Ryanodine receptors contribute to cGMP-induced late-phase LTP and CREB phosphorylation in the hippocampus

Differential binding of CREB and REST/NRSF to NMDAR1 promoter is associated with the sex-selective cognitive deficit following postnatal PBDE-209 exposure in mice

Neonatal exposure to decabromodiphenyl ether (PBDE-209), a widely used flame retardant, affects cognitive performances in the later stage of life in a sex-dependent manner. PBDE-209 interferes with glutamatergic signaling and N-methyl-D-aspartate receptor (NMDAR) subunits with unresolved regulatory mechanisms. This study exposed male and female mice pups through postnatal day (PND) 3-10 to PBDE-209 (oral dose: 0, 6, or 20 mg/kg body weight). The frontal cortex and hippocampus, collected from neonate (PND 11) and young (PND 60) mice, were analyzed for cAMP response element-binding protein (CREB) and RE1-silencing transcription factor/ Neuron-restrictive silencer factor (REST/NRSF) binding to NMDAR1 promoter and expression of NMDAR1 gene by electrophoretic mobility shift assay and semi-quantitative RT-PCR respectively. Behavioral changes were assessed using spontaneous alternation behavior and novel object recognition tests in young mice. In neonates, the binding of CREB was increased, while REST/NRSF was decreased significantly to their cognate NMDAR1 promoter sequences at the high dose of PBDE-209 in both the sexes. This reciprocal pattern of CREB and REST/NRSF interactions correlates with the up-regulation of NMDAR1 expression. Young males followed a similar pattern of CREB and REST/NRSF binding and NMDAR1 expression as in neonates. Surprisingly, young females did not show any alteration when compared to age-matched controls. Also, we found that only young males showed working and recognition memory deficits. These results indicate that early exposure to PBDE-209 interferes with CREB- and REST/NRSF-dependent regulation of the NMDAR1 gene in an acute setting. However, long-term effects persist only in young males that could be associated with cognitive impairment.

RyR2-dependent modulation of neuronal hyperactivity: A potential therapeutic target for treating Alzheimer's disease

Increasing evidence suggests that simply reducing β-amyloid (Aβ) plaques may not significantly affect the progression of Alzheimer's disease (AD). There is also increasing evidence indicating that AD progression is driven by a vicious cycle of soluble Aβ-induced neuronal hyperactivity. In support of this, it has recently been shown that genetically and pharmacologically limiting ryanodine receptor 2 (RyR2) open time prevents neuronal hyperactivity, memory impairment, dendritic spine loss and neuronal cell death in AD mouse models. By contrast, increased RyR2 open probability (Po) exacerbates the onset of familial AD-associated neuronal dysfunction and induces AD-like defects in the absence of AD-causing gene mutations. Thus, RyR2-dependent modulation of neuronal hyperactivity represents a promising new target for combating AD.

RyR-mediated calcium release in hippocampal health and disease

Hippocampal synaptic plasticity is widely considered the cellular basis of learning and spatial memory processes. This article highlights the central role of Ca2+ release from the endoplasmic reticulum (ER) in hippocampal synaptic plasticity and hippocampus-dependent memory in health and disease. The key participation of ryanodine receptor (RyR) channels, which are the principal Ca2+ release channels expressed in the hippocampus, in these processes is emphasized. It is proposed that the increased neuronal oxidative tone displayed by hippocampal neurons during aging or Alzheimer's disease (AD) leads to excessive activation of RyR-mediated Ca2+ release, a process that is highly redox-sensitive, and that this abnormal response contributes to and aggravates these deleterious conditions.

Subcellular localization and transcriptional regulation of brain ryanodine receptors. Functional implications

Ryanodine receptors (RyR) are intracellular Ca2+ channels localized in the endoplasmic reticulum, where they act as critical mediators of Ca2+-induced Ca2+ calcium release (CICR). In the brain, mammals express in both neurons, and non-neuronal cells, a combination of the three RyR-isoforms (RyR1-3). Pharmacological approaches, which do not distinguish between isoforms, have indicated that RyR-isoforms contribute to brain function. However, isoform-specific manipulations have revealed that RyR-isoforms display different subcellular localizations and are differentially associated with neuronal function. These findings raise the need to understand RyR-isoform specific transcriptional regulation, as this knowledge will help to elucidate the causes of neuronal dysfunction for a growing list of brain disorders that show altered RyR channel expression and function.

The Role of Ryanodine Receptor 2 in Drug-Associated Learning

Type-2 ryanodine receptor (RyR2) ion channels facilitate the release of Ca 2+ from stores and serve an important function in neuroplasticity. The role for RyR2 in hippocampal-dependent learning and memory is well established and chronic hyperphosphorylation of RyR2 (RyR2P) is associated with pathological calcium leakage and cognitive disorders, including Alzheimer's disease. By comparison, little is known about the role of RyR2 in the ventral medial prefrontal cortex (vmPFC) circuitry important for working memory, decision making, and reward seeking. Here, we evaluated the basal expression and localization of RyR2 and RyR2P in the vmPFC. Next, we employed an operant model of sucrose, cocaine, or morphine self-administration (SA) followed by a (reward-free) recall test, to reengage vmPFC neurons and reactivate reward-seeking and re-evaluated the expression and localization of RyR2 and RyR2P in vmPFC. Under basal conditions, RyR2 was expressed in pyramidal cells but not regularly detected in PV/SST interneurons. On the contrary, RyR2P was rarely observed in PFC somata and was restricted to a different subcompartment of the same neuron - the apical dendrites of layer-5 pyramidal cells. Chronic SA of drug (cocaine or morphine) and nondrug (sucrose) rewards produced comparable increases in RyR2 protein expression. However, recalling either drug reward impaired the usual localization of RyR2P in dendrites and markedly increased its expression in somata immunoreactive for Fos, a marker of highly activated neurons. These effects could not be explained by chronic stress or drug withdrawal and instead appeared to require a recall experience associated with prior drug SA. In addition to showing the differential distribution of RyR2/RyR2P and affirming the general role of vmPFC in reward learning, this study provides information on the propensity of addictive drugs to redistribute RyR2P ion channels in a neuronal population engaged in drug-seeking. Hence, focusing on the early impact of addictive drugs on RyR2 function may serve as a promising approach to finding a treatment for substance use disorders.

Associations between blood leukocyte DNA methylation and sustained attention in mid-to-late childhood

Aims: To identify associations between DNA methylation (DNAm) across the epigenome and symptoms related to attention-deficit/hyperactivity disorder in a population of Hispanic children. Materials & methods: Among 517 participants in the ELEMENT study aged 9-18 years, we conducted an epigenome-wide association study examining associations between blood leukocyte DNAm and performance on the Conners' continuous performance test (CPT3). Results: DNAm at loci in or near ZNF814, ELF4 and OR6K6 and functional enrichment for gene pathways pertaining to ferroptosis, inflammation, immune response and neurotransmission were significantly related to CPT3 scores. Conclusion: DNAm was associated with CPT3 performance. Further analysis is warranted to understand how these genes and enriched pathways contribute to attention-deficit/hyperactivity disorder.

De-glutathionylases: The resilient underdogs to keep neurodegeneration at bay

Proteins become S-glutathionylated as a result of the derivatization of their cysteine thiols with the thiolate anion derivative of glutathione; this process is frequently linked to diseases and protein misbehavior. Along with the other well-known oxidative modifications like S-nitrosylation, S-glutathionylation has quickly emerged as a major contributor to a number of diseases, with a focus on neurodegeneration. The immense clinical significance of S-glutathionylation in cell signaling and the genesis of diseases are progressively coming to light with advanced research, which is also creating new opportunities for prompt diagnostics that utilize this phenomenon. In-depth investigation in recent years has revealed other significant deglutathionylases in addition to glutaredoxin, necessitating the hunt for their specific substrates. The precise catalytic mechanisms of these enzymes must also be understood, along with how the intracellular environment affects their impact on protein conformation and function. These insights must then be extrapolated to the understanding of neurodegeneration and the introduction of novel and clever therapeutic approaches to clinics. Clarifying the importance of the functional overlap of glutaredoxin and other deglutathionylases and examining their complementary functions as defense systems in the face of stress are essential prerequisites for predicting and promoting cell survival under high oxidative/nitrosative stress.

