In addition, Caspr4 and NB2 associate in transfected cells in vitro ( Figures S3S and S3T). Together, these data provide evidence that NB2 and Caspr4 form an interaction complex in neural tissue. Based on these findings, we explored whether loss of Caspr4, like that of NB2, prevents high bouton-packing. We found that the overall trajectory of PvON proprioceptive axons was similar in p7 Caspr4 mutants as compared to wild-type mice ( Figures S3D and S3E). In addition, the number and size of vGluT1ON sensory terminals, as well as their alignment with motor neuron Shank1a plaques was similar in wild-type and Caspr4 mutant mice ( Figures S3F–S3K). Analysis of p21 Caspr4 Selleckchem GSK2656157 mutants revealed that
the number of GAD65ON/GAD67ON GABApre boutons on sensory terminals was reduced by 39% (t test, p < 0.0001) ( Figures 4J–4L and S3L). In contrast, analysis
of Caspr ( Gollan et al., 2003) and Caspr2 ( Poliak et al., 2003) mutants revealed no change in the density of GABApre boutons ( Figures 4G–4I; data not shown), indicating the specificity of Caspr4 function. If both NB2 and Caspr4 act in the same pathway, we might anticipate a similar severity of phenotype when ablating one or both components Bafilomycin A1 in vitro of this putative receptor complex. We therefore analyzed GABApre bouton formation in mice in which both NB2 and Caspr4 genes were inactivated. In NB2; Caspr4 double mutant mice, we detected a 34% reduction in GAD65ON/GAD67ON and Syt1ON/GAD67ON GABApre boutons in contact with vGluT1ON PDK4 sensory terminals (t test, p < 0.0001) ( Figures 4M–4O), a reduction similar in extent to that observed in NB2 and Caspr4 single mutant mice. These genetic data support the view that NB2 and Caspr4 act as coreceptors on sensory terminals to direct the formation of GABApre bouton synapses. Certain Schwann cell-axonal interactions in myelinated nerves are mediated by binding of contactins to various members of the L1 Ig superfamily (Poliak and Peles, 2003 and Shimoda and Watanabe, 2009). We therefore examined whether any of the four L1 family members—L1, close homolog of L1 (CHL1), neurofascin (NF), and NrCAM—might
function in GABApre neurons as ligands for sensory terminal-expressed NB2/Caspr4. We monitored expression of L1, CHL1, NF, and NrCAM in wild-type p5 to p7 DRG and spinal cord and found broad transcript expression by many spinal cord and DRG neurons ( Figures 5A–5D; for full spinal cord views see Figures S4A–S4D). In situ hybridization histochemistry revealed that neither L1, CHL1, nor NrCAM were expressed by PvON proprioceptive sensory neurons ( Figures 5A, 5B and 5D). To assess expression of L1 members in Ptf1a-derived dI4 interneurons that include the GABApre set we FACS-isolated YFPON interneurons from the spinal cord of p0 Ptf1a::Cre; Rosa26.lsl.YFP mice ( Betley et al., 2009 and Srinivas et al., 2001). All four L1 family transcripts were expressed by YFPON neurons, as detected in a RT-PCR analysis ( Figure 5E).