Familial prion protein mutants inhibit Hrd1-mediated retrotranslocation of misfolded proteins by depleting misfolded protein sensor BiP.

Similar to many proteins trafficking through the secretory pathway, cellular prion protein (PrP) partly retrotranslocates from the endoplasmic reticulum to the cytosol through the endoplasmic reticulum-associated degradation (ERAD) pathway in an attempt to alleviate accumulation of cellular misfolded PrP. Surprisingly, familial PrP mutants fail to retrotranslocate and simultaneously block normal cellular PrP retrotranslocation. That impairments in retrotranslocation of misfolded proteins could lead to global disruptions in cellular homeostasis prompted further investigations into PrP mutant retrotranslocation defects. A gain- and loss-of-function approach identified human E3 ubiquitin ligase, Hrd1, as a critical regulator of PrP retrotranslocation in mammalian cells. Expression of familial human PrP mutants, V210I(129V) and M232R(129V), not only abolished PrP retrotranslocation, but also that of Hrd1-dependent ERAD substrates, transthyretin TTR(D18G) and α1-anti-trypsin A1AT(NHK). Mutant PrP expression decreased binding immunoglobulin protein (BiP) levels by 50% and attenuated ER stress-induced BiP by increasing BiP turnover 6-fold. Overexpression of BiP with PrP mutants rescued retrotranslocation of PrP, TTR(D18G) and A1AT(NHK). PrP mutants-induced cell death was also rescued by co-expression of BiP. These results show that PrP mutants highjack the Hrd1-dependent ERAD pathway, an action that would result in misfolded protein accumulation especially in terminally differentiated neurons. This could explain the age-dependent neuronal degeneration in familial prion diseases.

Cyclin-dependent kinase 5 phosphorylation of familial prion protein mutants exacerbates conversion into amyloid structure.

Familial prion protein (PrP) mutants undergo conversion from soluble and protease-sensitive to insoluble and partially protease-resistant proteins. Cyclin-dependent kinase 5 (Cdk5) phosphorylation of wild type PrP (pPrP) at serine 43 induces a conversion of PrP into aggregates and fibrils. Here, we investigated whether familial PrP mutants are predisposed to Cdk5 phosphorylation and whether phosphorylation of familial PrP mutants increases conversion. PrP mutants representing three major familial PrP diseases and different PrP structural domains were studied. We developed a novel in vitro kinase reaction coupled with Thioflavin T binding to amyloid structure assay to monitor phosphorylation-dependent amyloid conversion. Although non-phosphorylated full-length wild type or PrP mutants did not convert into amyloid, Cdk5 phosphorylation rapidly converted these into Thioflavin T-positive structures following first order kinetics. Dephosphorylation partially reversed conversion. Phosphorylation-dependent conversion of PrP from α-helical structures into ß-sheet structures was confirmed by circular dichroism. Relative to wild type pPrP, most PrP mutants showed increased rate constants of conversion. In contrast, non-phosphorylated truncated PrP Y145X (where X represents a stop codon) and Q160X mutants converted spontaneously into Thioflavin T-positive fibrils after a lag phase of over 20 h, indicating nucleation-dependent polymerization. Phosphorylation reduced the lag phase by over 50% and thus accelerated the formation of the nucleating event. Consistently, phosphorylated Y145X and phosphorylated Q160X exacerbated conversion in a homologous seeding reaction, whereas WT pPrP could not seed WT PrP. These results demonstrate an influence of both the N terminus and the C terminus of PrP on conversion. We conclude that post-translational modifications of the flexible N terminus of PrP can cause or exacerbate PrP mutant conversion.

Identification of a novel endoplasmic reticulum stress response element regulated by XBP1.

