The η-secretase-derived APP fragment ηCTF is localized in Golgi, endosomes and extracellular vesicles and contributes to Aß production.

The processing of the amyloid precursor protein (APP) is one of the key events contributing to Alzheimer's disease (AD) etiology. Canonical cleavages by ß- and γ-secretases lead to Aß production which accumulate in amyloid plaques. Recently, the matrix metalloprotease MT5-MMP, referred to as η-secretase, has been identified as a novel APP cleaving enzyme producing a transmembrane fragment, ηCTF that undergoes subsequent cleavages by α- and ß-secretases yielding the Aηα and AÎ·ß peptides, respectively. The functions and contributions of ηCTF and its related fragments to AD pathology are poorly understood. In this study, we designed a novel immunological probe referred to as ηCTF-NTer antibody that specifically interacts with the N-terminal part of ηCTF targeting ηCTF, Aηα, AÎ·ß but not C99, C83 and Aß. We examined the fate and localization of ηCTF fragment in various cell models and in mice. We found that overexpressed ηCTF undergoes degradation in the proteasomal and autophagic pathways and accumulates mainly in the Golgi and in endosomes. Moreover, we observed the presence of ηCTF in small extracellular vesicles purified from neuroblastoma cells or from mouse brains expressing ηCTF. Importantly, the expression of ηCTF in fibroblasts devoid on APP leads to Aß production demonstrating its contribution to the amyloidogenic pathway. Finally, we observed an ηCTF-like immunoreactivity around amyloid plaques and an age-dependent accumulation of ηCTF in the triple-transgenic mouse AD model. Thus, our study suggests that the ηCTF fragment likely contributes to AD pathology by its exosomal spreading and involvement in Aß production.

The transcription factor XBP1 in memory and cognition: Implications in Alzheimer disease.

X-box binding protein 1 (XBP1) is a unique basic region leucine zipper transcription factor isolated two decades ago in a search for regulators of major histocompatibility complex class II gene expression. XBP1 is a very complex protein regulating many physiological functions, including immune system, inflammatory responses, and lipid metabolism. Evidence over the past few years suggests that XBP1 also plays important roles in pathological settings since its activity as transcription factor has profound effects on the prognosis and progression of diseases such as cancer, neurodegeneration, and diabetes. Here we provide an overview on recent advances in our understanding of this multifaceted molecule, particularly in regulating synaptic plasticity and memory function, and the implications in neurodegenerative diseases with emphasis on Alzheimer disease.

ER-stress-associated functional link between Parkin and DJ-1 via a transcriptional cascade involving the tumor suppressor p53 and the spliced X-box binding protein XBP-1.

Parkin and DJ-1 are two multi-functional proteins linked to autosomal recessive early-onset Parkinson's disease (PD) that have been shown to functionally interact by as-yet-unknown mechanisms. We have delineated the mechanisms by which parkin controls DJ-1. Parkin modulates DJ-1 transcription and protein levels via a signaling cascade involving p53 and the endoplasmic reticulum (ER)-stress-induced active X-box-binding protein-1S (XBP-1S). Parkin triggers the transcriptional repression of p53 while p53 downregulates DJ-1 protein and mRNA expressions. We show that parkin-mediated control of DJ-1 is fully p53-dependent. Furthermore, we establish that p53 lowers the protein and mRNA levels of XBP-1S. Accordingly, we show that parkin ultimately upregulates XBP-1 levels. Subsequently, XBP-1S physically interacts with the DJ-1 promoter, thereby enhancing its promoter trans-activation, mRNA levels and protein expression. This data was corroborated by the examination of DJ-1 in both parkin- and p53-null mice brains. This transcriptional cascade is abolished by pathogenic parkin mutations and is independent of its ubiquitin-ligase activity. Our data establish a parkin-dependent ER-stress-associated modulation of DJ-1 and identifies p53 and XBP-1 as two major actors acting downstream of parkin in this signaling cascade in cells and in vivo. This work provides a mechanistic explanation for the increase in the unfolded protein response observed in PD pathology, i.e. that it is due to a defect in parkin-associated control of DJ-1.

Nuclear factor-κB regulates ßAPP and ß- and γ-secretases differently at physiological and supraphysiological Aß concentrations.

