Drosophila Hearing Research Articles

The voltage-gated potassium channel Shal (Kv4) contributes to active hearing in Drosophila.

The full complement of ion channels which influence insect auditory mechanotransduction, and the mechanisms by which their influence is exerted, remain unclear. Shal (Kv4), a Shaker family member encoding voltage-gated potassium channels in Drosophila melanogaster, has been shown to localize to dendrites in some neuron types, suggesting a potential role for Shal in Drosophila hearing, including mechanotransduction. A GFP-protein trap was used to visualize the localization of the Shal channel in Johnston's organ neurons responsible for hearing in the antenna. Shal protein was localized strongly to the cell body and inner dendritic segment of sensory neurons. It was also detectable in the sensory cilium, suggesting its involvement not only in general auditory function, but specifically in mechanotransduction. Electrophysiological recordings to assess neural responses to auditory stimuli in mutant Shal flies revealed significant decreases in auditory responses. Laser Doppler vibrometer recordings indicated abnormal antennal free fluctuation frequencies in mutant lines, indicating an effect on active antennal tuning, and thus active transduction mechanisms. This suggests that Shal participates in coordinating energy-dependent antennal movements in Drosophila that are essential for tuning the antenna to courtship song frequencies.Significance Statement The study of fruit fly hearing has revealed mechanosensitive ion channels that participate in mechanotransduction, and as in mammalian hearing, energy-dependent mechanisms actively amplify and tune auditory processes. Identifying distinct roles played by different ion channels is essential to better understand this process. Here, we explore the influence of a specific voltage-gated potassium channel, Shal, on fly hearing, and find that it affects specific parts of the mechanotransduction process. Our research uncovers Shal's localization in sensory dendrite regions of auditory neurons, where it contributes to shaping mechanotransduction and active antennal tuning. Understanding Shal's involvement in auditory function and mechanotransduction deepens our knowledge of fly hearing and unveils a key player in the coordination of energy-dependent active antennal movements.

IFT88 maintains sensory function by localising signalling proteins along Drosophila cilia.

Ciliary defects cause several ciliopathies, some of which have late onset, suggesting cilia are actively maintained. Still, we have a poor understanding of the mechanisms underlying their maintenance. Here, we show Drosophila melanogaster IFT88 (DmIFT88/nompB) continues to move along fully formed sensory cilia. We further identify Inactive, a TRPV channel subunit involved in Drosophila hearing and negative-gravitaxis behaviour, and a yet uncharacterised Drosophila Guanylyl Cyclase 2d (DmGucy2d/CG34357) as DmIFT88 cargoes. We also show DmIFT88 binding to the cyclase´s intracellular part, which is evolutionarily conserved and mutated in several degenerative retinal diseases, is important for the ciliary localisation of DmGucy2d. Finally, acute knockdown of both DmIFT88 and DmGucy2d in ciliated neurons of adult flies caused defects in the maintenance of cilium function, impairing hearing and negative-gravitaxis behaviour, but did not significantly affect ciliary ultrastructure. We conclude that the sensory ciliary function underlying hearing in the adult fly requires an active maintenance program which involves DmIFT88 and at least two of its signalling transmembrane cargoes, DmGucy2d and Inactive.

Diverse Roles of Axonemal Dyneins in Drosophila Auditory Neuron Function and Mechanical Amplification in Hearing.

