PKA modulates GSK-3beta- and cdk5-catalyzed phosphorylation of tau in site- and kinase-specific manners

Cyclin-dependent Kinase 5 and Neurodegenerative Diseases

Neurodegenerative diseases are a group of diseases characterized by the progressive loss of neurons, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. These diseases have a high incidence and mortality rate globally, placing a heavy burden on patients and their families. The pathogenesis of neurodegenerative diseases is complex, and there are no effective treatments at present. Cyclin-dependent kinase 5 is a proline-directed serine/threonine protein kinase that is closely related to the development and function of the nervous system. Under physiological conditions, it is involved in regulating the process of neuronal proliferation, differentiation, migration, and synaptic plasticity. Moreover, there is increasing evidence that cyclin-dependent kinase 5 also plays an important role in the pathogenesis of neurodegenerative diseases. In this review, we address the biological characteristics of cyclin-dependent kinase 5 and its role in neurodegenerative diseases. In particular, this review highlights the underlying mechanistic linkages between cyclin-dependent kinase 5 and mitochondrial dysfunction, oxidative stress and neuroinflammation in the context of neurodegeneration. Finally, we also summarize the currently available cyclin-dependent kinase 5 inhibitors and their prospects for the treatment of neurodegenerative diseases. Taken together, a better understanding of the molecular mechanisms of cyclin-dependent kinase 5 involved in neurodegenerative diseases can lead to the development of new strategies for the prevention and treatment of these devastating diseases.

ERRα and ERRγ coordinate expression of genes associated with Alzheimer's disease, inhibiting DKK1 to suppress tau phosphorylation

Alzheimer's disease (AD) is a prevalent neurodegenerative disease characterized by cognitive decline and learning/memory impairment associated with neuronal cell loss. Estrogen-related receptor α (ERRα) and ERRγ, which are highly expressed in the brain, have emerged as potential AD regulators, with unelucidated underlying mechanisms. Here, we identified genome-wide binding sites for ERRα and ERRγ in human neuronal cells. They commonly target a subset of genes associated with neurodegenerative diseases, including AD. Notably, Dickkopf-1 (DKK1), a Wnt signaling pathway antagonist, was transcriptionally repressed by both ERRα and ERRγ in human neuronal cells and brain. ERRα and ERRγ repress RNA polymerase II (RNAP II) accessibility at the DKK1 promoter by modulating a specific active histone modification, histone H3 lysine acetylation (H3K9ac), with the potential contribution of their corepressor. This transcriptional repression maintains Wnt signaling activity, preventing tau phosphorylation and promoting a healthy neuronal state in the context of AD.

Effect of outer membrane vesicles of Lactobacillus pentosus on Tau phosphorylation and CDK5-Calpain pathway in mice

Alzheimer's disease (AD) stands as a neurodegenerative disorder causing cognitive decline, posing a significant health concern for the elderly population in China. This study explored the effects of outer membrane vesicles (OMVs) from the gut microbiota of AD patients on learning and memory abilities and Tau protein phosphorylation in mice. In contrast to the OMVs from healthy controls and the PBS treatment group, mice treated with AD-OMVs exhibited notable declines in learning and memory capabilities, as evidenced by results from the Morris water maze, Y-maze, and novel object recognition tests. Immunohistochemistry and Western blot assessments unveiled elevated levels of hyperphosphorylated Tau in the cortex and hippocampus of mice treated with AD-OMVs. However, there were no alterations observed in the total Tau levels. In addition, AD-OMVs treated mice showed increased neuroinflammation indicated by elevated astrocytes and microglia. Molecular mechanism studies demonstrated that AD-OMVs could activate GSK3β, CDK5-Calpain and NF-κB pathways in mice hippocampus. These studies suggest AD patient gut microbiota derived OMVs can promote host Tau phosphorylation and improved neuroinflammation.

Hiding in plain sight: Complex interaction patterns between Tau and 14-3-3ζ protein variants

Tau protein is an intrinsically disordered protein that plays a key role in Alzheimer's disease (AD). In brains of AD patients, Tau occurs abnormally phosphorylated and aggregated in neurofibrillary tangles (NFTs). Together with Tau, 14-3-3 proteins - abundant cytosolic dimeric proteins - were found colocalized in the NFTs. However, so far, the molecular mechanism of the process leading to pathological changes in Tau structure as well as the direct involvement of 14-3-3 proteins are not well understood. Here, we aimed to reveal the effects of phosphorylation by protein kinase A (PKA) on Tau structural preferences and provide better insight into the interaction between Tau and 14-3-3 proteins. We also addressed the impact of monomerization-inducing phosphorylation of 14-3-3 at S58 on the binding to Tau protein. Using multidimensional nuclear magnetic resonance spectroscopy (NMR), chemical cross-linking analyzed by mass spectrometry (MS) and PAGE, we unveiled differences in their binding affinity, stoichiometry, and interfaces with single-residue resolution. We revealed that the interaction between 14-3-3 and Tau proteins is mediated not only via the 14-3-3 amphipathic binding grooves, but also via less specific interactions with 14-3-3 protein surface and, in the case of monomeric 14-3-3, also partially via the exposed dimeric interface. In addition, the hyperphosphorylation of Tau changes its affinity to 14-3-3 proteins. In conclusion, we propose quite complex interaction mode between the Tau and 14-3-3 proteins.

Neuroprotection and Mechanism of Gas-miR36-5p from Gastrodia elata in an Alzheimer's Disease Model by Regulating Glycogen Synthase Kinase-3β

Alzheimer's disease (AD) is currently the most common neurodegenerative disease. Glycogen synthase kinase 3β (GSK-3β) is a pivotal factor in AD pathogenesis. Recent research has demonstrated that plant miRNAs exert cross-kingdom regulation on the target genes in animals. Gastrodia elata (G. elata) is a valuable traditional Chinese medicine that has significant pharmacological activity against diseases of the central nervous system (CNS). Our previous studies have indicated that G. elata-specific miRNA plays a cross-kingdom regulatory role for the NF-κB signaling pathway in mice. In this study, further bioinformatics analysis suggested that Gas-miR36-5p targets GSK-3β. Through western blot, RT-qPCR, and assessments of T-AOC, SOD, and MDA levels, Gas-miR36-5p demonstrated its neuroprotective effects in an AD cell model. Furthermore, Gas-miR36-5p was detected in the murine brain tissues. The results of the Morris water maze test and western blot analysis provided positive evidence for reversing the learning deficits and hyperphosphorylation of Tau in AD mice, elucidating significant neuroprotective effects in an AD model following G. elata RNA administration. Our research emphasizes Gas-miR36-5p as a novel G. elata-specific miRNA with neuroprotective properties in Alzheimer's disease by targeting GSK-3β. Consequently, our findings provide valuable insights into the cross-kingdom regulatory mechanisms underlying G. elata-specific miRNA, presenting a novel perspective for the treatment of Alzheimer's disease.

The hidden players: Shedding light on the significance of post-translational modifications and miRNAs in Alzheimer's disease development

Alzheimer's disease (AD) is the most prevalent, expensive, lethal, and burdening neurodegenerative disease of this century. The initial stages of this disease are characterized by a reduced ability to encode and store new memories. Subsequent cognitive and behavioral deterioration occurs during the later stages. Abnormal cleavage of amyloid precursor protein (APP) resulting in amyloid-beta (Aβ) accumulation along with hyperphosphorylation of tau protein are the two characteristic hallmarks of AD. Recently, several post-translational modifications (PTMs) have been identified on both Aβ as well as tau proteins. However, a complete understanding of how different PTMs influence the structure and function of proteins in both healthy and diseased conditions is still lacking. It has been speculated that these PTMs might play vital roles in the progression of AD. In addition, several short non-coding microRNA (miRNA) sequences have been found to be deregulated in the peripheral blood of Alzheimer patients. The miRNAs are single-stranded RNAs that control gene expression by causing mRNA degradation, deadenylation, or translational repression and have been implicated in the regulation of several neuronal and glial activities. The lack of comprehensive understanding regarding disease mechanisms, biomarkers, and therapeutic targets greatly hampers the development of effective strategies for early diagnosis and the identification of viable therapeutic targets. Moreover, existing treatment options for managing the disease have proven to be ineffective and provide only temporary relief. Therefore, understanding the role of miRNAs and PTMs in AD can provide valuable insights into disease mechanisms, aid in the identification of biomarkers, facilitate the discovery of novel therapeutic targets, and inspire innovative treatments for this challenging condition.