Anatomical lesions in Alzheimer disease-affected brains mainly consist of senile plaques, inflammation stigmata, and oxidative stress. The nuclear factor-κB (NF-κB) is a stress-activated transcription factor that is activated around senile plaques. We have assessed whether NF-κB could be differentially regulated at physiological or supraphysiological levels of amyloid ß (Aß) peptides. Under these experimental conditions, we delineated the putative NF-κB-dependent modulation of all cellular participants in Aß production, namely its precursor ßAPP (ß-amyloid precursor protein) and the ß- and γ-secretases, the two enzymatic machines involved in Aß genesis. Under physiological conditions, NF-κB lowers the transcriptional activity of the promoters of ßAPP, ß-secretase (ß-site APP-cleaving enzyme 1, BACE1), and of the four protein components (Aph-1, Pen-2, nicastrin, presenilin-1, or presenilin-2) of the γ-secretase in HEK293 cells. This was accompanied by a reduction of both protein levels and enzymatic activities, thereby ultimately yielding lower amounts of Aß and AICD (APP intracellular domain). In stably transfected Swedish ßAPP-expressing HEK293 cells triggering supraphysiological concentrations of Aß peptides, NF-κB activates the transcription of ßAPP, BACE1, and some of the γ-secretase members and increases protein expression and enzymatic activities, resulting in enhanced Aß production. Our pharmacological approach using distinct NF-κB kinase modulators indicates that both NF-κB canonical and alternative pathways are involved in the control of Aß production. Overall, our data demonstrate that under physiological conditions, NF-κB triggers a repressive effect on Aß production that contributes to maintaining its homeostasis, while NF-κB participates in a degenerative cycle where Aß would feed its own production under pathological conditions.

Potentially Pathogenic SORL1 Mutations Observed in Autosomal-Dominant Cases of Alzheimer's Disease Do Not Modulate APP Physiopathological Processing.

The SORL1 gene encodes LR11/SorLA, a protein that binds ß-amyloid precursor protein (APP) and drives its intracellular trafficking. SORL1 mutations, occurring frequently in a subset of familial cases of Alzheimer's disease (AD), have been documented, but their pathogenic potential is not yet clear and questions remain concerning their putative influence on the physiopathological processing of APP. We have assessed the influence of two SORL1 mutations that were described as likely disease-causing and that were associated with either benign (SorLA924) or severe (SorLA511) AD phenotypes. We examined the influence of wild-type and mutants SorLA in transiently transfected HEK293 cells expressing either wild-type or Swedish mutated APP on APP expression, secreted Aß and sAPPα levels, intracellular Aß 40 and Aß42 peptides, APP-CTFs (C99 and C83) expressions, α-, ß- and γ-secretases expressions and activities as well as Aß and CTFs-degrading enzymes. These paradigms were studied in control conditions or after pharmacological proteasomal modulation. We also established stably transfected CHO cells expressing wild-type SorLA and established the colocalization of APP and either wild-type or mutant SorLA. SorLA mutations partially disrupt co-localization of wild-type sorLA with APP. Overall, although we mostly confirmed previous data concerning the influence of wild-type SorLA on APP processing, we were unable to evidence significant alterations triggered by our set of SorLA mutants, whatever the cells or pharmacological conditions examined. Our study , however, does not rule out the possibility that other AD-linked SORL1 mutations could indeed affect APP processing, and that pathogenic mutations examined in the present study could interfere with other cellular pathways/triggers in AD.

The extracellular regulated kinase-1 (ERK1) controls regulated alpha-secretase-mediated processing, promoter transactivation, and mRNA levels of the cellular prion protein.

