Genetically targeted calcium sensors enhance the study of organelle function in living cells

Genetically Encoded Calcium Indicators as Probes to Assess the Role of Calcium Channels in Disease and for High-Throughput Drug Discovery

The calcium ion (Ca²⁺) is an important signaling molecule implicated in many cellular processes, and the remodeling of Ca²⁺ homeostasis is a feature of a variety of pathologies. Typical methods to assess Ca²⁺ signaling in cells often employ small molecule fluorescent dyes, which are sometimes poorly suited to certain applications such as assessment of cellular processes, which occur over long periods (hours or days) or in vivo experiments. Genetically encoded calcium indicators are a set of tools available for the measurement of Ca²⁺ changes in the cytosol and subcellular compartments, which circumvent some of the inherent limitations of small molecule Ca²⁺ probes. Recent advances in genetically encoded calcium sensors have greatly increased their ability to provide reliable monitoring of Ca²⁺ changes in mammalian cells. New genetically encoded calcium indicators have diverse options in terms of targeting, Ca²⁺ affinity and fluorescence spectra, and this will further enhance their potential use in high-throughput drug discovery and other assays. This review will outline the methods available for Ca²⁺ measurement in cells, with a focus on genetically encoded calcium sensors. How these sensors will improve our understanding of the deregulation of Ca²⁺ handling in disease and their application to high-throughput identification of drug leads will also be discussed.

Compartmentalizing genetically encoded calcium sensors

Within single cells there is a complex myriad of signaling which controls physiological process many of which are modulated, or signaled directly, by intracellular calcium ions. Understanding the exquisitely sensitive, and spatially restricted, changes in calcium has been of interest to the researcher for a number of years. Recent advances in this field have been driven by the development of genetically encoded calcium probes for detecting calcium changes within the cells specifically targeting organelles such as mitochondria, endoplasmic reticulum, and the nucleus. In this chapter the authors outline some of the available fluorescent probes, with particular emphasis on an endoplasmic reticulum targeted calcium biosensor in cell signaling studies with astrocytes, detailing experimental protocols and the interpretation of data from such probes.

Role of calcium in the regulation of mechanical power in insect flight

Most flying insect species use "asynchronous" indirect flight muscles (A-IFMs) that are specialized to generate high mechanical power at fast contraction frequencies. Unlike individual contractions of "synchronous" muscles, those of A-IFMs are not activated and deactivated in concert with neurogenically controlled cycling of myoplasmic [Ca(2+)] but rather are driven myogenically by oscillatory changes in length. The motor neurons of the A-IFMs, which fire at a rate much slower than contraction frequency, are thought to play the limited role of maintaining myoplasmic [Ca(2+)] above the critical threshold that maintains the muscle in a stretch-activatable state. Despite this asynchronous form of excitation-contraction coupling, animals can actively regulate power output as required for different flight behaviors, although the neurobiological and biophysical basis of this regulation is unknown. While presenting tethered flying fruit flies, Drosophila melanogaster, with visual stimuli, we recorded membrane potential spikes in identified A-IFM fibers. We show that mechanical power output rises and falls in concert with the firing frequency of all A-IFM fibers and cannot be explained by differential recruitment of separately innervated motor units. To explore the hypothesis that myoplasmic [Ca(2+)] might similarly rise and fall in concert with firing frequency, we genetically engineered Drosophila to express the FRET-based Ca(2+) indicator cameleon selectively within A-IFMs. The results show that Ca(2+) levels increase in proportion to muscle firing rate, both during spontaneous flight and when muscle spikes are elicited electrically. Collectively, these experiments on intact animals support an active role for [Ca(2+)] in regulating power output of stretch-activated A-IFM.