The nitric oxide-cyclic GMP pathway regulates FoxO and alters dopaminergic neuron survival in Drosophila

Function and regulation of nitric oxide signaling in Drosophila

Nitric oxide (NO) serves as an evolutionarily conserved signaling molecule that plays an important role in a wide variety of cellular processes. Extensive studies in Drosophila melanogaster have revealed that NO signaling is required for development, physiology, and stress responses in many different types of cells. In neuronal cells, multiple NO signaling pathways appear to operate in different combinations to regulate learning and memory formation, synaptic transmission, selective synaptic connections, axon degeneration, and axon regrowth. During organ development, elevated NO signaling suppresses cell cycle progression, whereas downregulated NO leads to an increase in larval body size via modulation of hormone signaling. The most striking feature of the Drosophila NO synthase is that various stressors, such as neuropeptides, aberrant proteins, hypoxia, bacterial infection, and mechanical injury, can activate Drosophila NO synthase, initially regulating cellular physiology to enable cells to survive. However, under severe stress or pathophysiological conditions, high levels of NO promote regulated cell death and the development of neurodegenerative diseases. In this review, I highlight and discuss the current understanding of molecular mechanisms by which NO signaling regulates distinct cellular functions and behaviors.

Nitric oxide-soluble guanylyl cyclase pathway as a contributor to age-related memory impairment in Drosophila

Age-related changes in the transcriptome lead to memory impairment. Several genes have been identified to cause age-dependent memory impairment (AMI) by changes in their expression, but genetic screens to identify genes critical for AMI have not been performed. The fruit fly is a useful model for studying AMI due to its short lifespan and the availability of consistent techniques and environments to assess its memory ability. We generated a list of candidate genes that act as AMI regulators by performing a comprehensive analysis of RNAsequencing data from young and aged fly heads and genome-wide RNAi screening data to identify memory-regulating genes. A candidate screen using temporal and panneuronal RNAi expression was performed to identify genes critical for AMI. We identified the guanylyl cyclase β-subunit at 100B (gycβ) gene, which encodes a subunit of soluble guanylyl cyclase (sGC), the only intracellular nitric oxide (NO) receptor in fruit flies, as a negative regulator of AMI. RNAi knockdown of gycβ in neurons and NO synthase (NOS) in glia or neurons enhanced the performance of intermediate-term memory (ITM) without apparent effects on memory acquisition. We also showed that pharmacological inhibition of sGC and NOS enhanced ITM in aged individuals, suggesting the possibility that age-related enhancement of the NO-sGC pathway causes memory impairment.

Role of FoxO transcription factors in aging and age-related metabolic and neurodegenerative diseases

Aging happens to all of us as we live. Thanks to the improved living standard and discovery of life-saving medicines, our life expectancy has increased substantially across the world in the past century. However, the rise in lifespan leads to unprecedented increases in both the number and the percentage of individuals 65 years and older, accompanied by the increased incidences of age-related diseases such as type 2 diabetes mellitus and Alzheimer’s disease. FoxO transcription factors are evolutionarily conserved molecules that play critical roles in diverse biological processes, in particular aging and metabolism. Their dysfunction is often found in the pathogenesis of many age-related diseases. Here, we summarize the signaling pathways and cellular functions of FoxO proteins. We also review the complex role of FoxO in aging and age-related diseases, with focus on type 2 diabetes and Alzheimer’s disease and discuss the possibility of FoxO as a molecular link between aging and disease risks.

FOXO and related transcription factors binding elements in the regulation of neurodegenerative disorders

