Identification of a multifunctional docking site on the catalytic unit of phosphodiesterase-4 (PDE4) that is utilised by multiple interaction partners

Phosphodiesterase 4D activity in acrodysostosis-associated neural pathology: too much or too little?

Members of the phosphodiesterase 4 (PDE4) enzyme family regulate the availability of the secondary messenger cyclic adenosine monophosphate (cAMP) and, by doing so, control cellular processes in health and disease. In particular, PDE4D has been associated with Alzheimer's disease and the intellectual disability seen in fragile X syndrome. Furthermore, single point mutations in critical PDE4D regions cause acrodysostosis type 2(ACRDYS2, also referred to as inactivating PTH/PTHrP signalling disorder 5 or iPPSD5), where intellectual disability is seen in ∼90% of patients alongside the skeletal dysmorphologies that are characteristic of acrodysostosis type 1 (ACRDYS1/iPPSD4) and ACRDYS2. Two contrasting mechanisms have been proposed to explain how mutations in PDE4D cause iPPSD5. The first mechanism, the 'over-activation hypothesis', suggests that cAMP/PKA (cyclic adenosine monophosphate/protein kinase A) signalling is reduced by the overactivity of mutant PDE4D, whilst the second, the 'over-compensation hypothesis' suggests that mutations reduce PDE4D activity. That reduction in activity is proposed to cause an increase in cellular cAMP, triggering the overexpression of other PDE isoforms. The resulting over-compensation then reduces cellular cAMP and the levels of cAMP/PKA signalling. However, neither of these proposed mechanisms accounts for the fine control of PDE activation and localization, which are likely to play a role in the development of iPPSD5. This review will draw together our understanding of the role of PDE4D in iPPSD5 and present a novel perspective on possible mechanisms of disease.

Stress and Inflammation Target Dorsolateral Prefrontal Cortex Function: Neural Mechanisms Underlying Weakened Cognitive Control

Most mental disorders involve dysfunction of the dorsolateral prefrontal cortex (dlPFC), a recently evolved brain region that subserves working memory, abstraction, and the thoughtful regulation of attention, action, and emotion. For example, schizophrenia, depression, long COVID, and Alzheimer's disease are all associated with dlPFC dysfunction, with neuropathology often being focused in layer III. The dlPFC has extensive top-down projections, e.g., to the posterior association cortices to regulate attention and to the subgenual cingulate cortex via the rostral and medial PFC to regulate emotional responses. However, the dlPFC is particularly dependent on arousal state and is very vulnerable to stress and inflammation, which are etiological and/or exacerbating factors for most mental disorders. The cellular mechanisms by which stress and inflammation impact the dlPFC are a topic of current research and are summarized in this review. For example, the layer III dlPFC circuits that generate working memory-related neuronal firing have unusual neurotransmission, depending on NMDA receptor and nicotinic α7 receptor actions that are blocked under inflammatory conditions by kynurenic acid. These circuits also have unusual neuromodulation, with the molecular machinery to magnify calcium signaling in spines needed to support persistent firing, which must be tightly regulated to prevent toxic calcium actions. Stress rapidly weakens layer III connectivity by driving feedforward calcium-cAMP (cyclic adenosine monophosphate) opening of potassium channels on spines. This is regulated by postsynaptic noradrenergic α2A adrenergic receptor and mGluR3 (metabotropic glutamate receptor 3) signaling but dysregulated by inflammation and/or chronic stress exposure, which contribute to spine loss. Treatments that strengthen the dlPFC via pharmacological (the α2A adrenergic receptor agonist, guanfacine) or repetitive transcranial magnetic stimulation manipulation provide a rational basis for therapy.

cAMP-phosphodiesterase 4D7 (PDE4D7) forms a cAMP signalosome complex with DHX9 and is implicated in prostate cancer progression

