Expression of truncated presenilin 2 splice variant in Alzheimer's disease, bipolar disorder, and schizophrenia brain cortex

Aberrant splicing of PSEN2, but not PSEN1, in individuals with sporadic Alzheimer's disease

Alzheimer's disease is the most common neurodegenerative disease, characterized by dementia and premature death. Early-onset familial Alzheimer's disease is caused in part by pathogenic variants in presenilin 1 (PSEN1) and presenilin 2 (PSEN2), and alternative splicing of these two genes has been implicated in both familial and sporadic Alzheimer's disease. Here, we leveraged targeted isoform-sequencing to characterize thousands of complete PSEN1 and PSEN2 transcripts in the prefrontal cortex of individuals with sporadic Alzheimer's disease, familial Alzheimer's disease (carrying PSEN1 and PSEN2 variants), and controls. Our results reveal alternative splicing patterns of PSEN2 specific to sporadic Alzheimer's disease, including a human-specific cryptic exon present in intron 9 of PSEN2 as well as a 77 bp intron retention product before exon 6 that are both significantly elevated in sporadic Alzheimer's disease samples, alongside a significantly lower percentage of canonical full-length PSEN2 transcripts versus familial Alzheimer's disease samples and controls. Both alternatively spliced products are predicted to generate a prematurely truncated PSEN2 protein and were corroborated in an independent cerebellum RNA-sequencing dataset. In addition, our data in PSEN variant carriers is consistent with the hypothesis that PSEN1 and PSEN2 variants need to produce full-length but variant proteins to contribute to the onset of Alzheimer's disease, although intriguingly there were far fewer full-length transcripts carrying pathogenic alleles versus wild-type alleles in PSEN2 variant carriers. Finally, we identify frequent RNA editing at Alu elements present in an extended 3' untranslated region in PSEN2. Overall, this work expands the understanding of PSEN1 and PSEN2 variants in Alzheimer's disease, shows that transcript differences in PSEN2 may play a role in sporadic Alzheimer's disease, and suggests novel mechanisms of Alzheimer's disease pathogenesis.

Preclinical Models for Alzheimer's Disease: Past, Present, and Future Approaches

A robust preclinical disease model is a primary requirement to understand the underlying mechanisms, signaling pathways, and drug screening for human diseases. Although various preclinical models are available for several diseases, clinical models for Alzheimer's disease (AD) remain underdeveloped and inaccurate. The pathophysiology of AD mainly includes the presence of amyloid plaques and neurofibrillary tangles (NFT). Furthermore, neuroinflammation and free radical generation also contribute to AD. Currently, there is a wide gap in scientific approaches to preventing AD progression. Most of the available drugs are limited to symptomatic relief and improve deteriorating cognitive functions. To mimic the pathogenesis of human AD, animal models like 3XTg-AD and 5XFAD are the primarily used mice models in AD therapeutics. Animal models for AD include intracerebroventricular-streptozotocin (ICV-STZ), amyloid beta-induced, colchicine-induced, etc., focusing on parameters such as cognitive decline and dementia. Unfortunately, the translational rate of the potential drug candidates in clinical trials is poor due to limitations in imitating human AD pathology in animal models. Therefore, the available preclinical models possess a gap in AD modeling. This paper presents an outline that critically assesses the applicability and limitations of the current approaches in disease modeling for AD. Also, we attempted to provide key suggestions for the best-fit model to evaluate potential therapies, which might improve therapy translation from preclinical studies to patients with AD.

CK2 and protein kinases of the CK1 superfamily as targets for neurodegenerative disorders

Casein kinases are involved in a variety of signaling pathways, and also in inflammation, cancer, and neurological diseases. Therefore, they are regarded as potential therapeutic targets for drug design. Recent studies have highlighted the importance of the casein kinase 1 superfamily as well as protein kinase CK2 in the development of several neurodegenerative pathologies, such as Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis. CK1 kinases and their closely related tau tubulin kinases as well as CK2 are found to be overexpressed in the mammalian brain. Numerous substrates have been detected which play crucial roles in neuronal and synaptic network functions and activities. The development of new substances for the treatment of these pathologies is in high demand. The impact of these kinases in the progress of neurodegenerative disorders, their bona fide substrates, and numerous natural and synthetic compounds which are able to inhibit CK1, TTBK, and CK2 are discussed in this review.

