Molecular isoforms of murine CD44 and evidence that the membrane proximal domain is not critical for hyaluronate recognition

The emerging role of the chondroitin sulfate proteoglycan family in neurodegenerative diseases

Chondroitin sulfate (CS) is a kind of linear polysaccharide that is covalently linked to proteins to form proteoglycans. Chondroitin sulfate proteoglycans (CSPGs) consist of a core protein, with one or more CS chains covalently attached. CSPGs are precisely regulated and they exert a variety of physiological functions by binding to adhesion molecules and growth factors. Widely distributed in the nervous system in human body, CSPGs contribute to the major component of extracellular matrix (ECM), where they play an important role in the development and maturation of the nervous system, as well as in the pathophysiological response to damage to the central nervous system (CNS). While there are more than 30 types of CSPGs, this review covers the roles of the most important ones, including versican, aggrecan, neurocan and NG2 in the pathogenesis of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and multiple sclerosis. The updated reports of the treatment of neurodegenerative diseases are involving CSPGs.

Dendritic cell entry to lymphatic capillaries is orchestrated by CD44 and the hyaluronan glycocalyx

CD44 anchors the hyaluronan glycocalyx on migrating dendritic cells to permit docking to the endothelial receptor LYVE-1, thus orchestrating lymphatic trafficking through modulating glycocalyx density.

DCs play a vital role in immunity by conveying antigens from peripheral tissues to draining lymph nodes, through afferent lymphatic vessels. Critical to the process is initial docking to the lymphatic endothelial receptor LYVE-1 via its ligand hyaluronan on the DC surface. How this relatively weak binding polymer is configured for specific adhesion to LYVE-1, however, is unknown. Here, we show that hyaluronan is anchored and spatially organized into a 400–500 nm dense glycocalyx by the leukocyte receptor CD44. Using gene knockout and by modulating CD44-hyaluronan interactions with monoclonal antibodies in vitro and in a mouse model of oxazolone-induced skin inflammation, we demonstrate that CD44 is required for DC adhesion and transmigration across lymphatic endothelium. In addition, we present evidence that CD44 can dynamically control the density of the hyaluronan glycocalyx, regulating the efficiency of DC trafficking to lymph nodes. Our findings define a previously unrecognized role for CD44 in lymphatic trafficking and highlight the importance of the CD44:HA:LYVE-1 axis in its regulation.

CD44 splice variant (CD44v3) promotes progression of urothelial carcinoma of bladder through Akt/ERK/STAT3 pathways: novel therapeutic approach

PURPOSE

The incidence of Urothelial carcinoma of bladder (UBC) is gradually increasing by changing lifestyle and environment. The development of a tumor has been noted to be accompanied by modifications in the extracellular matrix (ECM) consisting of CD44, hyaluronic acid (HA) and its family members. The importance of CD44 splice variants and HA family members has been studied in UBC.

METHODS

The cohort of study included 50 UBC patients undergoing radical cystectomy and 50 healthy subjects. The molecular expression of CD44 and HA family members was determined. Effect of CD44 variant-specific silencing on downstream signaling in HT1376 cells was investigated. Combinatorial treatment of 4-MU (4-methylumbelliferone) with cisplatin or doxorubicin on chemosensitivity was also explored.

RESULTS

Higher expression of HA, HAS2, and CD44 was observed in Indian UBC patients which also showed the trend with severity of disease. Splice variant assessment of CD44 demonstrated the distinct role of CD44v3 and CD44v6 in bladder cancer progression. shRNA-mediated downregulation of CD44v3 showed an increase effect on cell cycle, apoptosis and multiple downstream signaling cascade including pAkt, pERK and pSTAT3. Furthermore, 4-MU, an HA synthesis inhibitor, observed to complement the effect of Cisplatin or Doxorubicin by enhancing the chemosensitivity of bladder cancer cells.

CONCLUSIONS

Our findings exhibit involvement of CD44 splice variants and HA family members in UBC and significance of 4-MU in enhancing chemosensitivity suggesting their novel therapeutic importance in disease therapeutics.

Impact of structurally modifying hyaluronic acid on CD44 interaction

CD44 is a widely-distributed type I transmembrane glycoprotein that binds hyaluronic acid (HA) in most cell types, including primary tumor cells and cancer-initiating cells and has roles in cell migration, cell-cell, and cell-matrix adhesion. HA-derived conjugates and nanoparticles that target the CD44 receptor on cells have been reported for targeted delivery of therapeutics and imaging agents. Altering crucial interactions of HA with CD44 active sites holds significant importance in modulating targeting ability of hyaluronic acid to other cancer types that do not express the CD44 receptor or minimizing the interaction with CD44+ cells that are not target cells. The approach adopted here was deacetylation of the N-acetyl group and selective sulfation on the C6-OH on the HA polymer, which form critical interactions with the CD44 active site. Major interactions identified by molecular modeling were confirmed to be hydrogen bonding of the C6-OH with Tyr109 and hydrophobic interaction of the N-acetyl group with Tyr46, 83 and Ile 92. Modified HA was synthesized and characterized and its interactions were assessed by in vitro and molecular modeling approaches. In vitro techniques included flow cytometry and fluorescence polarization, while in silico approaches included docking and binding calculations by a MM-PBSA approach. These studies indicated that while both deacetylation and sulfation of HA individually decrease CD44 interaction, both chemical modifications are required to minimize interaction with CD44+ cells. The results of this study represent the first step to effective retargeting of HA-derived NPs for imaging and drug delivery.

Utilization of Glycosaminoglycans/Proteoglycans as Carriers for Targeted Therapy Delivery

The outcome of patients with cancer has improved significantly in the past decade with the incorporation of drugs targeting cell surface adhesive receptors, receptor tyrosine kinases, and modulation of several molecules of extracellular matrices (ECMs), the complex composite of collagens, glycoproteins, proteoglycans, and glycosaminoglycans that dictates tissue architecture. Cancer tissue invasive processes progress by various oncogenic strategies, including interfering with ECM molecules and their interactions with invasive cells. In this review, we describe how the ECM components, proteoglycans and glycosaminoglycans, influence tumor cell signaling. In particular this review describes how the glycosaminoglycan hyaluronan (HA) and its major receptor CD44 impact invasive behavior of tumor cells, and provides useful insight when designing new therapeutic strategies in the treatment of cancer.

Advances and advantages of nanomedicine in the pharmacological targeting of hyaluronan-CD44 interactions and signaling in cancer

Extensive experimental evidence in cell and animal tumor models show that hyaluronan-CD44 interactions are crucial in both malignancy and resistance to cancer therapy. Because of the intimate relationship between the hyaluronan-CD44 system and tumor cell survival and growth, it is an increasingly investigated area for applications to anticancer chemotherapeutics. Interference with the hyaluronan-CD44 interaction by targeting drugs to CD44, targeting drugs to the hyaluronan matrix, or interfering with hyaluronan matrix/tumor cell-associated CD44 interactions is a viable strategy for cancer treatment. Many of these methods can decrease tumor burden in animal models but have yet to show significant clinical utility. Recent advances in nanomedicine have offered new valuable tools for cancer detection, prevention, and treatment. The enhanced permeability and retention effect has served as key rationale for using nanoparticles to treat solid tumors. However, the targeted and uniform delivery of these particles to all regions of tumors in sufficient quantities requires optimization. An ideal nanocarrier should be equipped with selective ligands that are highly or exclusively expressed on target cells and thus endow the carriers with specific targeting capabilities. In this review, we describe how the hyaluronan-CD44 system may provide such an alternative in tumors expressing specific CD44 variants.

