Skeletal muscle LIM protein 1 (SLIM1/FHL1) induces {alpha}5{beta}1-integrin-dependent myocyte elongation

Department of Biochemistry and Molecular Biology, Monash University, Clayton, Victoria 3800, Australia Submitted 20 May 2003 ; accepted in final form 10 August 2003 Skeletal muscle LIM protein 1 (SLIM1/FHL1) contains four and a half LIM domains and is highly expressed in skeletal and cardiac muscle. Elevated SLIM1 mRNA expression has been associated with postnatal skeletal muscle growth and stretch-induced muscle hypertrophy in mice. Conversely, SLIM1 mRNA levels decrease during muscle atrophy. Together, these observations suggest a link between skeletal muscle growth and increased SLIM1 expression. However, the precise function of SLIM1 in skeletal muscle, specifically the role of SLIM1 during skeletal muscle differentiation, is not known. This study investigated the effect of increased SLIM1 expression during skeletal muscle differentiation. Western blot analysis showed an initial decrease followed by an increase in SLIM1 expression during differentiation. Overexpression of SLIM1 in Sol8 or C 2 C 12 skeletal muscle cell lines, at levels observed during hypertrophy, induced distinct effects in differentiating myocytes and undifferentiated reserve cells, which were distinguished by differential staining for two markers of differentiation, MyoD and myogenin. In differentiating skeletal myocytes, SLIM1 overexpression induced hyperelongation, which, by either plating cells on poly- L -lysine or using a series of peptide blockade experiments, was shown to be specifically dependent on ligand binding to the 5 1 -integrin, whereas in reserve cells, SLIM1 overexpression induced the formation of multiple cytoplasmic protrusions (branching), which was also integrin mediated. These results suggest that SLIM1 may play an important role during the early stages of skeletal muscle differentiation, specifically in 5 1 -integrin-mediated signaling pathways. myoblast; proteins and differentiation Address for reprint requests and other correspondence: S. Brown, Dept. of Biochemistry and Molecular Biology, Monash Univ., Wellington Rd., Clayton, VIC 3800, Australia (E-mail: susan.brown{at}med.monash.edu.au ).