dunce
Four human genes (DPDE1 through DPDE4) are closely related to the dnc locus of Drosophila. The deduced amino acid sequences of the
Drosophila and human proteins have considerable homology, extending beyond the putative catalytic
region to include two novel, highly conserved, upstream conserved regions (UCR1 and UCR2). The
upstream conserved regions are located in the amino-terminal regions of the proteins and appear to be
unique to these genes. Polymerase chain reaction analysis suggests that these genes encode the only
homologs of dnc in the human genome. Three of the four genes are expressed in Saccharomyces and have been shown to encode cyclic AMP-specific phosphodiesterases. The products of the
expressed genes display the pattern of sensitivity to inhibitors expected for members of the type IV
cyclic AMP-specific class of phosphodiesterases. Each of the four genes demonstrates a distinctive
pattern of expression in RNA from human cell lines (Bolger, 1993).
To study alternative splicing and tissue-specific expression of the mammalian genes encoding type-IV
cAMP-specific phosphodiesterases, seven cDNAs were cloned from four rat loci (PDE1, PDE2, PDE3 and
PDE4) homologous to dnc. The deduced amino-acid sequences of the proteins encoded by the rat loci
were shown to have a 1:1 correspondence with those encoded by the four human dnc homologs. The
proteins encoded by at least one cDNA from each of the four rat loci contain novel N-terminal
upstream conserved regions (UCR1 and UCR2), present in proteins encoded by the
human dnc homologs and by dnc. cDNAs from three of the rat loci (PDE2, PDE3 and PDE4) have a
structure consistent with alternative splicing of the 5' coding regions of their respective mRNAs.
UCR1, and in one case a portion of UCR2, are absent in one of the alternatively spliced transcripts
from these three loci. PDE3 and PDE4 loci are each
expressed at relatively constant levels in multiple regions of the brain, while PDE2 transcripts are
more abundant in the temporal cortex and brainstem. One of the alternatively spliced mRNAs from the
PDE4 locus is relatively more abundant in the temporal cortex and cerebellum. One alternatively spliced
transcript from the PDE3 locus is expressed more abundantly in parietal cortex. Both of the
alternatively spliced transcripts from the human DPDE4 locus (the homolog of rat PDE4) are
expressed in temporal cortex (Bolger, 1994).
The rat cAMP-specific phosphodiesterases (rPDEIV) are closely related to dunce. There are four known dunce-like cAMP PDE rat isogenes (rPDE-IV-A, -B, -C, -D). High expression of three of these
isogenes (rPDEIV-A, -B, -D) highlighted their involvement in regulation of cAMP in the brain. Distinct but overlapping expression patterns have been observed for rPDEIV-A, rPDEIV-B, and rPDEIV-D. Abundant expression of these subtypes have been observed in the olfactory system, the hippocampus and the cerebellum, while no specific signals could be detected in most areas of the
brain for the subtype rPDEIV-C (Engels, 1995).
Based on their relative abundance and regulation by Ca2+ and by phosphorylation in vitro, it is thought
that the Ca2+/calmodulin-dependent phosphodiesterases (CaM-PDEs) are important modulators of
cyclic nucleotide function in the brain. Two of the most abundant CaM-PDEs in the brain are the 61
kDa and 63 kDa isozymes. The 63 kDa
CaM-PDE mRNA has a wide-spread but uneven distribution. Very strong hybridization signals are
present in the caudate-putamen, nucleus accumbens, olfactory tubercle, and dentate gyrus of the
hippocampus. Somewhat lesser amounts of 63 kDa CaM-PDE mRNA are present in the olfactory
bulb and piriform cortex. Weaker but still easily discernible hybridization signals are seen in several
layers of the cerebral cortex, CA1 and CA3 regions of the hippocampus, amygdaloid nuclear complex,
thalamus, hypothalamus, midbrain, brainstem, cerebellum, and spinal cord. A weak hybridization signal
is detected in the globus pallidus of the basal ganglia. In general, the distribution of the 63 kDa
CaM-PDE is very similar to that of dopamine receptors, suggesting that it may modulate dopamine
function. In contrast, the 61 kDa CaM-PDE mRNA has a more limited and much different distribution,
with the highest level of expression in the cerebral cortex and in the pyramidal cells of the
hippocampus. A moderate hybridization signal is detected in the medial habenula and amygdaloid
nuclear complex. In addition, small subsets of neurons in several other regions show specific
hybridization. Both PDE mRNAs appear to be localized exclusively in neuronal cell bodies. Their
distinct distribution suggests important but different physiological roles for these two isozymes in the
regional regulation of cyclic nucleotides in the CNS. Since these two isozymes are differentially
phosphorylated by cAMP-dependent and Ca2+/CaM-dependent protein kinases, the differential
expression also provides a potential mechanism by which these PDEs can differentially regulate cAMP
and cGMP in different brain areas. The high expression levels in specific subsets of neurons also
suggest that agents increasing Ca2+ in these neurons will increase the rate of cyclic nucleotide
degradation (Yan, 1994).
