Inhibition of Gap Junction Activity through the Release of the C1B Domain of Protein Kinase Cγ (PKCγ) from 14-3-3

Special Issue: "Molecular Dynamics Simulations and Structural Analysis of Protein Domains"

The 3D protein structure is the basis for all their biological functions [...].

An Update on Protein Kinases as Therapeutic Targets-Part II: Peptides as Allosteric Protein Kinase C Modulators Targeting Protein-Protein Interactions

Human protein kinases are highly-sought-after drug targets, historically harnessed for treating cancer, cardiovascular disease, and an increasing number of autoimmune and inflammatory conditions. Most current treatments involve small molecule protein kinase inhibitors that interact orthosterically with the protein kinase ATP-binding pocket. As a result, these compounds are often poorly selective and highly toxic. Part I of this series reviews the role of PKC isoforms in various human diseases, featuring cancer and cardiovascular disease, as well as translational examples of PKC modulation applied to human health and disease. In the present Part II, we discuss alternative allosteric binding mechanisms for targeting PKC, as well as novel drug platforms, such as modified peptides. A major goal is to design protein kinase modulators with enhanced selectivity and improved pharmacological properties. To this end, we use molecular docking analysis to predict the mechanisms of action for inhibitor-kinase interactions that can facilitate the development of next-generation PKC modulators.

Investigation and Functional Enrichment Analysis of the Human Host Interaction Network with Common Gram-Negative Respiratory Pathogens Predicts Possible Association with Lung Adenocarcinoma

Haemophilus influenzae (Hi), Moraxella catarrhalis (MorCa) and Pseudomonas aeruginosa (Psa) are three of the most common gram-negative bacteria responsible for human respiratory diseases. In this study, we aimed to identify, using the functional enrichment analysis (FEA), the human gene interaction network with the aforementioned bacteria in order to elucidate the full spectrum of induced pathogenicity. The Human Pathogen Interaction Database (HPIDB 3.0) was used to identify the human proteins that interact with the three pathogens. FEA was performed via the ToppFun tool of the ToppGene Suite and the GeneCodis database so as to identify enriched gene ontologies (GO) of biological processes (BP), cellular components (CC) and diseases. In total, 11 human proteins were found to interact with the bacterial pathogens. FEA of BP GOs revealed associations with mitochondrial membrane permeability relative to apoptotic pathways. FEA of CC GOs revealed associations with focal adhesion, cell junctions and exosomes. The most significantly enriched annotations in diseases and pathways were lung adenocarcinoma and cell cycle, respectively. Our results suggest that the Hi, MorCa and Psa pathogens could be related to the pathogenesis and/or progression of lung adenocarcinoma via the targeting of the epithelial cellular junctions and the subsequent deregulation of the cell adhesion and apoptotic pathways. These hypotheses should be experimentally validated.

STAC proteins: The missing link in skeletal muscle EC coupling and new regulators of calcium channel function

Excitation-contraction coupling is the signaling process by which action potentials control calcium release and consequently the force of muscle contraction. Until recently, three triad proteins were known to be essential for skeletal muscle EC coupling: the voltage-gated calcium channel CaV1.1 acting as voltage sensor, the SR calcium release channel RyR1 representing the only relevant calcium source, and the auxiliary CaV β1a subunit. Whether CaV1.1 and RyR1 are directly coupled or whether their interaction is mediated by another triad protein is still unknown. The recent identification of the adaptor protein STAC3 as fourth essential component of skeletal muscle EC coupling prompted vigorous research to reveal its role in this signaling process. Accumulating evidence supports its possible involvement in linking CaV1.1 and RyR1 in skeletal muscle EC coupling, but also indicates a second, much broader role of STAC proteins in the regulation of calcium/calmodulin-dependent feedback regulation of L-type calcium channels.

Specific neural phase relation of serotonin and dopamine modulate the testicular activity in Japanese quail