A robust body of work has demonstrated that a reduction in cAMP-specific 3',5'-cyclic phosphodiesterase 4D isoform 7 (PDE4D7) is linked with negative prostate cancer outcomes; however, the exact molecular mechanism that underpins this relationship is unknown. Epigenetic profiling has shown that the PDE4D gene can be hyper-methylated in transmembrane serine protease 2 (TMPRSS2)-ETS transcriptional regulator ERG (ERG) gene-fusion-positive prostate cancer (PCa) tumours, and this inhibits messenger RNA (mRNA) expression, leading to a paucity of cellular PDE4D7 protein. In an attempt to understand how the resulting aberrant cAMP signalling drives PCa growth, we immunopurified PDE4D7 and identified binding proteins by mass spectrometry. We used peptide array technology and proximity ligation assay to confirm binding between PDE4D7 and ATP-dependent RNA helicase A (DHX9), and in the design of a novel cell-permeable disruptor peptide that mimics the DHX9-binding region on PDE4D7. We discovered that PDE4D7 forms a signalling complex with the DExD/H-box RNA helicase DHX9. Importantly, disruption of the PDE4D7-DHX9 complex reduced proliferation of LNCaP cells, suggesting the complex is pro-tumorigenic. Additionally, we have identified a novel protein kinase A (PKA) phosphorylation site on DHX9 that is regulated by PDE4D7 association. In summary, we report the existence of a newly identified PDE4D7-DHX9 signalling complex that may be crucial in PCa pathogenesis and could represent a potential therapeutic target.

The choreography of protein kinase PDK1 and its diverse substrate dance partners

The protein kinase PDK1 phosphorylates at least 24 distinct substrates, all of which belong to the AGC protein kinase group. Some substrates, such as conventional PKCs, undergo phosphorylation by PDK1 during their synthesis and subsequently get activated by DAG and Calcium. On the other hand, other substrates, including members of the Akt/PKB, S6K, SGK, and RSK families, undergo phosphorylation and activation downstream of PI3-kinase signaling. This review presents two accepted molecular mechanisms that determine the precise and timely phosphorylation of different substrates by PDK1. The first mechanism involves the colocalization of PDK1 with Akt/PKB in the presence of PIP3. The second mechanism involves the regulated docking interaction between the hydrophobic motif (HM) of substrates and the PIF-pocket of PDK1. This interaction, in trans, is equivalent to the molecular mechanism that governs the activity of AGC kinases through their HMs intramolecularly. PDK1 has been instrumental in illustrating the bi-directional allosteric communication between the PIF-pocket and the ATP-binding site and the potential of the system for drug discovery. PDK1's interaction with substrates is not solely regulated by the substrates themselves. Recent research indicates that full-length PDK1 can adopt various conformations based on the positioning of the PH domain relative to the catalytic domain. These distinct conformations of full-length PDK1 can influence the interaction and phosphorylation of substrates. Finally, we critically discuss recent findings proposing that PIP3 can directly regulate the activity of PDK1, which contradicts extensive in vitro and in vivo studies conducted over the years.

Unusual Molecular Regulation of Dorsolateral Prefrontal Cortex Layer III Synapses Increases Vulnerability to Genetic and Environmental Insults in Schizophrenia

Schizophrenia is associated with reduced numbers of spines and dendrites from layer III of the dorsolateral prefrontal cortex (dlPFC), the layer that houses the recurrent excitatory microcircuits that subserve working memory and abstract thought. Why are these synapses so vulnerable, while synapses in deeper or more superficial layers are little affected? This review describes the special molecular properties that govern layer III neurotransmission and neuromodulation in the primate dlPFC and how they may render these circuits particularly vulnerable to genetic and environmental insults. These properties include a reliance on NMDA receptor rather than AMPA receptor neurotransmission; cAMP (cyclic adenosine monophosphate) magnification of calcium signaling near the glutamatergic synapse of dendritic spines; and potassium channels opened by cAMP/PKA (protein kinase A) signaling that dynamically alter network strength, with built-in mechanisms to take dlPFC "offline" during stress. A variety of genetic and/or environmental insults can lead to the same phenotype of weakened layer III connectivity, in which mechanisms that normally strengthen connectivity are impaired and those that normally weaken connectivity are intensified. Inflammatory mechanisms, such as increased kynurenic acid and glutamate carboxypeptidase II expression, are especially detrimental to layer III dlPFC neurotransmission and modulation, mimicking genetic insults. The combination of genetic and inflammatory insults may cross the threshold into pathology.

The Complexity and Multiplicity of the Specific cAMP Phosphodiesterase Family: PDE4, Open New Adapted Therapeutic Approaches

