Muscle-regulated expression and determinants for neuromuscular junctional localization of the mouse RIalpha regulatory subunit of cAMP-dependent protein kinase

Myosin Va: Capturing cAMP for synaptic plasticity

The plus-end directed actin-dependent motor protein, myosin Va, is of particular relevance for outward vesicular protein trafficking and for restraining specific cargo vesicles within the actin cortex. The latter is a preferred site of cAMP production, and the specificity of cAMP signaling is largely mediated through the formation of microdomains that spatially couple localized metabotropic receptor activity and cAMP production to selected effectors and downstream targets. This review summarizes the core literature on the role of myosin Va for the creation of such a cAMP microdomain at the mammalian nerve-muscle synapse that serves the activity-dependent recycling of nicotinic acetylcholine receptors (nAChRs)-a principal ligand-gated ion channel which is imperative for voluntary muscle contraction. It is discussed that i) the nerve-muscle synapse is a site with a unique actin-dependent microstructure, ii) myosin Va and protein kinase A regulatory subunit Iα as well as nAChR and its constitutive binding partner, rapsyn, colocalize in endocytic/recycling vesicles near the postsynaptic membrane, and iii) impairment of myosin Va or displacement of protein kinase A regulatory subunit Iα leads to the loss of nAChR stability. Regulation of this signaling process and underlying basic pieces of machinery were covered in previous articles, to which the present review refers.

Pharmaceutical Agents for Contractile-Metabolic Dysfunction After Volumetric Muscle Loss

Volumetric muscle loss (VML) injuries represent a majority of military service member casualties and are common in civilian populations following blunt and/or penetrating traumas. Characterized as a skeletal muscle injury with permanent functional impairments, there is currently no standard for rehabilitation, leading to lifelong disability. Toward developing rehabilitative strategies, previous research demonstrates that the remaining muscle after a VML injury lacks similar levels of plasticity or adaptability as healthy, uninjured skeletal muscle. This may be due, in part, to impaired innervation and vascularization of the remaining muscle, as well as disrupted molecular signaling cascades commonly associated with muscle adaptation. The primary objective of this study was to assess the ability of four pharmacological agents with a strong record of modulating muscle contractile and metabolic function to improve functional deficits in a murine model of VML injury. Male C57BL/6 mice underwent a 15% multimuscle VML injury of the posterior hindlimb and were randomized into drug treatment groups (formoterol [FOR], 5-aminoimidazole-4-carboxamide riboside [AICAR], pioglitazone [PIO], or sildenafil [SIL]) or untreated VML group. At the end of 60 days, the injury model was first validated by comparison to age-matched injury-naive mice. Untreated VML mice had 22% less gastrocnemius muscle mass, 36% less peak-isometric torque, and 27% less maximal mitochondrial oxygen consumption rate compared to uninjured mice (p < 0.01). Experimental drug groups were, then, compared to VML untreated, and there was minimal evidence of efficacy for AICAR, PIO, or SIL in improving contractile and metabolic functional outcomes. However, FOR-treated VML mice had 18% greater peak isometric torque (p < 0.01) and permeabilized muscle fibers had 36% greater State III mitochondrial oxygen consumption rate (p < 0.01) compared to VML untreated mice, suggesting an overall improvement in muscle condition. There was minimal evidence that these benefits came from greater mitochondrial biogenesis and/or mitochondrial complex protein content, but could be due to greater enzyme activity levels for complex I and complex II. These findings suggest that FOR treatment is candidate to pair with a rehabilitative approach to maximize functional improvements in VML-injured muscle. Impact statement Volumetric muscle loss (VML) injuries result in deficiencies in strength and mobility, which have a severe impact on patient quality of life. Despite breakthroughs in tissue engineering, there are currently no treatments available that can restore function to the affected limb. Our data show that treatment of VML injuries with clinically available and FDA-approved formoterol (FOR), a beta-agonist, significantly improves strength and metabolism of VML-injured muscle. FOR is therefore a promising candidate for combined therapeutic approaches (i.e., regenerative rehabilitation) such as pairing FOR with structured rehabilitation or cell-seeded biomaterials as it may provide greater functional improvements than either strategy alone.

