Odorants induce the phosphorylation of the cAMP response element binding protein in olfactory receptor neurons

An Exploratory Application of Multilayer Networks and Pathway Analysis in Pharmacogenomics

Over the years, network analysis has become a promising strategy for analysing complex system, i.e., systems composed of a large number of interacting elements. In particular, multilayer networks have emerged as a powerful framework for modelling and analysing complex systems with multiple types of interactions. Network analysis can be applied to pharmacogenomics to gain insights into the interactions between genes, drugs, and diseases. By integrating network analysis techniques with pharmacogenomic data, the goal consists of uncovering complex relationships and identifying key genes to use in pathway enrichment analysis to figure out biological pathways involved in drug response and adverse reactions. In this study, we modelled omics, disease, and drug data together through multilayer network representation. Then, we mined the multilayer network with a community detection algorithm to obtain the top communities. After that, we used the identified list of genes from the communities to perform pathway enrichment analysis (PEA) to figure out the biological function affected by the selected genes. The results show that the genes forming the top community have multiple roles through different pathways.

Potential evidence of peripheral learning and memory in the arms of dwarf cuttlefish, Sepia bandensis

CREB (cAMP response element-binding) transcription factors are conserved markers of memory formation in the brain and peripheral circuits. We provide immunohistochemical evidence of CREB phosphorylation in the dwarf cuttlefish, Sepia bandensis, following the inaccessible prey (IP) memory experiment. During the IP experiment, cuttlefish are shown prey enclosed in a transparent tube, and tentacle strikes against the tube decrease over time as the cuttlefish learns the prey is inaccessible. The cues driving IP learning are unclear but may include sensory inputs from arms touching the tube. The neural activity marker, anti-phospho-CREB (anti-pCREB) was used to determine whether IP training stimulated cuttlefish arm sensory neurons. pCREB immunoreactivity occurred along the oral surface of the arms, including the suckers and epithelial folds surrounding the suckers. pCREB increased in the epithelial folds and suckers of trained cuttlefish. We found differential pCREB immunoreactivity along the distal-proximal axis of trained arms, with pCREB concentrated distally. Unequal CREB phosphorylation occurred among the 4 trained arm pairs, with arm pairs 1 and 2 containing more pCREB. The resulting patterns of pCREB in trained arms suggest that the arms obtain cues that may be salient for learning and memory of the IP experiment.

Olfactory receptor 10J5 responding to α-cedrene regulates hepatic steatosis via the cAMP-PKA pathway

Ectopic expression and functions of odorant receptors (ORs) in the human body have aroused much interest in the past decade. Mouse olfactory receptor 23 (MOR23, olfr16) and its human orthologue, OR10J5, have been found to be functionally expressed in several non-olfactory systems. Here, using MOR23- and OR10J5-expressing Hana3A cells, we identified α-cedrene, a natural compound that protects against hepatic steatosis in mice fed the high-fat diet, as a novel agonist of these receptors. In human hepatocytes, an RNA interference-mediated knockdown of OR10J5 increased intracellular lipid accumulation, along with upregulation of lipogenic genes and downregulation of genes related to fatty acid oxidation. α-Cedrene stimulation resulted in a significant reduction in lipid contents of human hepatocytes and reprogramming of metabolic signatures, which are mediated by OR10J5, as demonstrated by receptor knockdown experiments using RNA interference. Taken together, our findings show a crucial role of OR10J5 in the regulation of lipid accumulation in human hepatocytes.

Phosphoinositide and Erk signaling pathways mediate activity-driven rodent olfactory sensory neuronal survival and stress mitigation

