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

Phospholipase C beta 1 expression in the dorsolateral prefrontal cortex from patients with schizophrenia at different stages of illness

OBJECTIVE

Our recent microarray study detected decreases in the expression of phospholipase C beta 1 mRNA in the dorsolateral prefrontal cortex from subjects with schizophrenia at different stages of illness. Thus we aimed to validate and extend these findings.

METHOD

We measured levels of mRNA and protein for phospholipase C beta 1 variant a and b using real-time PCR and western blot analysis, respectively, in the dorsolateral prefrontal cortex from subjects with schizophrenia, who had a short (< 7 years) or long (> 22 years) duration of illness.

RESULTS

Compared to age/sex matched controls, levels of phospholipase C beta 1 variant a and b mRNAs were decreased (-33% and -50%, respectively) in short duration schizophrenia. By contrast, only variant a mRNA was decreased (-24%) in long duration schizophrenia. There was no significant difference in the protein levels of either phospholipase C beta 1 variant in schizophrenia, irrespective of duration of illness (variant a; P = 0.84, variant b; P = 0.73).

CONCLUSION

Our data confirm that phospholipase C beta 1 transcript levels are decreased in the dorsolateral prefrontal cortex from subjects with schizophrenia. However, the changes in levels of mRNA do not translate into a change at the level of protein. It is possible protein expression is regulated independently of mRNA and it remains to be determined whether there is a functional consequence of this change in mRNA relating to the pathophysiology of schizophrenia.

Proteomic analysis of glial fibrillary acidic protein in Alzheimer's disease and aging brain

Chronic inflammation is known to play an important role in the heterogeneous pathogenesis of Alzheimer's disease (AD). Activated astrocytes expressing glial fibrillary acidic protein (GFAP) are closely associated with AD pathology, such as tangles, neuritic plaques and amyloid depositions. Altogether, 46 soluble isoforms of GFAP were separated and most of them quantified by two-dimensional immunoblotting in frontal cortices of AD patients and age-matched controls. A 60% increase in the amount of more acidic isoforms of GFAP was observed in AD and these isoforms were both phosphorylated and N-glycosylated, while more basic isoforms were O-glycosylated and exhibited no quantitative differences between post-mortem AD and control brains. These data highlight the importance of exploring isoform-specific levels of proteins in pathophysiological conditions since modifications of proteins determine their activity state, localization, turnover and interaction with other molecules. Mechanisms, structures and functional consequences of modification of GFAP isoforms remain to be clarified.

Anesthesia and post-mortem interval profoundly influence the regulatory serine phosphorylation of glycogen synthase kinase-3 in mouse brain

Glycogen synthase kinase-3 (GSK3) is a crucial enzyme contributing to the regulation of neuronal structure, plasticity and survival, is implicated as a contributory factor in prevalent diseases such as Alzheimer's disease and mood disorders and is regulated by a wide range of signaling systems and pharmacological agents. Therefore, factors regulating GSK3 in vivo are currently of much interest. GSK3 is inhibited by phosphorylation of serine-9 or serine-21 in GSK3beta and GSK3alpha, respectively. This study found that accurate measurements of phospho-Ser-GSK3 in brain are confounded by a rapid post-mortem dephosphorylation, with approximately 90% dephosphorylation of both GSK3 isoforms occurring within 2 min post-mortem. Furthermore, three anesthetics, pentobarbital, halothane and chloral hydrate, each caused large in vivo increases in the serine phosphorylation of both GSK3beta and GSK3alpha in several regions of mouse brain. Thus, studies of the phosphorylation state of GSK3 in brain, and perhaps in other tissues, need to take into account post-mortem changes and the effects of anesthetics and there is a direct correlation between anesthesia and high levels of serine-phosphorylated GSK3.