Long-term potentiation and spatial memory training stimulate the hippocampal expression of RyR2 calcium release channels

Introduction: Neuronal Ca²⁺ signals generated through the activation of Ca²⁺-induced Ca²⁺ release in response to activity-generated Ca²⁺ influx play a significant role in hippocampal synaptic plasticity, spatial learning, and memory. We and others have previously reported that diverse stimulation protocols, or different memory-inducing procedures, enhance the expression of endoplasmic reticulum-resident Ca²⁺ release channels in rat primary hippocampal neuronal cells or hippocampal tissue. Methods and Results: Here, we report that induction of long-term potentiation (LTP) by Theta burst stimulation protocols of the CA3-CA1 hippocampal synapse increased the mRNA and protein levels of type-2 Ryanodine Receptor (RyR2) Ca²⁺ release channels in rat hippocampal slices. Suppression of RyR channel activity (1 h preincubation with 20 μM ryanodine) abolished both LTP induction and the enhanced expression of these channels; it also promoted an increase in the surface expression of the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor subunits GluR1 and GluR2 and caused a moderate but significant reduction of dendritic spine density. In addition, training rats in the Morris water maze induced memory consolidation, which lasted for several days after the end of the training period, accompanied by an increase in the mRNA levels and the protein content of the RyR2 channel isoform. Discussion: We confirm in this work that LTP induction by TBS protocols requires functional RyR channels. We propose that the increments in the protein content of RyR2 Ca²⁺ release channels, induced by LTP or spatial memory training, play a significant role in hippocampal synaptic plasticity and spatial memory consolidation.

Effect of biotin supplementation in infant formula: A multi-center study in Japan

BACKGROUND

This non-randomized intervention study aimed to evaluate the effect of supplementing infant formula with biotin on biotin metabolism and on development.

METHODS

We enrolled healthy Japanese infants (n = 84) and assigned them to groups offered Formula A (total biotin, 0.5 μg/100 kcal) or Formula B (total biotin, 2.4 μg/100 kcal) until they were 6 months of age, and completed an additional follow up to age 36 months. Urinary biotin concentrations were measured at 1 and 6 months, and were compared among breast-fed, Formula A-fed, and Formula B-fed infants at each age. In a follow-up subgroup analysis, we compared scores on the Ages and Stages Questionnaire, version 3 (ASQ-3), from 9 to 36 months among infants continuously fed Formula A, Formula B, or breastmilk.

RESULTS

No adverse events occurred during the intervention period. At 1 month, urinary biotin concentrations were highest in Formula B-fed infants and lowest in Formula A-fed infants. At 6 months, Formula B-fed infants retained higher biotin levels than Formula A-fed and breast-fed infants. Both differences were statistically significant (P < 0.05). The breast-fed, Formula A-fed, and Formula B-fed groups had similar ASQ scores at 9-36 months.

CONCLUSIONS

Biotin supplementation of infant formula contributed to improving biotin status in formula-fed infants. The results support the official approval of the use of biotin in infant formula by the government of Japan in 2014.

Combinatory Approaches Targeting Cognitive Impairments and Memory Enhancement: A Review

The objective of this paper is to look at how natural medicines can improve cognition and memory when used with sildenafil, a popular erectile dysfunction medicine that also has nootropic properties. Newer treatment strategies to treat the early stages of these diseases need to be developed. Multiple factors lead to complex pathophysiological conditions, which are responsible for various long-term complications. In this review, a combination of treatments targeting these pathologies is discussed. These combinations may help manage early and later phases of cognitive impairments. The purpose of this article is to discuss a link between these pathologies and a combinational approach with the objective of considering newer therapeutic strategies in the treatment of cognitive impairments. The natural drugs and their ingredients play a major role in the management of disease progression. Additionally, their combination with sildenafil allows for more efficacy and better response. Studies showing the effectiveness of natural drugs and sildenafil are mentioned, and how these combinations could be beneficial for the treatment of cognitive impairments and amnesia are summarised. Furthermore, preclinical and clinical trials are required to explore the medicinal potential of these drug combinations.

Reversal of spatial memory impairment by phosphodiesterase 3 inhibitor cilostazol is associated with reduced neuroinflammation and increased cerebral glucose uptake in aged male mice

The nucleotide second messenger 3', 5'-cyclic adenosine monophosphate (cAMP) and 3', 5'-cyclic guanosine monophosphate (cGMP) mediate fundamental functions of the brain, including learning and memory. Phosphodiesterase 3 (PDE3) can hydrolyze both cAMP and cGMP and appears to be involved in the regulation of their contents in cells. We previously demonstrated that long-term administration of cilostazol, a PDE3 inhibitor, maintained good memory performance in aging mice. Here, we report on studies aimed at determining whether cilostazol also reverses already-impaired memory in aged male mice. One month of oral 1.5% cilostazol administration in 22-month-old mice reversed age-related declines in hippocampus-dependent memory tasks, including the object recognition and the Morris water maze. Furthermore, cilostazol reduced neuroinflammation, as evidenced by immunohistochemical staining, and increased glucose uptake in the brain, as evidence by positron emission tomography (PET) with 2-deoxy-2-[¹⁸F]fluoro-d-glucose ([¹⁸F]FDG). These results suggest that already-expressed memory impairment in aged male mice that depend on cyclic nucleotide signaling can be reversed by inhibition of PDE3. The reversal of age-related memory impairments may occur in the central nervous system, either through cilostazol-enhanced recall or strengthening of weak memories that otherwise may be resistant to recall.

The Interplay between cGMP and Calcium Signaling in Alzheimer’s Disease

Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger and a key molecule in many important signaling cascades in the body and brain, including phototransduction, olfaction, vasodilation, and functional hyperemia. Additionally, cGMP is involved in long-term potentiation (LTP), a cellular correlate of learning and memory, and recent studies have identified the cGMP-increasing drug Sildenafil as a potential risk modifier in Alzheimer’s disease (AD). AD development is accompanied by a net increase in the expression of nitric oxide (NO) synthases but a decreased activity of soluble guanylate cyclases, so the exact sign and extent of AD-mediated imbalance remain unclear. Moreover, human patients and mouse models of the disease present with entangled deregulation of both cGMP and Ca²⁺ signaling, e.g., causing changes in cGMP-mediated Ca²⁺ release from the intracellular stores as well as Ca²⁺-mediated cGMP production. Still, the mechanisms governing such interplay are poorly understood. Here, we review the recent data on mechanisms underlying the brain cGMP signaling and its interconnection with Ca²⁺ signaling. We also discuss the recent evidence stressing the importance of such interplay for normal brain function as well as in Alzheimer’s disease.

Novel molecular targets and mechanisms for neuroprotective modulation in neurodegenerative disorders

BACKGROUND

Neuronal death underlies the symptoms of several human neurological disorders, including Alzheimer's, Parkinson's and Huntington's diseases, and amyotrophic lateral sclerosis that their precise pathophysiology have not yet been elucidated. According to various studies the prohibition is the best therapy with neuroprotective approaches which are advanced and safe methods.

METHODS

This review summarizes some of the already-known and newly emerged neuroprotective targets and strategies that their experimental effects have been reported. Accordingly, literature was studied from 2000 to 2021 and appropriate articles were searched in Google Scholar and Scopus with the keywords given in the Keywords section of the current review.

