Article

Photomechanical responses in Drosophila photoreceptors

Department of Physiology, Development and Neuroscience, University of Cambridge, Cambridge CB2 3EG, UK.
Science (Impact Factor: 31.48). 10/2012; 338(6104):260-3. DOI: 10.1126/science.1222376
Source: PubMed

ABSTRACT Phototransduction in Drosophila microvillar photoreceptor cells is mediated by a G protein-activated phospholipase C (PLC). PLC hydrolyzes the minor membrane lipid phosphatidylinositol 4,5-bisphosphate (PIP(2)), leading by an unknown mechanism to activation of the prototypical transient receptor potential (TRP) and TRP-like (TRPL) channels. We found that light exposure evoked rapid PLC-mediated contractions of the photoreceptor cells and modulated the activity of mechanosensitive channels introduced into photoreceptor cells. Furthermore, photoreceptor light responses were facilitated by membrane stretch and were inhibited by amphipaths, which alter lipid bilayer properties. These results indicate that, by cleaving PIP(2), PLC generates rapid physical changes in the lipid bilayer that lead to contractions of the microvilli, and suggest that the resultant mechanical forces contribute to gating the light-sensitive channels.

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Available from: Roger C Hardie, Aug 19, 2015
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    • "Another exciting possibility relates to the idea that mechanical force may act in the adult brain as an intermediate or 2nd messenger in metabotropic signal transduction (Hardie and Franze, 2012). In particular, phospholipase C (PLC) hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP 2 ) has been shown to cause rapid mechanical contraction of Drosophila photoreceptors and activation of endogenous transient receptor potential channels as well as an exogenous stretch-sensitive channel gramicidin (Martinac and Hamill, 2002) introduced into the photoreceptor cells (Hardie and Franze, 2012). Given the ubiquitous expression of PLC/PIP 2 signaling in cells, including neurons of the mammalian brain (Jang et al., 2013) MG cation channels described here may also play a role in transducing PLC/PIP 2 generated mechanical force. "
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    ABSTRACT: The mammalian brain is a mechanosensitive organ that responds to different mechanical forces ranging from intrinsic forces implicated in brain morphogenesis to extrinsic forces that can cause concussion and traumatic brain injury. However, little is known of the mechanosensors that transduce these forces. In this study we use cell-attached patch recording to measure single mechanically-gated (MG) channel currents and their affects on spike activity in identified neurons in neonatal mouse brain slices. We demonstrate that both neocortical and hippocampal pyramidal neurons express stretch-activated MG cation channels that are activated by suctions of~25mm Hg, have a single channel conductance for inward current of 50-70pS and show weak selectivity for alkali metal cations (i.e., Na(+)<K(+)<Cs(+)). Significantly, single MG channel currents activated on the soma trigger spiking/action potentials in both neocortical and hippocampal pyramidal neurons. Not all neuron types studied here expressed MG channel currents. In particular, locus coeruleus and cerebellar Purkinje neurons showed no detectable MG channel activity. Moreover their robust rhythmic spike activity was resistant to mechanical modulation. Our observation that a single MG channel current can trigger spiking predicates the need for reassessment of the long held view that the impulse output of central neurons depends only upon their intrinsic voltage-gated channels and/or their integrated synaptic input. Copyright © 2015. Published by Elsevier B.V.
    Brain Research 03/2015; 1608. DOI:10.1016/j.brainres.2015.02.051 · 2.83 Impact Factor
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    • "In Drosophila, photon absorption by rhodopsin causes photoisomerization to metarhodopsin, which activates a heterotrimeric Gq-protein, initiating a cascade leading to activation of IP 3 and diacylglycerol. Linkages from this cascade to opening of transient receptor potential (dTRP) and TRP-like (dTRPL) ion channels that carry the receptor current are still debated, and both chemical (Chyb et al., 1999; Huang et al., 2010) and mechanical (Hardie and Franze, 2012) intermediate steps have been proposed. In Drosophila, dTRP and dTRPL channels are thought to carry approximately equal parts of light-activated current under physiological conditions (Reuss et al., 1997). "
