Casein kinase II preferentially phosphorylates human tau isoforms containing an amino-terminal insert. Identification of threonine 39 as the primary phosphate acceptor

Baricitinib and tofacitinib off-target profile, with a focus on Alzheimer's disease

INTRODUCTION

Janus kinase (JAK) inhibitors were recently identified as promising drug candidates for repurposing in Alzheimer's disease (AD) due to their capacity to suppress inflammation via modulation of JAK/STAT signaling pathways. Besides interaction with primary therapeutic targets, JAK inhibitor drugs frequently interact with unintended, often unknown, biological off-targets, leading to associated effects. Nevertheless, the relevance of JAK inhibitors' off-target interactions in the context of AD remains unclear.

METHODS

Putative off-targets of baricitinib and tofacitinib were predicted using a machine learning (ML) approach. After screening scientific literature, off-targets were filtered based on their relevance to AD. Targets that had not been previously identified as off-targets of baricitinib or tofacitinib were subsequently tested using biochemical or cell-based assays. From those, active concentrations were compared to bioavailable concentrations in the brain predicted by physiologically based pharmacokinetic (PBPK) modeling.

RESULTS

With the aid of ML and in vitro activity assays, we identified two enzymes previously unknown to be inhibited by baricitinib, namely casein kinase 2 subunit alpha 2 (CK2-α2) and dual leucine zipper kinase (MAP3K12), both with binding constant (K d) values of 5.8 μM. Predicted maximum concentrations of baricitinib in brain tissue using PBPK modeling range from 1.3 to 23 nM, which is two to three orders of magnitude below the corresponding binding constant.

CONCLUSION

In this study, we extended the list of baricitinib off-targets that are potentially relevant for AD progression and predicted drug distribution in the brain. The results suggest a low likelihood of successful repurposing in AD due to low brain permeability, even at the maximum recommended daily dose. While additional research is needed to evaluate the potential impact of the off-target interaction on AD, the combined approach of ML-based target prediction, in vitro confirmation, and PBPK modeling may help prioritize drugs with a high likelihood of being effectively repurposed for AD.

Highlights

This study explored JAK inhibitors' off-targets in AD using a multidisciplinary approach.We combined machine learning, in vitro tests, and PBPK modelling to predict and validate new off-target interactions of tofacitinib and baricitinib in AD.Previously unknown inhibition of two enzymes (CK2-a2 and MAP3K12) by baricitinib were confirmed using in vitro experiments.Our PBPK model indicates that baricitinib low brain permeability limits AD repurposing.The proposed multidisciplinary approach optimizes drug repurposing efforts in AD research.

CK2 and protein kinases of the CK1 superfamily as targets for neurodegenerative disorders

Casein kinases are involved in a variety of signaling pathways, and also in inflammation, cancer, and neurological diseases. Therefore, they are regarded as potential therapeutic targets for drug design. Recent studies have highlighted the importance of the casein kinase 1 superfamily as well as protein kinase CK2 in the development of several neurodegenerative pathologies, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. CK1 kinases and their closely related tau tubulin kinases as well as CK2 are found to be overexpressed in the mammalian brain. Numerous substrates have been detected which play crucial roles in neuronal and synaptic network functions and activities. The development of new substances for the treatment of these pathologies is in high demand. The impact of these kinases in the progress of neurodegenerative disorders, their bona fide substrates, and numerous natural and synthetic compounds which are able to inhibit CK1, TTBK, and CK2 are discussed in this review.

CK2 alpha prime and alpha-synuclein pathogenic functional interaction mediates synaptic dysregulation in huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder caused by a CAG trinucleotide repeat expansion in the HTT gene for which no therapies are available. HTT mutation causes protein misfolding and aggregation, preferentially affecting medium spiny neurons (MSNs) of the basal ganglia. Transcriptional perturbations in synaptic genes and neuroinflammation are key processes that precede MSN dysfunction and motor symptom onset. Understanding the interplay between these processes is crucial to develop effective therapeutic strategies to treat HD. We investigated the role of protein kinase CK2α', a kinase upregulated in MSNs in HD and previously associated with Parkinson's disease (PD), in the regulation of neuroinflammation and synaptic function in HD. We used the heterozygous knock-in zQ175 HD mouse model and compared that to zQ175 mice lacking one allele of CK2α' (zQ175:CK2α'(±)). CK2α' haploinsufficiency in zQ175 mice resulted in decreased levels of pro-inflammatory cytokines, HTT aggregation, astrogliosis and transcriptional alterations of synaptic genes related to glutamatergic signaling. zQ175:CK2α'(±) mice also presented increased frequency of striatal miniature excitatory postsynaptic currents (mEPSCs), an indicator of synaptic activity, and improved motor coordination compared to zQ175 mice. Neuropathological and phenotypic changes mediated by CK2α' were connected to alpha-synuclein (α-syn) dysregulation and correlated with differences in α-syn serine 129 phosphorylation (pS129-α-syn), a post-translational modification involved in α-synucleinopathy and shown to be regulated by CK2 in PD. pS129-α-syn was increased in the nuclei of MSNs in zQ175 mice and in the striatum of patients with HD, and it decreased in zQ175:CK2α'(±) mice. Collectively, our data established a novel connection between CK2α', neuroinflammation and synaptic gene dysregulation with synucleinopathy in HD and suggested common molecular mechanisms of neurodegeneration between HD and PD. Our results also support CK2α' inhibition as a potential therapeutic strategy to modulate neuronal function and neuroprotection in HD.

Structural and Enzymological Evidence for an Altered Substrate Specificity in Okur-Chung Neurodevelopmental Syndrome Mutant CK2αLys198Arg

Specific de novo mutations in the CSNK2A1 gene, which encodes CK2α, the catalytic subunit of protein kinase CK2, are considered as causative for the Okur-Chung neurodevelopmental syndrome (OCNDS). OCNDS is a rare congenital disease with a high phenotypic diversity ranging from neurodevelopmental disabilities to multi-systemic problems and characteristic facial features. A frequent OCNDS mutation is the exchange of Lys198 to Arg at the center of CK2α′s P+1 loop, a key element of substrate recognition. According to preliminary data recently made available, this mutation causes a significant shift of the substrate specificity of the enzyme. We expressed the CK2α^Lys198Arg recombinantly and characterized it biophysically and structurally. Using isothermal titration calorimetry (ITC), fluorescence quenching and differential scanning fluorimetry (Thermofluor), we found that the mutation does not affect the interaction with CK2β, the non-catalytic CK2 subunit, and that the thermal stability of the protein is even slightly increased. However, a CK2α^Lys198Arg crystal structure and its comparison with wild-type structures revealed a significant shift of the anion binding site harboured by the P+1 loop. This observation supports the notion that the Lys198Arg mutation causes an alteration of substrate specificity which we underpinned here with enzymological data.