Regulation of choline transporter surface expression and phosphorylation by protein kinase C and protein phosphatase 1/2A

The Na(+)/Cl(-)-dependent, hemicholinium-3-sensitive choline transporter (CHT) provides choline for acetylcholine biosynthesis. Recent studies show that CHT contains canonical protein kinase C (PKC) serine and threonine residues. We examined the ability of PKC and serine/threonine protein phosphatase 1/2A (PP1/PP2A) to regulate CHT function, surface expression, and phosphorylation. In mouse crude striatal and hippocampal synaptosomes, PKC activators beta-phorbol 12-myristate 13-acetate (beta-PMA) and beta-phorbol 12,13-dibutyrate produced time- and concentration-dependent reductions in CHT function. PP1/PP2A inhibitors okadaic acid (OKA) and calyculin A (CL-A) produced a time- and concentration-dependent decrease in CHT function. However, tautomycin (PP1 inhibitor) and cyclosporin A (PP2B inhibitor) failed to alter CHT-mediated choline uptake. Choline transport kinetic studies following beta-PMA, OKA, and CL-A treatment revealed a reduction in the maximal choline transport velocity (V(max)) with no change in K(m) for choline. These modulators also produced no change in the total levels of CHT protein in the crude hippocampal and striatal synaptosomes; however, surface biotinylation studies using the membrane-impermeant N-hydroxysuccinimide-biotin in crude synaptosomes following treatment with beta-PMA, OKA, and CL-A indicate significant reductions of CHT levels in biotinylated fractions. Pretreatment with OKA alone, but not beta-PMA, significantly augmented the phosphorylation level of CHT proteins. Our findings suggest that neuronal PKC and PP1/PP2A activity may establish the level of function and surface expression of CHT. These studies also provide the first evidence that CHT is a phosphoprotein and that the basal PP1/PP2A activity may have a dominant role in controlling the levels of CHT phosphorylation.

Altered Striatal Function and Muscarinic Cholinergic Receptors in Acetylcholinesterase Knockout Mice

Cholinesterase inhibitors are commonly used to improve cognition and treat psychosis and other behavioral symptoms in Alzheimer's disease, Parkinson's disease, and other neuropsychiatric conditions. However, mechanisms may exist that down-regulate the synaptic response to altered cholinergic transmission, thus limiting the efficacy of cholinomimetics in treating disease. Acetylcholinesterase knockout (AChE-/-) mice were used to investigate the neuronal adaptations to diminished synaptic acetylcholine (ACh) metabolism. The striatum of AChE-/- mice showed no changes in choline acetyltransferase activity or levels of the vesicular ACh transporter but showed striking 60% increases in the levels of the highaffinity choline transporter. This transporter takes choline from the synapse into the neuron for resynthesis of ACh. In addition, the striata of AChE-/- mice showed dramatic reductions in levels of the M1, M2, and M4 muscarinic ACh receptors (mAChRs), but no alterations in dopamine receptors or the beta2 subunit of nicotinic receptors. M1, M2, and M4 also showed decreased dendritic and cell surface distributions and enhanced intracellular localizations in striatal neurons of AChE-/- mice. mAChR antagonist treatment reversed the shifts in mAChR distribution, indicating that internalized receptors in AChE-/- mice can recover to basal distributions. Finally, AChE-/- mice showed increased sensitivity to mAChR antagonist-induced increases in locomotor activity, demonstrating functional mAChR down-regulation. mAChR downregulation in AChE-/- mice has important implications for the long-term use of cholinesterase inhibitors and other cholinomimetics in treating disorders characterized by perturbed cholinergic function.

