RNA interference-mediated phosphodiesterase 4D splice variants knock-down in the prefrontal cortex produces antidepressant-like and cognition-enhancing effects

Ablation of specific long PDE4D isoforms increases neurite elongation and conveys protection against amyloid-β pathology

Inhibition of phosphodiesterase 4D (PDE4D) enzymes has been investigated as therapeutic strategy to treat memory problems in Alzheimer's disease (AD). Although PDE4D inhibitors are effective in enhancing memory processes in rodents and humans, severe side effects may hamper their clinical use. PDE4D enzymes comprise different isoforms, which, when targeted specifically, can increase treatment efficacy and safety. The function of PDE4D isoforms in AD and in molecular memory processes per se has remained unresolved. Here, we report the upregulation of specific PDE4D isoforms in transgenic AD mice and hippocampal neurons exposed to amyloid-β. Furthermore, by means of pharmacological inhibition and CRISPR-Cas9 knockdown, we show that the long-form PDE4D3, -D5, -D7, and -D9 isoforms regulate neuronal plasticity and convey resilience against amyloid-β in vitro. These results indicate that isoform-specific, next to non-selective, PDE4D inhibition is efficient in promoting neuroplasticity in an AD context. Therapeutic effects of non-selective PDE4D inhibitors are likely achieved through actions on long isoforms. Future research should identify which long PDE4D isoforms should be specifically targeted in vivo to both improve treatment efficacy and reduce side effects.

Protective effects of phosphodiesterase 2 inhibitor against Aβ1-42 induced neuronal toxicity

Our previous study suggested that inhibition of Phosphodiesterase 2 ameliorates memory loss upon exposure to oxidative stress. While whether memory enhancing effects of PDE2 inhibition on Alzheimer's disease mouse model are involved in antioxidant defense and neuronal remodeling, are largely unexplored. The present study addressed whether and how PDE2 inhibitor Bay 60-7550 rescued Aβ oligomers (Aβo)-induced neuronal damage and memory impairment. The results suggested that exposure of primary cortical neurons to Aβo induced neuronal cells damage and increased PDE2 expression, which were paralleled to an increase in the oxidative parameter malondialdehyde (MDA) level and cellular apoptosis. However, this Aβo-induced oxidative damage was blocked by pre-treatment with protein kinase A or G (PKA or PKG) inhibitor, suggesting the involvement of cAMP/cGMP signaling. Moreover, microinjection of Aβo into the prefrontal cortex of mice increased the MDA level; while Bay 60-7550 reversed this effect and increased antioxidant and anti-apoptotic factors, i.e. increased trolox-equivalent-antioxidant capacity and Bcl-2/Bax ratio. Bay 60-7550 also rescued Aβo-induced synaptic atrophy and memory deficits, as evidenced by the increased synaptic proteins' levels and spine density in the prefrontal cortex, and improved cognitive behaviors by decreased working memory errors in the eight-arm maze and increased discrimination index in the novel object recognition test. These findings suggest that inhibition of PDE2 contributes to antioxidant defense and neuronal remodeling by regulation of cAMP/cGMP signaling, which provide a theoretical basis for the future use of PDE2 inhibitors as the anti-AD drugs.

A Novel PDE4D Inhibitor BPN14770 Reverses Scopolamine-Induced Cognitive Deficits via cAMP/SIRT1/Akt/Bcl-2 Pathway

A global, quantitative proteomics/systems-biology analysis of the selective pharmacological inhibition of phosphodiesterase-4D (PDE4D) revealed the differential regulation of pathways associated with neuroplasticity in memory-associated brain regions. Subtype selective inhibitors of PDE4D bind in an allosteric site that differs between mice and humans in a single amino acid (tyrosine vs. phenylalanine, respectively). Therefore to study selective inhibition of PDE4D by BPN14770, a subtype selective allosteric inhibitor of PDE4D, we utilized a line of mice in which the PDE4D gene had been humanized by mutating the critical tyrosine to phenylalanine. Relatively low doses of BPN14770 were effective at reversing scopolamine-induced memory and cognitive deficits in humanized PDE4D mice. Inhibition of PDE4D alters the expression of protein kinase A (PKA), Sirt1, Akt, and Bcl-2/Bax which are components of signaling pathways for regulating endocrine response, stress resistance, neuronal autophagy, and apoptosis. Treatment with a series of antagonists, such as H89, sirtinol, and MK-2206, reversed the effect of BPN14770 as shown by behavioral tests and immunoblot analysis. These findings suggest that inhibition of PDE4D enhances signaling through the cAMP-PKA-SIRT1-Akt -Bcl-2/Bax pathway and thereby may provide therapeutic benefit in neurocognitive disorders.

Increased isoform-specific phosphodiesterase 4D expression is associated with pathology and cognitive impairment in Alzheimer's disease

Pharmacological phosphodiesterase 4D (PDE4D) inhibition shows therapeutic potential to restore memory function in Alzheimer's disease (AD), but will likely evoke adverse side effects. As PDE4D encodes multiple isoforms, targeting specific isoforms may improve treatment efficacy and safety. Here, we investigated whether PDE4D isoform expression and PDE4D DNA methylation is affected in AD and whether expression changes are associated with severity of pathology and cognitive impairment. In post-mortem temporal lobe brain material from AD patients (n = 42) and age-matched controls (n = 40), we measured PDE4D isoform expression and PDE4D DNA (hydroxy)methylation using quantitative polymerase chain reaction and Illumina 450k Beadarrays, respectively. Linear regression revealed increased PDE4D1, -D3, -D5, and -D8 expression in AD with concurrent (hydroxy)methylation changes in associated promoter regions. Moreover, increased PDE4D1 and -D3 expression was associated with higherplaque and tau pathology levels, higher Braak stages, and progressed cognitive impairment. Future studies should indicate functional roles of specific PDE4D isoforms and the efficacy and safety of their selective inhibition to restore memory function in AD.

Jiao-Tai-Wan Ameliorates Depressive-Like Behavior through the A1R Pathway in Ovariectomized Mice after Unpredictable Chronic Stress

Objective

This study was aimed at observing the effect Jiao-Tai-Wan in menopausal depression.

Methods

In this paper, we used ovariectomized mice subjected to chronic unpredictable stress as a menopausal depression model. After the chronic stress, mice were administrated with JTW (3.3 and 6.6mg/kg) and imipramine (10 mg/kg) for 14 days. On the 14th day, mice were subjected to the behavior test like the forced swim test, tail suspension test, and locomotor activity or were sacrificed to assess the protein changes in different brain regions.

Results

The administration of JTW at doses of 3.3 and 6.6mg/kg (p.o.) significantly shortened the duration of immobility in forced swim and tail suspension tests. There was no obvious difference in locomotor activity among all the groups. The western blot analysis data indicated that treatment with JTW (3.3 and 6.6 mg/kg, p.o.) prominently increased the A₁R protein and the downstream protein ERK1/2 levels in the prefrontal cortex and hippocampus. However, the administration of JTW did not influence c-Fos protein in either the prefrontal cortex or hippocampus.

Conclusion

Our findings suggest that JTW plays a vital role in ameliorating menopausal depression symptoms in the A₁R-ERK1/2 pathway in the prefrontal cortex and hippocampus.

Dominant-Negative Attenuation of cAMP-Selective Phosphodiesterase PDE4D Action Affects Learning and Behavior

PDE4 cyclic nucleotide phosphodiesterases reduce 3′, 5′ cAMP levels in the CNS and thereby regulate PKA activity and the phosphorylation of CREB, fundamental to depression, cognition, and learning and memory. The PDE4 isoform PDE4D5 interacts with the signaling proteins β-arrestin2 and RACK1, regulators of β₂-adrenergic and other signal transduction pathways. Mutations in PDE4D in humans predispose to acrodysostosis, associated with cognitive and behavioral deficits. To target PDE4D5, we developed mice that express a PDE4D5-D556A dominant-negative transgene in the brain. Male transgenic mice demonstrated significant deficits in hippocampus-dependent spatial learning, as assayed in the Morris water maze. In contrast, associative learning, as assayed in a fear conditioning assay, appeared to be unaffected. Male transgenic mice showed augmented activity in prolonged (2 h) open field testing, while female transgenic mice showed reduced activity in the same assay. Transgenic mice showed no demonstrable abnormalities in prepulse inhibition. There was also no detectable difference in anxiety-like behavior, as measured in the elevated plus-maze. These data support the use of a dominant-negative approach to the study of PDE4D5 function in the CNS and specifically in learning and memory.

