p35/Cdk5 pathway mediates soluble amyloid-beta peptide-induced tau phosphorylation in vitro

Early oxidative stress and DNA damage in Aβ-burdened hippocampal neurons in an Alzheimer's-like transgenic rat model

Oxidative stress is a key contributor to AD pathology. However, the earliest role of pre-plaque neuronal oxidative stress, remains elusive. Using laser microdissected hippocampal neurons extracted from McGill-R-Thy1-APP transgenic rats we found that intraneuronal amyloid beta (iAβ)-burdened neurons had increased expression of genes related to oxidative stress and DNA damage responses including Ercc2, Fancc, Sod2, Gsr, and Idh1. DNA damage was further evidenced by increased neuronal levels of XPD (Ercc2) and γH2AX foci, indicative of DNA double stranded breaks (DSBs), and by increased expression of Ercc6, Rad51, and Fen1, and decreased Sirt6 in hippocampal homogenates. We also found increased expression of synaptic plasticity genes (Grin2b (NR2B), CamkIIα, Bdnf, c-fos, and Homer1A) and increased protein levels of TOP2β. Our findings indicate that early accumulation of iAβ, prior to Aβ plaques, is accompanied by incipient oxidative stress and DSBs that may arise directly from oxidative stress or from maladaptive synaptic plasticity.

Truncated Dyrk1A aggravates neuronal apoptosis by inhibiting ASF-mediated Bcl-x exon 2b inclusion

AIM

Aggravated neuronal loss, caused mainly by neuronal apoptosis, is observed in the brain of patients with Alzheimer's disease (AD) and animal models of AD. A truncated form of Dual-specific and tyrosine phosphorylation-regulated protein kinase 1A (Dyrk1A) plays a vital role in AD pathogenesis. Downregulation of anti-apoptotic Bcl-xL is tightly correlated with neuronal loss in AD. However, the molecular regulation of neuronal apoptosis and Bcl-x expression by Dyrk1A in AD remains largely elusive. Here, we aimed to explore the role and molecular mechanism of Dyrk1A in apoptosis.

METHODS

Cell Counting Kit-8 (CCK8), flow cytometry, and TdT-mediated dUTP Nick-End Labeling (TUNEL) were used to check apoptosis. The cells, transfected with Dyrk1A or/and ASF with Bcl-x minigene, were used to assay Bcl-x expression by RT-PCR and Western blots. Co-immunoprecipitation, autoradiography, and immunofluorescence were conducted to check the interaction of ASF and Dyrk1A. Gene set enrichment analysis (GSEA) of apoptosis-related genes was performed in mice overexpressing Dyrk1A (TgDyrk1A) and AD model 5xFAD mice.

RESULTS

Dyrk1A promoted Bcl-xS expression and apoptosis. Splicing factor ASF promoted Bcl-x exon 2b inclusion, leading to increased Bcl-xL expression. Dyrk1A suppressed ASF-mediated Bcl-x exon 2b inclusion via phosphorylation. The C-terminus deletion of Dyrk1A facilitated its binding and kinase activity to ASF. Moreover, Dyrk1a1-483 further suppressed the ASF-mediated Bcl-x exon 2b inclusion and aggravated apoptosis. The truncated Dyrk1A, increased Bcl-xS, and enrichment of apoptosis-related genes was observed in the brain of 5xFAD mice.

CONCLUSIONS

We speculate that increased Dyrk1A and truncated Dyrk1A may aggravate neuronal apoptosis by decreasing the ratio of Bcl-xL/Bcl-xS via phosphorylating ASF in AD.

Types of memory, dementia, Alzheimer's disease, and their various pathological cascades as targets for potential pharmacological drugs

Alzheimer's disease (AD) is the most common type of dementia accounting for 90% of cases; however, frontotemporal dementia, vascular dementia, etc. prevails only in a minority of populations. The term dementia is defined as loss of memory which further takes several other categories of memories like working memory, spatial memory, fear memory, and long-term, and short-term memory into consideration. In this review, these memories have critically been elaborated based on context, duration, events, appearance, intensity, etc. The most important part and purpose of the review is the various pathological cascades as well as molecular levels of targets of AD, which have extracellular amyloid plaques and intracellular hyperphosphorylated tau protein as major disease hallmarks. There is another phenomenon that either leads to or arises from the above-mentioned hallmarks, such as oxidative stress, mitochondrial dysfunction, neuroinflammation, cholinergic dysfunction, and insulin resistance. Several potential drugs like antioxidants, anti-inflammatory drugs, acetylcholinesterase inhibitors, insulin mimetics or sensitizers, etc. studied in various previous preclinical or clinical reports were put as having the capacity to act on these pathological targets. Additionally, agents directly or indirectly targeting amyloid and tau were also discussed. This could be further investigated in future research.

Tau Protein Alterations Induced by Hypobaric Hypoxia Exposure

Tauopathies are a group of neurodegenerative diseases whose central feature is dysfunction of the microtubule-associated protein tau (MAPT). Although the exact etiology of tauopathies is still unknown, it has been hypothesized that their onset may occur up to twenty years before the clear emergence of symptoms, which has led to questions about whether the prognosis of these diseases can be improved by, for instance, targeting the factors that influence tauopathy development. One such factor is hypoxia, which is strongly linked to Alzheimer's disease because of its association with obstructive sleep apnea and has been reported to affect molecular pathways related to the dysfunction and aggregation of tau proteins and other biomarkers of neurological damage. In particular, hypobaric hypoxia exposure increases the activation of several kinases related to the hyperphosphorylation of tau in neuronal cells, such as ERK, GSK3β, and CDK5. In addition, hypoxia also increases the levels of inflammatory molecules (IL-β1, IL-6, and TNF-α), which are also associated with neurodegeneration. This review discusses the many remaining questions regarding the influence of hypoxia on tauopathies and the contribution of high-altitude exposure to the development of these diseases.

Tau-targeting therapies for Alzheimer disease: current status and future directions

Alzheimer disease (AD) is the most common cause of dementia in older individuals. AD is characterized pathologically by amyloid-β (Aβ) plaques and tau neurofibrillary tangles in the brain, with associated loss of synapses and neurons, which eventually results in dementia. Many of the early attempts to develop treatments for AD focused on Aβ, but a lack of efficacy of these treatments in terms of slowing disease progression led to a change of strategy towards targeting of tau pathology. Given that tau shows a stronger correlation with symptom severity than does Aβ, targeting of tau is more likely to be efficacious once cognitive decline begins. Anti-tau therapies initially focused on post-translational modifications, inhibition of tau aggregation and stabilization of microtubules. However, trials of many potential drugs were discontinued because of toxicity and/or lack of efficacy. Currently, the majority of tau-targeting agents in clinical trials are immunotherapies. In this Review, we provide an update on the results from the initial immunotherapy trials and an overview of new therapeutic candidates that are in clinical development, as well as considering future directions for tau-targeting therapies.

Pathophysiological Association of Alzheimer's Disease and Hypertension: A Clinical Concern for Elderly Population

Alzheimer's disease (AD), the most common cause of dementia and the fifth leading cause of death in the adult population has a complex pathophysiological link with hypertension (HTN). A growing volume of published literature on a parallel elevation of blood pressure (BP), amyloid plaques, and neurofibrillary tangles formation in post-middle of human brain cells has developed new, widely accepting foundations on this association. In particular, HTN in elderly life mediates cerebral blood flow dysfunction, neuronal dysfunction, and significant decline in cognitive impairment, primarily in the late-life populace, governing the onset of AD. Thus, HTN is an established risk factor for AD. Considering the impact of AD, 1.89 million deaths annually, and the failure of palliative therapies to cure AD, the scientific research community is looking to adopt integrated approaches to target early modified risk factors like HTN to reduce AD burden. The current review highlights the significance and impact of HTN-based prevention in lowering the AD burden in the elderly by providing a comprehensive overview of the physiological relationship between AD and HTN with an in-detail explanation of the role and applications of pathological biomarkers in this clinical association. The review will gain worth in presenting new insights and providing inclusive discussion on the correlation between HTN and cognitive impairment. It will increase across a wider scientific audience to expand understanding of this pathophysiological association.