Interrogating the Etiology of Sporadic Alzheimer's Disease Using Aging Rhesus Macaques: Cellular, Molecular, and Cortical Circuitry Perspectives

Aging is the most significant risk factor for neurodegenerative disorders such as Alzheimer's disease (AD) associated with profound socioeconomic and personal costs. Consequently, there is an urgent need for animal models that recapitulate the age-related spatial and temporal complexity and patterns of pathology identical to human AD. Our research in aging nonhuman primate models involving rhesus macaques has revealed naturally occurring amyloid and tau pathology, including the formation of amyloid plaques and neurofibrillary tangles comprising hyperphosphorylated tau. Moreover, rhesus macaques exhibit synaptic dysfunction in association cortices and cognitive impairments with advancing age, and thus can be used to interrogate the etiological mechanisms that generate neuropathological cascades in sporadic AD. Particularly, unique molecular mechanisms (eg, feedforward cyclic adenosine 3',5'-monophosphate [cAMP]-Protein kinase A (PKA)-calcium signaling) in the newly evolved primate dorsolateral prefrontal cortex are critical for persistent firing required for subserving higher-order cognition. For example, dendritic spines in primate dorsolateral prefrontal cortex contain a specialized repertoire of proteins to magnify feedforward cAMP-PKA-calcium signaling such as N-methyl-d-aspartic acid receptors and calcium channels on the smooth endoplasmic reticulum (eg, ryanodine receptors). This process is constrained by phosphodiesterases (eg, PDE4) that hydrolyze cAMP and calcium-buffering proteins (eg, calbindin) in the cytosol. However, genetic predispositions and age-related insults exacerbate feedforward cAMP-Protein kinase A-calcium signaling pathways that induce a myriad of downstream effects, including the opening of K+ channels to weaken network connectivity, calcium-mediated dysregulation of mitochondria, and activation of inflammatory cascades to eliminate synapses, thereby increasing susceptibility to atrophy. Therefore, aging rhesus macaques provide an invaluable model to explore novel therapeutic strategies in sporadic AD.

Identification of Novel Kinases of Tau Using Fluorescence Complementation Mass Spectrometry (FCMS)

Hyperphosphorylation of the microtubule-associated protein Tau is a major hallmark of Alzheimer's disease and other tauopathies. Understanding the protein kinases that phosphorylate Tau is critical for the development of new drugs that target Tau phosphorylation. At present, the repertoire of the Tau kinases remains incomplete, and methods to uncover novel upstream protein kinases are still limited. Here, we apply our newly developed proteomic strategy, fluorescence complementation mass spectrometry, to identify novel kinase candidates of Tau. By constructing Tau- and kinase-fluorescent fragment library, we detected 59 Tau-associated kinases, including 23 known kinases of Tau and 36 novel candidate kinases. In the validation phase using in vitro phosphorylation, among 15 candidate kinases we attempted to purify and test, four candidate kinases, OXSR1 (oxidative-stress responsive gene 1), DAPK2 (death-associated protein kinase 2), CSK (C-terminal SRC kinase), and ZAP70 (zeta chain of T-cell receptor-associated protein kinase 70), displayed the ability to phosphorylate Tau in time-course experiments. Furthermore, coexpression of these four kinases along with Tau increased the phosphorylation of Tau in human neuroglioma H4 cells. We demonstrate that fluorescence complementation mass spectrometry is a powerful proteomic strategy to systematically identify potential kinases that can phosphorylate Tau in cells. Our discovery of new candidate kinases of Tau can present new opportunities for developing Alzheimer's disease therapeutic strategies.

The Interplay between GSK3β and Tau Ser262 Phosphorylation during the Progression of Tau Pathology

Tau hyperphosphorylation has been linked directly to the formation of toxic neurofibrillary tangles (NFTs) in tauopathies, however, prior to NFT formation, the sequence of pathological events involving tau phosphorylation remains unclear. Here, the effect of glycogen synthase kinase 3β (GSK3β) on tau pathology was examined independently for each step of transcellular propagation; namely, tau intracellular aggregation, release, cellular uptake and seeding activity. We find that overexpression of GSK3β-induced phosphorylated 0N4R tau led to a higher level of tau oligomerization in SH-SY5Y neuroblastoma cells than wild type 0N4R, as determined by several orthogonal assays. Interestingly, the presence of GSK3β also enhanced tau release. Further, we demonstrated that cells endocytosed more monomeric tau protein when pre-phosphorylated by GSK3β. Using an extracellular vesicle (EVs)-assisted tau neuronal delivery system, we show that exosomal GSK3β-phosphorylated tau, when added to differentiated SH-SY5Y cells, induced more efficient tau transfer, showing much higher total tau levels and increased tau aggregate formation as compared to wild type exosomal tau. The role of a primary tau phosphorylation site targeted by microtubule-affinity regulating kinases (MARKs), Ser262, was tested by pseudo-phosphorylation using site-directed mutagenesis to aspartate (S262D). S262D tau overexpression significantly enhanced tau release and intracellular tau accumulation, which were concurrent with the increase of pathological states of tau, as determined by immunodetection. Importantly, phosphorylation-induced tau accumulation was augmented by co-transfecting S262D tau with GSK3β, suggesting a possible interplay between Ser262 phosphorylation and GSK3β activity in tau pathology. Lastly, we found that pre-treatment of cells with amyloid-β (Aβ) further tau phosphorylation and accumulation when Ser262 pre-phosphorylation was present, suggesting that S262 may be a primary mediator of Aβ-induced tau toxicity. These findings provide a potential therapeutic target for treating tau-related disorders by targeting specific phospho-tau isoforms and further elucidate the GSK3β-mediated pathological seeding mechanisms.

Selective inhibition of phosphodiesterase 4D increases tau phosphorylation at Ser214 residue

Tau is a protein that normally participates in the assembly and stability of microtubules. However, it can form intraneuronal hyperphosphorylated aggregates that are hallmarks of Alzheimer's disease and other neurodegenerative disorders known as tauopathies. Tau can be phosphorylated by multiple kinases at several sites. Among such kinases, the cAMP-dependent protein kinase A (PKA) phosphorylates tau at Ser214 (pTAU-S214), an event that was shown to reduce the pathological assembly of the protein. Given that the neuronal cAMP/PKA-activated cascade is involved in synaptic plasticity and memory, and that cAMP-enhancing strategies demonstrated promising therapeutic potential for the treatment of cognitive deficits, we investigated the impact of cAMP on pTAU-S214 in N2a cells and rat hippocampal slices. Our results confirm that the activation of adenylyl cyclase increases pTAU-S214 in both model systems and, more interestingly, this effect is mimicked by GEBR-7b, a phosphodiesterase 4D inhibitor with proven pro-cognitive efficacy in rodents.

Phosphorylation by GSK-3β increases the stability of SIRT6 to alleviate TGF-β-induced fibrotic response in renal tubular cells

AIMS

The deacetylase Sirtuin 6 (SIRT6) is up-regulated during fibrogenesis in renal tubular cells and post-ischemia/reperfusion kidneys. Hence, our aim was to investigate the mechanism of SIRT6 up-regulation upon profibrotic stress.

MAIN METHODS

Immunohistochemical staining was used to detect the expression of UBC9 in the kidney section. The interaction of GSK-3β and SIRT6, and phosphorylation level of SIRT6 were detected by the immunoprecipitation assay. The wild-type and phosphorylated site mutant plasmids of SIRT6 were constructed and stably transfected to BUMPT cells to evaluate the phosphorylation function of SIRT6 by immunoblotting assay.

KEY FINDINGS

The phosphorylation of SIRT6 is significantly increased during TGF-β treatment in mouse renal tubular cells. GSK-3β can physically interact with SIRT6 in renal tubular cells, and this interaction is enhanced by TGF-β treatment. Moreover, GSK-3β is the phosphorylation kinase for SIRT6, and phosphorylates SIRT6 at Serine 326 residue to prevent its ubiquitination-mediated proteasomal degradation. Non-phosphorylatable mutant, S326A, of SIRT6, restores β-catenin activation and fibrotic changes in renal tubular cells.