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis, and others, are characterized by progressive loss of neuronal cells, which causes memory impairment and cognitive decline. Mounting evidence demonstrated the possible implications of diverse biological processes, namely oxidative stress, mitochondrial dysfunction, aberrant cell cycle re-entry, post-translational modifications, protein aggregation, impaired proteasome dysfunction, autophagy, and many others that cause neuronal cell death. The condition worsens as there is no effective treatment for such diseases due to their complex pathogenesis and mechanism. Mounting evidence demonstrated the role of regulatory transcription factors, such as NFκβ, FoxO, Myc, CREB, and others that regulate the biological processes and diminish the disease progression and pathogenesis. Studies demonstrated that forkhead box O (FoxO) transcription factors had been implicated in the regulation of aging and longevity. Further, the functions of FoxO proteins are regulated by different post-translational modifications (PTMs), namely acetylation, and ubiquitination. Various studies concluded that FoxO proteins exert both neuroprotective and neurotoxic properties depending on their regulation mechanism and activity in the brain. Thus, understanding the nature of FoxO expression and activity in the brain will help develop effective therapeutic strategies. Herein, firstly, we discuss the role of FoxO protein in cell cycle regulation and cell proliferation, followed by the regulation of FoxO proteins through acetylation and ubiquitination. We also briefly explain the activity and expression pattern of FoxO proteins in the neuronal cells and explain the mechanism through which FoxO proteins are rescued from oxidative stress-induced neurotoxicity. Later on, we present a detailed view of the implication of FoxO proteins in neurodegenerative disease and FoxO proteins as an effective therapeutic target.

Soluble Guanylate Cyclase Inhibitors Discovered among Natural Compounds

Soluble guanylate cyclase (sGC) is the human receptor of nitric oxide (NO) in numerous kinds of cells and produces the second messenger 3',5'-cyclic guanosine monophosphate (cGMP) upon NO binding to its heme. sGC is involved in many cell signaling pathways both under healthy conditions and under pathological conditions, such as angiogenesis associated with tumor growth. Addressing the selective inhibition of the NO/cGMP pathway is a strategy worthwhile to be investigated for slowing down tumoral angiogenesis or for curing vasoplegia. However, sGC inhibitors are lacking investigation. We have explored a chemical library of various natural compounds and have discovered inhibitors of sGC. The selected compounds were evaluated for their inhibition of purified sGC in vitro and sGC in endothelial cells. Six natural compounds, from various organisms, have IC₅₀ in the range 0.2-1.5 μM for inhibiting the NO-activated synthesis of cGMP by sGC, and selected compounds exhibit a quantified antiangiogenic activity using an endothelial cell line. These sGC inhibitors can be used directly as tools to investigate angiogenesis and cell signaling or as templates for drug design.

Differential sensitivity of human islets from obese versus lean donors to chronic high glucose or palmitate

BACKGROUND

Due to the shortage of multi-organ donors, human pancreatic islet transplantation has now been extended to islets originating from obese subjects. In this study, our aim is to compare the respective sensitivity of human islets from lean vs obese donors to chronic high glucose or high palmitate.

METHODS

Human islets were isolated from pancreases harvested from brain-dead multi-organ donors. Islets were cultured during 72 hours in the presence of moderate (16.7 mmol/L) or high (28 mmoL/L) glucose concentrations, or glucose (5.6 mmoL/L) and palmitate (0.4 mmoL/L), before measurement of their response to glucose.

RESULTS

We first observed a greater insulin response in islets from obese donors under both basal and high-glucose conditions, confirming their hyperresponsiveness to glucose. When islets from obese donors were cultured in the presence of moderate or high glucose concentrations, insulin response to glucose remained unchanged or was slightly reduced, as opposed to that observed in lean subjects. Moreover, culturing islets from obese donors with high palmitate also induced less reduction in insulin response to glucose than in lean subjects. This partial protection of obese islets is associated with less induction of inducible nitric oxide synthase in islets, together with a greater expression of the transcription factor forkhead box O1 (FOXO1).

CONCLUSIONS

Our data suggest that in addition to an increased sensitivity to glucose, islets from obese subjects can be considered as more resistant to glucose and fatty acid excursions and are thus valuable candidates for transplantation.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Roles of forkhead box O (FoxO) transcription factors in neurodegenerative diseases: A panoramic view

