New Cre-dependent optogenetic reporter lines

Cre-dependent mouse strains that would express optogenetic control tools have been seen as the holy grail for the past 6 years. Not only such strains would simplify experimental procedures by eliminating the need for gene delivery (in utero electroporation or viral infection) but they would potentially yield more reproducible experiments by providing more homogeneous and predictable expression levels in a given neuronal population. Many labs gave it a try and failed. The challenge resided in achieving high expression using a ubiquitous promoter and a Cre-activated cassette (principle below).

The Cre/lox system. The driver line expresses Cre under the control of a gene-specific promoter (GS Pr), and the reporter line directs Cre-dependent expression of the transgene. Cre-mediated recombination between the two loxP sites deletes the STOP cassette and hence induces the transgene expression. UB Pr, ubiquitous promoter. From Zeng H & Madisen L, Prog. in Brain Res. 2012, 196:193-213.

Some time ago it seemed like the folks at the Allen Institute were on a promising track and their first ChR2 reporter strain called Ai27 leaked out to several labs. But many were discouraged by the insufficient expression level of this line and the small light-evoked responses obtained with it. But the Ai27 mouse was just the beginning. Last year the Allen Institute released a new line, codename Ai32, with significantly improved ChR2 expression. Apparently the same ones which were disappointed by the Ai27 line were pretty unanimous about the good results of this Ai32 mouse. The word spread quickly and the mouse was made available through the Jackson Laboratory together with 3 other similar strains (Ai35 for Arch, Ai39 for eNpHR3.0 and Ai38 for GCaMP3). But the ones which had not put their hands on these lines yet were anxiously waiting for their characterization. It’s nearly completely done now, with a 22-author paper published this month in Nature Neuroscience comparing the Ai27 vs 32 and Ai35 vs 39 lines. We can’t wait to see the paper about the Ai38 mouse.

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