The Phosphodiesterase Inhibitor Rolipram Promotes Survival of Newborn Hippocampal Neurons After Ischemia

Beyond PDE4 inhibition: A comprehensive review on downstream cAMP signaling in the central nervous system

Cyclic adenosine monophosphate (cAMP) is a key second messenger that regulates signal transduction pathways pivotal for numerous biological functions. Intracellular cAMP levels are spatiotemporally regulated by their hydrolyzing enzymes called phosphodiesterases (PDEs). It has been shown that increased cAMP levels in the central nervous system (CNS) promote neuroplasticity, neurotransmission, neuronal survival, and myelination while suppressing neuroinflammation. Thus, elevating cAMP levels through PDE inhibition provides a therapeutic approach for multiple CNS disorders, including multiple sclerosis, stroke, spinal cord injury, amyotrophic lateral sclerosis, traumatic brain injury, and Alzheimer's disease. In particular, inhibition of the cAMP-specific PDE4 subfamily is widely studied because of its high expression in the CNS. So far, the clinical translation of full PDE4 inhibitors has been hampered because of dose-limiting side effects. Hence, focusing on signaling cascades downstream activated upon PDE4 inhibition presents a promising strategy, offering novel and pharmacologically safe targets for treating CNS disorders. Yet, the underlying downstream signaling pathways activated upon PDE(4) inhibition remain partially elusive. This review provides a comprehensive overview of the existing knowledge regarding downstream mediators of cAMP signaling induced by PDE4 inhibition or cAMP stimulators. Furthermore, we highlight existing gaps and future perspectives that may incentivize additional downstream research concerning PDE(4) inhibition, thereby providing novel therapeutic approaches for CNS disorders.

cAMP-PKA cascade: An outdated topic for depression?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.

Angelica sinensis extract promotes neuronal survival by enhancing p38 MAPK-mediated hippocampal neurogenesis and dendritic growth in the chronic phase of transient global cerebral ischemia in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Angelica sinensis (Oliv.) Diels (ASD), commonly known as Dang Gui, is a popular Chinese herb that has long been used to treat ischemic stroke. However, the effects of ASD in chronic cerebral ischemia and its underlying mechanisms still remain unclear.

AIM OF THE STUDY

This study aimed to determine the effects of the ASD extract on hippocampal neuronal survival at 28 d after transient global cerebral ischemia (GCI) and to investigate the precise mechanisms underlying the p38 mitogen-activated protein kinase (MAPK)-related signaling pathway's involvement in hippocampal neurogenesis.

MATERIALS AND METHODS

Rats underwent 25 min of four-vessel occlusion. The ASD extract was intragastrically administered at doses of 0.25 g/kg (ASD-0.25 g), 0.5 g/kg (ASD-0.5 g), 1 g/kg (ASD-1 g), 1 g/kg after dimethyl sulfoxide administration (D + ASD-1 g), or 1 g/kg after SB203580 (a p38 MAPK inhibitor) administration (SB + ASD-1 g) at 1, 3, 7, 10, 14, 17, 21, and 24 d after transient GCI.

RESULTS

ASD-0.5 g, ASD-1 g, and D + ASD-1 g treatments had the following effects: upregulation of bromodeoxyuridine (BrdU) and Ki67 expression, and BrdU/neuronal nuclei (NeuN) and Ki67/nestin co-expression in the hippocampal dentate gyrus (DG); upregulation of microtubule-associated protein 2/NeuN co-expression, and NeuN and glial fibrillary acidic protein (GFAP) expression, and downregulation of tumor necrosis factor-α/GFAP co-expression in the hippocampal CA1 region; upregulation of phospho-p38 MAPK (p-p38 MAPK), phospho-cAMP response element-binding protein (p-CREB), brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), and vascular endothelial growth factor A (VEGF-A) expression in the hippocampus. SB + ASD-1 g treatment abrogated the effects of ASD-1 g on the expression of these proteins.

CONCLUSIONS

ASD-0.5 g and ASD-1 g treatments promotes neuronal survival by enhancing hippocampal neurogenesis. The effects of the ASD extract on astrocyte-associated hippocampal neurogenesis and dendritic growth are caused by the activation of p38 MAPK-mediated CREB/BDNF, GDNF, and VEGF-A signaling pathways in the hippocampus at 28 d after transient GCI.

Phosphodiesterase-4D Knockdown in the Prefrontal Cortex Alleviates Memory Deficits and Synaptic Failure in Mouse Model of Alzheimer's Disease

Phosphodiesterase 4 (PDE4)-dependent cAMP signaling plays a crucial role in cognitive impairment associated with Alzheimer’s disease (AD). However, whether inhibition of PDE4 subtypes or their splice variants in the prefrontal cortex positively regulates synaptic plasticity and antioxidative stress, and reverses β-amyloid 1–42 (Aβ1–42, Aβ42)-induced cognitive impairment still need to be clarified. The present study determined whether and how PDE4D knockdown by microinjection of lenti-PDE4D-miRNA into the prefrontal cortex reversed Aβ1–42-induced cognitive impairment in behavioral, neurochemical, and molecular biology assays. The results suggested that PDE4D knockdown increased time to explore the novel object and decreased latency to leave the platform in novel object recognition and step-down passive avoidance tests. Further study suggested that PDE4D knockdown decreased the number of working memory errors in the eight-arm maze test. These effects were prevented by PKA inhibitor H89. The subsequent experiment suggested that inhibition of PDE4D in the prefrontal cortex rescued the long-term potentiation (LTP) and synaptic proteins’ expression; it also increased antioxidant response by increasing superoxide dismutase (SOD) and decreasing malondialdehyde (MDA) levels. PDE4D knockdown also increased phosphorylated cAMP response element-binding protein (pCREB), brain-derived neurotrophic factor (BNDF), and anti-apoptotic proteins’ expression, i.e., the ratio of Bcl-2/Bax, and decreased caspase-3 level in the prefrontal cortex. These findings extend the previous findings and support the hypothesis that RNA interference-mediated PDE4D knockdown in the prefrontal cortex ameliorated memory loss associated with synaptic failure in an AD mouse model by its antioxidant, anti-apoptotic, and neuroprotective properties.

Positive effects of roflumilast on behavior, neuroinflammation, and white matter injury in mice with global cerebral ischemia

Inhibition of phosphodiesterase 4 (PDE4) is a promising pharmacological strategy for the treatment of cerebral ischemic conditions. To increase the relevance and increase the translational value of preclinical studies, it is important to conduct experiments using different animal species and strains, different animal models, and to evaluate long-term functional outcomes after cerebral ischemia. In the present study, the effects of the selective PDE4 inhibitor roflumilast were evaluated in vivo and in vitro. Balb/c mice were subjected to bilateral common carotid artery occlusion (BCCAO) and tested during 21 days in multiple behavioral tasks to investigate the long-term effects of roflumilast on functional recovery. The effects of roflumilast were also investigated on hippocampal cell loss, white matter injury, and expression of neuroinflammatory markers. Roflumilast prevented cognitive and emotional deficits induced by BCCAO in mice. Roflumilast also prevented neurodegeneration and reduced the white matter damage in the brain of ischemic animals. Besides, roflumilast decreased Iba-1 (microglia marker) levels and increased Arginase-1 (Arg-1; microglia M2 phenotype marker) levels in the hippocampus of these mice. Likewise, roflumilast suppressed inducible nitric oxide synthase (microglia M1 phenotype marker) expression and increased Arg-1 levels in a primary mouse microglia culture. These findings support evidence that PDE4 inhibition by roflumilast might be beneficial in cerebral ischemic conditions. The neuroprotective effects of roflumilast appear to be mediated by a decrease in neuroinflammation.

