Diffusional Mobility and Nanoscale Molecular Organization of the Muscular Dystrophy Related Nuclear Membrane Protein Emerin

Abstract

Emery-Dreifuss Muscular Dystrophy (EDMD) is caused by mutations in the nuclear membrane protein emerin and is part of a diverse group of muscular disorders characterized by progressive muscle weakness, cardiac failure, and early death. The reason why emerin mutations cause muscle disease remains unclear. To gain new insights into the molecular pathogenesis of EDMD at the nanoscale we used single molecule tracking and super-resolution optical microscopy in human cells. The diffusional mobility and the spatial distribution of emerin and a variety of clinically relevant emerin mutants were studied by single particle tracking PALM and dSTORM super-resolution imaging in rescued emerin-null cells. We identified different subpopulations of emerin associated with the endoplasmic reticulum, the outer or the inner nuclear membrane and show that emerin forms diffraction-limited clusters at the nuclear envelope. We further show that the dynamic interactions of emerin with the nuclear lamin network and its clustering state are differentially impacted by mutations causing EDMD. Emerin mutants with increased binding to lamin display slow diffusional mobility at the inner nuclear membrane and form nanoclusters 30% smaller than wild type emerin. Contrarily, an increased lateral mobility of emerin is observed for mutants having low binding affinity to lamin and for wild type emerin imaged during siRNA silencing of lamins. The reduced interactions between emerin and lamin additionally lead to significant changes in the spatial distribution and the overall number of emerin clusters at the nuclear membrane. Our data suggest that EDMD-inducing mutations of emerin critically affect its dynamic interactions with lamins, which results in the modification of its nanoscale spatial distribution and in a remodeling of the nuclear envelope architecture.