Does Intraneuronal Accumulation of Carboxyl-terminal Fragments of the Amyloid Precursor Protein Trigger Early Neurotoxicity in Alzheimer's Disease?

BACKGROUND: Alzheimer's disease (AD) is associated with extracellular accumulation and aggregation of amyloid ß (Aß) peptides ultimately seeding in senile plaques. Recent data show that their direct precursor C99 (ßCTF) also accumulates in AD-affected brain as well as in AD-like mouse models. C99 is consistently detected much earlier than Aß, suggesting that this metabolite could be an early contributor to AD pathology. C99 accumulates principally within endolysosomal and autophagic structures and its accumulation was described as both a consequence and one of the causes of endolysosomalautophagic pathology, the occurrence of which has been documented as an early defect in AD. C99 was also accompanied by C99-derived C83 (αCTF) accumulation occurring within the same intracellular organelles. Both these CTFs were found to dimerize leading to the generation of higher molecular weight CTFs, which were immunohistochemically characterized in situ by means of aggregate-specific antibodies. DISCUSSION: Here, we discuss studies demonstrating a direct link between the accumulation of C99 and C99-derived APP-CTFs and early neurotoxicity. We discuss the role of C99 in endosomal-lysosomalautophagic dysfunction, neuroinflammation, early brain network alterations and synaptic dysfunction as well as in memory-related behavioral alterations, in triple transgenic mice as well as in newly developed AD animal models. CONCLUSION: This review summarizes current evidence suggesting a potential role of the ß -secretasederived APP C-terminal fragment C99 in Alzheimer's disease etiology.

The transcription factor XBP1s restores hippocampal synaptic plasticity and memory by control of the Kalirin-7 pathway in Alzheimer model.

Neuronal network dysfunction and cognitive decline constitute the most prominent features of Alzheimer's disease (AD), although mechanisms causing such impairments are yet to be determined. Here we report that virus-mediated delivery of the active spliced transcription factor X-Box binding protein 1s (XBP1s) in the hippocampus rescued spine density, synaptic plasticity and memory function in a mouse model of AD. XBP1s transcriptionally activated Kalirin-7 (Kal7), a protein that controls synaptic plasticity. In addition, we found reduced levels of Kal7 in primary neurons exposed to Aß oligomers, transgenic mouse models and human AD brains. Short hairpin RNA-mediated knockdown of Kal7 altered synaptic plasticity and memory formation in naive mice. Further, reduction of endogenous Kal7 compromised the beneficial effects of XBP1s in Alzheimer's model. Hence, our findings reveal that XBP1s is neuroprotective through a mechanism that engages Kal7 pathway with therapeutic implications in AD pathology.

Influence of Genetic Background on Apathy-Like Behavior in Triple Transgenic AD Mice.

Apathy is an early and common neuropsychiatric syndrome in Alzheimer's disease (AD) patients. In clinical trials, apathy is associated with decreased motor activity that can be monitored by actigraphy. The triple transgenic mouse AD model (3xTgAD) has been shown to recapitulate the biochemical lesions as well as many of the synaptic and cognitive alterations associated with AD. In the present work we found that these mice also develop an early and consistent apathy-like behavior as evidenced by a drastic decrease in spontaneous activity measured by actimetry. We recently established that these mice also display an intraneuronal accumulation of the ß-secretase-derived ßAPP fragment (C99) appearing early, in absence of Aß. Interestingly, we found that the apathy-like behavior observed in 3xTgAD mice was temporally associated with C99 accumulation and synaptic alterations. Since it is well known that the genetic background can strongly influence behavior and can induce transcriptional variability in animal models, we decided to determine the influence of genetic background on the above-described alterations. We backcrossed 3xTgAD mice to C57BL/6 and found that the genetic background had no influence on either C99 accumulation or synaptic plasticity alterations, but strongly affected the apathy-like behavior.

Sox2 functionally interacts with ßAPP, the ßAPP intracellular domain and ADAM10 at a transcriptional level in human cells.

