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

Phenotypic plasticity in Periplaneta americana photoreceptors

Plasticity is a crucial aspect of neuronal physiology essential for proper development and continuous functional optimization of neurons and neural circuits. Despite extensive studies of different visual systems, little is known about plasticity in mature microvillar photoreceptors. Here we investigate changes in electrophysiological properties and gene expression in photoreceptors of the adult cockroach, Periplaneta americana, after exposure to constant light (CL) or constant dark (CD) for several months. After CL, we observed a decrease in mean whole-cell capacitance, a proxy for cell membrane area, from 362 ± 160 to 157 ± 58 pF, and a decrease in absolute sensitivity. However, after CD, we observed an increase in capacitance to 561 ± 155 pF and an increase in absolute sensitivity. Small changes in the expression of light-sensitive channels and signaling molecules were detected in CD retinas, together with a substantial increase in the expression of the primary green-sensitive opsin (GO1). Accordingly, light-induced currents became larger in CD photoreceptors. Even though normal levels of GO1 expression were retained in CL photoreceptors, light-induced currents became much smaller, suggesting that factors other than opsin are involved. Latency of phototransduction also decreased significantly in CL photoreceptors. Sustained voltage-activated K⁺ conductance was not significantly different between the experimental groups. The reduced capacitance of CL photoreceptors expanded their bandwidth, increasing the light-driven voltage signal at high frequencies. However, voltage noise was also amplified, probably because of unaltered expression of TRPL channels. Consequently, information transfer rates were lower in CL than in control or CD photoreceptors. These changes in whole-cell capacitance and electrophysiological parameters suggest that structural modifications can occur in the photoreceptors to adapt their function to altered environmental conditions. The opposing patterns of modifications in CL and CD photoreceptors differ profoundly from previous findings in Drosophila melanogaster photoreceptors.

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.

Downregulation of thymosin beta4 in neural progenitor grafts promotes spinal cord regeneration

Thymosin beta4 (Tbeta4) is an actin-binding peptide whose expression in developing brain correlates with migration and neurite extension of neurons. Here, we studied the effects of the downregulation of Tbeta4 expression on growth and differentiation of murine neural progenitor cells (NPCs), using an antisense lentiviral vector. In differentiation-promoting medium, we found twice the number of neurons derived from the Tbeta4-antisense-transduced NPCs, which showed enhanced neurite outgrowth accompanied by increased expression of the adhesion complex N-cadherin-beta-catenin and increased ERK activation. Importantly, when the Tbeta4-antisense-transduced NPCs were transplanted in vivo into a mouse model of spinal cord injury, they promoted a significantly greater functional recovery. Locomotory recovery correlated with increased expression of the regeneration-promoting cell adhesion molecule L1 by the grafted Tbeta4-antisense-transduced NPCs. This resulted in an increased number of regenerating axons and in sprouting of serotonergic fibers surrounding and contacting the Tbeta4-antisense-transduced NPCs grafted into the lesion site. In conclusion, our data identify a new role for Tbeta4 in neuronal differentiation of NPCs by regulating fate determination and process outgrowth. Moreover, NPCs with reduced Tbeta4 levels generate an L1-enriched environment in the lesioned spinal cord that favors growth and sprouting of spared host axons and enhances the endogenous tissue-repair processes.

Fear conditioning and extinction differentially modify the intrinsic excitability of infralimbic neurons

Extinction of conditioned fear is an active learning process involving inhibition of fear expression. It has been proposed that fear extinction potentiates neurons in the infralimbic (IL) prefrontal cortex, but the cellular mechanisms underlying this potentiation remain unknown. It is also not known whether this potentiation occurs locally in IL neurons as opposed to IL afferents. To determine whether extinction enhances the intrinsic excitability of IL pyramidal neurons in layers II/III and V, we performed whole-cell patch-clamp recordings in slices from naive, conditioned, or conditioned-extinguished rats. We observed that conditioning depressed IL excitability compared with slices from naive animals, as evidenced by a decreased number of spikes evoked by injected current and an increase in the slow afterhyperpolarizing potential (sAHP). Extinction reversed these conditioning-induced effects. Furthermore, IL neurons from extinguished rats showed increased burst spiking compared with naive rats, which was correlated with extinction recall. These changes were specific to IL prefrontal cortex and were not observed in prelimbic prefrontal cortex. Together, these findings suggest that IL intrinsic excitability is reduced to allow for expression of conditioning memory and enhanced for expression of extinction memory through the modulation of Ca(2+)-gated K(+) channels underlying the sAHP. Inappropriate modulation of these intrinsic mechanisms may underlie anxiety disorders, characterized by exaggerated fear and deficient extinction.

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.

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.

One-trial in vitro conditioning of hermissenda regulates phosphorylation of ser-122 of csp24

The regulation of the intrinsic excitability of a neuron is an important aspect of cellular and synaptic plasticity underlying learning and memory. Various voltage-dependent K(+) channels have been shown to be critical for the modification of membrane excitability. Components of the cytoskeleton have been proposed to contribute to the location, distribution, and function of diverse K(+) channels. However, the mechanisms underlying the regulation of the cytoskeleton by signaling pathways and the role of the cytoskeleton in the induction of intrinsic excitability is not understood. Hermissenda Csp24 is a beta-thymosin-like protein containing multiple actin-binding domains that contributes to intrinsic enhanced excitability produced by Pavlovian conditioning. One-trial in vitro conditioning produces a significant reduction in the A-type transient K(+) current (I(A)) and a depolarized shift in the steady-state activation curve of I(A). Intermediate and long-term enhanced excitability produced by one-trial conditioning is also dependent on the expression and phosphorylation of Csp24. Blocking the expression of Csp24 with an antisense oligonucleotide inhibits the development of intermediate-term enhanced excitability and the concomitant reduction in I(A) normally produced by one-trial in vitro conditioning. In this report using two-dimensional gel PAGE and electrospray mass spectrometry, we have identified two phosphorylation sites on Csp24. Using phospho-specific antibodies with Western blot analysis and immunoprecipitation procedures we show that one-trial in vitro conditioning results in an increase in the phosphorylation of Ser-122, but not Ser-49 of Csp24.