SIGNIFICANCE

The present study demonstrates that a new mechanism for GSK-3β-mediated anti-fibrotic function in renal fibrosis through phosphorylation of SIRT6 to prevent its proteasomal degradation.

Glucose Starvation Causes ptauS409 Increase in N2a Cells Through ATF3/PKAcα Signaling Pathway

In this work, we report that glucose starvation (GS) causes ptau^S409 increase, which may participate in GS-induced neurites retraction in neuro-2a (N2a) cells. Upon GS treatment, PKAcα was stimulated at mRNA and protein levels. Luciferase reporter gene assays indicated that GS regulated PKAcα expression through a core promoter (-345 to -95 bp upstream the transcription starting site) consisting of a cis-acting element of Activating Transcription Factor 3 (ATF3). Knockdown and over-expression experiments demonstrate that ATF3 transcriptionally regulated PKAcα expression. Moreover, GS stimulated ATF3 expression in a time-dependent manner. These findings reveal that glucose starvation induces ptau^S409 increase in N2a cells through an ATF3- PKAcα axis, which shed some light on the relationship between brain glucose metabolism and neurodegenerative diseases.

Alzheimer's disease associated AKAP9 I2558M mutation alters posttranslational modification and interactome of tau and cellular functions in CRISPR-edited human neuronal cells

Alzheimer's disease (AD) is a pervasive neurodegeneration disease with high heritability. In this study, we employed CRISPR-Cas9-engineered technology to investigate the effects of a rare mutation (rs144662445) in the A kinase anchoring protein 9 (AKAP9) gene, which is associated with AD in African Americans (AA), on tau pathology and the tau interactome in SH-SY5Y P301L neuron-like cells. The mutation significantly increased the level of phosphorylated tau, specifically at the site Ser396/Ser404. Moreover, analyses of the tau interactome measured by affinity purification-mass spectrometry revealed that differentially expressed tau-interacting proteins in AKAP9 mutant cells were associated with RNA translation, RNA localization and oxidative activity, recapitulating the tau interactome signature previously reported with human AD brain samples. Importantly, these results were further validated by functional studies showing a significant reduction in protein synthesis activity and excessive oxidative stress in AKAP9 mutant compared with wild type cells in a tau-dependent manner, which are mirrored with pathological phenotype frequently seen in AD. Our results demonstrated specific effects of rs14462445 on mis-processing of tau and suggest a potential role of AKAP9 in AD pathogenesis.

Molecular crowding and RNA synergize to promote phase separation, microtubule interaction, and seeding of Tau condensates

Biomolecular condensation of the neuronal microtubule-associated protein Tau (MAPT) can be induced by coacervation with polyanions like RNA, or by molecular crowding. Tau condensates have been linked to both functional microtubule binding and pathological aggregation in neurodegenerative diseases. We find that molecular crowding and coacervation with RNA, two conditions likely coexisting in the cytosol, synergize to enable Tau condensation at physiological buffer conditions and to produce condensates with a strong affinity to charged surfaces. During condensate-mediated microtubule polymerization, their synergy enhances bundling and spatial arrangement of microtubules. We further show that different Tau condensates efficiently induce pathological Tau aggregates in cells, including accumulations at the nuclear envelope that correlate with nucleocytoplasmic transport deficits. Fluorescent lifetime imaging reveals different molecular packing densities of Tau in cellular accumulations and a condensate-like density for nuclear-envelope Tau. These findings suggest that a complex interplay between interaction partners, post-translational modifications, and molecular crowding regulates the formation and function of Tau condensates. Conditions leading to prolonged existence of Tau condensates may induce the formation of seeding-competent Tau and lead to distinct cellular Tau accumulations.

Blood phospho-tau in Alzheimer disease: analysis, interpretation, and clinical utility

Well-authenticated biomarkers can provide critical insights into the biological basis of Alzheimer disease (AD) to enable timely and accurate diagnosis, estimate future burden and support therapeutic trials. Current cerebrospinal fluid and molecular neuroimaging biomarkers fulfil these criteria but lack the scalability and simplicity necessary for widespread application. Blood biomarkers of adequate effectiveness have the potential to act as first-line diagnostic and prognostic tools, and offer the possibility of extensive population screening and use that is not limited to specialized centres. Accelerated progress in our understanding of the biochemistry of brain-derived tau protein and advances in ultrasensitive technologies have enabled the development of AD-specific phosphorylated tau (p-tau) biomarkers in blood. In this Review we discuss how new information on the molecular processing of brain p-tau and secretion of specific fragments into biofluids is informing blood biomarker development, enabling the evaluation of preanalytical factors that affect quantification, and informing harmonized protocols for blood handling. We also review the performance of blood p-tau biomarkers in the context of AD and discuss their potential contexts of use for clinical and research purposes. Finally, we highlight outstanding ethical, clinical and analytical challenges, and outline the steps that need to be taken to standardize inter-laboratory and inter-assay measurements.

Hyperphosphorylated tau self-assembles into amorphous aggregates eliciting TLR4-dependent responses

Soluble aggregates of the microtubule-associated protein tau have been challenging to assemble and characterize, despite their important role in the development of tauopathies. We found that sequential hyperphosphorylation by protein kinase A in conjugation with either glycogen synthase kinase 3β or stress activated protein kinase 4 enabled recombinant wild-type tau of isoform 0N4R to spontaneously polymerize into small amorphous aggregates in vitro. We employed tandem mass spectrometry to determine the phosphorylation sites, high-resolution native mass spectrometry to measure the degree of phosphorylation, and super-resolution microscopy and electron microscopy to characterize the morphology of aggregates formed. Functionally, compared with the unmodified aggregates, which require heparin induction to assemble, these self-assembled hyperphosphorylated tau aggregates more efficiently disrupt membrane bilayers and induce Toll-like receptor 4-dependent responses in human macrophages. Together, our results demonstrate that hyperphosphorylated tau aggregates are potentially damaging to cells, suggesting a mechanism for how hyperphosphorylation could drive neuroinflammation in tauopathies.

Revisiting the grammar of Tau aggregation and pathology formation: how new insights from brain pathology are shaping how we study and target Tauopathies

Converging evidence continues to point towards Tau aggregation and pathology formation as central events in the pathogenesis of Alzheimer's disease and other Tauopathies. Despite significant advances in understanding the morphological and structural properties of Tau fibrils, many fundamental questions remain about what causes Tau to aggregate in the first place. The exact roles of cofactors, Tau post-translational modifications, and Tau interactome in regulating Tau aggregation, pathology formation, and toxicity remain unknown. Recent studies have put the spotlight on the wide gap between the complexity of Tau structures, aggregation, and pathology formation in the brain and the simplicity of experimental approaches used for modeling these processes in research laboratories. Embracing and deconstructing this complexity is an essential first step to understanding the role of Tau in health and disease. To help deconstruct this complexity and understand its implication for the development of effective Tau targeting diagnostics and therapies, we firstly review how our understanding of Tau aggregation and pathology formation has evolved over the past few decades. Secondly, we present an analysis of new findings and insights from recent studies illustrating the biochemical, structural, and functional heterogeneity of Tau aggregates. Thirdly, we discuss the importance of adopting new experimental approaches that embrace the complexity of Tau aggregation and pathology as an important first step towards developing mechanism- and structure-based therapies that account for the pathological and clinical heterogeneity of Alzheimer's disease and Tauopathies. We believe that this is essential to develop effective diagnostics and therapies to treat these devastating diseases.

Revisiting the Role of GSK3, A Modulator of Innate Immunity, in Idiopathic Inclusion Body Myositis

Idiopathic or sporadic inclusion body myositis (IBM) is the leading age-related (onset >50 years of age) autoimmune muscular pathology, resulting in significant debilitation in affected individuals. Once viewed as primarily a degenerative disorder, it is now evident that much like several other neuro-muscular degenerative disorders, IBM has a major autoinflammatory component resulting in chronic inflammation-induced muscle destruction. Thus, IBM is now considered primarily an inflammatory pathology. To date, there is no effective treatment for sporadic inclusion body myositis, and little is understood about the pathology at the molecular level, which would offer the best hopes of at least slowing down the degenerative process. Among the previously examined potential molecular players in IBM is glycogen synthase kinase (GSK)-3, whose role in promoting TAU phosphorylation and inclusion bodies in Alzheimer’s disease is well known. This review looks to re-examine the role of GSK3 in IBM, not strictly as a promoter of TAU and Abeta inclusions, but as a novel player in the innate immune system, discussing some of the recent roles discovered for this well-studied kinase in inflammatory-mediated pathology.

