Modulation of the phosphorylation state of tau in situ: the roles of calcium and cyclic AMP

Clinical relevance of biomarkers, new therapeutic approaches, and role of post-translational modifications in the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that causes progressive loss of cognitive functions like thinking, memory, reasoning, behavioral abilities, and social skills thus affecting the ability of a person to perform normal daily functions independently. There is no definitive cure for this disease, and treatment options available for the management of the disease are not very effective as well. Based on histopathology, AD is characterized by the accumulation of insoluble deposits of amyloid beta (Aβ) plaques and neurofibrillary tangles (NFTs). Although several molecular events contribute to the formation of these insoluble deposits, the aberrant post-translational modifications (PTMs) of AD-related proteins (like APP, Aβ, tau, and BACE1) are also known to be involved in the onset and progression of this disease. However, early diagnosis of the disease as well as the development of effective therapeutic approaches is impeded by lack of proper clinical biomarkers. In this review, we summarized the current status and clinical relevance of biomarkers from cerebrospinal fluid (CSF), blood and extracellular vesicles involved in onset and progression of AD. Moreover, we highlight the effects of several PTMs on the AD-related proteins, and provide an insight how these modifications impact the structure and function of proteins leading to AD pathology. Finally, for disease-modifying therapeutics, novel approaches, and targets are discussed for the successful treatment and management of AD.

HIV and FIV glycoproteins increase cellular tau pathology via cGMP-dependent kinase II activation

As the development of combination antiretroviral therapy (cART) against human immunodeficiency virus (HIV) drastically improves the lifespan of individuals with HIV, many are now entering the prime age when Alzheimer's disease (AD)-like symptoms begin to manifest. It has been shown that hyperphosphorylated tau, a known AD pathological characteristic, is prematurely increased in the brains of HIV-infected individuals as early as in their 30s and that its levels increase with age. This suggests that HIV infection might lead to accelerated AD phenotypes. However, whether HIV infection causes AD to develop more quickly in the brain is not yet fully determined. Interestingly, we have previously revealed that the viral glycoproteins HIV gp120 and feline immunodeficiency virus (FIV) gp95 induce neuronal hyperexcitation via cGMP-dependent kinase II (cGKII; also known as PRKG2) activation in cultured hippocampal neurons. Here, we use cultured mouse cortical neurons to demonstrate that the presence of HIV gp120 and FIV gp95 are sufficient to increase cellular tau pathology, including intracellular tau hyperphosphorylation and tau release to the extracellular space. We further reveal that viral glycoprotein-induced cellular tau pathology requires cGKII activation. Taken together, HIV infection likely accelerates AD-related tau pathology via cGKII activation.

N-methyl-D-aspartate receptor blockade reduces plasticity-related tau expression and phosphorylation of tau at Ser416 residue but not Thr231 residue

The molecular mechanisms regulating N-methyl-D-aspartate (NMDA) receptor-dependent synaptic plasticity are complex, and the contribution of Tau protein in the physiological process is not fully understood. Herein, we investigated whether the blockade of NMDA receptor activation might change Tau phosphorylation during long-term potentiation (LTP) and long-term depression (LTD) via contribution of GSK3β as a major Tau kinase. For this, we recorded two components (synaptic and population spike components) of hippocampal field potential, which is evoked by the stimulation of the perforant pathway with high- and low-frequency stimulation (HFS and LFS). We found under a 20-µl volume of D-AP5 infusion lasting 1 h that,HFS caused significant synaptic depression, whereas LFS induced a synaptic potentiation. Both the HFS and LFS protocols resulted in a significant increase in population spike component but were characterized by a slow increase in amplitude that occurred with the LFS. D-AP5 attenuated HFS-induced population spike potentiation, but augmented LFS-induced population spike potentiation. The enzymatic activity of GSK-3β was decreased by D-AP5 infusion in the hippocampus, indicating that NMDA receptor activity modulates the enzymatic activity of GSK-3β. In addition, NMDA receptor blockade reduced tau expression and phosphorylation of tau at Ser416 residue, but not Thr231 residue. These findings confirm previous studies that D-AP5 applied to the DG in vivo blocks HFS-induced LTP, but we further also showed that the same dose of D-AP5 resulted in a slowly rising LFS-induced LTP and HFS-induced LTD. The formation of such an LTP, together with reduced enzymatic activity of GSK-3β and tau phosphorylation at Ser⁴¹⁶ epitope, can make it a candidate mechanism for prevention of taupathies.

Axonopathy Likely Initiates Neuropathological Processes Via a Mechanism of Axonal Leakage in Alzheimer's Mouse Models

BACKGROUND

The formation of hyperphosphorylated tau and the production of β-amyloid are thought to be critical steps contributing to the pathological mechanisms in Alzheimer's disease (AD). However, there has been a long-lasting debate over their importance in the onset of AD. Recent studies have demonstrated that axonopathy is considered as an early neuropathological change of AD. However, the exact relationship between the development of axonopathy and the classic neuropathological changes such as senile plaques (SPs) and neurofibrillary tangles (NFTs) is unclear.

OBJECTIVE

The aim of this study was to investigate whether the formation of SPs and NFTs is associated with the development of axonal leakage.

METHOD AND RESULTS

Here we show that the formation and development of axonal leakage - a novel axonopathy is an age-dependent process, accompanied by swellings of axons and varicosities and associated with chronic oxidative stress induced by thiamine deficient (TD) diet in Kunming mice. In an APP/PS1 transgenic mouse model of AD, axonal leakage appears at 3 months, becomes more obvious at 6 months and severe, beyond 1 year. We also show that slight axonal leakage is related to the formation of hyperphosphorylated tau, but not plaques, and that only severe axonal leakage accompanied by the extensive swollen axons and varicosities, and overproduction of β-amyloid leads to the formation of SPs and hyperphosphorylated tau.

CONCLUSION

These data provide an explanation of the common origin and development of SPs and NFTs, and suggest that axonal leakage might be a key event in the development of the neuropathological processes in AD.

Free d-aspartate triggers NMDA receptor-dependent cell death in primary cortical neurons and perturbs JNK activation, Tau phosphorylation, and protein SUMOylation in the cerebral cortex of mice lacking d-aspartate oxidase activity

In mammals, free d-aspartate (D-Asp) is abundant in the embryonic brain, while levels remain very low during adulthood as a result of the postnatal expression and activity of the catabolizing enzyme d-aspartate oxidase (DDO). Previous studies have shown that long-lasting exposure to nonphysiological, higher D-Asp concentrations in Ddo knockout (Ddo^-/-) mice elicits a precocious decay of synaptic plasticity and cognitive functions, along with a dramatic age-dependent expression of active caspase 3, associated with increased cell death in different brain regions, including hippocampus, prefrontal cortex, and substantia nigra pars compacta. Here, we investigate the yet unclear molecular and cellular events associated with the exposure of abnormally high D-Asp concentrations in cortical primary neurons and in the brain of Ddo^-/- mice. For the first time, our in vitro findings document that D-Asp induces in a time-, dose-, and NMDA receptor-dependent manner alterations in JNK and Tau phosphorylation levels, associated with pronounced cell death in primary cortical neurons. Moreover, observations obtained in Ddo^-/- animals confirmed that high in vivo levels of D-Asp altered cortical JNK signaling, Tau phosphorylation and enhanced protein SUMOylation, indicating a robust indirect role of DDO activity in regulating these biochemical NMDA receptor-related processes. Finally, no gross modifications in D-Asp concentrations and DDO mRNA expression were detected in the cortex of patients with Alzheimer's disease when compared to age-matched healthy controls.