Screening of tau protein kinase inhibitors in a tauopathy-relevant cell-based model of tau hyperphosphorylation and oligomerization

Tauopathies are a class of neurodegenerative disorders characterized by abnormal deposition of post-translationally modified tau protein in the human brain. Tauopathies are associated with Alzheimer's disease (AD), chronic traumatic encephalopathy (CTE), and other diseases. Hyperphosphorylation increases tau tendency to aggregate and form neurofibrillary tangles (NFT), a pathological hallmark of AD. In this study, okadaic acid (OA, 100 nM), a protein phosphatase 1/2A inhibitor, was treated for 24h in mouse neuroblastoma (N2a) and differentiated rat primary neuronal cortical cell cultures (CTX) to induce tau-hyperphosphorylation and oligomerization as a cell-based tauopathy model. Following the treatments, the effectiveness of different kinase inhibitors was assessed using the tauopathy-relevant tau antibodies through tau-immunoblotting, including the sites: pSer202/pThr205 (AT8), pThr181 (AT270), pSer202 (CP13), pSer396/pSer404 (PHF-1), and pThr231 (RZ3). OA-treated samples induced tau phosphorylation and oligomerization at all tested epitopes, forming a monomeric band (46-67 kDa) and oligomeric bands (170 kDa and 240 kDa). We found that TBB (a casein kinase II inhibitor), AR and LiCl (GSK-3 inhibitors), cyclosporin A (calcineurin inhibitor), and Saracatinib (Fyn kinase inhibitor) caused robust inhibition of OA-induced monomeric and oligomeric p-tau in both N2a and CTX culture. Additionally, a cyclin-dependent kinase 5 inhibitor (Roscovitine) and a calcium chelator (EGTA) showed contrasting results between the two neuronal cultures. This study provides a comprehensive view of potential drug candidates (TBB, CsA, AR, and Saracatinib), and their efficacy against tau hyperphosphorylation and oligomerization processes. These findings warrant further experimentation, possibly including animal models of tauopathies, which may provide a putative Neurotherapy for AD, CTE, and other forms of tauopathy-induced neurodegenerative diseases.

Regulation of heat shock transcription factors and their roles in physiology and disease

The heat shock transcription factors (HSFs) were discovered over 30 years ago as direct transcriptional activators of genes regulated by thermal stress, encoding heat shock proteins. The accepted paradigm posited that HSFs exclusively activate the expression of protein chaperones in response to conditions that cause protein misfolding by recognizing a simple promoter binding site referred to as a heat shock element. However, we now realize that the mammalian family of HSFs comprises proteins that independently or in concert drive combinatorial gene regulation events that activate or repress transcription in different contexts. Advances in our understanding of HSF structure, post-translational modifications and the breadth of HSF-regulated target genes have revealed exciting new mechanisms that modulate HSFs and shed new light on their roles in physiology and pathology. For example, the ability of HSF1 to protect cells from proteotoxicity and cell death is impaired in neurodegenerative diseases but can be exploited by cancer cells to support their growth, survival and metastasis. These new insights into HSF structure, function and regulation should facilitate the development tof new disease therapeutics to manipulate this transcription factor family.

The Development of CK2 Inhibitors: From Traditional Pharmacology to in Silico Rational Drug Design

Casein kinase II (CK2) is an ubiquitous and pleiotropic serine/threonine protein kinase able to phosphorylate hundreds of substrates. Being implicated in several human diseases, from neurodegeneration to cancer, the biological roles of CK2 have been intensively studied. Upregulation of CK2 has been shown to be critical to tumor progression, making this kinase an attractive target for cancer therapy. Several CK2 inhibitors have been developed so far, the first being discovered by "trial and error testing". In the last decade, the development of in silico rational drug design has prompted the discovery, de novo design and optimization of several CK2 inhibitors, active in the low nanomolar range. The screening of big chemical libraries and the optimization of hit compounds by Structure Based Drug Design (SBDD) provide telling examples of a fruitful application of rational drug design to the development of CK2 inhibitors. Ligand Based Drug Design (LBDD) models have been also applied to CK2 drug discovery, however they were mainly focused on methodology improvements rather than being critical for de novo design and optimization. This manuscript provides detailed description of in silico methodologies whose applications to the design and development of CK2 inhibitors proved successful and promising.

CK2-An Emerging Target for Neurological and Psychiatric Disorders

Protein kinase CK2 has received a surge of attention in recent years due to the evidence of its overexpression in a variety of solid tumors and multiple myelomas as well as its participation in cell survival pathways. CK2 is also upregulated in the most prevalent and aggressive cancer of brain tissue, glioblastoma multiforme, and in preclinical models, pharmacological inhibition of the kinase has proven successful in reducing tumor size and animal mortality. CK2 is highly expressed in the mammalian brain and has many bona fide substrates that are crucial in neuronal or glial homeostasis and signaling processes across synapses. Full and conditional CK2 knockout mice have further elucidated the importance of CK2 in brain development, neuronal activity, and behavior. This review will discuss recent advances in the field that point to CK2 as a regulator of neuronal functions and as a potential novel target to treat neurological and psychiatric disorders.

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.

Tau protein kinases: involvement in Alzheimer's disease

Tau phosphorylation is regulated by a balance between tau kinase and phosphatase activities. Disruption of this equilibrium was suggested to be at the origin of abnormal tau phosphorylation and thereby might contribute to tau aggregation. Thus, understanding the regulation modes of tau phosphorylation is of high interest in determining the possible causes at the origin of the formation of tau aggregates in order to elaborate protection strategies to cope with these lesions in Alzheimer's disease. Among the possible and specific interventions that reverse tau phosphorylation is the inhibition of certain tau kinases. Here, we extensively reviewed tau protein kinases, their physiological roles and regulation, their involvement in tau phosphorylation and their relevance to AD. We also reviewed the most common inhibitory compounds acting on each tau kinase.