Chronotherapeutic neuroprotective effect of verapamil against lipopolysaccharide-induced neuroinflammation in mice through modulation of calcium-dependent genes

Background

Neuroinflammation is a major mechanism in neurodegenerative diseases such as Alzheimer’s disease (AD), which is a major healthcare problem. Notwithstanding of ample researches figured out possible molecular mechanisms underlying the pathophysiology of AD, there is no definitive therapeutics that aid in neuroprotection. Therefore, searching for new agents and potential targets is a critical demand. We aimed to investigate the neuroprotective effect of verapamil (VRP) against lipopolysaccharide (LPS)-induced neuroinflammation in mice and whether the time of VRP administration could affect its efficacy.

Methods

Forty male albino mice were used and were divided into normal control, LPS only, morning VRP, and evening VRP. Y-maze and pole climbing test were performed as behavioral tests. Hematoxylin and eosin together with Bielschowsky silver staining were done to visualize neuroinflammation and phosphorylated tau protein (pTAU); respectively. Additionally, the state of mitochondria, the levels of microglia-activation markers, inflammatory cytokines, intracellular Ca2+, pTAU, and Ca2+-dependent genes involving Ca2+/ calmodulin dependent kinase II (CAMKII) isoforms, protein kinase A (PKA), cAMP response element-binding protein (CREB), and brain-derived neurotrophic factor (BDNF), with the level of VRP in the brain tissue were measured.

Results

LPS successfully induced neuroinflammation and hyperphosphorylation of tau protein, which was indicated by elevated levels of microglia markers, inflammatory cytokines, and intracellular Ca2+ with compromised mitochondria and downregulated CAMKII isoforms, PKA, CREB and BDNF. Pretreatment with VRP showed significant enhancement in the architecture of the brain and in the behavioral tests as indicated by the measured parameters. Moreover, morning VRP exhibited better neuroprotective profile compared to the evening therapy.

Conclusions

VRP highlighted a multilevel of neuroprotection through anti-inflammatory activity, Ca2+ blockage, and regulation of Ca2+-dependent genes. Furthermore, chronotherapy of VRP administration should be consider to achieve best therapeutic efficacy.

Graphical Abstract

Inhibition of the CDK5/caspase-3 Pathway by p5-TAT Protects Hippocampal Neurogenesis and Alleviates Radiation-induced Cognitive Dysfunction

Radiation-induced cognitive dysfunction is a common complication associated with cranial radiation therapy. Inhibition of hippocampal neurogenesis and proliferation plays a critical role in this complication. Relieving hippocampal apoptosis may significantly protect hippocampal neurogenesis and proliferation. Previous studies have demonstrated that hyperactivity of cyclin-dependent kinase 5 (Cdk5) is closely related to apoptosis. The exact molecular changes and function of Cdk5 in radiation-induced cognitive dysfunction are still not clear. Whether inhibition of Cdk5 and the relevant caspase-3 could improve hippocampal neurogenesis and ameliorate radiation-induced cognitive dysfunction needs further exploration. We hypothesized that inhibition of the Cdk5/caspase-3 pathway by p5-TAT could protect hippocampal neurogenesis and alleviate radiation-induced cognitive dysfunction. In our study, we reported that radiation induced hyperactivity of Cdk5 accompanied by elevation of the levels of cleaved caspase-3, a marker of neuronal apoptosis. Inhibition of hippocampal neurogenesis and proliferation as well as cognitive dysfunction was also observed. p5-TAT, a specific inhibitor of Cdk5, decreased the overactivation of Cdk5 without affecting the levels of Cdk5 activators. Additionally, this treatment suppressed the expression of cleaved caspase-3. We further demonstrated that p5-TAT treatment reduced hippocampal dysfunction and improved behavioral performance. Therefore, Cdk5 inhibition by the small peptide p5-TAT is a promising therapeutic strategy for radiation-induced cognitive dysfunction.

Molecular mechanisms in Alzheimer's disease and the impact of physical exercise with advancements in therapeutic approaches

Alzheimer's disease (AD) is one of the most common, severe neurodegenerative brain disorder characterized by the accumulation of amyloid-beta plaques, neurofibrillary tangles in the brain causing neural disintegration, synaptic dysfunction, and neuronal death leading to dementia. Although many US-FDA-approved drugs like Donepezil, Rivastigmine, Galantamine are available in the market, their consumption reduces only the symptoms of the disease but fails in potency to cure the disease. This disease affects many individuals with aging. Combating the disease tends to be very expensive. This review focuses on biochemical mechanisms in the neuron both at normal and AD state with relevance to the tau hypothesis, amyloid hypothesis, the risk factors influencing dementia, oxidative stress, and neuroinflammation altogether integrated with neurodegeneration. A brief survey is carried out on available biomarkers in the diagnosis of the disease, drugs used for the treatment, and the challenges in approaching therapeutic targets in inhibiting the disease pathologies. This review conjointly assesses the demerits with the inefficiency of drugs to reach targets, their side effects, and toxicity. Optimistically, this review directs on the advantageous strategies in using nanotechnology-based drug delivery systems to cross the blood-brain barrier for improving the efficacy of drugs combined with a novel neuronal stem cell therapy approach. Determinately, this review aims at the natural, non-therapeutic healing impact of physical exercise on different model organisms and the effect of safe neuromodulation treatments using repetitive Transcranial Magnetic Stimulation (rTMS), transcranial Electrical Stimulation (tES) in humans to control the disease pathologies prominent in enhancing the synaptic function.

Regional correlation of biochemical measures of amyloid and tau phosphorylation in the brain

Alzheimer’s disease (AD) neuropathologic change is characterized by amyloid plaques and neurofibrillary tangles (NFTs) that consist of aggregated amyloid beta (Abeta) and hyperphosphorylated tau proteins (p-tau), respectively. Although the global relationship between Abeta and p-tau has been studied for decades, it is still unclear whether a regional correlation exists between Abeta and p-tau in the human brain. Recent studies in cerebrospinal fluid (CSF) have suggested that tau phosphorylation at specific sites such as T217 is modified at an early stage of AD when amyloid plaques become detectable. We applied biochemical and mass spectrometry methods in human brain samples with and without Abeta plaque pathology to measure site-specific phosphorylation occupancies in soluble and insoluble tau. Our quantitative results identified multiple residues specifically hyper-phosphorylated in AD, including at sites T111, T153, S184 (or S185), T205, S208, T217, S262, and S285 in brain soluble tau. In contrast, the most enriched phosphorylated residues in brain insoluble tau were T111, S113, T153, T181, S199, S202, T205, T217, T231, S262, and S396. Tau phosphorylation occupancies in the insoluble fraction were relatively constant across brain regions, suggesting that tau has a consistent phosphorylation pattern once it has aggregated into NFTs. We did not find regional association between Abeta42 and insoluble tau. However, the phosphorylation profile of soluble tau in AD brain was highly correlated to that in AD CSF, which was analyzed in a previous study. We also found a higher regional association between total Abeta42 and soluble tau phosphorylation occupancy at residues T111, T153 and T217 in the brain. This study provides insights into regional interactions between amyloidosis and specific tau phosphorylated residues in the human brain and may explain the specific increases of tau species phosphorylation observed in AD CSF.

