Mutational Landscape of Alzheimer's Disease and Frontotemporal Dementia: Regional Variances in Northern, Central, and Southern Italy.

Alzheimer's Disease (AD) and Frontotemporal Dementia (FTD) are the two major neurodegenerative diseases with distinct clinical and neuropathological profiles. The aim of this report is to conduct a population-based investigation in well-characterized APP, PSEN1, PSEN2, MAPT, GRN, and C9orf72 mutation carriers/pedigrees from the north, the center, and the south of Italy. We retrospectively analyzed the data of 467 Italian individuals. We identified 21 different GRN mutations, 20 PSEN1, 11 MAPT, 9 PSEN2, and 4 APP. Moreover, we observed geographical variability in mutation frequencies by looking at each cohort of participants, and we observed a significant difference in age at onset among the genetic groups. Our study provides evidence that age at onset is influenced by the genetic group. Further work in identifying both genetic and environmental factors that modify the phenotypes in all groups is needed. Our study reveals Italian regional differences among the most relevant AD/FTD causative genes and emphasizes how the collaborative studies in rare diseases can provide new insights to expand knowledge on genetic/epigenetic modulators of age at onset.

Presenilin Deficiency Results in Cellular Cholesterol Accumulation by Impairment of Protein Glycosylation and NPC1 Function.

Presenilin proteins (PS1 and PS2) represent the catalytic subunit of γ-secretase and play a critical role in the generation of the amyloid ß (Aß) peptide and the pathogenesis of Alzheimer disease (AD). However, PS proteins also exert multiple functions beyond Aß generation. In this study, we examine the individual roles of PS1 and PS2 in cellular cholesterol metabolism. Deletion of PS1 or PS2 in mouse models led to cholesterol accumulation in cerebral neurons. Cholesterol accumulation was also observed in the lysosomes of embryonic fibroblasts from Psen1-knockout (PS1-KO) and Psen2-KO (PS2-KO) mice and was associated with decreased expression of the Niemann-Pick type C1 (NPC1) protein involved in intracellular cholesterol transport in late endosomal/lysosomal compartments. Mass spectrometry and complementary biochemical analyses also revealed abnormal N-glycosylation of NPC1 and several other membrane proteins in PS1-KO and PS2-KO cells. Interestingly, pharmacological inhibition of N-glycosylation resulted in intracellular cholesterol accumulation prominently in lysosomes and decreased NPC1, thereby resembling the changes in PS1-KO and PS2-KO cells. In turn, treatment of PS1-KO and PS2-KO mouse embryonic fibroblasts (MEFs) with the chaperone inducer arimoclomol partially normalized NPC1 expression and rescued lysosomal cholesterol accumulation. Additionally, the intracellular cholesterol accumulation in PS1-KO and PS2-KO MEFs was prevented by overexpression of NPC1. Collectively, these data indicate that a loss of PS function results in impaired protein N-glycosylation, which eventually causes decreased expression of NPC1 and intracellular cholesterol accumulation. This mechanism could contribute to the neurodegeneration observed in PS KO mice and potentially to the pathogenesis of AD.

Generation and characterization of an iPS cell line (PUMCi006-A) from skin fibroblasts of a patient with an M239T mutation in PSEN2 gene.

Presenilin-2 (PSEN2) mutation is one of the pathogenic factors of autosomal dominant early-onset Alzheimer's disease (EOAD). We generated a human induced pluripotent stem cell (iPSC) line from fibroblasts of an EOAD patient carrying PSEN2 mutation (c.716 T > C) utilizing Sendai reprogramming kit. The resulting iPSC line carried patient-specific point mutation, exhibited typical iPSC morphology, retained a normal karyotype, expressed pluripotency markers, and could form embryoid bodies. Established iPSC line serve as valuable resource for EOAD disease pathogenesis modelling and drug screening.

Presenilin: A Multi-Functional Molecule in the Pathogenesis of Alzheimer's Disease and Other Neurodegenerative Diseases.