Sox2 (SRY (Sex-determining region Y)-related high mobility group (HMG) box 2) is a transcription factor that serves key roles in controlling the balance between stem cells maintenance and commitment to differentiated lineages throughout the lifetime. Importantly, Sox2 deficiency results in early embryonic lethality whereas the down-regulation of Sox2 expression triggers neurodegeneration in the adult mouse brain. Moreover, Sox2 is decreased in the brain of Alzheimer's disease (AD) patients and co localizes with the ß-amyloid precursor protein (ßAPP) in stem cells. Here we report the existence of functional interactions between Sox2 and ßAPP, the ßAPP intracellular domain AICD50 and the α-secretase ADAM10 in human cells. We first show, as observed in embryonic stem cells, that ßAPP overexpression in HEK293 cells results in an increase of Sox2 immunoreactivity and we further establish the transcriptional nature of this pathway. Moreover, overexpression of the pro-apoptotic C-terminal ßAPP-derived AICD50 metabolite leads to the down-regulation of Sox2 transcription whereas the pharmacological inhibition of endogenous AICD production increases Sox2 expression in both HEK293 and SH-SY5Y cell lines. In addition, we demonstrate that Sox2 is a potent activator of the non amyloidogenic processing of ßAPP as shown by the Sox2-dependent augmentation of ADAM10 catalytic activity, immunoreactivity, promoter transactivation and mRNA levels with no modification of the activity and the expression of the ß-secretase BACE1. Finally, the fact that γ-secretase inhibition induces an increase of ADAM10 protein levels in SH-SY5Y cells further supports the occurrence of functional AICD/Sox2/ADAM10 interactions. Altogether, our study identifies and characterizes new functional cross-talks between Sox2 and proteins involved in AD, thereby adding support to the view that Sox2 likely behaves as a protective factor during the development of this neurodegenerative disease.

Interplay between parkin and p53 governs a physiological homeostasis that is disrupted in Parkinson's disease and cerebral cancer.

Parkin is responsible for most autosomal juvenile recessive cases of Parkinson's disease (PD). Besides its well-characterized function as ubiquitin ligase, we previously established that parkin could repress p53 at the transcriptional level. Interestingly, p53 was recently shown to upregulate parkin, suggesting a feedback loop by which parkin and p53 interplay, thereby contributing to their physiological homeostasis. This equilibrium is disrupted in both PD and cerebral cancer. Thus, when parkin is mutated in PD, its transcriptional ability to repress p53 is abolished. Therefore, p53 elevation could likely contribute to the exacerbated cell death observed in PD-affected brains. Inversely, in brain-associated tumors linked to p53 mutations, the transcriptional control of parkin is reduced, and thereby, parkin expression is lowered. The reduction in parkin level could, in turn, contribute to an increase in the levels of transcriptionally inactive p53 that could explain, at least in part, the defect in cellular apoptotic commitment observed in cerebral cancer. Here, we discuss in detail the various studies demonstrating the importance of the functional interplay between parkin and p53 and its impairment by pathogenic mutations likely contributing to the etiology of PD and gliomas.

Glioma tumor grade correlates with parkin depletion in mutant p53-linked tumors and results from loss of function of p53 transcriptional activity.

Gliomas represent the most frequent form of primary brain tumors in adults, the prognosis of which remains extremely poor. Inactivating mutations on the tumor suppressor TP53 were proposed as a key etiological trigger of glioma development. p53 has been recently identified as a transcriptional target of parkin. Interestingly, somatic mutations on parkin have also been linked to glioma genesis. We examined the possibility that a disruption of a functional interaction between p53 and parkin could contribute to glioma development in samples devoid of somatic parkin mutations or genetic allele deletion. We show here that parkin levels inversely correlate to brain tumor grade and p53 levels in oligodendrogliomas, mixed gliomas and glioblastomas. We demonstrate that p53 levels negatively and positively correlate to bax and Bcl2 respectively, underlying a loss of p53 transcriptional activity in all types of glial tumors. Using various cell models lacking p53 or harboring either transcriptionally inactive or dominant negative p53, as well as in p53 knockout mice brain, we establish that p53 controls parkin promoter transactivation, mRNA and protein levels. Furthermore, we document an increase of parkin expression in mice brain after p53-bearing viral infection. Finally, both cancer-related p53 inactivating mutations and deletion of a consensus p53 binding sequence located on parkin promoter abolish p53-mediated control of parkin transcription, demonstrating that p53 regulates parkin transcription via its DNA binding properties. In conclusion, our work delineates a functional interplay between mutated p53 and parkin in glioma genesis that is disrupted by cancer-linked pathogenic mutations. It also allows envisioning parkin as a novel biomarker of glioma biopsies enabling to follow the progression of this type of cancers.

