Activated cAMP-response element-binding protein regulates neuronal expression of presenilin-1

The Pleiotropic Face of CREB Family Transcription Factors

cAMP responsive element-binding protein (CREB) is one of the most intensively studied phosphorylation-dependent transcription factors that provide evolutionarily conserved mechanisms of differential gene expression in vertebrates and invertebrates. Many cellular protein kinases that function downstream of distinct cell surface receptors are responsible for the activation of CREB. Upon functional dimerization of the activated CREB to cis-acting cAMP responsive elements within the promoters of target genes, it facilitates signal-dependent gene expression. From the discovery of CREB, which is ubiquitously expressed, it has been proven to be involved in a variety of cellular processes that include cell proliferation, adaptation, survival, differentiation, and physiology, through the control of target gene expression. In this review, we highlight the essential roles of CREB proteins in the nervous system, the immune system, cancer development, hepatic physiology, and cardiovascular function and further discuss a wide range of CREB-associated diseases and molecular mechanisms underlying the pathogenesis of these diseases.

BACE1 Mediates HIV-Associated and Excitotoxic Neuronal Damage Through an APP-Dependent Mechanism

HIV-associated neurocognitive disorders (HANDs) share common symptoms with Alzheimer's disease (AD), which is characterized by amyloid-β (Aβ) plaques. Plaques are formed by aggregation of Aβ oligomers, which may be the toxic species in AD pathogenesis, and oligomers are generated by cleavage of amyloid precursor protein (APP) by β-site amyloid precursor protein cleaving enzyme 1 (BACE1). BACE1 inhibitors reverse neuronal loss and cognitive decline in animal models of AD. Although studies have also found evidence of altered APP processing in HIV⁺ patients, it is unknown whether increased BACE1 expression or Aβ oligomer production is a common neuropathological feature of HAND. Moreover, it is unknown whether BACE1 or APP is involved in the excitotoxic, NMDAR-dependent component of HIV-associated neurotoxicity in vitro Herein, we hypothesize that HIV-associated neurotoxicity is mediated by NMDAR-dependent elevation of BACE1 and subsequent altered processing of APP. Supporting this, we observed elevated levels of BACE1 and Aβ oligomers in CNS of male and female HIV⁺ patients. In a model of HIV-associated neurotoxicity in which rat neurons are treated with supernatants from HIV-infected human monocyte-derived macrophages, we observed NMDAR-dependent elevation of BACE1 protein. NMDA treatment also increased BACE1 and both pharmacological BACE1 inhibition and genetic loss of APP were partially neuroprotective. Moreover, in APP knock-out (APP^-/-) mouse neurons, NMDA-induced toxicity was BACE1 independent, indicating that cytotoxicity of BACE1 is dependent upon APP cleavage. Our findings suggest that increased BACE1 and the resultant Aβ oligomer production may contribute to HIV-associated neuropathogenesis and inhibition of BACE1 could have therapeutic potential in HANDs.SIGNIFICANCE STATEMENT HIV-associated neurocognitive disorders (HANDs) represent a range of cognitive impairments affecting ∼50% of HIV⁺ individuals. The specific causes of HAND are unknown, but evidence suggests that HIV-infected macrophage infiltration into the brain may cause neuronal damage. Herein, we show that neurons treated with conditioned media from HIV-infected macrophages have increased expression of β-site amyloid precursor protein cleaving enzyme 1 (BACE1), a protein implicated in Alzheimer's disease pathogenesis. Moreover, inhibition of BACE1 prevented neuronal loss after conditioned media exposure, but had no effect on HIV-associated neurotoxicity in neurons lacking its cleavage target amyloid precursor protein. We also observed increased BACE1 expression in HIV⁺ patient brain tissue, confirming the potential relevance of BACE1 as a therapeutic target in HANDs.

CRTC1 mediates preferential transcription at neuronal activity-regulated CRE/TATA promoters

Gene expression mediated by the transcription factor cAMP-responsive element-binding protein (CREB) is essential for a wide range of brain processes. The transcriptional coactivartor CREB-regulated transcription coactivator-1 (CRTC1) is required for efficient induction of CREB target genes during neuronal activity. However, the mechanisms regulating induction of specific CREB/CRTC1-dependent genes during neuronal activity remain largely unclear. Here, we investigated the molecular mechanisms regulating activity-dependent gene transcription upon activation of the CREB/CRTC1 signaling pathway in neurons. Depolarization and cAMP signals induce preferential transcription of activity-dependent genes containing promoters with proximal CRE/TATA sequences, such as c-fos, Dusp1, Nr4a1, Nr4a2 and Ptgs2, but not genes with proximal CRE/TATA-less promoters (e.g. Nr4a3, Presenilin-1 and Presenilin-2). Notably, biochemical and chromatin immunoprecipitation analyses reveal constitutive binding of CREB to target gene promoters in the absence of neuronal activity, whereas recruitment of CRTC1 to proximal CRE/TATA promoters depends on neuronal activity. Neuronal activity induces rapid CRTC1 dephosphorylation, nuclear translocation and binding to endogenous CREB. These results indicate that neuronal activity induces a preferential binding of CRTC1 to the transcriptional complex in CRE/TATA-containing promoters to engage activity-dependent transcription in neurons.

