Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

CD44

  • Niharika Swain
  • Samapika Routray
  • Rashmi Maruti Hosalkar
Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101999

Synonyms

Historical Background

CD44 is a transmembrane glycoprotein and also major hyaluronan receptor, mediating cell responses to the extracellular microenvironment. In 1980s, it was first described as a surface molecule restricted to few immune cells like lymphocytes, thymocytes, and granulocytes (Dalchau et al. 1980), but later it was recognized as a novel human erythrocyte cell surface antigen, lymphocyte homing receptor, and leukocyte surface glycoprotein (Stefanová et al. 1989). Today, CD44 is being described as an adhesion molecule expressed in various cell types which includes both epithelial cells (even in keratinocytes) and connective tissue cells (endothelial cells, fibroblasts and leukocytes). It participates in a number of signaling pathway mediating proliferation, migration, hematopoiesis, lymphocyte activation and lymphnode homing (Jordan et al. 2015).

Gene Structure and Its Transcription

The coding gene of CD44 is located in 11p13 locus which has 20 exons and produces a number of isoforms through extensive alternative splicing. Among them, the most abundant form is standard CD44 (CD44s) which consists of various regions and domains like extracellular domain, transmembrane domain, and intracellular/cytoplasmic domain. The extracellular domain further has been divided into three regions such as constant region (encoded by exon 1–5), variable region (encoded by exon 6–15), and proximal region (encoded by exon 16 and 17). The transmembrane domain (268–289 aa) is encoded by exon 18. In addition, the exon 18 also encodes for the first three amino acids of cytoplasmic tail, whereas the next 67 amino acids are encoded by exon 20. Nearly always exon 19 splice out from CD44 cDNAs (Thorne et al. 2004; Dzwonek and Wilczynski 2015). However, the formation of isoforms of CD44 occurs via variable insertion of different combination of encoding exons through the process of alternative splicing. However, the recent cell line studies have found expression of another alternatively spliced “short tail” isoform of CD44 called CD44st by utilizing exon 19 as the terminating exon in place of exon 20. Thus the translation of CD44st mRNA results in a CD44 protein only having a cytoplasmic tail domain consisting three amino acids and, hence, it lacks the cytoplasmic domain-mediated cellular responses/functions like interaction with cytoskeletal components (Jiang et al. 2002) (Fig. 1).
CD44, Fig. 1

Structural details of CD44 with three domains and their coding exons. The constant region/link module is coded by exon 1–5 (red boxes), variable region coded by exon 6–15 (green boxes), and proximal/stem region is coded by exon 16–17 (blue boxes). Exon 18 (orange and violet filled box) contributes for coding of transmembrane domain and first three amino acids of cytoplasmic domain. Exon 19 (black box) is normally spliced out whereas the exon 20 codes the remaining cytoplasmic domain

Structure and Its Distribution

CD44 protein consists of three functional domains, including an extracellular domain (or ectodomain), a transmembrane (TM) domain, and an intracellular domain (ICD). The extracellular domain contains a highly conserved amino-terminal globular protein domain (~180 aa) that functions as ligand-binding receptor primarily for hyaluronic acid (HA), a negatively charged, nonsulfated glycosaminoglycan. This also contains six cysteine residues which are thought to form three disulfide bonds and sites for five conserved N-glycosylation located in the amino-terminal 120 aa of CD44. The nonconserved region of extracellular domain is mainly concerned with carbohydrate modification (O-glycosylation) and alternative splicing which allows for the insertion of extra aa sequence from variable exons of the CD44 gene. The transmembrane domain, which is 100% conserved in several mammals, is thought to be involved in posttranscriptional modification like palmitoylation as it contains one of two cysteine residue substrate Cys286 and helpful in mediating HA binding. The cytoplasmic tail (72 aa) serves diverse functions like site for palmitoylation (Cys295), phosphorylation, and binding to cytoskeletal proteins (Lesley and Hyman 1998).

CD44 in Embryogenesis

During embryogenesis, CD44 plays a general role through interaction. At early stages, the embryo contains large amount of HA as a pericellular coat, and thus increase in intercellular spaces prevents cell cohesion. During limb bud formation and reduction of HA concentration occurs via various mechanisms, i.e, its decreased synthesis, increased receptor-mediated degradation, and HA-mediated cross-bridging of mesonchymal cells. All the mechanisms are controlled by CD44. The cell surface CD44 expression mediates contrast mechanisms like both binding and degradation of HA, thus playing a pivotal role in embryogenesis.

