Time-dependent increase in protein phosphorylation following one-trial enhancement in Hermissenda

Serotonin regulates voltage-dependent currents in type I(e(A)) and I(i) interneurons of Hermissenda

Serotonin (5-HT) has both direct and modulatory actions on central neurons contributing to behavioral arousal and cellular-synaptic plasticity in diverse species. In Hermissenda, 5-HT produces changes in intrinsic excitability of different types of identified interneurons in the circumesophageal nervous system. Using whole cell patch-clamp techniques we have examined membrane conductance changes produced by 5-HT that contribute to intrinsic excitability in two identified classes of interneurons, types I(i) and I(eA). Whole cell currents were examined before and after 5-HT application to the isolated nervous system. A 4-aminopyridine-sensitive transient outward K(+) current [I(K(A))], a tetraethylammonium-sensitive delayed rectifier K(+) current [I(K(V))], an inward rectifier K(+) current [I(K(IR))], and a hyperpolarization-activated current (I(h)) were characterized. 5-HT decreased the amplitude of I(K(A)) and I(K(V)) in both type I(i) and I(eA) interneurons. However, differences in 5-HT's effects on the activation-inactivation kinetics were observed in different types of interneurons. 5-HT produced a depolarizing shift in the activation curve of I(K(V)) and a hyperpolarizing shift in the inactivation curve of I(K(A)) in type I(i) interneurons. In contrast, 5-HT produced a depolarizing shift in the activation curve and a hyperpolarizing shift in the inactivation curve of both I(K(V)) and I(K(A)) in type I(eA) interneurons. In addition, 5-HT decreased the amplitude of I(K(IR)) in type I(i) interneurons and increased the amplitude of I(h) in type I(eA) interneurons. These results indicate that 5-HT-dependent changes in I(K(A)), I(K(V)), I(K(IR)), and I(h) contribute to multiple mechanisms that synergistically support modulation of increased intrinsic excitability associated with different functional classes of identified type I interneurons.

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.

One-trial in vitro conditioning regulates an association between the beta-thymosin repeat protein Csp24 and actin

One-trial conditioning in Hermissenda results in enhanced intrinsic cellular excitability of sensory neurons in the conditioned stimulus pathway, and the phosphorylation of several proteins. Previous results demonstrated that the development of enhanced intrinsic excitability was dependent on the expression of conditioned stimulus pathway phosphoprotein-24 (Csp24), an intracellular protein containing four repeated beta-thymosin homology domains. Consistent with this, antisense oligonucleotide-mediated inhibition of Csp24 expression prevents the reduction in amplitude of the A-type transient K+ current (I(A)) and the depolarized shift in the steady-state activation curve normally produced by one-trial in vitro conditioning of isolated photoreceptors. One-trial conditioning also regulates Csp24 phosphorylation. We now show that purified recombinant Csp24 sequesters G-actin in vitro with an approximate K(d) value of 2.8 microM. We also observed a significant increase in the coprecipitation of actin with Csp24 after one-trial in vitro conditioning using antibodies directed toward either Csp24 or phospho-Csp24. Preincubation with protein kinase C (PKC) selective inhibitors attenuated the increase in Csp24 phosphorylation and coprecipitated actin observed after one-trial conditioning. Our findings indicate that the PKC signaling pathway contributes to the phosphorylation of Csp24 after one-trial conditioning, and that PKC activity modulates an association between Csp24 and actin. These data suggest Csp24 may influence intrinsic excitability by regulating cytoskeletal dynamics.

Inhibition of conditioned stimulus pathway phosphoprotein 24 expression blocks the reduction in A-type transient K+ current produced by one-trial in vitro conditioning of Hermissenda