Relevance of a Truncated PRESENILIN 2 Transcript to Alzheimer's Disease and Neurodegeneration

BACKGROUND

The PRESENILIN genes (PSEN1, PSEN2) encoding for their respective proteins have critical roles in many aspects of Alzheimer's disease (AD) pathogenesis. The PS2V transcript of PSEN2 encodes a truncated protein and is upregulated in AD brains; however, its relevance to AD and disease progression remains to be determined.

OBJECTIVE

Assess transcript levels in postmortem AD and non-AD brain tissue and in lymphocytes collected under the Australian Imaging Biomarker and Lifestyle (AIBL) study.

METHODS

Full length PSEN2 and PS2V transcript levels were assessed by quantitative digital PCR in postmortem brain tissue (frontal cortex and hippocampus) from control, AD, frontotemporal dementia (FTD), and Lewy body dementia (LBD). Transcript levels were also assessed in lymphocytes obtained from the Perth subset of the AIBL study (n = 160). Linear regression analysis was used to assess correlations between transcript copy number and brain volume and neocortical amyloid load.

RESULTS

PS2V levels increased in AD postmortem brain but PS2V was also present at significant levels in FTD and LBD brains. PS2V transcript was detected in lymphocytes and PS2V/PSEN2 ratios were increased in mild cognitive impairment (p = 0.024) and AD (p = 0.019) groups compared to control group. Increased ratios were significantly correlated with hippocampal volumes only (n = 62, β= -0.269, p = 0.03).

CONCLUSION

Taken together, these results suggest that PS2V may be a marker of overall neurodegeneration.

The Zebrafish Equivalent of Alzheimer's Disease-Associated PRESENILIN Isoform PS2V Regulates Inflammatory and Other Responses to Hypoxic Stress

Dominant mutations in the PRESENILIN genes PSEN1 and PSEN2 cause familial Alzheimer's disease (fAD) that usually shows onset before 65 years of age. In contrast, genetic variation at the PSEN1 and PSEN2 loci does not appear to contribute to risk for the sporadic, late onset form of the disease (sAD), leading to doubts that these genes play a role in the majority of AD cases. However, a truncated isoform of PSEN2, PS2V, is upregulated in sAD brains and is induced by hypoxia and high cholesterol intake. PS2V can increase γ-secretase activity and suppress the unfolded protein response (UPR), but detailed analysis of its function has been hindered by lack of a suitable, genetically manipulable animal model since mice and rats lack this PRESENILIN isoform. We recently showed that zebrafish possess an isoform, PS1IV, that is cognate to human PS2V. Using an antisense morpholino oligonucleotide, we can block specifically the induction of PS1IV that normally occurs under hypoxia. Here, we exploit this ability to identify gene regulatory networks that are modulated by PS1IV. When PS1IV is absent under hypoxia-like conditions, we observe changes in expression of genes controlling inflammation (particularly sAD-associated IL1B and CCR5), vascular development, the UPR, protein synthesis, calcium homeostasis, catecholamine biosynthesis, TOR signaling, and cell proliferation. Our results imply an important role for PS2V in sAD as a component of a pathological mechanism that includes hypoxia/oxidative stress and support investigation of the role of PS2V in other diseases, including schizophrenia, when these are implicated in the pathology.

Mutations in presenilin 2 and its implications in Alzheimer's disease and other dementia-associated disorders

Alzheimer's disease (AD) is the most common form of dementia. Mutations in the genes encoding presenilin 1 (PSEN1), presenilin 2 (PSEN2), and amyloid precursor protein have been identified as the main genetic causes of familial AD. To date, more than 200 mutations have been described worldwide in PSEN1, which is highly homologous with PSEN2, while mutations in PSEN2 have been rarely reported. We performed a systematic review of studies describing the mutations identified in PSEN2. Most PSEN2 mutations were detected in European and in African populations. Only two were found in Korean populations. Interestingly, PSEN2 mutations appeared not only in AD patients but also in patients with other disorders, including frontotemporal dementia, dementia with Lewy bodies, breast cancer, dilated cardiomyopathy, and Parkinson's disease with dementia. Here, we have summarized the PSEN2 mutations and the potential implications of these mutations in dementia-associated disorders.