Tau protein phosphatases in Alzheimer's disease: the leading role of PP2A

Tau phosphorylation is regulated by a balance between tau kinase and phosphatase activities. Disruption of this equilibrium was suggested to be at the origin of abnormal tau phosphorylation and thereby that might contributes to tau aggregation. Thus, understanding the regulation modes of tau dephosphorylation is of high interest in determining the possible causes at the origin of the formation of tau aggregates and to elaborate protection strategies to cope with these lesions in AD. Among the possible and relatively specific interventions that reverse tau phosphorylation is the stimulation of certain tau phosphatases. Here, we reviewed tau protein phosphatases, their physiological roles and regulation, their involvement in tau phosphorylation and the relevance to AD. We also reviewed the most common compounds acting on each tau phosphatase including PP2A.

Accelerated human mutant tau aggregation by knocking out murine tau in a transgenic mouse model

Many models of human tauopathies have been generated in mice by expression of a human mutant tau with maintained expression of mouse endogenous tau. Because murine tau might interfere with the toxic effects of human mutant tau, we generated a model in which a pathogenic human tau protein is expressed in the absence of wild-type tau protein, with the aim of facilitating the study of the pathogenic role of the mutant tau and to reproduce more faithfully a human tauopathy. The Tg30 line is a tau transgenic mouse model overexpressing human 1N4R double-mutant tau (P301S and G272V) that develops Alzheimer's disease-like neurofibrillary tangles in an age-dependent manner. By crossing Tg30 mice with mice invalidated for their endogenous tau gene, we obtained Tg30xtau(-/-) mice that express only exogenous human double-mutant 1N4R tau. Although Tg30xtau(-/-) mice express less tau protein compared with Tg30, they exhibit signs of decreased survival, increased proportion of sarkosyl-insoluble tau in the brain and in the spinal cord, increased number of Gallyas-positive neurofibrillary tangles in the hippocampus, increased number of inclusions in the spinal cord, and a more severe motor phenotype. Deletion of murine tau accelerated tau aggregation during aging of this mutant tau transgenic model, suggesting that murine tau could interfere with the development of tau pathology in transgenic models of human tauopathies.

Deletion of murine tau gene increases tau aggregation in a human mutant tau transgenic mouse model

We have reported previously a tau transgenic mouse model (Tg30tau) overexpressing human 4R1N double-mutant tau (P301S and G272V) and that develops AD (Alzheimer's disease)-like NFTs (neurofibrillary tangles) in an age-dependent manner. Since murine tau might interfere with the toxic effects of human mutant tau, we set out to analyse the phenotype of our Tg30tau model in the absence of endogenous murine tau with the aim to reproduce more faithfully a model of human tauopathy. By crossing the Tg30tau line with TauKO (tau-knockout) mice, we have obtained a new mouse line called Tg30×TauKO that expresses only exogenous human double-mutant 4R1N tau. Whereas Tg30×TauKO mice express fewer tau proteins compared with Tg30tau, they exhibit augmented sarkosyl-insoluble tau in the brain and an increased number of Gallyas-positive NFTs in the hippocampus. Taken together, exclusion of murine tau causes accelerated tau aggregation during aging of this mutant tau transgenic model.

How druggable is protein kinase CK2?

CK2 is a pleiotropic, ubiquitous, and constitutively active protein kinase (PK), with both cytosolic and nuclear localization in most mammalian cells. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. CK2 catalyzes the phosphorylation of more than 300 substrates characterized by multiple acidic residues surrounding the phosphor-acceptor amino acid, and, consequently, it plays a key role in several physiological and pathological processes. But how can one kinase orchestrate all these tasks faithfully? How is it possible that one kinase can, despite all pleiotropic characteristics of PKs in general, be involved in so many different biochemical events? Is CK2 a druggable target? Several questions are still to be clearly answered, and this review is an occasion for a fruitful discussion.

CK2 negatively regulates Galphas signaling

We present evidence, using biochemical and cellular approaches, that the kinase, CK2, negatively controls signaling via Galpha(s) (or Galpha(olf)) coupled to dopamine D1 and adenosine A2A receptors. Pharmacological inhibition of CK2 or CK2 knockdown by RNAi lead to elevated cAMP levels in dopamine D1 receptor-activated neuroblastoma cells. Phosphorylation levels of protein kinase A substrates were increased in the presence of CK2 inhibitors in mouse striatal slices. The effect of D1 receptor and A2A receptor agonists on the phosphorylation of protein kinase A sites was potentiated upon CK2 inhibition. Furthermore, in cell lines, we observed that reduction in CK2 activity, pharmacologically or genetically, reduced the amount of D1 receptor that was internalized in response to dopamine. Finally, the beta subunit of CK2 was found to interact specifically with the Galpha(s) subunit through protein interaction analyses. Thus CK2 can inhibit G protein-coupled receptor action by enabling faster receptor internalization, possibly through a direct association with Galpha(s).

Cytosolic prostaglandin E synthase: expression patterns in control and Alzheimer's disease brains

Anti-inflammatory drugs reduce the risk of Alzheimer's disease but fail to slow its progression. Studying the expression of prostaglandin E(2) synthases downstream of cyclooxygenase-2 is important. Here, the expression patterns of cytosolic prostaglandin E( 2) synthases, an immediate prostaglandin E(2) source was investigated. Sections taken from the middle frontal gyrus of brains of 10 patients with Alzheimer's and 5 age-matched controls were examined by immunostaining for the presence of the synthases. Immunofluorescence analysis of control brains showed that cytosolic prostaglandin E(2) synthases co-localize with microglia, neurons, and endothelium markers, but not with astrocytes or smooth muscle cells. Immunohistochemical staining for the synthases was positive in the pyramidal neurons of controls but barely detectable in the brain of Alzheimer's patients. These findings revealed that cytosolic prostaglandin E(2) synthases is found in microglia, neurons, and endothelium of control human middle frontal gyrus and that its levels decrease in pyramidal cells of Alzheimer's disease brains.

Biochemistry of Tau in Alzheimer's disease and related neurological disorders

Microtubule-associated Tau proteins belong to a family of factors that polymerize tubulin dimers and stabilize microtubules. Tau is strongly expressed in neurons, localized in the axon and is essential for neuronal plasticity and network. From the very beginning of Tau discovery, proteomics methods have been essential to the knowledge of Tau biochemistry and biology. In this review, we have summarized the main contributions of several proteomic methods in the understanding of Tau, including expression, post-translational modifications and structure, in both physiological and pathophysiological aspects. Finally, recent advances in proteomics technology are essential to develop further therapeutic targets and early predictive and discriminative diagnostic assays for Alzheimer's disease and related disorders.