cDNAs have been isolated corresponding to two human CaM-regulated 3',5'-cyclic nucleotide
phosphodiesterases (PDEs). One of these, Hcam1 (PDE1A3), corresponds to the bovine 61-kDa
CaM PDE (PDE1A2). The second, Hcam3 (PDE1C), represents a novel phosphodiesterase gene.
Hcam1 encodes a 535-amino acid protein that differs most notably from the bovine 61-kDa CaM PDE
by the presence of a 14-amino acid insertion and a divergent carboxyl terminus. Hcam1 is represented in human RNA from several tissues, including brain,
kidney, testes, and heart. Two carboxyl-terminal splice variants for Hcam3 were isolated. One,
Hcam3b (PDE1C1), encodes a protein 634 amino acids (72 kDa) in length. The other, Hcam3a
(PDE1C3), diverges from Hcam3b 4 amino acids from the carboxyl terminus of Hcam3b, and extends
an additional 79 amino acids. All the cDNAs isolated for Hcam3a are incomplete; they do not include
the 5'-end of the open reading frame. Both splice variants are
expressed in several tissues, including brain and heart. There may also be additional splice variants.
Hcam3a has a high affinity for both cAMP and cGMP and thus has distinctly different kinetic
parameters from Hcam1, which has a higher affinity for cGMP than for cAMP (Loughney, 1996).
The distributions in the rat brain of four different cyclic AMP-specific phosphodiesterase isoform mRNAs (APDE1-4) were examined and they were compared. by in situ hybridization histochemistry using specific radiolabeled oligonucleotides, with the distribution of the 63 kDa calmodulin-stimulated phosphodiesterase (CPDE) .
The distribution patterns were unique for each of the APDE isoforms examined. Although no significant signals for APDE1 could be detected anywhere in
the rat brain, all other isoforms are expressed ubiquitously but unevenly and show overlapping distribution patterns. Among all the APDE isoforms
studied, APDE3 shows the strongest and the most extensive expression. Its distribution pattern implies that it may modulate different cellular
processes associated with learning and memory. Compared to APDE3, the levels of expression of APDE2 and APDE4 are weaker, the latter showing
the weakest expression. This study suggests that different isoforms of APDE are expressed together in the same class of neurons implying complex
interactions among different signaling pathways, thereby mediating distinct and specific functions (Iwahashi, 1996).
Cyclic nucleotide phosphodiesterases (PDEs) catalyze the hydrolysis of cAMP and cGMP, thereby participating in regulation of the intracellular
concentrations of these second messengers. The PDE1 family is defined by regulation of activity by calcium and calmodulin. The mouse PDE1B gene was cloned and
characterized. PDE1B encodes the 63-kDa calcium/calmodulin-dependent PDE (CaM-PDE), an isozyme that is expressed in the
CNS in the olfactory tract, dentate gyrus, and striatum and may participate in learning, memory, and regulation of phosphorylation of DARPP-32 in
dopaminergic neurons. A mouse genomic library was screened and exons 2-13 of the PDE1B gene that span 8.4 kb of genomic
DNA were identified. Exons range from 67 to 205 nucleotides and introns from 91 to 2250 nucleotides in length. Exon 1 is not present in the 3 kb of genomic DNA 5'
to exon 2 in these clones. The mouse PDE1B gene shares many similar or identical exon boundaries as well as considerable sequence identity with the rat
PDE4B and PDE4D genes and the Drosophila dunce cAMP-specific PDE gene, suggesting that these genes all arose from a common ancestor. Using
fluorescence in situ hybridization, the PDE1B gene was localized to the distal tip of mouse chromosome 15 (Reed, 1998).
