Experiential input alters the phosphorylation of specific proteins in brain membranes

A Shift from a Pivotal to Supporting Role for the Growth-Associated Protein (GAP-43) in the Coordination of Axonal Structural and Functional Plasticity

In a number of animal species, the growth-associated protein (GAP), GAP-43 (aka: F1, neuromodulin, B-50, G50, pp46), has been implicated in the regulation of presynaptic vesicular function and axonal growth and plasticity via its own biochemical properties and interactions with a number of other presynaptic proteins. Changes in the expression of GAP-43 mRNA or distribution of the protein coincide with axonal outgrowth as a consequence of neuronal damage and presynaptic rearrangement that would occur following instances of elevated patterned neural activity including memory formation and development. While functional enhancement in GAP-43 mRNA and/or protein activity has historically been hypothesized as a central mediator of axonal neuroplastic and regenerative responses in the central nervous system, it does not appear to be the crucial substrate sufficient for driving these responses. This review explores the historical discovery of GAP-43 (and associated monikers), its transcriptional, post-transcriptional and post-translational regulation and current understanding of protein interactions and regulation with respect to its role in axonal function. While GAP-43 itself appears to have moved from a pivotal to a supporting factor, there is no doubt that investigations into its functions have provided a clearer understanding of the biochemical underpinnings of axonal plasticity.

Proteome rearrangements after auditory learning: high-resolution profiling of synapse-enriched protein fractions from mouse brain

Learning and memory processes are accompanied by rearrangements of synaptic protein networks. While various studies have demonstrated the regulation of individual synaptic proteins during these processes, much less is known about the complex regulation of synaptic proteomes. Recently, we reported that auditory discrimination learning in mice is associated with a relative down-regulation of proteins involved in the structural organization of synapses in various brain regions. Aiming at the identification of biological processes and signaling pathways involved in auditory memory formation, here, a label-free quantification approach was utilized to identify regulated synaptic junctional proteins and phosphoproteins in the auditory cortex, frontal cortex, hippocampus, and striatum of mice 24 h after the learning experiment. Twenty proteins, including postsynaptic scaffolds, actin-remodeling proteins, and RNA-binding proteins, were regulated in at least three brain regions pointing to common, cross-regional mechanisms. Most of the detected synaptic proteome changes were, however, restricted to individual brain regions. For example, several members of the Septin family of cytoskeletal proteins were up-regulated only in the hippocampus, while Septin-9 was down-regulated in the hippocampus, the frontal cortex, and the striatum. Meta analyses utilizing several databases were employed to identify underlying cellular functions and biological pathways. Data are available via ProteomeExchange with identifier PXD003089. How does the protein composition of synapses change in different brain areas upon auditory learning? We unravel discrete proteome changes in mouse auditory cortex, frontal cortex, hippocampus, and striatum functionally implicated in the learning process. We identify not only common but also area-specific biological pathways and cellular processes modulated 24 h after training, indicating individual contributions of the regions to memory processing.

Ectopic growth of hippocampal mossy fibers in a mutated GAP-43 transgenic mouse with impaired spatial memory retention

In a previous study, it was shown that transgenic mice, designated G-NonP, forget the location of a water maze hidden platform when tested 7 days after the last training day (Holahan and Routtenberg (2008) Hippocampus 18:1099-1102). The memory loss in G-NonP mice might be related to altered hippocampal architecture suggested by the fact that in the rat, 7 days after water maze training, there is discernible mossy fiber (MF) growth (Holahan et al. (2006) Hippocampus 16:560-570; Rekart et al. (2007) Learn Mem 14:416-421). In the present report, we studied the distribution of the MF system within the hippocampus of naïve, untrained, G-NonP mouse. In WT mice, the MF projection was restricted to the stratum lucidum of CA3 with no detectable MF innervation in distal stratum oriens (dSO). In G-NonP mice, in contrast, there was an ectopic projection terminating in the CA3 dSO. Unexpectedly, there was nearly a complete loss of immunostaining for the axonal marker Tau1 in the G-NonP transgenic mice in the MF terminal fields indicating that transgenesis itself leads to off-target consequences (Routtenberg (1996) Trends Neurosci 19:471-472). Because transgenic mice overexpressing nonmutated, wild type GAP-43 do not show this ectopic growth (Rekart et al., in press) and the G-NonP mice overexpress a mutated form of GAP-43 precluding its phosphorylation by protein kinase C (PKC), the possibility exists that permanently dephosphorylated GAP-43 disrupts normal axonal fasciculation which gives rise to the ectopic growth into dSO.

