Neurotoxic effects of dietary aluminium

Neurochemical responses to chronic oral aluminium administration have been studied in rats. Aluminium (0.3%) was added to drinking water of adult rats for four weeks or longer and weanling rats were given aluminium for eight weeks. Selective cognitive impairment was demonstrated in the adult rats. Aluminium inhibited calcium flux and phosphoinositide metabolism, one product of which (inositol 1,4,5-trisphosphate) modulates intracellular calcium levels. In weanling rats aluminium decreased the in vivo concentration of inositol 1,4,5-trisphosphate in the hippocampus. An increase in cyclic AMP concentrations by 30-70% in various brain regions in adult and weanling rats was found. Aluminium enhanced agonist-stimulated but not basal cyclic AMP production in vitro. It was postulated that aluminium inhibits the GTPase activity of the stimulatory G protein, Gs, leading to prolonged activation of Gs after receptor stimulation and increased cyclic AMP production. Aluminium treatment also increased the phosphorylation of microtubule-associated protein 2 (MAP-2) and the 200 kDa neurofilament protein (NF-H) but several other phosphoproteins were unaffected. Concentrations of seven structural proteins--MAP-2, tau, NF-H, NF-M (150 kDa), NF-L (68 kDa), tubulin and spectrin--were measured in rat brain regions by immunoblot methods. MAP-2 was most consistently decreased. These studies show that chronic oral aluminium administration to rats has significant neurochemical consequences. Three sites of action are implicated: altered calcium homeostasis, enhanced cyclic AMP production, and changes in cytoskeletal protein phosphorylation states and concentrations.

Inhibition of iron deficiency stress response in cucumber by rare earth elements

We investigated the influence of the trivalent scandium (Sc), chromium (Cr), gallium (Ga), yttrium (Y) and lanthanum (La) on both the function and activity of ferric chelate reductase (FCR) in cucumber (Cucumis sativus L.) roots. Cucumber seedlings were grown for 1week in a nutrient solution without Fe or in some experiments with 10microM FeEDTA. Intact root systems were assayed for FCR activity in a medium at pH 5.0 containing 100microM FeEDTA with the ferrous chelating agent Ferrozine. Addition of 100microM concentrations of the EDTA complexes of Sc, Cr, Ga, Y and La did not inhibit FCR in Fe-deficient roots. When Fe-deficient roots were grown with 10microM LaCl(3), ScCl(3), or YCl(3) for 3days, FCR activity decreased to 23%, 15% and 1%, respectively, of the activity of Fe-deficient plants grown without trivalent metal addition. Additionally, these trivalent metals suppressed proton secretion. Growth of Fe-deficient plants with 80microM Ga(2)(SO(4))(3) decreased FCR activity to 35% of the control activity while 80microM CrEDTA did not affect FCR activity. With the addition of either FeEDTA or YCl(3), FCR activity decreased to less than 5% of the activity of the Fe-deficient control roots in 3days. Addition of FeEDTA, but not Y, resulted in recovery from Fe deficiency as indicated by increasing chlorophyll content of leaves.

Multifocal osteonecrosis -- a case report

The pathogenesis of idiopathic multifocal osteonecrosis is poorly understood. It is difficult to diagnose with conventional radiography or computed tomography and poses a great management challenge. A case of idiopathic multifocal osteonecrosis is presented in a young boy illustrating the difficulties in the management of such patients.

Tau phosphorylation during apoptosis of human SH-SY5Y neuroblastoma cells

In Alzheimer's Disease brain, the microtubule-associated protein tau is hyperphosphorylated at specific epitopes and abnormally aggregates into filamentous structures. In addition, there is significant neurodegeneration in Alzheimer's disease brain, and there is data to suggest that apoptotic-like processes may contribute to the neurodegeneration. It has been demonstrated that in PC12 cells undergoing apoptosis due trophic factor removal, tau is hyperphosphorylated prior to chromatin condensation. To establish that increased tau phosphorylation is a generalized outcome of the apoptotic process, and to examine the involvement of the protein kinase in these events, apoptosis was induced in retinoic-acid differentiated human SH-SY5Y neuroblastoma cells using the topoisomerase-1 inhibitor camptothecin. Treatment of the differentiated SH-SY5Y cells with camptothecin resulted in a time and concentration dependent activation of caspase-3 with a concomitant increase in the presence of apoptotic nuclei. Immunoblotting revealed that camptothecin treatment resulted in a significant increase in tau phosphorylation. Addition of a cyclin-dependent kinase inhibitor reduced camptothecin-induced cell death in the differentiated SH-SY5Y cells and decreased the effects of camptothecin on tau phosphorylation. In contrast, a general caspase inhibitor decreased camptothecin-induced cell death, but did not significantly decrease the increases in tau phosphorylation. These results suggest that increased tau phosphorylation is likely a generalized outcome of apoptotic processes in neuron-related cells, and that cyclin-dependent kinases probably play a role in this process.

Tau and HMW tau phosphorylation and compartmentalization in apoptotic neuronal PC12 cells

In the Alzheimer disease brain, the microtubule-associated protein tau is hyperphosphorylated. There is also evidence that apoptotic-like processes may contribute to the neuronal loss in AD. In an apoptotic model that involves replating neuronal PC12 cells without serum and nerve growth factor (NGF), tau was hyperphosphorylated. During replating, however, neurites are removed. Here, differentiated cells were maintained in serum-free media before growth factor removal, thus maintaining neuritic processes during the apoptotic process and allowing for evaluation of neuritic changes. Tau phosphorylation, evaluated by immunoblotting and immunocytochemistry, was compared with various measures of cell death. Compared with control, NGF-deprived cells exhibited gradual and consistent increases of lactate dehydrogenase release over a 5-day period and a peak of caspase-3 activity at Day 2 after NGF removal. Nuclear staining demonstrated chromatin condensation in NGF-deprived cells. Apoptotic cells had thickened, tortuous, and shortened neuritic processes compared with control cells. Immunoblotting showed an increase in both tau and high molecular weight (HMW) tau phosphorylation during the apoptotic process. Immunoreactivity of both tau isoforms shifted from the detergent insoluble cytoskeleton to the detergent soluble compartment in the apoptotic cells. The microtubule binding of both tau isoforms from apoptotic cells also was impaired. Immunoblotting of purified plasma membrane showed preferential association of HMW tau with the plasma membrane during apoptosis. Also, plasma membrane-associated HMW tau was more phosphorylated during apoptosis. Immunocytochemistry demonstrated increased tau phosphorylation in most apoptotic cells, especially in the neurites. Tau was, however, dephosphorylated cells in the last stages of apoptosis.