Postmortem brain: an underutilized substrate for studying severe mental illness

We propose that postmortem tissue is an underutilized substrate that may be used to translate genetic and/or preclinical studies, particularly for neuropsychiatric illnesses with complex etiologies. Postmortem brain tissues from subjects with schizophrenia have been extensively studied, and thus serve as a useful vehicle for illustrating the challenges associated with this biological substrate. Schizophrenia is likely caused by a combination of genetic risk and environmental factors that combine to create a disease phenotype that is typically not apparent until late adolescence. The complexity of this illness creates challenges for hypothesis testing aimed at understanding the pathophysiology of the illness, as postmortem brain tissues collected from individuals with schizophrenia reflect neuroplastic changes from a lifetime of severe mental illness, as well as treatment with antipsychotic medications. While there are significant challenges with studying postmortem brain, such as the postmortem interval, it confers a translational element that is difficult to recapitulate in animal models. On the other hand, data derived from animal models typically provide specific mechanistic and behavioral measures that cannot be generated using human subjects. Convergence of these two approaches has led to important insights for understanding molecular deficits and their causes in this illness. In this review, we discuss the problem of schizophrenia, review the common challenges related to postmortem studies, discuss the application of biochemical approaches to this substrate, and present examples of postmortem schizophrenia studies that illustrate the role of the postmortem approach for generating important new leads for understanding the pathophysiology of severe mental illness.

The Guinea Pig as a Model for Sporadic Alzheimer's Disease (AD): The Impact of Cholesterol Intake on Expression of AD-Related Genes

We investigated the guinea pig, Cavia porcellus, as a model for Alzheimer’s disease (AD), both in terms of the conservation of genes involved in AD and the regulatory responses of these to a known AD risk factor - high cholesterol intake. Unlike rats and mice, guinea pigs possess an Aβ peptide sequence identical to human Aβ. Consistent with the commonality between cardiovascular and AD risk factors in humans, we saw that a high cholesterol diet leads to up-regulation of BACE1 (β-secretase) transcription and down-regulation of ADAM10 (α-secretase) transcription which should increase release of Aβ from APP. Significantly, guinea pigs possess isoforms of AD-related genes found in humans but not present in mice or rats. For example, we discovered that the truncated PS2V isoform of human PSEN2, that is found at raised levels in AD brains and that increases γ-secretase activity and Aβ synthesis, is not uniquely human or aberrant as previously believed. We show that PS2V formation is up-regulated by hypoxia and a high-cholesterol diet while, consistent with observations in humans, Aβ concentrations are raised in some brain regions but not others. Also like humans, but unlike mice, the guinea pig gene encoding tau, MAPT, encodes isoforms with both three and four microtubule binding domains, and cholesterol alters the ratio of these isoforms. We conclude that AD-related genes are highly conserved and more similar to human than the rat or mouse. Guinea pigs represent a superior rodent model for analysis of the impact of dietary factors such as cholesterol on the regulation of AD-related genes.

Γ-secretase modulators do not induce Aβ-rebound and accumulation of β-C-terminal fragment

γ-secretase inhibitors (GSIs) have been developed to reduce amyloid-β (Aβ) production for the treatment of Alzheimer's disease by inhibiting the cleavage of amyloid precursor protein (APP). However, cross-inhibitory activity on the processing of Notch can cause adverse reactions. To avoid these undesirable effects, γ-secretase modulators (GSMs) are being developed to selectively reduce toxic Aβ production without perturbing Notch signaling. As it is also known that GSIs can cause a paradoxical increase of plasma Aβ over the baseline after a transient reduction (known as Aβ-rebound), we asked if GSMs would cause a similar rebound and what the potential mechanism might be. Our studies were performed with one GSI (LY-450139) and two chemically distinct GSMs. Although LY-450139 caused Aβ-rebound as expected in rat plasma, the two GSMs did not. Inhibition of APP processing by LY-450139 induced an accumulation of γ-secretase substrates, α- and β-C-terminal fragments of APP, but neither GSM caused such an accumulation. In conclusion, we discover that GSMs, unlike GSIs, do not cause Aβ-rebound, possibly because of the lack of accumulation of β-C-terminal fragments. GSMs may be superior to GSIs in the treatment of Alzheimer's disease not only by sparing Notch signaling but also by avoiding Aβ-rebound.

