Sildenafil Reverses the Neuropathological Alzheimer's Disease Phenotype in Cholinergic-Like Neurons Carrying the Presenilin 1 E280A Mutation

Background

Familial Alzheimer's disease (FAD) presenilin 1 E280A (PSEN 1 E280A) is characterized by functional impairment and the death of cholinergic neurons as a consequence of amyloid-β (Aβ) accumulation and abnormal phosphorylation of the tau protein. Currently, there are no available therapies that can cure FAD. Therefore, new therapies are urgently needed for treating this disease.

Objective

To assess the effect of sildenafil (SIL) on cholinergic-like neurons (ChLNs) harboring the PSEN 1 E280A mutation.

Methods

Wild-type (WT) and PSEN 1 E280A ChLNs were cultured in the presence of SIL (25μM) for 24 h. Afterward, proteinopathy, cell signaling, and apoptosis markers were evaluated via flow cytometry and fluorescence microscopy.

Results

We found that SIL was innocuous toward WT PSEN 1 ChLNs but reduced the accumulation of intracellular Aβ fragments by 87%, decreased the non-physiological phosphorylation of the protein tau at residue Ser202/Thr205 by 35%, reduced the phosphorylation of the proapoptotic transcription factor c-JUN at residue Ser63/Ser73 by 63%, decreased oxidized DJ-1 at Cys106-SO3 by 32%, and downregulated transcription factor TP53 (tumor protein p53), BH-3-only protein PUMA (p53 upregulated modulator of apoptosis), and cleaved caspase 3 (CC3) expression by 20%, 32%, and 22%, respectively, compared with untreated mutant ChLNs. Interestingly, SIL also ameliorated the dysregulation of acetylcholine-induced calcium ion (Ca2+) influx in PSEN 1 E280A ChLNs.

Conclusions

Although SIL showed no antioxidant capacity in the oxygen radical absorbance capacity and ferric ion reducing antioxidant power assays, it might function as an anti-amyloid and antiapoptotic agent and functional neuronal enhancer in PSEN 1 E280A ChLNs. Therefore, the SIL has therapeutic potential for treating FAD.

Exploring the Multifaceted Potential of Sildenafil in Medicine

Phosphodiesterase type 5 (PDE5) is pivotal in cellular signalling, regulating cyclic guanosine monophosphate (cGMP) levels crucial for smooth muscle relaxation and vasodilation. By targeting cGMP for degradation, PDE5 inhibits sustained vasodilation. PDE5 operates in diverse anatomical regions, with its upregulation linked to various pathologies, including cancer and neurodegenerative diseases. Sildenafil, a selective PDE5 inhibitor, is prescribed for erectile dysfunction and pulmonary arterial hypertension. However, considering the extensive roles of PDE5, sildenafil might be useful in other pathologies. This review aims to comprehensively explore sildenafil's therapeutic potential across medicine, addressing a gap in the current literature. Recognising sildenafil's broader potential may unveil new treatment avenues, optimising existing approaches and broadening its clinical application.

The neurobiology and therapeutic potential of multi-targeting β-secretase, glycogen synthase kinase 3β and acetylcholinesterase in Alzheimer's disease

Alzheimer's Disease (AD) is the most common form of dementia, affecting almost 50 million of people around the world, characterized by a complex and age-related progressive pathology with projections to duplicate its incidence by the end of 2050. AD pathology has two major hallmarks, the amyloid beta (Aβ) peptides accumulation and tau hyperphosphorylation, alongside with several sub pathologies including neuroinflammation, oxidative stress, loss of neurogenesis and synaptic dysfunction. In recent years, extensive research pointed out several therapeutic targets which have shown promising effects on modifying the course of the disease in preclinical models of AD but with substantial failure when transposed to clinic trials, suggesting that modulating just an isolated feature of the pathology might not be sufficient to improve brain function and enhance cognition. In line with this, there is a growing consensus that an ideal disease modifying drug should address more than one feature of the pathology. Considering these evidence, β-secretase (BACE1), Glycogen synthase kinase 3β (GSK-3β) and acetylcholinesterase (AChE) has emerged as interesting therapeutic targets. BACE1 is the rate-limiting step in the Aβ production, GSK-3β is considered the main kinase responsible for Tau hyperphosphorylation, and AChE play an important role in modulating memory formation and learning. However, the effects underlying the modulation of these enzymes are not limited by its primarily functions, showing interesting effects in a wide range of impaired events secondary to AD pathology. In this sense, this review will summarize the involvement of BACE1, GSK-3β and AChE on synaptic function, neuroplasticity, neuroinflammation and oxidative stress. Additionally, we will present and discuss new perspectives on the modulation of these pathways on AD pathology and future directions on the development of drugs that concomitantly target these enzymes.