GAP-43 gene expression regulates information storage

Previous reports have shown that overexpression of the growth- and plasticity-associated protein GAP-43 improves memory. However, the relation between the levels of this protein to memory enhancement remains unknown. Here, we studied this issue in transgenic mice (G-Phos) overexpressing native, chick GAP-43. These G-Phos mice could be divided at the behavioral level into "spatial bright" and "spatial dull" groups based on their performance on two hidden platform water maze tasks. G-Phos dull mice showed both acquisition and retention deficits on the fixed hidden platform task, but were able to learn a visible platform task. G-Phos bright mice showed memory enhancement relative to wild type on the more difficult movable hidden platform spatial memory task. In the hippocampus, the G-Phos dull group showed a 50% greater transgenic GAP-43 protein level and a twofold elevated transgenic GAP-43 mRNA level than that measured in the G-Phos bright group. Unexpectedly, the dull group also showed an 80% reduction in hippocampal Tau1 staining. The high levels of GAP-43 seen here leading to memory impairment find its histochemical and behavioral parallel in the observation of Rekart et al. (Neuroscience 126: 579-584) who described elevated levels of GAP-43 protein in the hippocampus of Alzheimer's patients. The present data suggest that moderate overexpression of a phosphorylatable plasticity-related protein can enhance memory, while excessive overexpression may produce a "neuroplasticity burden" leading to degenerative and hypertrophic events culminating in memory dysfunction.

Nerve Ending “Signal” Proteins GAP‐43, MARCKS, and BASP1

Mechanisms of growth cone pathfinding in the course of neuronal net formation as well as mechanisms of learning and memory have been under intense investigation for the past 20 years, but many aspects of these phenomena remain unresolved and even mysterious. "Signal" proteins accumulated mainly in the axon endings (growth cones and the presynaptic area of synapses) participate in the main brain processes. These proteins are similar in several essential structural and functional properties. The most prominent similarities are N-terminal fatty acylation and the presence of an "effector domain" (ED) that dynamically binds to the plasma membrane, to calmodulin, and to actin fibrils. Reversible phosphorylation of ED by protein kinase C modulates these interactions. However, together with similarities, there are significant differences among the proteins, such as different conditions (Ca2+ contents) for calmodulin binding and different modes of interaction with the actin cytoskeleton. In light of these facts, we consider GAP-43, MARCKS, and BASP1 both separately and in conjunction. Special attention is devoted to a discussion of apparent inconsistencies in results and opinions of different authors concerning specific questions about the structure of proteins and their interactions.

Subfield-specific increase in brain growth protein in postmortem hippocampus of Alzheimer's patients

The neuropathology of Alzheimer's disease (AD) reflects a precarious balance between neurodegenerative phenomena and reactive events of neuroplasticity. This latter aspect of AD neuropathology has received less attention than it deserves and its contribution to memory loss is therefore not well understood. To monitor neuroplastic-related events we studied the distribution of the plasticity-associated, brain growth protein GAP-43 in AD subjects and age-matched controls. In tissue from AD patients, we observed a consistent elevation of GAP-43 in a subfield of the hippocampus, stratum lacunosum moleculare. This subfield contains inputs from multiple brain regions and is known to regulate declarative memory function. Levels of potentially aberrant sprouting, as marked by elevated growth protein, were positively correlated with the severity of AD suggesting that increased expression of GAP-43 leads to a miswiring of circuits critical for memory function. Our findings suggest a mechanism, aberrant neuroplasticity, that in concert with neurodegeneration may importantly contribute to the memory loss in AD.

