Shortfalls in the peptidyl-prolyl cis–trans isomerase protein Pin1 in neurons are associated with frontotemporal dementias

The role of the Pin1-cis P-tau axis in the development and treatment of vascular contribution to cognitive impairment and dementia and preeclampsia

Tauopathies are neurodegenerative diseases characterized by deposits of abnormal Tau protein in the brain. Conventional tauopathies are often defined by a limited number of Tau epitopes, notably neurofibrillary tangles, but emerging evidence suggests structural heterogeneity among tauopathies. The prolyl isomerase Pin1 isomerizes cis P-tau to inhibit the development of oligomers, tangles and neurodegeneration in multiple neurodegenerative diseases such as Alzheimer's disease, traumatic brain injury, vascular contribution to cognitive impairment and dementia (VCID) and preeclampsia (PE). Thus, cis P-tau has emerged as an early etiological driver, blood marker and therapeutic target for multiple neurodegenerative diseases, with clinical trials ongoing. The discovery of cis P-tau and other tau pathologies in VCID and PE calls attention for simplistic classification of tauopathy in neurodegenerative diseases. These recent advances have revealed the exciting novel role of the Pin1-cis P-tau axis in the development and treatment of vascular contribution to cognitive impairment and dementia and preeclampsia.

The regulatory role of Pin1 in neuronal death

Regulated cell death predominantly involves apoptosis, autophagy, and regulated necrosis. It is vital that we understand how key regulatory signals can control the process of cell death. Pin1 is a cis-trans isomerase that catalyzes the isomerization of phosphorylated serine or threonine-proline motifs of a protein, thereby acting as a crucial molecular switch and regulating the protein functionality and the signaling pathways involved. However, we know very little about how Pin1-associated pathways might play a role in regulated cell death. In this paper, we review the role of Pin1 in regulated cell death and related research progress and summarize Pin1-related pathways in regulated cell death. Aside from the involvement of Pin1 in the apoptosis that accompanies neurodegenerative diseases, accumulating evidence suggests that Pin1 also plays a role in regulated necrosis and autophagy, thereby exhibiting distinct effects, including both neurotoxic and neuroprotective effects. Gaining an enhanced understanding of Pin1 in neuronal death may provide us with new options for the development of therapeutic target for neurodegenerative disorders.

The Peptidyl-prolyl Isomerase Pin1 in Neuronal Signaling: from Neurodevelopment to Neurodegeneration

The peptidyl-prolyl isomerase Pin1 is a unique enzyme catalyzing the isomerization of the peptide bond between phosphorylated serine-proline or threonine-proline motifs in proteins, thereby regulating a wide spectrum of protein functions, including folding, intracellular signaling, transcription, cell cycle progression, and apoptosis. Pin1 has been reported to act as a key molecular switch inducing cell-type-specific effects, critically depending on the different phosphorylation patterns of its targets within different biological contexts. While its implication in proliferating cells, and, in particular, in the field of cancer, has been widely characterized, less is known about Pin1 biological functions in terminally differentiated and post-mitotic neurons. Notably, Pin1 is widely expressed in the central and peripheral nervous system, where it regulates a variety of neuronal processes, including neuronal development, apoptosis, and synaptic activity. However, despite studies reporting the interaction of Pin1 with neuronal substrates or its involvement in specific signaling pathways, a more comprehensive understanding of its biological functions at neuronal level is still lacking. Besides its implication in physiological processes, a growing body of evidence suggests the crucial involvement of Pin1 in aging and age-related and neurodegenerative diseases, including Alzheimer's disease, Parkinson disease, frontotemporal dementias, Huntington disease, and amyotrophic lateral sclerosis, where it mediates profoundly different effects, ranging from neuroprotective to neurotoxic. Therefore, a more detailed understanding of Pin1 neuronal functions may provide relevant information on the consequences of Pin1 deregulation in age-related and neurodegenerative disorders.

Pin1 Modulation in Physiological Status and Neurodegeneration. Any Contribution to the Pathogenesis of Type 3 Diabetes?

Prolyl isomerases (Peptidylprolyl isomerase, PPIases) are enzymes that catalyze the isomerization between the cis/trans Pro conformations. Three subclasses belong to the class: FKBP (FK506 binding protein family), Cyclophilin and Parvulin family (Pin1 and Par14). Among Prolyl isomerases, Pin1 presents as distinctive feature, the ability of binding to the motif pSer/pThr-Pro that is phosphorylated by kinases. Modulation of Pin1 is implicated in cellular processes such as mitosis, differentiation and metabolism: The enzyme is dysregulated in many diverse pathological conditions, i.e., cancer progression, neurodegenerative (i.e., Alzheimer’s diseases, AD) and metabolic disorders (i.e., type 2 diabetes, T2D). Indeed, Pin1 KO mice develop a complex phenotype of premature aging, cognitive impairment in elderly mice and neuronal degeneration resembling that of the AD in humans. In addition, since the molecule modulates glucose homeostasis in the brain and peripherally, Pin1 KO mice are resistant to diet-induced obesity, insulin resistance, peripheral glucose intolerance and diabetic vascular dysfunction. In this review, we revise first critically the role of Pin1 in neuronal development and differentiation and then focus on the in vivo studies that demonstrate its pivotal role in neurodegenerative processes and glucose homeostasis. We discuss evidence that enables us to speculate about the role of Pin1 as molecular link in the pathogenesis of type 3 diabetes i.e., the clinical association of dementia/AD and T2D.

