Activity-dependent feedback regulation of thalamocortical axon development by Lhx2 in cortical layer 4 neurons.

Establishing neuronal circuits requires interactions between pre- and postsynaptic neurons. While presynaptic neurons were shown to play instructive roles for the postsynaptic neurons, how postsynaptic neurons provide feedback to regulate the presynaptic neuronal development remains elusive. To elucidate the mechanisms for circuit formation, we study the development of barrel cortex (the primary sensory cortex, S1), whose development is instructed by presynaptic thalamocortical axons (TCAs). In the first postnatal weeks, TCA terminals arborize in layer (L) 4 to fill in the barrel center, but it is unclear how TCA development is regulated. Here, we reported that the deletion of Lhx2 specifically in the cortical neurons in the conditional knockout (cKO) leads to TCA arborization defects, which is accompanied with deficits in sensory-evoked and spontaneous cortical activities and impaired lesion-induced plasticity following early whisker follicle ablation. Reintroducing Lhx2 back in L4 neurons in cKO ameliorated TCA arborization and plasticity defects. By manipulating L4 neuronal activity, we further demonstrated that Lhx2 induces TCA arborization via an activity-dependent mechanism. Additionally, we identified the extracellular signaling protein Sema7a as an activity-dependent downstream target of Lhx2 in regulating TCA branching. Thus, we discovered a bottom-up feedback mechanism for the L4 neurons to regulate TCA development.

Lhx2 Expression in Postmitotic Cortical Neurons Initiates Assembly of the Thalamocortical Somatosensory Circuit.

Cortical neurons must be specified and make the correct connections during development. Here, we examine a mechanism initiating neuronal circuit formation in the barrel cortex, a circuit comprising thalamocortical axons (TCAs) and layer 4 (L4) neurons. When Lhx2 is selectively deleted in postmitotic cortical neurons using conditional knockout (cKO) mice, L4 neurons in the barrel cortex are initially specified but fail to form cellular barrels or develop polarized dendrites. In Lhx2 cKO mice, TCAs from the thalamic ventral posterior nucleus reach the barrel cortex but fail to further arborize to form barrels. Several activity-regulated genes and genes involved in regulating barrel formation are downregulated in the Lhx2 cKO somatosensory cortex. Among them, Btbd3, an activity-regulated gene controlling dendritic development, is a direct downstream target of Lhx2. We find that Lhx2 confers neuronal competency for activity-dependent dendritic development in L4 neurons by inducing the expression of Btbd3.