The Chemokine-like Receptor 1 Deficiency Improves Cognitive Deficits of AD Mice and Attenuates Tau Hyperphosphorylation via Regulating Tau Seeding

Pathologic features of Alzheimer's disease (AD) include accumulation of amyloid β (Aβ) and hyperphosphorylated tau protein. We have shown previously that the chemokine-like receptor 1 (CMKLR1) is a functional receptor for Aβ, and CMKLR1 contributes to the uptake of Aβ. However, it is unclear whether CMKLR1 ameliorates or aggravates the process of AD. Here, we show that deletion of the gene coding for CMKLR1 significantly increased Aβ deposits in brains of both male and female amyloid β precursor protein/presenilin-1 mice. However, it markedly decreased the mortality of these mice. Behavioral studies found that CMKLR1 deficiency improved cognitive impairment of male and female amyloid β precursor protein/presenilin-1 mice and intracerebroventricular-streptozotocin injection AD mice. We further explored the effect of CMKLR1 on tau pathology. We found that CMKLR1 deficiency or inhibition attenuated the hyperphosphorylation of tau in brains of AD mice in vivo and in the neuronal cells in vitro The expression of CMKLR1 on the neurons affected tau phosphorylation by participating in tau seeding. Together, these results uncover a novel mechanism of CMKLR1 in the pathologic process of AD and suggest that inhibiting the promotion effect of CMKLR1 on tau seeding may provide a new strategy for the treatment of AD.SIGNIFICANCE STATEMENT Evidence suggests that inflammation is involved in the pathologic progression of AD. The chemokine-like receptor 1 (CMKLR1), belonging to the family of GPCRs, is able to bind and uptake amyloid β. We show here, for the first time, that, although CMKLR1 deficiency increased amyloid β deposits in AD mice, it reduced the mortality and improved the cognitive deficits of AD mice. We furthermore show that CMKLR1 deficiency or inhibition attenuated tau hyperphosphorylation in brains of AD model mice in vivo and in neuronal cells in vitro Finally, we first discovered that the expression of CMKLR1 on neurons affected tau phosphorylation by participating in tau seeding. These findings suggest that inhibition of CMKLR1 may provide a new strategy for the treatment of AD.

Curcumin Downregulates GSK3 and Cdk5 in Scopolamine-Induced Alzheimer's Disease Rats Abrogating Aβ40/42 and Tau Hyperphosphorylation

Alzheimer's disease (AD) is the most common form of dementia. Extracellular amyloid-β (Aβ) aggregation and tau hyperphosphorylation are the key drivers of AD. Glycogen synthase kinase 3 (GSK3) and cyclin dependent kinase 5 (Cdk5) have been known as leading applicants arbitrating abnormal tau hyperphosphorylation. Thus, we evaluated the efficacy and underlying mechanism of action of curcumin in scopolamine-induced AD rats in our study. We found that curcumin-treated AD rats markedly reduced the levels of Aβ₄₀ and Aβ₄₂ in the brain and in the plasma in comparison to untreated AD rats. Moreover, the levels of phosphorylated tau at Ser396 (PHF13), Ser202/Thr205 (AT8), and Aβ_40/42 (MOAB2) were decreased significantly in AD rats treated with curcumin. Phospho-GSK3β (Tyr216), the active form of GSK3β, and total GSK3β were significantly decreased in AD rats treated with curcumin. Furthermore, Cdk5 and its activators p35 and p25 were significantly decreased in curcumin-treated AD rats. The reduced levels of Cdk5, p35, p25, and GSK3β in curcumin-treated AD rats may result decreased Aβ aggregation and tau hyperphosphorylation, thus ameliorating AD. Impaired spatial memory and locomotor activity in AD rats were partially reversed by curcumin. Therefore, curcumin, as a natural compound present in turmeric, may be a more effective therapeutic agent in the treatment of AD in humans.

Sex modulates the ApoE ε4 effect on brain tau deposition measured by 18F-AV-1451 PET in individuals with mild cognitive impairment

The strongest genetic risk factor for Alzheimer's disease (AD) is the Apolipoprotein E type 4 allele (ApoE ε4). The interaction between sex and ApoE ε4 carrier status on AD risk remains an area of intense investigation. We hypothesized that sex modulates the relationship between ApoE ε4 carrier status and brain tau deposition (a quantitative endophenotype in AD) in individuals with mild cognitive impairment (MCI).

Methods: Preprocessed ¹⁸F-AV-1451 tau and ¹⁸F-AV-45 amyloid PET images, T1-weighted structural magnetic resonance imaging (MRI) scans, demographic information, and cerebrospinal fluid (CSF) total tau (t-tau) and phosphorylated tau (p-tau) measurements from 108 MCI subjects in the Alzheimer's Disease Neuroimaging Initiative (ADNI) database were included. After downloading pre-processed images from ADNI, an iterative reblurred Van Cittertiteration partial volume correction (PVC) method was applied to all PET images. MRIs were used for PET spatial normalization. Regions of interest (ROIs) were defined in standard space, and standardized uptake value ratio (SUVR) images relative to cerebellum were computed. ApoE ε4 by sex interaction analyses on ¹⁸F-AV-1451 and CSF tau (t-tau, p-tau) were assessed using generalized linear models. The association between ¹⁸F-AV-1451 SUVR and CSF tau (t-tau, p-tau) was assessed.

Results: After applying PVC and controlling for age, education level and global cortical ¹⁸F-AV-45 SUVR, we found that the entorhinal cortex, amygdala, parahippocampal gyrus, posterior cingulate, and occipital ROIs exhibited a significant ApoE ε4 by sex interaction effect (false discovery rate P < 0.1) among MCI individuals. We also found a significant ApoE ε4 by sex interaction effect on CSF t-tau and p-tau. ¹⁸F-AV-1451 SUVR in the 5 ROIs with ApoE ε4 by sex interaction was significantly correlated with CSF p-tau and t-tau.

Conclusions: Our findings suggest that women are more susceptible to ApoE ε4-associated accumulation of neurofibrillary tangles in MCI compared to males. Both CSF tau (p-tau, t-tau) and brain tau PET are robust quantitative biomarkers for studying ApoE ε4 by sex effects on brain tau deposition in MCI participants.

In vivo induction of membrane damage by β-amyloid peptide oligomers

Exposure to the β-amyloid peptide (Aβ) is toxic to neurons and other cell types, but the mechanism(s) involved are still unresolved. Synthetic Aβ oligomers can induce ion-permeable pores in synthetic membranes, but whether this ability to damage membranes plays a role in the ability of Aβ oligomers to induce tau hyperphosphorylation, or other disease-relevant pathological changes, is unclear. To examine the cellular responses to Aβ exposure independent of possible receptor interactions, we have developed an in vivo C. elegans model that allows us to visualize these cellular responses in living animals. We find that feeding C. elegans E. coli expressing human Aβ induces a membrane repair response similar to that induced by exposure to the CRY5B, a known pore-forming toxin produced by B. thuringensis. This repair response does not occur when C. elegans is exposed to an Aβ Gly37Leu variant, which we have previously shown to be incapable of inducing tau phosphorylation in hippocampal neurons. The repair response is also blocked by loss of calpain function, and is altered by loss-of-function mutations in the C. elegans orthologs of BIN1 and PICALM, well-established risk genes for late onset Alzheimer's disease. To investigate the role of membrane repair on tau phosphorylation directly, we exposed hippocampal neurons to streptolysin O (SLO), a pore-forming toxin that induces a well-characterized membrane repair response. We find that SLO induces tau hyperphosphorylation, which is blocked by calpain inhibition. Finally, we use a novel biarsenical dye-tagging approach to show that the Gly37Leu substitution interferes with Aβ multimerization and thus the formation of potentially pore-forming oligomers. We propose that Aβ-induced tau hyperphosphorylation may be a downstream consequence of induction of a membrane repair process.

Preclinical, phase I, and phase II investigational clinical trials for treatment of progressive supranuclear palsy

INTRODUCTION

Our understanding of the pathological basis of progressive supranuclear palsy (PSP), as the most common atypical parkinsonian syndrome, has greatly increased in recent years and a number of disease-modifying therapies are under evaluation as a result of these advances.

AREAS COVERED

In this review, we discuss disease-modifying therapeutic options which are currently under evaluation or have been evaluated in preclinical or clinical trials based on their targeted pathophysiologic process. The pathophysiologic mechanisms are broadly divided into three main categories: genetic mechanisms, abnormal post-translational modifications of tau protein, and transcellular tau spread.

EXPERT OPINION

Once the best therapeutic approaches are identified, it is likely that some combination of interventions will need to be evaluated, but this will take time. It is critical to treat patients at early stages, and development of the Movement Disorder Society PSP diagnostic criteria is an important step in this direction. In addition, development of biological biomarkers such as tau PET and further refinement of tau ligands may help both diagnose early and measure disease progression. In the meantime, a comprehensive, personalized interdisciplinary approach to this disease is absolutely necessary.

