Probing roles of TRPML in Drosophila hearing

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Several transient receptor potential (TRP) channels have been implicated in Drosophila hearing, including the TRPN channel NOMPC, the TRPVs Nan and Iav, and the TRPA members Pyx and Pain. Here I report that fly hearing also involves TRPML mucolipin channels, mutations in which are implicated in the neurodegenerative disorder mucolipidosis type IV (TRPML1) in humans and hearing impairments in varitint-waddler mice (TRPML3) (Di Palma et al. 2002). The latter hearing impairments arise from a point mutation that render TRPML3 constitutively open (Grimm et al. 2007, Kim et al. 2007, Nagata et al. 2008, van Aken et al. 2008), yet hearing remains uncompromised by the loss of this channel (Jörs et al. 2010), leaving the auditory relevance of TRPMLs elusive. In contrast to the mammalian system, where different TRPML channels may compensate for each other, Drosophila has only one TRPML member (trpml; dTRPML), facilitating physiological investigations. Here, I show that Drosophila TRPML is expressed in auditory neurons and required for sensitive hearing. Investigations of the fly’s auditory mechanics revealed that mechanical amplification by auditory neurons is disrupted by null mutation in trpml, linearizing the mechanics of the fly’s antennal sound receiver, reducing its fluctuation power and its mechanical sensitivity, and increasing its mechanical best frequency. This loss of mechanical amplification is associated with a reduced sensitivity of auditory nerve responses to both sound stimuli and sound-induced receiver displacements. A genomic trpml rescue construct partially restored normal amplification and auditory sensitivity in mutant flies, and so did the selective expression of trpml in the auditory neurons. Promoter fusions revealed that trpml is expressed in the fly’s auditory neurons, and mutant defects in mechanotransduction were identified when I analyzed correlates of mechanical ion channel gating. Epistatic analyses place TRPML parallel to Nan-Iav TRPV channels, which impede mechanical amplification by auditory neurons whereas TRPML facilitates this amplification. This establishes a role of TRPML mucolipin channels in hearing, auditory organ physiology, and auditory neuron function. Results providing first hints on the relation between lysosomal defects and hearing deficits in trpml mutants are also presented and discussed.