Influence of myosin regulatory light chain and myosin light chain kinase on hair cells of the inner ear
In the receptor organs of the inner ear and lateral line, sensory hair cells detect mechanical stimuli such as sounds, accelerations, and water movements. In each instance a stimulus deflects the hair bundle, a hair cell's mechanically sensitive organelle. The bundle pivots upon the cell's apical surface, which includes an actin meshwork called the cuticular plate and is surrounded by a ring of filamentous actin and non-muscle myosin II (NM2). Myosin regulatory light chain (RLC) is expressed at the apical surfaces of hair cells and additionally occurs in hair bundles. NM2 and the phosphorylation of RLC by myosin light chain kinase (MLCK) have earlier been shown to regulate the sizes and shapes of hair cells' apical surfaces. We have found that inhibitors of NM2 and MLCK reduce the stiffness of hair bundles from the bullfrog's sacculus. Moreover, inhibition of MLCK arrests the spontaneous oscillation of hair bundles and increases the resting open probability of transduction channels. In addition, MLCK inhibition elevates hearing thresholds in mice. We conclude that NM2 and the phosphorylation of RLC modulate adaptation and thereby help to set the normal operating conditions of hair bundles.
Top: Immunofluorescence labeling demonstrates MYL12 in hair bundles and MYL9 in the apical cytoplasm of hair cells. Actin labeled with phalloidin marks hair bundles, cuticular plates, and cellular perimeters. Scale bar, 10 μm. Bottom left: In a measurement of the movement of an individual hair bundle with a photodiode, the MLCK inhibitor ML7 slows spontaneous hair-bundle oscillations significantly at a concentration of 2 μM. Washout of the substance leads to essentially complete recovery. Bottom right: Following the administration of 40 μM ML7 into the middle ear of the mouse, averaged auditory brainstem responses display significantly elevated thresholds at the four frequencies tested. The solid line marks responses from the ear exposed to the inhibitor, whereas the dotted line shows data for the control ear exposed to vehicle.