Depotentiation of Long-Term Potentiation Is Associated with Epitope-Specific Tau Hyper-/Hypophosphorylation in the Hippocampus of Adult Rats

It is well-known that some kinases which are involved in the induction of synaptic plasticity probably modulate tau phosphorylation. However, how depression of potentiated synaptic strength contributes to tau phosphorylation is unclear because of the lack of experiments in which depotentiation of LTP was induced. Field excitatory postsynaptic potential (fEPSP) and population spike (PS) were recorded from the dentate gyrus in response to the perforant pathway stimulation. To induce LTP, high-frequency stimulation (HFS) was used, while, for depotentiation of LTP, low-frequency stimulation (LFS) consisting of 900 pulses at 1 Hz was applied 5 min after tetanization. In some experiments, a neutral protocol at 0.033 Hz was applied throughout the experiment without any induction of synaptic plasticity. One-hertz depotentiation protocol was able to decrease fEPSP slope which was previously increased by HFS, whereas no significant change in fEPSP slope and PS amplitude was observed in neutral protocol experiments. Relative to saline infusion, LTP was lower in magnitude and was more reversed by subsequent LFS in the presence of ERK1/2 inhibitor. Western blot experiments indicated that tau protein was hyperphosphorylated at ser416 epitope but rather hypophosphorylated at thr231 epitope in the whole hippocampus upon depotentiation of LTP. These changes concomitantly occurred with a notable increase in the levels of total tau and in the levels of phosphorylated form of the extracellular signal-regulated protein kinases 1 and 2 (ERK1/2). ERK1/2 inhibition resulted in a decrease in phosphorylation of tau at p416Tau when ERK1/2 was inhibited. These findings indicate that some forms of long-term plastic changes might be related with epitope-specific tau phosphorylation and ERK1/2 activation in the hippocampus. Therefore, we emphasize that tau may be crucial for physiological learning as well as Alzheimer's disease pathology.

Insulin deficiency results in reversible protein kinase A activation and tau phosphorylation

Alzheimer's disease (AD) is a highly prevalent multifactorial disease for which Diabetes Mellitus (DM) is a risk factor. Abnormal phosphorylation and aggregation of tau is a key hallmark of AD. In animal models, DM induces or exacerbates the phosphorylation of tau, suggesting that DM may influence the risk at AD by directly facilitating tau pathology. Previously we reported that tau phosphorylation induced in response to metabolic stress is reversible. Since identification and understanding of early players in tau pathology is pivotal for therapeutic intervention, we here investigated the mechanism underlying tau phosphorylation in the diabetic brain and its potential for reversibility. To model DM we used streptozotocin-treatment to induce insulin deficiency in rats. Insulin depletion leads to increased tau phosphorylation in the brain and we investigated the activation status of known tau kinases and phosphatases in this model. We identified protein kinase A (PKA) as a tau kinase activated by DM in the brain. The potential relevance of this signaling pathway to AD pathogenesis is indicated by the increased level of active PKA in temporal cortex of early stage AD patients. Our data indicate that activation of PKA and tau phosphorylation are associated with insulin deficiency per se, rather than the downstream energy deprivation. In vitro studies confirm that insulin deficiency results in PKA activation and tau phosphorylation. Strikingly, both active PKA and induced tau phosphorylation are reversed upon insulin treatment in the steptozotocin animal model. Our data identify insulin deficiency as a direct trigger that induces the activity of the tau kinase PKA and results in tau phosphorylation. The reversibility upon insulin treatment underscores the potential of insulin as an early disease-modifying intervention in AD and other tauopathies.

Glycine Administration Alters MAPK Signaling Pathways and Causes Neuronal Damage in Rat Brain: Putative Mechanisms Involved in the Neurological Dysfunction in Nonketotic Hyperglycinemia

High glycine (GLY) levels have been suggested to induce neurotoxic effects in the central nervous system of patients with nonketotic hyperglycinemia (NKH). Since the mechanisms involved in the neuropathophysiology of NKH are not totally established, we evaluated the effect of a single intracerebroventricular administration of GLY on the content of proteins involved in neuronal damage and inflammatory response, as well as on the phosphorylation of the MAPK p38, ERK1/2, and JNK in rat striatum and cerebral cortex. We also examined glial fibrillary acidic protein (GFAP) staining, a marker of glial reactivity. The parameters were analyzed 30 min or 24 h after GLY administration. GLY decreased Tau phosphorylation in striatum and cerebral cortex 30 min and 24 h after its administration. On the other hand, synaptophysin levels were decreased in striatum at 30 min and in cerebral cortex at 24 h after GLY injection. GLY also decreased the phosphorylation of p38, ERK1/2, and JNK 30 min after its administration in both brain structures. Moreover, GLY-induced decrease of p38 phosphorylation in striatum was attenuated by N-methyl-D-aspartate receptor antagonist MK-801. In contrast, synuclein, NF-κB, iκB, inducible nitric oxide synthase and nitrotyrosine content, and GFAP immunostaining were not altered by GLY infusion. It may be presumed that the decreased phosphorylation of MAPK associated with alterations of markers of neuronal injury induced by GLY may contribute to the neurological dysfunction observed in NKH.

The Neurofibrillary Pathology of Alzheimer's Disease

Abundant neurofibrillary lesions in certain brain regions constitute one of the defining neuropathological characteristics of Alzheimer's disease, where their presence correlates with the degree of dementia. An understanding of the mechanisms that lead to the neurofibrillary pathology is critical for elucidating the pathogenesis of Alzheimer's disease and for developing effective therapeutic strategies. Neurofibrillary lesions consist of neurofibrillary tangles, neuropil threads, and abnormal neurites. Ultrastructurally, each of these lesions consists of abnormal paired helical and straight filaments. These filaments are made of the six brain isoforms of microtubule-associated protein tau in a hyperphosphorylated and an abnormally phosphorylated state. Several candidate protein kinases and protein phosphatases for the hyperphos phorylation of tau have been identified. Moreover, recent results suggest that an interaction between tau protein and sulfated glycosaminoglycans may play an important role in inducing both the hyperphosphor ylation of tau and the formation of paired helical and straight filaments. NEUROSCIENTIST 3:131-141, 1997

NMDA reduces Tau phosphorylation in rat hippocampal slices by targeting NR2A receptors, GSK3β, and PKC activities

The molecular mechanisms that regulate Tau phosphorylation are complex and currently incompletely understood. In the present study, pharmacological inhibitors were deployed to investigate potential processes by which the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors modulates Tau phosphorylation in rat hippocampal slices. Our results demonstrated that Tau phosphorylation at Ser199-202 residues was decreased in NMDA-treated hippocampal slices, an effect that was not reproduced at Ser262 and Ser404 epitopes. NMDA-induced reduction of Tau phosphorylation at Ser199-202 was further promoted when NR2A-containing receptors were pharmacologically isolated and were completely abrogated by the NR2A receptor antagonist NVP-AAM077. Compared with nontreated slices, we observed that NMDA receptor activation was reflected in high Ser9 and low Tyr216 phosphorylation of glycogen synthase kinase-3 beta (GSK3β), suggesting that NMDA receptor activation might diminish Tau phosphorylation via a pathway involving GSK3β inhibition. Accordingly, we found that GSK3β inactivation by a protein kinase C- (PKC-) dependent mechanism is involved in the NMDA-induced reduction of Tau phosphorylation at Ser199-202 epitopes. Taken together, these data indicate that NR2A receptor activation may be important in limiting Tau phosphorylation by a PKC/GSK3β pathway and strengthen the idea that these receptors might act as an important molecular device counteracting neuronal cell death mechanisms in various pathological conditions.