USF1 promotes the development of knee osteoarthritis by activating the NF-κB signaling pathway

The current study mainly aims to evaluate the expression pattern and underlying mechanism of upstream stimulating factor 1 (USF1) in the muscle tissues of knee osteoarthritis (KOA) patients. In accordance with previous findings, our data showed that muscle strength was significantly decreased in KOA patients compared with controls. Furthermore, several inflammatory factors, including tumor necrosis factor α (TNFα), IL-8, IL-6 and MCP-1, were associated with reduced muscle strength in KOA patients. Not surprisingly, NF-κB signaling was significantly activated in the muscle tissues of KOA patients compared with control individuals. Furthermore, we showed that USF1 was increased in the muscles of KOA patients compared with controls. More importantly, overexpression of USF1 in primary human skeletal muscle cells significantly increased the activation of NF-κB signaling as well as the levels of pro-inflammatory factors. In summary, we showed novel data that the upregulation of USF1 promoted NF-κB activation-induced inflammatory responses in muscle tissues of KOA patients.

PKA-type I selective constrained peptide disruptors of AKAP complexes

A-Kinase Anchoring Proteins (AKAPs) coordinate complex signaling events by serving as spatiotemporal modulators of cAMP-dependent protein kinase activity in cells. Although AKAPs organize a plethora of diverse pathways, their cellular roles are often elusive due to the dynamic nature of these signaling complexes. AKAPs can interact with the type I or type II PKA holoenzymes by virtue of high-affinity interactions with the R-subunits. As a means to delineate AKAP-mediated PKA signaling in cells, we sought to develop isoform-selective disruptors of AKAP signaling. Here, we report the development of conformationally constrained peptides named RI-STapled Anchoring Disruptors (RI-STADs) that target the docking/dimerization domain of the type 1 regulatory subunit of PKA. These high-affinity peptides are isoform-selective for the RI isoforms, can outcompete binding by the classical AKAP disruptor Ht31, and can selectively displace RIα, but not RIIα, from binding the dual-specific AKAP149 complex. Importantly, these peptides are cell-permeable and disrupt Type I PKA-mediated phosphorylation events in the context of live cells. Hence, RI-STAD peptides are versatile cellular tools to selectively probe anchored type I PKA signaling events.

Alterations of cAMP-dependent signaling in dystrophic skeletal muscle

Autonomic regulation processes in striated muscles are largely mediated by cAMP/PKA-signaling. In order to achieve specificity of signaling its spatial-temporal compartmentation plays a critical role. We discuss here how specificity of cAMP/PKA-signaling can be achieved in skeletal muscle by spatio-temporal compartmentation. While a microdomain containing PKA type I in the region of the neuromuscular junction (NMJ) is important for postsynaptic, activity-dependent stabilization of the nicotinic acetylcholine receptor (AChR), PKA type I and II microdomains in the sarcomeric part of skeletal muscle are likely to play different roles, including the regulation of muscle homeostasis. These microdomains are due to specific A-kinase anchoring proteins, like rapsyn and myospryn. Importantly, recent evidence indicates that compartmentation of the cAMP/PKA-dependent signaling pathway and pharmacological activation of cAMP production are aberrant in different skeletal muscles disorders. Thus, we discuss here their potential as targets for palliative treatment of certain forms of dystrophy and myasthenia. Under physiological conditions, the neuropeptide, α-calcitonin-related peptide, as well as catecholamines are the most-mentioned natural triggers for activating cAMP/PKA signaling in skeletal muscle. While the precise domains and functions of these first messengers are still under investigation, agonists of β₂-adrenoceptors clearly exhibit anabolic activity under normal conditions and reduce protein degradation during atrophic periods. Past and recent studies suggest direct sympathetic innervation of skeletal muscle fibers. In summary, the organization and roles of cAMP-dependent signaling in skeletal muscle are increasingly understood, revealing crucial functions in processes like nerve-muscle interaction and muscle trophicity.

