Overexpression of nPKC theta is inhibitory for agrin-induced nicotinic acetylcholine receptor clustering in C2C12 myotubes

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.

Lack of Fgf18 causes abnormal clustering of motor nerve terminals at the neuromuscular junction with reduced acetylcholine receptor clusters

FGF receptor 2 is involved in the formation of the neuromuscular junction (NMJ), but its in vivo ligand remains to be determined. Laser capture microdissection of the mouse spinal motor neurons (SMNs) revealed that Fgf18 mRNA is highly expressed in SMNs in adults. Expression of Fgf18 mRNA was the highest in the spinal cord at embryonic day (E) 15.5, which gradually decreased to postnatal day 7. FGF18 protein was localized at the NMJs of the tibialis anterior muscle at E18.5 and in adults. Fgf18−/− mice at E18.5 showed decreased expressions of the NMJ-specific Chrne and Colq genes in the diaphragm. In Fgf18−/− diaphragms, the synaptophysin-positive areas at the nerve terminals and the acetylcholine receptor (AChR)-positive areas at the motor endplates were both approximately one-third of those in wild-type embryos. Fgf18−/− diaphragms ultrastructurally showed abnormal aggregation of multiple nerve terminals making a gigantic presynapse with sparse synaptic vesicles, and simplified motor endplates. In Fgf18−/− diaphragms, miniature endplate potentials were low in amplitude with markedly reduced frequency. In C2C12 myotubes, FGF18 enhanced AChR clustering, which was blocked by inhibiting FGFRs or MEK1. We propose that FGF18 plays a pivotal role in AChR clustering and NMJ formation in mouse embryogenesis.

PKC and PKA regulate AChR dynamics at the neuromuscular junction of living mice

The steady state of the acetylcholine receptor (AChR) density at the neuromuscular junction (NMJ) is critical for efficient and reliable synaptic transmission. However, little is known about signaling molecules involved in regulating the equilibrium between the removal and insertion of AChRs that establishes a stable postsynaptic receptor density over time. In this work, we tested the effect of activities of two serine/threonine kinases, PKC and PKA, on the removal rate of AChRs from and the re-insertion rate of internalized recycled AChRs into synaptic sites of innervated and denervated NMJs of living mice. Using an in vivo time-lapse imaging approach and various pharmacological agents, we showed that PKC and PKA activities have antagonistic effects on the removal and recycling of AChRs. Inhibition of PKC activity or activation of PKA largely prevents the removal of pre-existing AChRs and promotes the recycling of internalized AChRs into the postsynaptic membrane. In contrast, stimulation of PKC or inactivation of PKA significantly accelerates the removal of postsynaptic AChRs and depresses AChR recycling. These results indicate that a balance between PKA and PKC activities may be critical for the maintenance of the postsynaptic receptor density.

Suppression of protein kinase C theta contributes to enhanced myogenesis in vitro via IRS1 and ERK1/2 phosphorylation

Background

Differentiation and fusion of skeletal muscle myoblasts into multi-nucleated myotubes is required for neonatal development and regeneration in adult skeletal muscle. Herein, we report novel findings that protein kinase C theta (PKCθ) regulates myoblast differentiation via phosphorylation of insulin receptor substrate-1 and ERK1/2.

Results

In this study, PKCθ knockdown (PKCθ^shRNA) myotubes had reduced inhibitory insulin receptor substrate-1 ser1095 phosphorylation, enhanced myoblast differentiation and cell fusion, and increased rates of protein synthesis as determined by [³H] phenylalanine incorporation. Phosphorylation of insulin receptor substrate-1 ser632/635 and extracellular signal-regulated kinase1/2 (ERK1/2) was increased in PKCθ^shRNA cells, with no change in ERK5 phosphorylation, highlighting a PKCθ-regulated myogenic pathway. Inhibition of PI3-kinase prevented cell differentiation and fusion in control cells, which was attenuated in PKCθ^shRNA cells. Thus, with reduced PKCθ, differentiation and fusion occur in the absence of PI3-kinase activity. Inhibition of the ERK kinase, MEK1/2, impaired differentiation and cell fusion in control cells. Differentiation was preserved in PKCθ^shRNA cells treated with a MEK1/2 inhibitor, although cell fusion was blunted, indicating PKCθ regulates differentiation via IRS1 and ERK1/2, and this occurs independently of MEK1/2 activation.

Conclusion

Cellular signaling regulating the myogenic program and protein synthesis are complex and intertwined. These studies suggest that PKCθ regulates myogenic and protein synthetic signaling via the modulation of IRS1and ERK1/2 phosphorylation. Myotubes lacking PKCθ had increased rates of protein synthesis and enhanced myotube development despite reduced activation of the canonical anabolic-signaling pathway. Further investigation of PKCθ regulated signaling may reveal important interactions regulating skeletal muscle health in an insulin resistant state.

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.