Long-term intrinsic enhanced excitability is a characteristic of cellular plasticity and learning-dependent modifications in the activity of neural networks. The regulation of voltage-dependent K+ channels by phosphorylation/dephosphorylation and their localization is proposed to be important in the control of cellular plasticity. One-trial conditioning in Hermissenda results in enhanced excitability in sensory neurons, type B photoreceptors, of the conditioned stimulus pathway. Conditioning also regulates the phosphorylation of conditioned stimulus pathway phosphoprotein 24 (Csp24), a cytoskeletal-related protein containing multiple beta-thymosin-like domains. Recently, it was shown that the downregulation of Csp24 expression mediated by an antisense oligonucleotide blocked the development of enhanced excitability in identified type B photoreceptors after one-trial conditioning without affecting short-term excitability. Here, we show using whole-cell patch recordings that one-trial in vitro conditioning applied to isolated photoreceptors produces a significant reduction in the amplitude of the A-type transient K+ current (I(A)) detected 1.5-16 h after conditioning. One-trial conditioning produced a depolarized shift in the steady-state activation curve of I(A) without altering the inactivation curve. The conditioning-dependent reduction in I(A) was blocked by preincubation of the photoreceptors with Csp antisense oligonucleotide. These results provide an important link between Csp24, a cytoskeletal protein, and regulation of voltage-gated ion channels associated with intrinsic enhanced excitability underlying pavlovian conditioning.

Sequestration of serum response factor in the hippocampus impairs long-term spatial memory

The formation of long-term memory has been shown to require protein kinase-mediated gene expression. One such kinase, mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK), can lead to the phosphorylation of serum response factor (SRF) and Elk-1, enhancing the expression of target genes. However, a direct involvement of these transcription factors in memory storage has not been demonstrated. We have employed an oligonucleotide decoy technique to interrogate SRF and Elk-1. Previously, it has been shown that intra-amygdalal infusion of small double-stranded decoy oligonucleotides for nuclear factor-kappaB (NFkappaB) can impair long-term memory for fear-potentiated startle. Using this approach, we found that intra-hippocampal infusion of NFkappaB decoy oligonucleotides also impairs long-term spatial memory, consistent with a role for this factor in long-term memory storage. Decoy oligonucleotides containing the binding site for SRF, as confirmed by shift-western, did not influence memory acquisition but impaired long-term spatial memory. Analysis of search behavior during the transfer test revealed deficits consistent with a loss of precise platform location information. In contrast, oligonucleotides with a binding site for either Elk-1 or another target of ERK activity, SMAD3/SMAD4, did not interfere with memory formation or storage. These findings suggest that SRF-mediated gene expression is required for long-term spatial memory.

Rho/ROCK and Cdk5 effects on phosphorylation of a β-thymosin repeat protein in Hermissenda

Rho GTPases acting through effector proteins regulate actin dynamics and cytoskeletal structure. In Hermissenda Csp24 is a cytoskeletal-related protein that contributes to the development of intermediate-term memory, and is homologous to other beta-thymosin-like repeat proteins containing multiple actin-binding domains. We have examined the role of Rho GTPase activity and its downstream target ROCK, and cyclin-dependent kinase 5 (Cdk5) on the phosphorylation of Csp24 using 32PO4 labeling of proteins separated with 2-D PAGE. The ROCK inhibitor Y-27632 significantly increased Csp24 phosphorylation, and the Rho activator lysophosphatidic acid (LPA) or the Cdk5 inhibitor butyrolactone significantly decreased Csp24 phosphorylation. Pretreatment with Y-27632 before LPA application significantly reduced the decreased phosphorylation of Csp24 normally detected in nervous systems exposed to LPA. Using a pull-down assay we found that LPA treatments activated Rho and exposure to 5-HT decreased Rho activity. Our results indicate that the Rho/ROCK and Cdk5 signaling pathways contribute to the regulation of Csp24 phosphorylation.

Inhibition of conditioned stimulus pathway phosphoprotein 24 expression blocks the development of intermediate-term memory in Hermissenda

Studies of memory consolidation have identified multiple phases or stages in the formation of memories. The multiple components of memory can be broadly divided into the three phases; short-term, intermediate-term, and long-term. Although molecular changes underlying short- and long-term memory have been examined extensively, the molecular mechanisms supporting the formation of intermediate-term memory are poorly understood. In several examples of cellular and synaptic plasticity, intermediate memory depends on translation but not transcription. One-trial conditioning in Hermissenda results in the development of intermediate memory that is associated with enhanced cellular excitability and the phosphorylation of a 24 kDa protein referred to as conditioned stimulus pathway phosphoprotein (Csp24). Using amino acid sequences derived from Csp24 peptide fragments, a full-length cDNA was cloned and shown to contain multiple beta-thymosin-like domains. The expression of Csp24 and the development of enhanced excitability, a characteristic of intermediate memory, were blocked by antisense oligonucleotide-mediated downregulation of Csp24 without affecting the induction of immediate enhanced excitability, a characteristic of short-term memory. These results demonstrate that the synthesis of Csp24 is required for the development and maintenance of intermediate memory.

