Mice lacking phosphatase PP2A subunit PR61/B'delta (Ppp2r5d) develop spatially restricted tauopathy by deregulation of CDK5 and GSK3beta

Functional diversity of protein phosphatase 2A (PP2A) enzymes mainly results from their association with distinct regulatory subunits. To analyze the functions of one such holoenzyme in vivo, we generated mice lacking PR61/B'δ (B56δ), a subunit highly expressed in neural tissues. In PR61/B'δ-null mice the microtubule-associated protein tau becomes progressively phosphorylated at pathological epitopes in restricted brain areas, with marked immunoreactivity for the misfolded MC1-conformation but without neurofibrillary tangle formation. Behavioral tests indicated impaired sensorimotor but normal cognitive functions. These phenotypical characteristics were further underscored in PR61/B'δ-null mice mildly overexpressing human tau. PR61/B'δ-containing PP2A (PP2A(T61δ)) poorly dephosphorylates tau in vitro, arguing against a direct dephosphorylation defect. Rather, the activity of glycogen synthase kinase-3β, a major tau kinase, was found increased, with decreased phosphorylation of Ser-9, a putative cyclin-dependent kinase 5 (CDK5) target. Accordingly, CDK5 activity is decreased, and its cellular activator p35, strikingly absent in the affected brain areas. As opposed to tau, p35 is an excellent PP2A(T61δ) substrate. Our data imply a nonredundant function for PR61/B'δ in phospho-tau homeostasis via an unexpected spatially restricted mechanism preventing p35 hyperphosphorylation and its subsequent degradation.     

Adenosine 3',5'-monophosphate-dependent protein kinase from normal human adrenal.

The protein kinase of normal human adrenal cytosol has been resolved by DEAE-cellulose chromatography into two major components, the protein kinases I and II, which are both adenosine 3',5'-monophosphate (cAMP) dependent. Both enzymes have similar substrate specificities, cAMP-dependency, and sensitivity to the stimulation by this nucleotide, but differ in their states of activation after preincubation with histone. The DEAE--cellulose charomatography of dissociated cytosol protein kinase reveals only one peak of kinase activity and two peaks of cAMP binding activity (A and B). Both binding proteins are able to inhibit the kinase activity of the catalytic subunit. Recombination experiments suggest that the regulatory subunit A originated from protein kinase I and subunit B from protein kinase II. The phosphorylation of histone by adrenal protein kinases is inhibited by a heat-stable protein inhibitor isolated from human fetal brain and human adult adrenal.     

Glucose-induced expression of the cyclin-dependent protein kinase 5 activator p35 involved in Alzheimer's disease regulates insulin gene transcription in pancreatic beta-cells

The deposition of amyloid within the insulin-producing islets of Langerhans in the pancreas is a common pathological finding in patients with type 2 diabetes. Its relationship with age and the progression of the disease resembles the pathological deposition of beta-amyloid in the brains of Alzheimer's patients. Endocrine cells of pancreatic islets and cells of neuronal lineages express a shared subset of specialized genes. The hyperactivity of the cyclin-dependent protein kinase CDK5, involved in the development and differentiation of the nervous system, is associated with Alzheimer's disease. Overactivity of CDK5 occurs by proteolytic cleavage and cellular mislocalization of its activator, p35. These alterations in p35/CDK5 signaling pathway may mediate, at least in part, the functional abnormalities characteristic of Alzheimer's disease. In this study we report that both the p35 and CDK5 genes are expressed in insulin-producing beta-cells of the pancreas. We detect in beta-cells the formation of an active p35/CDK5 complex with specific kinase activity. Notably, elevations of the extracellular concentration of glucose result in increases in p35 mRNA and protein levels that parallel elevations of p35/CDK5 activity. Functional studies show that p35 stimulates the activity of the insulin promoter and that the stimulation requires CDK5 because stimulation is blocked by roscovitine, an inhibitor of CDK5 activity, a dominant negative form of CDK5, and small interfering RNAs against p35. Our findings indicate that the expression of p35 and CDK5 in insulin-producing beta-cells ensembles a new signaling pathway, the activity of which is controlled by glucose, and its functional role may comprise the regulation of various biological processes in beta-cells, such as is the case for expression of the insulin gene.     

