Function of cGMP-dependent protein kinases in the nervous system

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

A GluR1-cGKII interaction regulates AMPA receptor trafficking

Trafficking of AMPA receptors (AMPARs) is regulated by specific interactions of the subunit intracellular C-terminal domains (CTDs) with other proteins, but the mechanisms involved in this process are still unclear. We have found that the GluR1 CTD binds to cGMP-dependent protein kinase II (cGKII) adjacent to the kinase catalytic site. Binding of GluR1 is increased when cGKII is activated by cGMP. cGKII and GluR1 form a complex in the brain, and cGKII in this complex phosphorylates GluR1 at S845, a site also phosphorylated by PKA. Activation of cGKII by cGMP increases the surface expression of AMPARs at extrasynaptic sites. Inhibition of cGKII activity blocks the surface increase of GluR1 during chemLTP and reduces LTP in the hippocampal slice. This work identifies a pathway, downstream from the NMDA receptor (NMDAR) and nitric oxide (NO), which stimulates GluR1 accumulation in the plasma membrane and plays an important role in synaptic plasticity.

Alpha-synuclein involvement in hippocampal synaptic plasticity: role of NO, cGMP, cGK and CaMKII

Synaptic plasticity involves a series of coordinate changes occurring both pre- and postsynaptically, of which alpha-synuclein is an integral part. We have investigated on mouse primary hippocampal neurons in culture whether redistribution of alpha-synuclein during plasticity involves retrograde signaling activation through nitric oxide (NO), cGMP, cGMP-dependent protein kinase (cGK) and calmodulin-dependent protein kinase II. We have found that deletion of the alpha-synuclein gene blocks both the long-lasting enhancement of evoked and miniature transmitter release and the increase in the number of functional presynaptic boutons evoked through the NO donor, DEA/NO, and the cGMP analog, 8-Br-cGMP. In agreement with these findings both DEA/NO and 8-Br-cGMP were capable of producing a long-lasting increase in number of clusters for alpha-synuclein through activation of soluble guanylyl cyclase, cGK and calcium/calmodulin-dependent protein kinase IIalpha. Thus, our results suggest that NO, cGMP, GMP-dependent protein kinase and calmodulin-dependent protein kinase II play a key role in the redistribution of alpha-synuclein during plasticity.

Natural polymorphism affecting learning and memory in Drosophila

Knowing which genes contribute to natural variation in learning and memory would help us understand how differences in these cognitive traits evolve among populations and species. We show that a natural polymorphism at the foraging (for) locus, which encodes a cGMP-dependent protein kinase (PKG), affects associative olfactory learning in Drosophila melanogaster. In an assay that tests the ability to associate an odor with mechanical shock, flies homozygous for one natural allelic variant of this gene (forR) showed better short-term but poorer long-term memory than flies homozygous for another natural allele (fors). The fors allele is characterized by reduced PKG activity. We showed that forR-like levels of both short-term learning and long-term memory can be induced in fors flies by selectively increasing the level of PKG in the mushroom bodies, which are centers of olfactory learning in the fly brain. Thus, the natural polymorphism at for may mediate an evolutionary tradeoff between short- and long-term memory. The respective strengths of learning performance of the two genotypes seem coadapted with their effects on foraging behavior: forR flies move more between food patches and so could particularly benefit from fast learning, whereas fors flies are more sedentary, which should favor good long-term memory.

Phosphodiesterase type 2 and the homeostasis of cyclic GMP in living thalamic neurons

The ubiquitous second messenger cyclic GMP (cGMP) is synthesized by soluble guanylate cyclases in response to nitric oxide (NO) and degraded by phosphodiesterases (PDE). We studied the homeostasis of cGMP in living thalamic neurons by using the genetically encoded fluorescence resonance energy transfer sensor Cygnet, expressed in brain slices through viral gene transfer. Natriuretic peptides had no effect on cGMP. Basal cGMP levels decreased upon inhibition of NO synthases or soluble guanylate cyclases and increased when PDEs were inhibited. Single cell RT-PCR analysis showed that thalamic neurons express PDE1, PDE2, PDE9, and PDE10. Basal cGMP levels were increased by the PDE2 inhibitors erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and BAY60-7550 but were unaffected by PDE1 or PDE10 inhibitors. We conclude that PDE2 regulates the basal cGMP concentration in thalamic neurons. In addition, in the presence of 3-isobutyl-1-methylxanthine (IBMX), cGMP still decreased after application of a NO donor. Probenecid, a blocker of cGMP transporters, had no effect on this decrease, leaving PDE9 as a possible candidate for decreasing cGMP concentration. Basal cGMP level is poised at an intermediate level from which it can be up or down-regulated according to the cyclase and PDE activities.