RESULTS

Lewy bodies are the histopathologic characteristics of neurodegenerative disorders and are protein-rich intracellular deposits in which Alpha-Synuclein is its major protein. Alpha-Synuclein's toxic potential provides a compelling rationale for therapeutic strategies aimed at decreasing its burden in neuronal cells through numerous pathways including ubiquitin-proteasome system and autophagy-lysosome Pathway, proteolytic breakdown via cathepsin D, kallikrein-6 (neurosin), calpain-1 or MMP9, heat shock proteins, and proteolysis targeting chimera which consists of a target protein ligand and an E3 ubiquitin ligase (E3) followed by target protein ubiquitination (PROTACs). Other targets that have been noticed recently are the mutant huntingtin, tau proteins and glycogen synthase kinase 3β that their accumulation proceeds extensive neuronal damage and up to the minute approach such as Proteolysis Targeting Chimera promotes its degradation in cells. As various studies demonstrated that Mendelian gene mutations can result into the neurodegenerative diseases, additional target that has gained much interest is epigenetics such as mutation, phosphodiesterase, RNA binding proteins and Nuclear respiratory factor 1.

CONCLUSION

The novel molecular targets and new strategies compiled and introduced here can be used by scientists to design and discover more efficient small molecule drugs against the neurodegenerative diseases. And also the genes in which their mutations can lead to the α-synuclein aggregation or accumulation are discussed and considered a valuable information of epigenetics in dementia.

Signaling Mechanism for Modulation by GLP-1 and Exendin-4 of GABA Receptors on Rat Retinal Ganglion Cells

Glucagon-like peptide-1 (GLP-1) is expressed in retinal neurons, but its role in the retina is largely unknown. Here, we demonstrated that GLP-1 or the GLP-1 receptor (GLP-1R; a G protein-coupled receptor) agonist exendin-4 suppressed γ-aminobutyric acid receptor (GABAR)-mediated currents through GLP-1Rs in isolated rat retinal ganglion cells (GCs). Pre-incubation with the stimulatory G protein (G_s) inhibitor NF 449 abolished the exendin-4 effect. The exendin-4-induced suppression was mimicked by perfusion with 8-Br-cAMP (a cAMP analog), but was eliminated by the protein kinase A (PKA) inhibitor Rp-cAMP/KT-5720. The exendin-4 effect was accompanied by an increase in [Ca²⁺]_i of GCs through the IP₃-sensitive pathway and was blocked in Ca²⁺-free solution. Furthermore, when the activity of calmodulin (CaM) and CaM-dependent protein kinase II (CaMKII) was inhibited, the exendin-4 effect was eliminated. Consistent with this, exendin-4 suppressed GABAR-mediated light-evoked inhibitory postsynaptic currents in GCs in rat retinal slices. These results suggest that exendin-4-induced suppression may be mediated by a distinct G_s/cAMP-PKA/IP₃/Ca²⁺/CaM/CaMKII signaling pathway, following the activation of GLP-1Rs.

Intracellular and extracelluar cyclic GMP in the brain and the hippocampus

Cyclic Guanosine-Monophosphate (cGMP) is implicated as second messenger in a plethora of pathways and its effects are executed mainly by cGMP-dependent protein kinases (PKG). It is involved in both peripheral (cardiovascular regulation, intestinal secretion, phototransduction, etc.) and brain (hippocampal synaptic plasticity, neuroinflammation, cognitive function, etc.) processes. Stimulation of hippocampal cGMP signaling have been proved to be beneficial in animal models of aging, Alzheimer's disease or hepatic encephalopathy, restoring different cognitive functions such as passive avoidance, object recognition or spatial memory. However, even when some inhibitors of cGMP-degrading enzymes (PDEs) are already used against peripheral pathologies, their utility as neurological treatments is still under clinical investigation. Additionally, it has been demonstrated a list of cGMP roles as not second but first messenger. The role of extracellular cGMP has been specially studied in hippocampal function and cognitive impairment in animal models and it has emerged as an important modulator of neuroinflammation-mediated cognitive alterations and hippocampal synaptic plasticity malfunction. Specifically, it has been demonstrated that extracellular cGMP decreases hippocampal IL-1β levels restoring membrane expression of glutamate receptors in the hippocampus and cognitive function in hyperammonemic rats. The mechanisms implicated are still unclear and might involve complex interactions between hippocampal neurons, astrocytes and microglia. Membrane targets for extracellular cGMP are still poorly understood and must be addressed in future studies.

Spatial Learning Is Associated with Antagonist Outcomes for DNA Methylation and DNA Hydroxymethylation in the Transcriptional Regulation of the Ryanodine Receptor 3

Increasing attention has been drawn to the role that intracellular calcium stores play in neuronal function. Ryr3 is an intracellular calcium channel that contributes to hippocampal long-term potentiation, dendritic spine function, and higher cognitive processes. Interestingly, stimuli that increase neuronal activity upregulate the transcriptional activity of Ryr3 and augment DNA methylation in its proximal promoter. However, if these observations are valid for complex behavioral tasks such as learning and memory remains being evaluated. Relative expression analysis revealed that spatial learning increased the hippocampal levels of Ryr3, whereas mice trained using a visible platform that resulted in no spatial association showed reduced expression. Interestingly, we also observed that specific DNA modifications accompanied these opposite transcriptional changes. Increased DNA methylation was observed in hippocampal samples from spatially trained mice, and increased DNA hydroxymethylation was found in samples from mice trained using a visible platform. Both DNA modifications were not altered in control regions, suggesting that these changes are not generalized, but rather specific modifications associated with this calcium channel's transcriptional regulation. Our two experimental groups underwent the same physical task differing only in the spatial learning component, highlighting the tight relationship between DNA modifications and transcriptional activity in a relevant context such as behavioral training. Our results complement previous observations and suggest that DNA modifications are a reliable signal for the transcriptional activity of Ryr3 and can be useful to understand how conditions such as aging and neuropathological diseases determine altered Ryr3 expression.

Characterization of a novel stimulus-induced glial calcium wave in Drosophila larval peripheral segmental nerves and its role in PKG-modulated thermoprotection

Insects, as poikilotherms, have adaptations to deal with wide ranges in temperature fluctuation. Allelic variations in the foraging gene that encodes a cGMP dependent protein kinase, were discovered to have effects on behavior in Drosophila by Dr. Marla Sokolowski in 1980. This single gene has many pleiotropic effects and influences feeding behavior, metabolic storage, learning and memory and has been shown to affect stress tolerance. PKG regulation affects motoneuronal thermotolerance in Drosophila larvae as well as adults. While the focus of thermotolerance studies has been on the modulation of neuronal function, other cell types have been overlooked. Because glia are vital to neuronal function and survival, we wanted to determine if glia play a role in thermotolerance as well. In our investigation, we discovered a novel calcium wave at the larval NMJ and set out to characterize the wave's dynamics and the potential mechanism underlying the wave prior to determining what effect, if any, PKG modulation has on the thermotolerance of glia cells. Using pharmacology, we determined that calcium buffering mechanisms of the mitochondria and endoplasmic reticulum play a role in the propagation of our novel glial calcium wave. By coupling pharmacology with genetic manipulation using RNA interference (RNAi), we found that PKG modulation in glia alters thermoprotection of function as well as glial calcium wave dynamics.

Nitrergic modulation of neuronal excitability in the mouse hippocampus is mediated via regulation of Kv2 and voltage-gated sodium channels

Regulation of neuronal activity is a necessity for communication and information transmission. Many regulatory processes which have been studied provide a complex picture of how neurons can respond to permanently changing functional requirements. One such activity-dependent mechanism involves signaling mediated by nitric oxide (NO). Within the brain, NO is generated in response to neuronal NO synthase (nNOS) activation but NO-dependent pathways regulating neuronal excitability in the hippocampus remain to be fully elucidated. This study was set out to systematically assess the effects of NO on ion channel activities and intrinsic excitabilities of pyramidal neurons within the CA1 region of the mouse hippocampus. We characterized whole-cell potassium and sodium currents, both involved in action potential (AP) shaping and propagation and determined NO-mediated changes in excitabilities and AP waveforms. Our data describe a novel signaling by which NO, in a cGMP-independent manner, suppresses voltage-gated Kv2 potassium and voltage-gated sodium channel activities, thereby widening AP waveforms and reducing depolarization-induced AP firing rates. Our data show that glutathione, which possesses denitrosylating activity, is sufficient to prevent the observed nitrergic effects on potassium and sodium channels, whereas inhibition of cGMP signaling is also sufficient to abolish NO modulation of sodium currents. We propose that NO suppresses both ion channel activities via redox signaling and that an additional cGMP-mediated component is required to exert effects on sodium currents. Both mechanisms result in a dampened excitability and firing ability providing new data on nitrergic activities in the context of activity-dependent regulation of neuronal function following nNOS activation.