Studies of aging nonhuman primates illuminate the etiology of early-stage Alzheimer's-like neuropathology: An evolutionary perspective

Tau pathology in Alzheimer's disease (AD) preferentially afflicts the limbic and recently enlarged association cortices, causing a progression of mnemonic and cognitive deficits. Although genetic mouse models have helped reveal mechanisms underlying the rare, autosomal-dominant forms of AD, the etiology of the more common, sporadic form of AD remains unknown, and is challenging to study in mice due to their limited association cortex and lifespan. It is also difficult to study in human brains, as early-stage tau phosphorylation can degrade postmortem. In contrast, rhesus monkeys have extensive association cortices, are long-lived, and can undergo perfusion fixation to capture early-stage tau phosphorylation in situ. Most importantly, rhesus monkeys naturally develop amyloid plaques, neurofibrillary tangles comprised of hyperphosphorylated tau, synaptic loss, and cognitive deficits with advancing age, and thus can be used to identify the early molecular events that initiate and propel neuropathology in the aging association cortices. Studies to date suggest that the particular molecular signaling events needed for higher cognition-for example, high levels of calcium to maintain persistent neuronal firing- lead to tau phosphorylation and inflammation when dysregulated with advancing age. The expression of NMDAR-NR2B (GluN2B)-the subunit that fluxes high levels of calcium-increases over the cortical hierarchy and with the expansion of association cortex in primate evolution, consistent with patterns of tau pathology. In the rhesus monkey dorsolateral prefrontal cortex, spines contain NMDAR-NR2B and the molecular machinery to magnify internal calcium release near the synapse, as well as phosphodiesterases, mGluR3, and calbindin to regulate calcium signaling. Loss of regulation with inflammation and/or aging appears to be a key factor in initiating tau pathology. The vast expansion in the numbers of these synapses over primate evolution is consistent with the degree of tau pathology seen across species: marmoset < rhesus monkey < chimpanzee < human, culminating in the vast neurodegeneration seen in humans with AD.

Cyclical phosphorylation of LRAP35a and CLASP2 by GSK3β and CK1δ regulates EB1-dependent MT dynamics in cell migration

Mammalian cell cytoskeletal reorganization for efficient directional movement requires tight coordination of actomyosin and microtubule networks. In this study, we show that LRAP35a potentiates microtubule stabilization by promoting CLASP2/EB1 interaction besides its complex formation with MRCK/MYO18A for retrograde actin flow. The alternate regulation of these two networks by LRAP35a is tightly regulated by a series of phosphorylation events that dictated its specificity. Sequential phosphorylation of LRAP35a by Protein Kinase A (PKA) and Glycogen Synthase Kinase-3β (GSK3β) initiates the association of LRAP35a with CLASP2, while subsequent binding and further phosphorylation by Casein Kinase 1δ (CK1δ) induce their dissociation, which facilitates LRAP35a/MRCK association in driving lamellar actomyosin flow. Importantly, microtubule dynamics is directly moderated by CK1δ activity on CLASP2 to regulate GSK3β phosphorylation of the SxIP motifs that blocks EB1 binding, an event countered by LRAP35a interaction and its competition for CK1δ activity. Overall this study reveals an essential role for LRAP35a in coordinating lamellar contractility and microtubule polarization in cell migration.

Intravenous administration of mesenchymal stem cells reduces Tau phosphorylation and inflammation in the 3xTg-AD mouse model of Alzheimer's disease

Mesenchymal stem cell (MSC) administration is a novel and promising therapeutic approach for Alzheimer's disease (AD). Focusing on an intervention easily translatable into clinical practice, we administered allogeneic bone marrow-derived MSCs intravenously in a mouse model of AD (3xTg-AD). We systematically evaluated the effects of a single-dose and multiple-doses of MSCs in young and old mice (5 or 10 months old), comparing the short-term and long-term effects after 1, 2, or 7 months of treatment. A single dose of MSCs in young mice attenuated neuroinflammation 1 and 7 months after injection, whereas multiple-doses did not show any effect. Multiple-doses of MSCs (administered at 5 to 12 mo, or 10 to 12 mo) reduced the β-secretase cleavage of the amyloid precursor protein, although levels of Aβ-42 did not change. Most interestingly, multiple doses of MSCs affected tau hyperphosphorylation. MSCs administered in young mice for 7 months decreased the pathological tau phosphorylation at T205, S214, and T231. MSCs administered in old mice for 2 months decreased tau phosphorylation at S396. Our findings show how different timing and frequency of MSC injections can affect and modulate several aspects of the AD-like neuropathology in the 3xTg-AD mouse model, strengthening the concept of fine-tuning MSC therapy for Alzheimer's disease.

Discovery of Novel Tacrine-Pyrimidone Hybrids as Potent Dual AChE/GSK-3 Inhibitors for the Treatment of Alzheimer's Disease

Based on a multitarget strategy, a series of novel tacrine-pyrimidone hybrids were identified for the potential treatment of Alzheimer's disease (AD). Biological evaluation results demonstrated that these hybrids exhibited significant inhibitory activities toward acetylcholinesterase (AChE) and glycogen synthase kinase 3 (GSK-3). The optimal compound 27g possessed excellent dual AChE/GSK-3 inhibition both in terms of potency and equilibrium (AChE: IC₅₀ = 51.1 nM; GSK-3β: IC₅₀ = 89.3 nM) and displayed significant amelioration on cognitive deficits in scopolamine-induced amnesia mice and efficient reduction against phosphorylation of tau protein on Ser-199 and Ser-396 sites in glyceraldehyde (GA)-stimulated differentiated SH-SY5Y cells. Furthermore, compound 27g exhibited eligible pharmacokinetic properties, good kinase selectivity, and moderate neuroprotection against GA-induced reduction in cell viability and neurite damage in SH-SY5Y-derived neurons. The multifunctional profiles of compound 27g suggest that it deserves further investigation as a promising lead for the prospective treatment of AD.

Plexin-A4 mediates amyloid-β-induced tau pathology in Alzheimer's disease animal model

Amyloid-β (Aβ) and tau are major pathological hallmarks of Alzheimer's disease (AD). Several studies have revealed that Aβ accelerates pathological tau transition and spreading during the disease progression, and that reducing tau can mitigate pathological features of AD. However, molecular links between Aβ and tau pathologies remain elusive. Here, we suggest a novel role for the plexin-A4 as an Aβ receptor that induces aggregated tau pathology. Plexin-A4, previously known as proteins involved in regulating axon guidance and synaptic plasticity, can bound to Aβ with co-receptor, neuropilin-2. Genetic downregulation of plexin-A4 in neurons was sufficient to prevent Aβ-induced activation of CDK5 and reduce tau hyperphosphorylation and aggregation, even in the presence of Aβ. In an AD mouse model that manifests both Aβ and tau pathologies, genetic downregulation of plexin-A4 in the hippocampus reduced tau pathology and ameliorated spatial memory impairment. Collectively, these results indicate that the plexin-A4 is capable of mediating Aβ-induced tau pathology in AD pathogenesis.

Exome sequencing revealed PDE11A as a novel candidate gene for early-onset Alzheimer's disease

To identify novel risk genes and better understand the molecular pathway underlying Alzheimer's disease (AD), whole-exome sequencing was performed in 215 early-onset AD (EOAD) patients and 255 unrelated healthy controls of Han Chinese ethnicity. Subsequent validation, computational annotation and in vitro functional studies were performed to evaluate the role of candidate variants in EOAD. We identified two rare missense variants in the phosphodiesterase 11A (PDE11A) gene in individuals with EOAD. Both variants are located in evolutionarily highly conserved amino acids, are predicted to alter the protein conformation and are classified as pathogenic. Furthermore, we found significantly decreased protein levels of PDE11A in brain samples of AD patients. Expression of PDE11A variants and knockdown experiments with specific short hairpin RNA (shRNA) for PDE11A both resulted in an increase of AD-associated Tau hyperphosphorylation at multiple epitopes in vitro. PDE11A variants or PDE11A shRNA also caused increased cyclic adenosine monophosphate (cAMP) levels, protein kinase A (PKA) activation and cAMP response element-binding protein phosphorylation. In addition, pretreatment with a PKA inhibitor (H89) suppressed PDE11A variant-induced Tau phosphorylation formation. This study offers insight into the involvement of Tau phosphorylation via the cAMP/PKA pathway in EOAD pathogenesis and provides a potential new target for intervention.