Hyperosmotic stress-induced apoptosis and tau phosphorylation in human neuroblastoma cells

A characteristic hallmark of Alzheimer's disease brain is the presence of hyperphosphorylated tau; however, the mechanisms responsible for the aberrant tau phosphorylation are unknown. Recently, it has been shown that apoptotic-like processes may be involved in some of the neuronal loss in Alzheimer's disease. In consideration of these findings, the relationship between tau phosphorylation and apoptosis was examined in human neuroblastoma SH-SY5Y cells that were subjected to hyperosmotic stress. In this model caspase 3 activity, which served as an indicator of apoptosis, was increased by 30 min of osmotic stress and remained elevated through 4 hr. Hyperosmotic stress also resulted in a robust increase in tau phosphorylation at both Ser/Pro and non-Ser/Pro sites. Phosphorylation of Ser262/356 (12E8) and Ser396/404 (PHF-1) increased by 5 min and remained elevated for at least 1 hr. In contrast, phosphorylation within the Tau-1 epitope did not increase (as evidenced by decreased immunoreactivity) until 30 min after treatment but remained elevated for a much greater period of time. Treatment with insulin-like growth factor-1 delayed but did not prevent apoptotic cell death induced by osmotic stress and attenuated the increase in phosphorylation at the Tau-1 epitope. Li(+), an inhibitor of glycogen synthase kinase 3 beta, had no effect on osmotic stress-induced caspase activation, but reduced phosphorylation at the Tau-1 epitope. Complete inhibition of osmotic stress-induced caspase activation with DEVD-CHO had no effect on the increases in tau phosphorylation. The results of these studies demonstrate that tau phosphorylation is increased at the specific epitopes during apoptosis. However, the changes in tau phosphorylation likely do not significantly impact the apoptotic process but rather occur concurrently as a result of inappropriate activation of specific protein kinases. Nonetheless, there is increasing evidence of a dysregulation of protein kinases that occurs in Alzheimer's disease brain that may be part of the events of apoptosis, which could contribute to aberrant increases in tau phosphorylation.

Complement activation by neurofibrillary tangles in Alzheimer's disease

Brain inflammation is widely documented to occur in Alzheimer's disease (AD), but its sources are still incompletely understood. Here, we present in vitro and in situ evidence that, like amyloid beta peptide (Abeta), tau, the major protein constituent of the neurofibrillary tangle, is a potent, antibody-independent activator of the classical complement pathway. Complement activation, in turn, is known to drive numerous inflammatory responses, including scavenger cell activation and cytokine production. Because Abeta deposits and extracellular tangles are present from early preclinical to terminal stages of AD, their ability to activate complement provides a ready mechanism for initiating and sustaining chronic, low-level inflammatory responses that may cumulate over the disease course.

Cholinergic- and stress-induced signaling activities in cells overexpressing wild-type and mutant presenilin-1

This study examined the effects of overexpression of presenilin-1 wild-type (PS1wt) or mutant L286V (PS1m) in human neuroblastoma SH-SY5Y cells on signal transduction systems. Oxotremorine-M-induced activation of AP-1 was 40--53% lower in PS1wt than control cells, and further impaired (63--76%) in PS1m cells. Heat shock (45 degrees C) activated Akt, increased heat shock factor-1 (HSF-1) DNA binding activity, and increased levels of heat shock protein 70, and these responses were not altered by overexpression of PS1wt or PS1m. H(2)O(2) also caused a time-dependent increase in HSF-1 DNA binding activity which was similar in all cell lines. Thus, overexpression of PS1wt reduced muscarinic receptor-mediated activation of AP-1, and PS1m overexpression caused greater inhibition, but stress-induced activation of Akt and HSF-1 was unaffected by either PS1wt or PS1m.

Glycogen synthase kinase 3beta is tyrosine phosphorylated by PYK2

Glycogen synthase kinase 3beta (GSK3beta) is a Ser/Thr kinase that is involved in numerous cellular activities. GSK3beta is activated by tyrosine phosphorylation. However, very little is known about the tyrosine kinases that are responsible for phosphorylating GSK3beta. In this report, we investigated the ability of the calcium-dependent tyrosine kinase, proline-rich tyrosine kinase 2 (PYK2) to tyrosine phosphorylate GSK3beta. In transfected CHO cells, it was demonstrated that PYK2 tyrosine phosphorylates GSK3beta in situ. The two kinases also coimmunoprecipitated. Furthermore, GSK3beta was tyrosine phosphorylated in vitro by an active, wild type PYK2, but not by the inactive, kinase dead form of PYK2. Therefore, this study is the first to demonstrate that GSK3beta is a substrate of PYK2 both in vitro and in situ.

Tissue transglutaminase selectively modifies proteins associated with truncated mutant huntingtin in intact cells

The cause of Huntington's disease (HD) is a pathological expansion of the polyglutamine domain within the N-terminal region of huntingtin. Neuronal intranuclear inclusions and cytoplasmic aggregates composed of the mutant huntingtin within certain neuronal populations are a characteristic hallmark of HD. However, how the expanded polyglutamine repeats of mutant huntingtin cause HD is not known. Because in vitro expanded polyglutamine repeats are excellent glutaminyl-donor substrates of tissue transglutaminase (tTG), it has been hypothesized that tTG may contribute to the formation of these aggregates in HD. However, an association between huntingtin and tTG or modification of huntingtin by tTG has not been demonstrated in cells. To examine the interactions between tTG and huntingtin human neuroblastoma SH-SY5Y cells were stably transfected with full-length huntingtin containing 23 (FL-Q23) (wild type) or 82 (FL-Q82) (mutant) glutamine repeats or a truncated N-terminal huntingtin construct containing 23 (Q23) (wild type) or 62 (Q62) (mutant) glutamine repeats. Aggregates were rarely observed in the cells expressing full-length mutant huntingtin, and no specific colocalization of full-length huntingtin and tTG was observed. In contrast, in cells expressing truncated mutant huntingtin (Q62) there were numerous complexes of truncated mutant huntingtin and many of these complexes co-localized with tTG. However, the complexes were not insoluble structures. Further, truncated huntingtin coimmunoprecipitated with tTG, and this association increased when tTG was activated. Activation of tTG did not result in the modification of either truncated or full-length huntingtin, however proteins that were associated with truncated mutant huntingtin were selectively modified by tTG. This study is the first to demonstrate that tTG specifically interacts with a truncated form of huntingtin, and that activated tTG selectively modifies mutant huntingtin-associated proteins. These data suggest that proteolysis of full-length mutant huntingtin likely precedes its interaction with tTG and this process may facilitate the modification of huntingtin-associated proteins and thus contribute to the etiology of HD.

