Not all that glitters is gold: off-target recombination in the somatostatin-IRES-Cre mouse line labels a subset of fast-spiking interneurons.

Neurons in the mammalian brain are a highly diverse population with a complex assortment of electrophysiological, morphological and molecular properties, which has hindered efforts to classify them into genetically and functionally meaningful subtypes and to understand their various roles in the normal or pathological brain. Nowhere is this issue more acutely felt than in the study of inhibitory cortical interneurons, whose classification is still a source of contention and confusion (Markram et al., 2004; Petilla Interneuron Nomenclature et al., 2008; Defelipe et al., 2013). A major boon to investigators has been the development of mouse lines in which genetically defined subsets of interneurons express fluorescent proteins, allowing their identification and targeting during electrophysiological recordings or imaging experiments (Oliva et al., 2000; Meyer et al., 2002; Chattopadhyaya et al., 2004; Ma et al., 2006). More recently, investigators funded by the NIH Neuroscience Blueprint project have developed a toolbox of “driver” lines in which the Cre recombinase gene is inserted immediately downstream to genes that are known markers of specific interneuron subsets (Taniguchi et al., 2011). These Cre lines can be bred with mice carrying floxed genes, to generate cell type-specific knockouts of genes of interest. In addition, by breeding these lines with mice from a parallel toolbox of Cre reporter lines in which a gene coding sequence is inserted after a lox-STOP-lox cassette, or by transfecting them with viral vectors carrying similar constructs, investigators can induce cell-type specific expression of any gene of interest, from inert fluorescent proteins to calcium probes and light-activated ion channels or pumps (Madisen et al., 2010, 2012; Zariwala et al., 2012). While these technologies carry great promise and have already enabled some important findings, the rush to use them also carries considerable risk, if the relevant expression patterns are not fully characterized. A case in point is the somatostatin–IRES-Cre (SOM-Cre) mouse line (Taniguchi et al., 2011), in which Cre expression was targeted to cells containing the neuropeptide somatostatin (SOM). In the cerebral cortex, SOM-containing neurons are a well-studied population of dendritic-targeting inhibitory interneurons (Ma et al., 2006; Silberberg and Markram, 2007; Fanselow et al., 2008; Tan et al., 2008; Ma et al., 2010; Fino and Yuste, 2011). The SOM-Cre line has already been used in several high-profile studies, and in most of these the authors tacitly assumed—but did not validate—that Cre-mediated recombination was restricted to SOM interneurons (Adesnik et al., 2012; Gentet et al., 2012; Lee et al., 2012; Wilson et al., 2012; Chiu et al., 2013; Kvitsiani et al., 2013; Xu et al., 2013). We found, however, that 6–10% of neurons expressing a Cre-dependent reporter in any given cortical layer were fast-spiking/ parvalbumin-expressing (FS/PV) interneurons, a subtype quite distinct from SOM interneurons in electrophysiological etc., morphological and molecular properties (Rudy et al., 2011) [Note that there is another SOM-Cre line reported in the literature (Lovett-Barron et al., 2012), which we did not test].