Combining calcium imaging with other optical techniques

Role for calcium signaling in manganese neurotoxicity

BACKGROUND

Calcium is an essential macronutrient that is involved in many cellular processes. Homeostatic control of intracellular levels of calcium ions [Ca²⁺] is vital to maintaining cellular structure and function. Several signaling molecules are involved in regulating Ca²⁺ levels in cells and perturbation of calcium signaling processes is implicated in several neurodegenerative and neurologic conditions. Manganese [Mn] is a metal which is essential for basic physiological functions. However, overexposure to Mn from environmental contamination and workplace hazards is a global concern. Mn overexposure leads to its accumulation in several human organs particularly the brain. Mn accumulation in the brain results in a manganism, a Parkinsonian-like syndrome. Additionally, Mn is a risk factor for several neurodegenerative diseases including Parkinson's disease and Alzheimer's disease. Mn neurotoxicity also affects several neurotransmitter systems including dopaminergic, cholinergic and GABAergic. The mechanisms of Mn neurotoxicity are still being elucidated.

AIM

The review will highlight a potential role for calcium signaling molecules in the mechanisms of Mn neurotoxicity.

CONCLUSION

Ca²⁺ regulation influences the neurodegenerative process and there is possible role for perturbed calcium signaling in Mn neurotoxicity. Mechanisms implicated in Mn-induced neurodegeneration include oxidative stress, generation of free radicals, and apoptosis. These are influenced by mitochondrial integrity which can be dependent on intracellular Ca²⁺ homeostasis. Nevertheless, further elucidation of the direct effects of calcium signaling dysfunction and calcium-binding proteins activities in Mn neurotoxicity is required.

Imaging GFP-based reporters in neurons with multiwavelength optogenetic control

To study the impact of neural activity on cellular physiology, one would like to combine precise control of firing patterns with highly sensitive probes of cellular physiology. Light-gated ion channels, e.g., Channelrhodopsin-2, enable precise control of firing patterns; green fluorescent protein-based reporters, e.g., the GCaMP6f Ca(2+) reporter, enable highly sensitive probing of cellular physiology. However, for most actuator-reporter combinations, spectral overlap prevents straightforward combination within a single cell. Here we explore multiwavelength control of channelrhodopsins to circumvent this limitation. The "stoplight" technique described in this article uses channelrhodopsin variants that are opened by blue light and closed by orange light. Cells are illuminated with constant blue light to excite fluorescence of a green fluorescent protein-based reporter. Modulated illumination with orange light negatively regulates activation of the channelrhodopsin. We performed detailed photophysical characterization and kinetic modeling of four candidate stoplight channelrhodopsins. The variant with the highest contrast, sdChR(C138S,E154A), enabled all-optical measurements of activity-induced calcium transients in cultured rat hippocampal neurons, although cell-to-cell variation in expression levels presents a challenge for quantification.

Combining Ca2+ and membrane potential imaging in single neurons

The ability to monitor Ca(2+) signals and membrane potential simultaneously at multiple locations on the same neuron facilitates further progress in our understanding of neuronal function. In particular, this method allows correlation of electrical and chemical signals from multiple sites, including those inaccessible to microelectrodes. This protocol describes a procedure for loading cells with two indicators, a Ca(2+)-sensitive Fura dye and voltage-sensitive JPW1114, together with the equipment required for detecting and imaging the two signals. Potential problems are discussed as well as the capabilities and limitations of the technique.

Combining Ca2+ imaging with -glutamate photorelease

This article describes simple configurations and methods for measuring optical Ca(2+) signals in response to photorelease of -glutamate. This photostimulation allows activation of postsynaptic glutamate receptors without activation of voltage-gated Ca(2+) channels, thereby permitting the separation and analysis of different Ca(2+) components. We give details of basic microscopy configurations and recommend tools for efficiently illuminating the preparation while preserving the healthy condition of the tissue. We also suggest methodological procedures and discuss linear optics for achieving simultaneous imaging and uncaging using two-photon illumination.