Control of membrane-receptor activity is required not only for the accuracy of sensory responses, but also to protect cells from excitotoxicity. Here we report the isolation of two noncomplementary fly mutants with slow termination of photoresponses. Genetic and electrophysiological analyses of the mutants revealed a defect in the deactivation of rhodopsin, a visual G protein-coupled receptor (GPCR). The mutant gene was identified as the calmodulin-binding transcription activator (dCAMTA). The known rhodopsin regulator Arr2 does not mediate this visual function of dCAMTA. A genome-wide screen identified five dCAMTA target genes. Of these, overexpression of the F box gene dFbxl4 rescued the mutant phenotypes. We further showed that dCAMTA is stimulated in vivo through interaction with the Ca(2+) sensor calmodulin. Our data suggest that calmodulin/CAMTA/Fbxl4 may mediate a long-term feedback regulation of the activity of Ca(2+)-stimulating GPCRs, which could prevent cell damage due to extra Ca(2+) influx.

Animals use circadian rhythms to anticipate daily environmental changes. Circadian clocks have a profound effect on behavior. In Drosophila, for example, brain pacemaker neurons dictate that flies are mostly active at dawn and dusk. miRNAs are small, regulatory RNAs (Ϸ22 nt) that play important roles in posttranscriptional regulation. Here, we identify miR-124 as an important regulator of Drosophila circadian locomotor rhythms. Under constant darkness, flies lacking miR-124 (miR-124 KO ) have a dramatically advanced circadian behavior phase. However, whereas a phase defect is usually caused by a change in the period of the circadian pacemaker, this is not the case in miR-124 KO flies. Moreover, the phase of the circadian pacemaker in the clock neurons that control rhythmic locomotion is not altered either. Therefore, miR-124 modulates the output of circadian clock neurons rather than controlling their molecular pacemaker. Circadian phase is also advanced under temperature cycles, but a light/dark cycle partially corrects the defects in miR-124 KO flies. Indeed, miR-124 KO shows a normal evening phase under the latter conditions, but morning behavioral activity is suppressed. In summary, miR-124 controls diurnal activity and determines the phase of circadian locomotor behavior without affecting circadian pacemaker function. It thus provides a potent entry point to elucidate the mechanisms by which the phase of circadian behavior is determined.

Background: Appropriate termination of photoresponse is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca 2ϩ -induced cell toxicity. Results: We isolated a novel G␣ q mutant allele and revealed that metarhodopsin/G q interaction affects Arr2-Rh1 binding. Conclusion: G q modulates the termination of phototransduction and prevents retinal degeneration. Significance: Our study revealed the novel role of G q in phototransduction deactivation and in retinal degeneration.Appropriate termination of the phototransduction cascade is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca 2؉ -induced cell toxicity. Using a genetic screen to identify defective photoresponse mutants in Drosophila, we isolated and identified a novel G␣ q mutant allele, which has defects in both activation and deactivation. We revealed that G q modulates the termination of the light response and that metarhodopsin/G q interaction affects subsequent arrestin-rhodopsin (Arr2-Rh1) binding, which mediates the deactivation of metarhodopsin. We further showed that the G␣ q mutant undergoes light-dependent retinal degeneration, which is due to the slow accumulation of stable Arr2-Rh1 complexes. Our study revealed the roles of G q in mediating photoresponse termination and in preventing retinal degeneration. This pathway may represent a general rapid feedback regulation of G protein-coupled receptor signaling.Heterotrimeric G proteins play pivotal roles in mediating extracellular signals from hormones, neurotransmitters, peptides, as well as sensory stimuli to intracellular signaling pathways (1, 2). In Drosophila photoreceptors, G proteins are essential for the activation of the phototransduction cascade (3, 4). Photon absorption leads to the photoisomerization of chromophores, resulting in the formation of activated metarhodopsin. In turn, metarhodopsin activates heterotrimeric G proteins and PLC.2 The activation of PLC leads to transient receptor potential and transient receptor potential-like channels opening and extracellular Ca 2ϩ influx (5-8). It is also critical for each step of the phototransduction cascade to be terminated appropriately, which is essential for the high temporal resolution of fly vision (9, 10). The most important step in phototransduction termination is the deactivation of metarhodopsin. During this step, arrestin (Arr2) plays an important role by displacing the G q ␣ subunit and allowing it to bind with rhodopsin (Rh1) (11,12). Unlike other G proteincoupled receptors (GPCRs), the phosphorylation of fly rhodopsin is not required for its deactivation (13) but is essential for its endocytosis (11). In contrast, the dephosphorylation of rhodopsin by retinal degeneration C (RDGC) is essential for receptor deactivation (14). Ca 2ϩ also plays critical roles in regulating the termination of the photoresponse in Drosophila (8,15,16). Several proteins that mediate this Ca 2ϩ -regulated termination have been identified, such as eye-specific protein kinase C...

Recycling of neurotransmitters is essential for sustained neuronal signaling, yet recycling pathways for various transmitters, including histamine, remain poorly understood. In the first visual ganglion (lamina) of Drosophila, photoreceptor-released histamine is taken up into perisynaptic glia, converted to carcinine, and delivered back to the photoreceptor for histamine regeneration. Here, identify an organic cation transporter, CarT (carcinine transporter), that transports carcinine into photoreceptors during histamine recycling. CarT mediated in vitro uptake of carcinine. Deletion of the CarT gene caused an accumulation of carcinine in laminar glia accompanied by a reduction in histamine, resulting in abolished photoreceptor signal transmission and blindness in behavioral assays. These defects were rescued by expression of CarT cDNA in photoreceptors, and were reproduced by photoreceptor-specific CarT knockdown. Our findings suggest a common role for the conserved family of CarT-like transporters in maintaining histamine homeostasis in both mammalian and fly brains.

The Drosophila photoreceptor is a model system for genetic study of retinal degeneration. Many gene mutations cause fly photoreceptor degeneration, either because of excessive stimulation of the visual transduction (phototransduction) cascade, or through apoptotic pathways that in many cases involve a visual arrestin Arr2. Here we report a gene named tadr (for torn and diminished rhabdomeres), which, when mutated, leads to photoreceptor degeneration through a different mechanism. Degeneration in the tadr mutant is characterized by shrunk and disrupted rhabdomeres, the light sensory organelles of photoreceptor. The TADR protein interacted in vitro with the major light receptor Rh1 rhodopsin, and genetic reduction of the Rh1 level suppressed the tadr mutation-caused degeneration, suggesting the degeneration is Rh1-dependent. Nonetheless, removal of phospholipase C (PLC), a key enzyme in phototransduction, and that of Arr2 failed to inhibit rhabdomeral degeneration in the tadr mutant background. Biochemical analyses revealed that, in the tadr mutant, the G q protein of Rh1 is defective in dissociation from the membrane during light stimulation. Importantly, reduction of G q level by introducing a hypomorphic allele of G ␣q gene greatly inhibited the tadr degeneration phenotype. These results may suggest that loss of a potential TADR-Rh1 interaction leads to an abnormality in the G q signaling, which in turn triggers rhabdomeral degeneration independent of the PLC phototransduction cascade. We propose that TADR-like proteins may also protect photoreceptors from degeneration in mammals including humans.