Soutenance de Kassandra Gérard

The nuclear pore complex (NPC) is an essential structure for the function of eukaryotic cells, enabling selective and bidirectional transport of macromolecules between the nucleus and the cytoplasm. Despite its central role, its physical properties remain largely unexplored, particularly under physiological conditions such as the circadian cycle (an endogenous rhythm optimizing various biological processes throughout the day) or during interactions with viral capsids. Yet, translocation through the NPC is a key step in the viral cycle of many viruses, including hepatitis B virus (HBV) and adeno-associated virus (AAV), which hijack this nuclear interface to deliver their genome into the host nucleus and initiate replication. This thesis adopts a biophysical approach to better understand the plasticity of the NPC under various physiological constraints. In this context, we used atomic force microscopy (AFM), which not only allows for nanoscopic imaging of biological samples but also enables probing of their mechanical properties. I developed an innovative protocol for studying, in liquid, intact isolated nu clei from mouse hepatocytes (the main cell type of the liver) using AFM. This protocol enabled us to analyze the mechanical properties of NPCs at different phases of the circadian cycle, as well as in its absence. Our results revealed significant variations in their size depending on the time of day, suggesting a cyclic modulation of their architecture or of their nuclear environment. This circadian plasticity may reflect an optimized functional adaptation of NPCs to fluctuating biological demands, such as transcriptional processes. Furthermore, we investigated the interaction between NPCs and two types of viral capsids: those of AAV and HBV. By comparing the mechanics of intact capsids and their behavior upon contact with NPCs, we observed differential responses from the nuclear pore, potentially linked to the intrinsic rigidity of each viral particle. These findings open new avenues toward a better understanding of viral nuclear import mechanisms in relation to viral mechanics. By combining AFM imaging with statistical analyses suited to non-Gaussian distribu tions, this work highlights an unsuspected dynamic behavior of the nuclear pore complex, both intrinsic (circadian rhythm) and induced (viral stress). It underscores the importance of a physical approach to the study of complex biological structures, as a complement to biochemical and molecular frameworks.