Apparent diffusion coefficients of normal uterus in premenopausal women with 3.0-T magnetic resonance imaging

PURPOSE

To investigate the apparent diffusion coefficients (ADCs) of the normal uterine zonal structures (myometrium, endometrium, and junctional zone) during different phases of the menstrual cycle among premenopausal women with different age groups.

MATERIALS AND METHODS

Magnetic resonance (MR) images of 67 healthy women were obtained during the midproliferative and midsecretory phases. They were further divided into 3 age groups: group A (age range, 20-29 years); group B (age range, 30-39 years), and group C (mean age, 44.62 years; age range, 40-49 years). All the women underwent 3.0-T MR scanning twice. The ADC values were compared among each uterine zonal structure and 3 age groups and were calculated between the midproliferative and midsecretory phase.

RESULTS

The ADC values among each uterine zonal structures were significantly different from one another (P < 0.001). The ADC values for endometrium in women in their 30s were higher than those in their 20s and in their 30s during the midproliferative and midsecretory phases (P < 0.05), and the ADC values for endometrium in each age groups were lower during the midproliferative phase than those during the midsecretory phase (P < 0.05), but there were no statistical differences in the myometrium and the junctional zone between the 2 phases or among age groups (P > 0.05).

CONCLUSION

These preliminary results suggested that the zone, age, and phase of the menstrual cycle in premenopausal women should be considered when interpreting the ADC values of uterine structures (especially for endometrium).

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.

Hyaluronan-CD44 interactions as potential targets for cancer therapy

It is becoming increasingly clear that signals generated in tumor microenvironments are crucial to tumor cell behavior, such as survival, progression and metastasis. The establishment of these malignant behaviors requires that tumor cells acquire novel adhesion and migration properties to detach from their original sites and to localize to distant organs. CD44, an adhesion/homing molecule, is a major receptor for the glycosaminoglycan hyaluronan, which is one of the major components of the tumor extracellular matrix. CD44, a multistructural and multifunctional molecule, detects changes in extracellular matrix components, and thus is well positioned to provide appropriate responses to changes in the microenvironment, i.e. engagement in cell-cell and cell-extracellular matrix interactions, cell trafficking, lymph node homing and the presentation of growth factors/cytokines/chemokines to co-ordinate signaling events that enable the cell responses that change in the tissue environment. The potential involvement of CD44 variants (CD44v), especially CD44v4-v7 and CD44v6-v9, in tumor progression has been confirmed for many tumor types in numerous clinical studies. The downregulation of the standard CD44 isoform (CD44s) in colon cancer is postulated to result in increased tumorigenicity. CD44v-specific functions could be caused by their higher binding affinity than CD44s for hyaluronan. Alternatively, CD44v-specific functions could be caused by differences in associating molecules, which may bind selectively to the CD44v exon. This minireview summarizes how the interaction between hyaluronan and CD44v can serve as a potential target for cancer therapy, in particular how silencing CD44v can target multiple metastatic tumors.

Stromal hyaluronan interaction with epithelial CD44 variants promotes prostate cancer invasiveness by augmenting expression and function of hepatocyte growth factor and androgen receptor

The main aim of our study is to determine the significance of the stromal microenvironment in the malignant behavior of prostate cancer. The stroma-derived growth factors/cytokines and hyaluronan act in autocrine/paracrine ways with their receptors, including receptor-tyrosine kinases and CD44 variants (CD44v), to potentiate and support tumor epithelial cell survival. Overexpression of hyaluronan, CD44v9 variants, and stroma-derived growth factors/cytokines are specific features in many cancers, including prostate cancer. Androgen/androgen receptor interaction has a critical role in regulating prostate cancer growth. Our previous study showed that 1) that increased synthesis of hyaluronan in normal epithelial cells promotes expression of CD44 variants; 2) hyaluronan interaction with CD44v6-v9 promotes activation of receptor-tyrosine kinase, which stimulates phosphatidylinositol 3-kinase-induced cell survival pathways; and 3) CD44v6/short hairpin RNA reduces colon tumor growth in vivo (Misra, S., Hascall, V. C., De Giovanni, C., Markwald, R. R., and Ghatak, S. (2009) J. Biol. Chem. 284, 12432–12446). Our results now show that hepatocyte growth factor synthesized by myofibroblasts associated with prostate cancer cells induces activation of HGF-receptor/cMet and stimulates hyaluronan/CD44v9 signaling. This, in turn, stabilizes the androgen receptor functions in prostate cancer cells. The stroma-derived HGF induces a lipid raft-associated signaling complex that contains CD44v9, cMet/phosphatidylinositol 3-kinase, HSP90 and androgen receptor. CD44v9/short hairpin RNA reverses the assembly of these components in the complex and inhibits androgen receptor function. Our results provide new insight into the hyaluronan/CD44v9-regulated androgen receptor function and the consequent malignant activities in prostate cancer cells. The present study describes a physiologically relevant in vitro model for studying the molecular mechanisms by which stroma-derived HGF and hyaluronan influence androgen receptor and CD44 functions in the secretory epithelia during prostate carcinogenesis.

CD44 in cancer progression: adhesion, migration and growth regulation

It is well established that the large array of functions that a tumour cell has to fulfil to settle as a metastasis in a distant organ requires cooperative activities between the tumour and the surrounding tissue and that several classes of molecules are involved, such as cell-cell and cell-matrix adhesion molecules and matrix degrading enzymes, to name only a few. Furthermore, metastasis formation requires concerted activities between tumour cells and surrounding cells as well as matrix elements and possibly concerted activities between individual molecules of the tumour cell itself. Adhesion molecules have originally been thought to be essential for the formation of multicellular organisms and to tether cells to the extracellular matrix or to neighbouring cells. CD44 transmembrane glycoproteins belong to the families of adhesion molecules and have originally been described to mediate lymphocyte homing to peripheral lymphoid tissues. It was soon recognized that the molecules, under selective conditions, may suffice to initiate metastatic spread of tumour cells. The question remained as to how a single adhesion molecule can fulfil that task. This review outlines that adhesion is by no means a passive task. Rather, ligand binding, as exemplified for CD44 and other similar adhesion molecules, initiates a cascade of events that can be started by adherence to the extracellular matrix. This leads to activation of the molecule itself, binding to additional ligands, such as growth factors and matrix degrading enzymes, complex formation with additional transmembrane molecules and association with cytoskeletal elements and signal transducing molecules. Thus, through the interplay of CD44 with its ligands and associating molecules CD44 modulates adhesiveness, motility, matrix degradation, proliferation and cell survival, features that together may well allow a tumour cell to proceed through all steps of the metastatic cascade.