Neurodegenerative diseases (NDDs), which are among the most important aging-related diseases, are typically characterized by neuronal damage and a progressive impairment in neurological function during aging. Few effective therapeutic targets for NDDs have been revealed; thus, an understanding of the pathogenesis of NDDs is important. Forkhead box O (FoxO) transcription factors have been implicated in the mechanisms regulating aging and longevity. The functions of FoxOs are regulated by diverse post-translational modifications (e.g., phosphorylation, acetylation, ubiquitination, methylation and glycosylation). FoxOs exert both detrimental and protective effects on NDDs. Therefore, an understanding of the precise function of FoxOs in NDDs will be helpful for developing appropriate treatment strategies. In this review, we first introduce the post-translational modifications of FoxOs. Next, the regulation of FoxO expression and post-translational modifications in the central nervous system (CNS) is described. Afterwards, we analyze and address the important roles of FoxOs in NDDs. Finally, novel potential directions of future FoxO research in NDDs are discussed. This review recapitulates essential facts and questions about the promise of FoxOs in treating NDDs, and it will likely be important for the design of further basic studies and to realize the potential for FoxOs as therapeutic targets in NDDs.

Japanese encephalitis virus induces apoptosis by inhibiting Foxo signaling pathway

Japanese encephalitis virus (JEV) infection induces brain tissue disease characterized by neuron death. however, little is known about the underlying mechanism. Using RNA sequencing, we profiled global mRNA expression changes in response to in vitro and in vivo JEV infection. Integration analysis of in vitro and in vivo mRNA transcriptome revealed that JEV infection regulated apoptosis-related Foxo signaling pathway. Foxo expression was reduced by JEV infection in vitro and in vivo. Knockdown of Foxo promoted apoptosis, while its overexpression reduced apoptosis in JEV-infected Neuro-2a cells. JEV infection in Neuro-2a cells decreased the expression of Foxo downstream genes including pro-apoptotic protein Bim, anti-apoptotic protein Bcl-6 and p21. Overexpression of anti-apoptotic proteins Bcl-6 and p21 repressed JEV-induced apoptosis. These findings suggest that Foxo primarily exerts an anti-apoptotic function via Bcl-6 and p21 in JEV-infected Neuro-2a cells. A STAT3 binding site was identified in the promoter region of Foxo by TFBIND software and confirmed by ChIP and reporter assays. JEV infection reduced STAT3 expression as well as its binding at the Foxo promoter compared to mock infection in Neuro-2a cells. Moreover, STAT3 knockdown reduced Foxo promoter activity and Foxo expression. Therefore, JEV reduced Foxo expression, at least in part, by downregulating STAT3. Taken together, we found that JEV induced cell apoptosis by inhibiting STAT3-Foxo-Bcl-6/p21 pathway, which provides a novel insight into JEV-caused encephalitis.

Feeding-Related Traits Are Affected by Dosage of the foraging Gene in Drosophila melanogaster

Nutrient acquisition and energy storage are critical parts of achieving metabolic homeostasis. The foraging gene in Drosophila melanogaster has previously been implicated in multiple feeding-related and metabolic traits. Before foraging's functions can be further dissected, we need a precise genetic null mutant to definitively map its amorphic phenotypes. We used homologous recombination to precisely delete foraging, generating the for⁰ null allele, and used recombineering to reintegrate a full copy of the gene, generating the {for^BAC} rescue allele. We show that a total loss of foraging expression in larvae results in reduced larval path length and food intake behavior, while conversely showing an increase in triglyceride levels. Furthermore, varying foraging gene dosage demonstrates a linear dose-response on these phenotypes in relation to foraging gene expression levels. These experiments have unequivocally proven a causal, dose-dependent relationship between the foraging gene and its pleiotropic influence on these feeding-related traits. Our analysis of foraging's transcription start sites, termination sites, and splicing patterns using rapid amplification of cDNA ends (RACE) and full-length cDNA sequencing, revealed four independent promoters, pr1-4, that produce 21 transcripts with nine distinct open reading frames (ORFs). The use of alternative promoters and alternative splicing at the foraging locus creates diversity and flexibility in the regulation of gene expression, and ultimately function. Future studies will exploit these genetic tools to precisely dissect the isoform- and tissue-specific requirements of foraging's functions and shed light on the genetic control of feeding-related traits involved in energy homeostasis.

Why is nitric oxide important for our brain?