Glutamate system, amyloid ß peptides and tau protein: functional interrelationships and relevance to Alzheimer disease pathology

Alzheimer disease is the most prevalent form of dementia globally and is characterized premortem by a gradual memory loss and deterioration of higher cognitive functions and postmortem by neuritic plaques containing amyloid ß peptide and neurofibrillary tangles containing phospho-tau protein. Glutamate is the most abundant neurotransmitter in the brain and is essential to memory formation through processes such as long-term potentiation and so might be pivotal to Alzheimer disease progression. This review discusses how the glutamatergic system is impaired in Alzheimer disease and how interactions of amyloid ß and glutamate influence synaptic function, tau phosphorylation and neurodegeneration. Interestingly, glutamate not only influences amyloid ß production, but also amyloid ß can alter the levels of glutamate at the synapse, indicating that small changes in the concentrations of both molecules could influence Alzheimer disease progression. Finally, we describe how the glutamate receptor antagonist, memantine, has been used in the treatment of individuals with Alzheimer disease and discuss its effectiveness.

The role of tau in neurodegenerative diseases and its potential as a therapeutic target

The abnormal deposition of proteins in and around neurons is a common pathological feature of many neurodegenerative diseases. Among these pathological proteins, the microtubule-associated protein tau forms intraneuronal filaments in a spectrum of neurological disorders. The discovery that dominant mutations in the MAPT gene encoding tau are associated with familial frontotemporal dementia strongly supports abnormal tau protein as directly involved in disease pathogenesis. This and other evidence suggest that tau is a worthwhile target for the prevention or treatment of tau-associated neurodegenerative diseases, collectively called tauopathies. However, it is critical to understand the normal biological roles of tau, the specific molecular events that induce tau to become neurotoxic, the biochemical nature of pathogenic tau, the means by which pathogenic tau exerts neurotoxicity, and how tau pathology propagates. Based on known differences between normal and abnormal tau, a number of approaches have been taken toward the discovery of potential therapeutics. Key questions still remain open, such as the nature of the connection between the amyloid- β protein of Alzheimer's disease and tau pathology. Answers to these questions should help better understand the nature of tauopathies and may also reveal new therapeutic targets and strategies.

The origin and development of plaques and phosphorylated tau are associated with axonopathy in Alzheimer's disease

OBJECTIVE

The production of neurotoxic β-amyloid and the formation of hyperphosphorylated tau are thought to be critical steps contributing to the neuropathological mechanisms in Alzheimer's disease (AD). However, there remains an argument as to their importance in the onset of AD. Recent studies have shown that axonopathy is considered as an early stage of AD. However, the exact relationship between axonopathy and the origin and development of classic neuropathological changes such as senile plaques (SPs) and neurofibrillary tangles (NFTs) is unclear. The present study aimed to investigate this relationship.

METHODS

Postmortem tracing, combined with the immunohistochemical or immunofluorescence staining, was used to detect axonopathy and the formation of SPs and NFTs.

RESULTS

Axonal leakage-a novel type of axonopathy, was usually accompanied with the extensive swollen axons and varicosities, and was associated with the origin and development of Aβ plaques and hyperphosphorylated tau in the brains of AD patients.

CONCLUSION

Axonopathy, particularly axonal leakage, might be a key event in the initiation of the neuropathological processes in AD.

Modulation of mitochondrial calcium as a pharmacological target for Alzheimer's disease

Perturbed neuronal calcium homeostasis is a prominent feature in Alzheimer's disease (AD). Mitochondria accumulate calcium ions (Ca(2+)) for cellular bioenergetic metabolism and suppression of mitochondrial motility within the cell. Excessive Ca(2+) uptake into mitochondria often leads to mitochondrial membrane permeabilization and induction of apoptosis. Ca(2+) is an interesting second messenger which can initiate both cellular life and death pathways in mitochondria. This review critically discusses the potential of manipulating mitochondrial Ca(2+) concentrations as a novel therapeutic opportunity for treating AD. This review also highlights the neuroprotective role of a number of currently available agents that modulate different mitochondrial Ca(2+) transport pathways. It is reasoned that these mitochondrial Ca(2+) modulators are most effective in combination with agents that increase the Ca(2+) buffering capacity of mitochondria. Modulation of mitochondrial Ca(2+) handling is a potential pharmacological target for future development of AD treatments.

Blockade of NR2A-containing NMDA receptors induces Tau phosphorylation in rat hippocampal slices

Physiological activation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors has been proposed to play a key role in both neuronal cell function and dysfunction. In the present study, we used selective NMDA receptor antagonists to investigate the involvement of NR2A and NR2B subunits in the modulatory effect of basal NMDA receptor activity on the phosphorylation of Tau proteins. We observed, in acute hippocampal slice preparations, that blockade of NR2A-containing NMDA receptors by the NR2A antagonist NVP-AAM077 provoked the hyperphosphorylation of a residue located in the proline-rich domain of Tau (i.e., Ser199). This effect seemed to be Ser199 specific as there was no increase in phosphorylation at Ser262 and Ser409 residues located in the microtubule-binding and C-terminal domains of Tau proteins, respectively. From a mechanistic perspective, our study revealed that blockade of NR2A-containing receptors influences Tau phosphorylation probably by increasing calcium influx into neurons, which seems to rely on accumulation of new NR1/NR2B receptors in neuronal membranes and could involve the cyclin-dependent kinase 5 pathway.

Communication breaks-Down: from neurodevelopment defects to cognitive disabilities in Down syndrome

Down syndrome (DS) is the leading cause of genetically-defined intellectual disability and congenital birth defects. Despite being one of the first genetic diseases identified, only recently, thanks to the phenotypic analysis of DS mouse genetic models, we have begun to understand how trisomy may impact cognitive function. Cognitive disabilities in DS appear to result mainly from two pathological processes: neurogenesis impairment and Alzheimer-like degeneration. In DS brain, suboptimal network architecture and altered synaptic communication arising from neurodevelopmental impairment are key determinants of cognitive defects. Hypocellularity and hypoplasia start at early developmental stages and likely depend upon impaired proliferation of neuronal precursors, resulting in reduction of numbers of neurons and synaptic contacts. The impairment of neuronal precursor proliferation extends to adult neurogenesis and may affect learning and memory. Neurodegenerative mechanisms also contribute to DS cognitive impairment. Early onset Alzheimer disease occurs with extremely high incidence in DS patients and is causally-related to overexpression of beta-amyloid precursor protein (betaAPP), which is one of the triplicated genes in DS. In this review, we will survey the available findings on neurodevelopmental and neurodegenerative changes occurring in DS throughout life. Moreover, we will discuss the potential mechanisms by which defects in neurogenesis and neurodegenerative processes lead to altered formation of neural circuits and impair cognitive function, in connection with findings on pharmacological treatments of potential benefit for DS.