Mitochondrial network genes in the skeletal muscle of amyotrophic lateral sclerosis patients

Recent evidence suggested that muscle degeneration might lead and/or contribute to neurodegeneration, thus it possibly play a key role in the etiopathogenesis and progression of amyotrophic lateral sclerosis (ALS). To test this hypothesis, this study attempted to categorize functionally relevant genes within the genome-wide expression profile of human ALS skeletal muscle, using microarray technology and gene regulatory network analysis. The correlation network structures significantly change between patients and controls, indicating an increased inter-gene connection in patients compared to controls. The gene network observed in the ALS group seems to reflect the perturbation of muscle homeostasis and metabolic balance occurring in affected individuals. In particular, the network observed in the ALS muscles includes genes (PRKR1A, FOXO1, TRIM32, ACTN3, among others), whose functions connect the sarcomere integrity to mitochondrial oxidative metabolism. Overall, the analytical approach used in this study offer the possibility to observe higher levels of correlation (i.e. common expression trends) among genes, whose function seems to be aberrantly activated during the progression of muscle atrophy.

Participation of myosin Va and Pka type I in the regeneration of neuromuscular junctions

Background

The unconventional motor protein, myosin Va, is crucial for the development of the mouse neuromuscular junction (NMJ) in the early postnatal phase. Furthermore, the cooperative action of protein kinase A (PKA) and myosin Va is essential to maintain the adult NMJ. We here assessed the involvement of myosin Va and PKA in NMJ recovery during muscle regeneration.

Methodology/Principal Findings

To address a putative role of myosin Va and PKA in the process of muscle regeneration, we used two experimental models the dystrophic mdx mouse and Notexin-induced muscle degeneration/regeneration. We found that in both systems myosin Va and PKA type I accumulate beneath the NMJs in a fiber maturation-dependent manner. Morphologically intact NMJs were found to express stable nicotinic acetylcholine receptors and to accumulate myosin Va and PKA type I in the subsynaptic region. Subsynaptic cAMP signaling was strongly altered in dystrophic muscle, particularly in fibers with severely subverted NMJ morphology.

Conclusions/Significance

Our data show a correlation between the subsynaptic accumulation of myosin Va and PKA type I on the one hand and NMJ regeneration status and morphology, AChR stability and specificity of subsynaptic cAMP handling on the other hand. This suggests an important role of myosin Va and PKA type I for the maturation of NMJs in regenerating muscle.

Rapsyn mediates subsynaptic anchoring of PKA type I and stabilisation of acetylcholine receptor in vivo

The stabilisation of acetylcholine receptors (AChRs) at the neuromuscular junction depends on muscle activity and the cooperative action of myosin Va and protein kinase A (PKA) type I. To execute its function, PKA has to be present in a subsynaptic microdomain where it is enriched by anchoring proteins. Here, we show that the AChR-associated protein, rapsyn, interacts with PKA type I in C2C12 and T-REx293 cells as well as in live mouse muscle beneath the neuromuscular junction. Molecular modelling, immunoprecipitation and bimolecular fluorescence complementation approaches identify an α-helical stretch of rapsyn to be crucial for binding to the dimerisation and docking domain of PKA type I. When expressed in live mouse muscle, a peptide encompassing the rapsyn α-helical sequence efficiently delocalises PKA type I from the neuromuscular junction. The same peptide, as well as a rapsyn construct lacking the α-helical domain, induces severe alteration of acetylcholine receptor turnover as well as fragmentation of synapses. This shows that rapsyn anchors PKA type I in close proximity to the postsynaptic membrane and suggests that this function is essential for synapse maintenance.