Understanding the regulatory mechanisms mediating PRNP gene expression is highly relevant to elucidating normal cellular prion protein (PrP) function(s) and the transmissibility of prion protein neurodegenerative diseases. Here, luciferase reporter assays showed that an endoplasmic reticulum stress element (ERSE)-like element, CCAAT-N26-CCACG in the human PRNP promoter, is regulated by ER stress and X-box-binding protein 1 (XBP1) but not by activating transcription factor 6 α (ATF6α). Bioinformatics identified the ERSE-26 motif in 37 other human genes in the absence of canonical ERSE sites except for three genes. Several of these genes are associated with a synaptic function or are involved in oxidative stress. Brefeldin A, tunicamycin, and thapsigargin ER stressors induced gene expression of PRNP and four randomly chosen ERSE-26-containing genes, ERLEC1, GADD45B, SESN2, and SLC38A5, in primary human neuron cultures or in the breast carcinoma MCF-7 cell line, although the level of the response depends on the gene analyzed, the genetic background of the cells, the cell type, and the ER stressor. Overexpression of XBP1 increased, whereas siRNA knockdown of XBP1 considerably reduced, PRNP and ERLEC1 mRNA levels in MCF-7 cells. Taken together, these results identify a novel ER stress regulator, which implicates the ER stress response in previously unrecognized cellular functions.

Endoplasmic reticulum stress induces PRNP prion protein gene expression in breast cancer.

INTRODUCTION: High prion protein (PrP) levels are associated with breast, colon and gastric cancer resistance to treatment and with a poor prognosis for the patients. However, little is known about the underlying molecular mechanism(s) regulating human PrP gene (PRNP) expression in cancers. Because endoplasmic reticulum (ER) stress is associated with solid tumors, we investigated a possible regulation of PRNP gene expression by ER stress. METHODS: Published microarray databases of breast cancer tissues and breast carcinoma cell lines were analyzed for PrP mRNA and ER stress marker immunoglobulin heavy chain binding protein (BiP) levels. Breast cancer tissue microarrays (TMA) were immunostained for BiP and PrP. Breast carcinoma MCF-7, MDA-MB-231, HS578T and HCC1500 cells were treated with three different ER stressors - Brefeldin A, Tunicamycin, Thapsigargin - and levels of PrP mRNA or protein assessed by RT-PCR and Western blot analyses. A human PRNP promoter-luciferase reporter was used to assess transcriptional activation by ER stressors. Site-directed mutagenesis identified the ER stress response elements (ERSE). Chromatin immunoprecipitation (ChIP) analyses were done to identify the ER stress-mediated transcriptional regulators. The role of cleaved activating transcription factor 6α (ΔATF6α) and spliced X-box protein-1 (sXBP1) in PRNP gene expression was assessed with over-expression or silencing techniques. The role of PrP protection against ER stress was assessed with PrP siRNA and by using Prnp null cell lines. RESULTS: We find that mRNA levels of BiP correlated with PrP transcript levels in breast cancer tissues and breast carcinoma cell lines. PrP mRNA levels were enriched in the basal subtype and were associated with poor prognosis in breast cancer patients. Higher PrP and BiP levels correlated with increasing tumor grade in TMA. ER stress was a positive regulator of PRNP gene transcription in MCF-7 cells and luciferase reporter assays identified one ER stress response element (ERSE) conserved among primates and rodents and three primate-specific ERSEs that regulated PRNP gene expression. Among the various transactivators of the ER stress-regulated unfolded protein response (UPR), ATF6α and XBP1 transactivated PRNP gene expression, but the ability of these varied in different cell types. Functionally, PrP delayed ER stress-induced cell death. CONCLUSIONS: These results establish PRNP as a novel ER stress-regulated gene that could increase survival in breast cancers.

Caspase-6 as a novel early target in the treatment of Alzheimer's disease.