Changes in protein kinase C (PKC) activity, isozyme translocation, and GAP-43 phosphorylation in the rat hippocampal formation after a single-trial contextual fear conditioning paradigm

The hippocampus plays an important role in spatial learning and memory. However, the biochemical alterations that subserve this function remain to be fully elucidated. In this study, rats were subjected to a single-trial contextual fear conditioning (CFC) paradigm; the activation of different protein kinase C (PKC) subtypes and the levels and phosphorylation of the plasticity-associated protein GAP-43 were assayed in the hippocampus at varying times after training. We observed a rapid activation of hippocampal PKC (15 min through 24 h), with differential translocation of the PKC isotypes studied. At early times after CFC (15-90 min), PKCalpha and PKCgamma translocated to the membrane, while PKCbetaII and PKCepsilon moved more transiently (15 to 30 min) to the cytosol. These PKC isotypes returned to the membrane at later time points after CFC. Correlating with these changes in PKC translocation and activity, there was an early decrease in GAP-43 phosphorylation followed by a more sustained increase from 1.5-72 h. GAP-43 protein levels were also increased after 3 h, and these levels remained elevated for at least 72 h. These changes in PKC and GAP-43 were specific to the CFC trained animals and no changes were seen in animals exposed to the same stimuli in a non-associative fashion. Comparison of translocation of different PKC isotypes with the changes in GAP-43 phosphorylation suggested that PKCbetaII and PKCepsilon may mediate both the early changes in the phosphorylation of this protein and the increases in GAP-43 expression at later times after CFC.

Enhanced learning after genetic overexpression of a brain growth protein

Ramón y Cajal proposed 100 years ago that memory formation requires the growth of nerve cell processes. One-half century later, Hebb suggested that growth of presynaptic axons and postsynaptic dendrites consequent to coactivity in these synaptic elements was essential for such information storage. In the past 25 years, candidate growth genes have been implicated in learning processes, but it has not been demonstrated that they in fact enhance them. Here, we show that genetic overexpression of the growth-associated protein GAP-43, the axonal protein kinase C substrate, dramatically enhanced learning and long-term potentiation in transgenic mice. If the overexpressed GAP-43 was mutated by a Ser --> Ala substitution to preclude its phosphorylation by protein kinase C, then no learning enhancement was found. These findings provide evidence that a growth-related gene regulates learning and memory and suggest an unheralded target, the GAP-43 phosphorylation site, for enhancing cognitive ability.

Alterations in hippocampal GAP-43 phosphorylation and protein level following contextual fear conditioning

C57BL/6 (B6) mice display better contextual learning than the DBA/2 (D2) mice. The possibility that GAP-43, is differentially affected as a function of strain and learning was investigated in the present study. No basal difference between C57BL/6J (B6) and DBA/2J (D2) mice in the amount of hippocampal GAP-43 was observed, but naive D2 mice have slightly lower basal levels of GAP-43 phosphorylation than do B6 mice. Interestingly, alterations in hippocampal GAP-43 protein levels and phosphorylation state in response to training for contextual learning were observed only in B6 mice. Immediate-shocked mice, serving as nonlearning controls, showed no GAP-43 alterations, nor did D2 mice subjected to either training condition. These results suggest that modulation of hippocampal GAP-43 may be important for contextual learning and that strain-specific alterations in GAP-43 may be part of a disrupted pathway in D2 mice that is essential for learning.

Long-term potentiation persistence greater in C57BL/6 than DBA/2 mice: predicted on basis of protein kinase C levels and learning performance

The gamma isoform of protein kinase C (gamma-PKC) activity is elevated and learning is superior in the inbred C57BL/6 mouse when compared to the DBA/2 mouse strain. Given the proposed link between PKC and long-term potentiation (LTP) on the one hand and PKC and learning on the other, it was predicted that LTP persistence would be greater in C57BL/6 mouse. When suprathreshold levels of tetanic stimulation were used, similar persistent LTP was observed in both C57BL/6 and DBA/2 strains. However, when tetanus was at threshold, the response in DBA/2 mice decayed to baseline in 30 min, similar to short-term potentiation (STP). Using this same paradigm with C57BL/6 mice, LTP persisted for 4 h, the longest time tested. The time course of the results parallels those observed in rat when phorbol ester, a potent PKC activator, converts STP to LTP. The present findings thus confirm the predicted difference between the two mouse strains. Moreover, the present findings are consistent with a role for gamma-PKC in LTP. Since such results call attention to the need for gamma-PKC interventive procedures, the relative utility of current PKC inhibitors, null mutants and antisense methods are discussed.

