Human TRPC5 channel activated by a multiplicity of signals in a single cell

Abstract

Here we explore the activation mechanisms of human TRPC5, a putative cationic channel that was cloned from a region of the X chromosome associated with mental retardation. No basal activity was evident but activity was induced by carbachol stimulation of muscarinic receptors independently of Ca 2+ release. This is 'receptor activation', as described for mouse TRPC5. In addition, and in the absence of receptor stimulation, extracellular gadolinium (0.1 mM) activated TRPC5, an effect that was mimicked by 5-20 mM extracellular Ca 2+ with intracellular Ca 2+ buffered. We refer to this as 'external ionic activation'. TRPC5 was also activated by modest elevation of [Ca 2+ ] i in the absence of GTP -'calcium activation'. A putative fourth activation mechanism is a signal from depleted intracellular Ca 2+ stores. Consistent with this idea, human TRPC5 was activated by a standard store-depletion/Ca 2+ re-entry protocol, an effect that was difficult to explain by calcium activation. Multiplicity of TRPC5 activation was demonstrated in single cells and thus not dependent on heterogeneity of expression levels or cellular context. Therefore, human TRPC5 is activated by a range of stimuli, avoiding dependence on a single critical activator as in many other ion channels. One of these stimuli would seem to be a change in Ca 2+ handling by the endoplasmic reticulum.

Potentiation of TRPC5 by Protons

“…Further decreases in pH reduced the maximum amplitudes. These effects of H ϩ show striking similarities to those of La 3ϩ and Gd 3ϩ , which also activate and modulate currents through TRPC5 in a biphasic manner with potentiatory and inhibitory components at micro-and millimolar concentrations, respectively (7,13). Further evidence for similar actions of Gd 3ϩ and H ϩ was obtained in the present study at the single channel level.…”

“…6). For TRPC5, additional activators have been described, the physiological significance of which is uncertain (7)(8)(9). A surprising feature of TRPC4 and TRPC5 and heteromultimeric combinations of TRPC1 and TRPC5 is their strong potentiation (10 -13) or activation (7) by micromolar concentrations of the trivalent lanthanoid cations La 3ϩ and Gd 3ϩ , effects also seen for TRPC5 with millimolar Ca 2ϩ (7,13).…”

“…For TRPC5, additional activators have been described, the physiological significance of which is uncertain (7)(8)(9). A surprising feature of TRPC4 and TRPC5 and heteromultimeric combinations of TRPC1 and TRPC5 is their strong potentiation (10 -13) or activation (7) by micromolar concentrations of the trivalent lanthanoid cations La 3ϩ and Gd 3ϩ , effects also seen for TRPC5 with millimolar Ca 2ϩ (7,13). Most other TRP channels, with the exception of TRPV1 (see below), and other Ca 2ϩ -permeable cation channels, including voltage-gated Ca 2ϩ channels (14), are inhibited by micromolar concentrations of lanthanoids.…”

Receptor-operated cation channels formed by TRPC4 and TRPC5

“…Interestingly, human TRPC5 shares greater similarity with the rabbit isoform, originally described to be store-dependent (Philipp et al 1998), than with the mouse variant. However, for human TRPC5, both store-dependent and storeindependent currents displayed similar IV relations (Zeng et al 2004). These reversed at around 0 mV, indicative of a nonselective cation channel, and displayed mainly outwardly rectifying IV relations with a weak hump positive to 0 mV.…”

“…In addition, this study revived the hypothesis that species variants can determine whether group 4 TRPCs can be regulated in a store-operated fashion. Using the human isoform of TRPC5, which had not been studied previously at a functional level, it was shown that the channel could operate in a receptor-operated, store-independent mode and in a store-operated mode (Zeng et al 2004). The channel was activated by muscarinic receptors following a long pretreatment with thapsigargin to empty intracellular Ca 2+ stores and was thus store-independent.…”