Cerebrospinal Aß11-x and 17-x levels as indicators of mild cognitive impairment and patients' stratification in Alzheimer's disease.

In the present work, the concentrations of Aß11-x and Aß17-x peptides (x=40 or 42), which result from the combined cleavages of ß-amyloid precursor protein (AßPP) by ß'/α or α/γ-secretases, respectively, were assessed in cerebrospinal fluid (CSF) samples from patients with Alzheimer's disease (AD) or mild cognitive impairment (MCI). Specific multiplexed assays were set up using new anti-40 and anti-42 monoclonal antibodies (mAbs) for the capture of these N-truncated Aß peptides and anti-11 or anti-17 mAbs for their detection. The specificity, sensitivity and reproducibility of such assays were assessed using synthetic peptides and human cell models. Aß11-x and Aß17-x were then measured in CSF samples from patients with AD (n=23), MCI (n=23) and controls with normal cognition (n=21). Aß11-x levels were significantly lower in patients with MCI than in controls. Compared with the combined quantification of Aß1-42, total Tau (T-Tau) and phosphorylated Tau (P-Tau; AlzBio3, Innogenetics), the association of Aß11-40, Aß17-40 and T-Tau improved the discrimination between MCI and controls. Furthermore, when patients with MCI were classified into two subgroups (MCI ≤1.5 or ≥2 based on their CDR-SB (Cognitive Dementia Rating-Sum of Boxes) score), the CSF Aß17-40/Aß11-40 ratio was significantly higher in patients with CDR-SB ≤1.5 than in controls, whereas neither Aß1-42, T-Tau nor P-Tau allowed the detection of this subpopulation. These results need to be confirmed in a larger clinical prospective cohort.

The caspase 6 derived N-terminal fragment of DJ-1 promotes apoptosis via increased ROS production.

In pathological conditions, the amount of DJ-1 determines whether a cell can survive or engage a cell death program. This is exemplified in epithelial cancers, in which DJ-1 expression is increased, while autosomal recessive early onset Parkinson's disease mutations of DJ-1 generally lead to decreased stability and expression of the protein. We have shown previously that DJ-1 is cleaved by caspase-6 during induction of apoptosis. We demonstrate here that the N-terminal cleaved fragment of DJ-1 (DJ-1 Nt) is specifically expressed in the nucleus and promotes apoptosis in SH-SY5Y neuroblastoma cell lines. In addition, overexpression of DJ-1 Nt in different cell lines leads to a loss of clonogenic potential and sensitizes to staurosporin and 1-methyl-4-phenylpyridinium (MPP+)-mediated caspase activation and apoptosis. Importantly, inhibition of endogenous DJ-1 expression with sh-RNA or DJ-1 deficiency mimics the effect of DJ-1 Nt on cell growth and apoptosis. Moreover, overexpression of DJ-1 Nt increases reactive oxygen species (ROS) production, and sensitizes to MPP+-mediated apoptosis and DJ-1 oxidation. Finally, specific exclusion of DJ-1 Nt from the nucleus abrogates its pro-apoptotic effect. Taken together, our findings identify an original pathway by which generation of a nuclear fragment of DJ-1 through caspase 6-mediated cleavage induces ROS-dependent amplification of apoptosis.

Galantamine-based hybrid molecules with acetylcholinesterase, butyrylcholinesterase and γ-secretase inhibition activities.

