Inhibition of Phosphodiesterase-4 Reverses Aβ-Induced Memory Impairment by Regulation of HPA Axis Related cAMP Signaling

Unraveling the Potential Underlying Mechanisms of Mild Behavioral Impairment: Focusing on Amyloid and Tau Pathology

The emergence of sustained neuropsychiatric symptoms (NPS) among non-demented individuals in later life, defined as mild behavioral impairment (MBI), is linked to a higher risk of cognitive decline. However, the underlying pathophysiological mechanisms remain largely unexplored. A growing body of evidence has shown that MBI is associated with alterations in structural and functional neuroimaging studies, higher genetic predisposition to clinical diagnosis of Alzheimer's disease (AD), as well as amyloid and tau pathology assessed in the blood, cerebrospinal fluid, positron-emission tomography (PET) imaging and neuropathological examination. These findings shed more light on the MBI-related potential neurobiological mechanisms, paving the way for the development of targeted pharmacological approaches. In this review, we aim to discuss the available clinical evidence on the role of amyloid and tau pathology in MBI and the potential underlying pathophysiological mechanisms. Dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis, disruption of neurotrophic factors, such as the brain-derived neurotrophic factor (BDNF), abnormal neuroinflammatory responses including the kynurenine pathway, dysregulation of transforming growth factor beta (TGF-β1), epigenetic alterations including micro-RNA (miR)-451a and miR-455-3p, synaptic dysfunction, imbalance in neurotransmitters including acetylcholine, dopamine, serotonin, gamma-aminobutyric acid (GABA) and norepinephrine, as well as altered locus coeruleus (LC) integrity are some of the potential mechanisms connecting MBI with amyloid and tau pathology. The elucidation of the underlying neurobiology of MBI would facilitate the design and efficacy of relative clinical trials, especially towards amyloid- or tau-related pathways. In addition, we provide insights for future research into our deeper understanding of its underlying pathophysiology of MBI, and discuss relative therapeutic implications.

Emerging phosphodiesterase inhibitors for treatment of neurodegenerative diseases

Neurodegenerative diseases (NDs) cause progressive loss of neuron structure and ultimately lead to neuronal cell death. Since the available drugs show only limited symptomatic relief, NDs are currently considered as incurable. This review will illustrate the principal roles of the signaling systems of cyclic adenosine and guanosine 3',5'-monophosphates (cAMP and cGMP) in the neuronal functions, and summarize expression/activity changes of the associated enzymes in the ND patients, including cyclases, protein kinases, and phosphodiesterases (PDEs). As the sole enzymes hydrolyzing cAMP and cGMP, PDEs are logical targets for modification of neurodegeneration. We will focus on PDE inhibitors and their potentials as disease-modifying therapeutics for the treatment of Alzheimer's disease, Parkinson's disease, and Huntington's disease. For the overlapped but distinct contributions of cAMP and cGMP to NDs, we hypothesize that dual PDE inhibitors, which simultaneously regulate both cAMP and cGMP signaling pathways, may have complementary and synergistic effects on modifying neurodegeneration and thus represent a new direction on the discovery of ND drugs.

Opportunities and perspectives of small molecular phosphodiesterase inhibitors in neurodegenerative diseases

Phosphodiesterase (PDE) is a superfamily of enzymes that are responsible for the hydrolysis of two second messengers: cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). PDE inhibition promotes the gene transcription by activating cAMP-response element binding protein (CREB), initiating gene transcription of brain-derived neurotrophic factor (BDNF). The procedure exerts neuroprotective profile, and motor and cognitive improving efficacy. From this point of view, PDE inhibition will provide a promising therapeutic strategy for treating neurodegenerative disorders. Herein, we summarized the PDE inhibitors that have entered the clinical trials or been discovered in recent five years. Well-designed clinical or preclinical investigations have confirmed the effectiveness of PDE inhibitors, such as decreasing Aβ oligomerization and tau phosphorylation, alleviating neuro-inflammation and oxidative stress, modulating neuronal plasticity and improving long-term cognitive impairment.

Maternal upbringing and selective breeding for voluntary exercise behavior modify patterns of DNA methylation and expression of genes in the mouse brain

Selective breeding has been utilized to study the genetic basis of exercise behavior, but research suggests that epigenetic mechanisms, such as DNA methylation, also contribute to this behavior. In a previous study, we demonstrated that the brains of mice from a genetically selected high runner (HR) line have sex-specific changes in DNA methylation patterns in genes known to be genomically imprinted compared to those from a non-selected control (C) line. Through cross-fostering, we also found that maternal upbringing can modify the DNA methylation patterns of additional genes. Here, we identify an additional set of genes in which DNA methylation patterns and gene expression may be altered by selection for increased wheel-running activity and maternal upbringing. We performed bisulfite sequencing and gene expression assays of 14 genes in the brain and found alterations in DNA methylation and gene expression for Bdnf, Pde4d and Grin2b. Decreases in Bdnf methylation correlated with significant increases in Bdnf gene expression in the hippocampus of HR compared to C mice. Cross-fostering also influenced the DNA methylation patterns for Pde4d in the cortex and Grin2b in the hippocampus, with associated changes in gene expression. We also found that the DNA methylation patterns for Atrx and Oxtr in the cortex and Atrx and Bdnf in the hippocampus were further modified by sex. Together with our previous study, these results suggest that DNA methylation and the resulting change in gene expression may interact with early-life influences to shape adult exercise behavior.