Synaptic therapy in Alzheimer's disease: a CREB-centric approach

Therapeutic attempts to cure Alzheimer's disease (AD) have failed, and new strategies are desperately needed. Motivated by this reality, many laboratories (including our own) have focused on synaptic dysfunction in AD because synaptic changes are highly correlated with the severity of clinical dementia. In particular, memory formation is accompanied by altered synaptic strength, and this phenomenon (and its dysfunction in AD) has been a recent focus for many laboratories. The molecule cyclic adenosine monophosphate response element-binding protein (CREB) is at a central converging point of pathways and mechanisms activated during the processes of synaptic strengthening and memory formation, as CREB phosphorylation leads to transcription of memory-associated genes. Disruption of these mechanisms in AD results in a reduction of CREB activation with accompanying memory impairment. Thus, it is likely that strategies aimed at these mechanisms will lead to future therapies for AD. In this review, we will summarize literature that investigates 5 possible therapeutic pathways for rescuing synaptic dysfunction in AD: 4 enzymatic pathways that lead to CREB phosphorylation (the cyclic adenosine monophosphate cascade, the serine/threonine kinases extracellular regulated kinases 1 and 2, the nitric oxide cascade, and the calpains), as well as histone acetyltransferases and histone deacetylases (2 enzymes that regulate the histone acetylation necessary for gene transcription).

Derailed intraneuronal signalling drives pathogenesis in sporadic and familial Alzheimer's disease

Although a wide variety of genetic and nongenetic Alzheimer's disease (AD) risk factors have been identified, their role in onset and/or progression of neuronal degeneration remains elusive. Systematic analysis of AD risk factors revealed that perturbations of intraneuronal signalling pathways comprise a common mechanistic denominator in both familial and sporadic AD and that such alterations lead to increases in Aβ oligomers (Aβo) formation and phosphorylation of TAU. Conversely, Aβo and TAU impact intracellular signalling directly. This feature entails binding of Aβo to membrane receptors, whereas TAU functionally interacts with downstream transducers. Accordingly, we postulate a positive feedback mechanism in which AD risk factors or genes trigger perturbations of intraneuronal signalling leading to enhanced Aβo formation and TAU phosphorylation which in turn further derange signalling. Ultimately intraneuronal signalling becomes deregulated to the extent that neuronal function and survival cannot be sustained, whereas the resulting elevated levels of amyloidogenic Aβo and phosphorylated TAU species self-polymerizes into the AD plaques and tangles, respectively.

Involvement of the Notch pathway in terminal astrocytic differentiation: role of PKA

The Notch pathway is a highly conserved signaling system essential for modulating neurogenesis and promoting astrogenesis. Similarly, the cAMP signaling cascade can promote astrocytic commitment in several cell culture models, such as the C6 glioma cell line. These cells have the capacity to differentiate into oligodendrocytes or astrocytes, characteristics that allow their use as a glial progenitor model. In this context, we explore here the plausible involvement of cAMP in Notch-dependent signal transactions. The exposure of C6 cells to a non-hydrolysable cAMP analogue resulted in a sustained augmentation of Notch activity, as detected by nuclear translocation of its intracellular domain portion (NICD) and transcriptional activity. The cAMP effect is mediated through the activation of the γ-secretase complex, responsible for Notch cleavage and is sensitive to inhibitors of the cAMP-dependent protein kinase, PKA. As expected, Notch cleavage and nuclear translocation resulted in the up-regulation of the mRNA levels of one of its target genes, the transcription factor Hair and enhancer of split 5. Moreover, the glutamate uptake activity, as well as the expression of astrocytic markers such as glial fibrillary acidic protein, S100β protein and GLAST was also enhanced in cAMP-exposed cells. Our results clearly suggest that during the process of C6 astrocytic differentiation, cAMP activates the PKA/γ-secretase/NICD/RBPJκ pathway and Notch1 expression, leading to transcriptional activation of the genes responsible for glial progenitor cell fate decision.

Transcriptional regulation and its misregulation in Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative disorder characterized by loss of memory and cognitive function. A key neuropathological event in AD is the accumulation of amyloid-β (Aβ) peptide. The production and clearance of Aβ in the brain are regulated by a large group of genes. The expression levels of these genes must be fine-tuned in the brain to keep Aβ at a balanced amount under physiological condition. Misregulation of AD genes has been found to either increase AD risk or accelerate the disease progression. In recent years, important progress has been made in uncovering the regulatory elements and transcriptional factors that guide the expression of these genes. In this review, we describe the mechanisms of transcriptional regulation for the known AD genes and the misregualtion that leads to AD susceptibility.

Alcohol and NMDA receptor: current research and future direction

The brain is one of the major targets of alcohol actions. Most of the excitatory synaptic transmission in the central nervous system is mediated by N-methyl-D-aspartate (NMDA) receptors. However, one of the most devastating effects of alcohol leads to brain shrinkage, loss of nerve cells at specific regions through a mechanism involving excitotoxicity, oxidative stress. Earlier studies have indicated that chronic exposure to ethanol both in vivo and in vitro, increases NR1 and NR2B gene expression and their polypeptide levels. The effect of alcohol and molecular changes on the regulatory process, which modulates NMDAR functions including factors altering transcription, translation, post-translational modifications, and protein expression, as well as those influencing their interactions with different regulatory proteins (downstream effectors) are incessantly increasing at the cellular level. Further, I discuss the various genetically altered mice approaches that have been used to study NMDA receptor subunits and their functional implication. In a recent countable review, epigenetic dimension (i.e., histone modification-induced chromatin remodeling and DNA methylation, in the process of alcohol related neuroadaptation) is one of the key molecular mechanisms in alcohol mediated NMDAR alteration. Here, I provide a recount on what has already been achieved, current trends and how the future research/studies of the NMDA receptor might lead to even greater engagement with many possible new insights into the neurobiology and treatment of alcoholism.

