Brain-derived neurotrophic factor signaling in the neuromuscular junction during developmental axonal competition and synapse elimination

Abstract

During the development of the nervous system, there is an overproduction of neurons and synapses. Hebbian competition between neighboring nerve endings and synapses performing different activity levels leads to their elimination or strengthening. We have extensively studied the involvement of the brain-derived neurotrophic factor-Tropomyosin-related kinase B receptor neurotrophic retrograde pathway, at the neuromuscular junction, in the axonal development and synapse elimination process versus the synapse consolidation. The purpose of this review is to describe the neurotrophic influence on developmental synapse elimination, in relation to other molecular pathways that we and others have found to regulate this process. In particular, we summarize our published results based on transmitter release analysis and axonal counts to show the different involvement of the presynaptic acetylcholine muscarinic autoreceptors, coupled to downstream serine-threonine protein kinases A and C (PKA and PKC) and voltage-gated calcium channels, at different nerve endings in developmental competition. The dynamic changes that occur simultaneously in several nerve terminals and synapses converge across a postsynaptic site, influence each other, and require careful studies to individualize the mechanisms of specific endings. We describe an activity-dependent balance (related to the extent of transmitter release) between the presynaptic muscarinic subtypes and the neurotrophin-mediated TrkB/p75NTR pathways that can influence the timing and fate of the competitive interactions between the different axon terminals. The downstream displacement of the PKA/PKC activity ratio to lower values, both in competing nerve terminals and at postsynaptic sites, plays a relevant role in controlling the elimination of supernumerary synapses. Finally, calcium entry through L- and P/Q- subtypes of voltage-gated calcium channels (both channels are present, together with the N-type channel in developing nerve terminals) contributes to reduce transmitter release and promote withdrawal of the most unfavorable nerve terminals during elimination (the weakest in acetylcholine release and those that have already become silent). The main findings contribute to a better understanding of punishment-rewarding interactions between nerve endings during development. Identifying the molecular targets and signaling pathways that allow synapse consolidation or withdrawal of synapses in different situations is important for potential therapies in neurodegenerative diseases.

Figure 1

Graphic representation of the neurotrophins BDNF, NT-4 and NT-3, and the receptors TrKB and p75^NTRand the muscarinic pathways (M1, M2 and M4 presynaptic autoreceptors) affecting different nerve endings in competition during synapse elimination (P7–P9) and in the adult. We studied the strong and weak endings in dual junctions (defined by the size of the EPP that each of them can evoke), and the almost eliminated silent endings that, however, can be transitorily recovered to release ACh. The solitary nerve terminal that wins the competition and the mature nerve endings in the adult (P30) are also discussed. The inhibitory (red arrows) and stimulatory effects (green arrows) of several downstream molecules have been also considered (PKC isoforms, PKA, and VGCC subtypes). Created with Adobe tools software. BDNF: Brain-derived neurotrophic factor; L: L-type; N: N-type; NT: neurotrophin; PKA: protein kinases A; PKC: protein kinases C.