Functional role of lipid raft microdomains in cyclic nucleotide-gated channel activation

Effects of the Molecular Structure of Malodor Substances and Their Masking on 1,2-Dioleoyl-sn-glycero-3-phosphocholine Molecular Layers

Certain odors have been shown not only to cause health problems and stress but also to affect skin barrier function. Therefore, it is important to understand olfactory masking to develop effective fragrances to mask malodors. However, olfaction and olfactory masking mechanisms are not yet fully understood. To understand the mechanism of the masking effect that has been studied, the responses of several target substance (TS) molecules-1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) mixed molecular layers to odorant (OD) molecules were examined as a simple experimental model of epithelial cellular membranes injured by TS molecules. Here, we examined trans-2-nonenal, 1-nonanal, trans-2-decenal, and 1-decanal as TS molecules to clarify the effects of double bonds and hydrocarbon chain lengths on the phospholipid molecular layer. In addition, benzaldehyde and cyclohexanecarboxaldehyde were utilized as OD molecules to clarify the masking effect of the aromatic ring. Surface pressure (Π)-area (A) isotherms were measured to clarify the adsorption or desorption of TS and OD molecules on the DOPC molecular layer. In addition, Fourier transform infrared spectroscopy was performed to clarify the interactions among DOPC, TS, and OD molecules. We found that TS molecules with and without double bonds had different effects on the DOPC molecular layer and that molecules with shorter chain lengths had greater effects on the DOPC molecular layer. Furthermore, OD molecules with aromatic rings counteracted the effects of the TS molecules. On the basis of this research, not only a detailed mechanism by which odor molecules affect lipid membranes without mediating olfactory receptors is elucidated but also more effective OD molecules with masking effects are proposed.

Dietary n-3 polyunsaturated fatty acid deficiency alters olfactory mucosa sensitivity in young mice but has no impact on olfactory behavior

BACKGROUND AND OBJECTIVE

We recently showed that perinatal exposure to diets with unbalanced n-6:n-3 polyunsaturated fatty acid (PUFA) ratios affects the olfactory mucosa (OM) fatty acid composition. To assess the repercussions of these modifications, we investigated the impact of diets unbalanced in n-3 PUFAs on the molecular composition and functionality of the OM in young mice.

METHODS

After mating, female mice were fed diets either deficient in α-linolenic acid (LOW diet) or supplemented with n-3 long-chain PUFAs (HIGH diet) during the perinatal period. Weaned male offspring were then fed ad libitum with the same experimental diets for 5 weeks. At 8 weeks of age, olfactory behavior tests were performed in young mice. The fatty acid composition of OM and olfactory cilia, as well as the expression of genes involved in different cellular pathways, were analyzed. The electroolfactograms induced by odorant stimuli were recorded to assess the impact of diets on OM functionality.

RESULTS AND CONCLUSION

Both diets significantly modified the fatty acid profiles of OM and olfactory cilia in young mice. They also induced changes in the expression of genes involved in olfactory signaling and in olfactory neuron maturation. The electroolfactogram amplitudes were reduced in mice fed the LOW diet. Nevertheless, the LOW diet and the HIGH diet did not affect mouse olfactory behavior. Our study demonstrated that consumption of diets deficient in or supplemented with n-3 PUFAs during the perinatal and postweaning periods caused significant changes in young mouse OM. However, these modifications did not impair their olfactory capacities.

Physiological roles of mammalian transmembrane adenylyl cyclase isoforms

Adenylyl cyclases (ACs) catalyze the conversion of ATP to the ubiquitous second messenger cAMP. Mammals possess nine isoforms of transmembrane ACs, dubbed AC1-9, that serve as major effector enzymes of G protein-coupled receptors (GPCRs). The transmembrane ACs display varying expression patterns across tissues, giving the potential for them to have a wide array of physiological roles. Cells express multiple AC isoforms, implying that ACs have redundant functions. Furthermore, all transmembrane ACs are activated by Gαs, so it was long assumed that all ACs are activated by Gαs-coupled GPCRs. AC isoforms partition to different microdomains of the plasma membrane and form prearranged signaling complexes with specific GPCRs that contribute to cAMP signaling compartments. This compartmentation allows for a diversity of cellular and physiological responses by enabling unique signaling events to be triggered by different pools of cAMP. Isoform-specific pharmacological activators or inhibitors are lacking for most ACs, making knockdown and overexpression the primary tools for examining the physiological roles of a given isoform. Much progress has been made in understanding the physiological effects mediated through individual transmembrane ACs. GPCR-AC-cAMP signaling pathways play significant roles in regulating functions of every cell and tissue, so understanding each AC isoform's role holds potential for uncovering new approaches for treating a vast array of pathophysiological conditions.

INPP5E controls ciliary localization of phospholipids and the odor response in olfactory sensory neurons

The lipid composition of the primary cilia membrane is emerging as a critical regulator of cilia formation, maintenance and function. Here, we show that conditional deletion of the phosphoinositide 5′-phosphatase gene Inpp5e, mutation of which is causative of Joubert syndrome, in terminally developed mouse olfactory sensory neurons (OSNs), leads to a dramatic remodeling of ciliary phospholipids that is accompanied by marked elongation of cilia. Phosphatidylinositol (4,5)-bisphosphate [PI(4,5)P₂], which is normally restricted to the proximal segment redistributed to the entire length of cilia in Inpp5e knockout mice with a reduction in phosphatidylinositol (3,4)-bisphosphate [PI(3,4)P₂] and elevation of phosphatidylinositol (3,4,5)-trisphosphate [PI(3,4,5)P₃] in the dendritic knob. The redistribution of phosphoinositides impaired odor adaptation, resulting in less efficient recovery and altered inactivation kinetics of the odor-evoked electrical response and the odor-induced elevation of cytoplasmic Ca²⁺. Gene replacement of Inpp5e through adenoviral expression restored the ciliary localization of PI(4,5)P₂ and odor response kinetics in OSNs. Our findings support the role of phosphoinositides as a modulator of the odor response and in ciliary biology of native multi-ciliated OSNs.

Summary: Cilia of olfactory sensory neurons have a unique lipid composition. Localization of phospholipids is controlled by the INPP5E phosphatase and is involved in modulation of the odor response.

Effects of trans-2-nonenal and olfactory masking odorants on proliferation of human keratinocytes

Malodorous compounds induce stress responses, mood changes, an increase of skin conductance, activation of the sympathetic nervous system and other physiological changes, and it has been suggested that sensing malodors could provide warning of danger to health. Furthermore, the human body secretes various malodorous compounds as waste products of metabolism, including trans-2-nonenal ((E)-2-nonenal), the amount of which increases with aging. In the present study, we examined the effects of some endogenous malodorous compounds ((E)-2-nonenal, nonanal, pentanal, hexanal, hexanoic acid, hexylamine and isovaleric acid) on cultured human keratinocytes. (E)-2-Nonenal decreased the viability and promoted apoptosis of cultured keratinocytes. It also reduced the thickness and the number of proliferative cells in a three-dimensional epidermal equivalent model. Co-application of masking odorants (dihydromycenol, benzaldehyde, linalool, phenethyl alcohol, benzyl acetate and anisaldehyde), but not non-masking odorants (1,8-cineol, β-damascone, and o-t-butylcyclohexyl acetate), reduced the effect of (E)-2-nonenal on keratinocyte proliferation, and restored the thickness and number of proliferative cells in a three-dimensional epidermal equivalent model.

