Targeting of aequorin for calcium monitoring in intracellular compartments

Measurement of cytoplasmic and cilioplasmic calcium in a single living cell

Cellular signaling represents an evolution of biological systems to sense external stimuli and communicate extracellular microenvironment to the intracellular compartments. The processes underlying molecular signaling have been widely studied due to their important cellular functions. There are numerous techniques available to quantitate the different molecules involved in cellular processes. Among them, calcium is a ubiquitous signaling molecule involved in many biological pathways. Over time the methods to measure intracellular calcium have advanced to better understand its role as a second messenger. In this chapter, we introduce a method to study a single cilium, a mechanosensor that elicits a calcium signaling cascade. To successfully observe the calcium changes in this thin cylindrical-like projection from the cell surface, we utilize a genetically encoded sensor with a high spatial and temporal resolution. In addition, the probe must be localized to the ciliary compartment in order to observe the intraciliary calcium signaling dynamics. To this end, a cilium targeting genetically encoded indicator is used to observe calcium fluxes in both cytoplasm and cilioplasm.

Genetically encoded Ca(2+) indicators: properties and evaluation

Genetically encoded calcium ion (Ca(2+)) indicators have become very useful and widely used tools for Ca(2+) imaging, not only in cellular models, but also in living organisms. However, the in vivo and in situ characterization of these indicators is tedious and time consuming, and it does not provide information regarding the suitability of an indicator for particular experimental environments. Thus, initial in vitro evaluation of these tools is typically performed to determine their properties. In this review, we examined the properties of dynamic range, affinity, selectivity, and kinetics for Ca(2+) indicators. Commonly used strategies for evaluating these properties are presented. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Strategies for the real-time detection of Ca2+ channel events of single cells: recent advances and new possibilities

Ca(2+) ion channels play key roles in cell physiology and they are important drug targets. The Ca(2+) channel events are mainly measurable by fluorescent and patch clamp methods. This review summarizes the recent advances of these techniques for the detection of Ca(2+) channel events and the prospect of their new directions in the near future. Conventional bulk fluorescent methods are amenable to high-throughput applications, but they are not real-time single-cell measurements, which provide kinetic data on individual cells and offer unparalleled sensitive data for rare cells. Recent advances on real-time single-cell fluorescent measurements are conducted on microfluidic chips with scalable cell-retention sites, integrated with electrical stimulation and fluorescent measuring features. Patch clamp techniques are real-time measurements conducted on single cells, but the measurements are of low throughput. Recent advances are conducted on microfluidic patch clamp chips for high-throughput applications. Future real-time single-cell Ca(2+) channel event measurements will be conducted in a multiparametric manner in an integrated and automated microfluidic chip.

Purinergic signalling mobilizes mitochondrial Ca²⁺ in mouse Sertoli cells

Intimate bidirectional communication between Sertoli cells and developing germ cells ensures the integrity and efficiency of spermatogenesis. Yet, a conceptual mechanistic understanding of the physiological principles that underlie Sertoli cell autocrine and paracrine signalling is lacking. Here, we characterize a purinergic Ca(2+) signalling network in immature mouse Sertoli cells that consists of both P2X2 and P2Y2 purinoceptor subtypes, the endoplasmic reticulum and, notably, mitochondria. By combining a transgenic mouse model with a dedicated bioluminescence imaging device, we describe a novel method to monitor mitochondrial Ca(2+) mobilization in Sertoli cells at subcellular spatial and millisecond temporal resolution. Our data identify mitochondria as essential components of the Sertoli cell signalling 'toolkit' that control the shape of purinergic Ca(2+) responses, and probably several other paracrine Ca(2+)-dependent signals.

N-methyl-D-aspartate receptors expressed in a nonneuronal cell line mediate subunit-specific increases in free intracellular calcium

N-methyl-D-aspartate (NMDA) receptors can mediate cell death in neurons and in non-neuronal cells that express recombinant NMDA receptors. In neurons, increases in intracellular calcium correlate with NMDA receptor-mediated death, supporting a key role for loss of cellular calcium homeostasis in excitotoxic cell death. In the present study, free intracellular calcium concentrations were examined in response to activation of recombinant NMDA receptors expressed in human embryonic kidney 293 cells. Intracellular calcium was measured in transfected cell populations by cotransfection with the calcium-sensitive, bioluminescent protein aequorin and by single cell imaging with the fluorescent calcium indicator fluo-3. Agonist application to NR1/2A or NR1/2B-transfected cells elicited robust rises in intracellular calcium. NR1/2A responses were inhibited by the noncompetitive antagonists MK-801 and dextromethorphan and were dependent on extracellular calcium but not on intracellular calcium stores. In contrast, no detectable intracellular calcium responses were observed in NR1/2C-transfected cells. These findings indicate that NMDA receptors in the absence of other neuron-specific factors can mediate increases in intracellular calcium with subunit specificity and extracellular calcium dependence.