PP2B isolated from human brain preferentially dephosphorylates Ser-262 and Ser-396 of the Alzheimer disease abnormally hyperphosphorylated tau

Updates in Alzheimer's disease: from basic research to diagnosis and therapies

Alzheimer's disease (AD) is the most common neurodegenerative disorder, characterized pathologically by extracellular deposition of β-amyloid (Aβ) into senile plaques and intracellular accumulation of hyperphosphorylated tau (pTau) as neurofibrillary tangles. Clinically, AD patients show memory deterioration with varying cognitive dysfunctions. The exact molecular mechanisms underlying AD are still not fully understood, and there are no efficient drugs to stop or reverse the disease progression. In this review, we first provide an update on how the risk factors, including APOE variants, infections and inflammation, contribute to AD; how Aβ and tau become abnormally accumulated and how this accumulation plays a role in AD neurodegeneration. Then we summarize the commonly used experimental models, diagnostic and prediction strategies, and advances in periphery biomarkers from high-risk populations for AD. Finally, we introduce current status of development of disease-modifying drugs, including the newly officially approved Aβ vaccines, as well as novel and promising strategies to target the abnormal pTau. Together, this paper was aimed to update AD research progress from fundamental mechanisms to the clinical diagnosis and therapies.

Single-cell transcriptomic and neuropathologic analysis reveals dysregulation of the integrated stress response in progressive supranuclear palsy

Progressive supranuclear palsy (PSP) is a sporadic neurodegenerative tauopathy variably affecting brainstem and cortical structures and characterized by tau inclusions in neurons and glia. The precise mechanism whereby these protein aggregates lead to cell death remains unclear. To investigate the contribution of these different cellular abnormalities to PSP pathogenesis, we performed single-nucleus RNA sequencing and analyzed 45,559 high quality nuclei targeting the subthalamic nucleus and adjacent structures from human post-mortem PSP brains with varying degrees of pathology compared to controls. Cell-type specific differential expression and pathway analysis identified both common and discrete changes in numerous pathways previously implicated in PSP and other neurodegenerative disorders. This included EIF2 signaling, an adaptive pathway activated in response to diverse stressors, which was the top activated pathway in vulnerable cell types. Using immunohistochemistry, we found that activated eIF2α was positively correlated with tau pathology burden in vulnerable brain regions. Multiplex immunofluorescence localized activated eIF2α positivity to hyperphosphorylated tau (p-tau) positive neurons and ALDH1L1-positive astrocytes, supporting the increased transcriptomic EIF2 activation observed in these vulnerable cell types. In conclusion, these data provide insights into cell-type-specific pathological changes in PSP and support the hypothesis that failure of adaptive stress pathways play a mechanistic role in the pathogenesis and progression of PSP.

Cytomegalovirus infection induces Alzheimer's disease-associated alterations in tau

Alzheimer's disease (AD) manifests with loss of neurons correlated with intercellular deposition of amyloid (amyloid plaques) and intracellular neurofibrillary tangles of hyperphosphorylated tau. However, targeting AD hallmarks has not as yet led to development of an effective treatment despite numerous clinical trials. A better understanding of the early stages of neurodegeneration may lead to development of more effective treatments. One underexplored area is the clinical correlation between infection with herpesviruses and increased risk of AD. We hypothesized that similar to work performed with herpes simplex virus 1 (HSV1), infection with the cytomegalovirus (CMV) herpesvirus increases levels and phosphorylation of tau, similar to AD tauopathy. We used murine CMV (MCMV) to infect mouse fibroblasts and rat neuronal cells to test our hypothesis. MCMV infection increased steady-state levels of primarily high molecular weight forms of tau and altered the patterns of tau phosphorylation. Both changes required viral late gene products. Glycogen synthase kinase 3 beta (GSK3β) was upregulated in the HSVI model, but inhibition with lithium chloride suggested that this enzyme is unlikely to be involved in MCMV infection mediated tau phosphorylation. Thus, we confirm that MCMV, a beta herpes virus, like alpha herpes viruses (e.g., HSV1), can promote tau pathology. This suggests that CMV infection can be useful as another model system to study mechanisms leading to neurodegeneration. Since MCMV infects both mice and rats as permissive hosts, our findings from tissue culture can likely be applied to a variety of AD models to study development of abnormal tau pathology.