Calcium dysregulation in Alzheimer's disease: from mechanisms to therapeutic opportunities

Calcium is involved in many facets of neuronal physiology, including activity, growth and differentiation, synaptic plasticity, and learning and memory, as well as pathophysiology, including necrosis, apoptosis, and degeneration. Though disturbances in calcium homeostasis in cells from Alzheimer's disease (AD) patients have been observed for many years, much more attention was focused on amyloid-beta (Abeta) and tau as key causative factors for the disease. Nevertheless, increasing lines of evidence have recently reported that calcium dysregulation plays a central role in AD pathogenesis. Systemic calcium changes accompany almost the whole brain pathology process that is observed in AD, including synaptic dysfunction, mitochondrial dysfunction, presenilins mutation, Abeta production and Tau phosphorylation. Given the early and ubiquitous involvement of calcium dysregulation in AD pathogenesis, it logically presents a variety of potential therapeutic targets for AD prevention and treatment, such as calcium channels in the plasma membrane, calcium channels in the endoplasmic reticulum membrane, Abeta-formed calcium channels, calcium-related proteins. The review aims to provide an overview of the current understanding of the molecular mechanisms involved in calcium dysregulation in AD, and an insight on how to exploit calcium regulation as therapeutic opportunities in AD.

Calcium signaling in neurodegeneration

Calcium is a key signaling ion involved in many different intracellular and extracellular processes ranging from synaptic activity to cell-cell communication and adhesion. The exact definition at the molecular level of the versatility of this ion has made overwhelming progress in the past several years and has been extensively reviewed. In the brain, calcium is fundamental in the control of synaptic activity and memory formation, a process that leads to the activation of specific calcium-dependent signal transduction pathways and implicates key protein effectors, such as CaMKs, MAPK/ERKs, and CREB. Properly controlled homeostasis of calcium signaling not only supports normal brain physiology but also maintains neuronal integrity and long-term cell survival. Emerging knowledge indicates that calcium homeostasis is not only critical for cell physiology and health, but also, when deregulated, can lead to neurodegeneration via complex and diverse mechanisms involved in selective neuronal impairments and death. The identification of several modulators of calcium homeostasis, such as presenilins and CALHM1, as potential factors involved in the pathogenesis of Alzheimer's disease, provides strong support for a role of calcium in neurodegeneration. These observations represent an important step towards understanding the molecular mechanisms of calcium signaling disturbances observed in different brain diseases such as Alzheimer's, Parkinson's, and Huntington's diseases.

Phosphorylation of neurofilament subunit NF-M is regulated by activation of NMDA receptors and modulates cytoskeleton stability and neuronal shape

The cytoskeleton is essential for the structural organization of neurons and is influenced during development by excitatory stimuli such as activation of glutamate receptors. In particular, NMDA receptors are known to modulate the function of several cytoskeletal proteins and to influence cell morphology, but the underlying molecular and cellular mechanisms remain unclear. Here, we characterized the neurofilament subunit NF-M in cultures of developing mouse cortical neurons chronically exposed to NMDA receptor antagonists. Western blots analysis showed that treatment of cortical neurons with MK801 or AP5 shifted the size of NF-M towards higher molecular weights. Dephosphorylation assay revealed that this increased size of NF-M observed after chronic exposure to NMDA receptor antagonists was due to phosphorylation. Neurons treated with cyclosporin, an inhibitor of the Ca(2+)-dependent phosphatase calcineurin, also showed increased levels of phosphorylated NF-M. Moreover, analysis of neurofilament stability revealed that the phosphorylation of NF-M, resulting from NMDA receptor inhibition, enhanced the solubility of NF-M. Finally, cortical neurons cultured in the presence of the NMDA receptor antagonists MK801 and AP5 grew longer neurites. Together, these data indicate that a blockade of NMDA receptors during development of cortical neurons increases the phosphorylation state and the solubility of NF-M, thereby favoring neurite outgrowth. This also underlines that dynamics of the neurofilament and microtubule cytoskeleton is fundamental for growth processes.

Cholinergic and glutamatergic alterations beginning at the early stages of Alzheimer disease: participation of the phospholipase A2 enzyme

RATIONALE

Alzheimer disease (AD), a progressive neurodegenerative disorder, is the leading cause of dementia in the elderly. A combination of cholinergic and glutamatergic dysfunction appears to underlie the symptomatology of AD, and thus, treatment strategies should address impairments in both systems. Evidence suggests the involvement of phospholipase A(2) (PLA(2)) enzyme in memory impairment and neurodegeneration in AD via actions on both cholinergic and glutamatergic systems.

OBJECTIVES

To review cholinergic and glutamatergic alterations underlying cognitive impairment and neuropathology in AD and attempt to link PLA(2) with such alterations.

METHODS

Medline databases were searched (no date restrictions) for published articles with links among the terms Alzheimer disease (mild, moderate, severe), mild cognitive impairment, choline acetyltransferase, acetylcholinesterase, NGF, NGF receptor, muscarinic receptor, nicotinic receptor, NMDA, AMPA, metabotropic glutamate receptor, atrophy, glucose metabolism, phospholipid metabolism, sphingolipid, membrane fluidity, phospholipase A(2), arachidonic acid, attention, memory, long-term potentiation, beta-amyloid, tau, inflammation, and reactive species. Reference lists of the identified articles were checked to identify additional studies of interest.

RESULTS

Overall, results suggest the hypothesis that persistent inhibition of cPLA(2) and iPLA(2) isoforms at early stages of AD may play a central role in memory deficits and beta-amyloid production through down-regulation of cholinergic and glutamate receptors. As the disease progresses, beta-amyloid induced up-regulation of cPLA(2) and sPLA(2) isoforms may play critical roles in inflammation and oxidative stress, thus participating in the neurodegenerative process.

CONCLUSION

Activation and inhibition of specific PLA(2) isoforms at different stages of AD could be of therapeutic importance and delay cognitive dysfunction and neurodegeneration.

Coupling of mammalian target of rapamycin with phosphoinositide 3-kinase signaling pathway regulates protein phosphatase 2A- and glycogen synthase kinase-3 -dependent phosphorylation of Tau

Tau is an important microtubule-stabilizing protein in neurons. In its hyperphosphorylated form, Tau protein loses its ability to bind to microtubules and then accumulates and is part of pathological lesions characterizing tauopathies, e.g. Alzheimer disease. Glycogen synthase kinase-3beta (GSK-3beta), antagonized by protein phosphatase 2A (PP2A), regulates Tau phosphorylation at many sites. Diabetes mellitus is linked to an increased risk of developing Alzheimer disease. This could be partially caused by dysregulated GSK-3beta. In a long term experiment (-16 h) using primary murine neuron cultures, we interfered in the insulin/phosphoinositide 3-kinase (PI3K) (LY294002 treatment and insulin boost) and mammalian target of rapamycin (mTor) (AICAR and rapamycin treatment) signaling pathways and examined consequent changes in the activities of PP2A, GSK-3beta, and Tau phosphorylation. We found that the coupling of PI3K with mTor signaling, in conjunction with a regulatory interaction between PP2A and GSK-3beta, changed activities of both enzymes always in the same direction. These balanced responses seem to ensure the steady Tau phosphorylation at GSK/PP2A-dependent sites observed over a long period of time (>/=6 h). This may help in preventing severe changes in Tau phosphorylation under conditions when neurons undergo transient fluctuations either in insulin or nutrient supply. On the other hand, the investigation of Tau protein at Ser-262 showed that interference in the insulin/PI3K and mTor signaling potentially influenced the Tau phosphorylation status at sites where only one of two enzymes (in this case PP2A) is involved in the regulation.

