Fig. 1. Conditional inactivation of TrkA. (A) Diagram of the replacement vector strategy used to generate the TrkA^neo allele and re-activation of TrkA expression by Cre recombinase. The pGKneobpA cassette flanked by loxP sites was placed into intron 11. (B) Southern blot analysis of tail DNA from three-week-old mice with a 5' external probe shows the switch of the wild-type (WT) BamHI fragment from 12 kb (WT) to a 7 kb restriction fragment (TrkA^neo) after neo insertion. (C) Northern blot analysis of total RNA extracted from Dorsal Root Ganglia (DRG) neurons of E13.5 embryos either WT (+/+), heterozygous for the neo insertion (+/neo), homozygous for the neo insertion (neo/neo), or homozygous for the allele after neo removal with ß-actin cre (loxP/loxP), hybridized with a TrkA-kinase domain-specific probe (exon 14-17). Note that neo insertion completely eliminates TrkA-specific transcripts whereas TrkA-transcription is restored to WT levels after neo-excision by Cre recombination. (D) RT-PCR analysis of the same samples analyzed by northern blot. Specific primers for actin, neo, and TrkA exon 10 and 14 were used. Samples were treated with (+) and without (–) RT. Note the complete absence of the 609 bp TrkA-specific PCR product in TrkA^neo/neo mice. (E) Wholemount lacZ staining of a Rosa-26 E11.5 embryo with the T1-cre transgene. Note the specific staining in both central and peripheral nervous system. (F) lacZ staining of sections from spleen, thymus and bone marrow of adult Rosa-26 mice carrying the T1-Cre (top panels) or a ß-actin-Cre transgene with mosaic expression used as control (bottom panels) (Ma et al., 2003). Cre recombinase under the control of the T-1 promoter is only expressed in the nervous system (E) and not in organs of the immune system (F).