Ai94(TITL-GCaMP6s)-D;Rosa26-ZtTA (or Ai94D;R26-ZtTA) mice have a Cre/Tet-dependent, fluorescent calcium indicator GCaMP6s (an ultrasensitive detector of single neuronal action potentials with slower decay and response kinetics) inserted into the Igs7 locus (TIGRE; an intergenic region on mouse chromosome 9 that allows reporter expression to be tightly regulated), as well as a Cre-dependent tetracycline-controlled transactivator protein (tTA) inserted into the Gt(ROSA)26Sor locus on chromosome 6.

In the absence of Cre recombinase, Ai94D;R26-ZtTA mice are designed to have widespread lacZ expression but the expression of both GCaMP6s (EGFP fluorescence) and tTA are blocked by a floxed STOP cassette in each allele.

Following exposure of Ai94D;R26-ZtTA mice to Cre recombinase, the floxed-STOP cassette in each allele is deleted; leading to expression of tTA and GCaMP6s in the cre-expressing cells (see additional details below). GCaMP6s expression can then be regulated with tetracycline or its analog doxycycline (dox).

When Ai94D;R26-ZtTA mice are bred with Cre-driver lines to create triple transgenic animals (GCaMP6s+/tTA+/Cre+), EGFP fluorescence in cre-expressing cells is low in the absence of calcium binding. Following calcium binding (such as neuronal activation), increased EGFP fluorescence is observed in those cells. See further phenotype considerations below.

Ai94D;R26-ZtTA mice heterozygous for both targeted mutations are expected to be viable and fertile with no reported gross physical or behavioral abnormalities. The Ai94D donating investigator reports that mice homozygous for Ai94D are viable and fertile with no reported gross physical or behavioral abnormalities. The R26-ZtTA donating investigator reports that mice heterozygous for R26-ZtTA are more healthy and fertile than mice homozygous for R26-ZtTA.

For characterization information of Ai94 with other tTA and Cre lines, see images at the Allen Institute for Brain Science website (Ai94(TITL-GCaMP6s) images).

Further phenotype considerations: Steinmetz et al. 2017 ENeuro reports aberrant neural activity is occasionally observed in Emx1-Cre;Camk2a-tTA;Ai94 mice. This paper is summarized below.

Steinmetz et al. 2017 ENeuro reports aberrant neural activity resembling interictal spikes were observed in some genotypes of GCaMP6f-expressing Ai93 mice. This activity was most prominent in Emx1-Cre;Camk2a-tTA;Ai93 mice and Emx1-Cre;Rosa26-ZtTA;Ai93 mice, and also in Camk2a-tTA;Ai93 mice that had undergone germline Cre recombination. Aberrant neural activity was also occasionally observed in Slc17a7-IRES-Cre;Camk2a-tTA;Ai93 mice and GCaMP6s-expressing Emx1-Cre;Camk2a-tTA;Ai94 mice. Several other combinations were absent of aberrant neural activity - including Emx1-CreERT2;Camk2a-tTA;Ai93 maintained on doxycycline, Camk2a-tTA;Ai93 with other Cre-drivers (Rorb-IRES2-Cre or Scnn1a-Tg3-Cre), Camk2-tTA with another GCaMP6s transgene (Camk2a-tTA;TRE-GCaMP6s line G6s2), Emx1-driven GCaMP6f (Emx1-Cre;Ai95) or GCaMP6s (Emx1-Cre;Ai96), excitatory neuron-expressing GCaMP6s strains (Snap25-2A-GCaMP6s and Thy1-GCaMP6s line GP4.3) and Pvalb-driven channelrhodopsin2 (Pvalb-Cre;Ai32).

Note on GCaMP6s: The GCaMP6 slow variant calcium indicator (GCaMP6s) is an ultrasensitive detector of single neuronal action potentials with slower decay and response kinetics (Chen et al. 2013 Nature 499:295), and is an improved version of GCaMP5G. The GCaMP6s fusion gene was designed by Drs. Douglas Kim and Loren Looger (Janelia Farm, HHMI). Compared to GCaMP6s, the slower kinetics render GCaMP6s more sensitive and slower to decay. GCaMP6s has a chicken smooth muscle M13 fragment of myosin light chain kinase, a circularly permutated EGFP (with several amino acid substitutions designed to optimize dynamic range, baseline fluorescence and sensitivity), and a rat calmodulin sequence modified to increase the fluorescence change for small calcium transients. Other modifications in the cpEGFP-to-CaM linker also improve sensor function. In the absence of calcium binding, dim EGFP fluorescence is expected as there is a pore from the outside of its barrel into the chromophore. Upon calcium binding, this pore becomes occupied and fluorescence is increased.