Cyclic nucleotide-gated ion channels in sensory transduction

Olfactory toxicity of tetrabromobisphenol A to the goldfish Carassius auratus

Tetrabromobisphenol A (TBBPA) is one of the most extensively used brominated flame retardants and its increasing use in consumer products has raised concerns about its ecotoxicity. Given the ubiquity of TBBPA in aquatic environments, it is inevitable that these chemicals will enter the olfactory chambers of fish via water currents. Nevertheless, the olfactory toxicity of TBBPA to aquatic organisms and the underlying toxic mechanisms have yet to be elucidated. Therefore, we investigated the olfactory toxicity of TBBPA in the goldfish Carassius auratus, a model organism widely used in sensory biology. Results showed that exposure to TBBPA resulted in abnormal olfactory-mediated behaviors and diminished electro-olfactogram (EOG) responses, indicating reduced olfactory acuity. To uncover the underlying mechanisms of action, we examined the structural integrity of the olfactory epithelium (OE), expression levels of olfactory G protein-coupled receptors (GPCRs), enzymatic activities of ion transporters, and fluctuations in neurotransmitters. Additionally, comparative transcriptomic analysis was employed to investigate the molecular mechanisms further. Our study demonstrates for the first time that TBBPA at environmentally relevant levels can adversely affect the olfactory sensitivity of aquatic organisms by interfering with the transmission of aqueous stimuli to olfactory receptors, impeding the binding of odorants to their receptors, disrupting the olfactory signal transduction pathway, and ultimately affecting the generation of action potentials.

Molecular and functional characterization of chemosensory genes from the root-knot nematode Meloidogyne graminicola

BACKGROUND

Root-knot nematode Meloidogyne graminicola has emerged as a major threat in rice agroecosystems owing to climate change-induced changes in cultivation practices. Synthetic nematicides are continually being withdrawn from the nematode management toolbox because of their ill effects on the environment. A sustainable strategy would be to develop novel nematicides or resistant plants that would target nematode sensory perception, which is a key step in the host finding biology of plant-parasitic nematodes (PPNs). However, compared to the extensive literature on the free-living nematode Caenorhabditis elegans, negligible research has been performed on PPN chemosensory biology.

RESULTS

The present study characterizes the five chemosensory genes (Mg-odr-7, Mg-tax-4, Mg-tax-4.1, Mg-osm-9, and Mg-ocr-2) from M. graminicola that are putatively associated with nematode host-finding biology. All the genes were highly transcribed in the early life stages, and RNA interference (RNAi)-induced downregulation of each candidate gene perturbed the normal behavioural phenotypes of M. graminicola, as determined by examining the tracking pattern of juveniles on Pluronic gel medium, attraction to and penetration in rice root tip, and developmental progression in rice root. In addition, a detrimental effect on nematode chemotaxis towards different volatile and nonvolatile organic compounds and host root exudates was documented.

CONCLUSION

Our findings enrich the existing literature on PPN chemosensory biology and can supplement future research aimed at identifying a comprehensive chemosensory signal transduction pathway in PPNs.

Shedding light on human olfaction: Electrophysiological recordings from sensory neurons in acute slices of olfactory epithelium

The COVID-19 pandemic brought attention to our limited understanding of human olfactory physiology. While the cellular composition of the human olfactory epithelium is similar to that of other vertebrates, its functional properties are largely unknown. We prepared acute slices of human olfactory epithelium from nasal biopsies and used the whole-cell patch-clamp technique to record electrical properties of cells. We measured voltage-gated currents in human olfactory sensory neurons and supporting cells, and action potentials in neurons. Additionally, neuronal inward current and action potentials responses to a phosphodiesterase inhibitor suggested a transduction cascade involving cAMP as a second messenger. Furthermore, responses to odorant mixtures demonstrated that the transduction cascade was intact in this preparation. This study provides the first electrophysiological characterization of olfactory sensory neurons in acute slices of the human olfactory epithelium, paving the way for future research to expand our knowledge of human olfactory physiology.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

Mathematical Modeling of Monovalent Permselectivity of a Bilayer Ion-Exchange Membrane as a Function of Current Density

Modification of an ion-exchange membrane with a thin layer, the charge of which is opposite to the charge of the substrate membrane, has proven to be an effective approach to obtaining a composite membrane with permselectivity towards monovalent ions. However, the mechanism of permselectivity is not clear enough. We report a 1D model based on the Nernst–Planck–Poisson equation system. Unlike other similar models, we introduce activity coefficients, which change when passing from one layer of the membrane to another. This makes it possible to accurately take into account the fact that the substrate membranes usually selectively sorb multiply charged counterions. We show that the main cause for the change in the permselectivity coefficient, P_1/2, with increasing current density, j, is the change in the membrane/solution layer, which controls the fluxes of the competing mono- and divalent ions. At low current densities, counterion fluxes are controlled by transfer through the substrate membrane, which causes selective divalent ion transfer. When the current increases, the kinetic control goes first to the modification layer (which leads to the predominant transfer of monovalent ions) and then, at currents close to the limiting current, to the depleted diffusion layer (which results in a complete loss of the permselectivity). Thus, the dependence P_1/2 − j passes through a maximum. An analytical solution is obtained for approximate assessment of the maximum value of P_1/2 and the corresponding fluxes of the competing ions. The maximum P_1/2 values, plotted as a function of the Na⁺ ion current density at which this maximum is reached, gives the theoretical trade-off curve between the membrane permselectivity and permeability of the bilayer monovalent selective ion-exchange membrane under consideration.

Intracellular and extracelluar cyclic GMP in the brain and the hippocampus

Cyclic Guanosine-Monophosphate (cGMP) is implicated as second messenger in a plethora of pathways and its effects are executed mainly by cGMP-dependent protein kinases (PKG). It is involved in both peripheral (cardiovascular regulation, intestinal secretion, phototransduction, etc.) and brain (hippocampal synaptic plasticity, neuroinflammation, cognitive function, etc.) processes. Stimulation of hippocampal cGMP signaling have been proved to be beneficial in animal models of aging, Alzheimer's disease or hepatic encephalopathy, restoring different cognitive functions such as passive avoidance, object recognition or spatial memory. However, even when some inhibitors of cGMP-degrading enzymes (PDEs) are already used against peripheral pathologies, their utility as neurological treatments is still under clinical investigation. Additionally, it has been demonstrated a list of cGMP roles as not second but first messenger. The role of extracellular cGMP has been specially studied in hippocampal function and cognitive impairment in animal models and it has emerged as an important modulator of neuroinflammation-mediated cognitive alterations and hippocampal synaptic plasticity malfunction. Specifically, it has been demonstrated that extracellular cGMP decreases hippocampal IL-1β levels restoring membrane expression of glutamate receptors in the hippocampus and cognitive function in hyperammonemic rats. The mechanisms implicated are still unclear and might involve complex interactions between hippocampal neurons, astrocytes and microglia. Membrane targets for extracellular cGMP are still poorly understood and must be addressed in future studies.

Mechanism of copper nanoparticle toxicity in rainbow trout olfactory mucosa

Chemosensory perception is crucial for fish reproduction and survival. Direct contact of olfactory neuroepithelium to the surrounding environment makes it vulnerable to contaminants in aquatic ecosystems. Copper nanoparticles (CuNPs), which are increasingly used in commercial and domestic applications due their exceptional properties, can impair fish olfactory function. However, the molecular events underlying olfactory toxicity of CuNPs are largely unexplored. Our results suggested that CuNPs were bioavailable to olfactory mucosal cells. Using RNA-seq, we compared the effect of CuNPs and copper ions (Cu²⁺) on gene transcript profiles of rainbow trout (Oncorhynchus mykiss) olfactory mucosa. The narrow overlap in differential gene expression between the CuNP- and Cu²⁺-exposed fish revealed that these two contaminants exert their effects through distinct mechanisms. We propose a transcript-based conceptual model that shows that olfactory signal transduction, calcium homeostasis, and synaptic vesicular signaling were affected by CuNPs in the olfactory sensory neurons (OSNs). Neuroregenerative pathways were also impaired by CuNPs. In contrast, Cu²⁺ did not induce toxicity pathways and rather upregulated regeneration pathways. Both Cu treatments reduced immune system pathway transcripts. However, suppression of transcripts that were associated with inflammatory signaling was only observed with CuNPs. Neither oxidative stress nor apoptosis were triggered by Cu²⁺ or CuNPs in mucosal cells. Dysregulation of transcripts that regulate function, maintenance, and reestablishment of damaged olfactory mucosa represents critical mechanisms of toxicity of CuNPs. The loss of olfaction by CuNPs may impact survival of rainbow trout and impose an ecological risk to fish populations in contaminated environments.

