The role ofDrosophilaLamin C in muscle function and gene expression

Abstract

SUMMARYThe inner side of the nuclear envelope (NE) is lined with lamins, a meshwork of intermediate filaments that provides structural support for the nucleus and plays roles in many nuclear processes. Lamins, classified as A-or B-types on the basis of biochemical properties, have a conserved globular head, central rod and C-terminal domain that includes an Ig-fold structural motif. In humans, mutations in A-type lamins give rise to diseases that exhibit tissue-specific defects, such as Emery-Dreifuss muscular dystrophy. Drosophila is being used as a model to determine tissue-specific functions of A-type lamins in development, with implications for understanding human disease mechanisms. The GAL4-UAS system was used to express wild-type and mutant forms of Lamin C (the presumed Drosophila A-type lamin), in an otherwise wild-type background. Larval muscle-specific expression of wild type Drosophila Lamin C caused no overt phenotype. By contrast, larval muscle-specific expression of a truncated form of Lamin C lacking the N-terminal head (Lamin C N) caused muscle defects and semi-lethality, with adult 'escapers' possessing malformed legs. The leg defects were due to a lack of larval muscle function and alterations in hormoneregulated gene expression. The consequences of Lamin C association at a gene were tested directly by targeting a Lamin C DNAbinding domain fusion protein upstream of a reporter gene. Association of Lamin C correlated with localization of the reporter gene at the nuclear periphery and gene repression. These data demonstrate connections among the Drosophila A-type lamin, hormone-induced gene expression and muscle function. DEVELOPMENT 3068 lethality when expressed in larval muscle but not in other tissues (Schulze et al., 2009). By contrast, alterations in the rod domain cause abnormal lamin aggregation within the nucleus but not lethality. Collectively, our studies support a role for the N-terminal head and C-terminal globular domain in larval muscle function.To better understand the role of the N-terminal head domain, we generated transgenic flies expressing a mutant form of Lamin C lacking the first 42 amino acids (Lamin C N) (Schulze et al., 2009). This mutant form localizes to the NE and causes semilethality when expressed in muscle but not in non-muscle tissues (Schulze et al., 2009). Here, we demonstrate that lethality is associated with larval body wall muscle defects that include nuclear and cytoplasmic abnormalities. 'Adult escapers' expressing Lamin C N have leg defects that are consistent with a loss of muscle function and disruptions in ecdysone hormone signaling.Collectively, these studies demonstrate a role for the N-terminal head domain of Lamin C in muscle function and gene expression. MATERIALS AND METHODS Drosophila stocks and genetic analysesDrosophila stocks were raised at room temperature on standard sucrose and cornmeal medium (Shaffer et al., 1994). Generation of transgenic stocks expressing wild type and Lamin C N was previously reported (Schulze et al., 2005;Schulze ...

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