Four cyclic AMP-specific, rolipram-inhibited phosphodiesterases (PDE4s) have been identified in mammals; all four are homologs of dunce, a gene
required for learning and memory in Drosophila. To determine the distribution of PDE4s in the mammalian brain, specific antibodies were generated against
the proteins encoded by each of three murine dunce homologs: PDE4A, PDE4B, and PDE4D. On Western blots, these antibodies recognize
multiple protein species in all brain regions studied. Immunohistochemical studies have shown that both cell bodies and neuropil are well labeled in selected
regions throughout the brain. Immunoreactivity for PDE4A is found predominantly in the anterior olfactory nucleus, subiculum, layer V pyramidal
neurons from the cerebral cortex, and corticospinal tracts. By contrast, anti-PDE4B-labeled neurons are observed in the inferior olive, the paraventricular
and supraoptic nuclei of the hypothalamus, and in the ventral striatum. Regions of neuropil containing high levels of PDE4B immunoreactivity include the
cerebellar molecular layer, globus pallidus, nucleus accumbens, and substantia nigra. Anti-PDE4D antibody distinctly labels cerebellar Purkinje cells as
well as neurons in the medial habenula and thalamic nuclei. Fibers in the fasciculus retroflexus, interpeduncular nuclei, and periaqueductal gray are also
stained with this antibody. These findings indicate that the distribution of PDE4s in the brain is remarkably segregated, and suggest that each of these
enzymes has a unique functional role. Furthermore, the data support the notion that rolipram, the PDE4-specific inhibitor that acts as an antidepressant in
humans, may mediate its behavioral effects through PDE4B, which is highly localized to neural pathways known to underlie reward and affect in mammals (Cherry, 1999).
The intron/exon and promoter
structures of the murine Pde4a gene were examined. Pde4a encodes at least two different transcripts, each generated by alternative mRNA splicing and the use of alternative promoters. The majority of Pde4a exons are tightly clustered at the 3' end of the gene. The 5' region of the gene contains at least one widely
separated exon, which encodes the 5' end of a distinct mRNA transcript and contains a separate promoter and transcriptional start site. Analysis of YAC
clones determined that the Pde4a gene maps to the 4-cM region of Chromosome (Chr) 9, close to Ldlr and Epor, in a region syntenic to human PDE4A (Olsen, 2000).
A cAMP-specific phosphodiesterase was found that is stimulated by binding to the regulatory subunit of cAMP-dependent protein kinase, PKA-R, from
either Dictyostelium or mammals. The phosphodiesterase is encoded by the regA gene of Dictyostelium, which was recovered in a mutant screen for
strains that sporulate in the absence of signals from prestalk cells. The sequence of RegA predicts that it will function as a member of a two-component
system. Genetic analyses indicate that inhibition of the phosphodiesterase results in an increase in the activity of PKA, which acts at a check point for
terminal differentiation. Conserved components known to affect memory, learning and differentiation in flies and vertebrates suggest that a similar circuitry
functions in higher eukaryotes (Shaulsky, 1998).
One of the
four mammalian dnc homologs (mPDE2) reveals high levels of expression in the olfactory
neuroepithelium. Anti-mPDE2 antibody confirms that this PDE protein is abundant in the axons and
dendrites of the olfactory receptor neurons but is conspicuously absent from the cilia, where the initial
events in olfactory signal transduction occur. Lower levels of mPDE2 were also detected throughout
the brain and in the testis. These findings suggest an important modulatory role for mPDE2 in
mammalian olfaction (Cherry, 1995).
Phosphorylation of the 61-kDa isoform of bovine calmodulin (CaM)-stimulated cyclic nucleotide
phosphodiesterase (CaM-PDE) by the catalytic subunit of cyclic AMP-dependent protein kinase A (PKA) results in a decrease in the affinity of the enzyme for calmodulin. Serine residues 120 and 138 have been
identified as the principal sites of phosphorylation. A cDNA encoding the 61-kDa CaM-PDE was used to generate point mutants in which either or both of these serines were replaced with alanine. The
mutants were expressed in COS-7 cells, purified, and phosphorylated. Phosphorylation of the mutant
Ser 138-->Ala results in a decrease in affinity for CaM that is comparable to that seen with the wild-type enzyme. In contrast, phosphorylation of the mutant Ser 120-->Ala has virtually no effect on
CaM affinity. It is concluded that phosphorylation of serine 120 by PKA is responsible for the reduction
in affinity of the 61-kDa CaM-PDE for CaM (Florio, 1994).