Presenilin enhancer-2 gene: identification of a novel promoter mutation in a patient with early-onset familial Alzheimer's disease

γ-Secretase proteins complex cleaves the amyloid precursor protein (APP) to generate amyloid-β (Aβ) peptides. Considerable evidence suggests that alterations in genes encoding these proteins exert their influence on the pathogenesis of familial Alzheimer's disease (FAD). Presenilin enhancer-2 gene (PEN-2) is a necessary component of the γ-Secretase complex. Recently, it has been shown that PEN-2 mutations could be involved in Alzheimer's disease (AD). We performed a mutational screening of all PEN-2 coding and promoter regions in a FAD cohort derived from Southern Italy. Four hundred and fifty-two subjects (FAD: 97;

CONTROLS

355) were recruited for this study. We identified for the first time in a key region necessary for the promoter activity a novel 3 bp deletion in a subject with early-FAD. Our genetic data demonstrate that the mutant allele may influence the transcriptional activity of the PEN-2 gene. Although the effective role of the PEN-2 promoter deletion in AD is not entirely clear, these findings might lead to more studies on its functional and genetic role.

Central angiotensin II stimulation promotes β amyloid production in Sprague Dawley rats

Background

Stress and various stress hormones, including catecholamines and glucocorticoids, have recently been implicated in the pathogenesis of Alzheimer's disease (AD), which represents the greatest unresolved medical challenge in neurology. Angiotensin receptor blockers have shown benefits in AD and prone-to-AD animals. However, the mechanisms responsible for their efficacy remain unknown, and no studies have directly addressed the role of central angiotensin II (Ang II), a fundamental stress hormone, in the pathogenesis of AD. The present study focused on the role of central Ang II in amyloidogenesis, the critical process in AD neuropathology, and aimed to provide direct evidence for the role of this stress hormone in the pathogenesis of AD.

Methodology/Principal Findings

Increased central Ang II levels during stress response were modeled by intracerebroventricular (ICV) administration of graded doses of Ang II (6 ng/hr low dose, 60 ng/hr medium dose, and 600 ng/hr high dose, all delivered at a rate of 0.25 µl/hr) to male Sprague Dawley rats (280–310 g) via osmotic pumps. After 1 week of continuous Ang II infusion, the stimulation of Ang II type 1 receptors was accompanied by the modulation of amyloid precursor protein, α-, β-and γ-secretase, and increased β amyloid production. These effects could be completely abolished by concomitant ICV infusion of losartan, indicating that central Ang II played a causative role in these alterations.

Conclusions/Significance

Central Ang II is essential to the stress response, and the results of this study suggest that increased central Ang II levels play an important role in amyloidogenesis during stress, and that central Ang II-directed stress prevention and treatment might represent a novel anti-AD strategy.

Regulation of Sertoli-germ cell adhesion and sperm release by FSH and nonclassical testosterone signaling

Testosterone and FSH act in synergy to produce the factors required to maximize the production of spermatozoa and male fertility. However, the molecular mechanisms by which these hormones support spermatogenesis are not well established. Recently, we identified a nonclassical mechanism of testosterone signaling in cultured rat Sertoli cells. We found that testosterone binding to the androgen receptor recruits and activates Src tyrosine kinase. Src then causes the activation of the epidermal growth factor receptor, which results in the phosphorylation and activation of the ERK MAPK and the cAMP response element-binding protein transcription factor. In this report, we find that FSH inhibits testosterone-mediated activation of ERK and the MAPK pathway in Sertoli cells via the protein kinase A-mediated inhibition of Raf kinase. In addition, FSH, as well as inhibitors of Src and ERK kinase activity, reduced germ cell attachment to Sertoli cells in culture. Using pathway-specific androgen receptor mutants we found that the nonclassical pathway is required for testosterone-mediated increases in germ cell attachment to Sertoli cells. Studies of seminiferous tubule explants determined that Src kinase, but not ERK kinase, activity is required for the release of sperm from seminiferous tubule explants. These findings suggest the nonclassical testosterone-signaling pathway acts via Src and ERK kinases to facilitate the adhesion of immature germ cells to Sertoli cells and through Src to permit the release of mature spermatozoa. In contrast, FSH acts to limit testosterone-mediated ERK kinase activity and germ cell attachment.

An oxidized extracellular oxidation-reduction state increases Nox1 expression and proliferation in vascular smooth muscle cells via epidermal growth factor receptor activation

OBJECTIVE

To examine the effect of an oxidized extracellular oxidation-reduction (redox) state (E(h)) on the expression of NADPH oxidases in vascular cells.

METHODS AND RESULTS

The generation of reactive oxygen species by NADPH oxidase (Nox)-based NADPH oxidases activates redox-dependent signaling pathways and contributes to the development of "oxidative stress" in vascular disease. An oxidized plasma redox state is associated with cardiovascular disease in humans; however, the cellular mechanisms by which the extracellular redox state may cause disease are not known. Aortic segments and cultured aortic smooth muscle cells were exposed to E(h) between -150 mV (reduced) and 0 mV (oxidized) by altering the concentration of cysteine and its disulfide, cystine, the predominant redox couple in plasma. A more oxidized E(h) increased the expression of Nox1 and resulted in Nox1-dependent proliferation of smooth muscle cells. Oxidized E(h) rapidly induced epidermal growth factor receptor phosphorylation via shedding of epidermal growth factor-like ligands from the plasma membrane and caused extracellular signal-regulated kinase 1/2-dependent phosphorylation of the transcription factors activating transcription factor-1 and cAMP-response element-binding protein. Inhibition of epidermal growth factor receptor or extracellular signal-regulated kinase 1/2 activation, or addition of small interference RNA to activating transcription factor-1, prevented the increase in Nox1 expression.