The action of phosphodiesterase-5 inhibitors on β-amyloid pathology and cognition in experimental Alzheimer's disease: A systematic review

Alzheimer's disease (AD) is the most frequent cause of dementia worldwide. The etiology of AD is partially explained by the deposition of β-amyloid in the brain. Despite extensive research on the pathogenesis of AD, the current treatments are ineffective. Here, we systematically reviewed studies that investigated whether phosphodiesterase 5 inhibitors (PDE5i) are efficient in reducing the β-amyloid load in hippocampi and improving cognitive decline in rodent models with β-amyloid accumulation. We identified ten original studies, which used rodent models with β-amyloid accumulation, were treated with PDE5i, and β-amyloid was measured in the hippocampi. PDE5i was efficient in reducing the β-amyloid levels, except for one study that exclusively used female rodents and the treatment did not affect β-amyloid levels. Interestingly, PDE5i prevented cognitive decline in all studies. This study supports the potential therapeutic use of PDE5i for the reduction of the β-amyloid load in hippocampi and cognitive decline. However, we highlight the importance of conducting additional experimental studies to evaluate the PDE5i-related molecular mechanisms involved in β-amyloid removal in male and female animals.

Trimethylamine N-Oxide (TMAO) drives insulin resistance and cognitive deficiencies in a senescence accelerated mouse model

It has been established that ageing is the major risk factor for cognitive deficiency and it is becoming increasingly evident that insulin resistance is another factor. Biological plausibility for a link between insulin resistance and dementia is relevant for understanding disease etiology, and to form bases for prevention efforts to decrease disease burden. In the present study, peripheral and central insulin resistance was found in SAMP8 mice (aging mouse model) accompanied by cognitive deficiencies. Furthermore, a marked peripheral inflammatory state was observed in SAMP8 mice, followed by neuroinflammation that could be due to a higher cytokine leaking into the brain across an aging-disrupted blood brain barrier. Moreover, aging-induced gut dysbiosis produces higher TMAO that could also contribute to the peripheral and central inflammatory tone as well as to the cognitive deficiencies observed in SAMP8 mice. All those alterations were reversed by DMB, a treatment that decreases TMAO levels. Data obtained from this project suggest that microbial dysbiosis and increased TMAO secretion could be a key link between aging, insulin resistance and dementia. Thus, pharmacological intervention that leads to decreased TMAO levels, such as DMB, could open a new avenue for the future treatment of neurodegenerative diseases.

SAMP8 Mice as a Model of Age-Related Cognition Decline with Underlying Mechanisms in Alzheimer's Disease

Alzheimer's disease (AD) is a highly age-related cognitive decline frequently attacking the elderly. Senescence-accelerated mouse-prone 8 (SAMP8) is an ideal model to study AD, displaying age-related learning and memory disorders. SAMP8 mice exhibit most features of pathogenesis of AD, including an abnormal expression of anti-aging factors, oxidative stress, inflammation, amyloid-β (Aβ) deposits, tau hyperphosphorylation, endoplasmic reticulum stress, abnormal autophagy activity, and disruption of intestinal flora. SAMP8 mice, therefore, have visualized the understanding of AD, and also provided effective ways to find new therapeutic targets. This review focused on the age-related pathogenesis in SAMP8 mice, to advance the understanding of age-related learning and memory decline and clarify the mechanisms. Furthermore, this review will provide extensive foundations for SAMP8 mice used in therapeutics for AD.

