In a complementary manner, two members of the L1 Ig family, CHL1 and NrCAM, are expressed by GABApre neurons and their function is required for the formation of high-density GABApre synapses with sensory terminals. Our findings pinpoint a molecular recognition system that helps to direct the formation of presynaptic inhibitory synapses. To define potential GABApre recognition molecules expressed by sensory neurons we screened 45 transcripts JQ1 cost encoding Ig domain-containing proteins for expression in dorsal

root ganglia (DRG) and spinal cord at postnatal days (p)5 to p6—the period at which GABApre axons form contacts with proprioceptive sensory terminals (Table S1 available online) (Betley et al., 2009). To explore the idea that incoming GABApre axons recognize receptors on sensory but not motor neurons, we focused our attention on transcripts expressed selectively by proprioceptive GSK1210151A supplier sensory neurons. This expression screen identified four transcripts, Contactin (Cntn) 3, Cntn5, Kirrel, and Kirrel-3, each of which was expressed by DRG neurons but not motor neurons. Only two of these, Kirrel-3 and Cntn5 were expressed by proprioceptors, as revealed by coexpression of Parvalbumin (Pv) ( Table S1) ( Arber et al., 2000). Analysis of Kirrel-3 mutant mice ( Prince et al., 2013) did not reveal a GABApre targeting phenotype (unpublished

observations), leading us to focus on the potential role of the contactin family ( Shimoda and Watanabe, 2009). We found that five of the six contactins, Cntn1, TAG-1 (Cntn2), BIG-1 (Cntn3), BIG-2 (Cntn4), and NB2 (Cntn5) ( Furley et al., 1990, Gennarini

et al., 1989, Ogawa et al., 1996, Yoshihara et al., 1994 and Yoshihara et al., 1995) were expressed Cell press by DRG neurons ( Figure 1A; Table S1). Of these, Cntn1, TAG-1, and BIG-2, were also expressed by motor neurons and, based on our design constraints were therefore excluded from further analysis ( Table S1). We failed to detect overlap in BIG-1 and Pv expression (data not shown), whereas Pv exhibited extensive overlap with NB2 transcript and protein ( Figures 1B–1C′). In addition, analysis of βgal expression in NB2::tauLacZ mice ( Li et al., 2003) revealed overlap in βgal expression and Pv-positive (PvON) proprioceptors, as well as expression in a subset of Pv-negative (PvOFF) sensory neurons ( Figures 1D and 1F). In spinal cord, we found that neither endogenous NB2, nor βgal were expressed by motor neurons, marked by choline acetyltransferase (ChAT) expression ( Figure 1E; for full spinal cord views and NB2 probe specificity see Figures S1A–S1D). These data establish that NB2 is expressed by proprioceptive sensory but not motor neurons. To test the involvement of NB2 in the formation of GABApre-sensory contacts, we assessed synaptic organization in NB2 heterozygous and homozygous null mutant mice ( Li et al., 2003). NB2 mutants survive, breed normally, and did not exhibit obvious locomotor abnormalities ( Li et al.