Presenilin, a transmembrane protein primarily known for its role in Alzheimer's disease (AD) as part of the γ-secretase complex, has garnered increased attention due to its multifaceted functions in various cellular processes. Recent investigations have unveiled a plethora of functions beyond its amyloidogenic role. This review aims to provide a comprehensive overview of presenilin's diverse roles in AD and other neurodegenerative disorders. It includes a summary of well-known substrates of presenilin, such as its involvement in amyloid precursor protein (APP) processing and Notch signaling, along with other functions. Additionally, it highlights newly discovered functions, such as trafficking function, regulation of ferritin expression, apolipoprotein E (ApoE) secretion, the interaction of ApoE and presenilin, and the Aß42-to-Aß40-converting activity of ACE. This updated perspective underscores the evolving landscape of presenilin research, emphasizing its broader impact beyond established pathways. The incorporation of these novel findings accentuates the dynamic nature of presenilin's involvement in cellular processes, further advancing our comprehension of its multifaceted roles in neurodegenerative disorders. By synthesizing evidence from a range of studies, this review sheds light on the intricate web of presenilin functions and their implications in health and disease.

Assessment of Mendelian and risk-factor genes in Alzheimer disease: A prospective nationwide clinical utility study and recommendations for genetic screening.

PURPOSE: To assess the likely pathogenic/pathogenic (LP/P) variants rates in Mendelian dementia genes and the moderate-to-strong risk factors rates in patients with Alzheimer disease (AD). METHODS: We included 700 patients in a prospective study and performed exome sequencing. A panel of 28 Mendelian and 6 risk-factor genes was interpreted and returned to patients. We built a framework for risk variant interpretation and risk gradation and assessed the detection rates among early-onset AD (EOAD, age of onset (AOO) ≤65 years, n = 608) depending on AOO and pedigree structure and late-onset AD (66 < AOO < 75, n = 92). RESULTS: Twenty-one patients carried a LP/P variant in a Mendelian gene (all with EOAD, 3.4%), 20 of 21 affected APP, PSEN1, or PSEN2. LP/P variant detection rates in EOAD ranged from 1.7% to 11.6% based on AOO and pedigree structure. Risk factors were found in 69.5% of the remaining 679 patients, including 83 (12.2%) being heterozygotes for rare risk variants, in decreasing order of frequency, in TREM2, ABCA7, ATP8B4, SORL1, and ABCA1, including 5 heterozygotes for multiple rare risk variants, suggesting non-monogenic inheritance, even in some autosomal-dominant-like pedigrees. CONCLUSION: We suggest that genetic screening should be proposed to all EOAD patients and should no longer be prioritized based on pedigree structure.

Presenilin Gene Mutation-associated Psychosis: Phenotypic Characteristics and Clinical Implications.

Although psychotic symptoms have been described in association with rare presenilin ( PSEN ) gene mutations underlying early-onset Alzheimer disease (AD), no contemporary reviews on this topic exist. The purpose of this review is to characterize the psychiatric phenotype (specifically with respect to psychosis) of PSEN1 and PSEN2 variant-associated AD. A PubMed search was completed in July 2023. Only articles that described individuals harboring a PSEN1 or PSEN2 mutation who experienced symptoms of psychosis were included in the review. Thirty-three articles describing 52 individuals were included in the review, as well as one other study that provided limited information pertaining to an additional 21 cases. While visual hallucinations were the most common psychotic symptom, followed by persecutory delusions, auditory hallucinations occurred in ~17% of individuals. In ~33% of the reviewed cases psychotic symptoms were present at or near disease onset, and 9 of these individuals experienced auditory hallucinations and/or delusions in the absence of visual hallucinations (~17% of all cases). In many cases, symptoms developed at a relatively young age. As presenilin gene variant-associated psychosis may resemble a primary psychotic disorder, clinicians should be vigilant with respect to screening for signs/symptoms suggestive of neurodegeneration in first-episode psychosis.

iPSC-derived PSEN2 (N141I) astrocytes and microglia exhibit a primed inflammatory phenotype.