CONCLUSIONS

Our results identify a novel mechanism by which extracellular oxidative stress increases expression and activity of Nox1 NADPH oxidase and contributes to vascular disease.

p53-dependent control of transactivation of the Pen2 promoter by presenilins

The senile plaques found in the brains of patients with Alzheimer's disease are mainly due to the accumulation of amyloid beta-peptides (A beta) that are liberated by gamma-secretase, a high molecular weight complex including presenilins, PEN-2, APH-1 and nicastrin. The depletion of each of these proteins disrupts the complex assembly into a functional protease. Here, we describe another level of regulation of this multimeric protease. The depletion of both presenilins drastically reduces Pen2 mRNA levels and its promoter transactivation. Furthermore, overexpression of presenilin-1 lowers Pen2 promoter transactivation, a phenotype abolished by a double mutation known to prevent presenilin-dependent gamma-secretase activity. PEN-2 expression is decreased by depletion of beta-amyloid precursor protein (APP) and increased by the APP intracellular domain (AICD). We show that AICD and APP complement for Pen2 mRNA levels in APP/APLP1-2 knockout fibroblasts. Interestingly, overexpression of presenilin-2 greatly increases Pen2 promoter transactivation. The opposite effect triggered by both presenilins was reminiscent of our previous study, which showed that these two proteins elicit antagonistic effects on p53. Therefore, we examined the contribution of p53 on Pen2 transcription. Pen2 promoter transactivation, and Pen2 mRNA and protein levels were drastically reduced in p53(-/-) fibroblasts. Furthermore, PEN-2 expression could be rescued by p53 complementation in p53- and APP-deficient cells. Interestingly, PEN-2 expression was also reduced in p53-deficient mouse brain. Overall, our study describes a p53-dependent regulation of PEN-2 expression by other members of the gamma-secretase complex, namely presenilins.

Signalling pathways in vasculogenic mimicry

Solid tumour growth is dependent on the development of an adequate blood supply. For years, sprouting angiogenesis has been considered an exclusive mechanism of tumour vascularization. However, over the last years, several other mechanisms have been identified, including vessel-co-option, intussusception, recruitment of endothelial precursor cells (EPCs) and even mechanisms that do not involve endothelial cells, a process called vasculogenic mimicry (VM). The latter describes a mechanism by which highly aggressive tumour cells can form vessel-like structures themselves, by virtue of their high plasticity. VM has been observed in several tumour types and its occurrence is strongly associated with a poor prognosis. This review will focus on signalling molecules and cascades involved in VM. In addition, we will discuss the presence of VM in relation to ongoing cancer research. Finally, we describe the clinical significance of VM regarding anti-angiogenesis treatment modalities.

AP-2alpha regulates migration of GN-11 neurons via a specific genetic programme involving the Axl receptor tyrosine kinase

Background

Neuronal migration is a crucial process that allows neurons to reach their correct target location to allow the nervous system to function properly. AP-2α is a transcription factor essential for neural crest cell migration and its mutation results in apoptosis within this cell population, as demonstrated by genetic models.

Results

We down-modulated AP-2α expression in GN-11 neurons by RNA interference and observe reduced neuron migration following the activation of a specific genetic programme including the Adhesion Related Kinase (Axl) gene. We prove that Axl is able to coordinate migration per se and by ChIP and promoter analysis we observe that its transcription is directly driven by AP-2α via the binding to one or more functional AP-2α binding sites present in its regulatory region. Analysis of migration in AP-2α null mouse embryo fibroblasts also reveals an essential role for AP-2α in cell movement via the activation of a distinct genetic programme.

Conclusion

We show that AP-2α plays an essential role in cell movement via the activation of cell-specific genetic programmes. Moreover, we demonstrate that the AP-2α regulated gene Axl is an essential player in GN-11 neuron migration.

Notch: from neural development to neurological disorders

Notch is an integral membrane protein that functions as receptor for ligands such as jagged and delta that are associated with the surface of neighboring cells. Upon ligand binding, notch is proteolytically cleaved within its transmembrane domain by presenilin-1 (the enzymatic component of the gamma-secretase complex) resulting in the release of a notch intracellular domain which translocates to the nucleus where it regulates gene expression. Notch signaling plays multiple roles in the development of the CNS including regulating neural stem cell (NSC) proliferation, survival, self-renewal and differentiation. Notch is also present in post-mitotic neurons in the adult CNS wherein its activation influences structural and functional plasticity including processes involved in learning and memory. Recent findings suggest that notch signaling in neurons, glia, and NSCs may be involved in pathological changes that occur in disorders such as stroke, Alzheimer's disease and CNS tumors. Studies of animal models suggest the potential of agents that target notch signaling as therapeutic interventions for several different CNS disorders.