Specific phase relation of serotonin and dopamine modulate the hypothalamo-hypophyseal-gonadal axis as well as photosexual responses in Japanese quail, but the effect of these specific phase relations on testicular activity and steroidogenesis is not yet been investigated. We hypothesized that temporal phase relation induced alteration in local testicular gonadotropin-releasing hormone (GnRH)-Gonadotropin-inhibitory hormone (GnIH) and their receptor system may modulate the testicular activity and steroidogenesis through local (paracrine and autocrine) action. To validate this hypothesis, we have checked the alterations in the expression of gonadotropin-releasing hormone receptor (GnRH-R), gonadotropin-inhibitory hormone receptor (GnIH-R) messenger RNA (mRNA), growth hormone receptor (GH-R), proliferating cell nuclear antigen (PCNA), cell communication and gap junctional proteins (14-3-3 and connexin-43 [Cnx-43]), steroidogenic factor-1 (SF-1), steroidogenic acute regulatory (StAR) protein, steroidogenic enzyme (3β-hydroxysteroid dehydrogenase [3β-HSD]) in testis as well as androgen receptor (AR) in testis and epididymis of control, 8-, and 12-hr quail. Experimental findings clearly indicate the increased expression of GnIH-R mRNA and suppression of GnRH-R, GH-R, PCNA, 14-3-3, Cnx-43, SF-1, StAR, 3β-HSD in testis as well as AR in testis and epididymis in 8-hr quail, while 12-hr quail exhibited the opposite results that is significantly decreased expression of GnIH-R mRNA and increased expression of GnRH-R, GH-R, PCNA, 14-3-3, Cnx-43, SF-1, StAR, 3β-HSD in testis as well as AR in testis and epididymis. The significantly increased intratesticular testosterone has been observed in the 12-hr quail while, 8-hr quail showed opposite result. Hence, it can be concluded that 12-hr quail showed significantly increased testicular activity and steroidogenesis while opposite pattern was observed in 8-hr quail.

Co-treatment with a C1B5 peptide of protein kinase Cγ and a low dose of gemcitabine strongly attenuated pancreatic cancer growth in mice through T cell activation

Although gemcitabine is an effective chemotherapeutic for pancreatic cancer, severe side effects often accompany its use. Since we have discovered that locally administered C1B domain peptides effectively control tumor growth without any side effects, the efficacy of co-treatment with this peptide and a low dose of gemcitabine on the growth of pancreatic cancer was examined. Two- and three-dimensional cell culture studies clarified that a co-treatment with C1B5 peptide and gemcitabine significantly attenuated growth of PAN02 mouse and PANC-1 human pancreatic cancer cells in 2D and 3D cultures. Although treatment with the low dose of gemcitabine alone (76%) or the C1B5 peptide alone (39%) inhibited tumor growth moderately, a co-treatment with C1B5 peptide and a low dose of gemcitabine markedly inhibited the growth of PAN02 autografts in the mouse peritoneal cavity (94% inhibition) without any noticeable adverse effect. The number of peritoneal cavity-infiltrating neutrophils and granzyme B⁺ lymphocytes was significantly higher in the co-treatment group than in the control group. A significant increase of granzyme B mRNA expression was also detected in human T cells by the co-treatment. Taken together, the current study suggests that C1B5 peptide offers a remarkably effective combination treatment strategy to reduce side effects associated with gemcitabine, without losing its tumoricidal effect.

STAC3 stably interacts through its C1 domain with CaV1.1 in skeletal muscle triads

The adaptor protein STAC3 is essential for skeletal muscle excitation-contraction (EC) coupling and a mutation in the STAC3 gene has been linked to a severe muscle disease, Native American myopathy (NAM). However the function of STAC3, its interaction partner, and the mode of interaction within the EC-coupling complex remained elusive. Here we demonstrate that STAC3 forms a stable interaction with the voltage-sensor of EC-coupling, Ca_V1.1, and that this interaction depends on a hitherto unidentified protein-protein binding pocket in the C1 domain of STAC3. While the NAM mutation does not affect the stability of the STAC3-Ca_V1.1 interaction, mutation of two crucial residues in the C1 binding pocket increases the turnover of STAC3 in skeletal muscle triads. Thus, the C1 domain of STAC3 is responsible for its stable incorporation into the Ca_V1.1 complex, whereas the SH3 domain containing the NAM mutation site may be involved in low-affinity functional interactions in EC-coupling.

Tuning the signalling output of protein kinase C

Precise control of the amplitude of protein kinase C (PKC) signalling is essential for cellular homoeostasis, and disruption of this control leads to pathophysiological states such as cancer, neurodegeneration and diabetes. For conventional and novel PKC, this amplitude is meticulously tuned by multiple inputs that regulate the amount of enzyme in the cell, its ability to sense its allosteric activator diacylglycerol, and protein scaffolds that co-ordinate access to substrates. Key to regulation of the signalling output of most PKC isoenzymes is the ability of cytosolic enzyme to respond to the membrane-embedded lipid second messenger, diacylglycerol, in a dynamic range that prevents signalling in the absence of agonists but allows efficient activation in response to small changes in diacylglycerol levels. The present review discusses the regulatory inputs that control the spatiotemporal dynamics of PKC signalling, with a focus on conventional and novel PKC isoenzymes.

