Mllt11 regulates the migration and neurite outgrowth of cortical projection neurons during development

The formation of connections within the mammalian neocortex is highly regulated by both extracellular guidance mechanisms and intrinsic gene expression programs. There are generally two types of cortical projection neurons: those that project locally and interhemispherically, and those that project to sub-cerebral structures such as the thalamus, hindbrain, and spinal cord. The regulation of cortical projection morphologies is not yet fully understood at the molecular level. Here we report a role for Mllt11 (Myeloid/lymphoid or mixed-lineage leukemia; translocated to chromosome 11/All1 Fused Gene From Chromosome 1q) in the migration and neurite outgrowth of callosal projection neurons during brain formation. We show that Mllt11 expression is exclusive to developing neurons and is enriched in the developing cortical plate, particularly during the formation of the upper or superficial cortical layers. In cultured primary cortical neurons, Mllt11 is detected in varicosities and growth cones as well as the soma. Using conditional loss-of-function and gain-of-function analysis we show that Mllt11 is a required for neuritogenesis and proper migration of upper layer cortical projection neurons. Loss of Mllt11 in the superficial cortex leads to a severe reduction in fibres crossing the corpus callosum, a progressive loss in the maintenance of upper layer projection neuron gene expression, and dysplasia of dendritic arborisation patterns. Proteomic analysis revealed that Mllt11 associates with cytoskeletal components including stabilized microtubules consistent with a role in neuronal migration and neuritogenesis. Taken together, our findings support a role for Mllt11 in promoting the formation of mature projection neuron morphologies and connectivity in the cerebral cortex.