Altered p59Fyn kinase expression accompanies disease progression in Alzheimer's disease: implications for its functional role

Tyrosine phosphorylation of tau by the SRC family kinases lck and fyn

Background

Tau protein is the principal component of the neurofibrillary tangles found in Alzheimer's disease, where it is hyperphosphorylated on serine and threonine residues, and recently phosphotyrosine has been demonstrated. The Src-family kinase Fyn has been linked circumstantially to the pathology of Alzheimer's disease, and shown to phosphorylate Tyr18. Recently another Src-family kinase, Lck, has been identified as a genetic risk factor for this disease.

Results

In this study we show that Lck is a tau kinase. In vitro, comparison of Lck and Fyn showed that while both kinases phosphorylated Tyr18 preferentially, Lck phosphorylated other tyrosines somewhat better than Fyn. In co-transfected COS-7 cells, mutating any one of the five tyrosines in tau to phenylalanine reduced the apparent level of tau tyrosine phosphorylation to 25-40% of that given by wild-type tau. Consistent with this, tau mutants with only one remaining tyrosine gave poor phosphorylation; however, Tyr18 was phosphorylated better than the others.

Conclusions

Fyn and Lck have subtle differences in their properties as tau kinases, and the phosphorylation of tau is one mechanism by which the genetic risk associated with Lck might be expressed pathogenically.

Tyrosine phosphorylation of tau accompanies disease progression in transgenic mouse models of tauopathy

AIM

Tau protein is a prominent component of paired helical filaments in Alzheimer's disease (AD) and other tauopathies. While the abnormal phosphorylation of tau on serine and threonine has been well established in the disease process, its phosphorylation on tyrosine has only recently been described. We previously showed that the Src family non-receptor tyrosine kinases (SFKs) Fyn and Src phosphorylate tau on Tyr18 and that phospho-Tyr18-tau was present in AD brain. In this study, we have investigated the appearance of phospho-Tyr18-tau, activated SFK and proliferating cell nuclear antigen (PCNA) during disease progression in a mouse model of human tauopathy.

METHODS

We have used JNPL3, which expresses human tau with P301L mutation, and antibodies specific for phospho-Tyr18-tau (9G3), ser/thr phosphorylated tau (AT8), activated SFK and PCNA. Antibody staining was viewed by either epifluorescence or confocal microscopy.

RESULTS

Phospho-Tyr18-tau appeared concurrently with AT8-reactive tau as early as 4 months in JNPL3. Some 9G3-positive cells also contained activated SFKs and PCNA. We also investigated the triple transgenic mouse model of AD and found that unlike the JNPL3 model, the appearance of 9G3 reactivity did not coincide with AT8 in the hippocampus, suggesting that the presence of APP/presenilin influences tau phosphorylation. Also, Thioflavin S-positive plaques were 9G3-negative, suggesting that phospho-Tyr18-tau is absent from the dystrophic neurites of the mouse triple transgenic brain.

CONCLUSIONS

Our results provide evidence for the association of tyrosine-phosphorylated tau with mechanisms of neuropathogenesis and indicate that SFK activation and cell cycle activation are also involved in JNPL3.

Molecular mechanisms of neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Beta-amyloid protein (25-35) disrupts hippocampal network activity: role of Fyn-kinase

Early cognitive deficit characteristic of early Alzheimer's disease seems to be produced by the soluble forms of beta-amyloid protein. Such cognitive deficit correlates with neuronal network dysfunction that is reflected as alterations in the electroencephalogram of both Alzheimer patients and transgenic murine models of such disease. Correspondingly, recent studies have demonstrated that chronic exposure to betaAP affects hippocampal oscillatory properties. However, it is still unclear if such neuronal network dysfunction results from a direct action of betaAP on the hippocampal circuit or it is secondary to the chronic presence of the protein in the brain. Therefore, we aimed to explore the effect of acute exposure to betaAP(25-35) on hippocampal network activity both in vitro and in vivo, as well as on intrinsic and synaptic properties of hippocampal neurons. We found that betaAP(25-35), reversibly, affects spontaneous hippocampal population activity in vitro. Such effect is not produced by the inverse sequence betaAP(35-25) and is reproduced by the full-length peptide betaAP(1-42). Correspondingly betaAP(25-35), but not the inverse sequence betaAP(35-25), reduces theta-like activity recorded from the hippocampus in vivo. The betaAP(25-35)-induced disruption in hippocampal network activity correlates with a reduction in spontaneous neuronal activity and synaptic transmission, as well as with an inhibition in the subthreshold oscillations produced by pyramidal neurons in vitro. Finally, we studied the involvement of Fyn-kinase on the betaAP(25-35)-induced disruption in hippocampal network activity in vitro. Interestingly, we found that such phenomenon is not observed in slices obtained from Fyn-knockout mice. In conclusion, our data suggest that betaAP acutely affects proper hippocampal function through a Fyn-dependent mechanism. We propose that such alteration might be related to the cognitive impairment observed, at least, during the early phases of Alzheimer's disease.