Olfactory sensory neurons (OSNs) are the initial site for olfactory signal transduction. Therefore, their survival is essential to olfactory function. In the current study, we demonstrated that while odorant stimulation promoted rodent OSN survival, it induced generation of reactive oxygen species in a dose- and time-dependent manner as well as loss of membrane potential and fragmentation of mitochondria. The MEK-Erk pathway played a critical role in mediating these events, as its inhibition decreased odorant stimulation-dependent OSN survival and exacerbated intracellular stress measured by reactive oxygen species generation and heat-shock protein 70 expression. The phosphoinositide pathway, rather than the cyclic AMP pathway, mediated the odorant-induced activation of the MEK-Erk pathway. These findings provide important insights into the mechanisms of activity-driven OSN survival, the role of the phosphoinositide pathway in odorant signaling, and demonstrate that odorant detection and odorant stimulation-mediated survival proceed via independent signaling pathways. This mechanism, which permits independent regulation of odorant detection from survival signaling, may be advantageous if not diminished by repeated or prolonged odor exposure. We investigated the role of odorant stimulation in generating cellular stress and the molecular mechanisms mitigating such stress and promoting neuronal survival. Odorant stimulation promoted olfactory sensory neuron (OSN) survival and also induced intracellular oxidative stress, which was exacerbated when MEK/Erks pathway was inhibited. Sensory stimulation simultaneously activated at least two parallel pathways, the AC/cAMP cascade responsible for odorant detection, and phosphoinositide hydrolysis to promote odorant stimulation-dependent neuronal survival odorants may activate parallel signaling cascades to mediate sensory detection and sensory stimulation-dependent survival. AC, adenylyl cyclase; cAMP, cyclic adenosine monophosphate; Erk, extracellular signal-regulated kinase; MEK, MAPK/ERK kinase.

MeCP2 is required for activity-dependent refinement of olfactory circuits

Methyl CpG binding protein 2 (MeCP2) is a structural chromosomal protein involved in the regulation of gene expression. Alterations in the levels of MeCP2 have been related to neurodevelopmental disorders. Studies in mouse models of MeCP2 deficiency have demonstrated that this protein is important for neuronal maturation, neurite complexity, synaptogenesis, and synaptic plasticity. However, the mechanisms by which MeCP2 dysfunction leads to neurodevelopmental defects, and the role of activity, remain unclear, as most studies examine the adult nervous system, which may obfuscate the primary consequences of MeCP2 mutation. We hypothesize that MeCP2 plays a role during the formation and activity-driven maturation of neural circuits at early postnatal stages. To test this hypothesis, we use the olfactory system as a neurodevelopmental model. This system undergoes postnatal neurogenesis; axons from olfactory neurons form highly stereotyped projections to higher-order neurons, facilitating the detection of possible defects in the establishment of connectivity. In vivo olfactory stimulation paradigms were used to produce physiological synaptic activity in gene-targeted mice in which specific olfactory circuits are visualized. Our results reveal defective postnatal refinement of olfactory circuits in Mecp2 knock out (KO) mice after sensory (odorant) stimulation. This failure in refinement was associated with deficits in the normal responses to odorants, including brain-derived neurotrophic factor (BDNF) production, as well as changes in adhesion molecules known to regulate axonal convergence. The defective refinement observed in Mecp2 KO mice was prevented by daily treatment with ampakine beginning after the first postnatal week. These observations indicate that increasing synaptic activity at early postnatal stage might circumvent the detrimental effect of MeCP2 deficiency on circuitry maturation. The present results provide in vivo evidence in real time for the role of MeCP2 in activity-dependent maturation of olfactory circuitry, with implications for understanding the mechanism of MeCP2 mutations in the development of neural connectivity.

Tbr2 deficiency in mitral and tufted cells disrupts excitatory-inhibitory balance of neural circuitry in the mouse olfactory bulb

The olfactory bulb (OB) is the first relay station in the brain where odor information from the olfactory epithelium is integrated, processed through its intrinsic neural circuitry, and conveyed to higher olfactory centers. Compared with profound mechanistic insights into olfactory axon wiring from the nose to the OB, little is known about the molecular mechanisms underlying the formation of functional neural circuitry among various types of neurons inside the OB. T-box transcription factor Tbr2 is expressed in various types of glutamatergic excitatory neurons in the brain including the OB projection neurons, mitral and tufted cells. Here we generated conditional knockout mice in which the Tbr2 gene is inactivated specifically in mitral and tufted cells from late embryonic stages. Tbr2 deficiency caused cell-autonomous changes in molecular expression including a compensatory increase of another T-box member, Tbr1, and a concomitant shift of vesicular glutamate transporter (VGluT) subtypes from VGluT1 to VGluT2. Tbr2-deficient mitral and tufted cells also exhibited anatomical abnormalities in their dendritic morphology and projection patterns. Additionally, several non-cell-autonomous phenotypes were observed in parvalbumin-, calbindin-, and 5T4-positive GABAergic interneurons. Furthermore, the number of dendrodendritic reciprocal synapses between mitral/tufted cells and GABAergic interneurons was significantly reduced. Upon stimulation with odorants, larger numbers of mitral and tufted cells were activated in Tbr2 conditional knockout mice. These results suggest that Tbr2 is required for not only the proper differentiation of mitral and tufted cells, but also for the establishment of functional neuronal circuitry in the OB and maintenance of excitatory-inhibitory balance crucial for odor information processing.

