Characterization of the in vitro phosphorylation of human tau by tau protein kinase II (cdk5/p20) using mass spectrometry

Biochemical approaches to assess the impact of post-translational modifications on pathogenic tau conformations using recombinant protein

Tau protein is associated with many neurodegenerative disorders known as tauopathies. Aggregates of tau are thought of as a main contributor to neurodegeneration in these diseases. Increasingly, evidence points to earlier, soluble conformations of abnormally modified monomers and multimeric tau as toxic forms of tau. The biological processes driving tau from physiological species to pathogenic conformations remain poorly understood, but certain avenues are currently under investigation including the functional consequences of various pathological tau changes (e.g. mutations, post-translational modifications (PTMs), and protein-protein interactions). PTMs can regulate several aspects of tau biology such as proteasomal and autophagic clearance, solubility, and aggregation. Moreover, PTMs can contribute to the transition of tau from normal to pathogenic conformations. However, our understating of how PTMs specifically regulate the transition of tau into pathogenic conformations is partly impeded by the relative lack of structured frameworks to assess and quantify these conformations. In this review, we describe a set of approaches that includes several in vitro assays to determine the contribution of PTMs to tau's transition into known pathogenic conformations. The approaches begin with different methods to create recombinant tau proteins carrying specific PTMs followed by validation of the PTMs status. Then, we describe a set of biochemical and biophysical assays that assess the contribution of a given PTM to different tau conformations, including aggregation, oligomerization, exposure of the phosphatase-activating domain, and seeding. Together, these approaches can facilitate the advancement of our understanding of the relationships between PTMs and tau conformations.

Tau Protein Biosensors in the Diagnosis of Neurodegenerative Diseases

Tau protein plays a crucial role in diagnosing neurodegenerative diseases. However, performing an assay to detect tau protein on a nanoscale is a great challenge for early diagnosis of diseases. Enzyme-linked immunosorbent assay (ELISA), western-blotting, and molecular-based methods, e.g., PCR and real-time PCR, are the most widely used methods for detecting tau protein. These methods are subject to certain limitations: the need for advanced equipment, low sensitivity, and specificity, to name a few. With the above said, it is necessary to discover advanced and novel methods for monitoring tau protein. Counted among remarkable approaches adopted by researchers, biosensors can largely eliminate the difficulties and limitations associated with conventional methods. The main objective of the present study is to review the latest biosensors developed to detect the tau protein. Furthermore, the problems and limitations of conventional diagnosis methods were discussed in detail.

Zinc in Regulating Protein Kinases and Phosphatases in Neurodegenerative Diseases

Zinc is essential for human growth and development. As a trace nutrient, zinc plays important roles in numerous signal transduction pathways involved in distinct physiologic or pathologic processes. Protein phosphorylation is a posttranslational modification which regulates protein activity, degradation, and interaction with other molecules. Protein kinases (PKs) and phosphatases (PPs), with their effects of adding phosphate to or removing phosphate from certain substrates, are master regulators in controlling the phosphorylation of proteins. In this review, we summarize the disturbance of zinc homeostasis and role of zinc disturbance in regulating protein kinases and protein phosphatases in neurodegenerative diseases, with the focus of that in Alzheimer’s disease, providing a new perspective for understanding the mechanisms of these neurologic diseases.

Amelioration of Tau pathology and memory deficits by targeting 5-HT7 receptor

Tauopathies comprise a heterogeneous family of neurodegenerative diseases characterized by pathological accumulation of hyperphosphorylated Tau protein. Pathological changes in serotonergic signaling have been associated with tauopathy etiology, but the underlying mechanisms remain poorly understood. Here, we studied the role of the serotonin receptor 7 (5-HT7R), in a mouse model of tauopathy induced by overexpressing the human Tau[R406W] mutant associated with inherited forms of frontotemporal dementia. We showed that the constitutive 5-HT7R activity is required for Tau hyperphosphorylation and formation of highly bundled Tau structures (HBTS) through G-protein-independent, CDK5-dependent mechanism. We also showed that 5-HT7R physically interacts with CDK5. At the systemic level, 5-HT7R-mediated CDK5 activation induces HBTS leading to neuronal death, reduced long-term potentiation (LTP), and impaired memory in mice. Specific blockade of constitutive 5-HT7R activity in neurons that overexpressed Tau[R406W] prevents Tau hyperphosphorylation, aggregation, and neurotoxicity. Moreover, 5-HT7R knockdown in the prefrontal cortex fully abrogates Tau[R406W]-induced LTP deficits and memory impairments. Thus, 5-HT7R/CDK5 signaling emerged as a new, promising target for tauopathy treatments.

It's complicated: The relationship between sleep and Alzheimer's disease in humans

Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by an asymptomatic period of amyloid-β (Aβ) deposition as insoluble extracellular plaque, intracellular tau aggregation, neuronal and synaptic loss, and subsequent cognitive dysfunction and dementia. A growing public health crisis, the worldwide prevalence of AD is expected to rise from 46.8 million individuals affected in 2015 to 131.5 million in 2050. Sleep disturbances have been associated with increased future risk of AD. A bi-directional relationship is hypothesized between sleep and AD with sleep disturbances as either markers for AD pathology and/or a mechanism mediating increased risk of AD. In this review, the evidence in humans supporting this complex relationship between sleep and AD will be discussed as well as the therapeutic potential and challenges of treating sleep disturbances to prevent or delay the onset of AD.

Fyn depletion ameliorates tauP301L-induced neuropathology

The Src family non-receptor tyrosine kinase Fyn has been implicated in neurodegeneration of Alzheimer's disease through interaction with amyloid β (Aβ). However, the role of Fyn in the pathogenesis of primary tauopathies such as FTDP-17, where Aβ plaques are absent, is poorly understood. In the current study, we used AAV2/8 vectors to deliver tauP301L to the brains of WT and Fyn KO mice, generating somatic transgenic tauopathy models with the presence or absence of Fyn. Although both genotypes developed tau pathology, Fyn KO developed fewer neurofibrillary tangles on Bielschowsky and Thioflavin S stained sections and showed lower levels of phosphorylated tau. In addition, tauP301L-induced behavior abnormalities and depletion of synaptic proteins were not observed in the Fyn KO model. Our work provides evidence for Fyn being a critical protein in the disease pathogenesis of FTDP-17.

A 3D human brain-like tissue model of herpes-induced Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder that causes cognitive decline, memory loss, and inability to perform everyday functions. Hallmark features of AD-including generation of amyloid plaques, neurofibrillary tangles, gliosis, and inflammation in the brain-are well defined; however, the cause of the disease remains elusive. Growing evidence implicates pathogens in AD development, with herpes simplex virus type I (HSV-1) gaining increasing attention as a potential causative agent. Here, we describe a multidisciplinary approach to produce physiologically relevant human tissues to study AD using human-induced neural stem cells (hiNSCs) and HSV-1 infection in a 3D bioengineered brain model. We report a herpes-induced tissue model of AD that mimics human disease with multicellular amyloid plaque-like formations, gliosis, neuroinflammation, and decreased functionality, completely in the absence of any exogenous mediators of AD. This model will allow for future studies to identify potential downstream drug targets for treating this devastating disease.