A recent paradigm shift appears to be underway on what scientists believe to be the cause of Alzheimer's disease (AD). The amyloid hypothesis has dominated the field of basic research for the last 25 years, and although these massive efforts have culminated in efficient removal of amyloid from the brains of patients, the absence of beneficial effects for the patient have been greatly disappointing. This has created a shift in the focus on amyloid to a much greater focus on Tau protein, in the hope that preventing tangle formation may inhibit or delay the progression of AD. Although there are promising developments in this area of research, diversifying our efforts to identify novel early targets by understanding the upstream molecular mechanisms that lead to, or occur with, neurofibrillary tangle and plaque formation may provide more efficient therapies against AD. Among many areas in development, an emphasis on the role of caspase-6 (Casp6) activity in early neurodegenerative mechanisms brings hope of a novel target against AD. Casp6 activity is intimately associated with the pathologies that define AD, correlates well with lower cognitive performance in aged individuals, and is involved in axonal degeneration in several cellular and in vivo animal models. This is a review of the evidence showing the relevance of Casp6 activation as an early event that could be inhibited to prevent the progression of AD.

Crystal structures of human caspase 6 reveal a new mechanism for intramolecular cleavage self-activation.

Dimeric effectors caspase 3 and caspase 7 are activated by initiator caspase processing. In this study, we report the crystal structures of effector caspase 6 (CASP6) zymogen and N-Acetyl-Val-Glu-Ile-Asp-al-inhibited CASP6. Both of these forms of CASP6 have a dimeric structure, and in CASP6 zymogen the intersubunit cleavage site (190)TEVD(193) is well structured and inserts into the active site. This positions residue Asp 193 to be easily attacked by the catalytic residue Cys 163. We demonstrate biochemically that intramolecular cleavage at Asp 193 is a prerequisite for CASP6 self-activation and that this activation mechanism is dependent on the length of the L2 loop. Our results indicate that CASP6 can be activated and regulated through intramolecular self-cleavage.

Caspase-1 inhibition improves cognition without significantly altering amyloid and inflammation in aged Alzheimer disease mice.

Human genetic and animal model studies indicate that brain microglial inflammation is a primary driver of cognitive impairment in Alzheimer Disease (AD). Inflammasome-activated Caspase-1 (Casp1) is associated with both AD microglial inflammation and neuronal degeneration. In mice, Casp1 genetic ablation or VX-765 small molecule inhibition of Casp1 given at onset of cognitive deficits strongly supports the association between microglial inflammation and cognitive impairment. Here, VX-765 significantly improved episodic and spatial memory impairment eight months after the onset of cognitive impairment in aged AD mice with significant amyloid beta peptide (Aß) accumulation and microglial inflammation. Unexpectedly, while cognitive improvement was associated with dendritic spine density and hippocampal synaptophysin level recovery, VX-765 only slightly decreased Aß deposition and did not alter biochemically-measured Aß levels. Furthermore, increased hippocampal Iba1+-microglia, GFAP+-astrocytes, IL-1ß, and TNF-α levels were unaltered by VX-765. These results support the hypothesis that neuronal degeneration, not Aß or microglial inflammation, drives cognitive impairment in AD.

Therapeutic potential of Nlrp1 inflammasome, Caspase-1, or Caspase-6 against Alzheimer disease cognitive impairment.