The 5-HT1A receptor as a serotonergic target for neuroprotection in cerebral ischemia

Cerebral ischemia due to stroke or cardiac arrest greatly affects daily functioning and the quality of life of patients and has a high socioeconomic impact due to the surge in their prevalence. Advances in the identification of an effective pharmacotherapy to promote neuroprotection and recovery after a cerebral ischemic insult are, however, limited. The serotonin 1A (5-HT_1A) receptor has been implicated in the regulation of several brain functions, including mood, emotions, memory, and neuroplasticity, all of which are deleteriously affected by cerebral ischemia. This review focuses on the specific roles and mechanisms of 5-HT_1A receptors in neuroprotection in experimental models of cerebral ischemia. We present experimental evidence that 5-HT_1A receptor agonists can prevent neuronal damage and promote functional recovery induced by focal and transient global ischemia in rodents. However, indiscriminate activation of pre-and postsynaptic by non-biased 5-HT_1A receptor agonists may be a limiting factor in the anti-ischemic clinical efficacy of these compounds since 5-HT_1A receptors in different brain regions can mediate diverging or even contradictory responses. Current insights are presented into the 'biased' 5-HT_1A post-synaptic heteroreceptor agonist NLX-101 (also known as F15599), a compound that preferentially and potently stimulates postsynaptic cortical pyramidal neurons without inhibiting firing of serotoninergic neurons, as a potential strategy providing neuroprotection in cerebral ischemic conditions.

Direct Neuronal Reprogramming: Bridging the Gap Between Basic Science and Clinical Application

Direct neuronal reprogramming is an innovative new technology that involves the conversion of somatic cells to induced neurons (iNs) without passing through a pluripotent state. The capacity to make new neurons in the brain, which previously was not achievable, has created great excitement in the field as it has opened the door for the potential treatment of incurable neurodegenerative diseases and brain injuries such as stroke. These neurological disorders are associated with frank neuronal loss, and as new neurons are not made in most of the adult brain, treatment options are limited. Developmental biologists have paved the way for the field of direct neuronal reprogramming by identifying both intrinsic cues, primarily transcription factors (TFs) and miRNAs, and extrinsic cues, including growth factors and other signaling molecules, that induce neurogenesis and specify neuronal subtype identities in the embryonic brain. The striking observation that postmitotic, terminally differentiated somatic cells can be converted to iNs by mis-expression of TFs or miRNAs involved in neural lineage development, and/or by exposure to growth factors or small molecule cocktails that recapitulate the signaling environment of the developing brain, has opened the door to the rapid expansion of new neuronal reprogramming methodologies. Furthermore, the more recent applications of neuronal lineage conversion strategies that target resident glial cells in situ has expanded the clinical potential of direct neuronal reprogramming techniques. Herein, we present an overview of the history, accomplishments, and therapeutic potential of direct neuronal reprogramming as revealed over the last two decades.

Neuroinflammation in Ischemic Stroke: Inhibition of cAMP-Specific Phosphodiesterases (PDEs) to the Rescue

Ischemic stroke is caused by a thromboembolic occlusion of a major cerebral artery, with the impaired blood flow triggering neuroinflammation and subsequent neuronal damage. Both the innate immune system (e.g., neutrophils, monocytes/macrophages) in the acute ischemic stroke phase and the adaptive immune system (e.g., T cells, B cells) in the chronic phase contribute to this neuroinflammatory process. Considering that the available therapeutic strategies are insufficiently successful, there is an urgent need for novel treatment options. It has been shown that increasing cAMP levels lowers neuroinflammation. By inhibiting cAMP-specific phosphodiesterases (PDEs), i.e., PDE4, 7, and 8, neuroinflammation can be tempered through elevating cAMP levels and, thereby, this can induce an improved functional recovery. This review discusses recent preclinical findings, clinical implications, and future perspectives of cAMP-specific PDE inhibition as a novel research interest for the treatment of ischemic stroke. In particular, PDE4 inhibition has been extensively studied, and is promising for the treatment of acute neuroinflammation following a stroke, whereas PDE7 and 8 inhibition more target the T cell component. In addition, more targeted PDE4 gene inhibition, or combined PDE4 and PDE7 or 8 inhibition, requires more extensive research.

Neuroprotective effect of roflumilast under cerebral ischaemia/reperfusion injury in juvenile rats through NLRP-mediated inflammatory response inhibition

This study aims to investigate the protective effect of roflumilast, a phosphodiesterase (PDE)-4 enzyme inhibitor, and demonstrate its possible role in the development prevention of cerebral ischemia/reperfusion injury (CI/RI) after stroke induced by carotid artery ligation in juvenile rats. The rats were randomly divided into five groups: healthy group without any treatment, healthy group administered with 1 mg/kg roflumilast, CI group not administered with roflumilast, CI group administered with 0.5 mg/kg roflumilast, and CI group administered with 1 mg/kg roflumilast. In the CI groups, reperfusion was achieved 2h after ischemia induction; in the roflumilast groups, this drug was intraperitoneally administered immediately after reperfusion and at the 12^th hour. At the end of 24h, the rats were sacrificed and their brain tissues removed for examination. The mRNA expressions obtained with real-time PCR of IL-1β, TNF-α, and NLRP3 significantly increased in the CI/RI-induced groups compared with the control group, and this increase was significantly lower in the groups administered with roflumilast compared with the CI/RI-induced groups. Moreover, ELISA revealed that both IL-1 β and IL-6 brain levels were significantly higher in the CI/RI-induced groups than in the controls. This increase was significantly lower in the groups administered with roflumilast compared with the CI/RI-induced groups. Histopathological studies revealed that the values closest to those of the healthy group were obtained from the roflumilast groups. Nissl staining revealed that the Nissl bodies manifested normal density in the healthy and roflumilast-administered healthy groups, but were rare in the CI/RI-induced groups. Roflumilast treatment increased these decreased Nissl bodies with increasing doses. Observations indicated that the Nissl body density was close to the value in the healthy group in the CI/RI-induced group administered with 1 mg/kg roflumilast. Overall, roflumilast reduced cellular damage caused by CI/RI in juvenile rats, and this effect may be mediated by NLRP3.

Roflumilast protects against spatial memory impairments and exerts anti-inflammatory effects after transient global cerebral ischemia

Phosphodiesterase 4 (PDE4) inhibitors have been shown to present beneficial effects in cerebral ischemic injury because of their ability to improve cognition and target different phases and mechanisms of cerebral ischemia, including apoptosis, neurogenesis, angiogenesis, and inflammation. The present study investigated whether repeated treatment with the PDE4 inhibitor roflumilast rescued memory loss and attenuated neuroinflammation in rats following transient global cerebral ischemia (TGCI). TGCI caused memory impairments, neuronal loss (reflected by Neuronal nuclei (NeuN) immunoreactivity), and compensatory neurogenesis (reflected by doublecortin (DCX) immunoreactivity) in the hippocampus. Also, increases in the protein expression of the phosphorylated response element-binding protein (pCREB) and inflammatory markers such as the glial fibrillary acidic protein (GFAP) and ionized calcium-binding adaptor molecule 1 (Iba-1), were detected in the hippocampus in TGCI rats. Repeated treatment with roflumilast (0.003 and 0.01 mg/kg) prevented spatial memory deficits without promoting hippocampal protection in ischemic animals. Roflumilast increased the levels of pCREB, arginase-1, interleukin (IL) 4, and IL-10 in the hippocampus 21 days after TGCI. These data suggest a protective effect of roflumilast against functional sequelae of cerebral ischemia, which might be related to its anti-inflammatory properties.