A new TAO kinase inhibitor reduces tau phosphorylation at sites associated with neurodegeneration in human tauopathies

In Alzheimer’s disease (AD) and related tauopathies, the microtubule-associated protein tau is highly phosphorylated and aggregates to form neurofibrillary tangles that are characteristic of these neurodegenerative diseases. Our previous work has demonstrated that the thousand-and-one amino acid kinases (TAOKs) 1 and 2 phosphorylate tau on more than 40 residues in vitro. Here we show that TAOKs are phosphorylated and active in AD brain sections displaying mild (Braak stage II), intermediate (Braak stage IV) and advanced (Braak stage VI) tau pathology and that active TAOKs co-localise with both pre-tangle and tangle structures. TAOK activity is also enriched in pathological tau containing sarkosyl-insoluble extracts prepared from AD brain. Two new phosphorylated tau residues (T123 and T427) were identified in AD brain, which appear to be targeted specifically by TAOKs. A new small molecule TAOK inhibitor (Compound 43) reduced tau phosphorylation on T123 and T427 and also on additional pathological sites (S262/S356 and S202/T205/S208) in vitro and in cell models. The TAOK inhibitor also decreased tau phosphorylation in differentiated primary cortical neurons without affecting markers of synapse and neuron health. Notably, TAOK activity also co-localised with tangles in post-mortem frontotemporal lobar degeneration (FTLD) brain tissue. Furthermore, the TAOK inhibitor decreased tau phosphorylation in induced pluripotent stem cell derived neurons from FTLD patients, as well as cortical neurons from a transgenic mouse model of tauopathy (Tau35 mice). Our results demonstrate that abnormal TAOK activity is present at pre-tangles and tangles in tauopathies and that TAOK inhibition effectively decreases tau phosphorylation on pathological sites. Thus, TAOKs may represent a novel target to reduce or prevent tau-associated neurodegeneration in tauopathies.

Molecular Mapping Alzheimer's Disease: MALDI Imaging of Formalin-fixed, Paraffin-embedded Human Hippocampal Tissue

A method for the molecular mapping of formalin-fixed, paraffin-embedded human hippocampal tissue affected by Alzheimer's disease (AD) is presented. This approach utilizes imaging mass spectrometry (IMS) with matrix-assisted laser desorption/ionization (MALDI). The usefulness of this technique in comparing diseased versus nor mal tissue at the molecular level while continuing to maintain topological and morphological integrity is evident in the preliminary findings. The critical correlation of the deparaffination, washing, matrix deposition, and analysis steps in handling the tissue sections and how these steps impact the successful mapping of human hippocampal tissue is clearly demonstrated. By use of this technique we have been able to identify several differences between the hippocampal AD tissue and the control hippocampal tissue. From the observed peptide clip masses we present preliminary identifications of the amyloid-beta peptides known to be prominent in the brains of those with AD. We have obtained high-resolution mass spectra and mass images with 100μm spatial resolution. Future experiments will couple this work with MALDI LIFT experiments to enable top down proteomics of fresh frozen tissue, which is not possible with paraffin-embedded tissues.

Effects of sodium bisulfite with or without procaine derivatives on axons of cultured mouse dorsal root ganglion neurons

BACKGROUND AND OBJECTIVES

Sodium bisulfite (NaHSO3) was clinically used as a preservative agent for local anesthetics but was later suspected to be neurotoxic. However, recent studies reported that NaHSO3 reduces the neurotoxicity of local anesthetics. The purpose of this study was to examine the effects of NaHSO3 with and without procaine on axonal transport in cultured mouse dorsal root ganglion (DRG) neurons.

METHODS

Experiment 1 served to determine the dose-dependent effects of NaHSO3 on axonal transport (DRG neurons were treated with 0.01, 0.1, 1, 10, or 20 mM of NaHSO3), whereas experiment 2 investigated the effect of 0.1 mM NaHSO3 on the action of local anesthetics on axonal transport (DRG neurons were treated with 1 mM procaine alone, or with 0.1 mM NaHSO3 plus 1 mM procaine). As an additional experiment, DRG neurons were also treated with 1 mM chloroprocaine alone, or with 0.1 mM NaHSO3 plus 1 mM chloroprocaine. In these experiments, we analyzed the percent change in the number of anterogradely and retrogradely transported organelles and recorded changes in neurite morphology using video-enhanced microscopy.

RESULTS

In experiment 1, NaHSO3 at more than 1 mM caused cell membrane damage and complete inhibition of axonal transport, whereas 0.1 mM NaHSO3 maintained axonal transport at 40% to 60% of control with intact cell membrane. In experiment 2, 1 mM procaine alone maintained axonal transport at 90% to 100%. However, application of 1 mM procaine-0.1 mM NaHSO3 solution resulted in deformation of neurites and with complete cessation of axonal transport. Likewise, although 1 mM chloroprocaine maintain axonal transport at 80% to 100%, 1 mM chloroprocaine-0.1 mM NaHSO3 arrested axonal transport.

CONCLUSIONS

NaHSO3 resulted in a dose-dependent damage to the cell membrane and axonal transport, especially when used in combination with procaine or chloroprocaine.

Calmodulin Binding Proteins and Alzheimer's Disease

The small, calcium-sensor protein, calmodulin, is ubiquitously expressed and central to cell function in all cell types. Here the literature linking calmodulin to Alzheimer's disease is reviewed. Several experimentally-verified calmodulin-binding proteins are involved in the formation of amyloid-β plaques including amyloid-β protein precursor, β-secretase, presenilin-1, and ADAM10. Many others possess potential calmodulin-binding domains that remain to be verified. Three calmodulin binding proteins are associated with the formation of neurofibrillary tangles: two kinases (CaMKII, CDK5) and one protein phosphatase (PP2B or calcineurin). Many of the genes recently identified by genome wide association studies and other studies encode proteins that contain putative calmodulin-binding domains but only a couple (e.g., APOE, BIN1) have been experimentally confirmed as calmodulin binding proteins. At least two receptors involved in calcium metabolism and linked to Alzheimer's disease (mAchR; NMDAR) have also been identified as calmodulin-binding proteins. In addition to this, many proteins that are involved in other cellular events intimately associated with Alzheimer's disease including calcium channel function, cholesterol metabolism, neuroinflammation, endocytosis, cell cycle events, and apoptosis have been tentatively or experimentally verified as calmodulin binding proteins. The use of calmodulin as a potential biomarker and as a therapeutic target is discussed.