Tissue transglutaminase is essential for neurite outgrowth in human neuroblastoma SH-SY5Y cells

Tissue transglutaminase is a normal constituent of the central and peripheral nervous systems and in rats transglutaminase activity in brain and spinal cord is highest during fetal stages when axonal outgrowth is occurring. Further, treatment of human neuroblastoma SH-SY5Y cells with retinoic acid results in the cells withdrawing from the cell cycle and extending neurites, in the same time frame that tissue transglutaminase expression significantly increases. Considering these and other previous findings, this study was carried out to determine whether tissue transglutaminase is involved in neuronal differentiation of SH-SY5Y cells. For these studies SH-SY5Y cells stably overexpressing wild-type tissue transglutaminase, an inactive tissue transglutaminase mutant (C277S) or an antisense tissue transglutaminase construct (which decreased endogenous tissue transglutaminase below detectable levels) were used. SH-SY5Y cells overexpressing wild-type tissue transglutaminase spontaneously differentiated into a neuronal phenotype when grown in low-serum media. In contrast, cells overexpressing inactive tissue transglutaminase or the antisense tissue transglutaminase continued to proliferate and exhibit a flat polygenic morphology even when maintained in low-serum conditions. In addition, increased tissue transglutaminase expression in response to retinoic acid was abolished in the antisense tissue transglutaminase cells, and antisense and mutant tissue transglutaminase expressing cells did not extend neurites in response to retinoic acid. Moreover, wild-type and inactive tissue transglutaminase exhibited differential intracellular localization. These data indicate that tissue transglutaminase is necessary and sufficient for neuronal differentiation of human neuroblastoma SH-SY5Y cells.

Three different human tau isoforms and rat neurofilament light, middle and heavy chain proteins are cellular substrates for transglutaminase

Abnormal aggregates of tau and neurofilaments are pathologies of Alzheimer's disease. Some of these aggregates are insoluble in chaotropic salts and ionic detergents but the mechanisms that lead to this are not clear. One suggestion is that it is due to crosslinking by tissue transglutaminase. Both tau and neurofilaments can be crosslinked by transglutaminase in vitro and one isoform of tau is now known to be a cellular transglutaminase substrate. However there is no evidence to demonstrate that neurofilaments are cellular substrates for transglutaminase and it is not clear whether other isoforms of tau are equally susceptible to transglutaminase crosslinking. Here, we demonstrate that three different tau isoforms and neurofilament light, middle and heavy chain proteins are all cellular substrates for transglutaminase.

Tau protein is hyperphosphorylated in a site-specific manner in apoptotic neuronal PC12 cells

Alterations in the status of microtubules contribute to the cytoskeletal rearrangements that occur during apoptosis. The microtubule-associated protein tau regulates microtubule dynamics and thus is likely to play an important role in the cytoskeletal changes that occur in apoptotic cells. Previously, we demonstrated that the phosphorylation of tau at the Tau-1 epitope was increased during neuronal PC12 cell apoptosis, and further that the microtubule binding of tau from apoptotic cells was significantly impaired because of altered phosphorylation. The fact that the microtubule-binding capacity of tau from apoptotic cells was reduced to approximately 30% of control values indicated that sites in addition to those within the Tau-1 epitope were hyperphosphorylated during apoptosis. In this study using a combination of immunological and biochemical approaches, numerous sites were found to be hyperphosphorylated on tau isolated from apoptotic cells. Further, during apoptosis, the activities of cell division control protein kinase (cdc2) and cyclin-dependent kinase 5 (cdk5) were selectively and significantly increased. The association of these two protein kinases with tau was also increased during apoptosis. These findings are intriguing because many of the sites found to be hyperphosphorylated on tau during apoptosis are also hyperphosphorylated on tau from Alzheimer's disease brain. Likewise, there are data indicating that in Alzheimer's disease the activities of cdc2 and cdk5 are also increased.

Impaired mitochondrial function results in increased tissue transglutaminase activity in situ

Tissue transglutaminase (tTG) is a transamidating enzyme that is elevated in Huntington's disease (HD) brain and may be involved in the etiology of the disease. Further, there is evidence of impaired mitochondrial function in HD. Therefore, in this study, we examined the effects of mitochondrial dysfunction on the transamidating activity of tTG. Neuroblastoma SH-SY5Y cells stably overexpressing human tTG or mutated inactive tTG were treated with 3-nitropropionic acid (3-NP), an irreversible inhibitor of succinate dehydrogenase. 3-NP treatment of tTG-expressing cells resulted in a significant increase of TG activity in situ. In vitro measurements demonstrated that 3-NP had no direct effect on tTG activity. However, 3-NP treatment resulted in a significant decrease of the levels of GTP and ATP, two potent inhibitors of the transamidating activity of tTG. No significant changes in the intracellular levels of calcium were observed in 3-NP-treated cells. Treatment with 3-NP in combination with antioxidants significantly reduced the 3-NP-induced increase in in situ TG activity, demonstrating that oxidative stress is a contributing factor to the increase of TG activity. This study demonstrates for the first time that impairment of mitochondrial function significantly increases TG activity in situ, a finding that may have important relevance to the etiology of HD.

Insulin-like growth factor-1 and insulin mediate transient site-selective increases in tau phosphorylation in primary cortical neurons

The modulation of tau phosphorylation and localization in response to insulin-like growth factor-1 or insulin was examined in primary cultures of rat cortical neurons. Insulin and insulin-like growth factor-1 treatment resulted in a rapid and transient increase in tau phosphorylation at specific epitopes. These effects were completely inhibited by lithium, revealing that the insulin and insulin-like growth factor-1 induced changes in tau phosphorylation were mediated by glycogen synthase kinase-3beta. In addition, the increase in tau phosphorylation directly correlated with a transient dissociation of tau from the cytoskeleton, indicating that insulin and insulin-like growth factor-1 treatment resulted in a change in tau localization. Using immunocytochemistry, it was also demonstrated that treatment of neurons with insulin-like growth factor-1 for 3 min resulted in a redistribution of tau to the growth cone and the distal segment of the axons. Further, insulin-like growth factor-1 treatment resulted in an increased immunoreactivity with the phospho-dependent antibody AT8 in the same areas of the axons. Thus, the phosphorylation state and distribution of tau can be modulated by insulin and insulin-like growth factor-1 signaling pathways involving glycogen synthase kinase-3beta. We propose that by transiently increasing tau phosphorylation, insulin and insulin-like growth factor-1 may contribute to the reorganization of the cytoskeleton necessary for the development and growth of the neurites.

Transient oxidative stress in SH-SY5Y human neuroblastoma cells results in caspase dependent and independent cell death and tau proteolysis

The effects of an oxidative insult on cell survival and tau metabolism were investigated in human neuroblastoma SH-SY5Y cells. In this treatment paradigm cells were exposed to the membrane permeant oxidant tert-butylhydroperoxide (tBHP) for 40 min, returned to fresh media and cell survival/death was monitored during the post-treatment period. Cell viability decreased significantly by 6 hr after tBHP exposure, and by 24 hr lactate dehydrogenase (LDH) release was 40.1 +/- 8.8% in tBHP treated cells compared to 8.1 +/- 4.7% in control cells. This oxidative stress paradigm also resulted in significant activation of caspase-3 by 2 hr post-treatment and nuclear apoptotic morphology. Furthermore, tBHP treatment also resulted in delayed tau proteolysis that was first evident 2 hr post-treatment. Treatment of the cells with the general caspase inhibitor Boc-Asp(OMe)-Fluoromethylketone (BAF) completely inhibited caspase-3 activation in response to tBHP, and delayed, but did not prevent cell death. BAF treatment also decreased tau proteolysis. In vitro, recombinant tau was readily proteolyzed by active recombinant caspase-3 into a stable breakdown product. Further tau in the cell lysates was cleaved by active recombinant caspase-3 at a rate, and to an extent similar to that observed for the well-established caspase-3 substrate poly(ADP-ribose)polymerase (PARP). These results suggest that oxidative stress-induced cell death occurs through both caspase-dependent and-independent pathways, and that tau is likely an in situ substrate of caspase-3.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Microtubule/MAP-affinity regulating kinase (MARK) is activated by phenylarsine oxide in situ and phosphorylates tau within its microtubule-binding domain