Calcium dysregulation in Alzheimer's disease

Alzheimer disease (AD) is the most common form of adult dementia. Its pathological hallmarks are synaptic degeneration, deposition of amyloid plaques and neurofibrillary tangles, leading to neuronal loss. A few hypotheses have been proposed to explain AD pathogenesis. The beta-amyloid (Abeta) and hyperphosphorylated tau hypotheses suggest that these proteins are the main players in AD development. Another hypothesis proposes that the dysregulation of calcium homeostasis may be a key factor in accelerating other pathological changes. Although Abeta and tau have been extensively studied, recently published data provide a growing body of evidence supporting the critical role of calcium signalling in AD. For example, presenilins, which are mutated in familial cases of AD, were demonstrated to form low conductance calcium channels in the ER and elevated cytosolic calcium concentration increases amyloid generation. Moreover, memantine, an antagonist of the NMDA-calcium channel receptor, has been found to have a beneficial effect for AD patients offering novel possibilities for a calcium signalling targeted therapy of AD. This review underscores the growing importance of calcium ions in AD development and focuses on the relevant aspects of calcium homeostasis.

Calcineurin dephosphorylates the C-terminal region of filamin in an important regulatory site: A possible mechanism for filamin mobilization and cell signaling

Filamin is a phosphoprotein that organizes actin filaments into networks. We report that a purified C-terminal recombinant region of filamin is a suitable substrate for calcineurin in vitro. Furthermore, 1 microM cyclosporin A (CsA), a specific calcineurin inhibitor, reduced the dephosphorylation of the recombinant fragment in 293FT cells. Mutagenesis analysis showed that a dephosphorylation step occurred in Ser 2152, which was previously shown to provide resistance to calpain cleavage when endogenous PKA is activated. In contrast, phosphorylation of Ser 2152 was recently reported to be necessary for membrane dynamic changes. In this regard, we found that CsA protects filamin in platelets from calpain degradation. Results could be combined with available information in a single model, assuming that some of the peptide fragments released by calcineurin-regulated calpain action could mediate actions in downstream pathways, which may help to resolve the controversies reported on the role of filamin phosphorylation in actin dynamics.

NMDA neuroprotection against a phosphatidylinositol-3 kinase inhibitor, LY294002 by NR2B-mediated suppression of glycogen synthase kinase-3beta-induced apoptosis

To identify the intracellular signaling pathways that mediate the pro-survival activity of NMDA receptors (NMDARs), we studied effects of exogenous NMDA on cultured rat cortical and hippocampal neurons that were treated with a phosphatidylinositol-3-kinase (PI3K) inhibitor, LY294002. NMDA at 5 or 10 microm protected against LY294002-induced apoptosis, suggesting NMDAR-mediated activation of a survival signaling pathway that is PI3K-independent. NR2B-specific NMDAR blockers antagonized anti-apoptotic effects of NMDA, indicating a critical role of NR2B NMDARs in the neuroprotection. NMDA at 10 microm suppressed LY294002-induced activation of a pro-apoptotic kinase, glycogen synthase kinase 3beta (GSK3beta). GSK3beta activation by LY294002 was associated with decreased levels of inhibitory GSK3beta phosphorylation at the Ser9 residue. However, NMDA did not prevent the LY294002-mediated decline of phospho-Ser9 levels. In addition, NMDA inhibited cortical neuron apoptosis induced by the overexpression of either wild type (wt) or Ser9Ala mutant form of GSK3beta, suggesting that NMDA suppressed GSK3beta in a Ser9-independent manner. Finally, inhibition of NR2B NMDARs reduced the NMDA protection against overexpression of GSK3betawt. These data indicate that moderate stimulation of NR2B NMDAR protects against inhibition of PI3K by a Ser9-independent inhibition of the pro-apoptotic activity of GSK3beta. Hence, the activation of NR2B and the Ser9-independent inhibition of GSK3beta are two newly identified elements of the signaling network that mediates the pro-survival effects of NMDA.

A positive feedback loop between glycogen synthase kinase 3beta and protein phosphatase 1 after stimulation of NR2B NMDA receptors in forebrain neurons

N-methyl-D-aspartate receptors (NMDARs) are critical for neuronal plasticity and survival, whereas their excessive activation produces excitotoxicity and may accelerate neurodegeneration. Here, we report that stimulation of NMDARs in cultured rat hippocampal or cortical neurons and in the adult mouse brain in vivo disinhibited glycogen synthase kinase 3beta (GSK3beta) by protein phosphatase 1(PP1)-mediated dephosphorylation of GSK3beta at the serine 9 residue. NMDA-triggered GSK3beta activation was mediated by NMDAR that contained the NR2B subunit. Interestingly, GSK3beta inhibition reduced inhibitory phosphorylation of the PP1 inhibitor 2 (I2) and attenuated serine 9 dephosphorylation by PP1. These data suggest existence of a feedback loop between GSK3beta and PP1 that results in amplification of PP1 activation by GSK3beta. In addition, GSK3beta inhibition decreased PP1-mediated dephosphorylation of the cAMP-response element-binding protein (CREB) at the serine 133 residue in NMDA-stimulated neurons. Conversely, overexpression of GSK3beta abolished non-NR2B-mediated activation of CRE-driven transcription. These data suggest that cross-talk between GSK3beta and PP1 contributes to NR2B NMDAR-induced inhibition of CREB signaling by non-NR2B NMDAR. The excessive activation of NR2B-PP1-GSK3beta-PP1 circuitry may contribute to the deficits of CREB-dependent neuronal plasticity in neurodegenerative diseases.

Tau phosphorylation in neuronal cell function and dysfunction

Tau is a group of neuronal microtubule-associated proteins that are formed by alternative mRNA splicing and accumulate in neurofibrillary tangles in Alzheimer's disease (AD) brain. Tau plays a key role in regulating microtubule dynamics, axonal transport and neurite outgrowth, and all these functions of tau are modulated by site-specific phosphorylation. There is significant evidence that a disruption of normal phosphorylation events results in tau dysfunction in neurodegenerative diseases, such as AD, and is a contributing factor to the pathogenic processes. Indeed, the abnormal tau phosphorylation that occurs in neurodegenerative conditions not only results in a toxic loss of function (e.g. decreased microtubule binding) but probably also a toxic gain of function (e.g. increased tau-tau interactions). Although tau is phosphorylated in vitro by numerous protein kinases, how many of these actually phosphorylate tau in vivo is unclear. Identification of the protein kinases that phosphorylate tau in vivo in both physiological and pathological processes could provide potential therapeutic targets for the treatment of AD and other neurodegenerative diseases in which there is tau pathology.

Bilateral injection of isoproterenol into hippocampus induces Alzheimer-like hyperphosphorylation of tau and spatial memory deficit in rat

The abnormal hyperphosphorylation of tau protein is one of the hallmarks of Alzheimer disease and other tauopathies; as yet the exact role of various tau kinases in this pathology is not fully understood. Here, we show that injection of isoproterenol, an activator of cAMP-dependent kinase (PKA), into rat hippocampus bilaterally results in the activation of PKA, calcium/calmodulin-dependent kinase II and cyclin-dependent kinase-5, inhibition of protein phosphatase-2A, hyperphosphorylation of tau at several Alzheimer-like epitopes and a disturbance of spatial memory retention 48 h after the drug injection. These findings suggest the involvement of PKA and PKA-mediated signaling pathway in the Alzheimer-like tau hyperphosphorylation and memory impairment.