The sequential activation of Nucleotide-binding oligomerization domain, Leucine rich Repeat and Pyrin domain containing protein 1 (Nlrp1) inflammasome, Caspase-1 (Casp1), and Caspase-6 (Casp6) is implicated in primary human neuron cultures and Alzheimer Disease (AD) neurodegeneration. To validate the Nlrp1-Casp1-Casp6 pathway in vivo, the APPSwedish/Indiana J20 AD transgenic mouse model was generated on either a Nlrp1, Casp1 or Casp6 null genetic background and mice were studied at 4-5 months of age. Episodic memory deficits assessed with novel object recognition were normalized by genetic ablation of Nlrp1, Casp1, or Casp6 in J20 mice. Spatial learning deficits, assessed with the Barnes Maze, were normalized in genetically ablated J20, whereas memory recall was normalized in J20/Casp1-/- and J20/Casp6-/-, and improved in J20/Nlrp1-/- mice. Hippocampal CA1 dendritic spine density of the mushroom subtype was reduced in J20, and normalized in genetically ablated J20 mice. Reduced J20 hippocampal dentate gyrus and CA3 synaptophysin levels were normalized in genetically ablated J20. Increased Iba1+-microglia in the hippocampus and cortex of J20 brains were normalized by Casp1 and Casp6 ablation and reduced by Nlrp1 ablation. Increased pro-inflammatory cytokines, TNF-α and CXCL1, in the J20 hippocampus were normalized by Nlrp1 or Casp1 genetic ablation. CXCL1 was also normalized by Casp6 genetic ablation. IFN-γ was increased and total amyloid ß peptide was decreased in genetically ablated Nlrp1, Casp1 or Casp6 J20 hippocampi. We conclude that Nlrp1, Casp1, or Casp6 are implicated in AD-related cognitive impairment, inflammation, and amyloidogenesis. These results indicate that Nlrp1, Casp1, and Casp6 represent rational therapeutic targets against cognitive impairment and inflammation in AD.

Identification of Caspase-6-mediated processing of the valosin containing protein (p97) in Alzheimer's disease: a novel link to dysfunction in ubiquitin proteasome system-mediated protein degradation.

The valosin-containing protein (p97) is a ubiquitin-dependent ATPase that plays central roles in ubiquitin proteasome system (UPS)-mediated protein degradation pathways. p97 has been recently identified as a putative substrate of active Caspase-6 (Casp6) in primary human neurons. Since Casp6 is activated in mild cognitive impairment (MCI) and Alzheimer's disease (AD) patients' brains, the targeting of p97 by Casp6 may represent an important step that leads to UPS impairment in AD. Here, we show that p97 is a Casp6 substrate in vitro and in vivo. Casp6 cleavage of recombinant p97 generated two N-terminal fragments of 28 and 20 kDa, which were not generated by the other two effector caspases, Caspase-3 and Caspase-7. ATP binding to the D1 ATPase ring of p97 reduced the susceptibility of the N-domain to caspase-mediated proteolysis. Mass spectrometric analysis identified VAPD(179) as a Casp6 cleavage site within p97's N-domain. An anti-neoepitope serum immunohistochemically detected p97 cleaved at VAPD(179) in the cytoplasm of the cell soma and neurites of hippocampal neurons in MCI and AD. Overexpression of p97 (1-179) fragment, representing p97 cleaved at D179, impaired the degradation of model substrates in the ubiquitin-fusion degradation and the N-end rule pathways, and destabilized endogenous p97. Collectively, these results show that p97 is cleaved by Casp6 in AD and suggest p97 cleavage as an important mechanism for UPS impairment.

Alternatively spliced caspase-6B isoform inhibits the activation of caspase-6A.

Caspase-6 (Casp6) is activated early in Alzheimer disease and involved in axonal degeneration, but the regulation of Casp6 activity has not been explored. Several alternatively spliced forms of caspases act as inhibitors of caspase activation. The CASP6 gene generates an alternatively spliced transcript known as CASP6ß in addition to the CASP6α that encodes pro-Casp6a. Here, we show that the CASP6ß transcript and the pro-Casp6b protein are present in many cell lines, in primary human neurons, and in human brains. Unlike most other alternatively spliced caspase transcripts, pro-Casp6b contains a catalytic site. However, purified pro-Casp6b did not have caspase activity, nor did it inhibit already activated Casp6a. Pro-Casp6b prevented the proteolytic activation of pro-Casp6a in vitro and in cells. Pro-Casp6b interacts directly with pro-Casp6a. This work shows that pro-Casp6b is an inhibitor of pro-Casp6a activation. These results imply that pro-Casp6b could negatively regulate pro-Casp6a activation in neurons and prevent Casp6a-mediated axonal degeneration.

