The C. elegans ion channel complex formed by the DEG/ENaC MEC-4 and MEC-10 subunits, exclusively assembled in six specialized touch sensing neurons, is thought to constitute the core of a voltage-independent Na + -selective mechanosensory ion channel. In a screen for touch insensitive nematodes, a very interesting
mec-4 mutant was isolated. This
mec-4(d) mutation specified a large side-chain amino acid substitution near the channel pore and induces degeneration of touch neurons through a mechanism that appears to involve channel hyperactivation . Analysis of
mec-4(d)- induced neurodegeneration , however, indicated that large increase of [Ca 2+ ] i is one of the molecular requirements for cell death. While Ca 2+ release from ER-based stores is certainly occurring in
mec-4(d) -induced cell-death, the triggering factor remains unknown. We speculated that Ca 2+ could permeate through MEC-4(d) channels to perhaps induce Ca 2+ -induced Ca 2+ release from stores. P> P> We analyzed Ca 2+ permeability of MEC-4(d) channels reconstituted in Xenopus oocytes and found that they are permeable to Ca 2+ ( P Ca /P Na ~ 0.22). Interestingly, Ca 2+ currents through MEC-4(d) channels display the same sensitivity to amiloride that Na + currents do, with Ki for the drug being reduced by co-expression with MEC-10(d). We also found that Ca 2+ permeability is independent of co-expression with MEC-10(d), MEC-2 or MEC-6, and appears to be determined by MEC-4(d) residues. We are currently searching for residues involved in Ca 2+ permeability, by introducing second site mutations in and near the pore, identified in our screen for suppressors of
mec-4(d) -induced cell death. In addition we are evaluating the intracellular pathway activated by entry of Ca 2+ through MEC-4(d) that leads to massive release of Ca 2+ from the stores that ultimately results in cell death.