Hsp90-interacting Co-chaperones and their Family Proteins in Tau Regulation: Introducing a Novel Role for Cdc37L1

Tau is a microtubule-associated protein that serves as a promoter of microtubule assembly and stability in neuron cells. In a collective group of neurodegenerative diseases called tauopathies, tau processing is altered as a result of gene mutations and post-translational modifications. In particular, in Alzheimer's disease (AD) or AD-like conditions, tau becomes hyperphosphorylated and forms toxic aggregates inside the cell. The chaperone heat shock protein 90 (Hsp90) plays an important role in the proper folding, degradation, and recycling of tau proteins and tau kinases. Hsp90 has many co-chaperones that aid in tau processing. In particular, a few of these co-chaperones, such as FK506-binding protein (FKBP) 51, protein phosphatase (PP) 5, cell division cycle 37 (Cdc37), and S100A1 have family members that are reported to affect Hsp90-mediated tau processing in either a similar or an opposite manner. Here, we provide a holistic review of these selected co-chaperones and their family proteins and introduce a novel Hsp90-binding Cdc37 relative, Cdc37-like-1 (Cdc37L1 or L1) in tau regulation. Overall, the proteins discussed here highlight the importance of studying family proteins in order to fully understand the mechanism of tau pathogenesis and to establish drug targets for the treatment of tauopathies.

Chronic Presence of Oligomeric Aβ Differentially Modulates Spine Parameters in the Hippocampus and Cortex of Mice With Low APP Transgene Expression

Alzheimer’s disease is regarded as a synaptopathy with a long presymptomatic phase. Soluble, oligomeric amyloid-β (Aβ) is thought to play a causative role in this disease, which eventually leads to cognitive decline. However, most animal studies have employed mice expressing high levels of the Aβ precursor protein (APP) transgene to drive pathology. Here, to understand how the principal neurons in different brain regions cope with moderate, chronically present levels of Aβ, we employed transgenic mice expressing equal levels of mouse and human APP carrying a combination of three familial AD (FAD)-linked mutations (Swedish, Dutch, and London), that develop plaques only in old age. We analyzed dendritic spine parameters in hippocampal and cortical brain regions after targeted expression of EGFP to allow high-resolution imaging, followed by algorithm-based evaluation of mice of both sexes from adolescence to old age. We report that Aβ species gradually accumulated throughout the life of APP_SDL mice, but not the oligomeric forms, and that the amount of membrane-associated oligomers decreased at the onset of plaque formation. We observed an age-dependent loss of thin spines under most conditions as an indicator of a loss of synaptic plasticity in older mice. We further found that hippocampal pyramidal neurons respond to increased Aβ levels by lowering spine density and shifting spine morphology, which reached significance in the CA1 subfield. In contrast, the spine density in cortical pyramidal neurons of APP_SDL mice was unchanged. We also observed an increase in the protein levels of PSD-95 and Arc in the hippocampus and cortex, respectively. Our data demonstrated that increased concentrations of Aβ have diverse effects on dendritic spines in the brain and suggest that hippocampal and cortical neurons have different adaptive and compensatory capacity during their lifetime. Our data also indicated that spine morphology differs between sexes in a region-specific manner.

Tau, Diabetes and Insulin

Tau protein which was discovered in 1975 [310] became of great interest when it was identified as the main component of neurofibrillary tangles (NFT), a pathological feature in the brain of patients with Alzheimer's disease (AD) [39, 110, 232]. Tau protein is expressed mainly in the brain as six isoforms generated by alternative splicing [46, 97]. Tau is a microtubule associated proteins (MAPs) and plays a role in microtubules assembly and stability, as well as diverse cellular processes such as cell morphogenesis, cell division, and intracellular trafficking [49]. Additionally, Tau is involved in much larger neuronal functions particularly at the level of synapses and nuclei [11, 133, 280]. Tau is also physiologically released by neurons [233] even if the natural function of extracellular Tau remains to be uncovered (see other chapters of the present book).