The α-secretases A disintegrin and metalloprotease 10 (ADAM10) and ADAM17 trigger constitutive and regulated processing of the cellular prion protein (PrP(c)) yielding N1 fragment. The latter depends on protein kinase C (PKC)-coupled M1/M3 muscarinic receptor activation and subsequent phosphorylation of ADAM17 on its intracytoplasmic threonine 735. Here we show that regulated PrP(c) processing and ADAM17 phosphorylation and activation are controlled by the extracellular-regulated kinase-1/MAP-ERK kinase (ERK1/MEK) cascade. Thus, reductions of ERK1 or MEK activities by dominant-negative analogs, pharmacological inhibition, or genetic ablation all impair N1 secretion, whereas constitutively active proteins increase N1 recovery in the conditioned medium. Interestingly, we also observed an ERK1-mediated enhanced expression of PrP(c). We demonstrate that the ERK1-associated increase in PrP(c) promoter transactivation and mRNA levels involve transcription factor AP-1 as a downstream effector. Altogether, our data identify ERK1 as an important regulator of PrP(c) cellular homeostasis and indicate that this kinase exerts a dual control of PrP(c) levels through transcriptional and post-transcriptional mechanisms.

Therapeutic potential of parkin as a tumor suppressor via transcriptional control of cyclins in glioblastoma cell and animal models.

Parkin (PK) is an E3-ligase harboring tumor suppressor properties that has been associated to various cancer types including glioblastoma (GBM). However, PK is also a transcription factor (TF), the contribution of which to GBM etiology remains to be established. Methods: The impact of PK on GBM cells proliferation was analyzed by real-time impedance measurement and flow cytometry. Cyclins A and B proteins, promoter activities and mRNA levels were measured by western blot, luciferase assay and quantitative real-time PCR. Protein-protein and protein-promoter interactions were performed by co-immunoprecipitation and by ChIP approaches. The contribution of endogenous PK to tumor progression in vivo was performed by allografts of GL261 GBM cells in wild-type and PK knockout mice. Results: We show that overexpressed and endogenous PK control GBM cells proliferation by modulating the S and G2/M phases of the cell cycle via the trans-repression of cyclin A and cyclin B genes. We establish that cyclin B is regulated by both E3-ligase and TF PK functions while cyclin A is exclusively regulated by PK TF function. PK invalidation leads to enhanced tumor progression in immunocompetent mice suggesting an impact of PK-dependent tumor environment to tumor development. We show that PK is secreted by neuronal cells and recaptured by tumor cells. Recaptured PK lowered cyclins levels and decreased GBM cells proliferation. Further, PK expression is decreased in human GBM biopsies and its expression is inversely correlated to both cyclins A and B expressions. Conclusion: Our work demonstrates that PK tumor suppressor function contributes to the control of tumor by its cellular environment. It also shows a key role of PK TF function in GBM development via the control of cyclins in vitro and in vivo. It suggests that therapeutic strategies aimed at controlling PK shuttling to the nucleus may prove useful to treat GBM.

Transcription- and phosphorylation-dependent control of a functional interplay between XBP1s and PINK1 governs mitophagy and potentially impacts Parkinson disease pathophysiology.

Parkinson disease (PD)-affected brains show consistent endoplasmic reticulum (ER) stress and mitophagic dysfunctions. The mechanisms underlying these perturbations and how they are directly linked remain a matter of questions. XBP1 is a transcription factor activated upon ER stress after unconventional splicing by the nuclease ERN1/IREα thereby yielding XBP1s, whereas PINK1 is a kinase considered as the sensor of mitochondrial physiology and a master gatekeeper of mitophagy process. We showed that XBP1s transactivates PINK1 in human cells, primary cultured neurons and mice brain, and triggered a pro-mitophagic phenotype that was fully dependent of endogenous PINK1. We also unraveled a PINK1-dependent phosphorylation of XBP1s that conditioned its nuclear localization and thereby, governed its transcriptional activity. PINK1-induced XBP1s phosphorylation occurred at residues reminiscent of, and correlated to, those phosphorylated in substantia nigra of sporadic PD-affected brains. Overall, our study delineated a functional loop between XBP1s and PINK1 governing mitophagy that was disrupted in PD condition.Abbreviations: 6OHDA: 6-hydroxydopamine; baf: bafilomycin A1; BECN1: beclin 1; CALCOCO2/NDP52: calcium binding and coiled-coil domain 2; CASP3: caspase 3; CCCP: carbonyl cyanide chlorophenylhydrazone; COX8A: cytochrome c oxidase subunit 8A; DDIT3/CHOP: DNA damage inducible transcript 3; EGFP: enhanced green fluorescent protein; ER: endoplasmic reticulum; ERN1/IRE1α: endoplasmic reticulum to nucleus signaling 1; FACS: fluorescence-activated cell sorting; HSPD1/HSP60: heat shock protein family D (Hsp60) member 1; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; MFN2: mitofusin 2; OPTN: optineurin; PD: Parkinson disease; PINK1: PTEN-induced kinase 1; PCR: polymerase chain reaction:; PRKN: parkin RBR E3 ubiquitin protein ligase; XBP1s [p-S61A]: XBP1s phosphorylated at serine 61; XBP1s [p-T48A]: XBP1s phosphorylated at threonine 48; shRNA: short hairpin RNA, SQSTM1/p62: sequestosome 1; TIMM23: translocase of inner mitochondrial membrane 23; TM: tunicamycin; TMRM: tetramethyl rhodamine methylester; TOMM20: translocase of outer mitochondrial membrane 20; Toy: toyocamycin; TP: thapsigargin; UB: ubiquitin; UB (S65): ubiquitin phosphorylated at serine 65; UPR: unfolded protein response, XBP1: X-box binding protein 1; XBP1s: spliced X-box binding protein 1.

