Effects of cholesterol alterations are mediated via G-protein-related pathways in outer hair cells

Abstract

Cholesterol is an essential component of cell membranes, and determines their rigidity and fluidity. Alterations in membrane cholesterol by MβCD or water-soluble cholesterol affect the stiffness, capacitance, motility, and cell length of outer hair cells (OHCs). This suggests that reconstruction of the cytoskeleton may be induced by cholesterol alterations. In this study, we investigated intracellular signaling pathways involving G proteins to determine whether they modulate the changes in voltage-dependent capacitance caused by cholesterol alterations. Membrane capacitance of isolated guinea pig OHCs were assessed using a two-sine voltage stimulus protocol superimposed onto a voltage ramp (200 ms duration) from -150 to +140 mV. One group of OHCs was treated with 100 μM guanosine 5'-O-(3-thiotriphosphate) tetralithium salt (GTPγS), the GTP analog, administrated into individual cells via patch pipettes. Another group of OHCs was internally perfused with 600 μM guanosine 5'-(β-thio) diphosphate trilithium salt (GDPβS), the GDP analog. A third group was perfused with internal solution only as a control. Application of 1 mM MβCD shifted non-linear capacitance curves to the depolarized direction of the control group with reduction of the peak capacitance (C mpeak). After the 10-min application of MβCD, shifts of voltage at C mpeak (V cmpeak) and reduction of C mpeak were 73.32 ± 11.09 mV and 9.09 ± 2.10 pF, respectively (n = 4). On the other hand, in the GTPγS-treated group, the shift of V cmpeak and reduction of C mpeak were attenuated remarkably. The shift of V cmpeak and reduction of C mpeak in the 10-min application of MβCD were 9.73 ± 10.92 mV and 3.08 ± 1.91 pF, respectively (n = 7). MβCD decreased the cell length by 16.53 ± 4.27 % in the control group and by 6.45 ± 6.22 % in the GTPγS group. In addition, we investigated the effects of GDPβS on cholesterol-treated OHCs. One millimolar cholesterol was externally applied after the 4-min application of 1 mM MβCD because the shift of V-C m function caused by cholesterol alone was small. Application of cholesterol shifted V-C m curves of the control group to the hyperpolarized direction with increase of the C mpeak. After the 10-min application of cholesterol, changes of V cmpeak and C mpeak were -9.19 ± 6.68 mV and 2.14 ± 0.44 pF, respectively (n = 4). On the other hand, in the GDPβS-treated OHCs, the shift of V cmpeak and increase of C mpeak were attenuated markedly. The shift of V cmpeak and increase of C mpeak after 10 min were 5.13 ± 10.46 mV and -0.55 ± 1.39 pF, respectively (n = 6). This study demonstrated that internally perfused GTPγS inhibited the MβCD effects and GDPβS inhibited the cholesterol effects, raising the possibility that G proteins may be involved in outer hair cell homeostasis as well as the possibility that cholesterol response may be G protein mediated. More study is required to clarify the detailed role of G proteins in the relation between cholesterol and the OHC cytoskeleton.