Sphingomyelin synthase (SMS) catalyzes the conversion of phosphatidylcholine and ceramide to sphingomyelin and diacylglycerol. We previously showed that SMS1 deficiency leads to a reduction in expression of the K ϩ channel KCNQ1 in the inner ear (Lu MH, Takemoto M, Watanabe K, Luo H, Nishimura M, Yano M, Tomimoto H, Okazaki T, Oike Y, and Song WJ. J Physiol 590: 4029 -4044, 2012), causing hearing loss. However, it remains unknown whether this change in expression is attributable to a cellular process or a systemic effect in the knockout animal. Here, we examined whether manipulation of SMS1 activity affects KCNQ1/KCNE1 currents in individual cells. To this end, we expressed the KCNQ1/KCNE1 channel in human embryonic kidney 293T cells and evaluated the effect of SMS1 manipulations on the channel using whole cell recording. Application of tricyclodecan-9-ylxanthogenate, a nonspecific inhibitor of SMSs, significantly reduced current density and altered channel voltage dependence. Knockdown of SMS1 by a short hairpin RNA, however, reduced current density alone. Consistent with this, overexpression of SMS1 increased the current density without changing channel properties. Furthermore, application of protein kinase D inhibitors also suppressed current density without changing channel properties; this effect was nonadditive with that of SMS1 short hairpin RNA. These results suggest that SMS1 positively regulates KCNQ1/KCNE1 channel density in a protein kinase D-dependent manner.KCNQ1; KCNE1; sphingomyelin synthase; PKD THE SLOWLY ACTIVATING, DELAYED rectifier potassium channel encoded by KCNQ1 (␣-subunit, also known as Kv7.1) and KCNE1 (␤-subunit, also known as Isk or minK) plays important roles in a number of organs, including the inner ear, heart, pancreas, and brain (reviewed in Refs. 1,15,19). In the heart, the KCNQ1/KCNE1 channel [also known as the cardiac slowly activating K ϩ current (I Ks ) channel] is necessary for the termination of action potentials (5,30,42), and loss of function of the I Ks channel causes prolongation of the QT interval in the ECG (26). In the inner ear, KCNQ1/KCNE1 channels are expressed exclusively on the apical surface of marginal cells of the stria vascularis (14, 29), maintaining the endocochlear potential and thereby the high sensitivity of the inner ear to sound. Dysfunction of KCNQ1/KCNE1 channels in the inner ear causes hearing impairment or congenital deafness (3,15,27). Understanding how KCNQ1/KCNE1 channels are regulated is, therefore, of profound biological significance.We previously showed that KCNQ1 expression in the inner ear may be subject to regulation by sphingomyelin synthase 1 (SMS1) (21). SMS1 is one of the two isoforms of the enzyme that catalyzes the conversion of phosphatidylcholine and ceramide to sphingomyelin and diacylglycerol (DAG) (reviewed in Refs. 18, 38). We showed that SMS1 deficiency results in hearing impairment, which is attributable to a reduction in endocochlear potential caused by aberrant KCNQ1 protein expression in the marginal cells of the stria ...

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