The Endoplasmic Reticulum Stress/Unfolded Protein Response and Their Contributions to Parkinson's Disease Physiopathology.

Parkinson's disease (PD) is a multifactorial age-related movement disorder in which defects of both mitochondria and the endoplasmic reticulum (ER) have been reported. The unfolded protein response (UPR) has emerged as a key cellular dysfunction associated with the etiology of the disease. The UPR involves a coordinated response initiated in the endoplasmic reticulum that grants the correct folding of proteins. This review gives insights on the ER and its functioning; the UPR signaling cascades; and the link between ER stress, UPR activation, and physiopathology of PD. Thus, post-mortem studies and data obtained by either in vitro and in vivo pharmacological approaches or by genetic modulation of PD causative genes are described. Further, we discuss the relevance and impact of the UPR to sporadic and genetic PD pathology.

The Transcription Factor EB Reduces the Intraneuronal Accumulation of the Beta-Secretase-Derived APP Fragment C99 in Cellular and Mouse Alzheimer's Disease Models.

: Brains that are affected by Alzheimer's disease (AD) are characterized by the overload of extracellular amyloid ß (Aß) peptides, but recent data from cellular and animal models propose that Aß deposition is preceded by intraneuronal accumulation of the direct precursor of Aß, C99. These studies indicate that C99 accumulation firstly occurs within endosomal and lysosomal compartments and that it contributes to early-stage AD-related endosomal-lysosomal-autophagic defects. Our previous work also suggests that C99 accumulation itself could be a consequence of defective lysosomal-autophagic degradation. Thus, in the present study, we analyzed the influence of the overexpression of the transcription factor EB (TFEB), a master regulator of autophagy and lysosome biogenesis, on C99 accumulation occurring in both AD cellular models and in the triple-transgenic mouse model (3xTgAD). In the in vivo experiments, TFEB overexpression was induced via adeno-associated viruses (AAVs), which were injected either into the cerebral ventricles of newborn mice or administrated at later stages (3 months of age) by stereotaxic injection into the subiculum. In both cells and the 3xTgAD mouse model, exogenous TFEB strongly reduced C99 load and concomitantly increased the levels of many lysosomal and autophagic proteins, including cathepsins, key proteases involved in C99 degradation. Our data indicate that TFEB activation is a relevant strategy to prevent the accumulation of this early neurotoxic catabolite.

Upregulation of the Sarco-Endoplasmic Reticulum Calcium ATPase 1 Truncated Isoform Plays a Pathogenic Role in Alzheimer's Disease.

Dysregulation of the Endoplasmic Reticulum (ER) Ca2+ homeostasis and subsequent ER stress activation occur in Alzheimer Disease (AD). We studied the contribution of the human truncated isoform of the sarco-endoplasmic reticulum Ca2+ ATPase 1 (S1T) to AD. We examined S1T expression in human AD-affected brains and its functional consequences in cellular and transgenic mice AD models. S1T expression is increased in sporadic AD brains and correlates with amyloid ß (Aß) and ER stress chaperone protein levels. Increased S1T expression was also observed in human neuroblastoma cells expressing Swedish-mutated ß-amyloid precursor protein (ßAPP) or treated with Aß oligomers. Lentiviral overexpression of S1T enhances in return the production of APP C-terminal fragments and Aß through specific increases of ß-secretase expression and activity, and triggers neuroinflammation. We describe a molecular interplay between S1T-dependent ER Ca2+ leak, ER stress and ßAPP-derived fragments that could contribute to AD setting and/or progression.

