A developmental switch

Using growth cone turning assays, the authors showed that embryonic dorsal root ganglia (DRG) neurons, which are normally attracted towards an extracellular cAMP gradient, were repulsed by cAMP when transfected with small interfering RNA (siRNA) against EPAC, a cAMP-dependent guanine nucleotide exchange factor. Activation of protein kinase A (PKA), which has previously been implicated in mediating both attraction and repulsion to cAMP, led to repulsion in this assay, and the response was blocked with siRNAs against PKA but not blocked with siRNAs against EPAC. Furthermore, the authors showed that EPAC, but not PKA, was required for growth cone attraction to both a netrin 1 and MAG gradient.

Next, the authors investigated adult DRG neurons, which contain low levels of cAMP and are repelled by netrin 1 and MAG gradients. Bath application of an EPAC-selective activator blocked the repulsive response to these guidance cues. Knocking down EPAC with siRNA did not affect the repulsive response, but knockdown of PKA switched growth cone repulsion to attraction, indicating that PKA is a key mediator of adult growth cone repulsion from both netrin 1 and MAG gradients.