Cyclic nucleotides (cAMP, cGMP) play a major role in normal and pathologic signaling. Beyond receptors, cyclic nucleotide phosphodiesterases; (PDEs) rapidly convert the cyclic nucleotide in its respective 5'-nucleotide to control intracellular cAMP and/or cGMP levels to maintain a normal physiological state. However, in many pathologies, dysregulations of various PDEs (PDE1-PDE11) contribute mainly to organs and tissue failures related to uncontrolled phosphorylation cascade. Among these, PDE4 represents the greatest family, since it is constituted by 4 genes with multiple variants differently distributed at tissue, cellular and subcellular levels, allowing different fine-tuned regulations. Since the 1980s, pharmaceutical companies have developed PDE4 inhibitors (PDE4-I) to overcome cardiovascular diseases. Since, they have encountered many undesired problems, (emesis), they focused their research on other PDEs. Today, increases in the knowledge of complex PDE4 regulations in various tissues and pathologies, and the evolution in drug design, resulted in a renewal of PDE4-I development. The present review describes the recent PDE4-I development targeting cardiovascular diseases, obesity, diabetes, ulcerative colitis, and Crohn's disease, malignancies, fatty liver disease, osteoporosis, depression, as well as COVID-19. Today, the direct therapeutic approach of PDE4 is extended by developing allosteric inhibitors and protein/protein interactions allowing to act on the PDE interactome.

Osthole-Mediated Inhibition of Neurotoxicity Induced by Ropivacaine via Amplification of the Cyclic Adenosine Monophosphate Signaling Pathway

Background

Ropivacaine is widely used for clinical anesthesia and postoperative analgesia. However, the neurotoxicity induced by ropivacaine in a concentration- and duration-dependent manner, and it is difficult to prevent neurotoxicity. Osthole inhibits phosphodiesterase-4 activity by binding to its catalytic site to prevent cAMP hydrolysis. The aim of this present study is to explore the precise molecular mechanism of osthole-mediated inhibition of neurotoxicity induced by ropivacaine.

Methods:

SH-SY5Y cell viability and apoptosis were measured in different concentration and duration. Protein concentration was determined in each signaling pathway. The molecular mechanism of osthole-mediated inhibition of ropivacaine-caused neurotoxicity was evaluated.

Results

The study demonstrated that osthole inhibits SH-SY5Y cells neurotoxicity in a duration- and concentration-dependent manner. Moreover, ropivacaine significantly increased the expression of caspase-3 by promoting the phosphorylation of p38. Osthole-induced upregulation of cAMP activated cAMP-dependent signaling pathway, sequentially leading to elevated cyclic nucleotide response element-binding protein levels, which inhibits P38-dependent signaling and decreases apoptosis of SH-SY5Y.

Conclusions

This study display the evidence confirmed the molecular mechanism by which osthole amplification of cAMP-dependent signaling pathway, and overexpression of cyclic nucleotide response element-binding protein inhibits P38-dependent signaling and decreases ropivacaine-induced SH-SY5Y apoptosis.

Phosphodiesterase type 4 anchoring regulates cAMP signaling to Popeye domain-containing proteins

Cyclic AMP is a ubiquitous second messenger used to transduce intracellular signals from a variety of Gs-coupled receptors. Compartmentalisation of protein intermediates within the cAMP signaling pathway underpins receptor-specific responses. The cAMP effector proteins protein-kinase A and EPAC are found in complexes that also contain phosphodiesterases whose presence ensures a coordinated cellular response to receptor activation events. Popeye domain containing (POPDC) proteins are the most recent class of cAMP effectors to be identified and have crucial roles in cardiac pacemaking and conduction. We report the first observation that POPDC proteins exist in complexes with members of the PDE4 family in cardiac myocytes. We show that POPDC1 preferentially binds the PDE4A sub-family via a specificity motif in the PDE4 UCR1 region and that PDE4s bind to the Popeye domain of POPDC1 in a region known to be susceptible to a mutation that causes human disease. Using a cell-permeable disruptor peptide that displaces the POPDC1-PDE4 complex we show that PDE4 activity localized to POPDC1 modulates cycle length of spontaneous Ca²⁺ transients firing in intact mouse sinoatrial nodes.

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POPDC1 forms a complex with type 4 phosphodiesterases (PDE4s) in cardiac myocytes.

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POPDC1 binds PDE4 enzymes in the Upstream Conserved Region 1 (UCR1) domain.

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The PDE4 binding motif within the Popeye domain lies in a region that harbours a mutation, which underpins human disease.

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Disruption of the POPDC1-PDE4 complex modulates the cycle length of spontaneous Ca2+ transients in the sinoatrial node.

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Disruption of the POPDC1-PDE4 complex causes a significant prolongation of the action potential repolarization phase.