CD44 in Adult Tissue

Since its discovery, it has been found that a wide range of tissues both epithelial and mesenchymal cells showed CD44 expression whereas its isoforms like CD44v appear to have a much more restricted distribution as it is expressed only in a few cells (keratinocytes, activated lymphocytes, macrophages, and some epithelial cells of bladder, stomach, and uterine cervix) suggesting that the process of alternative splicing is normally tightly regulated. In keratinocytes, varied and controversial expression of CD44 has been reported. Keratinocytes express CD44s and at least five isoforms containing variable exons including v2–v10, v3–v10, v4–v10, v6–v10, and v8–v10, among which the isoform v3–v10 is predominant in normal human keratinocytes. Intraepithelial variation in CD44 expression also is observed as it is reported that CD44v6 expression was limited to the stratum basale and the stratum spinosum of normal uterine cervical squamous epithelium (Sneath and Mangham 1998).

Physiological Role

Wide tissue distribution and a number of isoforms enable CD44 to play diverse biological functions like cellular and cell-extracellular interactions through ligand binding, lymphnode homing, lymphocyte maturation and activation, hemopoiesis (erythropoiesis and lymphopoiesis), and hyaluronic acid degradation (Fig. 2).
CD44, Fig. 2

Multipotent face of CD44 molecule in physiology

Cell-Cell and Cell-Extracellular Interaction

HA binding to CD44 has a larger contribution cell-cell interaction mainly of aggregation of cells (macrophages, lymphocytes, and fibroblasts). In addition, other ligands like chondrotin sulfate showed preference to exon v10 of CD44, a serine glycine motif. Exon v10 also promotes both homotypic and heterotypic cellular adhesion. The attachment of ligands like H blood group antigen and heparan sulfate seems to be more exon specific as they bind to exon v6 and exon v3, respectively. The glycosylation of exon v3 with heparan sulfate enables binding a number of growth factors (fibroblast growth factor, epidermal growth factor) and cytokines which could be a reservoir for these molecules. The interaction between CD44 and extracellular matrix involves its binding capacity with ligands like hyaluronate, chondroitin, collagen, laminin, and fibronectin. The amino-terminal domain of CD44 and its isoforms is mainly composed of two clusters of positively charged amino acids that bind to six-sugar sequence of hyaluronate through a highly regulated mechanism. Binding with hyaluronate is dependent on various factors like presence of an intact cytoplasmic domain, type of isoforms, and type and degree of glycosylation (N- or O-). The physiological cellular responses mediated by CD44 and hyaluronate include cell-to-cell and cell-substrate adhesion, cell migration , cell proliferation, and cell survival through various intracellular signaling pathways (Solis et al. 2012). Furthermore, recently, another key regulatory event like proteolytic cleavages of CD44 plays an important role in the CD44-dependent cell matrix interaction and signaling pathway. Sequential proteolytic cleavage of ectodomain of CD44 by matrix metalloproteinases (ADAM 10 and 17) and intramembranous domain by γ-secretase results in the release of a CD44 intracellular domain (ICD) fragment. The ectodomain cleavage of CD44 is triggered by multiple stimulations and contributes to the regulation of which acts as a signal transduction molecule; it is translocated to the nucleus and activates transcription (Nagano and Saya 2004) (Fig. 3).
CD44, Fig. 3

Cascade of proteolytic cleavage of ectodomain and intramembrane domain of CD44 leads to various cellular responses like cell motility and cell proliferation

Lymphnode Homing, Lymphocyte Maturation and Activation

Lymphnode homing , an another important function of CD44, is mainly orchestrated by its middle domain and a protein called mucosal addressin mainly present in endothelial cells of Peyer’s patches and lymphnodes. Whereas CD44v6 plays a crucial role in activation of B and T lymphocytes and maturation of stem cell progenitors by its cross-linking and adhesion molecules. Though the exact mechanism is not clearly elucidated, some authors also suggested that the costimulatory signal of CD44 is largely dependent on signaling through CD2. Transient expression of variants CD44v3 to CD44v6 in early event on the blast cells of lymphnodes and the spleen after activation of lymphocytes support contributory role of CD44 in immune-related function.

