A new genetically-encoded voltage sensor paper is out from a friend and former mentor of mine, Atsushi Miyawaki. One memorable moment when working in his lab during the RIKEN summer program of 2002 was when Atsushi took me into his office and whipped out a custom green laser pointer. These had been banned in Japan, as fans would shine their powerful light into the eyes of pitchers and batters at baseball games. Atsushi was really proud of his. He smiled and then started sweeping the light point over the rocks in his fishtank. Each ‘rock’ was actually coral his lab had collected from fluorescent protein hunting trips, and each glowed a different color when the green light hit it. He has been putting these novel discoveries to good use.
In Improving membrane voltage measurements using FRET with new fluorescent proteins, Tsutsui et. al take two fluorescent proteins discovered and engineered by the Miyawaki lab, mUKG and mKOk, and graft them onto the Ci-VSP scaffold used in VSFP2.1 (also developed at RIKEN). The green and orange fluorescent proteins undergo significant FRET transfer which is voltage dependent. They get 40% dR/R per 100mV with a 2 component association rate of around 10 and 200ms. Unsurprisingly, the kinetics speed up at physiological temperatures to 5-20ms on and off. They are able to pick up single pseudo-action potentials in Neuro2A cells, though the response is highly filtered. They are also able to see very clear spontaneous waves of potential change in cardiomyocytes (23% dR/R) and single spikes in cultured neurons (2% dR/R for 1AP). They dub this voltage sensor “Mermaid”.
The authors state that they used the new FPs due to their improved photostability and especially pH resistance.
Additionally, because Aequorea GFP variants are pH-sensitive, and neuronal activity causes considerable acidification, the responses of sensors to depolarization in intact neurons may be overwhelmed by sustained changes resulting from acidification.
Granted that mOrange2 is pretty pH-sensitive, but I’m not sure this is a real issue, or a potential issue to justify using their new FPs. From the spectra of mUKG vs. EGFP, it would seem that EGFP’s 10nm further redshifted emission would be a superior FRET pair for mKOk. It smells like there may be a bit of bundling of various independent projects into this paper. However, they do make a good point that this pair will have a different preferred dipole orientation than existing FRET pairs, which could lead to improved performance in some constructs.
Things I’m still wondering :
- Have they tried using the improved VSFP3.1 scaffold? This was shown to be much faster than 2.1. I suspect the mUKG is not as tolerant to C-terminal truncation than CFP and GFP.
- What about using EGFP as the donor? Could you then use the VSFP3.1 scaffold?
- Is there a rapid non-FRET quenching of the donor upon depolarization as seen in VSFP3.1?
- Why is the single wavelength fluorescence increasing in both channels in figure 2d? Is there some photoactivation going on?
- I’d love to see a head to head comparison of VSFP3.1 and Mermaid under identical conditions. Also responses in brain slice at physiological temperatures.