Myosin Va cooperates with PKA RIalpha to mediate maintenance of the endplate in vivo

Myosin V motor proteins facilitate recycling of synaptic receptors, including AMPA and acetylcholine receptors, in central and peripheral synapses, respectively. To shed light on the regulation of receptor recycling, we employed in vivo imaging of mouse neuromuscular synapses. We found that myosin Va cooperates with PKA on the postsynapse to maintain size and integrity of the synapse; this cooperation also regulated the lifetime of acetylcholine receptors. Myosin Va and PKA colocalized in subsynaptic enrichments. These accumulations were crucial for synaptic integrity and proper cAMP signaling, and were dependent on AKAP function, myosin Va, and an intact actin cytoskeleton. The neuropeptide and cAMP agonist, calcitonin-gene related peptide, rescued fragmentation of synapses upon denervation. We hypothesize that neuronal ligands trigger local activation of PKA, which in turn controls synaptic integrity and turnover of receptors. To this end, myosin Va mediates correct positioning of PKA in a postsynaptic microdomain, presumably by tethering PKA to the actin cytoskeleton.

Design of proteolytically stable RI-anchoring disruptor peptidomimetics for in vivo studies of anchored type I protein kinase A-mediated signalling

We have reported previously the design of a RIAD (RI-anchoring disruptor) peptide that specifically displaces PKA (protein kinase A) type I from the AKAP (A-kinase-anchoring protein) ezrin, which is present in the immunological synapse of T-cells. This increases immune reactivity by reducing the threshold for activation and may prove a feasible approach for improving immune function in patients with cAMP-mediated T-cell dysfunction. However, the use of RIAD in biological systems is restricted by its susceptibility to enzymatic cleavage and, consequently, its short half-life in presence of the ubiquitous serum peptidases. In the present study, carefully selected non-natural amino acids were employed in the design of RIAD analogues with improved stability. The resulting peptidomimetics demonstrated up to 50-fold increased half-lives in serum compared with RIAD, while maintaining similar or improved specificity and potency with respect to disruption of PKA type I-AKAP interactions.

Data recovery and integration from public databases uncovers transformation-specific transcriptional downregulation of cAMP-PKA pathway-encoding genes

BACKGROUND

The integration of data from multiple genome-wide assays is essential for understanding dynamic spatio-temporal interactions within cells. Such integration, which leads to a more complete view of cellular processes, offers the opportunity to rationalize better the high amount of "omics" data freely available in several public databases.In particular, integration of microarray-derived transcriptome data with other high-throughput analyses (genomic and mutational analysis, promoter analysis) may allow us to unravel transcriptional regulatory networks under a variety of physio-pathological situations, such as the alteration in the cross-talk between signal transduction pathways in transformed cells.

RESULTS

Here we sequentially apply web-based and statistical tools to a case study: the role of oncogenic activation of different signal transduction pathways in the transcriptional regulation of genes encoding proteins involved in the cAMP-PKA pathway. To this end, we first re-analyzed available genome-wide expression data for genes encoding proteins of the downstream branch of the PKA pathway in normal tissues and human tumor cell lines. Then, in order to identify mutation-dependent transcriptional signatures, we classified cancer cells as a function of their mutational state. The results of such procedure were used as a starting point to analyze the structure of PKA pathway-encoding genes promoters, leading to identification of specific combinations of transcription factor binding sites, which are neatly consistent with available experimental data and help to clarify the relation between gene expression, transcriptional factors and oncogenes in our case study.

CONCLUSIONS

Genome-wide, large-scale "omics" experimental technologies give different, complementary perspectives on the structure and regulatory properties of complex systems. Even the relatively simple, integrated workflow presented here offers opportunities not only for filtering data noise intrinsic in high throughput data, but also to progressively extract novel information that would have remained hidden otherwise. In fact we have been able to detect a strong transcriptional repression of genes encoding proteins of cAMP/PKA pathway in cancer cells of different genetic origins. The basic workflow presented herein may be easily extended by incorporating other tools and can be applied even by researchers with poor bioinformatics skills.