Neuromodulation of prefrontal cortex cognitive function in primates: the powerful roles of monoamines and acetylcholine

The primate prefrontal cortex (PFC) subserves our highest order cognitive operations, and yet is tremendously dependent on a precise neurochemical environment for proper functioning. Depletion of noradrenaline and dopamine, or of acetylcholine from the dorsolateral PFC (dlPFC), is as devastating as removing the cortex itself, and serotonergic influences are also critical to proper functioning of the orbital and medial PFC. Most neuromodulators have a narrow inverted U dose response, which coordinates arousal state with cognitive state, and contributes to cognitive deficits with fatigue or uncontrollable stress. Studies in monkeys have revealed the molecular signaling mechanisms that govern the generation and modulation of mental representations by the dlPFC, allowing dynamic regulation of network strength, a process that requires tight regulation to prevent toxic actions, e.g., as occurs with advanced age. Brain imaging studies in humans have observed drug and genotype influences on a range of cognitive tasks and on PFC circuit functional connectivity, e.g., showing that catecholamines stabilize representations in a baseline-dependent manner. Research in monkeys has already led to new treatments for cognitive disorders in humans, encouraging future research in this important field.

Studies of aging nonhuman primates illuminate the etiology of early-stage Alzheimer's-like neuropathology: An evolutionary perspective

Tau pathology in Alzheimer's disease (AD) preferentially afflicts the limbic and recently enlarged association cortices, causing a progression of mnemonic and cognitive deficits. Although genetic mouse models have helped reveal mechanisms underlying the rare, autosomal-dominant forms of AD, the etiology of the more common, sporadic form of AD remains unknown, and is challenging to study in mice due to their limited association cortex and lifespan. It is also difficult to study in human brains, as early-stage tau phosphorylation can degrade postmortem. In contrast, rhesus monkeys have extensive association cortices, are long-lived, and can undergo perfusion fixation to capture early-stage tau phosphorylation in situ. Most importantly, rhesus monkeys naturally develop amyloid plaques, neurofibrillary tangles comprised of hyperphosphorylated tau, synaptic loss, and cognitive deficits with advancing age, and thus can be used to identify the early molecular events that initiate and propel neuropathology in the aging association cortices. Studies to date suggest that the particular molecular signaling events needed for higher cognition-for example, high levels of calcium to maintain persistent neuronal firing- lead to tau phosphorylation and inflammation when dysregulated with advancing age. The expression of NMDAR-NR2B (GluN2B)-the subunit that fluxes high levels of calcium-increases over the cortical hierarchy and with the expansion of association cortex in primate evolution, consistent with patterns of tau pathology. In the rhesus monkey dorsolateral prefrontal cortex, spines contain NMDAR-NR2B and the molecular machinery to magnify internal calcium release near the synapse, as well as phosphodiesterases, mGluR3, and calbindin to regulate calcium signaling. Loss of regulation with inflammation and/or aging appears to be a key factor in initiating tau pathology. The vast expansion in the numbers of these synapses over primate evolution is consistent with the degree of tau pathology seen across species: marmoset < rhesus monkey < chimpanzee < human, culminating in the vast neurodegeneration seen in humans with AD.

Mitogen-activated protein kinase-activated protein kinase-2 (MK2) and its role in cell survival, inflammatory signaling, and migration in promoting cancer

Cancer and the immune system share an intimate relationship. Chronic inflammation increases the risk of cancer occurrence and can also drive inflammatory mediators into the tumor microenvironment enhancing tumor growth and survival. The p38 MAPK pathway is activated both acutely and chronically by stress, inflammatory chemokines, chronic inflammatory conditions, and cancer. These properties have led to extensive efforts to find effective drugs targeting p38, which have been unsuccessful. The immediate downstream serine/threonine kinase and substrate of p38 MAPK, mitogen-activated-protein-kinase-activated-protein-kinase-2 (MK2) protects cells against stressors by regulating the DNA damage response, transcription, protein and messenger RNA stability, and motility. The phosphorylation of downstream substrates by MK2 increases inflammatory cytokine production, drives an immune response, and contributes to wound healing. By binding directly to p38 MAPK, MK2 is responsible for the export of p38 MAPK from the nucleus which gives MK2 properties that make it unique among the large number of p38 MAPK substrates. Many of the substrates of both p38 MAPK and MK2 are separated between the cytosol and nucleus and interfering with MK2 and altering this intracellular translocation has implications for the actions of both p38 MAPK and MK2. The inhibition of MK2 has shown promise in combination with both chemotherapy and radiotherapy as a method for controlling cancer growth and metastasis in a variety of cancers. Whereas the current data are encouraging the field requires the development of selective and well tolerated drugs to target MK2 and a better understanding of its effects for effective clinical use.