Indirect regulation of presenilins in CREB-mediated transcription

Presenilins are essential for synaptic function, memory formation, and neuronal survival. Previously, we reported that expression of cAMP response element-binding protein (CREB) target genes is reduced in the cerebral cortex of presenilin (PS) conditional double knock-out (cDKO) mice. To determine whether the reduced expression of the CREB target genes in these mutant mice is due to loss of presenilin directly or secondary to the impaired neuronal activity, we established a sensitive luciferase reporter system to assess direct transcriptional regulation in cultured cells. We first used immortalized PS-deficient mouse embryonic fibroblasts (MEFs), and found that both CREB-mediated transcription and Notch-mediated HES1 transcription are decreased. However, the ubiquitin-C promoter-mediated transcription is also reduced, and among these three reporters, transfection of exogenous PS1 can rescue only the Notch-mediated HES1 transcription. Further Northern analysis revealed transcriptional alterations of Creb, ubiquitin-C, and other housekeeping genes in PS-deficient MEFs, indicating transcriptional dysregulation in these cells. We then used the Cre/loxP system to develop a postnatal PS-deficient cortical neuronal culture. Surprisingly, in these PS-null neurons, CREB-mediated transcription is not significantly decreased, and levels of total and phosphorylated CREB proteins are unchanged as well. Notch-mediated HES1 transcription is markedly reduced, and this reduction can be rescued by exogenous PS1. Together, our findings suggest that CREB-mediated transcription is regulated indirectly by PS in the adult cerebral cortex, and that attenuation of CREB target gene expression in PS cDKO mice is likely due to reduced neuronal activity in these mutant brains.

CREB binding and activity in brain: regional specificity and induction by electroconvulsive seizure

BACKGROUND

The transcription factor cyclic adenosine monophosphate response element binding protein (CREB) orchestrates diverse neurobiological processes including cell differentiation, survival, and plasticity. Alterations in CREB-mediated transcription have been implicated in numerous central nervous system (CNS) disorders including depression, anxiety, addiction, and cognitive decline. However, it remains unclear how CREB contributes to normal and aberrant CNS function, as the identity of CREB-regulated genes in brain and the regional and temporal dynamics of CREB function remain largely undetermined.

METHODS

We combined microarray and chromatin immunoprecipitation technology to analyze CREB-DNA interactions in brain. We compared the occupancy and activity of CREB at gene promoters in rat frontal cortex, hippocampus, and striatum before and after a rodent model of electroconvulsive therapy.

RESULTS

Our analysis identified >860 CREB binding sites in rat brain. We identified multiple genomic loci enriched with CREB binding sites and find that CREB-occupied transcripts interact extensively to promote cell proliferation, plasticity, and resiliency. We discovered regional differences in CREB occupancy and activity that explain, in part, the diverse biological and behavioral outputs of CREB activity in frontal cortex, hippocampus, and striatum. Electroconvulsive seizure rapidly increased CREB occupancy and/or phosphorylation at select promoters, demonstrating that both events contribute to the temporal regulation of the CREB transcriptome.

CONCLUSIONS

Our data provide a mechanistic basis for CREB's ability to integrate regional and temporal cues to orchestrate state-specific patterns of transcription in the brain, indicate that CREB is an important mediator of the biological responses to electroconvulsive seizure, and provide global mechanistic insights into CREB's role in psychiatric and cognitive function.

Age and Sex Dependent Alteration in Presenilin Expression in Mouse Cerebral Cortex

(1) Presenilin (PS) expression is regulated by several cellular and extracellular factors which change with age and sex. Both age and sex are key risk factors for Alzheimer's disease (AD), which is linked to mutations in PS genes. (2) We have analyzed the effect of age and sex on PS expression by northern hybridization and western blot analysis using the cerebral cortex of adult (24 +/- 2 weeks) and old (65 +/- 5 weeks) mice. (3) Our results demonstrate that PS1 was downregulated and PS 2 was upregulated in old mice of both sexes. The level of PS 1 was relatively higher and that of PS 2 was lower in female than male mice of same age group. Taken together, these findings show age and sex dependent alteration in PS expression, which in turn may influence the signal transduction pathways and consequently brain functions.

Current insights into molecular mechanisms of Alzheimer disease and their implications for therapeutic approaches

During the last 10 years, a lot of progress has been made in unraveling the pathogenic cascade leading to Alzheimer disease (AD). According to the most widely accepted hypothesis, production and aggregation of the amyloid beta (Abeta) peptide plays a key role in AD, and thus therapeutic interference with these processes is the subject of intense research. However, some important aspects of the disease mechanism are not yet fully understood. There is no consensus as yet on whether the disease acts through a loss- (LOF) or a gain-of-function (GOF) mechanism. While for many years, an increased production of Abeta42 was considered to be the prime culprit for the initiation of the disease process, and accordingly Abeta42 is elevated by AD-related presenilin(PS) mutations, recent data strongly suggest that PS mutations also lead to a LOF of PS towards a plethora of its substrates including amyloid precursor protein. How this PS LOF, especially decreased Abeta40 secretion due to mutant PS, impacts on the disease pathogenesis is yet to be elucidated. Secondly, vascular abnormalities--frequently observed to co-occur with AD--might also play a critical role in the initiation and aggravation of AD pathology given that the elimination of Abeta through a vascular route is an important brain Abeta clearance mechanism and its failure leads to formation of vascular amyloidosis and dense-core plaques. In this review, we will first focus on the important issue of a LOF versus a GOF mechanism for AD due to mutant PS, as well as on the possible role of vascular damage and reduced perfusion in AD. Special emphasis will be given to some of the AD mouse models that have helped to gain insights into the disease mechanism. Secondly, considering these mechanistic insights, we will discuss some therapeutic strategies which are currently in clinical or preclinical trials for AD.

