Exchange protein directly activated by cAMP (EPAC) regulates neuronal polarization through Rap1B

Abstract

Acquisition of neuronal polarity is a complex process involving cellular and molecular events. The second messenger cyclic adenosine monophosphate (cAMP) is involved in axonal specification through activation of protein kinase A (PKA). However, an alternative cAMP‐dependent mechanism involves the exchange protein directly activated by cAMP (EPAC), which also responds to physiological changes in cAMP concentration, promoting activation of the small Rap GTPases. Here we present evidence that EPAC signaling contributes to axon specification and elongation. In primary rat hippocampal neurons, EPAC isoforms were differentially expressed during axon specification. Furthermore, 8‐pCPT, an EPAC pharmacological activation and genetic manipulations of EPAC in neurons induced supernumerary axons, indicative of Rap1b activation. Moreover, 8‐pCPT–treated neurons expressed ankyrin G and other markers of mature axons such as synaptophysin and axonal accumulation of vGLUT1. In contrast, pharmacological inhibition of EPAC delayed neuronal polarity. Genetic manipulations to inactivate EPAC1 using either shRNA or neurons derived from EPAC1 knock‐out mice lead to axon elongation and polarization defects. Interestingly, multiaxonic neurons generated by 8‐pCPT treatments in wild type neurons were not found in EPAC1 knock‐out mice neurons. Altogether these results propose that EPAC signaling is an alternative and complementary mechanism for cAMP‐dependent axon determination.