CNG channel structure, function, and gating: a tale of conformational flexibility

Cyclic nucleotide-gated (CNG) channels are key to the signal transduction machinery of certain sensory modalities both in vertebrate and invertebrate organisms. They translate a chemical change in cyclic nucleotide concentration into an electrical signal that can spread through sensory cells. Despite CNG and voltage-gated potassium channels sharing a remarkable amino acid sequence homology and basic architectural plan, their functional properties are dramatically different. While voltage-gated potassium channels are highly selective and require membrane depolarization to open, CNG channels have low ion selectivity and are not very sensitive to voltage. In the last few years, many high-resolution structures of intact CNG channels have been released. This wealth of new structural information has provided enormous progress toward the understanding of the molecular mechanisms and driving forces underpinning CNG channel activation. In this review, we report on the current understanding and controversies surrounding the gating mechanism in CNG channels, as well as the deep intertwining existing between gating, the ion permeation process, and its modulation by membrane voltage. While the existence of this powerful coupling was recognized many decades ago, its direct structural demonstration, and ties to the CNG channel inherent pore flexibility, is a recent achievement.

Modulation of olfactory signal detection in the olfactory epithelium: focus on the internal and external environment, and the emerging role of the immune system

Detection and discrimination of odorants by the olfactory system plays a pivotal role in animal survival. Olfactory-based behaviors must be adapted to an ever-changing environment. Part of these adaptations includes changes of odorant detection by olfactory sensory neurons localized in the olfactory epithelium. It is now well established that internal signals such as hormones, neurotransmitters, or paracrine signals directly affect the electric activity of olfactory neurons. Furthermore, recent data have shown that activity-dependent survival of olfactory neurons is important in the olfactory epithelium. Finally, as olfactory neurons are directly exposed to environmental toxicants and pathogens, the olfactory epithelium also interacts closely with the immune system leading to neuroimmune modulations. Here, we review how detection of odorants can be modulated in the vertebrate olfactory epithelium. We choose to focus on three cellular types of the olfactory epithelium (the olfactory sensory neuron, the sustentacular and microvillar cells) to present the diversity of modulation of the detection of odorant in the olfactory epithelium. We also present some of the growing literature on the importance of immune cells in the functioning of the olfactory epithelium, although their impact on odorant detection is only just beginning to be unravelled.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

The cyclic AMP signaling pathway in the rodent main olfactory system

Odor perception begins with the detection of odorant molecules by the main olfactory epithelium located in the nasal cavity. Odorant molecules bind to and activate a large family of G-protein-coupled odorant receptors and trigger a cAMP-mediated transduction cascade that converts the chemical stimulus into an electrical signal transmitted to the brain. Morever, odorant receptors and cAMP signaling plays a relevant role in olfactory sensory neuron development and axonal targeting to the olfactory bulb. This review will first explore the physiological response of olfactory sensory neurons to odorants and then analyze the different components of cAMP signaling and their different roles in odorant detection and olfactory sensory neuron development.

Alzheimer's Disease: What Can We Learn From the Peripheral Olfactory System?

The sense of smell has been shown to deteriorate in patients with some neurodegenerative disorders. In Parkinson's disease (PD) and Alzheimer's disease (AD), decreased ability to smell is associated with early disease stages. Thus, olfactory neurons in the nose and olfactory bulb (OB) may provide a window into brain physiology and pathophysiology to address the pathogenesis of neurodegenerative diseases. Because nasal olfactory receptor neurons regenerate throughout life, the olfactory system offers a broad variety of cellular mechanisms that could be altered in AD, including odorant receptor expression, neurogenesis and neurodegeneration in the olfactory epithelium, axonal targeting to the OB, and synaptogenesis and neurogenesis in the OB. This review focuses on pathophysiological changes in the periphery of the olfactory system during the progression of AD in mice, highlighting how the olfactory epithelium and the OB are particularly sensitive to changes in proteins and enzymes involved in AD pathogenesis. Evidence reviewed here in the context of the emergence of other typical pathological changes in AD suggests that olfactory impairments could be used to understand the molecular mechanisms involved in the early phases of the pathology.

AAV-mediated human CNGB3 restores cone function in an all-cone mouse model of CNGB3 achromatopsia

Complete congenital achromatopsia is a devastating hereditary visual disorder. Mutations in the CNGB3 gene account for more than 50% of all known cases of achromatopsia. This work investigated the efficiency of subretinal (SR) delivered AAV8 (Y447, 733F) vector containing a human PR2.1 promoter and a human CNGB3 cDNA in Cngb3 ^-/-/ Nrl ^-/- mice. The Cngb3 ^-/-/ Nrl ^-/- mouse was a cone-dominant model with Cngb3 channel deficiency, which partially mimicked the all-cone foveal structure of human achromatopsia with CNGB3 mutations. Following SR delivery of the vector, AAV-mediated CNGB3 expression restored cone function which was assessed by the restoration of the cone-mediated electroretinogram (ERG) and immunohistochemistry. This therapeutic rescue resulted in long-term improvement of retinal function with the restoration of cone ERG amplitude. This study demonstrated an AAV-mediated gene therapy in a cone-dominant mouse model using a human gene construct and provided the potential to be utilized in clinical trials.

A perspective profile of ADCY1 in cAMP signaling with drug-resistance in lung cancer

Adenylate cyclase 1 (ADCY1 or AC1) is a member of ADCY superfamily and was primarily found to be expressed in the brain. ADCY1 is responsible for catalyzing ATP to cyclic AMP (cAMP). As a secondary messenger, cAMP can regulate plenty of cellular activities. cAMP can perform its regulation in cellular transport through the binding to cAMP dependent protein kinases (PKAs), cAMP-activated guanine exchange factors (EPACs) and cyclic nucleotide-gated channels functioning in transduction of sensory signals (CNGs). Lung cancer is one of the leading factors of cancer-related death worldwide. Platinum-based chemotherapy is the first-line treatment for advanced lung cancer patients. In addition, surgical treatment, radiation treatment, and molecular targeted therapy are also therapeutic options for lung cancer patients in clinical settings. However, drug resistance and toxicity are the major obstacles that affect chemotherapy outcome and prognosis of lung cancer patients. And the therapeutic efficiency and adverse effects are varying with each individual. In recent years, investigations based on genetic sequencing have revealed the emerging role of ADCY1 mutations in affecting drug efficiency in various cancers such as lung cancer, esophageal cancer and colorectal cancer. The potential function of ADCY1 in chemotherapy resistance is of great importance to be noticed and investigated.

Role of cAMP and cGMP Signaling in Brown Fat

Cold-induced activation of brown adipose tissue (BAT) is mediated by norepinephrine and adenosine that are released during sympathetic nerve activation. Both signaling molecules induce an increase in intracellular levels of 3',5'-cyclic adenosine monophosphate (cAMP) in murine and human BAT. In brown adipocytes, cAMP plays a central role, because it activates lipolysis, glucose uptake, and thermogenesis. Another well-studied intracellular second messenger is 3',5'-cyclic guanosine monophosphate (cGMP), which closely resembles cAMP. Several studies have shown that intact cGMP signaling is essential for normal adipogenic differentiation and BAT-mediated thermogenesis in mice. This chapter highlights recent observations, demonstrating the physiological significance of cyclic nucleotide signaling in BAT as well as their potential to induce browning of white adipose tissue (WAT) in mice and humans.