Icaritin Improves Memory and Learning Ability by Decreasing BACE-1 Expression and the Bax/Bcl-2 Ratio in Senescence-Accelerated Mouse Prone 8 (SAMP8) Mice

Icaritin (ICT) is the main component in the traditional Chinese herb Epimedium, and it has been shown to have anti-Alzheimer's disease (AD) effects, but its neuroprotective effects and the pharmacological mechanisms are unclear. In the present study, senescence-accelerated mouse prone 8 (SAMP8) mice were randomly divided into a model group and an ICT-treated group. Learning and memory abilities were detected by the Morris water maze assay, and the expression of amyloid beta protein (Aβ) and β-site APP cleavage enzyme 1 (BACE1) was determined by Western blotting and polymerase chain reaction (PCR). Histological changes in CA1 and CA3 were detected by hematoxylin-eosin staining (H&E staining), and the immunohistochemical analysis was used to detect the expression and localization of Bax and Bcl-2. The results showed that compared with the SAMP8 mice, the ICT-treated SAMP8 mice showed improvements in spatial learning and memory retention. In addition, the number of necrotic cells and the morphological changes in CA1 and CA3 areas were significantly alleviated in the group of ICT-treated SAMP8 mice, and the expression of BACE1, Aβ_1-42 levels, and the Bax/Bcl-2 ratio in the hippocampus was obviously decreased in the ICT-treated group compared with the control group. The results demonstrated that ICT reduced BACE-1 levels, the contents of Aβ_1-42, and the Bax/Bcl-2 ratio, suggesting that ICT might have potential therapeutic benefits by delaying or modifying the progression of AD.

Therapeutic Potential of Phosphodiesterase Inhibitors against Neurodegeneration: The Perspective of the Medicinal Chemist

Increasing human life expectancy prompts the development of novel remedies for cognitive decline: 44 million people worldwide are affected by dementia, and this number is predicted to triple by 2050. Acetylcholinesterase and N-methyl-d-aspartate receptors represent the targets of currently available drugs for Alzheimer’s disease, which are characterized by limited efficacy. Thus, the search for therapeutic agents with alternative or combined mechanisms of action is wide open. Since variations in 3′,5′-cyclic adenosine monophosphate, 3′,5′-cyclic guanosine monophosphate, and/or nitric oxide levels interfere with downstream pathways involved in memory processes, evidence supporting the potential of phosphodiesterase (PDE) inhibitors in contrasting neurodegeneration should be critically considered. For the preparation of this Review, more than 140 scientific papers were retrieved by searching PubMed and Scopus databases. A systematic approach was adopted when overviewing the different PDE isoforms, taking into account details on brain localization, downstream molecular mechanisms, and inhibitors currently under study, according to available in vitro and in vivo data. In the context of drug repurposing, a section focusing on PDE5 was introduced. Original computational studies were performed to rationalize the emerging evidence that suggests the role of PDE5 inhibitors as multi-target agents against neurodegeneration. Moreover, since such compounds must cross the blood–brain barrier and reach inhibitory concentrations in the central nervous system to exert their therapeutic activity, physicochemical parameters were analyzed and discussed. Taken together, literature and computational data suggest that some PDE5 inhibitors, such as tadalafil, represent promising candidates.

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.

Comparison of the Protective Effects of Ginsenosides Rb1 and Rg1 on Improving Cognitive Deficits in SAMP8 Mice Based on Anti-Neuroinflammation Mechanism

This present study was designed to investigate the different effects of ginsenosides Rb1 and Rg1 on improving cognitive deficits in 4-month-old SAMP8 mice. Mice were divided into six groups, including the SAMP8 group, the SAMP8 + Donepezil (1.6 mg/kg) group, the SAMP8 + Rb1 (30 and 60 µmol/kg), and SAMP8 + Rg1 (30 and 60 µmol/kg) groups. SAMR1 mice of the same age were used as the control group. Ginsenosides and donepezil were administrated orally to animals for 8 weeks, then the learning and memory ability of mice were measured by using Morris water maze (MWM) test, object recognition test and passive avoidance experiments. The possible mechanisms were studied including the anti-glial inflammation of Rb1 and Rg1 using HE staining, immunohistochemistry and western blot experiments. Results revealed that Rb1 and Rg1 treatment significantly improved the discrimination index of SAMP8 mice in the object recognition test. Rb1 (60 µmol/kg) and Rg1 (30, 60 µmol/kg) could significantly shorten the escape latency in the acquisition test of the MWM test in SAMP8 mice. Furthermore, Rb1 and Rg1 treatments effectively reduced the number of errors in the passive avoidance task in SAMP8 mice. Western blot experiments revealed that Rb1 showed higher effect than Rg1 in decreasing protein expression levels of ASC, caspase-1 and Aβ in the hippocampus of SAMP8 mice, while Rg1 was more effective than Rb1 in decreasing the protein levels of iNOS. In addition, although Rb1 and Rg1 treatments showed significant protective effects in repairing neuronal cells loss and inhibiting the activation of astrocyte and microglia in hippocampus of SAMP8 mice, Rb1 was more effective than Rg1. These results suggest that Rb1 and Rg1 could improve the cognitive impairment in SAMP8 mice, and they have different mechanisms for the treatment of 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.

