Fezl regulates the differentiation and axon targeting of layer 5 subcortical projection neurons in cerebral cortex

During the development of the cerebral cortex, progenitor cells produce neurons that migrate to laminar positions appropriate for their birth dates, adopt specific neuronal identities, and form appropriate local and long-distance axonal connections. Here, we report that forebrain embryonic zinc-finger-like protein (Fezl), a putative zinc-finger transcriptional repressor, is required for the differentiation of projection neurons in cortical layer 5. In Fezl-deficient mice, these neurons display molecular, morphological, and axonal targeting defects. The corticospinal tract was absent in Fezl(-/-) mice, corticotectal and pontine projections were severely reduced, and Fezl-expressing neurons formed aberrant axonal projections. The expression of many molecular markers for deep-layer neurons was reduced or absent in the Fezl(-/-) cerebral cortex. Most strikingly, Ctip2, a transcription factor required for the formation of the corticospinal tract, was not expressed in the Fezl-deficient cortex. These results suggest that Fezl regulates the differentiation of layer 5 subcortical projection neurons.