L-type calcium channels play a critical role in maintaining lens transparency by regulating phosphorylation of aquaporin-0 and myosin light chain and expression of connexins

Homeostasis of intracellular calcium is crucial for lens cytoarchitecture and transparency, however, the identity of specific channel proteins regulating calcium influx within the lens is not completely understood. Here we examined the expression and distribution profiles of L-type calcium channels (LTCCs) and explored their role in morphological integrity and transparency of the mouse lens, using cDNA microarray, RT-PCR, immunoblot, pharmacological inhibitors and immunofluorescence analyses. The results revealed that Ca (V) 1.2 and 1.3 channels are expressed and distributed in both the epithelium and cortical fiber cells in mouse lens. Inhibition of LTCCs with felodipine or nifedipine induces progressive cortical cataract formation with time, in association with decreased lens weight in ex-vivo mouse lenses. Histological analyses of felodipine treated lenses revealed extensive disorganization and swelling of cortical fiber cells resembling the phenotype reported for altered aquaporin-0 activity without detectable cytotoxic effects. Analysis of both soluble and membrane rich fractions from felodipine treated lenses by SDS-PAGE in conjunction with mass spectrometry and immunoblot analyses revealed decreases in β-B1-crystallin, Hsp-90, spectrin and filensin. Significantly, loss of transparency in the felodipine treated lenses was preceded by an increase in aquaporin-0 serine-235 phosphorylation and levels of connexin-50, together with decreases in myosin light chain phosphorylation and the levels of 14-3-3ε, a phosphoprotein-binding regulatory protein. Felodipine treatment led to a significant increase in gene expression of connexin-50 and 46 in the mouse lens. Additionally, felodipine inhibition of LTCCs in primary cultures of mouse lens epithelial cells resulted in decreased intracellular calcium, and decreased actin stress fibers and myosin light chain phosphorylation, without detectable cytotoxic response. Taken together, these observations reveal a crucial role for LTCCs in regulation of expression, activity and stability of aquaporin-0, connexins, cytoskeletal proteins, and the mechanical properties of lens, all of which have a vital role in maintaining lens function and cytoarchitecture.

Insight into hypoxic preconditioning and ischemic injury through determination of nPKCε-interacting proteins in mouse brain

Cerebral hypoxic preconditioning (HPC) provides neuroprotection by intracellular signaling pathways. We previously demonstrated that novel protein kinase Cε (nPKCε) activation participated in cerebral HPC development. In this study, we explore the role of nPKCε in HPC-induced neuroprotection against middle cerebral artery occlusion (MCAO)-induced ischemic injury and identify its possible signaling molecules. A total of 131 adult male BALB/c mice were divided into eight groups: normoxic control (n=9), HPC (n=9), HPC+εV1-2 (n=13), Sham (n=19), HPC+sham (n=6), Ischemia (I, 6h MCAO, n=31), HPC+I (n=25) and HPC+εV1-2+I (n=19). nPKCε specific inhibitor εV1-2 was administered via intracerebroventricular injection. Western blot, 2,3,5-triphenyltetrazolium chloride staining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling were applied to determine nPKCε membrane translocation, infarction volume and programmed cell death (PCD), respectively. Two-dimensional gel electrophoresis (2-De) and matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) were used to identify nPKCε-interacting proteins, followed by bioinformatics analysis of genee ontology (GO) to predict nPKCε-specific signaling pathways. Our results showed that HPC attenuates MCAO-induced brain injuries and stabilized nPKCεmembrane translocation in peri-infarct region, which was abolished by nPKCε-speecific inhibitor εV1-2. Proteomics analysis revealed 8 up- and 3 down-regulated nPKCε-interacting proteins both in cytosolic and particulate fractions of HPC mouse brain. GO analysis predicted 25 significant nPKCε-specific signaling pathways among the 16 identified nPKCε-interacting proteins in brain of HPC mice. This study is the first to report multiple nPKCε-interacting proteins and their signaling pathways in HPC mouse brain, suggesting that nPKCε signaling molecules is responsible for HPC-induced neuroprotection against cerebral ischemic injuries of mice.