Caspase-6-cleaved Tau fails to induce Tau hyperphosphorylation and aggregation, neurodegeneration, glial inflammation, and cognitive deficits.

Active Caspase-6 (Casp6) and Tau cleaved by Casp6 at amino acids 402 (Tau∆D402) and 421 (Tau∆D421) are present in early Alzheimer disease intraneuronal neurofibrillary tangles, which are made primarily of filamentous Tau aggregates. To assess whether Casp6 cleavage of Tau contributes to Tau pathology and Casp6-mediated age-dependent cognitive impairment, we generated transgenic knock-in mouse models that conditionally express full-length human Tau (hTau) 0N4R only (CTO) or together with human Casp6 (hCasp6) (CTC). Region-specific hippocampal and cortical hCasp6 and hTau expression were confirmed with western blot and immunohistochemistry in 2-25-month-old brains. Casp6 activity was confirmed with Tau∆D421 and Tubulin cleaved by Casp6 immunopositivity in 3-25-month-old CTC, but not in CTO, brains. Immunoprecipitated Tau∆D402 was detected in both CTC and CTO brains, but was more abundant in CTC brains. Intraneuronal hippocampal Tau hyperphosphorylation at S202/T205, S422, and T231, and Tau conformational change were absent in both CTC and CTO brains. A slight accumulation of Tau phosphorylated at S396/404 and S202 was observed in Cornu Ammonis 1 (CA1) hippocampal neuron soma of CTC compared to CTO brains. Eighteen-month-old CTC brains showed rare argentophilic deposits that increased by 25 months, whereas CTO brains only displayed them sparsely at 25 months. Tau microtubule binding was equivalent in CTC and CTO hippocampi. Episodic and spatial memory measured with novel object recognition and Barnes maze, respectively, remained normal in 3-25-month-old CTC and CTO mice, in contrast to previously observed impairments in ACL mice expressing equivalent levels of hCasp6 only. Consistently, the CTC and CTO hippocampal CA1 region displayed equivalent dendritic spine density and no glial inflammation. Together, these results reveal that active hCasp6 co-expression with hTau generates Tau cleavage and rare age-dependent argentophilic deposits but fails to induce cognitive deficits, neuroinflammation, and Tau pathology.

Rare CASP6N73T variant associated with hippocampal volume exhibits decreased proteolytic activity, synaptic transmission defect, and neurodegeneration.

Caspase-6 (Casp6) is implicated in Alzheimer disease (AD) cognitive impairment and pathology. Hippocampal atrophy is associated with cognitive impairment in AD. Here, a rare functional exonic missense CASP6 single nucleotide polymorphism (SNP), causing the substitution of asparagine with threonine at amino acid 73 in Casp6 (Casp6N73T), was associated with hippocampal subfield CA1 volume preservation. Compared to wild type Casp6 (Casp6WT), recombinant Casp6N73T altered Casp6 proteolysis of natural substrates Lamin A/C and α-Tubulin, but did not alter cleavage of the Ac-VEID-AFC Casp6 peptide substrate. Casp6N73T-transfected HEK293T cells showed elevated Casp6 mRNA levels similar to Casp6WT-transfected cells, but, in contrast to Casp6WT, did not accumulate active Casp6 subunits nor show increased Casp6 enzymatic activity. Electrophysiological and morphological assessments showed that Casp6N73T recombinant protein caused less neurofunctional damage and neurodegeneration in hippocampal CA1 pyramidal neurons than Casp6WT. Lastly, CASP6 mRNA levels were increased in several AD brain regions confirming the implication of Casp6 in AD. These studies suggest that the rare Casp6N73T variant may protect against hippocampal atrophy due to its altered catalysis of natural protein substrates and intracellular instability thus leading to less Casp6-mediated damage to neuronal structure and function.

Phosphorylation of prion protein at serine 43 induces prion protein conformational change.