Adaptive alternative splicing correlates with less environmental risk of parkinsonism

BACKGROUND/OBJECTIVE

Environmental exposure to anti-acetylcholinesterases (AChEs) aggravates the risk of Parkinsonism due to currently unclear mechanism(s). We explored the possibility that the brain's capacity to induce a widespread adaptive alternative splicing response to such exposure may be involved.

METHODS

Following exposure to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), brain region transcriptome profiles were tested.

RESULTS

Changes in transcript profiles, alternative splicing patterns and splicing-related gene categories were identified. Engineered mice over-expressing the protective AChE-R splice variant showed less total changes but more splicing-related ones than hypersensitive AChE-S over-expressors with similarly increased hydrolytic activities. Following MPTP exposure, the substantia nigra and prefrontal cortex (PFC) of both strains showed a nuclear increase in the splicing factor ASF/SF2 protein. Furthermore, intravenous injection with highly purified recombinant human AChE-R changed transcript profiles in the striatum.

CONCLUSIONS

Our findings are compatible with the working hypothesis that inherited or acquired alternative splicing deficits may promote parkinsonism, and we propose adaptive alternative splicing as a strategy for attenuating its progression.

The response of HMGA1 to changes in oxygen availability is evolutionarily conserved

Zebrafish embryos have evolved to cope with hypoxia during development. This includes the ability to completely suspend embryo development for extended periods until normoxia is restored. However, only a limited number of studies have examined the gene regulatory responses of zebrafish embryos to hypoxia. The High Mobility Group A1 protein encoded by the mammalian gene HMGA1 is widely expressed during embryo development but not in adults. Its expression can be induced in adult neurons by hypoxia/oxidative stress and it is commonly reactivated in many types of cancer. We report the identification by phylogenetic and conserved synteny analyses of an HMGA1 orthologue in zebrafish, hmga1 (hmg-i/y) and analysis of sodium azide as a chemical agent for inducing hypoxia-like responses in zebrafish embryos including temporary suspension of development ("suspended animation"). Evidence was only found for the existence of the "a" isoform of HMGA1 in fish. The "b" and "c" isoforms were not detected. We show that zebrafish hmga1 is expressed in a manner similar to in mammals including its induction by hypoxia during hatching stage and in adult zebrafish brain. However, earlier during development, hypoxia causes a decrease in hmga1 transcript levels. By analysis of conservation of the HMGA1a isoform binding site in zebrafish psen2 gene transcripts, we predict that a zebrafish equivalent of the PS2V isoform of human PSEN2 is not formed and we support this by RT-PCR analyses. Thus, analysis of hmga1 function in zebrafish embryogenesis may be valuable for understanding its wider role in vertebrate development, cancer and cellular responses to hypoxia but not for analysis of the action of HMGA1 in PS2V formation.

It all starts at the ends: multifaceted involvement of C- and N-terminally modified cholinesterases in Alzheimer's disease

In Alzheimer's disease (AD), premature demise of acetylcholine-producing neurons and the consequent decline of cholinergic transmission associate with the prominent cognitive impairments of affected individuals. However, the enzymatic activities of acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) are altered rather late in the disease progress. This raised questions regarding the causal involvement of AChE and BChE in AD. Importantly, single nucleotide polymorphisms (SNPs), alternative splicing, and alternate promoter usage generate complex expression of combinatorial cholinesterase (ChE) variants, which called for testing the roles of specific variants in AD pathogenesis. We found accelerated amyloid fibril formation in engineered mice with enforced over-expression of the AChE-S splice variant which includes a helical C-terminus. In contrast, the AChE-R variant, which includes a naturally unfolded C-terminus, attenuated the oligomerization of amyloid fibrils and reduced amyloid plaque formation and toxicity. An extended N-terminus generated by an upstream promoter enhanced the damage caused by N-AChE-S, which in cell cultures induced caspases and GSK3 activation, tau hyperphosphorylation, and apoptosis. In the post-mortem AD brain, we found reduced levels of the neuroprotective AChE-R and increased levels of the neurotoxic N-AChE-S, suggesting bimodal contribution to AD progress. Finally, local unwinding of the α-helical C-terminal BChE peptide and loss of function of the pivotal tryptophan at its position 541 impair amyloid fibril attenuation by the common BChE-K variant carrying the A539T substitution, in vitro. Together, our results point to causal yet diverse involvement of the different ChEs in the early stages of AD pathogenesis. Harnessing the neuroprotective variants while reducing the levels of damaging ones may hence underlie the development of novel therapeutics.