Advances in high-speed atomic force microscopy (HS-AFM) reveal dynamics of transmembrane channels and transporters

Recent advances in high-speed atomic force microscopy (HS-AFM) have made it possible to study the conformational dynamics of single unlabeled transmembrane channels and transporters. Improving environmental control with the integration of a non-disturbing buffer exchange system, which in turn allows the gradual change of conditions during HS-AFM operation, has provided a breakthrough toward the performance of structural titration experiments. Further advancements in temporal resolution with the use of line scanning and height spectroscopy techniques show how high-speed atomic force microscopy can measure millisecond to microsecond dynamics, pushing this method beyond current spatial and temporal limits offered by less direct techniques.

HIV-1 Envelope Glycoproteins Induce the Production of TNF-α and IL-10 in Human Monocytes by Activating Calcium Pathway

Human HIV-1 infection leads inevitably to a chronic hyper-immune-activation. However, the nature of the targeted receptors and the pathways involved remain to be fully elucidated. We demonstrate that X4-tropic gp120 induced the production of TNF-α and IL-10 by monocytes through activation of a cell membrane receptor, distinct from the CD4, CXCR4, and MR receptors. Gp120 failed to stimulate IL-10 and TNF-α production by monocytes in Ca²⁺ free medium. This failure was total for IL-10 and partial for TNF-α. However, IL-10 and TNF-α production was fully restored following the addition of exogenous calcium. Accordingly, addition of BAPTA-AM and cyclosporine-A, fully and partially inhibited IL-10 and TNF-α respectively. The PKA pathway was crucial for IL-10 production but only partially involved in gp120-induced TNF-α. The PLC pathway was partially and equivalently involved in gp120-induced TNF-α and IL-10. Moreover, the inhibition of PI3K, ERK1/2, p38 MAP-kinases and NF-κB pathways totally abolished the production of both cytokines. In conclusion, this study revealed the crucial calcium signaling pathway triggered by HIV-1 gp120 to control the production of these two cytokines: TNF-α and IL-10. The finding could help in the development of a new therapeutic strategy to alleviate the chronic hyper-immune-activation observed in HIV-1 infected patients.

An iris diaphragm mechanism to gate a cyclic nucleotide-gated ion channel

Cyclic nucleotide-gated (CNG) ion channels are non-selective cation channels key to signal transduction. The free energy difference of cyclic-nucleotide (cAMP/cGMP) binding/unbinding is translated into mechanical work to modulate the open/closed probability of the pore, i.e., gating. Despite the recent advances in structural determination of CNG channels, the conformational changes associated with gating remain unknown. Here we examine the conformational dynamics of a prokaryotic homolog of CNG channels, SthK, using high-speed atomic force microscopy (HS-AFM). HS-AFM of SthK in lipid bilayers shows that the CNBDs undergo dramatic conformational changes during the interconversion between the resting (apo and cGMP) and the activated (cAMP) states: the CNBDs approach the membrane and splay away from the 4-fold channel axis accompanied by a clockwise rotation with respect to the pore domain. We propose that these movements may be converted by the C-linker to pull the pore helices open in an iris diaphragm-like mechanism.

Cyclic nucleotide-gated (CNG) ion channels are non-selective cation channels key to signal transduction, but conformational changes associated with gating remained unknown. Here authors use high-speed atomic force microscopy to visualize SthK channels dynamics in response to cyclic nucleotides.

Bidirectional adaptive introgression between two ecologically divergent sparrow species

Natural hybrid zones can be used to dissect the mechanisms driving key evolutionary processes by allowing us to identify genomic regions important for establishing reproductive isolation and that allow for transfer of adaptive variation. We leverage whole-genome data in a system where two bird species, the saltmarsh (Ammospiza caudacuta) and Nelson's (A. nelsoni) sparrow, hybridize despite their relatively high background genetic differentiation and past ecological divergence. Adaptive introgression is plausible in this system because Nelson's sparrows are recent colonists of saltwater marshes, compared to the specialized saltmarsh sparrow that has a longer history of saltmarsh adaptation. Comparisons among whole-genome sequences of 34 individuals from allopatric and sympatric populations show that ongoing gene flow is shaping the genomic landscape, with allopatric populations exhibiting genome-wide F_ST estimates close to double of that observed in sympatry. We characterized patterns of introgression across the genome and identify regions that exhibit biased introgression into hybrids from one parental species. These regions offer compelling candidates for genes related to tidal marsh adaptations suggesting that adaptive introgression may be an important consequence of hybridization. These findings highlight the value of considering the landscapes of both genome-wide introgression and divergence when characterizing the evolutionary forces that drive speciation.

Sensory Adaptation to Chemical Cues by Vomeronasal Sensory Neurons

Sensory adaptation is a source of experience-dependent feedback that impacts responses to environmental cues. In the mammalian main olfactory system (MOS), adaptation influences sensory coding at its earliest processing stages. Sensory adaptation in the accessory olfactory system (AOS) remains incompletely explored, leaving many aspects of the phenomenon unclear. We investigated sensory adaptation in vomeronasal sensory neurons (VSNs) using a combination of in situ Ca²⁺ imaging and electrophysiology. Parallel studies revealed prominent short-term sensory adaptation in VSNs upon repeated stimulation with mouse urine and monomolecular bile acid ligands at interstimulus intervals (ISIs) less than 30 s. In such conditions, Ca²⁺ signals and spike rates were often reduced by more than 50%, leading to dramatically reduced chemosensory sensitivity. Short-term adaptation was reversible over the course of minutes. Population Ca²⁺ imaging experiments revealed the presence of a slower form of VSN adaptation that accumulated over dozens of stimulus presentations delivered over tens of minutes. Most VSNs showed strong adaptation, but in a substantial VSN subpopulation adaptation was diminished or absent. Investigation of same- and opposite-sex urine responses in male and female VSNs revealed that adaptation to same-sex cues occurred at ISIs up to 180 s, conditions that did not induce adaptation to opposite-sex cues. This result suggests that VSN sensory adaptation can be modulated by sensory experience. These studies comprehensively establish the presence of VSN sensory adaptation and provide a foundation for future inquiries into the molecular and cellular mechanisms of this phenomenon and its impact on mammalian behavior.

Retinal Cyclic Nucleotide-Gated Channels: From Pathophysiology to Therapy

The first step in vision is the absorption of photons by the photopigments in cone and rod photoreceptors. After initial amplification within the phototransduction cascade the signal is translated into an electrical signal by the action of cyclic nucleotide-gated (CNG) channels. CNG channels are ligand-gated ion channels that are activated by the binding of cyclic guanosine monophosphate (cGMP) or cyclic adenosine monophosphate (cAMP). Retinal CNG channels transduce changes in intracellular concentrations of cGMP into changes of the membrane potential and the Ca²⁺ concentration. Structurally, the CNG channels belong to the superfamily of pore-loop cation channels and share a common gross structure with hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and voltage-gated potassium channels (KCN). In this review, we provide an overview on the molecular properties of CNG channels and describe their physiological role in the phototransduction pathways. We also discuss insights into the pathophysiological role of CNG channel proteins that have emerged from the analysis of CNG channel-deficient animal models and human CNG channelopathies. Finally, we summarize recent gene therapy activities and provide an outlook for future clinical application.