Serine/Threonine Phosphatases in LTP: Two B or Not to Be the Protein Synthesis Blocker-Induced Impairment of Early Phase

Dephosphorylation of target proteins at serine/threonine residues is one of the most crucial mechanisms regulating their activity and, consequently, the cellular functions. The role of phosphatases in synaptic plasticity, especially in long-term depression or depotentiation, has been reported. We studied serine/threonine phosphatase activity during the protein synthesis blocker (PSB)-induced impairment of long-term potentiation (LTP). Established protein phosphatase 2B (PP2B, calcineurin) inhibitor cyclosporin A prevented the LTP early phase (E-LTP) decline produced by pretreatment of hippocampal slices with cycloheximide or anisomycin. For the first time, we directly measured serine/threonine phosphatase activity during E-LTP, and its significant increase in PSB-treated slices was demonstrated. Nitric oxide (NO) donor SNAP also heightened phosphatase activity in the same manner as PSB, and simultaneous application of anisomycin + SNAP had no synergistic effect. Direct measurement of the NO production in hippocampal slices by the NO-specific fluorescent probe DAF-FM revealed that PSBs strongly stimulate the NO concentration in all studied brain areas: CA1, CA3, and dentate gyrus (DG). Cyclosporin A fully abolished the PSB-induced NO production in the hippocampus, suggesting a close relationship between nNOS and PP2B activity. Surprisingly, cyclosporin A alone impaired short-term plasticity in CA1 by decreasing paired-pulse facilitation, which suggests bi-directionality of the influences of PP2B in the hippocampus. In conclusion, we proposed a minimal model of signaling events that occur during LTP induction in normal conditions and the PSB-treated slices.

Discovery of effective phosphodiesterase 2 inhibitors with antioxidant activities for the treatment of Alzheimer's disease

The multi-target-directed-ligand (MTDL) strategy has been widely applied in the discovery of novel drugs for the treatment of Alzheimer's disease (AD) because of the multifactorial pathological mechanisms of AD. Phosphodiesterase-2 (PDE2) has been identified to be a novel and promising target for AD. However, MTDL combining with the inhibitory activity against PDE2A and other anti-AD factors such as antioxidants has not been developed yet. Herein, a novel series of PDE2 inhibitors with antioxidant capacities were designed, synthesized, and evaluated. Most compounds showed remarkable inhibitory activities against PDE2A as well as antioxidant activities. Compound 6d was selected, which showed good IC₅₀ of 6.1 nM against PDE2A, good antioxidant activity (ORAC (Trolox) = 8.4 eq.) and no cytotoxicity to SH-SY5Y cells. Molecular docking and dynamics simulations were applied for the rational design and explanation of structure-activity relationship (SAR) of lead compounds.

The biological pathways of Alzheimer disease: a review

Alzheimer disease is a progressive neurodegenerative disorder, mainly affecting older people, which severely impairs patients' quality of life. In the recent years, the number of affected individuals has seen a rapid increase. It is estimated that up to 107 million subjects will be affected by 2050 worldwide. Research in this area has revealed a lot about the biological and environmental underpinnings of Alzheimer, especially its correlation with β-Amyloid and Tau related mechanics; however, the precise molecular events and biological pathways behind the disease are yet to be discovered. In this review, we focus our attention on the biological mechanics that may lie behind Alzheimer development. In particular, we briefly describe the genetic elements and discuss about specific biological processes potentially associated with the disease.

Cyclic Nucleotides Signaling and Phosphodiesterase Inhibition: Defying Alzheimer's Disease

Defects in brain functions associated with aging and neurodegenerative diseases benefit insignificantly from existing options, suggesting that there is a lack of understanding of pathological mechanisms. Alzheimer's disease (AD) is such a nearly untreatable, allied to age neurological deterioration for which only the symptomatic cure is available and the agents able to mould progression of the disease, is still far away. The altered expression of phosphodiesterases (PDE) and deregulated cyclic nucleotide signaling in AD has provoked a new thought of targeting cyclic nucleotide signaling in AD. Targeting cyclic nucleotides as an intracellular messenger seems to be a viable approach for certain biological processes in the brain and controlling substantial. Whereas, the synthesis, execution, and/or degradation of cyclic nucleotides has been closely linked to cognitive deficits. In relation to cognition, the cyclic nucleotides (cAMP and cGMP) have an imperative execution in different phases of memory, including gene transcription, neurogenesis, neuronal circuitry, synaptic plasticity and neuronal survival, etc. AD is witnessed by impairments of these basic processes underlying cognition, suggesting a crucial role of cAMP/cGMP signaling in AD populations. Phosphodiesterase inhibitors are the exclusive set of enzymes to facilitate hydrolysis and degradation of cAMP and cGMP thereby, maintains their optimum levels initiating it as an interesting target to explore. The present work reviews a neuroprotective and substantial influence of PDE inhibition on physiological status, pathological progression and neurobiological markers of AD in consonance with the intensities of cAMP and cGMP.

Phosphodiesterase Inhibitors for Alzheimer's Disease: A Systematic Review of Clinical Trials and Epidemiology with a Mechanistic Rationale

BACKGROUND

Preclinical studies, clinical trials, and reviews suggest increasing 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP) with phosphodiesterase inhibitors is disease-modifying in Alzheimer's disease (AD). cAMP/protein kinase A (PKA) and cGMP/protein kinase G (PKG) signaling are disrupted in AD. cAMP/PKA and cGMP/PKG activate cAMP response element binding protein (CREB). CREB binds mitochondrial and nuclear DNA, inducing synaptogenesis, memory, and neuronal survival gene (e.g., brain-derived neurotrophic factor) and peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α). cAMP/PKA and cGMP/PKG activate Sirtuin-1, which activates PGC1α. PGC1α induces mitochondrial biogenesis and antioxidant genes (e.g.,Nrf2) and represses BACE1. cAMP and cGMP inhibit BACE1-inducing NFκB and tau-phosphorylating GSK3β.

OBJECTIVE AND METHODS

We review efficacy-testing clinical trials, epidemiology, and meta-analyses to critically investigate whether phosphodiesteraseinhibitors prevent or treat AD.

RESULTS

Caffeine and cilostazol may lower AD risk. Denbufylline and sildenafil clinical trials are promising but preliminary and inconclusive. PF-04447943 and BI 409,306 are ineffective. Vinpocetine, cilostazol, and nicergoline trials are mixed. Deprenyl/selegiline trials show only short-term benefits. Broad-spectrum phosphodiesterase inhibitor propentofylline has been shown in five phase III trials to improve cognition, dementia severity, activities of daily living, and global assessment in mild-to-moderate AD patients on multiple scales, including the ADAS-Cogand the CIBIC-Plus in an 18-month phase III clinical trial. However, two books claimed based on a MedScape article an 18-month phase III trial failed, so propentofylline was discontinued. Now, propentofylline is used to treat canine cognitive dysfunction, which, like AD, involves age-associated wild-type Aβ deposition.

CONCLUSION

Phosphodiesterase inhibitors may prevent and treat AD.

Redox modifications in synaptic components as biomarkers of cognitive status, in brain aging and disease

Aging is a natural process that includes several changes that gradually make organisms degenerate and die. Harman's theory proposes that aging is a consequence of the progressive accumulation of oxidative modifications mediated by reactive oxygen/nitrogen species, which plays an essential role in the development and progression of many neurodegenerative diseases. This review will focus on how abnormal redox modifications induced by age impair the functionality of neuronal redox-sensitive proteins involved in axonal elongation and guidance, synaptic plasticity, and intercellular communication. We will discuss post-transcriptional regulation of gene expression by microRNAs as a mechanism that controls the neuronal redox state. Finally, we will discuss how some brain-permeant antioxidants from the diet have a beneficial effect on cognition. Taken together, the evidence revised here indicates that oxidative-driven modifications of specific proteins and changes in microRNA expression may be useful biomarkers for aging and neurodegenerative diseases. Also, some specific antioxidant therapies have undoubtedly beneficial neuroprotective effects when administered in the correct doses, in the ideal formulation combination, and during the appropriate therapeutic window. The use of some antioxidants is, therefore, still poorly explored for the treatment of neurodegenerative diseases such as Alzheimer's disease.