Cyclin-dependent kinase 5 regulates dopaminergic and glutamatergic transmission in the striatum

Dopaminergic and glutamatergic neurotransmissions in the striatum play an essential role in motor- and reward-related behaviors. Dysfunction of these neurotransmitter systems has been found in Parkinson's disease, schizophrenia, and drug addiction. Cyclin-dependent kinase 5 (CDK5) negatively regulates postsynaptic signaling of dopamine in the striatum. This kinase also reduces the behavioral effects of cocaine. Here we demonstrate that, in addition to a postsynaptic role, CDK5 negatively regulates dopamine release in the striatum. Inhibitors of CDK5 increase evoked dopamine release in a way that is additive to that of cocaine. This presynaptic action of CDK5 also regulates glutamatergic transmission. Indeed, inhibition of CDK5 increases the activity and phosphorylation of N-methyl-d-aspartate receptors, and these effects are reduced by a dopamine D1 receptor antagonist. Using mice with a point mutation of the CDK5 site of the postsynaptic protein DARPP-32 (dopamine- and cAMP-regulated phosphoprotein, molecular mass of 32 kDa), in the absence or in the presence of a dopamine D1 receptor antagonist, we provide evidence that CDK5 inhibitors potentiate dopaminergic transmission at both presynaptic and postsynaptic locations. These findings, together with the known ability of CDK5 inhibitors to prevent degeneration of dopaminergic neurons, suggest that this class of compounds could potentially be used as a novel treatment for disorders associated with dopamine deficiency, such as Parkinson's disease.     

S-Nitrosylation activates Cdk5 and contributes to synaptic spine loss induced by beta-amyloid peptide

The activity of Cdk5 and its regulatory subunit p35 is thought to be important in both normal brain function and neurodegenerative disease pathogenesis. Increased Cdk5 activity, via proteolytic cleavage of p35 to a p25 fragment by the calcium-activated protease calpain or by phosphorylation at Cdk5(Tyr15), can contribute to neurotoxicity. Nonetheless, our knowledge of regulation of Cdk5 activity in disease states is still emerging. Here we demonstrate that Cdk5 is activated by S-nitrosylation or reaction of nitric oxide (NO)-related species with the thiol groups of cysteine residues 83 and 157, to form SNO-Cdk5. We then show that S-nitrosylation of Cdk5 contributes to amyloid-β (Aβ) peptide-induced dendritic spine loss. Furthermore, we observed significant levels of SNO-Cdk5 in postmortem Alzheimer's disease (AD) but not in normal human brains. These findings suggest that S-nitrosylation of Cdk5 is an aberrant regulatory mechanism of enzyme activity that may contribute to the pathogenesis of AD.     

Regulation of cyclin-dependent kinase 5 catalytic activity by phosphorylation

Cyclin-dependent kinase 5 (cdk5) is found in an active form only in neuronal cells. Activation by virtue of association with the cyclin-like neuronal proteins p35 (or its truncated form p25) and p39 is the only mechanism currently shown to regulate cdk5 catalytic activity. In addition to cyclin binding, other members of the cdk family require for maximal activation phosphorylation of a Ser/Thr residue (Thr(160) in the case of cdk-2) that is conserved in all cdks except cdk8. This site is phosphorylated by cdk-activating kinases, which, however, do not phosphorylate cdk5. To examine the possible existence of a phosphorylation-dependent regulatory mechanism in the case of cdk5, we have metabolically labeled PC12 cells with (32)P(i) and shown that the endogenous cdk5 is phosphorylated. Bacterially expressed cdk5 also can be phosphorylated by PC12 cell lysates. Phosphorylation of cdk5 by a PC12 cell lysate results in a significant increase in cdk5/p25 catalytic activity. Ser(159) in cdk5 is homologous to the regulatory Thr(160) in cdk2. A Ser(159)-to-Ala (S159A) cdk5 mutant did not show similar activation, which suggests that cdk5 is also regulated by phosphorylation at this site. Like other members of the cdk family, cdk5 catalytic activity is influenced by both p25 binding and phosphorylation. We show that the cdk5-activating kinase (cdk5AK) is distinct from the cdk-activating kinase (cyclin H/cdk7) that was reported previously to neither phosphorylate cdk5 nor affect its activity. We also show that casein kinase I, but not casein kinase II, can phosphorylate and activate cdk5 in vitro.     

