In mammalian cells, A- and B-type lamins form a two-dimensional protein meshwork, the lamina, at the nucleoplasmic face of the nuclear envelope. Mutations scattered along the LMNA gene, which encodes A-type lamins, as well as mutations within other nuclear envelope proteins are associated with a broad range of human diseases collectively called laminopathies. The molecular etiology of these diseases remains unknown. Emery-Dreifuss muscular dystrophy (EDMD), the most prominent laminopathy, is an incurable, devastating muscular wasting disease, caused by mutations in either, the inner nuclear membrane (INM) protein emerin, laminA, or the outer nuclear membrane (ONM) KASH-proteins nesprins 1 and 2. The four proteins are connected via LINC complexes, evolutionary-conserved protein complexes between INM SUN-proteins and ONM KASH-proteins that bridge the faces of the nuclear envelope and physically connect the nuclear lamina to the cytoskeleton of mammalian cells. Taken together, the data suggests that EDMD is the result of aberrant nuclear positioning or, alternatively, aberrant mechanical signaling through the LINC complex. Furthermore, it has been shown that the over-accumulation of Sun1 at the INM is the pathological effector of EDMD. With funding through an exploratory R21 grant we have determined the core structure of the SUN-KASH complex in 2012, providing the first molecular insight into LINC complexes. Here, we build on this data and suggest a research program that should aid in the discovery of drug targets that hopefully will translate into a medication strateg for EDMD patients. This proposal outlines experiments that will lead A) to a comprehensive structural and biochemical understanding of the human SUN-KASH interactome, B) a structural basis for LINC complex anchorage to the lamin layer, and C) insight into the regulation of LINC complex assembly and disassembly. We expect that the pursuit of these three aims will yield a much better molecular description of the protein network that forms the basis of EDMD, and consequently will unveil possible drug targets that disrupt these processes. We further anticipate that this research will advance our understanding of the nuclear envelope in general, which will have a tangible impact on the vast array of pathological nuclear envelope disorders.
Laminopathies include a wide array of diverse diseases (muscular dystrophies, cardiomyopathies, premature aging, and cancer) that are caused by alterations in nuclear envelope proteins, notably lamin itself. Muscular dystrophies collectively have a high impact on human health, affecting tens of thousands of people in the United States alone. The work in this project is designed to be translational and provide a structural basis to identify molecular targets that can lead to specific treatments for patients in the foreseeable future.