Phosphatase inhibition in human neuroblastoma cells alters tau antigenicity and renders it incompetent to associate with exogenous microtubules

Use of okadaic acid to identify relevant phosphoepitopes in pathology: a focus on neurodegeneration

Protein phosphorylation is involved in the regulation of a wide variety of physiological processes and is the result of a balance between protein kinase and phosphatase activities. Biologically active marine derived compounds have been shown to represent an interesting source of novel compounds that could modify that balance. Among them, the marine toxin and tumor promoter, okadaic acid (OA), has been shown as an inhibitor of two of the main cytosolic, broad-specificity protein phosphatases, PP1 and PP2A, thus providing an excellent cell-permeable probe for examining the role of protein phosphorylation, and PP1 and PP2A in particular, in any physiological or pathological process. In the present work, we review the use of okadaic acid to identify specific phosphoepitopes mainly in proteins relevant for neurodegeneration. We will specifically highlight those cases of highly dynamic phosphorylation-dephosphorylation events and the ability of OA to block the high turnover phosphorylation, thus allowing the detection of modified residues that could be otherwise difficult to identify. Finally, its effect on tau hyperhosphorylation and its relevance in neurodegenerative pathologies such as Alzheimer’s disease and related dementia will be discussed.

Regulation of tau proteolysis by phosphatases

One pathological hallmark of Alzheimer's disease is the accumulation of highly phosphorylated tau. Since tau phosphorylation inhibits its proteolysis, we examined the impact of endogenous phosphatase activities on tau proteolysis by homogenization of cultured cells and 3xTg-AD mouse brain followed by incubation with or without phosphatase inhibitors. Incubation without phosphatase inhibitors significantly increased tau immunoreactivity against antibody C3 (which reacts with tau truncated at D421), and increased the generation of tau breakdown products. These changes were augmented by lithium treatment and inhibited by constitutively active GSK3β. These findings underscore that tau proteolysis is regulated by a balance of kinase and phosphatase activities.

Anesthesia-induced hyperphosphorylation detaches 3-repeat tau from microtubules without affecting their stability in vivo

In Alzheimer's disease, tau is hyperphosphorylated, which is thought to detach it from microtubules (MTs), induce MT destabilization, and promote aggregation. Using a previously described in vivo model, we investigated whether hyperphosphorylation impacts tau function in wild-type and transgenic mice. We found that after anesthesia-induced hypothermia, MT-free tau was hyperphosphorylated, which impaired its ability to bind MTs and promote MT assembly. MT-bound tau was more resistant to hyperphosphorylation compared with free tau and tau did not dissociate from MTs in wild-type mice. However, 3-repeat tau detached from MT in the transgenic mice. Surprisingly, dissociation of tau from MTs did not lead to overt depolymerization of tubulin, and there was no collapse, or disturbance of axonal MT networks. These results indicate that, in vivo, a subpopulation of tau bound to MTs does not easily dissociate under conditions that extensively phosphorylate tau. Tau remaining on the MTs under these conditions is sufficient to maintain MT network integrity.

Postsynaptic density protein PSD-95 expression in Alzheimer's disease and okadaic acid induced neuritic retraction

In order to understand how plasticity is related to neurodegeneration, we studied synaptic proteins with quantitative immunohistochemistry in the entorhinal cortex from Alzheimer patients and age-matched controls. We observed a significant decrease in presynaptic synaptophysin and an increase in postsynaptic density protein PSD-95, positively correlated with beta amyloid and phosphorylated Tau proteins in Alzheimer cases. Furthermore, Alzheimer-like neuritic retraction was generated in okadaic acid (OA) treated SH-SY5Y neuroblastoma cells with no decrease in PSD-95 expression. However, in a SH-SY5Y clone with decreased expression of transcription regulator LMO4 (as observed in Alzheimer's disease) and increased neuritic length, PSD-95 expression was enhanced but did not change with OA treatment. Therefore, increased PSD-95 immunoreactivity in the entorhinal cortex might result from compensatory mechanisms, as in the SH-SY5Y clone, whereas increased Alzheimer-like Tau phosphorylation is not related to PSD-95 expression, as suggested by the OA-treated cell models.

