Endogenous inhibitors of InsP 3-induced Ca 2+ release in neuroblastoma cells

Abstract

Cerebellar Purkinje neurons and neuroblastoma N1E-115 cells require 10–50 times more InsP 3 to induce Ca 2+ release than do a variety of non-neuronal cells (including astrocytes, hepatocytes, endothelial cells, or smooth muscle cells). Given the importance of InsP 3-induced Ca 2+ release for the development of synaptic plasticity in Purkinje neurons, a low InsP 3 sensitivity may facilitate the integration of numerous synaptic inputs before initiating a change in synaptic strength. In the present study, attention is directed at the mechanism underlying this low InsP 3 sensitivity of Ca 2+ release. We show that permeabilization of neuroblastoma cells with saponin increased InsP 3 sensitivity of Ca 2+ release, indicating the presence of a diffusible, cytosolic inhibitor(s) of Ca 2+ release. Consistent with this hypothesis, gel filtration of the neuroblastoma cytosol yielded three peaks that inhibited InsP 3-induced Ca 2+ release from permeabilized cells. The prominent inhibitory peak decreased the InsP 3 sensitivity of Ca 2+ release from permeabilized cells, did not bind 3H-InsP 3, and was present in sufficient levels to account for the low InsP 3 sensitivity of Ca 2+ release in intact neuroblastoma cells. Purification of this prominent inhibitory fraction yielded a protein band that was identified by mass spectrometry as stress-induced phosphoprotein 1 (mSTI1). Furthermore, immunoprecipitation of mSTI1 decreased the inhibitory activity of N1E-115 cytosol, indicating that mSTI1 contributes to the inhibition of InsP 3-induced Ca 2+ release. Thus, the low InsP 3 sensitivity of Ca 2+ release in neuroblastoma cells can be explained by the presence of cytosolic inhibitors of Ca 2+ release and include stress-induced phosphoprotein 1.