Voltage-dependent inhibition of thermo-TRPV channels by intracellular spermine.

Abstract

The Vanilloid TRP (TRPV1-4) subfamily of channels is involved in thermoregulation, osmoregulation, itch and pain perception, immune response and (neuro)inflammation, and tight control of channel activity is required for perception of pain and noxious stimuli. TRPV1, 2, 4 have been shown to be overexpressed in various cancer cell types and suggested to serve as a marker of cancer progression and, in some cases, as an anti-cancer drug target. The polyamines (PAs) putrescine, spermidine and spermine, are present in sub-millimolar concentrations in the cytoplasm of virtually all cells and in the extracellular fluid. Intracellular PA levels decrease with age, while pathological alterations of PA levels affect cognitive functions and have been linked to brain ageing and degeneration, as well as to cancer. We have discovered a previously unrecognized physiological modulation of human TRPV1, 3 and 4 by the endogenous intracellular polyamine spermine. As in inward rectifier K channels, currents are blocked in a strongly voltage-dependent manner, but, as in cyclic nucleotide-gated channels, the block is substantially relieved at more positive voltages. Consistent with block relief resulting from spermine permeation, there is much weaker relief of block by the analog 1-naphthyl-acetyl spermine, with a bulky aromatic group at one end of the molecule. A kinetic model of inhibition suggests two independent spermine binding sites with different affinities as well as significantly different degrees of PA permeability in TRPV1, 3, and 4. Since block, relief, and permeation occur over the physiological voltage range of action potentials and PA concentrations, voltage-dependent PA block may be a potent modulator of TRPV-dependent excitability in multiple cell types, and at least some TRPV channels may contribute to polyamine transport. Thermo-TRPV structural insights and available literature also hint at a common mechanism of PA block of different TRP channel sub-families.