44 is the receptor for hyaluronan (HA) and mediates cell rolling under fluid shear stress. The HA-binding domain (HABD) of CD44 interconverts between a low-affinity, ordered (O) state and a high-affinity, partially disordered (PD) state, by the conformational change of the C-terminal region, which is connected to the plasma membrane. To examine the role of tensile force on CD44-mediated rolling, we used a cell-free rolling system, in which recombinant HABDs were attached to beads through a C-terminal or N-terminal tag. We found that the rolling behavior was stabilized only at high shear stress, when the HABD was attached through the C-terminal tag. In contrast, no difference was observed for the beads coated with HABD mutants that constitutively adopt either the O state or the PD state. Steered molecular dynamics simulations suggested that the force from the C terminus disrupts the interaction between the C-terminal region and the core of the domain, thus providing structural insights into how the mechanical force triggers the allosteric O-to-PD transition. Based on these results, we propose that the force applied from the C terminus enhances the HABD-HA interactions by inducing the conformational change to the high-affinity PD transition more rapidly, thereby enabling CD44 to mediate lymphocyte trafficking and hematopoietic progenitor cell homing under high-shear conditions.

Terminal sialic acids on CD44 N-glycans can block hyaluronan binding by forming competing intramolecular contacts with arginine sidechains

Specific sugar residues and their linkages form the basis of molecular recognition for interactions of glycoproteins with other biomolecules. Seemingly small changes, like the addition of a single monosaccharide in the covalently attached glycan component of glycoproteins, can greatly affect these interactions. For instance, the sialic acid capping of glycans affects protein-ligand binding involved in cell-cell and cell-matrix interactions. CD44 is a single-pass transmembrane glycoprotein whose binding with its carbohydrate ligand hyaluronan (HA), an extracellular matrix component, mediates processes such as leukocyte homing, cell adhesion, and tumor metastasis. This binding is highly regulated by glycosylation of the N-terminal extracellular hyaluronan-binding domain (HABD); specifically, sialic acid capped N-glycans of HABD inhibit ligand binding. However, the molecular mechanism behind this sialic acid mediated regulation has remained unknown. Two of the five N-glycosyation sites of HABD have been previously identified as having the greatest inhibitory effect on HA binding, but only if the glycans contain terminal sialic acid residues. These two sites, Asn25 and Asn120, were chosen for in silico glycosylation in this study. Here, from extensive standard molecular dynamics simulations and biased simulations, we propose a molecular mechanism for this behavior based on spontaneously-formed charge-paired hydrogen bonding interactions between the negatively-charged sialic acid residues and positively-charged Arg sidechains known to be critically important for binding to HA, which itself is negatively charged. Such intramolecular hydrogen bonds would preclude associations critical to hyaluronan binding. This observation suggests how CD44 and related glycoprotein binding is regulated by sialylation as cellular environments fluctuate.

High-resolution crystal structures of alternate forms of the human CD44 hyaluronan-binding domain reveal a site for protein interaction

Two new crystal structures of the extracellular hyaluronan-binding domain of human CD44 are described at high resolution. A hexagonal crystal form at 1.60 Å resolution and a monoclinic form at 1.08 Å resolution both have two molecules in the asymmetric unit arranged about a similar noncrystallographic twofold axis of symmetry. These structures are compared with those previously reported at 2.20 Å resolution to show that the fold is quite resistant to structural deformation in different crystal environments. Unexpectedly, a short peptide is found in the monoclinic crystals at a site remote from the known hyaluronan-binding groove. The peptide with a valine at the carboxy-terminus must have co-purified from the bacterial expression host and binds on the opposite side of the domain from the known hyaluronan-binding groove. This opportunistic binding may identify a site of interaction used as CD44 assembles with other proteins to accomplish effective signaling regarding changes to the extracellular environment.

CD44 receptor unfolding enhances binding by freeing basic amino acids to contact carbohydrate ligand

The extracellular carbohydrate-binding domain of the Type I transmembrane receptor CD44 is known to undergo affinity switching, where change in conformation leads to enhanced binding of its carbohydrate ligand hyaluronan. Separate x-ray crystallographic and NMR experiments have led to competing explanations, with the former supporting minor conformational changes at the binding site and the latter a major order-to-disorder unfolding transition distant from the binding site. Here, all-atom explicit-solvent molecular dynamics studies employing adaptive biasing force sampling revealed a substantial favorable free-energy change associated with contact formation between the Arg(41) side chain and hyaluronan at the binding site, independent of whether the distant site was ordered or disordered. Analogous computational experiments on Arg(41)Ala mutants showed loss of this favorable free-energy change, consistent with existing experimental data. More provocatively, the simulation data revealed the molecular mechanism by which the order-to-disorder transition enhances hyaluronan binding: in the disordered state, a number of basic residues gain sufficient conformational freedom-lacking in the ordered state-to spontaneously form side-chain contacts with hyaluronan. Mutation of these residues to Ala had been known to decrease binding affinity, but there had previously been no structural explanation, given their lack of proximity to the carbohydrate-binding site in existing structures of the complex.