Rolipram Ameliorates Memory Deficits and Depression-Like Behavior in APP/PS1/tau Triple Transgenic Mice: Involvement of Neuroinflammation and Apoptosis via cAMP Signaling

BACKGROUND

Alzheimer disease (AD) and depression often cooccur, and inhibition of phosphodiesterase-4 (PDE4) has been shown to ameliorate neurodegenerative illness. Therefore, we explored whether PDE4 inhibitor rolipram might also improve the symptoms of comorbid AD and depression.

METHODS

APP/PS1/tau mice (10 months old) were treated with or without daily i.p. injections of rolipram for 10 days. The animal groups were compared in behavioral tests related to learning, memory, anxiety, and depression. Neurochemical measures were conducted to explore the underlying mechanism of rolipram.

RESULTS

Rolipram attenuated cognitive decline as well as anxiety- and depression-like behaviors. These benefits were attributed at least partly to the downregulation of amyloid-β, Amyloid precursor protein (APP), and Presenilin 1 (PS1); lower tau phosphorylation; greater neuronal survival; and normalized glial cell function following rolipram treatment. In addition, rolipram upregulated B-cell lymphoma-2 (Bcl-2) and downregulated Bcl-2-associated X protein (Bax) to reduce apoptosis; it also downregulated interleukin-1β, interleukin-6, and tumor necrosis factor-α to restrain neuroinflammation. Furthermore, rolipram increased cAMP, PKA, 26S proteasome, EPAC2, and phosphorylation of ERK1/2 while decreasing EPAC1.

CONCLUSIONS

Rolipram may mitigate cognitive deficits and depression-like behavior by reducing amyloid-β pathology, tau phosphorylation, neuroinflammation, and apoptosis. These effects may be mediated by stimulating cAMP/PKA/26S and cAMP/exchange protein directly activated by cAMP (EPAC)/ERK signaling pathways. This study suggests that PDE4 inhibitor rolipram can be an effective target for treatment of comorbid AD and depression.

Involvement of brain-derived neurotrophic factor signaling in the pathogenesis of stress-related brain diseases

Neurotrophins including brain-derived neurotrophic factor, BDNF, have critical roles in neuronal differentiation, cell survival, and synaptic function in the peripheral and central nervous system. It is well known that a variety of intracellular signaling stimulated by TrkB, a high-affinity receptor for BDNF, is involved in the physiological and pathological neuronal aspects via affecting cell viability, synaptic function, neurogenesis, and cognitive function. As expected, an alteration of the BDNF/TrkB system is suspected to be one of the molecular mechanisms underlying cognitive decline in cognitive diseases and mental disorders. Recent evidence has also highlighted a possible link between the alteration of TrkB signaling and chronic stress. Furthermore, it has been demonstrated that downregulation of the BDNF/TrkB system and chronic stress have a role in the pathogenesis of Alzheimer's disease (AD) and mental disorders. In this review, we introduce current evidence showing a close relationship between the BDNF/TrkB system and the development of cognition impairment in stress-related disorders, and the possible contribution of the upregulation of the BDNF/TrkB system in a therapeutic approach against these brain diseases.

Moderate-Intensity Intermittent Training Alters the DNA Methylation Pattern of PDE4D Gene in Hippocampus to Improve the Ability of Spatial Learning and Memory in Aging Rats Reduced by D-Galactose

(1) Background: Aging is the main risk factor for most neurodegenerative diseases, and the inhibition of Phosphodiesterase 4(PDE4) is considered a potential target for the treatment of neurological diseases. The purpose of this study was to investigate the inhibitory effect of moderate-intensity intermittent training (MIIT) on PDE4 in the hippocampus of rats with D-galactose (D-gal)-induced cognitive impairment, and the possible mechanism of improving spatial learning and memory. (2) Methods: the aging rats were treated with D-Gal (150 mg/kg/day, for 6 weeks). The aging rats were treated with MIIT for exercise intervention (45 min/day, 5 days/week, for 8 weeks). The Morris water maze test was performed before and after MIIT to evaluate the spatial learning and memory ability, then to observe the synaptic ultrastructure of the hippocampus CA1 region, to detect the expression of synaptic-related protein synaptophysin (SYP) and postsynaptic density protein 95 (PSD95), and to detect the expression of PDE4 subtypes, cAMP, and its signal pathway protein kinase A (PKA)/cAMP response element binding protein (CREB)/brain-derived neurotrophic factor (BDNF), and the PDE4 methylation level. (3) Results: we found that MIIT for 8 weeks alleviated the decline in spatial learning and memory ability, and improved the synaptic structure of the hippocampus and the expression of synaptic protein SYP and PSD95 in D-Gal aging rats. To elucidate the mechanism of MIIT, we analyzed the expression of PDE4 isoforms PDE4A/PDE4B/PDE4D, cAMP, and the signaling pathway PKA/CREB/BDNF, which play an important role in memory consolidation and maintenance. The results showed that 8 weeks of MIIT significantly up-regulated cAMP, PKA, p-CREB, and BDNF protein expression, and down-regulated PDE4D mRNA and protein expression. Methylation analysis of the PDE4D gene showed that several CG sites in the promoter and exon1 regions were significantly up-regulated. (4) Conclusions: MIIT can improve the synaptic structure of the hippocampus CA1 area and improve the spatial learning and memory ability of aging rats, which may be related to the specific regulation of the PDE4D gene methylation level and inhibition of PDE4D expression.

