GTP-dependent secretion from neutrophils is regulated by Cdk5

We have previously shown evidence for the existence of a calcium-independent, GTP-regulated mechanism of secretion from neutrophils, but this secretory mechanism remains to be fully elucidated. Cyclin-dependent kinase 5 (Cdk5), the various substrates of which include Munc18 and synapsin 1, has been implicated in neuronal secretion. Although the Cdk5 activator, p35, and Cdk5p35 activity are primarily associated with neurons, we report here that p35 also exists in neutrophils and that an active Cdk5-p35 complex is present in these cells. Cdk5-p35 activity in human neutrophils is mostly localized in secretory granules, which show an increase in Cdk5-p35 level and activity upon GTP stimulation. The potent Cdk5 inhibitor, roscovitine, completely blocks GTP-stimulated granule Cdk5 activity, which accompanies lactoferrin secretion from neutrophil-specific granules. Roscovitine also inhibits GTP-induced lactoferrin secretion and surface localization of the secretion markers, CD63 and CD66b, to a certain extent. Furthermore, neutrophils from wild-type mice treated with roscovitine and neutrophils from p35(-/-) mice exhibit comparable surface expression levels of both CD63 and CD66b upon GTP stimulation. Although our data suggest that other molecules control GTP-induced secretion from neutrophils, it is clear that Cdk5-p35 is required to elicit the maximum GTP-induced secretory response. Our observation that multiple proteins in neutrophil granules serve as specific substrates of Cdk5 further supports the premise that the kinase is a key component of the GTP-regulated secretory apparatus in neutrophils.