Tau is a microtubule-associated protein (MAP) that is functionally modulated by phosphorylation and that is hyperphosphorylated in several neurodegenerative diseases. Because phosphorylation regulates both normal and pathological tau functioning, it is of interest to identify the signaling pathways and enzymes capable of modulating tau phosphorylation in vivo. Previously, it was demonstrated that in SH-SY5Y human neuroblastoma cells and rat primary cortical cultures tau is phosphorylated at Ser262/356, within its microtubule-binding domain, by a staurosporine-sensitive protein kinase in response to the vicinal thiol-directed agent phenylarsine oxide. The current study demonstrates the presence of a 100-kDa protein kinase activity in SH-SY5Y cells that associates with microtubules, phosphorylates tau at Ser262/356, is activated by phenylarsine oxide, and is inhibited by the protein kinase inhibitor staurosporine. Isolation of individual protein bands from a polyacrylamide gel revealed two closely spaced proteins containing Ser262/356-directed protein kinase activity. Mass spectrometry analysis indicated that these protein bands correspond to the 100-kDa microtubule/MAP-affinity regulating kinase (MARK), which has been shown previously to phosphorylate tau within its microtubule-binding domain. Immunoblot analysis of the protein kinase bands confirmed this finding, providing the first demonstration that activation of endogenous MARK results in increased tau phosphorylation within its microtubule-binding domain in situ.

Glycogen synthase kinase-3beta, beta-catenin, and tau in postmortem bipolar brain

Therapeutic concentrations of the anti-bipolar drug lithium inhibit the activity of glycogen synthase kinase-3beta, which raises the possibility that this enzyme and its substrates may be altered in the brain of subjects with bipolar disorder. Therefore, in prefrontal cortical samples from subjects with bipolar disorder and age-matched control subjects, we examined the levels of glycogen synthase kinase 3beta and of two proteins modified by it, beta-catenin and the microtubule associated protein tau. There were no significant differences between subject groups among these measurements, but there was a tendency for the tau isoform profile to be modified in bipolar tissue. Thus, while there are no differences between bipolars and controls in prefrontal cortical levels of glycogen synthase kinase-3beta, beta-catenin, or tau, tau isoform levels or phosphorylation states may be modified in bipolar disorder.

Modulation of tau phosphorylation and intracellular localization by cellular stress

Tau is a microtubule-associated protein that is functionally modulated by phosphorylation and hyperphosphorylated in several neurodegenerative diseases. Because phosphorylation regulates both normal and pathological tau functioning, it is of great interest to identify the signalling pathways and enzymes capable of modulating tau phosphorylation in vivo. The present study examined changes in tau phosphorylation and localization in response to osmotic stress, which activates the stress-activated protein kinases (SAPKs), a family of proline-directed protein kinases shown to phosphorylate tau in vitro and hypothesized to phosphorylate tau in Alzheimer's disease. Immunoblot analysis with phosphorylation-dependent antibodies revealed that osmotic stress increased tau phosphorylation at the non-Ser/Thr-Pro sites Ser-262/356, within the microtubule-binding domain, as well as Ser/Thr-Pro sites outside of tau's microtubule-binding domain. Although all SAPKs examined were activated by osmotic stress, none of the endogenous SAPKs mediated the increase in tau phosphorylation. However, when transfected into SH-SY5Y cells, SAPK3, but not the other SAPKs examined, phosphorylated tau in situ in response to activation by osmotic stress. Osmotic-stress-induced tau phosphorylation correlated with a decrease in the amount of tau associated with the cytoskeleton and an increase in the amount of soluble tau. This stress-induced alteration in tau localization was only partially due to phosphorylation at Ser-262/356 by a staurosporine-sensitive, non-proline-directed, protein kinase. Taken together, these results suggest that osmotic stress activates at least two tau-directed protein kinases, one proline-directed and one non-proline-directed, that SAPK3 can phosphorylate tau on Ser/Thr-Pro residues in situ, and that Ser-262/356 phosphorylation only partially regulates tau localization in the cell.

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.

Tau protein in normal and Alzheimer's disease brain: an update

Tau is a microtubule-associated protein that, in a hyperphosphorylated form, comprises the main component of the paired helical filaments and neurofibrillary tangles found in Alzheimer's Disease (AD) brain. It is therefore important to understand the normal functioning and processing of tau protein, and the abnormal posttranslational processing of tau in AD pathology. In 1996, Johnson and Jenkins reviewed the literature on the biochemistry, function, and phosphorylation of tau in normal and AD brain. Since that time, numerous publications have come out further elucidating the properties of tau. The present review updates the topics originally covered in the 1996 review, as well as presents a number of new topics. For example, mutations in the tau gene have been found in several non-AD, autosomal dominant neurodegenerative disorders that exhibit extensive neurofibrillary pathology. In addition, there is increasing evidence that tau may be involved in signal transduction, organelle transport, and cell growth, independent of its microtubule-binding functions. Taken together, the research reviewed here demonstrates that tau is a very complex protein with various functions that are intricately regulated. It is clear that more research is required to completely understand the functions and regulation of tau in normal and AD brain.

Modulation of tau phosphorylation within its microtubule-binding domain by cellular thiols

Tau is a microtubule-stabilizing protein that is functionally modulated by alterations in its phosphorylation state. Because phosphorylation regulates both normal and pathological tau functioning, it is of importance to identify the signaling pathways that regulate tau phosphorylation in vivo. The present study examined changes in tau phosphorylation and function in response to modulation of cellular thiol content. Treatment of cells with phenylarsine oxide, which reacts with vicinal thiols, selectively increased tau phosphorylation within its microtubule-binding domain, at the non-Ser/Thr-Pro sites Ser262/356, while decreasing tau phosphorylation at Ser/ Thr-Pro sites outside this region. This increase in tau phosphorylation correlated with a decrease in the amount of tau associated with the cytoskeleton and decreased microtubule stability. Phenylarsine oxide-induced tau phosphorylation was inhibited by oxidants and by the protein kinase inhibitor staurosporine. Although staurosporine completely eliminated the increase in tau phosphorylation at Ser262/356, as detected by immunostaining with 12E8, it had a comparatively minor effect on the changes in tau localization induced by phenylarsine oxide. The results suggest that regulation of cellular thiols is important for modulating tau phosphorylation and function in situ. Additionally, although phosphorylation of Ser262/356 decreases tau's interaction with the cytoskeleton, phosphorylation of these residues alone is not sufficient for the phenylarsine oxide-induced changes in tau localization.