Surface protein phosphorylation by ecto-protein kinase is required for the maintenance of hippocampal long-term potentiation

During the induction of long-term potentiation (LTP) in hippocampal slices adenosine triphosphate (ATP) is secreted into the synaptic cleft, and a 48 kDa/50 kDa protein duplex becomes phosphorylated by extracellular ATP. All the criteria required as evidence that these two proteins serve as principal substrates of ecto-protein kinase activity on the surface of hippocampal pyramidal neurons have been fulfilled. This phosphorylation activity was detected on the surface of pyramidal neurons assayed after synaptogenesis, but not in immature neurons nor in glial cells. Addition to the extracellular medium of a monoclonal antibody termed mAb 1.9, directed to the catalytic domain of protein kinase C (PKC), inhibited selectively this surface protein phosphorylation activity and blocked the stabilization of LTP induced by high frequency stimulation (HFS) in hippocampal slices. This antibody did not interfere with routine synaptic transmission nor prevent the initial enhancement of synaptic responses observed during the 1-5 min period immediately after the application of HFS (the induction phase of LTP). However, the initial increase in the slope of excitatory postsynaptic potentials, as well as the elevated amplitude of the population spike induced by HFS, both declined gradually and returned to prestimulus values within 30-40 min after HFS was applied in the presence of mAb 1.9. A control antibody that binds to PKC but does not inhibit its activity had no effect on LTP. The selective inhibitory effects observed with mAb 1.9 provide the first direct evidence of a causal role for ecto-PK in the maintenance of stable LTP, an event implicated in the process of learning and the formation of memory in the brain.

Passive avoidance learning induced change in GAP43 phosphorylation in day-old chicks

Day-old chicks trained on a single trial passive discriminated avoidance task demonstrated a significant increase in in vitro phosphorylation of a 50 kDa protein in P2M fractions of total forebrain. The increase occurred 30 min posttraining, at a time when previous reports suggest that mechanisms for triggering protein synthesis-dependent long-term memory consolidation are activated. These changes in phosphorylation rates were accompanied by a substantial enhancement of total kinase activity. Immunoblotting studies with monoclonal anti-GAP43 antibody indicate that this protein is GAP43. These results contradict previous reports of a decrease in in vitro GAP43 phosphorylation following the same learning paradigm. A number of procedural differences may account for this discrepancy. The results suggest that changes in the phosphorylation state may be associated with mechanisms triggering long-term memory consolidation.

Effect of PKC inhibitors and activators on memory

Changes in the activity of the enzyme protein kinase C (PKC) have been implicated in learning and memory consolidation, and in the induction of long-term potentiation. The precise role of PKC in memory processing is still unknown. Using 1-day-old chicks trained on a single-trial passive avoidance task, we demonstrate that inhibition of PKC activity by melittin induced retention loss, in a dose-dependent manner, in the second stage of a three-stage sequence of memory processing. The effect was lateralized to the left hemisphere of the chick forebrain. This effect of melittin was prevented by high concentrations (16-320 microM) of the PKC activator, phorbol 12-myristate 13-acetate (PMA). Furthermore, concentrations of PMA in the range 1.6 to 40 microM were shown to induce long-term memory consolidation following a weakly reinforced version of the learning task, which normally does not lead to formation of long-term memory. That these actions of PMA are attributable to PKC activation is supported by the further finding that the inactive phorbol ester 4 alpha-PDD had no effect either on melittin-induced amnesia or on memory consolidation following weakly reinforced learning. Paradoxically, concentrations of 16 microM or higher of PMA inhibited memory consolidation for the normal strongly reinforced learning trial, an effect again not observed with 40 alpha-PDD. The results are consistent with the view that PKC activity may be implicated in a pre-long-term stage of memory processing.