The Enigmatic Role of Lipids in Cilia Signaling

Primary cilia are specialized cellular structures that project from the surface of most cell types in metazoans and mediate transduction of major signaling pathways. The ciliary membrane is contiguous with the plasma membrane, yet it exhibits distinct protein and lipid composition, which is essential for ciliary function. Diffusion barriers at the base of a cilium are responsible for establishing unique molecular composition of this organelle. Although considerable progress has been made in identifying mechanisms of ciliary protein trafficking in and out of cilia, it remains largely unknown how the distinct lipid identity of the ciliary membrane is achieved. In this mini review, I summarize recent developments in characterizing lipid composition and organization of the ciliary membrane and discuss the emerging roles of lipids in modulating activity of ciliary signaling components including ion channels and G protein-coupled receptors.

Axelrod Symposium 2019: Phosphoproteomic Analysis of G-Protein-Coupled Pathways

By limiting unrestricted activation of intracellular effectors, compartmentalized signaling of cyclic nucleotides confers specificity to extracellular stimuli and is critical for the development and health of cells and organisms. Dissecting the molecular mechanisms that allow local control of cyclic nucleotide signaling is essential for our understanding of physiology and pathophysiology, but mapping the dynamics and regulation of compartmentalized signaling is a challenge. In this minireview we summarize advanced imaging and proteomics techniques that have been successfully used to probe compartmentalized cAMP signaling in eukaryotic cells. Subcellularly targeted fluorescence resonance energy transfer sensors can precisely locate and measure compartmentalized cAMP, and this allows us to estimate the range of effector activation. Because cAMP effector proteins often cluster together with their targets and cAMP regulatory proteins to form discrete cAMP signalosomes, proteomics and phosphoproteomics analysis have more recently been used to identify additional players in the cAMP-signaling cascade. We propose that the synergistic use of the techniques discussed could prove fruitful in generating a detailed map of cAMP signalosomes and reveal new details of compartmentalized signaling. Compiling a dynamic map of cAMP nanodomains in defined cell types would establish a blueprint for better understanding the alteration of signaling compartments associated with disease and would provide a molecular basis for targeted therapeutic strategies. SIGNIFICANCE STATEMENT: cAMP signaling is compartmentalized. Some functionally important cellular signaling compartments operate on a nanometer scale, and their integrity is essential to maintain cellular function and appropriate responses to extracellular stimuli. Compartmentalized signaling provides an opportunity for precision medicine interventions. Our detailed understanding of the composition, function, and regulation of cAMP-signaling nanodomains in health and disease is essential and will benefit from harnessing the right combination of advanced biochemical and imaging techniques.

Local and Relayed Effects of Deep Brain Stimulation of the Pedunculopontine Nucleus

Our discovery of low-threshold stimulation-induced locomotion in the pedunculopontine nucleus (PPN) led to the clinical use of deep brain stimulation (DBS) for the treatment of disorders such as Parkinson's disease (PD) that manifest gait and postural disorders. Three additional major discoveries on the properties of PPN neurons have opened new areas of research for the treatment of motor and arousal disorders. The description of (a) electrical coupling, (b) intrinsic gamma oscillations, and (c) gene regulation in the PPN has identified a number of novel therapeutic targets and methods for the treatment of a number of neurological and psychiatric disorders. We first delve into the circuit, cellular, intracellular, and molecular organization of the PPN, and then consider the clinical results to date on PPN DBS. This comprehensive review will provide valuable information to explain the network effects of PPN DBS, point to new directions for treatment, and highlight a number of issues related to PPN DBS.

Bottom-up gamma maintenance in various disorders

Maintained gamma band activity is a key element of higher brain function, participating in perception, executive function, and memory. The pedunculopontine nucleus (PPN), as part of the reticular activating system (RAS), is a major source of the "bottom-up" flow of gamma activity to higher regions. However, interruption of gamma band activity is associated with a number of neurological and psychiatric disorders. This review will focus on the role of the PPN in activating higher regions to induce arousal and descending pathways to modulate posture and locomotion. As such, PPN deep brain stimulation (DBS) can not only help regulate arousal and stepping, but continuous application may help maintain necessary levels of gamma band activity for a host of other brain processes. We will explore the potential future applications of PPN DBS for a number of disorders that are characterized by disturbances in gamma band maintenance.

Arousal and drug abuse

The reticular activating system (RAS) is not an amorphous region but distinct nuclei with specific membrane properties that dictate their firing during waking and sleep. The locus coeruleus and raphe nucleus fire during waking and slow wave sleep, with the pedunculopontine nucleus (PPN) firing during both waking and REM sleep, the states manifesting arousal-related EEG activity. Two important discoveries in the PPN in the last 10 years are, 1) that some PPN cells are electrically coupled, and 2) every PPN cell manifests high threshold calcium channels that allow them to oscillate at beta/gamma band frequencies. The role of arousal in drug abuse is considered here in terms of the effects of drugs of abuse on these two mechanisms. Drug abuse and the perception of withdrawal/relapse are mediated by neurobiological processes that occur only when we are awake, not when we are asleep. These relationships focus on the potential role of arousal, more specifically of RAS electrical coupling and gamma band activity, in the addictive process as well as the relapse to drug use.

Intramembrane congestion effects on lysenin channel voltage-induced gating

All cell membranes are packed with proteins. The ability to investigate the regulatory mechanisms of protein channels in experimental conditions mimicking their congested native environment is crucial for understanding the environmental physicochemical cues that may fundamentally contribute to their functionality in natural membranes. Here we report on investigations of the voltage-induced gating of lysenin channels in congested conditions experimentally achieved by increasing the number of channels inserted into planar lipid membranes. Typical electrophysiology measurements reveal congestion-induced changes to the voltage-induced gating, manifested as a significant reduction of the response to external voltage stimuli. Furthermore, we demonstrate a similar diminished voltage sensitivity for smaller populations of channels by reducing the amount of sphingomyelin in the membrane. Given lysenin's preference for targeting lipid rafts, this result indicates the potential role of the heterogeneous organization of the membrane in modulating channel functionality. Our work indicates that local congestion within membranes may alter the energy landscape and the kinetics of conformational changes of lysenin channels in response to voltage stimuli. This level of understanding may be extended to better characterize the role of the specific membrane environment in modulating the biological functionality of protein channels in health and disease.

Store-operated Ca²⁺-entry and adenylyl cyclase

One of the longest-standing effects of SOCE is in its selective regulation of Ca(2+)-sensitive adenylyl cyclase (AC) activity in non-excitable cells. Remarkably it was this source of Ca(2+) (SOCE) rather than the apparent magnitude of the Ca(2+)-rise that conferred AC responsiveness. The molecular basis for this dependence is now resolved in the case of adenylyl cyclase 8 (AC8). Sensors for Ca(2+) and cAMP targeted to ACs have been particularly useful in dissecting the influences upon and composition of what turn out to be signalling microdomains centred on ACs. A number of physiological processes depend on the regulation by SOCE of ACs, but the issue is under-studied. Here I will expand on these topics and point to some immediate unresolved questions.

