Correction: Abolishing Tau cleavage by caspases at Aspartate421 causes memory/synaptic plasticity deficits and pre-pathological Tau alterations.

Text for Correction.

Abolishing Tau cleavage by caspases at Aspartate421 causes memory/synaptic plasticity deficits and pre-pathological Tau alterations.

TAU mutations are genetically linked to fronto-temporal dementia (FTD) and hyper-phosphorylated aggregates of Tau form neurofibrillary tangles (NFTs) that constitute a pathological hallmark of Alzheimer disease (AD) and FTD. These observations indicate that Tau has a pivotal role in the pathogenesis of neurodegenerative disorders. Tau is cleaved by caspases at Aspartate421, to form a Tau metabolite known as δTau; δTau is increased in AD, due to the hyper-activation of caspases in AD brains. δTau is considered a critical toxic moiety underlying neurodegeneration, which initiates and facilitates NFT formation. As Tau is a therapeutic target in neurodegeneration, it is important to rigorously determine whether δTau is a toxic Tau species that should be pharmacologically attacked. To directly address these questions, we have generated a knock-in (KI) mouse called TauDN-that expresses a Tau mutant that cannot be cleaved by caspases. TauDN mice present short-term memory deficits and synaptic plasticity defects. Moreover, mice carrying two mutant Tau alleles show increased total insoluble hyper-phosphorylated Tau in the forebrain. These data are in contrast with the concept that δTau is a critical toxic moiety underlying neurodegeneration, and suggest that cleavage of Tau by caspases represents a negative feedback mechanism aimed to eliminate toxic Tau species. Alternatively, it is possible that either a reduction or an increase in δTau leads to synaptic dysfunction, memory impairments and Tau pathology. Both possibilities will have to be considered when targeting caspase cleavage of Tau in AD therapy.

Extracellular Tau Oligomers Produce An Immediate Impairment of LTP and Memory.

Non-fibrillar soluble oligomeric forms of amyloid-ß peptide (oAß) and tau proteins are likely to play a major role in Alzheimer's disease (AD). The prevailing hypothesis on the disease etiopathogenesis is that oAß initiates tau pathology that slowly spreads throughout the medial temporal cortex and neocortices independently of Aß, eventually leading to memory loss. Here we show that a brief exposure to extracellular recombinant human tau oligomers (oTau), but not monomers, produces an impairment of long-term potentiation (LTP) and memory, independent of the presence of high oAß levels. The impairment is immediate as it raises as soon as 20 min after exposure to the oligomers. These effects are reproduced either by oTau extracted from AD human specimens, or naturally produced in mice overexpressing human tau. Finally, we found that oTau could also act in combination with oAß to produce these effects, as sub-toxic doses of the two peptides combined lead to LTP and memory impairment. These findings provide a novel view of the effects of tau and Aß on memory loss, offering new therapeutic opportunities in the therapy of AD and other neurodegenerative diseases associated with Aß and tau pathology.

SEPT5_v2 is a parkin-binding protein.

Mutations in parkin are associated with various inherited forms of Parkinson's disease (PD). Parkin is a ubiquitin ligase enzyme that catalyzes the covalent attachment of ubiquitin moieties onto substrate proteins destined for proteasomal degradation. The substrates of parkin-mediated ubiquitination have yet to be completely identified. Using a yeast two-hybrid screen, we isolated the septin, human SEPT5_v2 (also known as cell division control-related protein 2), as a putative parkin-binding protein. SEPT5_v2 is highly homologous to another septin, SEPT5, which was recently identified as a target for parkin-mediated ubiquitination. SEPT5_v2 binds to parkin at the amino terminus and in the ring finger domains. Several lines of evidence have validated the putative link between parkin and SEPT5_v2. Parkin co-precipitates with SEPT5_v2 from human substantia nigra lysates. Parkin ubiquitinates SEPT5_v2 in vitro, and both SEPT5_v1 and SEPT5_v2 accumulate in brains of patients with ARJP, suggesting that parkin is essential for the normal metabolism of these proteins. These findings suggest that an important relationship exists between parkin and septins.

Fas, ceramide and serum withdrawal induce apoptosis via a common pathway in a type II Jurkat cell line.

