LHFPL5 is a key element to the gating spring of cochlear hair cells: comparison of mechanotransduction channel activation in presence or absence of LHFPL5

During auditory transduction, sound-evoked vibrations of the hair cell stereociliary bundles open mechanotransducer (MET) ion channels via tip links extending from one stereocilium to its neighbor. How tension in the tip link is delivered to the channel is not fully understood.  The MET channel comprises a pore-forming subunit, transmembrane channel-like protein (TMC1 or TMC2), aided by several accessory proteins, including LHFPL5 (lipoma HMGIC fusion partner-like 5). We investigated the role of LHFPL5 in transduction by comparing MET channel activation in outer hair cells of Lhfpl5-/- knockout mice with those in Lhfpl5+/- heterozygotes. The 10-90 percent working range of transduction in Tmc1+/+; Lhfpl5+/- was 52 nm, from which the single-channel gating force, Z was evaluated as 0.34 pN. However, in Tmc1+/+; Lhfpl5‑/- mice, the working range increased to 123 nm and Z more than halved to 0.13 pN, indicating reduced sensitivity. Tip link tension is thought to activate the channel via a gating spring, whose stiffness is inferred from the stiffness change on tip link destruction. The gating stiffness was ~40 percent of the total bundle stiffness in wild-type but was virtually abolished in Lhfpl5-/-, implicating LHFPL5 as a principal component of the gating spring.  The mutation Tmc1 p.D569N reduced the LHFPL5 immunolabeling in the stereocilia and like Lhfpl5-/- doubled the MET working range but other deafness mutations had no effect on the dynamic range.  We conclude that tip-link tension is transmitted to the channel primarily via LHFPL5; residual activation without LHFPL5 may occur by direct interaction between PCDH15 and TMC1.

Methods
MET currents were recorded from outer hair cells (OHCs) in isolated organs of Corti of mice between P2 and P7. All whole cell recordings were performed at room temperature, ~23ºC.

All data included in the published article are presented in an Excel file, giving all values obtained to construct the mean +/- 1 SD, for every recorded hair cell in all the different mouse genotypes used in the study.

The Excel file is organized in 3 sheets, describing the MET current bundle displacement results, hair bundle stiffness data, and the hair bundle height measurements.

Stereociliary bundles were stimulated with a fluid jet, and the bundle deflections calibrated by projecting the bundle image onto a pair of photodiodes and measuring the change in photocurrent. The Excel file gives all values obtained for the relationship between the MET current, I, and bundle displacement, X. The I-X relationship was fitted with a Boltzmann equation: I = IMAX /(1 + exp((XO -X)/XS), where IMAX is the maximum current, XO the half-saturating displacement and XS the slope factor.  The 10 to 90 percent working range of transduction (WR) is 4.4*XS.

Hair bundle stiffness was determined by calibrating the force generated by the fluid jet against a flexible glass fiber of 1 mN/m and plotting against bundle displacement. The Excel file shows all hair bundle stiffness measurements used to construct our mean +/- SD, for all cells and mouse genotype used in the study.

Bundle height measurement was performed on solitary outer hair cells and the hair bundle length was measured from the root to the tip of the longest stereocilia.