The freely diffusible gaseous compound nitric oxide (NO) has been shown to be an important messenger in many organ systems throughout the body, and particularly in the central nervous system (CNS). The importance of NO as an intermediary in cell communication in the brain is highlighted by the fact that the excitatory amino acid glutamate, the most abundant CNS neurotransmitter, is an initiator of the reaction that forms NO. Because of its numerous physiological and pathophysiological roles, the impact of NO on clinical medicine is developing. NO can act as a "double-edged sword" and it has been demonstrated that clarification of the dual effect of NO might have implications for clinical medicine, and could lead to the emergence of therapeutic opportunities. Accordingly, NO was proclaimed "Mole cule of the Year" in 1992 by the journal Science, while discovery of the pathways and roles of NO was acknowledged with the Nobel Prize in 1998. Additionally, the ubiquity of NO in the CNS implies that drugs designed to modify the biological activity of NO may have distinct effects. Thus, further clinical applications of NO, of its analogs or of newly developed NOS inhibitors are forthcoming. The therapeutic challenge would be to succeed in manipulating the NO pathways selectively.

The Gyc76C Receptor Guanylyl Cyclase and the Foraging cGMP-Dependent Kinase Regulate Extracellular Matrix Organization and BMP Signaling in the Developing Wing of Drosophila melanogaster

The developing crossveins of the wing of Drosophila melanogaster are specified by long-range BMP signaling and are especially sensitive to loss of extracellular modulators of BMP signaling such as the Chordin homolog Short gastrulation (Sog). However, the role of the extracellular matrix in BMP signaling and Sog activity in the crossveins has been poorly explored. Using a genetic mosaic screen for mutations that disrupt BMP signaling and posterior crossvein development, we identify Gyc76C, a member of the receptor guanylyl cyclase family that includes mammalian natriuretic peptide receptors. We show that Gyc76C and the soluble cGMP-dependent kinase Foraging, likely linked by cGMP, are necessary for normal refinement and maintenance of long-range BMP signaling in the posterior crossvein. This does not occur through cell-autonomous crosstalk between cGMP and BMP signal transduction, but likely through altered extracellular activity of Sog. We identify a novel pathway leading from Gyc76C to the organization of the wing extracellular matrix by matrix metalloproteinases, and show that both the extracellular matrix and BMP signaling effects are largely mediated by changes in the activity of matrix metalloproteinases. We discuss parallels and differences between this pathway and other examples of cGMP activity in both Drosophila melanogaster and mammalian cells and tissues.

Signaling between cells regulates many processes, including the choices cells make between different fates during development and regeneration, and misregulation of such signaling underlies many human pathologies. To understand how such signals control developmental decisions, it is necessary to elucidate both how cells regulate and respond to different levels of signaling, and how different types of signals combine and regulate each other. We have used genetic screening in the fruitfly Drosophila melanogaster to identify mutations that reduce or eliminate signals carried by Bone Morphogenetic Proteins (BMPs), and show that BMP signaling is sensitive Gyc76C, a peptide receptor that stimulates the production of cGMP in cells. We identify downstream intracellular effectors of this cGMP activity, but provide evidence that the effects on the BMP pathway are not mediated at the intracellular level, but rather through cGMP’s effects upon the extracellular matrix and matrix-remodeling proteinases, which in turn affects the activity of extracellular BMP-binding proteins. We discuss differences and parallels with other examples of cGMP activity in Drosophila melanogaster and mammals.

The Parkinson's Disease-Associated Protein Kinase LRRK2 Modulates Notch Signaling through the Endosomal Pathway

Leucine-rich repeat kinase 2 (LRRK2) is a key molecule in the pathogenesis of familial and idiopathic Parkinson’s disease (PD). We have identified two novel LRRK2-associated proteins, a HECT-type ubiquitin ligase, HERC2, and an adaptor-like protein with six repeated Neuralized domains, NEURL4. LRRK2 binds to NEURL4 and HERC2 via the LRRK2 Ras of complex proteins (ROC) domain and NEURL4, respectively. HERC2 and NEURL4 link LRRK2 to the cellular vesicle transport pathway and Notch signaling, through which the LRRK2 complex promotes the recycling of the Notch ligand Delta-like 1 (Dll1)/Delta (Dl) through the modulation of endosomal trafficking. This process negatively regulates Notch signaling through cis-inhibition by stabilizing Dll1/Dl, which accelerates neural stem cell differentiation and modulates the function and survival of differentiated dopaminergic neurons. These effects are strengthened by the R1441G ROC domain-mutant of LRRK2. These findings suggest that the alteration of Notch signaling in mature neurons is a component of PD etiology linked to LRRK2.