Physiological and pathological phosphorylation of tau by Cdk5

Hyperphosphorylation of microtubule-associated protein tau is one of the major pathological events in Alzheimer’s disease (AD) and other related neurodegenerative diseases, including frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP-17). Mutations in the tau gene MAPT are a cause of FTDP-17, and the mutated tau proteins are hyperphosphorylated in patient brains. Thus, it is important to determine the molecular mechanism of hyperphosphorylation of tau to understand the pathology of these diseases collectively called tauopathy. Tau is phosphorylated at many sites via several protein kinases, and a characteristic is phosphorylation at Ser/Thr residues in Ser/Thr-Pro sequences, which are targeted by proline-directed protein kinases such as ERK, GSK3β, and Cdk5. Among these kinases, Cdk5 is particularly interesting because it could be abnormally activated in AD. Cdk5 is a member of the cyclin-dependent kinases (Cdks), but in contrast to the major Cdks, which promote cell cycle progression in proliferating cells, Cdk5 is activated in post-mitotic neurons via the neuron-specific activator p35. Cdk5-p35 plays a critical role in brain development and physiological synaptic activity. In contrast, in disease brains, Cdk5 is thought to be hyperactivated by p25, which is the N-terminal truncated form of p35 and is generated by cleavage with calpain. Several reports have indicated that tau is hyperphosphorylated by Cdk5-p25. However, normal and abnormal phosphorylation of tau by Cdk5 is still not completely understood. In this article, we summarize the physiological and pathological phosphorylation of tau via Cdk5.

Electrochemical investigations into kinase-catalyzed transformations of tau protein

The formation of neurofibrillary tangles by hyperphosphorylated tau is a well-recognized hallmark of Alzheimer's disease. Resulting from malfunctioning protein kinases, hyperphosphorylated tau is unable to bind microtubules properly, causing it to self-associate and aggregate. The effects of tau phosphorylation on tau conformation and aggregation are still largely unexplored. The conformational analysis of tau and its hyperphosphorylated forms is usually performed by a variety of spectroscopic techniques, all of which require ample sample concentrations and/or volumes. Here we report on the use of surface based electrochemical techniques that allow for detection of conformational changes and orientation of tau protein as a function of tau phosphorylation by tyrosine and serine/threonine kinases. The electrochemical methods utilize 5'-γ-ferrocenyl adenosine triphosphate (Fc-ATP) derivative as a cosubstrate and tau immobilized on gold surface to probe the role of the following protein kinases: Sarcoma related kinase (Src), Abelson tyrosine kinase (Abl), tau-tubulin kinase (TTBK), proto-oncogene tyrosine protein kinase Fyn (Fyn), and glycogen synthase kinase 3-β (Gsk-3β). The single kinase and sequential kinase-catalyzed Fc-phosphorylations modulate the electrochemical signal, pointing to the dramatic changes around the Fc group in the Fc-phosphorylated tau films. The location and orientation of the Fc-group in Fc-tau film was investigated by the surface plasmon resonance based on antiferrocene antibodies. Additional surface characterization of the Fc-tau films by time-of-flight secondary ion-mass spectrometry and X-ray photoelectron spectroscopy revealed that Fc-phosphorylations influence the tau orientation and conformation on surfaces. When Fc-phosphorylations were performed in solution, the subsequently immobilized Fc-tau exhibited similar trends. This study illustrates the validity and the utility of the labeled electrochemical approach for probing the changes in protein film properties, conformation, and orientation as a function of the enzymatically catalyzed modifications.

Global analysis of phosphorylation of tau by the checkpoint kinases Chk1 and Chk2 in vitro

Hyperphosphorylation of microtubule-associated protein tau is thought to contribute to Alzheimer's disease (AD) pathogenesis. We previously showed that DNA damage-activated cell cycle checkpoint kinases Chk1 and Chk2 phosphorylate tau at an AD-related site and enhance tau toxicity, suggesting potential roles of these kinases in AD. The purpose of this study is to systematically identify which sites in tau are directly phosphorylated by Chk1 and Chk2. Using recombinant human tau phosphorylated by Chk1 and Chk2 in vitro, we first analyzed tau phosphorylation at the AD-related sites by Western blot with phospho-tau-specific antibodies. Second, to globally identify phosphorylated sites in tau, liquid chromatography-tandem mass spectrometry (LC-MS(3)) was employed. These systematic analyses identified a total of 27 Ser/Thr residues as Chk1- or Chk2- target sites. None of them were proline-directed kinase targets. Many of these sites are located within the microtubule-binding domain and C-terminal domain, whose phosphorylation has been shown to reduce tau binding to microtubules and/or has been implicated in tau toxicity. Among these 27 sites, 13 sites have been identified to be phosphorylated in AD brains. Since DNA damage is accumulated in diseased brains, Chk1 and Chk2 may be involved in tau phosphorylation and toxicity in AD pathogenesis.

Transgenic models of Alzheimer's disease: better utilization of existing models through viral transgenesis

Animal models have been used for decades in the Alzheimer's disease (AD) research field and have been crucial for the advancement of our understanding of the disease. Most models are based on familial AD mutations of genes involved in the amyloidogenic process, such as the amyloid precursor protein (APP) and presenilin 1 (PS1). Some models also incorporate mutations in tau (MAPT) known to cause frontotemporal dementia, a neurodegenerative disease that shares some elements of neuropathology with AD. While these models are complex, they fail to display pathology that perfectly recapitulates that of the human disease. Unfortunately, this level of pre-existing complexity creates a barrier to the further modification and improvement of these models. However, as the efficacy and safety of viral vectors improves, their use as an alternative to germline genetic modification is becoming a widely used research tool. In this review we discuss how this approach can be used to better utilize common mouse models in AD research. This article is part of a Special Issue entitled: Animal Models of Disease.

Intensive protein synthesis in neurons and phosphorylation of beta-amyloid precursor protein and tau-protein are triggering factors of neuronal amyloidosis and Alzheimer's disease

Recently the studies of Alzheimer’s disease have become particularly actual and have attracted scientists from all over the world to this problem as a result of dissemination of this dangerous disorder. The reason for such pathogenesis is not known, but the final image, for the first time obtained on microscopic brain sections from patients with this disease more than a hundred years ago, is well known to clinicists. This is the deposition of Ab amyloid in the brain tissue of senile plaques and fibrils. Many authors suppose that the deposition of beta-amyloid provokes secondary neuronal changes which are the reason of neuron death. Other authors associate the death of neurons with hyperphosphorylation of tau-proteins which form neurofibrillar coils inside nerve cells and lead to their death. For creation of methods of preclinical diagnostics and effective treatment of Alzheimer’s disease novel knowledge is required on the nature of triggering factors of sporadic isoforms of Alzheimer’s disease, on cause-effect relationships of phosphorylation of amyloid precursor protein with formation of pathogenic beta-amyloids, on the relationship with these factors of hyperphosphorylation of tau-protein and neuron death. In this review we analyze the papers describing the increasing of intensity of biosynthesis in neurons in normal conditions and under the stress, the possibility of development of energetic unbalanced neurons and activation of their protective systems. Phosphorylation and hyperphosphorylation of tau-proteins is also tightly connected with protective mechanisms of cells and with processes of evacuation of phosphates, adenosine mono-phosphates and pyrophosphates from the region of protein synthesis. Upon long and high intensity of protein synthesis the protective mechanisms are overloaded and the complementarity of metabolitic processes is disturbed. This results in dysfunction of neurons, transport collapse, and neuron death.