One-trial in vitro conditioning regulates a cytoskeletal-related protein (CSP24) in the conditioned stimulus pathway of Hermissenda

Hermissenda CSP24 (cytoskeletal-related protein 24) is a 24 kDa beta-thymosin-like protein that is associated with intermediate memory. We showed previously that one-trial conditioning resulted in a significant increase in the phosphorylation of CSP24 detected in lysates of the pathway supporting the conditioned stimulus (CS). Here we report the association of the protein with the actin cytoskeleton and the distribution of CSP24-immunoreactive neurons in two sensory structures and the circumesophageal nervous system. Identified photoreceptors, hair cells, and neurons in the cerebropleural and pedal ganglia were immunoreactive for CSP24. Immunoprecipitation experiments with 32PO4-labeled lysates of the circumesophageal nervous system identified a 44 kDa protein band (consistent with actin) that coprecipitates with CSP24. An analysis of immunoprecipitates on Western blots probed with anti-actin antibody also showed that actin coprecipitates with CSP24. Laser confocal microscopy of photoreceptors costained with fluorescently labeled anti-actin antibody and anti-CSP24 antibody, or fluorescent phalloidin and anti-CSP24 antibody showed that CSP24 is localized with actin in the cytosol of photoreceptor cell bodies and colocalized with presumed G-actin, but not F-actin, in regions adjacent to the plasma membrane. Although CSP24 is widely distributed in the Hermissenda nervous system, its regulation by one-trial conditioning was observed only in the CS pathway. Our findings suggest that CSP24 may interact with components of the actin cytoskeleton that contribute to structural changes underlying the formation and maintenance of enduring forms of memory.

Concentration-dependent effects of protein phosphatase (PP) inhibitors implicate PP1 and PP2A in different stages of memory formation

Numerous studies have demonstrated roles for protein phosphorylation and for specific kinases in memory formation; however, a role for specific protein phosphatases has not been established. Previous studies using pharmacobehavioral methods to implicate protein phosphatase activity in memory formation have been unable to discriminate between protein phosphatases 1 (PP1) and 2A (PP2A), as available cell-permeable agents generally inhibit both enzyme classes. To address this difficulty the present study exploited differences in the potency of the selective phosphatase inhibitor, okadaic acid, toward PP1 and PP2A. Within the context of a temporally precise animal model of memory, developed using the day-old chick (Gallus domesticus), acute administration of various concentrations of okadaic acid was found to disrupt two temporally distinct stages of memory formation. When administered bilaterally into an area of the chick brain implicated in memory formation, concentrations of okadaic acid known to selectively inhibit PP2A in vitro disrupted memory from 50 min posttraining. Higher concentrations, reported to inhibit both PP2A and PP1 in vitro, produced significant retention deficits from 20 min posttraining. Identical temporally specific effects were also obtained by varying the concentration and time of administration of calyculin A, a phosphatase inhibitor with equal potency toward both enzyme classes. Hence, different phosphatase enzymes may contribute to different stages of the enzymatic cascade believed to underlie memory formation.

Protein synthesis-dependent and mRNA synthesis-independent intermediate phase of memory in Hermissenda

The conditioned stimulus pathway in Hermissenda has been used to examine the time-dependent mechanisms of memory consolidation following one-trial conditioning. Here we report an intermediate phase of memory consolidation following one-trial conditioning that requires protein synthesis, but not mRNA synthesis. In conditioned animals, enhanced excitability normally expressed during an intermediate phase of memory was reversed by the protein synthesis inhibitor anisomycin, but not by the mRNA synthesis inhibitor 5, 6-dichloro-1-beta-D-ribobenzimidazole (DRB). Associated with the intermediate phase of memory is an increase in the phosphorylation of a 24-kDa protein. Anisomycin present during the intermediate phase blocked the increased phosphorylation of the 24-kDa phosphoprotein, but did not block the increased phosphorylation of other proteins associated with conditioning or significantly change their baseline phosphorylation. DRB did not reverse enhanced excitability or decrease protein phosphorylation expressed during the intermediate phase of memory formation, but it did reverse enhanced excitability 3.5 h after conditioning. Phosphorylation of the 24-kDa protein may support enhanced excitability during the intermediate phase, in the transition period between short- and long-term memory.