Sildenafil for the Treatment of Alzheimer's Disease: A Systematic Review

Nitric oxide/cyclic guanosine monophosphate (cGMP) signaling is compromised in Alzheimer’s disease (AD), and phosphodiesterase 5 (PDE5), which degrades cGMP, is upregulated. Sildenafil inhibits PDE5 and increases cGMP levels. Integrating previous findings, we determine that most doses of sildenafil (especially low doses) likely activate peroxisome proliferator-activated receptor-γ coactivator 1α (PGC1α) via protein kinase G-mediated cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) phosphorylation and/or Sirtuin-1 activation and PGC1α deacetylation. Via PGC1α signaling, low-dose sildenafil likely suppresses β-secretase 1 expression and amyloid-β (Aβ) generation, upregulates antioxidant enzymes, and induces mitochondrial biogenesis. Plus, sildenafil should increase brain perfusion, insulin sensitivity, long-term potentiation, and neurogenesis while suppressing neural apoptosis and inflammation. A systematic review of sildenafil in AD was undertaken. In vitro, sildenafil protected neural mitochondria from Aβ and advanced glycation end products. In transgenic AD mice, sildenafil was found to rescue deficits in CREB phosphorylation and memory, upregulate brain-derived neurotrophic factor, reduce reactive astrocytes and microglia, decrease interleukin-1β, interleukin-6, and tumor necrosis factor-α, decrease neural apoptosis, increase neurogenesis, and reduce tau hyperphosphorylation. All studies that tested Aβ levels reported significant improvements except the two that used the highest dosage, consistent with the dose-limiting effect of cGMP-induced phosphodiesterase 2 (PDE2) activation and cAMP depletion on PGC1α signaling. In AD patients, a single dose of sildenafil decreased spontaneous neural activity, increased cerebral blood flow, and increased the cerebral metabolic rate of oxygen. A randomized control trial of sildenafil (ideally with a PDE2 inhibitor) in AD patients is warranted.

Carbon Monoxide, a Retrograde Messenger Generated in Postsynaptic Mushroom Body Neurons, Evokes Noncanonical Dopamine Release

Dopaminergic neurons innervate extensive areas of the brain and release dopamine (DA) onto a wide range of target neurons. However, DA release is also precisely regulated. In Drosophila melanogaster brain explant preparations, DA is released specifically onto α3/α'3 compartments of mushroom body (MB) neurons that have been coincidentally activated by cholinergic and glutamatergic inputs. The mechanism for this precise release has been unclear. Here we found that coincidentally activated MB neurons generate carbon monoxide (CO), which functions as a retrograde signal evoking local DA release from presynaptic terminals. CO production depends on activity of heme oxygenase in postsynaptic MB neurons, and CO-evoked DA release requires Ca²⁺ efflux through ryanodine receptors in DA terminals. CO is only produced in MB areas receiving coincident activation, and removal of CO using scavengers blocks DA release. We propose that DA neurons use two distinct modes of transmission to produce global and local DA signaling.SIGNIFICANCE STATEMENT Dopamine (DA) is needed for various higher brain functions, including memory formation. However, DA neurons form extensive synaptic connections, while memory formation requires highly specific and localized DA release. Here we identify a mechanism through which DA release from presynaptic terminals is controlled by postsynaptic activity. Postsynaptic neurons activated by cholinergic and glutamatergic inputs generate carbon monoxide, which acts as a retrograde messenger inducing presynaptic DA release. Released DA is required for memory-associated plasticity. Our work identifies a novel mechanism that restricts DA release to the specific postsynaptic sites that require DA during memory formation.

Role of cyclic nucleotides and their downstream signaling cascades in memory function: Being at the right time at the right spot

A plethora of studies indicate the important role of cAMP and cGMP cascades in neuronal plasticity and memory function. As a result, altered cyclic nucleotide signaling has been implicated in the pathophysiology of mnemonic dysfunction encountered in several diseases. In the present review we provide a wide overview of studies regarding the involvement of cyclic nucleotides, as well as their upstream and downstream molecules, in physiological and pathological mnemonic processes. Next, we discuss the regulation of the intracellular concentration of cyclic nucleotides via phosphodiesterases, the enzymes that degrade cAMP and/or cGMP, and via A-kinase-anchoring proteins that refine signal compartmentalization of cAMP signaling. We also provide an overview of the available data pointing to the existence of specific time windows in cyclic nucleotide signaling during neuroplasticity and memory formation and the significance to target these specific time phases for improving memory formation. Finally, we highlight the importance of emerging imaging tools like Förster resonance energy transfer imaging and optogenetics in detecting, measuring and manipulating the action of cyclic nucleotide signaling cascades.

Pro-cognitive effect of upregulating cyclic guanosine monophosphate signalling during memory acquisition or early consolidation is mediated by increased AMPA receptor trafficking

BACKGROUND

Episodic memory consists of different mnemonic phases, including acquisition and early and late consolidation. Each of these phases is characterised by distinct molecular processes. Although both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) are implicated in the acquisition phase, early consolidation only depends on cGMP, whereas late consolidation is mediated by cAMP. Accordingly, the cGMP-selective phosphodiesterase 5 (PDE5) inhibitor vardenafil or the cAMP-selective PDE4 inhibitor rolipram can improve memory acquisition or consolidation when applied during their respective time windows.

AIMS

Considering the important role of glutamatergic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR) during normal memory function, we aimed to investigate whether the differential actions of these PDE inhibitors are mediated through AMPAR dynamics.

METHODS

For biochemical analysis, mice were treated with either vardenafil or rolipram and sacrificed shortly after injection. For the behavioural studies, mice received either of the inhibitors during the different mnemonic phases, while their spatial memory was tested using the object location task, and they were sacrificed 24 hours later.

RESULTS

Administration of either vardenafil or rolipram causes rapid changes in AMPARs. Moreover, treatment with vardenafil during the acquisition or early consolidation of spatial memory resulted in increased surface levels of AMPARs which were still augmented 24 hours after learning. Membrane levels of AMPARs were not affected anymore 24 hours after learning when rolipram was administrated at either the acquisition or late consolidation phase.

CONCLUSIONS

These results suggest that dissociative molecular mechanisms could mediate the pro-cognitive function of different classes of PDE inhibitors, and in the case of vardenafil, this phenomenon could be explained by changes in AMPAR dynamics.

Two types of functionally distinct Ca2+ stores in hippocampal neurons

It is widely assumed that inositol trisphosphate (IP3) and ryanodine (Ry) receptors share the same Ca2+ pool in central mammalian neurons. We now demonstrate that in hippocampal CA1 pyramidal neurons IP3- and Ry-receptors are associated with two functionally distinct intracellular Ca2+ stores, respectively. While the IP3-sensitive Ca2+ store refilling requires Orai2 channels, Ry-sensitive Ca2+ store refilling involves voltage-gated Ca2+ channels (VGCCs). Our findings have direct implications for the understanding of function and plasticity in these central mammalian neurons.

