Lamin A/C and emerin regulate MKL1-SRF activity by modulating actin dynamics.

Laminopathies, caused by mutations in the LMNA gene encoding the nuclear envelope proteins lamins A and C, represent a diverse group of diseases that include Emery-Dreifuss muscular dystrophy (EDMD), dilated cardiomyopathy (DCM), limb-girdle muscular dystrophy, and Hutchison-Gilford progeria syndrome. Most LMNA mutations affect skeletal and cardiac muscle by mechanisms that remain incompletely understood. Loss of structural function and altered interaction of mutant lamins with (tissue-specific) transcription factors have been proposed to explain the tissue-specific phenotypes. Here we report in mice that lamin-A/C-deficient (Lmna(-/-)) and Lmna(N195K/N195K) mutant cells have impaired nuclear translocation and downstream signalling of the mechanosensitive transcription factor megakaryoblastic leukaemia 1 (MKL1), a myocardin family member that is pivotal in cardiac development and function. Altered nucleo-cytoplasmic shuttling of MKL1 was caused by altered actin dynamics in Lmna(-/-) and Lmna(N195K/N195K) mutant cells. Ectopic expression of the nuclear envelope protein emerin, which is mislocalized in Lmna mutant cells and also linked to EDMD and DCM, restored MKL1 nuclear translocation and rescued actin dynamics in mutant cells. These findings present a novel mechanism that could provide insight into the disease aetiology for the cardiac phenotype in many laminopathies, whereby lamin A/C and emerin regulate gene expression through modulation of nuclear and cytoskeletal actin polymerization.

Nuclear mechanics in disease.

Over the past two decades, the biomechanical properties of cells have emerged as key players in a broad range of cellular functions, including migration, proliferation, and differentiation. Although much of the attention has focused on the cytoskeletal networks and the cell's microenvironment, relatively little is known about the contribution of the cell nucleus. Here, we present an overview of the structural elements that determine the physical properties of the nucleus and discuss how changes in the expression of nuclear components or mutations in nuclear proteins can not only affect nuclear mechanics but also modulate cytoskeletal organization and diverse cellular functions. These findings illustrate that the nucleus is tightly integrated into the surrounding cellular structure. Consequently, changes in nuclear structure and composition are highly relevant to normal development and physiology and can contribute to many human diseases, such as muscular dystrophy, dilated cardiomyopathy, (premature) aging, and cancer.

Apoptotic histone modification inhibits nuclear transport by regulating RCC1.

A number of signalling pathways have been identified that regulate apoptosis, but the mechanism that initiates apoptosis remains incompletely understood. We have found that the nuclear RanGTP level is diminished during the early stages of apoptosis, which correlates with immobilization of RCC1 on the chromosomes. Furthermore, the expression of phosphomimetic histone H2B or caspase-activated Mst1 immobilizes RCC1 and causes reduction of nuclear RanGTP levels, which leads to inactivation of the nuclear transport machinery. As a consequence, nuclear localization signal (NLS)-containing proteins, including NF-kappaB-p65, remain bound to importins alpha and beta in the cytoplasm. Knocking down Mst1 allows resumption of nuclear transport and the nuclear entry of NF-kappaB-p65, which have important roles in rescuing cells from apoptosis. Therefore, we propose that RCC1 reads the histone code created by caspase-activated Mst1 to initiate apoptosis by reducing the level of RanGTP in the nucleus.

Perturbation of the chromosomal binding of RCC1, Mad2 and survivin causes spindle assembly defects and mitotic catastrophe.

Mitotic catastrophe is a form of cell death that results from aberrant mitosis. Currently, the mechanisms involved in this form of cell death remain poorly understood. We found that actinomycin D induces mitotic catastrophe with severe spindle assembly defects. We have studied the nature of three groups of chromosome binding proteins in mitotic cells treated with actinomycin D. We found that actinomycin D reduced the binding affinity of RCC1 to the mitotic chromosome, which led to a reduction of RanGTP level. In addition, Mad2 was not concentrated at the kinetochores, indicating that the mitotic spindle checkpoint was affected. Furthermore, the localization of survivin was altered in cells. These data suggested that chromosomal binding of the mitotic regulators such as RCC1, Mad2 and survivin is essential for mitotic progression. Mitotic chromosomes not only carry the genetic material needed for the newly synthesized daughter cells, but also serve as docking sites for some of the mitotic regulators. Perturbation of their binding to the mitotic chromosome by actinomycin D could affect their functions in regulating mitotic progression thus leading to severe spindle defects and mitotic catastrophe.