Characterization of a von Hippel Lindau pathway involved in extracellular matrix remodeling, cell invasion, and angiogenesis.

Inactivation of the von Hippel-Lindau (VHL) tumor suppressor gene results in highly vascularized tumors, making the VHL tumor syndrome an ideal system to study the mechanisms of angiogenesis. VHL operates along two pathways with the first involving hypoxia-inducible factor-alpha degradation and down-regulation of its proangiogenic target genes vascular endothelial growth factor and platelet-derived growth factor-beta, and the second pathway promoting extracellular matrix (ECM) assembly. Secretion of proangiogenic factors was shown to be a primary inducer of angiogenesis. Here, we show that loss of ECM assembly correlates with tumor angiogenesis in VHL disease. Upon inactivation of the VHL-ECM assembly pathway, we observe tumors that are highly vascularized, have a disrupted ECM, and show increased matrix metalloproteinase-2 activity. Loss of the VHL pathway leading to hypoxia-inducible factor-alpha degradation results in tumors with increased vascular endothelial growth factor levels but with surprisingly low microvessel density, a tightly assembled ECM and low invasive ability. We conclude that loss of ECM integrity could promote and maintain tumor angiogenesis by providing a route for blood vessels to infiltrate tumors.

The Transcription Factor Function of Parkin: Breaking the Dogma.

PRKN (PARK2) is a key gene involved in both familial and sporadic Parkinson's disease that encodes parkin (PK). Since its discovery by the end of the 90s, both functional and more recently, structural studies led to a consensual view of PK as an E3 ligase only. It is generally considered that this function conditions the cellular load of a subset of cytosolic proteins prone to proteasomal degradation and that a loss of E3 ligase function triggers an accumulation of potentially toxic substrates and, consequently, a neuronal loss. Furthermore, PK molecular interplay with PTEN-induced kinase 1 (PINK1), a serine threonine kinase also involved in recessive cases of Parkinson's disease, is considered to underlie the mitophagy process. Thus, since mitochondrial homeostasis significantly governs cell health, there is a huge interest of the scientific community centered on PK function. In 2009, we have demonstrated that PK could also act as a transcription factor (TF) and induces neuroprotection via the downregulation of the pro-apoptotic and tumor suppressor factor, p53. Importantly, the DNA-binding properties of PK and its nuclear localization suggested an important role in the control of several genes. The duality of PK subcellular localization and of its associated ubiquitin ligase and TF functions suggests that PK could behave as a key molecular modulator of various physiological cellular signaling pathways that could be disrupted in pathological contexts. Here, we update the current knowledge on PK direct and indirect TF-mediated control of gene expression.

Nuclear p53-mediated repression of autophagy involves PINK1 transcriptional down-regulation.

p53 is a transcription factor that is implicated in the control of both apoptotic and autophagic cell death. This tumor suppressor elicits both pro-autophagic and anti-autophagic phenotypes depending of its intracellular localization. The ability of p53 to repress autophagy has been exclusively associated to its cytoplasmic localization. Here, we show that transcriptional activity of p53 also contributes to autophagy down-regulation. Thus, nuclear p53 controls PINK1, a key protein involved in the control of mitophagy, by repressing its promoter activity, protein and mRNA levels, ex-vivo and in vivo. We establish that deletion of an identified p53 responsive element on PINK1 promoter impacts p53-mediated PINK1 transcriptional repression and we demonstrate a p53-PINK1 physical interaction by chromatin immunoprecipitation. Accordingly, we show that only nuclear p53 accounts for its ability to repress PINK1 gene transcription. Further, we demonstrate ex-vivo and in vivo that p53 invalidation in human cells increases LC3 maturation as well as optineurin and NDP52 autophagy receptors expression and down-regulates TIM23, TOM20 and HSP60 mitophagy markers. Importantly, this phenotype is mimicked by TP53 invalidation in mice brain. Finally, by combining pharmacological and genetic approaches, we show that the p53-mediated negative regulation of autophagy is PINK1-dependent. Thus pifithrin-α-mediated blockade of p53 transcriptional activity enhances LC3 maturation and reduces p62, TIM23, TOM20 and HSP60 protein levels. This pifithrin-α-associated pro-mitophagy phenotype is fully abolished by PINK1 depletion. This data unravels a novel pathway by which nuclear p53 can repress autophagy/mitophagy that could underlie important dysfunctions in both neurodegenerative and cancer diseases.