Oxidative stress promotes tau dephosphorylation in neuronal cells: the roles of cdk5 and PP1

Oxidative stress has been demonstrated to produce modifications in several intracellular proteins that lead to alterations in their activities. Alzheimer's disease is related to an increase of oxidative stress markers, which may be an early event in the progression of the disease and neurofibrillary tangles formation. Abnormal phosphorylation of tau has been implicated in the etiopathogenesis of Alzheimer's disease. By using phospho-specific antibodies, we analyzed the changes in tau phosphorylation patterns after treatment of rat hippocampal and SHSY5Y human neuroblastoma cells with H2O2. We found that tau isoforms were hypophosphorylated at the Tau1 epitope after 2 h in the presence of H2O2. The decrease in the phosphorylation levels of tau protein were prevented by pretreatment with N-acetyl-L-cysteine. These changes were shown to depend on the activity of the cdk5/p35 complex, since a 3-fold increase in substrate phosphorylation and a 2-fold increase for the complex association were observed. Also, a decrease in the amount of inhibitor-2 bound to phosphatase PP1 was found in SHSY5Y cells under oxidative stress conditions. This decrease of inhibitor-2 bound to PP1 is due to an increased phosphorylation of the inhibitor-2 protein, thus leading to increased PP1 activity. Therefore, we propose that oxidative stress-induced activation of cdk5 leads to inhibitor-2 phosphorylation, relieving its inhibitory effect on PP1.

Protein phosphorylation networks in motor neuron death

The disorder amyotrophic lateral sclerosis (ALS) is characterized by the death of specific groups of neurons, especially motor neurons, which innervate skeletal muscle, and neurons connecting the cerebral cortex with motor neurons, such as corticospinal tract neurons. There have been numerous attempts to elucidate why there is selective involvement of motor neurons in ALS. Recent observations have demonstrated altered activities and protein levels of diverse kinases in the brain and spinal cord of transgenic mice that overexpress a mutant superoxide dismutase (mSOD) gene that is found in patients with the familial form of ALS, as well as in patients who have died with ALS. These results suggest that the alteration of protein phosphorylation may be involved in the pathogenesis of ALS. The changes in protein kinase and phosphatase expression and activity can affect the activation of important neuronal neurotransmitter receptors such as NMDA receptors or other signaling proteins and can trigger, or modify, the process producing neuronal loss in ALS. These various kinases, phosphatases and signaling proteins are involved in many signaling pathways; however, they have close interactions with each other. Therefore, an understanding of the role of protein kinases and protein phosphatases and the molecular organization of protein phosphorylation networks are useful to determine the mechanisms of selective motor neuron death.

Cleavage of the cyclin-dependent kinase 5 activator p35 to p25 does not induce tau hyperphosphorylation

Hyperphosphorylated tau protein is the primary component of neurofibrillary tangles observed in several neurodegenerative disorders. It has been hypothesized that in certain pathological conditions, the calcium activated protease, calpain, would cleave the cyclin-dependent kinase 5 (cdk5) activator p35 to a p25 fragment, which would lead to augmented cdk5 activity, and cdk5-mediated tau hyperphosphorylation. To test this hypothesis, we induced calpain-mediated p35 cleavage in rat hippocampal neuronal cultures and studied the relationship between p25 production, cdk5 activity, and tau phosphorylation. In glutamate-treated cells p35 was cleaved to p25 and this was associated with elevated cdk5 activity. However, tau phosphorylation was concomitantly decreased at multiple sites. The calpain inhibitor MDL28170 prevented the cleavage of p35 but had no effect on tau phosphorylation, suggesting that calpain-mediated processes, i.e., the cleavage of p35 to p25 and cdk5 activation, do not contribute to tau phosphorylation in these conditions. Treatment of the neuronal cultures with N-methyl-D-aspartic acid or with calcium ionophores resulted in an outcome highly similar to that of glutamate. We conclude that, in neuronal cells, the cleavage of p35 to p25 is associated with increased activity of cdk5 but not with tau hyperphosphorylation.

Experimental approaches and drugs in development for the treatment of dementia

Treatment of dementia can be divided as symptomatic treatment of cognitive or non-cognitive symptoms and the treatment of underlying pathology. In the last decade the thrust of symptomatic treatment of Alzheimer's disease (AD) has been enhancement of cholinergic transmission. Besides the acetycholinesterase inhibitors (AChE-I) currently in use, cholinergic agonists and enhancers are in development. Other therapeutic approaches directed towards neurotransmitter substitution or modulation include serotoninergic, noradrenergic substances, neuropeptides and those acting via excitatory amino acid receptors, such as ampakines or NMDA antagonists. Introduction of atypical neuroleptics represents the most recent development in the treatment of behavioural symptoms. Efforts to treat the underlying pathology are based on modulation of APP processing in order to decrease the accumulation of beta-amyloid, those to decrease tau hyperphosphorylation, use of nerve growth factors and those based on Apo-E modulation. Potential use of oestrogens and NSAIDs are also under investigation. Recently, vaccination with amyloid-beta peptide has been reported to be effective in an animal model of AD, this putative vaccine is now in clinical trials. Likewise, recent studies suggest that some statins may have a prophylactic effect.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Targeted neutralization of calcineurin, by expression of an inhibitor peptide under the control of a cholinergic specific promoter in PC12 cells, promotes neurite outgrowth in the presence of NGF

We have characterized a region of the mouse vesicular acetylcholine transporter(VAChT)/choline acetyltransferase (ChAT) gene locus that serves as a cholinergic-specific promoter for the expression of both VAChT and ChAT genes, as well as a reporter gene (LacZ) in vivo. We have used this promoter to direct the expression of an inhibitor peptide, derived from the calcineurin (CalN) autoregulatory domain, to directly neutralize the function of CalN to define the role of this Ca2+/Calmodulin regulated phosphatase in neurite outgrowth. Targeted inhibition of CalN promotes neurite outgrowth in PC12 cells in the presence of NGF, as early as 24 h after transfection. Inhibition of CalN-mediated enhancement of neurite outgrowth in PC12 cells reaches a maximum effect within the first 4 to 6 days after transfection, and does not cause adverse effects when highly expressed for up to 12 days. Cyclosporin A, a nontargeted CalN inhibitor, increases the number of neurites in mock transfected cells by 1.5 fold, while in transfected PC12 cells, the expression of the CalN inhibitor peptide increases the neurite number by 1.8 fold. These data demonstrate that CalN is an important regulator of the neurotrophic response in cholinergic cells and may prove valuable in developing treatment strategies to promote recovery from neurological injury.

Amyloid-beta injection in rat amygdala alters tau protein but not mRNA expression

Previously we demonstrated local and distant changes in tau protein immunoreactivity reminiscent of those seen in Alzheimer's disease (AD) following a unilateral injection of amyloid-beta (Abeta)(25-35) into the rat amygdala. To explore the relevance of these findings to AD, we compared the effects of Abeta(1-42) to those of Abeta(25-35). Injections of both Abeta(1-42) and Abeta(25-35) into rat amygdala resulted in increased tau-2 immunolabeling in neurons. To determine whether these alterations were due to changes in the expression of tau, we measured tau protein expression by Western blotting and tau mRNA isoform expression by the reverse transcription-polymerase chain reaction in the amygdala, hippocampus, and cerebellum following a unilateral injection of Abeta(25-35) or vehicle into the amygdala. The levels of tau proteins were increased bilaterally in the amygdala of Abeta(25-35)- compared to vehicle-treated animals 8 and 16 days following treatment. The molecular weights of tau proteins were decreased in the Abeta(25-35)-treated (59-69 kDa) compared to the vehicle-treated (67-72 kDa) animals 8 days following treatment. There were no changes in tau mRNA expression in any brain region examined. In this model, just as in AD, there is an increase in tau protein levels without a change in tau mRNA expression, suggesting that Abeta peptides may influence tau protein stability in both the rat and the human brain.

