"Matière et Systèmes Complexes"
3 décembre 2008 à 11h30
Bâtiment Condorcet, 4ème étage, salle 454 A.
(Dept. of Biomedical Engineering, Pittsburgh)
The structural proteins of the nucleus primarily reside at the nuclear envelope and are responsible for maintaining the nuclear integrity. Nuclear lamins A, B and C form the nuclear lamina network along with many lamin binding proteins. Both mechanical stability and mechano-sensitive gene expression are defective in cells with mutations in lamin proteins, but the underlying nature of the mechanical properties of the lamina are poorly understood. We use micromanipulation to quantify the viscoelastic properties of isolated nuclei from human fibroblasts, including those from Hutchinson-Gilford Progeria Syndrome (HGPS) patients. HGPS, a premature aging disease, is caused by mutations in the gene encoding A-type lamins. Nuclei from HGPS patients show significant quantitative reduction in the ability to rearrange, and the HGPS nuclei collapse along major axes, suggesting catastrophic failure to distribute applied forces across the entire lamina. The HGPS lamin network appears to have locally-ordered microdomains, which can explain most of the mechanical differences seen here between HGPS and normal cells and may have functional consequences in disease.
While lamins provide a majority of the structure and mechanical rigidity of the nucleus, there are other putative structural elements which interconnect these filaments. Previous mechanical studies of Xenopus oocyte and mammalian cell nuclear lamina networks demonstrated 'spring-like' memory, not accountable by the properties of lamin filaments alone. To test the hypothesis that aII-spectrin contributes to nuclear lamina structure, we downregulated aII-spectrin (SPTAN1) in HeLa cells. Cells deficient for II-spectrin had grossly misshapen nuclei, defective organization of both the A- and B-type lamina networks, emerin mislocalization and altered chromatin organization. We also show that nuclei lacking aII-spectrin are unable to recoil from applied stress. We conclude that aII-spectrin is an essential component of the nuclear lamina infrastructure, and future work will examine mechanical changes associated with the loss and overexpression of aII-spectrin.