The genie in the bottle-magnified calcium signaling in dorsolateral prefrontal cortex

Neurons in the association cortices are particularly vulnerable in cognitive disorders such as schizophrenia and Alzheimer's disease, while those in primary visual cortex remain relatively resilient. This review proposes that the special molecular mechanisms needed for higher cognitive operations confer vulnerability to dysfunction, atrophy, and neurodegeneration when regulation is lost due to genetic and/or environmental insults. Accumulating data suggest that higher cortical circuits rely on magnified levels of calcium (from NMDAR, calcium channels, and/or internal release from the smooth endoplasmic reticulum) near the postsynaptic density to promote the persistent firing needed to maintain, manipulate, and store information without "bottom-up" sensory stimulation. For example, dendritic spines in the primate dorsolateral prefrontal cortex (dlPFC) express the molecular machinery for feedforward, cAMP-PKA-calcium signaling. PKA can drive internal calcium release and promote calcium flow through NMDAR and calcium channels, while in turn, calcium activates adenylyl cyclases to produce more cAMP-PKA signaling. Excessive levels of cAMP-calcium signaling can have a number of detrimental effects: for example, opening nearby K+ channels to weaken synaptic efficacy and reduce neuronal firing, and over a longer timeframe, driving calcium overload of mitochondria to induce inflammation and dendritic atrophy. Thus, calcium-cAMP signaling must be tightly regulated, e.g., by agents that catabolize cAMP or inhibit its production (PDE4, mGluR3), and by proteins that bind calcium in the cytosol (calbindin). Many genetic or inflammatory insults early in life weaken the regulation of calcium-cAMP signaling and are associated with increased risk of schizophrenia (e.g., GRM3). Age-related loss of regulatory proteins which result in elevated calcium-cAMP signaling over a long lifespan can additionally drive tau phosphorylation, amyloid pathology, and neurodegeneration, especially when protective calcium binding proteins are lost from the cytosol. Thus, the "genie" we need for our remarkable cognitive abilities may make us vulnerable to cognitive disorders when we lose essential regulation.

The Molecular Biology of Phosphodiesterase 4 Enzymes as Pharmacological Targets: An Interplay of Isoforms, Conformational States, and Inhibitors

The phosphodiesterase 4 (PDE4) enzyme family plays a pivotal role in regulating levels of the second messenger cAMP. Consequently, PDE4 inhibitors have been investigated as a therapeutic strategy to enhance cAMP signaling in a broad range of diseases, including several types of cancers, as well as in various neurologic, dermatological, and inflammatory diseases. Despite their widespread therapeutic potential, the progression of PDE4 inhibitors into the clinic has been hampered because of their related relatively small therapeutic window, which increases the chance of producing adverse side effects. Interestingly, the PDE4 enzyme family consists of several subtypes and isoforms that can be modified post-translationally or can engage in specific protein-protein interactions to yield a variety of conformational states. Inhibition of specific PDE4 subtypes, isoforms, or conformational states may lead to more precise effects and hence improve the safety profile of PDE4 inhibition. In this review, we provide an overview of the variety of PDE4 isoforms and how their activity and inhibition is influenced by post-translational modifications and interactions with partner proteins. Furthermore, we describe the importance of screening potential PDE4 inhibitors in view of different PDE4 subtypes, isoforms, and conformational states rather than testing compounds directed toward a specific PDE4 catalytic domain. Lastly, potential mechanisms underlying PDE4-mediated adverse effects are outlined. In this review, we illustrate that PDE4 inhibitors retain their therapeutic potential in myriad diseases, but target identification should be more precise to establish selective inhibition of disease-affected PDE4 isoforms while avoiding isoforms involved in adverse effects. SIGNIFICANCE STATEMENT: Although the PDE4 enzyme family is a therapeutic target in an extensive range of disorders, clinical use of PDE4 inhibitors has been hindered because of the adverse side effects. This review elaborately shows that safer and more effective PDE4 targeting is possible by characterizing 1) which PDE4 subtypes and isoforms exist, 2) how PDE4 isoforms can adopt specific conformations upon post-translational modifications and protein-protein interactions, and 3) which PDE4 inhibitors can selectively bind specific PDE4 subtypes, isoforms, and/or conformations.