Deregulated Cdk5 activity is involved in inducing Alzheimer's disease

Alzheimer's disease (AD), the most devastating chronic neurodegenerative disease in adults, causes dementia and eventually, death of the affected individuals. Clinically, AD is characterized as late-onset, age-dependent cognitive decline due to loss of neurons in cortex and hippocampus. The pathologic corollary of these symptoms is the formation of senile plaques and neurofibrillary tangles. Senile plaques are formed due to accumulation of oligomeric amyloid beta (Aβ) forming plaques. This occurs due to the amyloidogenic processing of the amyloid precursor protein (APP) by various secretases. On the other hand, neurofibrillary tangles are formed due to hyperphosphorylation of cytoskeleton proteins like tau and neurofilament. Both are hyperphosphorylated by cyclin-dependent kinase-5 (Cdk5) and are part of the paired helical filament (PHF), an integral part of neurofibrillary tangles. Unlike other cyclin-dependent kinases, Cdk5 plays a very important role in the neuronal development. Cdk5 gets activated by its neuronal activators p35 and p39. Upon stress, p35 and p39 are cleaved by calpain resulting in truncated products as p25 and p29. Association of Cdk5/p25 is longer and uncontrolled causing aberrant hyperphosphorylation of various substrates of Cdk5 like APP, tau and neurofilament, leading to neurodegenerative pathology like AD. Additionally recent evidence has shown increased levels of p25, Aβ, hyperactivity of Cdk5, phosphorylated tau and neurofilament in human AD brains. This review briefly describes the above-mentioned aspects of involvement of Cdk5 in the pathology of AD and at the end summarizes the advances in Cdk5 as a therapeutic target.

Tau as a therapeutic target for Alzheimer's disease

Neurofibrillary tangles (NFTs) are one of the pathological hallmarks of Alzheimer's disease (AD) and are primarily composed of aggregates of hyperphosphorylated forms of the microtubule associated protein tau. It is likely that an imbalance of kinase and phosphatase activities leads to the abnormal phosphorylation of tau and subsequent aggregation. The wide ranging therapeutic approaches that are being developed include to inhibit tau kinases, to enhance phosphatase activity, to promote microtubule stability, and to reduce tau aggregate formation and/or enhance their clearance with small molecule drugs or by immunotherapeutic means. Most of these promising approaches are still in preclinical development whilst some have progressed to Phase II clinical trials. By pursuing these lines of study, a viable therapy for AD and related tauopathies may be obtained.

Comparison of CID, ETD and metastable atom-activated dissociation (MAD) of doubly and triply charged phosphorylated tau peptides

The fragmentation behavior of the 2+ and 3+ charge states of eleven different phosphorylated tau peptides was studied using collision-induced dissociation (CID), electron transfer dissociation (ETD) and metastable atom-activated dissociation (MAD). The synthetic peptides studied contain up to two known phosphorylation sites on serine or threonine residues, at least two basic residues, and between four and eight potential sites of phosphorylation. CID produced mainly b-/y-type ions with abundant neutral losses of the phosphorylation modification. ETD produced c-/z-type ions in highest abundance but also showed numerous y-type ions at a frequency about 50% that of the z-type ions. The major peaks observed in the ETD spectra correspond to the charge-reduced product ions and small neutral losses from the charge-reduced peaks. ETD of the 2+ charge state of each peptide generally produced fewer backbone cleavages than the 3+ charge state, consistent with previous reports. Regardless of charge state, MAD achieved more extensive backbone cleavage than CID or ETD, while retaining the modification(s) in most cases. In all but one case, unambiguous modification site determination was achieved with MAD. MAD produced 15-20% better sequence coverage than CID and ETD for both the 2+ and 3+ charge states and very different fragmentation products indicating that the mechanism of fragmentation in MAD is unique and complementary to CID and ETD.

Molecular chaperones and regulation of tau quality control: strategies for drug discovery in tauopathies

Tau is a microtubule-associated protein that accumulates in at least 15 different neurodegenerative disorders, which are collectively referred to as tauopathies. In these diseases, tau is often hyperphosphorylated and found in aggregates, including paired helical filaments, neurofibrillary tangles and other abnormal oligomers. Tau aggregates are associated with neuron loss and cognitive decline, which suggests that this protein can somehow evade normal quality control allowing it to aberrantly accumulate and become proteotoxic. Consistent with this idea, recent studies have shown that molecular chaperones, such as heat shock protein 70 and heat shock protein 90, counteract tau accumulation and neurodegeneration in disease models. These molecular chaperones are major components of the protein quality control systems and they are specifically involved in the decision to retain or degrade many proteins, including tau and its modified variants. Thus, one potential way to treat tauopathies might be to either accelerate interactions of abnormal tau with these quality control factors or tip the balance of triage towards tau degradation. In this review, we summarize recent findings and suggest models for therapeutic intervention.

Tau phosphorylation and neuronal apoptosis induced by the blockade of PP2A preferentially involve GSK3β

Overactivation of GSK3β (glycogen synthase kinase-3β) and downregulation of PP2A (protein phosphatase-2A) have been proposed to be involved in the abnormal tau phosphorylation and aggregation in Alzheimer's disease (AD). GSK3β and PP2A signaling pathways were reported to be interconnected. Targeting tau kinases was suggested to represent a therapeutic strategy for AD. Here, tau phosphorylation and neuronal apoptosis were induced in cortical cultured neurons by the inhibition of PP2A by okadaic acid (OKA). In this in vitro model of 'tau pathology' and neurodegeneration, we tested whether GSK3β and other tau kinases including DYRK1A and CDK5 were implicated. Our results show that the inhibitors of GSK3β, lithium and 6-BIO (6-bromoindirubin-3'-oxime), prevented OKA-induced tau phosphorylation and neuronal apoptosis. The implication of GSK3β in these OKA-induced effects was confirmed by its silencing by hairpin siRNA. By contrast, inhibition of DYRK1A (dual-specificity tyrosine-phosphorylation regulated kinase-1A) and CDK5 (cyclin-dependent kinase-5) reversed OKA-induced tau phosphorylation at certain sites but failed to prevent neuronal apoptosis. These results indicate that OKA-induced effects, especially neuronal apoptosis, are preferentially mediated by GSK3β. Furthermore, since chronic exposure to lithium and 6-BIO might be deleterious for neurons, we tested the effect of a new 6-BIO derivative, 6-BIBEO (6-bromoindirubin-3'-(2-bromoethyl)-oxime), which is much less cytotoxic and more selectively inhibits GSK3β compared to lithium and 6-BIO. We show that 6-BIBEO efficiently reversed OKA-induced tau phosphorylation and neuronal apoptosis. It will be interesting to test neuroprotection by 6-BIBEO in an in vivo model of tau pathology and neurodegeneration.