A role for CK2 upon interkinetic nuclear migration in the cell cycle of retinal progenitor cells

In developing retina, the nucleus of the elongated neuroepithelial cells undergoes interkinetic nuclear migration (INM), that is it migrates back and forth across the proliferative layer during the cell cycle. S-phase occurs at the basal side, while M-phase occurs at the apical margin of the retinal progenitors. G1 and G2-phases occur along the nuclear migration pathway. We tested whether this feature of the retinal cell cycle is controlled by CK2, which, among its many substrates, phosphorylates both molecular motors and cytoskeletal components. Double immunolabeling showed that CK2 is contained in BrdU-labeled retinal progenitors. INM was examined after pulse labeling the retina of newborn rats with BrdU, by plotting nuclear movement from basal to apical sides of the retinal progenitors during G2. The CK2 specific inhibitor 4,5,6,7-tetrabromobenzotriazole inhibited the activity of rat retinal CK2, and blocked nuclear movement proper in a dose-dependent way. No apoptosis was detected, and total numbers of BrdU-labeled nuclei remained constant following treatment. Immunohistochemistry showed that, following inhibition of CK2, the tubulin cytoskeleton is disorganized, with reduced acetylated and increased tyrosinated tubulin. This indicates a reduction in stable microtubules, with accumulation of free tubulin dimers. The results show that CK2 activity is required for INM in retinal progenitor cells.

Novel phosphorylation sites in tau from Alzheimer brain support a role for casein kinase 1 in disease pathogenesis

Tau in Alzheimer disease brain is highly phosphorylated and aggregated into paired helical filaments comprising characteristic neurofibrillary tangles. Here we have analyzed insoluble Tau (PHF-tau) extracted from Alzheimer brain by mass spectrometry and identified 11 novel phosphorylation sites, 10 of which were assigned unambiguously to specific amino acid residues. This brings the number of directly identified sites in PHF-tau to 39, with an additional six sites indicated by reactivity with phosphospecific antibodies to Tau. We also identified five new phosphorylation sites in soluble Tau from control adult human brain, bringing the total number of reported sites to nine. To assess which kinases might be responsible for Tau phosphorylation, we used mass spectrometry to determine which sites were phosphorylated in vitro by several kinases. Casein kinase 1delta and glycogen synthase kinase-3beta were each found to phosphorylate numerous sites, and each kinase phosphorylated at least 15 sites that are also phosphorylated in PHF-tau from Alzheimer brain. A combination of casein kinase 1delta and glycogen synthase kinase-3beta activities could account for over three-quarters of the serine/threonine phosphorylation sites identified in PHF-tau, indicating that casein kinase 1delta may have a role, together with glycogen synthase kinase-3beta, in the pathogenesis of Alzheimer disease.

Label-free kinase profiling using phosphate affinity polyacrylamide gel electrophoresis

Herein we describe three applications of label-free kinase profiling using a novel type of phosphate affinity polyacrylamide gel electrophoresis. The phosphate affinity site is a polyacrylamide-bound dinuclear Mn2+ complex that enables the mobility shift detection of phosphorylated proteins from their nonphosphorylated counterpart. The first application is in vitro kinase activity profiling for the analysis of varied phosphoprotein isotypes in phosphorylation status. The activity profiles of six kinds of kinases, glycogen synthase kinase-3beta, cyclin-dependent kinase 5/p35, protein kinase A, mitogen-activated protein kinase (MAPK), casein kinase II, and calmodulin-dependent protein kinase II, were determined using a substrate protein, Tau, which has a number of phosphorylation sites. Each kinase demonstrated characteristic multiple electrophoresis migration bands up-shifted from the nonphosphorylated Tau due to differences in the phosphorylation sites and stoichiometry. The second application is in vivo kinase activity profiling for the analysis of protein phosphorylation involved in intracellular signal transduction. The time course changes in the epidermal growth factor-induced phosphorylation levels of Shc and MAPK in A431 cells were visualized as highly up-shifted migration bands by subsequent immunoblotting with anti-Shc and anti-MAPK antibodies. The third application is in vitro kinase inhibition profiling for the quantitative screening of kinase-specific inhibitors. The inhibition profile of a tyrosine kinase, Abl (a histidine-tagged recombinant mouse Abl kinase), was determined using the substrate Abltide-GST (a fusion protein consisting of a specific substrate peptide for Abl and glutathione S-transferase) and the approved drug Glivec (an ATP competitor). In the kinase assay, the slower migration band, monophosphorylated Abltide-GST, increased time-dependently, whereas the faster migration band, nonphosphorylated Abltide-GST, decreased. The dose-dependent inhibition of Glivec was determined by a change in the ratio of the faster and slower migration bands, which showed an IC50 value of 1.6 microM in the presence of 0.10 mM ATP.

A transitory activation of protein kinase-A induces a sustained tau hyperphosphorylation at multiple sites in N2a cells-imply a new mechanism in Alzheimer pathology

Overactivation of protein kinase in the end stage of Alzheimer's disease brain has not been established. The purpose of the present study was to explore the possible mechanism for protein kinases in leading to Alzheimer-like tau hyperphosphorylation. We found that incubation of N2a/tau441 with forskolin, a specific activator of cAMP-dependent protein kinase (PKA), induced an increased phosphorylation level of tau at both PKA and non-PKA sites in a dose- and time-dependent manner, and the hyperphosphorylation of tau was positively correlated with the elevation of PKA activity. When the cells were transitorily incubated with forskolin, a temporary activation of PKA with a sustained and almost equally graded tau hyperphosphorylation at some non-PKA sites was observed. In either case, the activity of glycogen synthase kinase-3 (GSK-3) was not changed. It is suggested that only transitory activation of PKA in early stage of Alzheimer disease may result in a sustained tau hyperphosphorylation at multiple sites, implying a new mechanism to Alzheimer-like tau hyperphosphorylation.

