Morassut, I. | Search bioRxiv

bioRxiv2024

Cell-extrinsic controls over neocortical neuron fate and diversity

Cellular diversity in the neocortex emerges gradually during prenatal and postnatal development. While environmental interactions occur during this extended maturation period, the impact of extrinsic cues on determining the fate of distinct neuron types remains unknown. To address this question, we exposed developing neocortical cells to various environmental conditions and examined how this affects cell fate and diversity. Our developmental analyses reveal a hierarchical molecular program in which cell class-distinguishing features emerge first, followed by subclass- and type-related characteristics, with distinct developmental paces among cell populations. Environmental contribution was assessed in vivo, using genetically modified mice models in which position or innervation are altered, and in vitro using two-dimensional cultures. Acquisition of cellular identity and diversity remained stable across in vivo models. In contrast, in vitro glutamatergic neurons showed decreased expression of identity-defining genes, reduced diversity and alterations in canonical cortical connectivity. Cellular identity and diversity were restored towards in vivo values in organotypic slice cultures. These findings reveal cell population-specific responses to environmental conditions and highlight the role of extracellular context in shaping cell diversity in the maturing neocortex.

neuroscience↗

bioRxiv2023

Developmental molecular controls over arealization of descending cortical motor pathways

Layer 5 extratelencephalic (ET) neurons are a main class of neocortical projection neurons that predominate in the motor cortex and send their axon to the pons and spinal cord, and collaterals to the thalamus and multiple deep subcerebral structures1-3. Precise connectivity of ET neurons is critical for fine motor control; they are central to loss of function upon spinal cord injury and specifically degenerate in select neurodegenerative disorders4, 5. ET neurons consist of several types of cells with distinct laminar and areal locations, molecular identities, connectivities, and functions6, 7. Within layer 5 of the cortex, two cardinal subtypes of ET neurons have been identified: "ETlower" neurons, which express Slco2a1 and project to distal targets including the spinal cord, "ETupper" neurons, which express Nprs1 or Hpgd and project more proximally to the pons and thalamus6. Despite their critical function, how these neuronal subtypes emerge during development and acquire their area-specific distributions remains unaddressed. Here, using combinations of anatomical labeling, MAPseq mapping8, and single-nucleus transcriptomics across developing cortical areas, we reveal that these two subtypes of ET neurons are present at birth along opposite antero-posterior cortical gradients. We first characterize area-specific developmental axonal dynamics of ETlower and ETupper neurons and find that the latter can emerge by pruning of subsets of ETlower neurons. We next identify area- and ET neuron type-specific developmental transcriptional programs to identify key target genes in vivo. Finally, we reprogram ET neuron area-specific connectivity from motor to visual by postnatal in vivo combinatorial knockout of three key type-specific transcription factors. Together, these findings delineate the functional transcriptional programs controlling ET neuron diversity across cortical areas and provide a molecular blueprint to investigate and direct the developmental emergence of corticospinal motor control.

neuroscience↗