Yohimbine increases impulsivity through activation of cAMP response element binding in the orbitofrontal cortex

BACKGROUND

Stress can increase impulsivity and has a negative impact on psychiatric outcome. Norepinephrine is heavily implicated in responses to stress, and the alpha(2) antagonist yohimbine is used clinically to study this aspect of the stress response. Yohimbine induces mild anxiety and increases impulsivity in healthy volunteers but has more detrimental effects in some psychiatric populations, triggering mania in bipolar patients and drug craving in substance-dependent individuals. Understanding the mechanism by which yohimbine affects brain function could provide insight into the heightened reaction to stress in these patients.

METHODS

Yohimbine's effects were assessed in rats using the five-choice serial reaction time test of attention and impulse control. We then examined whether yohimbine altered activity of cyclic adenosine monophosphate response element binding (CREB) protein-a transcription factor implicated in the stress response-in brain areas that regulate impulsivity. The behavioral consequences of any changes in CREB activity were subsequently assessed using viral-mediated gene transfer to regionally overexpress CREB or the dominant negative antagonist mCREB.

RESULTS

Yohimbine increased impulsive responding in rats and selectively increased CREB phosphorylation within the orbitofrontal cortex but not medial prefrontal cortex or nucleus accumbens. Overexpressing mCREB within the orbitofrontal cortex blocked yohimbine's effects on impulsivity, whereas overexpressing CREB in this region increased impulsive responding and potentiated the proimpulsive actions of yohimbine.

DISCUSSION

These data suggest a novel molecular mechanism contributing to impulsivity that may be sensitive to stress. Such findings may improve our understanding of the neurobiological pathways linking the response to stress and impulsivity in both healthy and psychiatric populations.

Molecular pathology of Lewy body diseases

Lewy body diseases are characterized by the presence of Lewy bodies, alpha-synuclein(AS)-positive inclusions in the brain. Since their main component is conformationally modified AS, aggregation of the latter is thought to be a key pathogenic event in these diseases. The analysis of inclusion body constituents gives additional information about pathways also involved in the pathology of synucleinopathies. Widespread mitochondrial dysfunction is very closely related to disease development. The impairment of protein degradation pathways, including both the ubiquitin-proteasome system and the autophagy-lysosome pathway also play an important role during the development of Lewy body diseases. Finally, differential expression changes of isoforms corresponding to genes primarily involved in Lewy body formation point to alternative splicing as another important mechanism in the development of Parkinson’s disease, as well as dementia with Lewy bodies. The present paper attempts to give an overview of recent molecular findings related to the pathogenesis of Lewy body diseases.

The N-terminus of presenilin-2 increases single channel activity of brain ryanodine receptors through direct protein-protein interaction

Presenilin-1 (PS1) and presenilin-2 (PS2) form the catalytic core in gamma-secretase complexes and mutations in these proteins result in aberrant cleavage of amyloid precursor protein leading to accumulation of the beta-amyloid in the brain of familial Alzheimer Disease patients. PS2 possesses a hydrophilic cytoplasmic N-terminal domain (PS2 NTF1-87) dispensable for gamma-secretase activity with physiological functions yet to be determined. The effects of this soluble 87 amino acid fragment of mouse PS2 on single channel activity of mouse brain ryanodine receptors (RyR) were determined. PS2 NTF1-87 application to the cytoplasmic side of the RyR significantly increased single channel activity by favoring higher sublevel openings. The Ca(2+) activation and desensitization ranges for RyRs were unchanged. We demonstrate facilitation of RyR gating by PS2 NTF1-87, which might represent a general mechanism of RyR regulation by presenilins potentially prone to be affected by mutations or external stimuli contributing to the development of neurodegenerative diseases.