Melatonin ameliorates Aβ42 -induced alteration of βAPP-processing secretases via the melatonin receptor through the Pin1/GSK3β/NF-κB pathway in SH-SY5Y cells

Melatonin is involved in the physiological regulation of the β-amyloid precursor protein (βAPP)-cleaving secretases which are responsible for generation of the neurotoxic amyloid beta (Aβ) peptide, one of the hallmarks of Alzheimer's disease (AD) pathology. In this study, we aimed to determine the underlying mechanisms of this regulation under pathological conditions. We establish that melatonin prevents Aβ₄₂ -induced downregulation of a disintegrin and metalloproteinase domain-containing protein 10 (ADAM10) as well as upregulation of β-site APP-cleaving enzyme 1 (BACE1) and presenilin 1 (PS1) in SH-SY5Y cell cultures. We also demonstrate that the intrinsic mechanisms of the observed effects occurred via regulation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) and glycogen synthase kinase (GSK)-3β as melatonin reversed Aβ₄₂ -induced upregulation and nuclear translocation of NF-κBp65 as well as activation of GSK3β via its receptor activation. Furthermore, specific blocking of the NF-κB and GSK3β pathways partially abrogated the Aβ₄₂ -induced reduction in the BACE1 and PS1 levels. In addition, GSK3β blockage affected α-secretase cleavage and modulated nuclear translocation of NF-κB. Importantly, our study for the first time shows that peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1) is a crucial target of melatonin. The compromised levels and/or genetic variation of Pin1 are associated with age-dependent tau and Aβ pathologies and neuronal degeneration. Interestingly, melatonin alleviated the Aβ₄₂ -induced reduction of nuclear Pin1 levels and preserved the functional integrity of this isomerase. Our findings illustrate that melatonin attenuates Aβ₄₂ -induced alterations of βAPP-cleaving secretases possibly via the Pin1/GSK3β/NF-κB pathway.

Dynamic regulation of Pin1 expression and function during zebrafish development

The prolyl isomerase Pin1 plays a key role in the modulation of proline-directed phosphorylation signaling by inducing local conformational changes in phosphorylated protein substrates. Extensive studies showed different roles for Pin1 in physiological processes and pathological conditions such as cancer and neurodegenerative diseases. However, there are still several unanswered questions regarding its biological role. Notably, despite evidences from cultured cells showing that Pin1 expression and activity may be regulated by different mechanisms, little is known on their relevance in vivo. Using Danio rerio (zebrafish) as a vertebrate model organism we showed that pin1 expression is regulated during embryogenesis to achieve specific mRNA and protein distribution patterns. Moreover, we found different subcellular distribution in particular stages and cell types and we extended the study of Pin1 expression to the adult zebrafish brain. The analysis of Pin1 overexpression showed alterations on zebrafish development and the presence of p53-dependent apoptosis. Collectively, our results suggest that specific mechanisms are operated in different cell types to regulate Pin1 function.

Gene promoter methylation and expression of Pin1 differ between patients with frontotemporal dementia and Alzheimer's disease

Frontotemporal Dementia (FTD) and Alzheimer's Disease (AD) share the accumulation of fibrillar aggregates of misfolded proteins. To better understand these neurodegenerative diseases and identify biomarkers in easily accessible cells, we investigated DNA methylation at Pin1 gene promoter and its expression in peripheral blood mononuclear cells of FTD patients. We found a lower gene expression of Pin1 with a higher DNA methylation in three CpG sites at Pin1 gene promoter analysed in FTD subjects, in contrast to a higher gene expression with a lower methylation in AD subjects and controls. These data suggest an important and distinct involvement of Pin1 in these two types of dementia.

Pin1 promotes degradation of Smad proteins and their interaction with phosphorylated tau in Alzheimer's disease

AIMS

Neurodegeneration in Alzheimer's disease (AD) is characterized by pathological protein aggregates and inadequate activation of cell cycle regulating proteins. Recently, Smad proteins were identified to control the expression of AD relevant proteins such as APP, CDK4 and CDK inhibitors, both critical regulators of cell cycle activation. This might indicate a central role for Smads in AD pathology where they show a substantial deficiency and disturbed subcellular distribution in neurones. Still, the mechanisms driving relocation and decrease of neuronal Smad in AD are not well understood. However, Pin1, a peptidyl-prolyl-cis/trans-isomerase, which allows isomerization of tau protein, was recently identified also controlling the fate of Smads. Here we analyse a possible role of Pin1 for Smad disturbances in AD.

