PDE4B as a microglia target to reduce neuroinflammation

Highlights lecture EANM 2016: "Embracing molecular imaging and multi-modal imaging: a smart move for nuclear medicine towards personalized medicine"

The 2016 EANM Congress took place in Barcelona, Spain, from 15 to 19 October under the leadership of Prof. Wim Oyen, chair of the EANM Scientific Committee. With more than 6,000 participants, this congress was the most important European event in nuclear medicine, bringing together a multidisciplinary community involved in the different fields of nuclear medicine. There were over 600 oral and 1,200 poster or e-Poster presentations with an overwhelming focus on development and application of imaging for personalized care, which is timely for the community. Beyond FDG PET, major highlights included progress in the use of PSMA and SSTR receptor-targeted radiopharmaceuticals and associated theranostics in oncology. Innovations in radiopharmaceuticals for imaging pathologies of the brain and cardiovascular system, as well as infection and inflammation, were also highlighted. In the areas of physics and instrumentation, multimodality imaging and radiomics were highlighted as promising areas of research.

Drug discovery for remyelination and treatment of MS

Glia constitute the majority of the cells in our nervous system, yet there are currently no drugs that target glia for the treatment of disease. Given ongoing discoveries of the many roles of glia in numerous diseases of the nervous system, this is likely to change in years to come. Here we focus on the possibility that targeting the oligodendrocyte lineage to promote regeneration of myelin (remyelination) represents a therapeutic strategy for the treatment of the demyelinating disease multiple sclerosis, MS. We discuss how hypothesis driven studies have identified multiple targets and pathways that can be manipulated to promote remyelination in vivo, and how this work has led to the first ever remyelination clinical trials. We also highlight how recent chemical discovery screens have identified a host of small molecule compounds that promote oligodendrocyte differentiation in vitro. Some of these compounds have also been shown to promote myelin regeneration in vivo, with one already being trialled in humans. Promoting oligodendrocyte differentiation and remyelination represents just one potential strategy for the treatment of MS. The pathology of MS is complex, and its complete amelioration may require targeting multiple biological processes in parallel. Therefore, we present an overview of new technologies and models for phenotypic analyses and screening that can be exploited to study complex cell-cell interactions in in vitro and in vivo systems. Such technological platforms will provide insight into fundamental mechanisms and increase capacities for drug-discovery of relevance to glia and currently intractable disorders of the CNS.

Phosphodiesterase 4B negatively regulates endotoxin-activated interleukin-1 receptor antagonist responses in macrophages

Activation of TLR4 by lipopolysaccharide (LPS) induces both pro-inflammatory and anti-inflammatory cytokine production in macrophages. Type 4 phosphodiesterases (PDE4) are key cAMP-hydrolyzing enzymes, and PDE4 inhibitors are considered as immunosuppressors to various inflammatory responses. We demonstrate here that PDE4 inhibitors enhance the anti-inflammatory cytokine interleukin-1 receptor antagonist (IL-1Ra) secretion in LPS-activated mouse peritoneal macrophages, and this response was regulated at the transcriptional level rather than an increased IL-1Ra mRNA stability. Studies with PDE4-deficient macrophages revealed that the IL-1Ra upregulation elicited by LPS alone is PKA-independent, whereas the rolipram-enhanced response was mediated by inhibition of only PDE4B, one of the three PDE4 isoforms expressed in macrophages, and it requires PKA but not Epac activity. However, both pathways activate CREB to induce IL-1Ra expression. PDE4B ablation also promoted STAT3 phosphorylation (Tyr⁷⁰⁵) to LPS stimulation, but this STAT3 activation is not entirely responsible for the IL-1Ra upregulation in PDE4B-deficient macrophages. In a model of LPS-induced sepsis, only PDE4B-deficient mice displayed an increased circulating IL-1Ra, suggesting a protective role of PDE4B inactivation in vivo. These findings demonstrate that PDE4B negatively modulates anti-inflammatory cytokine expression in innate immune cells, and selectively targeting PDE4B should retain the therapeutic benefits of nonselective PDE4 inhibitors.

Phosphodiesterase Inhibitors as a Therapeutic Approach to Neuroprotection and Repair

A wide diversity of perturbations of the central nervous system (CNS) result in structural damage to the neuroarchitecture and cellular defects, which in turn are accompanied by neurological dysfunction and abortive endogenous neurorepair. Altering intracellular signaling pathways involved in inflammation and immune regulation, neural cell death, axon plasticity and remyelination has shown therapeutic benefit in experimental models of neurological disease and trauma. The second messengers, cyclic adenosine monophosphate (cyclic AMP) and cyclic guanosine monophosphate (cyclic GMP), are two such intracellular signaling targets, the elevation of which has produced beneficial cellular effects within a range of CNS pathologies. The only known negative regulators of cyclic nucleotides are a family of enzymes called phosphodiesterases (PDEs) that hydrolyze cyclic nucleotides into adenosine monophosphate (AMP) or guanylate monophosphate (GMP). Herein, we discuss the structure and physiological function as well as the roles PDEs play in pathological processes of the diseased or injured CNS. Further we review the approaches that have been employed therapeutically in experimental paradigms to block PDE expression or activity and in turn elevate cyclic nucleotide levels to mediate neuroprotection or neurorepair as well as discuss both the translational pathway and current limitations in moving new PDE-targeted therapies to the clinic.

