Protein phosphatase inhibitors induce modification of synapse structure and tau hyperphosphorylation in cultured rat hippocampal neurons

Modulation of dendritic spines by protein phosphatase-1

Protein phosphatase-1 (PP-1), a highly conserved multifunctional serine/threonine phosphatase, is enriched in dendritic spines where it plays a major role in modulating excitatory synaptic activity. In addition to established functions in spine maturation and development, multi-subunit holoenzyme forms of PP-1 modulate higher-order cognitive functions such learning and memory. Mechanisms involved in regulating PP-1 activity and localization in spines include interactions with neurabin and spinophilin, structurally related synaptic scaffolding proteins associated with the actin cytoskeleton. Since PP-1 is a critical element in synaptic development, signaling, and plasticity, alterations in PP-1 signaling in dendritic spines are implicated in various neurological and psychiatric disorders. The effects of PP-1 depend on its isoform-specific association with regulatory proteins and activation of downstream signaling pathways. Here we review the role of PP-1 and its binding proteins neurabin and spinophilin in both developing and established dendritic spines, as well as some of the disorders that result from its dysregulation.

Stress-Induced Synaptic Dysfunction and Neurotransmitter Release in Alzheimer's Disease: Can Neurotransmitters and Neuromodulators be Potential Therapeutic Targets?

The communication between neurons at synaptic junctions is an intriguing process that monitors the transmission of various electro-chemical signals in the central nervous system. Albeit any aberration in the mechanisms associated with transmission of these signals leads to loss of synaptic contacts in both the neocortex and hippocampus thereby causing insidious cognitive decline and memory dysfunction. Compelling evidence suggests that soluble amyloid-β (Aβ) and hyperphosphorylated tau serve as toxins in the dysfunction of synaptic plasticity and aberrant neurotransmitter (NT) release at synapses consequently causing a cognitive decline in Alzheimer's disease (AD). Further, an imbalance between excitatory and inhibitory neurotransmission systems induced by impaired redox signaling and altered mitochondrial integrity is also amenable for such abnormalities. Defective NT release at the synaptic junction causes several detrimental effects associated with altered activity of synaptic proteins, transcription factors, Ca2+ homeostasis, and other molecules critical for neuronal plasticity. These detrimental effects further disrupt the normal homeostasis of neuronal cells and thereby causing synaptic loss. Moreover, the precise mechanistic role played by impaired NTs and neuromodulators (NMs) and altered redox signaling in synaptic dysfunction remains mysterious, and their possible interlink still needs to be investigated. Therefore, this review elucidates the intricate role played by both defective NTs/NMs and altered redox signaling in synaptopathy. Further, the involvement of numerous pharmacological approaches to compensate neurotransmission imbalance has also been discussed, which may be considered as a potential therapeutic approach in synaptopathy associated with AD.

Comparison of neurons derived from mouse P19, rat PC12 and human SH-SY5Y cells in the assessment of chemical- and toxin-induced neurotoxicity

Background

Exposure to chemicals might be toxic to the developing brain. There is a need for simple and robust in vitro cellular models for evaluation of chemical-induced neurotoxicity as a complement to traditional studies on animals. In this study, neuronally differentiated mouse embryonal carcinoma P19 cells (P19 neurons) were compared with human neuroblastoma SH-SY5Y cells and rat adrenal pheochromocytoma PC12 cells for their ability to detect toxicity of methylmercury (MeHg), okadaic acid and acrylamide.

Methods

Retinoic acid-treated P19 and SH-SY5Y cells and nerve growth factor-stimulated PC12 cells, allowed to differentiate for 6 days, were exposed to MeHg, okadaic acid and acrylamide for 48 h. Cell survival and neurite outgrowth were assessed with the calcein-AM assay and fluorescence detection of antibodies against the cytoskeletal neuron-specific protein βIII-tubulin, respectively. The effects of glutathione (GSH) and the potent inhibitor of GSH synthesis buthionine sulfoximine (BSO) on the MeHg induced-toxicity were assessed using the PrestoBlue™ cell viability assay and the TMRE mitochondrial membrane potential assay.

Results

Differentiated P19 cells developed the most extensive neuronal network among the three cell models and were the most sensitive neuronal model to detect neurotoxic effects of the test compounds. MeHg produced a concentration-dependent toxicity in differentiated P19 cells and SH-SY5Y cells, with statistically significant effects at concentrations from 0.1 μM in the P19 neurons and 1 μM in the SH-SY5Y cells. MeHg induced a decrease in the cellular metabolic activity and mitochondrial membrane potential (ΔΨm) in the differentiated P19 cells and SH-SY5Y cells, that were attenuated by GSH. Okadaic acid and acrylamide also showed statistically significant toxicity in the P19 neurons, but not in the SH-SY5Y cells or the P12 cells.