Much like vertebrate hair cells, the chordotonal sensory neurons that mediate hearing in Drosophila are motile and amplify the mechanical input of the ear. Because the neurons bear mechanosensory primary cilia whose microtubule axonemes display dynein arms, we hypothesized that their motility is powered by dyneins. Here, we describe two axonemal dynein proteins that are required for Drosophila auditory neuron function, localize to their primary cilia, and differently contribute to mechanical amplification in hearing. Promoter fusions revealed that the two axonemal dynein genes Dmdnah3 (=CG17150) and Dmdnai2 (=CG6053) are expressed in chordotonal neurons, including the auditory ones in the fly’s ear. Null alleles of both dyneins equally abolished electrical auditory neuron responses, yet whereas mutations in Dmdnah3 facilitated mechanical amplification, amplification was abolished by mutations in Dmdnai2. Epistasis analysis revealed that Dmdnah3 acts downstream of Nan-Iav channels in controlling the amplificatory gain. Dmdnai2, in addition to being required for amplification, was essential for outer dynein arms in auditory neuron cilia. This establishes diverse roles of axonemal dyneins in Drosophila auditory neuron function and links auditory neuron motility to primary cilia and axonemal dyneins. Mutant defects in sperm competition suggest that both dyneins also function in sperm motility.

Prestin is an anion transporter dispensable for mechanical feedback amplification in Drosophila hearing.

In mammals, the membrane-based protein Prestin confers unique electromotile properties to cochlear outer hair cells, which contribute to the cochlear amplifier. Like mammals, the ears of insects, such as those of Drosophila melanogaster, mechanically amplify sound stimuli and have also been reported to express Prestin homologs. To determine whether the D. melanogaster Prestin homolog (dpres) is required for auditory amplification, we generated and analyzed dpres mutant flies. We found that dpres is robustly expressed in the fly’s antennal ear. However, dpres mutant flies show normal auditory nerve responses, and intact non-linear amplification. Thus we conclude that, in D. melanogaster, auditory amplification is independent of Prestin. This finding resonates with prior phylogenetic analyses, which suggest that the derived motor function of mammalian Prestin replaced, or amended, an ancestral transport function. Indeed, we show that dpres encodes a functional anion transporter. Interestingly, the acquired new motor function in the phylogenetic lineage leading to birds and mammals coincides with loss of the mechanotransducer channel NompC (=TRPN1), which has been shown to be required for auditory amplification in flies. The advent of Prestin (or loss of NompC, respectively) may thus mark an evolutionary transition from a transducer-based to a Prestin-based mechanism of auditory amplification.Electronic supplementary materialThe online version of this article (doi:10.1007/s00359-014-0960-9) contains supplementary material, which is available to authorized users.

Mechanisms and genes in Drosophila hearing

Abstract The fruit fly Drosophila melanogaster com­municates acoustically and hears with its an­tennae. Fundamental aspects of hearing can be studied in these antennal ears, the audi­tory sensory cells of which are evolutionarily related to vertebrate hair cells and are spec­ified developmentally by homologous tran­scription factors. Like vertebrate hair cells, Drosophila auditory sensory cells are also mo­tile and actively amplify the mechanical vi­brations they transduce. The transduction and amplification mechanisms rely on the in­terplay between mechanically activated ion channels and motor proteins, whose move­ment impacts upon the macroscopic perfor­mance of the ear. The first molecular trans­ducer components have been identified and various auditory system-relevant proteins have been described. Several of these pro­teins are conserved components of cilia, sug­gesting the fly’s ear as a model for human cil­iopathies. The evolution of sensory signaling cascades can also be studied using the fly’s ear, as the fly employs key chemo- and pho­toreceptor proteins to hear. Evidence is al­so accumulating that the fly’s ear is a multi­functional sensory organ, which, in addition to mediating hearing, serves to detect wind, gravity and presumably temperature.

Mechanisms and genes in Drosophila hearing

The fruit fly Drosophila melanogaster communicates acoustically and hears with its antennae. Fundamental aspects of hearing can be studied in these antennal ears, the auditory sensory cells of which are evolutionarily related to vertebrate hair cells and are specified developmentally by homologous transcription factors. Like vertebrate hair cells, Drosophila auditory sensory cells are also motile and actively amplify the mechanical vibrations they transduce. The transduction and amplification mechanisms rely on the interplay between mechanically activated ion channels and motor proteins, whose movement impacts upon the macroscopic performance of the ear. The first molecular transducer components have been identified and various auditory system-relevant proteins have been described. Several of these proteins are conserved components of cilia, suggesting the fly’s ear as a model for human ciliopathies. The evolution of sensory signaling cascades can also be studied using the fly’s ear, as the fly employs key chemo- and photoreceptor proteins to hear. Evidence is also accumulating that the fly’s ear is a multifunctional sensory organ, which, in addition to mediating hearing, serves to detect wind, gravity and presumably temperature.