Early sirtuin 2 inhibition prevents age-related cognitive decline in a senescence-accelerated mouse model

The senescence-accelerated mouse prone-8 (SAMP8) model has been considered as a good model for aged-related cognitive decline and Alzheimer's disease (AD). Since epigenetic alterations represent a crucial mechanism during aging, in the present study we tested whether the inhibition of the histone deacetylase sirtuin 2 (SIRT2) could ameliorate the age-dependent cognitive impairments and associated neuropathology shown by SAMP8 mice. To this end, the potent SIRT2-selective inhibitor, 33i (5 mg/kg i.p. 8 weeks) was administered to 5-month-old (early treatment) and 8-month-old (late treatment) SAMP8 and aged matched control, senescence-accelerated mouse resistant-1 (SAMR1) mice. 33i administration to 5-month-old SAMP8 mice improved spatial learning and memory impairments shown by this strain in the Morris water maze. SAMP8 showed hyperphosphorylation of tau protein and decrease levels of SIRT1 in the hippocampus, which were not altered by 33i treatment. However, this treatment upregulated the glutamate receptor subunits GluN2A, GluN2B, and GluA1 in both SAMR1 and SAMP8. Moreover, early SIRT2 inhibition prevented neuroinflammation evidenced by reduced levels of GFAP, IL-1β, Il-6, and Tnf-α, providing a plausible explanation for the improvement of cognitive deficits shown by 33i-treated SAMP8 mice. When 33i was administered to 8-month-old SAMP8 with a severe established pathology, increases in GluN2A, GluN2B, and GluA1 were observed; however, it was not able to reverse the cognitive decline or the neuroinflammation. These results suggest that early SIRT2 inhibition might be beneficial in preventing age-related cognitive deficits, neuroinflammation, and AD progression and could be an emerging candidate for the treatment of other diseases linked to dementia.

Phosphodiesterase inhibitors say NO to Alzheimer's disease

Phosphodiesterases (PDEs) consisted of 11 subtypes (PDE1 to PDE11) and over 40 isoforms that regulate levels of cyclic guanosine monophosphate (cGMP) and cyclic adenosine monophosphate (cAMP), the second messengers in cell functions. PDE inhibitors (PDEIs) have been attractive therapeutic targets due to their involvement in diverse medical conditions, e.g. cardiovascular diseases, autoimmune diseases, Alzheimer's disease (AD), etc. Among them; AD with a complex pathology is a progressive neurodegenerative disorder which affect mostly senile people in the world and only symptomatic treatment particularly using cholinesterase inhibitors in clinic is available at the moment for AD. Consequently, novel treatment strategies towards AD are still searched extensively. Since PDEs are broadly expressed in the brain, PDEIs are considered to modulate neurodegenerative conditions through regulating cAMP and cGMP in the brain. In this sense, several synthetic or natural molecules inhibiting various PDE subtypes such as rolipram and roflumilast (PDE4 inhibitors), vinpocetine (PDE1 inhibitor), cilostazol and milrinone (PDE3 inhibitors), sildenafil and tadalafil (PDE5 inhibitors), etc have been reported showing encouraging results for the treatment of AD. In this review, PDE superfamily will be scrutinized from the view point of structural features, isoforms, functions and pharmacology particularly attributed to PDEs as target for AD therapy.

