LDV FITC

Conformational mAb as a tool for integrin ligand discovery.[Pubmed:19754304]

Assay Drug Dev Technol. 2009 Oct;7(5):507-15.

alpha(4)beta(1)-Integrin (very late antigen-4 (VLA-4)) mediates cell adhesion to cell surface ligands (VCAM-1). Binding of VLA-4 to VCAM-1 initiates rolling and firm adhesion of leukocytes to vascular endothelium followed by the extravasation into the tissue. VLA-4-dependent adhesion plays a key role in controlling leukocyte adhesive events. Small molecules that bind to the integrin ligand-binding site and block its interaction with natural ligands represent promising candidates for treatment of several diseases. Following a flow cytometric screen for small molecule discovery, we took advantage of a conformationally sensitive anti-beta(1)-integrin antibody (HUTS-21) and a small LDV-containing ligand (LDV-FITC) with known affinity to study binding affinities of several known and recently discovered integrin ligands. We found that binding of the LDV-containing small molecule induced exposure of HUTS-21 epitope and that the EC(50) for antibody binding was equal to previously reported K(d) for fluorescent LDV (LDV-FITC). Thus, binding of HUTS-21 can be used to report ligand-binding site occupancy. We studied binding of two known integrin ligands (YLDV and TR14035), as well as of two novel compounds. EC(50) values for HUTS-21 binding showed good correlation with K(i)s determined in the competition assay with LDV-FITC for all ligands. A docking model suggests a common mode of binding for the small molecule VLA-4 ligands. This novel approach described here can be used to determine ligand-binding affinities for unlabeled integrin ligands, and can be adapted to a high-throughput screening format for identification of unknown integrin ligands.

FRET detection of cellular alpha4-integrin conformational activation.[Pubmed:14645084]

Biophys J. 2003 Dec;85(6):3951-62.

Integrins are cell adhesion receptors, expressed on every cell type, that have been postulated to undergo conformational changes upon activation. Here, different affinity states were generated by exposing alpha4-integrins to divalent ions or by inside-out activation using a chemokine receptor. We probed the dynamic structural transformation of the integrin on live cells using fluorescence resonance energy transfer (FRET) between a peptide donor, which specifically binds to the alpha4-integrin, and octadecyl rhodamine B acceptors incorporated into the plasma membrane. We analyzed the data using a model that describes FRET between a random distribution of donors and acceptors in an infinite plane. The distance of closest approach was found to vary with the affinity of the integrin. The change in distance of closest approach was approximately 50 A between resting and Mn2+ activated receptors and approximately 25 A after chemokine activation. We used confocal microscopy to probe the lateral organization of donors and acceptors subsequent to integrin activation. Taken together, FRET and confocal results suggest that changes in FRET efficiencies are primarily due to the vertical extension of the integrin. The coordination between the extension of alpha4-integrin and its affinity provides a mechanism for Dembo's catch-bond concept.

Real time analysis of the affinity regulation of alpha 4-integrin. The physiologically activated receptor is intermediate in affinity between resting and Mn(2+) or antibody activation.[Pubmed:11641394]

J Biol Chem. 2001 Dec 28;276(52):48670-8.

This work examines the affinity of alpha(4)beta(1)-integrin and whether affinity regulation by G protein-coupled receptor (GPCR) and chemokines receptors is compatible with cell adhesion mediated between alpha(4)-integrin and vascular cell adhesion molecule-1. We used flow cytometry to examine the binding of a fluorescent derivative of an LDV peptide (Chen, L. L., Whitty, A., Lobb, R. R., Adams, S. P., and Pepinsky, R. B. (1999) J. Biol. Chem. 274, 13167-13175) to several cell lines and leukocytes with alpha(4)-integrin ranging from about 2,000 to 100,000 sites/cell. The results support the idea that alpha(4)-integrins exhibit multiple affinities and that affinity changes are regulated by the dissociation rate and conformation. The affinity varies by 3 orders of magnitude with the affinity induced by binding mAb TS2/16 plus Mn(2+) > Mn(2+) ' TS2/16 > activation because of occupancy of GPCR or chemokines receptor > resting receptors. A significant fraction of the receptors respond to the activating process. The change in alpha(4)-integrin affinity and the corresponding change in off rates mediated by GPCR receptor activation are rapid and transient, and their duration depends on GPCR desensitization. The affinity changes mediated by IgE receptor or interleukin-5 receptor persist longer. It appears that the physiologically active state of the alpha(4)-integrin, determined by inside-out signaling, has similar affinity in several cell types.