Ceramide is a key mediator of apoptosis, yet its role in Fas-mediated apoptosis is controversial. Some reports have indicated that ceramide is either a primary signaling molecule in Fas-induced cell death, or that it functions upstream of Fas by increasing FasL expression. Other studies have suggested that ceramide is not relevant to Fas-induced cell death. We have approached this problem by studying ceramide-induced apoptosis in unique Jurkat cell clones selected for resistance to membrane-bound FasL-induced death. Resistance of the mutant Jurkat cells was specific for FasL killing, since the mutant clones were sensitive to other apoptotic stimuli such as cycloheximide and staurosporine. We tested the effects of serum withdrawal, one of the strongest inducers of ceramide, and of exogenous ceramide on apoptosis of both wild-type and FasL-resistant clones. Wild-type Jurkat cells were remarkably sensitive to serum withdrawal and to exogenous ceramide. In contrast all FasL-resistant mutant clones were resistant to these apoptosis-inducing conditions. In contrast to previous work, we did not detect an increase in FasL in either wild-type or mutant clones. Moreover activation of stress-activated protein kinases (JNK/SAPKs) after serum withdrawal and exogenous ceramide treatment was detected only in the wild-type and not in the resistant clones. Because of the parallel resistance of the mutant clones to Fas and to ceramide-induced apoptosis, our data support the notion that ceramide is a second messenger for the Fas/FasL pathway and that serum withdrawal, through production of ceramide, shares a common step with the Fas-mediated apoptotic pathway. Finally, our data suggest that activation of JNK/SAPKs is a common mediator of the three pathways tested.

Generation of an apoptotic intracellular peptide by gamma-secretase cleavage of Alzheimer's amyloid beta protein precursor.

The amyloid beta protein precursor (AbetaPP) is sequentially processed by beta- and gamma-secretases to generate the Abeta peptide. The biochemical path leading to Abeta formation has been extensively studied since extracellular aggregates of amyloidogenic forms of Abeta peptide (Abeta42) are considered the culprit of Alzheimer's disease. Aside from its pathological relevance, the biological role of AbetaPP proteolysis is unknown. Although never previously described, cleavage of AbetaPP by gamma-secretase should release, together with Abeta, a COOH-terminal AbetaPP Intracellular Domain, herein termed AID. We have now identified AID-like peptides in brain tissue of normal control and patients with sporadic Alzheimer's disease and demonstrate that AID acts as a positive regulator of apoptosis. Thus, overproduction of AID may add to the toxic effect of Abeta42 aggregates and further accelerate neurodegeneration.

Interaction of Alzheimer's presenilin-1 and presenilin-2 with Bcl-X(L). A potential role in modulating the threshold of cell death.

The familial Alzheimer's disease gene products, presenilin-1 and presenilin-2, have been reported to be functionally involved in amyloid precursor protein processing, notch receptor signaling, and programmed cell death or apoptosis. However, the molecular mechanisms by which presenilins regulate these processes remain unknown. With regard to the latter, we describe a molecular link between presenilins and the apoptotic pathway. Bcl-X(L), an anti-apoptotic member of the Bcl-2 family was shown to interact with the carboxyl-terminal fragments of PS1 and PS2 by the yeast two-hybrid system. In vivo interaction analysis revealed that both PS2 and its naturally occurring carboxyl-terminal products, PS2short and PS2Ccas, associated with Bcl-X(L), whereas the caspase-3-generated amino-terminal PS2Ncas fragment did not. This interaction was corroborated by demonstrating that Bcl-X(L) and PS2 partially co-localized to sites of the vesicular transport system. Functional analysis revealed that presenilins can influence mitochondrial-dependent apoptotic activities, such as cytochrome c release and Bax-mediated apoptosis. Together, these data support a possible role of the Alzheimer's presenilins in modulating the anti-apoptotic effects of Bcl-X(L).

Alternative, non-secretase processing of Alzheimer's beta-amyloid precursor protein during apoptosis by caspase-6 and -8.

