The role of the cytoplasmic domain in regulating CD44 function

CD44 Intracellular Domain: A Long Tale of a Short Tail

CD44 is a single-chain transmembrane receptor that exists in multiple forms due to alternative mRNA splicing and post-translational modifications. CD44 is the main cell surface receptor of hyaluronan as well as other extracellular matrix molecules, cytokines, and growth factors that play important roles in physiological processes (such as hematopoiesis and lymphocyte homing) and the progression of various diseases, the predominant one being cancer. Currently, CD44 is an established cancer stem cell marker in several tumors, implying a central functional role in tumor biology. The present review aims to highlight the contribution of the CD44 short cytoplasmic tail, which is devoid of any enzymatic activity, in the extraordinary functional diversity of the receptor. The interactions of CD44 with cytoskeletal proteins through specific structural motifs within its intracellular domain drives cytoskeleton rearrangements and affects the distribution of organelles and transport of molecules. Moreover, the CD44 intracellular domain specifically interacts with various cytoplasmic effectors regulating cell-trafficking machinery, signal transduction pathways, the transcriptome, and vital cell metabolic pathways. Understanding the cell type- and context-specificity of these interactions may unravel the high complexity of CD44 functions and lead to novel improved therapeutic interventions.

Intranasal administration of vitamin D attenuates blood-brain barrier disruption through endogenous upregulation of osteopontin and activation of CD44/P-gp glycosylation signaling after subarachnoid hemorrhage in rats

In this study, we investigated the role of vitamin D3 (VitD3) on endogenous osteopontin (OPN), a neuroprotective glycoprotein, after subarachnoid hemorrhage (SAH). The endovascular perforation SAH model in Sprague-Dawley rats was used to study the effect of intranasal VitD3 (30 ng/kg) before (Pre-SAH + VitD3) and after (Post-SAH + VitD3) subarachnoid hemorrhage. Vitamin D3 (30, 60, 120 ng/kg/day) increased more than one fold endogenous OPN expression in astrocytes and endothelial cells of rat brain. Vitamin D3 significantly decreased brain edema and Evans blue extravasation. In addition, neurobehavioral scores were significantly higher in Pre-SAH + VitD3, but partly higher in Post-SAH + VitD3, group compared with SAH group. These protective effects of vitamin D3 were completely attenuated by intracerebroventricular injection of transcription inhibitor Actinomycin D and significantly inhibited by small interfering ribonucleic acid (siRNA) for vitamin D receptor and OPN in Pre-SAH + VitD3 rats. OPN expression was significantly higher in Pre-SAH + VitD3 rats, specifically A and C, but not B, isomers were upregulated in the astrocytes, leading to CD44 splicing, and P-gp glycosylation in brain endothelial cells. The results show that intranasal vitamin D3 attenuates blood-brain barrier (BBB) disruption through endogenous upregulation of OPN and subsequent CD44 and P-gp glycosylation signals in brain endothelial cells. Furthermore, this study identifies a novel strategy for the cost-effective management of subarachnoid hemorrhage.

Osteopontin inhibits HIF-2α mRNA expression in osteoarthritic chondrocytes

The aim of the present study was to investigate the in vitro effect of osteopontin (OPN) on the expression of hypoxia-inducible factor-2α (HIF-2α) in chondrocytes and the role of OPN in osteoarthritis (OA). Cartilage was purified from the tibial surfaces of patients with OA of the knee and cultured in vitro to obtain chondrocytes. Recombinant human OPN (rhOPN) and OPN small interfering RNA (siRNA) were used to treat the chondrocytes, and the changes in the expression levels of the HIF-2α gene were measured. An anti-CD44 blocking monoclonal antibody (mAb) was used to determine the probable ligand-receptor interactions. Reverse transcription-quantitative polymerase chain reaction assays were designed and validated with SYBR® Green dyes for the simultaneous quantification of the mRNA expression levels of OPN and HIF-2α. The mRNA expression level of HIF-2α was markedly decreased in the rhOPN-treated group compared with that in the control group; by contrast, OPN siRNA increased HIF-2α gene expression. CD44 blocking mAb suppressed the inhibitory effect of OPN on HIF-2α mRNA expression. The results of the present study suggest that OPN may play a protective role in OA by inhibiting HIF-2α gene expression in osteoarthritic chondrocytes through CD44 interaction.

Expression of CD44 in articular cartilage is associated with disease severity in knee osteoarthritis

OBJECTIVES

To investigate CD44 levels in articular cartilage of knee osteoarthritis (OA) and the relationship between CD44 and severity of the disease.

METHODS

All 50 cartilage tissues included normal and OA cartilage, and were ascribed to the following four groups on the basis of modified Mankin score: normal, mild lesions, moderate lesions and severe lesions. CD44 levels in articular cartilage were assessed by immunohistochemical methods.

RESULTS

CD44 levels were detected in all four groups. The difference in average gray value of CD44 expression showed statistical significance when compared between each group (P < 0.05). In addition, CD44 expression in each group correlated with disease severity, according to the modified Mankin score (ρ = -0.848, P < 0.01).

CONCLUSIONS

CD44 in articular cartilage is associated with progressive knee OA joint damage.

Receptor protein tyrosine phosphatases are novel components of a polycystin complex

Autosomal dominant polycystic kidney disease (ADPKD) is caused by mutation of PKD1 and PKD2 that encode polycystin-1 and polycystin-2. Polycystin-1 is tyrosine phosphorylated and modulates multiple signaling pathways including AP-1, and the identity of the phosphatases regulating polycystin-1 are previously uncharacterized. Here we identify members of the LAR protein tyrosine phosphatase (RPTP) superfamily as members of the polycystin-1complex mediated through extra- and intracellular interactions. The first extracellular PKD1 domain of polycystin-1 interacts with the first Ig domain of RPTPσ, while the polycystin-1 C-terminus of polycystin-1 interacts with the regulatory D2 phosphatase domain of RPTPγ. Additional homo- and heterotypic interactions between RPTPs recruit RPTPδ. The multimeric polycystin protein complex is found localised in cilia. RPTPσ and RPTPδ are also part of a polycystin-1/E-cadherin complex known to be important for early events in adherens junction stabilisation. The interaction between polycystin-1 and RPTPγ is disrupted in ADPKD cells, while RPTPσ and RPTPδ remain closely associated with E-cadherin, largely in an intracellular location. The polycystin-1 C-terminus is an in vitro substrate of RPTPγ, which dephosphorylates the c-Src phosphorylated Y4237 residue and activates AP1-mediated transcription. The data identify RPTPs as novel interacting partners of the polycystins both in cilia and at adhesion complexes and demonstrate RPTPγ phosphatase activity is central to the molecular mechanisms governing polycystin-dependent signaling. This article is part of a Special Issue entitled: Polycystic Kidney Disease.