Conclusions

P19 neurons are more sensitive to detect cytotoxicity of MeHg, okadaic acid and acrylamide than retinoic acid-differentiated SH-SY5Y cells and nerve growth factor-treated PC12 cells. P19 neurons are at least as sensitive as differentiated SH-SY5Y cells to detect the loss of mitochondrial membrane potential produced by MeHg and the protective effects of extracellular GSH on MeHg toxicity. P19 neurons may be a useful model to study neurotoxic effects of chemicals.

Polymeric alkylpyridinium salts permit intracellular delivery of human Tau in rat hippocampal neurons: requirement of Tau phosphorylation for functional deficits

Patients suffering from tauopathies including frontotemporal dementia (FTD) and Alzheimer's disease (AD) present with intra-neuronal aggregation of microtubule-associated protein Tau. During the disease process, Tau undergoes excessive phosphorylation, dissociates from microtubules and aggregates into insoluble neurofibrillary tangles (NFTs), accumulating in the soma. While many aspects of the disease pathology have been replicated in transgenic mouse models, a region-specific non-transgenic expression model is missing. Complementing existing models, we here report a novel region-specific approach to modelling Tau pathology. Local co-administration of the pore-former polymeric 1,3-alkylpyridinium salts (Poly-APS) extracted from marine sponges, and synthetic full-length 4R recombinant human Tau (hTau) was performed in vitro and in vivo. At low doses, Poly-APS was non-toxic and cultured cells exposed to Poly-APS (0.5 µg/ml) and hTau (1 µg/ml; ~22 µM) had normal input resistance, resting-state membrane potentials and Ca(2+) transients induced either by glutamate or KCl, as did cells exposed to a low concentration of the phosphatase inhibitor Okadaic acid (OA; 1 nM, 24 h). Combined hTau loading and phosphatase inhibition resulted in a collapse of the membrane potential, suppressed excitation and diminished glutamate and KCl-stimulated Ca(2+) transients. Stereotaxic infusions of Poly-APS (0.005 µg/ml) and hTau (1 µg/ml) bilaterally into the dorsal hippocampus at multiple sites resulted in hTau loading of neurons in rats. A separate cohort received an additional 7-day minipump infusion of OA (1.2 nM) intrahippocampally. When tested 2 weeks after surgery, rats treated with Poly-APS+hTau+OA presented with subtle learning deficits, but were also impaired in cognitive flexibility and recall. Hippocampal plasticity recorded from slices ex vivo was diminished in Poly-APS+hTau+OA subjects, but not in other treatment groups. Histological sections confirmed the intracellular accumulation of hTau in CA1 pyramidal cells and along their processes; phosphorylated Tau was present only within somata. This study demonstrates that cognitive, physiological and pathological symptoms reminiscent of tauopathies can be induced following non-mutant hTau delivery into CA1 in rats, but functional consequences hinge on increased Tau phosphorylation. Collectively, these data validate a novel model of locally infused recombinant hTau protein as an inducer of Tau pathology in the hippocampus of normal rats; future studies will provide insights into the pathological spread and maturation of Tau pathology.

Prion-like nanofibrils of small molecules (PriSM) selectively inhibit cancer cells by impeding cytoskeleton dynamics

Emerging evidence reveals that prion-like structures play important roles to maintain the well-being of cells. Although self-assembly of small molecules also affords prion-like nanofibrils (PriSM), little is known about the functions and mechanisms of PriSM. Previous works demonstrated that PriSM formed by a dipeptide derivative selectively inhibiting the growth of glioblastoma cells over neuronal cells and effectively inhibiting xenograft tumor in animal models. Here we examine the protein targets, the internalization, and the cytotoxicity pathway of the PriSM. The results show that the PriSM selectively accumulate in cancer cells via macropinocytosis to impede the dynamics of cytoskeletal filaments via promiscuous interactions with cytoskeletal proteins, thus inducing apoptosis. Intriguingly, Tau proteins are able to alleviate the effect of the PriSM, thus protecting neuronal cells. This work illustrates PriSM as a new paradigm for developing polypharmacological agents that promiscuously interact with multiple proteins yet result in a primary phenotype, such as cancer inhibition.

