Potassium rhythms couple the circadian clock to the cell cycle.

Circadian (~24 h) rhythms are a fundamental feature of life, and their disruption increases the risk of infectious diseases, metabolic disorders, and cancer1-6. Circadian rhythms couple to the cell cycle across eukaryotes7,8 but the underlying mechanism is unknown. We previously identified an evolutionarily conserved circadian oscillation in intracellular potassium concentration, [K+]i9,10. As critical events in the cell cycle are regulated by intracellular potassium11,12, an enticing hypothesis is that circadian rhythms in [K+]i form the basis of this coupling. We used a minimal model cell, the alga Ostreococcus tauri, to uncover the role of potassium in linking these two cycles. We found direct reciprocal feedback between [K+]i and circadian gene expression. Inhibition of proliferation by manipulating potassium rhythms was dependent on the phase of the circadian cycle. Furthermore, we observed a total inhibition of cell proliferation when circadian gene expression is inhibited. Strikingly, under these conditions a sudden enforced gradient of extracellular potassium was sufficient to induce a round of cell division. Finally, we provide evidence that interactions between potassium and circadian rhythms also influence proliferation in mammalian cells. These results establish circadian regulation of intracellular potassium levels as a primary factor coupling the cell- and circadian cycles across diverse organisms.

PDE2A2 regulates mitochondria morphology and apoptotic cell death via local modulation of cAMP/PKA signalling.

cAMP/PKA signalling is compartmentalised with tight spatial and temporal control of signal propagation underpinning specificity of response. The cAMP-degrading enzymes, phosphodiesterases (PDEs), localise to specific subcellular domains within which they control local cAMP levels and are key regulators of signal compartmentalisation. Several components of the cAMP/PKA cascade are located to different mitochondrial sub-compartments, suggesting the presence of multiple cAMP/PKA signalling domains within the organelle. The function and regulation of these domains remain largely unknown. Here, we describe a novel cAMP/PKA signalling domain localised at mitochondrial membranes and regulated by PDE2A2. Using pharmacological and genetic approaches combined with real-time FRET imaging and high resolution microscopy, we demonstrate that in rat cardiac myocytes and other cell types mitochondrial PDE2A2 regulates local cAMP levels and PKA-dependent phosphorylation of Drp1. We further demonstrate that inhibition of PDE2A, by enhancing the hormone-dependent cAMP response locally, affects mitochondria dynamics and protects from apoptotic cell death.

Cell autonomous regulation of herpes and influenza virus infection by the circadian clock.

Viruses are intracellular pathogens that hijack host cell machinery and resources to replicate. Rather than being constant, host physiology is rhythmic, undergoing circadian (∼24 h) oscillations in many virus-relevant pathways, but whether daily rhythms impact on viral replication is unknown. We find that the time of day of host infection regulates virus progression in live mice and individual cells. Furthermore, we demonstrate that herpes and influenza A virus infections are enhanced when host circadian rhythms are abolished by disrupting the key clock gene transcription factor Bmal1. Intracellular trafficking, biosynthetic processes, protein synthesis, and chromatin assembly all contribute to circadian regulation of virus infection. Moreover, herpesviruses differentially target components of the molecular circadian clockwork. Our work demonstrates that viruses exploit the clockwork for their own gain and that the clock represents a novel target for modulating viral replication that extends beyond any single family of these ubiquitous pathogens.

Cardiac Hypertrophy Is Inhibited by a Local Pool of cAMP Regulated by Phosphodiesterase 2.