Cyclic guanosine monophosphate signalling pathway plays a role in neural cell adhesion molecule-mediated neurite outgrowth and survival

The neural cell adhesion molecule (NCAM) plays a crucial role in neuronal development, regeneration, and synaptic plasticity associated with learning and memory consolidation. Homophilic binding of NCAM leads to neurite extension and neuroprotection in various types of primary neurons through activation of a complex network of signalling cascades, including fibroblast growth factor receptor, Src-family kinases, the mitogen-activated protein kinase pathway, protein kinase C, phosphatidylinositol-3 kinase, and an increase in intracellular Ca(2+). Here we present data indicating an involvement of cyclic GMP in NCAM-mediated neurite outgrowth in both hippocampal and dopaminergic neurons and in NCAM-mediated neuroprotection of dopaminergic neurons. In addition, evidence is presented suggesting that NCAM mediates activation of cGMP via synthesis of nitric oxide (NO) by NO synthase (NOS) and activation of soluble guanylyl cyclase by NO, leading to an increased synthesis of cGMP and activation by cGMP of protein kinase G.

Natural variation in Drosophila larval reward learning and memory due to a cGMP-dependent protein kinase

Animals must be able to find and evaluate food to ensure survival. The ability to associate a cue with the presence of food is advantageous because it allows an animal to quickly identify a situation associated with a good, bad, or even harmful food. Identifying genes underlying these natural learned responses is essential to understanding this ability. Here, we investigate whether natural variation in the foraging (for) gene in Drosophila melanogaster larvae is important in mediating associations between either an odor or a light stimulus and food reward. We found that for influences olfactory conditioning and that the mushroom bodies play a role in this for-mediated olfactory learning. Genotypes associated with high activity of the product of for, cGMP-dependent protein kinase (PKG), showed greater memory acquisition and retention compared with genotypes associated with low activity of PKG when trained with three conditioning trials. Interestingly, increasing the number of training trials resulted in decreased memory retention only in genotypes associated with high PKG activity. The difference in the dynamics of memory acquisition and retention between variants of for suggests that the ability to learn and retain an association may be linked to the foraging strategies of the two variants.

The hinge region operates as a stability switch in cGMP-dependent protein kinase I alpha

The molecular mechanism of cGMP-dependent protein kinase activation by its allosteric regulator cyclic-3',5'-guanosine monophosphate (cGMP) has been intensely studied. However, the structural as well as thermodynamic changes upon binding of cGMP to type I cGMP-dependent protein kinase are not fully understood. Here we report a cGMP-induced shift of Gibbs free enthalpy (DeltaDeltaGD) of 2.5 kJ.mol-1 as determined from changes in tryptophan fluorescence using urea-induced unfolding for bovine PKG Ialpha. However, this apparent increase in overall stability specifically excluded the N-terminal region of the kinase. Analyses of tryptic cleavage patterns using liquid chromatography-coupled ESI-TOF mass spectrometry and SDS/PAGE revealed that cGMP binding destabilizes the N-terminus at the hinge region, centered around residue 77, while the C-terminus was protected from degradation. Furthermore, two recombinantly expressed mutants: the deletion fragment Delta1-77 and the trypsin resistant mutant Arg77Leu (R77L) revealed that the labile nature of the N-terminus is primarily associated with the hinge region. The R77L mutation not only stabilized the N-terminus but extended a stabilizing effect on the remaining domains of the enzyme as well. These findings support the concept that the hinge region of PKG acts as a stability switch.

TRPC, cGMP-dependent protein kinases and cytosolic Ca2+

Ca2+, nitric oxide (NO), and protein kinase G (PKG) are important signaling molecules that play pivotal roles in many physiological processes such as vascular tone control, platelet activation, and synaptic plasticity. TRPC channels allow Ca2+ influx, thus contributing to the production of NO, which subsequently stimulates PKG. It has been demonstrated that PKG can phosphorylate human TRPC3 at Thr-11 and Ser-263 and that this phosphorylation inactivates TRPC3. These two PKG phosphorylation sites, Thr-11 and Ser-263 in human TRPC3, are conserved in other members of the TRPC3/6/7 subfamily, suggesting that PKG may also phosphorylate TRPC6 and TRPC7. In addition, protein kinase C (PKC) also inactivates TRPC3, partly through activating PKG. The PKG-mediated inhibition of TRPC channels may provide a feedback control for the fine tuning of [Ca2+]i levels and protect the cells from the detrimental effects of excessive [Ca2+]i and/or NO.