Reactivation from latency displays HIV particle budding at plasma membrane, accompanying CD44 upregulation and recruitment

BACKGROUND

It has been accepted that HIV buds from the cell surface in T lymphocytes, whereas in macrophages it buds into intracellular endosomes. Recent studies, on the other hand, suggest that HIV preferentially buds from the cell surface even in monocytic cells. However, most studies are based on observations in acutely infected cells and little is known about HIV budding concomitant with reactivation from latency. Such studies would provide a better understanding of a reservoir for HIV.

RESULTS

We observed HIV budding in latently infected T lymphocytic and monocytic cell lines following TNF-alpha stimulation and examined the upregulation of host factors that may be involved in particle production. Electron microscopy analysis revealed that reactivation of latently infected J1.1 cells (latently infected Jurkat cells with HIV-1) and U1 cells (latently infected U937 cells with HIV-1) displayed HIV particle budding predominantly at the plasma membrane, a morphology that is similar to particle budding in acutely infected Jurkat and U937 cells. When mRNA expression levels were quantified by qRT-PCR, we found that particle production from reactivated J1.1 and U1 cells was accompanied by CD44 upregulation. This upregulation was similarly observed when Jurkat and U937 cells were acutely infected with HIV-1 but not when just stimulated with TNF-alpha, suggesting that CD44 upregulation was linked with HIV production but not with cell stimulation. The molecules in endocytic pathways such as CD63 and HRS were also upregulated when U1 cells were reactivated and U937 cells were acutely infected with HIV-1. Confocal microscopy revealed that these upregulated host molecules were recruited to and accumulated at the sites where mature particles were formed at the plasma membrane.

CONCLUSION

Our study indicates that HIV particles are budded at the plasma membrane upon reactivation from latency, a morphology that is similar to particle budding in acute infection. Our data also suggest that HIV expression may lead to the upregulation of certain host cell molecules that are recruited to sites of particle assembly, possibly coordinating particle production.

The effect of glycosaminoglycans on rat gametes in vitro and the associated signal pathway

The effects of adding the extracellular glycosaminoglycans (GAGs), hyaluronic acid (HA) and chondroitin sulphate (CS) to ratin vitrofertilisation (IVF) media were assessed. Metaphase II (MII) oocytes were also incubated in GAG-supplemented modified rat 1-cell embryo culture medium (mR1ECM+BSA) for 3 days. Cytoplasmic fragmentation was significantly reduced in mR1ECM+BSA with HA (39.0–48.0%) compared with the control (82.0%). In IVF experiments, neither HA (8.0–30.8%) nor CS (9.7–42.5%) improved fertilisation rates compared with controls fertilised in M16 (47.2%) or enriched Krebs–Ringer bicarbonate solution (61.5%). RT-PCR and Western blot were used to probe for CD44 mRNA and protein in Sprague–Dawley gametes and cumulus cells. CD44 was identified in cumulus cells, suggesting a role for oocyte maturation and cumulus expansion. The CD44 protein was also present on caudal epididymal spermatozoa that were highly stimulated by CSin vitroimplicating a role in fertilisation for CS and CD44.

Regulation of hyaluronan binding by F-actin and colocalization of CD44 and phosphorylated ezrin/radixin/moesin (ERM) proteins in myeloid cells

Proinflammatory cytokines such as TNF-alpha up-regulate the expression of the cell adhesion molecule, CD44, and induce hyaluronan (HA) binding in peripheral blood monocytes (PBM). Here we show that in PBM, TNF-alpha induced cytoskeletal rearrangement, increased threonine phosphorylation of ERM proteins, and induced the redistribution and colocalization of phospho-ERM proteins (P-ERM) with CD44. In the myeloid progenitor cell line, KG1a, hyaluronan binding occurred in the pseudopod where CD44, P-ERM, and F-actin were highly localized. Hyaluronan binding correlated with high expression of both CD44 and P-ERM clustered in a single pseudopod. Disruption of polymerized actin reduced hyaluronan binding in both PBM and KG1a cells and abolished CD44 clustering and the pseudopod in KG1a cells. The pseudopod was not required for the clustering of CD44, the colocalization with P-ERM, or hyaluronan binding. However, treatment with a kinase inhibitor abolished ERM phosphorylation and reduced hyaluronan binding. Furthermore, expression of CD44 lacking the putative ERM binding site resulted in reduced hyaluronan binding. Taken together, these data suggest that CD44-mediated hyaluronan binding in human myeloid cells is regulated by P-ERM and the actin cytoskeleton.

Hyaluronan cell surface binding is induced by type I collagen and regulated by caveolae in glioma cells

Hyaluronan (HA) is a component of the brain extracellular matrix environment that is synthesized and secreted by glioma cells. The primary cell surface receptor for HA is CD44, a membrane glycoprotein that is functionally regulated by a membrane type 1 matrix metalloproteinase (MT1-MMP). Both CD44 and MT1-MMP are partially located in Triton X-100-insoluble domains, but no functional link has yet been established between them. In the present study, we studied the regulation of HA cell surface binding in U-87 glioma cells. We show that an MMP-dependent mechanism regulates the intrinsic cell surface binding of HA as ilomastat, a broad MMP inhibitor, increased HA binding to glioma cells. HA binding was also rapidly and specifically up-regulated by 3-fold by type I collagen in U-87 cells, which also induced a significant morphological reorganization associated with the activation of a latent form of MMP-2 through a MT1-MMP-mediated mechanism. Interestingly, caveolae depletion with a cell surface cholesterol-depleting agent beta-cyclodextrin triggered an additional increase (9-fold) in the binding of HA, in synergy with type I collagen. On the other hand, HA cell surface binding was diminished by the MEK inhibitor PD98059 and by the overexpression of a recombinant, wild type MT1-MMP, whereas its cytoplasmic-deleted form had no effect. Taken together, our results suggest that MT1-MMP regulates, through its cytoplasmic domain, the cell surface functions of CD44 in a collagen-rich pericellular environment. Additionally, we describe a new molecular mechanism regulating the invasive potential of glioma cells involving a MT1-MMP/CD44/caveolin interaction, which could represent a potential target for anti-cancer therapies.