Recent Advances in the Study of Bipolar/Rod-Shaped Microglia and their Roles in Neurodegeneration

Microglia are the resident immune cells of the central nervous system (CNS) and they contribute to primary inflammatory responses following CNS injuries. The morphology of microglia is closely associated with their functional activities. Most previous research efforts have attempted to delineate the role of ramified and amoeboid microglia in the pathogenesis of neurodegenerative diseases. In addition to ramified and amoeboid microglia, bipolar/rod-shaped microglia were first described by Franz Nissl in 1899 and their presence in the brain was closely associated with the pathology of infectious diseases and sleeping disorders. However, studies relating to bipolar/rod-shaped microglia are very limited, largely due to the lack of appropriate in vitro and in vivo experimental models. Recent studies have reported the formation of bipolar/rod-shaped microglia trains in in vivo models of CNS injury, including diffuse brain injury, focal transient ischemia, optic nerve transection and laser-induced ocular hypertension (OHT). These bipolar/rod-shaped microglia formed end-to-end alignments in close proximity to the adjacent injured axons, but they showed no interactions with blood vessels or other types of glial cell. Recent studies have also reported on a highly reproducible in vitro culture model system to enrich bipolar/rod-shaped microglia that acts as a powerful tool with which to characterize this form of microglia. The molecular aspects of bipolar/rod-shaped microglia are of great interest in the field of CNS repair. This review article focuses on studies relating to the morphology and transformation of microglia into the bipolar/rod-shaped form, along with the differential gene expression and spatial distribution of bipolar/rod-shaped microglia in normal and pathological CNSs. The spatial arrangement of bipolar/rod-shaped microglia is crucial in the reorganization and remodeling of neuronal and synaptic circuitry following CNS injuries. Finally, we discuss the potential neuroprotective roles of bipolar/rod-shaped microglia, and the possibility of transforming ramified/amoeboid microglia into bipolar/rod-shaped microglia. This will be of considerable clinical benefit in the development of novel therapeutic strategies for treating various neurodegenerative diseases and promoting CNS repair after injury.

Effect of Huperzine A on Aβ-induced p65 of astrocyte in vitro

Alzheimer's disease (AD) is the most common cause of dementia. Its pathology often accompanies inflammatory action, and astrocytes play important roles in such procedure. Rela(p65) is one of significant message factors in NF-κB pathway which has been reported high expression in astrocyte treated by Aβ. HupA, an alkaloid isolated from Chinese herb Huperzia serrata, has been widely used to treat AD and observations reflected that it improves memory and cognitive capacity of AD patients. To reveal its molecular mechanisms on p65, we cultured astrocytes, built Aβ-induced AD model, treated astrocytes with HupA at different concentrations, assayed cell viability with MTT, and detected p65 expression by immunohistochemistry and PCR. Our results revealed that treatment with 10 μM Aβ1-42 for 24 h induced a significant increase of NF-κB in astrocytes; HupA significantly down-regulated p65 expression induced by Aβ in astrocytes. This study infers that HupA can regulate NF-κB pathway to treat AD.

Cdk5/p25 specific inhibitory peptide TFP5 rescues the loss of dopaminergic neurons in a sub-acute MPTP induced PD mouse model

Parkinson's disease (PD) is pathologically characterized by progressively loss of dopaminergic (DA) neurons in the substantia nigra pars compacta (SNpc) and the formation of Lewy bodies. In 1-methyl-4-phenyl-1, 2, 3, 6-tetrahydropyridine (MPTP) induced PD mice models, the calpain- cyclin-dependent kinase 5 (Cdk5)-myocyte enhancer factor 2 (MEF2) signaling has been proven in governing dopaminergic neuronal death. Under MPTP insult, p35 is cleaved by calpain into p25, which binds to Cdk5 and exhibits hyperactivity of Cdk5/p25. Cdk5/p25 inactivates MEF2, a survivor factor, which is critical for DA neuronal death. In this study, neuroprotective effect of the Cdk5/p25 specific peptide, TFP5, was evaluated in sub-acute MPTP induced PD mouse model by intraperitoneal (i.p.) injection of MPTP for five consecutive days. The results indicated that the levels of p35 and p25, and p25/p35 ratio increased in the sub-acute MPTP mice. TFP5 broadly reached cortex neuron, hippocampus and SNpc areas after i.p. injections. Pretreatment with 45mg/kg/day TFP5, as well as 10mgkg/day Cdk5 inhibitor roscovitine, for three days significantly rescued DA neuronal loss up to 9.8% or 9.7% respectively compared to the saline treated group. Treatment of TFP5 and roscovitine reduced the levels of inactive form of MEF2 and cleaved caspase 3, thus protected apoptosis of DA neurons against MPTP insult. Our results propose that TFP5 might be a potential therapeutic candidate for PD.

Upregulation of calpain activity precedes tau phosphorylation and loss of synaptic proteins in Alzheimer's disease brain

Alterations in calcium homeostasis are widely reported to contribute to synaptic degeneration and neuronal loss in Alzheimer’s disease. Elevated cytosolic calcium concentrations lead to activation of the calcium-sensitive cysteine protease, calpain, which has a number of substrates known to be abnormally regulated in disease. Analysis of human brain has shown that calpain activity is elevated in AD compared to controls, and that calpain-mediated proteolysis regulates the activity of important disease-associated proteins including the tau kinases cyclin-dependent kinase 5 and glycogen kinase synthase-3. Here, we sought to investigate the likely temporal association between these changes during the development of sporadic AD using Braak staged post-mortem brain. Quantification of protein amounts in these tissues showed increased activity of calpain-1 from Braak stage III onwards in comparison to controls, extending previous findings that calpain-1 is upregulated at end-stage disease, and suggesting that activation of calcium-sensitive signalling pathways are sustained from early stages of disease development. Increases in calpain-1 activity were associated with elevated activity of the endogenous calpain inhibitor, calpastatin, itself a known calpain substrate. Activation of the tau kinases, glycogen-kinase synthase-3 and cyclin-dependent kinase 5 were also found to occur in Braak stage II-III brain, and these preceded global elevations in tau phosphorylation and the loss of post-synaptic markers. In addition, we identified transient increases in total amyloid precursor protein and pre-synaptic markers in Braak stage II-III brain, that were lost by end stage Alzheimer's disease, that may be indicative of endogenous compensatory responses to the initial stages of neurodegeneration. These findings provide insight into the molecular events that underpin the progression of Alzheimer's disease, and further highlight the rationale for investigating novel treatment strategies that are based on preventing abnormal calcium homeostasis or blocking increases in the activity of calpain or important calpain substrates.

Behind the curtain of tauopathy: a show of multiple players orchestrating tau toxicity

tau, a microtubule-associated protein, directly binds with microtubules to dynamically regulate the organization of cellular cytoskeletons, and is especially abundant in neurons of the central nervous system. Under disease conditions such as Pick's disease, progressive supranuclear palsy, frontotemporal dementia, parkinsonism linked to chromosome 17 and Alzheimer's disease, tau proteins can self-assemble to paired helical filaments progressing to neurofibrillary tangles. In these diseases, collectively referred to as "tauopathies", alterations of diverse tau modifications including phosphorylation, metal ion binding, glycosylation, as well as structural changes of tau proteins have all been observed, indicating the complexity and variability of factors in the regulation of tau toxicity. Here, we review our current knowledge and hypotheses from relevant studies on tau toxicity, emphasizing the roles of phosphorylations, metal ions, folding and clearance control underlining tau etiology and their regulations. A summary of clinical efforts and associated findings of drug candidates under development is also presented. It is hoped that a more comprehensive understanding of tau regulation will provide us with a better blueprint of tau networking in neuronal cells and offer hints for the design of more efficient strategies to tackle tau-related diseases in the future.