[Neuronal isoforms of Src, Fyn and Lck tyrosine kinases: A specific role for p56lckN in neuron protection]

The two main tyrosine kinases (TK) in the brain are p60Src and p59Fyn, expressed as specific isoforms (p60SrcNI, p60SrcNI+NII and p59fynB). They play a pivotal role in some major processes such as neuronal growth and myelinisation. Another member of this TK family was then reported in brain, the p56lck. Its name Lck (lymphocyte cell kinase) indicates its cellular specificity observed initially, so its presence in the brain was intriguing. But no further studies were performed to understand its role in brain until recent clinical studies on Alzheimer patients' brains. One study reveals a decreased p56lck level in the brains of these patients while another study shows an association between one peculiar SNP (single nucleotide polymorphism) of the lck gene and some cases of the disease. These new data prompt us to reinvestigate the original biochemical data and to confront them with the present knowledge. This analysis suggests some hypothesis concerning both the Lck protein expressed in the brain (rather an isoform than the lymphocyte protein itself) and its role (to maintain the neuronal survival presumably by protecting them from inflammation, the main pathway that leads to neuron degeneracy).

Mediators of tau phosphorylation in the pathogenesis of Alzheimer's disease

The need for disease-modifying drugs for Alzheimer's disease has become increasingly important owing to escalating disease prevalence and the associated socio-economic burden. Until recently, reducing brain amyloid accumulation has been the main therapeutic focus; however, increasing evidence suggests that targeting abnormal tau phosphorylation could be beneficial. Tau is phosphorylated by several protein kinases and this is balanced by dephosphorylation by protein phosphatases. Phosphorylation at specific sites can influence the physiological functions of tau, including its role in binding to and stabilizing the neuronal cytoskeleton. aberrant phosphorylation of tau could render it susceptible to potentially pathogenic alterations, including conformational changes, proteolytic cleavage and aggregation. While strategies that reduce tau phosphorylation in transgenic models of disease have been promising, our understanding of the mechanisms through which tau becomes abnormally phosphorylated in disease is lacking.

Lovastatin ameliorates alpha-synuclein accumulation and oxidation in transgenic mouse models of alpha-synucleinopathies

Alpha-synuclein (alpha-syn) aggregation is a neuropathological hallmark of many diseases including Dementia with Lewy Bodies (DLB) and Parkinson's Disease (PD), collectively termed the alpha-synucleinopathies. The mechanisms underlying alpha-syn aggregation remain elusive though emerging science has hypothesized that the interaction between cholesterol and alpha-syn may play a role. Cholesterol has been linked to alpha-synucleinopathies by recent work suggesting cholesterol metabolites appear to accelerate alpha-syn fibrillization. Consistent with these findings, cholesterol-lowering agents have been demonstrated to reduce alpha-syn accumulation and the associated neuronal pathology in vitro. In this context, this study sought to investigate the in vivo effects of the cholesterol synthesis inhibitor lovastatin on alpha-syn aggregation in two different transgenic (Tg) mouse models that neuronally overexpress human alpha-syn. Lovastatin-treated mice displayed significantly reduced plasma cholesterol levels and levels of oxidized cholesterol metabolites in the brain in comparison to saline-treated controls. Immunohistochemical analysis demonstrated a significant reduction of neuronal alpha-syn aggregates and alpha-syn immunoreactive neuropil in the temporal cortex of lovastatin-treated Tg mice in comparison to saline-treated alpha-syn Tg controls. Consistently, immunoblot analysis of mouse brain homogenates showed a reduction in levels of total and oxidized alpha-syn in lovastatin-treated alpha-syn Tg mice in comparison to saline-treated alpha-syn Tg controls. The reduced alpha-syn accumulation in lovastatin-treated mice was associated with abrogation of neuronal pathology. The results from this study demonstrate that lovastatin administration can reduce alpha-syn aggregation and associated neuropathology and support the possibility that treatment with cholesterol-lowering agents may be beneficial for patients with PD and/or DLB.

