The Dominance of Intracellular Maturation of Brain-Derived Neurotrophic Factor Ensures Its Retrograde Effect in Newly Formed Mouse Motor Synapses

Miniature endplate potentials (MEPPs) and multiquantal endplate potentials (EPPs) caused by short rhythmic nerve stimulation were recorded in newly formed neuromuscular synapses of mice using intracellular microelectrode technique. In this work, we investigated which pathway of maturation of mature brain-derived neurotrophic factor (BDNF) from its precursor proBDNF dominates in muscle fibers during their reinnervation—extracellular or intracellular. Matrix metalloprotease 3 (MMP-3) or intracellular proconvertase furin were selectively inhibited in combination with the release of endogenous neurotrophin from muscle fibers upon stimulation of protease-activated receptors (PAR1). It was confirmed that PAR1 stimulation causes an increase in the amplitude of MEPPs due to the release of endogenous BDNF from muscle fibers and its retrograde effect aimed at increasing the quantal size of the acetylcholine (ACh). MMP-3 does not participate in the maturation of BDNF. Inhibition of furin led to a change in the synaptic effect upon stimulation of PAR1. An increase in the amplitude of MEPPs upon activation of PAR1 changes to a decrease in the frequency of MEPPs, which is characteristic of the effect of proBDNF in newly formed synapses. Thus, it has been shown that it is possible to stop the maturation of muscle BDNF by inhibiting the activity of furin at the stage of proneurotrophin in weakened regenerating synapses and eventually ensure the appearance of proBDNF in the synaptic cleft with its spectrum of effects. This may change the balance of the retrograde effect of BDNF and its proneurotrophin on the functioning of newly formed motor synapses. Moreover, a change in this balance can potentially affect not only the regulation of quantal ACh release, but also the rate and severity of reinnervation, since BDNF and proBDNF have a multidirectional effect on the elimination of excessive synaptic contacts in embryogenesis and post-traumatic muscle reinnervation.