Fibroblast invasive migration into fibronectin/fibrin gels requires a previously uncharacterized dermatan sulfate-CD44 proteoglycan

After tissue injury, fibroblast migration from the peri-wound collagenous stroma into the fibrin-laden wound is critical for granulation tissue formation and subsequent healing. Recently we found that fibroblast transmigration from a collagen matrix into a fibrin matrix required the presence of fibronectin. Several integrins-alpha 4 beta 1, alpha 5 beta 1, and alpha v beta 3-with known fibronectin binding affinity were necessary for this invasive migration. Here we examined another family of cell surface receptors: the proteoglycans. We found that dermatan sulfate was required for fibroblast migration into a fibronectin/fibrin gel. This conclusion was based on beta-xyloside inhibition of glycanation and specific glycosaminoglycan degradation. CD44, a cell surface receptor known to bind hyaluronan, not infrequently exists as a proteoglycan, decorated with various glycosaminoglycan chains including heparan sulfate and chondroitin sulfate, and as such can bind fibronectin. We found that CD44H, the non-spliced isoform of CD44, was necessary for fibroblast invasion into fibronectin/fibrin gels. Resting fibroblasts expressed mostly nonglycanated CD44H core protein, which became glycanated with chondroitin sulfate and dermatan sulfate, but not heparan sulfate, after a 24 h incubation with platelet-derived growth factor, the stimulus used in the migration assay. These results demonstrate that dermatan sulfate-CD44H proteoglycan is essential for fibroblast migration into fibrin clots and that platelet-derived growth factor, the stimulus for migration, induces the production of chondroitin-sulfate- and dermatan-sulfate-glycanated CD44H.

TSG-6 modulates the interaction between hyaluronan and cell surface CD44

Interactions between CD44 and hyaluronan are implicated in the primary adhesion of lymphocytes to endothelium at inflammatory locations. Here we show that preincubation of hyaluronan with full-length recombinant TSG-6 or its Link module domain (Link_TSG6) enhances or induces the binding of hyaluronan to cell surface CD44 on constitutive and inducible cell backgrounds, respectively. These effects are blocked by CD44-specific antibodies and are absent in CD44-negative cells. Enhancement of CD44-mediated interactions of lymphoid cells with hyaluronan by TSG-6 proteins was seen under conditions of flow at shear forces that occur in post-capillary venules. Increases in the number of rolling cells were observed on substrates comprising TSG-6-hyaluronan complexes as compared with a substrate containing hyaluronan alone. In ligand competition experiments, cell surface-bound TSG-6-hyaluronan complexes were more potent than hyaluronan alone in inhibiting cell adhesion to immobilized hyaluronan. Link_TSG6 mutants with impaired hyaluronan binding function had a reduced ability to modulate ligand binding by cell surface CD44. However, some mutants that exhibited close to wild-type hyaluronan binding were found to have either reduced or increased activity, suggesting that some amino acid residues outside of the hyaluronan binding site might be involved in protein self-association, potentially leading to the formation of cross-linked hyaluronan fibers. In turn, cross-linked hyaluronan could increase the binding avidity of CD44 by inducing receptor clustering. The ability of TSG-6 to modulate the interaction of hyaluronan with CD44 has important implications for CD44-mediated cell activity at sites of inflammation, where TSG-6 is expressed.

Role of the extracellular and cytoplasmic domains of CD44 in the rolling interaction of lymphoid cells with hyaluronan under physiologic flow

CD44 can function as an adhesion receptor that mediates leukocyte rolling on hyaluronan (HA). To study the contributions of different domains of the standard isoform of CD44 to cell rolling, a CD44-negative mouse T lymphoma AKR1 was transfected with wild type (WT) or mutated cDNA constructs. A parallel flow chamber was used to study the rolling behavior of CD44 transfectants on immobilized HA. For CD44WT transfectants, the fraction of cells that rolled and the rolling velocity was inversely proportional to the amount of cell surface CD44. When the cytoplasmic domain distal to Gly(305) or sequences that serve as binding sites for cytoskeletal linker proteins, were deleted or replaced with foreign sequences, no significant changes in the rolling behavior of mutant cells, compared with the transfectant expressing CD44WT, were observed. Transfectants lacking 64 amino acids of the cytoplasmic tail distal to Cys(295) adhered to HA but showed enhanced rolling at low shear forces. When 83 amino acids from the "non-conserved" membrane-proximal region of the CD44 extracellular domain were deleted, cells adhered firmly to the HA substrate and did not roll at any fluid shear force tested. Unlike wild type cells that exhibited a nearly homogeneous distribution of CD44 on a smooth cell surface, cells expressing the non-conserved region deletion mutant accumulated CD44 in membrane protrusions. Disruption of the actin cytoskeleton with cytochalasin B precluded the formation of membrane protrusions, however, treated cells still adhered firmly to HA and did not roll. We conclude that interaction between the cytoplasmic domain of CD44 and the cytoskeleton is not required for cell rolling on immobilized ligand. The strong effect of deletion of the non-conserved region of the extracellular domain argues for a critical role of this region in CD44-dependent rolling and adhesion interactions with HA under flow.

Identification of sequence motifs responsible for the adhesive interaction between exon v10-containing CD44 isoforms

Previous studies have demonstrated that CD44 isoforms containing the alternatively spliced exon v10 promote cell-cell adhesion via a mechanism that involves the recognition of chondroitin sulfate side chains presented on the surface of interacting cells in association with other CD44 molecules. Sequence analysis revealed the presence within exon v10 of two motifs that may be relevant to this interaction, a B[X(7)]B motif that may contribute to the recognition and binding of chondroitin sulfate and a serine-glycine motif that may serve as a site of chondroitin sulfate attachment. To determine whether either of these two motifs explain the unique adhesive activity of exon v10-containing CD44 isoforms, each was targeted by site-directed mutagenesis, and the adhesive activity of the resultant mutants was determined using a quantitative cell-cell binding assay. The data obtained demonstrate conclusively that it is the exon v10-encoded B[X(7)]B motif that is solely responsible for the enhanced adhesive activity of exon v10-containing CD44 isoforms.

Hyaluronan binding properties of a CD44 chimera containing the link module of TSG-6

CD44, a cell-surface receptor for the extracellular matrix glycosaminoglycan hyaluronan, can mediate leukocyte rolling on hyaluronan substrates and has been implicated in leukocyte migration to sites of inflammation. CD44-mediated binding to hyaluronan is of low affinity, and effective cell/matrix interaction depends on multiple interactions with the multivalent ligand. We replaced the Link module of CD44 with the homologous region of TSG-6, a hyaluronan-binding protein secreted in response to inflammation whose Link module has a higher affinity for ligand. Monoclonal antibodies raised against the CD44/TSG-6 chimera recognized recombinant human TSG-6 and native mouse TSG-6 and blocked hyaluronan binding to these proteins. Cells expressing the CD44/TSG-6 molecule bound hyaluronan with higher avidity than cells expressing CD44. This resulted in changes in the hyaluronan binding properties characteristic of cells expressing CD44 such as requirements for threshold levels of receptor expression and for hyaluronan of high molecular mass. In parallel plate flow assays used to model leukocyte rolling, cells expressing CD44/TSG-6 failed to roll on hyaluronan. Instead, they stuck and remained "tethered" to the substrate under fluid flow. This result argues that the low affinity of CD44 for its ligand is important for rolling, an early phase of leukocyte extravasation from the blood.

Homing-associated cell adhesion molecule (H-CAM/CD44) on human CD34+ hematopoietic progenitor cells

Human CD34+ hematopoietic progenitor cells (HPCs) express CD44 and can directly adhere to hyaluronate (HA) via CD44. Furthermore, CD44 may also be involved in the regulation of CD34+ HPC proliferation and development. The expression of CD44 molecules on CD34+ hematopoietic progenitor cells is significantly lower on bone marrow (BM) CD34+ cells compared with circulating CD34+ cells in cord blood and peripheral blood. Myeloid and erythroid progenitor cells are found predominantly in CD34+ CD44+ cell fractions. More interestingly, CD34+ CD44- cells expressing B-lymphocyte-associated CD10 and CD19 would represent unique B-lymphocyte committed precursors in the BM, which might undergo apoptotic cell death in the early steps of B-cell differentiation.