Interplay between Obesity-Induced Inflammation and cGMP Signaling in White Adipose Tissue

Current worldwide figures suggest that obesity is pandemic. Understanding the underlying molecular mechanisms would help develop viable anti-obesity therapies. Here, we assess the influence of obesity-induced inflammation on white adipocyte cyclic guanosine monophosphate (cGMP) signaling, which is beneficial for adipocyte differentiation and thermogenesis. We find that murine gonadal and not inguinal fat is prone to obesity-induced inflammation. Correspondingly, the cGMP cascade is dysregulated in gonadal but not in inguinal fat of obese mice. Analysis of two independent human cohorts reveals a defective cGMP pathway only in visceral fat of obese subjects. Congruently, cGMP signaling is dysregulated in tumor necrosis factor α (TNF-α)-treated primary white adipocytes. TNF-α-mediated suppression of sGCβ₁ is mediated via NF-κB, whereas PKG is repressed by JNK signaling. Additionally, TNF-α-activated JNK signaling suppresses PPARγ and aP2. Taken together, the intensity of obesity-induced inflammation dictates the amplitude of cGMP signaling dysregulation in white adipocytes through distinct pathways.

UDP/P2Y6 receptor signaling regulates IgE-dependent degranulation in human basophils

BACKGROUND

P2Y purinergic receptors (P2YR) are G protein-coupled receptors that are stimulated by extracellular nucleotides. They mediate cellular effects by regulating cAMP production, protein kinase C activation, inositol trisphosphate generation, and Ca²⁺ release from intracellular stores. The P2Y6 receptor of this family is selectively stimulated by UDP, and selectively inhibited by MRS2578. In the present study, we examined the effect of UDP/P2Y6 receptor signaling on IgE-dependent degranulation in human basophils.

METHODS

Basophils were purified from human peripheral blood. The mRNA expression of genes encoding P2YR and ecto-nucleoside triphosphate diphosphohydrolase (ENTPDase) was measured by RT-PCR. Intracellular Ca²⁺ influx via UDP/P2Y6 receptor signaling in basophils was detected using a calcium probe. The effect of UDP/P2Y6 receptor signaling on IgE-dependent degranulation in basophils was confirmed by measuring CD63 expression by flow cytometry. Autocrine secretion of nucleotides was detected by HPLC analysis.

RESULTS

We showed that purified basophils express P2Y6 mRNA and that UDP increased intracellular Ca²⁺, which was reduced by MRS2578 treatment. UDP promoted IgE-dependent degranulation. Furthermore, MRS2578 inhibited IgE-dependent degranulation in basophils. HPLC analysis indicated that basophils spontaneously secrete UTP. In addition, basophils expressed the extracellular nucleotide hydrolases ENTPDase2, ENTPDase3, and ENTPDase8.

CONCLUSIONS

This study showed that UDP/P2Y6 receptor signaling is involved in the regulation of IgE-dependent degranulation in basophils, which might stimulate the P2Y6 receptor via the autocrine secretion of UTP. Thus, this receptor represents a potential target to regulate IgE-dependent degranulation in basophils during allergic diseases.

Modulation of cyclic nucleotide-mediated cellular signaling and gene expression using photoactivated adenylyl cyclase as an optogenetic tool

Cyclic nucleotide signaling pathway plays a significant role in various biological processes such as cell growth, transcription, inflammation, in microbial pathogenesis, etc. Modulation of cyclic nucleotide levels by optogenetic tools has overcome certain limitations of studying transduction cascade by pharmacological agents and has allowed several ways to modulate biological processes in a spatiotemporal manner. Here, we have shown the optogenetic modulation of the cyclooxygenase 2 (Cox-2) gene expression and their downstream effector molecule (PGE₂) in HEK-293T cells and the development process of Dictyostelium discoideum via modulating the cyclic nucleotide (cAMP) signaling pathway utilizing photoactivated adenylyl cyclases (PACs) as an optogenetic tool. Light-induced activation of PACs in HEK-293T cells increases the cAMP level that leads to activation of cAMP response element-binding protein (CREB) transcription factor and further upregulates downstream Cox-2 gene expression and their downstream effector molecule prostaglandin E2. In D. discoideum, the light-regulated increase in cAMP level affects the starvation-induced developmental process. These PACs could modulate the cAMP levels in a light-dependent manner and have a potential to control gene expression and their downstream effector molecules with varying magnitude. It would enable one to utilize PAC as a tool to decipher cyclic nucleotide mediated signaling pathway regulations and their mechanism.

Voltage-gated calcium channels of Paramecium cilia

Paramecium cells swim by beating their cilia, and make turns by transiently reversing their power stroke. Reversal is caused by Ca²⁺ entering the cilium through voltage-gated Ca²⁺ (Ca_V) channels that are found exclusively in the cilia. As ciliary Ca²⁺ levels return to normal, the cell pivots and swims forward in a new direction. Thus, the activation of the Ca_V channels causes cells to make a turn in their swimming paths. For 45 years, the physiological characteristics of the Paramecium ciliary Ca_V channels have been known, but the proteins were not identified until recently, when the P. tetraurelia ciliary membrane proteome was determined. Three Ca_Vα1 subunits that were identified among the proteins were cloned and confirmed to be expressed in the cilia. We demonstrate using RNA interference that these channels function as the ciliary Ca_V channels that are responsible for the reversal of ciliary beating. Furthermore, we show that Pawn (pw) mutants of Paramecium that cannot swim backward for lack of Ca_V channel activity do not express any of the three Ca_V1 channels in their ciliary membrane, until they are rescued from the mutant phenotype by expression of the wild-type PW gene. These results reinforce the correlation of the three Ca_V channels with backward swimming through ciliary reversal. The PwB protein, found in endoplasmic reticulum fractions, co-immunoprecipitates with the Ca_V1c channel and perhaps functions in trafficking. The PwA protein does not appear to have an interaction with the channel proteins but affects their appearance in the cilia.

The long tale of the calcium activated Cl- channels in olfactory transduction

Ca²⁺-activated Cl^- currents have been implicated in many cellular processes in different cells, but for many years, their molecular identity remained unknown. Particularly intriguing are Ca²⁺-activated Cl^- currents in olfactory transduction, first described in the early 90s. Well characterized electrophysiologically, they carry most of the odorant-induced receptor current in the cilia of olfactory sensory neurons (OSNs). After many attempts to determine their molecular identity, TMEM16B was found to be abundantly expressed in the cilia of OSNs in 2009 and having biophysical properties like those of the native olfactory channel. A TMEM16B knockout mouse confirmed that TMEM16B was indeed the olfactory Cl^- channel but also suggested a limited role in olfactory physiology and behavior. The question then arises of what the precise role of TMEM16b in olfaction is. Here we review the long story of this channel and its possible roles.

The Ca2+-activated Cl- channel TMEM16B regulates action potential firing and axonal targeting in olfactory sensory neurons

TMEM16B is expressed in olfactory sensory neurons, but previous attempts to establish a physiological role in olfaction have been unsuccessful. Pietra et al. find that genetic ablation of TMEM16B results in defects in the olfactory behavior of mice and the cellular physiology of olfactory sensory neurons.

The Ca²⁺-activated Cl⁻ channel TMEM16B is highly expressed in the cilia of olfactory sensory neurons (OSNs). Although a large portion of the odor-evoked transduction current is carried by Ca²⁺-activated Cl⁻ channels, their role in olfaction is still controversial. A previous report (Billig et al. 2011. Nat. Neurosci.http://dx.doi.org/10.1038/nn.2821) showed that disruption of the TMEM16b/Ano2 gene in mice abolished Ca²⁺-activated Cl⁻ currents in OSNs but did not produce any major change in olfactory behavior. Here we readdress the role of TMEM16B in olfaction and show that TMEM16B knockout (KO) mice have behavioral deficits in odor-guided food-finding ability. Moreover, as the role of TMEM16B in action potential (AP) firing has not yet been studied, we use electrophysiological recording methods to measure the firing activity of OSNs. Suction electrode recordings from isolated olfactory neurons and on-cell loose-patch recordings from dendritic knobs of neurons in the olfactory epithelium show that randomly selected neurons from TMEM16B KO mice respond to stimulation with increased firing activity than those from wild-type (WT) mice. Because OSNs express different odorant receptors (ORs), we restrict variability by using a mouse line that expresses a GFP-tagged I7 OR, which is known to be activated by heptanal. In response to heptanal, we measure dramatic changes in the firing pattern of I7-expressing neurons from TMEM16B KO mice compared with WT: responses are prolonged and display a higher number of APs. Moreover, lack of TMEM16B causes a markedly reduced basal spiking activity in I7-expressing neurons, together with an alteration of axonal targeting to the olfactory bulb, leading to the appearance of supernumerary I7 glomeruli. Thus, TMEM16B controls AP firing and ensures correct glomerular targeting of OSNs expressing I7. Altogether, these results show that TMEM16B does have a relevant role in normal olfaction.

