Miniature endplate potentials (MEPPs) and multiquantal endplate potentials (EPPs) evoked by short rhythmic nerve stimulation were recorded in newly formed mouse neuromuscular junctions using intracellular microelectrode technique. The pathways of proteolysis (extra- or intracellular) of proBDNF to brain-derived neurotrophic factor (BDNF) in muscle fibers during their reinnervation were investigated. 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 leads to an increase of MEPP amplitude due to the release of endogenous BDNF from muscle fibers and its retrograde action which increases the size of acetylcholine (ACh) quanta. MMP-3 does not participate in BDNF maturation. Inhibition of furin resulted in a change in the synaptic effect upon PAR1 stimulation: an increase of MEPP amplitude was substituted by a decrease in MEPP frequency, which is characteristic of the action of proBDNF in newly formed synapses. Thus, it has been shown that by inhibiting furin activity, it is possible to stop the maturation of muscle BDNF at the proneurotrophin stage in weakened regenerating synapses and ultimately ensure the appearance of proBDNF in the synaptic cleft with its own spectrum of effects. This can change the balance of the retrograde influence of BDNF and its proneurotrophin on the signal transmission in newly formed motor synapses. Moreover, a change in such a balance can potentially influence not only the regulation of quantal ACh secretion, but also the rate and severity of reinnervation, since BDNF and proBDNF have opposite effects on the elimination of excessive synaptic contacts during embryogenesis or during post-traumatic muscle reinnervation.
P. O. Bogacheva (2025) studied this question.