Free PKC catalytic subunits (PKM) phosphorylate tau via a pathway distinct from that utilized by intact PKC

Protein kinase C (PKC) is reversibly activated at the plasma membrane by the generation of diacylglycerol (DAG) coupled with the release of Ca2+ from intracellular stores. PKC is also irreversibly activated by calpain-mediated PKC cleavage of the regulatory and catalytic subunits; resultant free PKC catalytic subunits are termed "PKM". Unlike PKC, PKM is co-factor-independent, remains active following diffusion away from the membrane, and can theoretically phosphorylate targets inaccessible to, and inappropriate for, PKC. We examined the downstream consequences of PKC activation by the phorbol ester TPA and by ionophore A23187-mediated calcium influx (which experimentally correspond to DAG-mediated and calpain-mediated activation, respectively) on phosphorylation of the microtubule-associated protein tau. Both methods increased phospho-tau immunoreactivity, and neither was inhibited by lithium or olomoucin (inhibitors of tau kinases GSK-3 beta and cdk5, respectively). The TPA-mediated increase, and not the ionophore-mediated increase, was blocked by co-treatment with the mitogen-activated protein (MAP) kinase kinase inhibitor PD98059. These findings indicate that PKC phosphorylates tau via the MAP kinase pathway, but that PKM can bypass this requirement, therefore demonstrating that distinct intracellular pathways can be mediated by PKC and PKM. PKM generation may therefore trigger one or more additional pathways contributing to tau phosphorylation following inappropriate calcium influx.

The microtubule binding of Tau and high molecular weight Tau in apoptotic PC12 cells is impaired because of altered phosphorylation

Although the importance of the microtubule network throughout cell life is well established, the dynamics of microtubules during apoptosis, a regulated cell death process, is unclear. In a previous study (Davis, P. K., and Johnson, G. V. (1999) Biochem. J. 340, 51-58) we demonstrated that the phosphorylation of the microtubule-associated protein tau was increased during neuronal PC12 cell apoptosis. The purpose of this study was to determine whether the increased tau phosphorylation that occurred during apoptosis impaired the microtubule binding capacity of tau. This study is the first demonstration that microtubule-binding by tau and high molecular weight tau is significantly impaired as a result of altered phosphorylation during a naturally occurring process, apoptosis. Furthermore, co-immunofluorescence studies reveal for the first time that tau populations within an apoptotic neuronal PC12 cell exhibit differential phosphorylation. In control PC12 cells, Tau-1 staining (Tau-1 recognizes an unphosphorylated epitope) is evident throughout the entire cell body. In contrast, Tau-1 immunoreactivity in apoptotic PC12 cells is retained in the nuclear/perinuclear region but is significantly decreased in the cytoplasm up to the plasma membrane. The selective distribution of phosphorylated tau in apoptotic PC12 cells indicates that tau likely plays a significant role in the cytoskeletal changes that occur during apoptosis.

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.

Transient increases in intracellular calcium result in prolonged site-selective increases in Tau phosphorylation through a glycogen synthase kinase 3beta-dependent pathway

Calcium is a universal intracellular signaling molecule. Through variations in both the amplitude and frequency of intracellular calcium increases, the same calcium ion can elicit different responses. In this report, we investigated the effect of a calcium transient, lasting 2-5 min, on alterations in the phosphorylation state of the cytoskeletal protein, tau. Transient increases in calcium result in a prolonged (1-4 h) approximately 60% increase in tau phosphorylation at the Tau-1 epitope. These increases in tau phosphorylation appear to be more dependent upon the duration of the increase in intracellular calcium and less on the amplitude. The calcium-induced increases in tau phosphorylation are not dependent upon protein synthesis, nor are protein kinase C or calcium/calmodulin-dependent protein kinase II involved in the response. However, the calcium-induced increase in tau phosphorylation was inhibited by lithium, a noncompetitive inhibitor of glycogen synthase kinase-3beta (GSK-3beta), and by the tyrosine kinase inhibitor, genistein. Furthermore, transient increases in calcium resulted in a prolonged increase in GSK-3beta tyrosine phosphorylation concomitant with the increase in tau phosphorylation. Therefore, this study is the first to indicate that transient increases in intracellular calcium result in increased tyrosine phosphorylation and activation of GSK-3beta which subsequently results in a sustained increase in the phosphorylation state of tau.

Activities of calcineurin and phosphatase 2A in the hippocampus after transient forebrain ischemia

We investigated the changes in the enzyme activity and immunoreactivity of calcineurin in the rat hippocampus after transient forebrain ischemia. Immediately after 20-min transient forebrain ischemia, calcineurin activity decreased to about 40% of the control in the CA1 region and to about 55% in other regions. Protein phosphatase 2A activity showed no remarkable changes. By 12 h after ischemia, calcineurin activity recovered, more in the CA1 region than in other regions. At 24 h it decreased again, but only in the CA1 region. Immunohistochemical- and immunoblot analyses showed no remarkable change in calcineurin in any region of the hippocampus within 12 h after ischemia. Thus, the activity of calcineurin is dissociated from its immunoreactivity and quantity. Several studies have suggested that unknown inhibitory factor(s) and/or reversible changes in calcineurin act to modify enzyme activity after ischemia. In contrast, phosphatase 2A activity underwent no obvious changes during the post-ischemia period we examined. This unique time course of calcineurin activity may contribute to the mechanism of ischemic neuronal injury.

Energy metabolism and protein phosphorylation during apoptosis: a phosphorylation study of tau and high-molecular-weight tau in differentiated PC12 cells

Apoptosis has been characterized as a regulated, energy-dependent process. Specific protein-phosphorylation events have been demonstrated previously to occur during apoptosis and may play an important role in the regulation of this death process. In this study, energy metabolism and protein phosphorylation during apoptosis of neuronal PC12 cells induced by nerve growth factor and serum deprivation was examined using [32P]Pi-labelling techniques. Although ATP levels were maintained at control levels during apoptosis, [32P]Pi incorporation into ATP was decreased significantly, coinciding with an almost identical decrease in Na+-dependent phosphate uptake. During neuronal PC12-cell apoptosis, increased phosphorylation of tau and high-molecular-weight (HMW) tau was observed within the epitope of Tau-1, a phosphate-dependent tau antibody that only recognizes the unphosphorylated form of its epitope. In addition, based on two-dimensional phosphopeptide mapping, [32P]Pi incorporation into a phosphopeptide of tau and HMW tau from apoptotic cells increased. Whereas [32P]Pi incorporation into total protein decreased to 23% of the control during apoptosis, [32P]Pi incorporation into tau and HMW tau was significantly higher, indicating a preferential phosphorylation of specific proteins during the apoptotic process. This study provides novel information about phosphate uptake, incorporation of [32P]Pi into ATP, and protein phosphorylation events during apoptosis.