BACKGROUND: Widescale evidence points to the involvement of glia and immune pathways in the progression of Alzheimer's disease (AD). AD-associated iPSC-derived glial cells show a diverse range of AD-related phenotypic states encompassing cytokine/chemokine release, phagocytosis and morphological profiles, but to date studies are limited to cells derived from PSEN1, APOE and APP mutations or sporadic patients. The aim of the current study was to successfully differentiate iPSC-derived microglia and astrocytes from patients harbouring an AD-causative PSEN2 (N141I) mutation and characterise the inflammatory and morphological profile of these cells. METHODS: iPSCs from three healthy control individuals and three familial AD patients harbouring a heterozygous PSEN2 (N141I) mutation were used to derive astrocytes and microglia-like cells and cell identity and morphology were characterised through immunofluorescent microscopy. Cellular characterisation involved the stimulation of these cells by LPS and Aß42 and analysis of cytokine/chemokine release was conducted through ELISAs and multi-cytokine arrays. The phagocytic capacity of these cells was then indexed by the uptake of fluorescently-labelled fibrillar Aß42. RESULTS: AD-derived astrocytes and microglia-like cells exhibited an atrophied and less complex morphological appearance than healthy controls. AD-derived astrocytes showed increased basal expression of GFAP, S100ß and increased secretion and phagocytosis of Aß42 while AD-derived microglia-like cells showed decreased IL-8 secretion compared to healthy controls. Upon immunological challenge AD-derived astrocytes and microglia-like cells showed exaggerated secretion of the pro-inflammatory IL-6, CXCL1, ICAM-1 and IL-8 from astrocytes and IL-18 and MIF from microglia. CONCLUSION: Our study showed, for the first time, the differentiation and characterisation of iPSC-derived astrocytes and microglia-like cells harbouring a PSEN2 (N141I) mutation. PSEN2 (N141I)-mutant astrocytes and microglia-like cells presented with a 'primed' phenotype characterised by reduced morphological complexity, exaggerated pro-inflammatory cytokine secretion and altered Aß42 production and phagocytosis.

Presenilin-2 knock-In mice show severe depressive behavior via DVL3 downregulation.

INTRODUCTION: Alzheimer's disease (AD) is the most common form of dementia. Depression is one of the most critical psychiatric complications of AD, and 20%-30% of patients with AD experience symptoms of depression. Phospho-glycogen synthase kinase-3 beta (GSK3ß) is known to be associated with AD and depression. Furthermore, the role of disheveled (DVL) is known to regulate GSK3ß. Moreover, presenilin-2 (PS2) and DVL have cross-talk with each other. Also, it is widely hypothesized that stress leads to hypersecretion of cortisol and is thus associated with depression. Dickkopf WNT signaling pathway inhibitor-1 (DKK-1) is a crucial factor regulating depression and both amyloid beta (Aß) and phosphorylation of tau are widely known as a biomarker of AD. METHODS: To investigate the relationship between AD and depression, and possible pathways connecting the two diseases, we examined memory function and depression-related behavior test results in PS2 knock-in AD mice (PS2 MT). Next, we confirmed that there are relationships between DVL, depression, and cognitive disease through the comparative toxicogenomics database (https://ctdbase.org) and STRING (https://string-db.org) database. RESULTS: PS2 knock-in mice showed much more severe memory impairment and depression than PS2 wild-type mice (PS2 WT). In AD-related behavioral experiments, PS2 MT mice showed more memory dysfunction compared with PS2 WT group mice. Moreover, Aß and phosphorylation of tau showed higher expression in PS2 MT mice than in PS2 WT mice. Depression-related behavioral tests showed that PS2 MT mice exhibited more depressive behaviors than PS2 WT mice. Furthermore, both higher cortisol levels and higher expression of DKK-1 were found in PS2 MT mice relative to PS2 WT mice. The results indicated that there is a relationship between DVL and the release of AD-related mediators and expression of the depression-related glucocorticoid receptor and DKK-1. In the PS2 knock-in group, DVL was significantly decreased compared with the PS2 WT group. CONCLUSION: Depression increases the risk of developing AD and other forms of dementia. Recent evidence indicates that depression symptoms could trigger changes in memory and thinking over time. However, it is recognized that there are no drugs to facilitate a full recovery for both AD and depression. However, our results suggest that AD and depression could be associated, and DVL could be a significant target for the association between AD and depression.

