Stereocilia bundle stiffness: effects of neomycin sulphate, A23187 and concanavalin A

The effects that the aminoglycoside antibiotic neomycin sulphate, the ionophore A23187 and the lectin Concanavalin A (Con A) have on the steady state stiffness of the hair-cell stereocilia bundle have been studied using organotypic cultures of the early postnatal mouse cochlea. In normal saline, stereocilia bundle stiffness is increased 1.5-2.0 fold by neomycin sulphate, 1.3-1.7 fold by the ionophore A23187 and 3.0-5.0 fold by Con A. In low-calcium saline neither neomycin sulphate nor A23187 cause increases in stiffness, and the stiffness increases elicited by these two agents in normal saline are reversed on washout with low-calcium saline. In normal saline neomycin sulphate has two independent effects on hair cells; one effect is a reversible inhibition of transduction and the other effect is to cause an irreversible increase in bundle stiffness. Neither Con A nor the ionophore A23187 block transduction. No obvious changes in stereocilia bundle morphology are associated with the increases in stiffness caused by neomycin sulphate, A23187 and Con A. Succinylated Con A binds to stereocilia bundles but does not cause an increase in stiffness, suggesting that the stiffness increase caused by native Con A results from receptor cross-linking. The effects of Con A and neomycin are non-additive, saturating concentrations of neomycin sulphate block the effects of low doses of Con A, and pretreatment of cells with succinylated Con A prevents subsequent neomycin sulphate treatment from causing an increase in stiffness suggesting that neomycin sulphate and Con A are acting via a similar mechanism at the cell surface. Ionophore treatment prevents the subsequent application of neomycin sulphate from causing a further increase in stiffness, but when cells are treated with neomycin sulphate followed by ionophore the effects of the two drugs are additive indicating that they are operating via different mechanisms. The possible nature of these mechanisms and their role in the control of steady state stereocilia bundle stiffness are discussed.