Adenylate cyclase-centred microdomains

Recent advances in the AC (adenylate cyclase)/cAMP field reveal overarching roles for the ACs. Whereas few processes are unaffected by cAMP in eukaryotes, ranging from the rapid modulation of ion channel kinetics to the slowest developmental effects, the large number of cellular processes modulated by only three intermediaries, i.e. PKA (protein kinase A), Epacs (exchange proteins directly activated by cAMP) and CNG (cyclic nucleotide-gated) channels, poses the question of how selectivity and fine control is achieved by cAMP. One answer rests on the number of differently regulated and distinctly expressed AC species. Specific ACs are implicated in processes such as insulin secretion, immunological responses, sino-atrial node pulsatility and memory formation, and specific ACs are linked with particular diseased conditions or predispositions, such as cystic fibrosis, Type 2 diabetes and dysrhythmias. However, much of the selectivity and control exerted by cAMP lies in the sophisticated properties of individual ACs, in terms of their coincident responsiveness, dynamic protein scaffolding and organization of cellular microassemblies. The ACs appear to be the centre of highly organized microdomains, where both cAMP and Ca2+, the other major influence on ACs, change in patterns quite discrete from the broad cellular milieu. How these microdomains are organized is beginning to become clear, so that ACs may now be viewed as fundamental signalling centres, whose properties exceed their production of cAMP. In the present review, we summarize how ACs are multiply regulated and the steps that are put in place to ensure discrimination in their signalling. This includes scaffolding of targets and modulators by the ACs and assembling of signalling nexuses in discrete cellular domains. We also stress how these assemblies are cell-specific, context-specific and dynamic, and may be best addressed by targeted biosensors. These perspectives on the organization of ACs uncover new strategies for intervention in systems mediated by cAMP, which promise far more informed specificity than traditional approaches.

Localization and proteomic characterization of cholesterol-rich membrane microdomains in the inner ear

Biological membranes organize and compartmentalize cell signaling into discrete microdomains, a process that often involves stable, cholesterol-rich platforms that facilitate protein-protein interactions. Polarized cells with distinct apical and basolateral cell processes rely on such compartmentalization to maintain proper function. In the cochlea, a variety of highly polarized sensory and non-sensory cells are responsible for the early stages of sound processing in the ear, yet little is known about the mechanisms that traffic and organize signaling complexes within these cells. We sought to determine the prevalence, localization, and protein composition of cholesterol-rich lipid microdomains in the cochlea. Lipid raft components, including the scaffolding protein caveolin and the ganglioside GM1, were found in sensory, neural, and glial cells. Mass spectrometry of detergent-resistant membrane (DRM) fractions revealed over 600 putative raft proteins associated with subcellular localization, trafficking, and metabolism. Among the DRM constituents were several proteins involved in human forms of deafness including those involved in ion homeostasis, such as the potassium channel KCNQ1, the co-transporter SLC12A2, and gap junction proteins GJA1 and GJB6. The presence of caveolin in the cochlea and the abundance of proteins in cholesterol-rich DRM suggest that lipid microdomains play a significant role in cochlear physiology.

BIOLOGICAL SIGNIFICANCE

Although mechanisms underlying cholesterol synthesis, homeostasis, and compartmentalization in the ear are poorly understood, there are several lines of evidence indicating that cholesterol is a key modulator of cochlear function. Depletion of cholesterol in mature sensory cells alters calcium signaling, changes excitability during development, and affects the biomechanical processes in outer hair cells that are responsible for hearing acuity. More recently, we have established that the cholesterol-modulator beta-cyclodextrin is capable of inducing significant and permanent hearing loss when delivered subcutaneously at high doses. We hypothesize that proteins involved in cochlear homeostasis and otopathology are partitioned into cholesterol-rich domains. The results of a large-scale proteomic analysis point to metabolic processes, scaffolding/trafficking, and ion homeostasis as particularly associated with cholesterol microdomains. These data offer insight into the proteins and protein families that may underlie cholesterol-mediated effects in sensory cell excitability and cyclodextrin ototoxicity.

Primary cilia and dendritic spines: different but similar signaling compartments

Primary non-motile cilia and dendritic spines are cellular compartments that are specialized to sense and transduce environmental cues and presynaptic signals, respectively. Despite their unique cellular roles, both compartments exhibit remarkable parallels in the general principles, as well as molecular mechanisms, by which their protein composition, membrane domain architecture, cellular interactions, and structural and functional plasticity are regulated. We compare and contrast the pathways required for the generation and function of cilia and dendritic spines, and suggest that insights from the study of one may inform investigations into the other of these critically important signaling structures.

Identification of cyclic GMP-activated nonselective Ca2+-permeable cation channels and associated CNGC5 and CNGC6 genes in Arabidopsis guard cells

Cytosolic Ca(2+) in guard cells plays an important role in stomatal movement responses to environmental stimuli. These cytosolic Ca(2+) increases result from Ca(2+) influx through Ca(2+)-permeable channels in the plasma membrane and Ca(2+) release from intracellular organelles in guard cells. However, the genes encoding defined plasma membrane Ca(2+)-permeable channel activity remain unknown in guard cells and, with some exceptions, largely unknown in higher plant cells. Here, we report the identification of two Arabidopsis (Arabidopsis thaliana) cation channel genes, CNGC5 and CNGC6, that are highly expressed in guard cells. Cytosolic application of cyclic GMP (cGMP) and extracellularly applied membrane-permeable 8-Bromoguanosine 3',5'-cyclic monophosphate-cGMP both activated hyperpolarization-induced inward-conducting currents in wild-type guard cells using Mg(2+) as the main charge carrier. The cGMP-activated currents were strongly blocked by lanthanum and gadolinium and also conducted Ba(2+), Ca(2+), and Na(+) ions. cngc5 cngc6 double mutant guard cells exhibited dramatically impaired cGMP-activated currents. In contrast, mutations in CNGC1, CNGC2, and CNGC20 did not disrupt these cGMP-activated currents. The yellow fluorescent protein-CNGC5 and yellow fluorescent protein-CNGC6 proteins localize in the cell periphery. Cyclic AMP activated modest inward currents in both wild-type and cngc5cngc6 mutant guard cells. Moreover, cngc5 cngc6 double mutant guard cells exhibited functional abscisic acid (ABA)-activated hyperpolarization-dependent Ca(2+)-permeable cation channel currents, intact ABA-induced stomatal closing responses, and whole-plant stomatal conductance responses to darkness and changes in CO2 concentration. Furthermore, cGMP-activated currents remained intact in the growth controlled by abscisic acid2 and abscisic acid insensitive1 mutants. This research demonstrates that the CNGC5 and CNGC6 genes encode unique cGMP-activated nonselective Ca(2+)-permeable cation channels in the plasma membrane of Arabidopsis guard cells.

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.

Lipid raft integrity affects GABAA receptor, but not NMDA receptor modulation by psychopharmacological compounds

Lipid rafts have been shown to play an important role for G-protein mediated signal transduction and the function of ligand-gated ion channels including their modulation by psychopharmacological compounds. In this study, we investigated the functional significance of the membrane distribution of NMDA and GABAA receptor subunits in relation to the accumulation of the tricyclic antidepressant desipramine (DMI) and the benzodiazepine diazepam (Diaz). In the presence of Triton X-100, which allowed proper separation of the lipid raft marker proteins caveolin-1 and flotillin-1 from the transferrin receptor, all receptor subunits were shifted to the non-raft fractions. In contrast, under detergent-free conditions, NMDA and GABAA receptor subunits were detected both in raft and non-raft fractions. Diaz was enriched in non-raft fractions without Triton X-100 in contrast to DMI, which preferentially accumulated in lipid rafts. Impairment of lipid raft integrity by methyl-β-cyclodextrine (MβCD)-induced cholesterol depletion did not change the inhibitory effect of DMI at the NMDA receptor, whereas it enhanced the potentiating effect of Diaz at the GABAA receptor at non-saturating concentrations of GABA. These results support the hypothesis that the interaction of benzodiazepines with the GABAA receptor likely occurs outside of lipid rafts while the antidepressant DMI acts on ionotropic receptors both within and outside these membrane microdomains.

