TPRN deficiency has little effect on MET currents and stereocilia bundle stiffness in young postnatal OHCs. (A) Bright field image on an OC explant with a patch-clamp pipette (right) and fluid-jet deflecting the OHC stereocilia bundles (bottom). Inset shows a representative frame from a high-speed video recording sequence used to quantify stereocilia bundle movement and to determine positioning of the fluid-jet relative to the hair bundle (see Materials and methods). (B–D) Representative MET currents to fluid-jet stimuli evoked by step-like voltage commands in OHCs of control Tprn^+/+ (B), Tprn^−/− (C), and Tprn^N259/N259 (D) mice. (E and F) Maximal MET current (E) and open MET channel probability at resting bundle position, P_OPEN control Tprn^+/+ (open black circles), Tprn^−/− (closed magenta circles), and Tprn^N259/N259 (closed red circles) mice. Data from individual cells and mean ± SEM are shown. Asterisk shows statistical significance: *P < 0.05; unpaired two-sided t test. (G) Relationship between stereocilia bundle compliance and the tip area of the fluid-jet pipette that determines the force applied to the bundle. Hair bundle compliances were calculated from linear fits of relationships between hair bundle displacement and fluid-jet pressure around resting bundle positions (0–100 nm deflections). All data show the same parabolic function. Age of the cells: P4–P7, location along the cochlea: middle of the apical turn, number of tested cells/mice: WT 12/8; Tprn^−/− 6/2; Tprn^N259/N259 3/3.