Alzheimer's disease (AD) is a progressive neurodegenerative disorder. Although the pathogenesis of AD is unknown, it is widely accepted that AD is caused by extracellular accumulation of a neurotoxic peptide, known as Abeta. Mutations in the beta-amyloid precursor protein (APP), from which Abeta arises by proteolysis, are associated with some forms of familial AD (FAD) and result in increased Abeta production. Two other FAD genes, presenilin-1 and -2, have also been shown to regulate Abeta production; however, studies examining the biological role of these FAD genes suggest an alternative theory for the pathogenesis of AD. In fact, all three genes have been shown to regulate programmed cell death, hinting at the possibility that dysregulation of apoptosis plays a primary role in causing neuronal loss in AD. In an attempt to reconcile these two hypotheses, we investigated APP processing during apoptosis and found that APP is processed by the cell death proteases caspase-6 and -8. APP is cleaved by caspases in the intracellular portion of the protein, in a site distinct from those processed by secretases. Moreover, it represents a general effect of apoptosis, because it occurs during cell death induced by several stimuli both in T cells and in neuronal cells.

A role for neutral sphingomyelinase-mediated ceramide production in T cell receptor-induced apoptosis and mitogen-activated protein kinase-mediated signal transduction.

Studying apoptosis induced by T cell receptor (TCR) cross-linking in the T cell hybridoma, 3DO, we found both neutral sphingomyelinase activation and production of ceramide upon receptor engagement. Pharmacological inhibition of ceramide production by the fungal toxin, fumonisin B1, impaired TCR-induced interleukin (IL)-2 production and programmed cell death. Addition of either exogenous ceramide or bacterial sphingomyelinase reconstituted both responses. Moreover, specific inactivation of neutral sphingomyelinase by antisense RNA inhibited IL-2 production and mitogen-activated protein kinase activation after TCR triggering. These results suggest that ceramide production by activation of neutral sphingomyelinase is an essential component of the TCR signaling machinery.

Regulation of Fas ligand expression and cell death by apoptosis-linked gene 4.

Programmed cell death is a process required for the normal development of an organism. One of the best understood apoptotic pathways occurs in T lymphocytes and is mediated by Fas/Fas ligand (FasL) interaction. During studies of apoptosis induced by T cell-receptor engagement, we identified ALG-4F, a truncated transcript that prevents T cell-receptor-induced FasL upregulation and cell death. Overexpression of full-length ALG-4 induced transcription of FasL and, consequently, apoptosis. These results indicate that ALG-4 is necessary and sufficient for FasL expression. Fas/FasL interaction initiates cell death in many other systems, and its dysregulation is a mechanism by which several pathologic conditions arise. Understanding the molecular mechanisms of FasL regulation could be very useful in elucidating how these diseases develop and in identifying potential therapeutic targets.

Cloning of AIP1, a novel protein that associates with the apoptosis-linked gene ALG-2 in a Ca2+-dependent reaction.

ALG-2 is a 22-kDa calcium-binding protein necessary for cell death induced by different stimuli in 3DO T-cell hybridoma. 3DO cell clones depleted of ALG-2 protein exhibit normal caspases activation, suggesting that ALG-2 function is required downstream or is independent of caspase proteases activity for apoptosis to occur. Using the yeast two-hybrid screening system, we have isolated and characterized the mouse cDNA encoding for ALG-2 interacting protein 1 (AIP1), a novel protein that interacts with ALG-2. ALG-2 and AIP1 colocalize in the cytosol and the presence of calcium is an indispensable requisite for their association. Sequence alignment shows that AIP1 is highly similar to BRO1, a yeast protein related to components of the Pkc1p-MAP kinase cascade. Overexpression of a truncated form of AIP1 protects two different cell types from death induced by trophic factors withdrawal; thus, our data indicate that AIP1 cooperates with ALG-2 in executing the calcium-dependent requirements along the cell death pathway.

Generation of anti-apoptotic presenilin-2 polypeptides by alternative transcription, proteolysis, and caspase-3 cleavage.