Okadaic acid mediates tau phosphorylation via sustained activation of the L-voltage-sensitive calcium channel

Accumulation of phosphorylated isoforms of the microtubule-associated protein tau is one hallmark of affected neurons in Alzheimer's disease (AD). This increase has been attributed to increased kinase or decreased phosphatase activity. Prior studies indicate that one of the kinases that phosphorylates tau (mitogen-activated protein kinase, or MAP kinase) does so at least in part indirectly within intact neuronal cells by phosphorylating and activating the L-voltage-sensitive calcium channel. Resultant calcium influx then fosters tau phosphorylation via one or more calcium-activated kinases. We demonstrate herein that treatment of differentiated SH-SY-5Y human neuroblastoma with the phosphatase inhibitor okadaic acid (OA) similarly may increase tau phosphorylation via sustained activation of the L-voltage-sensitive calcium channel. OA increased phospho-tau as indicated by increased immunoreactivity towards an antibody (PHF-1) directed against paired helical filaments from AD brain. This increase was blocked by co-treatment with the channel antagonist nimodipine. OA treatment increased channel phosphorylation. The increases in calcium influx, PHF-1 immunoreactivity and channel phosphorylation were all attenuated by co-treatment with PD98059, which inhibits MAP kinase activity, suggesting that OA mediates these effects at least in part via sustained activation of MAP kinase. These findings underscore that divergent and convergent kinase and phosphatase activities regulate tau phosphorylation.

Naringin-sensitive phosphorylation of plectin, a cytoskeletal cross-linking protein, in isolated rat hepatocytes

To identify phosphoproteins that might play a role in naringin-sensitive hepatocellular cytoskeletal disruption and apoptosis induced by algal toxins, hepatocyte extracts were separated by gel electrophoresis and immunostained with a phosphothreonine-directed antibody. Use of dilute (5%) polyacrylamide gels containing 6 m urea allowed the resolution of one very large (approximately 500-kDa) okadaic acid- and naringin-sensitive phosphoprotein, identified by tryptic fingerprinting, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry, and immunostaining as the cytolinker protein, plectin. The naringin-sensitive phosphorylation induced by okadaic acid and microcystin-LR probably reflected inhibition of a type 2A protein phosphatase, whereas the naringin-resistant phosphorylation induced by calyculin A, tautomycin, and cantharidin probably involved a type 1 phosphatase. Okadaic acid caused a collapse of the plectin-immunostaining bile canalicular sheaths and the general cytoskeletal plectin network into numerous medium-sized plectin aggregates. Inhibitors of protein kinase C, cAMP-dependent protein kinase, or Ca(2+)/calmodulin-dependent kinase II had moderate or no protective effects on plectin network disruption, whereas naringin offered 86% protection. Okadaic acid induced a naringin-sensitive phosphorylation of AMP-activated protein kinase (AMPK), the stress-activated protein kinases SEK1 and JNK, and S6 kinase. The AMPK-activating kinase (AMPKK) is likely to be the target of inhibition by naringin, the other kinases serving as downstream components of an AMPKK-initiated signaling pathway.

Selective changes of calcineurin (protein phosphatase 2B) activity in Alzheimer's disease cerebral cortex

Neurofibrillary tangles, which contain abnormally hyperphosphorylated forms of tau protein, are one of the neuropathological hallmarks of Alzheimer's disease (AD). This altered phosphorylation state of tau protein may be due to increased kinase activity or/and decreased phosphatase activity. In the present study, we characterized human calcineurin phosphatase activity in postmortem superior frontal cortex and sensorimotor cortex and measured calcineurin phosphatase activity in samples from individuals with moderate to severe AD (n = 7) and age-matched controls (n = 5). Basal phosphatase activity was reduced by 25% (P < 0.05) in AD frontal cortex. Nickel-stimulated calcineurin activity was decreased by 52% (P < 0.05) and 30% (P < 0.05) in P2 and total cell homogenate, respectively, compared to age-matched controls. No differences in phosphatase activities were detected in the sensorimotor cortex. The decrease in nickel-stimulated calcineurin phosphatase activity in frontal lobe correlated with the neurofibrillary tangle pathology (total cell homogenate, r = -0.77, P < 0.05; P2 fraction, r = -0.76, P < 0.02), but not with diffuse or neuritic plaques. Despite the changes in calcineurin phosphatase activity in the superior frontal cortex, calcineurin protein levels determined by immunoblot were similar in control and AD cases. In addition, no changes in calcineurin regulatory proteins (cyclophilin A and FKBP12) levels were observed. These studies suggest that decrease of calcineurin activity may play a role in paired-helical filament formation and/or stabilization, and the decrease of activity was not accompanied by a decrease of calcineurin protein expression.