Direct measurement of extracellular electrical signals from mammalian olfactory sensory neurons in planar triode devices

An artificial nose was developed to mimic aspects of sensory transduction of the peripheral mammalian olfactory system. We directly cultured and differentiated rat olfactory sensory neurons (OSNs) on indium-tin oxide electrodes of planar triode substrates without a coupling agent. Direct voltage (~50 μV) and current (~250 nA) signals were measured simultaneously when OSNs on the planar triode substrates were exposed to odorant mixtures. The response signals were sensitive to the concentration of the odorant mixture, with a typical lifetime, shape, and adaptation profile as seen in responses upon repeated stimulation in vivo. We found that the rising time to the peak current was ~161 ms, while the signal back to baseline was in 1.8 s, which are in agreement with the natural intracellular electrophysiological responses. These results provide the first evidence that mature OSNs grown in a planar triode device are able to detect direct electrophysiological responses to odorants.

Interplay among cGMP, cAMP, and Ca2+ in living olfactory sensory neurons in vitro and in vivo

The mechanism of cGMP production in olfactory sensory neurons (OSNs) is poorly understood, although this messenger takes part in several key processes such as adaptation, neuronal development, and long-term cellular responses to odorant stimulation. Many aspects of the regulation of cGMP in OSNs are still unknown or highly controversial, such as its subcellular heterogeneity, mechanism of coupling to odorant receptors and downstream targets. Here, we have investigated the dynamics and the intracellular distribution of cGMP in living rat OSNs in culture transfected with a genetically encoded sensor for cGMP. We demonstrate that OSNs treated with pharmacological stimuli able to activate membrane or soluble guanylyl cyclase (sGC) presented an increase in cGMP in the entire neuron, from cilia-dendrite to the axon terminus-growth cone. Upon odorant stimulation, a rise in cGMP was again found in the entire neuron, including the axon terminus, where it is locally synthesized. The odorant-dependent rise in cGMP is due to sGC activation by nitric oxide (NO) and requires an increase of cAMP. The link between cAMP and NO synthase appears to be the rise in cytosolic Ca(2+) concentration elicited by either plasma membrane Ca(2+) channel activation or Ca(2+) mobilization from stores via the guanine nucleotide exchange factor Epac. Finally, we show that a cGMP rise can elicit both in vitro and in vivo the phosphorylation of nuclear CREB, suggesting that this signaling pathway may be relevant for both local events (pathfinding, neurotransmitter release) and more distal processes involving gene expression regulation.

Caenorhabditis elegans TRPV channels function in a modality-specific pathway to regulate response to aberrant sensory signaling

Olfaction and some forms of taste (including bitter) are mediated by G protein-coupled signal transduction pathways. Olfactory and gustatory ligands bind to chemosensory G protein-coupled receptors (GPCRs) in specialized sensory cells to activate intracellular signal transduction cascades. G protein-coupled receptor kinases (GRKs) are negative regulators of signaling that specifically phosphorylate activated GPCRs to terminate signaling. Although loss of GRK function usually results in enhanced cellular signaling, Caenorhabditis elegans lacking GRK-2 function are not hypersensitive to chemosensory stimuli. Instead, grk-2 mutant animals do not chemotax toward attractive olfactory stimuli or avoid aversive tastes and smells. We show here that loss-of-function mutations in the transient receptor potential vanilloid (TRPV) channels OSM-9 and OCR-2 selectively restore grk-2 behavioral avoidance of bitter tastants, revealing modality-specific mechanisms for TRPV channel function in the regulation of C. elegans chemosensation. Additionally, a single amino acid point mutation in OCR-2 that disrupts TRPV channel-mediated gene expression, but does not decrease channel function in chemosensory primary signal transduction, also restores grk-2 bitter taste avoidance. Thus, loss of GRK-2 function may lead to changes in gene expression, via OSM-9/OCR-2, to selectively alter the levels of signaling components that transduce or regulate bitter taste responses. Our results suggest a novel mechanism and multiple modality-specific pathways that sensory cells employ in response to aberrant signal transduction.

