A cAMP-dependent Ca2+ signalling pathway at the endplate provided by the gamma to epsilon subunit switch in ACh receptors

Opposed Actions of PKA Isozymes (RI and RII) and PKC Isoforms (cPKCβI and nPKCε) in Neuromuscular Developmental Synapse Elimination

Background: During neuromuscular junction (NMJ) development, synapses are produced in excess. By sensing the activity-dependent release of ACh, adenosine, and neurotrophins, presynaptic receptors prompt axonal competition and loss of the unnecessary axons. The receptor action is mediated by synergistic and antagonistic relations when they couple to downstream kinases (mainly protein kinases A and C (PKA and PKC)), which phosphorylate targets involved in axonal disconnection. Here, we directly investigated the involvement of PKA subunits and PKC isoforms in synapse elimination. Methods: Selective PKA and PKC peptide modulators were applied daily to the Levator auris longus (LAL) muscle surface of P5–P8 transgenic B6.Cg-Tg (Thy1-YFP) 16 Jrs/J (and also C57BL/6J) mice, and the number of axons and the postsynaptic receptor cluster morphology were evaluated in P9 NMJ. Results: PKA (PKA-I and PKA-II isozymes) acts at the pre- and postsynaptic sites to delay both axonal elimination and nAChR cluster differentiation, PKC activity promotes both axonal loss (a cPKCβI and nPKCε isoform action), and postsynaptic nAChR cluster maturation (a possible role for PKCθ). Moreover, PKC-induced changes in axon number indirectly influence postsynaptic maturation. Conclusions: PKC and PKA have opposed actions, which suggests that changes in the balance of these kinases may play a major role in the mechanism of developmental synapse elimination.

Presynaptic Membrane Receptors Modulate ACh Release, Axonal Competition and Synapse Elimination during Neuromuscular Junction Development

During the histogenesis of the nervous system a lush production of neurons, which establish an excessive number of synapses, is followed by a drop in both neurons and synaptic contacts as maturation proceeds. Hebbian competition between axons with different activities leads to the loss of roughly half of the neurons initially produced so connectivity is refined and specificity gained. The skeletal muscle fibers in the newborn neuromuscular junction (NMJ) are polyinnervated but by the end of the competition, 2 weeks later, the NMJ are innervated by only one axon. This peripheral synapse has long been used as a convenient model for synapse development. In the last few years, we have studied transmitter release and the local involvement of the presynaptic muscarinic acetylcholine autoreceptors (mAChR), adenosine autoreceptors (AR) and trophic factor receptors (TFR, for neurotrophins and trophic cytokines) during the development of NMJ and in the adult. This review article brings together previously published data and proposes a molecular background for developmental axonal competition and loss. At the end of the first week postnatal, these receptors modulate transmitter release in the various nerve terminals on polyinnervated NMJ and contribute to axonal competition and synapse elimination.

Presynaptic muscarinic acetylcholine autoreceptors (M1, M2 and M4 subtypes), adenosine receptors (A1 and A2A) and tropomyosin-related kinase B receptor (TrkB) modulate the developmental synapse elimination process at the neuromuscular junction

Background

The development of the nervous system involves an initially exuberant production of neurons that make an excessive number of synaptic contacts. The initial overproduction of synapses promotes connectivity. Hebbian competition between axons with different activities (the least active are punished) leads to the loss of roughly half of the overproduced elements and this refines connectivity and increases specificity. The neuromuscular junction is innervated by a single axon at the end of the synapse elimination process and, because of its relative simplicity, has long been used as a model for studying the general principles of synapse development. The involvement of the presynaptic muscarinic ACh autoreceptors may allow for the direct competitive interaction between nerve endings through differential activity-dependent acetylcholine release in the synaptic cleft. Then, the most active ending may directly punish the less active ones. Our previous results indicate the existence in the weakest axons on the polyinnervated neonatal NMJ of an ACh release inhibition mechanism based on mAChR coupled to protein kinase C and voltage-dependent calcium channels. We suggest that this mechanism plays a role in the elimination of redundant neonatal synapses.

Results

Here we used confocal microscopy and quantitative morphological analysis to count the number of brightly fluorescent axons per endplate in P7, P9 and P15 transgenic B6.Cg-Tg (Thy1-YFP)16 Jrs/J mice. We investigate the involvement of individual mAChR M₁-, M₂- and M₄-subtypes in the control of axonal elimination after the Levator auris longus muscle had been exposed to agonist and antagonist in vivo. We also analysed the role of adenosine receptor subtypes (A₁ and A_2A) and the tropomyosin-related kinase B receptor. The data show that postnatal axonal elimination is a regulated multireceptor mechanism that guaranteed the monoinnervation of the neuromuscular synapses.

