Pavlovian conditioning-specific increases of the Ca2+- and GTP-binding protein, calexcitin in identified Hermissenda visual cells

Mechanistic insights on the antibacterial action of the kyotorphin peptide derivatives revealed by in vitro studies and Galleria mellonella proteomic analysis

OBJECTIVES

The selected kyotorphin derivatives were tested to improve their antimicrobial and antibiofilm activity. The antimicrobial screening of the KTP derivatives were ascertained in the representative strains of bacteria, including Streptococcus pneumoniae, Streptococcus pyogenes, Escherichia coli and Pseudomonas aeruginosa.

METHODS

Kyotorphin derivatives, KTP-NH2, KTP-NH2-DL, IbKTP, IbKTP-NH2, MetKTP-DL, MetKTP-LD, were designed and synthesized to improve lipophilicity and resistance to enzymatic degradation. Peptides were synthesized by standard solution or solid-phase peptide synthesis and purified using RP-HPLC, which resulted in >95 % purity, and were fully characterized by mass spectrometry and 1H NMR. The minimum inhibitory concentrations (MIC) determined for bacterial strains were between 20 and 419 μM. The direct effect of IbKTP-NH2 on bacterial cells was imaged using scanning electron microscopy. The absence of toxicity, high survival after infection and an increase in the hemocytes count was evaluated by injections of derivatives in Galleria mellonella larvae. Proteomics analyses of G. mellonella hemolymph were performed to investigate the underlying mechanism of antibacterial activity of IbKTP-NH2 at MIC.

RESULTS

IbKTP-NH2 induces morphological changes in bacterial cell, many differentially expressed proteins involved in DNA replication, synthesis of cell wall, and virulence were up-regulated after the treatment of G. mellonella with IbKTP-NH2.

CONCLUSION

We suggest that this derivative, in addition to its physical activity on the bacterial membranes, can elicit a cellular and humoral immune response, therefore, it could be considered for biomedical applications.

Binding of Gd(3+) to the neuronal signalling protein calexcitin identifies an exchangeable Ca(2+)-binding site

Calexcitin was first identified in the marine snail Hermissenda crassicornis as a neuronal-specific protein that becomes upregulated and phosphorylated in associative learning. Calexcitin possesses four EF-hand motifs, but only the first three (EF-1 to EF-3) are involved in binding metal ions. Past work has indicated that under physiological conditions EF-1 and EF-2 bind Mg(2+) and Ca(2+), while EF-3 is likely to bind only Ca(2+). The fourth EF-hand is nonfunctional owing to a lack of key metal-binding residues. The aim of this study was to use a crystallographic approach to determine which of the three metal-binding sites of calexcitin is most readily replaced by exogenous metal ions, potentially shedding light on which of the EF-hands play a `sensory' role in neuronal calcium signalling. By co-crystallizing recombinant calexcitin with equimolar Gd(3+) in the presence of trace Ca(2+), EF-1 was shown to become fully occupied by Gd(3+) ions, while the other two sites remain fully occupied by Ca(2+). The structure of the Gd(3+)-calexcitin complex has been refined to an R factor of 21.5% and an Rfree of 30.4% at 2.2 Å resolution. These findings suggest that EF-1 of calexcitin is the Ca(2+)-binding site with the lowest selectivity for Ca(2+), and the implications of this finding for calcium sensing in neuronal signalling pathways are discussed.

Mushroom body-preferential expression of proteins/genes involved in endoplasmic reticulum Ca(2+)-transport in the worker honeybee (Apis mellifera L.) brain

To identify the molecular characteristics specific to the mushroom body (MB, a higher processing centre) neurones in the honeybee brain, we previously used proteomics to identify proteins that are preferentially expressed in these MBs. Here we continued our proteomic analysis to show that reticulocalbin, which is involved in endoplasmic reticulum (ER) Ca(2+) transport, is also preferentially expressed in the MBs in the honeybee brain. Gene expression analysis revealed that reticulocalbin is preferentially expressed in the large-type Kenyon cells, which are MB-intrinsic neurones. In addition, the gene for the ryanodine receptor, which is also involved in ER Ca(2+) transport, was also preferentially expressed in the large-type Kenyon cells. In contrast, the expression of three other ER-related genes, protein disulphide isomerase, sec61 and erp60, was not enriched in the MBs. These findings further support the notion that the function of ER Ca(2+)-signalling, but not the mere intracellular density of ER, is specifically enhanced in the large-type Kenyon cells in the honeybee brain.

