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Regulation of the avidity of integrin alpha 4 beta 7 by the beta 7 cytoplasmic domain
1Department of Pathology, Stanford University School of Medicine, California 94305.
Abstract:
Integrins are cell-surface heterodimeric receptors with adhesive and transmembrane signaling properties. Their cytoplasmic domains can affect receptor avidity, cytoskeletal association, and post-receptor occupancy events. The alpha 4 beta 7 integrin mediates cell adhesion to Peyer's patch high walled endothelial venules (HEV), VCAM, and CS-1/fibronectin. To determine the role of the beta 7 cytoplasmic domain in these interactions, wild-type and truncated beta 7 subunits were stably expressed in the alpha 4+/beta 1-/beta 7- B cell lymphoma 38C13. The cell line delta 727 lacks the entire beta 7 cytoplasmic domain, delta 753 lacks the 34 C-terminal residues, and LXSN is vector-transfected 38C13. Cells expressing wild-type beta 7 bound Peyer's patch HEV, fibronectin, and immobilized VCAM constitutively and did not require prior activation with phorbol esters. Interestingly, delta 753 displayed no ligand binding activity, while delta 727 was constitutively active for all ligands and displayed greater avidity for fibronectin and Peyer's patch HEV than the wild-type beta 7. beta 7, delta 753, delta 727, and LXSN were also tested for the ability to bind soluble VCAM in the presence of various divalent cations. In the presence of Ca2+, but not Mg2+, delta 727 constitutively bound soluble VCAM, whereas beta 7, delta 753, and LXSN did not. beta 7 and delta 753 could bind soluble VCAM if first activated with Mn2+. The results suggest that: (i) alpha 4 beta 7 can be expressed in a constitutively active state, (ii) the beta 7 cytoplasmic domain regulates the avidity of alpha 4 beta 7, and (iii) 38C13 cell lines expressing wild-type and truncated beta 7 subunits define three stable activation states of alpha 4 beta 7: inactive (delta 753), partially active (beta 7), and fully active (delta 727) receptors.
Insights
The beta 7 cytoplasmic domain regulates alpha 4 beta 7 integrin activity. Truncated versions show distinct activation states, from inactive to fully active, impacting cell adhesion and signaling.
Area of Science:
- Immunology
- Cell Biology
- Molecular Biology
Background:
- Integrins are crucial cell-surface receptors involved in cell adhesion and signal transduction.
- The alpha 4 beta 7 integrin plays a key role in lymphocyte homing to Peyer's patches by binding to VCAM and fibronectin.
- The cytoplasmic domain of integrins influences receptor avidity and downstream signaling events.
Purpose of the Study:
- To investigate the role of the beta 7 cytoplasmic domain in alpha 4 beta 7 integrin-mediated cell adhesion.
- To characterize the functional consequences of truncating the beta 7 cytoplasmic domain on alpha 4 beta 7 avidity and activation.
- To define distinct activation states of alpha 4 beta 7 based on beta 7 cytoplasmic domain modifications.
Main Methods:
- Stable expression of wild-type and truncated beta 7 subunits in alpha 4+/beta 1-/beta 7- B cell lymphoma 38C13.
- Assessment of ligand binding to Peyer's patch high endothelial venules (HEV), VCAM, and fibronectin.
- Analysis of soluble VCAM binding in the presence of divalent cations (Ca2+, Mg2+, Mn2+).
Main Results:
- Cells expressing wild-type beta 7 showed constitutive binding to HEV, fibronectin, and VCAM.
- A beta 7 truncation lacking the entire cytoplasmic domain (delta 727) resulted in constitutive high avidity binding to all ligands.
- A beta 7 truncation lacking 34 C-terminal residues (delta 753) exhibited no ligand binding activity, indicating an inactive state.
Conclusions:
- Alpha 4 beta 7 integrin can exist in constitutively active states.
- The beta 7 cytoplasmic domain is a critical regulator of alpha 4 beta 7 integrin avidity and activation.
- The study defines three stable activation states of alpha 4 beta 7: inactive (delta 753), partially active (wild-type beta 7), and fully active (delta 727).