M1 and M2 mAChRs activate PDK1 and regulate PKC βI and ε and the exocytotic apparatus at the NMJ

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

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.

Higher levels of AKT-interacting protein in the frontal pole from people with schizophrenia are limited to a sub-group who have a marked deficit in cortical muscarinic M1 receptors

We are studying the molecular pathology of a sub-group within schizophrenia (∼ 25 %: termed Muscarinic Receptor Deficit subgroup of Schizophrenia (MRDS)) who can be separated because they have very low levels of cortical muscarinic M1 receptors (CHRM1). Based on our transcriptomic data from Brodmann's area ((BA) 9, 10 and 33 (controls, schizophrenia and mood disorders) and the cortex of the CHRM1-/- mouse (a molecular model of aberrant CHRM1 signaling), we predicted levels of AKT interacting protein (AKTIP), but not tubulin alpha 1b (TUBA1B) or AKT serine/threonine kinase 1 (AKT1) and pyruvate dehydrogenase kinase 1 (PDK1) (two AKTIP-functionally associated proteins), would be changed in MRDS. Hence, we used Western blotting to measure AKTIP (BA 10: controls, schizophrenia and mood disorders; BA 9: controls and schizophrenia) plus TUBA1B, AKT1 and PDK1 (BA 10: controls and schizophrenia) proteins. The only significant change with diagnosis was higher levels of AKTIP protein in BA 10 (Cohen's d = 0.73; p = 0.02) in schizophrenia compared to controls due to higher levels of AKTIP only in people with MRDS (Cohen's d = 0.80; p = 0.03). As AKTIP is involved in AKT1 signaling, our data suggests that signaling pathway is particularly disturbed in BA 10 in MRDS.

BDNF/TrkB signalling, in cooperation with muscarinic signalling, retrogradely regulates PKA pathway to phosphorylate SNAP-25 and Synapsin-1 at the neuromuscular junction

BACKGROUND

Protein kinase A (PKA) enhances neurotransmission at the neuromuscular junction (NMJ), which is retrogradely regulated by nerve-induced muscle contraction to promote Acetylcholine (ACh) release through the phosphorylation of molecules involved in synaptic vesicle exocytosis (SNAP-25 and Synapsin-1). However, the molecular mechanism of the retrograde regulation of PKA subunits and its targets by BDNF/TrkB pathway and muscarinic signalling has not been demonstrated until now. At the NMJ, retrograde control is mainly associated with BDNF/TrkB signalling as muscle contraction enhances BDNF levels and controls specific kinases involved in the neurotransmission. Neurotransmission at the NMJ is also highly modulated by muscarinic receptors M1 and M2 (mAChRs), which are related to PKA and TrkB signallings. Here, we investigated the hypothesis that TrkB, in cooperation with mAChRs, regulates the activity-dependent dynamics of PKA subunits to phosphorylate SNAP-25 and Synapsin-1.

METHODS

To explore this, we stimulated the rat phrenic nerve at 1Hz (30 minutes), with or without subsequent contraction (abolished by µ-conotoxin GIIIB). Pharmacological treatments were conducted with the anti-TrkB antibody clone 47/TrkB for TrkB inhibition and exogenous h-BDNF; muscarinic inhibition with Pirenzepine-dihydrochloride and Methoctramine-tetrahydrochloride for M1 and M2 mAChRs, respectively. Diaphragm protein levels and phosphorylation' changes were detected by Western blotting. Location of the target proteins was demonstrated using immunohistochemistry.

RESULTS

While TrkB does not directly impact the levels of PKA catalytic subunits Cα and Cβ, it regulates PKA regulatory subunits RIα and RIIβ, facilitating the phosphorylation of critical exocytotic targets such as SNAP-25 and Synapsin-1. Furthermore, the muscarinic receptors pathway maintains a delicate balance in this regulatory process. These findings explain the dynamic interplay of PKA subunits influenced by BDNF/TrkB signalling, M1 and M2 mAChRs pathways, that are differently regulated by pre- and postsynaptic activity, demonstrating the specific roles of the BDNF/TrkB and muscarinic receptors pathway in retrograde regulation.

CONCLUSION

This complex molecular interplay has the relevance of interrelating two fundamental pathways in PKA-synaptic modulation: one retrograde (neurotrophic) and the other autocrine (muscarinic). This deepens the fundamental understanding of neuromuscular physiology of neurotransmission that gives plasticity to synapses and holds the potential for identifying therapeutic strategies in conditions characterized by impaired neuromuscular communication.

Synaptic retrograde regulation of the PKA-induced SNAP-25 and Synapsin-1 phosphorylation

BACKGROUND

Bidirectional communication between presynaptic and postsynaptic components contribute to the homeostasis of the synapse. In the neuromuscular synapse, the arrival of the nerve impulse at the presynaptic terminal triggers the molecular mechanisms associated with ACh release, which can be retrogradely regulated by the resulting muscle contraction. This retrograde regulation, however, has been poorly studied. At the neuromuscular junction (NMJ), protein kinase A (PKA) enhances neurotransmitter release, and the phosphorylation of the molecules of the release machinery including synaptosomal associated protein of 25 kDa (SNAP-25) and Synapsin-1 could be involved.

METHODS

Accordingly, to study the effect of synaptic retrograde regulation of the PKA subunits and its activity, we stimulated the rat phrenic nerve (1 Hz, 30 min) resulting or not in contraction (abolished by µ-conotoxin GIIIB). Changes in protein levels and phosphorylation were detected by western blotting and cytosol/membrane translocation by subcellular fractionation. Synapsin-1 was localized in the levator auris longus (LAL) muscle by immunohistochemistry.