Intermittent treatment with Apremilast, a phosphodiesterase-4 inhibitor, ameliorates Alzheimer's-like pathology and symptoms through multiple targeting actions in aged T2D rats

BACKGROUND

Apremilast (Apre), a novel phosphodiesterase-4 (PDE4) inhibitor, has been shown to have anti-inflammatory, immunomodulator, neuroprotective and senolytic properties, therefore, Apre like other PDE4 inhibitors may be a promising candidate for treatment of Alzheimer's disease (AD).

OBJECTIVE

To evaluate the effectiveness of Apre on Alzheimer's like pathology and symptoms in an animal model.

METHODS

The effects of Apre and cilostazol, a reference drug, on the behavioral, biochemical, and pathological features of Alzheimer's disease induced by a high-fat/high-fructose diet combined with low-dose streptozotocin (HF/HFr/l-STZ) were investigated.

RESULT

Apre 5 mg/kg IP/day for 3 consecutive days per week for 8 weeks attenuated memory and learning deficits tested by novel object recognition, Morris water maze and passive avoidance tests. Apre treatment significantly decreased the number of degenerating cells, and abnormal suppression of gene expression of AMPA and NMDA receptor subunits in the cortex and hippocampus of the AD rat model compared to rats that received vehicle. A significant decrease in elevated levels of hippocampal amyloid beta, tau-positive cell count, cholinesterase activity, and hippocampal caspase-3, a biomarker of neurodegeneration, was also observed after treatment with Apre in AD rats compared to rats that received placebo. Furthermore, a significant decrease in pro-inflammatory cytokines, oxidative stress, insulin resistance and GSK-3 was demonstrated in AD aged rats treated by Apre.

CONCLUSION

Our findings demonstrate that intermittent treatment with Apre can enhance cognitive function in HF/HFr/l-STZ rats which may be related to decreased pro-inflammatory cytokines, oxidative stress, insulin resistance and GSK-3β.

Drug Discovery and Development Targeting Dementia

Dementia, most often associated with neurodegenerative diseases, affects millions of people worldwide, predominantly the elderly. Unfortunately, no treatment is still available. Therefore, there is an urgent need to address this situation. This review presents the state of the art of drug discovery and developments in targeting dementia. Several approaches are discussed, such as drug repurposing, the use of small molecules, and phosphodiesterase inhibitors. Furthermore, the review also provides insights into clinical trials of these molecules. Emphasis has been placed on small molecules and multi-target-directed ligands, as well as disease-modifying therapies. Finally, attention is drawn to the possibilities of applications of nanotechnology in managing dementia.

Roflumilast, a Phosphodiesterase-4 Inhibitor, Ameliorates Sleep Deprivation-Induced Cognitive Dysfunction in C57BL/6J Mice

Sleep deprivation (SD) interferes with long-term memory and cognitive functions by overactivation of phosphodiesterase (PDEs) enzymes. PDE4, a nonredundant regulator of the cyclic nucleotides (cAMP), is densely expressed in the hippocampus and is involved in learning and memory processes. In the present study, we investigated the effects of Roflumilast (ROF), a PDE4B inhibitor, on sleep deprivation-induced cognitive dysfunction in a mouse model. Memory assessment was performed using a novel object recognition task, and the hippocampal cAMP level was estimated by the ELISA method. The alterations in the expressions of PDE4B, amyloid-β (Aβ), CREB, BDNF, and synaptic proteins (Synapsin I, SAP 97, PSD 95) were assessed to gain insights into the possible mechanisms of action of ROF using the Western blot technique. Results show that ROF reversed SD-induced cognitive decline in mice. ROF downregulated PDE4B and Aβ expressions in the brain. Additionally, ROF improved the cAMP level and the protein expressions of synapsin I, SAP 97, and PSD 95 in the hippocampal region of SD mice. Taken together, these results suggest that ROF can suppress the deleterious effects of SD-induced cognitive dysfunction via the PDE4B-mediated cAMP/CREB/BDNF signaling cascade.

cAMP-PKA cascade: An outdated topic for depression?

Depression is a common neuropsychiatric disorder characterized by persistent depressed mood and causes serious socioeconomic burden worldwide. Hyperactivity of the hypothalamic-pituitary-adrenal (HPA) axis, deficiency of monoamine transmitters, neuroinflammation and abnormalities of the gut flora are strongly associated with the onset of depression. The cyclic AMP (cAMP)/protein kinase A (PKA) cascade, a major cross-species cellular signaling pathway, is supposed as important player and regulator of depression onset by controlling synaptic plasticity, cytokinesis, transcriptional regulation and HPA axis. In the central nervous system, the cAMP-PKA cascade can dynamically shape neural circuits by enhancing synaptic plasticity, and affect K⁺ channels by phosphorylating Kir4.1, thereby regulating neuronal excitation. The reduction of cAMP-PKA cascade affects neuronal excitation as well as synaptic plasticity, ultimately leading to pathological outcome of depression, while activation of cAMP-PKA cascade would provide a rapid antidepressant effect. In this review, we proposed to reconsider the function of cAMP-PKA cascade, especially in the rapid antidepressant effect. Local activation or indirect activation of PKA through adjusting anchor proteins would provide new idea for acute treatment of depression.

