Thalamus-derived glutamate is required for early specification of layer 4 neurons in the sensory cortex

Elucidating the mechanisms that control the formation of the mammalian neocortex is crucial for understanding brain functions. Synaptic activity of thalamocortical axons (TCAs), mediated by glutamate, exerts a major extrinsic influence on the maturation of their target layer 4 neurons in postnatal primary sensory cortex. However, TCAs reach the sensory cortex during mid-embryonic stages in mice, when neurons of future superficial layers, including layer 4, are still being generated from radial glia (RGs) or intermediate progenitor cells (IPCs), well before the formation of direct synapses. We previously showed that TCAs are required for the production and specification of the proper number of layer 4 neurons in sensory areas, and that part of these area-specific roles is played by the thalamus-derived molecule VGF. However, the role of TCA-derived glutamate prior to synapse formation has remained unclear. In this study, we used mutant mice lacking vGluT2 , a vesicular glutamate transporter expressed in the embryonic thalamus, and found that vesicular release of thalamus-derived glutamate is required for the proper production and specification of layer 4 neurons in the sensory cortex by the neonatal stage, through mechanism distinct from those involving VGF. Our findings reveal that multiple molecular cues produced by incoming TCAs play distinct roles in the production and specification of layer 4 neurons in the sensory cortex.