Identification of p10 as a neurotoxic product generated from the proteolytic cleavage of the neuronal Cdk5 activator

The involvement of cyclin-dependent kinase-5 (Cdk5) and p25, the proteolytic fragment of activator p35, has long been implicated in the development of neuron-fibrillary tangles (NFTs), a hallmark of Alzheimer's disease (AD). Findings in this area over the past decade have been highly controversial and inconclusive. Here we report unprecedented detection of endogenous p10, the smaller proteolytic fragment of the Cdk5 activator p35 in treated primary cortical neurons that underwent significant apoptosis, triggered by proteasome inhibitors MG132 and lactacystin, and protein kinase inhibitor staurosporine (STS). p10 appeared exclusively in the detergent-resistant fraction made up of nuclear matrix, membrane-bound organelles, insoluble membrane proteins, and cytoskeletal components. Intriguingly, transient overexpression of p10 in neural cells induced apoptotic morphologies, suggesting that p10 may play an important role in mediating neuronal cell death in neurodegenerative diseases. We demonstrated for the first time that p10-mediated apoptosis occurred via a caspases-independent pathway. Furthermore, as p10 may contain the myristoylation signal for p35 which is responsible for binding p35 to several intracellular components and the membrane, all in all these novel results present that the accumulation of p10 to the detergent-insoluble fraction may be a crucial pathological event to triggering neuronal cell death.

No difference in kinetics of tau or histone phosphorylation by CDK5/p25 versus CDK5/p35 in vitro

CDK5/p35 is a cyclin-dependent kinase essential for normal neuron function. Proteolysis of the p35 subunit in vivo results in CDK5/p25 that causes neurotoxicity associated with a number of neurodegenerative diseases. Whereas the mechanism by which conversion of p35 to p25 leads to toxicity is unknown, there is common belief that CDK5/p25 is catalytically hyperactive compared to CDK5/p35. Here, we have compared the steady-state kinetic parameters of CDK5/p35 and CDK5/p25 towards both histone H1, the best known substrate for both enzymes, and the microtubule-associated protein, tau, a physiological substrate whose in vivo phosphorylation is relevant to Alzheimer's disease. We show that the kinetics of both enzymes are the same towards either substrate in vitro. Furthermore, both enzymes display virtually identical kinetics towards individual phosphorylation sites in tau monitored by NMR. We conclude that conversion of p35 to p25 does not alter the catalytic efficiency of the CDK5 catalytic subunit by using histone H1 or tau as substrates, and that neurotoxicity associated with CDK5/p25 is unlikely attributable to CDK5 hyperactivation, as measured in vitro.

Cyclin-dependent kinase 5 is associated with risk for Alzheimer's disease in a Dutch population-based study

Although the role of the Cdk5 protein in Alzheimer's disease (AD) is well recognized, there have been relatively few studies investigating genetic variants in the CDK5 gene in AD. In this study, we assessed the association between five previously described single nucleotide polymorphisms (SNPs) in the CDK5 gene and late onset AD by means of logistic regression and haplotype association analyses. Including all prevalent and incident AD cases, we found a significantly increased risk of AD for carriers of the GG genotype of SNP rs2069442 (OR = 1.79, 95 % CI 1.16-2.79, p = 0.001) in those without APOE*4. When limiting the analysis to incident cases without APOE*4, carriers of the GG genotype showed a 1.9-fold increased risk of AD (95 % CI 1.16-3.10, p = 0.003). Variations in the CDK5 gene can be described in 5 haplotype blocks. In our analysis, the haplotype tagged by the G allele of SNP rs2069442 was significantly associated with AD (p = 0.05). In conclusion, our study suggests that CDK5 may be associated with AD.

Lithium reduces tau phosphorylation but not A beta or working memory deficits in a transgenic model with both plaques and tangles

Glycogen synthase kinase 3 (GSK-3) is a major kinase implicated in the pathogenesis of Alzheimer's disease (AD), and reducing its activity may have therapeutic efficacy. Two variants exist, referred to as GSK-3 alpha and GSK-3beta. In addition to the latter's well-described role in the phosphorylation of tau, reports also suggest that GSK-3 alpha may regulate amyloid precursor protein processing and Abeta formation. The activities of both GSK-3 alpha and GSK-3beta are reduced by lithium, a well-tolerated drug used in humans to combat bipolar disorder. Here, we investigate the therapeutic efficacy of chronic lithium administration in aged 3xTg-AD mice that harbor both plaques and tangles. We found that lithium reduced tau phosphorylation but did not significantly alter the A beta load. Despite the reduction in phosphotau, lithium treatment did not improve deficits in working memory. Although other studies have investigated the effects of lithium on tau biochemistry, this study represents the first to address comprehensively its therapeutic potential on other critical aspects of AD including its effect on A beta and learning and memory. It remains to be determined from human clinical trials whether lithium treatment alone will improve the clinical outcome in AD patients. These results, however, suggest that the most efficacious treatment will be combining lithium with other anti-A beta interventions.

Proteomics of Alzheimer's disease: understanding mechanisms and seeking biomarkers

Alzheimer's disease is the scourge of the modern, aging world: a costly, damaging disease that robs the elderly of their ability to function as well as their memories. Three decades of progress have resulted in a deep understanding of the pathological processes and a range of targets for therapy, many of which have advanced to late-stage clinical trials. Proteomics has contributed greatly to these advances and will continue to have a growing role in determining the nature of the pathological lesions in the brain. In addition, proteomics (both gel based and gel free, mass spectrometry based), is likely to play an increasing role in identifying biomarkers that may assist in early diagnosis and in monitoring progression and, most importantly, response to therapy.

Tau Aggregation and Toxicity in a Cell Culture Model of Tauopathy

Intracellular aggregation of the microtubule-associated protein tau into filamentous inclusions is a defining characteristic of Alzheimer disease. Because appearance of tau-aggregate bearing lesions correlates with both cognitive decline and neurodegeneration, it has been hypothesized that tau aggregation may be directly toxic to cells that harbor them. Testing this hypothesis in cell culture has been complicated by the resistance of full-length tau isoforms to aggregation over experimentally tractable time periods. To overcome this limitation, a small-molecule agonist of the tau aggregation reaction, Congo red, was used to drive aggregation within HEK-293 cells expressing full-length tau isoform htau40. Formation of detergent-insoluble aggregates was both time and agonist concentration dependent. At 10 microM Congo red, detergent-insoluble aggregates appeared with pseudo-first order kinetics and a half-life of approximately 5 days. By 7 days in culture, total tau levels increased 2-fold, with approximately 30% of total tau converted into detergent-insoluble aggregates. Agonist addition also led to rapid losses in the tubulin binding activity of tau, although tau was not hyperphosphorylated as judged by occupancy of phosphorylation sites Ser396/Ser404. Tau aggregation was associated with decreased viability as detected by ToPro-3 uptake. The results, which establish a new approach for analysis of tau aggregation in cells independent of tau hyperphosphorylation, suggest that conformational changes associated with aggregation are incompatible with microtubule binding, and that toxicity associated with intracellular tau aggregation is not acute but develops over a period of days.

Hyperphosphorylation of tau and neurofilaments and activation of CDK5 and ERK1/2 in PTEN-deficient cerebella

Inherited mutations to the tumor suppressor PTEN sporadically lead to cerebellar gangliocytoma characterized by migration defects. This has been modeled by CNS-specific PTEN ablation in mice, but the underlying mechanism cannot be explained by the known role of PTEN in Akt/PKB inactivation. Here we show that the loss of PTEN in mouse cerebellar neurons causes neurodegeneration by hyperphosphorylation of tau and neurofilaments, and activation of Cdk5 and pERK1/2, suggesting that dysregulation of the PTEN/pAkt pathway can mediate neurodegeneration.

