Molecular Basis for the Homophilic Activated Leukocyte Cell Adhesion Molecule (ALCAM)-ALCAM Interaction

Abstract

Activated leukocyte cell adhesion molecule (ALCAM/ CD166), a member of the immunoglobulin superfamily with five extracellular immunoglobulin-like domains, facilitates heterophilic (ALCAM-CD6) and homophilic (ALCAM-ALCAM) cell-cell interactions. While expressed in a wide variety of tissues and cells, ALCAM is restricted to subsets of cells usually involved in dynamic growth and/or migration processes. A structure-function analysis, using two monoclonal anti-ALCAM antibodies and a series of amino-terminally deleted ALCAM constructs, revealed that homophilic cell adhesion depended on ligand binding mediated by the membranedistal amino-terminal immunoglobulin domain and on avidity controlled by ALCAM clustering at the cell surface involving membrane-proximal immunoglobulin domains. Co-expression of a transmembrane ALCAM deletion mutant, which lacks the ligand binding domain, and endogenous wild-type ALCAM inhibited homophilic cell-cell interactions by interference with ALCAM avidity, while homophilic, soluble ligand binding remained unaltered. The extracellular structures of ALCAM thus provide two structurally and functionally distinguishable modules, one involved in ligand binding and the other in avidity. Functionality of both modules is required for stable homophilic ALCAM-ALCAM cell-cell adhesion.Adhesion molecules play an important role in development, leukocyte function, and homeostasis in multicellular organisms, which are mainly governed by inter-and intracellular communication via cell-cell interactions. Alterations in cellular adhesion and communication can contribute to uncontrolled cell growth (1) and life-threatening syndromes like leukocyte adhesion deficiency (2). Activation of adhesion molecules generally involves both modulation of affinity and avidity. The affinity of adhesion molecules often reflects a specific conformation of the extracellular ligand-binding domain. Avidity modulation involves changes in the cell surface distribution of adhesion molecules (e.g. lateral oligomerization), which leads to clusters of molecules and thereby specifically increases the number of available receptors at the site of cell-cell interaction.Activated leukocyte cell adhesion molecule (ALCAM/MEMD/ CD166) 1 is a type I transmembrane protein and a member of the Ig superfamily. It has over 90% homology with the chicken adhesion molecule BEN/SC1/DM-GRASP (3-5), and it has 30% identity and 50% similarity with the human melanoma cell adhesion molecule Mel-CAM/MUC18/CD146 (6). Furthermore, ALCAM has 93% sequence identity with the candidate liver high density lipoprotein receptor HB2 (7). ALCAM is involved in various physiological processes including hematopoiesis (8, 9), thymus development (10), the immune response (11), neurite extension (12), neural cell migration (13), and osteogenesis (14).ALCAM has a short cytoplasmic tail and its extracellular part comprises five Ig domains: two amino-terminal variable (V) type Ig domains followed by three constant (C) type Ig domains (V 1 V 2 C 1 C 2 C 3 ). ALCAM was first ident...

Frontline: Optimal T cell activation requires the engagement of CD6 and CD166

“…In addition to its heterophilic engagement, CD166 interacts in a homophilic manner in cis or in trans [27,28]. There is evidence that CD166 clusters on the cell surface enhancing avidity for CD166 on an opposing cell and that this is reduced using a CD166 blocking mAb [27].…”

“…There is evidence that CD166 clusters on the cell surface enhancing avidity for CD166 on an opposing cell and that this is reduced using a CD166 blocking mAb [27]. The membrane proximal IgSF domains are involved in the cis clustering effect [27] while the trans homophilic interaction is mediated through the N-terminal domain [28]. The cis effect is dependent on cell surface expression thus it is the affinity of the trans homophilic interaction between the N-terminal domains we attempted to measure in solution to compare the kinetics of the interaction of CD166 with itself and with CD6.…”

“…The use of whole antibodies have not been sufficient to address this as both nonblocking and blocking [12] CD6 mAb enhance the mixed lymphocyte response consistent with the mAb exerting their effects by cross-linking CD6 and influencing signaling. In addition to its ability to bind CD6, CD166 mediates homotypic CD166/CD166 interactions [27,28].…”

Associations between Markers of Colorectal Cancer Stem Cells, Mutations, Mirna, and Clinical Characteristics of Ulcerative Colitis

“…Another important factor is that most of our patients received regular medication for UC. MiRNAs are noncoding RNAs that are 21 to 25 nucleotides in length and which induce the degradation of the target mRNA or repress mRNA translation by imperfectly binding to their 3'-untranslated region [28][29][30][31][32][33][34]. Functional studies of dysregulated miRNAs indicate that they regulate molecular pathways in cancer via targeting different oncogenes and/or tumor suppressors.…”

Ubiquitination and Endocytosis of Cell Adhesion Molecule DM-GRASP Regulate Its Cell Surface Presence and Affect Its Role for Axon Navigation

“…Highly conserved mammalian orthologs of DM-GRASP have been found in human (ALCAM/CD166 (15)), rodents (ALCAM/CD166 (16,17)), and bovine (CD166 (18)). DM-GRASP interacts homophilically with itself and heterophilically with the IgSF-CAM L1/NgCAM (5,6,19,20). DM-GRASP also interacts with CD6, which is, however, not present in the visual system (18,21).…”