Induction of Alzheimer-specific Tau epitope AT100 in apoptotic human fetal astrocytes

In Alzheimer's and other neurodegenerative diseases, hyperphosphorylated tau accumulates in affected neuronal and glial cells in the form of paired helical filaments (PHFs). This tau binds antibody AT100, which recognizes the double phosphorylation site (Thr212/Ser214) that is not present in normal biopsy tau. In primary cultures, highly enriched (>98%) in astrocytes of human fetal brain, three polypeptides of 52, 64, and 70 kD showed immunoreactivity with tau antibodies against non-phosphorylated epitopes, accounting for 88, 12, and <1%, respectively, of the total reactivity. All three polypeptides were phosphorylated at the PHF-1 epitope but not at the epitopes Tau-1, 12E8, AT8, and AT100. Treatment of cultures with okadaic acid resulted in apoptosis characterized by the blebbing of the plasma membrane, condensation of nuclear chromatin, and fragmentation of the nucleus. This treatment also resulted in a 3- to 5-fold increase in the content of both tau protein and phosphorylation. The increases were observed in all phosphorylation sites examined, and included the AT100 site. The AT100 site has been proposed to be generated by protein kinase B/Akt and Cdc2. Since okadaic acid can induce an AD-like hyperphosphorylated state of normal tau in primary cultures of human brain cells, a simple cellular model is available permitting study of self-aggregation of tau and phosphorylation events characteristic of neurodegeneration.

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.

Alzheimer-specific epitope of AT100 in transfected cell lines with tau: toward an efficient cell model of tau abnormal phosphorylation

Intraneuronal aggregation of specific hyperphosphorylated tau isoforms in subsets of neurons may explain many neurodegenerative processes. Only some antibodies including AP422 and AT100 are specific to the abnormal phosphorylation of tau proteins in these processes. AT100-immunoreactivity was never observed in cell models with the exception of Sf9 cells. In the present study, we developed a way to induce AT100-immunoreactivity in different cell types including COS and SY5Y cells after tau cDNA transfection and treatment by okadaic acid. This represents a useful model to study abnormal tau phosphorylation in situ.

Phosphorylation of tau, Abeta-formation, and apoptosis after in vivo inhibition of PP-1 and PP-2A

Chronic inhibition of protein phosphatases 1 and 2A in vivo was induced by infusion of okadaic acid into lateral ventricles of rat brain for up to 4 months. Cytoskeletal pathology, alterations of the amyloid precursor protein, and apoptotic cell death induced by this treatment followed a certain sequence and spatial distribution. Changes in the expression, phosphorylation, and subcellular distribution of neurofilament proteins and tau, as well as first signs of apoptotic cell death, occurred already after about 2 weeks. The distribution of apoptotic cells, however, was different from those revealing a high accumulation of hyperphosphorylated tau, indicating that those cytoskeletal pathology had no obvious sequelae for the viability of these neurones. A continuation of treatment for longer than 2 weeks induced diffuse deposits of both hyperphosphorylated tau and A beta-amyloid-immunoreactive material in white matter areas that increased in size and number over time. Because tau-phosphorylation is a regulator of the dynamic stability of microtubules, the pathology observed in the present experimental paradigm in the white matter might be viewed as an indication of a disturbed axonal transport. It is hypothesized that perturbations of the axonal transport might also be critically involved in the formation of paired helical filaments and amyloid deposits in Alzheimer's disease.