Phosphorylation of the tubulin-binding protein, stathmin, by Cdk5 and MAP kinases in the brain

Regulation of cytoskeletal dynamics is essential to neuronal plasticity during development and adulthood. Dysregulation of these mechanisms may contribute to neuropsychiatric and neurodegenerative diseases. The neuronal protein kinase, cyclin-dependent kinase 5 (Cdk5), is involved in multiple aspects of neuronal function, including regulation of cytoskeleton. A neuroproteomic search identified the tubulin-binding protein, stathmin, as a novel Cdk5 substrate. Stathmin was phosphorylated by Cdk5 in vitro at Ser25 and Ser38, previously identified as mitogen-activated protein kinase (MAPK) and p38 MAPKdelta sites. Cdk5 predominantly phosphorylated Ser38, while MAPK and p38 MAPKdelta predominantly phosphorylated Ser25. Stathmin was phosphorylated at both sites in mouse brain, with higher levels in cortex and striatum. Cdk5 knockout mice exhibited decreased phospho-Ser38 levels. During development, phospho-Ser25 and -Ser38 levels peaked at post-natal day 7, followed by reduction in total stathmin. Inhibition of protein phosphatases in striatal slices caused an increase in phospho-Ser25 and a decrease in total stathmin. Interestingly, the prefrontal cortex of schizophrenic patients had increased phospho-Ser25 levels. In contrast, total and phospho-Ser25 stoichiometries were decreased in the hippocampus of Alzheimer's patients. Thus, microtubule regulatory mechanisms involving the phosphorylation of stathmin may contribute to developmental synaptic pruning and structural plasticity, and may be involved in neuropsychiatric and neurodegenerative disorders.

Hyperphosphorylation at serine 199/202 of tau factor in the gerbil hippocampus after transient forebrain ischemia

We examined the phosphorylation state of tau factor in hippocampal delayed neuronal death (DND) after transient forebrain ischemia. A transient phosphorylation increase at serine 199/202 but not serine 396 of tau factor after transient ischemia was clearly observed. Intraventricular injections of olomoucine and U-0126 (CDK5 and MAP kinase inhibitors, respectively) inhibited hyperphosphorylation. In contrast, wortmannin (PI3 kinase inhibitor) increased phosphorylation at serine 199/202 and corresponded with an increase in GSK3 phosphorylation. Our findings suggest that CDK5, MAP kinase, and GSK3 phosphorylate these sites after ischemia. We prepared recombinant normal human tau (N-Tau40) with TAT-HA protein and dephosphorylated-form human Tau-40 (D-tau40) in which 199/202 serines were changed to alanine by site-directed mutagenesis. Intraventricularly injected D-tau40 protected somewhat against DND while N-Tau40 did not. These data suggest that hyperphosphorylation at serine 199/202 of tau factor is induced by MAP kinase, CDK5, and GSK3, and contributes to ischemic neuronal injury.

Lipopolysaccharide-induced inflammation exacerbates tau pathology by a cyclin-dependent kinase 5-mediated pathway in a transgenic model of Alzheimer's disease

Inflammation is a critical component of the pathogenesis of Alzheimer's disease (AD). Although not an initiator of this disorder, inflammation nonetheless plays a pivotal role as a driving force that can modulate the neuropathology. Here, we characterized the time course of microglia activation in the brains of a transgenic model of AD (3xTg-AD) and discerned its relationship to the plaque and tangle pathology. We find that microglia became activated in a progressive and age-dependent manner, and this activation correlated with the onset of fibrillar amyloidbeta-peptide plaque accumulation and tau hyperphosphorylation. To determine whether microglial activation can exacerbate the pathology, we exposed young 3xTg-AD mice to lipopolysaccharide (LPS), a known inducer of CNS inflammation. Although amyloid precursor protein processing appeared unaffected, we find that LPS significantly induced tau hyperphosphorylation at specific sites that were mediated by the activation of cyclin-dependent kinase 5 (cdk5) through increased formation of the p25 fragment. We further show that administration of roscovitine, a selective and potent inhibitor of cdk5, markedly blocked the LPS-induced tau phosphorylation in the hippocampus. Therefore, this study clearly demonstrates that microglial activation exacerbates key neuropathological features such as tangle formation.

Hyperphosphorylation of JNK-interacting Protein 1, a Protein Associated with Alzheimer Disease

The c-Jun N-terminal kinase (JNK) group of mitogen-activated protein (MAP) kinases are activated by pleiotropic signals including environmental stresses, growth factors, and hormones. JNK-interacting protein 1 (JIP1) is a scaffold protein that assembles and facilitates the activation of the mixed lineage kinase-dependent JNK module and also establishes an interaction with beta-amyloid precursor protein that has been partially characterized. Here we show that, similarly to other proteins involved in various neurological diseases, JIP1 becomes hyperphosphorylated following activation of stress-activated and MAP kinases. By immobilized metal affinity chromatography and a combined microcapillary LC/MALDI-TOF/ESI-ion trap mass spectrometry approach, we identified 35 sites of mitotic phosphorylation within JIP1, among which eight were present within (Ser/Thr)-Pro sequence. This motif is modified by various kinases in aggregates of the microtubule-associated protein tau, which generates typical intraneuronal lesions occurring in Alzheimer disease. Most of the post-translational modifications found were located within the JNK, MAP kinase kinase, and RAC-alpha Ser/Thr protein kinase binding regions; no modifications occurred in protein Src homology 3 and phosphotyrosine interaction domains, which are essential for binding to kinesin, beta-amyloid precursor protein, and MAP kinase kinase kinase. Protein phosphorylation is known to affect stability and protein-protein interactions. Thus, the findings that JIP1 is extensively phosphorylated after activation of stress-activated and MAP kinases indicate that these signaling pathways might modulate JIP1 signaling by regulating its stability and association with some, but not all, interacting proteins.

Phosphoproteomic analysis with a solid-phase capture-release-tag approach

A comprehensive study of global phosphorylation events in biological systems is critical. We report a chemistry-based capture-release-tag method for isolation of complex phospho-Ser/Thr-containing peptides by liquid beta-elimination combined with solid-phase Michael addition. The free thiol groups of 6-(mercapto-acetylamino)-hexanoic acid functionalized resin are used as immobilized Michael donors to capture dehydro-serine/threonine peptides. After an acid-mediated release step, phospho-peptides are labeled with a 6-(2-mercapto-acetylamine)-hexanoic amide tag at phosphorylated sites. We applied this method to analyze the phosphorylation status of microtubule-associated proteins. We find that a CDK5 substrate microtubule-associated protein 2 (MAP2) is phosphorylated on residues that are within a homologous region of Tau. The chemical method corroborates previous results and suggests that Tau and MAP2 may contain a CDK5 phosphorylation motif.