Mapping Phosphodiesterase 4D (PDE4D) in Macaque Dorsolateral Prefrontal Cortex: Postsynaptic Compartmentalization in Layer III Pyramidal Cell Circuits

cAMP signaling has powerful, negative effects on cognitive functions of the primate dorsolateral prefrontal cortex (dlPFC), opening potassium channels to reduce firing and impair working memory, and increasing tau phosphorylation in aging neurons. This contrasts with cAMP actions in classic circuits, where it enhances plasticity and transmitter release. PDE4 isozymes regulate cAMP actions, and thus have been a focus of research and drug discovery. Previous work has focused on the localization of PDE4A and PDE4B in dlPFC, but PDE4D is also of great interest, as it is the predominant PDE4 isoform in primate association cortex, and PDE4D expression decreases with aging in human dlPFC. Here we used laser-capture microdissection transcriptomics and found that PDE4D message is enriched in pyramidal cells compared to GABAergic PV-interneurons in layer III of the human dlPFC. A parallel study in rhesus macaques using high-spatial resolution immunoelectron microscopy revealed the ultrastructural locations of PDE4D in primate dlPFC with clarity not possible in human post-mortem tissue. PDE4D was especially prominent in dendrites associated with microtubules, mitochondria, and likely smooth endoplasmic reticulum (SER). There was substantial postsynaptic labeling in dendritic spines, associated with the SER spine-apparatus near glutamatergic-like axospinous synapses, but sparse labeling in axon terminals. We also observed dense PDE4D labeling perisynaptically in astroglial leaflets ensheathing glutamatergic connections. These data suggest that PDE4D is strategically positioned to regulate cAMP signaling in dlPFC glutamatergic synapses and circuits, especially in postsynaptic compartments where it is localized to influence cAMP actions on intracellular trafficking, mitochondrial physiology, and internal calcium release.

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.

Reshaping cAMP nanodomains through targeted disruption of compartmentalised phosphodiesterase signalosomes

Spatio-temporal regulation of localised cAMP nanodomains is highly dependent upon the compartmentalised activity of phosphodiesterase (PDE) cyclic nucleotide degrading enzymes. Strategically positioned PDE-protein complexes are pivotal to the homeostatic control of cAMP-effector protein activity that in turn orchestrate a wide range of cellular signalling cascades in a variety of cells and tissue types. Unsurprisingly, dysregulated PDE activity is central to the pathophysiology of many diseases warranting the need for effective therapies that target PDEs selectively. This short review focuses on the importance of activating compartmentalised cAMP signalling by displacing the PDE component of signalling complexes using cell-permeable peptide disrupters.

Cyclic nucleotide phosphodiesterases: New targets in the metabolic syndrome?

Metabolic diseases have a tremendous impact on human morbidity and mortality. Numerous targets regulating adenosine monophosphate kinase (AMPK) have been identified for treating the metabolic syndrome (MetS), and many compounds are being used or developed to increase AMPK activity. In parallel, the cyclic nucleotide phosphodiesterase families (PDEs) have emerged as new therapeutic targets in cardiovascular diseases, as well as in non-resolved pathologies. Since some PDE subfamilies inactivate cAMP into 5'-AMP, while the beneficial effects in MetS are related to 5'-AMP-dependent activation of AMPK, an analysis of the various controversial relationships between PDEs and AMPK in MetS appears interesting. The present review will describe the various PDE families, AMPK and molecular mechanisms in the MetS and discuss the PDEs/PDE modulators related to the tissues involved, thus supporting the discovery of original molecules and the design of new therapeutic approaches in MetS.

Alzheimer's-like pathology in aging rhesus macaques: Unique opportunity to study the etiology and treatment of Alzheimer's disease

Although mouse models of Alzheimer's disease (AD) have provided tremendous breakthroughs, the etiology of later onset AD remains unknown. In particular, tau pathology in the association cortex is poorly replicated in mouse models. Aging rhesus monkeys naturally develop cognitive deficits, amyloid plaques, and the same qualitative pattern and sequence of tau pathology as humans, with tangles in the oldest animals. Thus, aging rhesus monkeys can play a key role in AD research. For example, aging monkeys can help reveal how synapses in the prefrontal association cortex are uniquely regulated compared to the primary sensory cortex in ways that render them vulnerable to calcium dysregulation and tau phosphorylation, resulting in the selective localization of tau pathology observed in AD. The ability to assay early tau phosphorylation states and perform high-quality immunoelectron microscopy in monkeys is a great advantage, as one can capture early-stage degeneration as it naturally occurs in situ. Our immunoelectron microscopy studies show that phosphorylated tau can induce an "endosomal traffic jam" that drives amyloid precursor protein cleavage to amyloid-β in endosomes. As amyloid-β increases tau phosphorylation, this creates a vicious cycle where varied precipitating factors all lead to a similar phenotype. These data may help explain why circuits with aggressive tau pathology (e.g., entorhinal cortex) may degenerate prior to producing significant amyloid pathology. Aging monkeys therefore can play an important role in identifying and testing potential therapeutics to protect the association cortex, including preventive therapies that are challenging to test in humans.