Ageing and amyloidosis underlie the molecular and pathological alterations of tau in a mouse model of familial Alzheimer's disease

Despite compelling evidence that the accumulation of amyloid-beta (Aβ) promotes neocortical MAPT (tau) aggregation in familial and idiopathic Alzheimer's disease (AD), murine models of cerebral amyloidosis are not considered to develop tau-associated pathology. In the present study, we show that tau can accumulate spontaneously in aged transgenic APP_swe/PS1_ΔE9 mice. Tau pathology is abundant around Aβ deposits, and further characterized by accumulation of Gallyas and thioflavin-S-positive inclusions, which were detected in the APP_swe/PS1_ΔE9 brain at 18 months of age. Age-dependent increases in argyrophilia correlated positively with binding levels of the paired helical filament (PHF) tracer [¹⁸F]Flortaucipir, in all brain areas examined. Sarkosyl-insoluble PHFs were visualized by electron microscopy. Quantitative proteomics identified sequences of hyperphosphorylated and three-repeat tau in transgenic mice, along with signs of RNA missplicing, ribosomal dysregulation and disturbed energy metabolism. Tissue from the frontal gyrus of human subjects was used to validate these findings, revealing primarily quantitative differences between the tau pathology observed in AD patient vs. transgenic mouse tissue. As physiological levels of endogenous, 'wild-type' tau aggregate secondarily to Aβ in APP_swe/PS1_ΔE9 mice, this study suggests that amyloidosis is both necessary and sufficient to drive tauopathy in experimental models of familial AD.

It's all about tau

Tau is a protein that is highly enriched in neurons and was originally defined by its ability to bind and stabilize microtubules. However, it is now becoming evident that the functions of tau extend beyond its ability to modulate microtubule dynamics. Tau plays a role in mediating axonal transport, synaptic structure and function, and neuronal signaling pathways. Although tau plays important physiological roles in neurons, its involvement in neurodegenerative diseases, and most prominently in the pathogenesis of Alzheimer disease (AD), has directed the majority of tau studies. However, a thorough knowledge of the physiological functions of tau and its post-translational modifications under normal conditions are necessary to provide the foundation for understanding its role in pathological settings. In this review, we will focus on human tau, summarizing tau structure and organization, as well as its posttranslational modifications associated with physiological processes. We will highlight possible mechanisms involved in mediating the turnover of tau and finally discuss newly elucidated tau functions in a physiological context.

Protein Phosphatase 2A and Its Methylation Modulating Enzymes LCMT-1 and PME-1 Are Dysregulated in Tauopathies of Progressive Supranuclear Palsy and Alzheimer Disease

Hyperphosphorylated tau aggregates are characteristic of tauopathies including progressive supranuclear palsy (PSP) and Alzheimer disease (AD), but factors contributing to pathologic tau phosphorylation are not well understood. Here, we studied the regulation of the major tau phosphatase, the heterotrimeric AB55αC protein phosphatase 2 A (PP2A), in PSP and AD. The assembly and activity of this PP2A isoform are regulated by reversible carboxyl methylation of its catalytic C subunit, while the B subunit confers substrate specificity. We sought to address whether the decreases in PP2A methylation and its methylating enzyme, leucine carboxyl methyltransferase (LCMT-1), which are reported in AD, relate to tau pathology or to concomitant amyloid pathology by comparing them in the relatively pure tauopathy PSP. Immunohistochemical analysis of frontal cortices showed that methyl-PP2A is reduced while demethyl-PP2A is increased, with no changes in total PP2A or B55α subunit, resulting in a reduction in the methyl/demethyl PP2A ratio of 63% in PSP and 75% in AD compared to controls. Similarly, Western blot analyses showed a decrease of methyl-PP2A and an increase of demethyl-PP2A with a concomitant reduction in the methyl/demethyl PP2A ratio in both PSP (74%) and AD (76%) brains. This was associated with a decrease in LCMT-1 and an increase in the demethylating enzyme, protein phosphatase methylesterase (PME-1), in both diseases. These findings suggest that PP2A dysregulation in tauopathies may contribute to the accumulation of hyperphosphorylated tau and to neurodegeneration.