ß-Amyloid Precursor Protein Intracellular Domain Controls Mitochondrial Function by Modulating Phosphatase and Tensin Homolog-Induced Kinase 1 Transcription in Cells and in Alzheimer Mice Models.

BACKGROUND: Mitophagy and mitochondrial dynamics alterations are two major hallmarks of neurodegenerative diseases. Dysfunctional mitochondria accumulate in Alzheimer's disease-affected brains by yet unexplained mechanisms. METHODS: We combined cell biology, molecular biology, and pharmacological approaches to unravel a novel molecular pathway by which presenilins control phosphatase and tensin homolog-induced kinase 1 (Pink-1) expression and transcription. In vivo approaches were carried out on various transgenic and knockout animals as well as in adeno-associated virus-infected mice. Functional readout and mitochondrial physiology (mitochondrial potential) were assessed by combined procedures including flow cytometry, live imaging analysis, and immunohistochemistry. RESULTS: We show that presenilins 1 and 2 trigger opposite effects on promoter transactivation, messenger RNA, and protein expression of Pink-1. This control is linked to γ-secretase activity and ß-amyloid precursor protein but is independent of phosphatase and tensin homolog. We show that amyloid precursor protein intracellular domain (AICD) accounts for presenilin-dependent phenotype and upregulates Pink-1 transactivation in cells as well as in vivo in a Forkhead box O3a-dependent manner. Interestingly, the modulation of γ-secretase activity or AICD expression affects Pink-1-related control of mitophagy and mitochondrial dynamics. Finally, we show that parkin acts upstream of presenilins to control Pink-1 promoter transactivation and protein expression. CONCLUSIONS: Overall, we delineate a molecular cascade presenilins-AICD-Forkhead box O3a linking parkin to Pink-1. Our study demonstrates AICD-mediated Pink-1-dependent control of mitochondrial physiology by presenilins. Furthermore, it unravels a parkin-Pink-1 feedback loop controlling mitochondrial physiology that could be disrupted in neurodegenerative conditions.

Analysis of the hypoxia-sensing pathway in Drosophila melanogaster.

The mechanism by which hypoxia induces gene transcription involves the inhibition of HIF-1alpha (hypoxia-inducible factor-1 alpha subunit) PHD (prolyl hydroxylase) activity, which prevents the VHL (von Hippel-Lindau)-dependent targeting of HIF-1alpha to the ubiquitin/proteasome pathway. HIF-1alpha thus accumulates and promotes gene transcription. In the present study, first we provide direct biochemical evidence for the presence of a conserved hypoxic signalling pathway in Drosophila melanogaster. An assay for 2-oxoglutarate-dependent dioxygenases was developed using Drosophila embryonic and larval homogenates as a source of enzyme. Drosophila PHD has a low substrate specificity and hydroxylates key proline residues in the ODD (oxygen-dependent degradation) domains of human HIF-1alpha and Similar, the Drosophila homologue of HIF-1alpha. The enzyme promotes human and Drosophila [(35)S]VHL binding to GST (glutathione S-transferase)-ODD-domain fusion protein. Hydroxylation is enhanced by proteasomal inhibitors and was ascertained using an anti-hydroxyproline antibody. Secondly, by using transgenic flies expressing a fusion protein that combined an ODD domain and the green fluorescent protein (ODD-GFP), we analysed the hypoxic cascade in different embryonic and larval tissues. Hypoxic accumulation of the reporter protein was observed in the whole tracheal tree, but not in the ectoderm. Hypoxic stabilization of ODD-GFP in the ectoderm was restored by inducing VHL expression in these cells. These results show that Drosophila tissues exhibit different sensitivities to hypoxia.