Berberine attenuates axonal transport impairment and axonopathy induced by Calyculin A in N2a cells

Berberine is a primary component of the most functional extracts of Coptidis rhizome used in traditional Chinese medicine for centuries. Recent reports indicate that Berberine has the potential to prevent and treat Alzheimer's disease (AD). The previous studies reported that Calyculin A (CA) impaired the axonal transport in neuroblastoma-2a (N2a) cells. Berberine attenuated tau hyperphosphorylation and cytotoxicity induced by CA. Our study aimed at investigating the effects of Berberine on the axonal transport impairment induced by CA in N2a cells. The results showed that Berberine could protect the cell from CA -induced toxicity in metabolism and viability, as well as hyperphosphorylation of tau and neurofilaments (NFs). Furthermore, Berberine could reverse CA-induced axonal transport impairment significantly. Berberine also partially reversed the phosphorylation of the catalytic subunit of PP-2A at Tyrosine 307, a crucial site negatively regulating the activity of PP-2A, and reduced the levels of malondialdehyde and the activity of superoxide dismutase, markers of oxidative stress, induced by CA. The present work for the first time demonstrates that Berberine may play a role in protecting against CA-induced axonal transport impairment by modulating the activity of PP-2A and oxidative stress. Our findings also suggest that Berberine may be a potential therapeutic drug for AD.

Recovery of vesicular storage and release parameters after high frequency stimulation in rat hippocampus

The replenishment rates estimated from the recovery of synaptic efficacy following synaptic depression are known to be widely scattered. Given the importance of the replenishment during stimulation, especially if it is prolonged, it is important to better understand what influences the recovery of the synaptic efficacy following stimulation. We fit a two-pool model of vesicular secretion to the changes of the excitatory post-synaptic currents recorded in CA1 neurons of rat hippocampal slices to determine how the model parameters change during, and following, long stimulation. The replenishment rate at the end of stimulation inducing synaptic depression differs greatly from that at the beginning of stimulation. It decreases progressively and rapidly (by ~75 % and with a time constant of <10 s) during stimulation, and this is followed by a similarly fast recovery (time constant of ~10 s), but to a steady-state that is approximately twice as large as its pre-stimulation value. Both [Ca(++)]o and the duration of long stimulation influence the recovery of the replenishment rate. Its new steady-state is significantly higher, if either [Ca(++)]o is higher or stimulation longer, but the recovery of the replenishment rate becomes clearly slower if [Ca(++)]o is higher, and faster if stimulation is longer. Many factors thus influence the recovery of the replenishment rate and of the synaptic efficacy, but the stimulation induced [Ca(++)]i accumulation cannot explain the change of the replenishment rate during recovery. Finally, okadaic acid, which speeds up vesicular trafficking, does not alter the recovery of the replenishment rate. The vesicular replenishment of the RRP following stimulation is thus not likely to be associated with significant vesicular movement.

Melatonin ameliorates Alzheimer-like pathological changes and spatial memory retention impairment induced by calyculin A

We have reported recently that inhibition of protein phosphatase (PP)-2A and PP-1 by calyculin A, a specific inhibitor of PP-2A and PP-1, induced Alzheimer-like hyperphosphorylation of tau and spatial memory retention impairment. In this study, we tested the in vivo effects of melatonin on these Alzheimer-like changes. We found that administration of melatonin intraperitoneally for 9 consecutive days before injection of calyculin A could prevent calyculin A-induced synaptophysin loss, memory retention deficits, as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the phosphorylation of the catalytic subunit of PP-2A at Tyrosine 307 (Y307), a crucial site negatively regulating the activity of PP-2A, and reduced the levels of malondialdehyde, a marker of oxidative stress, induced by calyculin A. These results suggest that melatonin could serve as a potential therapeutic agent for preventing Alzheimer-like pathological changes and behavioral abnormality via modulating the activity of PP-2A and oxidative stress.

Melatonin reduces the impairment of axonal transport and axonopathy induced by calyculin A

Previous studies have reported that calyculin A (CA), a selective inhibitor of protein phosphatase (PP)-2A and PP-1, impairs axonal transport in neuroblastoma N2a cells. Melatonin prevents Alzheimer-like hyperphosphorylation of cytoskeletal proteins and the impairment of spatial memory retention induced by CA. In this study, we tested the effects of melatonin on the impairment of axonal transport induced by CA in neuroblastoma N2a cells. We found that melatonin protected the cells from CA-induced toxicity in metabolism and viability as well as hyperphosphorylation of tau and neurofilaments. Furthermore, melatonin partially reversed the CA-induced phosphorylation of the catalytic subunit of PP-2A at tyrosine 307, a crucial site that negatively regulates the activity of PP-2A, and reduced the levels of malondialdehyde and the activity of superoxide dismutase, which are markers of oxidative stress. Melatonin also significantly reversed the CA-induced impairment of axonal transport. These results suggest that melatonin may have a role in protecting against the CA-induced impairment of axonal transport by modulating the activity of PP-2A and oxidative stress.

