Imaging Calcium Dynamics Using Targeted Recombinant Aequorins

INTRODUCTION

Aequorin is a small protein produced by the genus Aequorea that was widely used in the 1960s and 1970s as a probe to measure Ca²⁺ in living cells. The invention of the carboxylate Ca²⁺ indicators, which are much simpler to load into intact living cells and to calibrate and image at the single-cell level, has led most groups to abandon aequorin. Yet, this latter Ca²⁺ indicator still offers some advantages over the fluorescent probes. In particular, the use of molecular biological techniques for expressing recombinant aequorin in mammalian cells, thus eliminating the need for microinjection, has opened new possibilities for this probe. Among the new uses of aequorin, one of the most interesting is the potential for targeting it specifically to different cellular locations, thus opening the possibility of monitoring selectively the dynamics of [Ca²⁺] with unprecedented spatial resolution. This article briefly discusses the problems concerned with targeting aequorin to different locations, the advantages and disadvantages offered by the steep dependence of luminescence on [Ca²⁺], and the instruments needed to obtain reliable measurements.