Direct α-synuclein promoter transactivation by the tumor suppressor p53.

BACKGROUND: Parkinson's disease (PD) is a motor disease associated with the degeneration of dopaminergic neurons of the substantia nigra pars compacta. p53 is a major neuronal pro-apoptotic factor that is at the center of gravity of multiple physiological and pathological cascades, some of which implying several key PD-linked proteins. Since p53 is up-regulated in PD-affected brain, we have examined its ability to regulate the transcription of α-synuclein, a key protein that accumulates in PD-related Lewy bodies. RESULTS: We show that pharmacological and genetic up-regulation of p53 expression lead to a strong increase of α-synuclein protein, promoter activity and mRNA levels. Several lines of evidence indicate that this transcriptional control is due to the DNA-binding properties of p53. Firstly, p53 DNA-binding dead mutations abolish p53 regulation of α-synuclein. Secondly, the deletion of p53 responsive element from α-synuclein promoter abrogates p53-mediated α-synuclein regulation. Thirdly, gel shift and chromatin immunoprecipitation studies indicate that p53 interacts physically with α-synuclein promoter both in vitro and in a physiological context. Furthermore, we show that the depletion of endogenous p53 in cells as well as in knockout mice down-regulates α-synuclein transcription. CONCLUSIONS: Overall, we have identified α-synuclein as a new transcriptional target of p53 and delineated a cellular mechanism feeding the accumulation of toxic aggregated α-synuclein in PD. This original α-syn regulatory mechanism may be central to PD-related cell death and may lead to novel opportunities to design alternative neuroprotective strategies in PD.

The transcription factor X-box binding protein-1 in neurodegenerative diseases.

Endoplasmic reticulum (ER) is the cellular compartment where secreted and integral membrane proteins are folded and matured. The accumulation of unfolded or misfolded proteins triggers a stress that is physiologically controlled by an adaptative protective response called Unfolded Protein Response (UPR). UPR is primordial to induce a quality control response and to restore ER homeostasis. When this adaptative response is defective, protein aggregates overwhelm cells and affect, among other mechanisms, synaptic function, signaling transduction and cell survival. Such dysfunction likely contributes to several neurodegenerative diseases that are indeed characterized by exacerbated protein aggregation, protein folding impairment, increased ER stress and UPR activation. This review briefly documents various aspects of the biology of the transcription factor XBP-1 (X-box Binding Protein-1) and summarizes recent findings concerning its putative contribution to the altered UPR response observed in various neurodegenerative disorders including Parkinson's and Alzheimer's diseases.

Parkin differently regulates presenilin-1 and presenilin-2 functions by direct control of their promoter transcription.

We previously established that besides its canonical function as E3-ubiquitin ligase, parkin also behaves as a transcriptional repressor of p53. Here we show that parkin differently modulates presenilin-1 and presenilin-2 expression and functions at transcriptional level. Thus, parkin enhances/reduces the protein expression, promoter activity and mRNA levels of presenilin-1 and presenilin-2, respectively, in cells and in vivo. This parkin-associated function is independent of its ubiquitin-ligase activity and remains unrelated to its capacity to repress p53. Accordingly, physical interaction of endogenous or overexpressed parkin with presenilins promoters is demonstrated by chromatin immunoprecipitation assays (ChIP). Furthermore, we identify a consensus sequence, the deletion of which abolishes parkin-dependent modulation of presenilins-1/2 and p53 promoter activities. Interestingly, electrophoretic mobility shift assays (EMSA) revealed a physical interaction between this consensus sequence and wild-type but not mutated parkin. Finally, we demonstrate that the RING1-IBR-RING2 domain of parkin harbors parkin's potential to modulate presenilins promoters. This transcriptional control impacts on presenilins-associated phenotypes, since parkin increases presenilin-1-associated γ-secretase activity and reduces presenilin-2-linked caspase-3 activation. Overall, our data delineate a promoter responsive element targeted by parkin that drives differential regulation of presenilin-1 and presenilin-2 transcription with functional consequences for γ-secretase activity and cell death.