Increased insulin sensitivity in mice lacking p85β subunit of phosphoinositide 3-kinase

On the basis of ex vivo studies using insulin-responsive cells, activation of a Class IA phosphoinositide 3-kinase (PI3K) seems to be required for a wide variety of cellular responses downstream of insulin. The Class IA PI3K enzymes are heterodimers of catalytic and regulatory subunits. In mammals, insulin-responsive tissues express both the p85alpha and p85beta isoforms of the regulatory subunit. Surprisingly, recent studies have revealed that disruption of the p85alpha gene in the mouse (p85alpha(-/-) mice) results in hypoglycemia with decreased plasma insulin, and the p85alpha(+/-) mice exhibit significantly increased insulin sensitivity. These results suggest either that p85alpha negatively regulates insulin signaling, or that p85beta, which mediates the major fraction of Class IA PI3K signaling in the absence of p85alpha, is more efficient than p85alpha in mediating insulin responses. To address this question, we have generated mice in which the p85beta gene is deleted (p85beta(-/-) mice). As with the p85alpha(-/-) mice, the p85beta(-/-) mice showed hypoinsulinemia, hypoglycemia, and improved insulin sensitivity. At the molecular level, PI3K activity associated with phosphotyrosine complexes was preserved despite a 20-30% reduction in the total protein level of the regulatory subunits. Moreover, insulin-induced activation of AKT was significantly up-regulated in muscle from the p85beta(-/-) mice. In addition, insulin-dependent tyrosine phosphorylation of insulin receptor substrate-2 was enhanced in the p85beta(-/-) mice, a phenotype not observed in the p85alpha(-/-) mice. These results indicate that in addition to their roles in recruiting the catalytic subunit of PI3K to the insulin receptor substrate proteins, both p85alpha and p85beta play negative roles in insulin signaling.     

Regulation of cell cycle molecules by the Ras effector system

Eukaryotic cell cycle progression is driven by an ordered array of phosphorylation events that are specifically catalyzed by members of CDK (cyclin-dependent kinase) family serine/threonine protein kinases, each consisting of a catalytic subunit CDK and a positive regulatory subunit cyclin. In mammalian somatic cells extracellular cues act mainly during the G1 phase to regulate the activity of D type cyclin-dependent CDKs, which, in turn, serve as key regulators of G1--S phase progression by phosphorylating and functionally inactivating the tumor suppressor retinoblastoma (Rb) protein. The small molecular weight G protein Ras has been implicated as a crucial molecule that transduces extracellular growth stimuli into intracellular signals. Recent studies, including our own, have demonstrated that maintained cellular Ras activity is required until late in the G1 phase for inactivation of the Rb protein and the G1/S transition and mediates both upregulation of cyclin D1 and downregulation of p27kip1 CDK inhibitor.     

The role of p27(Kip1) phosphorylation at serine 10 in the migration of malignant glioma cells in vitro

