Lamin AC and sumoylation: Implications of laminopathic mutant lamin AC expression on the dynamics of sumo1

Abstract

Laminopathies are a collection of over ten distinct and overlapping disease phenotypes resulting from over 340 known mutations in the LMNA gene. This gene encodes the alternatively spliced nuclear intermediate filament proteins, lamins A and C, that localize to the nuclear envelope and the nucleoplasm. These proteins are part of the nuclear lamina which is a complex meshwork of over 80 proteins underlying the inner nuclear membrane. Lamins have proposed roles in nuclear structural support and in multiple cell processes that include gene regulation, chromatin organization, and protein localization/scaffolding. Previous research has shown that in vitro expression of a laminopathic mutant lamin C resulted in nuclear aggregation of lamin C and subsequently sequestration of a co-expressed post-translational modification protein, sumo1. Sumoylation is the attachment of the s&barbelow;mall u&barbelow;biquitin-like modifier 1&barbelow; (sumo1) to a target lysine(s) on substrate proteins which serves to regulate various protein characteristics such as transcriptional activity, localization, protein interaction, and stability. In this thesis, I investigated the sumolyation of lamin NC and the effect of laminopathic mutant lamin NC on sumoylation and sumo1 localization. It was demonstrated that although not modified by sumo1, mutant lamin NC elicits a mutation-dependent alteration of sumo1 localization in vitro in cultured mouse myoblasts overexpressing lamin NC and primary mouse myoblasts expressing endogenous mutant lamins, as well as in vivo in affected muscle tissue from a laminopathy knock-in mouse model. Furthermore, in cultured myoblasts, this correlated with a mutation-dependent significant increase in the steady-state level of protein sumoylation by sumo1. Taken together, these results suggest that the alteration of sumo1 localization and sumoylation may be contributing factors in the pathophysiological mechanisms underlying the development of laminopathies.