Making and Using Calcium-Selective Mini- and Microelectrodes

Probing the interstitial calcium compartment

Calcium in interstitial fluids is a crucial ion pool for entry into cells through a plethora of calcium-permeable channels. It is also sensed actively by dedicated receptors. While the mechanisms of global calcium homeostasis and regulation in body fluids appear well understood, more efforts and new technology are needed to elucidate local calcium handling in the small and relatively isolated interstitial spaces between cells. Here we review current methodology for monitoring interstitial calcium and highlight the potential of new approaches for its study. In particular, new generations of high-performance low-affinity genetically encoded calcium indicators could allow imaging of calcium in relatively inaccessible intercellular structures in live tissues and organisms.

In vivo monitoring of calcium ions in rat cerebrospinal fluid using an all-solid-state acupuncture needle based potentiometric microelectrode

Acupuncture needles are regarded as ideal materiel for the development of microelectrodes for in vivo sensing. In this work, an all-solid-state ion-selective microelectrode (ISμE) has been developed by coating a calcium ion-selective membrane on an acupuncture needle tip with a diameter of less than 80 μm, which is modified with poly(3,4-ethylenedioxythiophene)-poly(sodium 4-styrenesulfonate) as solid contact. The proposed Ca²⁺-ISμE shows a Nernstian response toward Ca²⁺ in the range from 1.0 × 10^-6 to 3.1 × 10^-3 M with a slope of 30.8 ± 0.9 mV/decade (R² = 0.999), and the detection limit is 1.2 × 10^-7 M. The Ca²⁺-ISμE has been used for in vivo monitoring of the calcium changes in rat cerebrospinal fluid (CSF) under the injury of spinal cord transection. The results demonstrate that the calcium concentration in CSF increases sharply from the normal level of 20.6 ± 1.72 μM (n = 3) to 133.2 ± 7.63 μM (n = 3) with a severe fluctuation after spinal cord damage. Thus, the proposed Ca²⁺-ISμE is available for in vivo monitoring of calcium ions with high temporal resolution and flexibility. The detection system can be extended to measure other ions in CSF by changing different ion-selective membranes.

Molecular Basis for Synaptotagmin-1-Associated Neurodevelopmental Disorder

At neuronal synapses, synaptotagmin-1 (syt1) acts as a Ca²⁺ sensor that synchronizes neurotransmitter release with Ca²⁺ influx during action potential firing. Heterozygous missense mutations in syt1 have recently been associated with a severe but heterogeneous developmental syndrome, termed syt1-associated neurodevelopmental disorder. Well-defined pathogenic mechanisms, and the basis for phenotypic heterogeneity in this disorder, remain unknown. Here, we report the clinical, physiological, and biophysical characterization of three syt1 mutations from human patients. Synaptic transmission was impaired in neurons expressing mutant variants, which demonstrated potent, graded dominant-negative effects. Biophysical interrogation of the mutant variants revealed novel mechanistic features concerning the cooperative action, and functional specialization, of the tandem Ca²⁺-sensing domains of syt1. These mechanistic studies led to the discovery that a clinically approved K⁺ channel antagonist is able to rescue the dominant-negative heterozygous phenotype. Our results establish a molecular cause, basis for phenotypic heterogeneity, and potential treatment approach for syt1-associated neurodevelopmental disorder.

Recent advances in solid-contact ion-selective electrodes: functional materials, transduction mechanisms, and development trends

This article presents a comprehensive overview of recent progress in the design and applications of solid-contact ion-selective electrodes (SC-ISEs).

Measuring Ca2+ in Living Cells

Measuring free Ca²⁺ concentration ([Ca²⁺]) in the cytosol or organelles is routine in many fields of research. The availability of membrane permeant forms of indicators coupled with the relative ease of transfecting cell lines with biological Ca²⁺ sensors have led to the situation where cellular and subcellular [Ca²⁺] is examined by many non-specialists. In this chapter, we evaluate the most used Ca²⁺ indicators and highlight what their major advantages and disadvantages are. We stress the potential pitfalls of non-ratiometric techniques for measuring Ca²⁺ and the clear advantages of ratiometric methods. Likely improvements and new directions for Ca²⁺ measurement are discussed.