The cause of the conformational change of normal cellular prion protein (PrP) into its disease-associated form is unknown. Posttranslational modifications, such as glycosylation, acetylation, S-nitrosylation, and phosphorylation, are known to induce protein conformational changes. Here, we investigated whether phosphorylation could induce the conformational change of PrP because PrP contains several kinase motifs and has been found recently in the cytosol, in which kinases generally reside. Neuronal cyclin-dependent kinase 5 (Cdk5) phosphorylated recombinant PrP(23-231) at serine 43 (S43) in an in vitro kinase assay. Cdk5-phosphorylated PrP became proteinase K resistant, formed Congo Red-positive fibrils, and formed aggregates that were immunostained with anti-PrP and anti-phospho-PrP(S43) (anti-pPrP(S43)). pPrP(S43) was detected in PrP/Cdk5/p25 cotransfected N2a cells. Roscovitine inhibition of Cdk5 activity or transfection of N2a cells with mutant PrP S43A eliminated the anti-pPrP(S43)-immunopositive protein. Alkaline phosphatase-sensitive and proteinase K-resistant pPrP(S43) immunoreactivity was observed in scrapie-infected but not control-injected mice brains. These results raise the possibility that phosphorylation could represent a physiological mechanism of PrP conversion in vivo.

Self-activation of Caspase-6 in vitro and in vivo: Caspase-6 activation does not induce cell death in HEK293T cells.

Caspase-6 (Casp6) is a short pro-domain caspase that is activated early in Alzheimer disease, yet, little is known on the mechanism of activation of this caspase. In this study, critical proteolytic processing events required for Casp6 activation in vitro and in vivo were evaluated by site directed mutagenesis of the D23 pro-domain, and D179 and D193 linker processing sites. We found that (1) Casp6 was self-processed and activated in vitro and in vivo, (2) uncleavable Casp6 possessed low activity in vitro but not in vivo, (3) the pro-domain of Casp6 entirely prevented self-processing and activation in vivo but not in vitro, (4) removal of the pro-domain promoted Casp6 activation, (5) cleavage at either D179 or D193 was sufficient to generate activity in vitro and in vivo, and (6) Casp6 activity did not induce cell death in HEK293T cells. We conclude that the Casp6 is activated through proteolytic cleavage, as are the effector Caspase-3 and -7. However, unlike other effector caspases, Casp6 can be entirely self-activated and its activation does not necessarily induce cell death.

Targets of caspase-6 activity in human neurons and Alzheimer disease.

Caspase-6 activation occurs early in Alzheimer disease and sometimes precedes the clinical manifestation of the disease in aged individuals. The active Caspase-6 is localized in neuritic plaques, in neuropil threads, and in neurofibrillary tangles containing neurons that are not morphologically apoptotic in nature. To investigate the potential consequences of the activation of Caspase-6 in neurons, we conducted a proteomics analysis of Caspase-6-mediated cleavage of human neuronal proteins. Proteins from the cytosolic and membrane subcellular compartments were treated with recombinant active Caspase-6 and compared with undigested proteins by two-dimensional gel electrophoresis. LC/MS/MS analyses of the proteins that were cleaved identified 24 different potential protein substrates. Of these, 40% were cytoskeleton or cytoskeleton-associated proteins. We focused on the cytoskeleton proteins because these are critical for neuronal structure and function. Caspase-6 cleavage of alpha-Tubulin, alpha-Actinin-4, Spinophilin, and Drebrin was confirmed. At least one Caspase-6 cleavage site was identified for Drebrin, Spinophilin, and alpha-Tubulin. A neoepitope antiserum to alpha-Tubulin cleaved by Caspase-6 immunostained neurons, neurofibrillary tangles, neuropil threads, and neuritic plaques in Alzheimer disease and co-localized with active Caspase-6. These results imply that the early and neuritic activation of Caspase-6 in Alzheimer disease could disrupt the cytoskeleton network of neurons, resulting in impaired neuronal structure and function in the absence of cell death. This study provides novel insights into the pathophysiology of Alzheimer disease.