Posthoc phosphorylation of proteins derived from ischemic rat hippocampus, striatum and neocortex

Disruption of the brain's protein phosphorylation system by ischemia may cause irreversible metabolic and structural alterations leading eventually to cell death. To examine the effect of ischemia on the phosphorylation state of brain proteins, tissue homogenates derived from the hippocampus, striatum and neocortex of normal rats and rats subjected to severe forebrain ischemia were phosphorylated with [gamma-32P]ATP. The phosphorylated proteins were separated by two-dimensional polyacrylamide gel electrophoresis and changes were assessed by autoradiography. Cerebral ischemia caused marked alterations of the phosphorylation state of many brain proteins; phosphorylation of some proteins was increased while phosphorylation of others was decreased. Despite differences in the sensitivity of the hippocampus, striatum and neocortex to ischemic injury the direction and approximate magnitude of protein phosphorylation changes caused by ischemia were similar in all three regions. Since the pattern of protein phosphorylation in the ischemia-vulnerable hippocampus was identical to that in the ischemia-resistant paramedian neocortex we conclude that abnormalities of protein phosphorylation may be necessary for ischemic injury to neurons but none are sufficient to explain the selective vulnerability of certain brain regions to ischemic damage.

Stimulation by dopamine of protein phosphorylation in rat striatal slices

Incubation of rat striatal slices with dopamine enhanced the phosphorylation of two proteins with mol. wts. of 64,000 and 43,000. Although dopamine did increase cAMP levels in striatal slices, the addition of cAMP to striatal slices did not mimic the effects of dopamine on protein phosphorylation. The present results suggest that cAMP-independent protein kinases may mediate some of the effects of dopamine within the corpus striatum.

Characterization of protein F1 (47 kDa, 4.5 pI): a kinase C substrate directly related to neural plasticity

We recently demonstrated that long-term potentiation in rat hippocampal formation leads to a selective increase in the phosphorylation of a 47-kDa protein band (F1). The present report provided evidence, using two-dimensional gel electrophoresis, that only one major phosphoprotein in rat is found at 47 kDa under conditions identical to those used in that earlier study. This protein, which we also term F1, has an isoelectric point of 4.5 and is increased in phosphorylation after long-term potentiation. In addition to this identification, we demonstrated in two-dimensional gels that protein F1 is a membrane-enriched kinase C substrate whose phosphorylation is stimulated by Ca2+ and phosphatidylserine. Protein F1 may be equivalent to several reported proteins: a brain-specific, synaptically enriched protein (B-50), a major membrane-bound growth cone protein (pp46), and a fast axonally transported "growth-associated protein" (GAP43; 44- to 49-kDa goldfish optic nerve protein). Protein F1 participation in neural plasticity may thus involve growth occurring at synaptic loci.

Brain protein phosphorylation in vitro: selective substrate action of insulin

Insulin action on [32P]-phosphate incorporation into brain membranes was determined. Hippocampal homogenate tissue was phosphorylated with [32P]-ATP, and insulin was introduced at various times before or after ATP addition. With 50 microM Mg++ in the medium, insulin selectively stimulated the phosphorylation of a 47kD phosphoprotein, Protein F1. This effect required the prior presence of ATP. No effect of insulin on other phosphoproteins, or on [32P]-phosphate incorporation into TCA-precipitated material, was observed under these conditions. At 1 mM Mg++, insulin selectively decreased the phosphorylation of the alpha-subunit of pyruvate dehydrogenase. Insulin had no effect on other phosphoproteins, or on [32P]-phosphate incorporation into TCA-precipitated material under these conditions. The present study suggests a role for insulin in the modulation of brain protein phosphorylation. Since Protein F1 is phosphorylated by exogenous C kinase, and is likely the CNS-specific B-50 protein, these data also indicate a brain-specific function for insulin, possibly by action on a Ca++/phospholipid protein kinase.