Functional dynamics of cell surface membrane proteins

Cell surface receptors are integral membrane proteins that receive external stimuli, and transmit signals across plasma membranes. In the conventional view of receptor activation, ligand binding to the extracellular side of the receptor induces conformational changes, which convert the structure of the receptor into an active conformation. However, recent NMR studies of cell surface membrane proteins have revealed that their structures are more dynamic than previously envisioned, and they fluctuate between multiple conformations in an equilibrium on various timescales. In addition, NMR analyses, along with biochemical and cell biological experiments indicated that such dynamical properties are critical for the proper functions of the receptors. In this review, we will describe several NMR studies that revealed direct linkage between the structural dynamics and the functions of the cell surface membrane proteins, such as G-protein coupled receptors (GPCRs), ion channels, membrane transporters, and cell adhesion molecules.

Fragment-based identification of an inducible binding site on cell surface receptor CD44 for the design of protein-carbohydrate interaction inhibitors

Selective inhibitors of hyaluronan (HA) binding to the cell surface receptor CD44 will have value as probes of CD44-mediated signaling and have potential as therapeutic agents in chronic inflammation, cardiovascular disease, and cancer. Using biophysical binding assays, fragment screening, and crystallographic characterization of complexes with the CD44 HA binding domain, we have discovered an inducible pocket adjacent to the HA binding groove into which small molecules may bind. Iterations of fragment combination and structure-driven design have allowed identification of a series of 1,2,3,4-tetrahydroisoquinolines as the first nonglycosidic inhibitors of the CD44-HA interaction. The affinity of these molecules for the CD44 HA binding domain parallels their ability to interfere with CD44 binding to polymeric HA in vitro. X-ray crystallographic complexes of lead compounds are described and compared to a new complex with a short HA tetrasaccharide, to establish the tetrahydroisoquinoline pharmacophore as an attractive starting point for lead optimization.

Three-dimensional structural aspects of protein-polysaccharide interactions

Linear polysaccharides are typically composed of repeating mono- or disaccharide units and are ubiquitous among living organisms. Polysaccharide diversity arises from chain-length variation, branching, and additional modifications. Structural diversity is associated with various physiological functions, which are often regulated by cognate polysaccharide-binding proteins. Proteins that interact with linear polysaccharides have been identified or developed, such as galectins and polysaccharide-specific antibodies, respectively. Currently, data is accumulating on the three-dimensional structure of polysaccharide-binding proteins. These proteins are classified into two types: exo-type and endo-type. The former group specifically interacts with the terminal units of polysaccharides, whereas the latter with internal units. In this review, we describe the structural aspects of exo-type and endo-type protein-polysaccharide interactions. Further, we discuss the structural basis for affinity and specificity enhancement in the face of inherently weak binding interactions.

The role of flexibility and conformational selection in the binding promiscuity of PDZ domains

In molecular recognition, it is often the case that ligand binding is coupled to conformational change in one or both of the binding partners. Two hypotheses describe the limiting cases involved; the first is the induced fit and the second is the conformational selection model. The conformational selection model requires that the protein adopts conformations that are similar to the ligand-bound conformation in the absence of ligand, whilst the induced-fit model predicts that the ligand-bound conformation of the protein is only accessible when the ligand is actually bound. The flexibility of the apo protein clearly plays a major role in these interpretations. For many proteins involved in signaling pathways there is the added complication that they are often promiscuous in that they are capable of binding to different ligand partners. The relationship between protein flexibility and promiscuity is an area of active research and is perhaps best exemplified by the PDZ domain family of proteins. In this study we use molecular dynamics simulations to examine the relationship between flexibility and promiscuity in five PDZ domains: the human Dvl2 (Dishevelled-2) PDZ domain, the human Erbin PDZ domain, the PDZ1 domain of InaD (inactivation no after-potential D protein) from fruit fly, the PDZ7 domain of GRIP1 (glutamate receptor interacting protein 1) from rat and the PDZ2 domain of PTP-BL (protein tyrosine phosphatase) from mouse. We show that despite their high structural similarity, the PDZ binding sites have significantly different dynamics. Importantly, the degree of binding pocket flexibility was found to be closely related to the various characteristics of peptide binding specificity and promiscuity of the five PDZ domains. Our findings suggest that the intrinsic motions of the apo structures play a key role in distinguishing functional properties of different PDZ domains and allow us to make predictions that can be experimentally tested.