Molecular pathogenesis of Alzheimer's disease: reductionist versus expansionist approaches

Alzheimer's disease (AD) is characterized clinically by dementia and pathologically by two hallmark lesions, senile plaques and neurofibrillary tangles. About a quarter century ago these hallmark lesions were purified and their protein constituents identified, precipitating an avalanche of molecular studies as well as substantial optimism about successful therapeutic intervention. In 2009, we now have copious knowledge on the biochemical cascades that produce these proteins, the different modifications and forms in which these proteins exist, and the ability to selectively target these proteins for therapeutic intervention on an experimental basis. At the same time, there has been no discernible alteration in the natural course of AD in humans. While it may be that the complexity of AD will exceed our capacity to make significant treatment progress for decades or more, a paradigm shift from the reductionism that defines amyloid-beta and tau hypotheses, to one that more accurately reflects the meaning of neuropathological changes, may be warranted. We and others have demonstrated that AD pathology is a manifestation of cellular adaptation, specifically as a defense against oxidative injury. As such, AD pathology is therefore a host response rather than a manifestation of cytotoxic protein injury, and is unlikely to be a fruitful target for therapeutic intervention. An "expansionist" view of the disease, we believe, with oxidative stress as a pleiotropic and upstream process, more aptly describes the relationship between various and numerous molecular alterations and clinical disease.

An aluminum-based rat model for Alzheimer's disease exhibits oxidative damage, inhibition of PP2A activity, hyperphosphorylated tau, and granulovacuolar degeneration

In Alzheimer's disease (AD), oxidative damage leads to the formation of amyloid plaques while low PP2A activity results in hyperphosphorylated tau that polymerizes to form neurofibrillary tangles. We probed these early events, using brain tissue from a rat model for AD that develops memory deterioration and AD-like behaviors in old age after chronically ingesting 1.6 mg aluminum/kg bodyweight/day, equivalent to the high end of the human dietary aluminum range. A control group consumed 0.4 mg aluminum/kg/day. We stained brain sections from the cognitively-damaged rats for evidence of amyloid plaques, neurofibrillary tangles, aluminum, oxidative damage, and hyperphosphorylated tau. PP2A activity levels measured 238.71+/-17.56 pmol P(i)/microg protein and 580.67+/-111.70 pmol P(i)/microg protein (p<0.05) in neocortical/limbic homogenates prepared from cognitively-damaged and control rat brains, respectively. Thus, PP2A activity in cognitively-damaged brains was 41% of control value. Staining results showed: (1) aluminum-loading occurs in some aged rat neurons as in some aged human neurons; (2) aluminum-loading in rat neurons is accompanied by oxidative damage, hyperphosphorylated tau, neuropil threads, and granulovacuolar degeneration; and (3) amyloid plaques and neurofibrillary tangles were absent from all rat brain sections examined. Known species difference can reasonably explain why plaques and tangles are unable to form in brains of genetically-normal rats despite developing the same pathological changes that lead to their formation in human brain. As neuronal aluminum can account for early stages of plaque and tangle formation in an animal model for AD, neuronal aluminum could also initiate plaque and tangle formation in humans with AD.

BACE inhibitor reduces APP-beta-C-terminal fragment accumulation in axonal swellings of okadaic acid-induced neurodegeneration

Emerging evidence suggests that not only beta-amyloid but also other amyloid precursor protein (APP) fragments, such as the beta-C-terminal fragment (betaCTF), might be involved in Alzheimer's disease (AD). Treatment of neurons with okadaic acid (OA), a protein phosphatase-2A inhibitor, has been used to induce tau phosphorylation and neuronal death to create a research model of AD. In this study, we analyzed axonopathy and APP regulation in cultured rat neurons treated with OA. After OA treatment, the neurons presented with axonal swellings filled with vesicles, microtubule fragments, and transport molecules such as kinesin and synapsin-I. Western blotting showed that intracellular APP levels were increased and immunocytochemistry using antibodies against the APP C-terminus showed that APP accumulated in the axonal swellings. This APP C-terminus immunoreactivity disappeared when neurons were cotreated with a beta-secretase inhibitor, but not with alpha- or gamma-secretase inhibitors, indicating that the accumulation was primarily composed of APP-betaCTF. These findings provide the first evidence that APP-betaCTF can accumulate in the axons of OA-treated neurons, and may suggest that APP-betaCTF is involved in the pathogenesis of AD.