Caspase-dependent degradation of MDMx/MDM4 cell cycle regulatory protein in amyloid β-induced neuronal damage

MDMx/MDM4 is a negative regulator of the p53 tumor suppressor protein and is necessary for survival in dividing cells. MDMx is also expressed in postmitotic neurons, with prosurvival roles that are independent of its extensively described roles in carcinogenesis. We and others have shown a role for MDMx loss in neuronal death in vitro and in vivo in several neurodegenerative diseases. Further, we have recently shown that MDMx is targeted for proteolytic degradation by calcium-dependent proteases, calpains, in neurons in vitro, and that MDMx overexpression provided partial neuroprotection in a model of HIV-associated neurodegeneration. Here, we assessed whether amyloid β (Aβ)-induced MDMx degradation occurred in Alzheimer's Disease (AD) models. Our data shows an age-dependent reduction in MDMx levels in cholinergic neurons within the cortex of adult mice expressing the swedish mutant of the amyloid precursor protein, APP in the Tg2576 murine model of AD. In vitro, Aβ treatment of primary cortical neurons led to the caspase-dependent MDMx degradation. Our findings suggest that MDMx degradation associated with neuronal death occurs via caspase activation in neurons, and that the progressive loss of MDMx protein represents a potential mechanism of Aβ-induced neuronal death during disease progression in AD.

Cdk5 at crossroads of protein oligomerization in neurodegenerative diseases: facts and hypotheses

Cyclin-dependent kinase 5 (Cdk5) is involved in proper neurodevelopment and brain function and serves as a switch between neuronal survival and death. Overactivation of Cdk5 is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important link between disease-initiating factors and cell death effectors. A common hallmark of neurodegenerative disorders is incorrect folding of specific proteins, thus leading to their intra- and extracellular accumulation in the nervous system. Abnormal Cdk5 signaling contributes to dysfunction of individual proteins and has a substantial role in either direct or indirect interactions of proteins common to, and critical in, different neurodegenerative diseases. While the roles of Cdk5 in α-synuclein (ASN) - tau or β-amyloid peptide (Aβ) - tau interactions are well documented, its contribution to many other pertinent interactions, such as that of ASN with Aβ, or interactions of the Aβ - ASN - tau triad with prion proteins, did not get beyond plausible hypotheses and remains to be proven. Understanding of the exact position of Cdk5 in the deleterious feed-forward loop critical for development and progression of neurodegenerative diseases may help designing successful therapeutic strategies of several fatal neurodegenerative diseases. Cyclin-dependent kinase 5 (Cdk5) is associated with many neurodegenerative disorders such as Alzheimer's or Parkinson's diseases. It is believed that in those diseases Cdk5 may be an important factor involved in protein misfolding, toxicity and interaction. We suggest that Cdk5 may contribute to the vicious circle of neurotoxic events involved in the pathogenesis of different neurodegenerative diseases.

Atorvastatin prevents Aβ oligomer-induced neurotoxicity in cultured rat hippocampal neurons by inhibiting Tau cleavage

AIM

The proteolytic cleavage of Tau is involved in Aβ-induced neuronal dysfunction and cell death. In this study, we investigated whether atorvastatin could prevent Tau cleavage and hence prevent Aβ1-42 oligomer (AβO)-induced neurotoxicity in cultured cortical neurons.

METHODS

Cultured rat hippocampal neurons were incubated in the presence of AβOs (1.25 μmol/L) with or without atorvastatin pretreatment. ATP content and LDH in the culture medium were measured to assess the neuronal viability. Caspase-3/7 and calpain protease activities were detected. The levels of phospho-Akt, phospho-Erk1/2, phospho-GSK3β, p35 and Tau proteins were measured using Western blotting.

RESULTS

Treatment of the neurons with AβO significantly decreased the neuronal viability, induced rapid activation of calpain and caspase-3/7 proteases, accompanied by Tau degradation and relatively stable fragments generated in the neurons. AβO also suppressed Akt and Erk1/2 kinase activity, while increased GSK3β and Cdk5 activity in the neurons. Pretreatment with atorvastatin (0.5, 1, 2.5 μmol/L) dose-dependently inhibited AβO-induced activation of calpain and caspase-3/7 proteases, and effectively diminished the generation of Tau fragments, attenuated synaptic damage and increased neuronal survival. Atorvastatin pretreatment also prevented AβO-induced decreases in Akt and Erk1/2 kinase activity and the increases in GSK3β and Cdk5 kinase activity.

CONCLUSION

Atorvastatin prevents AβO-induced neurotoxicity in cultured rat hippocampal neurons by inhibiting calpain- and caspase-mediated Tau cleavage.

Cyclin I and p35 determine the subcellular distribution of Cdk5

The atypical cyclin-dependent kinase 5 (Cdk5) serves an array of different functions in cell biology. Among these are axonal guidance, regulation of intercellular contacts, cell differentiation, and prosurvival signaling. The variance of these functions suggests that Cdk5 activation comes to pass in different cellular compartments. The kinase activity, half-life, and substrate specificity of Cdk5 largely depend on specific activators, such as p25, p35, p39, and cyclin I. We hypothesized that the subcellular distribution of Cdk5 activators also determines the localization of the Cdk5 protein and sets the stage for targeted kinase activity within distinct cellular compartments to suit the varying roles of Cdk5. Cdk5 localization was analyzed in murine kidney and brain slices of wild-type and cyclin I- and/or p35-null mice by immunohistochemistry and in cultured mouse podocytes using immunofluorescence labeling, as well as cell fractionation experiments. The predominance of cyclin I mediates the nuclear localization of Cdk5, whereas the predominance of p35 results in a membranous localization of Cdk5. These findings were further substantiated by overexpression of cyclin I and p35 with altered targeting characteristics in human embryonic kidney 293T cells. These studies reveal that the subcellular localization of Cdk5 is determined by its specific activators. This results in the directed Cdk5 kinase activity in specific cellular compartments dependent on the activator present and allows Cdk5 to serve multiple independent roles.

Aβ Influences Cytoskeletal Signaling Cascades with Consequences to Alzheimer's Disease

Abnormal signal transduction events can impact upon the cytoskeleton, affecting the actin and microtubule networks with direct relevance to Alzheimer's disease (AD). Cytoskeletal anomalies, in turn, promote atypical neuronal responses, with consequences for cellular organization and function. Neuronal cytoskeletal modifications in AD include neurofibrillary tangles, which result from aggregates of hyperphosphorylated tau protein. The latter is a microtubule (MT)-binding protein, whose abnormal phosphorylation leads to MT instability and consequently provokes irregularities in the neuronal trafficking pathways. Early stages of AD are also characterized by synaptic dysfunction and loss of dendritic spines, which correlate with cognitive deficit and impaired brain function. Actin dynamics has a prominent role in maintaining spine plasticity and integrity, thus providing the basis for memory and learning processes. Hence, factors that disrupt both actin and MT network dynamics will compromise neuronal function and survival. The peptide Aβ is the major component of senile plaques and has been described as a pivotal mediator of neuronal dystrophy and synaptic loss in AD. Here, we review Aβ-mediated effects on both MT and actin networks and focus on the relevance of the elicited cytoskeletal signaling events targeted in AD pathology.

Bioactive marine drugs and marine biomaterials for brain diseases

Marine invertebrates produce a plethora of bioactive compounds, which serve as inspiration for marine biotechnology, particularly in drug discovery programs and biomaterials development. This review aims to summarize the potential of drugs derived from marine invertebrates in the field of neuroscience. Therefore, some examples of neuroprotective drugs and neurotoxins will be discussed. Their role in neuroscience research and development of new therapies targeting the central nervous system will be addressed, with particular focus on neuroinflammation and neurodegeneration. In addition, the neuronal growth promoted by marine drugs, as well as the recent advances in neural tissue engineering, will be highlighted.

Innate immunity in Alzheimer's disease: a complex affair

Alzheimer's disease (AD) is characterized by three major histopathological hallmarks: β-amyloid deposits, neurofibrillary tangles and gliosis. While neglected for decades, the neuroinflammatory processes coordinated by microglia are now accepted as etiologic events in AD evolution. Microglial cells are found in close vicinity to amyloid plaques and display various activation phenotypes determined by the expression of a wide range of cytokines, chemokines, and innate immune surface receptors. During the development of AD pathology, microglia fail to restrict amyloid plaques and may contribute to neurotoxicity and cognitive deficit. Nevertheless, under specific activation states, microglia can participate in cerebral amyloid clearance. This review focuses on the complex relationship between microglia and Aβ pathology, and highlights both deleterious and beneficial roles of microglial activation states in the context of AD. A deeper understanding of microglial biology will hopefully pave the way for next-generation AD therapeutic approaches aimed at harnessing these enigmatic innate immune cells of the central nervous system.