A functional genomics screen in planarians reveals regulators of whole-brain regeneration

Planarians regenerate all body parts after injury, including the central nervous system (CNS). We capitalized on this distinctive trait and completed a gene expression-guided functional screen to identify factors that regulate diverse aspects of neural regeneration in Schmidtea mediterranea. Our screen revealed molecules that influence neural cell fates, support the formation of a major connective hub, and promote reestablishment of chemosensory behavior. We also identified genes that encode signaling molecules with roles in head regeneration, including some that are produced in a previously uncharacterized parenchymal population of cells. Finally, we explored genes downregulated during planarian regeneration and characterized, for the first time, glial cells in the planarian CNS that respond to injury by repressing several transcripts. Collectively, our studies revealed diverse molecules and cell types that underlie an animal’s ability to regenerate its brain.

DOI:http://dx.doi.org/10.7554/eLife.17002.001

Animals differ in the extent to which they can regenerate missing body parts after injury. Humans regenerate poorly after many injuries, especially when the brain becomes damaged after stroke, disease or trauma. On the other hand, planarians – small worms that live in fresh water – regenerate exceptionally well. A whole planarian can regenerate from small pieces of tissue.

The ability of planarians to regenerate their nervous system relies on stem cells called neoblasts, which can migrate through the body and divide to replace lost cells. However, the specific mechanisms responsible for regenerating nervous tissue are largely unknown. Roberts-Galbraith et al. carried out a screen to identify genes that tell planarians whether to regenerate a new brain, what cells to make and how to arrange them.

The study revealed over thirty genes that allow planarians to regenerate their brains after their heads have been amputated. These genes play several different roles in the animal. Some of the genes help neoblasts to make decisions about what kinds of cells they should become. One gene is needed to make an important connection in the planarian brain after injury. Another helps to restore the ability of the planarian to sense its food. The experiments also show that some key genes are switched on in a new cell type that might produce signals to support regeneration.

Lastly, Roberts-Galbraith et al. found that the planarian nervous system contains cells called glia. Previous studies have shown that many of the cells in the human brain are glia and that these cells help nerve cells to survive and work properly. The discovery of glia in planarians means that it will be possible to use these worms to study how glia support brain regeneration and how glia themselves are replaced after injury. In the long term, this work might lead to discoveries that shed light on how tissue regeneration could be improved in humans.

DOI:http://dx.doi.org/10.7554/eLife.17002.002

Deciphering the function of the CNGB1b subunit in olfactory CNG channels

Olfactory cyclic nucleotide-gated (CNG) ion channels are key players in the signal transduction cascade of olfactory sensory neurons. The second messengers cAMP and cGMP directly activate these channels, generating a depolarizing receptor potential. Olfactory CNG channels are composed of two CNGA2 subunits and two modulatory subunits, CNGA4, and CNGB1b. So far the exact role of the modulatory subunits for channel activation is not fully understood. By measuring ligand binding and channel activation simultaneously, we show that in functional heterotetrameric channels not only the CNGA2 subunits and the CNGA4 subunit but also the CNGB1b subunit binds cyclic nucleotides and, moreover, also alone translates this signal to open the pore. In addition, we show that the CNGB1b subunit is the most sensitive subunit in a heterotetrameric channel to cyclic nucleotides and that it accelerates deactivation to a similar extent as does the CNGA4 subunit. In conclusion, the CNGB1b subunit participates in ligand-gated activation of olfactory CNG channels and, particularly, contributes to rapid termination of odorant signal in an olfactory sensory neuron.

Single Ca(2+)-activated Cl(-) channel currents recorded from toad olfactory cilia

BACKGROUND

Odor transduction, occurring in the chemosensory cilia of vertebrate olfactory sensory neurons, is triggered by guanosine triphosphate-coupled odor receptors and mediated by a cyclic adenosine monophosphate (cAMP) signaling cascade, where cAMP opens cationic non-selective cyclic nucleotide-gated (CNG) channels. Calcium enters through CNG gates Ca(2+)-activated Cl(-) channels, allowing a Cl(-) inward current that enhances the depolarization initiated by the CNG-dependent inward current. The anoctamin channel 2, ANO2, is considered the main Ca(2+)-activated Cl(-) channel of olfactory transduction. Although Ca(2+)-activated Cl(-) channel-dependent currents in olfactory sensory neurons were reported to be suppressed in ANO2-knockout mice, field potentials from their olfactory epithelium were only modestly diminished and their smell-dependent behavior was unaffected, suggesting the participation of additional Ca(2+)-activated Cl(-) channel types. The Bestrophin channel 2, Best2, was also detected in mouse olfactory cilia and ClCa4l, belonging to the ClCa family of Ca(2+)-activated Cl(-) channels, were found in rat cilia. Best2 knock-out mice present no electrophysiological or behavioral impairment, while the ClCa channels have not been functionally studied; therefore, the overall participation of all these channels in olfactory transduction remains unresolved.

RESULTS

We explored the presence of detectable Ca(2+)-activated Cl(-) channels in toad olfactory cilia by recording from inside-out membrane patches excised from individual cilia and detected unitary Cl(-) current events with a pronounced Ca(2+) dependence, corresponding to 12 and 24 pS conductances, over tenfold higher than the aforementioned channels, and a approx. fivefold higher Ca(2+) affinity (K0.5 = 0.38 µM). Remarkably, we observed immunoreactivity to anti-ClCa and anti-ANO2 antibodies in the olfactory cilia, suggesting a possible cooperative function of both channel type in chemotransduction.

CONCLUSIONS

These results are consistent with a novel olfactory cilia channel, which might play a role in odor transduction.

Inherited macular degeneration-associated mutations in CNGB3 increase the ligand sensitivity and spontaneous open probability of cone cyclic nucleotide-gated channels

Cyclic nucleotide gated (CNG) channels are a critical component of the visual transduction cascade in the vertebrate retina. Mutations in the genes encoding these channels have been associated with a spectrum of inherited retinal disorders. To gain insight into their pathophysiological mechanisms, we have investigated the functional consequences of several CNGB3 mutations, previously associated with macular degeneration (Y469D and L595F) or complete achromatopsia (S156F, P309L, and G558C), by expressing these subunits in combination with wild-type CNGA3 in Xenopus oocytes and characterizing them using patch-clamp recordings in the inside-out configuration. These mutations did not prevent the formation of functional heteromeric channels, as indicated by sensitivity to block by L-cis-diltiazem. With the exception of S156F, each of the mutant channels displayed electrophysiological properties reflecting enhanced channel activity at physiological concentrations of cGMP (i.e., a gain-of-function phenotype). The increased channel activity produced by these mutations resulted from either increased functional expression levels, or increased sensitivity to cyclic nucleotides. Furthermore, L595F increased the spontaneous open probability in the absence of activating ligand, signifying a ligand independent gain-of-function change. In addition to the CNGB3 disease-associate mutations, we characterized the effects of several common CNGB3 and CNGA3 single-nucleotide polymorphisms (SNPs) on heteromeric CNGA3+CNGB3 channel function. Two of the SNPs examined (A3-T153M, and B3-W234C) produced decreased ligand sensitivity for heteromeric CNG channels. These changes may contribute to background disease susceptibility when combined with other genetic or non-genetic factors. Together, these studies help to define the underlying molecular phenotype for mutations relating to CNG channel disease pathogenesis.