Characterization of tau proteins in human neuroblastoma SH-SY5Y cell line

Here we report three experimental paradigms in which tau proteins are differentially localized and expressed in human neuroblastoma cells SH-SY5Y. We found that in undifferentiated cells, tau proteins were predominantly localized in the nucleus. Western blot analysis of nuclear extracts revealed, among the others, a high molecular weight tau isoform and evaluation of tau mRNA levels showed a single tau isoform. After differentiation, tau immunoreactivity was detected only in cytosol and along neuritic processes. The high molecular weight tau isoform disappeared and an additional tau mRNA species was detected. Treatment of differentiated cells with doxorubicin or okadaic acid resulted in an increase of tau immunoreactivity and in a subsequent cell loss. Our results indicate that both subcellular localization and pattern of expression of tau proteins vary depending on the developmental and functional state of the cells, thus suggesting different roles in cell function.

Phospholipids alter tau conformation, phosphorylation, proteolysis, and association with microtubules: implication for tau function under normal and degenerative conditions

Discerning the in situ functions of the microtubule-associated protein (MAP) tau is of interest both in terms of neuronal differentiation and homeostasis as well as in terms of neurodegenerative conditions such as Alzheimer's disease. In the present study, exposure to excess phosphatidyl serine (PS) for < 1 min induced antigenic alterations in multiple N-terminal, C-terminal and central epitopes of purified human brain tau. Notably, "AD-like" epitopes (PHF-1, ALZ-50, AT-8) were decreased by PS; other epitopes (e.g., 5E2, Tau-1) increased and others remained relatively unchanged. Inclusion of gamma-AT[32P] during incubations did not reveal any contaminating kinase activity. Direct addition of chloroform:methanol (CM; the initial PS solvent) demonstrated that these changes were not derived from CM-mediated tau denaturation. Phosphatidyl choline induced similar antigenic changes, while phosphatidyl inositol did not. PS inhibited MAP-kinase generation of phospho-dependent tau epitopes and incorporation of phosphates by tau. Inclusion of PS during coincubation of tau and tubulin reduced the extent of cosedimentation of tau with MTs. Finally, PS enhanced the ability of calpain-mediated tau proteolysis. These data suggest that tau antigenicity in situ may be derived from phospholipid-dependent alterations in tau conformation in addition to tau phosphorylation state. These data further suggest that disruption of the normal association of tau with phospholipids may foster accumulation of tau and, in doing so, render tau more susceptible to hyperphosphorylation.

Regulated phosphorylation and dephosphorylation of tau protein: effects on microtubule interaction, intracellular trafficking and neurodegeneration

This review attempts to summarize what is known about tau phosphorylation in the context of both normal cellular function and dysfunction. However, conceptions of tau function continue to evolve, and it is likely that the regulation of tau distribution and metabolism is complex. The roles of microtubule-associated kinases and phosphatases have yet to be fully described, but may afford insight into how tau phosphorylation at the distal end of the axon regulates cytoskeletal-membrane interactions. Finally, lipid and glycosaminoglycan modification of tau structure affords yet more complexity for regulation and aggregation. Continued work will help to determine what is causal and what is coincidental in Alzheimer's disease, and may lead to identification of therapeutic targets for halting the progression of paired helical filament formation.

Normal and pathological Tau proteins as factors for microtubule assembly

Tau proteins are microtubule-associated proteins. They regulate the dynamics of the microtubule network, especially involved in the axonal transport and neuronal plasticity. Tau proteins belong to a family of developmentally regulated isoforms generated by alternative splicing and phosphorylation. This generates several Tau variants that interact with tubulin and other proteins. Therefore, Tau proteins are influenced by many physiological regulations. Tau proteins are also powerful markers of the neuronal physiological state. Their degree of phosphorylation is a good marker of cell integrity. It is heavily disturbed in numerous neurodegenerative disorders, leading to a collapse of the microtubule network and the presence of intraneuronal lesions resulting from Tau aggregation. However, different biochemical and immunological patterns of pathological Tau proteins found among neurodegenerative disorders are useful markers for the understanding of the role of Tau protein isoforms and the diagnosis of these pathological conditions.