Age-related calcium dysregulation linked with tau pathology and impaired cognition in non-human primates

Introduction

The etiology of sporadic Alzheimer's disease (AD) requires non‐genetically modified animal models.

Methods

The relationship of tau phosphorylation to calcium‐cyclic adenosine monophosphate (cAMP)‐protein kinase A (PKA) dysregulation was analyzed in aging rhesus macaque dorsolateral prefrontal cortex (dlPFC) and rat primary cortical neurons using biochemistry and immuno‐electron microscopy. The influence of calcium leak from ryanodine receptors (RyRs) on neuronal firing and cognitive performance was examined in aged macaques.

Results

Aged monkeys naturally develop hyperphosphorylated tau, including AD biomarkers (AT8 (pS202/pT205) and pT217) and early tau pathology markers (pS214 and pS356) that correlated with evidence of increased calcium leak (pS2808‐RyR2). Calcium also regulated early tau phosphorylation in vitro. Age‐related reductions in the calcium‐binding protein, calbindin, and in phosphodiesterase PDE4D were seen within dlPFC pyramidal cell dendrites. Blocking RyRs with S107 improved neuronal firing and cognitive performance in aged macaques.

Discussion

Dysregulated calcium signaling confers risk for tau pathology and provides a potential therapeutic target.

Seeding-Competent Tau in Gray Matter Versus White Matter of Alzheimer's Disease Brain

BACKGROUND

Neurofibrillary pathology of abnormally hyperphosphorylated tau spreads along neuroanatomical connections, underlying the progression of Alzheimer's disease (AD). The propagation of tau pathology to axonally connected brain regions inevitably involves trafficking of seeding-competent tau within the axonal compartment of the neuron.

OBJECTIVE

To determine the seeding activity of tau in cerebral gray and white matters of AD.

METHODS

Levels of total tau, hyperphosphorylation of tau, and SDS- and β-mercaptoethanol-resistant high molecular weight tau (HMW-tau) in crude extracts from gray and white matters of AD frontal lobes were analyzed by immuno-blots. Tau seeding activity was quantitatively assessed by measuring RIPA buffer-insoluble tau in HEK-293FT/tau151-391 cells treated with brain extracts.

RESULTS

We found a comparable level of soluble tau in gray matter versus white matter of control brains, but a higher level of soluble tau in gray matter than white matter of AD brains. In AD brains, tau is hyperphosphorylated in both gray and white matters, with a higher level in the former. The extracts of both gray and white matters of AD brains seeded tau aggregation in HEK-293FT/tau151-391 cells but the white matter showed less potency. Seeding activity of tau in brain extracts was positively correlated with the levels of tau hyperphosphorylation and HMW-tau. RIPA-insoluble tau, but not RIPA-soluble tau, was hyperphosphorylated tau at multiple sites.

CONCLUSION

Both gray and white matters of AD brain contain seeding-competent tau that can template aggregation of hyperphosphorylated tau, but the seeding potency is markedly higher in gray matter than in white matter.

Targeting Tau Hyperphosphorylation via Kinase Inhibition: Strategy to Address Alzheimer's Disease

Microtubule-associated protein tau is involved in the tubulin binding leading to microtubule stabilization in neuronal cells which is essential for stabilization of neuron cytoskeleton. The regulation of tau activity is accommodated by several kinases which phosphorylate tau protein on specific sites. In pathological conditions, abnormal activity of tau kinases such as glycogen synthase kinase-3 β (GSK3β), cyclin-dependent kinase 5 (CDK5), c-Jun N-terminal kinases (JNKs), extracellular signal-regulated kinase 1 and 2 (ERK1/2) and microtubule affinity regulating kinase (MARK) lead to tau hyperphosphorylation. Hyperphosphorylation of tau protein leads to aggregation of tau into paired helical filaments like structures which are major constituents of neurofibrillary tangles, a hallmark of Alzheimer's disease. In this review, we discuss various tau protein kinases and their association with tau hyperphosphorylation. We also discuss various strategies and the advancements made in the area of Alzheimer's disease drug development by designing effective and specific inhibitors for such kinases using traditional in vitro/in vivo methods and state of the art in silico techniques.

Levistolide A Attenuates Alzheimer's Pathology Through Activation of the PPARγ Pathway

Alzheimer's disease (AD) is a neurodegenerative disease characterized by β-amyloid (Aβ) protein deposition, neurofibrillary tangle (NFT) formation, and neuronal loss in the brain. The current study was designed to investigate the potential mechanisms by which levistolide A affects the pathogenesis of AD in an amyloid precursor protein/presenilin 1 (APP/PS1) transgenic (Tg) mouse model of AD and N2a/APP695swe cells. Specifically, behavioral changes in levistolide A-treated APP/PS1 Tg mice were assessed by the nest-building and Morris water maze (MWM) tests. Levistolide A treatment clearly ameliorated memory deficits and cognitive decline in APP/PS1 Tg mice. Aβ generation and the inflammatory response in APP/PS1 Tg mouse brains were clearly reduced after long-term levistolide A application. Mechanistically, levistolide A concurrently stimulated the expression of α-secretase and decreased the generation of β- and γ-secretases. In addition, levistolide A inhibited the phosphorylation of tau in the brains of the Tg mice. Furthermore, in vitro and in vivo experiments suggested that peroxisome proliferator-activated receptor γ (PPARγ) is the key transcription factor that mediates the regulatory effects of levistolide A on the expression of α-, β-, and γ-secretases and phosphorylation of tau. Collectively, these findings show that levistolide A may be a candidate for the treatment of AD.

Protein kinase G phosphorylates the Alzheimer's disease-associated tau protein at distinct Ser/Thr sites

Intraneuronal accumulation of hyperphosphorylated tau is a pathological hallmark of several neurodegenerative disorders, including Alzheimer's disease. Phosphorylation plays a crucial role in modulating the tau-microtubule interaction and the ability of the protein to aggregate, but despite efforts during the past decades, the real identity of the kynases involved in vivo remains uncertain. Here, for the first time, we demonstrate that the cGMP-dependent protein kinase G (PKG) phosphorylates tau in both in vitro and in vivo models. More intriguingly, we provide evidence that PKG phosphorylates tau at Ser214 but not at Ser202, a condition that could reduce the pathological aggregation of the protein shifting tau from a pro-aggregant to a neuroprotective anti-aggregant conformation.

Mapping Phosphodiesterase 4D (PDE4D) in Macaque Dorsolateral Prefrontal Cortex: Postsynaptic Compartmentalization in Layer III Pyramidal Cell Circuits

cAMP signaling has powerful, negative effects on cognitive functions of the primate dorsolateral prefrontal cortex (dlPFC), opening potassium channels to reduce firing and impair working memory, and increasing tau phosphorylation in aging neurons. This contrasts with cAMP actions in classic circuits, where it enhances plasticity and transmitter release. PDE4 isozymes regulate cAMP actions, and thus have been a focus of research and drug discovery. Previous work has focused on the localization of PDE4A and PDE4B in dlPFC, but PDE4D is also of great interest, as it is the predominant PDE4 isoform in primate association cortex, and PDE4D expression decreases with aging in human dlPFC. Here we used laser-capture microdissection transcriptomics and found that PDE4D message is enriched in pyramidal cells compared to GABAergic PV-interneurons in layer III of the human dlPFC. A parallel study in rhesus macaques using high-spatial resolution immunoelectron microscopy revealed the ultrastructural locations of PDE4D in primate dlPFC with clarity not possible in human post-mortem tissue. PDE4D was especially prominent in dendrites associated with microtubules, mitochondria, and likely smooth endoplasmic reticulum (SER). There was substantial postsynaptic labeling in dendritic spines, associated with the SER spine-apparatus near glutamatergic-like axospinous synapses, but sparse labeling in axon terminals. We also observed dense PDE4D labeling perisynaptically in astroglial leaflets ensheathing glutamatergic connections. These data suggest that PDE4D is strategically positioned to regulate cAMP signaling in dlPFC glutamatergic synapses and circuits, especially in postsynaptic compartments where it is localized to influence cAMP actions on intracellular trafficking, mitochondrial physiology, and internal calcium release.