LTP suppression by protein synthesis inhibitors is NO-dependent

For several decades, the ability of protein synthesis inhibitors (PSI) to suppress the long-term potentiation (LTP) of hippocampal responses is known. It is considered that mechanisms of such impairment are related to a cessation of translation and a delayed depletion of the protein pool required for maintenance of synaptic plasticity. The present study demonstrates that cycloheximide or anisomycin applications reduce amplitudes of the field excitatory postsynaptic potentials as well as the presynaptically mediated form of plasticity, the paired-pulse facilitation after LTP induction in neurons of the CA1 area of hippocampus. We showed that nitric oxide signaling could be one of the pathways that cause the LTP decrease induced by cycloheximide or anisomycin. Inhibitor of the NO synthase, L-NNA or the NO scavenger, PTIO, rescued the late-phase LTP and restored the paired-pulse facilitation up to the control levels. For the first time we have directly measured the nitric oxide production induced by application of the translation blockers in hippocampal neurons using the NO-sensitive dye DAF-FM. Inhibitory analysis demonstrated that changes during protein synthesis blockade downstream the NO signaling cascade are cGMP-independent and apparently are implemented through degradation of target proteins. Prolonged application of the NO donor SNAP impaired the LTP maintenance in the same manner as PSI.

Ryanodine Receptor-Mediated Calcium Release Has a Key Role in Hippocampal LTD Induction

The induction of both long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission entails pre- and postsynaptic Ca²⁺ signals, which represent transient increments in cytoplasmic free Ca²⁺ concentration. In diverse synapse types, Ca²⁺ release from intracellular stores contributes to amplify the Ca²⁺ signals initially generated by activation of neuronal Ca²⁺ entry pathways. Here, we used hippocampal slices from young male rats to evaluate whether pharmacological activation or inhibition of Ca²⁺ release from the endoplasmic reticulum (ER) mediated by ryanodine receptor (RyR) channels modifies LTD induction at Schaffer collateral-CA1 synapses. Pre-incubation of slices with ryanodine (1 μM, 1 h) or caffeine (1 mM, 30 min) to promote RyR-mediated Ca²⁺ release facilitated LTD induction by low frequency stimulation (LFS), but did not affect the amplitude of synaptic transmission, the profiles of field excitatory postsynaptic potentials (fEPSP) or the paired-pulse (PP) responses. Conversely, treatment with inhibitory ryanodine (20 μM, 1 h) to suppress RyR-mediated Ca²⁺ release prevented LTD induction, but did not affect baseline synaptic transmission or PP responses. Previous literature reports indicate that LTD induction requires presynaptic CaMKII activity. We found that 1 h after applying the LTD induction protocol, slices displayed a significant increase in CaMKII phosphorylation relative to the levels exhibited by un-stimulated (naïve) slices. In addition, LTD induction (1 h) enhanced the phosphorylation of the presynaptic protein Synapsin I at a CaMKII-dependent phosphorylation site, indicating that LTD induction stimulates presynaptic CaMKII activity. Pre-incubation of slices with 20 μM ryanodine abolished the increased CaMKII and Synapsin I phosphorylation induced by LTD, whereas naïve slices pre-incubated with inhibitory ryanodine displayed similar CaMKII and Synapsin I phosphorylation levels as naïve control slices. We posit that inhibitory ryanodine suppressed LTD-induced presynaptic CaMKII activity, as evidenced by the suppression of Synapsin I phosphorylation induced by LTD. Accordingly, we propose that presynaptic RyR-mediated Ca²⁺ signals contribute to LTD induction at Schaffer collateral-CA1 synapses.

Calcium Release Mediated by Redox-Sensitive RyR2 Channels Has a Central Role in Hippocampal Structural Plasticity and Spatial Memory

AIMS

Previous studies indicate that hippocampal synaptic plasticity and spatial memory processes entail calcium release from intracellular stores mediated by ryanodine receptor (RyR) channels. In particular, RyR-mediated Ca²⁺ release is central for the dendritic spine remodeling induced by brain-derived neurotrophic factor (BDNF), a neurotrophin that stimulates complex signaling pathways leading to memory-associated protein synthesis and structural plasticity. To examine if upregulation of ryanodine receptor type-2 (RyR2) channels and the spine remodeling induced by BDNF entail reactive oxygen species (ROS) generation, and to test if RyR2 downregulation affects BDNF-induced spine remodeling and spatial memory.

RESULTS

Downregulation of RyR2 expression (short hairpin RNA [shRNA]) in primary hippocampal neurons, or inhibition of nitric oxide synthase (NOS) or NADPH oxidase, prevented agonist-mediated RyR-mediated Ca²⁺ release, whereas BDNF promoted cytoplasmic ROS generation. RyR2 downregulation or inhibitors of N-methyl-d-aspartate (NMDA) receptors, or NOS or of NADPH oxidase type-2 (NOX2) prevented RyR2 upregulation and the spine remodeling induced by BDNF, as did incubation with the antioxidant agent N-acetyl l-cysteine. In addition, intrahippocampal injection of RyR2-directed antisense oligodeoxynucleotides, which caused significant RyR2 downregulation, caused conspicuous defects in a memorized spatial memory task.

INNOVATION

The present novel results emphasize the key role of redox-sensitive Ca²⁺ release mediated by RyR2 channels for hippocampal structural plasticity and spatial memory.

CONCLUSION

Based on these combined results, we propose (i) that BDNF-induced RyR2-mediated Ca²⁺ release and ROS generation via NOS/NOX2 are strictly required for the dendritic spine remodeling and the RyR2 upregulation induced by BDNF, and (ii) that RyR2 channel expression is crucial for spatial memory processes. Antioxid. Redox Signal. 29, 1125-1146.

Signaling Pathways for Long-Term Memory Formation in the Cricket

Unraveling the molecular mechanisms underlying memory formation in insects and a comparison with those of mammals will contribute to a further understanding of the evolution of higher-brain functions. As it is for mammals, insect memory can be divided into at least two distinct phases: protein-independent short-term memory and protein-dependent long-term memory (LTM). We have been investigating the signaling pathway of LTM formation by behavioral-pharmacological experiments using the cricket Gryllus bimaculatus, whose olfactory learning and memory abilities are among the highest in insect species. Our studies revealed that the NO-cGMP signaling pathway, CaMKII and PKA play crucial roles in LTM formation in crickets. These LTM formation signaling pathways in crickets share a number of attributes with those of mammals, and thus we conclude that insects, with relatively simple brain structures and neural circuitry, will also be beneficial in exploratory experiments to predict the molecular mechanisms underlying memory formation in mammals.

Novel Phosphodiesterase Inhibitors for Cognitive Improvement in Alzheimer's Disease

Alzheimer's disease (AD) is one of the greatest public health challenges. Phosphodiesterases (PDEs) are a superenzyme family responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). Since several PDE subfamilies are highly expressed in the human brain, the inhibition of PDEs is involved in neurodegenerative processes by regulating the concentration of cAMP and/or cGMP. Currently, PDEs are considered as promising targets for the treatment of AD since many PDE inhibitors have exhibited remarkable cognitive improvement effects in preclinical studies and over 15 of them have been subjected to clinical trials. The aim of this review is to summarize the outstanding progress that has been made by PDE inhibitors as anti-AD agents with encouraging results in preclinical studies and clinical trials. The binding affinity, pharmacokinetics, underlying mechanisms, and limitations of these PDE inhibitors in the treatment of AD are also reviewed and discussed.

Aging Impairs Hippocampal- Dependent Recognition Memory and LTP and Prevents the Associated RyR Up-regulation

Recognition memory comprises recollection judgment and familiarity, two different processes that engage the hippocampus and the perirhinal cortex, respectively. Previous studies have shown that aged rodents display defective recognition memory and alterations in hippocampal synaptic plasticity. We report here that young rats efficiently performed at short-term (5 min) and long-term (24 h) hippocampus-associated object-location tasks and perirhinal cortex-related novel-object recognition tasks. In contrast, aged rats successfully performed the object-location and the novel-object recognition tasks only at short-term. In addition, aged rats displayed defective long-term potentiation (LTP) and enhanced long-term depression (LTD). Successful long-term performance of object-location but not of novel-object recognition tasks increased the protein levels of ryanodine receptor types-2/3 (RyR2/RyR3) and of IP₃R1 in young rat hippocampus. Likewise, sustained LTP induction (1 h) significantly increased RyR2, RyR3 and IP₃R1 protein levels in hippocampal slices from young rats. In contrast, LTD induction (1 h) did not modify the levels of these three proteins. Naïve (untrained) aged rats displayed higher RyR2/RyR3 hippocampal protein levels but similar IP₃R1 protein content relative to young rats; these levels did not change following exposure to either memory recognition task or after LTP or LTD induction. The perirhinal cortex from young or aged rats did not display changes in the protein contents of RyR2, RyR3, and IP₃R1 after exposure at long-term (24 h) to the object-location or the novel-object recognition tasks. Naïve aged rats displayed higher RyR2 channel oxidation levels in the hippocampus compared to naïve young rats. The RyR2/RyR3 up-regulation and the increased RyR2 oxidation levels exhibited by aged rat hippocampus are likely to generate anomalous calcium signals, which may contribute to the well-known impairments in hippocampal LTP and spatial memory that take place during aging.