Phosphodiesterase-4 enzyme as a therapeutic target in neurological disorders

Phosphodiesterases (PDE) are a diverse family of enzymes (11 isoforms so far identified) responsible for the degradation of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which are involved in several cellular and biochemical functions. Phosphodiesterase 4 (PDE4) is the major isoform within this group and is highly expressed in the mammalian brain. An inverse association between PDE4 and cAMP levels is the key mechanism in various pathophysiological conditions like airway inflammatory diseases-chronic obstruction pulmonary disease (COPD), asthma, psoriasis, rheumatoid arthritis, and neurological disorders etc. In 2011, roflumilast, a PDE4 inhibitor (PDE4I) was approved for the treatment of COPD. Subsequently, other PDE4 inhibitors (PDE4Is) like apremilast and crisaborole were approved by the Food and Drug Administration (FDA) for psoriasis, atopic dermatitis etc. Due to the adverse effects like unbearable nausea and vomiting, dose intolerance and diarrhoea, PDE4 inhibitors have very less clinical compliance. Efforts are being made to develop allosteric modulation with high specificity to PDE4 isoforms having better efficacy and lesser adverse effects. Interestingly, repositioning PDE4Is towards neurological disorders including Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD), multiple sclerosis (MS) and sleep disorders, is gaining attention. This review is an attempt to summarize the data on the effects of PDE4 overexpression in neurological disorders and the use of PDE4Is and newer allosteric modulators as therapeutic options. We have also compiled a list of on-going clinical trials on PDE4 inhibitors in neurological disorders.

Taxane-induced neurotoxicity: Pathophysiology and therapeutic perspectives

Taxane-derived drugs are antineoplastic agents used for the treatment of highly common malignancies. Paclitaxel and docetaxel are the most commonly used taxanes; however, other drugs and formulations have been used, such as cabazitaxel and nab-paclitaxel. Taxane treatment is associated with neurotoxicity, a well-known and relevant side effect, very prevalent amongst patients undergoing chemotherapy. Painful peripheral neuropathy is the most dose-limiting side effect of taxanes, affecting up to 97% of paclitaxel-treated patients. Central neurotoxicity is an emerging side effect of taxanes and it is characterized by cognitive impairment and encephalopathy. Besides impairing compliance to chemotherapy treatment, taxane-induced neurotoxicity (TIN) can adversely affect the patient's life quality on a long-term basis. Despite the clinical relevance, not many reviews have comprehensively addressed taxane-induced neurotoxicity when they are used therapeutically. This article provides an up-to-date review on the pathophysiology of TIN and the novel potential therapies to prevent or treat this side effect.

Neuroprotective Effects of Endogenous Secretory Receptor for Advanced Glycation End-products in Brain Ischemia

The receptor for advanced glycation end-products (RAGE) is expressed on human brain endothelial cells (HBEC) and is implicated in neuronal cell death after ischemia. We report that endogenous secretory RAGE (esRAGE) is a splicing variant form of RAGE that functions as a decoy against ischemia-induced neuronal cell damage. This study demonstrated that esRAGE was associated with heparan sulphate proteoglycans on HBEC. The parabiotic experiments between human esRAGE overexpressing transgenic (Tg), RAGE knockout (KO), and wild-type (WT) mice revealed a significant neuronal cell damage in the CA1 region of the WT side of parabiotic WT→WT mice, but not of Tg→WT mice, 7 days after bilateral common carotid artery occlusion. Human esRAGE was detected around the CA1 neurons in the WT side of the parabiotic Tg→WT pair, but not in the KO side of the Tg→KO pair. To elucidate the dynamic transfer of esRAGE into the brain, we used the blood-brain barrier (BBB) system (PharmaCo-Cell) with or without RAGE knockdown in endothelial cells. A RAGE-dependent transfer of esRAGE was demonstrated from the vascular to the brain side. These findings suggested that esRAGE is associated with heparan sulphate proteoglycans and is transferred into the brain via BBB to exert its neuroprotective effects in ischemia.

Cyclic adenosine monophosphate in acute ischemic stroke: some to update, more to explore

The development of effective treatment for ischemic stroke, which is a common cause of morbidity and mortality worldwide, remains an unmet goal because the current first-line treatment management interventional therapy has a strict time window and serious complications. In recent years, a growing body of evidence has shown that the elevation of intracellular and extracellular cyclic adenosine monophosphate (cAMP) alleviates brain damage after ischemic stroke by attenuating neuroinflammation in the central nervous system and peripheral immune system. In the central nervous system, upregulated intracellular cAMP signaling can alleviate immune-mediated damage by restoring neuronal morphology and function, inhibiting microglia migration and activation, stabilizing the membrane potential of astrocytes and improving the cellular functions of endothelial cells and oligodendrocytes. Enhancement of the extracellular cAMP signaling pathway can improve neurological function by activating the cAMP-adenosine pathway to reduce immune-mediated damage. In the peripheral immune system, cAMP can act on various immune cells to suppress peripheral immune function, which can alleviate the inflammatory response in the central nervous system and improve the prognosis of acute cerebral ischemic injury. Therefore, cAMP may play key roles in reducing post-stroke neuroinflammatory damage. The protective roles of the cAMP indicate that the cAMP enhancing drugs such as cAMP supplements, phosphodiesterase inhibitors, adenylate cyclase agonists, which are currently used in the treatment of heart and lung diseases. They are potentially able to be applied as a new therapeutic strategy in ischemic stroke. This review focuses on the immune-regulating roles and the clinical implication of cAMP in acute ischemic stroke.

Inhibition of PDE4 protects neurons against oxygen-glucose deprivation-induced endoplasmic reticulum stress through activation of the Nrf-2/HO-1 pathway

Inhibition of phosphodiesterase 4 (PDE4) produces neuroprotective effects against cerebral ischemia. However, the involved mechanism remains unclear. Augmentation of endoplasmic reticulum (ER) stress promotes neuronal apoptosis, and excessive oxidative stress is an inducer of ER stress. The present study aimed to determine whether suppression of ER stress is involved in the protective effects of PDE4 inhibition against cerebral ischemia. We found that exposing HT-22 cells to oxygen-glucose deprivation (OGD) significantly activated ER stress, as evidenced by increased expression of the 78-kDa glucose-regulated protein (GRP78), phosphorylated eukaryotic translation-initiation factor 2α (eIF2α), and C/EBP-homologous protein (CHOP). Overexpression of PDE4B increased ER stress, while knocking down PDE4B or treatment with the PDE4 inhibitor, FCPR03, prevented OGD-induced ER stress in HT-22 cells. Furthermore, FCPR03 promoted the translocation of nuclear factor erythroid 2-related factor 2 (Nrf-2) from the cytoplasm to the nucleus. Importantly, the Nrf-2 inhibitor, ML385, blocked the inhibitory role of FCPR03 on OGD-induced ER stress. ML385 also abolished the protective role of FCPR03 in HT-22 cells subjected to OGD. Knocking down heme oxygenase-1 (HO-1), which is a target of Nrf-2, also blocked the protective role of FCPR03, enhanced the level of reactive oxygen species (ROS), and increased ER stress and cell death. We then found that FCPR03 or the antioxidant, N-Acetyl-l-cysteine, reduced oxidative stress in cells exposed to OGD. This effect was accompanied by increased cell viability and decreased ER stress. In primary cultured neurons, we found that FCPR03 reduced OGD-induced production of ROS and phosphorylation of eIF2α. The neuroprotective effect of FCPR03 against OGD in neurons was blocked by ML385. These results demonstrate that inhibition of PDE4 activates Nrf-2/HO-1, attenuates the production of ROS, and thereby attenuates ER stress in neurons exposed to OGD. Additionally, we conclude that FCPR03 may represent a promising therapeutic agent for the treatment of ER stress-related disorders.

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Overexpression of PDE4 increased ER stress under both basal and OGD conditions.

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Inhibition of PDE4 reduced ER stress and neuronal apoptosis in neurons exposed to OGD.

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PDE4 inhibition activated Nrf-2, and increased the level of antioxidant enzyme HO-1.

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Inhibition of Nrf-2 attenuated the role of FCPR03 on ER stress and cell viability.

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HO-1 knockdown abolished the effects of FCPR03 on ER stress and ROS production.