Emerging Evidences for an Implication of the Neurodegeneration-Associated Protein TAU in Cancer

Neurodegenerative disorders and cancer may appear unrelated illnesses. Yet, epidemiologic studies indicate an inverse correlation between their respective incidences for specific cancers. Possibly explaining these findings, increasing evidence indicates that common molecular pathways are involved, often in opposite manner, in the pathogenesis of both disease families. Genetic mutations in the MAPT gene encoding for TAU protein cause an inherited form of frontotemporal dementia, a neurodegenerative disorder, but also increase the risk of developing cancer. Assigning TAU at the interface between cancer and neurodegenerative disorders, two major aging-linked disease families, offers a possible clue for the epidemiological observation inversely correlating these human illnesses. In addition, the expression level of TAU is recognized as a prognostic marker for cancer, as well as a modifier of cancer resistance to chemotherapy. Because of its microtubule-binding properties, TAU may interfere with the mechanism of action of taxanes, a class of chemotherapeutic drugs designed to stabilize the microtubule network and impair cell division. Indeed, a low TAU expression is associated to a better response to taxanes. Although TAU main binding partners are microtubules, TAU is able to relocate to subcellular sites devoid of microtubules and is also able to bind to cancer-linked proteins, suggesting a role of TAU in modulating microtubule-independent cellular pathways associated to oncogenesis. This concept is strengthened by experimental evidence linking TAU to P53 signaling, DNA stability and protection, processes that protect against cancer. This review aims at collecting literature data supporting the association between TAU and cancer. We will first summarize the evidence linking neurodegenerative disorders and cancer, then published data supporting a role of TAU as a modifier of the efficacy of chemotherapies and of the oncogenic process. We will finish by addressing from a mechanistic point of view the role of TAU in de-regulating critical cancer pathways, including the interaction of TAU with cancer-associated proteins.

Hypothesis: Tau pathology is an initiating factor in sporadic Alzheimer's disease

The etiology of the common, sporadic form of Alzheimer's disease (sAD) is unknown. We hypothesize that tau pathology within select projection neurons with susceptible microenvironments can initiate sAD. This postulate rests on extensive data demonstrating that in human brains tau pathology appears about a decade before the formation of Aβ plaques (Aβps), especially targeting glutamate projection neurons in the association cortex. Data from aging rhesus monkeys show abnormal tau phosphorylation within vulnerable neurons, associated with calcium dysregulation. Abnormally phosphorylated tau (pTau) on microtubules traps APP‐containing endosomes, which can increase Aβ production. As Aβ oligomers increase abnormal phosphorylation of tau, this would drive vicious cycles leading to sAD pathology over a long lifespan, with genetic and environmental factors that may accelerate pathological events. This hypothesis could be testable in the aged monkey association cortex that naturally expresses characteristics capable of promoting and sustaining abnormal tau phosphorylation and Aβ production.

Tau Ser208 phosphorylation promotes aggregation and reveals neuropathologic diversity in Alzheimer's disease and other tauopathies

Tau protein abnormally aggregates in tauopathies, a diverse group of neurologic diseases that includes Alzheimer’s disease (AD). In early stages of disease, tau becomes hyperphosphorylated and mislocalized, which can contribute to its aggregation and toxicity. We demonstrate that tau phosphorylation at Ser208 (pSer208) promotes microtubule dysfunction and tau aggregation in cultured cells. Comparative assessment of the epitopes recognized by antibodies AT8, CP13, and 7F2 demonstrates that CP13 and 7F2 are specific for tau phosphorylation at Ser202 and Thr205, respectively, independently of the phosphorylation state of adjacent phosphorylation sites. Supporting the involvement of pSer208 in tau pathology, a novel monoclonal antibody 3G12 specific for tau phosphorylation at Ser208 revealed strong reactivity of tau inclusions in the brains of PS19 and rTg4510 transgenic mouse models of tauopathy. 3G12 also labelled neurofibrillary tangles in brains of patients with AD but revealed differential staining compared to CP13 and 7F2 for other types of tau pathologies such as in neuropil threads and neuritic plaques in AD, tufted astrocytes in progressive supranuclear palsy and astrocytic plaques in corticobasal degeneration. These results support the hypothesis that tau phosphorylation at Ser208 strongly contributes to unique types of tau aggregation and may be a reliable marker for the presence of mature neurofibrillary tangles.

Glycogen synthase kinase (GSK)-3 and the double-strand RNA-dependent kinase, PKR: When two kinases for the common good turn bad

Glycogen synthase kinase (GSK)-3α/β and the double-stranded RNA-dependent kinase PKR are two sentinel kinases that carry-out multiple similar yet distinct functions in both the cytosol and the nucleus. While these kinases belong to separate signal transduction cascades, they demonstrate an uncanny propensity to regulate many of the same proteins either through direct phosphorylation or by altering transcription/translation, including: c-MYC, NF-κB, p53 and TAU, as well as each another. A significant number of studies centered on the GSK3 kinases have led to the identification of the GSK3 interactome and a number of substrates, which link GSK3 activity to metabolic control, translation, RNA splicing, ribosome biogenesis, cellular division, DNA repair and stress/inflammatory signaling. Interestingly, many of these same pathways and processes are controlled by PKR, but unlike the GSK3 kinases, a clear picture of proteins interacting with PKR and a complete listing of its substrates is still missing. In this review, we take a detailed look at what is known about the PKR and GSK3 kinases, how these kinases interact to influence common cellular processes (innate immunity, alternative splicing, translation, glucose metabolism) and how aberrant activation of these kinases leads to diseases such as Alzheimer's disease (AD), diabetes mellitus (DM) and cancer.

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GSK3α/β and PKR are major regulators of cellular homeostasis and the response to stress/inflammation and infection.

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GSK3α/β and PKR interact with and/or modify many of the same proteins and affect the expression of similar genes.

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A balance between AKT and PKR nuclear signaling may be responsible for regulating the activation of nuclear GSK3β.

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GSK3α/β- and PKR-dependent signaling influence major molecular mechanisms of the cell through similar intermediates.

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Aberrant activation of GSK3α/β and PKR is highly involved in cancer, metabolic disorders, and neurodegenerative diseases.

Physiological and Pathological Roles of Cdk5: Potential Directions for Therapeutic Targeting in Neurodegenerative Disease

Cyclin-dependent kinase 5 (Cdk5) is a proline-directed serine (ser)/threonine (Thr) kinase that has been demonstrated to be one of the most functionally diverse kinases within neurons. Cdk5 is regulated via binding with its neuron-specific regulatory subunits, p35 or p39. Cdk5-p35 activity is critical for a variety of developmental and cellular processes in the brain, including neuron migration, memory formation, microtubule regulation, and cell cycle suppression. Aberrant activation of Cdk5 via the truncated p35 byproduct, p25, is implicated in the pathogenesis of several neurodegenerative diseases. The present review highlights the importance of Cdk5 activity and function in the brain and demonstrates how deregulation of Cdk5 can contribute to the development of neurodegenerative conditions such as Alzheimer's and Parkinson's disease. Additionally, we cover past drug discovery attempts at inhibiting Cdk5-p25 activity and discuss which types of targeting strategies may prove to be the most successful moving forward.

The Implication of Androgens in the Presence of Protein Kinase C to Repair Alzheimer’s Disease-Induced Cognitive Dysfunction

Aging, as a major risk factor of memory deficiency, affects neural signaling pathways in hippocampus. In particular, age-dependent androgens deficiency causes cognitive impairments. Several enzymes like protein kinase C (PKC) are involved in memory deficiency. Indeed, PKC regulatory process mediates α-secretase activation to cleave APP in β-amyloid cascade and tau proteins phosphorylation mechanism. Androgens and cortisol regulate PKC signaling pathways, affecting the modulation of receptor for activated C kinase 1. Mitogen-activated protein kinase/ERK signaling pathway depends on CREB activity in hippocampal neurons and is involved in regulatory processes via PKC and androgens. Therefore, testosterone and PKC contribute in the neuronal apoptosis. The present review summarizes the current status of androgens, PKC, and their influence on cognitive learning. Inconsistencies in experimental investigations related to this fundamental correlation are also discussed, with emphasis on the mentioned contributors as the probable potent candidates for learning and memory improvement.