Interference with splicing of Presenilin transcripts has potent dominant negative effects on Presenilin activity

Missense mutations in the PRESENILIN1 (PSEN1) gene frequently underlie familial Alzheimer's disease (FAD). Nonsense and most splicing mutations result in the synthesis of truncated peptides, and it has been assumed that truncated PSEN1 protein is functionless so that heterozygotes for these mutations are unaffected. Some FAD mutations affecting PSEN1 mRNA splicing cause loss of exon 8 or 9 sequences while maintaining the reading frame. We attempted to model these exon-loss mutations in zebrafish embryos by injecting morpholino antisense oligonucleotides (morpholinos) directed against splice acceptor sites in zebrafish psen1 transcripts. However, this produced cryptic changes in splicing potentially forming mRNAs encoding truncated presenilin proteins. Aberrant splicing in the region between exons 6 and 8 produces potent dominant negative effects on Psen1 protein activity, including Notch signalling, and causes a hydrocephalus phenotype. Reductions in Psen1 activity feedback positively to increase psen1 transcription through a mechanism apparently independent of gamma-secretase. We present evidence that the dominant negative effects are mediated through production of truncated Psen1 peptides that interfere with the normal activity of both Psen1 and Psen2. Mutations causing such truncations would be dominant lethal in embryo development. Somatic cellular changes in ageing cells that interfere with PSEN1 splicing, or otherwise cause protein truncation, might contribute to sporadic Alzheimer's disease, cancer and other diseases.

Human MIP synthase splice variants in bipolar disorder

OBJECTIVES

Alternative splicing allows the production of multiple gene products with different functions from a given sequence, affecting cellular function control. Tissue-specific splicing is most prevalent in the brain. We therefore investigate whether splice variants contribute to complex psychiatric disorders. A database search suggested that the myo-inositol-1-phosphate (MIP) synthase gene, possibly involved in pathophysiology of bipolar disorder, has splice variants.

METHODS

Human RNA was purified from lymphocytes and postmortem brain. MIP synthase alternative splice variants were amplified using reverse transcription-polymerase chain reaction.

RESULTS

The bioinformatics finding was confirmed in both tissues. No difference in lymphocyte MIP synthase mRNA splice-variant levels was found between bipolar patients and controls. However, patients with family history of a major psychiatric disorder had significantly higher levels of the variant lacking exons 3 and 4 versus patients with no family history and controls.

CONCLUSIONS

As alternative splicing may be a mechanism by which the approximately 30,000 genes are amplified in mammalian brain, further studies with other candidate genes for psychiatric disorders are needed.

Identification and characterization of a new alpha-synuclein isoform and its role in Lewy body diseases

Alternative splicing is an important mechanism to generate a large number of mRNAs, thus increasing proteome diversity and tissue specificity. Three transcript variants of alpha-synuclein, a neuronal protein mainly involved in synapses, have been described so far. Whereas alpha-synuclein 140 is the whole and main transcript, alpha-synuclein 112 and 126 are short proteins that result from in-frame deletions of exons 3 and 5, respectively. Because the aforesaid alpha-synuclein isoforms show differential expression changes in Lewy body diseases (LBDs), in the present work, we searched for a fourth alpha-synuclein isoform and studied its expression levels in LBD brains. By using isoform-specific primers, isoform co-amplification and direct sequencing, we identified alpha-synuclein 98, which lacks exons 3 and 5. mRNA expression analyses in non-neuronal tissue revealed that alpha-synuclein 98 is a brain-specific splice variant with varying expression levels in different areas of fetal and adult brain. Additionally, we studied alpha-synuclein 98 expression levels by real-time semi-quantitative RT-PCR in the frontal cortices of LBD patients and compared them with those of Alzheimer disease (AD) patients and control subjects. Overexpression of alpha-synuclein 98 in LBD and AD brains would indicate its specific involvement in the pathogenesis of these neurodegenerative disorders.

HMGA1a: sequence-specific RNA-binding factor causing sporadic Alzheimer's disease-linked exon skipping of presenilin-2 pre-mRNA