Deep sequencing of the murine olfactory receptor neuron transcriptome

The ability of animals to sense and differentiate among thousands of odorants relies on a large set of olfactory receptors (OR) and a multitude of accessory proteins within the olfactory epithelium (OE). ORs and related signaling mechanisms have been the subject of intensive studies over the past years, but our knowledge regarding olfactory processing remains limited. The recent development of next generation sequencing (NGS) techniques encouraged us to assess the transcriptome of the murine OE. We analyzed RNA from OEs of female and male adult mice and from fluorescence-activated cell sorting (FACS)-sorted olfactory receptor neurons (ORNs) obtained from transgenic OMP-GFP mice. The Illumina RNA-Seq protocol was utilized to generate up to 86 million reads per transcriptome. In OE samples, nearly all OR and trace amine-associated receptor (TAAR) genes involved in the perception of volatile amines were detectably expressed. Other genes known to participate in olfactory signaling pathways were among the 200 genes with the highest expression levels in the OE. To identify OE-specific genes, we compared olfactory neuron expression profiles with RNA-Seq transcriptome data from different murine tissues. By analyzing different transcript classes, we detected the expression of non-olfactory GPCRs in ORNs and established an expression ranking for GPCRs detected in the OE. We also identified other previously undescribed membrane proteins as potential new players in olfaction. The quantitative and comprehensive transcriptome data provide a virtually complete catalogue of genes expressed in the OE and present a useful tool to uncover candidate genes involved in, for example, olfactory signaling, OR trafficking and recycling, and proliferation.

Co-expression of anoctamins in cilia of olfactory sensory neurons

Vertebrates can sense and identify a vast array of chemical cues. The molecular machinery involved in chemodetection and transduction is expressed within the cilia of olfactory sensory neurons. Currently, there is only limited information available on the distribution and density of individual signaling components within the ciliary compartment. Using super-resolution microscopy, we show here that cyclic-nucleotide-gated channels and calcium-activated chloride channels of the anoctamin family are localized to discrete microdomains in the ciliary membrane. In addition to ANO2, a second anoctamin, ANO6, also localizes to ciliary microdomains. This observation, together with the fact that ANO6 and ANO2 co-localize, indicates a role for ANO6 in olfactory signaling. We show that both ANO2 and ANO6 can form heteromultimers and that this heteromerization alters the recombinant channels' physiological properties. Thus, we provide evidence for interaction of ANO2 and ANO6 in olfactory cilia, with possible physiological relevance for olfactory signaling.

Central role of soluble adenylyl cyclase and cAMP in sperm physiology

Cyclic adenosine 3',5'-monophosphate (cAMP), the first second messenger to be described, plays a central role in cell signaling in a wide variety of cell types. Over the last decades, a wide body of literature addressed the different roles of cAMP in cell physiology, mainly in response to neurotransmitters and hormones. cAMP is synthesized by a wide variety of adenylyl cyclases that can generally be grouped in two types: transmembrane adenylyl cyclase and soluble adenylyl cyclases. In particular, several aspects of sperm physiology are regulated by cAMP produced by a single atypical adenylyl cyclase (Adcy10, aka sAC, SACY). The signature that identifies sAC among other ACs, is their direct stimulation by bicarbonate. The essential nature of cAMP in sperm function has been demonstrated using gain of function as well as loss of function approaches. This review unifies state of the art knowledge of the role of cAMP and those enzymes involved in cAMP signaling pathways required for the acquisition of fertilizing capacity of mammalian sperm. This article is part of a Special Issue entitled: The role of soluble adenylyl cyclase in health and disease.

Developmental expression of the calcium-activated chloride channels TMEM16A and TMEM16B in the mouse olfactory epithelium

Calcium-activated chloride channels are involved in several physiological processes including olfactory perception. TMEM16A and TMEM16B, members of the transmembrane protein 16 family (TMEM16), are responsible for calcium-activated chloride currents in several cells. Both are present in the olfactory epithelium of adult mice, but little is known about their expression during embryonic development. Using immunohistochemistry we studied their expression in the mouse olfactory epithelium at various stages of prenatal development from embryonic day (E) 12.5 to E18.5 as well as in postnatal mice. At E12.5, TMEM16A immunoreactivity was present at the apical surface of the entire olfactory epithelium, but from E16.5 became restricted to a region near the transition zone with the respiratory epithelium, where localized at the apical part of supporting cells and in their microvilli. In contrast, TMEM16B immunoreactivity was present at E14.5 at the apical surface of the entire olfactory epithelium, increased in subsequent days, and localized to the cilia of mature olfactory sensory neurons. These data suggest different functional roles for TMEM16A and TMEM16B in the developing as well as in the postnatal olfactory epithelium. The presence of TMEM16A at the apical part and in microvilli of supporting cells is consistent with a role in the regulation of the chloride ionic composition of the mucus covering the apical surface of the olfactory epithelium, whereas the localization of TMEM16B to the cilia of mature olfactory sensory neurons is consistent with a role in olfactory signal transduction.

Cyclic nucleotide-gated channel subunit glycosylation regulates matrix metalloproteinase-dependent changes in channel gating

Cyclic-nucleotide gated (CNG) channels are essential for phototransduction within retinal photoreceptors. We have demonstrated previously that the enzymatic activity of matrix metalloproteinase-2 and -9, members of the matrix metalloproteinase (MMP) family of extracellular, Ca(2+)- and Zn(2+)-dependent proteases, enhances the ligand sensitivity of both rod (CNGA1 and CNGB1) and cone (CNGA3 and CNGB3) CNG channels. Additionally, we have observed a decrease in the maximal CNG channel current (Imax) that begins late during MMP-directed gating changes. Here we demonstrate that CNG channels become nonconductive after prolonged MMP exposure. Concurrent with the loss of conductive channels is the increased relative contribution of channels exhibiting nonmodified gating properties, suggesting the presence of a subpopulation of channels that are protected from MMP-induced gating effects. CNGA subunits are known to possess one extracellular core glycosylation site, located at one of two possible positions within the turret loop near the pore-forming region. Our results indicate that CNGA glycosylation can impede MMP-dependent modification of CNG channels. Furthermore, the relative position of the glycosylation site within the pore turret influences the extent of MMP-dependent proteolysis. Glycosylation at the site found in CNGA3 subunits was found to be protective, while glycosylation at the bovine CNGA1 site was not. Relocating the glycosylation site in CNGA1 to the position found in CNGA3 recapitulated CNGA3-like protection from MMP-dependent processing. Taken together, these data indicate that CNGA glycosylation may protect CNG channels from MMP-dependent proteolysis, consistent with MMP modification of channel function having a requirement for physical access to the extracellular face of the channel.

PKA controls calcium influx into motor neurons during a rhythmic behavior

Cyclic adenosine monophosphate (cAMP) has been implicated in the execution of diverse rhythmic behaviors, but how cAMP functions in neurons to generate behavioral outputs remains unclear. During the defecation motor program in C. elegans, a peptide released from the pacemaker (the intestine) rhythmically excites the GABAergic neurons that control enteric muscle contractions by activating a G protein-coupled receptor (GPCR) signaling pathway that is dependent on cAMP. Here, we show that the C. elegans PKA catalytic subunit, KIN-1, is the sole cAMP target in this pathway and that PKA is essential for enteric muscle contractions. Genetic analysis using cell-specific expression of dominant negative or constitutively active PKA transgenes reveals that knockdown of PKA activity in the GABAergic neurons blocks enteric muscle contractions, whereas constitutive PKA activation restores enteric muscle contractions to mutants defective in the peptidergic signaling pathway. Using real-time, in vivo calcium imaging, we find that PKA activity in the GABAergic neurons is essential for the generation of synaptic calcium transients that drive GABA release. In addition, constitutively active PKA increases the duration of calcium transients and causes ectopic calcium transients that can trigger out-of-phase enteric muscle contractions. Finally, we show that the voltage-gated calcium channels UNC-2 and EGL-19, but not CCA-1 function downstream of PKA to promote enteric muscle contractions and rhythmic calcium influx in the GABAergic neurons. Thus, our results suggest that PKA activates neurons during a rhythmic behavior by promoting presynaptic calcium influx through specific voltage-gated calcium channels.