Efficacy of Cilostazol Administration in Alzheimer's Disease Patients with White Matter Lesions: A Positron-Emission Tomography Study

This study tested the efficacy of the phosphodiesterase type III inhibitor cilostazol in Alzheimer's disease patients with white matter lesions treated with donepezil in comparison with donepezil monotherapy using fluorodeoxyglucose (18F) positron-emission tomography (FDG PET). A 24-week, randomized, double-blind, placebo-controlled, parallel-group study was conducted. Thirty-six Alzheimer's disease patients with white matter lesions who received donepezil (n = 18 each in the cilostazol and placebo groups) were enrolled. Participants underwent pre and post FDG PET imaging scans and three rounds of clinical and neuropsychological tests. The cilostazol group did not show a significant decrease of regional glucose metabolism; however, regional glucose metabolism was significantly decreased in the parietal and frontal lobes of the placebo group. The repeated measures ANOVA measuring differences in uptake change revealed that regional glucose metabolism in the left inferior frontal gyrus was significantly more preserved in the cilostazol group than that in the placebo group (p < 0.005). Mean changes from baseline on the Mini-Mental State Exam, Alzheimer's Disease Assessment Scale-cognitive subscale, Alzheimer's Disease Cooperative Study-Activities of Daily Living Inventory, and the Clinical Dementia Rating Sum of Boxes did not differ between the two groups. In the cilostazol group, the increase of glucose metabolism correlated with the improvment of the Alzheimer's Disease Assessment Scale-cognitive score. We conclude that cilostazol treatment added to donepezil may delay the decline in regional cerebral metabolism in Alzheimer's disease with white matter lesions compared with donepezil monotherapy. In additon, our results verified the efficacy of cilostazol in improving or protecting cognitive function in Alzheimer's disease through increased glucose metabolism. However, the long-term effect of cilostazol on cognitive function and Alzheimer's disease modification must be tested in further studies with larger sample size and longer study period. Trial registration: http://clinicaltrials.gov : NCT01409564.

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.

A meta-analysis of PDE-gene polymorphism and cerebral infarction risk

Previous studies identified that phosphodiesterase 4D (PDE4D) gene polymorphism might be associated with cerebral infarction or ischemic stroke, and hemorrhagic stroke in human populations. However, as yet, no meta-analysis has revealed any detailed association. We retrospectively reviewed studies regarding the relationship of PDE4D gene polymorphism with ischemic stroke (IS) published during the period January 2003 to September 2012. According to the inclusion criteria, 9 of 105 initial studies were included in the subsequent analysis. The PubMed, Embase and CNKI of China were searched to identify the relevant studies. A total of 186 young patients with IS were included for the meta-analysis and 232 matched control subjects were enrolled and results were presented. The association of PDE4D gene polymorphism with IS in various populations was examined. The results suggested that single nucleotide polymorphism (SNP), SNP 83 in PDE4D gene was significantly related with susceptibility to IS. The meta-analysis also showed that PDE4D gene was associated with an enhanced risk of IS. The meta-analysis suggested that PDE4D SNP 87 constitutes an independent risk factor for IS development. To the best of our knowledge, the present meta-analysis reveals a number of possible associations between PDE4D gene polymorphism and IS.

Genetic Understanding of Stroke Treatment: Potential Role for Phosphodiesterase Inhibitors

Phosphodiesterase (PDE) gene family is a large family having at least 21 genes and multiple versions (isoforms) of the phosphodiesterase enzymes. These enzymes catalyze the inactivation of intracellular mediators of signal transduction such as cAMP and cGMP and therefore, play a pivotal role in various cellular functions. PDE inhibitors (PDEI) are drugs that block one or more of the five subtypes of the PDE family and thereby prevent inactivation of the intracellular cAMP and cGMP by the respective PDE-subtypes. The first clinical use of PDEI was reported almost three decades ago. Studies later found the ability of these compounds to increase the levels of ubiquitous secondary messenger molecules that can cause changes in vascular tone, cardiac function and other cellular events and thus these findings paved the way for their use in various medical emergencies. PDEs are found to be distributed in many tissues including brain. Therefore, new therapeutic agents in the form of PDEI are being explored in neurodegenerative diseases including stroke. Although studies have revealed their use in cerebral infarction prevention, their full-fledged application in times of neurological emergency or stroke in specific has been very limited so far. Nevertheless, recent investigations suggest PDE4 and PDE5 inhibitors to play a vital role in mitigating stroke symptoms by modulating signaling mechanisms in PDE pathway. Further, extensive research in terms of their pharmacological properties like dosing, drug specific activities, use of simultaneous medications, ancillary properties of these compounds and studies on adverse drug reactions needs to be carried out to set them as standard drugs of use in stroke.

The phosphodiesterase type 2 inhibitor BAY 60-7550 reverses functional impairments induced by brain ischemia by decreasing hippocampal neurodegeneration and enhancing hippocampal neuronal plasticity

Cognitive and affective impairments are the most characterized consequences following cerebral ischemia. BAY 60-7550, a selective phosphodiesterase type 2 inhibitor (PDE2-I), presents memory-enhancing and anxiolytic-like properties. The behavioral effects of BAY 60-7550 have been associated with its ability to prevent hydrolysis of both cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) thereby interfering with neuronal plasticity. Here, we hypothesize that PDE2-I treatment could promote functional recovery after brain ischemia. Mice C57Bl/6 were submitted to bilateral common carotid artery occlusion (BCCAO), an experimental model of transient brain ischemia, for 20 min. During 21 days after reperfusion, the animals were tested in a battery of behavioral tests including the elevated zero maze (EZM), object location task (OLT) and forced swim test (FST). The effects of BAY 60-7550 were evaluated on neuronal nuclei (NeuN), caspase-9, cAMP response element-binding protein (CREB), phosphorylated CREB (pCREB) and brain-derived neurotrophic factor (BDNF) expression in the hippocampus. BCCAO increased anxiety levels, impaired hippocampus-dependent cognitive function and induced despair-like behavior in mice. Hippocampal neurodegeneration was evidenced by a decrease in NeuN and increase incaspase-9 protein levels in BCCAO mice. Ischemic mice also showed low BDNF protein levels in the hippocampus. Repeated treatment with BAY 60-7550 attenuated the behavioral impairments induced by BCCAO in mice. Concomitantly, BAY 60-7550 enhanced expression of pCREB and BDNF protein levels in the hippocampus of ischemic mice. The present findings suggest that chronic inhibition of PDE2 provides functional recovery in BCCAO mice possibly by augmenting hippocampal neuronal plasticity.

Phosphodiesterase7 Inhibition Activates Adult Neurogenesis in Hippocampus and Subventricular Zone In Vitro and In Vivo

The phosphodiesterase 7 (PDE7) enzyme is one of the enzymes responsible for controlling intracellular levels of cyclic adenosine 3',5'-monophosphate in the immune and central nervous system. We have previously shown that inhibitors of this enzyme are potent neuroprotective and anti-inflammatory agents. In addition, we also demonstrated that PDE7 inhibition induces endogenous neuroregenerative processes toward a dopaminergic phenotype. Here, we show that PDE7 inhibition controls stem cell expansion in the subgranular zone of the dentate gyrus of the hippocampus (SGZ) and the subventricular zone (SVZ) in the adult rat brain. Neurospheres cultures obtained from SGZ and SVZ of adult rats treated with PDE7 inhibitors presented an increased proliferation and neuronal differentiation compared to control cultures. PDE7 inhibitors treatment of neurospheres cultures also resulted in an increase of the levels of phosphorylated cAMP response element binding protein, suggesting that their effects were indeed mediated through the activation of the cAMP/PKA signaling pathway. In addition, adult rats orally treated with S14, a specific inhibitor of PDE7, presented elevated numbers of proliferating progenitor cells, and migrating precursors in the SGZ and the SVZ. Moreover, long-term treatment with this PDE7 inhibitor shows a significant increase in newly generated neurons in the olfactory bulb and the hippocampus. Also a better performance in memory tests was observed in S14 treated rats, suggesting a functional relevance for the S14-induced increase in SGZ neurogenesis. Taken together, our results indicate for the first time that inhibition of PDE7 directly regulates proliferation, migration and differentiation of neural stem cells, improving spatial learning and memory tasks. Stem Cells 2017;35:458-472.