GSKIP-Mediated Anchoring Increases Phosphorylation of Tau by PKA but Not by GSK3beta via cAMP/PKA/GSKIP/GSK3/Tau Axis Signaling in Cerebrospinal Fluid and iPS Cells in Alzheimer Disease

Based on the protein kinase A (PKA)/GSK3β interaction protein (GSKIP)/glycogen synthase kinase 3β (GSK3β) axis, we hypothesized that these might play a role in Tau phosphorylation. Here, we report that the phosphorylation of Tau Ser409 in SHSY5Y cells was increased by overexpression of GSKIP WT more than by PKA- and GSK3β-binding defective mutants (V41/L45 and L130, respectively). We conducted in vitro assays of various kinase combinations to show that a combination of GSK3β with PKA but not Ca²⁺/calmodulin-dependent protein kinase II (CaMK II) might provide a conformational shelter to harbor Tau Ser409. Cerebrospinal fluid (CSF) was evaluated to extend the clinical significance of Tau phosphorylation status in Alzheimer's disease (AD), neurological disorders (NAD), and mild cognitive impairment (MCI). We found higher levels of different PKA-Tau phosphorylation sites (Ser214, Ser262, and Ser409) in AD than in NAD, MCI, and normal groups. Moreover, we used the CRISPR/Cas9 system to produce amyloid precursor protein (APP^WT/D678H) isogenic mutants. These results demonstrated an enhanced level of phosphorylation by PKA but not by the control. This study is the first to demonstrate a transient increase in phosphor-Tau caused by PKA, but not GSK3β, in the CSF and induced pluripotent stem cells (iPSCs) of AD, implying that both GSKIP and GSK3β function as anchoring proteins to strengthen the cAMP/PKA/Tau axis signaling during AD pathogenesis.

The Evolution of Tau Phosphorylation and Interactions

Tau is a neuronal microtubule-associated protein (MAP) that is involved in the regulation of axonal microtubule assembly. However, as a protein with intrinsically disordered regions (IDRs), tau also interacts with many other partners in addition to microtubules. Phosphorylation at selected sites modulates tau's various intracellular interactions and regulates the properties of IDRs. In Alzheimer's disease (AD) and other tauopathies, tau exhibits pathologically increased phosphorylation (hyperphosphorylation) at selected sites and aggregates into neurofibrillary tangles (NFTs). By bioinformatics means, we tested the hypothesis that the sequence of tau has changed during the vertebrate evolution in a way that novel interactions developed and also the phosphorylation pattern was affected, which made tau prone to the development of tauopathies. We report that distinct regions of tau show functional specialization in their molecular interactions. We found that tau's amino-terminal region, which is involved in biological processes related to "membrane organization" and "regulation of apoptosis," exhibited a strong evolutionary increase in protein disorder providing the basis for the development of novel interactions. We observed that the predicted phosphorylation sites have changed during evolution in a region-specific manner, and in some cases the overall number of phosphorylation sites increased owing to the formation of clusters of phosphorylatable residues. In contrast, disease-specific hyperphosphorylated sites remained highly conserved. The data indicate that novel, non-microtubule related tau interactions developed during evolution and suggest that the biological processes, which are mediated by these interactions, are of pathological relevance. Furthermore, the data indicate that predicted phosphorylation sites in some regions of tau, including a cluster of phosphorylatable residues in the alternatively spliced exon 2, have changed during evolution. In view of the "antagonistic pleiotropy hypothesis" it may be worth to take disease-associated phosphosites with low evolutionary conservation as relevant biomarkers into consideration.

MicroRNA-200a-3p Mediates Neuroprotection in Alzheimer-Related Deficits and Attenuates Amyloid-Beta Overproduction and Tau Hyperphosphorylation via Coregulating BACE1 and PRKACB

Alzheimer’s disease (AD) is characterized by two landmark pathologies, the overproduction of amyloid-beta peptides (Aβ), predominated by the β-amyloid protein precursor cleaving enzyme 1 (BACE1), and hyperphosphorylation of the microtubule protein, tau, because of an imbalance in a kinase/phosphatase system that involves the activation of the protein kinase A (PKA). Current evidence indicates that brain microRNAs participate in multiple aspects of AD pathology. Here, the role and underlying molecular mechanisms of microRNA-200a-3p (miR-200a-3p) in mediating neuroprotection against AD-related deficits were investigated. The expression of miR-200a-3p was measured in the hippocampus of APP/PS1 and SAMP8 mice and in an AD cell model in vitro, as well as in blood plasma extracted from AD patients. The targets of miR-200a-3p were determined using bioinformatics and dual-luciferase assay analyses. In addition, cell apoptosis was detected using flow cytometry, and related protein levels were measured using Western blot and enzyme-linked immunosorbent assay (ELISA) techniques. miR-200a-3p was confirmed to be depressed in microarray miRNA profile analysis in vitro and in vivo, suggesting that miR-200a-3p is a potential biomarker of AD. Subsequently, miR-200a-3p was demonstrated to inhibit cell apoptosis accompanied by the inactivation of the Bax/caspase-3 axis and downregulation of Aβ_1-42 and tau phosphorylation levels in vitro. Further mechanistic studies revealed that miR-200a-3p reduced the production of Aβ_1-42 and decreased hyperphosphorylation of tau by regulating the protein translocation of BACE1 and the protein kinase cAMP-activated catalytic subunit beta (PRKACB) associated with the three prime untranslated regions, respectively. Importantly, the function of miR-200a-3p was reversed by overexpression of BACE1 or PRKACB in cultured cells. This resulted in an elevation in cell apoptosis and increases in Aβ_1-42 and tau hyperphosphorylation levels, involving the epitopes threonine 205 and serine 202, 214, 396, and 356, the favorable phosphorylated sites of PKA. In conclusion, our study suggests that miR-200a-3p is implicated in the pathology of AD, exerting neuroprotective effects against Aβ-induced toxicity by two possible mechanisms: one involving the inhibition of Aβ overproduction via suppression of the expression of BACE1 and synergistically decreasing the hyperphosphorylation of tau via attenuation of the expression of PKA.

Pathological Tau From Alzheimer's Brain Induces Site-Specific Hyperphosphorylation and SDS- and Reducing Agent-Resistant Aggregation of Tau in vivo

Neurofibrillary tangles (NFTs) made up of hyperphosphorylated tau are a histopathological hallmark of Alzheimer’s disease (AD) and related tauopathies. Hyperphosphorylation of tau is responsible for its loss of normal physiological function, gain of toxicity and its aggregation to form NFTs. Injection of misfolded tau seeds into mouse brain induces tau aggregation, but the nature of tau phosphorylation in pathologic tau seeded pathology is unclear. In the present study, we injected hyperphosphorylated and oligomeric tau isolated from AD brain (AD P-tau) into hippocampus of human tau transgenic mice and found that in addition to tau aggregation/pathology, tau was hyperphosphorylated at Ser202/Thr205, Thr212, Ser214, Thr217, Ser262, and Ser422 in AD P-tau injected hippocampus and at Ser422 in the contralateral hippocampus and in the ipsilateral cortex. AD P-tau-induced AD-like high molecular weight aggregation of tau that was SDS- and reducing agent-resistant and site-specifically hyperphosphorylated in the ipsilateral hippocampus. There were no detectable alterations in levels of tau phosphatases or tau kinases in AD P-tau-injected brains. Furthermore, we found that hyperphosphorylated tau was easier to be captured by AD P-tau and that aggregated tau was more difficult to be dephosphorylated than the non-aggregated tau by protein phosphatase 2A (PP2A). Based on these findings, we speculate that AD P-tau seeds hyperphosphorylated tau to form aggregates, which resist to the dephosphorylation by PP2A, resulting in hyperphosphorylation and pathology of tau.

