Unraveling the developmental role of Cajal-Retzius neurons during the establishment of neocortical circuits in mice

• Applicant: Oriane Blanquie, Ph.D.
• Subject Area: Developmental Neurobiology ; Anatomy and Physiology
• Term: from 2020 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442680687

Cajal-Retzius neurons (CRNs) represent a transient population of neurons necessary for the laminar organization of the neocortex. CRNs populate the outer layer of the neocortex before being eliminated by apoptosis by the end of the second post-natal week. CRNs seem well integrated into the neocortex: they display complex and long-range axonal processes, receive GABAergic-mediated inputs and are able to fire action potentials. I have recently demonstrated that excitatory GABAA-receptor mediated inputs are necessary to trigger their cell death. However, whether CRNs participate into the typical in vivo activity patterns displayed by developing cortical networks and whether this activity is necessary for the establishment of sensory cortical circuits has not yet been investigated. Here, I will investigate whether CRNs display spontaneous and sensory-evoked activity in vivo, and whether their electrical activity then translates into a developmental function. Experiments will be performed in the barrel field of the primary somatosensory cortex (S1BF), a region that displays a topographic map of the whisker pad.If CRNs participate in the establishment of the topographic representation of somatosensory stimuli, I expect them to be responsive to sensory inputs. In the first part of the funding, the identity of presynaptic neurons sending inputs into S1BF CRNs will be established by combining monosynaptic retrograde viral tracing, clearing and immunostaining procedures on transgenic mice.In a second part of the project, I plan to investigate whether CRNs participate into the spontaneous, synchronous activity in vivo and assess the role of whisker stimulation in their electrical activity. To this aim, mouse pups will be injected in the outer layer of the somatosensory cortex with a calcium indicator. In vivo calcium imaging will then be performed with and without mechanical whisker stimulation.In the last part of the project, the effect of CRNs silencing on the developing network will be assessed. CRNs will be specifically silenced in newborn mice using Designer Receptors Exclusively Activated by Designer Drugs (DREADDs), and the effect of silencing CRNs on the activity patterns of S1BF neurons and S1BF architecture will be assessed.To conclude, the experiments described in this funding proposal will allow the description of the spontaneous- and sensory-evoked activity patterns of S1BF CRNs in vivo as well as their anatomical inputs. These experiments will further unravel a putative causal relationship between the activity of CRNs and establishment of functional cortical circuits.

• DFG Programme: Research Grants