Regulation of intracellular cyclic GMP levels in olfactory sensory neurons

Cyclic AMP is the primary second messenger mediating odorant signal transduction in mammals. A number of studies indicate that cyclic GMP is also involved in a variety of other olfactory signal transduction processes, including adaptation, neuronal development, and long-term cellular responses in the setting of odorant stimulation. However, the mechanisms that control the production and degradation of cGMP in olfactory sensory neurons (OSNs) remain unclear. Here, we investigate these mechanisms using primary cultures of OSNs. We demonstrate that odorants increase cGMP levels in intact OSNs in vitro. Different from the rapid and transient cAMP responses to odorants, the cGMP elevation is both delayed and sustained. Inhibition of soluble guanylyl cyclase and heme oxygenase blocks these odorant-induced cGMP increases, whereas inhibition of cGMP PDEs (phosphodiesterases) increases this response. cGMP PDE activity is increased by odorant stimulation, and is sensitive to both ambient calcium and cAMP concentrations. Calcium stimulates cGMP PDE activity, whereas cAMP and protein kinase A appears to inhibit it. These data demonstrate a mechanism by which odorant stimulation may regulate cGMP levels through the modulation of cAMP and calcium level in OSNs. Such interactions between odorants and second messenger systems may be important to the integration of immediate and long-term responses in the setting odorant stimulation.

Odorant-dependent, spatially restricted induction of c-fos in the olfactory epithelium of the mouse

Volatile odorous chemicals are detected by around a thousand different G protein-coupled odorant receptors in the mouse. We demonstrated that exposure of the behaving mouse to odorant for a few minutes led to induction of the immediate early gene c-fos for several hours in a fraction of the olfactory sensory neurones in the nasal cavity. Associated with this odorant-specific induction event was activation of extracellular-regulated kinase (ERK)1/2 that preceded increased c-fos expression. The distribution of odorant-activated neurones mimicked the scattered and spatially limited distribution of neurones expressing a single odorant receptor gene. A small change in odorant chemical structure caused a zonal shift in the spatial distribution of activated neurones, suggesting that the gene expression change resulted from specific receptor interaction. Repeated exposure to odorant or use of different concentrations did not change the pattern of c-fos induction. These results indicate that odorant-induced c-fos expression can be used to visualize odorant representations in the olfactory epithelium that reflect late cellular events regulated by adequate odorant receptor stimulation.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

Nitroprusside stimulates mitochondrial aconitase gene expression through the cyclic adenosine 3',5'-monosphosphate signal transduction pathway in human prostate carcinoma cells

BACKGROUND

Mitochondrial aconitase (mACON), an iron-requiring enzyme, is a major target of nitric oxide (NO) in cells, which causes the oxidant-mediated disruption of the [4Fe-4S] prosthetic group of the enzyme. In this study, the effect of NO on mACON enzymatic activity and gene expression were investigated.

METHODS

Three NO generators, sodium nitroprusside (SNP), S-nitoso-N-acetylpenicillamine (SNAP), and 3-morpholinosydnonimine (SIN) were used to determine the regulation of mACON enzymatic activity by NO. The effect of SNP on mACON, which modulates citrate secretion and cellular bioenergetics in PC-3 cells, was investigated by determining the effect of SNP on mACON gene expression using Western blot and transient gene expression assays.

RESULTS

SNP upregulated mACON enzymatic activity and gene expression in PC-3 cells. However, treating cells with other NO generators, SNAP and SIN, resulted in decreased mACON enzymatic activity. The addition of ascorbic acid to the SNP treatment resulted in a decrease in mACON enzymatic activity and gene expression. Our results showed that both SNP and dibutyryl-cAMP increased the mACON promoter activity 2-fold while the effect was blocked by adding H-89. Mutation of the cAMP response element (CRE) to the AGAGCT abolished the activating effects of SNP and dibutyryl-cAMP on mACON promoter activity.

