Transcriptional regulation of the murine Presenilin-2 gene reveals similarities and differences to its human orthologue

Tau mRNA Metabolism in Neurodegenerative Diseases: A Tangle Journey

Tau proteins are known to be mainly involved in regulation of microtubule dynamics. Besides this function, which is critical for axonal transport and signal transduction, tau proteins also have other roles in neurons. Moreover, tau proteins are turned into aggregates and consequently trigger many neurodegenerative diseases termed tauopathies, of which Alzheimer’s disease (AD) is the figurehead. Such pathological aggregation processes are critical for the onset of these diseases. Among the various causes of tau protein pathogenicity, abnormal tau mRNA metabolism, expression and dysregulation of tau post-translational modifications are critical steps. Moreover, the relevance of tau function to general mRNA metabolism has been highlighted recently in tauopathies. In this review, we mainly focus on how mRNA metabolism impacts the onset and development of tauopathies. Thus, we intend to portray how mRNA metabolism of, or mediated by, tau is associated with neurodegenerative diseases.

Alternative promoter usage generates novel shorter MAPT mRNA transcripts in Alzheimer's disease and progressive supranuclear palsy brains

Alternative promoter usage is an important mechanism for transcriptome diversity and the regulation of gene expression. Indeed, this alternative usage may influence tissue/subcellular specificity, protein translation and function of the proteins. The existence of an alternative promoter for MAPT gene was considered for a long time to explain differential tissue specificity and differential response to transcription and growth factors between mRNA transcripts. The alternative promoter usage could explain partly the different tau proteins expression patterns observed in tauopathies. Here, we report on our discovery of a functional alternative promoter for MAPT, located upstream of the gene's second exon (exon 1). By analyzing genome databases and brain tissue from control individuals and patients with Alzheimer's disease or progressive supranuclear palsy, we identified novel shorter transcripts derived from this alternative promoter. These transcripts are increased in patients' brain tissue as assessed by 5'RACE-PCR and qPCR. We suggest that these new MAPT isoforms can be translated into normal or amino-terminal-truncated tau proteins. We further suggest that activation of MAPT's alternative promoter under pathological conditions leads to the production of truncated proteins, changes in protein localization and function, and thus neurodegeneration.

Early Growth Response 1 (Egr-1) Is a Transcriptional Activator of β-Secretase 1 (BACE-1) in the Brain

Accumulation of amyloid-β peptide (Aβ) in the brain is regarded as central to Alzheimer's disease (AD) pathogenesis. Aβ is generated by a sequential cleavage of amyloid precursor protein (APP) by β-secretase 1 (BACE-1) followed by γ-secretase. BACE-1 cleavage of APP is the committed step in Aβ synthesis. Understanding the mechanism by which BACE-1 is activated leading to Aβ synthesis in the brain can provide better understanding of AD pathology and help to develop novel therapies. In this study, we found that the levels of Aβ and BACE-1 are significantly reduced in the brains of mice lacking transcription factor early growth response 1 (Egr-1) when compared with the WT. We demonstrate that in COS-7 cells, Egr-1 binds to the BACE-1 promoter and activates BACE-1 transcription. In rat hippocampal primary neurons, overexpression of Egr-1 induces BACE-1 expression, activates BACE-1, promotes amyloidogenic APP processing, and enhances Aβ synthesis. In mouse hippocampal primary neurons, knockdown of BACE-1 almost completely blocks Egr-1-induced amyloidogenic APP processing and Aβ synthesis. Our data indicate that Egr-1 promotes Aβ synthesis via transcriptional activation of BACE-1 and suggest that Egr-1 plays role in activation of BACE-1 and acceleration of Aβ synthesis in AD brain. Egr-1 is a potential therapeutic target for AD.

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.