C1B domain peptide of protein kinase Cγ significantly suppresses growth of human colon cancer cells in vitro and in an in vivo mouse xenograft model through induction of cell cycle arrest and apoptosis

Two peptides derived from the C1B domain of protein kinase Cγ (PKCγ) were shown to associate with classical PKC isozymes and modulate their activities. These C1B peptides are designated C1B1 (amino acid residues 101-112) and C1B5 (residues 141-151). Since PKC enzyme activity is shown to be involved in colon cancer development, the effect of C1B peptides on the growth of various human colon cancer cell lines was examined in vitro and in vivo. Sub-micromolar to micromolar levels of both C1B peptides induced approximately 60-70% growth attenuation in multiple colon cancer cell lines in a soft agar tumor colony assay; however, C1B5 peptide was not cytotoxic to normal colon epithelial cells in two dimensional culture. The effect of C1B5 peptide on colony growth of COLO205 cells was reversed by treatment with the PKCα/β inhibitor, Ro-32-0432. C1B peptide treatment attenuated COLO205 cells via two mechanisms: 1) cell cycle arrest and 2) stimulation of apoptosis. This is evident in G 2 arrest and increases in levels of cleaved caspase 3 and p53 phosphorylated at serine 20. Intratumoral injection of C1B5 peptide (20 mg/kg/day, every three days) markedly attenuated the growth of subcutaneous xenografts of COLO205 cells in SCID mice by 76% compared with the control. Taken together, these results strongly suggest that C1B peptides have negligible effects on normal tissues but are potentially effective chemotherapeutic agents for colon cancer.

p23/Tmp21 associates with protein kinase Cdelta (PKCdelta) and modulates its apoptotic function

There is emerging evidence that C1 domains, motifs originally identified in PKC isozymes and responsible for binding of phorbol esters and diacylglycerol, interact with the Golgi/endoplasmic reticulum protein p23 (Tmp21). In this study, we investigated whether PKCδ, a kinase widely implicated in apoptosis and inhibition of cell cycle progression, associates with p23 and determined the potential functional implications of this interaction. Using a yeast two-hybrid approach, we found that the PKCδ C1b domain associates with p23 and identified two key residues (Asp(245) and Met(266)) implicated in this interaction. Interestingly, silencing p23 from LNCaP prostate cancer cells using RNAi markedly enhanced PKCδ-dependent apoptosis and activation of PKCδ downstream effectors ROCK and JNK by phorbol 12-myristate 13-acetate. Moreover, translocation of PKCδ to the plasma membrane by phorbol 12-myristate 13-acetate was enhanced in p23-depleted LNCaP cells. Notably, a PKCδ mutant that failed to interact with p23 triggered a strong apoptotic response when expressed in LNCaP cells. In summary, our data compellingly support the concept that C1 domains have dual roles both in lipid and protein associations and provide strong evidence that p23 acts as an anchoring protein that retains PKCδ at the perinuclear region, thus limiting the availability of this kinase for activation in response to stimuli.

14-3-3epsilon contributes to tumour suppression in laryngeal carcinoma by affecting apoptosis and invasion

Background

14-3-3epsilon regulates a wide range of biological processes, including cell cycle control, proliferation, and apoptosis, and plays a significant role in neurogenesis and the formation of malignant tumours. However, the exact function and regulatory mechanism of 14-3-3epsilon in carcinogenesis have not been elucidated.

Methods

The expression of 14-3-3epsilon was assessed by RT-PCR and western blotting. The invasiveness and viability of Hep-2 cells were determined by the transwell migration assay and MTT assay, respectively. Cell cycle and apoptosis of Hep-2 cells were detected by flow cytometry.

Results

The mRNA and protein expression of 14-3-3epsilon in larynx squamous cell carcinoma (LSCC) tissues were significantly lower than those in clear surgical margin tissues. Statistical analysis showed that the 14-3-3epsilon protein level in metastatic lymph nodes was lower than that in paired tumour tissues. In addition, the protein level of 14-3-3epsilon in stage III or IV tumours was significantly lower than that in stage I or II tumours. Compared with control Hep-2 cells, the percentages of viable cells in the 14-3-3epsilon-GFP and negative control GFP groups were 36.68 ± 14.09% and 71.68 ± 12.10%, respectively. The proportions of S phase were 22.47 ± 3.36%, 28.17 ± 3.97% and 46.15 ± 6.82%, and the apoptotic sub-G1 populations were 1.23 ± 1.02%, 2.92 ± 1.59% and 13.72 ± 3.89% in the control, negative control GFP and 14-3-3epsilon-GFP groups, respectively. The percentages of the apoptotic cells were 0.84 ± 0.25%, 1.08 ± 0.24% and 2.93 ± 0.13% in the control, negative control GFP and 14-3-3epsilon-GFP groups, respectively. The numbers of cells that penetrated the filter membrane in the control, negative control GFP and 14-3-3epsilon-GFP groups were 20.65 ± 1.94, 17.63 ± 1.04 and 9.1 ± 0.24, respectively, indicating significant differences among the different groups.