The presenilin hypothesis of Alzheimer's disease: evidence for a loss-of-function pathogenic mechanism

Dominantly inherited mutations in the genes encoding presenilins (PS) and the amyloid precursor protein (APP) are the major causes of familial Alzheimer's disease (AD). The prevailing view of AD pathogenesis posits that accumulation of beta-amyloid (Abeta) peptides, particularly Abeta42, is the central event triggering neurodegeneration. Emerging evidence, however, suggests that loss of essential functions of PS could better explain dementia and neurodegeneration in AD. First, conditional inactivation of PS in the adult mouse brain causes progressive memory loss and neurodegeneration resembling AD, whereas mouse models based on overproduction of Abeta have failed to produce neurodegeneration. Second, whereas pathogenic PS mutations enhance Abeta42 production, they typically reduce Abeta40 generation and impair other PS-dependent activities. Third, gamma-secretase inhibitors can enhance the production of Abeta42 while blocking other gamma-secretase activities, thus mimicking the effects of PS mutations. Finally, PS mutations have been identified in frontotemporal dementia, which lacks amyloid pathology. Based on these and other observations, we propose that partial loss of PS function may underlie memory impairment and neurodegeneration in the pathogenesis of AD. We also speculate that Abeta42 may act primarily to antagonize PS-dependent functions, possibly by operating as an active site-directed inhibitor of gamma-secretase.

ATF4 regulates γ-secretase activity during amino acid imbalance

Accumulation of amyloid-beta (Abeta), which is generated from amyloid precursor protein by gamma-secretase, in cerebral cortex is common and critical incident in Alzheimer disease. Specifically, presenilin is an essential for gamma-secretase activity. However, the regulation of presenilin expression, affecting gamma-secretase activity, remains obscure. We investigated mechanism controlling the expression of presenilin-1 (PS1) and gamma-secretase activity. We showed that PS1 is induced by activating transcription factor 4 (ATF4), regulated by GCN2 eukaryotic initiation factor 2alpha (eIF2alpha) kinase. A chromatin immunoprecipitation analysis and an electrophoretic mobility shift assay demonstrated that ATF4 binds to the regulatory region of human PS1 gene. Through knockdown analysis, we observed that the secretion of Abeta (1-42) and induction of gamma-secretase cofactors are controlled by ATF4. These data indicate that ATF4 is critical for gamma-secretase activity, by which ATF4 mRNA is preferentially translated in response to eIF2alpha phosphorylation.

Synaptic dysfunction and oxidative stress in Alzheimer's disease: emerging mechanisms

In this paper, we review experimental advances in molecular neurobiology of Alzheimer's disease (AD), with special emphasis on analysis of neural function of proteins involved in AD pathogenesis, their relation with several signaling pathways and with oxidative stress in neurons. Molecular genetic studies have found that mutations in APP, PS1 and PS2 genes and polymorphisms in APOE gene are implicated in AD pathogenesis. Recent studies show that these proteins, in addition to its role in beta-amyloid processing, are involved in several neuroplasticity-signaling pathways (NMDA-PKA-CREB-BDNF, reelin, wingless, notch, among others). Genomic and proteomic studies show early synaptic protein alterations in AD brains and animal models. DNA damage caused by oxidative stress is not completely repaired in neurons and is accumulated in the genes of synaptic proteins. Several functional SNPs in synaptic genes may be interesting candidates to explore in AD as genetic correlates of this synaptopathy in a "synaptogenomics" approach. Thus, experimental evidence shows that proteins implicated in AD pathogenesis have differential roles in several signaling pathways related to neuromodulation and neurotransmission in adult and developing brain. Genomic and proteomic studies support these results. We suggest that oxidative stress effects on DNA and inherited variations in synaptic genes may explain in part the synaptic dysfunction seen in AD.

Adrenomedullin/cyclic AMP pathway induces Notch activation and differentiation of arterial endothelial cells from vascular progenitors

OBJECTIVE

The acquisition of arterial or venous identity is highlighted in vascular development. Previously, we have reported an embryonic stem (ES) cell differentiation system that exhibits early vascular development using vascular endothelial growth factor (VEGF) receptor-2 (VEGFR2)-positive cells as common vascular progenitors. In this study, we constructively induced differentiation of arterial and venous endothelial cells (ECs) in vitro to elucidate molecular mechanisms of arterial-venous specification.

METHODS AND RESULTS

ECs were induced from VEGFR2+ progenitor cells with various conditions. VEGF was essential to induce ECs. Addition of 8bromo-cAMP or adrenomedullin (AM), an endogenous ligand-elevating cAMP, enhanced VEGF-induced EC differentiation. Whereas VEGF alone mainly induced venous ECs, 8bromo-cAMP (or AM) with VEGF supported substantial induction of arterial ECs. Stimulation of cAMP pathway induced Notch signal activation in ECs. The arterializing effect of VEGF and cAMP was abolished in recombination recognition sequence binding protein at the Jkappa site deficient ES cells lacking Notch signal activation or in ES cells treated with gamma-secretase inhibitor. Nevertheless, forced Notch activation by the constitutively active Notch1 alone did not induce arterial ECs.

CONCLUSIONS

Adrenomedullin/cAMP is a novel signaling pathway to activate Notch signaling in differentiating ECs. Coordinated signaling of VEGF, Notch, and cAMP is required to induce arterial ECs from vascular progenitors.

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.

Signaling pathways regulating gene expression, neuroplasticity, and neurotrophic mechanisms in the action of antidepressants: a critical overview

Regulation of gene expression represents a major component in antidepressant drug action. The effect of antidepressant treatments on the function of cAMP-responsive element binding protein (CREB), a transcription factor that regulates expression of several genes involved in neuroplasticity, cell survival, and cognition, has been extensively studied. Although there is general agreement that chronic antidepressants stimulate CREB function, conflicting results suggest that different effects may depend on drug type, drug dosage, and different experimental paradigms. CREB function is activated by a vast array of physiological stimuli, conveyed through a number of signaling pathways acting in concert, but thus far the effects of antidepressants on CREB have been analyzed mostly with regard to the cAMP-protein kinase A pathway. A growing body of data shows that other major pathways, such as the calcium/calmodulin-dependent kinase and the mitogen-activated kinase cascades, are involved in activity-dependent regulation of gene expression and may also be implicated in the mechanism of action of antidepressants. In this article the available evidence is reviewed with an attempt to identify the reasons for experimental discrepancies and possible directions for future research. Particularemphasis is given to the regulation of brain-derived neurotrophic factor (BDNF), a CREB-regulated gene, which has been implicated in both the pathophysiology and pharmacology of mood disorders. The array of different results obtained by various groups is analyzed with an eye on recent advancements in the regulation of BDNF transcription, in an attempt to understand better the mechanisms of drug action and dissect molecular requirements for faster and more efficient antidepressant treatment.

