Understanding Molecular Mechanisms of Striated Muscle Laminopathies Using Cellular and Zebrafish Models

Abstract

The Lamin A/C (LMNA) gene codes for the A-type lamins which are nuclear intermediate filaments that provide structural support to the nucleus and help regulate various nuclear processes such as gene expression. Mutations in LMNA cause a group of diseases collectively known as laminopathies. Laminopathies present with a diverse set of phenotypes with 79% of the cases affecting cardiac and/or skeletal muscles (striated muscle laminopathies). Striated muscle laminopathies tend to be severe and have poor patient prognosis. The lamin A/C binding partner - Protein Kinase C alpha (PKC α), is a serine/threonine kinase that phosphorylates many proteins including other lamin A/C partners, and proteins associated with striated muscle laminopathies. Chapter 2 of this thesis determined PKC α involvement in striated muscle laminopathies by examining PKC α subcellular localization, lamin A/C interaction, and activation in various cellular models expressing WT or mutant lamin A/C. The results showed that in most of the mutations that we studied, cells expressing mutant lamin A/C presented with an abnormal increase in nuclear PKC α localization compared to WT. Patient and mice model myoblasts and/or fibroblasts also presented with altered PKC α-lamin A/C interaction and reduced PKC α activation compared to control. In parallel, activation of ERK 1/2, a downstream PKC α target associated with striated muscle laminopathies, was also decreased in patient myoblasts and mice model myoblasts. Altogether, these results suggested PKC α involvement in striated muscle laminopathies. Chapter 3 of this thesis describes the creation of zebrafish with permanent lmna disruption, and characterization of the lmna mutants’ phenotype. We induced a five base pair deletion in exon 2 of lmna that is predicted to result in a potential non-functional truncated protein of 180 amino acids lacking the PKC α binding site. Cardiac structure and function analyses of the lmna mutants showed mild and transient cardiac defects during the first week of development. In addition, the lmna mutants displayed skeletal muscle structure fibre defects that likely contributed to their swim impairment. Expression of jun and nfkb2 was decreased while activated Erk 1/2 was increased in mutants. Jun, NF-KB and Erk 1/2 are biomarkers for striated muscle laminopathies. Furthermore, activation of Pkc α, an upstream Erk 1/2 regulator, was increased in the lmna mutants compared to WT. Overall, the zebrafish lmna mutants presented a model of muscular laminopathies. These zebrafish lmna mutants can be used to test for drug efficacy or identify novel therapeutic targets to treat muscular laminopathies.