CD44v10 in hematopoiesis and stem cell mobilization

Transplantation of hematopoietic progenitor cells provides in many instances of malignant tumors an ultimate chance of curative therapy, whereby the transfer of peripheral blood stem cells (PBSC) may even be advantageous as compared to bone marrow cells. Yet, the transfer of PBSC requires mobilization of stem cells into the periphery, which is mostly achieved via hematopoietic growth factors like G-CSF. Although G-CSF has been found to efficiently mobilize stem cells in most instances, some patients do not or insufficiently respond to G-CSF treatment In addition, G-CSF treatment may by accompanied by maturation of the most primitive progenitors and this may have an impact on stem cell homing and recovery of hemopoiesis. Therefore, additional approaches for stem cell mobilization have been searched for, in particular mobilization via a blockade of an adhesion molecule expressed by CD34-positive cells, like VLA-4 (CD49d) and the hematopoietic isoform of CD44 (CD44s). We recently described that in the mouse one of the CD44 variant isoforms, CD44v10, is expressed on a subpopulation of bone marrow cells, whereas a CD44v10 receptor-globulin only binds to stromal elements. These features appeared promising for anti-CD44v10 as a means of stem cell mobilization. Indeed, treatment with anti-CD44v10 revealed promising results concerning the recovery of multilineage colony forming units in the spleen and the peripheral blood. We here summarize features of expression and function of CD44 in hematopoiesis an provide further evidence for anti-CD44v10 as a means to mobilize hematopoietic progenitor cells.

Upregulation of CD44 expression in the retina during the rds degeneration

In adult mouse retinas the standard form of the cell surface adhesion/receptor molecule CD44 is localized to Müller cell apical microvilli. In the rds (retinal degeneration slow) mouse, however, CD44 immunolabel is increased and distributed throughout the retina by 3 months postnatal. At present, it is unclear if this labeling pattern is due to the increased expression of standard CD44, the expression of variant CD44 isoforms, or an unmasking of CD44 antigenic sites. To further characterize this response, we have studied the expression of CD44 mRNA and protein in rds retinas of different ages. RT-PCR analysis demonstrated one product which represented the message for standard CD44 in adult BALB/c mouse retina and in all ages of rds retinas studied. Upon Southern blotting, this major product was detected along with two minor bands of larger size in all samples. Northern blot analysis demonstrated a major transcript of approximately 4.0 kb and a minor one of 3.0 kb in all BALB/c and rds retinas. By 3 months postnatal in rds retinas, the expression of CD44 message was increased by at least two-fold. Western blot analysis demonstrated the presence of only the standard form of CD44 protein in all BALB/c and rds retinas. An increased amount of this standard CD44 protein was observed in 2, 3, and 6 month rds retinas. Thus, this study demonstrates that the inherited retinal degeneration exhibited by the rds mouse does not cause an altered expression of retinal CD44 isoforms, but does lead to an upregulation in the expression of mRNA and protein for standard CD44.

The CD44 receptor of lymphoma cells: structure-function relationships and mechanism of activation

Migration of some tumor cells, and their lodgment in target organs, is dependent on the activation of cell surface CD44 receptor, usually detected by its ability to bind hyaluronic acid (HA) or other ligands. In an attempt to reveal the mechanism of tumor cell CD44 activation, we compared the physical and chemical properties of CD44 in nonactivated LB cell lymphoma with those in phorbol 12-myristate 13-acetate (PMA)-activated LB cells and of an LB cell subline (designated HA9) expressing constitutively-active CD44. In contrast to nonactivated LB cells, PMA-activated LB cells and HA9 cells displayed a CD44-dependent ability to bind HA. The ability of activated cell CD44 to bind HA was not dependent on microfilament or microtubule integrity or on changes in CD44 mobility on the membrane plane, indicating that the CD44 activation status is not associated with cytoskeleton function. Aside from the increased expression of CD44 on the surface of PMA-activated LB cells and HA9 cells, qualitative differences between the CD44 of nonactivated and activated LB cells were also detected: the CD44 of the activated lymphoma was (i) larger in molecular size, (ii) displayed a broader CD44 isoform repertoire, including a CD44 variant that binds HA, and (iii) its glycoprotein contained less sialic acid. Indeed, after removal of sialic acid from their cell surface by neuraminidase, LB cells acquired the ability to bind HA. However, a reduced dose of neuraminidase did not confer HA binding on LB cells, unless they were also activated by a low concentration of PMA, which by itself was ineffective. Similarly, under suboptimal conditions, a synergistic effect was obtained with tunicamycin and PMA: each one alone was ineffective but in combination they induced the acquisition of HA binding by the lymphoma cells, while their CD44 expression was not enhanced. Unveiling of the activation mechanism of CD44, by exposing the cells to PMA stimulation or to deglycosylation, is not only academically important, but it also has practical implications, as activated CD44 may be involved in the support of tumor progression.

Hyaluronic acid as drug delivery for sodium butyrate: improvement of the anti-proliferative activity on a breast-cancer cell line

The potential clinical utility of sodium butyrate, a natural compound known to inhibit tumor-cell growth, is hampered by the difficulty of achieving effective in-vivo concentrations. The short half-life (about 5 minutes) of sodium butyrate results in rapid metabolism and excretion. To increase the availability of sodium butyrate over a longer period of time, we co-valently linked it to hyaluronic acid (a component of the extracellular matrix). Its major advantages as a drug carrier consist in its high biocompatibility and its ability to bind CD44, a specific membrane receptor frequently over-expressed on the tumor-cell surface. The degree of substitution of hyaluronic acid with butyrate residues ranged from d.s.=0.10 to d.s.=2.24 (1.8-28.4% w/w). The biological activity of hyaluronic-acid-butyric-ester derivatives was evaluated in terms of the inhibition of the growth of the MCF7 cell line and compared with that of sodium butyrate. After 6 days of treatment, we observed a progressive improvement of the anti-proliferative activity up to d.s.=0.20; thereafter, the anti-proliferative effect of the ester derivatives decreased. Fluorescence microscopy showed that after 2 hr of treatment fluorescein-labelled compounds appeared to be almost completely internalized into MCF7 cells, expressing CD44 standard and variant isoforms. These findings indicate that hyaluronic acid could offer an important advantage in drug delivery, in addition to its biocompatibility: the ability to bind to CD44, which are known to be frequently over-expressed on the tumor-cell surface.