Differential localization of protein phosphatase-1alpha, beta and gamma1 isoforms in primate prefrontal cortex

Prefrontal cortical functioning depends on D1 family receptors and their complex signal transduction cascade, including protein phosphatase-1 (PP1). Three PP1 isoforms are prominent in the brain: PP1alpha, PP1beta and PP1gamma1. PP1 localization by a variety of scaffolding proteins is critical for dopamine-mediated modulation of glutamatergic neurotransmission. We have quantified the subcellular distribution of each isoform in primate prefrontal cortex using immunoelectron microscopy. All three are found in spines, dendrites, axon terminals, axons and glia. However, PP1alpha and PP1gamma1 labeling is enriched in spines, whereas PP1beta label is enriched in dendrites. Using post-embedding immunogold labeling, we further examined the distribution of PP1alpha and PP1gamma1 within spines. PP1gamma1 is highly and specifically concentrated in the postsynaptic density (PSD) of these spines, while PP1alpha is enriched in the PSD but also found subjacent to the PSD in moderate amounts. Thus, PP1 isoforms are heterogeneously distributed in the cortical neuropil and within spines. These results suggest that each PP1 isoform has access to a different set of substrates and, furthermore, they demonstrate that the composition of signal transduction proteins varies in different parts of the neuron and even in different regions of a dendritic spine in the primate PFC.

Inactivation of GSK-3?? in okadaic acid-induced neurodegeneration: relevance to Alzheimer??s disease

Hyperphosphorylation of tau is a characteristic feature of the neurodegenerative pathology in Alzheimer's disease (AD). Okadaic acid is used as a research model of AD to increase the tau phosphorylation and neuronal death. Using Western blotting, we found that the amounts of activated PKB[pS-473] and inactivated GSK-3beta[pS-9] were increased in proportion to the progress of okadaic acid induced tau phosphorylation. Immunocytochemistry showed that PKB[pS-473] and GSK-3beta[pS-9] immunoreactivity increased in dystrophic neurites and cell bodies in degenerating neurons after okadaic acid treatment. Double staining with phosphospecific tau antibodies showed that PKB[pS-473] and GSK-3beta[pS-9] were colocalized with phosphospecific tau in response to okadaic acid. Taken together, our data suggest that inhibition of protein phosphatase results in the hyperphosphorylation of tau without GSK-3beta overactivation.

Inactivation of integrin-linked kinase induces aberrant tau phosphorylation via sustained activation of glycogen synthase kinase 3beta in N1E-115 neuroblastoma cells

Integrin-linked kinase (ILK) is a focal adhesion serine/threonine protein kinase with an important role in integrin and growth factor signaling pathways. Recently, we demonstrated that ILK is expressed in N1E-115 neuroblastoma cells and controls integrin-dependent neurite outgrowth in serum-starved cells grown on laminin (Ishii, T., Satoh, E., and Nishimura, M. (2001) J. Biol. Chem. 276, 42994-43003). Here we report that ILK controls tau phosphorylation via regulation of glycogen synthase kinase-3beta (GSK-3beta) activity in N1E-115 cells. Stable transfection of a kinase-deficient ILK mutant (DN-ILK) resulted in aberrant tau phosphorylation in N1E-115 cells at sites recognized by the Tau-1 antibody that are identical to some of the phosphorylation sites in paired helical filaments, PHF-tau, in brains of patients with Alzheimer's disease. The tau phosphorylation levels in the DN-ILK-expressing cells are constant under normal and differentiating conditions. On the other hand, aberrant tau phosphorylation was not observed in the parental control cells. ILK inactivation resulted in an increase in the active form but a decrease in the inactive form of GSK-3beta, which is a candidate kinase involved in PHF-tau formation. Moreover, inhibition of GSK-3beta with lithium prevented aberrant tau phosphorylation in the DN-ILK-expressing cells. These results suggest that ILK inactivation results in aberrant tau phosphorylation via sustained activation of GSK-3beta in N1E-115 Cells. ILK directly phosphorylates GSK-3beta and inhibits its activity. Therefore, endogenous ILK protects against GSK-3beta-induced aberrant tau phosphorylation via inhibition of GSK-3beta activity in N1E-115 cells.