Androgen receptor immunoreactivity in skeletal muscle: enrichment at the neuromuscular junction

Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.

Related protein-protein interaction modules present drastically different surface topographies despite a conserved helical platform

The subcellular localization of cAMP-dependent protein kinase (PKA) occurs through interaction with A-Kinase Anchoring Proteins (AKAPs). AKAPs bind to the PKA regulatory subunit dimer of both type Ialpha and type IIalpha (RIalpha and RIIalpha). RIalpha and RIIalpha display characteristic localization within different cell types, which is maintained by interaction of AKAPs with the N-terminal dimerization and docking domain (D/D) of the respective regulatory subunit. Previously, we reported the solution structure of RIIa D/D module, both free and bound to AKAPs. We have now solved the solution structure of the dimerization and docking domain of the type Ialpha regulatory dimer subunit (RIalpha D/D). RIalpha D/D is a compact docking module, with unusual interchain disulfide bonds that help maintain the AKAP interaction surface. In contrast to the shallow hydrophobic groove for AKAP binding across the surface of the RIIalpha D/D dimeric interface, the RIalpha D/D module presents a deep cleft for proposed AKAP binding. RIalpha and RIIalpha D/D interaction modules present drastically differing dimeric topographies, despite a conserved X-type four-helix bundle structure.

Cyclic AMP-dependent protein kinase A and CREB are involved in neuregulin-induced synapse-specific expression of acetylcholine receptor gene

Neuregulin is reported to stimulate synapse-specific transcription of acetylcholine receptor (AChR) genes in the skeletal muscle fiber by multiple signaling pathways such as ERK, PI3K, and JNK. The co-localization of PKA mRNA with AChR and ErbBs, receptors for neuregulin, at the confined region of synapse implicates the putative role of PKA in neuregulin-induced AChR gene expression. In the present study, we found that mRNA and protein of a regulatory subunit of PKA (PKARIalpha) were concentrated at synaptic sites of the rat sternomastoid muscle fiber, while those of ERK and PI3K were uniformly distributed throughout the muscle fiber. Neuregulin (100 ng/ml) increased both PKA activity in the nucleus and AChRdelta subunit gene transcription in cultured Sol8 myotubes. These increases were significantly blocked by a specific PKA inhibitor H-89 (100 nM) and an adenylcyclase inhibitor SQ 22536 (200 microM) (72.5% and 60.1%, respectively). Furthermore, neuregulin phosphorylated CREB, a well-known down-stream transcription factor of PKA. While H-89 inhibited CREB phosphorylation, H-89 and PD098059 (50 microM), a specific MEK1/2 inhibitor, did not inhibit the phosphorylation of ERK and CREB, respectively, suggesting no cross-talk between PKA and ERK pathways. In conclusion, neuregulin increases AChRdelta subunit gene transcription, in part, by the activation of PKA/CREB, an alternative route to the previously reported ERK signaling pathway.

Designing isoform-specific peptide disruptors of protein kinase A localization

A kinase-anchoring proteins (AKAPs) coordinate cAMP-mediated signaling by binding and localizing cAMP-dependent protein kinase (PKA), using an amphipathic helical docking motif. Peptide disruptors of PKA localization that mimic this helix have been used successfully to assess the involvement of PKA in specific signaling pathways. However, these peptides were developed as disruptors for the type II regulatory subunit (RII) even though both RI and RII isoforms can bind to AKAPs and have discrete functions. To evaluate the effects of each localized isoform, we designed peptides that specifically bind to either RI or RII. Using a peptide array, we have defined the minimal binding sequence of dual specific-AKAP 2 (d-AKAP2), which binds tightly to both RI and RII. Side-chain requirements for affinity and isoform specificity were evaluated by using a peptide substitution array where each position along the A kinase binding domain of d-AKAP2 was substituted by the other 19 l-amino acids. This array comprises 513 single-site substitution analogs of the d-AKAP2 sequence. Peptides containing single and multiple mutations were evaluated in a quantitative fluorescence binding assay and a cell-based colocalization assay. This strategy has allowed us to design peptides with high affinity (K(D) = 1-2 nM) and high specificity for RIalpha versus RIIalpha. These isoform-specific peptides will be invaluable tools to evaluate functional differences between localized RI and RII PKA and are RIalpha-specific disruptors. This array-based analysis also provides a foundation for biophysical analysis of this docking motif.

