Endoplasmic reticulum calcium tunnels integrate signalling in polarised cells

Abstract

One of the most crucial aspects of Ca²⁺ signalling is the ability to generate highly localised transient elevations of the cytosolic Ca²⁺ concentration at specific strategically important target sites. Inevitably this necessitates a relatively high Ca²⁺ buffering power of the cytoplasm, which in turn makes movement of Ca²⁺ from one part of a cell to another difficult. Nature has evolved an elegant solution to this problem by creating operational Ca²⁺ tunnels through the endoplasmic reticulum. Very recently direct evidence that such tunnelling also occurs in neurons has been provided.

Introduction

The omnipotence of the Ca²⁺ signalling system is generally accepted [1]. The involvement of Ca²⁺ in the regulation of a wide range of cellular reactions has been demonstrated again and again since 1943, when Lewis Heilbrunn contemplated that “the reaction of this calcium with the protoplasm inside the cell is the most basic of all protoplasmic reactions” [1], [2]. Furthermore, Ca²⁺ signalling cascades are responsible for the execution of numerous cellular death programmes, ranging from the physiological apoptosis to the pathological necrosis [3], [4], [5], [6]. Nevertheless, a full understanding of the detailed behaviour of Ca²⁺ signalling systems, which allows omnipotence, is still far from having been achieved.