Forebrain commissure formation in zebrafish embryo requires the binding of KLC1 to CRMP2

Formation of the corpus callosum (CC), anterior commissure (AC), and postoptic commissure (POC), connecting the left and right cerebral hemispheres, is crucial for cerebral functioning. Collapsin response mediator protein 2 (CRMP2) has been suggested to be associated with the mechanisms governing this formation, based on knockout studies in mice and knockdown/knockout studies in zebrafish. Previously, we reported two cases of non-synonymous CRMP2 variants with S14R and R565C substitutions. Among the, the R565C substitution (p.R565C) was caused by the novel CRMP2 mutation c.1693C > T, and the patient presented with intellectual disability accompanied by CC hypoplasia. In this study, we demonstrate that crmp2 mRNA could rescue AC and POC formation in crmp2-knockdown zebrafish, whereas the mRNA with the R566C mutation could not. Zebrafish CRMP2 R566C corresponds to human CRMP2 R565C. Further experiments with transfected cultured cells indicated that CRMP2 with the R566C mutation could not bind to kinesin light chain 1 (KLC1). Knockdown of klc1a in zebrafish resulted in defective AC and POC formation, revealing a genetic interaction with crmp2. These findings suggest that the CRMP2 R566C mutant fails to bind to KLC1, preventing axonal elongation and leading to defective AC and POC formation in zebrafish and CC formation defects in humans. Our study highlights the importance of the interaction between CRMP2 and KLC1 in the formation of the forebrain commissures, revealing a novel mechanism associated with CRMP2 mutations underlying human neurodevelopmental abnormalities.