CONCLUSIONS

These results establish the function of nitroprusside as a signaling molecule for mACON gene expression through the cAMP signal transduction pathway in human prostatic carcinoma cells.

Odorant Stimulation Enhances Survival of Olfactory Sensory Neurons via MAPK and CREB

Olfactory sensory neurons (OSNs) can be sensitized to odorants by repeated exposure, suggesting that an animal's responsiveness to olfactory cues can be enhanced at the initial stage of detection. However, because OSNs undergo a regular cycle of apoptosis and replacement by ostensibly naive, precursor-derived neurons, the advantage of sensitization would be lost in the absence of a mechanism for odorant-enhanced survival of OSNs. Using recombinant adenoviruses in conjunction with surgical and electrophysiological techniques, we monitored OSN survival and function in vivo and find that odorant exposure selectively rescues populations of OSNs from apoptosis. We further demonstrate that odorant stimuli rescue OSNs in a cAMP-dependent manner by activating the MAPK/CREB-dependent transcriptional pathway, possibly as a result of expression of Bcl-2.

Heme oxygenase-1 and heme oxygenase-2 have distinct roles in the proliferation and survival of olfactory receptor neurons mediated by cGMP and bilirubin, respectively

Heme oxygenase (HO) is implicated in protection against oxidative stress, proliferation and apoptosis in many cell types, including neurons. We utilized olfactory receptor neurons (ORNs) as a model to define the roles of HO-1 and HO-2 in neuronal development and survival, and to determine the mediators of these effects. The olfactory system is a useful model as ORNs display neurogenesis post-natally and do not contain nitric oxide synthase (NOS) activity, which could confound results. HO isoforms were expressed in ORNs during embryogenesis and post-natally. Mice null for either HO-1 or HO-2 displayed decreased proliferation of neuronal precursors. However, apoptosis was increased only in HO-2 null mice. Cyclic GMP immunostaining was reduced in ORNs in both genotypes, providing direct evidence that HO mediates cGMP production in vivo. Bilirubin immunostaining was reduced only in HO-2 null mice. These roles for HO-1 and HO-2 were confirmed using detergent ablation of the epithelium to observe increased neurogenesis of ORNs after target disruption in HO null mice. Primary cultures of ORNs revealed that proliferative and survival effects of HO were mediated through cGMP and bilirubin, respectively. These results support a role for HO, the CO-cGMP signaling system and bilirubin in neurodevelopment and in response to injury.

Atrial natriuretic peptide type C induces a cell-cycle switch from proliferation to differentiation in brain-derived neurotrophic factor- or nerve growth factor-primed olfactory receptor neurons

With the discovery of postnatal stem cells within the brain, it has become important to understand how extracellular factors might affect the maturation of neuronal precursors in the postnatal brain. Neurotrophic factors are known to play a role in neuronal development but display pleiotrophic effects, in part because of their physiological interactions with other factors. One factor positioned to interact with neurotrophins in the brains of postnatal animals is atrial C-type natriuretic peptide (CNP). In this study, we used olfactory receptor neurons (ORNs) as a model, because their precursors demonstrate the most robust and functional postnatal neurogenesis of those systems thus far described. We examined the effects of brain-derived neurotrophic factor (BDNF) and nerve growth factor (NGF) and the interactions of these neurotrophins and CNP in postnatal olfactory neuronal precursors. Results obtained using mice with targeted deletion of the gene for BDNF indicated that BDNF is a neuroproliferation-inducing and survival factor for ORN precursors. These roles were confirmed in vitro using primary cultures of ORNs. NGF was found to be a proliferation-inducing factor but not a survival factor. The addition of CNP to either BDNF- or NGF-treated neuronal precursors resulted in an inhibition of proliferation and the promotion of maturation. These effects were accompanied by changes in cell-cycle proteins that suggest possible mechanisms for these effects. Thus, CNP may function in the postnatal brain to regulate the exit from the cell cycle in neuronal precursor cells.