Phosphoproteomics by mass spectrometry and classical protein chemistry approaches

The general fields of biological sciences have seen phenomenal transformations in the past two decades at the level of data acquisition, understanding biological processes, and technological developments. Those advances have been made partly because of the advent of molecular biology techniques (which led to genomics) coupled to the advances made in mass spectrometry (MS) to provide the current capabilities and developments in proteomics. However, our current knowledge that approximately 30,000 human genes may code for up to 1 million or more proteins disengage the interface between the genome sequence database algorithms and MS to generate a major interest in independent de novo MS/MS sequence determination. Significant progress has been made in this area through procedures to covalently modify peptide N- and C-terminal amino-acids by sulfonation and guanidination to permit rapid de novo sequence determination by MS/MS analysis. A number of strategies that have been developed to perform qualitative and quantitative proteomics range from 2D-gel electrophoresis, affinity tag reagents, and stable-isotope labeling. Those procedures, combined with MS/MS peptide sequence analysis at the subpicomole level, permit the rapid and effective identification and quantification of a large number of proteins within a given biological sample. The identification of proteins per se, however, is not always sufficient to interpret biological function because many of the naturally occurring proteins are post-translationally modified. One such modification is protein phosphorylation, which regulates a large array of cellular biochemical pathways of the biological system. Traditionally, the study of phosphoprotein structure-function relationships involved classical protein chemistry approaches that required protein purification, peptide mapping, and the identification of the phosphorylated peptide regions and sites by N-terminal sequence analysis. Recent advances made in mass spectrometry have clearly revolutionized the studies of phosphoprotein biochemistry, and include the development of specific strategies to preferentially enrich phosphoproteins by covalent-modifications that incorporate affinity tags that use the physicochemical properties of phosphoaminoacids. The phosphoserine/phosphothreonine-containing proteins/peptides are derivatized under base-catalyzed conditions by thiol agents; mono- and di-thiol reagents both have been used in such studies. The thiol agent may have: (i) an affinity tag for protein enrichment; (ii) stable-isotopic variants for relative quantitation; or (iii) a combination of the moieties in (i) and (ii). These strategies and techniques, together with others, are reviewed, including their practical application to the study of phosphoprotein biochemistry and structure-function. The consensus of how classical protein chemistry and current MS technology overlap into special case of proteomics, namely "phosphoproteomics," will be discussed.

Phosphorylation of Apolipoprotein-E at an Atypical Protein Kinase CK2 PSD/E Site in Vitro†

Apolipoprotein-E (apoE) plays an important role in neuronal lipid transport and is thought to stabilize microtubules by preventing tau hyperphosphorylation. ApoE is also associated with insoluble amyloid detected in Alzheimer disease brain lesions. The apoE C-terminal shares several physicochemical features with alpha-synuclein, another neuronal apolipoprotein-like protein. Alpha-synuclein is phosphorylated by protein kinase CK2 (CK2) at an atypical PSD/E motif in vivo and in vitro. We identified a similar PSD/E motif in apoE and therefore investigated its potential phosphorylation by CK2 in vitro. When a [(32)P]-labeling approach was used, CK2 readily phosphorylated purified human apoE as well as recombinant forms of human apoE3 and apoE4. Using liquid chromatography mass spectrometry techniques, we mapped the major apoE CK2 phosphorylation site to Ser296 within the apoE PSD/E motif. We also found that apoE potently activated CK2 as demonstrated by increased CK2beta subunit autophosphorylation and by increased phosphorylation of tau when the latter was added to the kinase reaction mixtures. Other proteins such as apolipoprotein A-I and albumin did not effectively activate CK2. The phosphorylation of apoE by CK2 as well as the activation of CK2 by apoE may be relevant in vivo where apoE, CK2, and tau are co-localized with additional CK2 targets on neuronal microtubules.

Tau protein as a differential biomarker of tauopathies

Microtubule-associated Tau proteins are the basic component of intraneuronal and glial inclusions observed in many neurological disorders, the so-called tauopathies. Many etiological factors, phosphorylation, splicing, and mutations, relate Tau proteins to neurodegeneration. Molecular analysis has revealed that hyperphosphorylation and abnormal phosphorylation might be one of the important events in the process leading to tau intracellular aggregation. Specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution, could characterize five main classes of tauopathies. A direct correlation has been established between the regional brain distribution of tau pathology and clinical symptoms; for instance progressive involvement of neocortical areas is well correlated to the severity of dementia in Alzheimer's disease, overall suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. Recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies. Overall, a better knowledge of the etiological factors responsible for the aggregation of tau proteins in brain diseases is essential for development of future differential diagnosis and therapeutic strategies. They would hopefully find their application against Alzheimer's disease but also in all neurological disorders for which a dysfunction of Tau biology has been identified.

Gene and protein expression profiling during differentiation of neuroblastoma cells triggered by 13-cis retinoic acid

PURPOSE

The precise changes in RNA and protein expression that accompany neuroblastoma differentiation remain unknown. The authors used microarray technologies to screen molecules associated with the differentiation of neuroblastoma (NB) cells induced by 13-cis retinoic acid.

METHODS

The authors quantified the expression of 2,061 RNA transcripts related to oncogenesis and of 380 proteins expressed in SK-N-SH and CHP-134 NB cell lines in the presence or absence of 13-cis retinoic acid.

RESULTS

Hierarchical clustering captured gene expression altered during neuroblastoma differentiation induced by 13-cis retinoic acid. Several genes were further abstracted based on P values below 0.0017 or protein chips observed in both NB cell lines. The altered expressions of gene products revealed by both DNA and protein chips were in agreement. The expressions of N-myc, cyclin D3, and Wnt10B were downregulated, whereas those of retinoblastoma (RB) and related genes (p107, RB2/p130, p300/CBP, E2F-1, DP-1) as well as others were upregulated.

CONCLUSIONS

These results suggest that microarray technology can screen for genes that are important in neuroblastoma differentiation.

Modulation of the membrane-binding projection domain of tau protein: splicing regulation of exon 3

Tau is a microtubule-associated protein whose transcript undergoes complex regulated splicing in the mammalian nervous system. The N-terminal domain of the protein interacts with the axonal membrane, and is modulated by differential inclusion of exons 2 and 3. These two tau exons are alternatively spliced cassettes, in which exon 3 never appears independently of exon 2. Previous work with tau minigene constructs indicated that exon 3 is intrinsically suboptimal and its primary regulator is a weak branch point. In this study, we confirm the role of the weak branch point in the regulation of exon 3 but also show that the exon is additionally regulated by a combination of exonic enhancers and silencers. Furthermore, we demonstrate that known splicing regulators affect the ratio of exon 3 isoforms, Lastly, we tentatively pinpoint the site of action of several splicing factors which regulate tau exon 3.

Regulation of microtubule-associated proteins

Microtubule-associated proteins (MAPs) function to regulate the assembly dynamics and organization of microtubule polymers. Upstream regulation of MAP activities is the major mechanism used by cells to modify and control microtubule assembly and organization. This review summarizes the functional activities of MAPs found in animal cells and discusses how these MAPs are regulated. Mechanisms controlling gene expression, isoform-specific expression, protein localization, phosphorylation, and degradation are discussed. Additional regulatory mechanisms include synergy or competition between MAPs and the activities of cofactors or binding partners. For each MAP it is likely that regulation in vivo reflects a composite of multiple regulatory mechanisms.