METHODS

Multiple immunofluorescence labelling and confocal laser-scanning microscopy were performed to examine the localization of Smad and Pin1 in human control and AD hippocampi. Ectopic Pin1 expression in neuronal cell cultures combined with Western blot analysis and immunoprecipitation allowed studying Smad level and subcellular distribution. Luciferase reporter assays, electromobility shift, RNAi-technique and qRT-PCR revealed a potential transcriptional impact of Smad on Pin1 promoter.

RESULTS

We report on a colocalization of phosphorylated Smad in AD with Pin1. Pin1 does not only affect Smad phosphorylation and stability but also regulates subcellular localization of Smad2 and supports its binding to phosphorylated tau protein. Smads, in turn, exert a negative feed-back regulation on Pin1.

CONCLUSION

Our data suggest both Smad proteins and Pin1 to be elements of a vicious circle with potential pathogenetic significance in AD.

Pin1 cysteine-113 oxidation inhibits its catalytic activity and cellular function in Alzheimer's disease

The unique proline isomerase Pin1 is pivotal for protecting against age-dependent neurodegeneration in Alzheimer's disease (AD), with its inhibition providing a molecular link between tangle and plaque pathologies. Pin1 is oxidatively modified in human AD brains, but little is known about its regulatory mechanisms and pathological significance of such Pin1 modification. In this paper, our determination of crystal structures of oxidized Pin1 reveals a series of Pin1 oxidative modifications on Cys113 in a sequential fashion. Cys113 oxidization is further confirmed by generating antibodies specifically recognizing oxidized Cys113 of Pin1. Furthermore, Pin1 oxidation on Cys113 inactivates its catalytic activity in vitro, and Ala point substitution of Cys113 inactivates the ability of Pin1 to isomerize tau as well as to promote protein turnover of tau and APP. Moreover, redox regulation affects Pin1 subcellular localization and Pin1-mediated neuronal survival in response to hypoxia treatment. Importantly, Cys113-oxidized Pin1 is significantly increased in human AD brain comparing to age-matched controls. These results not only identify a novel Pin1 oxidation site to be the critical catalytic residue Cys113, but also provide a novel oxidative regulation mechanism for inhibiting Pin1 activity in AD. These results suggest that preventing Pin1 oxidization might help to reduce the risk of AD.

Peripheral blood mononuclear cells as a laboratory to study dementia in the elderly

The steady and dramatic increase in the incidence of Alzheimer's disease (AD) and the lack of effective treatments have stimulated the search for strategies to prevent or delay its onset and/or progression. Since the diagnosis of dementia requires a number of established features that are present when the disease is fully developed, but not always in the early stages, the need for a biological marker has proven to be urgent, in terms of both diagnosis and monitoring of AD. AD has been shown to affect peripheral blood mononuclear cells (PBMCs) that are a critical component of the immune system which provide defence against infection. Although studies are continuously supplying additional data that emphasize the central role of inflammation in AD, PBMCs have not been sufficiently investigated in this context. Delineating biochemical alterations in AD blood constituents may prove valuable in identifying accessible footprints that reflect degenerative processes within the Central Nervous System (CNS). In this review, we address the role of biomarkers in AD with a focus on the notion that PBMCs may serve as a peripheral laboratory to find molecular signatures that could aid in differential diagnosis with other forms of dementia and in monitoring of disease progression.

Alternative splicing of Myb-related genes MYR1 and MYR2 may modulate activities through changes in dimerization, localization, or protein folding

Arabidopsis genes MYR1 and MYR2 are regulators of flowering time under low light intensity. These Myb-related genes are expressed as alternative splice variants affected in their coiled-coil and DNA-binding domains. We tested whether alternative splicing could affect dimerization and localization of MYR1 and MYR2, thereby potentially affecting their activity. Using MYR1 as a model for variants within the coiled-coil region, we detected 2 types of homodimers. For MYR2, alternative splicing in the DNA-binding Myb-like domain abolished the ability of MYR2 to dimerize. Alternative splicing in the coiled-coil domain did not affect nuclear localization, as determined by transient expression in tobacco, while alternative splicing in the DNA-binding domain of MYR2 yielded a distinct intranuclear localization pattern that may reflect changes in phosphorylation-dependent protein folding. Thus alternative splicing of these genes may result in changes in dimerization or protein folding resulting in changes in activity and abundance of MYR1 or MYR2 protein.