The role of the CD44 transmembrane and cytoplasmic domains in co-ordinating adhesive and signalling events

CD44 is a widely distributed type I transmembrane glycoprotein and functions as the major hyaluronan receptor on most cell types. Although alternative splicing can produce a large number of different isoforms, they all retain the hyaluronan-binding Link-homology region and a common transmembrane and cytoplasmic domain, which are highly conserved between species. The past decade has seen an extensive investigation of this receptor owing to its importance in mediating cell-cell and cell-matrix interactions in both normal and disease states. Although roles for alternative splicing and variable glycosylation in determining ligand-binding interactions are now well established, the mechanisms by which CD44 integrates structural and signalling events to elicit cellular responses have been less well understood. However, there is now increasing evidence that CD44 is assembled in a regulated manner into membrane-cytoskeletal junctional complexes and, through both direct and indirect interactions, serves to focus downstream signal transduction events.

Rho-dependent Rho kinase activation increases CD44 surface expression and bone resorption in osteoclasts

Osteoclasts from osteopontin-deficient mice exhibit decreased CD44 surface expression [corrected]. Osteopontin (OPN)/alphavbeta3 generated Rho signaling pathway is required for the surface expression of CD44. In this work we show the Rho effector, Rho kinase (ROK-alpha), to be a potent activator of CD44 surface expression. ROK-alpha activation was associated with autophosphorylation, leading to its translocation to the plasma membrane, as well as its association with CD44. ROK-alpha promoted CD44 surface expression through phosphorylation of CD44 and ezrin-radixin-moesin (ERM) proteins and CD44.ERM.actin complex formation. Osteoclasts from OPN-/- mice exhibited an approximately 55-60% decrease in basal level ROK-alpha phosphorylation as compared with wild type osteoclasts. Furthermore, RhoVal-14 transduction was only partially effective in stimulating ROK-alpha/CD44 phosphorylation, as well as CD44 surface expression, in these osteoclasts. Studies on the inhibition of Rho by C3 transferase or ROK-alpha by the specific inhibitor, Y-27632, showed a decrease in the phosphorylation mediated by ROK-alpha and CD44 surface expression. Neutralizing antibodies to alphav, beta3, or CD44 inhibited the migration and bone resorption of wild type osteoclasts. However, only anti-alphav or -beta3 antibodies blocked OPN-induced phosphorylation of ROK-alpha, CD44, and the ERM proteins. Our results strongly suggest a role for ROK-alpha in alphavbeta3-mediated Rho signaling, which is required for the phosphorylation events and CD44 surface expression. The functional deficiencies in the Rho effector(s) because of the lack of OPN were associated with decreased CD44 surface expression and hypomotility in the OPN-/- osteoclasts. Finally, we find that cooperativity exists between alphavbeta3 and CD44 for osteoclast motility and bone resorption.

Hyaluronan and CD44: strategic players for cell-matrix interactions during chondrogenesis and matrix assembly

Embryonic induction, soluble and insoluble factors, receptors, and signal transduction are orchestrated for the morphogenesis of the cartilage elements. The interaction of cells with the extracellular matrix (ECM) may lead to altered cellular response to morphogens based on the formation of new adhesive contacts, or the uncoupling of cell-matrix interactions. Hyaluronan's influence on cell behavior, and its intimate association with cells are accomplished by a wide variety of specific binding proteins for hyaluronan. The temporal expression of the hyaluronan receptor CD44 (which is expressed as several alternatively spliced variants) may be strategic to many of these cell-matrix interactions during chondrogenesis. CD44 expression is temporally coincident with the reduction of intercellular spaces at the regions of future cartilage deposition. The spatial organization of CD44 at the cell surface may function to establish or regulate the structure of the pericellular matrix dependent on a hyaluronan scaffold. As the ECM is modified during embryogenesis, the cellular response to inductive signals may be altered. An uncoupling of chondrocyte-hyaluronan interaction leads to chondrocytic chondrolysis. Thus, consideration of cell-matrix interactions during chondrogenesis, in the light of our current understanding of the temporal and spatial expression of signaling morphogens, should become a promising focus of future research endeavors.

TNF- increases the carbohydrate sulfation of CD44: induction of 6-sulfo N-acetyl lactosamine on N- and O-linked glycans

CD44 and sulfation have both been implicated in leukocyte adhesion. In monocytes, the inflammatory cytokine tumor necrosis factor alpha (TNF-alpha) stimulates CD44 sulfation, and this correlates with the induction of CD44-mediated adhesion events. However, little is known about the sulfation of CD44 or its induction by inflammatory cytokines. We determined that TNF-alpha induces the carbohydrate sulfation of CD44. CD44 was established as a major sulfated cell surface protein on myeloid cells. In the SR91 myeloid cell line, the majority of CD44 sulfation was attributed to the glycosaminoglycan chondroitin sulfate. However, TNF-alpha stimulation increased CD44 sulfation two- to threefold, largely attributed to the increased sulfation of N- and O-linked glycans on CD44. Therefore, TNF-alpha induced a decrease in the percentage of CD44 sulfation due to chondroitin sulfate and an increase due to N- and O-linked sulfation. Furthermore, TNF-alpha induced the expression of 6-sulfo N-acetyl lactosamine (LacNAc)/Lewis x on these cells, which was detected by a monoclonal antibody after neuraminidase treatment. This 6-sulfo LacNAc/Lewis x epitope was induced on N-linked and (to a lesser extent) on O-linked glycans present on CD44. This demonstrates that CD44 is modified by sulfated carbohydrates in myeloid cells and that TNF-alpha modifies both the type and amount of carbohydrate sulfation occurring on CD44. In addition, it demonstrates that TNF-alpha can induce the expression of 6-sulfo N-acetyl glucosamine on both N- and O-linked glycans of CD44 in myeloid cells.