Phosphoprotein isotope-coded affinity tags: application to the enrichment and identification of low-abundance phosphoproteins

The use of a phosphoprotein isotope-coded affinity tag (PhIAT), which employs differential isotopic labeling and biotinylation, has been shown capable of enriching and identifying mixtures of low-abundance phosphopeptides. A denatured solution of beta-casein was labeled using the PhIAT method, and after proteolytic digestion, the labeled peptides were isolated using immobilized avidin. The recovered peptides were separated by capillary reversed-phase liquid chromatography and identified by tandem mass spectrometry. PhIAT-labeled peptides corresponding to known O-phosphorylated peptides from beta-casein were identified along with the phosphorylated peptides from alphas1-casein and alphas2-casein, known low-level (<5%) contaminants of commercially available beta-casein. All of the casein-phosphorylated residues identified by the present PhIAT approach correspond to previously documented sites of phosphorylation. The results illustrate the efficacy of the PhIAT-labeling strategy to not only enrich mixtures for phosphopeptides but also, more importantly, permit the detection and identification of low-level phosphopeptides. In addition, the differences in the phosphorylation state could be determined between phosphopeptides in comparative samples by stoichiometric conversion using the light and heavy isotopic versions of the PhIAT reagents. Overall, our results exemplify the application of the PhIAT approach and demonstrate its utility for proteome-wide phosphoprotein identification and quantitation.

14-3-3zeta is an effector of tau protein phosphorylation

Neurofibrillary tangles associated with Alzheimer's disease are composed mainly of paired helical filaments that are formed by the aggregation of abnormally phosphorylated microtubule-associated protein tau. 14-3-3, a highly conserved protein family that exists as seven isoforms and regulates diverse cellular processes is present in neurofibrillary tangles (Layfield, R., Fergusson, J., Aitken, A., Lowe, J., Landon, M., Mayer, R. J. (1996) Neurosci. Lett. 209, 57-60). The role of 14-3-3 in Alzheimer's disease pathogenesis is not known. In this study, we found that the 14-3-3zeta isoform is associated with tau in brain extract and profoundly stimulates cAMP-dependent protein kinase catalyzed in vitro phosphorylation on Ser(262)/Ser(356) located within the microtubule-binding region of tau. 14-3-3zeta binds to both phosphorylated and nonphosphorylated tau, and the binding site is located within the microtubule-binding region of tau. From brain extract, 14-3-3zeta co-purifies with microtubules, and tubulin blocks 14-3-3zeta-tau binding. Among four 14-3-3 isoforms tested, beta and zeta but not gamma and epsilon associate with tau. Our data suggest that 14-3-3zeta is a tau protein effector and may be involved in the abnormal tau phosphorylation occurring during Alzheimer's disease ontogeny.

Tau protein isoforms, phosphorylation and role in neurodegenerative disorders

Tau proteins belong to the family of microtubule-associated proteins. They are mainly expressed in neurons where they play an important role in the assembly of tubulin monomers into microtubules to constitute the neuronal microtubules network. Microtubules are involved in maintaining the cell shape and serve as tracks for axonal transport. Tau proteins also establish some links between microtubules and other cytoskeletal elements or proteins. Tau proteins are translated from a single gene located on chromosome 17. Their expression is developmentally regulated by an alternative splicing mechanism and six different isoforms exist in the human adult brain. Tau proteins are the major constituents of intraneuronal and glial fibrillar lesions described in Alzheimer's disease and numerous neurodegenerative disorders referred to as 'tauopathies'. Molecular analysis has revealed that an abnormal phosphorylation might be one of the important events in the process leading to their aggregation. Moreover, a specific set of pathological tau proteins exhibiting a typical biochemical pattern, and a different regional and laminar distribution could characterize each of these disorders. Finally, a direct correlation has been established between the progressive involvement of the neocortical areas and the increasing severity of dementia, suggesting that pathological tau proteins are reliable marker of the neurodegenerative process. The recent discovery of tau gene mutations in frontotemporal dementia with parkinsonism linked to chromosome 17 has reinforced the predominant role attributed to tau proteins in the pathogenesis of neurodegenerative disorders, and underlined the fact that distinct sets of tau isoforms expressed in different neuronal populations could lead to different pathologies.

Casein kinase 2 as a potentially important enzyme in the nervous system

Protein kinase CK2 is a ubiquitous and pleiotropic seryl/threonyl protein kinase which is highly conserved in evolution indicating a vital cellular role for this kinase. The holoenzyme is generally composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits, but the free alpha/alpha' subunits are catalytically active by themselves and can be present in cells under some circumstances. Special attention has been devoted to phosphorylation status and structure of these enzymic molecules, however, their regulation and roles remain intriguing. Until recently, CK2 was believed to represent a kinase especially required for cell cycle progression in non-neural cells. At present, with respect to recent findings, four essential features suggest potentially important roles for this enzyme in specific neural functions: (1) CK2 is much more abundant in brain than in any other tissue; (2) there appear to be a myriad of substrates for CK2 in both synaptic and nuclear compartments that have clear implications in development, neuritogenesis, synaptic transmission, synaptic plasticity, information storage and survival; (3) CK2 seems to be associated with mechanisms underlying long-term potentiation in hippocampus; and (4) neurotrophins stimulate activity of CK2 in hippocampus. In addition, some data are suggestive that CK2 might play a role in processes underlying progressive disorders due to Alzheimer's disease, ischemia, chronic alcohol exposure or immunodeficiency virus HIV. The present review focuses mainly on the latest data concerning the regulatory mechanisms and the possible neurophysiological functions of this enzyme.

Specific binding of protein kinase CK2 catalytic subunits to tubulin

Protein kinase CK2 is composed of two regulatory beta-subunits and two catalytic alpha- or alpha'-subunits. To analyse these subunits individually we generated antibodies against unique peptides derived from the alpha-, alpha'- and beta-subunit. Immunofluorescence studies with these antibodies revealed the presence of all three CK2 subunits in the cytoplasm and weakly in the nucleus with strong signals around the nuclear membrane. Double staining experiments revealed a co-localisation of all three subunits with tubulin. A direct association between the CK2 alpha- and the alpha'-subunit and tubulin was confirmed by co-immunoprecipitation experiments as well as by Far Western analysis. There was no binding of the CK2 beta-subunit to tubulin. Thus, with tubulin we have identified a new binding partner specific for the catalytic subunits of CK2.