Peptidyl-prolyl cis–trans isomerase Pin1 in ageing, cancer and Alzheimer disease

Phosphorylation of proteins on serine or threonine residues preceding proline is a key signalling mechanism in diverse physiological and pathological processes. Pin1 (peptidyl-prolyl cis–trans isomerase) is the only enzyme known that can isomerise specific Ser/Thr-Pro peptide bonds after phosphorylation and regulate their conformational changes with high efficiency. These Pin1-catalysed conformational changes can have profound effects on phosphorylation signalling by regulating a spectrum of target activities. Interestingly, Pin1 deregulation is implicated in a number of diseases, notably ageing and age-related diseases, including cancer and Alzheimer disease. Pin1 is overexpressed in most human cancers; it activates numerous oncogenes or growth enhancers and also inactivates a large number of tumour suppressors or growth inhibitors. By contrast, ablation of Pin1 prevents cancer, but eventually leads to premature ageing and neurodegeneration. Consistent with its neuroprotective role, Pin1 has been shown to be inactivated in neurons of patients with Alzheimer disease. Therefore, Pin1-mediated phosphorylation-dependent prolyl isomerisation represents a unique signalling mechanism that has a pivotal role in the development of human diseases, and might offer an attractive new diagnostic and therapeutic target.

FUS immunogold labeling TEM analysis of the neuronal cytoplasmic inclusions of neuronal intermediate filament inclusion disease: a frontotemporal lobar degeneration with FUS proteinopathy

Fused in sarcoma (FUS)-immunoreactive neuronal and glial inclusions define a novel molecular pathology called FUS proteinopathy. FUS has been shown to be a component of inclusions of familial amyotrophic lateral sclerosis with FUS mutation and three frontotemporal lobar degeneration entities, including neuronal intermediate filament inclusion disease (NIFID). The pathogenic role of FUS is unknown. In addition to FUS, many neuronal cytoplasmic inclusions (NCI) of NIFID contain aggregates of α-internexin and neurofilament proteins. Herein, we have shown that: (1) FUS becomes relatively insoluble in NIFID and there are no apparent posttranslational modifications, (2) there are no pathogenic abnormalities in the FUS gene in NIFID, and (3) immunoelectron microscopy demonstrates the fine structural localization of FUS in NIFID which has not previously been described. FUS localized to euchromatin, and strongly with paraspeckles, in nuclei, consistent with its RNA/DNA-binding functions. NCI of varying morphologies were observed. Most frequent were the "loosely aggregated cytoplasmic inclusions," 81% of which had moderate or high levels of FUS immunoreactivity. Much rarer "compact cytoplasmic inclusions" and "tangled twine ball inclusions" were FUS-immunoreactive at their granular peripheries, or heavily FUS-positive throughout, respectively. Thus, FUS may aggregate in the cytoplasm and then admix with neuronal intermediate filament accumulations.

PIN1 gene variants in Alzheimer's disease

BACKGROUND

Peptidyl-prolyl isomerase, NIMA-interacting 1 (PIN1) plays a significant role in the brain and is implicated in numerous cellular processes related to Alzheimer's disease (AD) and other neurodegenerative conditions. There are confounding results concerning PIN1 activity in AD brains. Also PIN1 genetic variation was inconsistently associated with AD risk.

METHODS

We performed analysis of coding and promoter regions of PIN1 in early- and late-onset AD and frontotemporal dementia (FTD) patients in comparison with healthy controls.

RESULTS

Analysis of eighteen PIN1 common polymorphisms and their haplotypes in EOAD, LOAD and FTD individuals in comparison with the control group did not reveal their contribution to disease risk.In six unrelated familial AD patients four novel PIN1 sequence variants were detected. c.58+64C>T substitution that was identified in three patients, was located in an alternative exon. In silico analysis suggested that this variant highly increases a potential affinity for a splicing factor and introduces two intronic splicing enhancers. In the peripheral leukocytes of one living patient carrying the variant, a 2.82 fold decrease in PIN1 expression was observed.

CONCLUSION

Our data does not support the role of PIN1 common polymorphisms as AD risk factor. However, we suggest that the identified rare sequence variants could be directly connected with AD pathology, influencing PIN1 splicing and/or expression.

Immunoreactivity to Lys63-linked polyubiquitin is a feature of neurodegeneration

The major human neurodegenerative diseases are characterised by ubiquitin-positive intraneuronal inclusions, however the precise nature of the ubiquitin modifications in these structures is unclear. Using a monoclonal antibody specific for Lys63-linked polyubiquitin we have performed the first immunohistochemical analysis of linkage-specific ubiquitination in vivo associated with neurodegeneration. Immunoreactivity was detected within the pathological lesions of Alzheimer's, Huntington's and Parkinson's disease brains, although staining of Lewy bodies in the substantia nigra in Parkinson's disease was rare, indicating a selective involvement of Lys63-linked polyubiquitin in inclusion biogenesis in this disorder. Immunoreactivity was also a feature in neurons of proteasome-depleted mice, suggesting a proteasomal contribution to the degradation of Lys63-linked polyubiquitinated proteins in vivo.