Pre-symptomatic Caspase-1 inhibitor delays cognitive decline in a mouse model of Alzheimer disease and aging.

Early therapeutic interventions are essential to prevent Alzheimer Disease (AD). The association of several inflammation-related genetic markers with AD and the early activation of pro-inflammatory pathways in AD suggest inflammation as a plausible therapeutic target. Inflammatory Caspase-1 has a significant impact on AD-like pathophysiology and Caspase-1 inhibitor, VX-765, reverses cognitive deficits in AD mouse models. Here, a one-month pre-symptomatic treatment of Swedish/Indiana mutant amyloid precursor protein (APPSw/Ind) J20 and wild-type mice with VX-765 delays both APPSw/Ind- and age-induced episodic and spatial memory deficits. VX-765 delays inflammation without considerably affecting soluble and aggregated amyloid beta peptide (Aß) levels. Episodic memory scores correlate negatively with microglial activation. These results suggest that Caspase-1-mediated inflammation occurs early in the disease and raise hope that VX-765, a previously Food and Drug Administration-approved drug for human CNS clinical trials, may be a useful drug to prevent the onset of cognitive deficits and brain inflammation in AD.

Cytosolic prion protein is the predominant anti-Bax prion protein form: exclusion of transmembrane and secreted prion protein forms in the anti-Bax function.

Prion protein (PrP) prevents Bax-mediated cell death by inhibiting the initial Bax conformational change that converts cytosolic Bax into a pro-apoptotic protein. PrP is mostly a glycophosphatidylinositol-anchored cell surface protein but it is also retrotranslocated into cytosolic PrP (CyPrP) or can become a type 1 or type 2 transmembrane protein. To determine the form and subcellular location of the PrP that has anti-Bax function, we co-expressed various Syrian hamster PrP (SHaPrP) mutants that favour specific PrP topologies and subcellular localization with N-terminally green fluorescent protein tagged pro-apoptotic Bax (EGFP-Bax) in MCF-7 cells and primary human neurons. Mutants that generate both CyPrP and secreted PrP ((Sec)PrP) or only CyPrP have anti-Bax activity. Mutants that produce (Ctm)PrP or (Ntm)PrP lose the anti-Bax activity, despite their ability to also make (Sec)PrP. Transmembrane-generating mutants do not produce CyPrP and both normal and cognate mutant forms of CyPrP rescue against the loss of anti-Bax activity. (Sec)PrP-generating constructs also produce non-membrane attached (Sec)PrP. However, this form of PrP has minimal anti-Bax activity. We conclude that CyPrP is the predominant form of PrP with anti-Bax function. These results imply that the retrotranslocation of PrP encompasses a survival function and is not merely a pathway for the proteasomal degradation of misfolded protein.

Helix 3 is necessary and sufficient for prion protein's anti-Bax function.

To identify the structural elements of the prion protein (PrP) necessary for its protective function against Bcl-2 associated protein X (Bax), we performed structure-function analyses of the anti-Bax function of cytosolic PrP (CyPrP) in MCF-7 cells. Deletions of 1, 2, or 3 N-terminal Bcl-2 homology domain 2-like octapeptide repeats (BORs), but not deletion of all four BORs, abolish CyPrPs anti-Bax function. Deletion of alpha-helix 3 (PrP23-199) or further C-terminal deletions of alpha-helix 1 and 2, and beta-strand 1 and 2 (PrP23-172, PrP23-160, PrP23-143, and PrP23-127) eliminates CyPrPs protection against Bax-mediated cell death. The substitution of helix 3 amino acid residues K204, V210, and E219 by proline inhibits the anti-Bax function of CyPrP. The substitution of K204, but not V210 and E219, by alanine residues also prevents CyPrPs anti-Bax function. Expression of PrPs helix 3 displays anti-Bax activity in MCF-7 cells and in human neurons. Together, these results indicate that although the BOR domain has an influence on PrPs anti-Bax function, the helix 3 is necessary and sufficient for the anti-Bax function of CyPrP. Identification of helix 3 as the structural element for the anti-Bax function thus provides a molecular target to modulate PrPs anti-Bax function in cancer and neurodegeneration.