Until recently, Cip/Kip members were almost solely viewed as nuclear proteins with a principal function of inhibiting cyclin/cyclin dependent kinase (CDK) activity and hence, inhibiting cell cycle progression. P27(Kip1) (hereafter p27) belongs to the Cip/Kip family that binds and inhibits all the cyclin/CDK complexes, thus often referred as a universal CDK inhibitor. However, emerging studies now suggest that Cip/Kip proteins play additional roles outside of the nucleus. Indeed, previous reports have linked p27 to the regulation of actin dynamics and cell migration. In this study, we constructed a model of migration-activated glioma cells by using the migration-stimulating substrate, a kind of ECM, laminin in vitro. Our results present evidence that laminin drives glioma cell migration without altering cell proliferation. Further, actively migrating cells which expressioned high phosphorylation of p27 at Ser10, and induced its cytoplasmic localization. In this process, Jab1 and CRM1 were also involved. Thus phosphorylation of p27 at Ser10 is necessary for both cytoplasmic localization and induction of cell migration. These observations solidified a genetic role of p27 in cell migration and this was independent of cyclin/CDK inhibition. Eventually, we transiently transfected p27S10A into T98G glioma cells, found that overexpression of p27S10A inhibited cell migration but not cell proliferation. These data linked phosphorylation of p27 at Ser10 and cell motility. Therefore, the major phosphorylation site at Ser10 of p27 played a pivotal role in the migration of malignant glioma cells.     

PI3 kinase and not p42/p44 appears to be implicated in the protection conferred by ischemic preconditioning

Ischemic preconditioning results in an immediate phase of protection against lethal ischemia/reperfusion injury that is comprised of both irreversible necrosis and programmed cell death, apoptosis. We hypothesized that preconditioning may activate putative anti-apoptotic pathways, through the induction of either phosphatidyl inositol 3-OH kinase (PI3 kinase) or p42/p44 extracellular receptor kinase, attenuating total cell death. Isolated perfused rat hearts were preconditioned with two cycles of 5 min ischemia and 10 min reperfusion. Then they were frozen for Western blot analysis or subjected to 35 min regional ischemia and 120 min reperfusion prior to infarct size assessment. Selective PI3 kinase inhibitors, wortmannin (W, 100 n M) and LY294002 (LY, 15 microM) and the p42/p44 inhibitor, PD 98059 (PD, 10 and 50 microM), were individually infused during the preconditioning protocol. One further group of hearts received both inhibitors (W and PD). The results were expressed as percentage of infarction within the risk zone. Inhibition of PI3 kinase by either W or LY partially abrogated the infarct sparing effect of ischemic preconditioning (I/R%: 44.6+/-2.7 in C, 17.6+/-2.0 in IP, vs 32.2+/-4.2 in W, and 30.9+/-2.6 in LY, P<0.05). Inhibition of ERK phosphorylation however, had no significant effect upon infarct size reduction (17.6+/-2.0 in ischemic preconditioning vs 21.4+/-3.0 in IP+10 microM PD and 15.2+/-1.4 in IP+50 microM PD, P>0.05). Western blot analysis confirmed that PD abrogated the phosphorylation of p42/p44 and LY the phosphorylation of AKT. Combined inhibition with PD+W failed to further attenuate protection (27.6+/-1.3%, P>0.1). These data appear to demonstrate that the PI3 kinase, but not the p42/p44 cascade, is implicated in early ischemic preconditioning.     

Hyperphosphorylated tau and neurofilament and cytoskeletal disruptions in mice overexpressing human p25, an activator of cdk5

Hyperphosphorylation of microtubule-associated proteins such as tau and neurofilament may underlie the cytoskeletal abnormalities and neuronal death seen in several neurodegenerative diseases including Alzheimer's disease. One potential mechanism of microtubule-associated protein hyperphosphorylation is augmented activity of protein kinases known to associate with microtubules, such as cdk5 or GSK3beta. Here we show that tau and neurofilament are hyperphosphorylated in transgenic mice that overexpress human p25, an activator of cdk5. The p25 transgenic mice display silver-positive neurons using the Bielschowsky stain. Disturbances in neuronal cytoskeletal organization are apparent at the ultrastructural level. These changes are localized predominantly to the amygdala, thalamus/hypothalamus, and cortex. The p25 transgenic mice display increased spontaneous locomotor activity and differences from control in the elevated plus-maze test. The overexpression of an activator of cdk5 in transgenic mice results in increased cdk5 activity that is sufficient to produce hyperphosphorylation of tau and neurofilament as well as cytoskeletal disruptions reminiscent of Alzheimer's disease and other neurodegenerative diseases.     