Presenilin2 D439A Mutation Induces Dysfunction of Mitochondrial Fusion/Fission Dynamics and Abnormal Regulation of GTPase Activity.

Alzheimer's disease (AD) is an age-related progressive neurodegenerative disease, and approximately 10% of AD cases are early-onset familial AD (EOFAD), which is mainly linked to point mutations in genes encoding presenilins (PS1 and PS2). Mutations in PS2 are extremely rare and have not received enough attention. Recently, studies have found that Rho GTPase activity is closely related to the pathogenesis of AD. In this study, we used transcriptome sequencing in PS2 siRNA-transfected SH-SY5Y cells and found a group of differentially expressed genes (DEGs) related to the regulation of GTPase activity. Among those DEGs, the most significantly downregulated was Rho guanine nucleotide exchange factor 5 (ARHGEF5). GTPase activity in PS2 siRNA-transfected cells was significantly decreased. Then, we found that the expression of ARHGEF5 and the GTPase activity of Mitochondrial Rho GTPase 2 (Miro2) in PS2 D439A mutant SH-SY5Y cells were significantly decreased. We found for the first time that PS2 can bind to Miro2, and the PS2 D439A mutation reduced the binding between PS2 and Miro2, reduced the expression of Miro2, and resulted in an imbalance in mitochondrial fusion/fission dynamics. In conclusion, PS2 gene knockdown may participate in the pathogenesis of AD through the regulation of GTPase activity. The imbalance in mitochondrial dynamics mediated by the PS2 D439A mutation through regulation of the expression and GTPase activity of Miro2 may be a potential pathogenic mechanism of AD.

Quantitative comparison of presenilin protein expression reveals greater activity of PS2-γ-secretase.

γ-secretase processing of amyloid precursor protein (APP) has long been of interest in the pathological progression of Alzheimer's disease (AD) due to its role in the generation of amyloid-ß. The catalytic component of the enzyme is the presenilins of which there are two homologues, Presenilin-1 (PS1) and Presenilin-2 (PS2). The field has focussed on the PS1 form of this enzyme, as it is typically considered the more active at APP processing. However, much of this work has been completed without appropriate consideration of the specific levels of protein expression of PS1 and PS2. We propose that expression is an important factor in PS1- and PS2-γ-secretase activity, and that when this is considered, PS1 does not have greater activity than PS2. We developed and validated tools for quantitative assessment of PS1 and PS2 protein expression levels to enable the direct comparison of PS in exogenous and endogenous expression systems, in HEK-293 PS1 and/or PS2 knockout cells. We show that exogenous expression of Myc-PS1-NTF is 5.5-times higher than Myc-PS2-NTF. Quantitating endogenous PS protein levels, using a novel PS1/2 fusion standard we developed, showed similar results. When the marked difference in PS1 and PS2 protein levels is considered, we show that compared to PS1-γ-secretase, PS2-γ-secretase has equal or more activity on APP and Notch1. This study has implications for understanding the PS1- and PS2-specific contributions to substrate processing, and their potential influence in AD pathogenesis.

Cerebral organoids derived from patients with Alzheimer's disease with PSEN1/2 mutations have defective tissue patterning and altered development.

During the past two decades, induced pluripotent stem cells (iPSCs) have been widely used to study human neural development and disease. Especially in the field of Alzheimer's disease (AD), remarkable effort has been put into investigating molecular mechanisms behind this disease. Then, with the advent of 3D neuronal cultures and cerebral organoids (COs), several studies have demonstrated that this model can adequately mimic familial and sporadic AD. Therefore, we created an AD-CO model using iPSCs derived from patients with familial AD forms and explored early events and the progression of AD pathogenesis. Our study demonstrated that COs derived from three AD-iPSC lines with PSEN1(A246E) or PSEN2(N141I) mutations developed the AD-specific markers in vitro, yet they also uncover tissue patterning defects and altered development. These findings are complemented by single-cell sequencing data confirming this observation and uncovering that neurons in AD-COs likely differentiate prematurely.