Latrunculin and cytochalasin decrease chondrocyte matrix retention

The proteoglycan-rich extracellular matrix (ECM) directly associated with the cells of articular cartilage is anchored to the chondrocyte plasma membrane via interaction with the hyaluronan receptor CD44. The cytoplasmic tail of CD44 interacts with the cortical cytoskeleton. The objective of this study was to determine the role of the actin cytoskeleton in CD44-mediated matrix assembly by chondrocytes and cartilage matrix retention and homeostasis. Adult bovine articular cartilage tissue slices and isolated chondrocytes were treated with latrunculin or cytochalasin. Tissues were processed for histology and chondrocytes were examined for CD44 expression and pericellular matrix assembly. Treatments that disrupt the actin cytoskeleton reduced chondrocyte pericellular matrix assembly and the retention of proteoglycan within cartilage explants. There was enhanced detection of a neoepitope resulting from proteolysis of aggrecan. Cytoskeletal disruption did not reduce CD44 expression, as monitored by flow cytometry, but detergent extraction of CD44 was enhanced and hyaluronan binding was decreased. Thus, disruption of the cytoskeleton reduces the anchorage of CD44 in the chondrocyte membrane and the capacity of CD44 to bind its ligand. The results suggest that cytoskeletal disruption within cartilage uncouples chondrocytes from the matrix, resulting in altered metabolism and deleterious changes in matrix structure.

The C-type lectin receptor Endo180 displays internalization and recycling properties distinct from other members of the mannose receptor family

Endo180/urokinase plasminogen activator receptor-associated protein together with the mannose receptor, the phospholipase A(2) receptor, and DEC-205/MR6-gp200 comprise the four members of the mannose receptor family. These receptors have a unique structural composition due to the presence of multiple C-type lectin-like domains within a single polypeptide backbone. In addition, they are all constitutively internalized from the plasma membrane via clathrin-mediated endocytosis and recycled back to the cell surface. Endo180 is a multifunctional receptor displaying Ca(2+)-dependent lectin activity, collagen binding, and association with the urokinase plasminogen activator receptor, and it has a proposed role in extracellular matrix degradation and remodeling. Within their short cytoplasmic domains, all four receptors contain both a conserved tyrosine-based and dihydrophobic-based putative endocytosis motif. Unexpectedly, Endo180 was found to be distinct within the family in that the tyrosine-based motif is not required for efficient delivery to and recycling from early endosomes. By contrast, receptor internalization is completely dependent on the dihydrophobic motif and modulated by a conserved upstream acidic residue. Furthermore, unlike the mannose receptor, Endo180 does not function as a phagocytic receptor in vitro. These findings demonstrate that despite an overall structural similarity, members of this receptor family employ distinct trafficking mechanisms that may reflect important differences in their physiological functions.

CD44-mediated uptake and degradation of hyaluronan

Hyaluronan turnover occurs systemically from the lymph and serum as well as locally by the same cells responsible for its synthesis. Local turnover involves receptor-mediated uptake and delivery to lysosomes. Of the many hyaluronan binding proteins/receptors known, the participation of CD44 in the internalization of hyaluronan has been best characterized. Some fraction of the hyaluronan bound to CD44 becomes internalized and delivered to lysosomes by a mechanism that is not dependent on clatherin, caveolae or pinocytosis. In cells such as chondrocytes, anabolic and catabolic cytokines can alter the activity of CD44 toward hyaluronan internalization. However, the mechanism of cellular regulation remains unclear. Regulation may involve the participation of alternatively spliced isoforms of CD44, changes in CD44 phosphorylation, changes in cytoskeletal binding proteins or, the activity or extracellular proteolytic activity.

Differential expression of CD44 isoforms during liver regeneration in rats

BACKGROUND

CD44 is a transmembrane glycoprotein known to bind hyaluronic acid (HA). This molecule is a multifunctional cell surface glycoprotein involved in lymphocyte homing and activation, tumor growth and metastasis. We have investigated the qualitative modification of CD44 in the regenerating liver as a model for studying cellular proliferation in vivo. Molecules involved in cell adhesion and the extracellular matrix (ECM), which influence differentiation, growth, cell-cell interactions and cellular polarity, play an important role in the liver regeneration. We studied the modulation of CD44 gene expression and its post-transcriptional modifications, analyzing the expression of different isoforms containing exon v6 in the regenerating liver, in sham operated liver and in the hepatoma cells H-35.

METHODS

The expression of CD44 and CD44v6 were analyzed in RNA extracted from regenerating liver at different times after partial hepatectomy (PH), and H-35 hepatoma cells by Northern blot, RT-PCR and Southern blot, and in protein extracts from regenerating liver by Western blot. H-35 hepatoma cells were assayed with the antibody cross-linked technique with CD44 antibodies.

RESULTS

The standard CD44 form is expressed in regenerating liver and its levels were not modified following PH. However, our analysis revealed CD44 isoforms containing v6 in the first hours after PH as well as in the H-35 hepatoma cell line. H-35 cells treated with cross-linked anti-CD44 antibodies or HA show an increased rate of incorporation of [3H]thymidine (30 and 25%, respectively) with respect to the control.

CONCLUSION

These findings suggest that CD44 may play a role in the proliferation of residual hepatocytes following PH.