Rolipram stimulates angiogenesis and attenuates neuronal apoptosis through the cAMP/cAMP-responsive element binding protein pathway following ischemic stroke in rats

Rolipram, a phosphodiesterase-4 inhibitor, can activate the cyclic adenosine monophosphate (cAMP)/cAMP-responsive element binding protein (CREB) pathway to facilitate functional recovery following ischemic stroke. However, to date, the effects of rolipram on angiogenesis and cerebral ischemia-induced neuronal apoptosis are yet to be fully elucidated. In this study, the aim was to reveal the effect of rolipram on the angiogenesis and neuronal apoptosis following brain cerebral ischemia. Rat models of ischemic stroke were established following transient middle cerebral artery occlusion and rolipram was administered for three, seven and 14 days. The results were examined using behavioral tests, triphenyl tetrazolium chloride staining, immunostaining and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) to evaluate the effects of rolipram therapy on functional outcome, angiogenesis and apoptosis. Western blot analysis was used to show the phosphorylated- (p-)CREB protein level in the ischemic hemisphere. The rolipram treatment group exhibited a marked reduction in infarct size and modified neurological severity score compared with the vehicle group, and rolipram treatment significantly promoted the microvessel density in the ischemic boundary region and increased p-CREB protein levels in the ischemic hemisphere. Furthermore, a significant reduction in the number of TUNEL-positive cells was observed in the rolipram group compared with the vehicle group. These findings suggest that rolipram has the ability to attenuate cerebral ischemic injury, stimulate angiogenesis and reduce neuronal apoptosis though the cAMP/CREB pathway.

The effect of dopamine on adult hippocampal neurogenesis

Cumulative studies indicated that adult hippocampal neurogenesis might be involved in the action mechanism of antidepressant drugs and/or the pathophysiology of depression. Dopamine (DA) is involved in the regulation of motivation, volition, interest/pleasure, and attention/concentration, all of which are likely to be impaired in depressed patients. Several previous reports suggest that depression may often be accompanied by a relative hypo-dopaminergic state, and some DA receptor agonists are beneficial effects in the treatment for refractory and bipolar depression. In the present study, to clarify the direct effect of DA on neural progenitor cells, we examined the effect of DA on the proliferation of adult rat dentate gyrus-derived neural precursor cells (ADPs). In addition, we examined the effect of DA receptor agonists on adult rat hippocampal neurogenesis in vivo. Results showed that DA promoted the increase of ADPs via D1-like receptor and D1-like receptor agonist promoted the survival of newborn cells in the adult hippocampus. On the contrary, D2-like receptor agonist did not affect both proliferation and survival. These results suggested that DA might play, at least in part, a role in adult hippocampal neurogenesis via D1-like receptor and the activation of D1-like receptor has a therapeutic potential for depression.

Regulating the ubiquitin/proteasome pathway via cAMP-signaling: neuroprotective potential

The cAMP-signaling pathway has been under intensive investigation for decades. It is a wonder that such a small simple molecule like cAMP can modulate a vast number of diverse processes in different types of cells. The ubiquitous involvement of cAMP-signaling in a variety of cellular events requires tight spatial and temporal control of its generation, propagation, compartmentalization, and elimination. Among the various steps of the cAMP-signaling pathway, G-protein-coupled receptors, adenylate cyclases, phosphodiesterases, the two major cAMP targets, i.e., protein kinase A and exchange protein activated by cAMP, as well as the A-kinase anchoring proteins, are potential targets for drug development. Herein we review the recent progress on the regulation and manipulation of different steps of the cAMP-signaling pathway. We end by focusing on the emerging role of cAMP-signaling in modulating protein degradation via the ubiquitin/proteasome pathway. New discoveries on the regulation of the ubiquitin/proteasome pathway by cAMP-signaling support the development of new therapeutic approaches to prevent proteotoxicity in chronic neurodegenerative disorders and other human disease conditions associated with impaired protein turnover by the ubiquitin/proteasome pathway and the accumulation of ubiquitin-protein aggregates.

Phosphodiesterase type 4 inhibitor rolipram improves survival of spiral ganglion neurons in vitro

Sensorineural deafness is caused by damage of hair cells followed by degeneration of the spiral ganglion neurons and can be moderated by cochlear implants. However, the benefit of the cochlear implant depends on the excitability of the spiral ganglion neurons. Therefore, current research focuses on the identification of agents that will preserve their degeneration. In this project we investigated the neuroprotective effect of Rolipram as a promising agent to improve the viability of the auditory neurons. It is a pharmaceutical agent that acts by selective inhibition of the phosphodiesterase 4 leading to an increase in cyclic AMP. Different studies reported a neuroprotective effect of Rolipram. However, its significance for the survival of SGN has not been reported so far. Thus, we isolated spiral ganglion cells of neonatal rats for cultivation with different Rolipram concentrations and determined the neuronal survival rate. Furthermore, we examined immunocytologically distinct proteins that might be involved in the neuroprotective signalling pathway of Rolipram and determined endogenous BDNF by ELISA. When applied at a concentration of 0.1 nM, Rolipram improved the survival of SGN in vitro. According to previous studies, our immunocytological data showed that Rolipram application induces the phosphorylation and thereby activation of the transcription factor CREB. This activation can be mediated by the cAMP-PKA-signalling pathway as well as via ERK as a part of the MAP-kinase pathway. However, only in cultures pre-treated with BDNF, an endogenous increase of BDNF was detected. We conclude that Rolipram has the potential to improve the vitality of neonatal auditory nerve cells in vitro. Further investigations are necessary to prove the effect of Rolipram in vivo in the adult organism after lesion of the hair cells and insertion of cochlear implants.

Role of cyclic nucleotide-gated channels in the modulation of mouse hippocampal neurogenesis

Neural stem cells generate neurons in the hippocampal dentate gyrus in mammals, including humans, throughout adulthood. Adult hippocampal neurogenesis has been the focus of many studies due to its relevance in processes such as learning and memory and its documented impairment in some neurodegenerative diseases. However, we are still far from having a complete picture of the mechanism regulating this process. Our study focused on the possible role of cyclic nucleotide-gated (CNG) channels. These voltage-independent channels activated by cyclic nucleotides, first described in retinal and olfactory receptors, have been receiving increasing attention for their involvement in several brain functions. Here we show that the rod-type, CNGA1, and olfactory-type, CNGA2, subunits are expressed in hippocampal neural stem cells in culture and in situ in the hippocampal neurogenic niche of adult mice. Pharmacological blockade of CNG channels did not affect cultured neural stem cell proliferation but reduced their differentiation towards the neuronal phenotype. The membrane permeant cGMP analogue, 8-Br-cGMP, enhanced neural stem cell differentiation to neurons and this effect was prevented by CNG channel blockade. In addition, patch-clamp recording from neuron-like differentiating neural stem cells revealed cGMP-activated currents attributable to ion flow through CNG channels. The current work provides novel insights into the role of CNG channels in promoting hippocampal neurogenesis, which may prove to be relevant for stem cell-based treatment of cognitive impairment and brain damage.

Phosphodiesterases: Regulators of cyclic nucleotide signals and novel molecular target for movement disorders

Movement disorders rank among the most common neurological disorders. During the last two decades substantial progress has been made in understanding of the pathological basis of these disorders. Although, several mechanisms have been proposed, downregulation of cyclic nucleotide mediated signaling cascade has consistently been shown to contribute to the striatal dysfunctioning as seen in movement disorders. Thus, counteracting dysregulated cyclic nucleotide signaling has been considered to be beneficial in movement disorders. Cyclic nucleotide phosphodiesterases (PDEs) are the enzymes responsible for the breakdown of cyclic nucleotides and upregulation in PDE activity has been reported in various movement disorders. Thus, PDE inhibition is considered to be a novel strategy to restore cerebral cyclic nucleotide levels and their downstream signalling cascade. Indeed, various PDE inhibitors have been tested pre-clinically and were reported to be neuroprotective in various neurodegenerative disorders associated with movement disabilities. In this review, we have discussed a putative role of PDE inhibitors in movement disorders and associated abnormalities.