Epigenetic inactivation of INK4/CDK/RB cell cycle pathway in acute leukemias

Dysregulation of cell cycle is important in oncogenesis. We analyzed the inactivation of the INK4 family CKI/CDK/RB pathway by gene promoter hypermethylation in leukemogenesis. The methylation-specific polymerase chain reaction (MSP) with primers for methylated (M-MSP) and unmethylated (U-MSP) alleles of the p15, p16, p18, and RB genes was used to study five leukemic cell lines, 50 acute myeloid leukemia (AML) and 25 acute lymphoblastic leukemia (ALL) samples. None of the leukemic cell lines showed p18 and RB methylation. p15 was methylated in Raji, while p16 was methylated in U937 and Raji. In NB4 and Jurkat, both alleles of p15 and p16 appeared to be deleted. At diagnosis, p15 methylation occurred in 29 (58%) AML patients, and 10 (40.0%) ALL patients. p16 methylation occurred in two (4%) AML and two (8%) ALL patients. Only one each of AML and ALL patients had concurrent p15 and p16 methylation. None of the patients had methylation of p18 or RB. In AML, p15 methylation was associated with M2 subtype (p=0.018). Patients with and without p15 methylation had similar complete remission (CR) rates and projected 5-year overall survival (OS) or disease-free survival (DFS). Therefore, methylation inactivation of the INK4/CDK/RB pathway in leukemia involved primarily p15 and occasionally p16, but not p18 or RB. In AML, p15 gene methylation was associated with the M2 subtype, but was not prognostic for CR, OS, or DFS.     

Tranilast inhibits vascular smooth muscle cell growth and intimal hyperplasia by induction of p21(waf1/cip1/sdi1) and p53

Tranilast, which is an antiallergic drug, has a potent effect on preventing postangioplasty restenosis. To elucidate this mechanism, we studied the effect of tranilast on the proliferation of vascular smooth muscle cells (SMCs) in vitro and in vivo. Tranilast decreased the growth rate of SMCs stimulated by either 10% FBS or platelet-derived growth factor. The IC50 value, evaluated as cell number, was 100 micromol/L. These inhibitory effects were associated with inhibition of the retinoblastoma gene product (pRb) phosphorylation. Because pRb phosphorylation is regulated by cyclin-dependent kinases (CDK), we investigated CDK2 and CDK4 activities and the expression of CDK inhibitor p21(waf1/cip1/sdi1) (p21). When SMCs were stimulated by 10% FBS or platelet-derived growth factor, CDK2 and CDK4 activities reached a maximum near the G1/S transition. Tranilast suppressed their activities by >80% without reduction of CDK2/cyclin E and CDK4/cyclin D1 protein levels. These inhibitory effects were associated with enhanced expression of p21 and elevated complexing of p21 with CDK2/CDK4. Next, rat balloon-injured carotid artery was analyzed for intimal thickening and p21 expression. Tranilast-treated rats had a 70% (P<0.001) smaller neointima/media area ratio at 14 days after balloon injury compared with the controls. Immunohistochemical staining demonstrated that, in tranilast-treated rats, p21 was already present in the neointima at day 7 and strongly expressed throughout the neointima at day 14. In control rats, p21 was not observed in the neointima at day 7 but was sparsely expressed at day 14. These data demonstrate that inhibition of CDK2/CDK4 activities by the increased expression of p21 may be one mechanism by which tranilast inhibits SMC proliferation and prevents postangioplasty restenosis.     