A novel de novo PDE4D gene mutation identified in a Chinese patient with acrodysostosis

Acrodysostosis is an extremely rare disorder at birth, that is, characterized by skeletal dysplasia with short stature and midfacial hypoplasia, which has been reported to be caused by PDE4D and PRKAR1A gene mutations. Here, a Chinese boy with acrodysostosis, ventricular septal defect, and pulmonary hypertension was recruited for our study, and his clinical and biochemical characteristics were analyzed. A novel de novo heterozygous missense mutation (NM_001104631: c.2030A>C, p.Tyr677Ser) of the PDE4D gene was detected by whole exome sequencing and confirmed by Sanger sequencing. The c.2030A>C (p.Tyr677Ser) variant was located in exon 15 of the PDE4D gene, predicted to be damaging by a functional prediction program and shown to be highly conserved among many species. Further functional analysis showed that the p.Tyr677Ser substitution changes the function of the PDE4D protein, affects its subcellular localization in transfected cells, increases PDE4 activity in the regulation of cAMP signaling and affects cell proliferation. Our study identified a novel de novo PDE4D mutation in acrodysostosis of Chinese origin that not only contributes a deeper appreciation of the phenotypic characteristics of patients with PDE4D mutations but also expands the spectrum of PDE4D mutations.

Loss of Prefrontal Cortical Higher Cognition with Uncontrollable Stress: Molecular Mechanisms, Changes with Age, and Relevance to Treatment

The newly evolved prefrontal cortex (PFC) generates goals for "top-down" control of behavior, thought, and emotion. However, these circuits are especially vulnerable to uncontrollable stress, with powerful, intracellular mechanisms that rapidly take the PFC "off-line." High levels of norepinephrine and dopamine released during stress engage α1-AR and D1R, which activate feedforward calcium-cAMP signaling pathways that open nearby potassium channels to weaken connectivity and reduce PFC cell firing. Sustained weakening with chronic stress leads to atrophy of dendrites and spines. Understanding these signaling events helps to explain the increased susceptibility of the PFC to stress pathology during adolescence, when dopamine expression is increased in the PFC, and with advanced age, when the molecular "brakes" on stress signaling are diminished by loss of phosphodiesterases. These mechanisms have also led to pharmacological treatments for stress-related disorders, including guanfacine treatment of childhood trauma, and prazosin treatment of veterans and civilians with post-traumatic stress disorder.

A high-fat diet promotes depression-like behavior in mice by suppressing hypothalamic PKA signaling

Obesity is associated with an increased risk of depression. The aim of the present study was to investigate whether obesity is a causative factor for the development of depression and what is the molecular pathway(s) that link these two disorders. Using lipidomic and transcriptomic methods, we identified a mechanism that links exposure to a high-fat diet (HFD) in mice with alterations in hypothalamic function that lead to depression. Consumption of an HFD selectively induced accumulation of palmitic acid in the hypothalamus, suppressed the 3', 5'-cyclic AMP (cAMP)/protein kinase A (PKA) signaling pathway, and increased the concentration of free fatty acid receptor 1 (FFAR1). Deficiency of phosphodiesterase 4A (PDE4A), an enzyme that degrades cAMP and modulates stimulatory regulative G protein (Gs)-coupled G protein-coupled receptor signaling, protected animals either from genetic- or dietary-induced depression phenotype. These findings suggest that dietary intake of saturated fats disrupts hypothalamic functions by suppressing cAMP/PKA signaling through activation of PDE4A. FFAR1 inhibition and/or an increase of cAMP signaling in the hypothalamus could offer potential therapeutic targets to counteract the effects of dietary or genetically induced obesity on depression.

The Prognostic PDE4D7 Score in a Diagnostic Biopsy Prostate Cancer Patient Cohort with Longitudinal Biological Outcomes