Conclusions

Decreased expression of 14-3-3epsilon in LSCC tissues contributes to the initiation and progression of LSCC. 14-3-3epsilon can promote apoptosis and inhibit the invasiveness of LSCC.

p23/Tmp21 differentially targets the Rac-GAP beta2-chimaerin and protein kinase C via their C1 domains

The ER/Golgi protein p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β-chimaerin. C1 domains from PKC isozymes can also interact with p23/Tmp21. Our study highlights the relevance of C1 domains in protein-protein interactions in addition to their well-established lipid-binding properties.

The C1 domains in protein kinase C (PKC) isozymes and other signaling molecules are responsible for binding the lipid second messenger diacylglycerol and phorbol esters, and for mediating translocation to membranes. Previous studies revealed that the C1 domain in α- and β-chimaerins, diacylglycerol-regulated Rac-GAPs, interacts with the endoplasmic reticulum/Golgi protein p23/Tmp21. Here, we found that p23/Tmp21 acts as a C1 domain-docking protein that mediates perinuclear translocation of β2-chimaerin. Glu227 and Leu248 in the β2-chimaerin C1 domain are crucial for binding p23/Tmp21 and perinuclear targeting. Interestingly, isolated C1 domains from individual PKC isozymes differentially interact with p23/Tmp21. For PKCε, it interacts with p23/Tmp21 specifically via its C1b domain; however, this association is lost in response to phorbol esters. These results demonstrate that p23/Tmp21 acts as an anchor that distinctively modulates compartmentalization of C1 domain-containing proteins, and it plays an essential role in β2-chimaerin relocalization. Our study also highlights the relevance of C1 domains in protein–protein interactions in addition to their well-established lipid-binding properties.

Loss of protein kinase Cgamma in knockout mice and increased retinal sensitivity to hyperbaric oxygen

OBJECTIVE

To determine if loss of protein kinase Cgamma (PKCgamma) results in increased structural damage to the retina by hyperbaric oxygen (HBO), a treatment used for several ocular disorders.

METHODS

Six-week-old mice were exposed in vivo to 100% HBO 3 times a week for 8 weeks. Eyes were dissected, fixed, embedded in Epon, sectioned, stained with toluidine blue O, and examined by light microscopy.

RESULTS

The thicknesses of the inner nuclear and ganglion cell layers were increased. Destruction of the outer plexiform layer was observed in the retinas of the PKCgamma-knockout mice relative to control mice. Exposure to HBO caused significant degradation of the retina in knockout mice compared with control mice. Damage to the outer segments of the photoreceptor layer and ganglion cell layer was apparent in central retinas of HBO-treated knockout mice.

CONCLUSIONS

Protein kinase Cgamma-knockout mice had increased retinal sensitivity to HBO. Results demonstrate that PKCgamma protects retinas from HBO damage.

CLINICAL RELEVANCE

Care should be taken in treating patients with HBO, particularly if they have a genetic disease, such as spinocerebellar ataxia type 14, a condition in which the PKCgamma is mutated and nonfunctional.

Structural basis of protein kinase C isoform function

Protein kinase C (PKC) isoforms comprise a family of lipid-activated enzymes that have been implicated in a wide range of cellular functions. PKCs are modular enzymes comprised of a regulatory domain (that contains the membrane-targeting motifs that respond to lipid cofactors, and in the case of some PKCs calcium) and a relatively conserved catalytic domain that binds ATP and substrates. These enzymes are coexpressed and respond to similar stimulatory agonists in many cell types. However, there is growing evidence that individual PKC isoforms subserve unique (and in some cases opposing) functions in cells, at least in part as a result of isoform-specific subcellular compartmentalization patterns, protein-protein interactions, and posttranslational modifications that influence catalytic function. This review focuses on the structural basis for differences in lipid cofactor responsiveness for individual PKC isoforms, the regulatory phosphorylations that control the normal maturation, activation, signaling function, and downregulation of these enzymes, and the intra-/intermolecular interactions that control PKC isoform activation and subcellular targeting in cells. A detailed understanding of the unique molecular features that underlie isoform-specific posttranslational modification patterns, protein-protein interactions, and subcellular targeting (i.e., that impart functional specificity) should provide the basis for the design of novel PKC isoform-specific activator or inhibitor compounds that can achieve therapeutically useful changes in PKC signaling in cells.

