• Overview
• Leukocyte Transmigration
• JAMs
• Selectins
• Integrins
• VLA-4/VCAM-1
• LFA-1/ICAM-1

Adhesion Molecules of Leukocyte Trafficking :
VLA-4/VCAM-1 Interaction

Structural Basis for the Interaction between VLA-4 and VCAM-1.

VLA-4 is one of 25 a b heterodimeric integrin molecules formed by the non-covalent association of the a 4 (MW ~ 155 kDa) and b 1 (MW ~ 150 kDa) subunits (Hemler et al., 1990). VLA-4 has two known ligands, VCAM-1 and the extracellular matrix protein, fibronectin (FN). VCAM-1 is expressed on endothelial cells in two alternatively spliced forms, a major form consisting of 7 Ig-like domains (i.e., VCAM-1(7D)) and a minor form lacking domain 4 (i.e., VCAM-1(6D)) (Osborn et al., 1994). VCAM-1(7D) has two homologous VLA-4 binding sites. One binding site has been localized to domains 1 and 2 (i.e., D1-D2) and the second to D4-D5. The binding of VLA-4 to VCAM-1 involves the N-terminal domains of both a and b subunits of VLA-4. The N-terminal A domain of b 1 is homologous to the von Willebrand factor A domain and contains a metal ion dependent adhesion site (MIDAS) that contributes to VCAM-1 binding (Vonderheide et al., 1994). In addition, repeats 2-4 of the N-terminal 7 bladed b -propeller domain of a 4 have also been shown to be important for VCAM-1 binding. The most crucial interaction in the stabilization of the VLA-4/VCAM-1 complex appears to be the electrostatic interaction between Asp-40 of VCAM-1 and the chelated divalent cation (i.e., Mg 2+) of the b I A domain. Asp-40 is part of a loop that projects from b strains C and D of domain 1 of VCAM-1. Other important amino acid residues in this loop that are important in VLA-4 binding include the IDSPL sequence (Osborn et al., 1994). Recent studies have suggested that D143, S148, and E150 of domain 2 of VCAM-1 may also be involved in stabilizing the VLA-4/VCAM-1 complex (Newham et al., 1997). Like other integrins, VLA-4 can assume different conformations that bind to VCAM-1 with different affinities (Dustin and Springer, 1989; Diamond and Springer, 1994). The activation of VLA-4 occurs via inside-out signaling, initiated by the engagement of surface receptors. VLA-4 can also be activated in vitro by high levels of extracellular Mg 2+ (or Mn 2+) and by certain monoclonal antibodies (mAbs) that bind the b 1 subunit (Chen et al., 1999).

Affinity and Avidity modulations of the VLA-4/VCAM-1 interaction.

An important attribute of integrins is their ability to modulate the adhesive states of cells (Dustin and Springer, 1991; Diamond and Springer, 1994). In resting monocytes, VLA-4 is expressed in its inactive form. Following cell activation, the activated, high affinity form of VLA-4 is expressed, leading to stronger binding of the monocyte to the target cell (Chigaev et al., 2003a). The mechanism for affinity modulation involves an inside-out signal that acts on the cytoplasmic domain of the b chain of VLA-4. This change, which involves a conformational change of the VLA-4 receptor, can also be achieved experimentally by high concentrations of Mg 2+ and by certain antibodies directed against the b chain (Chigaev et al., 2001). Enhanced adhesion is also associated with a structural rearrangement of VLA-4 from a bent to an extended conformer (Chigaev et al., 2003b). In this so-called switchblade model, integrin activation augments adhesion by increasing the effective density of VLA-4 on the cell surface ( Takagi and Springer, 2002).. Avidity modulation is another mechanism for the regulation of VLA-4-mediated adhesion of monocytes (Carman and Spring, 2003). This mechanism remains poorly understood, but it has been suggested that dimerization and/or clustering/ redistribution of VLA-4 receptors on the cell surface play a key role in avidity modulation (Sanchez-Mateos et al., 1993; Weber et al., 1996). Avidity modulation takes place in the presence of endothelial chemokines (Grabovsky et al., 2000). Avidity modulation can also be achieved experimentally through stimulation with pharmaceutical agents such as phorbol myristate acetate (PMA) and the formyl peptide, fMLFF (Needham et al., 1994; Faull et al., 1994; Chigaev et al., 2001).