Distinct Roles of TRP Channels in Auditory Transduction and Amplification in Drosophila

Auditory receptor cells rely on mechanically gated channels to transform sound stimuli into neural activity. Several TRP channels have been implicated in Drosophila auditory transduction, but mechanistic studies have been hampered by the inability to record subthreshold signals from receptor neurons. Here, we develop a non-invasive method for measuring these signals by recording from a central neuron that is electrically coupled to a genetically defined population of auditory receptor cells. We find that the TRPN family member NompC, which is necessary for the active amplification of sound-evoked motion by the auditory organ, is not required for transduction in auditory receptor cells. Instead, NompC sensitizes the transduction complex to movement and precisely regulates the static forces on the complex. In contrast, the TRPV channels Nanchung and Inactive are required for responses to sound, suggesting they are components of the transduction complex. Thus, transduction and active amplification are genetically separable processes in Drosophila hearing.

Hearing in Drosophila Requires TilB, a Conserved Protein Associated With Ciliary Motility

Cilia were present in the earliest eukaryotic ancestor and underlie many biological processes ranging from cell motility and propulsion of extracellular fluids to sensory physiology. We investigated the contribution of the touch insensitive larva B (tilB) gene to cilia function in Drosophila melanogaster. Mutants of tilB exhibit dysfunction in sperm flagella and ciliated dendrites of chordotonal organs that mediate hearing and larval touch sensitivity. Mutant sperm axonemes as well as sensory neuron dendrites of Johnston's organ, the fly's auditory organ, lack dynein arms. Through deficiency mapping and sequencing candidate genes, we identified tilB mutations in the annotated gene CG14620. A genomic CG14620 transgene rescued deafness and male sterility of tilB mutants. TilB is a 395-amino-acid protein with a conserved N-terminal leucine-rich repeat region at residues 16-164 and a coiled-coil domain at residues 171-191. A tilB-Gal4 transgene driving fluorescently tagged TilB proteins elicits cytoplasmic expression in embryonic chordotonal organs, in Johnston's organ, and in sperm flagella. TilB does not appear to affect tubulin polyglutamylation or polyglycylation. The phenotypes and expression of tilB indicate function in cilia construction or maintenance, but not in intraflagellar transport. This is also consistent with phylogenetic association of tilB homologs with presence of genes encoding axonemal dynein arm components. Further elucidation of tilB functional mechanisms will provide greater understanding of cilia function and will facilitate understanding ciliary diseases.

Hearing in Drosophila: from sensory mechanisms to genes

Hearing in Drosophila: from sensory mechanisms to genes

Transducer-Based Force Generation Explains Active Process in Drosophila Hearing

Like vertebrate hair cells, Drosophila auditory neurons are endowed with an active, force-generating process that boosts the macroscopic performance of the ear. The underlying force generator may be the molecular apparatus for auditory transduction, which, in the fly as in vertebrates, seems to consist of force-gated channels that occur in series with adaptation motors and gating springs. This molecular arrangement explains the active properties of the sensory hair bundles of inner-ear hair cells, but whether it suffices to explain the active macroscopic performance of auditory systems is unclear. To relate transducer dynamics and auditory-system behavior, we have devised a simple model of the Drosophila hearing organ that consists only of transduction modules and a harmonic oscillator that represents the sound receiver. In vivo measurements show that this model explains the ear's active performance, quantitatively capturing displacement responses of the fly's antennal sound receiver to force steps, this receiver's free fluctuations, its response to sinusoidal stimuli, nonlinearity, and activity and cycle-by-cycle amplification, and properties of electrical compound responses in the afferent nerve. Our findings show that the interplay between transduction channels and adaptation motors accounts for the entire macroscopic phenomenology of the active process in the Drosophila auditory system, extending transducer-based amplification from hair cells to fly ears and demonstrating that forces generated by transduction modules can suffice to explain active processes in ears.