Transcriptional regulation of PEN-2, a key component of the gamma-secretase complex, by CREB

Gamma-secretase, which is responsible for the intramembranous cleavage of Alzheimer's beta-amyloid precursor protein (APP), the signaling receptor Notch, and many other substrates, is a multiprotein complex consisting of at least four components: presenilin (PS), nicastrin, APH-1, and PEN-2. Despite the fact that PEN-2 is known to mediate endoproteolytic cleavage of full-length PS and APH-1 and nicastrin are required for maintaining the stability of the complex, the detailed physiological function of each component remain elusive. Unlike that of PS, the transcriptional regulation of PEN-2, APH-1, and nicastrin has not been investigated. Here, we characterized the upstream regions of the human PEN-2 gene and identified a 238-bp fragment located 353 bp upstream of the translational start codon as the key region necessary for the promoter activity. Further analysis revealed a CREB binding site located in the 238-bp region that is essential for the transcriptional activity of the PEN-2 promoter. Mutation of the CREB site abolished the transcriptional activity of the PEN-2 promoter. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis showed the binding of CREB to the PEN-2 promoter region both in vitro and in vivo. Activation of the CREB transcriptional factor by forskolin dramatically promoted the expression of PEN-2 mRNA and protein, whereas the other components of the gamma-secretase complex remained unaffected. Forskolin treatment slightly increases the secretion of soluble APPalpha and Abeta without affecting Notch cleavage. These results demonstrate that expression of PEN-2 is regulated by CREB and suggest that the specific control of PEN-2 expression may imply additional physiological functions uniquely assigned to PEN-2.

Loss of presenilin function causes impairments of memory and synaptic plasticity followed by age-dependent neurodegeneration

Mutations in presenilins are the major cause of familial Alzheimer's disease, but the pathogenic mechanism by which presenilin mutations cause memory loss and neurodegeneration remains unclear. Here we demonstrate that conditional double knockout mice lacking both presenilins in the postnatal forebrain exhibit impairments in hippocampal memory and synaptic plasticity. These deficits are associated with specific reductions in NMDA receptor-mediated responses and synaptic levels of NMDA receptors and alphaCaMKII. Furthermore, loss of presenilins causes reduced expression of CBP and CREB/CBP target genes, such as c-fos and BDNF. With increasing age, mutant mice develop striking neurodegeneration of the cerebral cortex and worsening impairments of memory and synaptic function. Neurodegeneration is accompanied by increased levels of the Cdk5 activator p25 and hyperphosphorylated tau. These results define essential roles and molecular targets of presenilins in synaptic plasticity, learning and memory, and neuronal survival in the adult cerebral cortex.

NFkappaB regulates plasma apolipoprotein A-I and high density lipoprotein cholesterol through inhibition of peroxisome proliferator-activated receptor alpha

The levels of plasma HDL cholesterol and apoA-I in NFkappaB p50 subunit-deficient mice were significantly higher than those in wild-type mice under regular and high fat diets, without any significant difference in the level of total cholesterol. To examine the role of NFkappaBin lipid metabolism, we studied its effect on the regulation of apoA-I secretion from human hepatoma HepG2 cells. Lipopolysaccharide-induced activation of NFkappaB reduced the expression of apoA-I mRNA and protein, whereas adenovirus-mediated expression of IkappaBalpha super-repressor ameliorated the reduction. This IkappaBalpha-induced apoA-I increase was blocked by preincubation with MK886, a selective inhibitor of peroxisome proliferator-activated receptor alpha (PPARalpha), suggesting that NFkappaB inactivation induces apoA-I through activation of PPARalpha. To further support this idea, the expression of IkappaBalpha increased apoA-I promoter activity, and this increase was blocked by preincubation with MK886. Mutations in the putative PPARalpha-binding site in the apoA-I promoter or lack of the site abrogated these changes. Taking these results together, inhibition of NFkappaB increases apoA-I and HDL cholesterol through activation of PPARalpha in vivo and in vitro. Our data suggest a new aspect of lipid metabolism and may lead to a new paradigm for prevention and treatment of atherosclerotic disease.

Egr-1 upregulates the Alzheimer's disease presenilin-2 gene in neuronal cells

Regulation of the Alzheimer's disease (AD)-related gene, presenilin-2 (PSEN2), was analyzed in neuronal (SK-N-SH) and non-neuronal (human embryonic kidney 293, HEK293) cells. We show that the PSEN2 regulatory region includes two separate promoter elements, each located upstream of multiple transcription start sites in the first and second exons. The stronger upstream promoter, P1, has housekeeping characteristics: it resides in a CpG island, is TATA-less, and up to 83% of PSEN2-P1 activity depends on a stimulating protein 1 (Sp1) site at the most 5' initiation site. However, the downstream promoter P2 includes neuronal-specific elements and two sites for early growth response gene-1 (Egr-1), a transcription factor upregulated in learning paradigms and implicated in neuronal plasticity, in response to injury. We show that Egr-1 binds to PSEN-P2, and that PSEN-P2 activity is increased threefold by overexpression of Egr-1, and by 12-O-tetradecanoylphorbol-13-acetate (TPA), which induces physiological Egr-1 levels. Egr-1 represses PSEN2-P1 activity by 50% in neuronal cells, suggesting it partially shifts promoter usage from PSEN2-P1 to PSEN2-P2. This could lead to a relative increase in shorter exon 2 transcripts, which may be more efficiently translated than exon 1 transcripts. Identification of PSEN2 as an Egr-1 target suggests a link between PSEN2 expression and Egr-1-related processes, which may impact on understanding PSEN-2's physiological function and its role in Alzheimer's disease.