RATIONALE: Chronic elevation of 3'-5'-cyclic adenosine monophosphate (cAMP) levels has been associated with cardiac remodeling and cardiac hypertrophy. However, enhancement of particular aspects of cAMP/protein kinase A signaling seems to be beneficial for the failing heart. cAMP is a pleiotropic second messenger with the ability to generate multiple functional outcomes in response to different extracellular stimuli with strict fidelity, a feature that relies on the spatial segregation of the cAMP pathway components in signaling microdomains. OBJECTIVE: How individual cAMP microdomains affect cardiac pathophysiology remains largely to be established. The cAMP-degrading enzymes phosphodiesterases (PDEs) play a key role in shaping local changes in cAMP. Here we investigated the effect of specific inhibition of selected PDEs on cardiac myocyte hypertrophic growth. METHODS AND RESULTS: Using pharmacological and genetic manipulation of PDE activity, we found that the rise in cAMP resulting from inhibition of PDE3 and PDE4 induces hypertrophy, whereas increasing cAMP levels via PDE2 inhibition is antihypertrophic. By real-time imaging of cAMP levels in intact myocytes and selective displacement of protein kinase A isoforms, we demonstrate that the antihypertrophic effect of PDE2 inhibition involves the generation of a local pool of cAMP and activation of a protein kinase A type II subset, leading to phosphorylation of the nuclear factor of activated T cells. CONCLUSIONS: Different cAMP pools have opposing effects on cardiac myocyte cell size. PDE2 emerges as a novel key regulator of cardiac hypertrophy in vitro and in vivo, and its inhibition may have therapeutic applications.

Analysis of compartmentalized cAMP: a method to compare signals from differently targeted FRET reporters.

Förster resonance energy transfer (FRET)-based reporters are important tools to study the spatiotemporal compartmentalization of cyclic adenosine monophosphate (cAMP) in living cells. To increase the spatial resolution of cAMP detection, new reporters with specific intracellular targeting have been developed. Therefore it has become critical to be able to appropriately compare the signals revealed by the different sensors. Here we illustrate a protocol to calibrate the response detected by different targeted FRET reporters involving the generation of a dose-response curve to the cAMP raising agent forskolin. This method represents a general tool for the accurate analysis and interpretation of intracellular cAMP changes detected at the level of different subcellular compartments.

PKA and PDE4D3 anchoring to AKAP9 provides distinct regulation of cAMP signals at the centrosome.

Previous work has shown that the protein kinase A (PKA)-regulated phosphodiesterase (PDE) 4D3 binds to A kinase-anchoring proteins (AKAPs). One such protein, AKAP9, localizes to the centrosome. In this paper, we investigate whether a PKA-PDE4D3-AKAP9 complex can generate spatial compartmentalization of cyclic adenosine monophosphate (cAMP) signaling at the centrosome. Real-time imaging of fluorescence resonance energy transfer reporters shows that centrosomal PDE4D3 modulated a dynamic microdomain within which cAMP concentration selectively changed over the cell cycle. AKAP9-anchored, centrosomal PKA showed a reduced activation threshold as a consequence of increased autophosphorylation of its regulatory subunit at S114. Finally, disruption of the centrosomal cAMP microdomain by local displacement of PDE4D3 impaired cell cycle progression as a result of accumulation of cells in prophase. Our findings describe a novel mechanism of PKA activity regulation that relies on binding to AKAPs and consequent modulation of the enzyme activation threshold rather than on overall changes in cAMP levels. Further, we provide for the first time direct evidence that control of cell cycle progression relies on unique regulation of centrosomal cAMP/PKA signals.

cGMP-cAMP interplay in cardiac myocytes: a local affair with far-reaching consequences for heart function.

cAMP and cGMP signalling pathways are common targets in the pharmacological treatment of heart failure, and often drugs that modulate the level of these second messengers are simultaneously administered to patients. cGMP can potentially affect cAMP levels by modulating the activity of PDEs (phosphodiesterases), the enzymes that degrade cyclic nucleotides. This biochemical cross-talk provides the means for drugs that increase cGMP to concomitantly affect cAMP signals. Recent studies using FRET (fluorescence resonance energy transfer) reporters and real-time imaging show that, in cardiac myocytes, the interplay between cGMP and cAMP has different outcomes depending on the specific location where the cross-modulation occurs. cGMP can either increase or decrease the cAMP response to catecholamines, based on the cyclase that generates it and on the PDEs associated with each subcellular compartment. cGMP-mediated modulation of cAMP signals has functional relevance as it affects protein phosphorylation downstream of protein kinase A and myocyte contractility. The physical separation of positive and negative modulation of cAMP levels by cGMP offers the previously unrecognized possibility to selectively modulate local cAMP signals to improve the efficacy of therapy.