Tau protein in normal and Alzheimer's disease brain

In 1975, Weingarten and colleagues isolated a protein factor that was able to induce microtubule formation. They called this factor tau (t). Some ten years later a new era of research on this microtubule-associated protein was launched when several groups almost simultaneously discovered that tau was the predominant protein component of the paired helical filaments (PHFs) and neurofibrillary tangles (NFTs) which are characteristic pathological lesions of the Alzheimer's disease brain. Subsequent findings that PHF-tau isolated from Alzheimer's disease brain was phosphorylated to a greater extent than non-PHF tau, led to extensive investigation into the posttranslational modifications (mainly phosphorylation) of tau in normal and Alzheimer's disease brain. The present review highlights the literature concerning the normal functioning and processing of tau protein, and examines the evidence for the involvement of the abnormal posttranslational processing of tau in the pathology of Alzheimer's disease. Finally, speculation as to the relationship between abnormal processing of tau, other subcellular abnormalities seen in Alzheimer's disease, and the pathological causes of the disease are discussed.

Novel bimodal effects of the G-protein tissue transglutaminase on adrenoreceptor signalling

Tissue transglutaminase (tTG) is a novel G-protein that previous studies showed can couple ligand-bound activated alpha(1B) adrenoreceptors to phospholipase C-delta, resulting in phosphoinositide (PI) hydrolysis. In human neuroblastoma SH-SY5Y cells we found that although endogenous tTG can facilitate alpha(1B) adrenoreceptor-stimulated PI hydrolysis, its contribution is minor compared with the classical heterotrimeric G-protein G(q/11). Further, we show that the alpha(1B) adrenoreceptor recruits tTG to the membrane and that this recruitment is enhanced by agonist occupancy of the receptor. In addition, the effects of tTG on signalling are bimodal. At low expression levels, tTG enhanced alpha(1B) adrenoreceptor-stimulated PI hydrolysis, whereas at higher expression levels tTG attenuated significantly this response. These findings are the first to demonstrate that a protein can both facilitate and attenuate receptor-stimulated PI hydrolysis.

Tau is modified by tissue transglutaminase in situ: possible functional and metabolic effects of polyamination

Tissue transglutaminase (tTG) is up-regulated in Alzheimer's disease brain and localizes to neurofibrillary tangles with the tau protein. Tau is an in vitro tTG substrate, being cross-linked and/or polyaminated. Further, the Gln and Lys residues in tau that are modified by tTG in vitro are located primarily within or adjacent to the microtubule-binding domains. Considering these and other previous findings, this study was carried out to determine if tau is modified in situ by tTG in human neuroblastoma SH-SY5Y cells, and whether tTG-catalyzed tau polyamination modulates the function and/or metabolism of tau in vitro. For these studies, SH-SY5Y cells stably overexpressing tTG were used. tTG coimmunoprecipitated with tau, and elevating intracellular calcium levels with maitotoxin resulted in a 52 +/- 4% increase in the amount of tTG that coimmunoprecipitated with tau. The increase in association of tTG with tau after treatment with maitotoxin corresponded to a coimmunolocalization of tTG, tTG activity, and tau in the cells. Further, tau was modified by tTG in situ in response to maitotoxin treatment. In vitro polyaminated tau was significantly less susceptible to micro-calpain proteolysis; however, tTG-mediated polyamination of tau did not significantly alter the microtubule-binding capacity of tau. Thus, tau interacts with and is modified by tTG in situ, and modification of tau by tTG alters its metabolism. These data indicate that tau is likely to be modified physiologically and pathophysiologically by tTG, and tTG may play a role in Alzheimer's disease.

Tissue transglutaminase is increased in Huntington's disease brain

The polyglutamine-expanded N-terminal region of mutant huntingtin causes neurodegeneration in Huntington's disease (HD). Neuronal intranuclear and cytosolic inclusions composed of mutant huntingtin are found in brains of HD patients. Because tissue transglutaminase cross-links proteins into filamentous aggregates and polypeptide-bound glutamines are primary determining factors for tissue transglutaminase-catalyzed reactions, it has been hypothesized that tissue transglutaminase may contribute to the formation of these aggregates. In this report immunohistochemical and biochemical methods were used to demonstrate that tissue transglutaminase expression and transglutaminase activity are elevated in HD brains in a grade-dependent manner. In the striatum, tissue transglutaminase activity was significantly increased in the grade 3 HD cases compared with controls. When normalized to the neuronal marker calbindin D28k, immunoblot analysis revealed that in the striatum the levels of tissue transglutaminase were significantly increased in all HD cases compared with controls. Immunohistochemical staining of the HD striatum revealed that tissue transglutaminase immunoreactivity was markedly increased in all grades as compared with controls. In the superior frontal cortex, tissue transglutaminase activity was significantly higher in all HD cases as compared with controls. Quantitative analysis of immunoblots demonstrated that tissue transglutaminase levels were elevated in HD grades 2 and 3 cases. Tissue transglutaminase immunoreactivity within the superior frontal neocortex was also greater in all the HD cases compared with controls. These data clearly indicate that tissue transglutaminase is elevated in HD brain and may play a role in the disease process.

Rapid, single-step procedure for the identification of transglutaminase-mediated isopeptide crosslinks in amino acid digests

Tissue transglutaminase (tTG) is a calcium-activated enzyme which can covalently crosslink the epsilon-amino group of a peptide-bound lysine into the gamma-carboxamide group of a peptide-bound glutamine, forming a epsilon-(-gamma-glutamyl)lysine isopeptide bond. We have developed a sensitive, single-step method for the isolation and detection of tTG-mediated isopeptide bonds from purified proteins and tissue homogenates. This method offers significantly improved resolution over current techniques, and obviates the need for multi-column systems or costly fluorescence monitors. By using enzymatic proteolysis, derivatization with phenylisothiocyanate, and a simple elution gradient for HPLC, we were able to determine the frequency of crosslinks in purified fibrin (1.7 mol of isodipeptide per mol of fibrin), crosslinked tau proteins (0.75 mol of isodipeptide per mol of tau), and whole-tissue liver homogenates (0.5 nmol of isodipeptide per mg of total protein).

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.

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.

p38 kinase is activated in the Alzheimer's disease brain

The p38 mitogen-activated protein kinase is a stress-activated enzyme responsible for transducing inflammatory signals and initiating apoptosis. In the Alzheimer's disease (AD) brain, increased levels of phosphorylated (active) p38 were detected relative to age-matched normal brain. Intense phospho-p38 immunoreactivity was associated with neuritic plaques, neuropil threads, and neurofibrillary tangle-bearing neurons. The antibody against phosphorylated p38 recognized many of the same structures as an antibody against aberrantly phosphorylated, paired helical filament (PHF) tau, although PHF-positive tau did not cross-react with the phospho-p38 antibody. These findings suggest a neuroinflammatory mechanism in the AD brain, in which aberrant protein phosphorylation affects signal transduction elements, including the p38 kinase cascade, as well as cytoskeletal components.