Breathing, walking and sleeping, are examples of rhythmic behaviors that occur at regular time intervals. The time intervals are determined by pacemakers, which generate the rhythms, and the behaviors are carried out by different tissues such as neurons and muscles. How do timing signals from pacemakers get delivered to target tissues to ensure proper execution of these behaviors? To begin to address this question, we study a simple rhythmic behavior in the nematode C. elegans called the defecation motor program. In this behavior, enteric muscles contract every 50 seconds, allowing digested food to be expelled from the gut. The pacemaker is the gut itself, and here we identify a specific protein, PKA, that responds to the signal from the pacemaker by activating certain neurons that trigger enteric muscle contraction. We further demonstrate that PKA activates these neurons by controlling the entry of calcium into these neurons. We also identify two calcium channels that allow calcium to enter the neurons when PKA is activated by the signal from the pacemaker. Our results raise the possibility that PKA-mediated calcium entry might be a mechanism used in other organisms to regulate rhythmic behaviors.

Role of cyclic nucleotide-gated channels in the modulation of mouse hippocampal neurogenesis

Neural stem cells generate neurons in the hippocampal dentate gyrus in mammals, including humans, throughout adulthood. Adult hippocampal neurogenesis has been the focus of many studies due to its relevance in processes such as learning and memory and its documented impairment in some neurodegenerative diseases. However, we are still far from having a complete picture of the mechanism regulating this process. Our study focused on the possible role of cyclic nucleotide-gated (CNG) channels. These voltage-independent channels activated by cyclic nucleotides, first described in retinal and olfactory receptors, have been receiving increasing attention for their involvement in several brain functions. Here we show that the rod-type, CNGA1, and olfactory-type, CNGA2, subunits are expressed in hippocampal neural stem cells in culture and in situ in the hippocampal neurogenic niche of adult mice. Pharmacological blockade of CNG channels did not affect cultured neural stem cell proliferation but reduced their differentiation towards the neuronal phenotype. The membrane permeant cGMP analogue, 8-Br-cGMP, enhanced neural stem cell differentiation to neurons and this effect was prevented by CNG channel blockade. In addition, patch-clamp recording from neuron-like differentiating neural stem cells revealed cGMP-activated currents attributable to ion flow through CNG channels. The current work provides novel insights into the role of CNG channels in promoting hippocampal neurogenesis, which may prove to be relevant for stem cell-based treatment of cognitive impairment and brain damage.

Cell- and subunit-specific mechanisms of CNG channel ciliary trafficking and localization in C. elegans

Primary cilia are ubiquitous sensory organelles that concentrate transmembrane signaling proteins essential for sensing environmental cues. Mislocalization of crucial ciliary signaling proteins, such as the tetrameric cyclic nucleotide-gated (CNG) channels, can lead to cellular dysfunction and disease. Although several cis- and trans-acting factors required for ciliary protein trafficking and localization have been identified, whether these mechanisms act in a protein- and cell-specific manner is largely unknown. Here, we show that CNG channel subunits can be localized to discrete ciliary compartments in individual sensory neurons in C. elegans, suggesting that channel composition is heterogeneous across the cilium. We demonstrate that ciliary localization of CNG channel subunits is interdependent on different channel subunits in specific cells, and identify sequences required for efficient ciliary targeting and localization of the TAX-2 CNGB and TAX-4 CNGA subunits. Using a candidate gene approach, we show that Inversin, transition zone proteins, intraflagellar transport motors and a MYND-domain protein are required to traffic and/or localize CNG channel subunits in both a cell- and channel subunit-specific manner. We further find that TAX-2 and TAX-4 are relatively immobile in specific sensory cilia subcompartments, suggesting that these proteins undergo minimal turnover in these domains in mature cilia. Our results uncover unexpected diversity in the mechanisms that traffic and localize CNG channel subunits to cilia both within and across cell types, highlighting the essential contribution of this process to cellular functions.

A cyclic nucleotide-gated channel (CNGC16) in pollen is critical for stress tolerance in pollen reproductive development

Cyclic nucleotide-gated channels (CNGCs) have been implicated in diverse aspects of plant growth and development, including responses to biotic and abiotic stress, as well as pollen tube growth and fertility. Here, genetic evidence identifies CNGC16 in Arabidopsis (Arabidopsis thaliana) as critical for pollen fertility under conditions of heat stress and drought. Two independent transfer DNA disruptions of cngc16 resulted in a greater than 10-fold stress-dependent reduction in pollen fitness and seed set. This phenotype was fully rescued through pollen expression of a CNGC16 transgene, indicating that cngc16-1 and 16-2 were both loss-of-function null alleles. The most stress-sensitive period for cngc16 pollen was during germination and the initiation of pollen tube tip growth. Pollen viability assays indicate that mutant pollen are also hypersensitive to external calcium chloride, a phenomenon analogous to calcium chloride hypersensitivities observed in other cngc mutants. A heat stress was found to increase concentrations of 3',5'-cyclic guanyl monophosphate in both pollen and leaves, as detected using an antibody-binding assay. A quantitative PCR analysis indicates that cngc16 mutant pollen have attenuated expression of several heat-stress response genes, including two heat shock transcription factor genes, HsfA2 and HsfB1. Together, these results provide evidence for a heat stress response pathway in pollen that connects a cyclic nucleotide signal, a Ca(2+)-permeable ion channel, and a signaling network that activates a downstream transcriptional heat shock response.

Calcium-activated chloride channels in the apical region of mouse vomeronasal sensory neurons

The rodent vomeronasal organ plays a crucial role in several social behaviors. Detection of pheromones or other emitted signaling molecules occurs in the dendritic microvilli of vomeronasal sensory neurons, where the binding of molecules to vomeronasal receptors leads to the influx of sodium and calcium ions mainly through the transient receptor potential canonical 2 (TRPC2) channel. To investigate the physiological role played by the increase in intracellular calcium concentration in the apical region of these neurons, we produced localized, rapid, and reproducible increases in calcium concentration with flash photolysis of caged calcium and measured calcium-activated currents with the whole cell voltage-clamp technique. On average, a large inward calcium-activated current of −261 pA was measured at −50 mV, rising with a time constant of 13 ms. Ion substitution experiments showed that this current is anion selective. Moreover, the chloride channel blockers niflumic acid and 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid partially inhibited the calcium-activated current. These results directly demonstrate that a large chloride current can be activated by calcium in the apical region of mouse vomeronasal sensory neurons. Furthermore, we showed by immunohistochemistry that the calcium-activated chloride channels TMEM16A/anoctamin1 and TMEM16B/anoctamin2 are present in the apical layer of the vomeronasal epithelium, where they largely colocalize with the TRPC2 transduction channel. Immunocytochemistry on isolated vomeronasal sensory neurons showed that TMEM16A and TMEM16B coexpress in the neuronal microvilli. Therefore, we conclude that microvilli of mouse vomeronasal sensory neurons have a high density of calcium-activated chloride channels that may play an important role in vomeronasal transduction.