Inhibition of GPR17 with cangrelor improves cognitive impairment and synaptic deficits induced by Aβ1-42 through Nrf2/HO-1 and NF-κB signaling pathway in mice

The accumulation of amyloid beta (Aβ) in the brain is thought to be associated with cognitive deficits in Alzheimer's disease (AD). However, current methods to combat Aβ neurotoxicity are still lacking. G protein-coupled receptor 17 (GPR17) has become a target for treating inflammation in brain diseases, but it is unclear whether it has a role in AD. Here, we investigated the effects of cangrelor, a GPR17 antagonist, on neurotoxicity and memory impairment induced by intracerebroventricular (i.c.v.) injection of Aβ_1-42 in mice. The behavior results showed that cangrelor (2.0 or 4.0 μg/mouse, i.c.v.) treatment reversed the deficits in memory and learning ability induced by Aβ_1-42 in mice. Importantly, we demonstrated for the first time that GPR17 expression in the hippocampus and frontal cortex is increased in response to Aβ_1-42 exposures. We also found that cangrelor treatment reduced the activity of β-secretase 1 (BACE1) and the levels of soluble Aβ_1-42 in the hippocampus and frontal cortex. Meanwhile, cangrelor treatment suppressed oxidative stress induced by Aβ_1-42, as proved by reduced production of malondialdehyde (MDA), and increased glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT), and promoted the expression of nuclear factor E2-related factor 2 (Nrf2) and heme oxygenase 1 (HO-1). Furthermore, cangrelor also suppressed Aβ_1-42-induced neuroinflammation, characterized by suppressed activation of microglia, decreased the levels of pro-inflammatory cytokines, and nuclear translocation of NF-κB p65, as well as ameliorated synaptic deficits by promoting the upregulation of synaptic proteins, and increasing the number of Golgi-Cox stained dendritic spines. These results suggest that cangrelor may reverse Aβ_1-42-induced cognition deficits via inhibiting oxidative stress, neuroinflammation, and synaptic dysfunction mediated by Nrf2/HO-1 and NF-κB signaling.

Mechanisms Involved in Microglial-Interceded Alzheimer's Disease and Nanocarrier-Based Treatment Approaches

Alzheimer's disease (AD) is a common neurodegenerative disorder accountable for dementia and cognitive dysfunction. The etiology of AD is complex and multifactorial in origin. The formation and deposition of amyloid-beta (Aβ), hyperphosphorylated tau protein, neuroinflammation, persistent oxidative stress, and alteration in signaling pathways have been extensively explored among the various etiological hallmarks. However, more recently, the immunogenic regulation of AD has been identified, and macroglial activation is considered a limiting factor in its etiological cascade. Macroglial activation causes neuroinflammation via modulation of the NLRP3/NF-kB/p38 MAPKs pathway and is also involved in tau pathology via modulation of the GSK-3β/p38 MAPK pathways. Additionally, microglial activation contributes to the discrete release of neurotransmitters and an altered neuronal synaptic plasticity. Therefore, activated microglial cells appear to be an emerging target for managing and treating AD. This review article discussed the pathology of microglial activation in AD and the role of various nanocarrier-based anti-Alzeihmenr's therapeutic approaches that can either reverse or inhibit this activation. Thus, as a targeted drug delivery system, nanocarrier approaches could emerge as a novel means to overcome existing AD therapy limitations.

METTL3-dependent RNA m6A dysregulation contributes to neurodegeneration in Alzheimer's disease through aberrant cell cycle events

Background

N6-methyladenosine (m⁶A) modification of RNA influences fundamental aspects of RNA metabolism and m⁶A dysregulation is implicated in various human diseases. In this study, we explored the potential role of RNA m⁶A modification in the pathogenesis of Alzheimer disease (AD).

Methods

We investigated the m⁶A modification and the expression of m⁶A regulators in the brain tissues of AD patients and determined the impact and underlying mechanism of manipulated expression of m⁶A levels on AD-related deficits both in vitro and in vivo.

Results

We found decreased neuronal m⁶A levels along with significantly reduced expression of m⁶A methyltransferase like 3 (METTL3) in AD brains. Interestingly, reduced neuronal m⁶A modification in the hippocampus caused by METTL3 knockdown led to significant memory deficits, accompanied by extensive synaptic loss and neuronal death along with multiple AD-related cellular alterations including oxidative stress and aberrant cell cycle events in vivo. Inhibition of oxidative stress or cell cycle alleviated shMettl3-induced apoptotic activation and neuronal damage in primary neurons. Restored m⁶A modification by inhibiting its demethylation in vitro rescued abnormal cell cycle events, neuronal deficits and death induced by METTL3 knockdown. Soluble Aβ oligomers caused reduced METTL3 expression and METTL3 knockdown exacerbated while METTL3 overexpression rescued Aβ-induced synaptic PSD95 loss in vitro. Importantly, METTL3 overexpression rescued Aβ-induced synaptic damage and cognitive impairment in vivo.

Conclusions

Collectively, these data suggested that METTL3 reduction-mediated m⁶A dysregulation likely contributes to neurodegeneration in AD which may be a therapeutic target for AD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13024-021-00484-x.

Rolipram rescues memory consolidation deficits caused by sleep deprivation: Implication of the cAMP/PKA and cAMP/Epac pathways

BACKGROUND

Over the last few years, the number of people suffering from sleeping disorders has increased significantly despite negative effects on cognition and an association with brain inflammation.

OBJECTIVES

We assessed memory deficits caused by sleep deprivation (SD) to determine the therapeutic effect of phosphodiesterase 4 (PDE4) inhibitors on SD-induced memory deficits and to investigate whether the modulation of memory deficits by PDE4 inhibitors is mediated by a protein kinase A (PKA)-independent pathway in conjunction with a PKA-dependent pathway.