Potential role of calcineurin for brain ischemia and traumatic injury

Calcineurin belongs to the family of Ca2+/calmodulin-dependent protein phosphatase, protein phosphatase 2B. Calcineurin is the only protein phosphatase which is regulated by a second messenger, Ca2+. Furthermore, calcineurin is highly localized in the central nervous system, especially in those neurons vulnerable to ischemic and traumatic insults. For these reasons, calcineurin is considered to play important roles in neuron-specific functions. Recently, on the basis of the finding that FK506 and cyclosporin A serve as calcineurin-specific inhibitors, this enzyme has become the subject of much study. It is clear that calcineurin is involved in many neuronal (or non-neuronal) functions such as neurotransmitter release, regulation of receptor functions, signal transduction systems, neurite outgrowth, gene expression and neuronal cell death. In this review, we describe the calcineurin functions, functions of the substrates, and the pathogenesis of traumatic and ischemic insults, and we discuss the potential role of calcineurin. There are many similarities in traumatic and ischemic pathogenesis of the brain in which the release of excessive glutamate is followed by an intracellular Ca2+ increase. However, the intracellular cascade which leads to neuronal cell death after the release of excess Ca2+ is unclear. Although calcineurin is thought to be a key toxic enzyme on the basis of studies using immunosuppressants (FK506 or cyclosporin A), many of the functions of the substrates for calcineurin protect against neuronal cell death. We concluded that calcineurin is a bi-directional enzyme for neuronal cell death, having protective and toxic actions, and the balance of the bi-directional effects may be important in ischemic and traumatic pathogenesis.

Heparin-induced conformational change in microtubule-associated protein Tau as detected by chemical cross-linking and phosphopeptide mapping

In Alzheimer's disease, microtubule-associated protein tau becomes abnormally phosphorylated and aggregates into paired helical filaments. Sulfated glycosaminoglycans such as heparin and heparan sulfate were shown to accumulate in pretangle neurons, stimulate in vitro tau phosphorylation, and cause tau aggregation into paired helical filament-like filaments. The sulfated glycosaminoglycan-tau interaction was suggested to be the central event in the development of neuropathology in Alzheimer's disease brain (Goedert, M., Jakes, R., Spillantini, M. G., Hasegawa, M., Smith, M. J., and Crowther, R. A. (1996) Nature 383, 550-553). The biochemical mechanism by which sulfated glycosaminoglycans stimulate tau phosphorylation and cause tau aggregation remains unclear. In this study, disuccinimidyl suberate (DSS), a bifunctional chemical cross-linker, cross-linked tau dimers, tetramers, high molecular size aggregates, and two tau species of sizes 72 and 83 kDa in the presence of heparin. In the absence of heparin only dimeric tau was cross-linked by DSS. Fast protein liquid chromatography gel filtration revealed that 72- and 83-kDa species were formed by intramolecular cross-linking of tau by DSS. These observations indicate that heparin, in addition to causing aggregation, also induces a conformational change in tau in which reactive groups are unmasked or move closer leading to the DSS cross-linking of 72- and 83-kDa species. Heparin-induced structural changes in tau molecule depended on time of heparin exposure. Dimerization and tetramerization peaked at 48 h, whereas conformational change was completed within 30 min of heparin exposure. Heparin exposure beyond 48 h caused an abrupt aggregation of tau into high molecular size species. Heparin stimulated tau phosphorylation by neuronal cdc2-like kinase (NCLK) and cAMP-dependent protein kinase. Phosphopeptide mapping and phosphopeptide sequencing revealed that tau is phosphorylated by NCLK on Thr212 and Thr231 and by cAMP-dependent protein kinase on Ser262 only in the presence of heparin. Heparin stimulation of tau phosphorylation by NCLK showed dependence on time of heparin exposure and correlated with the heparin-induced conformational change of tau. Our data suggest that heparin-induced conformational change exposes new sites for phosphorylation within tau molecule.

The expression of casein kinase 2alpha' and phosphatase 2A activity

Protein phosphatase 2A (PP2A) activity may be differentially regulated by the expression of proteins containing a related amino acid sequence motif such as the casein kinase 2alpha (CK2alpha) subunit or SV40 small t antigen (SVt). Expression of CK2alpha increases PP2A activity whereas SVt decreases its activity. In this work we have tested for the effect of the expression of a third protein containing a similar motif that could be involved in PP2A regulation, the catalytic casein kinase 2alpha' subunit. Our results show that despite the structural similarity of this protein with the other CK2 catalytic (alpha) subunit, the function of the two subunits with respect to the modulation of PP2A activity is quite different: CK2alpha increases whereas CK2alpha' slightly decreases PP2A activity.

Tau interacts with src-family non-receptor tyrosine kinases

Tau and other microtubule-associated proteins promote the assembly and stabilization of neuronal microtubules. While each microtubule-associated protein has distinct properties, their in vivo roles remain largely unknown. Tau is important in neurite outgrowth and axonal development. Recently, we showed that the amino-terminal region of tau, which is not involved in microtubule interactions, is important in NGF induced neurite outgrowth in PC12 cells. Here we report that a proline rich sequence in the amino terminus of tau interacts with the SH3 domains of fyn and src non-receptor tyrosine kinases. Tau and fyn were co-immunoprecipitated from human neuroblastoma cells and co-localization of tau and fyn was visualized in co-transfected NIH3T3 cells. Co-transfection of tau and fyn also resulted in an alteration in NIH3T3 cell morphology, consistent with an in vivo interaction. Fyn-dependent tyrosine phosphorylation of tau occurred in transfected cells and tyrosine phosphorylated tau was identified in human neuroblastoma cells as well. Our data suggest that tau is involved in signal transduction pathways. An interaction between tau and fyn may serve as a mechanism by which extracellular signals influence the spatial distribution of microtubules. The tyrosine phosphorylation of tau by fyn may also have a role in neuropathogenesis, as fyn is upregulated in Alzheimer's disease.