LRRK2 is linked to autosomal dominant late-onset Parkinson’s disease, suggesting that LRRK2 gain-of-function mutations lead to age-dependent degeneration of the midbrain dopaminergic neurons. In this study, we describe two novel LRRK2-associated proteins HERC2 and NEURL4, which are a ubiquitin ligase and an adaptor-like protein, respectively. HERC2 and NEURL4 direct LRRK2 to Notch signaling pathway, in which the LRRK2-NEURL4-HERC2 complex promotes the recycling of the Notch ligand Delta-like 1 (Dll1)/Delta (Dl) through the modulation of endosomal trafficking. As a result, the amounts of Dll1/D1 on the plasma membrane are increased, which affects negatively Notch signaling through cis-inhibition. The effect is enhanced by a Parkinson’s-disease associated mutation of LRRK2. Inhibition of Notch signaling in adult dopaminergic neurons impairs its functions and survival. These findings indicate a possible link between Notch pathway and Parkinson’s disease.

Thymosin Beta-4, Actin-Sequestering Protein Regulates Vascular Endothelial Growth Factor Expression via Hypoxia-Inducible Nitric Oxide Production in HeLa Cervical Cancer Cells

Vascular endothelial growth factor (VEGF) is an important regulator of neovascularization. Hypoxia inducible nitric oxide (NO) enhanced the expression of VEGF and thymosin beta-4 (Tβ4), actin sequestering protein. Here, we investigated whether NO-mediated VEGF expression could be regulated by Tβ4 expression in HeLa cervical cancer cells. Hypoxia inducible NO production and VEGF expression were reduced by small interference (si) RNA of Tβ4. Hypoxia response element (HRE)-luciferase activity and VEGF expression were increased by the treatment with N-(β-D-Glucopyranosyl)-N2-acetyl-S-nitroso-D, L-penicillaminamide (SNAP-1), to generate NO, which was inhibited by the inhibition of Tβ4 expression with Tβ4-siRNA. In hypoxic condition, HRE-luciferase activity and VEGF expression were inhibited by the treatment with N^G-monomethyl-L-arginine (L-NMMA), an inhibitor to nitric oxide synthase (NOS), which is accompanied with a decrease in Tβ4 expression. VEGF expression inhibited by L-NMMA treatment was restored by the transfection with pCMV-Tβ4 plasmids for Tβ4 overexpression. Taken together, these results suggest that Tβ4 could be a regulator for the expression of VEGF via the maintenance of NOS activity.

Proteasome, but not autophagy, disruption results in severe eye and wing dysmorphia: a subunit- and regulator-dependent process in Drosophila

Proteasome-dependent and autophagy-mediated degradation of eukaryotic cellular proteins represent the two major proteostatic mechanisms that are critically implicated in a number of signaling pathways and cellular processes. Deregulation of functions engaged in protein elimination frequently leads to development of morbid states and diseases. In this context, and through the utilization of GAL4/UAS genetic tool, we herein examined the in vivo contribution of proteasome and autophagy systems in Drosophila eye and wing morphogenesis. By exploiting the ability of GAL4-ninaE. GMR and P{GawB}Bx^MS1096 genetic drivers to be strongly and preferentially expressed in the eye and wing discs, respectively, we proved that proteasomal integrity and ubiquitination proficiency essentially control fly’s eye and wing development. Indeed, subunit- and regulator-specific patterns of severe organ dysmorphia were obtained after the RNAi-induced downregulation of critical proteasome components (Rpn1, Rpn2, α5, β5 and β6) or distinct protein-ubiquitin conjugators (UbcD6, but not UbcD1 and UbcD4). Proteasome deficient eyes presented with either rough phenotypes or strongly dysmorphic shapes, while transgenic mutant wings were severely folded and carried blistered structures together with loss of vein differentiation. Moreover, transgenic fly eyes overexpressing the UBP2-yeast deubiquitinase enzyme were characterized by an eyeless-like phenotype. Therefore, the proteasome/ubiquitin proteolytic activities are undoubtedly required for the normal course of eye and wing development. In contrast, the RNAi-mediated downregulation of critical Atg (1, 4, 7, 9 and 18) autophagic proteins revealed their non-essential, or redundant, functional roles in Drosophila eye and wing formation under physiological growth conditions, since their reduced expression levels could only marginally disturb wing’s, but not eye’s, morphogenetic organization and architecture. However, Atg9 proved indispensable for the maintenance of structural integrity of adult wings in aged flies. In toto, our findings clearly demonstrate the gene-specific fundamental contribution of proteasome, but not autophagy, in invertebrate eye and wing organ development.