APP17肽对糖尿病小鼠微管结构和tau蛋白磷酸化有关酶类的影响

目的　观察ＡＰＰ１７ 肽对糖尿病小鼠微管结构及ｔａｕ 蛋白磷酸化有关酶类变化的影响。　方法　用链脲佐菌素（ＳＴＺ）诱发小鼠糖尿病模型,并皮下注射ＡＰＰ１７ 肽对糖尿病小鼠进行保护。４周后,小鼠灌注固定。取各组小鼠的海马ＣＡ１ 区组织进行光镜和电镜观察。行冰冻切片,做糖原合成激酶（ＧＳＫ３）、细胞周期动态激酶（ＣＤＫ５）、蛋白磷酸酶１（ＰＰ１）、蛋白磷酸酶２Ａ（ＰＰ２Ａ）、微管蛋白（ｔｕｂｕｌｉｎ）免疫组化染色。　结果　（１）糖尿病小鼠海马神经元的微管结构出现明显的断裂或溶解现象,而用ＡＰＰ１７ 肽保护可使这种现象得到明显改善;（２）糖尿病小鼠海马内表达磷酸化ｔａｕ 蛋白的两种蛋白激酶ＧＳＫ３、ＣＤＫ５ 及表达脱ｔａｕ 蛋白磷酸化的磷酸酯酶ＰＰ１、ＰＰ２Ａ 均明显减少,用ＡＰＰ１７ 肽保护的糖尿病小鼠上述酶类表达均恢复至正常水平。　结论　糖尿病小鼠脑组织代谢紊乱,蛋白质合成降低。ＡＰＰ１７ 肽可能是一种能量代谢激动剂,通过保持糖尿病小鼠海马神经元ｔａｕ 蛋白的正常磷酸化而维护微管的正常结构。     

Protein kinases potentially capable of catalyzing the phosphorylation of p47-phox in normal neutrophils and neutrophils of patients with chronic granulomatous disease

A procedure for uncovering novel protein kinases was used to search for enzymes in neutrophils that may catalyze the phosphorylation of the 47- Kd subunit of the NADPH oxidase system (p47-phox). This component of the oxidase can undergo phosphorylation on multiple sites. The method is based on the ability of renatured kinases to recognize exogenous substrates fixed in gels. We report that neutrophils contain several uncharacterized protein kinases that catalyze the phosphorylation of a peptide substrate that corresponds to amino acid residues 297 through 331 of p47-phox. Some of these enzymes are strongly activated on stimulation of the cells with phorbol 12-myristate 13-acetate (PMA). The results indicate that the phosphorylation of p47-phox in neutrophils may be more complicated than previously appreciated and may involve multiple protein kinases. In addition, we have examined both the renaturable protein kinases and the properties of protein kinase C (PKC) in neutrophils from patients with chronic granulomatous disease (CGD) who are deficient in cytochrome b558. Previous studies have shown that these cells exhibit incomplete phosphorylation of p47-phox on stimulation. In this study, we were unable to detect any alterations in the renaturable protein kinases or PKC in CGD neutrophils that could explain these defects in the phosphorylation of p47-phox.     