PS2, the chromosome 1 familial Alzheimer's disease gene, has been shown to be involved in programmed cell death by three complementary experimental approaches. Reduction of PS2 protein levels by antisense RNA protects from apoptosis, whereas overexpression of an Alzheimer's PS2 mutant increases cell death induced by several stimuli. In addition, ALG-3, a truncated PS2 cDNA, encodes an artificial COOH-terminal PS2 segment that dominantly inhibits apoptosis. Here we describe a physiological COOH-terminal PS2 polypeptide (PS2s, Met298-Ile448) generated by both an alternative PS2 transcript and proteolytic cleavage. We find that PS2s protects transfected cells from Fas- and tumor necrosis factor alpha (TNFalpha)-induced apoptosis. Furthermore, a similar anti-apoptotic COOH-terminal PS2 polypeptide (PS2Ccas) is generated by caspase-3 cleavage at Asp329. These results suggest that caspase-3 not only activates pro-apoptotic substrates but also generates a negative feedback signal in which PS2Ccas antagonizes the progression of cell death. Thus, whereas PS2 is required for apoptosis, PS2s and PS2Ccas oppose this process, and the balance between PS2 and these COOH-terminal fragments may dictate the cell fate.

Dissociation of apoptosis and activation of IL-1beta-converting enzyme/Ced-3 proteases by ALG-2 and the truncated Alzheimer's gene ALG-3.

Recent attention has been focused on the members of the IL-1beta-converting enzyme (ICE)/Ced-3 family of cysteine protease as the key components of programmed cell death. However, the molecular events that lead to protease activation and link it to the final apoptotic processes remain poorly characterized. We have shown recently that ALG-2 is a Ca2+-binding protein required for apoptosis. ALG-2 depletion protects the mouse T cell hybridoma 3DO from programmed cell death induced by several stimuli, such as synthetic glucocorticoids, TCR, and Fas triggering. In this work, we show that in the ALG-2-depleted clones the ICE/Ced-3 proteases are normally activated upon TCR, Fas, and dexamethasone stimulation, as determined by cleavage of the endogenous substrate poly(ADP-ribose) polymerase and of a fluorogenic substrate. ALG-3, a truncated form of the familial Alzheimer's disease gene PS2, confers resistance to TCR- and Fas-induced apoptosis. Of interest, it also reduces protease activity and inhibits poly(ADP-ribose) polymerase cleavage upon Fas triggering. Our results suggest that, during apoptosis, ALG-2 functions downstream of, and that ALG-3 interferes with the sequential activation of members of the ICE family proteases.

Functional cloning of genes involved in T-cell receptor-induced programmed cell death.

Programmed cell death (PCD) is a normal event under genetic control that regulates the life span of different cell types in multicellular organisms. Among other physiological processes, PCD plays a pivotal role in the regulation of the immune system. Using a functional selection strategy we have isolated and characterized genes involved in T-cell receptor-induced apoptosis. One, ALG-2, is a Ca(2+)-binding protein that participates in regulatory events that occur late in the apoptotic program, where several death signals converge. Another, ALG-3, is a mouse homologue of the chromosome 1 familial Alzheimer's disease gene PS2. ALG-3 codes for a truncated PS2 polypeptide that antagonizes the apoptotic role of PS2. A PS2 mutation associated with Alzheimer's disease generates a molecule with enhanced apoptotic activity indicating that it might accelerate the process of neurodegeneration that occurs in this disease.

Participation of presenilin 2 in apoptosis: enhanced basal activity conferred by an Alzheimer mutation.

Overexpression of the familial Alzheimer's disease gene Presenilin 2 (PS2) in nerve growth factor-differentiated PC12 cells increased apoptosis induced by trophic factor withdrawal or beta-amyloid. Transfection of antisense PS2 conferred protection against apoptosis induced by trophic withdrawal in nerve growth factor-differentiated or amyloid precursor protein-expressing PC12 cells. The apoptotic cell death induced by PS2 protein was sensitive to pertussis toxin, suggesting that heterotrimeric GTP-binding proteins are involved. A PS2 mutation associated with familial Alzheimer's disease was found to generate a molecule with enhanced basal apoptotic activity. This gain of function might accelerate the process of neurodegeneration that occurs in Alzheimer's disease, leading to the earlier age of onset characteristic of familial Alzheimer's disease.

Requirement of the familial Alzheimer's disease gene PS2 for apoptosis. Opposing effect of ALG-3.