Cyclic GMP-dependent pathways protect differentiated oligodendrocytes from multiple types of injury

The cyclic GMP analog 8-bromo-cyclic GMP (8-Br-cGMP) protects differentiated murine oligodendrocytes (OLs) from caspase-mediated death initiated by staurosporine, thapsigargin or kainate. Caspase-independent death caused by high levels of NO is also partially prevented by 8-Br-cGMP. Inhibitors of protein kinase G (cGMP-dependent protein kinase, cGK) reversed protection, supporting involvement of cGK. Since NO stimulates soluble guanylate cyclase, increasing cGMP, we treated OLs with low levels of NO and observed partial protection against thapsigargin, staurosporine and kainate. Two inhibitors of mitochondrial pore transition (MPT), cyclosporin A and bongkrekic acid, were poorly protective, indicating that cGMP is not acting primarily by blocking MPT. 8Br-cGMP was more effective than 8Br-cAMP in protecting against staurosporine or release of intracellular Ca(++) by thapsigargin. The cAMP analog exhibited little or no protection against kainate or high levels of NO. Thus cGK signaling is more effective than protein kinase A or phosphodiesterase 3 signaling in preventing OL death.

Tonic and phasic nitric oxide signals in hippocampal long-term potentiation

Nitric oxide (NO) participates in long-term potentiation (LTP) and other forms of synaptic plasticity in many different brain areas but where it comes from and how it acts remain controversial. Using rat and mouse hippocampal slices, we tested the hypothesis that tonic and phasic NO signals are needed and that they derive from different NO synthase isoforms. NMDA increased NO production in a manner that was potently inhibited by three different neuronal NO synthase (nNOS) inhibitors. Tonic NO could be monitored after sensitizing guanylyl cyclase-coupled NO receptors, allowing the very low ambient NO concentrations to be detected by cGMP measurement. The levels were unaffected by inhibition of NMDA receptors, nNOS, or the inducible NO synthase (iNOS). iNOS was also undetectable in protein or activity assays. Tonic NO was susceptible to agents inhibiting endothelial NO synthase (eNOS) and was missing in eNOS knock-out mice. The eNOS knock-outs exhibited a deficiency in LTP resembling that seen in wild-types treated with a NO synthase inhibitor. LTP in the knock-outs could be fully restored by supplying a low level of NO exogenously. Inhibition of nNOS also caused a major loss of LTP, particularly of late-LTP. Again, exogenous NO could compensate, but higher concentrations were needed compared with those restoring LTP in the eNOS knock-outs. It is concluded that tonic and phasic NO signals are both required for hippocampal LTP and the two are generated, respectively, by eNOS and nNOS, the former in blood vessels and the latter in neurons.

Nitric oxide-evoked transient kinetics of cyclic GMP in vascular smooth muscle cells

Cyclic-3',5'-guanosine monophosphate (cGMP) mediates the intracellular signaling cascade responsible for the nitric oxide (NO) initiated relaxation of vascular smooth muscle (VSM). However, the temporal dynamics, including the regulation of cGMP turnover, are largely unknown. Here we report new mechanistic insights into the kinetics of cGMP synthesis and hydrolysis in primary VSM cells by utilizing FRET-based cGMP-indicators [A. Honda, S.R. Adams, C.L. Sawyer, V. Lev-Ram, R.Y. Tsien, W.R. Dostmann, Proc. Natl. Acad. Sci. U S A 98 (5) (2001) 2437.]. First, 2-(N,N-Diethylamino)-diazenolate 2-oxide (DEA/NO) and 2,2'-(Hydroxynitrosohydrazono)-bis-ethanimine (DETA/NO) induced NO-concentration dependent, transient cGMP responses ("peaks") irrespective of their rates of NO release. The kinetic characteristics of these cGMP peaks were governed by the concerted action of the NO-sensitive guanylyl cyclase (GC) and phosphodiesterase type V (PDE5) as shown by their respective inhibition using 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) and Sildenafil. These responses occurred in the presence of moderately elevated cGMP (5-15% FRET ratio), and thus activated PKG and phosphorylated PDE5, suggesting a prominent role for GC in the maintenance and termination of cGMP peaks. Furthermore, cGMP transients could be elicited repeatedly without apparent desensitization of GC or by suppression of cGMP via long-term PDE5 activity. These results demonstrate a continuous sensitivity of the NO/cGMP signaling system, inherent to the phasic nature of smooth muscle physiology.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Regulation of cGMP-dependent protein kinase expression by Rho and Kruppel-like transcription factor-4