Phosphodiesterases and subcellular compartmentalized cAMP signaling in the cardiovascular system.

Phosphodiesterases are key enzymes in the cAMP signaling cascade. They convert cAMP in its inactive form 5'-AMP and critically regulate the intensity and the duration of cAMP-mediated signals. Multiple isoforms exist that possess different intracellular distributions, different affinities for cAMP, and different catalytic and regulatory properties. This complex repertoire of enzymes provides a multiplicity of ways to modulate cAMP levels, to integrate more signaling pathways, and to respond to the specific needs of the cell within distinct subcellular domains. In this review we summarize key findings on phosphodiesterase compartmentalization in the cardiovascular system.

Local termination of 3'-5'-cyclic adenosine monophosphate signals: the role of A kinase anchoring protein-tethered phosphodiesterases.

A kinase anchoring proteins (AKAPs) belong to a family of functionally related proteins capable of binding protein kinase A (PKA) and tether it to relevant targets. In this way, AKAPs organize macromolecular complexes to segregate PKA activity and retain signal specificity. In the heart, AKAP-PKA interaction is central to the regulation of cardiac contractility. Phosphodiesterases belong to a large superfamily of enzymes that degrade 3'-5'-cyclic adenosine monophosphate (cAMP). They possess diverse catalytic properties and multiple regulatory mechanisms and control the duration and amplitude of the cAMP signal, including its propagation in space. AKAPs, together with PKA, can also assemble phosphodiesterases thereby providing a means to locally control cAMP dynamics at the level of single macromolecular complexes. This allows for the fine tuning of the cAMP response to the specific demands of the cell.

Measuring spatiotemporal dynamics of cyclic AMP signaling in real-time using FRET-based biosensors.

Cyclic AMP governs many fundamental signaling events in eukaryotic cells. Although cAMP signaling has been a major research focus for a long time, recent technological developments are revealing novel aspects of this paradigmatic pathway. In this chapter, we give an overview over current fluorescence resonance energy transfer (FRET)-based sensors for detection of cAMP dynamics, and their application in monitoring local, compartmentalized cAMP signals within living cells. A basic step-by-step protocol is given for conducting a FRET experiment in primary cells with a unimolecular cAMP sensor, which can easily be adapted to a user's specific requirements.

cGMP signals modulate cAMP levels in a compartment-specific manner to regulate catecholamine-dependent signaling in cardiac myocytes.

RATIONALE: cAMP and cGMP are intracellular second messengers involved in heart pathophysiology. cGMP can potentially affect cAMP signals via cGMP-regulated phosphodiesterases (PDEs). OBJECTIVE: To study the effect of cGMP signals on the local cAMP response to catecholamines in specific subcellular compartments. METHODS AND RESULTS: We used real-time FRET imaging of living rat ventriculocytes expressing targeted cAMP and cGMP biosensors to detect cyclic nucleotides levels in specific locales. We found that the compartmentalized, but not the global, cAMP response to isoproterenol is profoundly affected by cGMP signals. The effect of cGMP is to increase cAMP levels in the compartment where the protein kinase (PK)A-RI isoforms reside but to decrease cAMP in the compartment where the PKA-RII isoforms reside. These opposing effects are determined by the cGMP-regulated PDEs, namely PDE2 and PDE3, with the local activity of these PDEs being critically important. The cGMP-mediated modulation of cAMP also affects the phosphorylation of PKA targets and myocyte contractility. CONCLUSIONS: cGMP signals exert opposing effects on local cAMP levels via different PDEs the activity of which is exerted in spatially distinct subcellular domains. Inhibition of PDE2 selectively abolishes the negative effects of cGMP on cAMP and may have therapeutic potential.