Pyramidal neurons proportionately alter the identity and survival of specific cortical interneuron subtypes
The mammalian cerebral cortex comprises a complex neuronal network that maintains a delicate balance between excitatory neurons and inhibitory interneurons. Previous studies, including our own research, have shown that specific interneuron subtypes are closely associated with particular pyramidal neuron types, forming stereotyped local inhibitory microcircuits. However, the developmental processes that establish these precise networks are not well understood. Here we show that pyramidal neuron types are instrumental in maintaining the terminal differentiation and survival of specific associated interneuron subtypes. In a wild-type cortex, the relative abundance of different interneuron subtypes aligns precisely with the pyramidal neuron types with which they synaptically target. In Fezf2 mutant cortex, characterized by the absence of layer 5 pyramidal tract neurons and an expansion of layer 6 intratelencephalic neurons, we observed a corresponding decrease in associated layer 5b interneurons and an increase in layer 6 subtypes. Interestingly, these shifts in composition are achieved through mechanisms that are specific to different interneuron types.  While SST interneurons adjust their abundance to the change in pyramidal neuron prevalence through the regulation of programmed cell death, parvalbumin cells alter their identity. These findings illustrate two key strategies by which the dynamic interplay between pyramidal neurons and interneurons allows local microcircuits to be sculpted precisely. These insights underscored the precise roles of extrinsic signals from pyramidal cells in the establishment of interneuron diversity and their subsequent integration into local cortical microcircuits. Overall design: To directly test the influence of pyramidal neurons (PNs) on interneuron development, we used genetic strategies to selectively alter the identity of specific PN types and demonstrated that the quantity and position of particular PVALB and SST interneuron subtypes change in accordance with this shift. Importantly, these changes occur through distinct mechanisms: PNs promote the survival of their partner SST interneuron subtypes, while inducing fate changes in PVALB interneuron population towards their preferred subtypes.