Proteomic analysis of short- and intermediate-term memory in Hermissenda

Changes in cellular and synaptic plasticity related to learning and memory are accompanied by both upregulation and downregulation of the expression levels of proteins. Both de novo protein synthesis and post-translational modification of existing proteins have been proposed to support the induction and maintenance of memory underlying learning. However, little is known regarding the identity of proteins regulated by learning that are associated with the early stages supporting the formation of memory over time. In this study we have examined changes in protein abundance at two different times following one-trial in vitro conditioning of Hermissenda using two-dimensional difference gel electrophoresis (2D-DIGE), quantification of differences in protein abundance between conditioned and unpaired controls, and protein identification with tandem mass spectrometry. Significant regulation of protein abundance following one-trial in vitro conditioning was detected 30 min and 3 h post-conditioning. Proteins were identified that exhibited statistically significant increased or decreased abundance at both 30 min and 3 h post-conditioning. Proteins were also identified that exhibited a significant increase in abundance only at 30 min, or only at 3 h post-conditioning. A few proteins were identified that expressed a significant decrease in abundance detected at both 30 min and 3 h post-conditioning, or a significant decrease in abundance only at 3 h post-conditioning. The proteomic analysis indicates that proteins involved in diverse cellular functions such as translational regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to the formation of memory produced by one-trial in vitro conditioning. These findings support the view that changes in protein abundance over time following one-trial in vitro conditioning involve dynamic and complex interactions of the proteome.

Proteomic analysis of post-translational modifications in conditioned Hermissenda

Post-translational modifications of proteins are a major determinant of biological function. Phosphorylation of proteins involved in signal transduction contributes to the induction and maintenance of several examples of cellular and synaptic plasticity. In this study we have identified phosphoproteins regulated by Pavlovian conditioning in lysates of Hermissenda nervous systems using two-dimensional electrophoresis (2DE) in conjunction with (32)P labeling, fluorescence based phosphoprotein in-gel staining, and mass spectrometry. Modification of protein phosphorylation regulated by conditioning was first assessed by densitometric analysis of (32)P labeled proteins resolved by 2DE from lysates of conditioned and pseudorandom control nervous systems. An independent assessment of phosphorylation regulated by conditioning was obtained from an examination of 2D gels stained with Pro-Q Diamond phosphoprotein dye. Mass spectrometric analysis of protein digests from phosphoprotein stained analytical gels or Coomassie Blue stained preparative gels provided for the identification of phosphoproteins that exhibited statistically significant increased phosphorylation in conditioned groups as compared to pseudorandom controls. A previously identified cytoskeletal related protein, Csp24 (24 kDa conditioned stimulus pathway phosphoprotein), involved in intermediate-term memory exhibited significantly increased phosphorylation detected 24 h post-conditioning. Our results show that proteins involved in diverse cellular functions such as transcriptional regulation, cell signaling, cytoskeletal regulation, metabolic activity, and protein degradation contribute to long-term post-translational modifications associated with Pavlovian conditioning.

Bryostatin enhancement of memory in Hermissenda

Bryostatin, a potent agonist of protein kinase C (PKC), when administered to Hermissenda was found to affect acquisition of an associative learning paradigm. Low bryostatin concentrations (0.1 to 0.5 ng/ml) enhanced memory acquisition, while concentrations higher than 1.0 ng/ml down-regulated the pathway and no recall of the associative training was exhibited. The extent of enhancement depended upon the conditioning regime used and the memory stage normally fostered by that regime. The effects of two training events (TEs) with paired conditioned and unconditioned stimuli, which standardly evoked only short-term memory (STM) lasting 7 min, were--when bryostatin was added concurrently--enhanced to a long-term memory (LTM) that lasted about 20 h. The effects of both 4- and 6-paired TEs (which by themselves did not generate LTM), were also enhanced by bryostatin to induce a consolidated memory (CM) that lasted at least 5 days. The standard positive 9-TE regime typically produced a CM lasting at least 6 days. Low concentrations of bryostatin (<0.5 ng/ml) elicited no demonstrable enhancement of CM from 9-TEs. However, animals exposed to bryostatin concentrations higher than 1.0 ng/ml exhibited no behavioral learning. Sharp-electrode intracellular recordings of type-B photoreceptors in the eyes from animals conditioned in vivo with bryostatin revealed changes in input resistance and an enhanced long-lasting depolarization (LLD) in response to light. Likewise, quantitative immunocytochemical measurements using an antibody specific for the PKC-activated Ca2+/GTP-binding protein calexcitin showed enhanced antibody labeling with bryostatin. Animals exposed to the PKC inhibitor bisindolylmaleimide-XI (Ro-32-0432) administered by immersion prior to 9-TE conditioning showed no training-induced changes with or without bryostatin exposure. However, if animals received bryostatin before Ro-32, the enhanced acquisition and demonstrated recall still occurred. Therefore, pathways responsible for the enhancement effects induced by bryostatin were putatively mediated by PKC. Overall, the data indicated that PKC activation occurred and calexcitin levels were raised during the acquisition phases of associative conditioning and memory initiation, and subsequently returned to baseline levels within 24 and 48 h, respectively. Therefore, the protracted recall measured by the testing regime used was probably due to bryostatin-induced changes during the acquisition and facilitated storage of memory, and not necessarily to enhanced recall of the stored memory when tested many days after training.