Hemopoiesis

Contribution of CD44 in hemoposis mainly concentrated around myelopoiesis and lymphopoiesis as its isoform CD44v6 is required for maturation of the stem cell population. CD44v6 behaves as a transducer of signal exchange between stromal cells and stem cells, and thereby initiates differentiation. Upon ligand interaction, CD44s is thought to provide proliferation signals for early progenitors of hematopoietic lineages.

Hyaluronic Acid Degradation

Hyaluronate degradation is an important functional role of CD44 as this mechanism is essential for some tissue-specific function like in alveoli of lungs (surface clearance for gas exchange), cell migration, and embryological development events. Hyaluronate degradation occurs in three steps: first, it binds the cell surface to form hyaluronate-CD44 complex; second, endocytosis of the complex; and third, degradation by acid hydrolases.

CD44 in Tumor Biology

CD44-mediated signaling has been much explored in context with tumorigenesis and can be understood by three primary domains like (1) hyluronate-independent signaling (interactions between intracellular domain of CD44 and cytoskeletal proteins or membrane-associated kinases), (2) HA-dependent signaling, and (3) its diverse role in cancer stem cell.

Hyluronate-Independent Signaling

Both tumor promoting and tumor inhibiting role of CD44 can be appreciated in observations of experimental studies of benign and malignant tumors by its ligation through various cofactors. By binding with LRP6 (low-density lipoprotein receptor-related protein 6) with its extracellular domain, CD44 can trigger Wnt-βcatenin-mediated cell proliferation pathway and also triggers the interaction of cytoplasmic domain with ERM protein to modulate the cytoskeletal rearrangement and hence the cell motility. Furthermore, through interaction with hepatocyte growth factor (HGF), CD44 has been observed to enhance cell proliferation and migration via c-Met activation-mediated MAPK-ERK pathway. In contrast, in association with PP2A (a phosphatase known to dephosphorylate Raf, MEK, and Akt) CD44 favors intrinsic apoptosis by decreasing ERK phosphorylation.

HA-Dependent Signaling

Apart from its ligand-binding capacity, putative role of CD44 in tumor biology largely depends on molecular weight of its major ligand, hyaluronate. Interaction with high molecular weight hyaluronate (˃950 kDa) mainly drives the tumor-promoting signaling pathways like PI3- AKT and MAPK-Ras, through enhancement of cell proliferation, migration, and invasion. In addition, HA binding to CD44 leading to close proximity to signaling receptors such as ErbB2, EGFR, and TGF-β also behaves as an additive this role. Whereas association with low-molecular weight HA blocks the CD44-mediated principal oncogenic pathways and hence favors tumor suppression (Louderbough and Schroeder 2011; Misra et al. 2011) (Fig. 4).
CD44, Fig. 4

Role of CD44-hyaluronic acid interaction in tumorigenesis. HA-mediated CD44 interaction largely depends on molecular weights of HA

Role in Cancer Stem Cell

Many clinical and experimental studies revealed the consistent expression of CD44 especially its isoform CD44v6 in cancer stem cells in various human cancers, suggesting its multivalent role in aggressive tumor behavior, drug resistance, and recurrences. Various putative roles of CD44 have been suggested like preservation of stemness, induction of epithelial mesenchymal transition, and protection against free radicals (Yan et al. 2015) (Fig. 5).
CD44, Fig. 5

Putative role of CD44 in cancer stem cell through preservation of stemness (growth factor-mediated kinase pathway and intramembranous cleavage), induction of EMT, and inhibition of reactive oxygen species (activation of antioxidant enzyme)

Summary

Multivalent role of CD44 can be attributed to its complex structure, varied isoforms, ligand-binding capacity, extensive posttranslational modifications, and ability interaction with various signaling molecules. Beyond its vital physiological roles, cumulative evidences support its contrast role in tumorigenesis. In addition, CD44 also gained much attention as an established cancer stem cell marker. Hence its multipotency could be utilized not only in the exploration of etiopathogenesis but also in interventional targeted therapy of various human cancers.

References

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • Niharika Swain
    • 1
  • Samapika Routray
    • 2
  • Rashmi Maruti Hosalkar
    • 3
    • 4
  1. 1.MGM Dental College and HospitalNavi MumbaiIndia
  2. 2.Department of Dental SurgeryAll India Institute of Medical SciencesBhubaneswarIndia
  3. 3.Indian Association of Oral and Maxillofacial PathologistsMumbaiIndia
  4. 4.Maharashtra State Dental CouncilMumbaiIndia