Capturing calcium moments with light

Calcium imaging is a common and established way of looking at brain signals during stimulation as an indirect method. If you take a neuron for example, calcium ions are always presenting in high amount at the extracellular matrix. Electrical signals that are passed through will cause calcium to go from the extracellular space into the intracellular space of a neuron. This event will signal release of neurotransmitters from the synaptic vesicles. The neurotransmitters travel through the synaptic cleft and are either degraded or taken up by the receptors at the post-synaptic cleft.

The changing calcium concentration of calcium ions outside and inside the neuron is the main feature for capturing such moments. Capturing these moments gives us invaluable information about brain signalling and its networks. Only then we are able to probe questions and correlate certain specific brain signalling with cognitive and sensory behaviour. In order to do so, one would require a calcium chelator. These are small molecules that are specific towards calcium ions against other internal ions such as potassium, magnesium, sodium and so forth. 

There are many types of already well-established calcium indicators. When I think of calcium indicators however, these four mainstream types that instantly comes to mind:

(A) Bioluminescent proteins. It works by binding of calcium ions to aequorin leading to oxidation coelenterazine C to colenteramide. The amide group relaxes to the ground state while emitting a photon.

Photograph by Denise Allen 'Crystal jelly - Aequorea victoria', 2007, online flick
Aequorea victoria Photograph by Denise Allen ‘Crystal jelly – Aequorea victoria’, 2007, online flickr

(B) Chemical calcium indicators. For example, fura-2 being excitable by UV light. Increasing concentration of calcium ion leads to changes in emitted fluorescence.

Rhodamine-2 dye
© Rhod-2, cell impermeant. Excibit fluorescence increase upon binding to calcium ions, just like Fura-2

(C) Fret-based genetically encoded calcium indicators (GECI). In the presence of calcium will bind to calmodulin and will result in the two fluorescent proteins ECPF (donor) and Venus (acceptor) approach. This will induce Förster resonance energy transfer (FRET) and the blue fluorescence of 480 nm will decrease and emit 530 nm instead.

(D) Single-fluorophore GECI. After binding to GCamp, an intramolecular conformational change leads to increase in the emitted fluorescence of 515 nm.

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© Sheryl Roberts, So you think you can grow crystals in a beaker, 2014 all rights reserved

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