ALG-3, a truncated mouse homologue of the chromosome 1 familial Alzheimer's disease gene PS2, rescues T hybridoma 3DO cells from T-cell receptor-induced apoptosis by inhibiting Fas ligand induction and Fas signaling. Here we show that ALG-3 transfected 3DO cells express a COOH-terminal PS2 polypeptide. Overexpression of PS2 in ALG-3 transfected 3DO cells reconstitutes sensitivity to receptor-induced cell death, suggesting that the artificial PS2 polypeptide functions as a dominant negative mutant of PS2. ALG-3 and antisense PS2 protect PC12 cells from glutamate-induced apoptosis but not from death induced by hydrogen peroxide or the free radical MPP+. Thus, the PS2 gene is required for some forms of cell death in diverse cell types, and its function is opposed by ALG-3.

Interfering with apoptosis: Ca(2+)-binding protein ALG-2 and Alzheimer's disease gene ALG-3.

Two apoptosis-linked genes, named ALG-2 and ALG-3, were identified by means of a functional selection strategy. ALG-2 codes for a Ca(2+)-binding protein required for T cell receptor-, Fas-, and glucocorticoid-induced cell death. ALG-3, a partial complementary DNA that is homologous to the familial Alzheimer's disease gene STM2, rescues a T cell hybridoma from T cell receptor- and Fas-induced apoptosis. These findings suggest that ALG-2 may mediate Ca(2+)-regulated signals along the death pathway and that cell death may play a role in Alzheimer's disease.

Co-receptor (CD4/CD8) engagement enhances CD3-induced apoptosis in thymocytes. Implications for negative selection.

Negative selection of self-reactive immature T cells is mediated by TCR engagement and is thought to occur via apoptosis (programmed cell death). The requirement for the co-receptors CD4 and CD8 in negative selection has been demonstrated, but the biochemical mechanisms underlying their involvement in this process remain undefined. Here we present evidence that co-receptor engagement dramatically enhances CD3-induced endonuclease activation and cell death characteristic of apoptosis in immature thymocytes. The responses are associated with increased tyrosine phosphorylation of a number of cellular substrates, including the gamma isoform of phospholipase C, and with increased association of tyrosine phosphoproteins, including the protein tyrosine kinase p56lck, with the TCR complex. Co-receptor engagement also potentiated CD3-mediated Ca2+ increases via a mechanism dependent upon tyrosine kinase activation. Sustained Ca2+ availability was found to be necessary for endonuclease activation and apoptosis to occur. We suggest that CD4 and CD8 may participate in negative selection by enhancing TCR/CD3-induced tyrosine kinase activation and sustained Ca2+ increases that lead to endonuclease activation and apoptosis in self-reactive CD4+ CD8+ thymocytes.

CD3 zeta/eta/theta locus is colinear with and transcribed antisense to the gene encoding the transcription factor Oct-1.

CD3 zeta and eta are signal-transducing components of the TCR and are derived from alternative splicing of transcripts from a single genetic locus that also encodes CD30 theta. We have isolated two murine cDNA clones that appear to result from antisense transcription through CD3 theta-specific exon 10 and CD3 eta-specific exon 9. The sequence of these clones shows no open reading frame. Northern analysis with single stranded probes confirms the existence of a ubiquitously expressed > 12-kb polyadenylated mRNA antisense to CD3 eta. A "genomic walk," which extended 32 kb distal to murine CD3 eta exon 9, provided genomic DNA containing a more 5' portion of the antisense transcript. This probe identified two murine thymic cDNA with 91% sequence homology to the human transcription factor Oct-1. Five exons of murine Oct-1 map in an antisense orientation to the CD3 zeta/eta/theta locus on the cloned genomic sequences. The murine Oct-1 cDNA and exon 9 of CD3 eta hybridize to the same > 12-kb mRNA. Similarly, human Oct-1 and previously characterized human genomic sequences homologous to murine CD3 eta exon 9 each hybridize to the same > 15-kb human mRNA. Thus, the CD3 zeta/eta/theta and Oct-1 gene loci are partially overlapping and transcribed in opposite directions. The potential functional implications of these findings are discussed.