Protein kinase B is obligatory for follicle-stimulating hormone-induced granulosa cell differentiation

Although FSH receptors are linked to the cAMP second messenger system, additional intracellular signaling pathways appear to be required for the induction of aromatase and the LH receptor during granulosa cell differentiation. We employed adenovirus vectors to modulate specific intracellular signaling systems in undifferentiated granulosa cells to identify the signaling pathway(s) that may be involved in the FSH-mediated induction of aromatase and the LH receptor. Expression of either the constitutively activated human LH receptor D578H or the constitutively active human G(s)alpha Q227L resulted in increased cAMP production without increasing aromatase activity or mRNA levels for the LH receptor. To explore the contributions of other pathways, we expressed the constitutively activated forms MAPK kinase (MEK) and protein kinase B (PKB). Neither MEK nor PKB alone increased estrogen or progesterone production by undifferentiated granulosa cells. Stimulation of granulosa cells by FSH in the presence of the constitutively active PKB, but not MEK, led to an amplification of FSH-induced aromatase and LH receptor mRNA levels, whereas a dominant negative PKB vector completely abolished the actions of FSH. The expression of the constitutively active PKB in combination with the constitutively active LH receptor D578H, the constitutively active G(s)alpha Q227L, or 8-bromo-cAMP led to an induction of aromatase as well as LH receptor mRNA comparable to that seen in cells stimulated with FSH alone. These results demonstrate that PKB is an essential component of the FSH-mediated granulosa cell differentiation and that both PKB and G(s)alpha signaling pathways are required.

Activation of transcription of the human presenilin 1 gene by 12-O-tetradecanoylphorbol 13-acetate

We have recently identified an Ets element controlling over 90% of the basal expression of the human presenilin 1 (PS1) gene. We have also shown that Ets1 and Ets2 act as transactivators of the PS1 gene by cotransfection experiments in SK-N-SH neuronal cells. The PS1 gene is widely but differentially expressed across tissues and the expression in brain appears to be restricted to neurons. To gain further insight into the regulation of the gene we have examined the regulation of PS1 by 12-O-tetradecanoylphorbol 13-acetate (TPA). SK-N-SH neuronal cells were treated with 0.2 micro m TPA for 30 min to 24 h and the level of expression of the endogenous PS1 gene was measured by Northern blot analysis. A two- to threefold increase in the level of PS1 mRNA appeared 4-8 h after the addition of TPA. A similar increase in transcription activity was observed in nuclear run off experiments, indicating that the increased mRNA level results from an activation in the initiation of transcription of PS1. Consistently, TPA also increased the level of PS1 protein. No activation of the PS1 endogenous gene by TPA was observed in hepatoma HepG2 cells. Next we tested the effect of TPA on the expression of the PS1 promoter by transfecting fusion genes including various fragments of the PS1 promoter linked to a CAT reporter into SK-N-SH cells. TPA also stimulated the expression of the PS1CAT constructs. Generally wild type constructs -687/+178, -118/+178, -22/+178 including the short -35/+6 fragment showed a minor two- to threefold activation by TPA. Point mutations eliminating the -10 Ets motif or the -6 CREB/AP1 motif did not decrease the stimulation by TPA. Thus TPA appears to activate the transcription of the PS1 gene by a mechanism which does not require these elements.

Cyclooxygenase-2 and presenilin-1 gene expression induced by interleukin-1beta and amyloid beta 42 peptide is potentiated by hypoxia in primary human neural cells

Lipid messengers and amyloid beta (Abeta) peptides generated by cyclooxygenase-2 (COX-2) and presenilin-1 (PS1) mediate pro-inflammatory signaling and neural degeneration in Alzheimer's disease (AD) brain. This study provides data showing that the COX-2 and PS1 genes each transcribe rare, highly labile RNA species that display early response gene behavior in human neural (HN) cells in primary culture, down-regulation during human neural development, and up-regulation in AD neocortex and hippocampal CA1. Together, interleukin-1beta and amyloid beta42 peptide [IL-1beta+Abeta42] synergistically activated COX-2 and PS1 gene expression preceded by increases in AP1-, STAT1alpha-, and in particular NF-kappaBp50/p65- and HIF-1alpha-DNA binding. These events were markedly potentiated by hypoxia and blocked by the antioxidant alpha-phenyl-N-tert-butyl nitrone. Broad transcription profiling further indicated that hypoxia-induced, [IL-1beta+Abeta42]-treated HN cells display robust induction of COX-2 and PS1 as well as a pro-inflammatory gene family that includes NF-kappaBp50/p105, IL-1beta precursor, and cytosolic phospholipase A2 genes. These findings indicate a novel [IL-1beta+Abeta42]-mediated, hypoxia-enhanced, free radical-triggered gene program that drives inflammatory gene signaling and suggest a mechanism by which hypoxia during aging contributes episodically to amyloidogenesis, inflammation, and AD pathophysiology.