Loss of floor plate Netrin-1 impairs midline crossing of corticospinal axons and leads to mirror movements

In human, execution of unimanual movements requires lateralized activation of the primary motor cortex, which then transmits the motor command to the contralateral hand through the crossed corticospinal tract (CST). Mutations in NETRIN-1 alter motor control lateralization, leading to congenital mirror movements. To address the role of midline Netrin-1 on CST development and subsequent motor control, we analyzed the morphological and functional consequences of floor-plate Netrin-1 depletion in conditional knock-out mice (Shh::cre;Ntn1lox/lox mice). Here, we show that depletion of floor plate Netrin-1 critically disrupts midline crossing of the CST, whereas the other commissural systems are mostly preserved. The CST defect results in abnormal but functional ipsilateral projections, and is associated with abnormal symmetric movements. Therefore, our study reveals a new role for Netrin-1 in CST development. It also describes a unique mouse model recapitulating characteristics of human congenital mirror movements, through abnormal CST decussation.