Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents.

Cyclic nucleotide-activated ion channels play a fundamental role in a variety of physiological processes. By opening in response to intracellular cyclic nucleotides, they translate changes in concentrations of signaling molecules to changes in membrane potential. These channels belong to two families: the cyclic nucleotide-gated (CNG) channels and the hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels. The two families exhibit high sequence similarity and belong to the superfamily of voltage-gated potassium channels. Whereas HCN channels are activated by voltage and CNG channels are virtually voltage independent, both channels are activated by cyclic nucleotide binding. Furthermore, the channels are thought to have similar channel structures, leading to similar mechanisms of activation by cyclic nucleotides. However, although these channels are structurally and behaviorally similar, they have evolved to perform distinct physiological functions. This review describes the physiological roles and biophysical behavior of CNG and HCN channels. We focus on how similarities in structure and activation mechanisms result in common biophysical models, allowing CNG and HCN channels to be viewed as a single genre.

CNG AND HCN CHANNELS: Two Peas, One Pod

Ion channels are membrane proteins, found in virtually all cells, that are of crucial physiological importance. In the past decade, an explosion in the number of crystal structures of ion channels has led to a marked increase in our understanding of how ion channels open and close, and select between permeant ions. There has been a parallel advance in research on channelopathies (diseases resulting from impaired channel function), and mutations in over 60 ion-channel genes are now known to cause human disease. Characterization of their functional consequences has afforded unprecedented and unexpected insights into ion-channel mechanisms and physiological roles.

From molecule to malady

Most riboswitches are metabolite-binding RNA structures located in bacterial messenger RNAs where they control gene expression. We have discovered a riboswitch class in many bacterial and archaeal species whose members are selectively triggered by fluoride but reject other small anions, including chloride. These fluoride riboswitches activate expression of genes that encode putative fluoride transporters, enzymes that are known to be inhibited by fluoride, and additional proteins of unknown function. Our findings indicate that most organisms are naturally exposed to toxic levels of fluoride and that many species use fluoride-sensing RNAs to control the expression of proteins that alleviate the deleterious effects of this anion.

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Widespread genetic switches and toxicity resistance proteins for fluoride.

Cyclic nucleotide phosphodiesterases (PDEs) that specifically inactivate the intracellular messengers cAMP and cGMP in a compartmentalized manner represent an important enzyme class constituted by 11 gene-related families of isozymes (PDE1 to PDE11). Downstream receptors, PDEs play a major role in controlling the signalosome at various levels of phosphorylations and protein/protein interactions. Due to the multiplicity of isozymes, their various intracellular regulations and their different cellular and subcellular distributions, PDEs represent interesting targets in intracellular pathways. Therefore, the investigation of PDE isozyme alterations related to various pathologies and the design of specific PDE inhibitors might lead to the development of new specific therapeutic strategies in numerous pathologies. This manuscript (i) overviews the different PDEs including their endogenous regulations and their specific inhibitors; (ii) analyses the intracellular implications of PDEs in regulating signalling cascades in pathogenesis, exemplified by two diseases affecting cell cycle and proliferation; and (iii) discusses perspectives for future therapeutic developments.

https://bpspubs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1476-5381.2011.01729.x

Cyclic nucleotide phosphodiesterase (PDE) isozymes as targets of the intracellular signalling network: benefits of PDE inhibitors in various diseases and perspectives for future therapeutic developments

▪ Abstract Cyclic nucleotide-gated (CNG) ion channels were first discovered in rod photoreceptors, where they are responsible for the primary electrical signal of the photoreceptor in response to light. CNG channels are highly specialized membrane proteins that open an ion-permeable pore across the membrane in response to the direct binding of intracellular cyclic nucleotides. CNG channels have been identified in a number of other tissues, including the brain, where their roles are only beginning to be appreciated. Recently, significant progress has been made in understanding the molecular mechanisms underlying their functional specializations. From these studies, a picture is beginning to emerge for how the binding of cyclic nucleotide is transduced into the opening of the pore and how this allosteric transition is modulated by various physiological effectors.

Cyclic Nucleotide-Gated Ion Channels

Side-Dependent Inhibition of a Prokaryotic ClC by DIDS

Anion-transport proteins are central to all of physiology, for processes ranging from regulating bone-density, muscle excitability, and blood pressure, to facilitating extreme-acid survival of pathogenic bacteria. 4,4-Diisothiocyanatostilbene-2,2′-disulfonic acid (DIDS) has been used as an anion-transport inhibitor for decades. In this study, we demonstrate that polythiourea products derived from DIDS hydrolysis inhibit three different CLC chloride-transport proteins, ClC-ec1, ClC-0, and ClC-Ka, more effectively than DIDS itself. The structures of the five major products were determined by NMR spectroscopy, mass spectrometry, and chemical synthesis. These compounds bind directly to the CLC proteins, as evidenced by the fact that inhibition of ClC-0 occurs only from the intracellular side and inhibition of ClC-Ka is prevented by the point mutation N68D. These polythioureas are the highest affinity inhibitors known for the CLCs and provide a new class of chemical probes for dissecting the molecular mechanis...

Discovery of Potent CLC Chloride Channel Inhibitors

Members of the CLC ‘chloride channel’ family play vital roles in a wide variety of physiological settings. Research on prokaryotic CLC homologues provided long-anticipated high-resolution structures as well as the unexpected discovery that some CLCs are not chloride channels, but rather are proton-chloride antiporters. Hence, CLCs encompass two functional classes of transport proteins once thought to be fundamentally different from one another. In this review, we discuss the structural features and molecular mechanisms of CLC channels and antiporters. We focus on ClC-0, the most thoroughly studied CLC channel, and ClC-ec1, the prokaryotic antiporter of known structure. We highlight some striking similarities between these CLCs and discuss compelling questions that remain to be addressed. Prokaryotic CLCs will undoubtedly continue to shed light upon this understudied family of proteins.

Kimberly Matulef

Papers

Cyclic Nucleotide-Gated Ion Channels

Side-Dependent Inhibition of a Prokaryotic ClC by DIDS

Discovery of Potent CLC Chloride Channel Inhibitors

The CLC 'chloride channel' family: revelations from prokaryotes.

A Designed Inhibitor of a CLC Antiporter Blocks Function through a Unique Binding Mode