Calpain cleavage and inactivation of the sodium calcium exchanger-3 occur downstream of Aβ in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by pathological deposits of β-amyloid (Aβ) in senile plaques, intracellular neurofibrillary tangles (NFTs) comprising hyperphosphorylated aggregated tau, synaptic dysfunction and neuronal death. Substantial evidence indicates that disrupted neuronal calcium homeostasis is an early event in AD that could mediate synaptic dysfunction and neuronal toxicity. Sodium calcium exchangers (NCXs) play important roles in regulating intracellular calcium, and accumulating data suggests that reduced NCX function, following aberrant proteolytic cleavage of these exchangers, may contribute to neurodegeneration. Here, we show that elevated calpain, but not caspase-3, activity is a prominent feature of AD brain. In addition, we observe increased calpain-mediated cleavage of NCX3, but not a related family member NCX1, in AD brain relative to unaffected tissue and that from other neurodegenerative conditions. Moreover, the extent of NCX3 proteolysis correlated significantly with amounts of Aβ1-42. We also show that exposure of primary cortical neurons to oligomeric Aβ1-42 results in calpain-dependent cleavage of NCX3, and we demonstrate that loss of NCX3 function is associated with Aβ toxicity. Our findings suggest that Aβ mediates calpain cleavage of NCX3 in AD brain and therefore that reduced NCX3 activity could contribute to the sustained increases in intraneuronal calcium concentrations that are associated with synaptic and neuronal dysfunction in AD.

Specific inhibition of p25/Cdk5 activity by the Cdk5 inhibitory peptide reduces neurodegeneration in vivo

The aberrant hyperactivation of Cyclin-dependent kinase 5 (Cdk5), by the production of its truncated activator p25, results in the formation of hyperphosphorylated tau, neuroinflammation, amyloid deposition, and neuronal death in vitro and in vivo. Mechanistically, this occurs as a result of a neurotoxic insult that invokes the intracellular elevation of calcium to activate calpain, which cleaves the Cdk5 activator p35 into p25. It has been shown previously that the p25 transgenic mouse as a model to investigate the mechanistic implications of p25 production in the brain, which recapitulates deregulated Cdk5-mediated neuropathological changes, such as hyperphosphorylated tau and neuronal death. To date, strategies to inhibit Cdk5 activity have not been successful in targeting selectively aberrant activity without affecting normal Cdk5 activity. Here we show that the selective inhibition of p25/Cdk5 hyperactivation in vivo, through overexpression of the Cdk5 inhibitory peptide (CIP), rescues against the neurodegenerative pathologies caused by p25/Cdk5 hyperactivation without affecting normal neurodevelopment afforded by normal p35/Cdk5 activity. Tau and amyloid pathologies as well as neuroinflammation are significantly reduced in the CIP-p25 tetra transgenic mice, whereas brain atrophy and subsequent cognitive decline are reversed in these mice. The findings reported here represent an important breakthrough in elucidating approaches to selectively inhibit the p25/Cdk5 hyperactivation as a potential therapeutic target to reduce neurodegeneration.

Special Issue on "Cdk5 and Brain Disorders": Prologue

Cyclin-dependent kinase 5 (Cdk5) was identified almost two decades ago as a Tau kinase specific to the nervous system. Shortly after its discovery, it was revealed that this atypical member of the CDK family does not partner with cyclins but with two other proteins, p35 and p39. P35 is predominantly expressed in post-mitotic neurons, whereas p39 is expressed in many different tissues including the brain, pancreas, muscle cells, neutrophils, and many other cell types. A proline-directed serine/threonine (S/T) kinase, predominantly active in the nervous system, Cdk5 regulates a multitude of functions including nervous system development, neuronal migration, cytoskeletal dynamics, axonal guidance, synaptic plasticity, neurotransmission, neuronal survival and death, to mention a few. In association with its ubiquitous expression in other tissues, Cdk5 is implicated in a wide range of functions, such as gene transcription, vesicular transport, apoptosis, cell adhesion, migration, exocytosis, etc. A focal point of investigation surrounding Cdk5 is its deregulation in pathogenic processes of neurodegenerative disorders, which has emphasized on its hyperactivation by p25, a calpain-cleaved product of p35 leading to Tau and neurofilament hyperphosphorylation followed by neuronal death. What has intrigued researchers about Cdk5 is its tight regulation in carrying out many normal physiological functions while its deregulation under pathological conditions, is linked to neurodegenerative diseases like amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD), Neiman Pick's Type C disease and others. Between these two so-called 'good Cdk5 (Cdk5/p35)' and 'bad Cdk5 (Cdk5/p25)', the latter has become the target for therapeutic intervention in neurodegenerative disorders.

Tau phosphorylation and μ-calpain activation mediate the dexamethasone-induced inhibition on the insulin-stimulated Akt phosphorylation

Evidence has suggested that insulin resistance (IR) or high levels of glucocorticoids (GCs) may be linked with the pathogenesis and/or progression of Alzheimer's disease (AD). Although studies have shown that a high level of GCs results in IR, little is known about the molecular details that link GCs and IR in the context of AD. Abnormal phosphorylation of tau and activation of μ-calpain are two key events in the pathology of AD. Importantly, these two events are also related with GCs and IR. We therefore speculate that tau phosphorylation and μ-calpain activation may mediate the GCs-induced IR. Akt phosphorylation at Ser-473 (pAkt) is commonly used as a marker for assessing IR. We employed two cell lines, wild-type HEK293 cells and HEK293 cells stably expressing the longest human tau isoform (tau-441; HEK293/tau441 cells). We examined whether DEX, a synthetic GCs, induces tau phosphorylation and μ-calpain activation. If so, we examined whether the DEX-induced tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. The results showed that DEX increased tau phosphorylation and induced tau-mediated μ-calpain activation. Furthermore, pre-treatment with LiCl prevented the effects of DEX on tau phosphorylation and μ-calpain activation. Finally, both LiCl pre-treatment and calpain inhibition prevented the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation. In conclusion, our study suggests that the tau phosphorylation and μ-calpain activation mediate the DEX-induced inhibition on the insulin-stimulated Akt phosphorylation.

Cdk5/p25-induced cytosolic PLA2-mediated lysophosphatidylcholine production regulates neuroinflammation and triggers neurodegeneration

The deregulation of cyclin-dependent kinase 5 (Cdk5) by p25 has been shown to contribute to the pathogenesis in a number of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD) and Alzheimer's disease (AD). In particular, p25/Cdk5 has been shown to produce hyperphosphorylated tau, neurofibrillary tangles as well as aberrant amyloid precursor protein processing found in AD. Neuroinflammation has been observed alongside the pathogenic process in these neurodegenerative diseases, however the precise mechanism behind the induction of neuroinflammation and the significance in the AD pathogenesis has not been fully elucidated. In this report, we uncover a novel pathway for p25-induced neuroinflammation where p25 expression induces an early trigger of neuroinflammation in vivo in mice. Lipidomic mass spectrometry, in vitro coculture and conditioned media transfer experiments show that the soluble lipid mediator lysophosphatidylcholine (LPC) is released by p25 overexpressing neurons to initiate astrogliosis, neuroinflammation and subsequent neurodegeneration. Reverse transcriptase PCR and gene silencing experiments show that cytosolic phospholipase 2 (cPLA2) is the key enzyme mediating the p25-induced LPC production and cPLA2 upregulation is critical in triggering the p25-mediated inflammatory and neurodegenerative process. Together, our findings delineate a potential therapeutic target for the reduction of neuroinflammation in neurodegenerative diseases including AD.