Phosphorylation of synaptic proteins in chick forebrain: changes with development and passive avoidance training

We have used synaptic plasma membranes (SPMs) and postsynaptic densities (PSDs) to study protein phosphorylation at the synapse in the developing chick forebrain and in 1-day-old chick forebrain following training on a passive avoidance task. Endogenous phosphorylation patterns in SPMs and PSDs prepared by extraction with n-octylglucoside isolated from chick forebrain were investigated by labelling with [32P]ATP. The phosphoprotein components of the SPM and PSD fractions were separated using sodium dodecyl sulphate gradient polyacrylamide gel electrophoresis. Autoradiography and densitometry of the Coomassie Blue protein staining pattern revealed phosphate incorporation into several SPM components including those of molecular mass 52, 37, and 29 kilodaltons (kDa). Bands of similar molecular mass were not phosphorylated in PSD fractions. This difference in phosphorylation between SPMs and PSDs was not due to the detergent n-octylglucoside. In a developmental study in which SPM and PSD fractions were prepared from 1-day-old, 14-day-old, and 21-day-old chickens, the phosphorylation patterns of SPMs were similar throughout, but striking differences occurred in PSDs, both in the level of phosphorylation and in the components phosphorylated. A time-course study was carried out in which phosphorylation of SPMs and PSDs from 1-day-old chicks trained on a passive avoidance task was compared with patterns from control chicks trained on a water-coated bead and untrained chicks. In SPMs prepared from forebrains removed 10 mins following training, a consistent but nonsignificant decrease (-21%) in phosphorylation of a 52 kDa band occurred in chicks with passive avoidance training compared with water-trained and untrained chicks.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of training, epinephrine, and glucose injections on plasma glucose levels in rats

Recent findings indicate that a post-training injection of glucose enhances memory storage, suggesting that release of glucose into plasma may mediate the effects of epinephrine and perhaps other treatments on memory. The present experiment examined the effects of handling, inhibitory (passive) avoidance training, epinephrine and glucose injections on plasma glucose levels in Sprague-Dawley rats. Handling produced a small, but significant, transient increase in plasma glucose above basal levels. Saline injections caused a similar increase in circulating glucose levels. Inhibitory avoidance training with high footshock (2.0 mA, 2.0 s) resulted in significant increases in plasma glucose levels above those of low (0.5 mA, 0.75 s) and unshocked animals suggesting that glucose release is responsive to inhibitory avoidance training. Subcutaneous injections of epinephrine (0.01-1.0 mg/kg), or glucose (10-1000 mg/kg) significantly elevated glucose levels above those of saline-injected animals in a dose-dependent manner. Memory facilitating doses of epinephrine and glucose resulted in increases in plasma glucose levels similar to those seen in rats trained with high footshock. Higher doses of epinephrine and glucose resulted in further increases in circulating glucose, to levels significantly greater than those of memory facilitating doses. These results suggest that memory modulation, both endogenous and in response to epinephrine injections, may be mediated in part by circulating glucose levels. Thus, the findings of these experiments support the view that circulating glucose levels regulate the efficacy of neural memory storage processes.

Protein kinase C activation leading to protein F1 phosphorylation may regulate synaptic plasticity by presynaptic terminal growth