Translocator protein-mediated fast-onset antidepressant-like and memory-enhancing effects in chronically stressed mice

BACKGROUND

Fast-acting and cognitive-enhancing antidepressants are desperately needed. Activation of translocator protein (18 kDa, TSPO) is a novel strategy for developing potential antidepressants, but there are no data available on the onset time of TSPO ligands. This study aimed to investigate the fast-onset antidepressant actions of AC-5216, a selective TSPO ligand, in TSPO knock-out (KO) mice.

METHODS

TSPO wild-type (WT) and KO mice were subjected to a six-week chronic unpredicted stress (CUS) paradigm. Then, the mice were treated with AC-5216 and tested with depressive and cognitive behaviours.

RESULTS

A single dose of AC-5216 (0.3 mg/kg) exerted anxiolytic- and antidepressant-like actions in TSPO WT mice. Moreover, in chronically stressed WT mice, two to four days of AC-5216 treatment (0.3 mg/kg, once per day) produced fast-onset antidepressant-like effects in the novelty-suppressed feeding and sucrose preference tests, as well as memory-enhancing effects in the novel object recognition test. In addition, a rapid (with five days of treatment) restoration of serum corticosterone levels and prefrontal cortex (PFC) allopregnanolone levels was found. Further studies showed that in these stress-exposed WT mice, AC-5216 significantly increased the levels of mTOR signalling-related proteins (mBDNF, p-mTOR, PSD-95, synapsin-1, GluR1), as well as the total dendritic length and branching points of pyramidal neurons in the PFC.

CONCLUSIONS

These results suggest that TSPO mediates the fast-onset antidepressant-like and memory-enhancing effects of AC-5216, possibly through the rapid activation of mTOR signalling and restoration of dendritic complexity in the PFC.

A systematic strategy for screening therapeutic constituents of Schisandra chinensis (Turcz.) Baill infiltrated blood-brain barrier oriented in lesions using ethanol and water extracts: a novel perspective for exploring chemical material basis of herb medicines

Schisandra chinensis, a widely used Chinese herbal medicine, was considered as central nervous system (CNS) drug for years. Both ethanol extracts (EES) and water extracts (WES) of it were applied clinically. Unfortunately, the difference of their efficacy and even effective material foundation of S. chinensis remains obscure. In this study, to explore the active constituents of S. chinensis, we compared pharmacodynamics and chemical profiles in vitro/in vivo of EES/WES for the first time using multiple chemical analysis, pharmacological and data processing approaches. It was proved that there was no significant difference in the anti-depressive effects between WES and EES. However, the contents of most components in vitro and in plasma were higher in EES than those in WES, which was unconvincing for their similar efficacy. Therefore, we further explored components of S. chinensis targeted onto brain and the results showed that 5 lignans were identified with definite absorptivity respectively both in EES and WES caused by the limitation of blood−brain barrier. Moreover, bioinformatic analysis predicted their anti-depressive action. Above all, the systematic strategy screened 5 brain-targeted effective substances of S. chinensis and it was suggested that exploring the components into nidi would promote the studies on herbs effective material basis.

Disregulation of Autophagy in the Transgenerational Cc2d1a Mouse Model of Autism

Autism spectrum disorder (ASD) is a heterogeneously childhood neurodevelopmental disorder, believed to be under development of various genetic and environmental factors. Autophagy and related pathways have also been implicated in the etiology of ASD. We aimed to investigate autophagic markers by generating the transgenerational inheritance of ASD-like behaviors in the Cc2d1a animal model of ASD. Cc2d1a (+/-) mouse model of ASD was built in two different groups by following three generations. After behavior test, bilateral hippocampus was sliced. Western Blot assay and quantitative real-time polymerase chain reaction (QRT-PCR) were used for measurement of LC3 and Beclin-1 as key regulators of autophagy. All of the animal and laboratory studies were conducted in the Erciyes University Genome and Stem Cell Center (GENKOK). Significant LC3 and Beclin-1 mRNA expression levels were observed in mouse hippocampus between groups and generations. Western blot confirmed the changes of the proteins in the hippocampus. LC3 expressions were increased for females and decreased for males compared to the control group. Beclin-1 expression levels were found to be significantly decreased in males and females compared to controls. This study could help explain a new pathway of autophagy in ASD mouse models. Future animal studies need to investigate sex differences in mouse modeling autism-relevant genes like CC2D1A. We anticipate our results to be a starting point for more comprehensive autophagy studies in this mouse model of ASD.

The faster-onset antidepressant effects of hypidone hydrochloride (YL-0919)

Hypidone hydrochloride (YL-0919), is a novel structural antidepressant candidate as a triple selective serotonin re-uptake inhibitor (SSRI), 5-HT_1A partial agonist and 5-HT₆ agonist. Here, we investigated the rapid onset antidepressant-like effects of YL-0919 and the possible mechanism in rats exposed to a chronic unpredictable stress (CUS) paradigm. In the CUS rats, it was found that fluoxetine (FLX, 10 mg/kg) treatment exerted antidepressant actions on 20-22d, while YL-0919 or vilazodone (VLZ, a dual 5-HT_1A partial agonist and SSRI) administrated once daily exerted faster antidepressant-like behaviors [4 days in the sucrose preference test (SPT) and 6 days in the novelty suppressed feeding test (NSF)]. Thereafter, the serum corticosterone (CORT) and adrenocorticotropic hormone (ACTH) levels were reversed by treatment with YL-0919 for 7 days. Furthermore, YL-0919 treatment for 5 days reversed the brain derived neurotrophic factor (BDNF)-mammalian target of rapamycin (mTOR) signaling and the key synaptic proteins, such as post-synaptic density (PSD95), GluR1 and presynaptic protein synapsin1. Meanwhile, the dendritic complexity of pyramidal neurons in prefrontal cortex (PFC) were also increased in the CUS rats. These data suggest that YL-0919 exerts a faster antidepressant-like effect on behaviors and this effect maybe at least partially mediated by the BDNF-mTOR signaling related dendritic complexity increase in the PFC.

Protection from Amyloid β Peptide-Induced Memory, Biochemical, and Morphological Deficits by a Phosphodiesterase-4D Allosteric Inhibitor

Recent imaging studies of amyloid and tau in cognitively normal elderly subjects imply that Alzheimer's pathology can be tolerated by the brain to some extent due to compensatory mechanisms operating at the cellular and synaptic levels. The present study investigated the effects of an allosteric inhibitor of phosphodiesterase-4D (PDE4D), known as BPN14770 (2-(4-((2-(3-Chlorophenyl)-6-(trifluoromethyl)pyridin-4-yl)methyl)phenyl)acetic Acid), on impairment of memory, dendritic structure, and synaptic proteins induced by bilateral microinjection of oligomeric amyloid beta (Aβ _1-42 into the hippocampus of humanized PDE4D (hPDE4D) mice. The hPDE4D mice provide a unique and powerful genetic tool for assessing PDE4D target engagement. Behavioral studies showed that treatment with BPN14770 significantly improved memory acquisition and retrieval in the Morris water maze test and the percentage of alternations in the Y-maze test in the model of Aβ impairment. Microinjection of oligomeric Aβ _1-42 caused decreases in the number of dendrites, dendritic length, and spine density of pyramid neurons in the hippocampus. These changes were prevented by BPN14770 in a dose-dependent manner. Furthermore, molecular studies showed that BPN14770 prevented Aβ-induced decreases in synaptophysin, postsynaptic density protein 95, phosphorylated cAMP-response element binding protein (CREB)/CREB, brain-derived neurotrophic factor, and nerve growth factor inducible protein levels in the hippocampus. The protective effects of BPN14770 against Aβ-induced memory deficits, synaptic damage, and the alteration in the cAMP-meditated cell signaling cascade were blocked by H-89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride), an inhibitor of protein kinase A. These results suggest that BPN14770 may activate compensatory mechanisms that support synaptic health even with the onset of amyloid pathology in Alzheimer's disease. SIGNIFICANCE STATEMENT: This study demonstrates that a phosphodiesterase-4D allosteric inhibitor, BPN14770, protects against memory loss and neuronal atrophy induced by oligomeric Aβ _1-42. The study provides useful insight into the potential role of compensatory mechanisms in Alzheimer's disease in a model of oligomeric Aβ _1-42 neurotoxicity.