Forkhead box(O) in control of reactive oxygen species and genomic stability to ensure healthy lifespan

SIGNIFICANCE

Transcription factors of the Forkhead box O class (FOXOs) are associated with lifespan and play a role in age-related diseases. FOXOs, therefore, serve as a paradigm for developing an understanding as to how age-related diseases, such as cancer and diabetes interconnect with lifespan. Understanding the regulatory inputs on FOXO may reveal how changes in these regulatory signaling pathways affect disease and lifespan.

RECENT ADVANCES

Numerous regulators of FOXO have now been described and a clear and evolutionary conserved role has emerged for phosphoinositide-3 kinase/protein kinase B (also known as c-Akt or AKT) signaling and c-jun N-terminal kinase signaling. Analysis of FOXO function in the context of these signaling pathways has shown the importance of FOXO-mediated transcriptional regulation on cell cycle progression and other cell fates, such as cell metabolism, stress resistance, and apoptosis in mediating disease and lifespan.

CRITICAL ISSUES

Persistent DNA damage is also tightly linked to disease and aging; yet, data on a possible link between DNA damage and FOXO have been limited. Here, we discuss possible connections between FOXO and the DNA damage response in the context of the broader role of connecting lifespan and disease.

FUTURE DIRECTIONS

Understanding the role of lifespan in diseases onset may provide unique and generic possibilities to intervene in disease processes to ensure a healthy lifespan.

Gene expression profiles in Parkinson disease prefrontal cortex implicate FOXO1 and genes under its transcriptional regulation

Parkinson disease (PD) is a complex neurodegenerative disorder with largely unknown genetic mechanisms. While the degeneration of dopaminergic neurons in PD mainly takes place in the substantia nigra pars compacta (SN) region, other brain areas, including the prefrontal cortex, develop Lewy bodies, the neuropathological hallmark of PD. We generated and analyzed expression data from the prefrontal cortex Brodmann Area 9 (BA9) of 27 PD and 26 control samples using the 44K One-Color Agilent 60-mer Whole Human Genome Microarray. All samples were male, without significant Alzheimer disease pathology and with extensive pathological annotation available. 507 of the 39,122 analyzed expression probes were different between PD and control samples at false discovery rate (FDR) of 5%. One of the genes with significantly increased expression in PD was the forkhead box O1 (FOXO1) transcription factor. Notably, genes carrying the FoxO1 binding site were significantly enriched in the FDR-significant group of genes (177 genes covered by 189 probes), suggesting a role for FoxO1 upstream of the observed expression changes. Single-nucleotide polymorphisms (SNPs) selected from a recent meta-analysis of PD genome-wide association studies (GWAS) were successfully genotyped in 50 out of the 53 microarray brains, allowing a targeted expression-SNP (eSNP) analysis for 52 SNPs associated with PD affection at genome-wide significance and the 189 probes from FoxO1 regulated genes. A significant association was observed between a SNP in the cyclin G associated kinase (GAK) gene and a probe in the spermine oxidase (SMOX) gene. Further examination of the FOXO1 region in a meta-analysis of six available GWAS showed two SNPs significantly associated with age at onset of PD. These results implicate FOXO1 as a PD-relevant gene and warrant further functional analyses of its transcriptional regulatory mechanisms.