GFAP reactivity, apolipoprotein E redistribution and cholesterol reduction in human astrocytes treated with alpha-synuclein

Alpha-synuclein (alpha-syn) is an abundant neuronal protein expressed at the synapse. In neurodegenerative disease alpha-syn accumulates in the extracellular space. Astrocytes present at neural synapses are thought to contribute to synaptogenesis through cholesterol release and normally exhibit increased glial fibrillary acid protein (GFAP) reactivity and apolipoprotein E (apoE) expression in neurodegenerative disease states. We proposed that extracellular alpha-syn treatment of human astrocytes would impact cholesterol levels and expression of GFAP and apolipoprotein E (apoE). Human astrocytes were treated with alpha-syn at different concentrations and time points to determine the effective membrane permeability of the peptide. After alpha-syn treatment, we analyzed apoE and cholesterol levels in the astrocyte membrane. Lastly, we performed immunocytochemistry for GFAP in control and alpha-syn treated cells. Our results indicate membrane apoE was reduced and redistributed from a nuclear and membranous dominated expression to the cytosol. Cholesterol levels were also reduced in the astrocyte cell membrane. GFAP expression was sharply increased in alpha-syn treated cells indicating that alpha-syn may contribute to reactive gliosis. Our results support the conclusion that astrocytes play a role in pathological mechanisms in synucleinopathies.

Nicotinic acetylcholine receptor signalling: roles in Alzheimer's disease and amyloid neuroprotection

Alzheimer's disease (AD), the major contributor to dementia in the elderly, involves accumulation in the brain of extracellular plaques containing the beta-amyloid protein (Abeta) and intracellular neurofibrillary tangles of hyperphosphorylated tau protein. AD is also characterized by a loss of neurons, particularly those expressing nicotinic acetylcholine receptors (nAChRs), thereby leading to a reduction in nAChR numbers. The Abeta(1-42) protein, which is toxic to neurons, is critical to the onset and progression of AD. The discovery of new drug therapies for AD is likely to be accelerated by an improved understanding of the mechanisms whereby Abeta causes neuronal death. We examine the evidence for a role in Abeta(1-42) toxicity of nAChRs; paradoxically, nAChRs can also protect neurons when activated by nicotinic ligands. Abeta peptides and nicotine differentially activate several intracellular signaling pathways, including the phosphatidylinositol 3-kinase/v-akt murine thymoma viral oncogene homolog pathway, the extracellular signal-regulated kinase/mitogen-activated protein kinase, and JAK-2/STAT-3 pathways. These pathways control cell death or survival and the secretion of Abeta peptides. We propose that understanding the differential activation of these pathways by nicotine and/or Abeta(1-42) may offer the prospect of new routes to therapy for AD.

The microtubule-associated protein tau is phosphorylated by Syk

Aberrant phosphorylation of tau protein on serine and threonine residues has been shown to be critical in neurodegenerative disorders called tauopathies. An increasing amount of data suggest that tyrosine phosphorylation of tau might play an equally important role in pathology, with at least three putative tyrosine kinases of tau identified to date. It was recently shown that the tyrosine kinase Syk could efficiently phosphorylate alpha-synuclein, the aggregated protein found in Parkinson's disease and other synucleinopathies. We report herein that Syk is also a tau kinase, phosphorylating tau in vitro and in CHO cells when both proteins are expressed exogenously. In CHO cells, we have also demonstrated by co-immunoprecipitation that Syk binds to tau. Finally, by site-directed mutagenesis substituting the tyrosine residues of tau with phenylalanine, we established that tyrosine 18 was the primary residue in tau phosphorylated by Syk. The identification of Syk as a common tyrosine kinase of both tau and alpha-synuclein may be of potential significance in neurodegenerative disorders and also in neuronal physiology. These results bring another clue to the intriguing overlaps between tauopathies and synucleinopathies and provide new insights into the role of Syk in neuronal physiology.