Hypoxia-regulated activity of PKCepsilon in the lens

PURPOSE

To show that hypoxia is necessary to prevent opacification of the lens. Protein kinase C (PKC)-epsilon serves a role that is distinct from PKC-gamma when both PKC isoforms are expressed in the lens. PKCepsilon serves a very important role in hypoxic conditions, helping to prevent opacification of the lens.

METHODS

Digital image analysis, confocal microscopy, dye transfer assay, coimmunoprecipitation, Western blot analysis, and enzyme activity assays were used, respectively, to study opacification of the lens, intercellular communications, cellular localization of connexin-43 (Cx43), and the interactions between PKCepsilon, PKCgamma, and Cx43 in the lens epithelial cells.

RESULTS

Hypoxic conditions (1%-5% of oxygen) were very important in maintaining clarity of the lenses of wild-type (WT) mice. Normoxic conditions induced opacification of the WT lens. Lenses from the PKCepsilon-knockout mice underwent rapid opacification, even in hypoxic conditions. Hypoxia did not induce apoptosis in the lens epithelial cells, judging by the absence of active caspase-3, and it did not change intercellular communication and did not affect the number and localization of junctional Cx43 plaques in the lens epithelial cell culture. Hypoxia activated PKCepsilon, whereas phorbol ester (TPA), oxidation (H(2)O(2)), and insulin-like growth factor-1 (IGF-1) activated PKCgamma and decreased the activity of PKCepsilon. Hypoxia did not induce the phosphorylation of the Cx43.

CONCLUSIONS

Hypoxia-induced activation of PKCepsilon is very important in surviving hypoxia and maintaining the clarity of the lens. However, PKCgamma is utilized in the control of Cx43 gap junctions.

The regulated assembly of a PKCepsilon complex controls the completion of cytokinesis

The cell cycle is exquisitely controlled by multiple sequential regulatory inputs to ensure fidelity. Here we demonstrate that the final step in division, the physical separation of daughter cells, is controlled by a member of the PKC gene superfamily. Specifically, we have identified three phosphorylation sites within PKCepsilon that control its association with 14-3-3. These phosphorylations are executed by p38 MAP kinase (Ser 350), GSK3 (Ser 346) and PKC itself (Ser 368). Integration of these signals is essential during mitosis because mutations that prevent phosphorylation of PKCepsilon and/or PKCepsilon binding to 14-3-3 also cause defects in the completion of cytokinesis. Using chemical genetic and dominant-negative approaches it is shown that selective inhibition of PKCepsilon halts cells at the final stages of separation. This arrest is associated with persistent RhoA activation at the midbody and a delay in actomyosin ring dissociation. This study therefore identifies a new regulatory mechanism that controls exit from cytokinesis, which has implications for carcinogenesis.

Protein kinase Cepsilon binds peripherin and induces its aggregation, which is accompanied by apoptosis of neuroblastoma cells

A hallmark of the afflicted nervous tissue in amyotrophic lateral sclerosis is the presence of protein aggregates, which to a large extent contain the intermediate filament protein peripherin. Here we show that activation of protein kinase C (PKC) or overexpression of PKCepsilon induces the aggregation of peripherin in cultured neuroblastoma cells with elevated amounts of peripherin. The formation of aggregates was coupled to an increased apoptosis, suggesting a functional link between these events. Both induction of aggregates and apoptosis were suppressed in cells that had been transfected with small interfering RNAs targeting PKCepsilon. PKCepsilon and peripherin associate as shown by co-immunoprecipitation, and the interaction is dependent on and mediated by the C1b domain of PKCepsilon. The interaction was specific for PKCepsilon since corresponding structures from other isoforms did not co-precipitate peripherin, with the exception for PKCeta and -, which pulled down minute amounts. PKCepsilon interacts with vimentin through the same structures but does not induce its aggregation. When the PKCepsilon C1b domain is expressed in neuroblastoma cells together with peripherin, both phorbol ester-induced peripherin aggregation and apoptosis are abolished, supporting a model in which PKCepsilon through its interaction with peripherin facilitates its aggregation and subsequent cell death. These events may be prevented by expressing molecules that bind peripherin at the same site as PKCepsilon.