A-kinase anchoring proteins regulate compartmentalized cAMP signaling in airway smooth muscle

A-kinase anchoring proteins (AKAPs) have emerged as important regulatory molecules that can compartmentalize cAMP signaling transduced by β2-adrenergic receptors (β(2)ARs); such compartmentalization ensures speed and fidelity of cAMP signaling and effects on cell function. This study aimed to assess the role of AKAPs in regulating global and compartmentalized β(2)AR signaling in human airway smooth muscle (ASM). Transcriptome and proteomic analyses were used to characterize AKAP expression in ASM. Stable expression or injection of peptides AKAP-IS or Ht31 was used to disrupt AKAP-PKA interactions, and global and compartmentalized cAMP accumulation stimulated by β-agonist was assessed by radioimmunoassay and membrane-delineated flow through cyclic nucleotide-gated channels, respectively. ASM expresses multiple AKAP family members, with gravin and ezrin among the most readily detected. AKAP-PKA disruption had minimal effects on whole-cell cAMP accumulation stimulated by β-agonist (EC(50) and B(max)) concentrations, but significantly increased the duration of plasma membrane-delineated cAMP (τ=251±51 s for scrambled peptide control vs. 399±79 s for Ht31). Direct PKA inhibition eliminated decay of membrane-delineated cAMP levels. AKAPs coordinate compartmentalized cAMP signaling in ASM cells by regulating multiple elements of β(2)AR-mediated cAMP accumulation, thereby representing a novel target for manipulating β(2)AR signaling and function in ASM.

Thy1 associates with the cation channel subunit HCN4 in adult rat retina

PURPOSE

The membrane expression and gene promoter of the glycosylphosphatidylinositol (GPI)-anchored protein Thy1 have been widely used to examine the morphology and distribution of retinal ganglion cells in normal eyes and disease models. However, it is not known how adult mammalian retinal neurons use Thy1. Because Thy1 is not a membrane-spanning protein and, instead, complexes with structural and signaling proteins in other tissues, the aim of this study was to find protein partners of retinal Thy1.

METHODS

Coimmunoprecipitation, immunohistochemistry, confocal imaging, and patch-clamp recording were used to test for association of Thy1 and HCN4, a cation channel subunit, in adult rat retina.

RESULTS

Hyperpolarization of cells immunopanned by an anti-Thy1 antibody activated HCN channels. Confocal imaging showed that individual somata in the ganglion cell layer bound antibodies against Thy1 and HCN4, that the majority of these bindings colocalized, and that some of the immunopositive cells also bound antibody against a ganglion cell marker (Brn3a). Consistent with these results, Thy1 and HCN4 were coimmunoprecipitated by magnetic beads coated with either anti-Thy1 antibody or anti-HCN4 antibody. In control experiments, beads coated with these antibodies did not immunoprecipitate a photoreceptor rim protein (ABCR) and uncoated beads did not immunoprecipitate either Thy1 or HCN4.

CONCLUSIONS

This is the first report that Thy1 colocalizes and coimmunoprecipitates with a membrane-spanning protein in retina, that Thy1 complexes with an ion channel protein in any tissue, and that a GPI-anchored protein associates with an HCN channel subunit protein.

Cholesterol influences voltage-gated calcium channels and BK-type potassium channels in auditory hair cells

The influence of membrane cholesterol content on a variety of ion channel conductances in numerous cell models has been shown, but studies exploring its role in auditory hair cell physiology are scarce. Recent evidence shows that cholesterol depletion affects outer hair cell electromotility and the voltage-gated potassium currents underlying tall hair cell development, but the effects of cholesterol on the major ionic currents governing auditory hair cell excitability are unknown. We investigated the effects of a cholesterol-depleting agent (methyl beta cyclodextrin, MβCD) on ion channels necessary for the early stages of sound processing. Large-conductance BK-type potassium channels underlie temporal processing and open in a voltage- and calcium-dependent manner. Voltage-gated calcium channels (VGCCs) are responsible for calcium-dependent exocytosis and synaptic transmission to the auditory nerve. Our results demonstrate that cholesterol depletion reduced peak steady-state calcium-sensitive (BK-type) potassium current by 50% in chick cochlear hair cells. In contrast, MβCD treatment increased peak inward calcium current (~30%), ruling out loss of calcium channel expression or function as a cause of reduced calcium-sensitive outward current. Changes in maximal conductance indicated a direct impact of cholesterol on channel number or unitary conductance. Immunoblotting following sucrose-gradient ultracentrifugation revealed BK expression in cholesterol-enriched microdomains. Both direct impacts of cholesterol on channel biophysics, as well as channel localization in the membrane, may contribute to the influence of cholesterol on hair cell physiology. Our results reveal a new role for cholesterol in the regulation of auditory calcium and calcium-activated potassium channels and add to the growing evidence that cholesterol is a key determinant in auditory physiology.

A2B adenosine receptors inhibit superoxide production from mitochondrial complex I in rabbit cardiomyocytes via a mechanism sensitive to Pertussis toxin

BACKGROUND AND PURPOSE

A(2B) adenosine receptors protect against ischaemia/reperfusion injury by activating survival kinases including extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K). However, the underlying mechanism(s) and signalling pathway(s) remain undefined.

EXPERIMENTAL APPROACH

HEK 293 cells stably transfected with human A(2B) adenosine receptors (HEK-A(2B) ) and isolated adult rabbit cardiomyocytes were used to assay phosphorylation of ERK by Western blot and cation flux through cAMP-gated channels by patch clamp methods. Generation of reactive oxygen species (ROS) by mitochondria was measured with a fluorescent dye.

KEY RESULTS

In HEK-A(2B) cells, the selective A(2B) receptor agonist Bay 60-6583 (Bay 60) increased ERK phosphorylation and cAMP levels, detected by current through cAMP-gated ion channels. However, increased cAMP or its downstream target protein kinase A was not involved in ERK phosphorylation. Pertussis toxin (PTX) blocked ERK phosphorylation, suggesting receptor coupling to G(i) or G(o) proteins. Phosphorylation was also blocked by inhibition of PI3K (with wortmannin) or of ERK kinase (MEK1/2, with PD 98059) but not by inhibition of NO synthase (NOS). In cardiomyocytes, Bay 60 did not affect cAMP levels but did block the increased superoxide generation induced by rotenone, a mitochondrial complex I inhibitor. This effect of Bay 60 was inhibited by PD 98059, wortmannin or PTX. Inhibition of NOS blocked superoxide production because NOS is downstream of ERK.

CONCLUSION AND IMPLICATIONS

Activation of A(2B) adenosine receptors reduced superoxide generation from mitochondrial complex I through G(i/o) , ERK, PI3K, and NOS, all of which have been implicated in ischaemic preconditioning.

Differential subcellular distribution of rat brain dopamine receptors and subtype-specific redistribution induced by cocaine

We investigated the subcellular distribution of dopamine D(1), D(2) and D(5) receptor subtypes in rat frontal cortex, and examined whether psychostimulant-induced elevation of synaptic dopamine could alter the receptor distribution. Differential detergent solubilization and density gradient centrifugation were used to separate various subcellular fractions, followed by semi-quantitative determination of the relative abundance of specific receptor proteins in each fraction. D(1) receptors were predominantly localized to detergent-resistant membranes, and a portion of these receptors also floated on sucrose gradients. These properties are characteristic of proteins found in lipid rafts and caveolae. D(2) receptors exhibited variable distribution between cytoplasmic, detergent-soluble and detergent-resistant membrane fractions, yet were not present in buoyant membranes. Most D(5) receptor immunoreactivity was distributed into the cytoplasmic fraction, failing to sediment at forces up to 300,000g, while the remainder was localized to detergent-soluble membranes in cortex. D(5) receptors were undetectable in detergent-resistant fractions or raft-like subdomains. Following daily cocaine administration for seven days, a significant portion of D(1) receptors translocated from detergent-resistant membranes to detergent-soluble membranes and the cytoplasmic fraction. The distributions of D(5) and D(2) receptor subtypes were not significantly altered by cocaine treatment. These data imply that D(5) receptors are predominantly cytoplasmic, D(2) receptors are diffusely distributed within the cell, whereas D(1) receptors are mostly localized to lipid rafts within the rat frontal cortex. Dopamine receptor subtype localization is susceptible to modulation by pharmacological manipulations that elevate synaptic dopamine, however the functional implications of such drug-induced receptor warrant further investigation.

