The phosphatidylinositol 4,5-bisphosphate (PIP 2 )-sensitive inward rectifier channel Kir2.1 was expressed in Drosophila photoreceptors and used to monitor in vivo PIP 2 levels. Since the wild-type (WT) Kir2.1 channel appeared to be saturated by the prevailing PIP 2 concentration, we made a single amino acid substitution (R228Q), which reduced the effective affinity for PIP 2 and yielded channels generating currents proportional to the PIP 2 levels relevant for phototransduction. To isolate Kir2.1 currents, recordings were made from mutants lacking both classes of light-sensitive transient receptor potential channels (TRP and TRPL). Light resulted in the effective depletion of PIP 2 by phospholipase C (PLC) in approximately three or four microvilli per absorbed photon at rates exceeding ϳ150% of total microvillar phosphoinositides per second. PIP 2 was resynthesized with a half-time of ϳ50 s. When PIP 2 resynthesis was prevented by depriving the cell of ATP, the Kir current spontaneously decayed at maximal rates representing a loss of ϳ40% loss of total PIP 2 per minute. This loss was attributed primarily to basal PLC activity, because it was greatly decreased in norpA mutants lacking PLC. We tried to confirm this by using the PLC inhibitor U73122; however, this was found to act as a novel inhibitor of the Kir2.1 channel. PIP 2 levels were reduced ϳ5-fold in the diacylglycerol kinase mutant (rdgA), but basal PLC activity was still pronounced, consistent with the suggestion that raised diacylglycerol levels are responsible for the constitutive TRP channel activity characteristic of this mutant.Phototransduction in Drosophila is mediated by a G-proteincoupled phospholipase C (PLC) 1 cascade, resulting in activation of two classes of light-sensitive channels TRP and TRPL. These are the prototypical members of the large and diverse family of "transient receptor potential" (TRP) channels many of which, including all of the most closely related "canonical" TRPC subfamily, are also regulated by PLC (reviewed in Refs.
Most insect species rely on the detection of olfactory cues for critical behaviors for the survival of the species, e.g., finding food, suitable mates and appropriate egg-laying sites. Although insects show a diverse array of molecular receptors dedicated to the detection of sensory cues, two main types of molecular receptors have been described as responsible for olfactory reception in Drosophila, the odorant receptors (ORs) and the ionotropic receptors (IRs). Although both receptor families share the role of being the first chemosensors in the insect olfactory system, they show distinct evolutionary origins and several distinct structural and functional characteristics. While ORs are seven-transmembrane-domain receptor proteins, IRs are related to the ionotropic glutamate receptor (iGluR) family. Both types of receptors are expressed on the olfactory sensory neurons (OSNs) of the main olfactory organ, the antenna, but they are housed in different types of sensilla, IRs in coeloconic sensilla and ORs in basiconic and trichoid sensilla. More importantly, from the functional point of view, they display different odorant specificity profiles. Research advances in the last decade have improved our understanding of the molecular basis, evolution and functional roles of these two families, but there are still controversies and unsolved key questions that remain to be answered. Here, we present an updated review on the advances of the genetic basis, evolution, structure, functional response and regulation of both types of chemosensory receptors. We use a comparative approach to highlight the similarities and differences among them. Moreover, we will discuss major open questions in the field of olfactory reception in insects. A comprehensive analysis of the structural and functional convergence and divergence of both types of receptors will help in elucidating the molecular basis of the function and regulation of chemoreception in insects.
In Drosophila photoreceptors, the amplification responsible for generating quantum bumps in response to photoisomerization of single rhodopsin molecules has been thought to be mediated downstream of phospholipase C (PLC), since bump amplitudes were reportedly unaffected in mutants with greatly reduced levels of either G protein or PLC. We now find that quantum bumps in such mutants are reduced approximately 3- to 5-fold but are restored to near wild-type values by mutations in the rdgA gene encoding diacylglycerol kinase (DGK) and also by depleting intracellular ATP. The results demonstrate that amplification requires activation of multiple G protein and PLC molecules, identify DGK as a key enzyme regulating amplification, and implicate diacylglycerol as a messenger of excitation in Drosophila phototransduction.
Light responses in Drosophila are reportedly abolished in severe mutants of the phospholipase C (PLC) gene, norpA. However, on establishing the whole-cell recording configuration in photoreceptors of the supposedly null allele, norpA P24 , we detected a small (ϳ15 pA) inward current that represented spontaneous light channel activity. The current decayed during ϳ20 min, after which tiny residual responses (<2 pA) were elicited by intense flashes. Both spontaneous currents and light responses appeared to be mediated by residual PLC activity, because they were enhanced by impairing diacylglycerol (DAG) kinase function by mutation (rdgA) or by restricting ATP but were reduced or abolished by a mutation of the PLC-specific G q ␣ subunit. It was reported recently that metabolic inhibition activated the light-sensitive transient receptor potential and transient receptor potential-like channels, even in norpA P24 , leading to the conclusion that this action was independent of PLC (Agam, K., von Campenhausen, M., Levy, S., Ben-Ami, H. C., Cook, B., Kirschfeld, K., and Minke, B. (2000) J. Neurosci. 20, 5748 -5755). However, we found that channel activation by metabolic inhibitors in norpA P24 was strictly dependent on the residual PLC activity underlying the spontaneous current, because the inhibitors failed to activate any channels after the spontaneous current had decayed. By contrast, polyunsaturated fatty acids invariably activated the channels independently of PLC. The results strongly support the obligatory requirement for PLC and DAG in Drosophila phototransduction, suggest that activation by metabolic inhibition is primarily because of the failure of diacylglycerol kinase, and are consistent with the proposal that polyunsaturated fatty acids, which are potential DAG metabolites, act directly on the channels.
Two main second messenger systems depending on IP3 and cAMP have been related to olfaction in vertebrates as well as invertebrates. In Drosophila melanogaster, the availability of mutations affecting one or the other pathway (rdgB and norpA or rut and dnc, respectively) allowed showing of abnormal olfactory behavior phenotypes associated with olfactory transduction in complete living animals. However, because rut and dnc genes showed ubiquitous expression at olfactory receptor organs and some brain locations, the mutant behavior cannot be assigned exclusively to olfactory reception. In this report, overexpression of the dnc gene directed specifically to different olfactory receptor neuron subsets was used to produce dominant mutants. Abnormal olfactory behavior was found in 62.5% of the 8 lines studied in response to some odorants, depending on the affected neuronal subset. These results suggest that even for a small number of tested odorants (5), cAMP cascade is involved in olfactory reception to an important extent.
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