Human Presenilin-1 delivered by AAV9 rescues impaired γ-secretase activity, memory deficits, and neurodegeneration in Psen mutant mice.

Mutations in the Presenilin (PSEN1 and PSEN2) genes are the major cause of early-onset familial Alzheimer's disease (FAD). Presenilin (PS) is the catalytic subunit of the γ-secretase complex, which cleaves type I transmembrane proteins, such as Notch and the amyloid precursor protein (APP), and plays an evolutionarily conserved role in the protection of neuronal survival during aging. FAD PSEN1 mutations exhibit impaired γ-secretase activity in cell culture, in vitro, and knockin (KI) mouse brains, and the L435F mutation is the most severe in reducing γ-secretase activity and is located closest to the active site of γ-secretase. Here, we report that introduction of the codon-optimized wild-type human PSEN1 cDNA by adeno-associated virus 9 (AAV9) results in broadly distributed, sustained, low to moderate levels of human PS1 (hPS1) expression and rescues impaired γ-secretase activity in the cerebral cortex of Psen mutant mice either lacking PS or expressing the Psen1 L435F KI allele, as evaluated by endogenous γ-secretase substrates of APP and recombinant γ-secretase products of Notch intracellular domain and Aß peptides. Furthermore, introduction of hPS1 by AAV9 alleviates impairments of synaptic plasticity and learning and memory in Psen mutant mice. Importantly, AAV9 delivery of hPS1 ameliorates neurodegeneration in the cerebral cortex of aged Psen mutant mice, as shown by the reversal of age-dependent loss of cortical neurons and elevated microgliosis and astrogliosis. These results together show that moderate hPS1 expression by AAV9 is sufficient to rescue impaired γ-secretase activity, synaptic and memory deficits, and neurodegeneration caused by Psen mutations in mouse models.

Pathogenic variants in the Longitudinal Early-onset Alzheimer's Disease Study cohort.

INTRODUCTION: One goal of the Longitudinal Early-onset Alzheimer's Disease Study (LEADS) is to investigate the genetic etiology of early onset (40-64 years) cognitive impairment. Toward this goal, LEADS participants are screened for known pathogenic variants. METHODS: LEADS amyloid-positive early-onset Alzheimer's disease (EOAD) or negative early-onset non-AD (EOnonAD) cases were whole exome sequenced (N = 299). Pathogenic variant frequency in APP, PSEN1, PSEN2, GRN, MAPT, and C9ORF72 was assessed for EOAD and EOnonAD. Gene burden testing was performed in cases compared to similar-age cognitively normal controls in the Parkinson's Progression Markers Initiative (PPMI) study. RESULTS: Previously reported pathogenic variants in the six genes were identified in 1.35% of EOAD (3/223) and 6.58% of EOnonAD (5/76). No genes showed enrichment for carriers of rare functional variants in LEADS cases. DISCUSSION: Results suggest that LEADS is enriched for novel genetic causative variants, as previously reported variants are not observed in most cases. HIGHLIGHTS: Sequencing identified eight cognitively impaired pathogenic variant carriers. Pathogenic variants were identified in PSEN1, GRN, MAPT, and C9ORF72. Rare variants were not enriched in APP, PSEN1/2, GRN, and MAPT. The Longitudinal Early-onset Alzheimer's Disease Study (LEADS) is a key resource for early-onset Alzheimer's genetic research.

Protein Structure Prediction for Disease-Related Insertions/Deletions in Presenilin 1 Gene.