Influence of hydrophobic mismatch and amino acid composition on the lateral diffusion of transmembrane peptides

We investigated the effect of amino acid composition and hydrophobic length of alpha-helical transmembrane peptides and the role of electrostatic interactions on the lateral diffusion of the peptides in lipid membranes. Model peptides of varying length and composition, and either tryptophans or lysines as flanking residues, were synthesized. The peptides were labeled with the fluorescent label Alexa Fluor 488 and incorporated into phospholipid bilayers of different hydrophobic thickness and composition. Giant unilamellar vesicles were formed by electroformation, and the lateral diffusion of the transmembrane peptides (and lipids) was determined by fluorescence correlation spectroscopy. In addition, we performed coarse-grained molecular-dynamics simulations of single peptides of different hydrophobic lengths embedded in planar membranes of different thicknesses. Both the experimental and simulation results indicate that lateral diffusion is sensitive to membrane thickness between the peptides and surrounding lipids. We did not observe a difference in the lateral diffusion of the peptides with respect to the presence of tryptophans or lysines as flanking residues. The specific lipid headgroup composition of the membrane has a much less pronounced impact on the diffusion of the peptides than does the hydrophobic thickness.

Stimulation of apical Cl⁻/HCO₃⁻(OH⁻) exchanger, SLC26A3 by neuropeptide Y is lipid raft dependent

Neuropeptide Y (NPY), an important proabsorptive hormone of the gastrointestinal tract has been shown to inhibit chloride secretion and stimulate NaCl absorption. However, mechanisms underlying the proabsorptive effects of NPY are not fully understood. The present studies were designed to examine the direct effects of NPY on apical Cl⁻/HCO₃⁻(OH⁻) exchange activity and the underlying mechanisms involved utilizing Caco2 cells. Our results showed that NPY (100 nM, 30 min) significantly increased Cl⁻/HCO₃⁻(OH⁻) exchange activity (∼2-fold). Selective NPY/Y1 or Y2 receptor agonists mimicked the effects of NPY. NPY-mediated stimulation of Cl⁻/HCO₃⁻(OH⁻) exchange activity involved the ERK1/2 MAP kinase-dependent pathway. Cell surface biotinylation studies showed that NPY does not alter DRA (apical Cl⁻/HCO₃⁻(OH⁻) exchanger) surface expression, ruling out the involvement of membrane trafficking events. Interestingly, DRA was found to be predominantly expressed in the detergent-insoluble (DI) and low-density fractions (LDF) of human colonic apical membrane vesicles (AMVs) representing lipid rafts. Depletion of membrane cholesterol by methyl-β-cyclodextrin (MβCD, 10 mM, 1 h) remarkably decreased DRA expression in the DI fractions. Similar results were obtained in Triton-X 100-treated Caco2 plasma membranes. DRA association with lipid rafts in the DI and LDF fractions of Caco2 cells was significantly enhanced (∼45%) by NPY compared with control. MβCD significantly decreased Cl⁻/HCO₃⁻(OH⁻) exchange activity in Caco2 cells as measured by DIDS- or niflumic acid-sensitive ³⁶Cl⁻ uptake (∼50%). Our results demonstrate that NPY modulates Cl⁻/HCO₃⁻(OH⁻) exchange activity by enhancing the association of DRA with lipid rafts, thereby resulting in an increase in Cl⁻/HCO₃⁻(OH⁻) exchange activity. Our findings suggest that the alteration in the association of DRA with lipid rafts may contribute to the proabsorptive effects of NPY in the human intestine.

Trafficking, lateral mobility and segregation of the plant K channel KAT1

Functioning of ion channels depends not only on the control of their activity but also on their density and lateral organisation in the membrane. The two latter aspects have recently attracted increasing attention. Here, we summarize studies on trafficking and plasma membrane distribution of the plant K(+) channel KAT1 from Arabidopsis thaliana. In guard cells, KAT1 was found to be subject to constitutive and pressure-driven turnover and ABA-stimulated endocytosis. These results point to a role of exo- and endocytosis in regulating KAT1 density and thus ion transport during guard cell functioning. Recent studies indicate that KAT1 density can also be adjusted at the site of ER export. Efficient ER export of KAT1 was shown to depend on an acidic motif that interacts with Sec24, a component of ER-derived vesicles. Surface expression of ER export mutants of KAT1 can be rescued through heterotetrameric assembly with wild-type KAT1, implying that not all subunits of the channel tetramer need to carry an ER export motif. Analysis of the distribution of KAT1 in the plasma membrane revealed segregation of the channel into microdomains, and low lateral mobility in both plant and mammalian cells. In plant cells, SNAREs have been shown to be involved in anchoring KAT1 in the plasma membrane. Studies on guard cells imply a role for the cell wall in organisation of KAT1 microdomains. Together, these findings underline the importance of investigating mechanisms of KAT1 trafficking and lateral organisation in order to fully understand channel functioning.

Cyclic nucleotide-gated channels: a familiar channel family with a new function?

The cyclic nucleotide-gated (CNG) channel is a family of nonselective cation channels that open in response to an elevated cyclic nucleotide level. Cyclic nucleotides, particularly cAMP and cGMP, govern a great diversity of cellular functions. While the pivotal roles of CNG channels in the visual and olfactory systems have been well established in the past decade, relatively few studies were performed regarding the functional roles of CNG channels in non-neuronal systems. Cyclic nucleotides and Ca2+ are key signaling molecules in cardiovascular systems. Given that CNG channels are expressed in vascular tissues, several recent studies have explored the possible functional role of CNG channels in cardiovascular systems. This article intends to summarize some recent developments regarding the expression and functional role of CNG channels in the cardiovascular system.

A novel mechanism of sequestering fibroblast growth factor 2 by glypican in lipid rafts, allowing skeletal muscle differentiation

Heparan sulfate proteoglycans (HSPGs) are critical modulators of growth factor activities. Skeletal muscle differentiation is strongly inhibited by fibroblast growth factor 2 (FGF-2). We have shown that HSPGs present at the plasma membrane are expressed in myoblasts and are downregulated during muscle differentiation. An exception is glypican-1, which is present throughout the myogenic process. Myoblasts that do not express glypican-1 exhibit defective differentiation, with an increase in the receptor binding of FGF-2, concomitant with increased signaling. Glypican-1-deficient myoblasts show decreased expression of myogenin, the master gene that controls myogenesis, myosin, and the myoblast fusion index. Reversion of these defects was induced by expression of rat glypican-1. Glypican-1 is the only HSPG localized in lipid raft domains in myoblasts, resulting in the sequestration of FGF-2 away from FGF-2 receptors (FGFRs) located in nonraft domains. A chimeric glypican-1, containing syndecan-1 transmembrane and cytoplasmic domains, is located in nonraft domains interacting with FGFR-IV- and enhanced FGF-2-dependent signaling. Thus, glypican-1 acts as a positive regulator of muscle differentiation by sequestering FGF-2 in lipid rafts and preventing its binding and dependent signaling.