METHODS

Adult male mice were divided into four groups. Three SD groups were deprived of Rapid eye movement (REM) sleep for 12 h a day for six consecutive days. They were tested daily in the Morris water maze to evaluate learning and memory. One of the SD groups was injected with a PDE4 inhibitor, rolipram (1 mg/kg ip), whereas another had rolipram co-administered with chlorogenic acid (CHA, 20 mg/kg ip), an inhibitor of PKA. After 6 days, the mice were sacrificed, and the hippocampi were evaluated for cyclic AMP (cAMP) and nuclear factor Nrf-2 levels. The hippocampal expression of PKA, phosphorylated cAMP response element-binding protein (CREB), and phosphorylated glycogen synthase 3β (Ser389) were also evaluated.

RESULTS

SD caused a significant decrease in cAMP levels in the brain and had a detrimental effect on learning and memory. The administration of rolipram or rolipram+CHA resulted in an improvement in cognitive function.

CONCLUSION

The present study provides evidence that restoration of memory with PDE4 inhibitors occurs through a dual mechanism involving the PKA and Epac pathways.

GLP-1 improves the neuronal supportive ability of astrocytes in Alzheimer's disease by regulating mitochondrial dysfunction via the cAMP/PKA pathway

The glucagon-like peptide-1 (GLP-1) was shown to have neuroprotective effects in Alzheimer's disease (AD). However, the underlying mechanism remains elusive. Astrocytic mitochondrial abnormalities have been revealed to constitute important pathologies. In the present study, we investigated the role of astrocytic mitochondria in the neuroprotective effect of GLP-1 in AD. To this end, 6-month-old 5 × FAD mice were subcutaneously treated with liraglutide, a GLP-1 analogue (25 nmol/kg/qd) for 8 weeks. Liraglutide ameliorated mitochondrial dysfunction and prevented neuronal loss with activation of the cyclic adenosine 3',5'-monophosphate (cAMP)/phosphorylate protein kinase A (PKA) pathway in the brain of 5 × FAD mice. Next, we exposed astrocytes to β-amyloid (Aβ) in vitro and treated them with GLP-1. By activating the cAMP/PKA pathway, GLP-1 increased the phosphorylation of DRP-1 at the s637 site and mitigated mitochondrial fragmentation in Aβ-treated astrocytes. GLP-1 further improved the Aβ-induced energy failure, mitochondrial reactive oxygen species (ROS) overproduction, mitochondrial membrane potential (MMP) collapse, and cell toxicity in astrocytes. Moreover, GLP-1 also promoted the neuronal supportive ability of Aβ-treated astrocytes via the cAMP/PKA pathway. This study revealed a new mechanism behind the neuroprotective effect of GLP-1 in AD.

Roflumilast and tadalafil improve learning and memory deficits in intracerebroventricular Aβ1-42 rat model of Alzheimer's disease through modulations of hippocampal cAMP/cGMP/BDNF signaling pathway

BACKGROUND

Alzheimer's disease (AD) is the most prevalent age-dependent neurodegenerative disease characterized by progressive impairment of memory and cognitive functions. Cyclic nucleotides like cAMP and cGMP are well-known to play an important role in learning and memory functions. Enhancement of cAMP and cGMP levels in the hippocampus by phosphodiesterase (PDE) inhibitors might be a novel therapeutic approach for AD. Thus, the present study was planned to explore the therapeutic potential of roflumilast (RFM) and tadalafil (TDF) phosphodiesterase inhibitors in intracerebroventricular (ICV) Aβ1-42 induced AD in rats.

METHODS

ICV Aβ1-42 was administered in rats followed by treatment with RFM (0.05 mg/kg) and TDF (0.51 mg/kg) for 15 days. Novel object recognition (NOR), and Morris water maze (MWM) test were performed during the drug treatment schedule. On the day, 22 rats were sacrificed, and hippocampus was separated for biochemical, neuroinflammation, and histopathological analysis.

RESULTS

Aβ1-42 infused rats were induce behavioral impairment and increased AChE, BACE-1, Aβ1-42, GSK-3β, phosphorylated tau (p-Tau), pro-inflammatory cytokines (TNF-α, IL-1β, and IL-6) levels, oxidative stress (increased MDA, Nitrite and decreased GSH), histopathological changes, and reduced cAMP, cGMP, and BDNF levels. RFM and TDF significantly attenuated Aβ1-42 induced memory deficits and neuropathological alterations in the hippocampus.

CONCLUSION

The outcomes of the current study indicate that RFM and TDF lead to memory enhancement through upregulation of cAMP/cGMP/BDNF pathway, thus they may have a therapeutic potential in cognitive deficits associated with AD.

Alterations in cyclic nucleotide signaling are implicated in healthy aging and age-related pathologies of the brain

It is not only important to consider how hormones may change with age, but also how downstream signaling pathways that couple to hormone receptors may change. Among these hormone-coupled signaling pathways are the 3',5'-cyclic guanosine monophosphate (cGMP) and 3',5'-cyclic adenosine monophosphate (cAMP) intracellular second messenger cascades. Here, we test the hypothesis that dysfunction of cAMP and/or cGMP synthesis, execution, and/or degradation occurs in the brain during healthy and pathological diseases such as Alzheimer's disease, Parkinson's disease, and Huntington's disease. Although most studies report lower cyclic nucleotide signaling in the aged brain, with further reductions noted in the context of age-related diseases, there are select examples where cAMP signaling may be elevated in select tissues. Thus, therapeutics would need to target cAMP/cGMP in a tissue-specific manner if efficacy for select symptoms is to be achieved without worsening others.