Genome-wide approaches reveal EGR1-controlled regulatory networks associated with neurodegeneration

Early growth response gene 1 (Egr1) is a member of the immediate early gene (IEG) family of transcription factors and plays a role in memory formation. To identify EGR1 target genes in brain of Alzheimer's disease (AD) model mice - APP23, we applied chromatin immunoprecipitation (ChIP) followed by high-throughput DNA sequencing (ChIP-seq). Functional annotation of genes associated with EGR1 binding revealed a set of related networks including synaptic vesicle transport, clathrin-mediated endocytosis (CME), intracellular membrane fusion and transmission of signals elicited by Ca(2+) influx. EGR1 binding is associated with significant enrichment of activating chromatin marks and appears enriched near genes that are up-regulated in the brains of APP23 mice. Among the putative EGR1 targets identified and validated in this study are genes related to synaptic plasticity and transport of proteins, such as Arc, Grin1, Syn2, Vamp2 and Stx6, and genes implicated in AD such as Picalm, Psen2 and App. We also demonstrate a potential regulatory link between EGR1 and its newly identified targets in vivo, since conditions that up-regulate Egr1 levels in brain, such as a spatial memory test, also lead to increased expression of the targets. On the other hand, protein levels of EGR1 and ARC, SYN2, STX6 and PICALM are significantly lower in the brain of adult APP mice than in age-matched wild type animals. The results of this study suggest that EGR1 regulates the expression of genes involved in CME, vesicular transport and synaptic transmission that may be critical for AD pathogenesis.

Parkin differently regulates presenilin-1 and presenilin-2 functions by direct control of their promoter transcription

We previously established that besides its canonical function as E3-ubiquitin ligase, parkin also behaves as a transcriptional repressor of p53. Here we show that parkin differently modulates presenilin-1 and presenilin-2 expression and functions at transcriptional level. Thus, parkin enhances/reduces the protein expression, promoter activity and mRNA levels of presenilin-1 and presenilin-2, respectively, in cells and in vivo. This parkin-associated function is independent of its ubiquitin-ligase activity and remains unrelated to its capacity to repress p53. Accordingly, physical interaction of endogenous or overexpressed parkin with presenilins promoters is demonstrated by chromatin immunoprecipitation assays (ChIP). Furthermore, we identify a consensus sequence, the deletion of which abolishes parkin-dependent modulation of presenilins-1/2 and p53 promoter activities. Interestingly, electrophoretic mobility shift assays (EMSA) revealed a physical interaction between this consensus sequence and wild-type but not mutated parkin. Finally, we demonstrate that the RING1-IBR-RING2 domain of parkin harbors parkin's potential to modulate presenilins promoters. This transcriptional control impacts on presenilins-associated phenotypes, since parkin increases presenilin-1-associated γ-secretase activity and reduces presenilin-2-linked caspase-3 activation. Overall, our data delineate a promoter responsive element targeted by parkin that drives differential regulation of presenilin-1 and presenilin-2 transcription with functional consequences for γ-secretase activity and cell death.

Specificity protein-1 as a critical regulator of human cystathionine gamma-lyase in smooth muscle cells

Cystathionine γ-lyase (CSE) is the major enzyme in vascular smooth muscle cells (SMCs) that catalyzes the endogenous production of H(2)S. Phenotypic switching of SMCs is affected by endogenous H(2)S level and alterations of this switching may result in vascular disorders. To date, the mechanisms underlying the alteration of CSE expression and H(2)S production in vascular proliferative diseases have been unclear. In the present study, we found that serum deprivation induced SMC differentiation marker gene expressions and increased CSE expression and H(2)S production in cultured human aorta SMCs (HASMCs). Carotid artery ligation in mice resulted in enhanced neointima formation and down-regulation of CSE expression, suggesting an important role of CSE in SMC differentiation. Transient transfection of HASMCs with human CSE (hCSE) promoter/luciferase reporter revealed that the region between -226 to +140 base pair contains the core promoter for the hCSE gene. Deletion and mutation analysis demonstrated that two specificity protein-1 (Sp1) consensus binding sites were present in the core promoter region of the hCSE gene. Incubation of HASMCs with Sp1 binding inhibitor mithramycin inhibited CSE mRNA expression in a dose-dependent manner. Overexpression of Sp1 alone was sufficient to increase the activity of the hCSE core promoter and CSE protein expression. Chromatin immunoprecipitation assay showed that the binding of Sp1 to the hCSE promoter was increased in differentiated HASMCs compared with that in proliferated HASMCs. Exogenously applied H(2)S at 100 μM stimulated SMC differentiation, which was reversed by p38 MAPK inhibitor SB203580. These results suggest that transcript factor Sp1 is a critical regulator of the hCSE expression during SMC differentiation, and CSE/H(2)S system is essential for maintenance of SMC phenotype.