Phosphodiesterase 5 inhibitors as novel agents for the treatment of Alzheimer's disease

Alzheimer's disease (AD), characterized by a progressive impairment of memory and cognition, is a major health problem in both developing and developed countries. Currently, no drugs can reverse the progression of AD. Phosphodiesterase 5 (PDE5) is a critical component of the cyclic guanosine monophosphate/protein kinase G (cGMP/PKG) signaling pathway in neurons, the inhibition of which has produced neuroprotective effects, and PDE5 inhibitors have recently been thought to be potential therapeutic agents for AD. In this paper, we summarized the outstanding progress that has been made in PDE5 inhibitors as anti-AD agents with encouraging results in animal studies, clinical trials and the investigations on the underlying mechanisms. The novel PDE5 inhibitors reported recently in the treatment of AD were also reviewed and discussed.

Molecular Mechanisms of Phosphodiesterase-5 Inhibitors on Neuronal Apoptosis

Phosphodiesterase-5 inhibitors (PDE5Is) have been shown to modulate cell death/cell survival in different in vivo and in vitro models of disease by activating many signaling pathways. This review aimed at elucidating how PDE5Is can inhibit apoptosis. In this study, we describe many signaling pathways involved with the mechanism of action of PDE5Is that ultimately inhibit apoptosis and thus promote cell survival.

Pigment Epithelium-Derived Factor Plays a Role in Alzheimer's Disease by Negatively Regulating Aβ42

Alzheimer's disease (AD) is the most common cause of dementia. Pigment epithelium-derived factor (PEDF), a unique neurotrophic protein, decreases with aging. Previous reports have conflicted regarding whether the PEDF concentration is altered in AD patients. In addition, the effect of PEDF on AD has not been documented. Here, we tested serum samples of 31 AD patients and 271 normal controls. We found that compared to PEDF levels in young and middle-aged control subjects, PEDF levels were reduced in old-aged controls and even more so in AD patients. Furthermore, we verified that PEDF expression was much lower and amyloid β-protein (Aβ)42 expression was much higher in senescence-accelerated mouse prone 8 (SAMP8) strain mice than in senescence-accelerated mouse resistant 1 (SAMR1) control strain mice. Accordingly, high levels of Aβ42 were also observed in PEDF knockout (KO) mice. PEDF notably reduced cognitive impairment in the Morris water maze (MWM) and significantly downregulated Aβ42 in SAMP8 mice. Mechanistically, PEDF downregulated presenilin-1 (PS1) expression by inhibiting the c-Jun N-terminal kinase (JNK) pathway. Taken together, our findings demonstrate for the first time that PEDF negatively regulates Aβ42 and that PEDF deficiency with aging might play a crucial role in the development of AD.

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.

Neuroprotective potential of high-dose biotin

A recent controlled trial has established that high-dose biotin supplementation - 100 mg, three times daily - has a stabilizing effect on progression of multiple sclerosis (MS). Although this effect has been attributed to an optimization of biotin's essential cofactor role in the brain, a case can be made that direct stimulation of soluble guanylate cyclase (sGC) by pharmacological concentrations of biotin plays a key role in this regard. The utility of high-dose biotin in MS might reflect an anti-inflammatory effect of cGMP on the cerebral microvasculature, as well on oligodendrocyte differentiation and on Schwann cell production of neurotrophic factors thought to have potential for managing MS. But biotin's ability to boost cGMP synthesis in the brain may have broader neuroprotective potential. In many types of neurons and neural cells, cGMP exerts neurotrophic-mimetic effects - entailing activation of the PI3K-Akt and Ras-ERK pathways - that promote neuron survival and plasticity. Hippocampal long term potentiation requires nitric oxide synthesis, which in turn promotes an activating phosphorylation of CREB via a pathway involving cGMP and protein kinase G (PKG). In Alzheimer's disease (AD), amyloid beta suppresses this mechanism by inhibiting sGC activity; agents which exert a countervailing effect by boosting cGMP levels tend to restore effective long-term potentiation in rodent models of AD. Moreover, NO/cGMP suppresses amyloid beta production within the brain by inhibiting expression of amyloid precursor protein and BACE1. In conjunction with cGMP's ability to oppose neuron apoptosis, these effects suggest that high-dose biotin might have potential for the prevention and management of AD. cGMP also promotes neurogenesis, and may lessen stroke risk by impeding atherogenesis and hypertrophic remodeling in the cerebral vasculature. The neuroprotective potential of high-dose biotin likely could be boosted by concurrent administration of brain-permeable phosphodiesterase-5 inhibitors.

