By amplifying sound-induced vibrations in an intensity- and frequency specific manner, ears can sharpen their resonantmechanics. Ears boost their mechanical input with motile auditory receptors. In Drosophila, this mechanical amplification iscontrolled by heteromeric TRPV channels formed by Nanchung (Nan) and Inactive (Iav) (Nan-Iav). Nan-Iav promote calcium entry into auditory receptors and can be activated in vitro by nicotinamide (NAM), an intermediate metabolite of the NAD-salvage pathway. We identify NAM as a direct, endogenous ligand of these TRPV channels, regulating their activity in vivo, in auditory receptors, and, thus, the mechanical amplification gain in the ear. NAM is enzymatically removedby nicotinamidase (Naam), allowing for a regulation of TRPV activity. Both, nicotinamidase and TRPV activity are matched to each other, with calcium modulating the activity of nicotinamidase by binding to its EF-hand motifs. Thus, we can establish in vivo ligand-gating of TRPV channels by a cell-autonomous mechanism that allows to couple auditorymechanics to the metabolic state of the auditory receptors. Here, NAM-activated TRPV channels result in calcium-influx that subsequently enhances NAM turnover, providing a molecular feedback loop that allows to adjust the mechanical feedback gain by regulating auditory receptor motility.

By amplifying sound-induced vibrations in an intensity- and frequency specific manner, ears can sharpen their resonantmechanics. Ears boost their mechanical input with motile auditory receptors. In Drosophila, this mechanical amplification iscontrolled by heteromeric TRPV channels formed by Nanchung (Nan) and Inactive (Iav) (Nan-Iav).Nan-Iav promote calcium entry into auditory receptors and can be activated in vitro by nicotinamide (NAM), an intermediatemetabolite of the NAD-salvage pathway. We identify NAM as a direct, endogenous ligand of these TRPV channels, regulatingtheir activity in vivo, in auditory receptors, and, thus, the mechanical amplification gain in the ear. NAM is enzymatically removedby nicotinamidase (Naam), allowing for a regulation of TRPV activity. Both, nicotinamidase and TRPV activity are matched toeach other, with calcium modulating the activity of nicotinamidase by binding to its EF-hand motifs.Thus, we can establish in vivo ligand-gating of TRPV channels by a cell-autonomous mechanism that allows to couple auditorymechanics to the metabolic state of the auditory receptors. Here, NAM-activated TRPV channels result in calcium-influx thatsubsequently enhances NAM turnover, providing a molecular feedback loop that allows to adjust the mechanical feedback gain byregulating auditory receptor motility.