Reduced cGMP levels in CSF of AD patients correlate with severity of dementia and current depression

BACKGROUND

Alzheimer's disease (AD) is a neurodegenerative disorder, primarily affecting memory. That disorder is thought to be a consequence of neuronal network disturbances and synapse loss. Decline in cognitive function is associated with a high burden of neuropsychiatric symptoms (NPSs) such as depression. The cyclic nucleotides cyclic adenosine-3',5'-monophosphate (cAMP) and cyclic guanosine-3',5'-monophosphate (cGMP) are essential second messengers that play a crucial role in memory processing as well as synaptic plasticity and are potential therapeutic targets. Biomarkers that are able to monitor potential treatment effects and that reflect the underlying pathology are of crucial interest.

METHODS

In this study, we measured cGMP and cAMP in cerebrospinal fluid (CSF) in a cohort of 133 subjects including 68 AD patients and 65 control subjects. To address the association with disease progression we correlated cognitive status with cyclic nucleotide levels. Because a high burden of NPSs is associated with decrease in cognitive function, we performed an exhaustive evaluation of AD-relevant marker combinations in a depressive subgroup.

RESULTS

We show that cGMP, but not cAMP, levels in the CSF of AD patients are significantly reduced compared with the control group. Reduced cGMP levels in AD patients correlate with memory impairment based on Mini-Mental State Examination score (r = 0.17, p = 0.048) and tau as a marker of neurodegeneration (r = -0.28, p = 0.001). Moreover, we were able to show that AD patients suffering from current depression show reduced cGMP levels (p = 0.07) and exhibit a higher degree of cognitive impairment than non-depressed AD patients.

CONCLUSION

These results provide further evidence for an involvement of cGMP in AD pathogenesis and accompanying co-morbidities, and may contribute to elucidating synaptic plasticity alterations during disease progression.

From Age-Related Cognitive Decline to Alzheimer's Disease: A Translational Overview of the Potential Role for Phosphodiesterases

Phosphodiesterase inhibitors (PDE-Is) are pharmacological compounds enhancing cAMP and/or cGMP signaling. Both these substrates affect neural communication by influencing presynaptic neurotransmitter release and postsynaptic intracellular pathways after neurotransmitter binding to its receptor. Both cAMP and cGMP play an important role in a variety of cellular functions including neuroplasticity and neuroprotection. This chapter provides a translational overview of the effects of different classes of PDE-Is on cognition enhancement in age-related cognitive decline and Alzheimer's disease (AD). The most effective PDE-Is in preclinical models of aging and AD appear to be PDE2-Is, PDE4-Is and PDE5-Is. Clinical studies are relatively sparse and so far PDE1-Is and PDE4-Is showed some promising results. In the future, the demonstration of clinical proof of concept and the generation of isoform selective PDE-Is are the hurdles to overcome in developing safe and efficacious novel PDE-Is for the treatment of age-related cognitive decline and cognitive dysfunction in AD.

A First-in-Class Small-Molecule that Acts as a Dual Inhibitor of HDAC and PDE5 and that Rescues Hippocampal Synaptic Impairment in Alzheimer's Disease Mice

The targeting of two independent but synergistic enzymatic activities, histone deacetylases (HDACs, class I and HDAC6) and phosphodiesterase 5 (PDE5), has recently been validated as a potentially novel therapeutic approach for Alzheimer's disease (AD). Here we report the discovery of a new first-in-class small-molecule (CM-414) that acts as a dual inhibitor of PDE5 and HDACs. We have used this compound as a chemical probe to validate this systems therapeutics strategy, where an increase in the activation of cAMP/cGMP-responsive element-binding protein (CREB) induced by PDE5 inhibition, combined with moderate HDAC class I inhibition, leads to efficient histone acetylation. This molecule rescued the impaired long-term potentiation evident in hippocampal slices from APP/PS1 mice. Chronic treatment of Tg2576 mice with CM-414 diminished brain Aβ and tau phosphorylation (pTau) levels, increased the inactive form of GSK3β, reverted the decrease in dendritic spine density on hippocampal neurons, and reversed their cognitive deficits, at least in part by inducing the expression of genes related to synaptic transmission. Thus, CM-414 may serve as the starting point to discover balanced dual inhibitors with an optimal efficacy and safety profile for clinical testing on AD patients.

Cellular and molecular mechanisms of the brain-derived neurotrophic factor in physiological and pathological conditions

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that plays a key role in the central nervous system, promoting synaptic plasticity, neurogenesis and neuroprotection. The BDNF gene structure is very complex and consists of multiple 5′-non-coding exons, which give rise to differently spliced transcripts, and one coding exon at the 3′-end. These multiple transcripts, together with the complex transcriptional regulatory machinery, lead to a complex and fine regulation of BDNF expression that can be tissue and stimulus specific. BDNF effects are mainly mediated by the high-affinity, tropomyosin-related, kinase B receptor and involve the activation of several downstream cascades, including the mitogen-activated protein kinase, phospholipase C-γ and phosphoinositide-3-kinase pathways. BDNF exerts a wide range of effects on neuronal function, including the modulation of activity-dependent synaptic plasticity and neurogenesis. Importantly, alterations in BDNF expression and function are involved in different brain disorders and represent a major downstream mechanism for stress response, which has important implications in psychiatric diseases, such as major depressive disorders and schizophrenia. In the present review, we have summarized the main features of BDNF in relation to neuronal plasticity, stress response and pathological conditions, and discussed the role of BDNF as a possible target for pharmacological and non-pharmacological treatments in the context of psychiatric illnesses.

Reactive Oxygen Species: Physiological and Physiopathological Effects on Synaptic Plasticity

In the mammalian central nervous system, reactive oxygen species (ROS) generation is counterbalanced by antioxidant defenses. When large amounts of ROS accumulate, antioxidant mechanisms become overwhelmed and oxidative cellular stress may occur. Therefore, ROS are typically characterized as toxic molecules, oxidizing membrane lipids, changing the conformation of proteins, damaging nucleic acids, and causing deficits in synaptic plasticity. High ROS concentrations are associated with a decline in cognitive functions, as observed in some neurodegenerative disorders and age-dependent decay of neuroplasticity. Nevertheless, controlled ROS production provides the optimal redox state for the activation of transductional pathways involved in synaptic changes. Since ROS may regulate neuronal activity and elicit negative effects at the same time, the distinction between beneficial and deleterious consequences is unclear. In this regard, this review assesses current research and describes the main sources of ROS in neurons, specifying their involvement in synaptic plasticity and distinguishing between physiological and pathological processes implicated.

Short-term environmental enrichment enhances synaptic plasticity in hippocampal slices from aged rats

Age-associated changes in cognition are mirrored by impairments in cellular models of memory and learning, such as long-term potentiation (LTP) and long-term depression (LTD). In young rodents, environmental enrichment (EE) can enhance memory, alter LTP and LTD, as well as reverse cognitive deficits induced by aging. Whether short-term EE can benefit cognition and synaptic plasticity in aged rodents is unclear. Here, we tested if short-term EE could overcome age-associated impairments in induction of LTP and LTD. LTP and LTD could not be induced in the CA1 region of hippocampal slices in control, aged rats using standard stimuli that are highly effective in young rats. However, exposure of aged littermates to EE for three weeks enabled successful induction of LTP and LTD. EE-facilitated LTP was dependent upon N-methyl-d-aspartate receptors (NMDARs). These alterations in synaptic plasticity occurred with elevated levels of phosphorylated cAMP response element-binding protein and vascular endothelial growth factor, but in the absence of changes in several other synaptic and cellular markers. Importantly, our study suggests that even a relatively short period of EE is sufficient to alter synaptic plasticity and molecular markers linked to cognitive function in aged animals.