Altered protein kinase C activation of Na+/Ca2+ exchange in mesangial cells from salt-sensitive rats

The purpose of these studies was to determine whether there is a defect in protein kinase C (PKC) regulation of the Na+/Ca2+ exchanger in cultured mesangial cells (MC) from Dahl/Rapp salt-sensitive (S) and salt-resistant (R) rats. R and S MCs were cultured, grown on coverslips, and loaded with fura 2 for measurement of single cell cytosolic calcium concentration ([Ca2+]i) in a microscope-based photometry system. Studies were performed in cells that were exposed to serum (serum fed) and in cells that were serum deprived for 24 h. Baseline [Ca2+]i values measured in a Ringer solution containing 150 mM NaCl were similar between R and S MCs in both serum-fed and serum-deprived groups, although baseline [Ca2+]i values were uniformly higher in the serum-deprived groups. Exchanger activity was assessed by reducing extracellular Na (Nae) from 150 to 2 mM, which resulted in movement of Na+ out of and Ca2+ into these cells (reverse-mode Na+/Ca2+ exchange). PKC was activated in these cells with 15-min exposure to 100 nM phorbol 12-myristate 13-acetate (PMA). In the absence of PMA, the change in [Ca2+]i (Delta[Ca2+]i) with reduction in Nae was similar between R and S MCs in both serum-fed and serum-deprived groups, although the magnitude of Delta[Ca2+]i was enhanced by serum deprivation. In both serum-fed and serum-deprived groups, PMA significantly increased Delta[Ca2+]i in R but not S MCs. Upregulation of exchanger activity in R MCs could be abolished by prior 24-h exposure to PMA, a maneuver that downregulates PKC activity. Other studies were performed to evaluate exchanger protein expression using monoclonal and polyclonal antibodies. Immunoblots of PMA-treated cells revealed an increase in the levels of 70- and 120-kDa proteins in the crude membrane fraction of R but not S MCs, an increase which was abrogated by prior 24-h PMA pretreatment and corresponded to reduction in the 70-kDa protein in the crude cytosolic fraction. These data demonstrate that PKC enhances Na+/Ca2+ exchange activity in MCs from R but not from S rats, suggesting that there may be a defect in the PKC-Na+/Ca2+ exchange regulation pathway in MCs of S rats.

Hippocampal microtubule-associated protein-2 alterations with contextual memory

Using immunohistochemistry and immunoblots, we show that alterations in hippocampal microtubule-associated protein-2 appear to be highly correlated with contextual memory as measured by significantly heightened fear responses. Compared to naive controls, rats trained in a novel context showed significantly increased immunostaining for the high molecular weight microtubule-associated protein-2a/b. This increase was observed 2 weeks after training and it was selective for hippocampal CA1 and CA2 pyramidal cells. Pre-exposure to the training context one month before training altered the hippocampal microtubule-associated protein-2 response; in these animals only the dentate gyrus showed significantly increased microtubule-associated protein-2a/b. Training-related increases in immunohistochemical staining for microtubule-associated protein-2 suggested that there was an increase in overall intact protein, an increase in immunoreactive breakdown products, or changes in protein compartmentalization. Immunoblots of hippocampal homogenates reacted with monoclonal antibodies to microtubule-associated protein-2a/b showed an increased presence of breakdown products in trained animals compared to untrained controls. Additional immunoblot studies demonstrated statistically significant decreases in the levels and/or phosphorylation state of the low molecular weight microtubule-associated protein-2c in the hippocampus of trained animals as compared to that of controls. These alterations in microtubule-associated protein-2 may reflect dendritic remodeling related to contextual memory storage.

Insulin transiently increases tau phosphorylation: involvement of glycogen synthase kinase-3beta and Fyn tyrosine kinase

The modulation of tau phosphorylation in response to insulin was examined in human neuroblastoma SH-SY5Y cells. Insulin treatment resulted in a transient increase in tau phosphorylation followed by a decrease in tau phosphorylation that correlated directly with a sequential activation and deactivation of glycogen synthase kinase-3beta (GSK-3beta). The insulin-induced increase in tau phosphorylation and concurrent activation of GSK-3beta was rapid (<2 min) and transient, and was associated with increased tyrosine phosphorylation of GSK-3beta. The increase in GSK-3beta tyrosine phosphorylation corresponded directly to an increase in the association of Fyn tyrosine kinase with GSK-3beta, and Fyn immunoprecipitated from cells treated with insulin for 1 min phosphorylated GSK-3beta to a significantly greater extent than Fyn immunoprecipitated from control cells. Subsequent to the increase in GSK-3beta activation and tau phosphorylation, treatment of cells with insulin for 60 min resulted in a dephosphorylation of tau and a decrease in GSK-3beta activity. Thus, insulin rapidly and transiently activated GSK-3beta and modulated tau phosphorylation, alterations that may contribute to neuronal plasticity.

Enhancement of peroxynitrite-induced apoptosis in PC12 cells by fibroblast growth factor-1 and nerve growth factor requires p21Ras activation and is suppressed by Bcl-2

Extracellular trophic factors can regulate whether cells subjected to oxidative stress will survive to proliferate or else undergo cell death. We have previously shown that about 35% of undifferentiated PC12 cells undergo apoptosis 18 h after exposure to peroxynitrite and that pretreatment with nerve growth factor (NGF) protects PC12 cells through activation of phosphatidylinositol (PI) 3-kinase. In contrast, pretreatment with acidic fibroblast growth factor (FGF-1) approximately doubled apoptosis. We report here that NGF added immediately after peroxynitrite treatment no longer protected against apoptosis, but instead enhanced apoptosis to the same extent as FGF. We further investigated which signaling pathways were involved in increasing the level of apoptosis. Overexpression of Bcl-2 blocked the increased apoptosis caused by NGF and FGF-1, but Bcl-2 did not prevent the induction of apoptosis by peroxynitrite alone. The increase in apoptosis caused by the trophic factors was also blocked by the expression of a dominant negative p21Ras mutant. Activation of PI 3-kinase by NGF pretreatment completely protected against both the enhanced apoptosis induced by FGF-1 pretreatment and NGF posttreatment and the apoptosis induced by peroxynitrite alone. Our results indicate that the enhancement of peroxynitrite-induced apoptosis caused by NGF and FGF-1 is dependent on the stimulation of a proapoptotic pathway involving p21Ras that can be suppressed by Bcl-2.