It has recently been proposed by the author that protein kinase C regulates the expression of synaptic plasticity. In the present review it is suggested that this regulation involves a growth of presynaptic terminals. This proposal was based on the discovery that one of the substrates of protein kinase C, protein F1 (molecular mass = 47 kDa, pI = 4.5) is increased in its phosphorylation 5 min, 1 hr, and 3 days following long-term potentiation (LTP) in the intact hippocampal formation. No other phosphoprotein studied was altered by LTP. The amplitude or persistence of synaptic plasticity was directly related to the extent of protein F1 phosphorylation. As a critical control, it was shown that protein F1 was unaltered following synaptic activation that did not alter synaptic strength. Protein F1 in the hippocampus was also altered in its phosphorylation after an experience involving memory of a spatial environment. Phosphorylation F1 may thus participate in both neurophysiological and behavioral events that evoke plasticity. The identification of the F1 substrate has recently been sought. The physical characteristics of protein F1 (mol wt., isoelectric point) indicate that it is the same as the B-50 protein and the growth protein, GAP-43. Protein F1 is then a brain-specific, synaptically enriched phosphoprotein. Recent evidence indicates that protein F1 is present in high concentration in growth cones of late embryonic rat brain in which postsynaptic specializations are not detected, suggesting a presynaptic locus. With respect to the identity of the F1 kinase, we have shown that protein F1, like B-50, is a substrate for protein kinase C, a Ca2+/phospholipid-dependent kinase. Activation of this enzyme by tumor-promoting phorbol esters can trigger cell growth and neurite extension. Recent evidence indicates a presynaptic localization of the enzyme. On the basis of the colocalization of enzyme and substrate in the presynaptic terminal it is proposed that protein kinase C control of the phosphorylation state of protein F1 may regulate the expression of synaptic plasticity via presynaptic terminal growth.

Effects of calcium, strontium, and barium ions on phosphorylation of hippocampal proteins in vitro

Calcium ion alone or in the presence of added calmodulin stimulated in vitro transfer of 32P from [gamma 32P]ATP into several proteins of mitochondrial and synaptosomal particulate fractions from rat brain. Strontium ion was capable of substituting for calcium ion in this stimulation, but barium ion lacked this capacity. These results bring into question the hypothesis that calcium-dependent protein phosphorylation of synaptic proteins is intrinsic to neurotransmitter release during neurotransmission, but they do not rule out that possibility.

Brain cyclic AMP and memory in mice

A phosphodiesterase inhibitor 4-(3-cyclopentyloxy-4-methoxyphenyl)-2-pyrrolidone (Rolipram, 10 mg/kg IP) administered immediately, but not 3 hr post-training, reversed an amnesia for an inhibitory avoidance response induced by the protein synthesis inhibitor anisomycin. Immediate post-training administration of Rolipram also enhanced retention for a weakly learned avoidance response. Unshocked animals did not show increased test latencies thus ruling out conditioned aversion as an explanation for the enhanced avoidance. Mice treated with Rolipram (10 mg/kg after training showed elevated cyclic AMP but not cyclic GMP in frontal cortex, thalamus, and hypothalamus. These results support the suggestion that cyclic AMP may play a role in memory processes.

Characterization of infant rat cerebral cortical membrane proteins phosphorylated in vivo: identification of the ACTH-sensitive phosphoprotein B-50

This study on the phosphorylation in vivo of membrane proteins in cerebral cortices of infant rats reports the identification of the adrenocorticotropin (ACTH)-sensitive phosphoprotein B-50 as one of the substrate proteins that are rapidly phosphorylated in vivo following intracisternal administration of 2 mCi [32P]orthophosphate. Rats were sacrificed 30 min after isotope injection. A fraction enriched in membranes, designated neural membranes (NM), was isolated from the cerebral cortices according to the procedure used for preparation of synaptic plasma membranes (SPM) from adult brain. This NM fraction was characterized by electron microscopy. The proteins of NM were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Numerous protein bands of NM in infant rat brain were phosphorylated in vivo. Attention was focussed on the 32P-labeled protein bands in the molecular weight range of 47K-67K. In this region one phosphoprotein band (MW 48K) was more highly labeled than the other bands. The electrophoretic behavior of three of these labeled bands, designated a, c, and e (MW 48K, 55K, and 62K, respectively) was compared with that of protein bands that were phosphorylated in vitro in cerebral membranes isolated from noninjected infant rats. The effects of ACTH1-24 and cyclic AMP in the in vitro system were also studied to probe for the presence of specific membrane proteins known to be sensitive to these modulators. On incubation of NM with [gamma-32P)ATP in the presence and absence of ACTH1-24 in vitro, phosphorylation of a 48K protein band was inhibited in a dose-dependent fashion by the neuropeptide. Two-dimensional electrophoretic separation of NM proteins labeled in vivo indicated that the 48K band had an isoelectric point of 4.5, identical to that of the ACTH-sensitive B-50 protein previously identified. Cyclic AMP stimulated phosphorylation in vitro of two protein bands (MW 55K and 59K) in NM preparations. This result indicates that the in vivo labeled band c may correspond to the cyclic AMP-sensitive 55K protein, whereas phosphoprotein band e, labeled in vivo, appears to be different from the cyclic AMP-sensitive 59K protein band. These observations indicate that neural membranes isolated from infant rat cerebral cortices contain a variety of proteins that can be phosphorylated in vivo. Several of these, for example, the 48K protein band, have the properties of synaptic plasma membrane proteins of adult rat brain that have been characterized by their sensitivity to neuromodulators in endogenous phosphorylating systems in vitro.

