sAPPα Inhibits Neurite Outgrowth in Primary Mouse Neurons via GABA B Receptor Subunit 1a.

Neurite outgrowth is essential for neural circuit formation and is tightly regulated by secreted factors and their receptors. The secreted extracellular domain of the amyloid precursor protein (sAPPα) has been shown to modulate neurite outgrowth. Recently, the gamma amino butyric acid receptor type-B subunit 1a (GABA(B)R1a) was identified as an sAPPα binding partner that mediates its effects on synaptic transmission. Here, we investigated whether this interaction also regulates neurite outgrowth. In mouse primary hippocampal neurons of either sex, the GABA(B)R agonist baclofen reduced axon length; whereas its antagonist CGP54626 increased axon length in primary hippocampal neurons. Moreover, GABA(B)R1a knock-out increased axon length and abolished the effect of baclofen. Application of sAPPα reduced axon length, an effect that required the presence of both GABA(B)R1a and the extension domain of sAPPα, which mediates its binding to GABA(B)R1a. Similarly, the APP 17mer peptide, which is sufficient to bind GABA(B)R1a and mimic the effects of sAPP on synaptic transmission, reduced axon outgrowth in wild-type but not in GABA(B)R1a-deficient neurons. Together, these findings indicate that the 1a isoform contributes to GABA(B)R-dependent suppression of neurite outgrowth and mediates the inhibitory effect of sAPPα on neurite outgrowth.