RESULTS

Here we show that synaptic PKA Cβ subunit regulated by RIIβ or RIIα subunits controls activity-dependent phosphorylation of SNAP-25 and Synapsin-1, respectively. Muscle contraction retrogradely downregulates presynaptic activity-induced pSynapsin-1 S9 while that enhances pSNAP-25 T138. Both actions could coordinately contribute to decreasing the neurotransmitter release at the NMJ.

CONCLUSION

This provides a molecular mechanism of the bidirectional communication between nerve terminals and muscle cells to balance the accurate process of ACh release, which could be important to characterize molecules as a therapy for neuromuscular diseases in which neuromuscular crosstalk is impaired.

Muscarinic Receptors in Developmental Axonal Competition at the Neuromuscular Junction

In recent years, we have studied by immunohistochemistry, intracellular recording, and western blotting the role of the muscarinic acetylcholine receptors (mAChRs; M₁, M₂, and M₄ subtypes) in the mammalian neuromuscular junction (NMJ) during development and in the adult. Here, we evaluate our published data to emphasize the mAChRs' relevance in developmental synaptic elimination and their crosstalk with other metabotropic receptors, downstream kinases, and voltage-gated calcium channels (VGCCs). The presence of mAChRs in the presynaptic membrane of motor nerve terminals allows an autocrine mechanism in which the secreted acetylcholine influences the cell itself in feedback. mAChR subtypes are coupled to different downstream pathways, so their feedback can move in a broad range between positive and negative. Moreover, mAChRs allow direct activity-dependent interaction through ACh release between the multiple competing axons during development. Additional regulation from pre- and postsynaptic sites (including neurotrophic retrograde control), the agonistic and antagonistic contributions of adenosine receptors (AR; A₁ and A_2A), and the tropomyosin-related kinase B receptor (TrkB) cooperate with mAChRs in the axonal competitive interactions which lead to supernumerary synapse elimination that achieves the optimized monoinnervation of musculoskeletal cells. The metabotropic receptor-driven balance between downstream PKA and PKC activities, coupled to developmentally regulated VGCC, explains much of how nerve terminals with different activities finally progress to their withdrawal or strengthening.

TrkB signaling is correlated with muscular fatigue resistance and less vulnerability to neurodegeneration

At the neuromuscular junction (NMJ), motor neurons and myocytes maintain a bidirectional communication that guarantees adequate functionality. Thus, motor neurons' firing pattern, which is influenced by retrograde muscle-derived neurotrophic factors, modulates myocyte contractibility. Myocytes can be fast-twitch fibers and become easily fatigued or slow-twitch fibers and resistant to fatigue. Extraocular muscles (EOM) show mixed properties that guarantee fast contraction speed and resistance to fatigue and the degeneration caused by Amyotrophic lateral sclerosis (ALS) disease. The TrkB signaling is an activity-dependent pathway implicated in the NMJ well-functioning. Therefore, it could mediate the differences between fast and slow myocytes' resistance to fatigue. The present study elucidates a specific protein expression profile concerning the TrkB signaling that correlates with higher resistance to fatigue and better neuroprotective capacity through time. The results unveil that Extra-ocular muscles (EOM) express lower levels of NT-4 that extend TrkB signaling, differential PKC expression, and a higher abundance of phosphorylated synaptic proteins that correlate with continuous neurotransmission requirements. Furthermore, common molecular features between EOM and slow soleus muscles including higher neurotrophic consumption and classic and novel PKC isoforms balance correlate with better preservation of these two muscles in ALS. Altogether, higher resistance of Soleus and EOM to fatigue and ALS seems to be associated with specific protein levels concerning the TrkB neurotrophic signaling.

Presynaptic Acetylcholine Receptors Modulate the Time Course of Action Potential-Evoked Acetylcholine Quanta Secretion at Neuromuscular Junctions

For effective transmission of excitation in neuromuscular junctions, the postsynaptic response amplitude must exceed a critical level of depolarization to trigger action potential spreading along the muscle-fiber membrane. At the presynaptic level, the end-plate potential amplitude depends not only on the acetylcholine quanta number released from the nerve terminals in response to the nerve impulse but also on a degree of synchronicity of quanta releases. The time course of stimulus-phasic synchronous quanta secretion is modulated by many extra- and intracellular factors. One of the pathways to regulate the neurosecretion kinetics of acetylcholine quanta is an activation of presynaptic autoreceptors. This review discusses the contribution of acetylcholine presynaptic receptors to the control of the kinetics of evoked acetylcholine release from nerve terminals at the neuromuscular junctions. The timing characteristics of neurotransmitter release is nowadays considered an essential factor determining the plasticity and efficacy of synaptic transmission.

Activation of Neuronal Nicotinic Receptors Inhibits Acetylcholine Release in the Neuromuscular Junction by Increasing Ca2+ Flux through Cav1 Channels

Cholinergic neurotransmission is a key signal pathway in the peripheral nervous system and in several branches of the central nervous system. Despite the fact that it has been studied extensively for a long period of time, some aspects of its regulation still have not yet been established. One is the relationship between the nicotine-induced autoregulation of acetylcholine (ACh) release with changes in the concentration of presynaptic calcium levels. The mouse neuromuscular junction of m. Levator Auris Longus was chosen as the model of the cholinergic synapse. ACh release was assessed by electrophysiological methods. Changes in calcium transients were recorded using a calcium-sensitive dye. Nicotine hydrogen tartrate salt application (10 μM) decreased the amount of evoked ACh release, while the calcium transient increased in the motor nerve terminal. Both of these effects of nicotine were abolished by the neuronal ACh receptor antagonist dihydro-beta-erythroidine and Cav1 blockers, verapamil, and nitrendipine. These data allow us to suggest that neuronal nicotinic ACh receptor activation decreases the number of ACh quanta released by boosting calcium influx through Cav1 channels.