Mutual Relationship between Tau and Central Insulin Signalling: Consequences for AD and Tauopathies?

Alzheimer disease (AD) is a progressive neurodegenerative disorder mainly characterized by cognitive deficits and neuropathological changes such as Tau lesions and amyloid plaques, but also associated with non-cognitive symptomatology. Metabolic and neuroendocrine abnormalities, such as alterations in body weight, brain insulin impairments, and lower brain glucose metabolism, which often precede clinical diagnosis, have been extensively reported in AD patients. However, the origin of these symptoms and their relation to pathology and cognitive impairments remain misunderstood. Insulin is a hormone involved in the control of energy homeostasis both peripherally and centrally, and insulin-resistant state has been linked to increased risk of dementia. It is now well established that insulin resistance can exacerbate Tau lesions, mainly by disrupting the balance between Tau kinases and phosphatases. On the other hand, the emerging literature indicates that Tau protein can also modulate insulin signalling in the brain, thus creating a detrimental vicious circle. The following review will highlight our current understanding of the role of insulin in the brain and its relation to Tau protein in the context of AD and tauopathies. Considering that insulin signalling is prone to be pharmacologically targeted at multiple levels, it constitutes an appealing approach to improve both insulin brain sensitivity and mitigate brain pathology with expected positive outcome in terms of cognition.

Transcriptome analyses of chronic traumatic encephalopathy show alterations in protein phosphatase expression associated with tauopathy

Chronic traumatic encephalopathy (CTE) is a progressive neurodegenerative disorder that is associated with repetitive head injury and has distinctive neuropathological features that differentiate this disease from other neurodegenerative diseases. Intraneuronal tau aggregates, although they occur in different patterns, are diagnostic neuropathological features of CTE, but the precise mechanism of tauopathy is not known in CTE. We performed whole RNA sequencing analysis of post-mortem brain tissue from patients with CTE and compared the results to normal controls to determine the transcriptome signature changes associated with CTE. The results showed that the genes related to the MAP kinase and calcium-signaling pathways were significantly downregulated in CTE. The altered expression of protein phosphatases (PPs) in these networks further suggested that the tauopathy observed in CTE involves common pathological mechanisms similar to Alzheimer's disease (AD). Using cell lines and animal models, we also showed that reduced PPP3CA/PP2B phosphatase activity is directly associated with increases in phosphorylated (p)-tau proteins. These findings provide important insights into PP-dependent neurodegeneration and may lead to novel therapeutic approaches to reduce the tauopathy associated with CTE.

Region-specific dendritic simplification induced by Aβ, mediated by tau via dysregulation of microtubule dynamics: a mechanistic distinct event from other neurodegenerative processes

Background

Dendritic simplification, a key feature of the neurodegenerative triad of Alzheimer’s disease (AD) in addition to spine changes and neuron loss, occurs in a region-specific manner. However, it is unknown how changes in dendritic complexity are mediated and how they relate to spine changes and neuron loss.

Results

To investigate the mechanisms of dendritic simplification in an authentic CNS environment we employed an ex vivo model, based on targeted expression of enhanced green fluorescent protein (EGFP)-tagged constructs in organotypic hippocampal slices of mice. Algorithm-based 3D reconstruction of whole neuron morphology in different hippocampal regions was performed on slices from APP_SDL-transgenic and control animals. We demonstrate that induction of dendritic simplification requires the combined action of amyloid beta (Aβ) and human tau. Simplification is restricted to principal neurons of the CA1 region, recapitulating the region specificity in AD patients, and occurs at sites of Schaffer collateral input. We report that γ-secretase inhibition and treatment with the NMDA-receptor antagonist, CPP, counteract dendritic simplification. The microtubule-stabilizing drug epothilone D (EpoD) induces simplification in control cultures per se. Similar morphological changes were induced by a phosphoblocking tau construct, which also increases microtubule stability. In fact, low nanomolar concentrations of naturally secreted Aβ decreased phosphorylation at S262 in a cellular model, a site which is known to directly modulate tau-microtubule interactions.

