LHX2 regulates dendritic morphogenesis in layer II/III of the neocortex via distinct pathways in progenitors and postmitotic neurons

In the mammalian neocortex, excitatory neurons that send projections via the corpus callosum are critical to integrating information across the two brain hemispheres. The molecular mechanisms governing the development of the dendritic arbours and spines of these callosal neurons are poorly understood, yet these features are critical to their physiological properties. LIM Homeodomain 2 (Lhx2), a regulator of fundamental processes in cortical development, is expressed in postmitotic callosal neurons occupying layer II/III of the neocortex and also in their progenitors in the embryonic day (E) 15.5 ventricular zone of the mouse neocortex. We tested whether this factor is essential for dendritic arbour configuration and spine morphogenesis of layer II/III neurons. Here, we report loss of Lhx2 either in postmitotic callosal neurons or their progenitors, resulting in shrunken dendritic arbours and perturbed spine morphology. In postmitotic neurons, we identified that LHX2 regulates dendritic and spine morphogenesis via the canonical Wnt/β Catenin signalling pathway. Constitutive activation of this pathway in postmitotic neurons recapitulates the Lhx2 loss-of-function phenotype. In E15.5 progenitors, we identified that bHLH transcription factor Neurog2 mediates LHX2 function in regulating dendritic and spine morphogenesis. We show that Neurog2 expression increases upon loss of Lhx2 and that shRNA-mediated Neurog2 knockdown rescues the loss of Lhx2 phenotype. Our study uncovers novel LHX2 functions consistent with its temporally dynamic and multifunctional roles in the cortical circuit assembly. To assess the implications of the loss of Lhx2 in the dendritogenesis of Layer II/III neurons, either EGFP (Control) or Cre-EGFP (Progenitor mutant) plasmids were transfected in E15.5 progenitors of the developing cerebral cortex of Lhx2lox/lox mice using in utero electroporation. The brain was isolated once the animal was of age Postnatal Day 5. The labelled layer II/III neurons were microdissected, and their RNA was isolated. For postmitotic deletion similar strategy was used to label layer II/III neurons by in utero electroporating EGFP in Lhx2lox/lox (Control) or NexCre/+; Lhx2lox/lox (Postmitotic mutant) mouse cortices.