Negative regulation of epithelium-neutrophil interactions via activation of CD44

Polymorphonuclear neutrophil (PMN) migration across epithelia is a common feature of active inflammation. Given the suggested role of carbohydrates in this process, we examined the receptor CD44. The standard CD44 isoform was expressed at the cell surface of PMN. PMN migration across model polarized intestinal epithelia was reduced (by 60%) if the CD44 receptor was activated by either a specific antibody (clone IM7) or the natural soluble ligand, hyaluronic acid. This inhibitory effect following receptor activation occurred with both basolateral-to-apical- and apical-to-basolateral-directed migration. The anti-CD44 antibody similarly reduced PMN migration through filters in the absence of epithelia, while preincubation of the antibody with the epithelium did not alter subsequent PMN transepithelial migration. These data suggest that PMN, rather than epithelial, CD44 is responsible for these effects. A similar inhibitory effect of anti-CD44 antibody was also observed on migration of intraepithelial lymphocytes. The molecular mechanism involved in such negative signaling following CD44 activation may include modulation of outside-in cell signaling. While neither the anti-CD44 antibody nor CD44 ligand affected PMN mobilization of intracellular Ca(2+), both led to increased adenylate cyclase activity, an inhibitory signal for PMN migration. Together, these results suggest that CD44 of PMN may potentially serve as a negative regulator of leukocyte migration across biological surfaces such as columnar epithelia.

Mucin MUC1 is seen in cell surface protrusions together with ezrin in immunoelectron tomography and is concentrated at tips of filopodial protrusions in MCF-7 breast carcinoma cells

MUC1, a transmembrane member of the mucin family, is believed to have anti-adhesive properties because of its highly sialylated, extended, and rigid rod-like conformation. The ERM proteins (ezrin, radixin, and moesin) function as membrane-cytoskeletal linkers. MUC1 and ezrin are enriched in microvilli in MCF-7az breast carcinoma cells. Similar localization was also found in peripheral membrane areas and in filopodium-like protrusions. Whereas ezrin was consistently detected in the cell-cell contact region, MUC1 was less frequently found there. MUC1 was distinctly expressed in long filopodial protrusions and was highly concentrated at their tips, which also contained ezrin, whereas F-actin was found along the stalk. This localization of MUC1 suggests a role for MUC1 in transient cell structures of migrating cells and transient cell adhesion. No direct association has yet been found between MUC1 and ezrin. However, both MUC1 and ezrin had a similar overall distribution pattern in microvilli and filopodium-like protrusions in immunoelectron tomography. In addition, MUC1 and ezrin showed spatial association, because several 10-nm gold particles used to decorate ezrin were seen in the vicinity close to the clusters of 5-nm gold particles decorating MUC1. Therefore, MUC1 appears to be associated with ezrin, but the nature of this association requires further study.

Reduced expression of CD44 variant 9 is related to lymph node metastasis and poor survival in squamous cell carcinoma of tongue

Expression of CD44v9 was immunohistochemically studied in 120 biopsy specimens from primary squamous cell carcinoma (SCC) of the tongue and correlated with clinicopathological findings of the SCCs. The tumors were classified into three groups according to immunostaining pattern of CD44v9; 53 cases with distinct positivity in all cancer cells except for those in the central part of nests (Group 1, non-reduced group), 42 cases with reduced expression in peripheral cells of nests (Group 2, reduced group), and 25 cases with complete disappearance of the expression in one or more nests (Group 3, negative group). Nineteen of 25 (76%) tumors in Group 3 and 14 of 42 (33%) in Group 2 exhibited lymph node metastasis, compared with only 8 of 53 (15%) in Group 1. The average survival time in Groups 1, 2 and 3 was 4496+/-204, 3866+/-379 and 2719+/-359 days, respectively and became shorter with the reduction of CD44v9 expression. These results suggest that the down-regulation of CD44v9 in SCC of the tongue may relate to the detachment of tumor cells from primary lesions, establishment of lymph node metastasis and consequently the death of patients.

The ankyrin-binding domain of CD44s is involved in regulating hyaluronic acid-mediated functions and prostate tumor cell transformation

CD44 isoforms, such as CD44s (the standard form), contain at least one ankyrin-binding site within the 70-amino acid (aa) cytoplasmic domain and several hyaluronic acid (HA)-binding sites within the extracellular domain. To study the role of CD44s-ankyrin interaction in regulating human prostate tumor cells, we have constructed several CD44s cytoplasmic deletion mutants that lack the ankyrin-binding site(s). These truncated cDNAs were stably transfected into CD44-negative human prostate tumor cells (LNCaP). Our results indicate that a critical region of 15-amino acids (aa) between aa 304 and aa 318 of CD44s is required for ankyrin binding. Biochemical analyses, using competition binding assays with a synthetic peptide containing the 15 aa between aa 304 and aa 318 (NSGNGAVEDRKPSGL), further support the conclusion that this region contains the ankyrin-binding domain of CD44s. Deletion of this 15-aa ankyrin-binding sequence from CD44s results in a drastic reduction of HA-mediated binding/cell adhesion, Src p60 kinase(s) interaction and anchorage-independent growth in soft agar. These findings suggest that the binding of cytoskeletal proteins, such as ankyrin, to the cytoplasmic domain of CD44s plays a pivotal role in regulating HA-mediated functions as well as Src kinase activity and prostate tumor cell transformation.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

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.

Analysis of CD44-containing lipid rafts: Recruitment of annexin II and stabilization by the actin cytoskeleton

CD44, the major cell surface receptor for hyaluronic acid (HA), was shown to localize to detergent-resistant cholesterol-rich microdomains, called lipid rafts, in fibroblasts and blood cells. Here, we have investigated the molecular environment of CD44 within the plane of the basolateral membrane of polarized mammary epithelial cells. We show that CD44 partitions into lipid rafts that contain annexin II at their cytoplasmic face. Both CD44 and annexin II were released from these lipid rafts by sequestration of plasma membrane cholesterol. Partition of annexin II and CD44 to the same type of lipid rafts was demonstrated by cross-linking experiments in living cells. First, when CD44 was clustered at the cell surface by anti-CD44 antibodies, annexin II was recruited into the cytoplasmic leaflet of CD44 clusters. Second, the formation of intracellular, submembranous annexin II-p11 aggregates caused by expression of a trans-dominant mutant of annexin II resulted in coclustering of CD44. Moreover, a frequent redirection of actin bundles to these clusters was observed. These basolateral CD44/annexin II-lipid raft complexes were stabilized by addition of GTPgammaS or phalloidin in a semipermeabilized and cholesterol-depleted cell system. The low lateral mobility of CD44 in the plasma membrane, as assessed with fluorescent recovery after photobleaching (FRAP), was dependent on the presence of plasma membrane cholesterol and an intact actin cytoskeleton. Disruption of the actin cytoskeleton dramatically increased the fraction of CD44 which could be recovered from the light detergent-insoluble membrane fraction. Taken together, our data indicate that in mammary epithelial cells the vast majority of CD44 interacts with annexin II in lipid rafts in a cholesterol-dependent manner. These CD44-containing lipid microdomains interact with the underlying actin cytoskeleton.