Statins reduce neuronal alpha-synuclein aggregation in in vitro models of Parkinson's disease

Aggregation of alpha-synuclein (alpha-syn) is believed to play a critical role in the pathogenesis of disorders such as dementia with Lewy bodies and Parkinson's disease. The function of alpha-syn remains unclear, although several lines of evidence suggest that alpha-syn is involved in synaptic vesicle trafficking probably via lipid binding. Moreover, interactions with cholesterol and lipids have been shown to be involved in alpha-syn aggregation. In this context, the main objective of this study was to determine if statins--cholesterol synthesis inhibitors--might interfere with alpha-syn accumulation in cellular models. For this purpose, we studied the effects of lovastatin, simvastatin, and pravastatin on the accumulation of alpha-syn in a stably transfected neuronal cell line and in primary human neurons. Statins reduced the levels of alpha-syn accumulation in the detergent insoluble fraction of the transfected cells. This was accompanied by a redistribution of alpha-syn in caveolar fractions, a reduction in oxidized alpha-syn, and enhanced neurite outgrowth. In contrast, supplementation of the media with cholesterol increased alpha-syn aggregation in detergent insoluble fractions of transfected cells and was accompanied by reduced neurite outgrowth. Taken together, these results suggest that regulation of cholesterol levels with cholesterol inhibitors might be a novel approach for the treatment of Parkinson's disease.

Fyn modulation of Dab1 effects on amyloid precursor protein and ApoE receptor 2 processing

Dab1 is an intracellular adaptor protein that interacts with amyloid precursor protein (APP) and apoE receptor 2 (apoEr2), increases their levels on the cell surface, and increases their cleavage by alpha-secretases. To investigate the mechanism underlying these alterations in processing and trafficking of APP and apoEr2, we examined the effect of Fyn, an Src family-tyrosine kinase known to interact with and phosphorylate Dab1. Co-immunoprecipitation, co-immunostaining, and fluorescence lifetime imaging demonstrated an association between Fyn and APP. Fyn induced phosphorylation of APP at Tyr-757 of the (757)YENPTY(762) motif and increased cell surface expression of APP. Overexpression of Fyn alone did not alter levels of sAPPalpha or cytoplasmic C-terminal fragments, although it significantly decreased production of Abeta. However, in the presence of Dab1, Fyn significantly increased sAPPalpha and C-terminal fragments. Fyn-induced APP phosphorylation and cell surface levels of APP were potentiated in the presence of Dab1. Fyn also induced phosphorylation of apoEr2 and increased its cell surface levels and, in the presence of Dab1, affected processing of its C-terminal fragment. In vivo studies showed that sAPPalpha was decreased in the Fyn knock-out, supporting a role for Fyn in APP processing. These data demonstrate that Fyn, due in part to its effects on Dab1, regulates the phosphorylation, trafficking, and processing of APP and apoEr2.

Tau impacts on growth-factor-stimulated actin remodeling

The microtubule-associated protein tau interacts with the SH3 domain of non-receptor Src family protein tyrosine kinases. A potential consequence of the SH3 interaction is the upregulation of tyrosine kinase activity. Here we investigated the activation of Src or Fyn by tau, both in vitro and in vivo. Tau increased the kinase activity in in vitro assays and in transfected COS7 cells. In platelet-derived growth factor (PDGF)-stimulated fibroblasts, tau appeared to prime Src for activation following PDGF stimulation, as reflected by changes in Src-mediated actin rearrangements. In addition, while fibroblasts normally recovered actin stress fibers by 5-7 hours after PDGF stimulation, tau-expressing cells showed sustained actin breakdown. Microtubule association by tau was not required for the observed changes in actin morphology. Inhibition of Src kinases or a mutant deficient in Src interaction reduced the effects, implicating Src family protein tyrosine kinases as a mediator of the effects of tau on actin rearrangements. Our results provide evidence that the interaction of tau with Src upregulates tyrosine kinase activity and that this interaction allows tau to impact on growth-factor-induced actin remodeling.