Tau pathology involves protein phosphatase 2A in Parkinsonism-dementia of Guam

Parkinsonism-dementia (PD) of Guam is a neurodegenerative disease with parkinsonism and early-onset Alzheimer-like dementia associated with neurofibrillary tangles composed of hyperphosphorylated microtubule-associated protein, tau. β-N-methylamino-l-alanine (BMAA) has been suspected of being involved in the etiology of PD, but the mechanism by which BMAA leads to tau hyperphosphorylation is not known. We found a decrease in protein phosphatase 2A (PP2A) activity associated with an increase in inhibitory phosphorylation of its catalytic subunit PP2Ac at Tyr³⁰⁷ and abnormal hyperphosphorylation of tau in brains of patients who had Guam PD. To test the possible involvement of BMAA in the etiopathogenesis of PD, we studied the effect of this environmental neurotoxin on PP2A activity and tau hyperphosphorylation in mouse primary neuronal cultures and metabolically active rat brain slices. BMAA treatment significantly decreased PP2A activity, with a concomitant increase in tau kinase activity resulting in elevated tau hyperphosphorylation at PP2A favorable sites. Moreover, we found an increase in the phosphorylation of PP2Ac at Tyr³⁰⁷ in BMAA-treated rat brains. Pretreatment with metabotropic glutamate receptor 5 (mGluR5) and Src antagonists blocked the BMAA-induced inhibition of PP2A and the abnormal hyperphosphorylation of tau, indicating the involvement of an Src-dependent PP2A pathway. Coimmunoprecipitation experiments showed that BMAA treatment dissociated PP2Ac from mGluR5, making it available for phosphorylation at Tyr³⁰⁷. These findings suggest a scenario in which BMAA can lead to tau pathology by inhibiting PP2A through the activation of mGluR5, the consequent release of PP2Ac from the mGluR5–PP2A complex, and its phosphorylation at Tyr³⁰⁷ by Src.Indigenous residents and immigrants of the Pacific Island of Guam suffer from a high incidence of a progressive and fatal neurodegenerative tauopathy called “parkinsonism-dementia” (PD). This is a long-latency disease with phenotypic characteristics of idiopathic parkinsonism with an early-onset dementia. The disorder is found together with a second high-incidence disease, amyotrophic lateral sclerosis (ALS). The incidence of both diseases has declined dramatically over the past half century, suggesting an environmental etiology (1). The neuropathology of PD is hallmarked by neurofibrillary tangles (NFTs) (2) of paired helical filaments (PHFs) composed of abnormally hyperphosphorylated forms of the microtubule-associated protein tau. The NFTs found in PD brains are ultra-structurally and biochemically similar to those in Alzheimer’s disease (AD) (3). In contrast to AD pathology, however, studies using postmortem brains confirmed the absence of amyloid β plaques in most PD cases (2). Many studies have demonstrated that abnormal hyperphosphorylation and aggregation of tau are crucial to neurodegeneration in AD and tauopathies (4). Although the mechanism leading to the formation of NFTs is still obscure, it has been well recognized that an imbalanced regulation in protein kinases and protein phosphatases can directly cause AD-like tau hyperphosphorylation (4). Among the various kinases, glycogen synthase kinase-3β (GSK-3β), cyclin-dependent kinase 5, Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), MAP kinase kinase (MEK 1/2), ERK 1/2, protein kinase A, casein kinase 1, and p70 S6 kinase have been most often implicated in tau phosphorylation (4). Among the phosphatases, protein phosphatase 2A (PP2A) accounts for ∼70% of tau phosphatase activity in the human brain (5). PP2A is responsible for dephosphorylating most of the hyperphosphorylated sites of tau, and its activity is compromised in the AD brain (6). Given the increasing evidence that PD histopathologically and genetically should be classified as a tauopathy (7, 8), identifying the protein kinases/phosphatases and the associated signaling cascades that regulate tau phosphorylation is of crucial importance.Several etiological factors, especially environmental factors, genetic susceptibility, and metabolic alterations, have been proposed to contribute to PD. The nonprotein amino acid β-N-methylamino-l-alanine (BMAA) has been implicated as a potential environmental factor in PD, ALS, AD, and other neurodegenerative disorders (9–11). Although the neurotoxic effects of BMAA are not conclusive, BMAA concentrations as low as 30 µM can cause selective death of motor neurons (12), and a 10-µM concentration can potentiate neuronal injury induced by exposure to amyloid-β, 1-methyl-4-phenylpyridinium, or methylmercury in mixed cortical cultures (13). The mechanism of BMAA toxicity is complex and several fold: (i) BMAA displays agonistic properties for NMDA (14), AMPA/kainate (12), and metabotropic glutamate receptor 5 (mGluR5) (14, 15) and for mGluR1 receptors (16) in a cell-specific manner, and (ii) it increases intracellular calcium levels (17) and oxidative stress (15). BMAA was reported to induce learning and memory deficits accompanied by neuronal cell death in rats (18), although the underlying molecular mechanism by which BMAA affects tau phosphorylation and eventually toxicity has not been established.In the present study, we report the involvement of PP2A signaling in PD and show that BMAA can produce similar changes by activating the mGluR5 receptor, leading to the dissociation of PP2A from the receptor, followed by its phosphorylation at Tyr³⁰⁷ by Src, a nonreceptor tyrosine kinase. Phosphorylation of PP2A at Tyr³⁰⁷ inhibits its activity and leads to hyperphosphorylation of tau. Together these data reveal an etiopathogenic mechanism of neurofibrillary pathology in PD, AD, and ALS involving mGluR5-dependent inactivation of PP2A.