Inhibition of protein phosphatase 2A- and protein phosphatase 1-induced tau hyperphosphorylation and impairment of spatial memory retention in rats

Tau hyperphosphorylation leads to formation of paired helical filament/neurofibrillary tangles, the hallmark lesion seen in Alzheimer's disease (AD) brain. An imbalanced regulation in protein kinases and protein phosphatases in the affected neurons is proposed to be a reasonable causative factor to the disease process. To verify the hypothesis, we have injected in the present study calyculin A, a potent and specific inhibitor of protein phosphatase (PP) 2A and PP1, into rat hippocampus bilaterally, thus reproduced an Alzheimer's-like deficiency in dephosphorylation system. It was found that calyculin A-injected rats developed lesions in spatial memory retention in Morris water maze test. At mean time, tau was hyperphosphorylated at Ser396/Ser404 (PHF-1) and Ser-262/Ser-356 (12E8) sites determined both by immunohistochemistry and Western blot. It is implicated that (1) PP2A and PP1 participate in the in vivo regulation of tau phosphorylation, and down-regulation of the two phosphatases will result in tau hyperphosphorylation; (2) hyperphosphorylation of tau at PHF-1 and 12E8 sites might be crucial to affect spatial memory in AD.

Current drugs and future hopes in the treatment of Alzheimer's disease

In spite of several years of experience with the use of cholinesterase inhibitors for treatment of symptoms of Alzheimer's disease their influence on disease progression remains still unclear. New cholinesterase inhibitors should provide an additional neuroprotective activity, because only substances which stop neuronal death can influence disease progression. New treatment strategies are focusing on amyloid processing, preventing the occurrence of toxic A beta(1-42) peptide. These procedures include the vaccination trials, but their clinical usefulness has to be proven. Also strategies focussing on neurofibrillary pathologies should be explored in detail. Drug development for Alzheimer's disease should include all pathological events associated with neurodegeneration, like oxidative stress, neuroinflammation or disturbances in growth factor signaling. Abnormal protein aggregation as a common feature of different neurodegenerative diseases might also be a promising drug target. Beside beta sheet breakers directed against beta-amyloid deposition the endogenous protein beta-synuclein or derivatives of it might be able to counteract aggregation of alpha-synuclein as well as of amyloid beta protein. Interaction with alpha-synuclein deserves special attention because it might be an early step of synaptic degeneration. Due to the complexity of the disease combination of different drugs might be the most promising way to go. The parallel development of early biological markers should enable intervention in pre-symptomatic disease stages.

Impaired conditioned taste aversion learning in spinophilin knockout mice

Plasticity in dendritic spines may underlie learning and memory. Spinophilin, a protein enriched in dendritic spines, has the properties of a scaffolding protein and is believed to regulate actin cytoskeletal dynamics affecting dendritic spine morphology. It also binds protein phosphatase-1 (PP-1), an enzyme that regulates dendritic spine physiology. In this study, we tested the role of spinophilin in conditioned taste aversion learning (CTA) using transgenic spinophilin knockout mice. CTA is a form of associative learning in which an animal rejects a food that has been paired previously with a toxic effect (e.g., a sucrose solution paired with a malaise-inducing injection of lithium chloride). Acquisition and extinction of CTA was tested in spinophilin knockout and wild-type mice using taste solutions (sucrose or sodium chloride) or flavors (Kool-Aid) paired with moderate or high doses of LiCl (0.15 M, 20 or 40 mL/kg). When sucrose or NaCl solutions were paired with a moderate dose of LiCl, spinophilin knockout mice were unable to learn a CTA. At the higher dose, knockout mice acquired a CTA but extinguished more rapidly than wild-type mice. A more salient flavor stimulus (taste plus odor) revealed similar CTA learning at both doses of LiCl in both knockouts and wild types. Sensory processing in the knockouts appeared normal because knockout mice and wild-type mice expressed identical unconditioned taste preferences in two-bottle tests, and identical lying-on-belly responses to acute LiCl. We conclude that spinophilin is a candidate molecule required for normal CTA learning.

Protein phosphatase 2A: the Trojan Horse of cellular signaling

Dynamic phosphorylation and dephosphorylation of proteins are fundamental mechanisms utilized by cells to transduce signals. Whereas transduction by protein kinases has been a major focus of studies in the last decade, protein phosphatase 2A (PP2A) enzymes emerge in this millenium as the most fashionable players in cellular signaling. Viral proteins target specific PP2A enzymes in order to deregulate chosen cellular pathways in the host and promote viral progeny. The observation that a variety of viruses utilize PP2A to alienate cellular behavior emphasizes the fundamental importance of PP2A in signal transduction. This review will primarily focus on discussing the uniqueness of PP2A regulation and uncovering the critical role played by protein-protein interactions in the modulation of PP2A signaling. Moreover, the place of PP2A in signaling pathways and its functional significance for human diseases will be discussed.