Conclusions

The data provide evidence that dendritic simplification is mechanistically distinct from other neurodegenerative events and involves microtubule stabilization by dendritic tau, which becomes dephosphorylated at certain sites. They imply that treatments leading to an overall decrease of tau phosphorylation might have a negative impact on neuronal connectivity.

Calmodulin Binding Proteins and Alzheimer's Disease

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

Phosphatases of α-synuclein, LRRK2, and tau: important players in the phosphorylation-dependent pathology of Parkinsonism

An important challenge in the field of Parkinson’s disease (PD) is to develop disease modifying therapies capable of stalling or even halting disease progression. Coupled to this challenge is the need to identify disease biomarkers, in order to identify pre-symptomatic hallmarks of disease and monitor disease progression. The answer to these challenges lies in the elucidation of the molecular causes underlying PD, for which important leads are disease genes identified in studies investigating the underlying genetic causes of PD. LRRK2 and α-syn have been both linked to familial forms of PD as well as associated to sporadic PD. Another gene, microtubule associated protein tau (MAPT), has been genetically linked to a dominant form of frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and genome-wide association studies report a strong association between MAPT and sporadic PD. Interestingly, LRRK2, α-syn, and tau are all phosphorylated proteins, and their phosphorylation patterns are linked to disease. In this review, we provide an overview of the evidence linking LRRK2, α-syn, and tau phosphorylation to PD pathology and focus on studies which have identified phosphatases responsible for dephosphorylation of pathology-related phosphorylations. We also discuss how the LRRK2, α-syn, and tau phosphatases may point to separate or cross-talking pathological pathways in PD. Finally, we will discuss how the study of phosphatases of dominant Parkinsonism proteins opens perspectives for targeting pathological phosphorylation events.

Tau protein modifications and interactions: their role in function and dysfunction

Tau protein is abundant in the central nervous system and involved in microtubule assembly and stabilization. It is predominantly associated with axonal microtubules and present at lower level in dendrites where it is engaged in signaling functions. Post-translational modifications of tau and its interaction with several proteins play an important regulatory role in the physiology of tau. As a consequence of abnormal modifications and expression, tau is redistributed from neuronal processes to the soma and forms toxic oligomers or aggregated deposits. The accumulation of tau protein is increasingly recognized as the neuropathological hallmark of a number of dementia disorders known as tauopathies. Dysfunction of tau protein may contribute to collapse of cytoskeleton, thereby causing improper anterograde and retrograde movement of motor proteins and their cargos on microtubules. These disturbances in intraneuronal signaling may compromise synaptic transmission as well as trophic support mechanisms in neurons.

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

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

Calcyclin binding protein and Siah-1 interacting protein in Alzheimer's disease pathology: neuronal localization and possible function

The calcyclin binding protein and Siah-1 interacting protein (CacyBP/SIP) protein was shown to play a role in the organization of microtubules. In this work we have examined the neuronal distribution and possible function of CacyBP/SIP in cytoskeletal pathophysiology. We have used brain tissue from Alzheimer's disease (AD) patients and from transgenic mice modeling 2 different pathologies characteristic for AD: amyloid and tau. In the brain from AD patients, CacyBP/SIP was found to be almost exclusively present in neuronal somata, and in control patients it was seen in the somata and neuronal processes. In mice doubly transgenic for amyloid precursor protein and presenilin 1 there was no difference in CacyBP/SIP neuronal localization in comparison with the nontransgenic animals. By contrast in tau transgenic mice, localization of CacyBP/SIP was similar to that observed for AD patients. To find the relation between CacyBP/SIP and tau we examined dephosphorylation of tau by CacyBP/SIP. We found that indeed it exhibited phosphatase activity toward tau. Altogether, our results suggest that CacyBP/SIP might play a role in AD pathology.

Unveiling clusters of RNA transcript pairs associated with markers of Alzheimer's disease progression

Background

One primary goal of transcriptomic studies is identifying gene expression patterns correlating with disease progression. This is usually achieved by considering transcripts that independently pass an arbitrary threshold (e.g. p<0.05). In diseases involving severe perturbations of multiple molecular systems, such as Alzheimer’s disease (AD), this univariate approach often results in a large list of seemingly unrelated transcripts. We utilised a powerful multivariate clustering approach to identify clusters of RNA biomarkers strongly associated with markers of AD progression. We discuss the value of considering pairs of transcripts which, in contrast to individual transcripts, helps avoid natural human transcriptome variation that can overshadow disease-related changes.