Conclusion

The three receptor sets considered (mAChR, AR and TrkB receptors) intervene in modulating the conditions of the competition between nerve endings, possibly helping to determine the winner or the lossers but, thereafter, the final elimination would occur with some autonomy and independently of postsynaptic maturation.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

Decreased phosphorylation of δ and ε subunits of the acetylcholine receptor coincides with delayed postsynaptic maturation in PKC θ deficient mouse

Protein kinase C (PKC) activity is involved in the nicotinic acetylcholine receptor (nAChR) redistribution at the neuromuscular junction in vivo during postnatal maturation. Here we studied, in PKC theta (PKCtheta) deficient mice (KO), how the theta isoform of PKC is involved in the nAChR cluster maturation that is accompanied by the developmental activity-dependent neuromuscular synapse elimination process. We found that axonal elimination and dispersion of nAChR from the postsynaptic plaques and its redistribution to form the mature postsynaptic apparatus were delayed but not totally suppressed in PKCtheta deficient mice. Moreover, the delay in the maturation of the morphology of the nAChR clusters during the early postnatal synapse elimination period in the PKCtheta deficient mice coincides with a reduction in the PKCtheta-mediated phosphorylation on the delta subunit of the nAChR. In addition, we show evidence for PKCtheta regulation of PKA in normally phosphorylating the epsilon subunit of nAChR. We have also found that the theta isoform of PKC is located on the postsynaptic component of the neuromuscular junction but is also expressed by motoneurons in the spinal cord and in the motor nerve terminals. The results allow us to hypothesize that a spatially specific and opposing action of PKCtheta and PKA may result in activity-dependent alterations to synaptic connectivity at both the nerve inputs and the postsynaptic nAChR clusters.

IP3 receptors and associated Ca2+ signals localize to satellite cells and to components of the neuromuscular junction in skeletal muscle

Recently, we described an inositol 1,4,5-trisphosphate (IP3) signaling system in cultured rodent skeletal muscle, triggered by high K+ and affecting gene transcription (Powell et al., 2001). Now, in a study of adult rodent skeletal muscle, using immunocytology and confocal microscopy, we have found a high level of IP3 receptor (IP3R) staining in satellite cells, which have been shown recently to contribute to nuclei in adult fibers after muscle exercise. These IP3R staining cells are positively identified as satellite cells by their position, morphology and staining with satellite-cell-specific antibodies such as desmin and neural cell adhesion molecule. IP3Rs are also localized to postsynaptic components of the neuromuscular junction (NMJ), in areas surrounding the nuclei of the motor end plate, and in perisynaptic Schwann cells, and localized close to nicotinic acetylcholine receptors of the endplate gutters. Ca2+ imaging experiments show calcium release at the motor endplate upon K+ depolarization precisely in these IP3R-rich regions. We suggest that electrical activity stimulates IP3-associated Ca2+ signals that may be involved in gene regulation in satellite cells and in elements of the NMJ, contributing both to muscle fiber growth and stabilization of the NMJ.

Positive- and negative-regulation of Ca2+ entry through ACh receptor channels by phosphorylation

During development of mammalian neuromuscular junctions, the expression of ACh receptors (AChRs) switches from an embryonic form to an adult form, which provides a higher Ca2+ permeable channel. The lack of this switch can prevent normal development of neuromuscular junctions. In non-mammalian species, however, the switch appears not to occur. The results of the present study show that Ca2+ entry through non-mammalian AChR channels was controlled by differential phosphorylation rather than a subunit switch. This finding provides support for the hypothesis that localized Ca2+ influx through AChR channels has a critical role in the formation and maintenance of neuromuscular junctions.

Nicotinic receptors are regulated by protein kinase C activated via a nicotinic receptors-mediated signaling pathway

The present study was conducted to examine the effect of protein kinase C (PKC) on nicotinic acetylcholine (ACh) receptors expressed in Xenopus oocytes by monitoring single-channel currents. In an outside-out patch-clamp configuration, ACh (1 microM) elicited single channel currents with a slope conductance of 31 pS (control) in normal Torpedo ACh receptors. Activation of PKC via an endogenous phosphatidylinositol signaling pathway elevated the slope conductance to 41 pS, which effect was blocked by the selective PKC inhibitor, staurosporine. Mutant ACh receptor channels, which mimic PKC phosphorylation of the receptors, exhibited a slope conductance of 41 pS. Notably, pretreatment with a higher concentration of ACh (100 microM) caused an increase in the slope conductance of the channels for 1 microM ACh (43 pS), which was the same level as obtained with either PKC activation or mutant ACh receptors, and this effect was also inhibited by staurosporine. In addition, the control slope conductance was reduced by PKC inhibitor peptide (24 pS), which corresponded to that obtained with another mutant ACh receptors lacking PKC phosphorylation sites (18 pS). Mouse muscle ACh receptors were also regulated by the same mechanism. The results of the present study suggest that ACh activates PKC via nicotinic ACh receptors, which alternatively, modulates the properties of the receptor channels.

Calcium-dependent desensitizing function of the postsynaptic neuronal-type nicotinic acetylcholine receptors at the neuromuscular junction

Several subunits that commonly have been regarded as neuronal-type nicotinic acetylcholine receptor (nAChR) subtypes, have been found in the postjunctional endplate membrane of adult skeletal muscle fibres. The postsynaptic function of these neuronal-type nAChR subtypes at the neuromuscular junction has been investigated by using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The fast and slow Ca2+ components are operated by a postsynaptic muscle- and colocalized neuronal-type nAChR, respectively. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain neuronal-type nAChR may be endowed with the same functional role in the central nervous system also.

Calcium permeability of glutamate-gated channels in the central nervous system

Molecular cloning of ionotropic glutamate receptors and the development of new measurement techniques have significantly advanced our understanding of the molecular mechanisms controlling ligand-mediated entry of Ca2+ into neurons of the mammalian CNS. Recent studies have demonstrated that various types of glutamate receptors expressed in different nerve cells are permeable to Ca2+ to variable extents, depending on the structural peculiarities of the subunits and their composition in a particular cell. This diversity provides a regulable pathway for Ca2+ entry during synaptic transmission. The fractional contribution of this Ca2+ to the total synaptic current might be a substantial means of elevating the intracellular Ca2+ concentration over a wide temporal range.