Methylene blue inhibits Caspase-6 activity, and reverses Caspase-6-induced cognitive impairment and neuroinflammation in aged mice.

Activated Caspase-6 (Casp6) is associated with age-dependent cognitive impairment and Alzheimer disease (AD). Mice expressing human Caspase-6 in hippocampal CA1 neurons develop age-dependent cognitive deficits, neurodegeneration and neuroinflammation. This study assessed if methylene blue (MB), a phenothiazine that inhibits caspases, alters Caspase-6-induced neurodegeneration and cognitive impairment in mice. Aged cognitively impaired Casp6-overexpressing mice were treated with methylene blue in drinking water for 1 month. Methylene blue treatment did not alter Caspase-6 levels, assessed by RT-PCR, western blot and immunohistochemistry, but inhibited fluorescently-labelled Caspase-6 activity in acute brain slice intact neurons. Methylene blue treatment rescued Caspase-6-induced episodic and spatial memory deficits measured by novel object recognition and Barnes maze, respectively. Methylene blue improved synaptic function of hippocampal CA1 neurons since theta-burst long-term potentiation (LTP), measured by field excitatory postsynaptic potentials (fEPSPs) in acute brain slices, was successfully induced in the Schaffer collateral-CA1 pathway in methylene blue-treated, but not in vehicle-treated, Caspase-6 mice. Increased neuroinflammation, measured by ionized calcium binding adaptor molecule 1 (Iba1)-positive microglia numbers and subtypes, and glial fibrillary acidic protein (GFAP)-positive astrocytes, were decreased by methylene blue treatment. Therefore, methylene blue reverses Caspase-6-induced cognitive deficits by inhibiting Caspase-6, and Caspase-6-mediated neurodegeneration and neuroinflammation. Our results indicate that Caspase-6-mediated damage is reversible months after the onset of cognitive deficits and suggest that methylene blue could benefit Alzheimer disease patients by reversing Caspase-6-mediated cognitive decline.

Stem Cell-Derived Neurons as Cellular Models of Sporadic Alzheimer's Disease.

Alzheimer's disease (AD) occurs as either an autosomal dominant inherited disease or sporadically. While familial mutant genes can be expressed in cells or in animal models to assess dysregulated functions, sporadic AD cannot be replicated in models given our lack of understanding of causality. Furthermore, the study of sporadic forms of AD is difficult given the inaccessibility of brain tissues in living individuals and the manifestation of symptoms years after the onset of disease. Here, the objective was to assess if induced pluripotent stem cell-derived neurons from well-ascertained sporadic AD individuals could represent potential cellular models to determine the underlying molecular mechanisms of disease. We used cryopreserved peripheral blood mononuclear cells from three well-ascertained sporadic AD and three non-cognitively impaired (NCI) individuals of the CIMA-Q cohort to obtain iPSC-derived neurons. Microtubule associated protein 2 was decreased in AD neurons, whereas expression of AD-associated amyloid precursor protein, tau, and amyloid-ß peptide was similar in AD and NCI individuals. RNA sequencing identified several upregulated and downregulated mRNAs in AD relative to NCI neurons. Of these, complement Factor H (CFH), signal regulatory protein beta1 (SIRPB1), and insulin like growth factor binding protein 5 (IGFBP5) were previously associated with AD. In addition, several transcription factors not previously associated with AD, but involved in neuronal proliferation and differentiation were differentially expressed. The results identify novel avenues for the study of the underlying causes of sporadic AD and support the establishment of additional lines to identify mechanisms of disease in sporadic AD individuals.