Overexpression of phosphodiesterase-4 subtypes involved in surgery-induced neuroinflammation and cognitive dysfunction in mice

Postoperative cognitive dysfunction (POCD) is characterized by cognitive impairments in patients after surgery. Hippocampal neuroinflammation induced by surgery is highly associated with POCD. Phosphodiesterase-4 (PDE4) is an enzyme that specifically hydrolyses cyclic adenosine monophosphate (cAMP), which plays an important role during neuroinflammation and the process of learning and memory. However, the role of PDE4 in the development of POCD remains unclear. Male 14-month-old C57BL/6 mice received carotid artery exposure to mimic POCD. First, we evaluated cognitive performance by a Morris water maze (MWM) and fear conditioning system (FCS) test after surgery. The expression of PDE4 subtypes, pro-inflammatory cytokines, p-CREB and PSD95 as well as cAMP levels were investigated. Then, we used rolipram, a PDE4 inhibitor, to block the effects of PDE4. The cognitive performance of the mice and the expression of PDE4 subtypes, pro-inflammatory cytokines, p-CREB and PSD95 as well as cAMP levels were examined again. Mice displayed learning and memory impairment, overexpression of PDE4B and PDE4D, elevation of pro-inflammatory cytokines, and reduction in the expression of p-CREB, PSD95 and cAMP levels after surgery. The expression of PDE4B and PDE4D in the hippocampus decreased following blocking of PDE4 by rolipram. Meanwhile, rolipram attenuated the cognitive impairment and the elevation of pro-inflammatory cytokines induced by surgery. Moreover, rolipram reversed the reduction of p-CREB and PSD95. These results indicate that PDE4 subtype overexpression may be involved in the development of surgery-induced cognitive dysfunction in mice.

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.

Increase of Intracellular Cyclic AMP by PDE4 Inhibitors Affects HepG2 Cell Cycle Progression and Survival

Type 4 cyclic nucleotide phosphodiesterases (PDE4) are major members of a superfamily of enzymes (PDE) involved in modulation of intracellular signaling mediated by cAMP. Broadly expressed in most human tissues and present in large amounts in the liver, PDEs have in the last decade been key therapeutic targets for several inflammatory diseases. Recently, a significant body of work has underscored their involvement in different kinds of cancer, but with no attention paid to liver cancer. The present study investigated the effects of two PDE4 inhibitors, rolipram and DC-TA-46, on the growth of human hepatoma HepG2 cells. Treatment with these inhibitors caused a marked increase of intracellular cAMP level and a dose- and time-dependent effect on cell growth. The concentrations of inhibitors that halved cell proliferation to about 50% were used for cell cycle experiments. Rolipram (10 μM) and DC-TA-46 (0.5 μM) produced a decrease of cyclin expression, in particular of cyclin A, as well as an increase in p21, p27 and p53, as evaluated by Western blot analysis. Changes in the intracellular localization of cyclin D1 were also observed after treatments. In addition, both inhibitors caused apoptosis, as demonstrated by an Annexin-V cytofluorimetric assay and analysis of caspase-3/7 activity. Results demonstrated that treatment with PDE4 inhibitors affected HepG2 cell cycle and survival, suggesting that they might be useful as potential adjuvant, chemotherapeutic or chemopreventive agents in hepatocellular carcinoma. J. Cell. Biochem. 118: 1401-1411, 2017. © 2016 Wiley Periodicals, Inc.

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.

siRNA-mediated silencing of phosphodiesterase 4B expression affects the production of cytokines in endotoxin-stimulated primary cultured microglia

Phosphodiesterase 4 (PDE4) has four subtypes: PDE4A, PDE4B, PDE4C and PDE4D. The expression of PDE4 subtypes in microglial cells and the specific contribution of each subtype to inflammation remain unclear. In this study, the expression of PDE4 subtypes in primary microglial cells was assayed. Primary microglial cells were then transfected with specific small interfering RNA (siRNA) against each PDE4 subtype. PDE4 subtype A-D knockdown was confirmed by quantitative polymerase chain reaction. Secreted cytokines in the supernatant and intracellular cyclic adenosine monophosphate (cAMP) levels of transfected cells were measured. The effect of PDE4B siRNA on the activation of extracellular regulated protein kinase (ERK) induced by lipopolysaccharide (LPS) in microglia was further tested by western blotting. Results showed that the primary microglial cells expressed all four types of PDE4s at the protein level. Transfection with the four siRNAs inhibited PDE4 subtype A-D mRNA expression, respectively. In primary microglial cells, treatment with PDE4B siRNA significantly inhibited the expression of tumor necrosis factor-α and interleukin (IL)-1β, and enhanced the expression of cAMP, while siRNAs to other subtypes had no significant effects. However, none of the four siRNAs had any significant effect on the expression of IL-10. Furthermore, in the PDE4B group, the level of phosphorylated ERK was reduced. Among the four PDE4 subtypes, PDE4B plays an important role in regulating inflammatory responses in microglia, potentially through initially regulating the intracellular cAMP concentration.