Fine structural analysis of the neuronal inclusions of frontotemporal lobar degeneration with TDP-43 proteinopathy

TAR DNA-binding protein of 43 kDa (TDP-43) is a major component of the pathological inclusions of frontotemporal lobar degeneration with TDP-43 proteinopathy, also called FTLD with ubiquitin-positive, tau-negative inclusions (FTLD-U), and motor neuron disease (MND). TDP-43 is predominantly expressed in the nucleus and regulates gene expression and splicing. In FTLD with TDP-43 proteinopathy, neuronal inclusions present variably as cytoplasmic inclusions (NCIs), dystrophic neurites (DNs), and intranuclear inclusions (NIIs), leading to a fourfold neuropathological classification correlating with genotype. There have been few fine structural studies of these inclusions. Thus, we undertook an immunoelectron microscopic study of FTLD with TDP-43 proteinopathy, including sporadic and familial cases with progranulin (GRN) mutation. TDP-43-immunoreactive inclusions comprised two components: granular and filamentous. Filament widths, expressed as mean (range) were: NCI, 9 nm (4-16 nm); DN, 10 nm (5-16 nm); NII, 18 nm (9-50 nm). Morphologically distinct inclusion components may reflect the process of TDP-43 aggregation and interaction with other proteins: determining these latter may contribute towards understanding the heterogeneous pathogenesis of FTLD with TDP-43 proteinopathy.

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

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

Novel strategies for Alzheimer's disease treatment

Considerable progress has been made in recent years towards better understanding the pathogenesis of Alzheimer's disease (AD), a dementing neurodegenerative disorder that affects > 10 million individuals in the US and Europe combined. Recent studies suggest that alterations in the processing of amyloid precursor protein (APP), resulting in the accumulation of amyloid-beta protein (Abeta) and the formation of oligomers leads to synaptic damage and neurodegeneration. Therefore, strategies for treatment development have been focused on reducing Abeta accumulation using, among other approaches, antiaggregation molecules, regulators of the APP proteolysis and processing, reducing APP production (e.g., small-interfering RNA), and increasing Abeta clearance with antibodies, apolipoprotein E and Abeta-degrading enzymes (e.g., neprilysin). The main focus of this review is on novel treatments for AD with a special emphasis on delivering neuroprotective and antiamyloidogenic molecules by gene therapy and by promoting neurogenesis.

Prolyl cis-trans isomerization as a molecular timer

Proline is unique in the realm of amino acids in its ability to adopt completely distinct cis and trans conformations, which allows it to act as a backbone switch that is controlled by prolyl cis-trans isomerization. This intrinsically slow interconversion can be catalyzed by the evolutionarily conserved group of peptidyl prolyl cis-trans isomerase enzymes. These enzymes include cyclophilins and FK506-binding proteins, which are well known for their isomerization-independent role as cellular targets for immunosuppressive drugs. The significance of enzyme-catalyzed prolyl cis-trans isomerization as an important regulatory mechanism in human physiology and pathology was not recognized until the discovery of the phosphorylation-specific prolyl isomerase Pin1. Recent studies indicate that both phosphorylation-dependent and phosphorylation-independent prolyl cis-trans isomerization can act as a novel molecular timer to help control the amplitude and duration of a cellular process, and prolyl cis-trans isomerization might be a new target for therapeutic interventions.

The prolyl isomerase PIN1: a pivotal new twist in phosphorylation signalling and disease

Protein phosphorylation regulates many cellular processes by causing changes in protein conformation. The prolyl isomerase PIN1 has been identified as a regulator of phosphorylation signalling that catalyses the conversion of specific phosphorylated motifs between the two completely distinct conformations in a subset of proteins. PIN1 regulates diverse cellular processes, including growth-signal responses, cell-cycle progression, cellular stress responses, neuronal function and immune responses. In line with the diverse physiological roles of PIN1, it has also been linked to several diseases that include cancer, Alzheimer's disease and asthma, and thus it might represent a novel therapeutic target.