Differential effect of membrane cholesterol removal on mu- and delta-opioid receptors: a parallel comparison of acute and chronic signaling to adenylyl cyclase

According to the lipid raft theory, the plasma membrane contains small domains enriched in cholesterol and sphingolipid, which may serve as platforms to organize membrane proteins. Using methyl-beta-cyclodextrin (MbetaCD) to deplete membrane cholesterol, many G protein-coupled receptors have been shown to depend on putative lipid rafts for proper signaling. Here we examine the hypothesis that treatment of HEK293 cells stably expressing FLAG-tagged mu-opioid receptors (HEK FLAG-mu) or delta-opioid receptors (HEK FLAG-delta) with MbetaCD will reduce opioid receptor signaling to adenylyl cyclase. The ability of the mu-opioid agonist [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin to acutely inhibit adenylyl cyclase or to cause sensitization of adenylyl cyclase following chronic treatment was attenuated with MbetaCD. These effects were due to removal of cholesterol, because replenishment of cholesterol restored [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin responses back to control values, and were confirmed in SH-SY5Y cells endogenously expressing mu-opioid receptors. The effects of MbetaCD may be due to uncoupling of the mu receptor from G proteins but were not because of decreases in receptor number and were not mimicked by cytoskeleton disruption. In contrast to the results in HEK FLAG-mu cells, MbetaCD treatment of HEK FLAG-delta cells had no effect on acute inhibition or sensitization of adenylyl cyclase by delta-opioid agonists. The differential responses of mu- and delta-opioid agonists to cholesterol depletion suggest that mu-opioid receptors are more dependent on cholesterol for efficient signaling than delta receptors and can be partly explained by localization of mu- but not delta-opioid receptors in cholesterol- and caveolin-enriched membrane domains.

The role of membrane microdomains in shaping beta2-adrenergic receptor-mediated cAMP dynamics

Recently, membrane rafts and caveolae have received much attention for their role as signaling platforms, particularly due to their involvement in the pathogenesis of a number of diseases, including HIV as well as neurological and cardiovascular conditions. Signaling mediated by the beta-adrenergic receptor (beta-AR), a member of the large family of G-protein coupled receptors (GPCRs) that transduce extracellular messages via the ubiquitous second messenger, cAMP, has been a focus of raft studies since multiple components of the pathway are compartmentalized by these membrane microdomains. However, how these membrane rafts behave and regulate signaling dynamics in a cellular context is poorly understood. Here, we describe a live-cell assay based on single-cell, real-time fluorescence imaging, via an improved FRET-based cAMP biosensor, to monitor raft regulation of second messenger dynamics. Upon cholesterol depletion with methyl-beta-cyclodextrin (MbetaCD), beta(2)-AR-mediated cAMP accumulation was enhanced and prolonged in HEK-293 cells, demonstrating that membrane raft integrity helps shape beta-AR signaling. Single-cell imaging in parallel with fractionation studies reveal that the enhancement and change of dynamics are mediated by the receptor and correlated with its redistribution. Finally, the effect of cholesterol depletion is receptor-type specific as MbetaCD treatment did not show the same effect when the raft-excluded prostaglandin E receptor was stimulated. This study highlights the potential of a live-cell, real-time imaging assay for studying membrane rafts, including high sensitivity and spatiotemporal resolution, to achieve a better understanding of the nuances of membrane microdomains in both healthy and diseased states.

Olfactory cilia: linking sensory cilia function and human disease

The olfactory system gives us an awareness of our immediate environment by allowing us to detect airborne stimuli. The components necessary for detection of these odorants are compartmentalized in the cilia of olfactory sensory neurons. Cilia are microtubule-based organelles, which can be found projecting from the surface of almost any mammalian cell, and are critical for proper olfactory function. Mislocalization of ciliary proteins and/or the loss of cilia cause impaired olfactory function, which is now recognized as a clinical manifestation of a broad class of human diseases, termed ciliopathies. Future work investigating the mechanisms of olfactory cilia function will provide us important new information regarding the pathogenesis of human sensory perception diseases.

The electrochemical basis of odor transduction in vertebrate olfactory cilia

Most vertebrate olfactory receptor neurons share a common G-protein-coupled pathway for transducing the binding of odorant into depolarization. The depolarization involves 2 currents: an influx of cations (including Ca2+) through cyclic nucleotide-gated channels and a secondary efflux of Cl- through Ca2+-gated Cl- channels. The relation between stimulus strength and receptor current shows positive cooperativity that is attributed to the channel properties. This cooperativity amplifies the responses to sufficiently strong stimuli but reduces sensitivity and dynamic range. The odor response is transient, and prolonged or repeated stimulation causes adaptation and desensitization. At least 10 mechanisms may contribute to termination of the response; several of these result from an increase in intraciliary Ca2+. It is not known to what extent regulation of ionic concentrations in the cilium depends on the dendrite and soma. Although many of the major mechanisms have been identified, odor transduction is not well understood at a quantitative level.

Cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the hawkmoth Manduca sexta

Moth pheromones cause rises in intracellular Ca(2+) concentrations that activate Ca(2+)-dependent cation channels in antennal olfactory receptor neurons. In addition, mechanisms of adaptation and sensitization depend on changes in cyclic nucleotide concentrations. Here, cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the moth Manduca sexta are described, which share properties with currents through vertebrate cyclic nucleotide-gated channels. The cyclic nucleotide-activated currents of M. sexta carried Ca(2+) and monovalent cations. They were directly activated by cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), modulated by Ca(2+)/calmodulin, and inhibited by lanthanum. M. sexta cyclic nucleotide-activated currents developed in an all-or-none manner, which suggests that the underlying channels are coupled and act coordinately. At least one cAMP- and two cGMP-activated nonselective cation currents could be distinguished. Compared with the cAMP-activated current, both cGMP-activated currents appeared to conduct more Ca(2+) and showed a stronger down-regulation by Ca(2+)/calmodulin-dependent negative feedback. Furthermore, both cGMP-activated currents differed in their Ca(2+)-dependent inhibition. Thus M. sexta olfactory receptor neurons, like vertebrate sensory neurons, appear to express nonselective cyclic nucleotide-activated cation channels with different subunit compositions. Besides the nonselective cyclic nucleotide-activated cation currents, olfactory receptor neurons express a cAMP-dependent current. This current resembled a protein kinase-modulated low-voltage-activated Ca(2+) current.

Cyclic GMP-gated CNG channels function in Sema3A-induced growth cone repulsion

Cyclic nucleotide-gated channels (CNGCs) transduce external signals required for sensory processes, e.g., photoreception, olfaction, and taste. Nerve growth cone guidance by diffusible attractive and repulsive molecules is regulated by differential growth cone Ca2+ signaling. However, the Ca2+-conducting ion channels that transduce guidance molecule signals are largely unknown. We show that rod-type CNGC-like channels function in the repulsion of cultured Xenopus spinal neuron growth cones by Sema3A, which triggers the production of the cGMP that activates the Xenopus CNGA1 (xCNGA1) subunit-containing channels in interneurons. Downregulation of xCNGA1 or overexpression of a mutant xCNGA1 incapable of binding cGMP abolished CNG currents and converted growth cone repulsion to attraction in response to Sema3A. We also show that Ca2+ entry through xCNGCs is required to mediate the repulsive Sema3A signal. These studies extend our knowledge of the function of CNGCs by demonstrating their requirement for signal transduction in growth cone guidance.