Akt phosphorylates Connexin43 on Ser373, a "mode-1" binding site for 14-3-3

Connexin43 (Cx43) is a membrane-spanning protein that forms channels that bridge the gap between adjacent cells and this allows for the intercellular exchange of information. Cx43 is regulated by phosphorylation and by interacting proteins. "Mode-1" interaction with 14-3-3 requires phosphorylation of Ser373 on Cx43 (Park et al. 2006). Akt phosphorylates and targets a number of proteins to interactions with 14-3-3. Here we demonstrate that Akt phosphorylates Cx43 on Ser373 and Ser369; antibodies recognizing Akt-phosphorylated sites or phospho-Ser "mode-1" 14-3-3-binding sites recognize a protein from EGF-treated cells that migrates as Cx43, and GST-14-3-3 binds to Cx43 phosphorylated endogenously in EGF-treated cells. Confocal microscopy supports the co-localization of Cx43 with Akt and with 14-3-3 at the outer edges of gap junctional plaques. These data suggest that Akt could target Cx43 to an interaction with 14-3-3 that may play a role in the forward trafficking of Cx43 multimers and/or their incorporation into existing gap junctional plaques.

Protection from ataxia-linked apoptosis by gap junction inhibitors

Mutations in the protein kinase C gamma (PKCgamma) gene cause spinocerebellar ataxia type 14 (SCA14), a heterogeneous neurodegenerative disorder. Synthetic peptides (C1B1) serve as gap junction inhibitors through activation of PKCgamma control of gap junctions. We investigated the neuroprotective potential of these peptides against SCA14 mutation-induced cell death using neuronal HT22 cells. The C1B1 synthetic peptides completely restored PKCgamma enzyme activity and subsequent control of gap junctions. PKCgamma SCA14 mutant proteins were shown to cause aggregation which initially resulted in endoplasmic reticulum (ER) stress and cell apoptosis as demonstrated by phosphorylation of PERK on Thr981, activation of caspase-12, increases in BiP/GRP78 protein levels, and consequent activation of caspase-3. Pre-incubation with C1B1 peptides completely abolished these SCA14 effects on ER stress and caspase-3 activation, suggesting that C1B1 peptides protect cells from apoptosis through inhibition of gap junctions by restoration of PKCgamma control of gap junctions, which may result in neuroprotection in SCA14.

14-3-3 proteins interact with the beta-thymosin repeat protein Csp24

Conditioned stimulus pathway protein 24 (Csp24) is a beta-thymosin-like protein that is homologous to other members of the family of beta-thymosin repeat proteins that contain multiple actin binding domains. Actin co-precipitates with Csp24 and co-localizes with it in the cytosol of type-B photoreceptor cell bodies. Several signal transduction pathways have been shown to regulate the phosphorylation of Csp24 and contribute to cellular plasticity. Here, we report the identification of the adapter protein 14-3-3 in lysates of the Hermissenda circumesophageal nervous system and its interaction with Csp24. Immunoprecipitation experiments using an antibody that is broadly reactive with several isoforms of the 14-3-3 family of proteins showed that Csp24 co-precipitates with 14-3-3 protein, and nervous systems stimulated with 5-HT exhibited a significant increase in co-precipitated Csp24 probed with a phosphospecific antibody as compared with controls. These results indicate that post-translational modifications of Csp24 regulate its interaction with 14-3-3 protein, and suggest that this mechanism may contribute to the control of intrinsic enhanced excitability.

Protein kinase C as a stress sensor

While there are many reviews which examine the group of proteins known as protein kinase C (PKC), the focus of this article is to examine the cellular roles of two PKCs that are important for stress responses in neurological tissues (PKC gamma and epsilon) and in cardiac tissues (PKC epsilon). These two kinases, in particular, seem to have overlapping functions and interact with an identical target, connexin 43 (Cx43), a gap junction protein which is central to proper control of signals in both tissues. While PKC gamma and PKC epsilon both help protect neural tissue from ischemia, PKC epsilon is the primary PKC isoform responsible for responding to decreased oxygen, or ischemia, in the heart. Both do this through Cx43. It is clear that both PKC gamma and PKC epsilon are necessary for protection from ischemia. However, the importance of these kinases has been inferred from preconditioning experiments which demonstrate that brief periods of hypoxia protect neurological and cardiac tissues from future insults, and that this depends on the activation, translocation, or ability for PKC gamma and/or PKC epsilon to interact with distinct cellular targets, especially Cx43. This review summarizes the recent findings which define the roles of PKC gamma and PKC epsilon in cardiac and neurological functions and their relationships to ischemia/reperfusion injury. In addition, a biochemical comparison of PKC gamma and PKC epsilon and a proposed argument for why both forms are present in neurological tissue while only PKC epsilon is present in heart, are discussed. Finally, the biochemistry of PKCs and future directions for the field are discussed, in light of this new information.