USF2 inhibits C/EBP-mediated transcriptional regulation of the RIIbeta subunit of cAMP-dependent protein kinase

BACKGROUND

Cyclic AMP-dependent protein kinase (PKA) plays a central role in regulation of energy metabolism. Upon stimulation of testicular Sertoli cells by follicle stimulating hormone (FSH), glycolysis is activated to increase the production of nutrients for the germ cells, and a new regulatory subunit of cAMP-dependent protein kinase, RIIbeta, is induced. We have previously shown that production of the transcription factor C/EBPbeta is rapidly increased by FSH and cAMP in primary Sertoli cell cultures, and that C/EBPbeta induces the RIIbeta promoter.

RESULTS

In this work we show that USF1, USF2 and truncated USF isoforms bind to a conserved E-box in the RIIbeta gene. Interestingly, overexpression of USF2, but not USF1, led to inhibition of both cAMP- and C/EBPbeta-mediated induction of RIIbeta. Furthermore, Western blots show that a novel USF1 isoform is induced by cAMP in Sertoli cells.

CONCLUSIONS

These results indicate that the expression of various USF isoforms may be regulated by cAMP, and that the interplay between USF and C/EBPbeta is important for cAMP-mediated regulation of RIIbeta expression. The counteracting effects of USF2 and C/EBPbeta observed on the RIIbeta promoter is in accordance with the hypothesis that C/EBP and USF play opposite roles in regulation of glucose metabolism.

PKA, PKC, and AKAP localization in and around the neuromuscular junction

BACKGROUND

One mechanism that directs the action of the second messengers, cAMP and diacylglycerol, is the compartmentalization of protein kinase A (PKA) and protein kinase C (PKC). A-kinase anchoring proteins (AKAPs) can recruit both enzymes to specific subcellular locations via interactions with the various isoforms of each family of kinases. We found previously that a new class of AKAPs, dual-specific AKAPs, denoted D-AKAP1 and D-AKAP2, bind to RIalpha in addition to the RII subunits.

RESULTS

Immunohistochemistry and confocal microscopy were used here to determine that D-AKAP1 colocalizes with RIalpha at the postsynaptic membrane of the vertebrate neuromuscular junction (NMJ) and the adjacent muscle, but not in the presynaptic region. The labeling pattern for RIalpha and D-AKAP1 overlapped with mitochondrial staining in the muscle fibers, consistent with our previous work showing D-AKAP1 association with mitochondria in cultured cells. The immunoreactivity of D-AKAP2 was distinct from that of D-AKAP1. We also report here that even though the PKA type II subunits (RIIalpha and RIIbeta) are localized at the NMJ, their patterns are distinctive and differ from the other R and D-AKAP patterns examined. PKCbeta appeared to colocalize with the AKAP, gravin, at the postsynaptic membrane.

CONCLUSIONS

The kinases and AKAPs investigated have distinct patterns of colocalization, which suggest a complex arrangement of signaling micro-environments. Because the labeling patterns for RIalpha and D-AKAP 1 are similar in the muscle fibers and at the postsynaptic membrane, it may be that this AKAP anchors RIalpha in these regions. Likewise, gravin may be an anchor of PKCbeta at the NMJ.