TDP-43 in familial and sporadic frontotemporal lobar degeneration with ubiquitin inclusions

TAR DNA-binding protein 43 (TDP-43) is a major pathological protein of sporadic and familial frontotemporal lobar degeneration with ubiquitin-positive, tau-negative inclusions (FTLD-U) with or without motor neuron disease (MND). Thus, TDP-43 defines a novel class of neurodegenerative diseases called TDP-43 proteinopathies. We performed ubiquitin and TDP-43 immunohistochemistry on 193 cases of familial and sporadic FTLD with or without MND. On selected cases, immunoelectron microscopy and biochemistry were performed. Clinically defined frontotemporal dementias (FTDs) included four groups: 1) familial FTD with mutations in progranulin (n = 36), valosin-containing protein (n = 5), charged multivesicular body protein 2B (n = 4), and linked to chromosome 9p (n = 7); 2) familial cases of FTD with unknown gene association (n = 29); 3) sporadic FTD (n = 72); and 4) familial and sporadic FTD with MND (n = 40). Our studies confirm that the spectrum of TDP-43 proteinopathies includes most cases of sporadic and familial FTLD-U with and without MND and expand this disease spectrum to include reported families with FTD linked to chromosome 9p but not FTD with charged multivesicular body protein 2B mutations. Thus, despite significant clinical, genetic, and neuropathological heterogeneity of FTLD-U, TDP-43 is a common pathological substrate underlying a large subset of these disorders, thereby implicating TDP-43 in novel and unifying mechanisms of FTLD pathogenesis.

Inhibition of Pin1 reduces glutamate-induced perikaryal accumulation of phosphorylated neurofilament-H in neurons

Under normal conditions, the proline-directed serine/threonine residues of neurofilament tail-domain repeats are exclusively phosphorylated in axons. In pathological conditions such as amyotrophic lateral sclerosis (ALS), motor neurons contain abnormal perikaryal accumulations of phosphorylated neurofilament proteins. The precise mechanisms for this compartment-specific phosphorylation of neurofilaments are not completely understood. Although localization of kinases and phosphatases is certainly implicated, another possibility involves Pin1 modulation of phosphorylation of the proline-directed serine/threonine residues. Pin1, a prolyl isomerase, selectively binds to phosphorylated proline-directed serine/threonine residues in target proteins and isomerizes cis isomers to more stable trans configurations. In this study we show that Pin1 associates with phosphorylated neurofilament-H (p-NF-H) in neurons and is colocalized in ALS-affected spinal cord neuronal inclusions. To mimic the pathology of neurodegeneration, we studied glutamate-stressed neurons that displayed increased p-NF-H in perikaryal accumulations that colocalized with Pin1 and led to cell death. Both effects were reduced upon inhibition of Pin1 activity by the use of an inhibitor juglone and down-regulating Pin1 levels through the use of Pin1 small interfering RNA. Thus, isomerization of lys-ser-pro repeat residues that are abundant in NF-H tail domains by Pin1 can regulate NF-H phosphorylation, which suggests that Pin1 inhibition may be an attractive therapeutic target to reduce pathological accumulations of p-NF-H.

Pin1 in Alzheimer's disease: multiple substrates, one regulatory mechanism?

Presence of neuritic plaques and neurofibrillary tangles in the brain are two neuropathological hallmarks of Alzheimer's disease (AD), although the molecular basis of their coexistence remains elusive. The neurofibrillary tangles are composed of microtubule binding protein Tau, whereas neuritic plaques consist of amyloid-beta peptides derived from amyloid precursor protein (APP). Recently, the peptidyl-prolyl cis/trans isomerase Pin1 has been identified to regulate the function of certain proteins after phosphorylation and to play an important role in cell cycle regulation and cancer development. New data indicate that Pin1 also regulates the function and processing of Tau and APP, respectively, and is important for protecting against age-dependent neurodegeneration. Furthermore, Pin1 is the only gene known so far that, when deleted in mice, can cause both Tau and Abeta-related pathologies in an age-dependent manner, resembling many aspects of human Alzheimer's disease. Moreover, in the human AD brain Pin1 is downregulated or inhibited by oxidative modifications and/or genetic changes. These results suggest that Pin1 deregulation may provide a link between formation of tangles and plaques in AD.

Post-phosphorylation prolyl isomerisation of gephyrin represents a mechanism to modulate glycine receptors function

The microtubule binding protein gephyrin plays a prominent role in establishing and maintaining a high concentration of inhibitory glycine receptors juxtaposed to presynaptic releasing sites. Here, we show that endogenous gephyrin undergoes proline-directed phosphorylation, which is followed by the recruitment of the peptidyl-prolyl isomerase Pin1. The interaction between gephyrin and Pin1 is strictly dependent on gephyrin phosphorylation and requires serine-proline consensus sites encompassing the gephyrin proline-rich domain. Upon binding, Pin1 triggers conformational changes in the gephyrin molecule, thus enhancing its ability to bind the beta subunit of GlyRs. Consistently, a downregulation of GlyR clusters was detected in hippocampal neurons derived from Pin1 knockout mice, which was paralleled by a reduction in the amplitude of glycine-evoked currents. Our results suggest that phosphorylation-dependent prolyl isomerisation of gephyrin represents a mechanism for regulating GlyRs function.