Effects of prenatal exposure to cocaine on the developing brain: anatomical, chemical, physiological and behavioral consequences

Earlier studies of human infants and studies employing animal models had indicated that prenatal exposure to cocaine produced developmental changes in the behavior of the offspring. The present paper reports on the results obtained in a rabbit model of in utero exposure to cocaine using intravenous injections (4 mg/kg, twice daily) that mimic the pharmacokinetics of crack cocaine in humans. At this dose, cocaine had no effect on the body weight gain of dams, time to delivery, litter size and body weight or other physical characteristics of the offspring. In spite of an otherwise normal appearance, cocaine-exposed neonates displayed a permanent impairment in signal transduction via the D1 dopamine receptor in caudate nucleus, frontal cortex and cingulate cortex due to an uncoupling of the receptor from its associated Gs protein. This uncoupling in the caudate nucleus was shown to have behavioral consequences in that young or adult rabbits, exposed to cocaine in utero, failed to demonstrate amphetamine-elicited motor responses normally seen after activation of D1 receptors in the caudate. The cocaine progeny also demonstrated permanent morphological abnormalities in the anterior cingulate cortex due to uncoupling of the D1 receptor and the consequent inability of dopamine to regulate neurite outgrowth during neuronal development. Consistent with the known functions of the anterior cingulate cortex, adult cocaine progeny demonstrated deficits in attentional processes. This was reflected by impairment in discrimination learning during classical conditioning that was due to an inability to ignore salient stimuli even when these were not relevant to the task. The impairment in discrimination learning also occurred in an instrumental avoidance task and could be shown to be due to an impairment of cingulothalamic learning-related neuronal coding. It was proposed that the selective loss of D1-related neurotransmission in the anterior cingulate cortex prevented an appropriate activation of GABA neurons and thus a loss of inhibitory regulation that is necessary for processes involved in associative attention. Taken together, these findings suggest that the uncoupling of the D1 receptor from its G protein may be the fundamental source of the anatomic, cognitive and motor disturbances seen in rabbits exposed to cocaine in utero. Moreover, the long-term cognitive and motor deficits observed in the rabbit model are in agreement with the recent reports indicating that persistent attentional and other behavioral deficits may be evident in cocaine-exposed children as they grow older and are challenged to master more complex cognitive tasks.

Okadaic acid induces cycloheximide and caspase sensitive apoptosis in immature neurons

Previous studies have shown that okadaic acid (OA) evokes tau phosphorylation and neurofibrillary changes in vivo, and in cultured neurons, that resemble Alzheimer's disease pathogenesis. In order to investigate the mechanism of OA-neurotoxicity, we treated cultured rat neurons with OA and examined nuclear morphology, phosphatidylserine (PS) externalization, alpha-fodrin cleavage, and the effects of cell death inhibitors. Our results demonstrated that cycloheximide (CHX) and the broad-spectrum caspase inhibitor, ZVAD, significantly reduced cell death in a dose-dependent manner. Nuclear fragmentation, a hallmark of apoptosis, occurred after OA treatment and was inhibited by CHX or ZVAD. PS externalization was apparent in 6-12 h in neurites and in cell bodies, and peaked at 24 h after OA treatment. Cleavage of alpha-fodrin as visualized by the appearance of 150- and 120-kDa bands appeared with a time course similar to PS externalization. These results suggest that OA induce CHX and caspase sensitive neuronal apoptosis.

Phosphorylation of microtubule-associated protein tau is regulated by protein phosphatase 2A in mammalian brain. Implications for neurofibrillary degeneration in Alzheimer's disease

Hyperphosphorylated tau, which is the major protein of the neurofibrillary tangles in Alzheimer's disease brain, is most probably the result of an imbalance of tau kinase and phosphatase activities in the affected neurons. By using metabolically competent rat brain slices as a model, we found that selective inhibition of protein phosphatase 2A by okadaic acid induced an Alzheimer-like hyperphosphorylation and accumulation of tau. The hyperphosphorylated tau had a reduced ability to bind to microtubules and to promote microtubule assembly in vitro. Immunocytochemical staining revealed hyperphosphorylated tau accumulation in pyramidal neurons in cornu ammonis and in neocortical neurons. The topography of these changes recalls the distribution of neurofibrillary tangles in Alzheimer's disease brain. Selective inhibition of protein phosphatase 2B with cyclosporin A did not have any significant effect on tau phosphorylation, accumulation, or function. These studies suggest that protein phosphatase 2A participates in regulation of tau phosphorylation, processing, and function in vivo. A down-regulation of protein phosphatase 2A activity can lead to Alzheimer-like abnormal hyperphosphorylation of tau.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Comparison of A68 levels in Alzheimer diseased and non-Alzheimer's diseased brain by two ALZ50 based methods