Proteins that are capable of binding to many different ligands are said to have broad specificity. This is sometimes also referred to as promiscuity. Whether a protein is promiscuous or not can sometimes be readily explained by the structure of the protein and the ligand in terms of electrostatic and steric effects. Sometimes however, this simple interpretation can struggle to explain the experimentally observed data. A prominent case in point is the PDZ domains. These small protein domains bind to unstructured regions of other proteins and are involved in many signaling pathways. Some PDZ domains appear to be more promiscuous than others, but this has been difficult to explain purely on the basis of the composition of residues in the binding groove. In this work we examine the dynamics and conformational flexibility of five key PDZ domains and demonstrate that despite similar folds, these proteins can exhibit quite different dynamics. Furthermore the difference in the dynamic behavior appears to correlate with the observed promiscuity. Our findings suggest that knowledge of the dynamic behavior of the PDZs can be used to rationalize the extent of expected promiscuity. Such knowledge will be critical for drug design against PDZ domains.

Functional dynamics of proteins revealed by solution NMR

Solution NMR spectroscopy can analyze the dynamics of proteins on a wide range of timescales, from picoseconds to even days, in a site-specific manner, and thus its results are complementary to the detailed but largely static structural information obtained by X-ray crystallography. We review recent progresses in a variety of NMR techniques, including relaxation dispersion and paramagnetic relaxation enhancement (PRE), that permit the observation of the low-populated states, which had been 'invisible' with other techniques. In addition, we review how NMR spectroscopy can be used to elucidate functionally relevant protein dynamics.

Regulation of Antigen-Experienced T Cells: Lessons from the Quintessential Memory Marker CD44

Despite the widespread use of the cell-surface receptor CD44 as a marker for antigen (Ag)-experienced, effector and memory T cells, surprisingly little is known regarding its function on these cells. The best-established function of CD44 is the regulation of cell adhesion and migration. As such, the interactions of CD44, primarily with its major ligand, the extracellular matrix (ECM) component hyaluronic acid (HA), can be crucial for the recruitment and function of effector and memory T cells into/within inflamed tissues. However, little is known about the signaling events following engagement of CD44 on T cells and how cooperative interactions of CD44 with other surface receptors affect T cell responses. Recent evidence suggests that the CD44 signaling pathway(s) may be shared with those of other adhesion receptors, and that these provide contextual signals at different anatomical sites to ensure the correct T cell effector responses. Furthermore, CD44 ligation may augment T cell activation after Ag encounter and promote T cell survival, as well as contribute to regulation of the contraction phase of an immune response and the maintenance of tolerance. Once the memory phase is established, CD44 may have a role in ensuring the functional fitness of memory T cells. Thus, the summation of potential signals after CD44 ligation on T cells highlights that migration and adhesion to the ECM can critically impact the development and homeostasis of memory T cells, and may differentially affect subsets of T cells. These aspects of CD44 biology on T cells and how they might be modulated for translational purposes are discussed.

Retinoic acid induces HL-60 cell differentiation via the upregulation of miR-663

BACKGROUND

Differentiation of the acute myeloid leukemia (AML) cell line HL-60 can be induced by all trans-retinoic acid (ATRA); however, the mechanism regulating this process has not been fully characterized.

METHODS

Using bioinformatics and in vitro experiments, we identified the microRNA gene expression profile of HL-60 cells during ATRA induced granulocytic differentiation.

RESULTS

Six microRNAs were upregulated by ATRA treatment, miR-663, miR-494, miR-145, miR-22, miR-363* and miR-223; and three microRNAs were downregulated, miR-10a, miR-181 and miR-612. Additionally, miR-663 expression was regulated by ATRA. We used a lentivirus (LV) backbone incorporating the spleen focus forming virus (SFFV-F) promoter to drive miR-663 expression, as the CMV (Cytomegalovirus) promoter is ineffective in some lymphocyte cells. Transfection of LV-miR-663 induced significant HL-60 cell differentiation in vitro.

CONCLUSIONS

Our results show miR-663 may play an important role in ATRA induced HL-60 cell differentiation. Lentivirus delivery of miR-663 could potentially be used directly as an anticancer treatment in hematological malignancies.

Combinatorial selection of DNA thioaptamers targeted to the HA binding domain of human CD44

CD44, the primary receptor for hyaluronic acid, plays an important role in tumor growth and metastasis. CD44-hyaluronic acid interactions can be exploited for targeted delivery of anticancer agents specifically to cancer cells. Although various splicing variants of CD44 are expressed on the plasma membrane of cancer cells, the hyaluronic acid binding domain (HABD) is highly conserved among the CD44 splicing variants. Using a novel two-step process, we have identified monothiophosphate-modified aptamers (thioaptamers) that specifically bind to the CD44's HABD with high affinities. Binding affinities of the selected thioaptamers for the HABD were in the range of 180-295 nM, an affinity significantly higher than that of hyaluronic acid (K(d) above the micromolar range). The selected thioaptamers bound to CD44 positive human ovarian cancer cell lines (SKOV3, IGROV, and A2780) but failed to bind the CD44 negative NIH3T3 cell line. Our results indicated that thio substitution at specific positions of the DNA phosphate backbone results in specific and high-affinity binding of thioaptamers to CD44. The selected thioaptamers will be of great interest for further development as a targeting or imaging agent for the delivery of therapeutic payloads for cancer tissues.