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    ABSTRACT: Our current understanding of insect phototransduction is based on a small number of species, but insects occupy many different visual environments. We created the retinal transcriptome of a nocturnal insect, the cockroach, Periplaneta americana to identify proteins involved in the earliest stages of compound eye phototransduction, and test the hypothesis that different visual environments are reflected in different molecular contributions to function. We assembled five novel mRNAs: two green opsins, one UV opsin, and one each TRP and TRPL ion channel homologs. One green opsin mRNA (pGO1) was 100-1000 times more abundant than the other opsins (pGO2 and pUVO), while pTRPL mRNA was 10 times more abundant than pTRP, estimated by transcriptome analysis or quantitative PCR (qPCR). Electroretinograms were used to record photoreceptor responses. Gene-specific in vivo RNA interference (RNAi) was achieved by injecting long (596-708 bp) double-stranded RNA into head hemolymph, and verified by qPCR. RNAi of the most abundant green opsin reduced both green opsins by more than 97% without affecting UV opsin, and gave a maximal reduction of 75% in ERG amplitude 7 days after injection that persisted for at least 19 days. RNAi of pTRP and pTRPL genes each specifically reduced the corresponding mRNA by 90%. Electroretinogram (ERG) reduction by pTRPL RNAi was slower than for opsin, reaching 75% attenuation by 21 days, without recovery at 29 days. pTRP RNAi attenuated ERG much less; only 30% after 21 days. Combined pTRP plus pTRPL RNAi gave only weak evidence of any cooperative interactions. We conclude that silencing retinal genes by in vivo RNAi using long dsRNA is effective, that visible light transduction in Periplaneta is dominated by pGO1, and that pTRPL plays a major role in cockroach phototransduction.
    Frontiers in Physiology 01/2015; 6:207. DOI:10.3389/fphys.2015.00207 · 3.50 Impact Factor
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    • "The smallest LICs, namely quantum bumps (Wu and Pak, 1975), result from single photon absorptions and the opening of ∼15 TRP and TRPL channels, producing peak responses of ∼10 pA (Henderson et al., 2000). The influx of Ca 2+ is particularly important, as it is not only the predominant mediator of the LIC but also a necessary modulator of the whole phototransduction machinery (Hardie, 1991, 1995a; Henderson et al., 2000; Hardie et al., 2001, 2012; Gu et al., 2005; Liu et al., 2008). It is widely believed that microvillar photoreceptors generally rely on the phosphoinositide cascade (Hardie and Postma, 2009). "
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    ABSTRACT: In a microvillar photoreceptor, absorption of an incident photon initiates a phototransduction reaction that generates a depolarizing light-induced current (LIC) in the microvillus. Although in-depth knowledge about these processes in photoreceptors of the fruitfly Drosophila is available, not much is known about their nature in other insect species. Here, we present description of some basic properties of both elementary and macroscopic LICs and their Ca(2+)-dependence in the photoreceptors of a dark-active species, the cockroach Periplaneta americana. Cockroach photoreceptors respond to single photon absorptions by generating quantum bumps with about 5-fold larger amplitudes than in Drosophila. At the macroscopic current level, cockroach photoreceptors responded to light with variable sensitivity and current waveform. This variability could be partially attributed to differences in whole-cell capacitance. Transient LICs, both elementary and macroscopic, showed only moderate dependence on extracellular Ca(2+). However, with long light pulses, response inactivation was largely abolished and the overall size of LICs increased when extracellular Ca(2+) was omitted. Finally, by determining relative ionic permeabilities from reversals of LICs, we demonstrate that when compared to Drosophila, cockroach light-gated channels are only moderately Ca(2+)-selective.
    Frontiers in Physiology 04/2014; 5:153. DOI:10.3389/fphys.2014.00153 · 3.50 Impact Factor
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