Phosphorylation of mitogen-activated protein kinase by one-trial and multi-trial classical conditioning

The pathway supporting the conditioned stimulus (CS) is one site of plasticity that has been studied extensively in conditioned Hermissenda. Several signal transduction pathways have been implicated in classical conditioning of this preparation, although the major emphasis has been on protein kinase C. Here we provide evidence for the activation and phosphorylation of a mitogen-activated protein kinase (MAPK) pathway by one-trial and multi-trial conditioning. A one-trial in vitro conditioning procedure consisting of light (CS) paired with the application of 5-HT results in the increased incorporation of 32PO4 into proteins detected with two-dimensional gel electrophoresis. Two of the phosphoproteins have molecular weights of 44 and 42 kDa, consistent with extracellular signal-regulated protein kinases (ERK1 and ERK2). Phosphorylation of the 44 and 42 kDa proteins by one-trial conditioning was inhibited by pretreatment with PD098059, A MEK1 (ERK-Activating kinase) inhibitor. Assays of ERK activity with brain myelin basic protein as a substrate revealed greater ERK activity for the group that received one-trial conditioning compared with an unpaired control group. Western blot analysis of phosphorylated ERK using antibodies recognizing the dually phosphorylated forms of ERK1 and ERK2 showed an increase in phosphorylation after one-trial conditioning compared with unpaired controls. The increased phosphorylation of ERK after one-trial conditioning was blocked by pretreatment with PD098059. Hermissenda that received 10 or 15 conditioning trials showed significant behavioral suppression compared with pseudo-random controls. After conditioning and behavioral testing, the conditioned animals showed significantly greater phosphorylation of ERK compared with the pseudo-random controls. These results show that the ERK-MAPK signaling pathway is activated in Pavlovian conditioning of Hermissenda.

Time-dependent changes in excitability after one-trial conditioning of Hermissenda

The visual system of Hermissenda has been studied extensively as a site of cellular plasticity produced by classical conditioning. A one-trial conditioning procedure consisting of light paired with the application of serotonin (5-HT) to the exposed, but otherwise intact, nervous system produces suppression of phototactic behavior tested 24 h after conditioning. Short- and long-term enhancement (STE and LTE) of excitability in identified type B photoreceptors is a cellular correlate of one-trial conditioning. LTE can be expressed in the absence of STE suggesting that STE and LTE may be parallel processes. To examine the development of enhancement, we studied its time-dependent alterations after one-trial conditioning. Intracellular recordings from identified type B photoreceptors of independent groups collected at different times after conditioning revealed that enhanced excitability follows a biphasic pattern in its development. The analysis of spikes elicited by 2 and 30 s extrinsic current pulses at different levels of depolarization showed that enhancement reached a peak 3 h after conditioning. From its peak, excitability decreased toward baseline control levels 5-6 h after conditioning followed by an increase to a stable plateau at 16 to 24 h postconditioning. Excitability changes measured in cells from unpaired control groups showed maximal changes 1 h posttreatment that rapidly decremented within 2 h. The conditioned stimulus (CS) elicited significantly more spikes 24 h postconditioning for the conditioned group as compared with the unpaired control group. The analysis of the time-dependent development of enhancement may reveal the processes underlying different stages of memory for this associative experience.

Long-term enhancement but not short-term in Hermissenda is dependent upon mRNA synthesis

The reversible transcription inhibitor 5,6-dichloro-1-beta-D-ribobenzimidazole (DRB) was used to examine the contribution of mRNA synthesis to long-term enhancement (LTE) following one-trial conditioning of Hermissenda. Inhibition of mRNA synthesis by DRB or inhibition of protein synthesis by anisomycin did not significantly affect the induction and maintenance of short-term enhancement (STE) examined 1 h after one-trial conditioning. In contrast to the absence of an effect of the inhibitors on STE, LTE was blocked by DRB or anisomycin applied shortly before and during the presentation of the conditioning trial. Consistent with previous reports, animals that received an unpaired CS and US did not exhibit LTE. In addition, a control group that received a concentration of DRB (10(-7) M) that does not significantly affect mRNA synthesis exhibited typical LTE when tested 24 h postconditioning. These results demonstrate that the induction of LTE produced by one-trial conditioning is dependent upon transcription and the regulation of gene expression.