Methodology/Principal Findings

We re-analysed a dataset of hippocampal transcript levels in nine controls and 22 patients with varying degrees of AD. A large-scale clustering approach determined groups of transcript probe sets that correlate strongly with measures of AD progression, including both clinical and neuropathological measures and quantifiers of the characteristic transcriptome shift from control to severe AD. This enabled identification of restricted groups of highly correlated probe sets from an initial list of 1,372 previously published by our group. We repeated this analysis on an expanded dataset that included all pair-wise combinations of the 1,372 probe sets. As clustering of this massive dataset is unfeasible using standard computational tools, we adapted and re-implemented a clustering algorithm that uses external memory algorithmic approach. This identified various pairs that strongly correlated with markers of AD progression and highlighted important biological pathways potentially involved in AD pathogenesis.

Conclusions/Significance

Our analyses demonstrate that, although there exists a relatively large molecular signature of AD progression, only a small number of transcripts recurrently cluster with different markers of AD progression. Furthermore, considering the relationship between two transcripts can highlight important biological relationships that are missed when considering either transcript in isolation.

A role for calcineurin in Alzheimer's disease

Alzheimer's disease (AD) is an incurable age-related neurodegenerative disorder characterized by profound memory dysfunction. This bellwether symptom suggests involvement of the hippocampus -- a brain region responsible for memory formation -- and coincidentally an area heavily burdened by hyperphosphorylated tau and neuritic plaques of amyloid beta (Aβ). Recent evidence suggests that pre-fibrillar soluble Aβ underlies an early, progressive loss of synapses that is a hallmark of AD. One of the downstream effects of soluble Aβ aggregates is the activation of the phosphatase calcineurin (CaN). This review details the evidence of CaN hyperactivity in 'normal' aging, models of AD, and actual disease pathogenesis; elaborates on how this could manifest as memory impairment, neuroinflammation, hyperphosphorylated tau, and neuronal death.

Calcineurin regulation of cytoskeleton organization: a new paradigm to analyse the effects of calcineurin inhibitors on the kidney

Calcineurin is a serine/threonine phosphatase originally involved in the immune response but is also known for its role as a central mediator in various non-immunological intracellular signals. The nuclear factor of activated T cell (NFAT) proteins are the most widely described substrates of calcineurin, but ongoing work has uncovered other substrates among which are the cytoskeleton organizing proteins (i.e. cofilin, synaptopodin, WAVE-1). Control over cytoskeletal proteins is of outmost interest because the phenotypic properties of cells are dependent on cytoskeleton architecture integrity, while rearrangements of the cytoskeleton are implicated in both physiological and pathological processes. Previous works investigating the role of calcineurin on the cytoskeleton have focused on neurite elongation, myocyte hypertrophic response and recently in kidney cells structure. Nuclear factor of activated T cell activation is expectedly identified in the signalling pathways for calcineurin-induced cytoskeleton organization, however new NFAT-independent pathways have also been uncovered. The aim of this review is to summarize the current knowledge on the effects of calcineurin on cytoskeletal proteins and related intracellular pathways. These newly described properties of calcineurin on cytoskeletal proteins may explain some of the beneficial or deleterious effects observed in kidney cells associated with the use of the calcineurin inhibitors, cyclosporine and tacrolimus.

Protein phosphatases and Alzheimer's disease

Alzheimer's Disease (AD) is characterized by progressive loss of cognitive function, linked to marked neuronal loss. Pathological hallmarks of the disease are the accumulation of the amyloid-β (Aβ) peptide in the form of amyloid plaques and the intracellular formation of neurofibrillary tangles (NFTs). Accumulating evidence supports a key role for protein phosphorylation in both the normal and pathological actions of Aβ as well as the formation of NFTs. NFTs contain hyperphosphorylated forms of the microtubule-binding protein tau, and phosphorylation of tau by several different kinases leads to its aggregation. The protein kinases involved in the generation and/or actions of tau or Aβ are viable drug targets to prevent or alleviate AD pathology. However, it has also been recognized that the protein phosphatases that reverse the actions of these protein kinases are equally important. Here, we review recent advances in our understanding of serine/threonine and tyrosine protein phosphatases in the pathology of AD.