Phosphorylation of FTDP-17 mutant tau by cyclin-dependent kinase 5 complexed with p35, p25, or p39

One of the major pathological hallmarks of Alzheimer disease is neurofibrillary tangles. Neurofibrillary tangles are bundles of paired helical filaments composed of hyperphosphorylated tau. Cyclin-dependent kinase 5 (Cdk5) is one of the tau protein kinases that increase paired helical filament epitopes in tau by phosphorylation. Recently, various mutations of tau have been identified in frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17). Here, we investigated the phosphorylation of FTDP-17 mutant tau proteins, K257T, P301L, P301S, and R406W, by Cdk5 complexed with p35, p25, or p39 in vitro and in cultured cells. The extent of phosphorylation by all Cdk5 species was slightly lower in mutant tau than in wild-type tau. Major phosphorylation sites, including Ser202, Ser235, and Ser404, were the same among the wild-type, K257T, P301L, and P301S tau proteins phosphorylated by any Cdk5. On the other hand, R406W tau was less phosphorylated at Ser404 than were the other variants. This was not due to the simple replacement of amino acid Arg406 with Trp close to the phosphorylation site, because Ser404 in a R406W peptide was equally phosphorylated in a wild-type peptide. The decreased phosphorylation of mutant tau by Cdk5s was canceled when tau protein bound to microtubules was phosphorylated. These results indicate that FTDP-17 mutations do not affect the phosphorylatability of tau by Cdk5 complexed with p35, p25, or p39 and may explain part of the discrepancy reported previously between in vivo and in vitro phosphorylation of FTDP-17 tau mutants.

Tau phosphorylation in neuronal cell function and dysfunction

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

Association of cyclin-dependent kinase 5 and neuronal activators p35 and p39 complex in early-onset Alzheimer's disease

Malfunctioning of cyclin-dependent kinase 5 (CDK5) through aberrant proteolytic cleavage of its neuronal activators p35 and p39 is involved in neurodegeneration in Alzheimer's disease (AD) and other neurodegenerative brain diseases. By extensive genetic analysis of the genes encoding CDK5 (CDK5), p35 (CDK5R1) and p39 (CDK5R2), we excluded causal mutations in 70 familial early-onset AD patients. We performed an association study with five informative SNPs in CDK5 in two independent samples of early-onset AD patients and matched control individuals from The Netherlands and northern Sweden. Association was observed with g.149800G>C in intron 5 of CDK5, and a two times increased risk was observed in both patient samples for carriers of the C-allele. Our data are indicative for a role of the CDK5 molecular complex in the genetic etiology of early-onset AD, and suggest that a yet unknown functional variant in CDK5 or in a nearby gene might lead to increased susceptibility for early-onset AD.

Cdk5, a therapeutic target for Alzheimer's disease?

Alzheimer's disease (AD) represents the leading cause for senile dementia affecting more than 4 million people worldwide. AD patients display a triad of pathological features including brain atrophy caused by neuronal loss, beta-amyloid plaque and neurofibrillary tangles. We previously show that Cyclin-dependent kinase 5 (Cdk5) is deregulated in AD brains and may contribute to the pathogenesis of AD. In AD brains, a calpain cleavage product of its physiological regulator p35, p25 is elevated. p25 causes prolonged activation of Cdk5 and alteration of its substrate specificity. The implications of p25/Cdk5 in neurotoxicity, beta-amyloid plaque and neurofibrillary tangle pathology will be discussed.

Effects of cyclin-dependent kinase-5 activity on apoptosis and tau phosphorylation in immortalized mouse brain cortical cells

Cyclin-dependent kinase-5 (CDK5), a unique CDK family member, is active primarily in the central nervous system (CNS). Previous studies suggest that CDK5 is proapoptotic and contributes to tau hyperphosphorylation and neurodegeneration in Alzheimer's disease. The objective of this study was to examine CDK5 effects on apoptotic progression and tau phosphorylation. Immortalized embryonic mouse brain cortical cells were used to establish a stable cell line that overexpressed wild-type human tau. In these studies, thapsigargin, which induces endoplasmic reticulum stress and can cause accumulation of misfolded proteins, was used to induce apoptosis. Caspase-3 activity and poly-(ADP-ribose)-polymerase (PARP) cleavage, as measures of apoptosis, were significantly increased 24 and 48 hr after thapsigargin treatment, and these events were unaffected by tau expression. Although transient coexpression of CDK5 and its activator, p25, increased CDK5 activity greater than tenfold, increases in caspase-3 activity in response to thapsigargin treatment were unaffected by the presence of CDK5/p25. Tau phosphorylation at the PHF-1 epitope, but not the Tau-1 epitope, was increased significantly in CDK5/p25-transfected cells compared to cells transfected with dominant negative CDK5 (DNCDK5). The PHF-1 epitope remained phosphorylated until 48 hr after thapsigargin treatment in the CDK5/p25-transfected cells. Over the course of apoptosis in this model, phosphorylation of the Tau-1 epitope was unaffected in cells transfected with DNCDK5, vector, or CDK5/p25. In summary, these results demonstrate that CDK5 does not have a significant impact on tau phosphorylation and thapsigargin-induced apoptosis in this neuronal cell model.

Cyclin-dependent kinase-5 in neurodegeneration

Cyclin-dependent kinase-5 (CDK5) is predominantly active in the nervous system and it is well established that CDK5 is essential in neuronal development. In addition to its recognized role in development, there is increasing evidence that CDK5 may be involved in the pathogenesis of several neurodegenerative disorders. Although studies have shown that CDK5 can modulate cell death and survival, controversy still exists as to the exact role CDK5 may play in neurodegenerative processes. This review will highlight recent data on the possible roles of CDK5 in neurodegeneration.

Casein kinase 1 delta phosphorylates tau and disrupts its binding to microtubules

Tau hyperphosphorylation precedes neuritic lesion formation in Alzheimer's disease, suggesting it participates in the tau fibrillization reaction pathway. Candidate tau protein kinases include members of the casein kinase 1 (CK1) family of phosphotransferases, which are highly overexpressed in Alzheimer's disease brain and colocalize with neuritic and granulovacuolar lesions. Here we characterized the contribution of one CK1 isoform, Ckidelta, to the phosphorylation of tau at residues Ser202/Thr205 and Ser396/Ser404 in human embryonic kidney 293 cells using immunodetection and fluorescence microscopy. Treatment of cells with membrane permeable CK1 inhibitor 3-[(2,3,6-trimethoxyphenyl)methylidenyl]-indolin-2-one (IC261) lowered occupancy of Ser396/Ser404 phosphorylation sites by >70% at saturation, suggesting that endogenous CK1 was the major source of basal phosphorylation activity at these sites. Overexpression of Ckidelta increased CK1 enzyme activity and further raised tau phosphorylation at residues Ser202/Thr205 and Ser396/Ser404 in situ. Inhibitor IC261 reversed tau hyperphosphorylation induced by Ckidelta overexpression. Co-immunoprecipitation assays showed direct association of tau and Ckidelta in situ, consistent with tau being a Ckidelta substrate. Ckidelta overexpression also produced a decrease in the fraction of bulk tau bound to detergent-insoluble microtubules. These results suggest that Ckidelta phosphorylates tau at sites that modulate tau/microtubule binding, and that the expression pattern of Ckidelta in Alzheimer's disease is consistent with it playing an important role in tau aggregation.

