The research in the Lammerding lab is focused on subcellular mechanics and the cellular signaling response to mechanical stimulation. In particular, we are studying how mutations in nuclear envelope proteins such as lamin can render cells more sensitive to mechanical stress and affect their mechanotransduction signaling. Insights gained from this work can lead to a better understanding of the molecular mechanism underlying laminopathies, a diverse group of diseases including Emery-Dreifuss muscular dystrophy, Hutchison-Gilford progeria syndrome, and familial partial lipodystrophy.

Cells from laminopathy patients are often characterized by abnormal nuclear shape and structure. We are developing and applying novel experimental techniques to examine how these changes affect the mechanical properties and function of the nucleus. Impaired nuclear mechanics and increased nuclear envelope fragility could be at least in part be responsible for the muscular phenotypes found in many of the laminopathies, e.g. Emery-Dreifuss muscular dystrophy or limb-girdle muscular dystrophy. Increased cellular sensitivity to mechanical strain and fluid shear stress could also contribute to atheriosclerosis, the leading cause of death in Hutchinson-Gilford progeria syndrome patients. We have previously demonstrated that lamin A/C-deficient mouse embryo fibroblasts have decreased nuclear stiffness and increased nuclear fragility, and that these cells have increased rates of necrosis and apopotis under strain. Techniques available in our lab include cellular strain devices to quantify nuclear deformations under strain, a microinjection system to measure nuclear envelope fragility, an automated microscope shutter system to perform time-lapse videomicroscopy to monitor dynamic changes in nuclear shape over time, and a micropipette aspiration set-up to probe the mechanical properties of isolated nuclei in cells from human laminopathy patients, healthy control subjects, and mouse models of diverse laminopathies.

Mechanotransduction can be defined as the process by which cell convert biological stimuli into biochemical signals. Examples include hair cells in the inner ear responsible for hearing, muscle cells that respond to exercise with muscle growth, bone cells that adjust bone density in response to changes in mechanical load, and endothelial cells exposed to fluid shear stress that regulate blood pressure and local circulation. Mechanotransduction plays a critical role in maintaining physiological cell function, and genetic mutations that interfere with normal cellular mechanics or mechanotransduction can lead to a variety of human diseases including muscular dystrophy and cardiomyopathy. The precise molecular mechanisms involved in mechanotransduction are still unclear, but intracellular mechanosensors may be activated by conformational changes induced by local cellular deformations. Since these mechanosensors are located in several subcellular domains (the cell membrane, the cytoskeleton and the nucleus), it is necessary to achieve a detailed understanding of subcellular mechanics in response to mechanical stimulation. We are applying novel experimental methods to independently quantify cytoskeletal displacements, mechanical coupling between the cytoskeleton and the extracellular matrix (see below), and nuclear mechanics (see above) based on high resolution tracking of cellular structures and receptor-bound magnetic beads in response to applied strain and microscopic forces. Our experiments aim to provide new insights into the role of subcellular biomechanics on mechanotransduction in normal and mutant cells, leading to improved understanding of disease mechanisms associated with altered cell mechanics.

When studying the nuclear response to mechanical stimulation, the nucleus a cannot be viewed in isolation from the rest of the cell, as the nucleus (specifically the nuclear envelope) is physically connected to the surrounding cytoskeleton, which is in turn connected to the extra cellular matrix through cell membrane receptors such as integrins. These physical connections allows for force transmission from the extracellular matrix all the way to the nucleus and the chromatin. The precise molecular components involved in the force transmission from the cytoskeleton across the perinuclear space to the inner nuclear membrane and the lamina are only slowly emerging. The main players involved appear to be members of the sun and nesprin protein families, as well as the nuclear lamins which interact with these proteins. Loss of lamin A/C has been previously demonstrated to affect cytoskeletal organization around the nucleus, and we have previously reported that lamin A/C-deficient mouse embryo fibroblasts have impaired cytoskeletal mechanics. Currently, we are developing novel experimental tools to investigate the physical connections between the nuclear envelope and the cytoskeleton in more detail. Mutations in nuclear envelope proteins such as lamin or emerin could interrupt some of these connections and result in impaired nuclear/cytoskeletal coupling, altered cytoskeletal mechanics, and defective mechanotransduction, which could then contribute to the pathophysiology of laminopathies.

Experiments on single cells and larger cell populations are an important tool to study the function of specific proteins, but to understand the full physiological significance of specific proteins or mutations, these experiments have to be complemented by in vivo studies in the whole animal. Through collaboration with Dr. Colin Stewart at the National Cancer Institute, we have access to several mouse models for human laminopathies, including mice that develop muscular dystrophy, dilated cardiomyopathy, or progeria-like premature aging. Using these mice, we study the in vivo consequences of impaired nuclear mechanics and abnormal mechanotransduction signaling, in particular on cardiac muscle. At the same time, we are exploring some of the subtle differences in the disease mechanism between mice and humans, e.g. the fact that most lamin A/C mutations are autosomal dominant in humans, but require homozygous expression of the mutant protein to trigger a phenotype in the mice.