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

Identification of the oxidative stress proteome in the brain

The redox proteomics technique normally combines two-dimensional gel electrophoresis, mass spectrometry, and protein databases to analyze the cell proteome from various samples, thereby leading to the identification of specific targets of oxidative modification. Oxidative stress that occurs because of increased levels of reactive oxygen species and reactive nitrogen species can target most biomolecules, consequently leading to altered physiological function of the cells. Redox proteomics has identified oxidatively modified protein targets in various pathological conditions, consequently providing insight into the pathways involved in the pathogenesis of these conditions. This approach also can be used to identify possible protective mechanisms to prevent or delay these disorders.

Tyr(682) in the intracellular domain of APP regulates amyloidogenic APP processing in vivo

BACKGROUND

The pathogenesis of Alzheimer's disease is attributed to misfolding of Amyloid-β (Aβ) peptides. Aβ is generated during amyloidogenic processing of Aβ-precursor protein (APP). Another characteristic of the AD brain is increased phosphorylation of APP amino acid Tyr(682). Tyr(682) is part of the Y(682)ENPTY(687) motif, a docking site for interaction with cytosolic proteins that regulate APP metabolism and signaling. For example, normal Aβ generation and secretion are dependent upon Tyr(682) in vitro. However, physiological functions of Tyr(682) are unknown.

METHODOLOGY/PRINCIPAL FINDINGS

To this end, we have generated an APP Y682G knock-in (KI) mouse to help dissect the role of APP Tyr(682) in vivo. We have analyzed proteolytic products from both the amyloidogenic and non-amyloidogenic processing of APP and measure a profound shift towards non-amyloidogenic processing in APP KI mice. In addition, we demonstrate the essential nature of amino acid Tyr(682) for the APP/Fe65 interaction in vivo.

CONCLUSIONS/SIGNIFICANCE

Together, these observations point to an essential role of APP intracellular domain for normal APP processing and function in vivo, and provide rationale for further studies into physiological functions associated with this important phosphorylation site.

Hsp90 regulates tau pathology through co-chaperone complexes in Alzheimer's disease

Alzheimer's disease is a tauopathy which involves the deposition of microtubule-associated tau proteins into neurofibrillary tangles. Post-translational modifications, in particular site-specific phosphorylations, affect the conformation of tau protein which is an intrinsically disordered protein. These structural changes significantly increase the affinity of tau protein for certain molecular chaperones. Hsp90 is a major cellular chaperone which assembles large complexes with a variety of co-chaperones. The main function of Hsp90 complexes is to maintain protein quality control and assist in protein degradation via proteasomal and autophagic-lysosomal pathways. Tau protein is a client protein for these Hsp90 complexes. If the tau protein is in an abnormal or modified form, then it can trigger the recruitment of CHIP protein, a co-chaperone with E3 activity, to the complex which induces the ubiquitination of tau protein and activates its downstream degradation processes. Large immunophilins, FKBP51 and FKBP52 are also co-chaperones of Hsp90-tau complexes. These proteins contain peptidylprolyl cis/trans isomerase activity which catalyzes phosphorylation-dependent rotation in pSer/Thr-Pro peptide bond. The proline switch in the tau conformation triggers dephosphorylation of Ser/Thr residues phosphorylated, e.g. by two well-known tau kinases Cdk5 and GSK-3β. Binding of PP5 protein phosphatase to Hsp90 complex, can also dephosphorylate tau protein. Subsequently, dephosphorylated tau protein can be shuttled back to the microtubules. It seems that high-affinity binding of abnormal tau to Hsp90 complexes may have some counteracting effects on the aggregation process, since Hsp90 inhibitors can ameliorate the aggregation process in several neurodegenerative diseases. We will review the role of Hsp90 chaperone network in the regulation of tau biology and pathology in Alzheimer's disease.

Pseudophosphorylation of tau protein directly modulates its aggregation kinetics

Hyperphosphorylation of tau protein is associated with neurofibrillary lesion formation in Alzheimer's disease and other tauopathic neurodegenerative diseases. It fosters lesion formation by increasing the concentration of free tau available for aggregation and by directly modulating the tau aggregation reaction. To clarify how negative charge incorporation into tau directly affects aggregation behavior, the fibrillization of pseudophosphorylation mutant T212E prepared in a full-length four-repeat tau background was examined in vitro as a function of time and submicromolar tau concentrations using electron microscopy assay methods. Kinetic constants for nucleation and extension phases of aggregation were then estimated by direct measurement and mathematical simulation. Kinetic analysis revealed that pseudophosphorylation increased tau aggregation rate by increasing the rate of filament nucleation. In addition, it increased aggregation propensity by stabilizing mature filaments against disaggregation. The data suggest that incorporation of negative charge into the T212 site can directly promote tau filament formation at multiple steps in the aggregation pathway.