More than 450 mutations, some of which have unknown toxicity, have been reported in the presenilin 1 gene, which is the most common cause of Alzheimer's disease (AD) with an early onset. PSEN1 mutations are thought to be responsible for approximately 80% of cases of monogenic AD, which are characterized by complete penetrance and an early age of onset. It is still unknown exactly how mutations in the presenilin 1 gene can cause dementia and neurodegeneration; however, both conditions have been linked to these changes. In this chapter, well-known computational analysis servers and accessible databases such as Uniprot, iTASSER, and PDBeFold are examined for their ability to predict the functional domains of mutant proteins and quantify the effect that these mutations have on the three-dimensional structure of the protein.

Presenilin-1 (PSEN1) Mutations: Clinical Phenotypes beyond Alzheimer's Disease.

Presenilin 1 (PSEN1) is a part of the gamma secretase complex with several interacting substrates, including amyloid precursor protein (APP), Notch, adhesion proteins and beta catenin. PSEN1 has been extensively studied in neurodegeneration, and more than 300 PSEN1 mutations have been discovered to date. In addition to the classical early onset Alzheimer's disease (EOAD) phenotypes, PSEN1 mutations were discovered in several atypical AD or non-AD phenotypes, such as frontotemporal dementia (FTD), Parkinson's disease (PD), dementia with Lewy bodies (DLB) or spastic paraparesis (SP). For example, Leu113Pro, Leu226Phe, Met233Leu and an Arg352 duplication were discovered in patients with FTD, while Pro436Gln, Arg278Gln and Pro284Leu mutations were also reported in patients with motor dysfunctions. Interestingly, PSEN1 mutations may also impact non-neurodegenerative phenotypes, including PSEN1 Pro242fs, which could cause acne inversa, while Asp333Gly was reported in a family with dilated cardiomyopathy. The phenotypic diversity suggests that PSEN1 may be responsible for atypical disease phenotypes or types of disease other than AD. Taken together, neurodegenerative diseases such as AD, PD, DLB and FTD may share several common hallmarks (cognitive and motor impairment, associated with abnormal protein aggregates). These findings suggested that PSEN1 may interact with risk modifiers, which may result in alternative disease phenotypes such as DLB or FTD phenotypes, or through less-dominant amyloid pathways. Next-generation sequencing and/or biomarker analysis may be essential in clearly differentiating the possible disease phenotypes and pathways associated with non-AD phenotypes.

Double Mutations in a Patient with Early-Onset Alzheimer's Disease in Korea: An APP Val551Met and a PSEN2 His169Asn.

The etiology of early-onset Alzheimer's disease (EOAD) is associated with alterations in the production of amyloid beta (Aß) species caused by mutations in the APP, PSEN1, and PSEN2 genes. Mutations affect intra- or inter-molecular interactions and processes between the γ-secretase complex and amyloid precursor protein (APP), leading to the aberrant sequential cleavage of Aß species. A 64-year-old woman presented with progressive memory decline, mild right hippocampal atrophy, and a family history of Alzheimer's dementia (AD). Whole exome sequencing was performed to evaluate AD-related gene mutations, which were verified by Sanger sequencing. A mutation-caused structural alteration of APP was predicted using in silico prediction programs. Two AD-related mutations, in APP (rs761339914; c.G1651A; p.V551M) and PSEN2 (rs533813519; c.C505A; p.H169N), were identified. The APP Val551Met mutation in the E2 domain may influence APP homodimerization through changes in intramolecular interactions between adjacent amino acids, altering Aß production. The second mutation was PSEN2 His169Asn mutation, which was previously reported in five EOAD patients from Korea and China, with a relatively high frequency in the East Asian population. According to a previous report, the presenilin 2 protein was predicted to result in a major helical torsion by PSEN2 His169Asn mutation. Notably, the co-existence of APP Val551Met and PSEN2 His169Asn may induce a synergistic effect by both mutations. Future functional studies are needed to clarify the pathological effects of these double mutations.

Transmembrane BAX inhibitor motif containing 6 suppresses presenilin-2 to preserve mitochondrial integrity after myocardial ischemia-reperfusion injury.

