Mucin 5AC (MUC5AC) is secreted by goblet cells of the respiratory tract and, surprisingly, also expressed de novo in mucus secreting cancer lines. siRNA-mediated knockdown of 7343 human gene products in a human colonic cancer goblet cell line (HT29-18N2) revealed new proteins, including a Ca(2+)-activated channel TRPM5, for MUC5AC secretion. TRPM5 was required for PMA and ATP-induced secretion of MUC5AC from the post-Golgi secretory granules. Stable knockdown of TRPM5 reduced a TRPM5-like current and ATP-mediated Ca(2+) signal. ATP-induced MUC5AC secretion depended strongly on Ca(2+) influx, which was markedly reduced in TRPM5 knockdown cells. The difference in ATP-induced Ca(2+) entry between control and TRPM5 knockdown cells was abrogated in the absence of extracellular Ca(2+) and by inhibition of the Na(+)/Ca(2+) exchanger (NCX). Accordingly, MUC5AC secretion was reduced by inhibition of NCX. Thus TRPM5 activation by ATP couples TRPM5-mediated Na(+) entry to promote Ca(2+) uptake via an NCX to trigger MUC5AC secretion

Funding: Vivek Malhotra: Sandler Foundation for Asthma Research; Institucio Catalana de Recerca i Estudis Avancats (ICREA); Plan Nacional (BFU2008-00414); Consolider (CSD2009-00016); Agencia de Gestio d’ Ajuts Universitaris i de Recerca (AGAUR) Grups de Recerca Emergents (SGR2009-1488); European Research Council (268692). Miguel A. Valverde: ICREA Academia Award; Spanish Ministry of Economy and Competitiveness (SAF2012-38140); Fondo de Investigacion Sanitaria (Red HERACLES RD12/0042/0014); FEDER Funds; SGR09-1369. José M. Fernández Fernández: Spanish Ministry of Economy and Competitiveness (SAF2012-31089)