Transcriptomic analysis of human retinal detachment reveals both inflammatory response and photoreceptor death

BACKGROUND

Retinal detachment often leads to a severe and permanent loss of vision and its therapeutic management remains to this day exclusively surgical. We have used surgical specimens to perform a differential analysis of the transcriptome of human retinal tissues following detachment in order to identify new potential pharmacological targets that could be used in combination with surgery to further improve final outcome.

METHODOLOGY/PRINCIPAL FINDINGS

Statistical analysis reveals major involvement of the immune response in the disease. Interestingly, using a novel approach relying on coordinated expression, the interindividual variation was monitored to unravel a second crucial aspect of the pathological process: the death of photoreceptor cells. Within the genes identified, the expression of the major histocompatibility complex I gene HLA-C enables diagnosis of the disease, while PKD2L1 and SLCO4A1 -which are both down-regulated- act synergistically to provide an estimate of the duration of the retinal detachment process. Our analysis thus reveals the two complementary cellular and molecular aspects linked to retinal detachment: an immune response and the degeneration of photoreceptor cells. We also reveal that the human specimens have a higher clinical value as compared to artificial models that point to IL6 and oxidative stress, not implicated in the surgical specimens studied here.

CONCLUSIONS/SIGNIFICANCE

This systematic analysis confirmed the occurrence of both neurodegeneration and inflammation during retinal detachment, and further identifies precisely the modification of expression of the different genes implicated in these two phenomena. Our data henceforth give a new insight into the disease process and provide a rationale for therapeutic strategies aimed at limiting inflammation and photoreceptor damage associated with retinal detachment and, in turn, improving visual prognosis after retinal surgery.

Effect of cortistatin on tau phosphorylation at Ser262 site

The development of intraneuronal lesions as a result of the progressive deposition of hyperphosphorylated tau at specific brain regions (such as hippocampus and cortex) plays a key role in the pathological process of Alzheimer's disease. However, the mechanisms by which tau phosphorylation is regulated, mainly in the pathology found in the cortex, are still poorly understood. Here, we analyzed the effect of cortistatin, a cortical neuropeptide related to somatostatin, on tau phosphorylation at Ser262 in cultures of murine cortical neurons. Both somatostatin and cortistatin induce tau phosphorylation at Ser262, a site modified in Alzheimer's disease, although with different kinetics in cortex. The effect of cortistatin likely is mediated by heterodimeric receptors composed of somatostatin receptor subtypes 2 and 4 and also by protein kinase C signaling. Cortistatin-deficient mice show decreased tau phosphorylation at Ser262 in the cortex but not in other brain regions tested. Our results suggest an important role for cortistatin in the regulation of tau phosphorylation that may be associated with the pathophysiology of Alzheimer's disease in regions such as the cerebral cortex.

Retinal tau pathology in human glaucomas

BACKGROUND

Tau protein is a microtubule-associated protein critical to neuron structure and integrity. The abnormal hyperphosphorylated tau protein AT8 disrupts microtubules, interferes with axonal transport, and is associated with neuron injury in neurodegenerative diseases such as Alzheimer's disease. The purpose of this study was to assess the presence of tau protein and abnormal tau protein AT8 in human glaucomas and to determine whether abnormal tau protein plays a role in glaucomatous neural degeneration.

METHODS

Sections from 11 surgical eye specimens with glaucoma from elevated intraocular pressure causes and 10 age-matched control eye specimens were immunostained for normal tau protein (BT2) and hyperphosphorylated tau protein (AT8). Postmortem specimens with incidental open-angle glaucoma (n = 6) were compared with controls (n = 3). Measurements of immunofluorescence intensity in glaucoma retinas were compared with those in control retinas. Abnormal tau AT8 and parvalbumin, a horizontal cell-specific marker, were studied with double-immunofluorescence techniques to determine colocalization.