Genetic polymorphism in PDE4D gene and risk of ischemic stroke in Chinese population: a meta-analysis

Background

Stroke is the second most common cause of death and major cause of disability worldwide. The SNP 83 in PDE4D gene has been suggested as a risk factor in ischemic stroke, but direct evidence from genetic association studies remains inconclusive even in Chinese population.

Methods

Meta-analysis of case-control studies on the relationship between SNP 83 in PDE4D gene and susceptibility to ischemic stroke in Chinese population published domestically and abroad from January 2003 to September 2012.

Results

9 case-control studies were selected. Meta-analysis results showed that the significant association between SNP 83 and ischemic stroke was found under the dominant model (OR = 1.34, 95% CI: 1.20–1.49) and recessive model (OR = 1.45, 95% CI: 1.19–1.76) in Chinese population. In subgroup meta-analysis, SNP 83 and atherothrombotic stroke, rather than lacunar stroke, showed the significant association under the dominant model (OR = 1.69, 95% CI: 1.41–2.01) and recessive model (OR = 1.47, 95% CI: 1.04–2.06).

Conclusions

The results suggest that SNP 83 in PDE4D gene is significantly associated with susceptibility to ischemic stroke in Chinese population.

Effects of combining methylprednisolone with rolipram on functional recovery in adult rats following spinal cord injury

Methylprednisolone (MP) has been widely used as a standard therapeutic agent for the treatment of spinal cord injury (SCI). Because of its controversial beneficial effects, the combination of MP and other pharmacological agents aimed at enhancing functional recovery is desirable. The phosphodiesterase 4 (PDE4) inhibitor rolipram has been implicated in promotion of regeneration due to elevating cAMP. In the present study, we sought to determine the effects of MP and rolipram, administered in combination, after spinal cord injury (SCI) in adult rats. Here we show that in vitro administration of rolipram and MP significantly increased neuron survival and promoted neurite outgrowth of neurons on the inhibitory substrate CSPGs by upregulation of MMP-2 expression; in vivo administration of rolipram and MP inhibited CSPG expression and increase CSPG digestion after rat SCI. Rolipram and MP combining treatment promoted significant neuroprotection through reduced motoneuron death, minimized lesion cavity, and increased regeneration of lesioned corticospinal tract (CST) axons beyond the lesion site after SCI. Enhanced functional recovery was also observed. Overall, our study strongly suggested that the combination treatment of MP and rolipram may represent a promising strategy for clinically applicable pharmacological therapy for rapid initiation of neuroprotection after SCI.

Association of SNP41, SNP56 and a novel SNP in PDE4D gene with stroke and its subtypes

An association between phosphodiesterase 4D (PDE4D) gene and risk of stroke has been suggested by deCODE group in an Icelandic population. In the present case-control study we investigated the association of SNP41 (rs12153798) and SNP56 (rs702553) with ischemic stroke and stroke subtypes. Five hundred and sixteen ischemic stroke patients and 513 healthy age and sex matched controls were included in the study. The genotypes were determined by subjecting the PCR products to sequencing. Both the SNPs 56 and 41 associated significantly with stroke [adjusted OR=1.97; 95% CI (1.262-3.082); p=0.003: adjusted OR=5.42; 95% CI (3.45-8.5); p<0.001 respectively]. In addition to this, a novel SNP at position 59736747 T>G was found while sequencing the PCR products including SNP56. This novel SNP was found in patients as well as controls but did not show a significant association with the disease. We found significant association of SNPs 56 and 41 with large artery atherosclerosis, lacunar and cardioembolic stroke. In conclusion PDE4D gene plays a key part in the pathogenesis of ischemic stroke in the South Indian population from Andhra Pradesh.

G-protein-coupled receptors in adult neurogenesis

The importance of adult neurogenesis has only recently been accepted, resulting in a completely new field of investigation within stem cell biology. The regulation and functional significance of adult neurogenesis is currently an area of highly active research. G-protein-coupled receptors (GPCRs) have emerged as potential modulators of adult neurogenesis. GPCRs represent a class of proteins with significant clinical importance, because approximately 30% of all modern therapeutic treatments target these receptors. GPCRs bind to a large class of neurotransmitters and neuromodulators such as norepinephrine, dopamine, and serotonin. Besides their typical role in cellular communication, GPCRs are expressed on adult neural stem cells and their progenitors that relay specific signals to regulate the neurogenic process. This review summarizes the field of adult neurogenesis and its methods and specifies the roles of various GPCRs and their signal transduction pathways that are involved in the regulation of adult neural stem cells and their progenitors. Current evidence supporting adult neurogenesis as a model for self-repair in neuropathologic conditions, adult neural stem cell therapeutic strategies, and potential avenues for GPCR-based therapeutics are also discussed.

Dual contradictory roles of cAMP signaling pathways in hydroxyl radical production in the rat striatum

Studies have suggested that cAMP signaling pathways may be associated with the production of reactive oxygen species. In this study, we examined how modifications in cAMP signaling affected the production of hydroxyl radicals in rat striatum using microdialysis to measure extracellular 2,3-dihydroxybenzoic acid (2,3-DHBA), which is a hydroxyl radical adduct of salicylate. Up to 50 nmol of the cell-permeative cAMP mimetic 8-bromo-cAMP (8-Br-cAMP) increased 2,3-DHBA in a dose-dependent manner (there was no additional increase in 2,3-DHBA at 100 nmol). Another cAMP mimetic, dibutyryl cAMP (db-cAMP), caused a nonsignificant increase in 2,3-DHBA at 50 nmol and a significant decrease at 100 nmol. Up to 20 nmol of forskolin, which is a direct activator of adenylyl cyclase, increased 2,3-DHBA, similar to the effect of 8-Br-cAMP; however, forskolin resulted in a much greater increase in 2,3-DHBA. A potent inhibitor of protein kinase A (PKA), H89 (500 μM), potentiated the 8-Br-cAMP- and forskolin-induced increases in 2,3-DHBA and antagonized the inhibitory effect of 100 nmol of db-cAMP. Interestingly, the administration of 100 nmol of 8-bromo-cGMP alone or in combination with H89 had no significant effect on 2,3-DHBA levels. Doses of 100 nmol of a preferential PKA activator (6-phenyl-cAMP) or a preferential PKA inhibitor (8-bromoadenosine-3',5'-cyclic monophosphorothionate, Rp-isomer; Rp-8-Br-cAMPS), which also inhibits the cAMP-mediated activation of Epac (the exchange protein directly activated by cAMP), suppressed or enhanced, respectively, the formation of 2,3-DHBA. Up to 100 nmol of 8-(4-chlorophenylthio)-2'-O-methyladenosine-cAMP, which is a selective activator of Epac, dose-dependently stimulated the formation of 2,3-DHBA. These findings suggest that cAMP signaling plays contradictory roles (stimulation and inhibition) in the production of hydroxyl radicals in rat striatum by differential actions of Epac and PKA. These roles might contribute to the production of hydroxyl radicals concomitant with cAMP in carbon monoxide poisoning, because the formation of 2,3-DHBA was potentiated by the PKA inhibitor H89 and suppressed by Rp-8-Br-cAMPS, which inhibits PKA and Epac.

Remote ischemic preconditioning protects neurocognitive function of rats following cerebral hypoperfusion

BACKGROUND

Protection of remote ischemic preconditioning on neurocognitive function caused by bilateral common carotid artery occlusion has been investigated in rats.

MATERIAL/METHODS

Thirty-six male Sprague-Dawley rats were divided into 3 groups - control group (Group C, n=12), bilateral carotid arteries occlusion group (Group B, n=12) and remote ischemic precondition group (Group P, n=12). In Group P, remote ischemic preconditioning (RIPC) was performed on the right femoral artery with 3 cycles (10 min) of occlusion/perfusion. After 3 cycles of preconditioning, bilateral carotid arteries were occluded immediately for 60 min. In Group B, ischemic insults were conducted without RIPC. Sham surgeries were performed in Group C. Evaluation of memory and learning capacity was performed on days 5-8 after surgery by Morris water maze testing of spatial learning capacity (n=6 for each group). Apoptosis of cells in the hippocampus region was determined by TUNEL tests and Bcl-2 at this region was determined by ELISA 24 h and 9 days after vessel occlusion (n=6 for each group).

