Classification of cortical GABAergic neurons has long been contentious (Ascoli et al., 2008). A useful criterion is the pattern of axon projection along with cellular and subcellular targets of innervation (Figure 1) (Somogyi
et al., 1998 and Markram et al., 2004). PLX3397 manufacturer For the purpose of genetic targeting, we parse cortical GABAergic populations based on their gene expression. Although gene expression profiles correlate and likely contribute to cell phenotype and identity (Nelson et al., 2006 and Sugino et al., 2006), there is often no simple relationship between the expression of a single gene and a morphologically and functionally defined cell type. However, current methods of genetic targeting restrict our approach to cell types based on expression of one or two genes. As a first step, we selected over a dozen genes to target major GABAergic populations and lineages. These included Selleck Galunisertib broadly expressed GABA synthetic enzyme and
transcription factors, as well as neuropeptides, enzymes, and calcium binding proteins with more restricted expression that correlates with subpopulations (Figure 1). We used the Cre/loxP binary gene expression system (Dymecki and Kim, 2007) to target GABAergic neurons. In order to faithfully engage the genetic mechanisms that specify and maintain cell identity, we aimed to generate driver lines in which Cre activity precisely and reliably recapitulates the endogenous gene expression. We therefore used gene targeting in embryonic stem (ES) cells to insert Cre coding cassettes either at the translation initiation codon or immediately after the
translation STOP codon of an endogenous gene (Figure 1 and Table 1; see Figure S1 and Table S1 available online). We used four reporter alleles, all generated at the Rosa26 locus, to assay recombination patterns: (1) RCE-LoxP is a loxP-STOP-loxP-GFP reporter ( Miyoshi et al., 2010), (2) RCE-Frt is an frt-STOP-frt-GFP reporter ( Miyoshi et al., 2010), (3) Ai9 is a loxP-STOP-loxP- tdTomado reporter ( Madisen et al., 2010), and (4) RCE-dual is a loxP-STOP-loxP- frt-STOP-frt-GFP reporter which expresses GFP upon the intersection of Cre and Flp recombination ( Miyoshi et al., 2010). Our current characterizations have focused on neocortex 17-DMAG (Alvespimycin) HCl and hippocampus, but most GABA driver lines also show Cre activities throughout the brain (Table 2) from the retina to the spinal cord. A broader characterization of these GABA drivers in the CNS including atlases of Cre-dependent reporter expression is presented at the Cre Driver website http://credrivermice.org. Cre activities in the peripheral nervous system and nonneuronal tissues have not been examined. These GABA driver lines are being distributed by the Jackson Laboratory (http://www.jax.org/). Genetic fate mapping using transcription factors that define progenitor pools should provide insight into the specification and development of GABAergic subtypes.