Tau is phosphorylated by GSK-3 at several sites found in Alzheimer disease and its biological activity markedly inhibited only after it is prephosphorylated by A-kinase

Alzheimer disease is characterized by a specific type of neuronal degeneration in which the microtubule associated protein tau is abnormally hyperphosphorylated causing the disruption of the microtubule network. We have found that the phosphorylation of human tau (tau3L) by A-kinase, GSK-3 or CK-1 inhibits its microtubule assembly-promoting and microtubule-binding activities. However, the inhibition of these activities of tau by GSK-3 is significantly increased if tau is prephosphorylated by A-kinase or CK-1. The most potent inhibition is observed by combination phosphorylation of tau with A-kinase and GSK-3. Under these conditions, only very few microtubules are seen by electron microscopy. Sequencing of 32P-labeled trypsin phosphopeptides from tau prephosphorylated by A-kinase (using unlabeled ATP) and further phosphorylated by GSK-3 in the presence of [gamma-32P]ATP revealed that Ser-195, Ser-198, Ser-199, Ser-202, Thr-205, Thr-231, Ser-235, Ser-262, Ser-356 and Ser-404 are phosphorylated, whereas if tau is not prephosphorylated by A-kinase, GSK-3 phosphorylates it at Thr-181, Ser-184, Ser-262, Ser-356 and Ser-400. These data suggest that (i) prephosphorylation of tau by A-kinase makes additional and different sites accessible for phosphorylation by GSK-3; (ii) phosphorylation of tau at these additional sites further inhibits the biological activity of tau in its ability to bind to microtubules and promote microtubule assembly. Thus a combined role of A-kinase and GSK-3 should be considered in Alzheimer neurofibrillary degeneration.

Protein kinase CK2 ("casein kinase-2") and its implication in cell division and proliferation

Protein kinase CK2 (also termed casein kinase-2 or -II) is a ubiquitous Ser/Thr-specific protein kinase required for viability and for cell cycle progression. CK2 is especially elevated in proliferating tissues, either normal or transformed, and the expression of its catalytic subunit in transgenic mice is causative of lymphomas. CK2 is highly pleiotropic: more than 160 proteins phosphorylated by it at sites specified by multiple acidic residues are known. Despite its heterotetrameric structure generally composed by two catalytic (alpha and/or alpha') and two non catalytic beta-subunits, the regulation of CK2 is still enigmatic. A number of functional features of the beta-subunit which could cooperate to the modulation of CK2 targeting/activity will be discussed.

Distribution of the messenger RNA for the extracellularly regulated kinases 1, 2 and 3 in rat brain: effects of excitotoxic hippocampal lesions

Neurofibrillary tangles in Alzheimer's disease are composed of hyperphosphorylated forms of the microtubule-associated protein tau. Based on biochemical criteria, several enzymes have emerged as potential tau protein kinases, including the extracellularly regulated kinases 1, 2 and 3. In situ hybridization was used to map the messenger RNA distribution of extracellularly regulated kinase 1, 2 and 3 in the adult rat brain and their response to excitotoxic hippocampal lesions was examined. Extracellularly regulated kinase 1 messenger RNA was uniformly expressed by glia, but was also present in the dentate gyrus and some other neuronal populations. Extracellularly regulated kinase 2 was exclusively neuronal and concentrated within the cortical laminae and the CA subfields of the hippocampal formation. Extracellularly regulated kinase 3 messenger RNA expression was similar to extracellularly regulated kinase 2 and was also present in neurons but the level of expression was lower. Extracellularly regulated kinases 2 and 3 messenger RNA expression was lost following excitotoxic injury, further supporting a neuronal localization. Extracellularly regulated kinase 1 messenger RNA expression appeared unaltered, suggesting a non-neuronal localization and lack of responsiveness to lesion at the level of transcription. By contrast, messenger RNA of sgk, a recently described serine/threonine kinase, was up-regulated by glial cells following excitotoxic injury. Based on their messenger RNA distribution, cellular localization and response to lesion, it is clear that each kinase may function differently in various signaling pathways. Extracellularly regulated kinase 2, however, is the only kinase with the proper messenger RNA distribution to contribute to neurofibrillary tangle formation in Alzheimer's disease.

Normal and pathological Tau proteins as factors for microtubule assembly

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

Non-proline-dependent protein kinases phosphorylate several sites found in tau from Alzheimer disease brain

Of 21 phosphorylation sites identified in PHF-tau 11 are on ser/thr-X motifs and are probably phosphorylated by non-proline-dependent protein kinases (non-PDPKs). The identities of the non-PDPKs and how they interact to hyperphosphorylate PHF-tau are still unclear. In a previous study we have shown that the rate of phosphorylation of human tau 39 by a PDPK (GSK-3) was increased several fold if tau were first prephosphorylated by non-PDPKs (Singh et al., FEBS Lett 358: 267-272, 1995). In this study we have examined how the specificity of a non-PDPK for different sites on human tau 39 is modulated when tau is prephosphorylated by other non-PDPKs (A-kinase, C-kinase, CK-1, CaM kinase II) as well as a PDPK (GSK-3). We found that the rate of phosphorylation of tau 39 by a non-PDPK can be stimulated if tau were first prephosphorylated by other non-PDPKs. Of the four non-PDPKs only CK-1 can phosphorylate sites (thr 231, ser 396, ser 404) known to be present in PHF-tau. Further, these sites were phosphorylated more rapidly and to a greater extent by CK-1 if tau 39 were first prephosphorylated by A-kinase, CaM kinase II or GSK-3. These results suggest that the site specificities of the non-PDPKs that participate in PHF-tau hyperphosphorylation can be modulated at the substrate level by the phosphorylation state of tau.

Rapid Alzheimer-like phosphorylation of tau by the synergistic actions of non-proline-dependent protein kinases and GSK-3

Tau protein from Alzheimer disease (AD) brain is phosphorylated at eleven Ser/Thr-Pro and nine Ser/Thr-X sites. The former sites are phosphorylated by proline-dependent protein kinases (PDPKs), the latter by non-PDPKs. The identities of both the PDPKs and non-PDPKs involved in AD tau hyperphosphorylation are still to be established. In this study we have analyzed the interactions between a PDPK (GSK-3) and several non-PDPKs (A-kinase, C-kinase, CK-1, CaM kinase II) in the phosphorylation of one isoform (tau 39) of human tau. We found that the rate of phosphorylation of tau 39 by GSK-3 was increased several-fold if tau were first prephosphorylated by the non-PDPKs. Further, several Alzheimer-like epitopes in tau can be induced only slowly after phosphorylation of tau by GSK-3 alone. After a prephosphorylation of tau by the non-PDPKs, however, the rate of induction of these epitopes by GSK-3 is increased several-fold. These results suggest that one role of non-PDPK-catalyzed phosphorylation is the modulation of PDPK-catalyzed phosphorylation of tau in AD brain.