Type I cGMP-dependent protein kinase (PKG I) plays a major role in vascular homeostasis by mediating smooth muscle relaxation in response to nitric oxide, but little is known about the regulation of PKG I expression in smooth muscle cells. We found opposing effects of RhoA and Rac1 on cellular PKG I expression: (i) cell density-dependent changes in PKG I expression varied directly with Rac1 activity and inversely with RhoA activity; (ii) RhoA activation by calpeptin suppressed PKG I, whereas RhoA down-regulation by small interfering RNA increased PKG I expression; and (iii) PKG I promoter activity was suppressed in cells expressing active RhoA or Rho-kinase but was enhanced in cells expressing active Rac1 or a dominant negative RhoA. Sp1 consensus sequences in the PKG I promoter were required for Rho regulation and bound nuclear proteins in a cell density-dependent manner, including the Krüppel-like factor 4 (KLF4). KLF4 was identified as a major trans-acting factor at two proximal Sp1 sites; active RhoA suppressed KLF4 DNA binding and trans-activation potential on the PKG I promoter. Experiments with actin-binding agents suggested that RhoA could regulate KLF4 via its ability to induce actin polymerization. Regulation of PKG I expression by RhoA may explain decreased PKG I levels in vascular smooth muscle cells found in some models of hypertension and vascular injury.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

The neurogenetics and evolution of food-related behaviour

All organisms must acquire nutrients from the ambient environment to survive. In animals, the need to eat has driven the evolution of a rich array of complex food-related behaviours that ensure appropriate nutrient intake in diverse niches. Here, we review some of the neural and genetic components that contribute to the regulation of food-related behaviour in invertebrates, with emphasis on mechanisms that are conserved throughout various taxa and activities. We focus on synthesizing neurobiological and genetic approaches into a neurogenetic framework that explains food-related behaviour as the product of interactions between neural substrates, genes and internal and external environments.

cGMP-dependent protein kinases in drug discovery

Cyclic guanosine-3', 5'-monophosphate (cGMP)-dependent protein kinases (cGKs) are key enzymes of nitric oxide-cGMP and natriuretic peptide signalling cascades. These kinases mediate most of the effects of cGMP-elevating drugs, such as nitrates and phosphodiesterase inhibitors. cGKs modulate smooth muscle relaxation (e.g. the vasculature, gastrointestinal tract, bladder and penile), platelet aggregation, renin release, intestinal secretion, learning and memory. Furthermore, several cGK substrates have been identified. Isozyme-specific inhibitors and activators of cGK and its downstream substrates might act more specifically than upstream signalling activators, such as organic nitrates and phosphodiesterase inhibitors.

Distribution of cGMP-dependent protein kinase type I and its isoforms in the mouse brain and retina

Nitric oxide (NO) modulates a variety of processes in the mammalian brain, but the mechanisms of neuronal NO signaling are poorly understood. In the periphery, many effects of NO are mediated via the generation of the second messenger cyclic guanosine-3',5'-monophosphate (cGMP) and activation of the cGMP-dependent protein kinase type I (cGKI). However, previous studies suggested that the expression of cGKI in the nervous system is rather restricted, thus, questioning the functional significance of the cGMP/cGKI pathway as a mediator of NO signaling in the brain. Here we have performed a detailed immunohistochemical study to elucidate the distribution of cGKI in the CNS and eye of the mouse. Expression of cGKI protein was detected not only in the previously described areas (cerebellum, hippocampus, dorsomedial hypothalamus) but also in a number of additional regions, such as medulla, subcommissural organ, cerebral cortex, amygdala, habenulae, various hypothalamic regions, olfactory bulb, pituitary gland, and retina. Immunoblotting with isoform-specific antibodies indicated that the cGKIalpha isoform is prominent in the cerebellum and medulla, whereas the cGKIbeta isoform is predominant in the cortex, hippocampus, hypothalamus, and olfactory bulb. Similar levels of the isoforms were detected in the pituitary gland and eye. Thus, it appears that distinct brain regions express distinct cGKI isoforms that signal via distinct pathways. Together, these results improve our understanding of the cellular and molecular mechanisms of NO/cGMP/cGKI signaling and indicate that the distribution and functional relevance of this pathway in the mammalian brain is broader than previously thought.