Leukemia inhibitory factor inhibits neuronal terminal differentiation through STAT3 activation

The discovery of stem cells in the adult central nervous system raises questions concerning the neurotrophic factors that regulate postnatal neuronal development. Olfactory receptor neurons (ORNs) are a useful model, because they are capable of robust neurogenesis throughout adulthood. We have investigated the role of leukemia inhibitory factor (LIF) in postnatal neuronal development by using ORNs as a model. LIF is a multifunctional cytokine implicated in various aspects of neuronal development, including phenotype determination, survival, and in response to nerve injury. LIF-deficient mice display significant increases, both in the absolute amount and in the number of cells expressing olfactory marker protein, a marker of mature ORNs. The maturation of ORNs was significantly inhibited by LIF in vitro. LIF activated the STAT3 pathway in ORNs, and transfection of ORNs with a dominant negative form of STAT3 abolished the effect of LIF. These findings demonstrate that LIF negatively regulates ORN maturation via the STAT3 pathway. Thus, LIF plays a critical role in controlling the transition of ORNs to maturity. Consequently, a population of ORNs is maintained in an immature state to facilitate the rapid repopulation of the olfactory epithelium with mature neurons during normal cell turnover or after injury.

G proteins and olfactory signal transduction

The olfactory system sits at the interface of the environment and the nervous system and is responsible for correctly coding sensory information from thousands of odorous stimuli. Many theories existed regarding the signal transduction mechanism that mediates this difficult task. The discovery that odorant transduction utilizes a unique variation (a novel family of G protein-coupled receptors) based upon a very common theme (the G protein-coupled adenylyl cyclase cascade) to accomplish its vital task emphasized the power and versatility of this motif. We now must understand the downstream consequences of this cascade that regulates multiple second messengers and perhaps even gene transcription in response to the initial interaction of ligand with G protein-coupled receptor.

Phosphorylation of voltage-gated ion channels in rat olfactory receptor neurons

In olfactory receptor neurons (ORNs), ligand-odorant receptor interactions cause G protein-mediated activation of adenylate cyclase and a subsequent increase in concentration of the intracellular messenger cAMP. Odorant-evoked elevation in cAMP is thought to directly activate a cation-selective cyclic nucleotide-gated channel, which causes external Ca2+ influx, leading to membrane depolarization and the generation of action potentials. Our data show that in freshly dissociated rat ORNs, odorant-induced elevation in cAMP also activates cAMP-dependent protein kinase (PKA), which is then able to phosphorylate various protein targets in the olfactory signal transduction pathway, specifically voltage-gated sodium and calcium channels. The presence of PKI (PKA inhibitor peptide) blocked the modulatory action of cAMP on voltage-gated ion channels. By modulating the input/output properties of the sensory neurons, this mechanism could take part in the complex adaptation process in odorant perception. In addition, we found modulation of voltage-gated sodium and calcium channel currents by 5-hydroxytryptamine and the dopamine D1 receptor agonist SKF 38393. These findings suggest that in situ ORNs might also be a target for efferent modulation.

Odorants stimulate the ERK/mitogen-activated protein kinase pathway and activate cAMP-response element-mediated transcription in olfactory sensory neurons

Olfactory sensory neurons (OSNs) respond acutely to volatile molecules and exhibit adaptive responses including desensitization to odorant exposure. Although mechanisms for short term adaptation have been described, there is little evidence that odorants cause long lasting, transcription-dependent changes in OSNs. Here we report that odorants stimulate cAMP-response element (CRE)-mediated transcription in OSNs through Ca2+ activation of the ERK/MAPK/p90rsk pathway. Odorant stimulation of ERK phosphorylation was ablated by inhibition of calmodulin-dependent protein kinase II suggesting that odorant activation of ERK is mediated through this kinase. Moreover, a brief exposure in vivo to an odorant in vapor phase stimulated CRE-mediated gene transcription in discrete populations of OSNs. These data suggest that like central nervous system neurons, OSNs may undergo long term adaptive changes mediated through CRE-mediated transcription.