Effects of the cholesterol-lowering compound methyl-beta-cyclodextrin in models of alpha-synucleinopathy

The aggregation of alpha-synuclein (alpha-syn) is believed to play a critical role in the pathogenesis of disorders such as dementia with Lewy bodies and Parkinson's disease. The function of alpha-syn remains unclear, although several lines of evidence suggest that alpha-syn is involved in synaptic vesicle trafficking, probably via lipid binding, and interactions with lipids have been shown to regulate alpha-syn aggregation. In this context, the main objective of this study was to determine whether methyl-beta-cyclodextrin (MbetaCD), a cholesterol-extracting agent, interfered with alpha-syn accumulation in models of synucleinopathy. For this purpose, we studied the effects of MbetaCD on the accumulation of alpha-syn in a transfected neuronal cell line and in transgenic mice. Immunoblot analysis showed that MbetaCD reduced the level of alpha-syn in the membrane fraction and detergent-insoluble fraction of transfected cells. In agreement with the in vitro studies, treatment of mice with MbetaCD resulted in decreased levels of alpha-syn in membrane fractions and reduced accumulation of alpha-syn in the neuronal cell body and synapses. Taken together, these results suggest that changes in cholesterol and lipid composition using cholesterol-lowering agents may be used as a tool for the treatment of synucleinopathies.

Interleukins, inflammation, and mechanisms of Alzheimer's disease

Alzheimer's disease (AD) is the most common progressive neurodegenerative form of dementia in the elderly and is characterized neuropathologically by neurofibrillary tangles (NFT), amyloid neuritic plaques (NP), and prominent synaptic and eventually neuronal loss. Although the molecular basis of AD is not clearly understood, a neuroinflammatory process, triggered by Abeta42, plays a central role in the neurodegenerative process. This inflammatory process is driven by activated microglia, astrocytes and the induction of proinflammatory molecules and related signaling pathways, leading to both synaptic and neuronal damage as well as further inflammatory cell activation. Epidemiologic data as well as clinical trial evidence suggest that nonsteroidal anti-inflammatory drug (NSAID) use may decrease the incidence of AD, further supporting a role for inflammation in AD pathogenesis. Although the precise molecular and cellular relationship between AD and inflammation remains unclear, interleukins and cytokines might induce activation of signaling pathways leading to futher inflammation and neuronal injury. This chapter will discuss the association between interleukins and neurodegeneration in AD and highlight the significance of genetic and clinical aspects of interleukins in disease expression and progression. As part of an emerging inflammatory signaling network underlying AD pathogenesis, beta-amyloid (Abeta) stimulates the glial and microglial production of interleukins and other cytokines, leading to an ongoing inflammatory cascade and contributing to synaptic dysfunction and loss, and later, neuronal death. Inflammatory pathways involving interleukin and cytokine signaling might suggest potential targets for intervention and influence the development of novel therapies to circumvent synaptic and neuronal dysfunction ultimately leading to AD neurodegeneration.

Disease-related modifications in tau affect the interaction between Fyn and Tau

Microtubule-associated protein tau is the major component of the neurofibrillary tangles of Alzheimer disease (AD) and is genetically linked to frontotemporal dementias (FTDP-17). We have recently shown that tau interacts with the SH3 domain of Fyn, an Src family non-receptor tyrosine kinase, and is tyrosine-phosphorylated by Fyn on Tyr-18. Also, tyrosine-phosphorylated tau is present in the neuropathology of AD. To determine whether alterations in the tau-Fyn interaction might correlate with disease-related factors in AD and FTDP-17, we have performed real-time surface plasmon resonance studies on a panel of 21 tau constructs with Fyn SH3. We report that the interaction between Fyn SH3 and 3R-tau was 20-fold higher than that with 4R-tau. In addition, the affinity between 4R-tau and Fyn SH3 was increased 25-45-fold by phosphorylation-mimicking mutations or by FTDP-17 mutations. In vitro kinase reactions show that tau, with lower affinity SH3 interactions, exhibited a lower level of Tyr-18 phosphorylation under our reaction conditions. Lastly, we have demonstrated that tau is phosphorylated on Tyr-18 in the tau P301L mouse model for tauopathy (JNPL3). In summary, our results suggest that disease-related phosphorylation and missense mutations of tau increase association of tau with Fyn. Because these effects are mediated through the 4R component of the tau population, these results also have implications for the FTDP-17 diseases caused by increased expression of 4R-tau. Our data support a role for the Fyn-tau interaction in neurodegeneration.