Calmodulin (Ca2+/CaM or CaM)-stimulated cyclic nucleotide phosphodiesterases are a genetically diverse class of cAMP and cGMP hydrolyzing activities that are activated by calcium and calmodulin. In addition to Ca2+/CaM, certain CaM-PDEs may be subject to a secondary form of regulation via phosphorylation. For example, PDE1A1 and PDE1A2 are bovine phosphodiesterases that are phosphorylated by cAMP-dependent protein kinase (PKA) (Link to the Drosophila homolog) while CaM-dependent protein kinase II (Link to the Drosophila homolog) catalyzes the phosphorylation of a third PDE, PDE1B1. Each of these genes are subject to alternative splicing to generate distinctive gene products. CaM-PDE genes possess a segment of about 250 residues near the C-terminal end of the molecule that is conserved among all PDE isoforms and is thought to encode a catalytic domain. PDE1B1 and PDE1A2 are products of different genes, while PDE1A1 and PDE1A2 are nearly identical except for a short amino-terminal segment, suggesting that these isozymes are alternatively spliced products of the same gene. Interestingly, CaM is a 10-fold more potent activator of PDE1A1 than the PDE1A2 isoform. Mutational deletion of a previously identified, putatitive CaM-binding domain (residues 4-46) produce a polypeptide that is still activated 3-fold by CaM. However, complete CaM-independent activation occurs when residues 4-98 are deleted. Synthetic peptides were used to locate an additional CaM-binding domain. One peptide spanning amino acids 114-137 of PDE1A2 appears to be the most potent inhibitor of CaM-stimulated activity. A PKA phosphorylation site is present in the center of this inhibitory peptide. Moreover, a discrete segment important for holding these CaM-PDEs in a less active state at low Ca2+ concentrations is located between the two CaM-binding domains (Sonnenberg, 1995).
The cAMP phosphodiesterase (PDE) activity of CHO cells is unaffected by the addition of Ca2++/calmodulin (CaM), indicating the absence of any PDE1 (Ca2+/CaM-stimulated PDE) activity.
Treatment with the tumour promoting phorbol ester leads to the
rapid transient induction of PDE1 activity, attaining a maximum value after about 13 h before
slowly decreasing. Such induction is attenuated by actinomycin D. RT-PCR using degenerate primers allow an approx. 600
bp fragment to be amplified from RNA preparations of rat brain but not from CHO cells unless they
had been treated with PMA. CHO cells transfected to overexpress protein kinase C (PKC)-alpha (See Drosophila PKC) and
PKC-epsilon, but not those transfected to overexpress PKC-beta I or PKC-gamma, exhibit a
twofold higher PDE activity. They also express a PDE1 activity, with Ca2+/CaM effecting a
1.8-2.8-fold increase in total PDE activity. RT-PCR, with PDE1-specific primers, identify an approx.
600 bp product in CHO cells transfected to overexpress PKC-alpha and PKC-epsilon, but not in those
overexpressing PKC-beta I or PKC-gamma. Treatment of PKC-alpha transfected cells with PMA
causes a rapid, albeit transient, increase in PDE1 activity,reaching a maximum approximately 1 h after
PMA challenge, before returning to resting levels some 2 h later. The residual isobutylmethylxanthine
(IBMX)-insensitive PDE activity is dramatically reduced (approx. 4-fold) in the PKC-gamma
transfectants, suggesting that the activity of the cyclic AMP-specific IBMX-insensitive PDE7 activity
is selectively reduced by overexpression of this particular PKC isoform. These data identify a novel
point of 'cross-talk' between the lipid and cyclic AMP signaling systems, where the action of specific
PKC isoforms is shown to cause the induction of Ca2+/CaM-stimulated PDE (PDE1) activity. It is
suggested that this protein kinase C-mediated process might involve regulation of PDE1 gene
expression by the AP-1 (fos/jun) system (Spence, 1995).