Transcriptional profiling of Alzheimer blood mononuclear cells by microarray

We evaluated pathomechanisms and systemic manifestations of Alzheimer disease (AD), an aging-related dementing neurodegenerative disorder, by expression profiling. Blood mononuclear cell (BMC) transcriptomes of sporadic AD subjects and aged-matched normal elderly controls (NEC) were compared using the human NIA microarray. Relative to the NEC samples, the Alzheimer BMC exhibited a significant decline in the expression of genes concerned with cytoskeletal maintenance, cellular trafficking, cellular stress response, redox homeostasis, transcription and DNA repair. We observed decreased expression of several genes which may impact amyloid-beta production and the processing of the microtubule-associated protein tau. The microarray results were validated by quantitative real time PCR and revealed gender differences in the levels of altered gene expression. Our findings attest to the systemic nature of gene dys-regulation in sporadic AD, implicate disruption of cytoskeletal integrity, DNA repair mechanisms and cellular defenses in this condition, and suggest novel pathways of beta-amyloid deposition in this disease. BMC are highly accessible and may reflect molecular events germane to the neuropathophysiology of AD.

REVIEW: Pin1 in Alzheimer's disease

Proteolytic processing and phosphorylation of amyloid precursor protein (APP), and hyperphosphorylation of tau protein, have been shown to be increased in Alzheimer's disease (AD) brains, leading to increased production of beta-amyloid (Abeta) peptides and neurofibrillary tangles, respectively. These observations suggest that phosphorylation events are critical to the understanding of the pathogenesis and treatment of this devastating disease. Pin-1, one of the peptidyl-prolyl isomerases (PPIase), catalyzes the isomerization of the peptide bond between pSer/Thr-Pro in proteins, thereby regulating their biological functions which include protein assembly, folding, intracellular transport, intracellular signaling, transcription, cell cycle progression and apoptosis. A number of previous studies have shown that Pin1 is co-localized with phosphorylated tau in AD brain, and shows an inverse relationship to the expression of tau. Pin1 protects neurons under in vitro conditions. Moreover, recent studies demonstrate that APP is a target for Pin1 and thus, in Abeta production. Furthermore, Pin1 was found to be oxidatively modified and to have reduced activity in the hippocampus in mild cognitive impairment (MCI) and AD. Because of the diverse functions of Pin1, and the discovery that this protein is one of the oxidized proteins common to both MCI and AD brain, the question arises as to whether Pin1 is one of the driving forces for the initiation or progression of AD pathogenesis, finally leading to neurodegeneration and neuronal apoptosis. In the present review, we discuss the role of Pin1 with respect to Alzheimer's disease.

Pin1 allows for differential Tau dephosphorylation in neuronal cells

Neurofibrillary degeneration is likely to be related to abnormal Tau phosphorylation and aggregation. Among abnormal Tau phosphorylation sites, pThr231 is of particular interest since it is associated with early stages of Alzheimer's disease and is a binding site of Pin1, a peptidyl-prolyl cis/trans isomerase mainly involved in cell cycle regulation. In the present work, Pin1 level was found strongly increased during neuronal differentiation and tightly correlated with Tau dephosphorylation at Thr231. Likewise, we showed in cellular model that Pin1 allowed for specific Tau dephosphorylation at Thr231, whereas other phosphorylation sites were unchanged. Moreover, cells displaying Tau phosphorylation at Thr231 did not show any Pin1 nuclear depletion. Altogether, these data indicate that Pin1 has key function(s) in neuron and is at least involved in the regulation of Tau phosphorylation at relevant sites. Hence, Pin1 dysfunction, unlikely by nuclear depletion, may have critical consequences on Tau pathological aggregation and neuronal death.

Pin1 protein associates with neuronal lipofuscin: potential consequences in age-related neurodegeneration

Pin1 protein is a peptidyl-prolyl cis-trans isomerase that modulates the activity of a range of proteins involved in cell function. We and others have demonstrated neuronal Pin1 deficits in Alzheimer's disease (AD) and have shown similar deficits in frontotemporal dementia and in aging. Pin1 may, in fact, be a susceptibility factor; others have shown that Pin1 depletion causes apoptosis in HeLa cells. Further, patterns of AD pathology correlate with regions of lower Pin1 expression in normal human brain; Pin1 knockout mice suffer neurodegeneration; and Pin1 can ameliorate p-tau pathology by isomerizing p-tau, facilitating its trans-specific dephosphorylation and restoring its ability to bind to and restabilize microtubules and thence cytoskeletal integrity. Here, we report a novel localization of high levels of Pin1 with lipofuscin in aging neurons. This association could progressively drain available Pin1 and be deleterious to neuronal function during aging. We also show that Pin1 associates with lipofuscin when lipofuscin accumulations become marked and correlate with susceptibility to neurodegenerative disease. Our data are consistent with the possibility that neuronal Pin1 deficits may be a contributory factor in neurodegeneration associated with aging.

