PKA and PKC Balance in Synapse Elimination during Neuromuscular Junction Development

Abstract

During the development of the nervous system, synaptogenesis occurs in excess though only the appropriate connections consolidate. At the neuromuscular junction, competition between several motor nerve terminals results in the maturation of a single axon and the elimination of the others. The activity-dependent release of transmitter, cotransmitters, and neurotrophic factors allows the direct mutual influence between motor axon terminals through receptors such as presynaptic muscarinic ACh autoreceptors and the tropomyosin-related kinase B neurotrophin receptor. In previous studies, we investigated the synergistic and antagonistic relations between these receptors and their downstream coupling to PKA and PKC pathways and observed a metabotropic receptor-driven balance between PKA (stabilizes multinnervation) and PKC (promotes developmental axonal loss). However, how much does each kinase contribute in the developmental synapse elimination process? A detailed statistical analysis of the differences between the PKA and PKC effects in the synapse elimination could help to explore this point. The present short communication provides this analysis and results show that a similar level of PKA inhibition and PKC potentiation would be required during development to promote synapse loss.

Involvement of the Voltage-Gated Calcium Channels L- P/Q- and N-Types in Synapse Elimination During Neuromuscular Junction Development

“…In previous studies, we found that during the first postnatal days (P5–P9), PKA and PKC have opposed effects in delaying and favoring, respectively, synapse maturation [ 3 , 35 , 45 , 49 ]. Here, we evaluate the strength of the hypothesis of the close relation between the effect of these serine/threonine kinases and VGCCs for developmental axonal competition and loss.…”

“…Here, we evaluate the strength of the hypothesis of the close relation between the effect of these serine/threonine kinases and VGCCs for developmental axonal competition and loss. We repeated some experiments of PKA and PKC stimulation or inhibition of the previously cited paper Garcia et al [ 49 ] and compared them with those obtained after VGCC stimulation or inhibition. PKA and PKC modulation of VDCCs has been determined, and phosphorylation sites of functional relevance are known for the three L-, P/Q-, and N-type VDCCs [ 50 – 52 ].…”

“…Contrarily, PKC activity, through cPKCβI and nPKCε isoforms action, promotes axonal loss [ 24 ]. Moreover, a similar level of PKC potentiation and PKA inhibition is required during developmental synaptic elimination [ 49 ] and no significant differences exist between the effects of PKA activators and PKC inhibitors or PKA inhibitors and PKC activators on the developmental axon loss rate, which indicates the complementarity of the kinases [ 49 ]. Changes in the phosphorylation of PKA and PKC targets involved in transmitter release and nerve terminal stability could realize the final molecular mechanism of synapse loss.…”

Muscarinic Receptors in Developmental Axonal Competition at the Neuromuscular Junction

“…Recently, we found [ 88 ] that PKC favors axon loss through cPKCβI and nPKCε isoform activity (as judging by the effect of their general [Bry-1 or PMA] and specific activators [dPPA, FR236924] and inhibitors [βIV 5–3 and εV 1–2 ] respectively) whereas PKA-I and II activity (as judging by the effect of their specific blockers [H-89, Rp8-Br, and Rp-cAMPs] and activator [Sp8Br], respectively) delay axonal loss in P9 mice. Furthermore, no significant differences exist between the effects of PKA activators and PKC inhibitors, or between PKA inhibitors and PKC activators, on changing axon loss rate [ 25 ]. Moreover, a similar level of PKA inhibition and PKC potentiation (mainly of the cPKCβI and nPKCε isoforms that are strictly localized on the presynaptic site [ 89 – 91 ]) seems to be required to advance in axonal loss, clearly suggesting the complementarity of these kinases.…”

“…During development, skeletal myocytes start polyinnervated by several axons [15][16][17], but after their competitive interactions, neuromuscular junctions (NMJ) finally become innervated by only one axon [6,13,14,18]. There are many reviews about synapse elimination mainly focused on the NMJ [1,[18][19][20][21][22][23][24][25][26][27]. Several relevant cues of the molecular and cellular mechanisms involved in the elimination of supernumerary nerve terminals have been investigated and collected in the cited reviews.…”