TNFalpha and IL-4 regulation of hyaluronan binding to monocyte CD44 involves posttranslational modification of CD44

Our previous studies have identified TNFalpha as a positive regulator and IL-4 as a negative regulator of human monocyte CD44-HA binding. In order to determine the mechanisms of IL-4- and TNFalpha-mediated regulation of monocyte HA binding, we measured HA binding and CD44 expression on peripheral blood monocytes following monocyte treatment with TNFalpha or IL-4, as well as following monocyte treatment with inhibitors of protein synthesis, N- and O-linked glycosylation, and chondroitin sulfation. IL-4 decreased CD44-HA binding on monocytes initially treated with TNFalpha. Similarly, pretreatment of monocytes with IL-4 prevented subsequent TNFalpha-mediated HA binding. Cycloheximide (protein synthesis inhibitor), tunicamycin (N-linked glycosylation inhibitor), and beta-d-xyloside (chondroitin sulfation inhibitor) all inhibited IL-4-mediated downregulation of TNFalpha-induced monocyte HA binding. Western blot analysis of CD44 from TNFalpha-treated monocytes revealed a 5-10 Mr decrease in the standard isoform of CD44. In contrast, IL-4 treatment of monocytes inhibited CD44-HA binding and reversed the 5- to 10-kDa decrease in monocyte CD44 Mr. Finally, studies with F10.44.2, a CD44 mab that enhances CD44-HA binding, indicated that IL-4 treatment of monocytes not only diminished constitutive HA binding, but also diminished CD44 mab-induced HA binding. Taken together, these data suggested that IL-4-mediated inhibition of TNFalpha-induced monocyte HA binding was dependent not only on protein synthesis, but also on N-linked glycosylation and chondroitin-sulfate modification of either CD44 or, alternatively, another monocyte protein(s) that may regulate the ability of CD44 to bind HA.

Growth and differentiation regulate CD44 expression on human keratinocytes

Several members of the CD44 family of hyaluronan receptors are expressed on keratinocytes. To identify factors that might be important in regulating CD44 expression, we studied CD44 expression on keratinocytes growing in vitro under a variety of conditions and on cells isolated directly from epidermis. Using Western immunoblots and metabolic labeling, we showed that the pattern of CD44 proteins expressed by keratinocytes was strongly influenced by growth and differentiation. Many protein forms of CD44 are expressed on proliferating keratinocytes in preconfluent cultures, whereas only a few forms are expressed on differentiated cells and in confluent cultures. In preconfluent monolayers, at least four splice variants were identified, including epican, CD44H, CD44E, and a 180-kDa variant. In differentiated cells or in confluent cultures, by contrast, only epican and the 180-kDa protein variant were found. Synthesis of all variants is strongly downregulated when keratinocytes become confluent or when they differentiate. Epican is the predominant form of CD44 on keratinocytes under all conditions and is expressed as a heparan, chondroitin, or keratan sulfate proteoglycan. Preconfluent basal keratinocytes, but not confluent or differentiated keratinocytes, also express chondroitin sulfate proteoglycan forms of CD44E and of the 180-kDa core protein. The modal size of the epican expressed on differentiated keratinocytes is smaller than the size of the epican expressed on basal keratinocytes. Thus, cell confluence and differentiation regulate several aspects of CD44 expression on keratinocytes, suggesting nuances in function for the different protein forms.

Differences in the migration capacity of primary human colon carcinoma cells (SW480) and their lymph node metastatic derivatives (SW620)

Two human cell lines, one established from a colon carcinoma (SW480) and the other from its lymph node metastasis (SW620), were compared with respect to their migration capacity employing a three-dimensional collagen matrix and time-lapse video recording. Non-motile cells were characterized by a round shape, whereas motile cells appeared in an elongated form with pseudopodia. The primary tumor cells showed a higher spontaneous locomoting activity than the cells from the metastasis. Using single cell analysis, the distance migrated within 15 h was slightly increased in the presence of hyaluronic acid (HA) in both cell lines. An investigation of the amount of CD44 on the cell surface using the anti-CD44 antibody Hermes-1 showed only minor concentrations of this glycoprotein on cells from the metastasis, whereas a much higher amount was found on cells derived from the primary tumor. The distribution of CD44 on the cell surfaces of HA-treated and untreated cells did not differ as shown by confocal laser scanning microscopy in SW480. The results indicate a restricted influence of HA on migration in the two cell lines.

CD44 isoform expression follows two alternative splicing pathways in breast tissue

The repertoire of distinct CD44 protein isoforms is generated by means of alternative pre-mRNA splicing of 10 variable exons located in the central region of the CD44 gene. We have used human breast ductal carcinoma as a model to identify two alternative splicing pathways of the CD44 pre-mRNA variable region that account for the generation of all of the CD44 isoforms described in breast tissue. An alternative splicing pathway that reflects inclusion of variable exons in a gradual 3'-to-5' fashion is evidenced in breast ductal carcinoma and its lymph node metastases. This pathway is compatible with a mechanism that generates the standard form of CD44 (devoid of variable exons) and is distinguishable from an alternative splicing pathway that involves exclusively variant exon 3 and is observable in both normal and carcinoma breast tissue. We show that both pathways are detectable in the same cell type in the breast and provide a speculative model by which these splicing routes could take place.

Differential Effects of Chondroitin Sulfates A and B on Monocyte and B-Cell Activation: Evidence for B-Cell Activation Via a CD44-Dependent Pathway

At inflammatory sites, proteoglycans are both secreted by activated mononuclear leukocytes and released as a consequence of extracellular matrix degradation. Chondroitin 4-sulfate proteoglycans constitute the predominant ones produced by activated human monocytes/macrophages. In this study, we show that two chondroitin 4-sulfate forms, CSA and CSB, can activate distinct peripheral blood mononuclear cell types. Whereas CSA activates monocytes (to secrete monokines), CSB activates B-cells (to proliferate). In contrast, the chondroitin 6-sulfate CSC and heparin do not exert these functional effects. We further show that CD44 monoclonal antibodies block CSB-induced B-cell proliferation. These findings point to glycosaminoglycans, and specifically chondroitin 4-sulfates, as a novel class of immunological mediators at inflammatory sites. Furthermore, the data link CD44 to B-cell activation, paralleling the established roles of CD44 in T-cell and monocyte activation.

Hyaluronan-dependent cell migration can be blocked by a CD44 cytoplasmic domain peptide containing a phosphoserine at position 325

CD44 is the principle transmembrane receptor for the extracellular matrix glycosaminoglycan hyaluronan. This receptor:ligand interaction plays an essential role in a number of physiological events including tumour progression, lymphocyte homing into inflammatory sites and tissue morphogenesis during development. In previous studies we have shown that serine phosphorylation is a critical control mechanism for CD44-dependent cell migration. Here we have investigated the target phosphorylation residues by mutating them individually or in combination. These studies demonstrate that Ser325 is the principle CD44 phosphorylation site and that mutation of this residue blocks CD44-mediated cell migration but not hyaluronan binding. In addition, we show that an upstream Ser323 residue is required as part of the kinase consensus site. To further characterize the role of CD44 phosphorylation, phosphorylated and non-phosphorylated peptides spanning the Ser325 region were synthesised and linked to a 16 amino acid Penetratin sequence to mediate efficient plasma membrane translocation. Peptides containing a phosphoserine at residue 325 are efficient blockers of CD44-mediated cell migration but do not reduce CD44 expression or its ability to bind hyaluronan. These data strongly argue that CD44 adhesion and migration are regulated by distinct mechanisms and that migration requires the specific interaction of intracellular component(s) with phosphorylated CD44 receptors.