Tau and GSK3beta dephosphorylations are required for regulating Pin1 phosphorylation

Pin1 binds mitotically phosphorylated Thr231-Pro232 and Thr212-Pro213 sites on tau, and a Pin1 deficiency in mice leads to tau hyperphosphorylation. The aim of this study was to determine if the dephosphorylation or inhibition of tau and GSK3beta phosphorylation induces the Pin1 phosphorylation. To test this, human SK-N-MC cells were stably transfected with a fusion gene containing neuron-specific enolase (NSE)-controlled APPsw gene(NSE/APPsw), to induce Abeta-42. The stable transfectants were then transiently transfected with NSE/Splice, lacking human tau (NSE/Splice), or NSE/hTau, containing human tau, into the cells. The NSE/Splice- and NSE/hTau-cells were then treated with lithium. We concluded that (i) there was more C99-beta APP accumulation than C83-betaAPP in APPsw-tansfectant and thereby promoted Abeta-42 production in transfectants. (ii) the inhibition of tau and GSK3beta phosphorylations correlated with increase in Pin1 activation in NSE/hTau- cells. Thus, these observations suggest that Pin1 might have an inhibitive role in phosphorylating tau and GSK3beta for protecting against Alzheimer's disease.

Pharmacological targeting of catalyzed protein folding: the example of peptide bond cis/trans isomerases

Peptide bond isomerases are involved in important physiological processes that can be targeted in order to treat neurodegenerative disease, cancer, diseases of the immune system, allergies, and many others. The folding helper enzyme class of Peptidyl-Prolyl-cis/trans Isomerases (PPIases) contains the three enzyme families of cyclophilins (Cyps), FK506 binding proteins (FKBPs), and parvulins (Pars). Although they are structurally unrelated, all PPIases catalyze the cis/trans isomerization of the peptide bond preceding the proline in a polypeptide chain. This process not only plays an important role in de novo protein folding, but also in isomerization of native proteins. The native state isomerization plays a role in physiological processes by influencing receptor ligand recognition or isomer-specific enzyme reaction or by regulating protein function by catalyzing the switch between native isomers differing in their activity, e.g., ion channel regulation. Therefore elucidating PPIase involvement in physiological processes and development of specific inhibitors will be a suitable attempt to design therapies for fatal and deadly diseases.

DNA sequence variations in the prolyl isomerase Pin1 gene and Alzheimer's disease

Senile plaques and neurofibrillary tangles (NFT) are the prominent lesions in the brain of Alzheimer's disease (AD) patients. NFT are mainly composed of an abnormally phosphorylated form of tau protein, which has lost its function to bind microtubules and promote their assembly. Tau hyperphosphorylation critically decreases tau function and precedes neurodegeneration. The majority of tau phosphorylation sites are Ser/Thr-Pro motifs, which are known to exist in two distinct cis and trans conformations. The prolyl isomerase Pin1 catalyses the conversion of those conformations. Pin1 binds to tau specifically at the Thr231-Pro site and restores tau function, either by inducing conformational changes or facilitating dephosphorylation. It has been shown that Pin1 expression levels inversely correlate with the predicted vulnerability of different brain areas to neurodegeneration and soluble Pin1 is depleted in neurons from AD brains; furthermore, Pin1 knock-out mice develop signs and symptoms of tau-related pathologies late in life. It seems that Pin1 plays an important role in maintaining tau function, thereby preserving neuronal homeostasis and preventing age-dependent neurodegeneration. DNA sequence variations in Pin1 gene may affect its expression level or function and influence the individual risk for developing AD. We screened by denaturing high performance liquid chromatography the genomic DNA of 120 AD subjects and 134 age-matched controls and we found very few and rare sequence variations in the promoter region and in exons 2 and 3. We conclude that Pin1 is a very well conserved gene, whose rare nucleotide variations have no effect on the individual genetic risk for AD.

Phosphorylation-specific prolyl isomerization: is there an underlying theme?

The prolyl isomerase Pin1 is a conserved enzyme that is intimately involved in diverse biological processes and pathological conditions such as cancer and Alzheimer's disease. By catalysing cis-trans interconversion of certain motifs containing phosphorylated serine or threonine residues followed by a proline residue (pSer/Thr-Pro), Pin1 can have profound effects on phosphorylation signalling. The structural and functional differences that result from cis-trans isomerization of specific pSer/Thr-Pro motifs probably underlie most, if not all, Pin1-dependent actions. Phosphorylation-dependent prolyl isomerization by Pin1 remains a unique mode for the modulation of signal transduction. Here, we provide an overview of the plethora of regulatory events that involve this unique enzyme, with a particular focus on oncogenic signalling and neurodegeneration.