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The GCaMP3 reporter strain (Ai38)
A couple months ago the latest Cre-dependent optogenetic reporter mouse lines were published in a Nature Neuroscience paper. But one particular line, the GCaMP3 reporter strain, was missing at roll call. This line was published almost at the same time in the Journal of Neuroscience. In the February 29 issue of J. Neurosci, Zariwala et al. show that when crossed with Cre lines the Ai38 line yields stable GCaMP3 expression without the typical toxicity observed with AAV infections (which correlates with GCaMP3 diffusing into the nucleus and is observed as early as 4-6 weeks after infection). For a side-by-side comparison of GCaMP3 and Oregon Green BAPTA-1 (OGB-1), the authors looked at the visual cortex of anesthetized mice. GCaMP3-expressing pyramidal cells of the visual cortex (obtained using a Wfs1-Tg2-Cre line) showed preserved orientation selectivity to moving oriented gratings. Interestingly, the maximum ΔF/F achieved in GCaMP3-expressing neurons was substantially higher than with OGB-1. On the opposite, OGB-1 gave higher ΔF/F for low responder cells. One downside is that imaging from the Ai38 line required about 3 x more laser power than when using AAVs or OGB-1. But overall it looks like GcaMP reporter strains are on the right track and already experiment-ready. Now the question is will you get the Ai38 mouse from the Jackson Lab (#014538, link here) or will you wait for the GCaMP5 reporter strain which is announced for the summer?
Stable expression levels in the Ai38 mouse over months. Native GCaMP3 fluorescence in layer 2/3 of visual cortex from Wfs1-Tg2-CreERT2:Ai38 mice (A, B) and adult wild-type mice infected with AAV-syn-GCaMP3 (C, D). Previous GECI delivery using in utero electroporation or viral infection produced a subpopulation of cells (typically at the site of injection) with strong nuclear fluorescence (arrows in the above pictures). These filled cells displayed abnormal physiology and attenuated stimulus-evoked responses. The number of filled neurons increased with time, which fundamentally limits the duration and stability of chronic GECI imaging. With the Ai38 mouse, Zariwala et al found virtually no cells with filled nuclei. From Zariwala et al.
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