Unravelling region-specific responses to embryonic DNA damage in the developing brain

DNA damage during early neurogenesis, e.g. resulting from DNA repair deficiencies or exposure to moderate and high doses of ionizing radiation, can lead to neurodevelopmental disorders such as microcephaly. The DNA damage response in dorsal neural progenitor cells (NPCs), which are responsible for generating excitatory neurons, has been well-studied, with outcomes including p53-mediated apoptosis and premature neuronal differentiation. However, little is known about the response of ventral NPCs, which give rise to inhibitory interneurons. Given the critical role of the balance between excitation and inhibition in maintaining proper brain function, disruptions in the development of these neuron types are linked to various neurodevelopmental disorders. This thesis aims to fill the knowledge gap regarding the effects of DNA damage on both ventral and dorsal NPC populations. For this, we used ionizing radiation as a mean
to induce acute DNA damage in a controlled way. Our study focused on the effects of prenatal ionizing radiation on both ventral and dorsal NPCs, specifically from the medial ganglionic eminence (MGE) and neocortex (NCX), important regions involved in the generation of inhibitory and excitatory neurons, respectively. We exposed mouse fetuses to ionizing radiation during an early stage of forebrain neurogenesis to compare the DNA damage response in the NCX and MGE. Both regions exhibited a p53-mediated DNA damage response, involving cell cycle
arrest, DNA repair, and apoptosis. However, the response differed between both regions: NCX cells experienced prolonged cell cycle arrest, while MGE cells exhibited more sustained apoptosis. Furthermore, MGE cells displayed reduced interneuron migration speed in acute living brain slices and MGE explants, the latter indicating a potential cell-intrinsic defect resulting from irradiation. This
defect was not seen in explants from p53-deficient mice, indicating it is independent from the canonical DNA damage response. RNA sequencing and protein analyses revealed that disruptions in cytoskeletal-related machinery, particularly involving actin and microtubules, were more prominent in MGE cells. Despite these acute defects, irradiated mice did not show an increased susceptibility to pentylenetetrazole-induced seizures and the overall number of cortical interneurons in the young adult brain was unaffected. These findings suggest a remarkable plasticity of the developing brain in response to acute embryonic DNA damage. Overall, this study highlights region-specific responses
to DNA damage in the embryonic forebrain and enhances our understanding of how early developmental insults impact brain development and may contribute to or mitigate the risk of neurodevelopmental disorders.