The prolyl isomerase Pin1 acts as a novel molecular switch for TNF-alpha-induced priming of the NADPH oxidase in human neutrophils

Neutrophils play a key role in host defense by releasing reactive oxygen species (ROS). However, excessive ROS production by neutrophil nicotinamide adenine dinucleotide phosphate (NADPH) oxidase can damage bystander tissues, thereby contributing to inflammatory diseases. Tumor necrosis factor-α (TNF-α), a major mediator of inflammation, does not activate NADPH oxidase but induces a state of hyperresponsiveness to subsequent stimuli, an action known as priming. The molecular mechanisms by which TNF-α primes the NADPH oxidase are unknown. Here we show that Pin1, a unique cis-trans prolyl isomerase, is a previously unrecognized regulator of TNF-α-induced NADPH oxidase hyperactivation. We first showed that Pin1 is expressed in neutrophil cytosol and that its activity is markedly enhanced by TNF-α. Inhibition of Pin1 activity with juglone or with a specific peptide inhibitor abrogated TNF-α-induced priming of neutrophil ROS production induced by N-formyl-methionyl-leucyl-phenylalanine peptide (fMLF). TNF-α enhanced fMLF-induced Pin1 and p47phox translocation to the membranes and juglone inhibited this process. Pin1 binds to p47phox via phosphorylated Ser345, thereby inducing conformational changes that facilitate p47phox phosphorylation on other sites by protein kinase C. These findings indicate that Pin1 is critical for TNF-α-induced priming of NADPH oxidase and for excessive ROS production. Pin1 inhibition could potentially represent a novel anti-inflammatory strategy.

An update on clinical proteomics in Alzheimer's research

Alzheimer's disease (AD) is a pathologically complex and aetiologically multifactorial dementing disorder affecting millions of people worldwide. The pathological brain changes are assumed to occur decades prior to the onset of clinical symptoms. The diagnosis of early AD remains problematic and is mainly based on clinical and neuropsychological findings after the onset of symptoms. Currently available drugs are able to delay the symptom progression of the disease but not to attenuate the progression of pathological brain changes. Many studies exploring AD proteomes have been conducted as the middle of 1990s as a consequence of recent advances in the development of both gel-based and gel-free proteomics approaches. It is hoped that proteomics can contribute to improving the understanding, diagnosis, and follow-up of the progression of AD. In this review, we summarise the present status of proteome alterations, with emphasis on quantitative approaches, in AD brain, CSF and blood, and their relevance to dementia research.

Novel role of Pin1 induction in type II collagen-mediated rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease characterized by chronic inflammation in joints and subsequent destruction of cartilage and bone. Inflammatory mediators such as PGs and proinflammatory cytokines contribute to RA progress. Pin1, a peptidyl prolyl isomerase, plays important pathophysiological roles in several diseases, including cancer and neurodegeneration. We found that both Pin1 and cyclooxygenase-2 (COX-2) were highly expressed in ankle tissues of type II collagen-induced RA mice. HTB-94 cells overexpressing Pin1 and primary cultured human chondrocytes showed increased basal expression of proinflammatory proteins (COX-2, inducible NO synthase, TNF-alpha, and IL-1beta). Site-directed mutagenesis revealed that Pin1-mediated transcriptional activation of COX-2 was coordinately regulated by NF-kappaB, CREB, and C/EBP. Gel shift, reporter gene, and Western blot analyses confirmed that NF-kappaB, CREB, and C/EBP were consistently activated in chondrocytes overexpressing Pin1. Treatment of RA mice with juglone, a chemical inhibitor of Pin1, significantly reduced RA progress and COX-2 expression in the ankle tissues. Moreover, juglone dose dependently decreased the basal COX-2 expression in primary cultured chondrocytes from RA patients. These results demonstrate that Pin1 induction during RA progress stimulates proinflammatory protein expression by activating NF-kappaB, CREB, and C/EBP, and suggest that Pin1 is a potential therapeutic target of RA.

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.

The interactome of the amyloid beta precursor protein family members is shaped by phosphorylation of their intracellular domains

BACKGROUND

Brain tissue from patients with Alzheimer's disease has shown an increase of phosphorylation of Tyr-682, located on the conserved Y682ENPTY motif, and Thr-668 residues, both in the intracellular domain (AID) of amyloid beta precursor protein (APP), although the role of these two residues is not yet known.