Calcineurin inhibition prevents calpain-mediated proteolysis of tau in differentiated PC12 cells

The effects of calcium influx on tau levels and phosphorylation were examined in differentiated PC12 cells. Maitotoxin-induced calcium influx resulted in time- and concentration-dependent tau dephosphorylation and degradation. Incubation of PC12 cells with a membrane-permeable calpain inhibitor blocked maitotoxin-induced tau degradation, suggesting the involvement of calpain in calcium-stimulated tau turnover. Okadaic acid or the calcineurin inhibitor FK520 partially inhibited maitotoxin-induced tau dephosphorylation at the Tau-1 epitope, indicating both phosphatase 2A/1 and calcineurin were involved. In addition, FK520, but not okadaic acid, blocked the maitotoxin-induced tau degradation, demonstrating that dephosphorylation of specific tau epitopes by was essential for calpain-mediated tau degradation. Moreover, maitotoxin effects were likely independent of tau association with microtubules because maitotoxin induced tau degradation and dephosphorylation in the presence of either nocodazole or taxol. These data provide evidence that calpain is involved in tau turnover in situ and calcineurin plays an important role in modulating tau susceptibility to calpain.

The interrelationship between selective tau phosphorylation and microtubule association

The purpose of this study was to examine the modulation of tau phosphorylation mediated by protein kinase A, a kinase with low intrinsic activity, and by the constitutively active glycogen synthase kinase, as well as to examine the subsequent effects on tau-microtubule association in differentiated human SH-SY5Y neuroblastoma cells. Activation of protein kinase A with forskolin and rolipram significantly increased tau phosphorylation at Ser262/356 only in the presence of okadaic acid, indicating that phosphates at these sites are normally turned over rapidly. In contrast, glycogen synthase kinase appears to maintain tau phosphorylation at Thr181 and Ser396/404 since inhibition of glycogen synthase kinase with lithium reduced phosphorylation at these sites. Lithium treatment also significantly decreased tau and tyrosinated alpha-tubulin levels. Perturbation of microtubules with nocodazole or taxol induced tau dephosphorylation at Tau-1 sites, Thr181 and Ser396/404, indicating that both constitutive kinase activity and microtubule state modulate tau phosphorylation at these sites. Nocodazole- or taxol-induced tau dephosphorylation was blocked by the protein phosphatase 2A/1 inhibitor okadaic acid, but not by the protein phosphatase 2B inhibitor cyclosporin A. In addition, osmotic stress, such as treatment with 20 mM NaCl, selectively increased tau phosphorylation at the Tau-1 epitope. To investigate the effect of phosphorylation on tau association with microtubules and microtubule stability in situ, a Triton X-100 extraction assay was utilized to separate the detergent-soluble cytosolic components from the detergent-insoluble cytoskeletal components. In control cells or cells treated with lithium very little tau was detected in the cytosolic fraction. Activation of protein kinase A in the presence of okadaic acid elevated tau levels in the detergent-soluble fraction, which contained all the tau phosphorylated at Ser262/356, and also decreased microtubule stability, as indicated by decreased acetylated alpha-tubulin levels. In conclusion, the phosphorylation state of tau in differentiated SH-SY5Y cells is regulated by glycogen synthase kinase, microtubule dynamics and osmotic stress at overlapping sites which apparently have little influence on tau-microtubule association. In contrast, phosphorylation of tau at Ser262/356 within the microtubule-binding, which was mediated in part by protein kinase A, prevented the association of tau with microtubules in situ.

Tissue transglutaminase is an in situ substrate of calpain: regulation of activity

Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the transamidation of specific polypeptide-bound glutamine residues, a reaction that is inhibited by GTP. There is also preliminary evidence that, in situ, calpain and GTP may regulate tTG indirectly by modulating its turnover by the calcium-activated protease calpain. In the present study, the in vitro and in situ proteolysis of tTG by calpain, and modulation of this process by GTP, was examined. tTG is an excellent substrate for calpain and is rapidly degraded. Previously it has been demonstrated that GTP binding protects tTG from degradation by trypsin. In a similar manner, guanosine-5'-O-(3-thiotriphosphate) protects tTG against proteolysis by calpain. Treatment of SH-SY5Y cells with 1 nM maitotoxin, which increases intracellular calcium levels, resulted in a significant increase in in situ TG activity, with only a slight decrease in tTG protein levels. In contrast, when GTP levels were depleted by pretreating the cells with tiazofurin, maitotoxin treatment resulted in an approximately 50% decrease in tTG protein levels, and a significant decrease in TG activity, compared with maitotoxin treatment alone. Addition of calpain inhibitors inhibited the degradation of tTG in response to the combined treatment of maitotoxin and tiazofurin and resulted in a significant increase in in situ TG activity. These studies indicate that tTG is an endogenous substrate of calpain and that GTP selectively inhibits the degradation of tTG by calpain.

Oxidative stress inhibits calpain activity in situ

In this study, the effects of oxidative stress on calpain-mediated proteolysis and calpain I autolysis in situ were examined. Calpain activity was stimulated in SH-SY5Y human neuroblastoma cells with the calcium ionophore, ionomycin. Calpain-mediated proteolysis of the membrane-permeable fluorescent substrate N-succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosine-7-amido-4-methylcouma rin, as well as the endogenous protein substrates microtubule-associated protein 2, tau and spectrin, was measured. Oxidative stress, induced by addition of either doxorubicin or 2-mercaptopyridine N-oxide, resulted in a significant decrease in the extent of ionophore-stimulated calpain activity of both the fluorescent compound and the endogenous substrates compared with control, normoxic conditions. Addition of glutathione ethyl ester, as well as other antioxidants, resulted in the retention/recovery of calpain activity, indicating that oxidation-induced calpain inactivation was preventable/reversible. The rate of autolytic conversion of the large subunit of calpain I from 80 to 78 to 76 kDa was decreased during oxidative stress; however, the extent of calpain autolysis was not altered. These data indicate that oxidative stress may reversibly inactivate calpain I in vivo.

Distinct nuclear localization and activity of tissue transglutaminase

Tissue transglutaminase is a calcium-dependent transamidating enzyme that has been postulated to play a role in the pathology of expanded CAG repeat disorders with polyglutamine expansions expressed within the affected proteins. Because intranuclear inclusions have recently been shown to be a common feature of many of these codon reiteration diseases, the nuclear localization and activity of tissue transglutaminase was examined. Subcellular fractionation of human neuroblastoma SH-SY5Y cells demonstrated that 93% of tissue transglutaminase is localized to the cytosol. Of the 7% found in the nucleus, 6% copurified with the chromatin-associated proteins, and the remaining 1% was in the nuclear matrix fraction. In situ transglutaminase activity was measured in the cytosolic and nuclear compartments of control cells, as well as cells treated with the calcium-mobilizing agent maitotoxin to increase endogenous tissue transglutaminase activity. These studies revealed that tissue transglutaminase was activated in the nucleus, a finding that was further supported by cytochemical analysis. Immunofluorescence studies revealed that nuclear proteins modified by transglutaminase exhibited a discrete punctate, as well as a diffuse staining pattern. Furthermore, different proteins were modified by transglutaminase in the nucleus compared with the cytosol. The results of these experiments clearly demonstrate localization of tissue transglutaminase in the nucleus that can be activated. These findings may have important implications in the formation of the insoluble nuclear inclusions, which are characteristic of codon reiteration diseases such as Huntington's disease and the spinocerebellar ataxias.

