PAR1-Mediated Retrograde Action of Brain-Derived Neurotrophic Factor and Its Prodomain in Mature Mouse Motor Synapses is Provided by Furin Activity

We recorded miniature and evoked endplate multiquantal potentials (MEPPs and EPPs, respectively) at mature diaphragm motor synapses of mice. The protease-activated receptor (PAR1) agonist TFLLR-NH2 increased MEPP amplitude, which was prevented by inhibition of the vesicular acetylcholine transporter with vesamicol. TFLLR-NH2 increased not only the amplitude of MEPPs, but also their frequency, when LM11A-31 (100 nM) inhibited p75 receptors for the BDNF prodomain. This indicates that after activation of PAR1, the BDNF prodomain is released from muscle fibers along with mature BDNF, whose action dominates in mature motor synapses. The extracellular matrix metalloprotease MMP-3 inhibitor NNGH (10 μM) did not prevent the potentiating effect of TFLLR-NH2 on MEPP amplitude. Inhibition of the intracellular protease furin BOS-318 (0.5 μM) for two hours did not affect the parameters of spontaneous and evoked ACh release and was unable to prevent the TFLLR-NH2-induced increase in MEPP amplitude. When BOS-318 was administered orally (10 mg/kg) one day before electrophysiological experiments, the PAR1 agonist lost its ability to potentiate neuromuscular transmission, which may indicate the release of proBDNF from muscle fibers under such conditions. It was concluded that intracellular furin-mediated maturation of BDNF occurs in muscle fibers with the formation of both the brain-derived neurotrophic factor itself and its prodomain, ensuring their combined retrograde presynaptic action after PAR1 activation.