Stable mutated tau441 transfected SH-SY5Y cells as screening tool for Alzheimer's disease drug candidates

The role of hyperphosphorylation of the microtubule-associated protein tau in the pathological processes of several neurodegenerative diseases is becoming better understood. Consequently, development of new compounds capable of preventing tau hyperphosphorylation is an increasingly hot topic. For this reason, dependable in vitro and in vivo models that reflect tau hyperphosphorylation in human diseases are needed. In this study, we generated and validated an in vitro model appropriate to test potential curative and preventive compound effects on tau phosphorylation. For this purpose, a stably transfected SH-SY5Y cell line was constructed over-expressing mutant human tau441 (SH-SY5Y-TMHT441). Analyses of expression levels and tau phosphorylation status in untreated cells confirmed relevance to human diseases. Subsequently, the effect of different established kinase inhibitors on tau phosphorylation (e.g., residues Thr231, Thr181, and Ser396) was examined. It was shown with several methods including immunosorbent assays and mass spectrometry that the phosphorylation pattern of tau in SH-SY5Y-TMHT441 cells can be reliably modulated by these compounds, specifically targeting JNK, GSK-3, CDK1/5, and CK1. These four protein kinases are known to be involved in in vivo tau phosphorylation and are therefore authentic indicators for the suitability of this new cell culture model for tauopathies.

Astrocytes are important mediators of Aβ-induced neurotoxicity and tau phosphorylation in primary culture

Alzheimer's disease (AD) is pathologically characterised by the age-dependent deposition of β-amyloid (Aβ) in senile plaques, intraneuronal accumulation of tau as neurofibrillary tangles, synaptic dysfunction and neuronal death. Neuroinflammation, typified by the accumulation of activated microglia and reactive astrocytes, is believed to modulate the development and/or progression of AD. We have used primary rat neuronal, astrocytic and mixed cortical cultures to investigate the contribution of astrocyte-mediated inflammatory responses during Aβ-induced neuronal loss. We report that the presence of small numbers of astrocytes exacerbate Aβ-induced neuronal death, caspase-3 activation and the production of caspase-3-cleaved tau. Furthermore, we show that astrocytes are essential for the Aβ-induced tau phosphorylation observed in primary neurons. The release of soluble inflammatory factor(s) from astrocytes accompanies these events, and inhibition of astrocyte activation with the anti-inflammatory agent, minocycline, reduces astrocytic inflammatory responses and the associated neuronal loss. Aβ-induced increases in caspase-3 activation and the production of caspase-3-truncated tau species in neurons were reduced when the astrocytic response was attenuated with minocycline. Taken together, these results show that astrocytes are important mediators of the neurotoxic events downstream of elevated Aβ in models of AD, and suggest that mechanisms underlying pro-inflammatory cytokine release might be an important target for therapy.

Amyloid beta-mediated cell death of cultured hippocampal neurons reveals extensive Tau fragmentation without increased full-length tau phosphorylation

A variety of genetic and biochemical evidence suggests that amyloid β (Aβ) oligomers promote downstream errors in Tau action, in turn inducing neuronal dysfunction and cell death in Alzheimer and related dementias. To better understand molecular mechanisms involved in Aβ-mediated neuronal cell death, we have treated primary rat hippocampal cultures with Aβ oligomers and examined the resulting cellular changes occurring before and during the induction of cell death with a focus on altered Tau biochemistry. The most rapid neuronal responses upon Aβ administration are activation of caspase 3/7 and calpain proteases. Aβ also appears to reduce Akt and Erk1/2 kinase activities while increasing GSK3β and Cdk5 activities. Shortly thereafter, substantial Tau degradation begins, generating relatively stable Tau fragments. Only a very small fraction of full-length Tau remains intact after 4 h of Aβ treatment. In conflict with expectations based on suggested increases of GSK3β and Cdk5 activities, Aβ does not cause any major increases in phosphorylation of full-length Tau as assayed by immunoblotting one-dimensional gels with 11 independent site- and phospho-specific anti-Tau antibodies as well as by immunoblotting two-dimensional gels probed with a pan-Tau antibody. There are, however, subtle and transient increases in Tau phosphorylation at 3-4 specific sites before its degradation. Taken together, these data are consistent with the notion that Aβ-mediated neuronal cell death involves the loss of full-length Tau and/or the generation of toxic fragments but does not involve or require hyperphosphorylation of full-length Tau.

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

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

Neurobiological aspects of Alzheimer's disease

INTRODUCTION

The molecular pathogenesis of Alzheimer's disease (AD) includes a variety of risk factors, extracellular deposition of β-amyloid, accumulation of intracellular neurofibrillary tangles, oxidative neuronal damage and inflammatory cascades. Although amyloid-β-containing senile plaques and phospho-tau-containing neurofibrillary tangles are hallmark lesions of AD, neither is specific to nor even a marker of the disease. From a biochemical point of view the most consistent finding is a decreased level of choline acetyltransferase. In recent years, cumulative evidence has been gained on the involvement of neuronal lipoprotein activity, and on the role of cholesterol and other lipids in pathogenesis. Although basic research has made remarkable progress in the past two decades, currently available drugs are only able to improve cognitive symptoms temporarily and no treatment can reverse, stop or even slow this inexorable neurodegenerative process.

AREAS COVERED

The various neurobiological events associated with development of AD and the multiple treatment approaches for combating this disorder.

EXPERT OPINION

AD is a complex multifactorial disorder and thus a single target or pathogenic pathway is unlikely to be identified. Developing therapeutic interventions demands a greater understanding of the processes and the differential involvement of the various mediators. Effective therapeutics are urgently needed, and it is hoped that anti-amyloid strategies will offer a significant step towards a causal therapy.

A 24-residue peptide (p5), derived from p35, the Cdk5 neuronal activator, specifically inhibits Cdk5-p25 hyperactivity and tau hyperphosphorylation

The activity of Cdk5-p35 is tightly regulated in the developing and mature nervous system. Stress-induced cleavage of the activator p35 to p25 and a p10 N-terminal domain induces deregulated Cdk5 hyperactivity and perikaryal aggregations of hyperphosphorylated Tau and neurofilaments, pathogenic hallmarks in neurodegenerative diseases, such as Alzheimer disease and amyotrophic lateral sclerosis, respectively. Previously, we identified a 125-residue truncated fragment of p35 called CIP that effectively and specifically inhibited Cdk5-p25 activity and Tau hyperphosphorylation induced by Aβ peptides in vitro, in HEK293 cells, and in neuronal cells. Although these results offer a possible therapeutic approach to those neurodegenerative diseases assumed to derive from Cdk5-p25 hyperactivity and/or Aβ induced pathology, CIP is too large for successful therapeutic regimens. To identify a smaller, more effective peptide, in this study we prepared a 24-residue peptide, p5, spanning CIP residues Lys(245)-Ala(277). p5 more effectively inhibited Cdk5-p25 activity than did CIP in vitro. In neuron cells, p5 inhibited deregulated Cdk5-p25 activity but had no effect on the activity of endogenous Cdk5-p35 or on any related endogenous cyclin-dependent kinases in HEK293 cells. Specificity of p5 inhibition in cortical neurons may depend on the p10 domain in p35, which is absent in p25. Furthermore, we have demonstrated that p5 reduced Aβ(1-42)-induced Tau hyperphosphorylation and apoptosis in cortical neurons. These results suggest that p5 peptide may be a unique and useful candidate for therapeutic studies of certain neurodegenerative diseases.

Abeta promotes Alzheimer's disease-like cytoskeleton abnormalities with consequences to APP processing in neurons

Abeta is proteolytically produced from the Alzheimer's amyloid precursor protein (APP). Major properties attributed to Abeta include neurotoxic effects that contribute to Alzheimer's disease neurodegeneration. However, Abeta can also affect APP processing and trafficking that, in neurons, is anterogradelly transported via microtubules in a kinesin-associated manner. Herein we show that Abeta can induce accumulation of intracellular sAPP in primary neuronal cultures. Subcellular fractionation studies and immunofluorescence analysis revealed that upon Abeta exposure sAPP retention was localized to cytoskeleton associated vesicular structures along the neurite processes, positive for an APP N-terminal antibody and negative for an APP C-terminal antibody. These vesicular structures were also positive for kinesin light chain 1 (KLC). We confirm that Abeta alters both actin and microtubule networks. It increases F-actin polymerization and we report for the first time that Abeta decreases alpha-tubulin acetylation. The use of cytoskeleton associated drugs partially reversed the Abeta-induced effects on sAPP secretion. The data here presented show that Abeta causes intracellular sAPP retention by inducing alterations in the cytoskeleton network, thus contributing to impaired APP/sAPP vesicular transport. Moreover, the data strengthens the hypothesis that Abeta-induces neurodegeneration and provides a potential mechanism of action, as impaired vesicular and axonal transport have been linked to Alzheimer's disease pathology.