Protein kinase C gamma mutations in the C1B domain cause caspase-3-linked apoptosis in lens epithelial cells through gap junctions

Failure to control oxidative stress is closely related to aging and to a diverse range of human diseases. We have reported that protein kinase C gamma (PKCgamma) acts as a primary oxidative stress sensor in the lens. PKCgamma has a Zn-finger C1B stress switch domain, residues 101-150. Mutation, H101Y, in the C1B domain of PKCgamma proteins causes a failure of the PKCgamma oxidative stress response [Lin, D., Takemoto, D.J., 2005. Oxidative activation of protein kinase Cgamma through the C1 domain. Effects on gap junctions. J. Biol. Chem. 280, 13682-13693]. Some human neurodegenerative spinocerebellar ataxia type 14 are caused by mutations in the PKCgamma C1B domain. In the current study we have investigated the effects of these mutations on lens epithelial cell responses to oxidative stress. The results demonstrate that PKCgamma C1B mutants had lower basal enzyme activities and were not activated by H(2)O(2). Furthermore, the PKCgamma mutations caused a failure of endogenous wild type PKCgamma to be activated by H(2)O(2). These PKCgamma mutations abolished the effect of H(2)O(2) on phosphorylation of Cx43 and Cx50 by H(2)O(2) activation of PKCgamma. The cells with PKCgamma C1B mutations had more Cx43 and/or Cx50 gap junction plaques which were not decreased by H(2)O(2). Since open gap junctions could have a bystander effect this could cause apoptosis to occur. H(2)O(2) (100 microM, 3 h) activated a caspase-3 apoptotic pathway in the lens epithelial cells but was more severe in cells expressing PKCgamma mutations. The presence of 18alpha-glycyrrhetinic acid (AGA), an inhibitor of gap junctions, decreased Cx43 and Cx50 protein levels and gap junction plaque number. This reduction in gap junctions by AGA resulted in inhibition of H(2)O(2)-induced apoptosis. Our results demonstrate that there is a dominant negative effect of PKCgamma C1B mutations on endogenous PKCgamma which results in loss of control of gap junctions. Modeled structures suggest that the severity of C1B mutation effects may be related to the extent of loss of C1B structure. Mutations in the C1B domain of PKCgamma result in increased apoptosis in lens epithelial cells. This can be prevented by a gap junction inhibitor. Thus, propagation of apoptosis from cell-to-cell in lens epithelial cells may be through open gap junctions. The control of gap junctions requires PKCgamma.

Comparison of the PKCalpha and the PKCepsilon C1b domains: identification of residues critical for PKCepsilon-mediated neurite induction

We showed earlier that over-expression of protein kinase C (PKC) epsilon induces neurite outgrowth. The effect is mediated by a region (PKCepsilonPSC1V3) encompassing the pseudosubstrate, the two C1 domains and part of the V3 region, and is independent of the catalytic activity of the enzyme. In this region, residues immediately N-terminal of the C1b domain are crucial for neurite outgrowth. However, in this study we show that the PKCepsilon C1b domain itself is necessary for neurite induction, since a mutant in which the PKCepsilon C1b domain has been replaced with the C1b domain from PKCalpha, PKCepsilonPSC1a(alphaC1b)V3 lacks neurite-inducing capacity. The molecular basis for the importance of the PKCepsilon C1b domain was investigated by mutation studies of the PKCalpha C1b domain. Point mutations were done in the PKCalpha C1b domain of the PKCepsilonPSC1a(alphaC1b)V3 construct, in which the PKCalpha residues were mutated into the corresponding residues in PKCepsilon. This highlighted residues in the C-terminal part of the primary sequence of the C1b domain, located in the base of the C1b domain, as important for neurite outgrowth. The mutations S48P, D32K and L49N all influenced neurite induction positively. Furthermore, the mutation of L49N alone was sufficient to make PKCepsilonPSC1a(alphaC1b)V3 neuritogenic in phorbol ester-stimulated cells, and mutation of this residue in full-length PKCepsilon into the corresponding residue in PKCalpha, N291L reduced the neurite-inducing effect of PKCepsilon. In conclusion, we have identified residues in the PKCepsilon C1b domain, in particular Asn49, that are essential for neurite outgrowth.