Responses to reviewers' comments

The authors of this study investigated a novel role for the protein Cerebellin-1 (Cbln1) in axon guidance during development. Using microarray analysis of dorsal spinal cord neurons, Cbln1 was identified as one of the differentially expressed gene candidates and verified by in situ hybridization that its mRNA is expressed both in dorsal spinal commissural neurons and the ventral floor plate. In order to study the cell autonomous (dorsal commissural neurons) and non-cell autonomous (floor plate) roles of Cbln1, a conditional flox Cbln1 mouse was generated and crossed with tissue-specific Cre lines. Using in vivo mouse genetics for loss-of-function, in utero electroporation in the chick for gain-of-function and in vitro tissue culture assays, it was shown that Cbln1 promoted commissural axon growth cell autonomously in commissural neurons and guidance by the floor plate in a non-cell autonomous manner. The investigators go on to show that Cbln1 most likely mediates these axon guidance events through its receptor Neurexin-2. It is also interesting that Cbln1 signaling mediates axonal growth and guidance in other neuronal populations including cerebellar granule cells and retinal ganglionic cells, of which these results suggest that Cbln1 is used widely by the nervous system for axon growth and guidance besides its previously known role in synaptogenesis. This study adds new knowledge to the complex and diverse molecular mechanisms involved in axon guidance during early nervous system development. However, this study falls short of demonstrating the downstream signaling mechanism employed by Cbln1 in axon guidance. It is also not known whether the same or different signaling mechanisms are used by Cbln1 in synaptogenesis versus axon guidance, which would add substantial significance to this work.