Dynamic regulation of neutrophil polarity and migration by the heterotrimeric G protein subunits Gα i -GTP and Gβγ

Opposing effects of Gi protein α and βγ subunits on the abundance of the second messenger cAMP enable neutrophil migration. Control cAMP to control migration Activation of the G protein–coupled receptors (GPCRs) that stimulate cellular migration generates active G protein α and βγ subunits, which interact with distinct effector molecules. Using a small molecule that activates βγ subunits without activating α subunits in neutrophils, Surve et al. determined that active βγ subunits alone increased the intracellular concentration of the second messenger cAMP so much that the cells stuck to coated surfaces. Active G protein αi subunits balanced this βγ signal, reducing cAMP sufficiently to enable the cells to move. Activation of the Gi family of heterotrimeric guanine nucleotide–binding proteins (G proteins) releases βγ subunits, which are the major transducers of chemotactic G protein–coupled receptor (GPCR)–dependent cell migration. The small molecule 12155 binds directly to Gβγ and activates Gβγ signaling without activating the Gαi subunit in the Gi heterotrimer. We used 12155 to examine the relative roles of Gαi and Gβγ activation in the migration of neutrophils on surfaces coated with the integrin ligand intercellular adhesion molecule–1 (ICAM-1). We found that 12155 suppressed basal migration by inhibiting the polarization of neutrophils and increasing their adhesion to ICAM-1–coated surfaces. GPCR-independent activation of endogenous Gαi and Gβγ with the mastoparan analog Mas7 resulted in normal migration. Furthermore, 12155-treated cells expressing a constitutively active form of Gαi1 became polarized and migrated. The extent and duration of signaling by the second messenger cyclic adenosine monophosphate (cAMP) were enhanced by 12155. Inhibiting the activity of cAMP-dependent protein kinase (PKA) restored the polarity of 12155-treated cells but did not decrease their adhesion to ICAM-1 and failed to restore migration. Together, these data provide evidence for a direct role of activated Gαi in promoting cell polarization through a cAMP-dependent mechanism and in inhibiting adhesion through a cAMP-independent mechanism.