RESULTS

Here, we report that the phosphorylation status of Tyr-682, and in some cases Thr-668, shapes the APP interactome. It creates a docking site for SH2-domain containing proteins, such as ShcA, ShcB, ShcC, Grb7, Grb2, as well as adapter proteins, such as Crk and Nck, that regulate important biological processes, cytosolic tyrosine kinases, such as Abl, Lyn and Src, which regulate signal transduction pathways, and enzymes that control phosphatidylinositols levels and signaling, such as PLC-gamma. At the same time, it either reduces (like for JIP1, NUMB, NUMBL and ARH) or abolishes (like for Fe65, Fe65L1 and Fe65L2) binding of other APP interactors. Phosphorylation of Thr-668, unlike Tyr-682, does not seem to affect APP's ability to interact with the various proteins, with Pin1 and X11 being the exclusions. We also found that there are some differences between the interactions to AID and to ALID1 and ALID2, its two homologues.

CONCLUSION

Our data indicates that APP can regulate diverse cellular processes and that, vice versa, a network of signaling events can impact APP processing. Our results also suggest that phosphorylation of the APP Intracellular Domain will dramatically shape the APP interactome and, consequently, will regulate APP processing, APP transport and APP/AID-mediated functions.

Peptidylproline cis-trans-isomerase Pin1 interacts with human T-cell leukemia virus type 1 tax and modulates its activation of NF-kappaB

Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus etiologically causal of adult T-cell leukemia (ATL). The virus encodes a Tax oncoprotein that functions in transcriptional regulation, cell cycle control, and transformation. ATL is a highly virulent cancer that is resistant to chemotherapeutic treatments. To understand this disease better, it is important to comprehend how HTLV-1 promotes cellular growth and survival. Tax activation of NF-kappaB is important for the proliferation and transformation of virus-infected cells. We show here that prolyl isomerase Pin1 is over expressed in HTLV-1 cell lines; Pin1 binds Tax and regulates Tax-induced NF-kappaB activation.

Nuclear import of Pin1 is mediated by a novel sequence in the PPIase domain

Pin1 actively regulates diverse biological/pathological processes, but little is known about the regulatory mechanisms of its cellular localization. In this study, we report that the endogenous Pin1 is distributed in both nucleus and cytoplasm. We found that point mutations of several basic amino acids in the PPIase domain of Pin1 significantly compromise its nuclear localization. Such inhibition is independent of Pin1 enzymatic activity, and is mainly due to the defects in the nuclear import. A novel sequence harboring these residues was identified as a putative nuclear localization signal (NLS) of Pin1. Importin alpha5 of the nuclear import machinery was found to interact with Pin1.

The dual histidine motif in the active site of Pin1 has a structural rather than catalytic role

The catalytic domain of the peptidyl-prolyl cis/ trans isomerase Pin1 is a member of the FKBP superfold family. Within its active site are two highly conserved histidine residues, H59 and H157. Despite their sequence conservation in parvulin PPIase domains, the role of these histidine residues remains unclear. Our previous work (Behrsin et al. (2007) J. Mol. Biol. 365, 1143- 1162.) was consistent with a model where one or both histidines had critical roles in a hydrogen bonding network in the active site. Here, we test this model by looking at the effect of mutations to H59 and H157 on Pin1 function, activity, and protein stability. Using a yeast complementation assay, we show that both H59 and H157 can be mutated to non-hydrogen bonding residues and still support viability. Surprisingly, a nonfunctional H59L mutation can be rescued by a mutation of H157, to leucine. This double mutation (H59L/H157L) also had about 5-fold greater isomerase activity than the H59L mutation with a phosphorylated substrate. Structural analyses suggest that rescue of function and activity results from partial rescue of protein stability. Our findings indicate that H59 and H157 are not required for hydrogen bonding within the active site, and in contrast to the active site C113, they do not participate directly in catalysis. Instead, we suggest these histidines play a key role in domain structure or stability.