Clinical implication and mitotic effect of CD44 cleavage in relation to osteopontin/CD44 interaction and dysregulated cell cycle protein in gastrointestinal stromal tumor

BACKGROUND

CD44 and osteopontin (OPN) are functionally related molecules that, alone or in combination, play miscellaneous biological and pathophysiologic roles. CD44 cleavage, one unique feature of CD44, occurs in human cancers, but its function remains unclear. This study aimed to assess the clinicopathologic significance and mechanism of CD44 cleavage in gastrointestinal stromal tumor (GIST) with respect to OPN and OPN/CD44 interaction.

MATERIALS AND METHODS

CD44 cleavage was evaluated by immunoblotting in 31 primary GIST tumor specimens with paired normal tissues. OPN/CD44 interaction was examined by in situ proximity ligation assay. The associations of CD44 cleavage activity with clinicopathologic parameters, cyclin D1 expression, beta-catenin expression, OPN expression, and OPN/CD44 interaction were analyzed.

RESULTS

CD44 cleavage activity was demonstrated in 87.1% of GIST, in contrast to its absence in normal tissues. Increased CD44 cleavage activity was significantly associated with enhanced mitosis by multivariate analysis, in addition to being related to tumor size, recurrence, high-risk status, and poor survival by univariate analysis. Mitosis was significantly higher in GIST with increased CD44 cleavage activity, which also positively correlated with tumor-specific beta-catenin and cyclin D1 overexpression, indicating a mitotic effect through aberrant cell cycle. Both OPN and OPN/CD44 interactions were significantly associated with CD44 cleavage.

CONCLUSION

Our study demonstrates the clinicopathological significance of CD44 cleavage in GIST. There is a significantly increased mitosis associated with CD44 cleavage in relation to OPN/CD44 interaction and dysregulated cell cycle in GIST.

Two missense mutations in the CD44 gene encode two new antigens of the Indian blood group system

BACKGROUND

Blood samples were referred over a 10-year period from five patients whose serum samples contained antibodies to unidentified high-incidence antigens. Three patients (A, B, C) were of Moroccan origin and their antibodies and red blood cells (RBCs) were mutually compatible, but incompatible with those of the other two patients (D, E), who were of Pakistani origin. The antibodies and RBCs of D and E were mutually compatible, but incompatible with those of Patients A, B, and C. All the antibodies were detected during pregnancy.

STUDY DESIGN AND METHODS

Serologic tests, including the use of enzyme-treated and chemically modified RBCs, suggested a relationship to CD44 (Indian blood group system). The monoclonal antibody immobilization of erythrocyte antigens (MAIEA) assay with monoclonal CD44 antibodies, immunoblotting of RBC membranes, and CD44 gene sequencing were carried out.

RESULTS

Positive reactions in the MAIEA assay confirmed that the patients' antibodies are directed at CD44. Immunoblotting with two of the antibodies gave positive reactions of identical size to monoclonal anti-CD44 and failed to react with the RBCs of a CD44-deficient patient. One of the antibodies reacted with purified CD44. Sequencing of Exons 1 to 5 of CD44 revealed 255C>G in Exon 3 for A, B, and C encoding H85Q and 488C>A in Exon 5 for D and E encoding T163K [corrected]

CONCLUSION

Two novel CD44 antigens of high incidence have been identified: IN3 (INFI) and IN4 (INJA) in the IN (Indian) blood group system. Lack of IN3 and IN4 results from homozygosity for mutations encoding H85Q and T163R in CD44, respectively.

Structures of the Cd44-hyaluronan complex provide insight into a fundamental carbohydrate-protein interaction

Regulation of transient interactions between cells and the ubiquitous matrix glycosaminoglycan hyaluronan is crucial to such fundamental processes as embryonic development and leukocyte homing. Cd44, the primary cell surface receptor for hyaluronan, binds ligand via a lectin-like fold termed the Link module, but only after appropriate functional activation. The molecular details of the Cd44-hyaluronan interaction and hence the structural basis for this activation are unknown. Here we present the first crystal structure of Cd44 complexed with hyaluronan. This reveals that the interaction with hyaluronan is dominated by shape and hydrogen-bonding complementarity and identifies two conformational forms of the receptor that differ in orientation of a crucial hyaluronan-binding residue (Arg45, equivalent to Arg41 in human CD44). Measurements by NMR indicate that the conformational transition can be induced by hyaluronan binding, providing further insight into possible mechanisms for regulation of Cd44.