Aberrant exon 5 skipping of presenilin-2 (PS2) pre-mRNA produces a deleterious protein isoform PS2V, which is almost exclusively observed in the brains of sporadic Alzheimer's disease patients. PS2V over-expression in vivo enhances susceptibility to various endoplasmic reticulum (ER) stresses and increases production of amyloid-beta peptides. We previously purified and identified high mobility group A protein 1a (HMGA1a) as a trans-acting factor responsible for aberrant exon 5 skipping. Using heterologous pre-mRNAs, here we demonstrate that a specific HMGA1a-binding sequence in exon 5 adjacent to the 5' splice site is necessary for HMGA1a to inactivate the 5' splice site. An aberrant HMGA1a-U1 snRNP complex was detected on the HMGA1a-binding site adjacent to the 5' splice site during the early splicing reaction. A competitor 2'-O-methyl RNA (2'-O-Me RNA) consisting of the HMGA1a-binding sequence markedly repressed exon 5 skipping of PS2 pre-mRNA in vitro and in vivo. Finally, HMGA1a-induced cell death under ER stress was prevented by transfection of the competitor 2'-O-Me RNA. These results provide insights into the molecular basis for PS2V-associated neurodegenerative diseases that are initiated by specific RNA binding of HMGA1a.

Alzheimer disease: amyloidogenesis, the presenilins and animal models

Alzheimer's disease is the most prevalent form of dementia. Neuropathogenesis is proposed to be a result of the accumulation of amyloid beta peptides in the brain together with oxidative stress mechanisms and neuroinflammation. The presenilin proteins are central to the gamma-secretase cleavage of the amyloid prescursor protein (APP), releasing the amyloid beta peptide. Point mutations in the presenilin genes lead to cases of familial Alzheimer's disease by increasing APP cleavage resulting in excess amyloid beta formation. This review discusses the molecular mechanism of Alzheimer's disease with a focus on the presenilin genes. Alternative splicing of transcripts from these genes and how these may function in several disease states is discussed. There is an emphasis on the importance of animal models in elucidating the molecular mechanisms behind the development of Alzheimer's disease and how the zebrafish, Danio rerio, can be used as a model organism for analysis of presenilin function and Alzheimer's disease pathogenesis.

Mitochondrial damage modulates alternative splicing in neuronal cells: implications for neurodegeneration

Mitochondrial damage is linked to many neurodegenerative conditions, such as Parkinson's disease, Alzheimer's disease and amyotrophic lateral sclerosis. These diseases are associated with changes in the splicing pattern of individual mRNAs. Here, we tested the hypothesis that mitochondrial damage modulates alternative splicing, not only of a few mRNAs, but in a general manner. We incubated cultured human neuroblastoma cells with the chemical agent paraquat (a neurotoxin that interferes with mitochondrial function, causing energy deficit and oxidative stress) and analysed the splicing pattern of 13 genes by RT-PCR. For all mRNAs that are alternatively spliced, we observed a dose- and time-dependent increase of the smaller isoforms. In contrast, splicing of all constitutive splicing exons that we monitored did not change. Using other drugs, we show that the modulation of alternative splicing correlates with ATP depletion, not with oxidative stress. Such drastic changes in alternative splicing are not observed in cell lines of non-neuronal origin, suggesting a selective susceptibility of neuronal cells to modulation of splicing. As a significant percentage of all mammalian mRNAs undergo alternative splicing, we predict that mitochondrial failure will unbalance a vast number of isoform equilibriums, which would give an important contribution to neurodegeneration.

Transcriptional regulation of APH-1A and increased gamma-secretase cleavage of APP and Notch by HIF-1 and hypoxia

The proteolytic cleavage of Alzheimer beta-amyloid precursor protein (APP) and signaling receptor Notch is mediated by the PS/gamma-secretase complex, which consists of presenilins, nicastrin, APH-1, and PEN-2. Although the four components are known to coordinately regulate each other at the protein level, information regarding their transcription regulation is scarce. Here we characterized the 5'-flanking region of the human APH-1A gene and identified a 271-bp fragment containing the transcription initiation site for the promoter activity. Sequence analysis, mutagenesis, and gel shift studies revealed a binding of AP4 and HIF-1 to the promoter, which affects the promoter activity. Activation of HIF-1 by short-term NiCl2 treatments (a condition of chemical hypoxia) dramatically increased APH-1A mRNA and protein expression, accompanied by increased secretion of Abeta and decreased APP CTFs formation, indicative of an increase in gamma-secretase activity. NiCl2 treatments had little effect on APP and the other three components of the gamma-secretase complex. The cellular concentration of Notch intracellular domain (NICD) was also increased by the hypoxic treatment. Our results demonstrate that APH-1A expression and the gamma-secretase mediated Abeta and Notch NICD generation are regulated by HIF-1, and the specific control of APH-1A expression may imply physiological functions uniquely assigned to APH-1A.