Apparent identity of alpha-subunit of pyruvate dehydrogenase and the protein phosphorylated in the presence of glutamate in P2-fractions of rat cerebral cortex

Addition of L-glutamate or several citric acid cycle intermediates stimulated the phosphorylation of a protein with apparent molecular weight of 43,000 ( P43 ) in P2-fractions from rat cerebral cortex, and this phosphorylation was inhibited by dichloroacetic acid, a specific inhibitor of pyruvate dehydrogenase kinase. Comparison of several molecular properties of phosphorylated P43 and the phosphorylated alpha-subunit of pyruvate dehydrogenase indicated that both proteins are extracted by a similar procedure and have an identical apparent molecular weight and isoelectric point. Furthermore, digestion of both phosphorylated proteins by several different proteases in the presence of SDS yielded a similar pattern of phosphorylated peptides indicating that these proteins have a considerable sequence homology. Thus, various pieces of evidence indicate that P43 and the alpha-chain of pyruvate dehydrogenase are very similar if not identical. The possible implication of a glutamate stimulated phosphorylation of pyruvate dehydrogenase for long term potentiation and epilepsy is discussed.

Neonatal handling selectively alters the phosphorylation of a 47,000 mol. wt. protein in male rat hippocampus

Beginning on postnatal day 1, rat pups were handled for 7 min daily for 23-30 days. This treatment diminished the in vitro phosphorylation of phosphoprotein F-1 [mol. wt. approximately 47 K daltons, protein B-50] in the hippocampus of male rats. Other major phosphoproteins (D-1-2. mol. wt. 80-86 K daltons: E-2-3. mol. wt. 50-55 K daltons) were not influenced by handling. These findings confirm and extend the results of Holmes et al. who observed a decrease in ECS-induced protein kinase activity subsequent to handling.

Entorhinal cortex lesions induce a decreased calcium transport in hippocampal mitochondria

Lesions to the entorhinal afferent of the hippocampus in rats caused marked changes in calcium transport into mitochondria. Pyruvate-supported calcium transport into mitochondria from the denervated hippocampus was decreased to a larger extent than succinate-supported transport, and adenosine triphosphate-supported transport was not significantly modified. Although cytochrome oxidase and succinate dehydrogenase activities were not significantly changed by entorhinal lesions, pyruvate flux through pyruvate dehydrogenase was significantly decreased, and this effect was correlated with changes in pyruvate-supported calcium transport. The active portion of pyruvate dehydrogenase decreased, whereas total pyruvate dehydrogenase was not modified. These data suggest that denervation might initiate dendritic atrophy and subsequent growth responses by modifying calcium regulation through a change in the phosphorylation of pyruvate dehydrogenase.

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.

Human brain protein phosphorylation in vitro: cyclic AMP stimulation of electrophoretically-separated substrates

In vitro phosphorylation of electrophoretically-separated brain proteins was studied in human frontal cortex obtained 3-16 h post-mortem from 13 patients ages 3 days-82 years with extensive, mild or no neuropathological involvement. In 12 of the 13 cases, cyclic AMP increased incorporation of phosphate into acid-precipitable protein. Analysis of the autoradiographic profiles of separate proteins indicated that phosphorylation of the doublet of molecular weight 86-80,000 was stimulated by cyclic AMP in certain samples. This doublet corresponded to the cyclic AMP stimulated doublet from rat frontal cortex we have termed band D-1,2 (proteins Ia and Ib of Ueda and Greengard). Of special interest was the fact that, while co-migration was observed in the other phosphoprotein bands studied, band D-1,2 of humans consistently migrated slightly less than rat protein band D-1,2. This difference was not a function of post-mortem time, subcellular fraction or buffer used in the reaction phosphorylation assay. The use of post-mortem tissue was not a contributing factor as the retardation in band D-1,2 migration was still observed when post-mortem rat brain was used for comparison. In two human post-mortem samples, there was no measureable band D-1,2 phosphorylation even in the presence of cyclic AMP. This was the case in both homogenate and crude synaptosome/mitochondrial preparations. Band F-1 (mol. wt. = 47,000) was not observed in any of the human samples studied. This is consistent with prior studies in rat which show that band F-1 phosphorylation is not detected in post-mortem brain, Band F-2 (mol. wt. 41,000) recently identified as pyruvate dehydrogenase, was lightly phosphorylated under the reaction conditions used in this study.

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.