Ezrin/radixin/moesin (ERM) proteins bind to a positively charged amino acid cluster in the juxta-membrane cytoplasmic domain of CD44, CD43, and ICAM-2

CD44 has been identified as a membrane-binding partner for ezrin/radixin/moesin (ERM) proteins, plasma membrane/actin filament cross-linkers. ERM proteins, however, are not necessarily colocalized with CD44 in tissues, but with CD43 and ICAM-2 in some types of cells. We found that glutathione-S-transferase fusion proteins with the cytoplasmic domain of CD43 and ICAM-2, as well as CD44, bound to moesin in vitro. The regions responsible for the in vitro binding of CD43 and CD44 to moesin were narrowed down to their juxta-membrane 20-30-amino acid sequences in the cytoplasmic domain. These sequences and the cytoplasmic domain of ICAM-2 (28 amino acids) were all characterized by the positively charged amino acid clusters. When E-cadherin chimeric molecules bearing these positively charged amino acid clusters of CD44, CD43, or ICAM-2 were expressed in mouse L fibroblasts, they were co-concentrated with ERM proteins at microvilli, whereas those lacking these clusters were diffusely distributed on the cell surface. The specific binding of ERM proteins to the juxta-membrane positively charged amino acid clusters of CD44, CD43, and ICAM-2 was confirmed by immunoprecipitation and site-directed mutagenesis. From these findings, we conclude that ERM proteins bind to integral membrane proteins bearing a positively charged amino acid cluster in their juxta-membrane cytoplasmic domain.

Glycosylation provides both stimulatory and inhibitory effects on cell surface and soluble CD44 binding to hyaluronan

Glycosylation has been implicated in the regulation of CD44-mediated cell binding of hyaluronan (HA). However, neither the relative contribution of N- and O-linked glycans nor the oligosaccharide structures that alter CD44 affinity for HA have been elucidated. To determine the effect of selective alteration of CD44 oligosaccharide composition on the affinity of CD44 for HA, we developed a novel strategy based on the use of affinity capillary electrophoresis (ACE). Soluble recombinant CD44-immunoglobulin fusion proteins were overproduced in the mutant CHO cell line ldl-D, which has reversible defects in both N- and O-linked oligosaccharide synthesis. Using this cell line, a panel of recombinant glycosidases, and metabolic glycosidase inhibitors, CD44 glycoforms with defined oligosaccharide structures were generated and tested for HA affinity by ACE. Because ldl-D cells express endogenous cell surface CD44, the effect of any given glycosylation change on the ability of cell surface and soluble CD44 to bind HA could be compared. Four distinct oligosaccharide structures were found to effect CD44-mediated HA binding: (a) the terminal alpha2,3-linked sialic acid on N-linked oligosaccharides inhibited binding; (b) the first N-linked N-acetylglucosamine residue enhanced binding; (c) O-linked glycans on N-deglycosylated CD44 enhanced binding; and (d) N-acetylgalactosamine incorporation into non-N-linked glycans augmented HA binding by cell surface CD44. The first three structures induced up to a 30-fold alteration in the intrinsic CD44 affinity for HA (Kd = 5 to >150 microM). The fourth augmented CD44-mediated cellular HA avidity without changing the intrinsic HA affinity of soluble CD44.

CD44 isoform expression in periodontal tissues: cell-type specific regulation of alternative splicing

CD44 functions as a receptor for various extracellular matrices and plays crucial roles in homotypic and heterotypic cell-cell interactions. Recently, the molecular structure of CD44 has been extensively analyzed and multiple isoforms produced by alternative splicing of messenger RNA have been identified. In this study, we examined the expression of CD44 isoforms on different cell types isolated from periodontal tissue. In order to examine tissue differences in CD44 isoform expression, we established in vitro cell culture of human gingival fibroblasts (HGF), human periodontal ligament cells (HPDL) and human gingival epithelial cells (HGEC). These cells all expressed CD44 protein and messenger RNA. However, immunoprecipitation and Northern blot analysis revealed that HGEC expressed larger CD44 isoforms than HGF and HPDL. Reverse transcription-polymerase chain reaction with primers flanking the insertion site of alternatively spliced exons was used to study details of the heterogeneity. All cells examined expressed a major band in the absence of alternatively spliced exons and additional larger bands. In particular, HGEC contained more abundant high molecular mass species. In vitro stimulation by IL-1 beta, TNF alpha or phorbol 12-myristate 13-acetate induced an increase in total CD44 messenger RNA in HGF but not change in overall patterns of CD44 isoform expression. However, the isoform expression of HGEC was sensitive to cell density. The amount of larger isoform was decreased by culturing cells beyond confluence. These findings suggest that CD44 isoform expression is cell type-specifically regulated in periodontium and altered according to growth phase of HGEC.

Hyaluronan is a prerequisite for ductal branching morphogenesis

Hyaluronan, a macromolecular carbohydrate polymer of the extracellular matrix is prominent early in embryogenesis, coinciding with rapid tissue growth. CD44, the predominant receptor for hyaluronan on vertebrate cells, is a variably expressed transmembrane glycoprotein. Mouse anterior prostate glands obtained at various postnatal time points were examined for the expression of hyaluronan and CD44. Reverse transcriptase polymerase chain reaction analysis was used to map the temporal regulation of specific CD44 variant isoforms. In each age group, hyaluronan was localized exclusively in the stromal matrix. Hyaluronan was greatly reduced in the later ages and was entirely absent around the developmentally quiescent proximal regions of the ducts. Early in prostate development, CD44 was prominent in the mesenchyme. However, in the later phases, CD44 expression became associated with membranes of epithelial cells. The role of hyaluronan-CD44 interactions in ductal branching morphogenesis was studied by serum-free organ culture of mouse anterior prostate. In the presence of optimal levels of testosterone, the organs underwent ductal branching morphogenesis. Treatment with either neutralizing anti-CD44 antibodies, hyaluronan hexasaccharides or the enzyme hyaluronidase inhibited androgen-stimulated ductal branching morphogenesis. These results are suggestive of the significant role played by hyaluronan-CD44 interactions in mediating androgen-induced prostatic growth and morphogenesis.

Influence of hyaluronic acid or phorbol 12-myristate 13-acetate on the migration capacity of a murine lymphoma cell line (Eb) and its metastatic variant (ESb)

The in vitro migration of two murine T cell lymphoma cell lines (Eb and ESb) was studied employing a three-dimensional collagen matrix and time-lapse video recording. In the highly metastatic cell line ESb, which had a low spontaneous locomoting activity, migration could clearly be stimulated by hyaluronic acid (HA) whereas only a small increase was found after incubation with phorbol myristate acetate (PMA). The observed stimulation could be attributed to an increase in recruitment of locomoting cells and not to changes in migration parameters of motile individual cells such as percentage of time locomoting, velocity or distance migrated. Incubation of the low metastatic cell line Eb with HA led to a decrease in migration but blocking of CD44, the principle ligand for HA, by preincubation with an anti-CD44 mAb (KM114), followed by HA exposure increased the locomoting activity significantly. The effect was based on both an increase in recruitment as well as in all migration parameters regarding motile individual Eb cells.

The importance of cellular environment to function of the CD44 matrix receptor

Much has been learned recently by experimental manipulation of the structure of CD44 and assessment of the resulting functions. However, even greater structural variation is naturally introduced by CD44-bearing cells. A structural model is now available for the portion of CD44 that recognizes hyaluronan, but it is clear that all domains of the molecule influence CD44 functions.