Age-dependent differences in glutamate-induced phosphorylation systems in rat hippocampal slices

Glutamate receptor induced changes in the activity of different phosphorylation systems were measured in hippocampal slices from 12- and 56-day-old rats, by determining the endogenous phosphorylation of 2.5% perchloric acid (PCA) soluble proteins. We identified among these proteins an 85, 80 kDa and the tau protein as specific substrates for protein kinase A (PKA), MARCKS, and neurogranin as specific substrates for protein kinase C (PKC), and prostaglandin-D-synthase as substrate for casein kinase II (CKII). In addition, a 35 kDa protein was phosphorylated by calcium/calmodulin dependent kinase II and protein kinase C and a 21 kDa protein was a substrate for all investigated kinases. The basal endogenous phosphorylation of 2.5% PCA soluble proteins changed during development qualitatively and quantitatively. Thus, the phosphorylation degree of nearly all proteins declines during maturation. Activation of mGluR induced an increased phosphorylation of PKA, PKC, and CKII substrates in hippocampal slices from 12-day-old rats, but in slices of 56-day-old rats only PKA and to a lower extent PKC substrates were affected. In contrast, stimulation of NMDA receptors led to an enhancement of CKII and PKA dependent phosphorylation only in slices of young animals, whereas the endogenous phosphorylation of some proteins in adult slices was actually decreased. These data showing developmental changes in the coupling of metabotropic and ionotropic glutamate receptors to different phosphorylation systems are discussed in the light of altered physiological properties of the mature hippocampus.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Axonal cytoskeleton changes in experimental optic neuritis

Axonal loss and degeneration in multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE) have been suggested by brain imaging, pathological and axonal transport studies. Further elucidation of the processes and mechanisms of axonal degeneration in demyelinating diseases is therefore of potential importance in order to alleviate the permanent disabilities of MS patients. However, detailed studies in this area are impeded by the small number of reliable models in which the onset and location of demyelination can be well-controlled. In this study, microinjection of polyclonal rabbit anti-galactocerebroside (anti-Gal C) antibody and guinea pig complement was used to induce local demyelination in the rat optic nerve. We found that treatment with appropriate volumes of the antibody and complement could induce local demyelination with minimal pressure- or trauma-induced damage. Local changes in neurofilaments (NFs) and microtubules (MTs) were examined with both immunohistochemistry (IHC) and electron microscopy (EM). On day 1 after microinjection, we observed moderate NF and MT disassembly in the local demyelinated area, although in most cases, no apparent inflammatory cell infiltration was seen. The NF and MT changes became more apparent on days 3, 5, 7 after microinjection, along with gradually increased inflammatory cell infiltration. These results suggested that acute demyelination itself may induce local cytoskeleton changes in the demyelinated axons, and that the ensuing local inflammation may further enhance the axonal damage. When the lesions were stained with specific antibodies for T lymphocytes, macrophages, and astrocytes, we found that most of the cells were macrophages, suggesting that macrophages may play a greater role in inflammation-related axonal degeneration and axonal loss. These results were confirmed and further characterized on the ultrastructural level.

Helicity of alpha(404-451) and beta(394-445) tubulin C-terminal recombinant peptides

We have investigated the solution conformation of the functionally relevant C-terminal extremes of alpha- and beta-tubulin, employing the model recombinant peptides RL52alpha3 and RL33beta6, which correspond to the amino acid sequences 404-451(end) and 394-445(end) of the main vertebrate isotypes of alpha- and beta-tubulin, respectively, and synthetic peptides with the alpha-tubulin(430-443) and beta-tubulin(412-431) internal sequences. Alpha(404-451) and beta(394-445) are monomeric in neutral aqueous solution (as indicated by sedimentation equilibrium), and have circular dichroism (CD) spectra characteristic of nearly disordered conformation, consistent with low scores in peptide helicity prediction. Limited proteolysis of beta(394-445) with subtilisin, instead of giving extensive degradation, resulted in main cleavages at positions Thr409-Glu410 and Tyr422-Gln423-Gln424, defining the proteolysis resistant segment 410-422, which corresponds to the central part of the predicted beta-tubulin C-terminal helix. Both recombinant peptides inhibited microtubule assembly, probably due to sequestration of the microtubule stabilizing associated proteins. Trifluoroethanol (TFE)-induced markedly helical CD spectra in alpha(404-451) and beta(394-445). A substantial part of the helicity of beta(394-445) was found to be in the CD spectrum of the shorter peptide beta(412-431) with TFE. Two-dimensional 1H-NMR parameters (nonsequential nuclear Overhauser effects (NOE) and conformational C alphaH shifts) in 30% TFE permitted to conclude that about 25% of alpha(404-451) and 40% of beta(394-451) form well-defined helices encompassing residues 418-432 and 408-431, respectively, flanked by disordered N- and C-segments. The side chains of beta(394-451) residues Leu418, Val419, Ser420, Tyr422, Tyr425, and Gln426 are well defined in structure calculations from the NOE distance constraints. The apolar faces of the helix in both alpha and beta chains share a characteristic sequence of conserved residues Ala,Met(+4),Leu(+7),Tyr(+11). The helical segment of alpha(404-451) is the same as that described in the electron crystallographic model structure of alphabeta-tubulin, while in beta(394-451) it extends for nine residues more, supporting the possibility of a functional coil --> helix transition at the C-terminus of beta-tubulin. These peptides may be employed to construct model complexes with microtubule associated protein binding sites.

Calcineurin. Structure, function, and inhibition

Calcineurin is a serine-threonine specific Ca(2+)-calmodulin-activated protein phosphatase that is conserved from yeast to humans. Remarkably, this enzyme is the common target for two novel and structurally unrelated immunosuppressive antifungal drugs, cyclosporin A and FK506. Both drugs form complexes with abundant intracellular binding proteins, cyclosporin A with cyclophilin A and FK506 with FKBP 12, which bind to and inhibit calcineurin. The X-ray structure of an FKPB12-FK506-calcineurin AB ternary complex reveals that FKBP12-FK506 binds in a hydophobic groove between the calcineurin A catalytic and the regulatory B subunit, in accord with biochemical and genetic studies on inhibitor action. Calcineurin plays a key role in regulating the transcription factor NF-AT during T-cell activation, and in mediating responses of microorganisms to cation stress. These findings highlight the potential of yeast genetic studies to define novel drug targets and elucidate conserved elements of signal transduction cascades.

Postmortem changes in the phosphorylation state of tau-protein in the rat brain

The phosphorylation state of tau-protein is crucial for the regulation of neuronal microtubule organization. Functional conclusions on tau-protein require an accurate assessment of phosphorylated sites. Therefore, the in vivo distribution and postmortem preservation of some phospho-epitopes on tau-protein were examined in the rat brain under different fixation and preparation conditions. Detection of tau-protein with a phosphorylation-independent antiserum revealed both axonal and somatodendritic localizations, which were not influenced by a postmortem interval of 30 min. The phospho-epitopes recognized by 12E8, AT8, and PHF-1 were mainly localized in the somatodendritic compartment. The binding sites of AT8 and PHF-1 were rapidly dephosphorylated postmortem, whereas the Tau-1 epitope was unmasked in the somatodendritic region. The axonally located phospho-epitope of AT270 and the nuclear epitope of AT100 were still detectable after a postmortem interval of 30 min. Postmortem dephosphorylation and inhibition of this process by PP1 and/or PP2A was further demonstrated on Western blot. In conclusion, rapid processing of tau-protein is essential for the correct assessment of investigations on phospho-isoforms.