The antidepressant- and anxiolytic-like effects of resveratrol: Involvement of phosphodiesterase-4D inhibition

Resveratrol is a natural non-flavonoid polyphenol found in red wine, which has numerous pharmacological properties including anti-stress and antidepressant-like abilities. However, whether the antidepressant- and anxiolytic-like effects of resveratrol are related to the inhibition of phosphodiesterase 4 (PDE4) and its subtypes remains unknown. The same holds true for the subsequent cAMP-dependent pathway. The first set of studies investigated whether resveratrol exhibited neuroprotective effects against corticosterone-induced cell lesion as well as its underlying mechanism. We found that 100 μM corticosterone induced PDE2A, PDE3B, PDE4A, PDE4D, PDE10 and PDE11 expression in HT-22 cells, which results in significant cell lesion. However, treatment with resveratrol increased cell viability in a dose- and time-dependent manner. These effects seem related to the inhibition of PDE4D, as evidenced by resveratrol dose-dependently decreasing PDE4D expression. In addition, the PKA inhibitor H89 reversed resveratrol's effects on cell viability. Resveratrol prevented corticosterone-induced reduction in cAMP, pVASP(s157), pCREB, and BDNF levels, indicating that cAMP signaling is involved in resveratrol-induced neuroprotective effects. Not to mention, PDE4D knockdown by PDE4D siRNA potentiated the effect of low dose of resveratrol on cAMP, pVASP, pCREB, and BDNF expression, while PDE4D overexpression reversed the effect of high dose of resveratrol on the expression of the above proteins. Finally, the subsequent in vivo data supports the in vitro findings, suggesting that resveratrol-induced antidepressant- and anxiolytic-like effects are mediated by PDE4D. Overall, these findings support the hypothesis that PDE4D-mediated cAMP signaling plays an important role in resveratrol's protective effects on stress-induced depression- and anxiety-like behavior.

Inhibition of Phosphodiesterase 4 by FCPR03 Alleviates Chronic Unpredictable Mild Stress-Induced Depressive-Like Behaviors and Prevents Dendritic Spine Loss in Mice Hippocampi

BACKGROUND

Phosphodiesterase 4 is a promising target for developing novel antidepressants. However, prototype phosphodiesterase 4 inhibitors show severe side effects, including nausea and vomiting. N-Isopropyl-3-(cyclopropylmethoxy)-4-difluoromethoxy benzamide (FCPR03) is a novel phosphodiesterase 4 inhibitor with little emetic potential. In the present study, we investigated the inhibitory effect of FCPR03 on chronic unpredictable mild stress-induced, depressive-like behaviors in mice and explored the underlying mechanisms.

METHODS

The depression model of mice was established by chronic unpredictable mild stress. Forced swim test, tail suspension test, and sucrose preference test were used to assess depressive-like behaviors. Golgi-staining was utilized to analyze dendritic morphology and spine density. The level of cAMP was measured by enzyme-linked immnosorbent assay assay. Western blot was used to evaluate protein levels of phosphorylated cAMP-response element binding protein, protein kinase B, glycogen synthase kinase-3β, and brain derived neurotrophic factor in both hippocampus and prefrontal cortex. Postsynaptic density protein 95 and synapsin 1 were also detected by western blot in the hippocampi.

RESULTS

Treatment with FCPR03 (0.5-1.0 mg/kg, i.p.) increased consumption of sucrose in the sucrose preference test in mice exposed to chronic unpredictable mild stress. FCPR03 shortened the immobility time in forced swim test and tail suspension test without affecting locomotor activity. Furthermore, chronic unpredictable mild stress decreased the dendritic spine density and dendritic length in the hippocampus. This change was accompanied by decreased expression of postsynaptic density protein 95 and synapsin 1. Interestingly, FCPR03 prevented dendritic spine loss and increased synaptic protein levels. Moreover, the levels of cAMP, phosphorylated cAMP-response element binding protein, and brain derived neurotrophic factor were elevated in chronic unpredictable mild stress-challenged mice after treatment with FCPR03. In addition, FCPR03 also enhanced the phosphorylation of both protein kinase B and glycogen synthase kinase-3β in mice exposed to chronic unpredictable mild stress.

CONCLUSION

The present study suggests that FCPR03 could prevent both depressive-like behaviors and spine loss induced by chronic unpredictable mild stress in the mice hippocampi.

Memory enhancing effects of BPN14770, an allosteric inhibitor of phosphodiesterase-4D, in wild-type and humanized mice

Inhibitors of phosphodiesterase-4 (PDE4) have beneficial effects on memory in preclinical and clinical studies. Development of these drugs has stalled due to dose-limiting side effects of nausea and emesis. While use of subtype-selective inhibitors (i.e., for PDE4A, B, or D) could overcome this issue, conservation of the catalytic region, to which classical inhibitors bind, limits this approach. The present study examined the effects of BPN14770, an allosteric inhibitor of PDE4D, which binds to a primate-specific, N-terminal region. In mice engineered to express PDE4D with this primate-specific sequence, BPN14770 was 100-fold more potent for improving memory than in wild-type mice; meanwhile, it exhibited low potency in a mouse surrogate model for emesis. BPN14770 also antagonized the amnesic effects of scopolamine, increased cAMP signaling in brain, and increased BDNF and markers of neuronal plasticity associated with memory. These data establish a relationship between PDE4D target engagement and effects on memory for BPN14770 and suggest clinical potential for PDE4D-selective inhibitors.

trans-Resveratrol Ameliorates Stress-Induced Irritable Bowel Syndrome-Like Behaviors by Regulation of Brain-Gut Axis

Background: Irritable bowel syndrome (IBS) is a functional disorder characterized by abdominal pain and abnormalities in defecation associated with psychiatric disorders such as depression and anxiety due to the dysfunction of brain-gut axis. This study aims to determine whether trans-Resveratrol affects chronic-acute combined stress (CACS)-induced IBS-like symptoms including depression, anxiety and intestinal dysfunction. Methods: ICR male mice were exposed to the CACS for 3 weeks. trans-Resveratrol were administrated daily (2.5, 5, and 10 mg/kg, i.g.) 30 min before CACS. Behavioral tests were performed to evaluate the treatment effects of trans-Resveratrol on IBS. Hippocampus tissues were collected and processed Golgi staining and immuno-blot analysis. Ileum and colon tissues were collected and processed Hematoxylin and Eosin staining and immuno-blot analysis. Results: Administration with trans-Resveratrol before CACS for 3 weeks significantly reversed CACS-induced depression- and anxiety-like behaviors and intestinal dysfunction in mice, which implied a crucial role of trans-Resveratrol in treatment of IBS-like disorder. Furthermore, trans-Resveratrol improved hippocampal neuronal remodeling, protected ileal and colonic epithelial barrier structure against CACS insults. The further study suggested that trans-Resveratrol normalized phosphodiesterases 4A (PDE4A) expression and CREB-BDNF signaling that were disturbed by CACS. The increased pCREB and BDNF expression in the hippocampus were found, while decreased pCREB and BDNF levels were observed after treatment with trans-Resveratrol. Conclusions: The dual effects of trans-Resveratrol on stress-induced psychiatric and intestinal dysfunction may be related to normalization of PDE4A expression and subsequent pCREB-BDNF signaling in the hippocampus, ileum and colon.