Use of quantitative clinical pharmacology to improve early clinical development success in neurodegenerative diseases

INTRODUCTION

The success rate of pharmaceutical Research & Development (R&D) is much lower compared to other industries such as micro-electronics or aeronautics with the probability of a successful clinical development to approval in central nervous system (CNS) disorders hovering in the single digits (7%). Areas covered: Inspired by adjacent engineering-based industries, we argue that quantitative modeling in CNS R&D might improve success rates. We will focus on quantitative techniques in early clinical development, such as PharmacoKinetic-PharmacoDynamic modeling, clinical trial simulation, model-based meta-analysis and the mechanism-based physiology-based pharmacokinetic modeling, and quantitative systems pharmacology. Expert commentary: Mechanism-based computer modeling rely less on existing clinical datasets, therefore can better generalize than Big Data analytics, including prospectively and quantitatively predicting the clinical outcome of new drugs. More specifically, exhaustive post-hoc analysis of failed trials using individual virtual human trial simulation could illuminate underlying causes such as lack of sufficient functional target engagement, negative pharmacodynamic interactions with comedications and genotypes, and mismatched patient population. These insights are beyond the capacity of artificial intelligence (AI) methods as they are many more possible combinations than subjects. Unlike 'black box' approaches in AI, mechanism-based platforms are transparent and based on biologically sound assumptions that can be interrogated.

Tau Phosphorylation is Impacted by Rare AKAP9 Mutations Associated with Alzheimer Disease in African Americans

We studied the effect of two rare mutations (rs144662445 and rs149979685) in the A-kinase anchoring protein 9 (AKAP9) gene, previously associated with Alzheimer disease (AD) in African Americans (AA), on post-translational modifications of AD-related pathogenic molecules, amyloid precursor protein (APP) and microtubule-associated protein Tau using lymphoblastoid cell lines (LCLs) from 11 AA subjects with at least one AKAP9 mutation and 17 AA subjects lacking these mutations. LCLs were transduced by viral vectors expressing causative AD mutations in APP or human full-length wild type Tau. Cell lysates were analyzed for total APP, Aβ₄₀, and total and T181 phospho-Tau (pTau). AKAP9 mutations had no effect on Aβ₄₀/APP, but significantly increased pTau/Tau ratio in LCLs treated with phosphodiesterase-4 inhibitor rolipram, which activates protein kinase A. Proteomic analysis of Tau interactome revealed enrichment of RNA binding proteins and decrease of proteasomal molecules in rolipram-treated cells with AKAP9 mutations. This study shows the impact of rare functional AKAP9 mutations on Tau, a central mechanism of AD pathogenesis, in LCLs derived from AD and control subjects.

A mathematical model of multisite phosphorylation of tau protein

Abnormal tau metabolism followed by formation of tau deposits causes a number of neurodegenerative diseases called tauopathies including Alzheimer's disease. Hyperphosphorylation of tau protein precedes tau aggregation and is a topic of interest for the development of pharmacological interventions to prevent pathology progression at early stages. The development of a mathematical model of multisite phosphorylation of tau would be helpful for searching for the targets of pharmacological interventions and candidates for biomarkers of pathology progression. In the present study, we for the first time developed a model of multisite phosphorylation of tau protein and elucidated the relative contribution of kinases to phosphorylation of distinct sites. The model describes phosphorylation of tau or PKA-prephosphorylated tau by GSK3β and CDK5 and dephosphorylation by PP2A, accurately reproducing the data for short-term kinetics of tau (de)phosphorylation. Our results suggest that kinase inhibition may more specifically prevent tau hyperphosphorylation, e.g., on PHF sites, which are key biomarkers of pathological changes in Alzheimer's disease. The main features of our model are partial phosphorylation of tau residues and merging of random and sequential mechanisms of multisite phosphorylation within the framework of the probability-based approach assuming independent phosphorylation events.

Inhibition of glycogen synthase kinase-3 by BTA-EG4 reduces tau abnormalities in an organotypic brain slice culture model of Alzheimer's disease

Organotypic brain slice culture models provide an alternative to early stage in vivo studies as an integrated tissue system that can recapitulate key disease features, thereby providing an excellent platform for drug screening. We recently described a novel organotypic 3xTg-AD mouse brain slice culture model with key Alzheimer’s disease-like changes. We now highlight the potential of this model for testing disease-modifying agents and show that results obtained following in vivo treatment are replicated in brain slice cultures from 3xTg-AD mice. Moreover, we describe novel effects of the amyloid-binding tetra (ethylene glycol) derivative of benzothiazole aniline, BTA-EG₄, on tau. BTA-EG₄ significantly reduced tau phosphorylation in the absence of any changes in the amounts of amyloid precursor protein, amyloid-β or synaptic proteins. The reduction in tau phosphorylation was associated with inactivation of the Alzheimer’s disease-relevant major tau kinase, GSK-3. These findings highlight the utility of 3xTg-AD brain slice cultures as a rapid and reliable in vitro method for drug screening prior to in vivo testing. Furthermore, we demonstrate novel tau-directed effects of BTA-EG₄ that are likely related to the ability of this agent to inactivate GSK-3. Our findings support the further exploration of BTA-EG₄ as a candidate therapeutic for Alzheimer’s disease.

Deletion of Type-2 Cannabinoid Receptor Induces Alzheimer's Disease-Like Tau Pathology and Memory Impairment Through AMPK/GSK3β Pathway

Although several studies have shown that type-2 cannabinoid receptor (CB2R) is involved in Alzheimer's disease (AD) pathology, the effects of CB2R on AD-like tau abnormal phosphorylation and its underlying mechanism remain unclear. Herein, we employed the CB2R^-/- mice as the animal model to explore roles of CB2R in regulating tau phosphorylation and brain function. We found that CB2R^-/- mice display AD-like tau hyperphosphorylation, hippocampus-dependent memory impairment, increase of GSK3β activity, decrease of AMPK and Sirt1 activity and mitochondria dysfunction. Interestingly, AICAR or resveratrol (AMPK agonist) could efficiently rescue most alternations caused by solo deletion of CB2R in CB2R^-/- mice. Moreover, JWH133, a selective agonist of CB2R, reduces phosphorylation of tau and GSK3β activity in HEK293 tau cells, but the effects of JWH133 on phosphorylation of tau and GSK3β disappeared while blocking AMPK activity with compound C or Prkaa2-RNAi. Taken together, our study indicated that deletion of CB2R induces behavior damage and AD-like pathological alternation via AMPK/GSK3β pathway. These findings proved that CB2R/AMPK/GSK3β pathway can be a promising new drug target for AD.

Insulin deficiency results in reversible protein kinase A activation and tau phosphorylation

Alzheimer's disease (AD) is a highly prevalent multifactorial disease for which Diabetes Mellitus (DM) is a risk factor. Abnormal phosphorylation and aggregation of tau is a key hallmark of AD. In animal models, DM induces or exacerbates the phosphorylation of tau, suggesting that DM may influence the risk at AD by directly facilitating tau pathology. Previously we reported that tau phosphorylation induced in response to metabolic stress is reversible. Since identification and understanding of early players in tau pathology is pivotal for therapeutic intervention, we here investigated the mechanism underlying tau phosphorylation in the diabetic brain and its potential for reversibility. To model DM we used streptozotocin-treatment to induce insulin deficiency in rats. Insulin depletion leads to increased tau phosphorylation in the brain and we investigated the activation status of known tau kinases and phosphatases in this model. We identified protein kinase A (PKA) as a tau kinase activated by DM in the brain. The potential relevance of this signaling pathway to AD pathogenesis is indicated by the increased level of active PKA in temporal cortex of early stage AD patients. Our data indicate that activation of PKA and tau phosphorylation are associated with insulin deficiency per se, rather than the downstream energy deprivation. In vitro studies confirm that insulin deficiency results in PKA activation and tau phosphorylation. Strikingly, both active PKA and induced tau phosphorylation are reversed upon insulin treatment in the steptozotocin animal model. Our data identify insulin deficiency as a direct trigger that induces the activity of the tau kinase PKA and results in tau phosphorylation. The reversibility upon insulin treatment underscores the potential of insulin as an early disease-modifying intervention in AD and other tauopathies.