We previously designed novel peptides-containing galantamine analogues. These compounds we analyzed for their putative inhibitory effect towards acetylcholinesterase, butyrylcholinesterase and γ-secretase, three activities of which could be central to various neurodegenerative pathologies including Alzheimer's disease. These pharmacological agents were virtually equipotent on acetylcholinesterase activity but display drastically higher inhibitory activities towards butyrylcholinesterase with several compounds displaying an about 100-fold higher activity than that harboured by galantamine. Strikingly, two of the galantamine amides that displayed low activity towards acetylcholinesterase exhibited the highest inhibitory potency towards butyrylcholinesterase (106 to 133 times more active than galantamine). Interestingly, five compounds show a rather good γ-secretase inhibitory potency while they retain their ability to inhibit AChE and/or BuChE activity. Thus, we have been able to design novel compounds with significant inhibitory activity against several of the enzymes responsible for key dysfunctions taking place in several neurodegenerative diseases. These mixed inhibitors could therefore be envisioned as potential pharmacological tools aimed at circumventing the degenerative processes taking place in these major pathologies.

α-Secretase-derived cleavage of cellular prion yields biologically active catabolites with distinct functions.

The cellular prion protein (PrP(c)) undergoes α-secretase-derived processing by disintegrins. This cleavage occurs within the 106-126 putative toxic domain of PrP(c), yielding two complementary N- and C-terminal fragments referred to as N1 and C1, respectively. Here we review our recent data showing that these two PrP(c)-derived products harbor distinct p53-dependent functions. Thus, C1 potentiates staurosporine (STS)-induced caspase-3 activation by upregulating p53 transcription, mRNA levels and activity. Conversely, N1 is protective both in vitro and in vivo. Thus, N1 inhibits STS-induced caspase-3 activation by downregulating p53 in various cell systems and protects rat retinal ganglion cells from hypoxia-induced apoptosis. Furthermore, N1 protects cells against C1-induced toxicity. Therefore, our data show that disintegrin-associated processing of PrP(c) gives rise to two fragments that display opposite effects on p53-dependent cell death and that can functionally interact.

Cellular prion and its catabolites in the brain: production and function.

During the last thirty years, part of the scientific community focused on the mechanisms by which a naturally occurring protein called cellular prion (PrP(c)) converts into a protease-resistant isoform (PrP(sc)) responsible for fatal Transmissible Spongiform Encephalopathies (TSE). Concomitantly, the physiology of PrP(c) has also been studied. PrP(c) undergoes proteolytic attacks leading to both membrane-attached and secreted fragments, the nature of which differs in normal and TSE-affected human brains. Does proteolysis of PrP(c) correspond to an inactivating mechanism impairing the biological function of the protein, or alternatively, does it represent a maturation process allowing the produced fragments to trigger their own physiological function? Here we review the mechanisms involved in the production of PrP(c) catabolites and we focus on the function of PrP(c) and its derived fragments in the cell death/ survival regulation in the nervous system.

Parkin: much more than a simple ubiquitin ligase.

Parkin is mainly a cytosolic protein involved in a subset of Parkinson's disease (PD) cases referred to as autosomal juvenile recessive forms of PD. Most studies have established as a dogma that parkin function could be resumed as an ubiquitin ligase activity. Accordingly, several cellular functions ascribed to parkin derive from its ability to ubiquitinate a series of proteins, thereby rendering them prone to proteasomal degradation. Several lines of data indicated that parkin could display antiapoptotic properties and we demonstrated that indeed, parkin could downregulate the p53-dependent pathway. However, we showed that such function remained independent of parkin's ability to act as an ubiquitin ligase. Thus, we established that parkin repressed p53 transcription by physically interacting with its promoter. Here, we describe this novel parkin-associated transcription factor function and we speculate on putative additional transcriptional targets.

p53, a pivotal effector of a functional cross-talk linking presenilins and Pen-2.