Dopamine D1 receptor-mediated inhibition of NADPH oxidase activity in human kidney cells occurs via protein kinase A-protein kinase C cross talk

Dopamine cellular signaling via the D(1) receptor (D(1)R) involves both protein kinase A (PKA) and protein kinase C (PKC), but the PKC isoform involved has not been determined. Therefore, we tested the hypothesis that the D(1)R-mediated inhibition of NADPH oxidase activity involves cross talk between PKA and a specific PKC isoform(s). In HEK-293 cells heterologously expressing human D(1)R (HEK-hD(1)), fenoldopam, a D(1)R agonist, and phorbol 12-myristate 13-acetate (PMA), a PKC activator, inhibited oxidase activity in a time- and concentration-dependent manner. The D(1)R-mediated inhibition of oxidase activity (68.1±3.6%) was attenuated by two PKA inhibitors, H89 (10μmol/L; 88±8.1%) and Rp-cAMP (10μmol/L; 97.7±6.7%), and two PKC inhibitors, bisindolylmaleimide I (1μmol/L; 94±6%) and staurosporine (10nmol/L; 93±8%), which by themselves had no effect (n=4-8/group). The inhibitory effect of PMA (1μmol/L) on oxidase activity (73±3.2%) was blocked by H89 (100±7.8%; n=5 or 6/group). The PMA-mediated inhibition of NADPH oxidase activity was accompanied by an increase in PKCθ(S676), an effect that was also blocked by H89. Fenoldopam (1μmol/L) also increased PKCθ(S676) in HEK-hD(1) and human renal proximal tubule (RPT) cells. Knockdown of PKCθ with siRNA in RPT cells prevented the inhibitory effect of fenoldopam on NADPH oxidase activity. Our studies demonstrate for the first time that cross talk between PKA and PKCθ plays an important role in the D(1)R-mediated negative regulation of NADPH oxidase activity in human kidney cells.

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.

Developing skeletal muscle cells express functional muscarinic acetylcholine receptors coupled to different intracellular signaling systems

This study analyzed the expression of muscarinic acetylcholine receptors (mAChRs) in the rat cultured skeletal muscle cells and their coupling to G protein, phospholipase C and adenylyl cyclase (AC). Our results showed the presence of a homogeneous population of [(3)H]methyl-quinuclidinyl benzilate-binding sites in the membrane fraction from the rat cultured muscle (K(D) = 0.4 nM, B(max) = 8.9 fmol mg protein(-1)). Specific muscarinic binding sites were also detected in denervated diaphragm muscles from adult rats and in myoblasts isolated from newborn rats. Activation of mAChRs with carbachol induced specific [(35)S]GTPgammaS binding to cultured muscle membranes and potentiated the forskolin-dependent stimulation of AC. These effects were totally inhibited by 0.1-1 microM atropine. In addition, mAChRs were able to stimulate generation of diacylglycerol (DAG) in response to acetylcholine, carbachol or selective mAChR agonist oxotremorine-M. The carbachol-dependent increase in DAG was inhibited in a concentration-dependent manner by mAChR antagonists atropine, pirenzepine and 4-DAMP mustard. Finally, activation of these receptors was correlated with increased synthesis of acetylcholinesterase, via a PKC-dependent pathway. Taken together, these results indicate that expression of mAChRs, coupled to G protein and distinct intracellular signaling systems, is a characteristic of noninnervated skeletal muscle cells and may be responsible for trophic influences of acetylcholine during formation of the neuromuscular synapse.

Agrin signaling in cortical neurons is mediated by a tyrosine kinase-dependent increase in intracellular Ca2+ that engages both CaMKII and MAPK signal pathways

Agrin has been implicated in multiple aspects of central nervous system (CNS) neuron differentiation and function including neurite formation, synaptogenesis, and synaptic transmission. However, little is known about the signaling mechanisms whereby agrin exerts its effects. We have recently identified a neuronal receptor for agrin, whose activation induces expression of c-fos, and provided evidence that agrin binding to this receptor is associated with a rise in intracellular Ca2+, a ubiquitous second messenger capable of mediating a wide range of effects. To gain further insight into agrin's role in brain, we used Ca2+ imaging to explore agrin signal transduction in cultured cortical neurons. Bath application of either z+ or z-agrin isoforms resulted in marked changes in intracellular Ca2+ concentration specifically in neurons. Propagation of the Ca2+ response was a two-step process characterized by an initial increase in intracellular Ca2+ mediated by ryanodine receptor (RyR) release from intracellular stores, supplemented by influx through voltage-gated calcium channels (VGCCs). Agrin-induced increases in intracellular Ca2+ were blocked by genistein and herbimycin, suggesting that the agrin receptor is a tyrosine kinase. Ca2+ release from intracellular stores activates both calcium/calmodulin-dependent kinase II (CaMKII) and mitogen activated protein kinase (MAPK). Activation of CaMKII is required for propagation of the Ca2+ wave itself, whereas both MAPK and CaMKII play a role in mediating long latency responses such as induction of c-fos. These results suggest that an agrin-dependent tyrosine kinase could play a critical role in modulating levels of intracellular Ca2+ and activity of MAPK and CaMKII in CNS neurons.

Biolistic transfection of cultured myotubes

Transfection of cells in culture with cDNA constructs is a powerful tool in cell biology, but postmitotic cells, including myotubes, can be hard to transfect with classic methods. Biolistics provides an alternative. We have used this biolistic technique to introduce cDNAs into cultured rat, chick, and C2C12 myotubes. This protocol results in efficient (20 to 70%, depending on cell type) transfection of myotubes, high levels of cDNA expression in individual myotubes, and little cellular damage. Using this procedure, we have expressed different muscle-specific cDNAs as green fluorescent protein (GFP) fusions. This technique is rapid, reliable, uses minimal amounts of reagent per transfection, and yields high transfection rates in a previously hard-to-transfect cell type. Its efficiency and reliability are high, regardless of plasmid size or epitope tag. Muscle cell biologists may now perform experiments in mature myotubes rather than relying on transfection of myoblast cultures or heterologous expression systems.