Cilomilast Ameliorates Renal Tubulointerstitial Fibrosis by Inhibiting the TGF-β1-Smad2/3 Signaling Pathway

Background: Renal tubulointerstitial fibrosis is the key pathological feature in chronic kidney diseases (CKDs) with no satisfactory therapies in clinic. Cilomilast is a second-generation, selective phosphodiesterase-4 inhibitor, but its role in renal tubulointerstitial fibrosis in CKD remains unclear.

Material and Methods: Cilomilast was applied to the mice with unilateral ureteric obstruction (UUO) and renal fibroblast cells (NRK-49F) stimulated by TGF-β1. Renal tubulointerstitial fibrosis and inflammation after UUO or TGF-β1 stimulation were examined by histology, Western blotting, real-time PCR and immunohistochemistry. KIM-1 and NGAL were detected to evaluate tubular injury in UUO mice.

Results:In vivo, immunohistochemistry and western blot data demonstrated that cilomilast treatment inhibited extracellular matrix deposition, profibrotic gene expression, and the inflammatory response. Furthermore, cilomilast prevented tubular injury in UUO mice, as manifested by reduced expression of KIM-1 and NGAL in the kidney. In vitro, cilomilast attenuated the activation of fibroblast cells stimulated by TGF-β1, as shown by the reduced expression of fibronectin, α-SMA, collagen I, and collagen III. Cilomilast also inhibited the activation of TGF-β1-Smad2/3 signaling in TGF-β1-treated fibroblast cells.

Conclusion: The findings of this study suggest that cilomilast is protective against renal tubulointerstitial fibrosis in CKD, possibly through the inhibition of TGF-β1-Smad2/3 signaling, indicating the translational potential of this drug in treating CKD.

Phosphodiesterase-2 inhibitor reverses post-traumatic stress induced fear memory deficits and behavioral changes via cAMP/cGMP pathway

Phosphodiesterase 2 is one of the phosphodiesterase (PDEs) family members that regulate cyclic nucleotide (namely cAMP and cGMP) concentrations. The present study determined whether PDE2 inhibition could rescue post-traumatic stress disorder (PTSD)-like symptoms. Mice were subjected to single prolonged stress (SPS) and treated with selective PDE2 inhibitor Bay 60-7550 (0.3, 1, or 3 mg/kg, i.p.). The behavioral tests such as forced swimming, sucrose preference test, open field, elevated plus maze, and contextual fear paradigm were conducted to determine the effects of Bay 60-7550 on SPS-induced depression- and anxiety-like behavior and fear memory deficits. The results suggested that Bay 60-7550 reversed SPS-induced depression- and anxiety-like behavior and fear memory deficits. Moreover, Bay 60-7550 prevented SPS-induced changes in the adrenal gland index, synaptic proteins synaptophysin and PSD95 expression, PKA, PKG, pCREB, and BDNF levels in the hippocampus and amygdala. These effects were completely prevented by PKG inhibitor KT5823. While PKA inhibitor H89 also prevented Bay 60-7550-induced pCREB and BDNF expression, but only partially prevented the effects on PSD95 expression in the hippocampus. These findings suggest that Bay 60-7550 protects mice against PTSD-like stress induced traumatic injury by activation of cGMP- or cAMP-related neuroprotective molecules, such as synaptic proteins, pCREB and BDNF.

Role of phosphodiesterases in the pathophysiology of neurodevelopmental disorders

Phosphodiesterases (PDEs) are enzymes involved in the homeostasis of both cAMP and cGMP. They are members of a family of proteins that includes 11 subfamilies with different substrate specificities. Their main function is to catalyze the hydrolysis of cAMP, cGMP, or both. cAMP and cGMP are two key second messengers that modulate a wide array of intracellular processes and neurobehavioral functions, including memory and cognition. Even if these enzymes are present in all tissues, we focused on those PDEs that are expressed in the brain. We took into consideration genetic variants in patients affected by neurodevelopmental disorders, phenotypes of animal models, and pharmacological effects of PDE inhibitors, a class of drugs in rapid evolution and increasing application to brain disorders. Collectively, these data indicate the potential of PDE modulators to treat neurodevelopmental diseases characterized by learning and memory impairment, alteration of behaviors associated with depression, and deficits in social interaction. Indeed, clinical trials are in progress to treat patients with Alzheimer's disease, schizophrenia, depression, and autism spectrum disorders. Among the most recent results, the application of some PDE inhibitors (PDE2A, PDE3, PDE4/4D, and PDE10A) to treat neurodevelopmental diseases, including autism spectrum disorders and intellectual disability, is a significant advance, since no specific therapies are available for these disorders that have a large prevalence. In addition, to highlight the role of several PDEs in normal and pathological neurodevelopment, we focused here on the deregulation of cAMP and/or cGMP in Down Syndrome, Fragile X Syndrome, Rett Syndrome, and intellectual disability associated with the CC2D1A gene.