Potential Role for Hyaluronan and the Hyaluronan Receptor RHAMM in Mobilization and Trafficking of Hematopoietic Progenitor Cells

Although the mechanism(s) underlying mobilization of hematopoietic progenitor cells (HPCs) is unknown, detachment from the bone marrow (BM) microenvironment and motility are likely to play a role. This work analyzes the motile behavior of HPCs and the receptors involved. CD34+45lo/medScatterlo/med HPCs from granulocyte colony-stimulating factor (G-CSF)–mobilized blood and mobilized BM were compared with steady-state BM for their ability to bind hyaluronan (HA), their expression of the HA receptors RHAMM and CD44, and their motogenic behavior. Although RHAMM and CD44 are expressed by mobilized blood HPCs, function blocking monoclonal antibodies (MoAbs) identified RHAMM as a major HA binding receptor, with a less consistent participation by CD44. Permeabilization of mobilized blood HPCs showed a pool of intracellular (ic) RHAMM and a smaller pool of icCD44. In contrast, steady-state BM HPCs have significantly larger pools of icRHAMM and icCD44. Also, in contrast to mobilized blood HPCs, for steady-state BM HPCs, MoAbs to RHAMM and CD44 act as agonists to upregulate HA binding. The comparison between mobilized and steady-state BM HPCs suggests that G-CSF mobilization is associated with depletion of intracellular stores of HA receptors and modulates HA receptor usage. To confirm that mobilization alters the HA receptor distribution and usage by HPCs, samples of BM were collected at the peak of G-CSF mobilization in parallel with mobilized blood samples. HA receptor distribution of mobilized BM HPCs was closely matched with mobilized blood HPCs and different from steady-state BM HPCs. Mobilized BM HPCs had lower pools of icHA receptors, similar to those of mobilized blood HPCs. Treatment of mobilized BM HPCs with anti-RHAMM MoAb decreased HA binding, in contrast to steady-state BM HPCs. Thus, G-CSF mobilization may stimulate an autocrine stimulatory loop for HPCs in which HA interacts with basal levels of RHAMM and/or CD44 to stimulate receptor recycling. Consistent with this, treatment of HPCs with azide, nystatin, or cytochalasin B increased HA binding, implicating an energy-dependent process involving lipid rafts and the cytoskeleton. Of the sorted HPCs, 66% were adherent and 27% were motile on fibronectin plus HA. HPC adherence was inhibited by MoAbs to β1 integrin and CD44, but not to RHAMM, whereas HPC motility was inhibited by MoAb to RHAMM and β1 integrin, but not to CD44. This finding suggests that RHAMM and CD44 play reciprocal roles in adhesion and motility by HPCs. The G-CSF–associated alterations in RHAMM distribution and the RHAMM-dependent motility of HPCs suggest a potential role for HA and RHAMM in trafficking of HPCs and the possible use of HA as a mobilizing agent in vivo.

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.

Potential Role for Hyaluronan and the Hyaluronan Receptor RHAMM in Mobilization and Trafficking of Hematopoietic Progenitor Cells

Although the mechanism(s) underlying mobilization of hematopoietic progenitor cells (HPCs) is unknown, detachment from the bone marrow (BM) microenvironment and motility are likely to play a role. This work analyzes the motile behavior of HPCs and the receptors involved. CD34+45lo/medScatterlo/med HPCs from granulocyte colony-stimulating factor (G-CSF)–mobilized blood and mobilized BM were compared with steady-state BM for their ability to bind hyaluronan (HA), their expression of the HA receptors RHAMM and CD44, and their motogenic behavior. Although RHAMM and CD44 are expressed by mobilized blood HPCs, function blocking monoclonal antibodies (MoAbs) identified RHAMM as a major HA binding receptor, with a less consistent participation by CD44. Permeabilization of mobilized blood HPCs showed a pool of intracellular (ic) RHAMM and a smaller pool of icCD44. In contrast, steady-state BM HPCs have significantly larger pools of icRHAMM and icCD44. Also, in contrast to mobilized blood HPCs, for steady-state BM HPCs, MoAbs to RHAMM and CD44 act as agonists to upregulate HA binding. The comparison between mobilized and steady-state BM HPCs suggests that G-CSF mobilization is associated with depletion of intracellular stores of HA receptors and modulates HA receptor usage. To confirm that mobilization alters the HA receptor distribution and usage by HPCs, samples of BM were collected at the peak of G-CSF mobilization in parallel with mobilized blood samples. HA receptor distribution of mobilized BM HPCs was closely matched with mobilized blood HPCs and different from steady-state BM HPCs. Mobilized BM HPCs had lower pools of icHA receptors, similar to those of mobilized blood HPCs. Treatment of mobilized BM HPCs with anti-RHAMM MoAb decreased HA binding, in contrast to steady-state BM HPCs. Thus, G-CSF mobilization may stimulate an autocrine stimulatory loop for HPCs in which HA interacts with basal levels of RHAMM and/or CD44 to stimulate receptor recycling. Consistent with this, treatment of HPCs with azide, nystatin, or cytochalasin B increased HA binding, implicating an energy-dependent process involving lipid rafts and the cytoskeleton. Of the sorted HPCs, 66% were adherent and 27% were motile on fibronectin plus HA. HPC adherence was inhibited by MoAbs to β1 integrin and CD44, but not to RHAMM, whereas HPC motility was inhibited by MoAb to RHAMM and β1 integrin, but not to CD44. This finding suggests that RHAMM and CD44 play reciprocal roles in adhesion and motility by HPCs. The G-CSF–associated alterations in RHAMM distribution and the RHAMM-dependent motility of HPCs suggest a potential role for HA and RHAMM in trafficking of HPCs and the possible use of HA as a mobilizing agent in vivo.