Calcium, Reactive Oxygen Species, and Synaptic Plasticity

In this review article, we address how activity-dependent Ca2+ signaling is crucial for hippocampal synaptic/structural plasticity and discuss how changes in neuronal oxidative state affect Ca2+ signaling and synaptic plasticity. We also analyze current evidence indicating that oxidative stress and abnormal Ca2+ signaling contribute to age-related synaptic plasticity deterioration.

Re-engineering a neuroprotective, clinical drug as a procognitive agent with high in vivo potency and with GABAA potentiating activity for use in dementia

Background

Synaptic dysfunction is a key event in pathogenesis of neurodegenerative diseases such as Alzheimer’s disease (AD) where synapse loss pathologically correlates with cognitive decline and dementia. Although evidence suggests that aberrant protein production and aggregation are the causative factors in familial subsets of such diseases, drugs singularly targeting these hallmark proteins, such as amyloid-β, have failed in late stage clinical trials. Therefore, to provide a successful disease-modifying compound and address synaptic dysfunction and memory loss in AD and mixed pathology dementia, we repurposed a clinically proven drug, CMZ, with neuroprotective and anti-inflammatory properties via addition of nitric oxide (NO) and cGMP signaling property.

Results

The novel compound, NMZ, was shown to retain the GABA_A potentiating actions of CMZ in vitro and sedative activity in vivo. Importantly, NMZ restored LTP in hippocampal slices from AD transgenic mice, whereas CMZ was without effect. NMZ reversed amnestic blockade of acetylcholine receptors by scopolamine as well as NMDA receptor blockade by a benzodiazepine and a NO synthase inhibitor in the step-through passive avoidance (STPA) test of learning and working memory. A PK/PD relationship was developed based on STPA analysis coupled with pharmacokinetic measures of drug levels in the brain: at 1 nM concentration in brain and plasma, NMZ was able to restore memory consolidation in mice.

Conclusion

Our findings show that NMZ embodies a promising pharmacological approach targeting synaptic dysfunction and opens new avenues for neuroprotective intervention strategies in mixed pathology AD, neurodegeneration, and dementia.

Electronic supplementary material

The online version of this article (doi:10.1186/s12868-015-0208-9) contains supplementary material, which is available to authorized users.

Ryanodine Receptor Activation Induces Long-Term Plasticity of Spine Calcium Dynamics

A key feature of signalling in dendritic spines is the synapse-specific transduction of short electrical signals into biochemical responses. Ca²⁺ is a major upstream effector in this transduction cascade, serving both as a depolarising electrical charge carrier at the membrane and an intracellular second messenger. Upon action potential firing, the majority of spines are subject to global back-propagating action potential (bAP) Ca²⁺ transients. These transients translate neuronal suprathreshold activation into intracellular biochemical events. Using a combination of electrophysiology, two-photon Ca²⁺ imaging, and modelling, we demonstrate that bAPs are electrochemically coupled to Ca²⁺ release from intracellular stores via ryanodine receptors (RyRs). We describe a new function mediated by spine RyRs: the activity-dependent long-term enhancement of the bAP-Ca²⁺ transient. Spines regulate bAP Ca²⁺ influx independent of each other, as bAP-Ca²⁺ transient enhancement is compartmentalized and independent of the dendritic Ca²⁺ transient. Furthermore, this functional state change depends exclusively on bAPs travelling antidromically into dendrites and spines. Induction, but not expression, of bAP-Ca²⁺ transient enhancement is a spine-specific function of the RyR. We demonstrate that RyRs can form specific Ca²⁺ signalling nanodomains within single spines. Functionally, RyR mediated Ca²⁺ release in these nanodomains induces a new form of Ca²⁺ transient plasticity that constitutes a spine specific storage mechanism of neuronal suprathreshold activity patterns.

A combination of two-photon calcium imaging, electrophysiology, and modelling shows how ryanodine receptors (a type of intracellular calcium channel) generate a signalling nanodomain within individual dendritic spines, enabling compartmentalized plasticity of calcium dynamics.

Experiences change neuronal circuits, and these circuit changes outlast the initial experiences. This means that, in neurons, the fast electrical activity encoding experiences needs to be transduced into longer-lived biochemical and structural changes. A key mediator between these two timescales of neuronal activity is the Ca²⁺ ion. Ca²⁺ serves both as an electric charge carrier mediating fast voltage changes at the membrane and as a second messenger activating intracellular signalling cascades. Even within the spatial confines of dendritic spines, the specialized domains of dendrites that receive synaptic connections, Ca²⁺ encodes a versatile array of specific functions. In this study, we first demonstrate that voltage-gated Ca²⁺ channels and ryanodine receptors, intracellular channels located on the membrane of the endoplasmic reticulum through which Ca²⁺ can be released into the cytosol, are electrochemically coupled in single dendritic spines. We identify how ryanodine receptors induce enhancement of the Ca²⁺ influx, mediated by the opening of voltage-gated Ca²⁺ channels, induced by action potentials in a compartmentalized, spine-specific manner. Within the femtoliter volume of a single spine, specificity of this route of Ca²⁺-signalling is achieved by a signalling nanodomain centred on the ryanodine receptor. Our work stresses the role of the ryanodine receptor not only as an ion channel releasing Ca²⁺ from the endoplasmic reticulum but also as a macromolecular complex generating specificity of Ca²⁺-signalling within the spatial constraints of a single spine.

Object memory enhancement by combining sub-efficacious doses of specific phosphodiesterase inhibitors

The second messengers cGMP and cAMP have a vital role in synaptic plasticity and memory processes. As such, phosphodiesterases inhibitors (PDE-Is), which prevent the breakdown of these cyclic nucleotides, represent a potential treatment strategy in memory decline. Recently it has been demonstrated that cGMP and cAMP signaling act in sequence during memory consolidation, with early cGMP signaling requiring subsequent cAMP signaling. Here, we sought to confirm this relationship, and to evaluate its therapeutic implications. Combining sub-efficacious doses of the cGMP-specific PDE type 5 inhibitor vardenafil (0.1 mg/kg) and cAMP-specific PDE type 4 inhibitor rolipram (0.01 mg/kg) during the early and late memory consolidation phase, respectively, led to improved memory performance in a 24 h interval object recognition task. Similarly, such a sub-efficacious combination treatment enhanced the transition of early-phase long-term potentiation (LTP) to late-phase LTP in hippocampal slices. In addition, both object memory and LTP were improved after administration of two sub-efficacious doses of the dual substrate PDE type 2 inhibitor BAY60 7550 (0.3 mg/kg) at the early and late consolidation phase, respectively. Taken together, combinations of sub-efficacious doses of cAMP- and cGMP-specific PDE-Is have an additive effect on long-term synaptic plasticity and memory formation and might prove a superior alternative to single PDE-I treatment.

Phosphodiesterase-5 Inhibitors: Action on the Signaling Pathways of Neuroinflammation, Neurodegeneration, and Cognition

Phosphodiesterase type 5 inhibitors (PDE5-Is) have recently emerged as a potential therapeutic strategy for neuroinflammatory, neurodegenerative, and memory loss diseases. Mechanistically, PDE5-Is produce an anti-inflammatory and neuroprotection effect by increasing expression of nitric oxide synthases and accumulation of cGMP and activating protein kinase G (PKG), the signaling pathway of which is thought to play an important role in the development of several neurodiseases, such as Alzheimer's disease (AD), Parkinson's disease (PD), and multiple sclerosis (MS). The aim of this paper was to review present knowledge of the signaling pathways that underlie the use of PDE5-Is in neuroinflammation, neurogenesis, learning, and memory.