An ATP-activated, ligand-gated ion channel on a cholinergic presynaptic nerve terminal.

ATP has recently been identified as a fast neurotransmitter in both the central and peripheral nervous systems. Several studies have suggested that ATP can also affect the release of classical neurotransmitters, including acetylcholine with which it is co-released. We have searched for ATP receptors on a cholinergic presynaptic nerve terminal using the calyx-type synapse of the chicken ciliary ganglion. ATP was pulsed onto the terminals under voltage clamp and induced a short latency cation current that exhibited inward rectification and marked desensitization. This current was not seen with adenosine but was mimicked by several sterically restricted ATP analogs and was blocked by suramin. ATP-activated single ion channels exhibited prominent flickering and had a conductance of approximately 17 pS. Our results demonstrate a ligand-gated P2X-like purinergic receptor on a cholinergic presynaptic nerve terminal.

Molecular cloning and characterization of SCaMPER, a sphingolipid Ca2+ release-mediating protein from endoplasmic reticulum.

Release of Ca2+ stored in endoplasmic reticulum is a ubiquitous mechanism involved in cellular signal transduction, proliferation, and apoptosis. Recently, sphingolipid metabolites have been recognized as mediators of intracellular Ca2+ release, through their action at a previously undescribed intracellular Ca2+ channel. Here we describe the molecular cloning and characterization of a protein that causes the expression of sphingosyl-phosphocholine-mediated Ca2+ release when its complementary RNA is injected into Xenopus oocytes. SCaMPER (for sphingolipid Ca2+ release-mediating protein of endoplasmic reticulum) is an 181 amino acid protein with two putative membrane-spanning domains. SCaMPER is incorporated into microsomes upon expression in SO cells or after translation in vitro. It mediates Ca2+ release at 4 degrees C as well as 22 degrees C, consistent with having ion channel function. The EC50 for Ca2+ release from Xenopus oocytes is 40 microM, similar to sphingosyl-phosphocholine-mediated Ca2+ release from permeabilized mammalian cells. Because Ca2+ release is not blocked by ryanodine or La3+, the activity described here is distinct from the Ca2+ release activity of the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor. The properties of SCaMPER are identical to those of the sphingolipid-gated Ca2+ channel that we have previously described. These findings suggest that SCaMPER is a sphingolipid-gated Ca2+-permeable channel and support its role as a mediator of this pathway for intracellular Ca2+ signal transduction.