New roles for lamins, nuclear envelope proteins and actin in the nucleus

To face the problem of the pathogenesis of different laminopathies, the regulation of the expression of nuclear structural proteins in various tissues has been investigated, mainly in muscle tissue that is affected in some of them. Developmental regulation of lamin A/C has been demonstrated during myoblast differentiation. An interaction specifically occurring in muscle cells is likely to provide lamin A/C with a muscle tissue-specific reactivity. A possible candidate for this interaction is actin, whose expression of cytoskeletal isoforms is down-regulated during muscle differentiation, while sarcomeric actin expression is induced. In this study, aimed to demonstrate a developmental regulation of the interaction among these nuclear structural proteins, we provide evidence that lamin A/C and emerin are bound to nuclear actin isoforms mainly at the late stages of myotube differentiation and in mature muscle cells. In cycling myoblasts, emerin binding is obtained by a serine-threonine phosphatase activity added to either whole extracts or nuclear extracts. On the contrary, this binding is prevented by active protein phosphatase 1 (PP1), a phosphatase that associates with lamin A/C. These data provide evidence of a modulation of emerin-lamin A/C-actin interactions in muscle cells, possibly through differentiation-related stimuli and indicate that nuclear proteins, including lamins, inner nuclear membrane-associated proteins and actin, once considered to play pure structural roles, are responsible for the modulation of key nuclear functions.