Protein synthesis required for long-term memory is induced by PKC activation on days before associative learning

Protein synthesis has long been known to be required for associative learning to consolidate into long-term memory. Here we demonstrate that PKC isozyme activation on days before training can induce the synthesis of proteins necessary and sufficient for subsequent long-term memory consolidation. Bryostatin (Bryo), a macrolide lactone with efficacy in subnanomolar concentrations and a potential therapeutic for Alzheimer's disease, is a potent activator of PKC, some of whose isozymes undergo prolonged activation after associative learning. Under normal conditions, two training events with paired visual and vestibular stimuli cause short-term memory of the mollusc Hermissenda that lasts approximately 7 min. However, after 4-h exposures to Bryo (0.25 ng/ml) on two preceding days, the same two training events produced long-term conditioning that lasted >1 week and that was not blocked by anisomycin (1 mug/ml). Anisomycin, however, eliminated long-term memory lasting at least 1 week after nine training events. Both the nine training events alone and two Bryo exposures plus two training event regimens caused comparably increased levels of the PKC alpha-isozyme substrate calexcitin in identified type B neurons and enhanced PKC activity in the membrane fractions. Furthermore, Bryo increased overall protein synthesis in cultured mammalian neurons by up to 60% for >3 days. The specific PKC antagonist Ro-32-0432 blocked much of this Bryo-induced protein synthesis as well as the Bryo-induced enhancement of the behavioral conditioning. Thus, Bryo-induced PKC activation produces those proteins necessary and sufficient for long-term memory on days in advance of the training events themselves.

Effects of N-acetylcysteine on lead-exposed PC-12 cells

The neurotoxicity of lead has been well established through numerous studies. However, the cellular processes of lead neurotoxicity, as well as techniques to prevent or reverse cellular damage after lead exposure, remain unknown. If oxidative stress plays a primary role in lead-induced neurotoxicity, antioxidants should assist in reviving lead-exposed cells. The present study explores N-acetylcysteine (NAC) as an antioxidant agent in PC-12 cells after lead exposure. Selective oxidative stress parameters, including glutathione (GSH), glutathione disulfide (GSSG), and malondialdehyde (MDA), were measured in PC-12 cells exposed to various concentrations of lead acetate. Administering NAC after lead exposure improved cell survival as measured by Trypan Blue exclusion. NAC treatment also increased the GSH/GSSG ratio compared to the lead-only group, and reduced MDA to near control levels. These results imply that NAC protects cells from lead-induced oxidative damage by boosting the PC-12 cells' antioxidant defense mechanisms.

Pavlovian conditioning of Hermissenda: current cellular, molecular, and circuit perspectives

The less-complex central nervous system of many invertebrates make them attractive for not only the molecular analysis of the associative learning and memory, but also in determining how neural circuits are modified by learning to generate changes in behavior. The nudibranch mollusk Hermissenda crassicornis is a preparation that has contributed to an understanding of cellular and molecular mechanisms of Pavlovian conditioning. Identified neurons in the conditioned stimulus (CS) pathway have been studied in detail using biophysical, biochemical, and molecular techniques. These studies have resulted in the identification and characterization of specific membrane conductances contributing to enhanced excitability and synaptic facilitation in the CS pathway of conditioned animals. Second-messenger systems activated by the CS and US have been examined, and proteins that are regulated by one-trial and multi-trial Pavlovian conditioning have been identified in the CS pathway. The recent progress that has been made in the identification of the neural circuitry supporting the unconditioned response (UR) and conditioned response (CR) now provides for the opportunity to understand how Pavlovian conditioning is expressed in behavior.

Calcium-regulated GTPase activity in the calcium-binding protein calexcitin

Calexcitin (CE) is a calcium-binding protein, closely related to sarcoplasmic calcium-binding proteins, that is involved in invertebrate learning and memory. Early reports indicated that both Hermissenda and squid CE also could bind GTP; however, the biochemical significance of GTP-binding and its relationship to calcium binding have remained unclear. Here, we report that the GTPase activity of CE is strongly regulated by calcium. CE possessed a P-loop-like structure near the C-terminal similar to the phosphate-binding regions in other GTP-binding proteins. Site-directed mutagenesis of this region showed that Gly(182), Phe(186) and Gly(187) are required for maximum affinity, suggesting that the GTP-binding motif is G-N-x-x-[FM]-G. CE cloned from Drosophila CNS possessed a similar C-terminal sequence and also bound and hydrolyzed GTP. GTPase activity in Drosophila CE was also strongly regulated by Ca(2+), exhibiting over 23-fold higher activity in the presence of 0.3 microM calcium. Analysis of the conserved protein motifs defines a new family of Ca(2+)-binding proteins representing the first example of proteins endowed with both EF-hand calcium binding domains and a C-terminal, P-loop-like GTP-binding motif. These results establish that, in the absence of calcium, both squid and Drosophila CE bind GTP at near-physiological concentrations and hydrolyze GTP at rates comparable to unactivated ras. Calcium functions to increase GTP-binding and GTPase activity in CE, similar to the effect of GTPase activating proteins in other low-MW GTP-binding proteins. CE may, therefore, act as a molecular interface between Ca(2+) cytosolic oscillations and the G protein-coupled signal transduction.