RESULTS

Neurocognitive tests showed that latency time was significantly longer in Group B than in Group P on day 7 (p=0.016) and day 8 (p=0.036). Moreover, frequency of platform crossings was significant less in group B than in the other 2 groups on day 9. Bcl-2 level was significantly increased in the hippocampal region of rats in Group P on days 1 and 9 after vessel occlusion. TUNEL test showed that apoptosis could be observed at 24 h after occlusion in Group B, but not in Group P and Group C. No apoptosis was observed on day 9.

CONCLUSIONS

Our results suggest that RIPC can protect neurocognitive function of rats after bilateral carotid occlusions, and that Bcl-2 may play an important role in this protective effect.

Time-course of changes in phosphorylated CREB in neuroblasts and BDNF in the mouse dentate gyrus at early postnatal stages

Cyclic AMP (cAMP) response element-binding protein (CREB) is involved in memory, learning, and synaptic transmission. In this study, we observed changes of phosphorylated CREB (pCREB) immunoreactivity and its protein levels as well as brain-derived neurotrophic factor (BDNF) levels in the hippocampal dentate gyrus at postnatal (P) 1, 7, 14, and 21 in mice. In addition, we also investigated pCREB expression in doublecortin (DCX, a marker for neuronal progenitors) immunoreactive neuroblasts at P21. pCREB immunoreaction at P1 was detected in most of cells in the dentate gyrus, thereafter pCREB immunoreactivity was decreased in all the layers of the dentate gyrus with time, however, strong pCREB immunoreactivity was shown in cells confined to the subgranular zone of the dentate gyrus at P21. In this group, many pCREB immunoreactive cells were co-localized with DCX immunoreactive neuroblasts. In addition, pCREB protein levels were decreased with age, showing that their levels were very low at P21, while BDNF protein levels were increased with age. These results suggest that pCREB may play important roles in functional maturity of granule cells in mice.

Prevention of cerebral ischemia-induced memory deficits by inhibition of phosphodiesterase-4 in rats

Inhibition of phosphodiesterase-4 (PDE4) by rolipram, a prototypical PDE4 inhibitor, reverses memory impairment produced pharmacologically or genetically. Comparably, much less is known about the effect of rolipram on cerebral ischemia-induced memory deficits. The objective of this study was to determine the effects of rolipram on ischemia-induced memory deficit, neuronal damage, and alteration of PDE4 activity in the hippocampus. Memory was examined using Morris water-maze and step-through passive avoidance tests in rats subjected to global cerebral ischemia with or without repeated treatment with rolipram (0.3 or 1 mg/kg, i.p.); neuronal damage in the hippocampus and PDE4 activity in hippocampal tissues were determined using Nissl staining and HPLC, respectively. In the water-maze test, cerebral ischemia significantly increased the escape latency to reach the platform during acquisition training and decreased the exploration time in the target quadrant in the probe trial test; these were blocked by rolipram in a dose-dependent manner. Rolipram also reduced the distracted platform searches induced by cerebral ischemia. In the passive avoidance test, ischemia decreased the 24-h latency to the dark compartment, which was also blocked by rolipram treatment. In addition, Nissl staining revealed ischemia-induced neuron loss in hippocampal CA1; this was blocked by rolipram. Further, cerebral ischemia led to increases in activity of PDE, primarily PDE4, in the hippocampus, which also was antagonized by rolipram. These results suggest that rolipram prevents cerebral ischemia-induced memory deficits via inhibition of increased PDE4 activity and attenuation of hippocampal, neuronal damages induced by ischemia. PDE4 may be a target for treatment of cognitive disorders associated with cerebral ischemia.

Phosphodiesterase-4D knock-out and RNA interference-mediated knock-down enhance memory and increase hippocampal neurogenesis via increased cAMP signaling

Phosphodiesterase-4 (PDE4) plays an important role in mediating memory via the control of intracellular cAMP signaling; inhibition of PDE4 enhances memory. However, development of PDE4 inhibitors as memory enhancers has been hampered by their major side effect of emesis. PDE4 has four subtypes (PDE4A-D) consisting of 25 splice variants. Mice deficient in PDE4D displayed memory enhancement in radial arm maze, water maze, and object recognition tests. These effects were mimicked by repeated treatment with rolipram in wild-type mice. In addition, similarly as rolipram-treated wild-type mice, PDE4D-deficient mice also displayed increased hippocampal neurogenesis and phosphorylated cAMP response element-binding protein (pCREB). Furthermore, microinfusion of lentiviral vectors that contained microRNAs (miRNAs) targeting long-form PDE4D isoforms into bilateral dentate gyri of the mouse hippocampus downregulated PDE4D4 and PDE4D5, enhanced memory, and increased hippocampal neurogenesis and pCREB. Finally, while rolipram and PDE4D deficiency shortened α2 adrenergic receptor-mediated anesthesia, a surrogate measure of emesis, miRNA-mediated PDE4D knock-down in the hippocampus did not. The present results suggest that PDE4D, in particular long-form PDE4D, plays a critical role in the mediation of memory and hippocampal neurogenesis, which are mediated by cAMP/CREB signaling; reduced expression of PDE4D, or at least PDE4D4 and PDE4D5, in the hippocampus enhances memory but appears not to cause emesis. These novel findings will aid in the development of PDE4 subtype- or variant-selective inhibitors for treatment of disorders involving impaired cognition, including Alzheimer's disease.

Contribution of constitutively proliferating precursor cell subtypes to dentate neurogenesis after cortical infarcts

Background

It is well known that focal ischemia increases neurogenesis in the adult dentate gyrus of the hippocampal formation but the cellular mechanisms underlying this proliferative response are only poorly understood. We here investigated whether precursor cells which constitutively proliferate before the ischemic infarct contribute to post-ischemic neurogenesis. To this purpose, transgenic mice expressing green fluorescent protein (GFP) under the control of the nestin promoter received repetitive injections of the proliferation marker bromodeoxyuridine (BrdU) prior to induction of cortical infarcts. We then immunocytochemically analyzed the fate of these BrdU-positive precursor cell subtypes from day 4 to day 28 after the lesion.

Results

Quantification of BrdU-expressing precursor cell populations revealed no alteration in number of radial glia-like type 1 cells but a sequential increase of later precursor cell subtypes in lesioned animals (type 2a cells at day 7, type 3 cells/immature neurons at day 14). These alterations result in an enhanced survival of mature neurons 4 weeks postinfarct.

Conclusions

Focal cortical infarcts recruit dentate precursor cells generated already before the infarct and significantly contribute to an enhanced neurogenesis. Our findings thereby increase our understanding of the complex cellular mechanisms of postlesional neurogenesis.

Effects of electroacupuncture at Zusanli and Baihui on brain-derived neurotrophic factor and cyclic AMP response element-binding protein in the hippocampal dentate gyrus

Previously, we observed that electroacupuncture (EA) at ST36 (Zusanli) and GV20 (Baihui) enhanced cell proliferation and neuroblast differentiation in the rat dentate gyrus. In this study, we investigated the possible mechanisms of EA in this effect. For this, we applied EA at ST36 and GV20 of Wistar rats (13-week-old) once a day for 3 weeks. Application of EA at these acupoints significantly increased the number of phosphorylated cyclic AMP response element-binding protein (pCREB)-immunoreactive cells in the dentate gyrus. In addition, EA significantly increased the levels of brain-derived neurotrophic factor (BDNF) and pCREB protein in the dentate gyrus. The administration of K252a, an inhibitor of BDNF receptor, significantly reduced cell proliferation in the subgranular zone of dentate gyrus. These results suggest that EA significantly increased neuroblast plasticity via pCREB and BDNF activation in the dentate gyrus.