Growth as a solid tumor or reduced glucose concentrations in culture reversibly induce CD44-mediated hyaluronan recognition by Chinese hamster ovary cells

The density, molecular isoform, and posttranslational modifications of CD44 can markedly influence growth and metastatic behavior of tumors. Many CD44 functions, including some involving tumors, have been attributed to its ability to recognize hyaluronan (HA). However, only certain CD44-bearing cells bind soluble or immobilized HA. We now show that CD44 made by wild-type Chinese hamster ovary (CHO-K1) cells and a ligand-binding subclone differ with respect to N-linked glycosylation. While both bear CD44 with highly branched, complex-type glycoforms, CD44 expressed by the wild type was more extensively sialylated. CHO-K1 cells which failed to recognize HA when grown in culture gained this ability when grown as a solid tumor and reverted to a non-HA-binding state when returned to culture. The ability of CHO-K1 cells to recognize HA was also reversibly induced when glucose concentrations in the medium were reduced. Glucose restriction influenced CD44-mediated HA binding by many but not all, of a series of murine tumors. Glucose concentrations and glycosylation inhibitors only partially influenced CD44 receptor function on resting murine B lymphocytes. These observations suggest that glucose levels or other local environmental conditions may markedly influence glycosylation pathways used by some tumor cells, resulting in dramatic alteration of CD44-mediated functions.

Coexpression of CD44 variant (v10/ex14) and CD44S in human mammary epithelial cells promotes tumorigenesis

CD44 is the major hyaluronan cell surface receptor and functions as an adhesion molecule in many different cell types, including human breast epithelial cells. The coexpression of certain CD44 variants (CD44v), such as CD44v (v10/ex14), with CD44s (standard form) appears to be closely associated with human breast tumor metastasis. In this study we have established a stable transfection of CD44v (v10/ex14) cDNA into nontumorigenic human breast epithelial cells (HBL100) which contain endogenous CD44s. Our results indicate that coexpression of both CD44v (v10/ex14) and CD44s alters the following important biological properties of these cells: 1) there is a significant reduction in hyaluronic acid (HA)-mediated cell adhesion; 2) there is an increased migration capability in collagen-matrix gel; and 3) these cells constitutively produce certain angiogenic factors and effectively promote tumorigenesis in athymic nude mice. These findings suggest that coexpression of CD44v (v10/ex14) and CD44s may trigger the onset of cell transformation required for breast cancer development.

CD44 and the adhesion of neoplastic cells

CD44 is a family of transmembrane glycoproteins that act mainly as a receptor for hyaluronan. It can also bind some other extracellular matrix ligands (chondroitin sulphate, heparan sulphate, fibronectin, serglycin, osteopontin) with lower affinity. CD44 is encoded by a single gene containing 20 exons, 10 of which (v1-v10) are variant exons inserted by alternative splicing. The standard, ubiquitously expressed isoform of CD44, does not contain sequences encoded by these variant exons. Numerous variant isoforms of CD44 containing different combinations of exons v1-v10 inserted into the extracellular domain can be expressed in proliferating epithelial cells and activated lymphocytes. CD44 plays a significant role in lymphocyte homing. Both alternative splicing and glycosylation influence receptor function of the molecule, usually reducing its affinity to hyaluronan. The cytoplasmic domain of CD44 communicates with the cytoskeleton via ankyrin and proteins belonging to the ezrin-moesin-radixin family. Relatively little is known about the intracellular events following interactions of CD44 with its ligands. Some variant isoforms, especially those containing sequences encoded by v6-v10, are overexpressed in both human and animal neoplasms. In a rat pancreatic adenocarcinoma model one of the variant CD44 isoforms was proved to be determinant in the metastatic process. For some human neoplasms (carcinomas of the digestive tract, non-Hodgkin's lymphomas, thyroid carcinomas, and others) correlations have been made between the particular pattern of CD44 variants produced by neoplastic cells and clinicopathological parameters of tumours, such as grade, stage, presence of metastases, and survival. In vitro studies indicate that modifications of CD44 expression result in different ligand recognition and influence cell motility, invasive properties, and metastatic potential of experimental tumours. Investigation of CD44 neoexpression can be useful both in early cancer diagnosis and in predicting tumour behaviour. It can also contribute to better understanding of molecular mechanisms leading to neoplastic transformation.

CD44: structure, function, and association with the malignant process

CD44 is a ubiquitous multistructural and multifunctional cells surface adhesion molecule involved in cell-cell and cell-matrix interactions. Twenty exons are involved in the genomic organization of this molecule. The first five and the last 5 exons are constant, whereas the 10 exons located between these regions are subjected to alternative splicing, resulting in the generation of a variable region. Differential utilization of the 10 variable region exons, as well as variations in N-glycosylation, O-glycosylation, and glycosaminoglycanation (by heparan sulfate or chondroitin sulfate), generate multiple isoforms (at least 20 are known) of different molecular sizes (85-230 kDa). The smallest CD44 molecule (85-95 kDa), which lacks the entire variable region, is standard CD44 (CD44s). As it is expressed mainly on cells of lymphohematopoietic origin, CD44s is also known as hematopoietic CD44 (CD44H). CD44s is a single-chain molecule composed of a distal extracellular domain (containing, the ligand-binding sites), a membrane-proximal region, a transmembrane-spanning domain, and a cytoplasmic tail. The molecular sequence (with the exception of the membrane-proximal region) displays high interspecies homology. After immunological activation, T lymphocytes and other leukocytes transiently upregulate CD44 isoforms expressing variant exons (designated CD44v). A CD44 isform containing the last 3 exon products of the variable region (CD44V8-10, also known as epithelial CD44 or CD44E), is preferentially expressed on epithelial cells. The longest CD44 isoform expressing in tandem eight exons of the variable region (CD44V3-10) was detected in keratinocytes. Hyaluronic acid (HA), an important component of the extracellular matrix (ECM), is the principal, but by no means the only, ligand of CD44. Other CD44 ligands include the ECM components collagen, fibronectin, laminin, and chondroitin sulfate. Mucosal addressin, serglycin, osteopontin, and the class II invariant chain (Ii) are additional, ECM-unrelated, ligands of the molecule. In many, but not in all cases, CD44 does not bind HA unless it is stimulated by phorbol esters, activated by agonistic anti-CD44 antibody, or deglycosylated (e.g., by tunicamycin). CD44 is a multifunctional receptor involved in cell-cell and cell-ECM interactions, cell traffic, lymph node homing, presentation of chemokines and growth factors to traveling cells, and transmission of growth signals. CD44 also participates in the uptake and intracellular degradation of HA, as well as in transmission of signals mediating hematopoiesis and apoptosis. Many cancer cell types as well as their metastases express high levels of CD44. Whereas some tumors, such as gliomas, exclusively express standard CD44, other neoplasms, including gastrointestinal cancer, bladder cancer, uterine cervical cancer, breast cancer and non-Hodgkin's lymphomas, also express CD44 variants. Hence CD44, particularly its variants, may be used as diagnostic or prognostic markers of at least some human malignant diseases. Furthermore, it has been shown in animal models that injection of reagents interfering with CD44-ligand interaction (e.g., CD44s- or CD44v-specific antibodies) inhibit local tumor growth and metastatic spread. These findings suggest that CD44 may confer a growth advantage on some neoplastic cells and, therefore, could be used as a target for cancer therapy. It is hoped that identification of CD44 variants expressed on cancer but not on normal cells will lead to the development of anti-CD44 reagents restricted to the neoplastic growth.