Purpose. To further validate the prognostic power of the biomarker PDE4D7, we investigated the correlation of PDE4D7 scores adjusted for presurgical clinical variables with longitudinal postsurgical biological outcomes. Methods. RNA was extracted from biopsy punches of resected tumors (550 patients; RP cohort) and diagnostic needle biopsies (168 patients; DB cohort). Cox regression and survival were applied to correlate PDE4D7 scores with patient outcomes. Logistic regression was used to combine the clinical CAPRA score with PDE4D7. Results. In univariate analysis, the PDE4D7 score was significantly associated with PSA recurrence after prostatectomy in both studied patient cohorts' analysis (HR 0.53; 95% CI 0.41-0.67; p<1.0E-04 and HR 0.47; 95% CI 0.33-0.65; p<1.0E-04, respectively). After adjustment for the presurgical clinical variables preoperative PSA, PSA density, biopsy Gleason, clinical stage, percentage tumor in the biopsy (data only available for RP cohort), and percentage of positive biopsies, the HR was 0.49 (95% CI 0.38-0.64; p<1.0E-04) and 0.43 (95% CI 0.29-0.63; p<1.0E-04), respectively. The addition of the PDE4D7 to the clinical CAPRA score increased the AUC by 5% over the CAPRA score alone (0.82 versus 0.77; p=0.004). This combination model stratified 14.6% patients of the DB cohort to no risk of biochemical relapse (NPV 100%) over a follow-up period of up to 15 years. Conclusions. The PDE4D7 score provides independent risk information for pretreatment risk stratification. Combining CAPRA with PDE4D7 scores significantly improved the clinical risk stratification before surgery.

Mechanisms underlying the role of DISC1 in synaptic plasticity

Disrupted in schizophrenia 1 (DISC1) is an important hub protein, forming multimeric complexes by self-association and interacting with a large number of synaptic and cytoskeletal molecules. The synaptic location of DISC1 in the adult brain suggests a role in synaptic plasticity, and indeed, a number of studies have discovered synaptic plasticity impairments in a variety of different DISC1 mutants. This review explores the possibility that DISC1 is an important molecule for organizing proteins involved in synaptic plasticity and examines why mutations in DISC1 impair plasticity. It concentrates on DISC1's role in interacting with synaptic proteins, controlling dendritic structure and cellular trafficking of mRNA, synaptic vesicles and mitochondria. N-terminal directed mutations appear to impair synaptic plasticity through interactions with phosphodiesterase 4B (PDE4B) and hence protein kinase A (PKA)/GluA1 and PKA/cAMP response element-binding protein (CREB) signalling pathways, and affect spine structure through interactions with kalirin 7 (Kal-7) and Rac1. C-terminal directed mutations also impair plasticity possibly through altered interactions with lissencephaly protein 1 (LIS1) and nuclear distribution protein nudE-like 1 (NDEL1), thereby affecting developmental processes such as dendritic structure and spine maturation. Many of the same molecules involved in DISC1's cytoskeletal interactions are also involved in intracellular trafficking, raising the possibility that impairments in intracellular trafficking affect cytoskeletal development and vice versa. While the multiplicity of DISC1 protein interactions makes it difficult to pinpoint a single causal signalling pathway, we suggest that the immediate-term effects of N-terminal influences on GluA1, Rac1 and CREB, coupled with the developmental effects of C-terminal influences on trafficking and the cytoskeleton make up the two main branches of DISC1's effect on synaptic plasticity and dendritic spine stability.

PDE4-Mediated cAMP Signalling

cAMP is the archetypal and ubiquitous second messenger utilised for the fine control of many cardiovascular cell signalling systems. The ability of cAMP to elicit cell surface receptor-specific responses relies on its compartmentalisation by cAMP hydrolysing enzymes known as phosphodiesterases. One family of these enzymes, PDE4, is particularly important in the cardiovascular system, where it has been extensively studied and shown to orchestrate complex, localised signalling that underpins many crucial functions of the heart. In the cardiac myocyte, cAMP activates PKA, which phosphorylates a small subset of mostly sarcoplasmic substrate proteins that drive β-adrenergic enhancement of cardiac function. The phosphorylation of these substrates, many of which are involved in cardiac excitation-contraction coupling, has been shown to be tightly regulated by highly localised pools of individual PDE4 isoforms. The spatial and temporal regulation of cardiac signalling is made possible by the formation of macromolecular “signalosomes”, which often include a cAMP effector, such as PKA, its substrate, PDE4 and an anchoring protein such as an AKAP. Studies described in the present review highlight the importance of this relationship for individual cardiac PKA substrates and we provide an overview of how this signalling paradigm is coordinated to promote efficient adrenergic enhancement of cardiac function. The role of PDE4 also extends to the vascular endothelium, where it regulates vascular permeability and barrier function. In this distinct location, PDE4 interacts with adherens junctions to regulate their stability. These highly specific, non-redundant roles for PDE4 isoforms have far reaching therapeutic potential. PDE inhibitors in the clinic have been plagued with problems due to the active site-directed nature of the compounds which concomitantly attenuate PDE activity in all highly localised “signalosomes”.