Can we increase the speed and efficacy of antidepressant treatments? Part II. Glutamatergic and RNA interference strategies

In the second part we focus on two treatment strategies that may overcome the main limitations of current antidepressant drugs. First, we review the experimental and clinical evidence supporting the use of glutamatergic drugs as fast-acting antidepressants. Secondly, we review the involvement of microRNAs (miRNAs) in the pathophysiology of major depressive disorder (MDD) and the use of small RNAs (e.g.., small interfering RNAs or siRNAs) to knockdown genes in monoaminergic and non-monoaminergic neurons and induce antidepressant-like responses in experimental animals. The development of glutamatergic agents is a promising venue for antidepressant drug development, given the antidepressant properties of the non-competitive NMDA receptor antagonist ketamine. Its unique properties appear to result from the activation of AMPA receptors by a metabolite [(2S,6S;2R,6R)-hydroxynorketamine (HNK)] and mTOR signaling. These effects increase synaptogenesis in prefrontal cortical pyramidal neurons and enhance serotonergic neurotransmission via descending inputs to the raphe nuclei. This view is supported by the cancellation of ketamine's antidepressant-like effects by inhibition of serotonin synthesis. We also review existing evidence supporting the involvement of miRNAs in MDD and the preclinical use of RNA interference (RNAi) strategies to target genes involved in antidepressant response. Many miRNAs have been associated to MDD, some of which e.g., miR-135 targets genes involved in antidepressant actions. Likewise, SSRI-conjugated siRNA evokes faster and/or more effective antidepressant-like responses. Intranasal application of sertraline-conjugated siRNAs directed to 5-HT_1A receptors and SERT evoked much faster changes of pre- and postsynaptic antidepressant markers than those produced by fluoxetine.

A negative allosteric modulator of PDE4D enhances learning after traumatic brain injury

Traumatic brain injury (TBI) significantly decreases cyclic AMP (cAMP) signaling which produces long-term synaptic plasticity deficits and chronic learning and memory impairments. Phosphodiesterase 4 (PDE4) is a major family of cAMP hydrolyzing enzymes in the brain and of the four PDE4 subtypes, PDE4D in particular has been found to be involved in memory formation. Although most PDE4 inhibitors target all PDE4 subtypes, PDE4D can be targeted with a selective, negative allosteric modulator, D159687. In this study, we hypothesized that treating animals with D159687 could reverse the cognitive deficits caused by TBI. To test this hypothesis, adult male Sprague Dawley rats received sham surgery or moderate parasagittal fluid-percussion brain injury. After 3 months of recovery, animals were treated with D159687 (0.3 mg/kg, intraperitoneally) at 30 min prior to cue and contextual fear conditioning, acquisition in the water maze or during a spatial working memory task. Treatment with D159687 had no significant effect on these behavioral tasks in non-injured, sham animals, but did reverse the learning and memory deficits in chronic TBI animals. Assessment of hippocampal slices at 3 months post-TBI revealed that D159687 reversed both the depression in basal synaptic transmission in area CA1 as well as the late-phase of long-term potentiation. These results demonstrate that a negative allosteric modulator of PDE4D may be a potential therapeutic to improve chronic cognitive dysfunction following TBI.

Peripheral administration of lactate produces antidepressant-like effects

In addition to its role as metabolic substrate that can sustain neuronal function and viability, emerging evidence supports a role for l-lactate as an intercellular signaling molecule involved in synaptic plasticity. Clinical and basic research studies have shown that major depression and chronic stress are associated with alterations in structural and functional plasticity. These findings led us to investigate the role of l-lactate as a potential novel antidepressant. Here we show that peripheral administration of l-lactate produces antidepressant-like effects in different animal models of depression that respond to acute and chronic antidepressant treatment. The antidepressant-like effects of l-lactate are associated with increases in hippocampal lactate levels and with changes in the expression of target genes involved in serotonin receptor trafficking, astrocyte functions, neurogenesis, nitric oxide synthesis and cAMP signaling. Further elucidation of the mechanisms underlying the antidepressant effects of l-lactate may help to identify novel therapeutic targets for the treatment of depression.

Interaction of Cdk5 and cAMP/PKA Signaling in the Mediation of Neuropsychiatric and Neurodegenerative Diseases

Both cyclin-dependent kinase 5 (Cdk5) and cyclic AMP (cAMP)/protein kinase A (PKA) regulate fundamental central nervous system (CNS) functions including neuronal survival, neurite and axonal outgrowth, neuron development and cognition. Cdk5, a serine/threonine kinase, is activated by p35 or p39 and phosphorylates multiple signaling components of various pathways, including cAMP/PKA signaling. Here, we review the recent literature on the interaction between Cdk5 and cAMP/PKA signaling and their role in the mediation of CNS functions and neuropsychiatric and neurodegenerative diseases.

The Past, Present, and Future of Phosphodiesterase-4 Modulation for Age-Induced Memory Loss

The purpose of this chapter is to highlight the state of progress for phosphodiesterase-4 (PDE4) modulation as a potential therapeutic for psychiatric illness, and to draw attention to particular hurdles and obstacles that must be overcome in future studies to develop PDE4-mediated therapeutics. Pathological and non-pathological related memory loss will be the focus of the chapter; however, we will at times also touch upon other psychiatric illnesses like anxiety and depression. First, we will provide a brief background of PDE4, and the rationale for its extensive study in cognition. Second, we will explore fundamental differences in individual PDE4 subtypes, and then begin to address differences between pathological and non-pathological aging. Alterations of cAMP/PDE4 signaling that occur within normal vs. pathological aging, and the potential for PDE4 modulation to combat these alterations within each context will be described. Finally, we will finish the chapter with obstacles that have hindered the field, and future studies and alternative viewpoints that need to be addressed. Overall, we hope this chapter will demonstrate the incredible complexity of PDE4 signaling in the brain, and will be useful in forming a strategy to develop future PDE4-mediated therapeutics for psychiatric illnesses.

Altered phosphorylation, electrophysiology, and behavior on attenuation of PDE4B action in hippocampus

Background

PDE4 cyclic nucleotide phosphodiesterases regulate 3′, 5′ cAMP abundance in the CNS and thereby regulate PKA activity and phosphorylation of CREB, which has been implicated in learning and memory, depression and other functions. The PDE4 isoform PDE4B1 also interacts with the DISC1 protein, implicated in neural development and behavioral disorders. The cellular functions of PDE4B1 have been investigated extensively, but its function(s) in the intact organism remained unexplored.

Results

To specifically disrupt PDE4B1, we developed mice that express a PDE4B1-D564A transgene in the hippocampus and forebrain. The transgenic mice showed enhanced phosphorylation of CREB and ERK1/2 in hippocampus. Hippocampal neurogenesis was increased in the transgenic mice. Hippocampal electrophysiological studies showed increased baseline synaptic transmission and enhanced LTP in male transgenic mice. Behaviorally, male transgenic mice showed increased activity in prolonged open field testing, but neither male nor female transgenic mice showed detectable anxiety-like behavior or antidepressant effects in the elevated plus-maze, tail-suspension or forced-swim tests. Neither sex showed any significant differences in associative fear conditioning or showed any demonstrable abnormalities in pre-pulse inhibition.

Conclusions

These data support the use of an isoform-selective approach to the study of PDE4B1 function in the CNS and suggest a probable role of PDE4B1 in synaptic plasticity and behavior. They also provide additional rationale and a refined approach to the development of small-molecule PDE4B1-selective inhibitors, which have potential functions in disorders of cognition, memory, mood and affect.