Roles for H+ /K+ -ATPase and zinc transporter 3 in cAMP-mediated lysosomal acidification in bafilomycin A1-treated astrocytes

Vacuolar ATPase (v-ATPase) is the main proton pump that acidifies vesicles such as lysosomes. Disruption in the lysosomal localization of v-ATPase leads to lysosomal dysfunction, thus contributing to the pathogenesis of lysosomal storage disorders and neurodegenerative diseases such as Alzheimer's disease. Recent studies showed that increases in cyclic AMP (cAMP) levels acidify lysosomes and consequently enhance autophagy flux. Although the upregulation of v-ATPase function may be the key mechanism underlying the cAMP-mediated lysosomal acidification, it is unknown whether a mechanism independent of v-ATPase may be contributing to this phenomenon. In the present study, we modeled v-ATPase dysfunction in brain cells by blocking lysosomal acidification in cortical astrocytes through treatment with bafilomycin A1, a selective v-ATPase inhibitor. We observed that cAMP reversed the pH changes via the activation of protein kinase A; interestingly, cAMP also increased autophagy flux even in the presence of bafilomycin A1, suggesting the presence of an alternative route of proton entry. Notably, pharmacological inhibitors and siRNAs of H⁺ /K⁺ -ATPase markedly shifted the lysosomal pH toward more alkaline values in bafilomycin A1/cAMP-treated astrocytes, suggesting that H⁺ /K⁺ -ATPase may be the alternative route of proton entry for lysosomal acidification. Furthermore, the cAMP-mediated reversal of lysosomal pH was nullified in the absence of ZnT3 that interacts with H⁺ /K⁺ -ATPase. Our results suggest that the H⁺ /K⁺ -ATPase/ZnT3 complex is recruited to lysosomes in a cAMP-dependent manner and functions as an alternative proton pump for lysosomes when the v-ATPase function is downregulated, thus providing insight into the potential development of a new class of lysosome-targeted therapeutics in neurodegenerative diseases.

Beta amyloid-induced time-dependent learning and memory impairment: involvement of HPA axis dysfunction

Aβ aggregation is one of the pathological biomarkers of Alzheimer's disease (AD). However, the possible mechanism related to Aβ-induced pathological signaling pathway is still unknown. In the present study, Aβ1-42-induced time-dependent memory impairment and its possible relationship to hypothalamic-pituitary-adrenal (HPA) axis hyperactivity were examined. Aβ1-42-treated mice significantly impaired acquisition activity in the learning curve at 10 days, 1 and 4 months in the Morris water-maze (MWM) task. This learning activity was back to normal at 8 months after Aβ1-42 treatment. In the probe trial test, Aβ1-42-treated mice needed longer latencies to touch the precious platform location and fewer numbers of crossing from 10 days to 4 months after microinjection. This Aβ1-42 induced memory loss was consistent with the results of the step-down passive avoidance test. The HPA axis related parameters, such as corticosterone (CORT) level in the serum, glucocorticoid receptor (GR) and corticotropin-releasing factor receptor (CRF-R) expression in the frontal cortex and hippocampus increased in Aβ1-42-treated mice from 10 days to 4 months. While the downstream molecules phosphorylation of cyclic AMP response element binding (pCREB) and brain-derived neurotrophic factor (BDNF) expression decreased during this time. These effects were back to normal 8 months after treatment with Aβ1-42. Altogether, our results suggested that Aβ1-42 induced significant learning and memory impairment, which is involved in HPA axis dysfunction.

GW9508 ameliorates cognitive impairment via the cAMP-CREB and JNK pathways in APPswe/PS1dE9 mouse model of Alzheimer's disease

GPR40 was utilized as the drug target to the treatment of diabetes, but the function and mechanisms ameliorating the Alzheimer's disease (AD) remain unknown. In present study, the typical APP/PS1 mouse model was applied to explore the function and mechanism of GPR40 in AD. GPR40 agonist GW9508 and antagonist GW1100 were respectively given by i.c.v. injection to activate/inhibit the GPR40 in the brain of APP/PS1 mice which illustrated the function and mechanism of GPR40 in ameliorating AD symptoms. Morris water maze test, step-through test, Y-maze spontaneous alternation test, open field test and new object recognition test were used to test the cognitive function and memory ability of mice, while molecular biology experiments such as Western blot, immunofluorescence, JC-1 were used to detect the corresponding changes of signal pathways. The results revealed that treatment with GW9508 could significantly ameliorate cognitive deficits of APP/PS1 mice, upregulate the expression levels of cAMP, p-CREB and neurotrophic factors in vivo, while GW9508 also ameliorate Aβ_1-42-induced neuron damage and downregulate the expression levels of pathological protein such as p-JNK, JNK and apoptosis-related proteins such as IL-6, IL-1β, TNF-α and caspase-3 in vitro. Meanwhile, high-content screening also showed that GW9508 promoted the cellular differentiation of SH-SY5Y cells, while GW1100 reversed the effects of GW9508. These results suggested that GPR40 was an underlying therapeutic target for the treatment of AD and GPR40 agonist could be explored as the emerging AD therapeutic drug.

Pharmacological Targeting of Microglial Activation: New Therapeutic Approach

Mounting evidence suggests that neuroinflammation is not just a consequence but a vital contributor to the development and progression of Parkinson’s disease (PD). Microglia in particular, may contribute to the induction and modulation of inflammation in PD. Upon stimulation, microglia convert into activated phenotypes, which exist along a dynamic continuum and bear different immune properties depending on the disease stage and severity. Activated microglia release various factors involved in neuroinflammation, such as cytokines, chemokines, growth factors, reactive oxygen species (ROS), reactive nitrogen species (RNS), and prostaglandins (PGs). Further, activated microglia interact with other cell types (e.g., neurons, astrocytes and mast cells) and are closely associated with α-synuclein (α-syn) pathophysiology and iron homeostasis disturbance. Taken together, microglial activation and microglia-mediated inflammatory responses play essential roles in the pathogenesis of PD and elucidation of the complexity and imbalance of microglial activation may shed light on novel therapeutic approaches for PD.