A generalization of the van-der-Pol oscillator underlies active signal amplification in Drosophila hearing

The antennal hearing organs of the fruit fly Drosophila melanogaster boost their sensitivity by an active mechanical process that, analogous to the cochlear amplifier of vertebrates, resides in the motility of mechanosensory cells. This process nonlinearly improves the sensitivity of hearing and occasionally gives rise to self-sustained oscillations in the absence of sound. Time series analysis of self-sustained oscillations now unveils that the underlying dynamical system is well described by a generalization of the van-der-Pol oscillator. From the dynamic equations, the underlying amplification dynamics can explicitly be derived. According to the model, oscillations emerge from a combination of negative damping, which reflects active amplification, and a nonlinear restoring force that dictates the amplitude of the oscillations. Hence, active amplification in fly hearing seems to rely on the negative damping mechanism initially proposed for the cochlear amplifier of vertebrates.

A TRPV family ion channel required for hearing in Drosophila.

The many types of insect ear share a common sensory element, the chordotonal organ, in which sound-induced antennal or tympanal vibrations are transmitted to ciliated sensory neurons and transduced to receptor potentials. However, the molecular identity of the transducing ion channels in chordotonal neurons, or in any auditory system, is still unknown. Drosophila that are mutant for NOMPC, a transient receptor potential (TRP) superfamily ion channel, lack receptor potentials and currents in tactile bristles but retain most of the antennal sound-evoked response, suggesting that a different channel is the primary transducer in chordotonal organs. Here we describe the Drosophila Nanchung (Nan) protein, an ion channel subunit similar to vanilloid-receptor-related (TRPV) channels of the TRP superfamily. Nan mediates hypo-osmotically activated calcium influx and cation currents in cultured cells. It is expressed in vivo exclusively in chordotonal neurons and is localized to their sensory cilia. Antennal sound-evoked potentials are completely absent in mutants lacking Nan, showing that it is an essential component of the chordotonal mechanotransducer.

Towards a molecular understanding of Drosophila hearing.

The Drosophila auditory system is presented as a powerful new genetic model system for understanding the molecular aspects of development and physiology of hearing organs. The fly's ear resides in the antenna, with Johnston's organ serving as the mechanoreceptor. New approaches using electrophysiology and laser vibrometry have provided useful tools to apply to the study of mutations that disrupt hearing. The fundamental developmental processes that generate the peripheral nervous system are fairly well understood, although specific variations of these processes for chordotonal organs (CHO) and especially for Johnston's organ require more scrutiny. In contrast, even the fundamental physiologic workings of mechanosensitive systems are still poorly understood, but rapid recent progress is beginning to shed light. The identification and analysis of mutations that affect auditory function are summarized here, and prospects for the role of the Drosophila auditory system in understanding both insect and vertebrate hearing are discussed.

Identifying human disease genes in Drosophila

Geneticists have long known that humans share many similar genes with the fruit fly, but just how alike we are was not clear until the completion of the Drosophila genome. Now, Ethan Bier and his group at the University of California at San Diego have identified 548 Drosophila genes that are so similar to genes involved in 714 human genetic disorders that they could have occurred by chance only one time in 10 billion. The diseases fell into essentially every major category, including neurological, immunological, cardiovascular, auditory, visual, developmental and metabolic disorders, as well as many forms of cancer, giving medical researchers a head start in searching for models for these disorders and a new way to investigate the function of the genes implicated. ‘Most people don't think of studying blindness, or hearing in Drosophila,’ says Lawrence Reiter, one of the researchers. ‘But human genetics has hit a wall with regards to function. And the fly turns out to be an ideal model genetic system for analyzing genes and placing them in the context of known genetic pathways.’ [Reiter, L.T. et al. (2001) Genome Res. 11, 1114–1125] PL