Local Inhibition of PERK Enhances Memory and Reverses Age-Related Deterioration of Cognitive and Neuronal Properties

Protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) is one of four known kinases that respond to cellular stress by deactivating the eukaryotic initiation factor 2 α (eIF2α) or other signal transduction cascades. Recently, both eIF2α and its kinases were found to play a role in normal and pathological brain function. Here, we show that reduction of either the amount or the activity of PERK, specifically in the CA1 region of the hippocampus in young adult male mice, enhances neuronal excitability and improves cognitive function. In addition, this manipulation rescues the age-dependent cellular phenotype of reduced excitability and memory decline. Specifically, the reduction of PERK expression in the CA1 region of the hippocampus of middle-aged male mice using a viral vector rejuvenates hippocampal function and improves hippocampal-dependent learning. These results delineate a mechanism for behavior and neuronal aging and position PERK as a promising therapeutic target for age-dependent brain malfunction.SIGNIFICANCE STATEMENT We found that local reduced protein kinase R (PKR)-like endoplasmic reticulum kinase (PERK) expression or activity in the hippocampus enhances neuronal excitability and cognitive function in young normal mice, that old CA1 pyramidal cells have reduced excitability and increased PERK expression that can be rescued by reducing PERK expression in the hippocampus, and that reducing PERK expression in the hippocampus of middle-aged mice enhances hippocampal-dependent learning and memory and restores it to normal performance levels of young mice. These findings uncover an entirely new biological link among PERK, neuronal intrinsic properties, aging, and cognitive function. Moreover, our findings propose a new way to fight mild cognitive impairment and aging-related cognitive deterioration.

Phosphodiesterase-1b deletion confers depression-like behavioral resistance separate from stress-related effects in mice

Phosphodiesterase-1b (Pde1b) is highly expressed in striatum, dentate gyrus, CA3 and substantia nigra. In a new Floxed Pde1b × Cre^CMV global knockout (KO) mouse model, we show an immobility-resistance phenotype that recapitulates that found in constitutive Pde1b KO mice. We use this new mouse model to show that the resistance to acute stress-induced depression-like phenotype is not the product of changes in locomotor activity or reactivity to other stressors (learned helplessness, novelty suppressed feeding or dexamethasone suppression), and is not associated with anhedonia using the sucrose preference test. Using tamoxifen inducible Cre, we show that the immobility-resistant phenotype depends on the age of induction. The effect is present when Pde1b is Reduced from conception, P0 or P32, but not if reduced as adults (P60). We also mapped regional brain expression of PDE1B protein and of the Cre driver. These data add to the suggestion that PDE1B may be a target for drug development with therapeutic potential in depression alone or in combination with existing antidepressants.

Phosphodiesterase-1b (Pde1b) knockout mice are resistant to forced swim and tail suspension induced immobility and show upregulation of Pde10a

RATIONALE

Major depressive disorder is a leading cause of suicide and disability. Despite this, current antidepressants provide insufficient efficacy in more than 60% of patients. Most current antidepressants are presynaptic reuptake inhibitors; postsynaptic signal regulation has not received as much attention as potential treatment targets.

OBJECTIVES

We examined the effects of disruption of the postsynaptic cyclic nucleotide hydrolyzing enzyme, phosphodiesterase (PDE) 1b, on depressive-like behavior and the effects on PDE1B protein in wild-type (WT) mice following stress.

METHODS

Littermate knockout (KO) and WT mice were tested in locomotor activity, tail suspension (TST), and forced swim tests (FST). FST was also used to compare the effects of two antidepressants, fluoxetine and bupropion, in KO versus WT mice. Messenger RNA (mRNA) expression changes were also determined. WT mice underwent acute or chronic stress and markers of stress and PDE1B expression were examined.

RESULTS

Pde1b KO mice exhibited decreased TST and FST immobility. When treated with antidepressants, both WT and KO mice showed decreased FST immobility and the effect was additive in KO mice. Mice lacking Pde1b had increased striatal Pde10a mRNA expression. In WT mice, acute and chronic stress upregulated PDE1B expression while PDE10A expression was downregulated after chronic but not acute stress.

CONCLUSIONS

PDE1B is a potential therapeutic target for depression treatment because of the antidepressant-like phenotype seen in Pde1b KO mice.

The effect of resveratrol on beta amyloid-induced memory impairment involves inhibition of phosphodiesterase-4 related signaling

Resveratrol, a natural polyphenol found in red wine, has wide spectrum of pharmacological properties including antioxidative and antiaging activities. Beta amyloid peptides (Aβ) are known to involve cognitive impairment, neuroinflammatory and apoptotic processes in Alzheimer's disease (AD). Activation of cAMP and/or cGMP activities can improve memory performance and decrease the neuroinflammation and apoptosis. However, it remains unknown whether the memory enhancing effect of resveratrol on AD associated cognitive disorders is related to the inhibition of phosphodiesterase 4 (PDE4) subtypes and subsequent increases in intracellular cAMP and/or cGMP activities. This study investigated the effect of resveratrol on Aβ1-42-induced cognitive impairment and the participation of PDE4 subtypes related cAMP or cGMP signaling. Mice microinfused with Aβ1-42 into bilateral CA1 subregions displayed learning and memory impairment, as evidenced by reduced memory acquisition and retrieval in the water maze and retention in the passive avoidance tasks; it was also significant that neuroinflammatory and pro-apoptotic factors were increased in Aβ1-42-treated mice. Aβ1-42-treated mice also increased in PDE4A, 4B and 4D expression, and decreased in PKA level. However, PKA inhibitor H89, but not PKG inhibitor KT5823, prevented resveratrol's effects on these parameters. Resveratrol also reversed Aβ1-42-induced decreases in phosphorylated cAMP response-element binding protein (pCREB), brain derived neurotrophic factor (BDNF) and anti-apoptotic factor BCl-2 expression, which were reversed by H89. These findings suggest that resveratrol reversing Aβ-induced learning and memory disorder may involve the regulation of neuronal inflammation and apoptosis via PDE4 subtypes related cAMP-CREB-BDNF signaling.

SB203580 reverses memory deficits and depression-like behavior induced by microinjection of Aβ1-42 into hippocampus of mice

A high co-morbidity between Alzheimer's disease (AD) and depression suggests there might be similar mechanisms underlying the course of these diseases. Previous studies have shown that p38MAPK plays a critical role in the pathophysiology of AD and depression. However, little is known about whether SB203580, a selective inhibitor of p38MAPK, may protect against AD-associated cognitive impairments and depression-like behavior, simultaneously. Herein, we have shown, for the first time, that SB203580 may reverse memory impairments and depression-like behavior induced by hippocampal infusion of β-amyloid 1-42 (Aβ_1-42), as measured by novel object recognition, Morris water maze, tail-suspension and forced-swimming tests. In addition, phorbol 12-myristate 13-acetate (PMA), a PKC activator which also activates p38MAPK, significantly abolished the effects of SB203580. Moreover, Aβ_1-42 causes increased phosphorylation of p38MAPK and decreased phosphorylation of Ser9-glycogen synthase kinase 3β (GSK3β) and cAMP-response element binding protein (CREB) in the hippocampus of mice, which could be significantly reversed by SB203580. Our results suggest that SB203580 reversed Aβ_1-42-induced cognitive impairments and depression-like behavior via inhibiting p38MAPK signaling pathway, which not only supports p38MAPK as a therapeutic target for AD-associated cognitive dysfunction and depression-like behavior, but also provides experimental basis for the use of SB203580 in co-morbidity of AD and depression.

Discovery of N-Alkyl Catecholamides as Selective Phosphodiesterase-4 Inhibitors with Anti-neuroinflammation Potential Exhibiting Antidepressant-like Effects at Non-emetic Doses

Depression involving neuroinflammation is one of the most common disabling and life-threatening psychiatric disorders. Phosphodiesterase 4 (PDE4) inhibitors produce potent antidepressant-like and cognition-enhancing effects. However, their clinical utility is limited by their major side effect of emesis. To obtain more selective PDE4 inhibitors with antidepressant and anti-neuroinflammation potential and less emesis, we designed and synthesized a series of N-alkyl catecholamides by modifying the 4-methoxybenzyl group of our hit compound, FCPE07, with an alkyl side chain. Among these compounds, 10 compounds displayed submicromolar IC₅₀ values in the mid- to low-nanomolar range. Moreover, 4-difluoromethoxybenzamides 10g and 10j, bearing isopropyl groups, exhibited the highest PDE4 inhibitory activities, with IC₅₀ values in the low-nanomolar range and with higher selectivities for PDE4 (approximately 5000-fold and 2100-fold over other PDEs, respectively). Furthermore, compound 10j displayed anti-neuroinflammation potential, promising antidepressant-like effects, and a zero incidence rate of emesis at 0.8 mg/kg within 180 min.