NO activation of fos promoter elements requires nuclear translocation of G-kinase I and CREB phosphorylation but is independent of MAP kinase activation

We have shown that nitric oxide (NO) regulates c-fos gene expression via cGMP-dependent protein kinase (G-kinase), but NO's precise mechanism of action is unclear. We now demonstrate that: (1) NO targets two transcriptional elements in the fos promoter, i.e., the fos AP-1 binding site and the cAMP-response element (CRE); (2) NO activation of these two enhancer elements requires the CRE binding protein CREB because a dominant negative CREB fully inhibits NO transactivation of reporter genes whereas dominant negative Fos or CCAAT enhancer binding proteins have no effect; (3) CREB is phosphorylated by G-kinase in vitro and its phosphorylation increases in vivo when G-kinase is activated either directly by cGMP or indirectly by NO via soluble guanylate cyclase; (4) NO activation of fos promoter elements requires nuclear translocation of G-kinase but not activation of mitogen-activated protein kinases.

The genomic structure of human olfactory receptor genes

The genomic and cDNA structures were studied for eight human olfactory receptor (OR) genes within the chromosome 17p13.3 cluster. A common gene structure was revealed, which included an approximately 1-kb intronless coding region terminated by a signal for polyadenylation and a variable number of upstream noncoding exons. The latter were found to be alternatively spliced, giving rise to different isoforms of OR mRNA. While the initial exons mostly agreed with previous computer predictions and were conserved within OR subfamilies, other upstream exons were novel and idiosyncratic. In some cases, repetitive sequences were involved in the generation of splice sites and putative transcription control elements. Such gene structure is consistent with early repertoire enhancement by retrogene generation, which was likely followed by extensive genomic duplication. Each OR gene had a unique signature of transcription factor elements, consistent with a combinatorial expression control mechanism.

AMPA receptor-mediated, calcium-dependent CREB phosphorylation in a subpopulation of auditory neurons surviving activity deprivation

Although dependence on afferent synaptic activity has been shown for central neurons in every sensory system, the mechanisms of afferent maintenance of target sensory neurons are not understood. Neurons in the cochlear nucleus (CN) require afferent activity for maintenance and survival. One of the earliest changes seen after activity deprivation is an increase in intracellular calcium that leads to the death of 30% of the neuronal population. Sixty minutes after deafferentation, the surviving neurons show increased phosphorylation of the transcription factor calcium/cAMP response element-binding protein (CREB). CREB phosphorylation in activity-deprived CN neurons is dependent on increased intracellular calcium resulting from influx through AMPA receptors and is mediated by calcium/calmodulin-dependent kinases and protein kinase A. We conclude that in CN neurons, the deafferentation-induced increase in calcium activates at least two kinase pathways that phosphorylate CREB in surviving neurons. We hypothesize that this phosphorylation results in the transcription of genes containing the calcium/cAMP response element within their promoter regions, and these genes code for proteins that allow the neurons to compensate for their hypercalcemic, activity-deprived state.

AMPA receptor-mediated, calcium-dependent CREB phosphorylation in a subpopulation of auditory neurons surviving activity deprivation

Although dependence on afferent synaptic activity has been shown for central neurons in every sensory system, the mechanisms of afferent maintenance of target sensory neurons are not understood. Neurons in the cochlear nucleus (CN) require afferent activity for maintenance and survival. One of the earliest changes seen after activity deprivation is an increase in intracellular calcium that leads to the death of 30% of the neuronal population. Sixty minutes after deafferentation, the surviving neurons show increased phosphorylation of the transcription factor calcium/cAMP response element-binding protein (CREB). CREB phosphorylation in activity-deprived CN neurons is dependent on increased intracellular calcium resulting from influx through AMPA receptors and is mediated by calcium/calmodulin-dependent kinases and protein kinase A. We conclude that in CN neurons, the deafferentation-induced increase in calcium activates at least two kinase pathways that phosphorylate CREB in surviving neurons. We hypothesize that this phosphorylation results in the transcription of genes containing the calcium/cAMP response element within their promoter regions, and these genes code for proteins that allow the neurons to compensate for their hypercalcemic, activity-deprived state.

Reconstructing smell

Odorant signal transduction and neurogenesis are fundamental properties of the olfactory epithelium. Many preparations have been used to elucidate some of the mechanisms underlying these properties. In this article, we briefly review these research areas and describe some of the techniques used to obtain the data. We focus specifically on the cell-culture paradigm and the data obtained from various immortal cell lines in their attempts to reconstruct the olfactory epithelium in vitro.