LYVE-1, a new homologue of the CD44 glycoprotein, is a lymph-specific receptor for hyaluronan

The extracellular matrix glycosaminoglycan hyaluronan (HA) is an abundant component of skin and mesenchymal tissues where it facilitates cell migration during wound healing, inflammation, and embryonic morphogenesis. Both during normal tissue homeostasis and particularly after tissue injury, HA is mobilized from these sites through lymphatic vessels to the lymph nodes where it is degraded before entering the circulation for rapid uptake by the liver. Currently, however, the identities of HA binding molecules which control this pathway are unknown. Here we describe the first such molecule, LYVE-1, which we have identified as a major receptor for HA on the lymph vessel wall. The deduced amino acid sequence of LYVE-1 predicts a 322-residue type I integral membrane polypeptide 41% similar to the CD44 HA receptor with a 212-residue extracellular domain containing a single Link module the prototypic HA binding domain of the Link protein superfamily. Like CD44, the LYVE-1 molecule binds both soluble and immobilized HA. However, unlike CD44, the LYVE-1 molecule colocalizes with HA on the luminal face of the lymph vessel wall and is completely absent from blood vessels. Hence, LYVE-1 is the first lymph-specific HA receptor to be characterized and is a uniquely powerful marker for lymph vessels themselves.

CD44 cleavage induced by a membrane-associated metalloprotease plays a critical role in tumor cell migration

CD44 is a cell surface receptor for hyaluronate, a component of the extracellular matrix (ECM). Although CD44 has been implicated in tumor invasion and metastasis, the molecular mechanisms remain to be elucidated. Here we find that CD44 expressed in cancer cells is cleaved at the membrane-proximal region of the ectodomain and the membrane-bound cleavage product can be detected using an antibody against the cytoplasmic domain of CD44. Furthermore, we report that CD44 cleavage is mediated by a membrane-associated metalloprotease expressed in cancer cells. A tissue inhibitor of metalloproteases-1 (TIMP-1), as well as metalloprotease inhibitors, inhibit CD44 cleavage in the cell-free assay. Contrary, serine protease inhibitors enhance CD44 cleavage, and the enhancement can be prevented by pretreatment with a metalloprotease inhibitor. Thus, CD44 cleavage is regulated by an intricate balance between some proteases and their inhibitors. Interestingly, treatment with the metalloprotease blocker 1,10-phenanthroline, which strongly prevent the CD44 cleavage, suppressed RERF-LC-OK lung cancer cell migration on a hyaluronate substrate, but not on several other substrates. These results suggest that CD44 cleavage plays a critical role in an efficient cell-detachment from a hyaluronate substrate during the cell migration and consequently promotes CD44-mediated cancer cell migration. Our present data indicate that CD44, not only ECM per se, is one of the targets of pericellular proteolysis involved in tumor invasion and metastasis.

Cell interaction molecules utilized in bone marrow

Many aspects of blood cell formation can now be modeled in culture and rapid progress is being made in understanding how blood cell precursors interact with unique components of their environment. This brief review considers some cell interaction molecules that may be important for controlling the position of cells within, as well as their egress from, bone marrow.

Discrete domains within the hyaluronan receptor CD44 regulate membrane localization and cell migration

CD44 is the principle transmembrane receptor for the extracellular matrix glycosaminoglycan, hyaluronan. This receptor: ligand interaction is required for many normal cellular processes including lymphocyte homing into inflammatory sites, assembly of a pericellular matrix during chondrogenesis, wound healing and tissue morphogenesis during development. In order to mediate these diverse events, CD44 expressing cells must be able to regulate, and respond to, interactions with hyaluronan. The mechanisms responsible have been subject to scrutiny over the past few years as it has become clear that their disruption can underlie the progression of both metastatic tumours and chronic inflammatory diseases. Here we describe recent data identifying discrete regions within the transmembrane and cytoplasmic domains of CD44 which regulate this important adhesion receptor.

Identification and functional analysis of the ezrin-binding site in the hyaluronan receptor, CD44

ERM (ezrin, radixin and moesin) proteins function as linkers between the actin cytoskeleton and the plasma membrane. In addition to this structural role, these proteins are highly regulatable making them ideal candidates to mediate important physiological events such as adhesion and membrane morphology and to control formation and breakdown of membrane-cytoskeletal junctions. Recently, a direct interaction in vitro has been demonstrated between ERM proteins and the hyaluronan receptor, CD44. We have mapped the ezrin-binding site to two clusters of basic amino acids in a membrane-proximal 9 amino-acid region within the CD44 cytoplasmic domain. To investigate the functional importance of this interaction in vivo, we created a number of mutations within full-length CD44 and expressed these mutants in human melanoma cells. We demonstrate here that mutations within the ezrin-binding site do not disrupt the plasma membrane localization of CD44 and, in addition, that this region is not required to mediate efficient hyaluronan binding. These studies suggest that ERM proteins mediate the outside-in, rather than inside-out, signalling of adhesion receptors.

An essential role for the interaction between hyaluronan and hyaluronan binding proteins during joint development

We studied the expression of hyaluronan binding proteins (HABPs) during the development of embryonic chick joints, using immunocytochemistry and biotinylated HA. The expression of actin capping proteins and of actin itself was also studied because the cytoskeleton is important in controlling HA-HABP interactions. Three cell surface HABPs were localized in the epiphyseal cartilage, articular fibrocartilage, and interzone that comprise the developing joint. Of these three HABPs, CD44 was associated with the articular fibrocartilages and interzone, whereas RHAMM and the IVd4 epitope were associated with all three tissues. Biotinylated HA was localized to interzone and articular fibrocartilages before cavity formation and within epiphyseal chondrocytes post cavitation. Actin filament bundles were observed at the developing joint line, as was the expression of the actin capping protein moesin. Manipulation of joint cavity development, using oligosaccharides of HA, disrupted joint formation and was associated with decreases in CD44 and actin filament expression as well as decreased hyaluronan synthetic capability. These results suggest that HA is actively bound by CD44 at the developing joint line and that HA-HABP interactions play a major role in the initial separation events occurring during joint formation.