Comparison of the Pharmacological Profiles of Selective PDE4B and PDE4D Inhibitors in the Central Nervous System

Inhibition of cyclic AMP (cAMP)-specific phosphodiesterase 4 (PDE4) has been proposed as a potential treatment for a series of neuropsychological conditions such as depression, anxiety and memory loss. However, the specific involvement of each of the PDE4 subtypes (PDE4A, 4B and 4C) in different categories of behavior has yet to be elucidated. In the present study, we compared the possible pharmacological effects of PDE4B and PDE4D selective inhibitors, A-33 and D159687, in mediating neurological function in mice. Both compounds were equally potent in stimulating cAMP signaling in the mouse hippocampal cell line HT-22 leading to an increase in CREB phosphorylation. In contrast, A-33 and D159687 displayed distinct neuropharmacological effects in mouse behavioral tests. A-33 has an antidepressant-like profile as indicated by reduced immobility time in the forced swim and tail suspension tasks, as well as reduced latency to feed in the novelty suppressed feeding test. D159687, on the other hand, had a procognitive profile as it improved memory in the novel object recognition test but had no antidepressant or anxiolytic benefit. The present data suggests that inhibitors targeting specific subtypes of PDE4 may exhibit differential pharmacological effects and aid a more efficient pharmacotherapy towards neuropsychological conditions.

The PDE4 cAMP-Specific Phosphodiesterases: Targets for Drugs with Antidepressant and Memory-Enhancing Action

The PDE4 cyclic nucleotide phosphodiesterases are essential regulators of cAMP abundance in the CNS through their ability to regulate PKA activity, the phosphorylation of CREB, and other important elements of signal transduction. In pre-clinical models and in early-stage clinical trials, PDE4 inhibitors have been shown to have antidepressant and memory-enhancing activity. However, the development of clinically-useful PDE4 inhibitors for CNS disorders has been limited by variable efficacy and significant side effects. Recent structural studies have greatly enhanced our understanding of the molecular configuration of PDE4 enzymes, especially the "long" PDE4 isoforms that are abundant in the CNS. The new structural data provide a rationale for the development of a new generation of PDE4 inhibitors that specifically act on long PDE4 isoforms. These next generation PDE4 inhibitors may also be capable of targeting the interactions of select long forms with their "partner" proteins, such as RACK1, β-arrestin, and DISC1. They would therefore have the ability to affect cAMP levels in specific cellular compartments and target localized cellular functions, such as synaptic plasticity. These new agents might also be able to target PDE4 populations in select regions of the CNS that are implicated in learning and memory, affect, and cognition. Potential therapeutic uses of these agents could include affective disorders, memory enhancement, and neurogenesis.

The analgesic effect of rolipram is associated with the inhibition of the activation of the spinal astrocytic JNK/CCL2 pathway in bone cancer pain

Bone cancer pain (BCP) is one of the most difficult and intractable tasks for pain management, which is associated with spinal 'neuron-astrocytic' activation. The activation of the c-Jun N-terminal kinase (JNK)/chemokine (C-C motif) ligand (CCL2) signaling pathway has been reported to be critical for neuropathic pain. Rolipram (ROL), a selective phosphodiesterase 4 inhibitor, possesses potent anti-inflammatory and anti-nociceptive activities. The present study aimed to investigate whether the intrathecal administration of ROL has an analgesic effect on BCP in rats, and to assess whether the inhibition of spinal JNK/CCL2 pathway and astrocytic activation are involved in the analgesic effects of ROL. The analgesic effects of ROL were evaluated using the Von Frey and Hargreaves tests. Immunofluorescence staining was used to determine the number of c-Fos immunoreactive neurons, and the expression of spinal astrocytes and microglial activation on day 14 after tumor cell inoculation. Enzyme-linked immunosorbent assay (ELISA) was used to detect the expression of pro-inflammatory cytokines [interleukin (IL)-1β, IL-6 and tumor necrosis factor (TNF)-α] and chemokines (CCL2), and western blot analysis was then used to examine the spinal phosphodiesterase 4 (PDE4), ionized calcium binding adapter molecule-1 (IBA-1) and JNK levels on day 14 after tumor cell inoculation. The results revealed that ROL exerted a short-term analgesic effect in a dose-dependent manner, and consecutive daily injections of ROL exerted continuous analgesic effects. In addition, spinal 'neuron-astrocytic' activation was suppressed and was associated with the downregulation of spinal IL-1β, IL-6 and TNF-α expression, and the inhibition of PDE4B and JNK levels in the spine was also observed. In addition, the level of CCL2 was decreased in the rats with BCP. The JNK inhibitor, SP600125, decreased CCL2 expression and attenuated pain behavior. Following co-treatment with ROL and SP600125, no significant increases in thermal hyperalgesia and CCL2 expression were observed compared with the ROL group. Thus, our findings suggest that the analgesic effects of ROL in BCP are mainly mediated through the inhibition of 'neuron-astrocytic' activation, which occurs via the suppression of spinal JNK/CCL2 signaling.

Electroacupuncture ameliorates post-stroke learning and memory through minimizing ultrastructural brain damage and inhibiting the expression of MMP-2 and MMP-9 in cerebral ischemia-reperfusion injured rats

The aim of the present study was to investigate the potential neuroprotective effects of electroacupuncture (EA) in the treatment of cerebral ischemia/reperfusion (I/R) injury, and to elucidate the association between this neuroprotective effect and brain ultrastructure and expression of matrix metalloproteinase (MMP)-2 and 9. Rats underwent focal cerebral I/R injury by arterial ligation and received in vivo therapeutic EA at the Baihui (DU20) and Shenting (DU24) acupoints. The therapeutic efficacy was then evaluated following the surgery. The results of the current study demonstrated that EA treatment significantly ameliorated neurological deficits and reduced cerebral infarct volume compared with I/R injured rats. Furthermore, EA improved the learning and memory ability of rats following I/R injury, inhibited blood brain barrier breakdown and reduced neuronal damage in the ischemic penumbra. Furthermore, EA attenuated ultrastructural changes in the brain tissue following ischemia and inhibited MMP-2/MMP-9 expression in cerebral I/R injured rats. The results suggest that EA ameliorates anatomical deterioration, and learning and memory deficits in rats with cerebral I/R injury.

cAMP/PKA/CREB/GLT1 signaling involved in the antidepressant-like effects of phosphodiesterase 4D inhibitor (GEBR-7b) in rats

OBJECTIVES

GEBR-7b, a potential phosphodiesterase 4D inhibitor, has been shown to have memory-enhancing effects in rodents. However, it is still unknown whether GEBR-7b also has the antidepressant-like effects in rats. Herein, we examined the potential of GEBR-7b to attenuate depression-like behaviors in the rat model of depression induced by chronic unpredictable stress (CUS). Next, we also investigated the alterations of cyclic adenosine monophosphate (cAMP), protein kinase A (PKA) catalytic subunit (PKAca), cAMP response element-binding (CREB), and glutamate transporter 1 (GLT1) levels produced by GEBR-7b in the rats model of depression.

METHODS

Effects of GEBR-7b on CUS (35 days)-induced depression-like behaviors were examined by measuring immobility time in the forced swimming test (FST). Hippocampal cAMP levels were examined by enzyme-linked immunosorbent assay, whereas PKAca, phosphorylation of CREB (pCREB), CREB, and GLT1 in the hippocampus of rats were subjected to Western blot analysis.

RESULTS

CUS exposure caused a depression-like behavior evidenced by the increased immobility time in FST. Depression-like behavior induced by CUS was accompanied by a significant increased GLT, decreased cAMP, PKAca, pCREB activities in hippocampus. However, repeated GEBR-7b administration significantly reversed CUS-induced depression-like behavior and changes of cAMP/PKA/CREB/GLT1 signaling. No alteration was observed in locomotor activity in open field test.

