Cyclic AMP mediates keratinocyte directional migration in an electric field

Re-epithelialization of wounded skin is necessary for wound closure and restoration of barrier function and requires directional keratinocyte migration towards the center of the wound. The electric field (EF) generated immediately upon wounding could be the earliest signal keratinocytes receive to initiate directional migration and healing. Keratinocytes express many 2-adrenergic receptors (2-ARs), but their role in the epidermis is unknown. We have previously shown that -AR agonists decrease keratinocyte migration in a cyclic AMP (cAMP) independent mechanism involving the activation of protein phosphatase 2A (PP2A). Here, we ask whether 2-ARs play a role in keratinocyte galvanotaxis.

We report a bimodal response. When keratinocytes were exposed to higher concentrations of -AR agonist (0.1 µM), their tracked migratory speed was inhibited, in both the presence (directional migration) and the absence (random migration) of a 100 mV mm^-1 EF, as expected. At lower agonist concentrations (0.1 pM to 0.1 nM), there was no effect on migratory speed; however, all directionality was lost - essentially, cells were 'blinded' to the directional cue. Preincubating the cells with -antagonist restored directional migration, demonstrating that the 'blindness' was 2-AR mediated. Incubation of keratinocytes with agents known to increase intracellular cAMP levels, such as sp-cAMP, pertussis toxin and forskolin, resulted in similar 'blinding' to the EF, whereas random migration was unaffected. The inactive cAMP analog rp-cAMP had no effect on keratinocyte migration, whether directional or random. However, rp-cAMP pretreatment before -agonist addition fully restored galvanotaxis, demonstrating the complete cAMP dependence of the attenuation of keratinocyte directional migration. This is the first report that cAMP is capable of mediating keratinocyte galvanotaxis. -AR agonists and antagonists could be valuable tools for modulating re-epithelialization, an essential step in the wound-healing process. Thus, -ARs regulate the two distinct components of keratinocyte directional migration differently: migration speed via a cAMP-independent mechanism and galvanotaxis by a cAMP-dependent one.