Pou4f2 knock-in Cre mouse: A multifaceted genetic tool for vision researchers

Purpose: A transgenic mouse that expresses Cre recombinase under control of the Pou4f2-promoter (also referred to as Brn-3b and Brn-3.2) was characterized. Pou4f2 expression has been reported in a subset of retinal ganglion cells (RGCs) in the retina, in the midbrain, and in the germline. In this study, we characterize the expression pattern of this Cre-recombinase line and report its utility in targeted deletion, temporal deletion, RGC depletion, and germline targeting, which can be regulated by the sex of the Cre-carrying mouse. Methods: Pou4f2(Cre) was mapped by using a combination of PCR and sequencing of PCR products to better understand the construct and to locate where it was inserted within the Pou4f2 locus. Cre expression patterns were examined by crossing Pou4f2(Cre/+) mice to Cre reporter mice. Immunohistochemistry was used to further define the pattern of Cre expression and Cre-mediated recombination within the retina, brain, and other tissues. Results: An internal ribosome entry site (IRES)-Cre cassette was inserted into the Pou4f2 gene disrupting normal gene function, as verified by the depletion of RGCs in mice homozygous for the insert. Pou4f2(Cre) expression was observed in the retina, brain, peripheral neurons, and male germ cells. Germline recombination was observed when the sire carried the Cre and the target for recombination. In all other breeding schemes, recombination was observed within subsets of cells within the retina, brain, intestines, heart, and gonads. In the retina, Cre efficiently targets recombination in neurons within the RGC layer (RGL), the inner nuclear layer (INL), and a small percentage of photoreceptors, activity that has not been previously reported. Unlike most other Cre lines active in the inner retina, recombination in Müller and other glia was not observed in mice carrying Pou4f2(Cre). Within the visual centers of the brain, Cre targets recombination in about 15% of cells within the superchiasmatic nucleus, lateral geniculate nucleus, and superior colliculus. Conclusions: Pou4f2(Cre) provides multiple uses for the vision researcher’s genetic toolkit. First, Pou4f2(Cre) is a knock-in allele that can be used to eliminate Pou4f2, resulting in depletion of RGCs. Second, expression of Cre in male germ cells makes this strain an efficient germline activator of recombination, for example, to target LoxP-flanked sequences in the whole mouse. Third, Pou4f2(Cre) efficiently targets RGCs, amacrine cells, bipolar cells, horizontal cells, and a small number of photoreceptors within the retina, as well as the visual centers in the brain. Unlike other Cre recombinase lines that target retinal neurons, no recombination was observed in Müller or other retinal glia. These properties make this Cre recombinase line a useful tool for vision researchers.