CONCLUSION

These findings indicate that GEBR-7b reversed the depression-like behaviors induced by CUS in rats, which is at least in part mediated by modulating cAMP, PKAca, pCREB, and GLT1 levels in the hippocampus of rats, supporting its neuroprotective potential against behavioral and biochemical dysfunctions induced by CUS.

Chronic corticosterone-mediated dysregulation of microRNA network in prefrontal cortex of rats: relevance to depression pathophysiology

Stress plays a major role in inducing depression, which may arise from interplay between complex cascades of molecular and cellular events that influence gene expression leading to altered connectivity and neural plasticity. In recent years, microRNAs (miRNAs) have carved their own niche owing to their innate ability to induce disease phenotype by regulating expression of a large number of genes in a cohesive and coordinated manner. In this study, we examined whether miRNAs and associated gene networks have a role in chronic corticosterone (CORT; 50 mg  kg(-1) × 21 days)-mediated depression in rats. Rats given chronic CORT showed key behavioral features that resembled depression phenotype. Expression analysis revealed differential regulation of 26 miRNAs (19 upregulated, 7 downregulated) in prefrontal cortex of CORT-treated rats. Interaction between altered miRNAs and target genes showed dense interconnected molecular network, in which multiple genes were predicated to be targeted by the same miRNA. A majority of altered miRNAs showed binding sites for glucocorticoid receptor element, suggesting that there may be a common regulatory mechanism of miRNA regulation by CORT. Functional clustering of predicated target genes yielded disorders such as developmental, inflammatory and psychological that could be relevant to depression. Prediction analysis of the two most prominently affected miRNAs miR-124 and miR-218 resulted into target genes that have been shown to be associated with depression and stress-related disorders. Altogether, our study suggests miRNA-mediated novel mechanism by which chronic CORT may be involved in depression pathophysiology.

Phosphodiesterase 4D: an enzyme to remember

Cyclic adenosine monophosphate (cAMP) is one of the second messengers critically involved in the molecular mechanisms underlying memory formation. In the CNS, the availability of cAMP is tightly controlled by phosphodiesterase 4 (PDE4), a family of enzymes that degrades the cyclic nucleotide to inactive AMP. Among the different PDE4 isoforms, in the last few years PDE4D has been hogging the limelight due to accumulating evidence for its crucial role in cognitive processes, which makes this enzyme a promising target for therapeutic interventions in a variety of pathological conditions characterized by memory impairment, such as Alzheimer's disease. In this article, we review the role of the cAMP signal transduction pathway in memory formation with a particular focus on the recent progress in PDE4D research.

Neuroglial Transmission

Neuroglia, the "glue" that fills the space between neurons in the central nervous system, takes active part in nerve cell signaling. Neuroglial cells, astroglia, oligodendroglia, and microglia, are together about as numerous as neurons in the brain as a whole, and in the cerebral cortex grey matter, but the proportion varies widely among brain regions. Glial volume, however, is less than one-fifth of the tissue volume in grey matter. When stimulated by neurons or other cells, neuroglial cells release gliotransmitters by exocytosis, similar to neurotransmitter release from nerve endings, or by carrier-mediated transport or channel flux through the plasma membrane. Gliotransmitters include the common neurotransmitters glutamate and GABA, the nonstandard amino acid d-serine, the high-energy phosphate ATP, and l-lactate. The latter molecule is a "buffer" between glycolytic and oxidative metabolism as well as a signaling substance recently shown to act on specific lactate receptors in the brain. Complementing neurotransmission at a synapse, neuroglial transmission often implies diffusion of the transmitter over a longer distance and concurs with the concept of volume transmission. Transmission from glia modulates synaptic neurotransmission based on energetic and other local conditions in a volume of tissue surrounding the individual synapse. Neuroglial transmission appears to contribute significantly to brain functions such as memory, as well as to prevalent neuropathologies.

Phosphodiesterase-4D Knock-down in the Prefrontal Cortex Alleviates Chronic Unpredictable Stress-Induced Depressive-Like Behaviors and Memory Deficits in Mice

Phosphodiesterase 4 (PDE4) has four isoforms (PDE4A-D) with at least 25 splice variants. PDE4 subtype nonselective inhibitors produce potent antidepressant-like and cognition-enhancing effects via increased intracellular cyclic AMP (cAMP) signaling in the brain. Our previous data have demonstrated that long-form PDE4Ds appear to be involved in these pharmacological properties of PDE4 inhibitors in the normal animals. However, it is not clear whether long-form PDE4Ds are critical for the behaviors and related cellular signaling/neuronal plasticity/neuroendocrine alterations in the depressed animals. In the present study, animals exposed to the chronic unpredictable stress (CUS), a rodent model of depression, exhibited elevated corticosterone, depressive-like behavior, memory deficits, accompanied with decreased cAMP-PKA-CREB and cAMP-ERK1/2-CREB signaling and neuroplasticity. These alterations induced by CUS were reversed by RNA interference (RNAi)-mediated prefrontal cortex long-form PDE4Ds (especially PDE4D4 and PDE4D5) knock-down, similar to the effects of the PDE4 subtype nonselective inhibitor rolipram. Furthermore, these effects of RNAi were not enhanced by rolipram. These data indicate a predominant role of long-form PDE4Ds in the pharmacotherapies of PDE4 inhibitors for depression and concomitant memory deficits. Long-form PDE4Ds, especially PDE4D4 and PDE4D5, appear to be the promising targets for the development of antidepressants with high therapeutic indices.

Synthesis, biological activities and pharmacokinetic properties of new fluorinated derivatives of selective PDE4D inhibitors

A new series of selective PDE4D inhibitors has been designed and synthesized by replacing 3-methoxy group with 3-difluoromethoxy isoster moiety in our previously reported cathecolic structures. All compounds showed a good PDE4D3 inhibitory activity, most of them being inactive toward other PDE4 isoforms (PDE4A4, PDE4B2 and PDE4C2). Compound 3b, chosen among the synthesized compounds as the most promising in terms of inhibitory activity, selectivity and safety, showed an improved pharmacokinetic profile compared to its non fluorinated analogue. Spontaneous locomotor activity, assessed in an open field apparatus, showed that, differently from rolipram and diazepam, selective PDE4D inhibitors, such as compounds 3b, 5b and 7b, did not affect locomotion, whereas compound 1b showed a tendency to reduce the distance traveled and to prolong the immobility period, possibly due to a poor selectivity.

5-HT7 receptor activation: procognitive and antiamnesic effects

RATIONALE

The serotonin (5-hydroxytryptamine (5-HT)) 5-HT7 receptor is localized in brain areas mediating memory; however, the role of this receptor on memory remains little explored.

OBJECTIVE

First, demonstrating the associative nature of Pavlovian/instrumental autoshaping (P/I-A) task, rats were exposed (three sessions) to CS-US (Pavlovian autoshaping), truly random control, free operant, and presentations of US or CS, and they were compared with rats trained-tested for one session to the P/I-A procedure. Also, effects of the 5-HT7 receptor agonist LP-211 administered intraperitoneally after training was determined on short- (1.5 h) and long-term memory 24 and 48 h) and on scopolamine-induced memory impairment and cAMP production.

METHODS

Autoshaping and its behavioral controls were studied. Other animals were subjected to an autoshaping training session and immediately afterwards were given (intraperitoneal) vehicle or LP-211 (0.1-10 mg/kg) and/or scopolamine (0.2 mg/kg) and tested for short-term memory (STM) and long-term memory (LTM); their brains were extracted for the cAMP ELISA immunoassay.

RESULTS

P/I-A group produced the higher %CR. LP-211 did not affect STM; nonetheless, at 0.5 and 1.0 mg/kg, it improved LTM. The 5-HT7 receptor antagonist SB-269970 (SB; 10.0 mg/kg) alone had no effect; nevertheless, the LP-211 (1.0 mg/kg) LTM facilitation was reversed by SB. The scopolamine (0.2 mg/kg) induced-decrement in CR was accompanied by significant increased cAMP production. The scopolamine-induced decrement in CR and increments in cAMP were significantly attenuated by LP-211.