A total of 61 human brain specimens were analyzed with both ELISA and Western Blot using the ALZ50 monoclonal antibody. The brain specimens included: Alzheimer's Disease (AD, n=31), AD/Down's (n=2), Normal (n=14), Parkinson's Disease (n=7), Huntington's chorea (n=2), Wernicke-Korsakov's Encephalopathy (n=3), and Motor Neuron Disease (n=2). The non-AD cases (n=28) had no detectable A68 by ELISA, and showed no A68 bands by Western blot. The AD cases (n=33), all were positive for A68 by the ELISA, but only 31 of 33 had visible A68 band by Western blot. Additionally, a method for solubilization of A68 is reported.

Regulation of neurabin I interaction with protein phosphatase 1 by phosphorylation

Neurabin I is a brain-specific actin-binding protein. Here we show that neurabin I binds protein phosphatase 1 (PP1) and inhibits PP1 activity. Neurabin I interacted with PP1alpha in an overlay assay, in yeast two-hybrid interaction analysis, and in coprecipitation and co-immunoprecipitation experiments. Neurabin I also copurified with both the alpha and gamma isoforms of PP1. A glutathione S-transferase (GST)-neurabin I fusion protein (residues 318-661) containing the putative PP1 binding domain (residues 456-460) inhibited PP1 activity (K(i) = 2.7 +/- 1.2 nM). This fusion protein was also rapidly phosphorylated in vitro by PKA (K(m) = 6 microM) to a stoichiomtry of 1 mol/mol. The phosphorylated residue was identified as serine 461 by HPLC-MS analysis of a tryptic digest. Phosphorylation of GST-neurabin I (residues 318-661) by PKA significantly reduced its binding to PP1 by overlay and by glutathione-Sepharose coprecipitation assays. A 35-fold decrease in inhibitory potency was also observed using a S461E mutant, which mimics phosphorylation of S461. These findings identify a signaling mechanism involving the regulation of PP1 activity and localization mediated by the cAMP pathway.

Sequence of neurodegeneration and accumulation of phosphorylated tau in cultured neurons after okadaic acid treatment

Within neurofibrillary tangles and dystrophic neurites of Alzheimer's disease (AD), the cytoskeletal protein tau is abnormally hyperphosphorylated. In the present study, we examined the effect of okadaic acid (OA), a protein phosphatase inhibitor, in rat cultured neurons. Low concentrations of OA induce degeneration of neurites, rounding of cell bodies, detachment from the substratum, and eventual neuronal death. During OA-induced degeneration, SMI-31 immunoreactivity became punctate in neurites at 6 h after OA treatment, and over time, accumulated in cell bodies and dystrophic neurites. Hyperphosphorylation of tau and marked loss of MAP-2-positive dendrites occurred after 6 h of treatment with OA. Thereafter, AT-8 and PHF-1 immunoreactivity accumulated in cell bodies and subsequently appeared in distal axon-like neurites. These results demonstrate that OA treatment induced hyperphosphorylation of tau and preferential dendritic damage, with subsequent accumulation of phosphorylated tau in cell bodies and dystrophic axon-like neurites. OA-induced neurodegeneration may provide a useful model to study AD.

Perfluorodecalin modifies the pattern of cell arrangement and induces loss of neurites in rat retinal cultures

Perfluorodecalin (PFD), a high specific weight, water-immiscible perfluorocarbon, previously studied as a potential blood substitute, now is used widely in the field of ophthalmic surgery as a tool for maneuvering intraocular tissues and as a short- or medium-term vitreous substitute. In in vivo experiments, several types of lesions in retinal tissue have been described in conjunction with long-term PFD treatment. To better evaluate the biological effects of PFD on retinal cells, we tested it on primary cultures of rat retina seeded on special cyclopore wells that allow the culture to be fed from the bottom side while the top side is in contact with the water-immiscible compound. We found that PFD changed the pattern of cell arrangement and induced loss of neurites. The modification of cell arrangement was less evident at the periphery of the wells where the amount of PFD, and consequently the pressure exerted, was lower. This observation suggests that the changes may be due more to a physical than to a toxic effect of PFD.