RESULTS

In surgical glaucoma specimens, normal tau protein was decreased in both the optic nerve and retina compared with age-matched controls. Abnormal tau AT8 was evident within the posterior retina, predominantly at the outer border of the inner nuclear layer in surgical glaucoma specimens, and this was not observed in controls or incidental glaucoma cases. Quantitative immunofluorescence techniques demonstrated significantly increased abnormal tau AT8 in surgical glaucoma specimens compared with controls. Abnormal tau AT8 colocalized with parvalbumin in horizontal cells of the retina.

INTERPRETATION

Abnormal tau AT8, a marker of injury in various neurological diseases, is present in human glaucomas with uncontrolled intraocular pressure. The finding of abnormal tau protein in retinal horizontal cells may relate to elevated intraocular pressure and (or) neural degeneration in glaucoma. Tau protein abnormality in glaucoma underscores shared pathways with other neurodegenerative diseases.

Microtubule-associated protein tau in development, degeneration and protection of neurons

As a principal neuronal microtubule-associated protein, tau has been recognized to play major roles in promoting microtubule assembly and stabilizing the microtubules and to maintain the normal morphology of the neurons. Recent studies suggest that tau, upon alternative mRNA splicing and multiple posttranslational modifications, may participate in the regulations of intracellular signal transduction, development and viability of the neurons. Furthermore, tau gene mutations, aberrant mRNA splicing and abnormal posttranslational modifications, such as hyperphosphorylation, have also been found in a number of neurodegenerative disorders, collectively known as tauopathies. Therefore, changes in expression of the tau gene, alternative splicing of its mRNA and its posttranslational modification can modulate the normal architecture and functions of neurons as well as in a situation of tauopathies, such as Alzheimer's disease. The primary aim of this review is to summarize the latest developments and perspectives in our understanding about the roles of tau, especially hyperphosphorylation, in the development, degeneration and protection of neurons.

Kinases and phosphatases and tau sites involved in Alzheimer neurofibrillary degeneration

Microtubule associated protein (MAP) tau is abnormally hyperphosphorylated in Alzheimer's disease (AD) and related tauopathies; in this form it is the major protein subunit of paired helical filaments (PHF)/neurofibrillary tangles. However, the nature of protein kinases and phosphatases and tau sites involved in this lesion has been elusive. We investigated self-assembly and microtubule assembly promoting activities of hyperphosphorylated tau isolated from Alzheimer disease brain cytosol, the AD abnormally hyperphosphorylated tau (AD P-tau) before and after dephosphorylation by phosphoseryl/phosphothreonyl protein phosphatase-2A (PP-2A), and then rephosphorylation by cyclic AMP-dependent protein kinase (PKA), calcium, calmodulin-dependent protein kinase II (CaMKII), glycogen synthase kinase-3beta (GSK-3beta) and cyclin-dependent protein kinase 5 (cdk5) in different kinase combinations. We found that (i) dephosphorylation of AD P-tau by PP-2A inhibits its polymerization into PHF/straight filaments (SF) and restores its binding and ability to promote assembly of tubulin into microtubules; (ii) rephosphorylation of PP-2A-dephosphorylated AD P-tau by sequential phosphorylation by PKA, CaMKII and GSK-3beta or cdk5, and as well as by cdk5 and GSK-3beta, promotes its self-assembly into tangles of PHF similar to those seen in Alzheimer brain, and (iii) phosphorylation of tau sites required for this pathology are Thr231 and Ser262, along with several sites flanking the microtubule binding repeat region. Phosphorylation of recombinant human brain tau(441) yielded similar results as the PP-2A dephosphorylated AD P-tau, except that mostly SF were formed. The conditions for the abnormal hyperphosphorylation of tau that promoted its self-assembly also induced the microtubule assembly inhibitory activity. These findings suggest that activation of PP-2A or inhibition of either both GSK-3beta and cdk5 or one of these two kinases plus PKA or CaMKII might be required to inhibit Alzheimer neurofibrillary degeneration.