Homeostatic Intracellular-Free Ca2+ Is Permissive for Rap1-Mediated Constitutive Activation of α4 Integrins on Eosinophils

• Published in: The Journal of immunology (1950) Vol. 180; no. 8; pp. 5512 - 5519
• Main Authors: Ulfman, Laurien H., Kamp, Vera M., van Aalst, Corneli W., Verhagen, Liesbeth P., Sanders, Marjolein E., Reedquist, Kris A., Buitenhuis, Miranda, Koenderman, Leo
• Format: Journal Article
• Language: English
• Published: 15-04-2008
• ISSN: 0022-1767; 1550-6606
• DOI: 10.4049/jimmunol.180.8.5512

Abstract
Although much progress has been made in understanding the molecular mechanisms underlying agonist-induced “inside-out” activation of integrins, little is known about how basal levels of integrin function are maintained. This is particularly important for nonactivated eosinophils, where intermediate activation of α4β1 integrin supports recruitment to endothelial cells under flow conditions. Depletion of intracellular Ca2+ and pharmacological inhibition of phospholipase C (but not other intracellular signaling molecules, including PI3K, ERK1/2, p38 MAPK, and tyrosine kinase activity) abrogated basal α4 integrin activity in nonactivated eosinophils. Basal α4 integrin activation was associated with activation of the small GTPase Rap1, a known regulator of agonist-induced integrin function. Basal Rap activation was dependent upon phospholipase C, but not intracellular Ca2+. However, depletion of intracellular Ca2+ in CD34+ hematopoietic progenitor cells abolished RapV12-mediated induction of α4 integrin activity. Thus, residual Rap activity or constitutively active Rap activity in Ca2+-depleted cells is not sufficient to induce α4 integrin activation. These data suggest that activation of functional α4 integrin activity in resting eosinophils is mediated by Rap1 provided that the intracellular-free Ca2+ is at a normal homeostatic concentration.