Einfluss von Laminproteinen auf die mechanischen Eigenschaften von Zellkernen - Eine Rasterkraft- und Elektronenmikroskopische Analyse

Abstract

Lamins are type V intermediate filament proteins. They are building blocks of the nuclear lamina, a filamentous meshwork underlying the inner nuclear membrane. The nuclear lamina provides structural support to maintain nuclear shape and stability. The lamina is also involved in transcription, the organisation of chromatin, DNA replication, DNA-repair and gene regulation. Mutations in lamin genes, especially in the LMNA gene, cause tissue-specific diseases, the laminopathies. Within the course of this work a method was established to quantify the contribution of the lamina to the mechanical properties of the nucleus. The ectopic expression of lamin A in Xenopus oocytes induces an additional filament layer. Nuclei of these oocytes were stiffer than control nuclei. Force curves of isolated nuclei were obtained with an atomic force microscope (AFM) to quantify the stiffness (spring constant) of the nuclei. The stiffness and the thickness of the lamina increased by a higher expression of lamin A. The thickness of the lamina was measured by transmission electron microscopy. With both, thickness of the lamina and spring constant the E-Modul for the lamina was calculated. Nuclei of Xenopus oocytes exhibit a sponge-like meshwork of actin, which contributes to the stability of the nucleus. Due to the expression of the specific export factor of actin, nuclei became extremely fragile. In contrast, nuclei with lamin A remained stiff, even when the acitn was not present. In further studies the effect of lamin B1 was analyzed. The results shown, that the expression of lamin B1 did not influence the stiffness of the nuclei. The different morphology of the nuclear envelope could be a cause for that. A-type lamins induce layers of filaments. In contrast, B-type lamins induce intranuclear membranes. The B-type lamins are located at these membranes and can not contribute to the stiffness of the lamina. Chimeras of A-type and B-type lamins have shown that the tail domain is responsible for the building of filament layers.