Impairment of adolescent hippocampal plasticity in a mouse model for Alzheimer's disease precedes disease phenotype

The amyloid precursor protein (APP) was assumed to be an important neuron-morphoregulatory protein and plays a central role in Alzheimer's disease (AD) pathology. In the study presented here, we analyzed the APP-transgenic mouse model APP23 using 2-dimensional gel electrophoresis technology in combination with DIGE and mass spectrometry. We investigated cortex and hippocampus of transgenic and wildtype mice at 1, 2, 7 and 15 months of age. Furthermore, cortices of 16 days old embryos were analyzed. When comparing the protein patterns of APP23 with wildtype mice, we detected a relatively large number of altered protein spots at all age stages and brain regions examined which largely preceded the occurrence of amyloid plaques. Interestingly, in hippocampus of adolescent, two-month old mice, a considerable peak in the number of protein changes was observed. Moreover, when protein patterns were compared longitudinally between age stages, we found that a large number of proteins were altered in wildtype mice. Those alterations were largely absent in hippocampus of APP23 mice at two months of age although not in other stages compared. Apparently, the large difference in the hippocampal protein patterns between two-month old APP23 and wildtype mice was caused by the absence of distinct developmental changes in the hippocampal proteome of APP23 mice. In summary, the absence of developmental proteome alterations as well as a down-regulation of proteins related to plasticity suggest the disturption of a normally occurring peak of hippocampal plasticity during adolescence in APP23 mice. Our findings are in line with the observation that AD is preceded by a clinically silent period of several years to decades. We also demonstrate that it is of utmost importance to analyze different brain regions and different age stages to obtain information about disease-causing mechanisms.

Pin1 promotes cell death in NGF-dependent neurons through a mechanism requiring c-Jun activity

Developing neurons deprived of trophic support undergo apoptosis mediated by activation of c-Jun N-terminal kinases (JNK) and c-Jun, induction of the Bcl-2 homology 3-only protein Bim(EL), Bax-dependent loss of mitochondrial cytochrome c, and caspase activation. However, the mechanisms that regulate each of these events are only partially understood. Here we show that the prolyl isomerase Pin1 functions as a positive regulator of neuronal death through a c-Jun-dependent mechanism. Ectopic Pin1 promoted caspase-dependent death of NGF-maintained neurons that was associated with an accumulation of Ser(63)-phosphorylated c-Jun in neuronal nuclei and was partially dependent on Bax. Downregulating Pin1 prior to NGF withdrawal suppressed the accumulation of phosphorylated c-Jun, inhibited the release of cytochrome c, and significantly delayed cell death. Pin1 knockdown inhibited NGF deprivation-induced death to a similar extent in Bim (+/+) and Bim (-/-) neurons. The protective effect of Pin1 knockdown was significantly greater than that caused by loss of Bim and nearly identical to that caused by a dominant negative form of c-Jun. Finally, cell death induced by ectopic Pin1 was largely blocked by expression of dominant negative c-Jun. These results suggest a novel mechanism by which Pin1 promotes cell death involving activation of c-Jun.

Small family with key contacts: par14 and par17 parvulin proteins, relatives of pin1, now emerge in biomedical research

The parvulin-type peptidyl-prolyl cis/trans isomerase Pin1 is subject of intense biochemical and clinical research as it seems to be involved in the pathogenesis of certain cancers and protein folding illnesses like Alzheimer's and Parkinson's disease. In addition to Pin1, the human genome only contains a single other parvulin locus encoding two protein species-Par14 and Par17. Much less is known about these enzymes although their sequences are highly conserved in all metazoans. Parvulin has been proposed to function as Pin1 complementing enzyme in cell cycle regulation and in chromatin remodelling. Pharmaceutical modulation of Par14 might therefore have benefits for certain types of cancer. Moreover, the Par17 protein that has been shown to be confined to anthropoid primate species only might provide a deeper understanding for human-specific brain development. This review aims at stimulating further research on Par14 and Par17 that are overlooked drug targets in the shadow of an overwhelming plethora of Pin1 literature by summarising all current knowledge on these parvulin proteins.

Alzheimer’s disease, oxidative stress and B-vitamin depletion

There is an association between cognitive function and vitamin B12 and folate status. Both vitamins participate in recycling the potentially toxic amino acid homocysteine to methionine and, ultimately, to the methyl donor S-adenosylmethionine (SAM). Consequently, B12 and folate indirectly influence glutathione synthesis – a major intracellular antioxidant. Neuroinflammation and oxidative stress are early features of Alzheimer’s disease (AD). Such stress impairs homocysteine recycling, degrades folate and decreases its cellular retention, resulting in limited SAM availability and increased homocysteine levels. Oxidized homocysteine derivatives, such as homocysteic acid, can initiate a vicious cycle by promoting free-radical formation. Decreased SAM also fosters development of characteristic AD neuropathologies – neurofibrillary tangles and amyloid plaques. The latter generate additional free radicals in a further feed-forward cascade. Future therapies should simultaneously halt neuroinflammation, restore redox homeostasis and replace depleted intracellular B vitamins. Developing early markers for these harmful processes will allow targeting of such therapy before irreversible cellular damage ensues.