Targeting Cdk5 activity in neuronal degeneration and regeneration

The major priming event in neurodegeneration is loss of neurons. Loss of neurons by apoptotic mechanisms is a theme for studies focused on determining therapeutic strategies. Neurons following an insult, activate a number of signal transduction pathways, of which, kinases are the leading members. Cyclin-dependent kinase 5 (Cdk5) is one of the kinases that have been linked to neurodegeneration. Cdk5 along with its principal activator p35 is involved in multiple cellular functions ranging from neuronal differentiation and migration to synaptic transmission. However, during neurotoxic stress, intracellular rise in Ca(2+) activates calpain, which cleaves p35 to generate p25. The long half-life of Cdk5/p25 results in a hyperactive, aberrant Cdk5 that hyperphosphorylates Tau, neurofilament and other cytoskeletal proteins. These hyperphosphorylated cytoskeletal proteins set the groundwork to forming neurofibrillary tangles and aggregates of phosphorylated proteins, hallmarks of neurodegenerative diseases like Alzheimer's disease, Parkinson's disease and Amyotropic Lateral Sclerosis. Attempts to selectively target Cdk5/p25 activity without affecting Cdk5/p35 have been largely unsuccessful. A polypeptide inhibitor, CIP (Cdk5 inhibitory peptide), developed in our laboratory, successfully inhibits Cdk5/p25 activity in vitro, in cultured primary neurons, and is currently undergoing validation tests in mouse models of neurodegeneration. Here, we discuss the therapeutic potential of CIP in regenerating neurons that are exposed to neurodegenerative stimuli.

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

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

Critical role of CDK5 and Polo-like kinase 2 in homeostatic synaptic plasticity during elevated activity

Homeostatic plasticity keeps neuronal spiking output within an optimal range in the face of chronically altered levels of network activity. Little is known about the underlying molecular mechanisms, particularly in response to elevated activity. We report that, in hippocampal neurons experiencing heightened activity, the activity-inducible protein kinase Polo-like kinase 2 (Plk2, also known as SNK) was required for synaptic scaling-a principal mechanism underlying homeostatic plasticity. Synaptic scaling also required CDK5, which acted as a "priming" kinase for the phospho-dependent binding of Plk2 to its substrate SPAR, a postsynaptic RapGAP and scaffolding molecule that is degraded following phosphorylation by Plk2. RNAi knockdown of SPAR weakened synapses, and overexpression of a SPAR mutant resistant to Plk2-dependent degradation prevented synaptic scaling. Thus, priming phosphorylation of the Plk2 binding site in SPAR by CDK5, followed by Plk2 recruitment and SPAR phosphorylation-degradation, constitutes a molecular pathway for neuronal homeostatic plasticity during chronically elevated activity.

Altered neuronal gene expression in brain regions differentially affected by Alzheimer's disease: a reference data set

Alzheimer's Disease (AD) is the most widespread form of dementia during the later stages of life. If improved therapeutics are not developed, the prevalence of AD will drastically increase in the coming years as the world's population ages. By identifying differences in neuronal gene expression profiles between healthy elderly persons and individuals diagnosed with AD, we may be able to better understand the molecular mechanisms that drive AD pathogenesis, including the formation of amyloid plaques and neurofibrillary tangles. In this study, we expression profiled histopathologically normal cortical neurons collected with laser capture microdissection (LCM) from six anatomically and functionally discrete postmortem brain regions in 34 AD-afflicted individuals, using Affymetrix Human Genome U133 Plus 2.0 microarrays. These regions include the entorhinal cortex, hippocampus, middle temporal gyrus, posterior cingulate cortex, superior frontal gyrus, and primary visual cortex. This study is predicated on previous parallel research on the postmortem brains of the same six regions in 14 healthy elderly individuals, for which LCM neurons were similarly processed for expression analysis. We identified significant regional differential expression in AD brains compared with control brains including expression changes of genes previously implicated in AD pathogenesis, particularly with regard to tangle and plaque formation. Pinpointing the expression of factors that may play a role in AD pathogenesis provides a foundation for future identification of new targets for improved AD therapeutics. We provide this carefully phenotyped, laser capture microdissected intraindividual brain region expression data set to the community as a public resource.

Mechanism of arsenic-induced neurotoxicity may be explained through cleavage of p35 to p25 by calpain

In recent studies we have demonstrated that arsenic (As) metabolites change the composition of neuronal cytoskeletal proteins in vivo and in vitro. To further examine the mechanism of arsenic-induced neurotoxicity with various arsenate metabolites (iAsV, MMAV and DMAV) and arsenite metabolites (iAsIII, MMAIII and DMAIII), we investigated the role of the proteolytic enzyme calpain and its involvement in the cleavage of p35 protein to p25, and also mRNA expression levels of calpain, cyclin-dependent kinase 5 (cdk5) and glycogen synthase kinase 3 beta (gsk3ss). A HeLa cell line transfected with a p35 construct (HeLa-p35) was used as a model, since all other proteins such as calpain, CDK5 and GSK3beta are already present in HeLa cells as they are in neuronal cells. HeLa-p35 cells were incubated with various As metabolites and concentrations of 0, 10 and 30 microM for duration of 4 h. Subsequently the cells were either lysed to study their relative quantification levels of these genes or to be examined on their p35-protein expression. P35-RNA expression levels were significantly (p<0.01) increased by arsenite metabolites, while p35 protein was cleaved to p25 (and p10) after incubation with these metabolites. The cleavage of p35 is caused by calcium (Ca2+) induced activation of calpain. Inhibition of calpain activity by calpeptin prevents cleavage of p35 to p25. These results suggest that cleavage of p35 to p25 by calpain, probably As-induced Ca2+-influx, may explain the mechanism by which arsenic induces its neurotoxic effects.

Untangling tau hyperphosphorylation in drug design for neurodegenerative diseases

Aggregation of hyperphosphorylated tau is one of the characteristic neuropathological lesions of Alzheimer's disease and other neurodegenerative disorders. Pharmacological modulation of tau hyperphosphorylation might represent a valid and feasible therapeutic strategy for such disorders. Here, we consider recent evidence supporting the validity of the three most relevant kinases affecting tau hyperphosphorylation - GSK3beta, CDK5 and ERK2 - as drug targets and describe progress in the design of inhibitors for these kinases.

p35/Cyclin-dependent kinase 5 is required for protection against beta-amyloid-induced cell death but not tau phosphorylation by ceramide

Ceramide is a bioactive sphingolipid that can prevent calpain activation and beta-amyloid (A beta) neurotoxicity in cortical neurons. Recent evidence supports A beta induction of a calpain-dependent cleavage of the cyclin-dependent kinase 5 (cdk5) regulatory protein p35 that contributes to tau hyperphosphorylation and neuronal death. Using cortical neurons isolated from wild-type and p35 knockout mice, we investigated whether ceramide required p35/cdk5 to protect against A beta-induced cell death and tau phosphorylation. Ceramide inhibited A beta-induced calpain activation and cdk5 activity in wild-type neurons and protected against neuronal death and tau hyperphosphorylation. Interestingly, A beta also increased cdk5 activity in p35-/- neurons, suggesting that the alternate cdk5 regulatory protein, p39, might mediate this effect. In p35 null neurons, ceramide blocked A beta-induced calpain activation but did not inhibit cdk5 activity or cell death. However, ceramide blocked tau hyperphosphorylation potentially via inhibition of glycogen synthase kinase-3beta. These data suggest that ceramide can regulate A beta cell toxicity in a p35/cdk5-dependent manner.