Voltage Dependence of the Ca2+-activated Cation Channel TRPM4

Abstract

TRPM4 is a Ca 2؉ -activated but Ca 2؉ -impermeable cation channel. An increase of [Ca 2؉ ] i induces activation and subsequent reduction of currents through TRPM4 channels. This inactivation is strikingly decreased in cell-free patches. In whole cell and cell-free configuration, currents through TRPM4 deactivate rapidly at negative potentials. Cation channels of the transient receptor potential (TRP) 1 superfamily have received much attention during the last years because of their pivotal role in various cell functions linked to the modulation of intracellular Ca 2ϩ signals, mostly in nonexcitable cells (1-3). More than 20 mammalian TRP members are known, which are classified into the TRPC (C for "canonical"), TRPV (V for "vanilloid"), and TRPM (M for "melastatin") subfamilies (1, 2). However, the functional properties of most members of this novel and fast growing channel family are not yet analyzed in detail. The predicted transmembrane topology of TRPs is similar to that of voltage-gated and cyclic nucleotide gated channels; they consist of six transmembrane-spanning helices (TM1 to -6), cytoplasmic N and C termini, and a pore region between TM5 and TM6 (2, 3). Since the fourth transmembrane helix is not positively charged, TRP channels were considered as voltage-independent. The voltage-sensing features of some members of the TRPV subfamily could be attributed to voltage-dependent block of the channel pore by intra-or extracellular divalent cations (4, 5).Members of the TRPM subfamily are much less studied at the functional level than those of the TRPV and TRPC family. They are characterized by relatively long N and C termini, and some of them have entire enzyme domains linked to their C termini: an ADP-ribose pyrophosphatase in TRPM2 (6) and an atypical ␣-kinase domain in TRPM6 and TRPM7 (7-11). Surprisingly, TRPM4b, which is a Ca 2ϩ -impermeable monovalent cation channel of 25-picosiemens unitary conductance belonging to the TRPM subfamily, is in contrast to other TRP channels not inactivated but activated by intracellular Ca 2ϩ , [Ca 2ϩ ] i (12). A short form of TRPM4, TRPM4a, is characterized in less detail and displays completely different properties with regard to Ca 2ϩ permeability and activation (13). In this study, we report cloning of the human and mouse TRPM4 cDNAs. Transcripts of these genes are expressed in heart, kidney, and endothelial cells, indicating that this channel plays a role in the cardiovascular system. We demonstrate that TRPM4 is a Ca 2ϩ -and voltage-dependent channel. It could therefore modulate the electrical activity of cells that generate action potentials. This is, to our knowledge, the first description of voltage-dependent properties of a TRP channel, suggesting a special role for this channel in excitable cells. MATERIALS AND METHODS Cloning of Human and Mouse TRPM4 cDNAs-The human expressed sequence tag 885075 (GenBank TM ), homologous to the human TRPM1 cDNA, was identified and sequenced on both strands; it contained a ϳ1500-bp DNA fragment, which represented part of the...

Increased IgE-dependent mast cell activation and anaphylactic responses in mice lacking the calcium-activated nonselective cation channel TRPM4

“…1 online). We confirmed recombination and Cre-mediated excision of exons 15 and 16, encoding the first transmembrane segment in TRPM4, which is present in all known TRPM4 splice variants 17 , by Southern blot analysis ( Supplementary Fig. 1).…”

“…TRPM4 and TRPM5 have been identified as two unique molecular candidates for the CAN class of ion channels [15][16][17] . Both proteins belong to the melastatin subfamily of transient receptor potential (TRP) membrane proteins 18 .…”

Structures of the calcium-activated, non-selective cation channel TRPM4

“…7 ). While TRPM4 is ubiquitously expressed in pancreas, heart, prostate, renal tubule and many other tissues and organs 9,10 , TRPM5 is mainly detected in taste receptor cells 11 .…”

“…This TRPM4 desensitization was attributed to the loss of phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P 2 ) in the membrane due to Ca 2+ -activated phospholipase C 13,14 . At the Ca 2+ -desensitized steady state, TRPM4 exhibits voltage-dependent gating with higher channel open probability at depolarizing membrane potential 10 . While PtdInsP 2 alone cannot activate the channel, its presence with Ca 2+ reverses channel desensitization, potentiates Ca 2+ sensitivity, and mitigates voltage dependence of the channel 13,14 .…”

Emerging roles of TRPM6/TRPM7 channel kinase signal transduction complexes

“…TRPM3 appears to be activated by hypoosmotic cell swelling, rises of intracellular Ca 2+ and by D-erythro-sphingosine (Grimm et al 2003(Grimm et al , 2004Lee et al 2003). Two other TRPM subfamily members, TRPM4 and TRPM5, are mainly permeable for monovalent cations, gated by increases in intracellular Ca 2+ and exhibit a pronounced voltage modulation (Hofmann et al 2003;Launay et al 2002;Liu and Liman, 2003;Nilius et al 2003;Prawitt et al 2003). Intriguingly, mice with a genetically disrupted TRPM5 gene display impaired sweet, bitter and umami taste perception (Zhang et al 2003b), but the precise role of TRPM5 in taste receptor cells is still under debate (Hofmann et al 2003;Perez et al 2002;Prawitt et al 2003;Zhang et al 2003b).…”