Cytoplasmic cAMP concentrations in intact cardiac myocytes

In cardiac myocytes there is evidence that activation of some receptors can regulate protein kinase A (PKA)-dependent responses by stimulating cAMP production that is limited to discrete intracellular domains. We previously developed a computational model of compartmentalized cAMP signaling to investigate the feasibility of this idea. The model was able to reproduce experimental results demonstrating that both beta(1)-adrenergic and M(2) muscarinic receptor-mediated cAMP changes occur in microdomains associated with PKA signaling. However, the model also suggested that the cAMP concentration throughout most of the cell could be significantly higher than that found in PKA-signaling domains. In the present study we tested this counterintuitive hypothesis using a freely diffusible fluorescence resonance energy transfer-based biosensor constructed from the type 2 exchange protein activated by cAMP (Epac2-camps). It was determined that in adult ventricular myocytes the basal cAMP concentration detected by the probe is approximately 1.2 muM, which is high enough to maximally activate PKA. Furthermore, the probe detected responses produced by both beta(1) and M(2) receptor activation. Modeling suggests that responses detected by Epac2-camps mainly reflect what is happening in a bulk cytosolic compartment with little contribution from microdomains where PKA signaling occurs. These results support the conclusion that even though beta(1) and M(2) receptor activation can produce global changes in cAMP, compartmentation plays an important role by maintaining microdomains where cAMP levels are significantly below that found throughout most of the cell. This allows receptor stimulation to regulate cAMP activity over concentration ranges appropriate for modulating both higher (e.g., PKA) and lower affinity (e.g., Epac) effectors.

Olfactory cilia: our direct neuronal connection to the external world

An organism's awareness of its surroundings is dependent on sensory function. As antennas to our external environment, cilia are involved in fundamental biological processes such as olfaction, photoreception, and touch. The olfactory system has adapted this organelle for its unique sensory function and optimized it for detection of external stimuli. The elongated and tapering structure of olfactory cilia and their organization into an overlapping meshwork bathed by the nasal mucosa is optimized to enhance odor absorption and detection. As many as 15-30 nonmotile, sensory cilia on dendritic endings of single olfactory sensory neurons (OSNs) compartmentalize signaling molecules necessary for odor detection allowing for efficient and spatially confined responses to sensory stimuli. Although the loss of olfactory cilia or deletion of selected components of the olfactory signaling cascade leads to anosmia, the mechanisms of ciliogenesis and the selected enrichment of signaling molecules remain poorly understood. Much of our current knowledge is the result of elegant electron microscopy studies describing the structure and organization of the olfactory epithelium and cilia. New genetic and cell biological approaches, which compliment these early studies, show promise in elucidating the mechanisms of olfactory cilia assembly, maintenance, and compartmentalization. Importantly, emerging evidence suggests that olfactory dysfunction represents a previously unrecognized clinical manifestation of multiple ciliary disorders. Future work investigating the mechanisms of olfactory dysfunction combining both clinical studies with basic science research will provide us important new information regarding the pathogenesis of human sensory perception diseases.

Calcium-signaling networks in olfactory receptor neurons

The olfactory neuroepithelium represents a unique interface between the brain and the external environment. Olfactory function comprises a distinct set of molecular tasks: sensory signal transduction, cytoprotection and adult neurogenesis. A multitude of biochemical studies has revealed the central role of Ca(2+) signaling in the function of olfactory receptor neurons (ORNs). We set out to establish Ca(2+)-dependent signaling networks in ORN cilia by proteomic analysis. We subjected a ciliary membrane preparation to Ca(2+)/calmodulin-affinity chromatography using mild detergent conditions in order to maintain functional protein complexes involved in olfactory Ca(2+) signaling. Thus, calmodulin serves as a valuable tool to gain access to novel Ca(2+)-regulated protein complexes. Tandem mass spectrometry (nanoscale liquid-chromatography-electrospray injection) identified 123 distinct proteins. Ninety-seven proteins (79%) could be assigned to specific olfactory functions, including 32 to sensory signal transduction and 40 to cytoprotection. We point out novel perspectives for research on the Ca(2+)-signaling networks in the olfactory system of the rat.

BK channels are linked to inositol 1,4,5-triphosphate receptors via lipid rafts: a novel mechanism for coupling [Ca(2+)](i) to ion channel activation

Glioma cells prominently express a unique splice variant of a large conductance, calcium-activated potassium channel (BK channel). These channels transduce changes in intracellular calcium to changes of K(+) conductance in the cells and have been implicated in growth control of normal and malignant cells. The Ca(2+) increase that facilitates channel activation is thought to occur via activation of intracellular calcium release pathways or influx of calcium through Ca(2+)-permeable ion channels. We show here that BK channel activation involves the activation of inositol 1,4,5-triphosphate receptors (IP(3)R), which localize near BK channels in specialized membrane domains called lipid rafts. Disruption of lipid rafts with methyl-beta-cyclodextrin disrupts the functional association of BK channel and calcium source resulting in a >50% reduction in K(+) conductance mediated by BK channels. The reduction of BK current by lipid raft disruption was overcome by the global elevation of intracellular calcium through inclusion of 750 nm Ca(2+) in the pipette solution, indicating that neither the calcium sensitivity of the channel nor their overall number was altered. Additionally, pretreatment of glioma cells with 2-aminoethoxydiphenyl borate to inhibit IP(3)Rs negated the effect of methyl-beta-cyclodextrin, providing further support that IP(3)Rs are the calcium source for BK channels. Taken together, these data suggest a privileged association of BK channels in lipid raft domains and provide evidence for a novel coupling of these Ca(2+)-sensitive channels to their second messenger source.

Differential distribution of proteins and lipids in detergent-resistant and detergent-soluble domains in rod outer segment plasma membranes and disks

Membrane heterogeneity plays a significant role in regulating signal transduction and other cellular activities. We examined the protein and lipid components associated with the detergent-resistant membrane (DRM) fractions from retinal rod outer segment (ROS) disk and plasma membrane-enriched preparations. Proteomics and correlative western blot analysis revealed the presence of alpha and beta subunits of the rod cGMP-gated ion channel and glucose transporter type 1, among other proteins. The glucose transporter was present exclusively in ROS plasma membrane (not disks) and was highly enriched in DRMs, as was the cGMP-gated channel beta-subunit. In contrast, the majority of rod opsin and ATP-binding cassette transporter A4 was localized to detergent-soluble domains in disks. As expected, the cholesterol : fatty acid mole ratio was higher in DRMs than in the corresponding parent membranes (disk and plasma membranes, respectively) and was also higher in disks compared to plasma membranes. Furthermore, the ratio of saturated : polyunsaturated fatty acids was also higher in DRMs compared to their respective parent membranes (disk and plasma membranes). These results confirm that DRMs prepared from both disks and plasma membranes are enriched in cholesterol and in saturated fatty acids compared to their parent membranes. The dominant fatty acids in DRMs were 16 : 0 and 18 : 0; 22 : 6n3 and 18 : 1 levels were threefold higher and twofold lower, respectively, in disk-derived DRMs compared to plasma membrane-derived DRMs. We estimate, based on fatty acid recovery that DRMs account for only approximately 8% of disks and approximately 12% of ROS plasma membrane.

Organization and Ca2+Regulation of Adenylyl Cyclases in cAMP Microdomains

The adenylyl cyclases are variously regulated by G protein subunits, a number of serine/threonine and tyrosine protein kinases, and Ca2+. In some physiological situations, this regulation can be readily incorporated into a hormonal cascade, controlling processes such as cardiac contractility or neurotransmitter release. However, the significance of some modes of regulation is obscure and is likely only to be apparent in explicit cellular contexts (or stages of the cell cycle). The regulation of many of the ACs by the ubiquitous second messenger Ca2+provides an overarching mechanism for integrating the activities of these two major signaling systems. Elaborate devices have been evolved to ensure that this interaction occurs, to guarantee the fidelity of the interaction, and to insulate the microenvironment in which it occurs. Subcellular targeting, as well as a variety of scaffolding devices, is used to promote interaction of the ACs with specific signaling proteins and regulatory factors to generate privileged domains for cAMP signaling. A direct consequence of this organization is that cAMP will exhibit distinct kinetics in discrete cellular domains. A variety of means are now available to study cAMP in these domains and to dissect their components in real time in live cells. These topics are explored within the present review.