A study was made of the influence of electroconvulsive seizure (ECS) and imipramine (IMI) treatment on the transcription and translation of cyclic nucleotide
phosphodiesterase type IV (PDE IV) isozymes in the rat brain. In situ hybridization studies reveal an increase of PDE IV-B mRNA level in various
brain regions after acute ECS. However, the increase of PDE IV activity is produced not by acute but by chronic ECS treatment in the frontal cortex.
Increased PDE IV-B mRNA expression in frontal but not in hippocampal subfields is induced also after chronic ECS treatment. Although an increase in
PDE IV-A mRNA expression of the dentate gyrus in the hippocampus is observed, no change of PDE IV activity is produced in the hippocampus by
acute or chronic ECS treatment. These results suggest that the repeated increases of PDE IV-B mRNA expression are attributable to the increase of PDE
IV translation. Increased PDE IV-B transcription and PDE IV translation in the frontal cortex are also produced after chronic IMI treatment. This is the
first report demonstrating an expressional regulation of Drosophila melanogaster dunce gene homolog PDE IV isozymes in the brain. Although no
pathophysiological conditions with reduced PDE IV activity in the nervous system are known except for a learning deficit in the mutant fly dnc-, these results
suggest possible treatments to cope with reduced PDE IV activity (Suda, 1998).
In eukaryotic cells, the inactivation of the cyclic nucleotide signal depends on a complex array of cyclic nucleotide phosphodiesterases
(PDEs). Although it has been established that multiple PDE isoenzymes with distinct catalytic properties and regulations coexist in the
same cell, the physiological significance of this remarkable complexity is poorly understood.
Although only one dunce PDE has been described in the fly, four orthologous genes (PDE4A, PDE4B, PDE4C, and PDE4D) are present in mice, rats,
and humans, and the encoded proteins share considerable homology in their catalytic and regulatory domains. The evolutionary significance for this
gene duplication is unknown. In vitro and in vivo studies have demonstrated that one of type 4 cAMP-specific PDEs (PDE4D) is activated either through protein
kinase A-dependent phosphorylation or through regulation of transcription. This dual regulation in endocrine, inflammatory, and neuronal cells determines
the intensity and the duration of the cAMP stimulus and promotes adaptive changes such as desensitization, at least in vitro. To examine the role of a PDE in cAMP
signaling in vivo, the type 4 cAMP-specific PDE (PDE4D) gene has been inactivated. This isoenzyme is involved in feedback regulation of cAMP levels. Mice deficient in PDE4D exhibit delayed growth as well as
reductions in viability and female fertility. The decrease in fertility of the null female is caused by impaired ovulation and diminished sensitivity of the granulosa cells to
gonadotropins. These pleiotropic phenotypes demonstrate that PDE4D plays a critical role in cAMP signaling and that the activity of this isoenzyme is required for
the regulation of growth and fertility (Jin, 1999).
It should be emphasized that in spite of the fact that PDE activity is reduced in granulosa cells of the PDE4D-/- mice, rather than increased, the
cAMP response to gonadotropin is significantly reduced. This apparent paradox can be reconciled by hypothesizing that PDE4D regulation in granulosa cells has a
protective effect on responsiveness, and that inactivation of this PDE4D negative feedback causes a permanent desensitized state of the gonadotropin signal
transduction. For instance, a chronic increase in cAMP resulting from the PDE4D inactivation may produce a condition similar to that produced by the targeted
disruption of the RIIbeta regulatory subunit of protein kinase A. In this latter case, loss of RII subunit also causes a decrease, rather than an increase, in
the C subunit activity because of an accelerated degradation. Moreover, an altered duration and intensity of a transient cAMP accumulation may have a major impact
in the ovarian follicle, where cAMP is thought to diffuse through gap junctions between granulosa cells themselves and between granulosa cells and oocytes. In
addition to compromising the normal dynamics of follicular differentiation, an altered half-life and diffusion of cAMP may abrogate differences between mural and
cumulus granulosa cell functions that are crucial for oocyte maturation and ovulation. The reduced viability of the oocytes that are ovulated in PDE4D-/- adult mice probably is caused by the uncoupling of granulosa cell and oocyte maturation. In view of the finding that PDE4 inhibitors cause oocyte
maturation in follicle culture in the absence of gonadotropin stimulation, it is possible that resumption of meiosis in the PDE4D null mice is no longer synchronized
with final maturation of the follicle. These findings are strikingly similar to the phenotype caused by mutations in the Drosophila dunce PDE. The defect in egg deposition in the mutant flies is caused by
an impaired function of both the egg and nursing cells. Thus, PDE4D plays a critical role in the ovarian follicle, and this function is conserved from Drosophila to
mammals (Jin, 1999).