CD44-stimulated dendrite formation ('spreading') in activated B cells

A rat monoclonal antibody (mAb) (NIM-R8), insolubilized by binding to plastic plates, induced a rapid and extensive formation of dendrite processes ('spreading') in B lymphocytes activated by anti-IgM and interleukin-4 (IL-4) or anti-CD38 and IL-4. In contrast, resting B cells were unable to spread similarly on the NIM-R8-coated plates. The NIM-R8 antibody recognized a 90,000 MW surface glycoprotein (gp90) present on both B and T lymphocytes. The expression of this molecule was greatly increased after polyclonal (lipopolysaccharide, anti-IgM plus IL-4 or concanavalin A) activation. The NIM-R8 mAb with or without IL-2 or IL-4 was unable to induce proliferation of splenic lymphocytes. Following the demonstration that the NIM-R8 mAb recognizes the murine equivalent of human CD44, the induction of spreading of activated B lymphocytes was studied using a panel of mAb recognizing different epitopes of murine CD44. All of these different mAb induced similar spreading of activated B cells. The ligand-inducible spreading of activated B lymphocytes may be an important mechanism for providing an increased cell-surface area for cell-cell or cell-matrix interactions, and thus may be an important factor controlling the response of activated lymphocytes.

Common pattern of CD44 isoforms is expressed in morphogenetically active epithelia

CD44 is a widely distributed cell surface glycoprotein that is expressed as many isoforms arising from a single gene by alternative splicing. An important ligand for the widespread CD44 isoform, CD44s (standard form of CD44), is hyaluronan but ligands for the numerous variant isoforms are not as well characterized. Although it has been documented that CD44 is present at critical sites and stages of morphogenesis of several organs, the nature of the isoforms expressed and their precise localization have not been described. In this study we have determined the identity and distribution of CD44 isoforms expressed during development of several embryonic mouse organs by a combination of immunohistochemistry, in situ hybridization, reverse transcription-polymerase chain reaction, and DNA sequencing. As expected from previous studies, numerous CD44 variants are expressed by actively proliferating and invaginating epithelia at sites of epithelial-mesenchymal interaction, e.g., in the developing tooth, nose and hair follicle. Our results show that most prominent amongst these variants at these sites are CD44(v3-v10), CD44(v4-v10), and CD44(v6-v10), as was also found previously in the apical ectodermal ridge of the developing limb (Yu et al. [1996]). The common pattern of expression of these three particular variants in active epithelia implies that they play an important role in morphogenesis. Also very prominent in morphogenetically active epithelia is CD44s; hyaluronan is uniformly absent from these epithelia and often also from associated mesenchyme with which they interact, supporting the previously documented role of CD44s in endocytic removal of hyaluronan.

Regulated clustering of variant CD44 proteins increases their hyaluronate binding capacity

Cell contact with the extracellular matrix component hyaluronic acid (HA) plays an important role in many developmental, physiological, and pathological processes, although the regulation of this contact is poorly understood. CD44 proteins carry an amino acid motif that mediates affinity to HA. Artificial clustering of the smallest 85-kD isoform of CD44 (CD44s) has previously been shown to promote binding of the protein to soluble HA (Lesley, J., R. Hyman, and P.W. Kincade. 1993. Adv. Immunol. 54:271-335; Persche, A., J. Lesley, N. English, I. Trowbridge, and R. Hyman. 1995. Eur. J. Immunol. 25:495-501). Here we show that in rat pancreatic carcinoma cells, splice variants of CD44 (CD44v), but not CD44s, form molecular aggregates in the plasma membrane. We demonstrate that reduction-sensitive dimerization of CD44v occurs, and also that larger aggregations of the protein can be stabilized by chemical cross-linking. Different CD44v proteins present on the same cell exclusively form homoaggregates. Molecular clustering does not require an intact cytoplasmic domain of the protein. The ability of cells to bind to soluble HA is upregulated more than one magnitude by the ectopic expression of CD44v4-v7, but only when the CD44v4-v7 protein forms intermolecular aggregates. Tunicamycin treatment inhibits HA binding by CD44v and at the same time destroys oligomerization. We propose that the regulation of clustering of CD44, mediated by factors including the presence of variant exons and glycosylation, allows cells in turn to regulate their HA binding properties.

Expression of multiple CD44 isoforms in the apical ectodermal ridge of the embryonic mouse limb

Previous immunohistochemical studies have shown that CD44 is highly enriched within the apical ectodermal ridge of the developing limb (Wheatley et al. [1993] Development 119: 295-306), but the particular isoforms of CD44 were not identified. We show here that CD44s (standard or "hemopoietic" isoform) and several CD44 variants, especially V3-V10, V4-V10, and V6-V10, are concentrated in the apical ectodermal ridge in the early mouse limb. Since CD44s is a major cell surface receptor for hyaluronan, we compared its localization with that of hyaluronan. In the early limb bud, hyaluronan is distributed throughout the mesoderm but is absent from all regions of the ectoderm. Hyaluronan is especially enriched in the basement membrane separating ectoderm and mesoderm, except beneath the apical ectodermal ridge where it is absent. Since CD44s is a known endocytic receptor for hyaluronan, its presence in ridge ectoderm could lead to degradation of hyaluronan destined for the neighboring region of basement membrane, thus facilitating interaction of the ridge with underlying mesoderm. The CD44 (V3-V10) isoform found in the ridge is expressed elsewhere as a proteoglycan with heparan sulfate chains that bind fibroblast growth factors. Since fibroblast growth factors are present in the ridge and are essential for limb morphogenesis, CD44 (V3-V10) is likely to act as a cofactor or modulator in the growth-promoting action or maintenance of the ridge.

Enhanced tubular epithelial CD44 expression in MRL-lpr lupus nephritis

The cell surface glycoprotein CD44 is expressed by cells of hematopoietic origin and constitutes a receptor for hyaluronic acid and matrix proteins. Because CD44 could play a role in recruiting inflammatory cells to sites of immune injury, we examined the renal CD44 expression in normal and in autoimmune MRL-lpr mice by immunohistochemistry and at a molecular level. In normal kidneys, immunoperoxidase staining for CD44 is restricted to interstitial cells and certain urothelial cells. In nephritic MRL-lpr, CD44 expression is prominent in perivascular inflammatory infiltrates and in glomerular crescents. Interestingly, CD44 is also focally expressed by cortical tubular epithelial cells (TEC) in nephritic MRL-lpr kidneys but not in normal kidneys. Reverse transcription-polymerase chain reaction (RT-PCR) as well as Northern blotting demonstrate that CD44 kidney mRNA levels are increased in nephritic MRL-lpr mice compared with normal mice. To further characterize the tubular CD44 expression, we examined cultured TEC (primary cultures and SV40-transformed TEC lines C1 and MCT). TEC constitutively express abundant cell surface CD44 that is modestly up-regulated in response to 18 hours stimulation with TNF-alpha (100 ng/ml), IFN-gamma (100 U/ml) and IL-1 (100 ng/ml). Northern analysis of TEC mRNA reveals a constitutive CD44 mRNA transcript at 3 kb. Stimulation with IFN-gamma or TNF-alpha for six hours markedly up-regulates CD44 mRNA expression in these cells. We conclude that mononuclear infiltration with CD44 positive cells and cytokine-induced up-regulation of CD44 by renal TEC is a prominent feature of MRL-lpr lupus nephritis. The contribution of CD44 induction on TEC to the pathogenesis of the autoimmune nephritic process in MRL-lpr remains to be determined.