Aequorin-based genetic approaches to visualize Ca2+ signaling in developing animal systems

The Expanding Toolbox of In Vivo Bioluminescent Imaging

In vivo bioluminescent imaging (BLI) permits the visualization of engineered bioluminescence from living cells and tissues to provide a unique perspective toward the understanding of biological processes as they occur within the framework of an authentic in vivo environment. The toolbox of in vivo BLI includes an inventory of luciferase compounds capable of generating bioluminescent light signals along with sophisticated and powerful instrumentation designed to detect and quantify these light signals non-invasively as they emit from the living subject. The information acquired reveals the dynamics of a wide range of biological functions that play key roles in the physiological and pathological control of disease and its therapeutic management. This mini review provides an overview of the tools and applications central to the evolution of in vivo BLI as a core technology in the preclinical imaging disciplines.

Methods to measure intracellular Ca(2+) fluxes with organelle-targeted aequorin-based probes

The photoprotein aequorin generates blue light upon binding of Ca(2+) ions. Together with its very low Ca(2+)-buffering capacity and the possibility to add specific targeting sequences, this property has rendered aequorin particularly suitable to monitor Ca(2+) concentrations in specific subcellular compartments. Recently, a new generation of genetically encoded Ca(2+) probes has been developed by fusing Ca(2+)-responsive elements with the green fluorescent protein (GFP). Aequorin has also been employed to this aim, resulting in an aequorin-GFP chimera with the Ca(2+) sensitivity of aequorin and the fluorescent properties of GFP. This setup has actually solved the major limitation of aequorin, for example, its poor ability to emit light, which rendered it inappropriate for the monitoring of Ca(2+) waves at the single-cell level by imaging. In spite of the numerous genetically encoded Ca(2+) indicators that are currently available, aequorin-based probes remain the method of election when an accurate quantification of Ca(2+) levels is required. Here, we describe currently available aequorin variants and their use for monitoring Ca(2+) waves in specific subcellular compartments. Among various applications, this method is relevant for the study of the alterations of Ca(2+) homeostasis that accompany oncogenesis, tumor progression, and response to therapy.

Sense in Pb2+ sensing

It is generally acknowledged that Pb(2+), which is sequestered by live cells from their direct environment, affects a large number of cellular processes at picomolar to micromolar concentrations. However, resolving the specific molecular targets and mechanisms responsible for the neurotoxic effects of this xenobiotic metal is hampered by the lack of suitable tools to investigate the intracellular dynamics of Pb(2+) at low concentrations. Fluorescent Ca(2+) indicators have been used as Pb(2+) sensors and have proven useful to detect cellular Pb(2+) entry and to estimate the overall intracellular free Pb(2+) concentration associated with adverse cellular effects. Despite the high affinity of these Ca(2+) indicators for Pb(2+), their utility for more advanced studies is limited. This is merely due to their moderate metal selectivity and uncertainties about the subcellular (co)localization of the indicators and the targets. Novel Pb(2+) sensors, specifically developed for this purpose, still lack affinity to sense toxicologically relevant intracellular concentrations of Pb(2+). Nonetheless, the development of genetically encoded protein sensors for Ca(2+), Zn(2+), and, recently, also for Pb(2+) opens a new and promising perspective to resolve spatiotemporal changes in intracellular Pb(2+) in relation to cellular signaling and intracellular divalent metal homeostasis. Such a development is required for enabling more systematic studies of the intracellular dynamics of Pb(2+), which are essential for progress in mechanistic knowledge and will ultimately reveal the critical toxic targets of Pb(2+) at the subcellular and molecular level.