Brain pyruvate dehydrogenase: phosphorylation and enzyme activity altered by a training experience

The active portion of the alpha subunit of pyruvate dehydrogenase in rat frontal cortex was elevated after a training experience. No change in total pyruvate dehydrogenase activity was observed. The phosphorylation in vitro of pyruvate dehydrogenase (band F-2) was also elevated after training. Since activation of pyruvate dehydrogenase requires its dephosphorylation, the following sequence is proposed. Training alters frontal cortex and reduces the phosphate content of pyruvate dehydrogenase in vivo; this leads to enzyme activation; and an increase in back-titration of sites available for phosphorylation in vitro.

In vivo phosphorylation following [32P]orthophosphate injection into neostriatum or hippocampus: selective and rapid labeling of electrophoretically separated brain proteins

Intracranial injections of [32P]orthophosphate readily label a number of brain phosphoproteins as resolved by polyacrylamide gel electrophoresis. The majority of these in vivo labeled phosphoproteins co-migrate with phosphoproteins that are labeled in vitro by incubation of brain membranes with [32P]ATP. Two of the major in vitro labeled phosphoproteins with apparent molecular weights of 47,000 (band F1) and 41,000 (band F2) are rapidly labeled in vivo. Since they are rapidly dephosphorylated in vitro, this suggests a high rate of phosphate turnover. The electrophoretic pattern of in vivo labeled phosphoproteins did not appear to be altered by the method of sacrifice (focused microwave irradiation, decapitation or liquid nitrogen immersion) or by the state of the animal at the time of labeling (awake or lightly anesthetized with pentobarbital). The reduction of phosphatase activity during tissue processing at 0 degree C may account for the similarities observed with different sacrifice methods. Removal of phospholipids or polynucleotides had little effect on the in vivo labeled 32P-containing bands. However, alkaline hydrolysis or protease treatment uniformly reduced the radioactivity in the labeled bands. These findings suggest that the 32P-containing bands consist of phosphoester linkages to serine or threonine residues. The present evidence emphasizes that previously characterized in vitro labeled brain phosphoproteins are, in fact, labeled in the awake, freely-moving animal.

Endogenous phosphorylation in vitro: differential effects of brain state (anesthesia, post-mortem) on electrophoretically separated brain proteins