The γ-secretase is a multiprotein complex responsible for the ultimate cut yielding amyloid-ß peptides and their N-terminal truncated species. This complex is composed of at least four distinct entities, namely presenilin-1 (PS1) or PS2, anterior pharynx defective-1, presenilin enhancer-2 (Pen-2) and nicastrin. Very few studies examined the transcriptional regulation of this complex, and more precisely, whether some of the members functionally interact. Here, we summarize our previous data documenting the fact that Pen-2 controls cell death in a p53-dependent manner and our recent demonstration of a pivotal role of p53 as a regulator of Pen-2 transcription. As PS trigger amyloid precursor protein intracellular domain-dependent regulation of p53, our studies delineate a feedback control mechanism by which PS and Pen-2 functionally interact in a p53-dependent manner.

Loss of function of DJ-1 triggered by Parkinson's disease-associated mutation is due to proteolytic resistance to caspase-6.

DJ-1 was recently identified as a gene product responsible for a subset of familial Parkinson's disease (PD). The mechanisms by which mutations in DJ-1 alter its function and account for PD-related pathology remained largely unknown. We show that DJ-1 is processed by caspase-6 and that the caspase-6-derived C-terminal fragment of DJ-1 fully accounts for associated p53-dependent cell death. In line with the above data, we show that a recently described early-onset PD-associated mutation (D149A) renders DJ-1 resistant to caspase-6 proteolysis and abolishes its protective phenotype. Unlike the D149A mutation, the L166P mutation that prevents DJ-1 dimerization does not impair its proteolysis by caspase-6 although it also abolishes DJ-1 antiapoptotic function. Therefore, we show here that DJ-1 loss of function could be due to impaired caspase-6 proteolysis and we document the fact that various DJ-1 mutations could lead to PD pathology through distinct molecular mechanisms.

[Fundamental data on the pathologies amyloid and Tau in Alzheimer's disease: which therapeutic perspectives?]. / Données fondamentales sur les pathologies amyloïde et Tau dans la maladie d'Alzheimer : quelles perspectives thérapeutiques ?

What is an innovative therapeutics for the Alzheimer's disease? An already used therapeutics which appeals to a recent and innovative concept or a therapeutic still putative based on tracks turned out experimentally but which still ask to be supported by man? Some therapeutic used at present are based on often former observations (anti-acetylcholinesterasic strategy) or more recent (antiglutamatergic strategy) but cannot be really considered as therapeutic innovative. They will be reviewed thus quickly because treated somewhere else. Potentially innovative therapeutics arise from recent headways and are there often only because of their stammerings. If the biology of Tau proteins is well-known, its therapeutic approach is little developed. On the contrary, therapeutics approaches turns essentially around the peptide amyloid, whether its training or the cellular consequences of its overproduction. This article is centred on the various therapeutic approaches which we can prospectively propose and which are very promising for some and for the others, collide with abstract or theoretical problems which will be approached here.

Neprilysin activity and expression are controlled by nicastrin.

We recently demonstrated that the presenilin-dependent gamma-secretase complex regulates the expression and activity of neprilysin, one of the main enzymes that degrade the amyloid beta-peptide (Abeta) which accumulates in Alzheimer's disease. Here, we examined the influence of endogenous nicastrin (NCT), a member of the gamma-secretase complex, on neprilysin physiology. We show that nicastrin deficiency drastically lowers neprilysin expression, membrane-bound activity and mRNA levels, but it did not modulate the expression of two other putative Abeta-cleaving enzymes, endothelin-converting enzyme and insulin-degrading enzyme. Furthermore, we show that nicastrin restores neprilysin activity and expression in nicastrin-deficient, but not presenilin-deficient fibroblasts, indicating that the control of neprilysin necessitates the complete gamma-secretase complex harbouring its four reported components. Finally, we show that NCT expression peaked 24 h after NCT cDNA transfection of wild-type and NCT-/- fibroblasts, while neprilysin expression drastically increased only after 36 h and was maximal at 48 h. This delayed effect on neprilysin expression correlates well with our demonstration of an indirect gamma-secretase-dependent modulation of neprilysin at its transcriptional level.