NR4A orphan nuclear receptors as mediators of CREB-dependent neuroprotection

Induced expression of neuroprotective genes is essential for maintaining neuronal integrity after stressful insults to the brain. Here we show that NR4A nuclear orphan receptors are induced after excitotoxic and oxidative stress in neurons, up-regulate neuroprotective genes, and increase neuronal survival. Moreover, we show that NR4A proteins are induced by cAMP response element binding protein (CREB) in neurons exposed to stressful insults and that they function as mediators of CREB-induced neuronal survival. Animals with null mutations in three of six NR4A alleles show increased oxidative damage, blunted induction of neuroprotective genes, and increased vulnerability in the hippocampus after treatment with kainic acid. We also demonstrate that NR4A and the transcriptional coactivator PGC-1alpha independently regulate distinct CREB-dependent neuroprotective gene programs. These data identify NR4A nuclear orphan receptors as essential mediators of neuroprotection after exposure to neuropathological stress.

A large-scale chemical screen for regulators of the arginase 1 promoter identifies the soy isoflavone daidzeinas a clinically approved small molecule that can promote neuronal protection or regeneration via a cAMP-independent pathway

An ideal therapeutic for stroke or spinal cord injury should promote survival and regeneration in the CNS. Arginase 1 (Arg1) has been shown to protect motor neurons from trophic factor deprivation and allow sensory neurons to overcome neurite outgrowth inhibition by myelin proteins. To identify small molecules that capture Arg1's protective and regenerative properties, we screened a hippocampal cell line stably expressing the proximal promoter region of the arginase 1 gene fused to a reporter gene against a library of compounds containing clinically approved drugs. This screen identified daidzein as a transcriptional inducer of Arg1. Both CNS and PNS neurons primed in vitro with daidzein overcame neurite outgrowth inhibition from myelin-associated glycoprotein, which was mirrored by acutely dissociated and cultured sensory neurons primed in vivo by intrathecal or subcutaneous daidzein infusion. Further, daidzein was effective in promoting axonal regeneration in vivo in an optic nerve crush model when given intraocularly without lens damage, or most importantly, when given subcutaneously after injury. Mechanistically, daidzein requires transcription and induction of Arg1 activity for its ability to overcome myelin inhibition. In contrast to canonical Arg1 activators, daidzein increases Arg1 without increasing CREB phosphorylation, suggesting its effects are cAMP-independent. Accordingly, it may circumvent known CNS side effects of some cAMP modulators. Indeed, daidzein appears to be safe as it has been widely consumed in soy products, crosses the blood-brain barrier, and is effective without pretreatment, making it an ideal candidate for development as a therapeutic for spinal cord injury or stroke.

The effects of the phosphodiesterase inhibitor olprinone on global cerebral ischemia

BACKGROUND

The phosphodiesterase III inhibitor olprinone has been confirmed to improve myocardial function and increase cerebral blood flow; therefore, if olprinone exerts direct neuroprotective effects against global cerebral ischemia to the same degree as cilostazol, olprinone could be useful for cerebral resuscitation after cardiac arrest. We examined whether olprinone directly protected neuronal cells from global cerebral ischemia both in vivo and in vitro.

METHODS

In a rat model of 10-minute global cerebral ischemia induced by 4-vessel occlusion, 0.3, 3, or 30 microg x kg(-1) x min(-1) olprinone or saline was infused for a periischemic period of 40 minutes (n = 6 for each group). Hippocampal CA1 neuronal cells were then counted 3 days after reperfusion, and the phosphorylation of cyclic adenosine 3'5'-monophosphate response element-binding protein was examined using Western blotting analyses of specimens obtained 15 minutes after reperfusion. In vitro, cultured cerebral neurons were exposed to 4 hours of hypoxia and glucose deprivation and then 24 hours of recovery in the absence or presence of olprinone (10(-11)-10(-5) mol x L(-1)). Cell viability was measured using the Cell Counting Kit-8 (Dojindo Molecular Technologies, Gaithersburg, MD).

RESULTS

In the rat model of global ischemia, the number of surviving CA1 neurons counted under a microscopic field in the 30 microg x kg(-1) x min(-1) olprinone-treated group (49.9 +/- 9.2) was significantly higher than that in the saline infusion control group (7.2 +/- 3.4), and olprinone treatment increased the phosphorylation of cyclic adenosine 3'5'-monophosphate response element-binding protein. The survival fraction of the neuronal cells cultured in the presence of olprinone was also significantly higher than that of cells cultured in the absence of olprinone in a dose-dependent manner.

CONCLUSIONS

Our study successfully demonstrated, for the first time, that olprinone had a protective effect on neuronal cells in vitro and in vivo, especially against global cerebral ischemia. These results suggest that olprinone might be useful for the treatment of patients experiencing global cerebral ischemia.

Antidepressant- and anxiolytic-like effects of the phosphodiesterase-4 inhibitor rolipram on behavior depend on cyclic AMP response element binding protein-mediated neurogenesis in the hippocampus

Inhibition of phosphodiesterase-4 (PDE4), an enzyme that catalyzes the hydrolysis of cyclic AMP (cAMP), increases phosphorylation of the cAMP response element binding protein (pCREB) and hippocampal neurogenesis, and produces antidepressant-like effects on behavior; however, causal links among these actions have not been established. In this study, chronic administration of rolipram (0.31-1.25 mg/kg, 16-23 days) produced antidepressant- and anxiolytic-like effects on behavior in mice. It also increased cAMP and pCREB levels in the hippocampus and prefrontal cortex, but increased Sox2, a marker for mitotic progenitor cells, only in the hippocampus. Chronic rolipram treatment also increased hippocampal neurogenesis, as evidenced by increased bromodeoxyuridine (BrdU)-positive cells in the hippocampal dentate gyrus. Methylazoxymethanol (MAM), which is toxic to proliferating cells, reversed rolipram-induced increases in BrdU-positive cells and pCREB in the hippocampus and partially blocked its behavioral effects. Approximately 84% of BrdU-positive cells became newborn neurons, 93% of which co-expressed pCREB; these proportions were not altered by rolipram or MAM, either alone or in combination. Finally, 3 weeks after the end of the MAM treatment, when neurogenesis was no longer inhibited, rolipram again increased hippocampal pCREB and its antidepressant- and anxiolytic-like effects were restored. Overall, these results suggest that rolipram produces its effects on behavior in a manner that at least partially depends on its neurogenic action in the hippocampus, targeting mitotic progenitor cells rather than newborn or mature neurons; cAMP/CREB signaling in hippocampal newborn neurons is critical for neurogenesis and contributes to the behavioral effects of rolipram.

Association between the PDE4D gene and ischaemic stroke in the Chinese Han population

Recent findings suggests that PDE4D (gene encoding phosphodiesterase 4D) is a stroke-related gene in the Icelandic population, but it is still very controversial as to whether it is a susceptible gene for stroke in other populations. In the present study, we attempted to explore the role of the gene in the pathogenesis of stroke in the Chinese Han population of eastern China. A total of 649 ischaemic stroke patients and 761 unrelated control individuals with no history of stroke or transient ischaemic attack were examined in a case-control study. Four SNPs (single nucleotide polymorphisms) rs152312 (C/T), SNP56 (A/T), SNP83 (C/T) and SNP87 (C/T) with a minor allele frequency over 5% were genotyped and the corresponding haplotypes were constructed. In an analysis of the combined cardiogenic and carotid stroke group, both the allele (P=0.0060) and genotype (P=0.0160) frequencies between cases and controls at SNP83 showed significant differences. However, no difference in haplotype frequencies was observed between cases and controls at rs152312 and SNP56. In the analysis of the small-artery-occlusive stroke group, no difference in allele or genotype frequencies was observed at any marker between cases and controls; the global haplotype frequency in rs152312 and SNP56 had a significant difference between cases and controls (P=0.0162); the frequency of haplotype C-A was higher in cases than in controls (P=0.0122). In conclusion, our present findings show that polymorphisms in the PDE4D gene are associated with an increased risk of ischaemic stroke in the Chinese Han population. The present study adds further support to the role of PDE4D in stroke.