Modulation of the in situ activity of tissue transglutaminase by calcium and GTP

Tissue transglutaminase (tTG) is a calcium-dependent enzyme that catalyzes the posttranslational modification of proteins by transamidation of specific polypeptide-bound glutamine residues. Previous in vitro studies have demonstrated that the transamidating activity of tTG requires calcium and is inhibited by GTP. To investigate the endogenous regulation of tTG, a quantitative in situ transglutaminase (TG) activity assay was developed. Treatment of human neuroblastoma SH-SY5Y cells with retinoic acid (RA) resulted in a significant increase in tTG levels and in vitro TG activity. In contrast, basal in situ TG activity did not increase concurrently with RA-induced increased tTG levels. However, stimulation of cells with the calcium-mobilizing drug maitotoxin (MTX) resulted in increases in in situ TG activity that correlated (r2 = 0.76) with increased tTG levels. To examine the effects of GTP on in situ TG activity, tiazofurin, a drug that selectively decreases GTP levels, was used. Depletion of GTP resulted in a significant increase in in situ TG activity; however, treatment of SH-SY5Y cells with a combination of MTX and tiazofurin resulted in significantly less in situ TG activity compared with treatment with MTX alone. This raised the possibility of calcium-dependent proteolysis due to the effects of tiazofurin, because in vitro GTP protects tTG against proteolysis by trypsin. Studies with a selective membrane permeable calpain inhibitor indicated that tTG is likely to be an endogenous substrate of calpain, and that depletion of GTP increases tTG degradation after elevation of intracellular calcium levels. TG activity was also increased in response to activation of muscarinic cholinergic receptors, which increases intracellular calcium through inositol 1,4,5-trisphosphate generation. The results of these experiments demonstrate that selective changes in calcium and GTP regulate the activity and levels of tTG in situ.

Tau complexes with phospholipase C-gamma in situ

Based on the results of recent in vitro studies, tau has been proposed to be involved in regulating signal transduction through the phospholipase C-gamma (PLC-gamma) signaling pathway. The present study provides support for the physiological relevance of this hypothesis by demonstrating the existence of a tau-PLC-gamma complex in situ in a human neuroblastoma cell line. Both PLC-gamma and PLC-delta, but not PLC-beta, co-purified with microtubule-associated proteins. PLC-gamma, but neither PLC-delta nor PLC-beta, co-immunoprecipitated with tau, and the PLC co-precipitating with tau was enzymatically active. Additionally, both tau and MAP-2 co-precipitated with PLC-gamma. These studies indicate that tau associates, either directly or indirectly, with PLC-gamma in situ, suggesting that tau may be appropriately localized to participate in the regulation of signal transduction through the PLC-gamma pathway in vivo.

Calpains: intact and active?

Calpains are a family of calcium-dependent thiol-proteases which are proposed to be involved in many physiological processes as well as pathological conditions. Calpains are likely to be involved in processing of numerous enzymes and cytoskeletal components, thereby linking their activity to a variety of intracellular events. Although widely studied, the precise mechanism(s) involved in calpain activation and activity in vivo remain poorly understood. Initial studies suggested that calpain exists primarily as an inactive proenzyme that required autolytic cleavage for activation. It was also hypothesized that calpain associated with membrane phospholipids, serving to increase calcium sensitivity, facilitating autolytic conversion and thus activating the enzyme. These hypotheses, however, have not been universally accepted and there is increasing evidence that intact, non-autolyzed calpain is the physiologically active calpain form.

Phosphorylation of microtubule-associated protein tau on Ser 262 by an embryonic 100 kDa protein kinase

This study examined the phosphorylation of tau on Ser 262, within the first microtubule-binding domain, by a developmentally regulated 100 kDa protein kinase exhibiting significantly greater activity in the embryonic rat brain than in the adult rat brain. This protein kinase co-purified with microtubules and co-immunoprecipitated with both tau and MAP-2. In addition to phosphorylating tau, MAP-2, and a Ser 262-containing peptide, the present protein kinase activity was shown to autophosphorylate as determined by the in-gel kinase assay in the absence of any protein or peptide polymerized into the matrix. Phosphorylation of tau with this protein kinase significantly reduced the tau-microtubule interaction, and the effect was significantly greater with microtubule-associated protein (MAP) preparations from embryonic brain than with preparations from the adult. Ser 262 is phosphorylated extensively in paired helical filament (PHF) tau from Alzheimer's disease (AD) brain, to a lesser extent in fetal tau, and only to a very minor extent in biopsy-derived human tau. Because the 100 kDa protein kinase activity phosphorylates Ser 262 and is higher in the fetal brain than the adult brain, it is hypothesized that an inappropriate re-expression and/or re-activation of this or a similar developmentally regulated protein kinase could contribute to the phosphorylation of Ser 262 in PHF-tau, and thus play a role in the pathogenesis of AD.

Ceramide selectively decreases tau levels in differentiated PC12 cells through modulation of calpain I

Ceramide has been recently proposed to be a signal mediator in several important physiological processes including apoptosis, cellular growth, and differentiation. Because the microtubule-associated protein tau plays an important role in the establishment and maintenance of neuronal morphology, the effects of ceramide on tau were examined. Treatment of differentiated PC12 cells with the cell-permeable ceramide derivative N-acetylsphingosine (C2) resulted in a significant reduction in tau levels. Significant decreases in tau levels were also observed when the cells were treated with another ceramide derivative, N-hexanoylsphingosine (C6). In addition, C2 treatment increased the levels of a calpain-derived spectrin breakdown product but did not alter the levels of two cytoskeletal proteins, alpha-actin and alpha-tubulin. Because both tau and spectrin are proteolyzed in vitro by the calcium-activated cysteine protease calpain, the effects of ceramide analogues on the activity of this protease were examined. Treatment of PC12 cells with C2 enhanced calcium-stimulated proteolytic activity significantly, as revealed by monitoring the hydrolysis of the membrane-permeable calpain-selective fluorescence probe N-succinyl-L-leucyl-L-leucyl-L-valyl-L-tyrosine-7-amido-4-methylcoumarin . This activity increase was not due to a direct effect of C2 on calpains, because C2 did not alter the activities of purified calpain I or II. In addition, C2 treatment of PC12 cells resulted in a significant increase in the levels of calpain I and, to a lesser extent, the levels of calpastatin (an endogenous calpain inhibitor protein), whereas the levels of calpain II were not changed. Moreover, treatment of the cells with the synthetic calpain-specific inhibitor N-carbobenzoxy-L-leucyl-L-leucyl-L-tyrosine diazomethyl ketone blocked the C2-induced decreases in tau levels. These results indicate that tau levels are regulated in response to a physiological factor and, thus, have implications for ceramide-mediated changes in normal and pathological neuronal processes.