Neuroprotective effects of sodium hydrosulfide against β-amyloid-induced neurotoxicity

Alzheimer's disease (AD) is known to be caused by the accumulation of amyloid-β peptide (Aβ). The accumulation of Aβ has been shown to cause learning and memory impairment in rats, and it has been shown that hydrogen sulfide donors, such as sodium hydrosulfide (NaHS) can attenuate these effects. However, the underlying mechanisms have not yet been fully eludicated. This study was designed to investigate whether NaHS attenuates the inflammation and apoptosis induced by Aβ. We demonstrated that NaHS attenuated Aβ_25–35-induced neuronal reduction and apoptosis, and inhibited the activation of pro-caspase-3. It also decreased the protein expresion of phosphodiesterase 5 (PDE5) in the hippocampus of the rats. In addition, NaHS upregulated the expression of peroxisome proliferator-activated receptor (PPAR)-α and PPAR-γ, but it did not affect the expression of PPAR-β. Moreover, the Aβ_25–35-exposed rats exhibited a decrease in IκB-α degradation and an increase in nuclear factor-κB (NF-κB) p65 phosphorylation levels, whereas these effects were attenuated by NaHS. Our data suggest that NaHS prevents Aβ-induced neurotoxicity via the upregulation of PPAR-α and PPAR-γ and the inhibition of PDE5. Hence NaHS may prove to be beneficial in the treatment of AD.

Melatonin administration reverses the alteration of amyloid precursor protein-cleaving secretases expression in aged mouse hippocampus

Beta-amyloid (Aβ) peptide is the pathological hallmark of Alzheimer's disease (AD). Interestingly, Aβ is normally synthesized in the brain of healthy people; however, during advanced aging, the level of Aβ peptides increases. As a result, the aggregation of Aβ peptides leads to trafficking problems, synaptic loss, inflammation, and cell death. Melatonin, the hormone primarily synthesized and secreted from the pineal gland, is decreased with progressing age, particularly in Alzheimer's disease patients. The loss of melatonin levels and the abnormal accumulation of some proteins, such as Aβ peptides in the brains of AD patients are considered important factors in the initiation of the cognitive symptoms of dementia. A previous study in mice reported that increased brain melatonin levels remarkably diminished the potentially toxic Aβ peptide levels. The present study showed that aged mice significantly impaired spatial memory in the Morris Water Maze task. We also showed that α-, β-, and γ-secretases, which are type-I membrane protein proteases responsible for Aβ production, showed alterations in both mRNA and protein expression in the hippocampus of aged mice. The long-term administration of melatonin, mice had shorter escape latencies and remained in the target quadrant longer compared to the aged group. Melatonin attenuated the reduction of α-secretase and inhibited the increase of β- and γ-secretases. Moreover, melatonin attenuated the upregulation of pNFkB and the reduction of sirtuin1 in the hippocampus of aged mice. These results suggested that melatonin protected against Aβ peptide production in aged mice. Hence, melatonin loss in aging could be recompensed through dietary supplementation as a beneficial therapeutic strategy for AD prevention and progression.

Electroacupuncture Reduces Aβ Production and BACE1 Expression in SAMP8 Mice

Electroacupuncture (EA) has been reported to have beneficial effects on Alzheimer’s disease (AD). BACE1 (β-site amyloid precursor protein-cleaving enzyme 1) is involved in the abnormal production of amyloid-β plaque (Aβ), a hallmark of AD pathophysiology. Thus, the present study investigated the effects of EA on memory impairment, Aβ production, and BACE1 expression in senescence-accelerated mouse prone 8 (SAMP8) mice. We found that EA improved spatial learning and memory impairment of SAMP8 mice. Furthermore, EA attenuated Aβ production and repressed the expression of BACE1 in the hippocampus of SAMP8 mice. Taken together, our results suggest that EA could have a potential therapeutic application in AD and that BACE1 may be an important target of EA in the treatment of AD.

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.