Early detection and monitoring of cerebral ischemia using calcium-responsive MRI probes

Cerebral ischemia is one of the leading causes of mortality and disability in infants and adults and its timely diagnosis is essential for an efficient treatment. We present a methodology for fast detection and real-time monitoring of fluctuations of calcium ions associated with focal ischemia using a molecular functional MRI approach. We used a dinuclear paramagnetic gadolinium(III) complex chelate that changes MR image contrast through its reversible interaction with extracellular calcium ions, while applying a remote transient middle cerebral artery occlusion as a model for ischemic stroke. Our method sensitively recognizes the onset and follows the dynamics of the ischemic core and penumbra with submillimeter spatial and second-scale temporal resolution, thus paving the way for noninvasive monitoring and development of targeted treatment strategies for cerebral ischemia.

Methods to Measure Intracellular Ca2+ Concentration Using Ca2+-Sensitive Dyes

Ca²⁺ ion is universally considered the most versatile second messenger responsible for decoding and regulating the majority of the signaling pathways within the cell. The study of intracellular Ca²⁺ concentration ([Ca²⁺]_i) dynamics is consequently of primary importance for the interpretation of cellular biology. This chapter will present a relatively simple, largely diffused, and nevertheless robust method to measure variations of [Ca²⁺]_i by the use of the Ca²⁺-sensitive chemical dye Fura-2. A general protocol for the assessment of [Ca²⁺]_i in adherent cells, applicable to a variety of cell systems, will be first presented. Then, the implementation of Fura-2 to detect [Ca²⁺]_i in two specific cell types, namely, human adrenocortical cells and primary skin fibroblasts, will be discussed in more particulars. Finally, the procedure to monitor Ca²⁺ influx through the plasma membrane using Fura-2 will be described.

How to make calcium-sensitive minielectrodes

In this protocol we describe how to make and use minielectrodes for measuring [Ca(2+)] in small volumes of solution. The minielectrodes are ~2 mm in diameter and have sufficiently low resistances to be used with a standard pH meter. They are made by dipping polyethylene or borosilicate glass tubes (~5 cm long) in a membrane solution. Although the chemicals used to make these Ca(2+)-sensitive minielectrodes are expensive, they can be used to make hundreds of electrodes, each with a useful life of several months.

Calcium-sensitive mini- and microelectrodes

It is widely agreed that the best method for measuring the ionized free calcium concentration ([Ca(2+)]) in large volumes of biological solutions is to use Ca(2+)-sensitive macroelectrodes. These are commercially available. To measure [Ca(2+)] in small volumes of solution, minielectrodes with 1-2-mm tips can easily be made and used, and may also be commercially available. Ca(2+)-sensitive microelectrodes (CaSMs, with 0.5-2-μm tips) can also be made and used extracellularly or intracellularly in robust cells, but interest in their use has recently been largely eclipsed. This is because of practical difficulties and the introduction of a large number of fluorescent and other optical calcium probes with calcium sensitivities varying from the nanomolar to the millimolar range, such as Fura-2, Indo-1, Fluo-4, and many others. In this article, we emphasize the utility of Ca(2+)-selective electrodes and show that their use is complementary to use of fluorescent and other optical methods. Each method has advantages and disadvantages. Because numerous reviews and books have been dedicated to the theoretical aspects of ion-selective electrode principles and technology, this article is mainly intended for investigators who have some degree of electrophysiological experience with ion-selective electrodes or microelectrodes.

How to make calcium-sensitive microelectrodes

Ca(2+)-sensitive microelectrodes (CaSMs) directly measure the pCa at their tip, which can be in a small extracellular space or inside a large and robust cell. They do not add to buffering and do not require expensive equipment. But they are time-consuming to make, require a reference electrode in the same location, and tend to create a leak around the point of insertion. In addition, CaSMs only work well with a tip diameter of >1 μm. In this protocol, we describe how to make and use the electrodes and briefly consider possible problems.