Mitotic-like tau phosphorylation by p25-Cdk5 kinase complex

Among tau phosphorylation sites, some phosphoepitopes referred to as abnormal ones are exclusively found on tau aggregated into filaments in Alzheimer's disease. Recent data suggested that molecular mechanisms similar to those encountered during mitosis may play a role in abnormal tau phosphorylation. In particular, TG-3 phosphoepitope is associated with early stages of neurofibrillary tangles (NFTs). In this study, we reported a suitable cell model consisting of SH-SY5Y cells stably transfected with an inducible p25 expression vector. It allows investigation of tau phosphorylation by p25-Cdk5 kinase complex in a neuronal context and avoiding p25-induced cytotoxicity. Immunoblotting analyses showed that p25-Cdk5 strongly phosphorylates tau protein not only at the AT8 epitope but also at the AT180 epitope and at the Alzheimer's mitotic epitope TG-3. Further biochemical analyses showed that abnormal phosphorylated tau accumulated in cytosol as a microtubule-free form, suggesting its impact on tau biological activity. Since tau abnormal phosphorylation occurred in dividing cells, TG-3 immunoreactivity was also investigated in differentiated neuronal ones, and both TG-3-immunoreactive tau and nucleolin, another early marker for NFT, were also generated. These data suggest that p25-Cdk5 is responsible for the mitotic-like phosphoepitopes present in NFT and argue for a critical role of Cdk5 in neurodegenerative mechanisms.

Proteomics in Alzheimer's disease: insights into potential mechanisms of neurodegeneration

Proteomics involves the identification of unknown proteins following their separation, often using two-dimensional electrophoresis, digestion of particular proteins of interest by trypsin, determination of the molecular weight of the resulting peptides, and database searching to make the identification of the proteins. Application of proteomics to Alzheimer's disease (AD), the major dementing disorder of the elderly, has just begun. Differences in protein expression and post-translational modification (mostly oxidative modification) of proteins from AD brain and peripheral tissue, as well as in brain from rodent models of AD, have yielded insights into potential molecular mechanisms of neurodegeneration in this dementing disorder. This review surveys the proteomics studies relevant to AD, from which new understandings of the pathology, biochemistry, and physiology of AD are beginning to emerge.

Partial rescue of the p35-/- brain phenotype by low expression of a neuronal-specific enolase p25 transgene

Cyclin-dependent kinase 5 (Cdk5) is activated on binding of activator proteins p35 and p39. A N-terminally truncated p35, termed p25, is generated through cleavage by the Ca(2+)-dependent protease calpain after induction of ischemia in rat brain. p25 has been shown to accumulate in brains of patients with Alzheimer's disease and may contribute to A-beta peptide-mediated toxicity. Studies from transfected neurons as well as p35 and p25 transgenic mice have indicated that Cdk5, when activated by p25, gains some toxic function compared with p35/Cdk5. It remains unclear, however, whether p25/Cdk5 signaling additionally channels into pathways usually used by p35/Cdk5 and whether p25 is associated with a loss of p35 function. To clarify these issues, we have generated p25-transgenic mice in a p35-null background. We find that low levels of p25 during development induce a partial rescue of the p35-/- phenotype in several brain regions analyzed, including a rescue of cell positioning of a subset of neurons in the neocortex. In accordance with the partial rescue of brain anatomy, phosphorylation of the Cdk5 substrate mouse disabled 1 is partially restored during development. Besides this, p25/Cdk5 fails to phosphorylate other substrates that are normally phosphorylated by p35/Cdk5. Our results show that p25 can substitute for p35/Cdk5 under certain circumstances during development. In addition, they suggest that p25 may have lost some functions of p35.

Cdk5 is a key factor in tau aggregation and tangle formation in vivo

Tau aggregation is a common feature of neurodegenerative diseases such as Alzheimer's disease, and hyperphosphorylation of tau has been implicated as a fundamental pathogenic mechanism in this process. To examine the impact of cdk5 in tau aggregation and tangle formation, we crossed transgenic mice overexpressing the cdk5 activator p25, with transgenic mice overexpressing mutant (P301L) human tau. Tau was hyperphosphorylated at several sites in the double transgenics, and there was a highly significant accumulation of aggregated tau in brainstem and cortex. This was accompanied by increased numbers of silver-stained neurofibrillary tangles (NFTs). Insoluble tau was also associated with active GSK. Thus, cdk5 can initiate a major impact on tau pathology progression that probably involves several kinases. Kinase inhibitors may thus be beneficial therapeutically.

Cdk5 as a drug target for the treatment of Alzheimer's disease

Cyclin-dependent kinase-5 (cdk5) is suggested to play a role in tau phosphorylation and contribute to the pathogenesis of Alzheimer's disease (AD). One of its activators, p25, is dramatically increased in AD brains where p25 and cdk5 are colocalized with neurofibrillary tangles. Several animal models have shown a correlation of p25/cdk5 activities with tau phosphorylation. Overexpression of p25/cdk5 in nueronal cultures not only leads to tau phosphorylation but also cytoskeletal abnormalities and neurodegeneration. Therefore, cdk5 kinase inhibitors are potential therapeutic agents for the treatment of AD. Availability of potent, selective, brain permeable cdk5 inhibitors and relevant animal models in which their efficacy can be treated will be critical in the development of these inhibitors.

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

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

Involvement of aberrant glycosylation in phosphorylation of tau by cdk5 and GSK-3beta

Microtubule-associated protein tau is abnormally hyperphosphorylated, glycosylated, and aggregated in affected neurons in the brains of individuals with Alzheimer's disease (AD). We recently found that the glycosylation might precede hyperphosphorylation of tau in AD. In this study, we investigated the effect of glycosylation on phosphorylation of tau catalyzed by cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase-3beta (GSK-3beta). The phosphorylation of the longest isoform of recombinant human brain tau, tau(441), at various sites was detected by Western blots and by radioimmuno-dot-blot assay with phosphorylation-dependent and site-specific tau antibodies. We found that cdk5 phosphorylated tau(441) at Thr-181, Ser-199, Ser-202, Thr-205, Thr-212, Ser-214, Thr-217, Thr-231, Ser-235, Ser-396, and Ser-404, but not at Ser-262, Ser-400, Thr-403, Ser-409, Ser-413, or Ser-422. GSK-3beta phosphorylated all the cdk5-catalyzed sites above except Ser-235. Deglycosylation by glycosidases depressed the subsequent phosphorylation of AD-tau (i) with cdk5 at Thr-181, Ser-199, Ser-202, Thr-205, and Ser-404, but not at Thr-212; and (ii) with GSK-3beta at Thr-181, Ser-202, Thr-205, Ser-217, and Ser-404, but not at Ser-199, Thr-212, Thr-231, or Ser-396. These data suggest that aberrant glycosylation of tau in AD might be involved in neurofibrillary degeneration by promoting abnormal hyperphosphorylation by cdk5 and GSK-3beta.