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.

Mechanism of impaired local hyaluronan turnover in bleomycin-induced lung injury in rat

Hyaluronan, a linear polysaccharide, is accumulated in lung interstitium during different pathological conditions, causing interstitial edema and thereby impaired lung function. We investigated the mechanism of local hyaluronan turnover during the early phase of bleomycin-induced fibrotic lung injury in rats. The binding of [3H]hyaluronan to alveolar macrophages (AM) established from bleomycin-treated rats 1 and 5 days after induction of injury was decreased 8- and 15-fold, respectively, compared with that of AM from saline-treated control counterparts, but at day 14 returned almost to the normal level. Data was confirmed by quantitative cytochemistry, using fluorescein-labeled hyaluronan. Analysis of the expression of CD44, a receptor for hyaluronan, by Western blotting revealed a 30% increase of CD44 molecules expressed on AM from bleomycin-treated rats at day 5 compared with control rats. In particular a lower molecular mass form of CD44 appeared. No expression of the receptor for hyaluronan-mediated motility (RHAMM) could be detected. The internalization and degradation of [3H]hyaluronan by AM, obtained from bleomycin-treated rats at days 1, 5, and 14, were decreased about 65%, 35%, and 30%, respectively, compared with AM from the control rats. The AM lysosomal hyaluronidase activity did not differ significantly between bleomycin-treated and control rats. Our results indicate that a decreased hyaluronan binding capacity of AM may account for the impairment of internalization and thereby degradation of excessive hyaluronan during the early phase of fibrotic lung injury.

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.

A di-hydrophobic Leu-Val motif regulates the basolateral localization of CD44 in polarized Madin-Darby canine kidney epithelial cells

Both in vivo and in vitro the distribution of the resident plasma membrane adhesion protein, CD44, is restricted to the basolateral domain of polarized epithelial cells, suggesting a role in interepithelial interactions. To determine how this localization might be regulated a range of CD44 cytoplasmic domain mutations were generated and a minimal 5 amino acid sequence, His330-Leu-Val-Asn-Lys334, was identified which when deleted results in expression of CD44 on the apical microvillal membrane. Further mutagenesis throughout this regions pinpointed a critical di-hydrophobic motif, Leu331/Val332. The ability of wild type but not mutant CD44 cytoplasmic domains to redirect an apically targeted protein, placental alkaline phosphatase, to the basolateral plasma membrane demonstrates that this sequence can function as a dominant localization signal. This His330-Lys334 sequence is spatially separate from other CD44 regulatory elements and as discussed here, a comparison with known basolateral sorting sequences identified in other transmembrane proteins suggests that a distinct mechanism operates to retain resident plasma membrane proteins in their correct plasma membrane subdomains.

Assembly of focal adhesions: progress, paradigms, and portents

Receptor-mediated assembly of an adhesion plaque occurs through an ordered series of steps, and intermediate assemblies can be identified. The recent demonstration of some of these partial reactions in permeabilized cells predicts that cell-free reconstitution of adhesion plaque assembly is an attainable goal. Newly discovered cryptic actin-binding sites in vinculin and ezrin, two proteins recruited to adhesion sites, suggest that actin-binding proteins are targets for the signals generated by adhesion receptors.

CD44 exhibits a cell type dependent interaction with triton X-100 insoluble, lipid rich, plasma membrane domains

CD44 is an abundant, widely expressed transmembrane glycoprotein which can act as a receptor for the extracellular matrix glycosaminoglycan, hyaluronan. Biochemical and morphological studies have demonstrated that in fibroblasts a significant of the CD44 population is resistant to Triton X-100 extraction and that the detergent insoluble protein is co-localized with components of the cortical cytoskeleton. Surprisingly, this distribution is not abrogated upon deletion of the CD44 cytoplasmic tail indicating that mechanisms other than a direct interaction with the cytoskeleton can regulate CD44. In this manuscript, the mechanisms underlying this detergent-insoluble association are further investigated. There was no evidence that the Triton X-100 insolubility of CD44 resulted from homotypic aggregation, an association with hyaluronan or from a direct, or indirect, association with the cytoskeleton. Instead, evidence is presented that the detergent insolubility of fibroblast CD44 at 4 degrees C results from an association of the CD44 transmembrane domain with Triton X-100 resistant, lipid rich, plasma membrane domains. The proportion of the CD44 found in these Triton X-100 insoluble structures is dependent upon cell type and cannot be altered by changing cell motility or extracellular matrix associations. These studies provide evidence for a novel mechanism regulating this adhesion protein in the plasma membrane.

Induction of a hyaluronan receptor, CD44, during embryonal carcinoma and embryonic stem cell differentiation

This paper describes the expression profile of the CD44 glycoprotein during differentiation of embryonal carcinoma (EC) and embryonic stem (ES) cells. We have recently shown that CD44 is expressed in discrete embryonic structures and, in view of this, we sought an in vitro differentiation model of development in which we could study more readily the structure and function of the CD44 molecule. The P19 EC and CGR8 ES cells were chosen as they have the capacity to develop down the cardiac muscle pathway and we have previously demonstrated that CD44 is expressed abundantly in the embryonic myocardium. The differentiation process in both cell types is accompanied by an induction of CD44 mRNA and protein. However, in differentiated cultures CD44 is not expressed in contractile cells, indicating that these P19 cells do not represent CD44-positive embryonic cardiomyocytes. Expression of CD44 is observed on fibroblast-like cells which appear to migrate over and out from the plated aggregates. Hyaluronan, the major ligand for CD44, is also associated with these CD44-positive fibroblast-like cells. It is suggested that expression of both receptor and ligand by the fibroblast cells is required for cell:matrix adhesion and cell motility. As CD44 is up-regulated in these cultures, P19 cells are now established as a useful model system to study the factors regulating expression of the CD44 gene.