In vitro phosphorylation of rat cerebral cortex synaptosomes was measured in animals that had been acutely treated with sodium pentobarbital. [32P]Labelled phosphoproteins were separated by SDS-slab gel electrophoresis, and the autoradiographs were analyzed by densitometry. We report here that Band F of our previous reports can be separated into two components, F1 and F2, using an improved gel system. This separation is particularly relevant in this report since these components appear to be differentially sensitive to the manipulations used. Specifically, we found that while F1 phosphorylation was markedly diminished by deep barbiturate anesthesia, F2 was relatively stable. While phosphorylation of F2 was also stable 24 h post-mortem, Band F1 phosphorylation was no longer detectable. Finally, while osmotic shock treatment of synaptosomes reduced phosphorylation of F2 somewhat, it eliminated the in vitro phosphorylation of Band F1. We found that under light barbiturate anesthesia, just at the time when the animals lost the righting reflex, the in vitro phosphorylation of Bands D (MR 78,000--80,000 daltons), F1 (MR 47,000--49,000) and F2 (MR 40,000--45,000) increased relative to unanesthetized controls. The in vitro labelling of Bands D and F1 was depressed in tissue prepared from animals that were deeply comatose. These effects of pentobarbital were more pronounced when animals were sacrificed by liquid nitrogen immersion, rather than by decapitation. Cyclic AMP-dependent phosphorylation of Band D exhibited remarkable stability 24 h post-morten (7 days in one case), even when brain tissue was left at room temperature (21--23 degrees C). Phosphorylation of Band F1, however, was not detectable in post-mortem tissue. The results of these studies indicate that phosphorylation of Band F1 is: (1) sensitive to pentobarbital, and (2) unstable post-mortem. Previous findings from our laboratory suggest that Band F1 is: (3) increased in phosphorylation in liquid nitrogen P2 preparations, and may be (4) cAMP-independent, (5) rapidly turning over its phosphate in vivo, and (6) altered by a training experience. Other evidence suggests that: (7) Band F1 phosphorylation may be Ca2+-dependent and that: (8) its phosphorylation is sensitive to osmotic lysis of synaptosomes. The results suggest an important and perhaps unique role for Band F1 in neuronal function.

Selective changes in the phosphorylation of endogenous proteins in subcellular fractions from cyclic AMP-induced differentiated neuroblastoma cells

The endogenous phosphorylation of specific proteins was studied in subcellular fractions from proliferating and cAMP-induced differentiated neuroblastoma cells. Fractions containing nuclear, membrane-bound, and cytosolic proteins were incubated with [gamma-32P]ATP, in the presence and absence of added cyclic nucleotides. Phosphate incorporation into specific proteins was determined by slab-gel electrophoresis of sodium dodecyl sulfate-solubilized reaction products. Cytosol fractions from differentiated cells demonstrated a twofold increase in cAMP-dependent phosphorylation of a specific protein with apparent mol wt of 59,000 daltons and a comparable decrease in cAMP-independent phosphorylation of another protein (97,000). The nuclear fraction of differentiated cells showed an increase in the cAMP-independent phosphorylation of two nonhistone proteins (110,000 and 102,000). Membrane fractions from differentiated cells exhibited a differential decrease in endogenous phosphorylation of specific proteins. Selective alterations in the phosphorylation of specific proteins in various subcellular components may be important biochemical events associated with the increased levels of differentiated functions in neuroblastoma cells in culture.

ACTH-induced inhibition of endogenous rat brain protein phosphorylation in vitro: structure activity

ACTH1--24 inhibits the endogenous phosphorylation in vitro of distinct SPM protein bands. Using N-terminal fragments of ACTH, the structure-activity requirements for this effect were studied. A rather complex interaction of the ACTH fragments with endogenous SPM phosphorylation was observed. The effects were not only dependent on the primary structure of the peptide used, but also on the protein band studied and the ATP/SPM ratio used in the incubation system. ACTH1--24 did not interfere with the ATP-hydrolyzing activity of the SPM preparation, nor did it influence the endogenous phosphatase activity. Therefore, a direct interaction of ACTH with SPM protein kinase(s) is likely to be responsible for its effect on phosphorylation.

Effects of decapitation-stress on the phosphorylation of cortical membrane proteins

The endogenous phosphorylation of membrane-bound proteins was studied in preparations from the cerebral cortex of rats sacrificed by immersion in liquid nitrogen or by decapitation. Compared to quick-frozen rats, samples from decapitated animals demonstrated a two-fold increase in 32P-phosphate incorporation into specific protein bands with apparent molecular weights of 56K, and 52K (designated E1 and E2) and a decreased incorporation into a phosphoprotein of 47K (designated F). The phosphorylation of two proteins (78K and 34K) in membranes from decapitated rats was found to be highly stimulated by exogenously added cyclic AMP. On the other hand, the phosphorylation of specific protein bands in preparations from quick frozen rats was minimally affected by addition of cyclic AMP. The results indicate that conditions which lead to increases in cyclic AMP levels in the brain in situ induce specific changes in phosphorylative activity, and these can be detected by assaying isolated membrane fragments in vitro.