Myocardial ischemia-reperfusion (I/R) damage is characterized by mitochondrial damage in cardiomyocytes. Transmembrane BAX inhibitor motif containing 6 (TMBIM6) and presenilin-2 (PS2) participate in multiple mitochondrial pathways; thus, we investigated the impact of these proteins on mitochondrial homeostasis during an acute reperfusion injury. Myocardial post-ischemic reperfusion stress impaired myocardial function, induced structural abnormalities and promoted cardiomyocyte death by disrupting the mitochondrial integrity in wild-type mice, but not in TMBIM6 transgenic mice. We found that TMBIM6 bound directly to PS2 and promoted its post-transcriptional degradation. Knocking out PS2 in mice reduced I/R injury-induced cardiac dysfunction, inflammatory responses, myocardial swelling and cardiomyocyte death by improving the mitochondrial integrity. These findings demonstrate that sufficient TMBIM6 expression can prevent PS2 accumulation during cardiac I/R injury, thus suppressing reperfusion-induced mitochondrial damage. Therefore, TMBIM6 and PS2 are promising therapeutic targets for the treatment of cardiac reperfusion damage.

Different transmembrane domains determine the specificity and efficiency of the cleavage activity of the γ-secretase subunit presenilin.

The γ-secretase complex catalyzes the intramembrane cleavage of C99, a carboxy-terminal fragment of the amyloid precursor protein. Two paralogs of its catalytic subunit presenilin (PS1 and PS2) are expressed which are autocatalytically cleaved into an N-terminal and a C-terminal fragment during maturation of γ-secretase. In this study, we compared the efficiency and specificity of C99 cleavage by PS1- and PS2-containing γ-secretases. Mass spectrometric analysis of cleavage products obtained in cell-free and cell-based assays revealed that the previously described lower amyloid-ß (Aß)38 generation by PS2 is accompanied by a reciprocal increase in Aß37 production. We further found PS1 and PS2 to show different preferences in the choice of the initial cleavage site of C99. However, the differences in Aß38 and Aß37 generation appear to mainly result from altered subsequent stepwise cleavage of Aß peptides. Apart from these differences in cleavage specificity, we confirmed a lower efficiency of initial C99 cleavage by PS2 using a detergent-solubilized γ-secretase system. By investigating chimeric PS1/2 molecules, we show that the membrane-embedded, nonconserved residues of the N-terminal fragment mainly account for the differential cleavage efficiency and specificity of both presenilins. At the level of individual transmembrane domains (TMDs), TMD3 was identified as a major modulator of initial cleavage site specificity. The efficiency of endoproteolysis strongly depends on nonconserved TMD6 residues at the interface to TMD2, i.e., at a putative gate of substrate entry. Taken together, our results highlight the role of individual presenilin TMDs in the cleavage of C99 and the generation of Aß peptides.

Unraveling Presenilin 2 Functions in a Knockout Zebrafish Line to Shed Light into Alzheimer's Disease Pathogenesis.

Mutations in presenilin 2 (PS2) have been causally linked to the development of inherited Alzheimer's disease (AD). Besides its role as part of the γ-secretase complex, mammalian PS2 is also involved, as an individual protein, in a growing number of cell processes, which result altered in AD. To gain more insight into PS2 (dys)functions, we have generated a presenilin2 (psen2) knockout zebrafish line. We found that the absence of the protein does not markedly influence Notch signaling at early developmental stages, suggesting a Psen2 dispensable role in the γ-secretase-mediated Notch processing. Instead, loss of Psen2 induces an exaggerated locomotor response to stimulation in fish larvae, a reduced number of ER-mitochondria contacts in zebrafish neurons, and an increased basal autophagy. Moreover, the protein is involved in mitochondrial axonal transport, since its acute downregulation reduces in vivo organelle flux in zebrafish sensory neurons. Importantly, the expression of a human AD-linked mutant of the protein increases this vital process. Overall, our results confirm zebrafish as a good model organism for investigating PS2 functions in vivo, representing an alternative tool for the characterization of new AD-linked defective cell pathways and the testing of possible correcting drugs.