CONCLUSIONS

Autoshaping is a reliable associative learning task whose consolidation is facilitated by the 5-HT7 receptor agonist LP-211.

Mice deficient in phosphodiesterase-4A display anxiogenic-like behavior

RATIONALE

Phosphodiesterases (PDEs) are a super family of enzymes responsible for the halting of intracellular cyclic nucleotide signaling and may represent novel therapeutic targets for treatment of cognitive disorders. PDE4 is of considerable interest to cognitive research because it is highly expressed in the brain, particularly in the cognition-related brain regions. Recently, the functional role of PDE4B and PDE4D, two of the four PDE4 subtypes (PDE4A, B, C, and D), in behavior has begun to be identified; however, the role of PDE4A in the regulation of behavior is still unknown.

OBJECTIVES

The purpose of this study was to characterize the functional role of PDE4A in behavior.

METHODS

The role of PDE4A in behavior was evaluated through a battery of behavioral tests using PDE4A knockout (KO) mice; urine corticosterone levels were also measured.

RESULTS

PDE4A KO mice exhibited improved memory in the step-through-passive-avoidance test. They also displayed anxiogenic-like behavior in elevated-plus maze, holeboard, light-dark transition, and novelty suppressed feeding tests. Consistent with the anxiety profile, PDE4A KO mice had elevated corticosterone levels compared with wild-type controls post-stress. Interestingly, PDE4A KO mice displayed no change in object recognition, Morris water maze, forced swim, tail suspension, and duration of anesthesia induced by co-administration of xylazine and ketamine (suggesting that PDE4A KO may not be emetic).

CONCLUSIONS

These results suggest that PDE4A may be important in the regulation of emotional memory and anxiety-like behavior, but not emesis. PDE4A could possibly represent a novel therapeutic target in the future for anxiety or disorders affecting memory.

Phosphodiesterase types 3 and 4 regulate the phasic contraction of neonatal rat bladder smooth myocytes via distinct mechanisms

Activation of the cyclic AMP (cAMP) pathway reduces bladder contractility. However, the role of phosphodiesterase (PDE) families in regulating this function is poorly understood. Here, we compared the contractile function of the cAMP hydrolyzing PDEs in neonatal rat bladder smooth myocytes. RT-PCR and Western blotting analysis revealed that several isoforms of PDE1-4 were expressed in neonatal rat bladder. While 8-methoxymethyl-3-isobutyl-1-methylxanthine (a PDE1 inhibitor) and BAY-60-7550 (a PDE2 inhibitor) had no effect on the carbachol-enhanced phasic contractions of bladder strips, cilostamide (Cil, a PDE3 inhibitor) and Ro-20-1724 (Ro, a PDE4 inhibitor) significantly reduced these contractions. This inhibitory effect of Ro was blunted by the PKA inhibitor H-89, while the inhibitory effect of Cil was strongly attenuated by the PKG inhibitor KT 5823. Application of Ro in single bladder smooth myocytes resulted in an increase in Ca(2+) spark frequency but a decrease both in Ca(2+) transients and in sarcoplasmic reticulum (SR) Ca(2+) content. In contrast, Cil had no effect on these events. Furthermore, Ro-induced inhibition of the phasic contractions was significantly blocked by ryanodine and iberiotoxin. Taken together, PDE3 and PDE4 are the main PDE isoforms in maintaining the phasic contractions of bladder smooth myocytes, with PDE4 being functionally more active than PDE3. However, their roles are mediated through different mechanisms.

Etazolate abrogates the lipopolysaccharide (LPS)-induced downregulation of the cAMP/pCREB/BDNF signaling, neuroinflammatory response and depressive-like behavior in mice

Increasing evidence has indicated that immune challenge by bacterial lipopolysaccharide (LPS) induces depressive-like behavior, neuroinflammatory response and upregulates phosphodiesterase-4 (PDE4), an enzyme that specifically hydrolyzes cyclic adenosine monophosphate (cAMP). However, whether the potential PDE4 inhibitor etazolate prevents the LPS-induced depressive-like behavior remains unclear. Here using a model of depression induced by the repeated administration of LPS during 16days, and then investigated the influence of LPS on the expression of PDE4, interleukin-1β (IL-1β) and antidepressant action of etazolate in mice through forced swimming, novelty suppressed feeding, sucrose preference and open-field tests. Our results showed that etazolate pretreatment facilitated the recovery from weight loss and prevented the depressive-like behavior induced by repeated LPS administration. Moreover, the antidepressant action of etazolate was paralleled by significantly reducing the expression levels of PDE4A, PDE4B, PDE4D and IL-1β and up-regulating the cAMP/phosphorylated cAMP response-element binding protein (pCREB)/brain-derived neurotrophic factor (BDNF) signaling in the hippocampus and prefrontal cortex of mice. These results indicate that the effects of etazolate on the depressive-like behavior induced by repeated LPS treatment may partially depend on the inhibition of PDE4 subtypes, the activation of the cAMP/pCREB/BDNF signaling and the anti-inflammatory responses in the hippocampus and prefrontal cortex.

PDE4 as a target for cognition enhancement

INTRODUCTION

The second messengers cAMP and cGMP mediate fundamental aspects of brain function relevant to memory, learning, and cognitive functions. Consequently, cyclic nucleotide phosphodiesterases (PDEs), the enzymes that inactivate the cyclic nucleotides, are promising targets for the development of cognition-enhancing drugs.

AREAS COVERED

PDE4 is the largest of the 11 mammalian PDE families. This review covers the properties and functions of the PDE4 family, highlighting procognitive and memory-enhancing effects associated with their inactivation.

EXPERT OPINION

PAN-selective PDE4 inhibitors exert a number of memory- and cognition-enhancing effects and have neuroprotective and neuroregenerative properties in preclinical models. The major hurdle for their clinical application is to target inhibitors to specific PDE4 isoforms relevant to particular cognitive disorders to realize the therapeutic potential while avoiding side effects, in particular emesis and nausea. The PDE4 family comprises four genes, PDE4A-D, each expressed as multiple variants. Progress to date stems from characterization of rodent models with selective ablation of individual PDE4 subtypes, revealing that individual subtypes exert unique and non-redundant functions in the brain. Thus, targeting specific PDE4 subtypes, as well as splicing variants or conformational states, represents a promising strategy to separate the therapeutic benefits from the side effects of PAN-PDE4 inhibitors.

Da-Bu-Yin-Wan and Qian-Zheng-San, two traditional Chinese herbal formulas, up-regulate the expression of mitochondrial subunit NADH dehydrogenase 1 synergistically in the mice model of Parkinson's disease

ETHNOPHARMACOLOGICAL RELEVANCE

Da-Bu-Yin-Wan (DBYW) and Qian-Zheng-San (QZS), two traditional Chinese herbal formulas, were clinically employed to treat Parkinson's disease (PD) for decades.

AIM OF THE STUDY

Our previous studies demonstrated neuroprotective effects of DBYW and QZS on mitochondrial function in mice model of PD induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In present research, we aimed to investigate the possible neuroprotective mechanisms of DBYW and QZS.

MATERIALS AND METHODS

The effects of DBYW and QZS on the behavioral changes (pole test), expression of tyrosine hydroxylase (TH) of substantia nigra by immunohistochemistry, monoaminergic contents and activity of striatum by high performance liquid chromatography, neuronal ultrastructure changes by transmission electron microscopy, mitochondrial DNA (mtDNA) damage by long-extension polymerase chain reaction (PCR), and mRNA expression of mitochondrial subunit NADH dehydrogenase 1(ND1) by qualitative real-time PCR were investigated.

RESULTS

Present study demonstrated that DBYW and QZS not only ameliorated the behavior induced by the administration of MPTP and synergistically prevented the decreasing of TH expression, but also increased monoaminergic contents and activity, improved the ultrastructural changes, decreased the mtDNA damage, and synergistically up-regulated the expression of ND1 in mRNA level.

CONCLUSIONS

These results suggest that DBYW and QZS possess anti-parkinsonism and neuroprotective properties.