Cullins function as scaffolds that, along with F-box/WD40-repeat-containing proteins, mediate the ubiquitination of proteins to target
them for degradation by the proteasome. The cullin CulA is required at several stages during Dictyostelium
development. culA null cells are defective in inducing cell-type-specific gene expression and exhibit defects during aggregation, including
reduced chemotaxis. PKA is an important regulator of Dictyostelium development. The levels of intracellular cAMP and PKA activity
are controlled by the rate of synthesis of cAMP and its degradation by the cAMP-specific phosphodiesterase RegA. Overexpression of the PKA catalytic subunit (PKAcat) rescues many of the culA null defects and those of cells lacking FbxA/ChtA, a previously described
F-box/WD40-repeat-containing protein, suggesting CulA and FbxA proteins are involved in regulating PKA function. Whereas RegA protein levels drop as the
multicellular organism forms in the wild-type strain, they remain high in culA null and fbxA null cells. Although PKA can suppress the culA and fbxA null
developmental phenotypes, it does not suppress the altered RegA degradation, suggesting that PKA lies downstream of RegA, CulA, and FbxA. CulA, FbxA, and RegA are found in a complex in vivo, and formation of this complex is dependent on the MAP kinase ERK2, which is also required for PKA
function. It is proposed that CulA and FbxA regulate multicellular development by targeting RegA for degradation via a pathway that requires ERK2 function, leading
to an increase in cAMP and PKA activity (Mohanty, 2001).
In an attempt to improve behavioral memory, a strategy was devised to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple high-frequency trains of electrical stimulation induce long-lasting long-term potentiation, a
form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a
single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of
cAMP-dependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response
element binding protein, are important in memory in vivo. These findings suggest that amplification of signals through the cAMP pathway might lower the
threshold for generating long-lasting long-term potentiation and increase behavioral memory. Therefore, the biochemical, physiological, and
behavioral effects were examined in mice subjected to partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase
inhibitor, rolipram, that have no significant effect on basal cAMP concentration, increase the cAMP response of hippocampal slices to stimulation with
forskolin and induce persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training
increases long- but not short-term retention in freezing to context (a measure of fear conditioning), a hippocampus-dependent memory task (Barad, 1998).
cGMP signaling is widespread in the nervous system. However, it has proved difficult to visualize and genetically probe endogenously evoked cGMP dynamics in neurons in vivo. This study combined cGMP and Ca2+ biosensors to image and dissect a cGMP signaling network in a C. elegans oxygen-sensing neuron. A rise in O2 can evoke a tonic increase in cGMP that requires an atypical 2-binding soluble guanylate cyclase and that is sustained until oxygen levels fall. Increased cGMP leads to a sustained Ca2+ response in the neuron that depends on cGMP-gated ion channels. Elevated levels of cGMP and Ca2+ stimulate competing negative feedback loops that shape cGMP dynamics. Ca2+-dependent negative feedback loops, including activation of phosphodiesterase-1 (PDE-1), dampen the rise of cGMP. A different negative feedback loop, mediated by phosphodiesterase-2 (PDE-2) and stimulated by cGMP-dependent kinase (PKG), unexpectedly promotes cGMP accumulation following a rise in O2, apparently by keeping in check gating of cGMP channels and limiting activation of Ca2+-dependent negative feedback loops. Simultaneous imaging of Ca2+ and cGMP suggests that cGMP levels can rise close to cGMP channels while falling elsewhere. O2-evoked cGMP and Ca2+ responses are highly reproducible when the same neuron in an individual animal is stimulated repeatedly, suggesting that cGMP transduction has high intrinsic reliability. However, responses vary substantially across individuals, despite animals being genetically identical and similarly reared. This variability may reflect stochastic differences in expression of cGMP signaling components. This work provides in vivo insights into the architecture of neuronal cGMP signaling (Couto, 2013).
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