A dynamical feedback model for adaptation in the olfactory transduction pathway

Olfactory transduction exhibits two distinct types of adaptation, which we denote multipulse and step adaptation. In terms of measured transduction current, multipulse adaptation appears as a decrease in the amplitude of the second of two consecutive responses when the olfactory neuron is stimulated with two brief pulses. Step adaptation occurs in response to a sustained steplike stimulation and is characterized by a return to a steady-state current amplitude close to the prestimulus value, after a transient peak. In this article, we formulate a dynamical model of the olfactory transduction pathway, which includes the kinetics of the CNG channels, the concentration of Ca ions flowing through them, and the Ca-complexes responsible for the regulation. Based on this model, a common dynamical explanation for the two types of adaptation is suggested. We show that both forms of adaptation can be well described using different time constants for the kinetics of Ca ions (faster) and the kinetics of the feedback mechanisms (slower). The model is validated on experimental data collected in voltage-clamp conditions using different techniques and animal species.

Expression of olfactory-type cyclic nucleotide-gated channels in rat cortical astrocytes

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels activated by cyclic AMP (cAMP) or cyclic GMP (cGMP). They were originally identified in retinal and olfactory receptors, but evidence has also emerged for their expression in several mammalian brain areas. Because cGMP and cAMP control important aspects of glial cell physiology, we wondered whether CNG channels are expressed in astrocytes, the most functionally relevant glial cells in the CNS. Immunoblot and immunofluorescence experiments demonstrated expression of the CNG channel olfactory-type A subunit, CNGA2, in cultured rat cortical astrocytes. In patch-clamp experiments, currents elicited in these cells by voltage ramps from -100 to +100 mV in the presence of the cGMP analogue, dB-cGMP, were significantly reduced by the CNG channel blockers, L-cis-diltiazem (LCD) and Cd(2+) . The reversal potentials of the LCD- and Cd(2+) -sensitive currents were more positive than that of K(+) , as expected for a mixed cation current. Noninactivating, voltage-independent currents were also elicited by extracellular application of the membrane permeant cGMP analogue, 8-Br-cGMP. These effects were blocked by LCD and were mimicked by natriuretic peptide receptor activation and inhibition of phosphodiesterase activity. Voltage-independent, LCD-sensitive currents were also elicited by 8-Br-cGMP in astrocytes of hippocampal and neocortical brain slices. Immunohistochemistry confirmed a broad distribution of CNG channels in astrocytes of the rat forebrain, midbrain, and hindbrain. These findings suggest that CNG channels are downstream targets of cyclic nucleotides in astrocytes, and they may be involved in the glial-mediated regulation of CNS functions under physiological and pathological conditions.

Matrix metalloproteinase-9 and -2 enhance the ligand sensitivity of photoreceptor cyclic nucleotide-gated channels

Photoreceptor cyclic nucleotide-gated (CNG) channels are the principal ion channels responsible for transduction of the light-induced change in cGMP concentration into an electrical signal. The ligand sensitivity of photoreceptor CNG channels is subject to regulation by intracellular signaling effectors, including calcium-calmodulin, tyrosine kinases and phosphoinositides. Little is known, however, about regulation of channel activity by modification to extracellular regions of CNG channel subunits. Extracellular proteases MMP9 and -2 are present in the interphotoreceptor matrix adjacent to photoreceptor outer segments. Given that MMPs have been implicated in retinal dysfunction and degeneration, we hypothesized that MMP activity may alter the functional properties of photoreceptor CNG channels. For heterologously expressed rod and cone CNG channels, extracellular exposure to MMPs dramatically increased the apparent affinity for cGMP and the efficacy of cAMP. These changes to ligand sensitivity were not prevented by destabilization of the actin cytoskeleton or by disruption of integrin mediated cell adhesion, but could be attenuated by inhibition of MMP catalytic activity. MMP-mediated gating changes exhibited saturable kinetic properties consistent with enzymatic processing of the CNG channels. In addition, exposure to MMPs decreased the abundance of full-length expressed CNGA3 subunits, with a concomitant increase in putative degradation products. Similar gating effects and apparent proteolysis were observed also for native rod photoreceptor CNG channels. Furthermore, constitutive apparent proteolysis of retinal CNGA1 and retinal MMP9 levels were both elevated in aged mice compared with young mice. Together, these results provide evidence that MMP-mediated proteolysis can regulate the ligand sensitivity of CNG channels.

Peripheral regulation by ecdysteroids of olfactory responsiveness in male Egyptian cotton leaf worms, Spodoptera littoralis

Physiological and behavioral plasticity allows animals to adapt to changes in external (environmental) and internal (physiological) factors. In insects, the physiological state modulates adult behavior in response to different odorant stimuli. Hormones have the potential to play a major role in the plasticity of the olfactory responses. To explore if peripheral olfactory processing could be regulated by steroid hormones, we characterized the molecular, electrophysiological, and behavioral response to changes in endogenous hormone levels in adult male Spodoptera littoralis. The expression of the receptor complex (EcR/USP) was localized by in situ hybridization in the olfactory sensilla of antennae. Injections of 20-hydroxyecdysone (20E) induced an ecdysteroid signaling pathway in antennae and increased expression of the nuclear receptors EcR, USP and E75. Diacylglycerol kinase (DGK) and CaM expression were also up-regulated by 20E. Taken together, these molecular, electrophysiological, and behavioral results suggest a hormonal regulation of the peripheral olfactory processing in S. littoralis.

Flash photolysis of caged compounds in the cilia of olfactory sensory neurons

Photolysis of caged compounds allows the production of rapid and localized increases in the concentration of various physiologically active compounds. Caged compounds are molecules made physiologically inactive by a chemical cage that can be broken by a flash of ultraviolet light. Here, we show how to obtain patch-clamp recordings combined with photolysis of caged compounds for the study of olfactory transduction in dissociated mouse olfactory sensory neurons. The process of olfactory transduction (Figure 1) takes place in the cilia of olfactory sensory neurons, where odorant binding to receptors leads to the increase of cAMP that opens cyclic nucleotide-gated (CNG) channels. Ca entry through CNG channels activates Ca-activated Cl channels. We show how to dissociate neurons from the mouse olfactory epithelium and how to activate CNG channels or Ca-activated Cl channels by photolysis of caged cAMP or caged Ca. We use a flash lamp to apply ultraviolet flashes to the ciliary region to uncage cAMP or Ca while patch-clamp recordings are taken to measure the current in the whole-cell voltage-clamp configuration.

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca2+ with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca2+ channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca2+ and SLO3 for K+, which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

Distinct binding properties distinguish LQ-type calmodulin-binding domains in cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels operate as transduction channels in photoreceptors and olfactory receptor neurons. Direct binding of cGMP or cAMP opens these channels which conduct a mixture of monovalent cations and Ca(2+). Upon activation, CNG channels generate intracellular Ca(2+) signals that play pivotal roles in the transduction cascades of the visual and olfactory systems. Channel activity is controlled by negative feedback mechanisms that involve Ca(2+)-calmodulin, for which all CNG channels possess binding sites. Here we compare the binding properties of the two LQ-type calmodulin binding sites, both of which are thought to be involved in channel regulation. They reside on the isoforms CNGB1 and CNGA4. The CNGB1 subunit is present in rod photoreceptors and olfactory receptor neurons. The CNGA4 subunit is only expressed in olfactory receptor neurons, and there are conflicting results as to its role in calmodulin-mediated feedback inhibition. We examined the interaction of Ca(2+)-calmodulin with two recombinant proteins that encompass either of the two LQ sites. Comparing binding properties, we found that the LQ site of CNGB1 binds Ca(2+)-calmodulin at 10-fold lower Ca(2+) levels than the LQ site of CNGA4. Our data provide biochemical evidence against a contribution of CNGA4 to feedback inhibition. In accordance with previous work on photoreceptor CNG channels, our results indicate that feedback control is the exclusive role of the B-subunits in photoreceptors and olfactory receptor neurons.