Modified Huang-Lian-Jie-Du Decoction Ameliorates Aβ Synaptotoxicity in a Murine Model of Alzheimer's Disease

Alzheimer's disease (AD) is a common neurodegenerative disease, characterized by cognitive dysfunction; however, the therapeutic strategies are not fully understood. Huang-Lian-Jie-Du-Decoction (HLJDD) is a famous traditional Chinese herbal formula that has been widely used clinically to treat dementia. Recently, according to previous study and our clinical practice, we generate a new modification of HLJDD (named modified-HLJDD). In this study, we indicated that modified-HLJDD attenuated learning and memory deficiencies in Aβ_1-42 oligomer-induced AD model, and we confirmed the exact metabolites in modified-HLJDD solution, as compared with HLJDD by UHPLC-Q-TOF-MS. Using GC-Q-TOF/MS-based metabolomics, we identified adenosine as the potential significant metabolite, responsible for modified-HLJDD regulating energy metabolism and synaptic plasticity in AD model. We also revealed that the potential underlying mechanism of modified-HLJDD in AD model may involve NMDA receptor-mediated glutamatergic transmission and adenosine/ATPase/AMPK cascade. Moreover, we also indicated the differential gut microbiota which mainly involved Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria at the phylum level upon modified-HLJDD treatment in AD model. Based on the correlation of metabolomic analysis with microbiome analysis, we clarified that Dorea is the most affected microbiota with adenosine upon modified-HLJDD treatment in AD model. Thus, our study suggests that modified-HLJDD may serve as a potential therapeutic drug in treating AD.

Phosphodiesterase-2 Inhibitor Bay 60-7550 Ameliorates Aβ-Induced Cognitive and Memory Impairment via Regulation of the HPA Axis

The dysfunction of the hypothalamus-pituitary-adrenal (HPA) axis is often seen in Alzheimer's disease (AD) patients with cognitive deficits. Selective inhibition of phosphodiesterase (PDE) 4 and 5 has already proven to be effective in reducing beta-amyloid 1-42 (Aβ1-42)-mediated pathology by regulating corticotropin-releasing factor (CRF) and glucocorticoid receptor (GR) expression, suggesting that PDE-dependent signaling is involved in Aβ1-42-induced HPA axis dysfunction. However, nausea and vomiting are the side effects of some PDE4 inhibitors, which turn our attention to other PDEs. PDE2 are highly expressed in the hippocampus and cortex, which associate with learning and memory, but not in the area postrema that would cause vomiting. The present study suggested that microinjection of Aβ1-42 to the intracerebroventricle induced learning and memory impairments and dysregulation of the HPA axis by increased expression of CRF and GR. However, the PDE2 inhibitor Bay 60-7550 significantly ameliorated the learning and memory impairment in the Morris water maze (MWM) and step-down passive avoidance tests. The Aβ1-42-induced increased CRF and GR levels were also reversed by the treatment with Bay 60-7550. These Bay 60-7550's effects were prevented by pretreatment with the PKG inhibitor KT5823. Moreover, the Bay 60-7550-induced downstream phosphorylation of cyclic AMP response element binding (pCREB) and brain-derived neurotrophic factor (BDNF) expression was also prevented (or partially prevented) by KT5823 or the PKA inhibitor H89. These results may lead to the discovery of novel strategies for the treatment of age-related cognitive disorders, such as AD, which affects approximately 44 million people worldwide.

Treadmill exercise restores high fat diet-induced disturbance of hippocampal neurogenesis through β2-adrenergic receptor-dependent induction of thioredoxin-1 and brain-derived neurotrophic factor

A high-fat diet (HFD) is known to induce metabolic disturbances that may lead to cognitive impairment. In the present study, we investigated whether a regular treadmill exercise program would improve HFD-induced hippocampal-dependent memory deficits in C57BL/6 mice. Weight gain and hepatic triglyceride levels were profoundly elevated following administration of a 60% HFD for 23 weeks, and this change was attenuated by 23-weeks of treadmill running. The exercise regimen attenuated impairments in memory function of HFD-fed mice in a water maze test and recovered HFD-induced anti-neurogenic effects as shown by immunohistochemistry data with Ki-67 and doublecortin (DCX) antibodies. Moreover, the treadmill exercise resulted in anti-inflammatory, antioxidant, and neuroprotective effects in the HFD-fed brain. The exercise inhibited HFD-induced microglial activation, expression of proinflammatory cytokines (tumor necrosis factor-α and interleukin-1β), and NF-κB activity in the dentate gyrus (DG) of the hippocampus. In addition, the exercise reduced malondialdehyde levels elevated by HFD and recovered antioxidant superoxide dismutase and glutathione levels in hippocampal DG of HFD-mice. The exercise also reduced the number of apoptotic cells induced by HFD, as shown by TUNEL staining in the DG region. Finally, we demonstrated that the thioredoxin-1 (TRX-1) and brain-derived neurotrophic factor (BDNF) levels were recovered by exercise, which was demonstrated to act via β2-adrenergic receptor enriched in synaptosomes of the DG. Therefore, our data collectively suggests that regular exercise may be a promising approach to preventing HFD-induced memory impairments via anti-inflammatory, antioxidant and neuroprotective mechanisms in the hippocampal DG region.