JLK isocoumarin inhibitors: selective gamma-secretase inhibitors that do not interfere with notch pathway in vitro or in vivo.

gamma-Secretase activity is involved in the generation of Abeta and therefore likely contributes to the pathology of Alzheimer's disease. Blocking this activity was seen as a major therapeutic target to slow down or arrest Abeta-related AD progression. This strategy seemed more doubtful when it was established that gamma-secretase also targets other substrates including Notch, a particularly important transmembrane protein involved in vital functions, at both embryonic and adulthood stages. We have described previously new non-peptidic inhibitors able to selectively inhibit Abeta cellular production in vitro without altering Notch pathway. We show here that in vivo, these inhibitors do not alter the Notch pathway responsible for somitogenesis in the zebrafish embryo. In addition, we document further the selectivity of JLK inhibitors by showing that, unlike other described gamma-secretase inhibitors, these agents do not affect E-cadherin processing. Finally, we establish that JLKs do not inhibit beta-site APP cleaving enzymes (BACE) 1 and BACE2, alpha-secretase, the proteasome, and GSK3beta kinase. Altogether, JLK inhibitors are the sole agents to date that are able to prevent Abeta production without triggering unwanted cleavages of other proteins.

Overexpression of PrPc triggers caspase 3 activation: potentiation by proteasome inhibitors and blockade by anti-PrP antibodies.

We examined the influence of cellular prion protein (PrPc) in the control of cell death in stably transfected HEK293 cell line and in the PrPc-inducible Rov9 cells. PrPc expression in stably transfected HEK293 human cells did not modify basal apoptotic tonus but drastically potentiated staurosporine-stimulated cellular toxicity and DNA fragmentation as well as caspase 3-like activity and immunoreactivity. An identical staurosporine-induced caspase 3 activation was observed after doxycycline in the PrPc-inducible Rov9 cell line. Interestingly, proteasome inhibitors increase PrPc-like immunoreactivity and unmasked a basal caspase 3 activation. Conversely, we show that anti-PrPc antibodies sequestrate PrPc at the cell surface and drastically lower PrPc-dependent caspase activation. We suggest that intracellular PrPc could sensitize human cells to pro-apoptotic phenotype and that blockade of PrPc internalization could be a track to prevent intracellular toxicity associated with PrPc overexpression.

Wild-type and mutated nicastrins do not display aminopeptidase M- and B-like activities.

Nicastrin is a recently discovered protein interacting with presenilins and the beta-amyloid precursor protein, the proteins playing key roles in Alzheimer's disease and which, when mutated, appear responsible for early-onset familial forms of Alzheimer's disease. Nicastrin was reported to modulate beta-amyloid production, a phenotype affected differently by missense mutations or deletions of a conserved hydrophilic domain. In addition to such a function, nicastrin was recently suggested to possess putative catalytic activity based on its sequence homology with enzymes of the aminopeptidase family. We set up stably transfected human HEK293 cells expressing either wild-type or mutated nicastrins and we show that these proteins do not exhibit aminopeptidase M- and B-like activities.

Endogenous beta-amyloid production in presenilin-deficient embryonic mouse fibroblasts.

Genetic and biochemical evidence have led to the suggestion that presenilins could be the long-searched-for gamma-secretase, the proteolytic activity that generates the carboxy terminus of amyloid beta-peptides. This activity is also thought to be responsible for the release of the Notch intracellular domain (NICD) from Notch. Here, we report the production of endogenous secreted and intracellular 40- and 42-amino-acid Abeta peptides in mouse fibroblasts deficient in presenilin 1, presenilin 2 or both. We show that the endogenous production of Abeta40 and Abeta42 was not altered by presenilin deficiency. By contrast, inactivating presenilin genes fully abolished NICD production. These data indicate that Abeta and NICD production are distinct catabolic events. Also, even though NICD formation is indeed presenilin dependent, endogenous secreted and intracellular beta-amyloid peptides are still generated in absence of presenilins, indicating that there is a gamma-secretase activity distinct from presenilins, at least in murine fibroblasts.