Compartmentation of cAMP signaling in cardiac myocytes: a computational study

Receptor-mediated changes in cAMP production play an essential role in sympathetic and parasympathetic regulation of the electrical, mechanical, and metabolic activity of cardiac myocytes. However, responses to receptor activation cannot be easily ascribed to a uniform increase or decrease in cAMP activity throughout the entire cell. In this study, we used a computational approach to test the hypothesis that in cardiac ventricular myocytes the effects of beta(1)-adrenergic receptor (beta(1)AR) and M(2) muscarinic receptor (M(2)R) activation involve compartmentation of cAMP. A model consisting of two submembrane (caveolar and extracaveolar) microdomains and one bulk cytosolic domain was created using published information on the location of beta(1)ARs and M(2)Rs, as well as the location of stimulatory (G(s)) and inhibitory (G(i)) G-proteins, adenylyl cyclase isoforms inhibited (AC5/6) and stimulated (AC4/7) by G(i), and multiple phosphodiesterase isoforms (PDE2, PDE3, and PDE4). Results obtained with the model indicate that: 1), bulk basal cAMP can be high ( approximately 1 microM) and only modestly stimulated by beta(1)AR activation ( approximately 2 microM), but caveolar cAMP varies in a range more appropriate for regulation of protein kinase A ( approximately 100 nM to approximately 2 microM); 2), M(2)R activation strongly reduces the beta(1)AR-induced increases in caveolar cAMP, with less effect on bulk cAMP; and 3), during weak beta(1)AR stimulation, M(2)R activation not only reduces caveolar cAMP, but also produces a rebound increase in caveolar cAMP following termination of M(2)R activity. We conclude that compartmentation of cAMP can provide a quantitative explanation for several aspects of cardiac signaling.

Lipid raft microdomains and neurotransmitter signalling

Lipid rafts are specialized structures on the plasma membrane that have an altered lipid composition as well as links to the cytoskeleton. It has been proposed that these structures are membrane domains in which neurotransmitter signalling might occur through a clustering of receptors and components of receptor-activated signalling cascades. The localization of these proteins in lipid rafts, which is affected by the cytoskeleton, also influences the potency and efficacy of neurotransmitter receptors and transporters. The effect of lipid rafts on neurotransmitter signalling has also been implicated in neurological and psychiatric diseases.

Interplay between PIP3 and calmodulin regulation of olfactory cyclic nucleotide-gated channels

Phosphatidylinositol-3,4,5-trisphosphate (PIP3) has been proposed to modulate the odorant sensitivity of olfactory sensory neurons by inhibiting activation of cyclic nucleotide-gated (CNG) channels in the cilia. When applied to the intracellular face of excised patches, PIP3 has been shown to inhibit activation of heteromeric olfactory CNG channels, composed of CNGA2, CNGA4, and CNGB1b subunits, and homomeric CNGA2 channels. In contrast, we discovered that channels formed by CNGA3 subunits from cone photoreceptors were unaffected by PIP3. Using chimeric channels and a deletion mutant, we determined that residues 61-90 within the N terminus of CNGA2 are necessary for PIP3 regulation, and a biochemical "pulldown" assay suggests that PIP3 directly binds this region. The N terminus of CNGA2 contains a previously identified calcium-calmodulin (Ca2+/CaM)-binding domain (residues 68-81) that mediates Ca2+/CaM inhibition of homomeric CNGA2 channels but is functionally silent in heteromeric channels. We discovered, however, that this region is required for PIP3 regulation of both homomeric and heteromeric channels. Furthermore, PIP3 occluded the action of Ca2+/CaM on both homomeric and heteromeric channels, in part by blocking Ca2+/CaM binding. Our results establish the importance of the CNGA2 N terminus for PIP3 inhibition of olfactory CNG channels and suggest that PIP3 inhibits channel activation by disrupting an autoexcitatory interaction between the N and C termini of adjacent subunits. By dramatically suppressing channel currents, PIP3 may generate a shift in odorant sensitivity that does not require prior channel activity.

Functional localization of cAMP signalling in cardiac myocytes

The cAMP pathway is of cardinal importance for heart physiology and pathology. The spatial organization of the various components of the cAMP pathway is thought to allow the segregation of functional responses triggered by the different neuromediators and hormones that use this pathway. PDEs (phosphodiesterases) hydrolyse cAMP (and cGMP) and play a major role in this process by preventing cAMP diffusion to the whole cytosol and inadequate target activation. The development of olfactory cyclic nucleotide-gated channels to directly monitor cAMP beneath the plasma membrane in real time allows us to gain new insights into the molecular mechanisms responsible for cAMP homoeostasis and hormonal specificity in cardiac cells. The present review summarizes the recent results we obtained using this approach in adult rat ventricular myocytes. In particular, the role of PDEs in the maintenance of specific cAMP signals generated by beta-adrenergic receptors and other G(s)-coupled receptors will be discussed.

Higher-order organization and regulation of adenylyl cyclases

There is increasing awareness of the compartmentalization of cAMP signalling--the means by which cAMP levels change in discrete domains of the cell with discrete local consequences. Current developments in understanding the organization of adenylyl cyclases in the plasma membrane are illuminating how the earliest part of cAMP compartmentalization could occur. This review focuses on recent findings regarding three levels of adenylyl cyclase organization--oligomerization, positioning to lipid rafts and participation in multiprotein signalling complexes. This organization, coupled with the role of scaffolding proteins in arranging the downstream effectors of cAMP, helps to identify complexes that greatly facilitate the translation of enzyme activation into local consequences.

Natriuretic peptides and nitric oxide stimulate cGMP synthesis in different cellular compartments

Cyclic nucleotide-gated (CNG) channels are a family of ion channels activated by the binding of cyclic nucleotides. Endogenous channels have been used to measure cyclic nucleotide signals in photoreceptor outer segments and olfactory cilia for decades. Here we have investigated the subcellular localization of cGMP signals by monitoring CNG channel activity in response to agonists that activate either particulate or soluble guanylyl cyclase. CNG channels were heterologously expressed in either human embryonic kidney (HEK)-293 cells that stably overexpress a particulate guanylyl cyclase (HEK-NPRA cells), or cultured vascular smooth muscle cells (VSMCs). Atrial natriuretic peptide (ANP) was used to activate the particulate guanylyl cyclase and the nitric oxide donor S-nitroso-n-acetylpenicillamine (SNAP) was used to activate the soluble guanylyl cyclase. CNG channel activity was monitored by measuring Ca2+ or Mn2+ influx through the channels using the fluorescent dye, fura-2. We found that in HEK-NPRA cells, ANP-induced increases in cGMP levels activated CNG channels in a dose-dependent manner (0.05-10 nM), whereas SNAP (0.01-100 microM) induced increases in cGMP levels triggered little or no activation of CNG channels (P < 0.01). After pretreatment with 100 microM 3-isobutyl-1-methylxanthine (IBMX), a nonspecific phosphodiesterase inhibitor, ANP-induced Mn2+ influx through CNG channels was significantly enhanced, while SNAP-induced Mn2+ influx remained small. In contrast, we found that in the presence of IBMX, both 1 nM ANP and 100 microM SNAP triggered similar increases in total cGMP levels. We next sought to determine if cGMP signals are compartmentalized in VSMCs, which endogenously express particulate and soluble guanylyl cyclase. We found that 10 nM ANP induced activation of CNG channels more readily than 100 muM SNAP; whereas 100 microM SNAP triggered higher levels of total cellular cGMP accumulation. These results suggest that cGMP signals are spatially segregated within cells, and that the functional compartmentalization of cGMP signals may underlie the unique actions of ANP and nitric oxide.