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Lepidopteran DALP, and its mammalian ortholog HIC-5, function as negative regulators of muscle differentiation
1Molecular and Cellular Biology Program, University of Massachusetts, Amherst, MA 01003, USA.
Abstract:
During myogenesis, reductions in trophic factor availability signal most myoblasts to fuse, up-regulate the expression of muscle-specific genes, and form myotubes. Those cells failing to differentiate into myotubes initiate apoptosis and rapidly die. At present, the signal-transduction molecules that determine whether myoblasts should differentiate or die are largely unknown. In this report, we describe the cloning and characterization of DALP, a small LIM-only type zinc-finger protein that is induced when the intersegmental muscles (ISMs) of the moth Manduca sexta become committed to die at the end of metamorphosis. Forced expression of death-associated LIM-only protein (DALP) in Drosophila results in skeletal muscle atrophy. Ectopic expression of DALP, or its mammalian ortholog Hic-5, blocks differentiation and induces apoptosis in mouse C(2)C(12) myoblasts. Both of these effects can be overcome by contact with normal myoblasts or by ectopic expression of the muscle-specific transcription factor MyoD. Hic-5 expression is specifically and dramatically induced in normal myoblasts that die after removal of trophic support. Taken together, these data suggest that DALP and Hic-5 act upstream of MyoD and function as phylogenetically conserved "switches" to block muscle differentiation and induce death.
Insights
Death-associated LIM-only protein (DALP) and its mammalian ortholog Hic-5 block muscle differentiation and induce apoptosis. These proteins act as conserved switches, regulating myoblast fate and skeletal muscle atrophy.
Area of Science:
- Muscle development and cell death
- Molecular biology
- Insect and mammalian muscle physiology
Background:
- Myogenesis involves myoblast differentiation into myotubes or apoptosis when trophic support is low.
- Signal transduction pathways governing myoblast differentiation versus apoptosis remain largely uncharacterized.
Purpose of the Study:
- To identify and characterize novel signal-transduction molecules involved in myoblast fate determination.
- To investigate the role of the LIM-only protein DALP and its ortholog Hic-5 in muscle differentiation and apoptosis.
Main Methods:
- Cloning and characterization of the DALP gene.
- Forced expression of DALP in Drosophila to study muscle atrophy.
- Ectopic expression of DALP and Hic-5 in mouse C(2)C(12) myoblasts.
- Investigating the rescue effects of myoblast contact and MyoD expression.
- Analyzing Hic-5 expression in dying myoblasts.
Main Results:
- DALP, a LIM-only zinc-finger protein, is induced during the programmed death of moth intersegmental muscles.
- Forced expression of DALP in Drosophila causes skeletal muscle atrophy.
- Ectopic DALP or Hic-5 expression inhibits differentiation and induces apoptosis in mouse myoblasts.
- These inhibitory effects are reversible by myoblast contact or MyoD expression.
- Hic-5 is significantly upregulated in myoblasts undergoing apoptosis due to trophic factor withdrawal.
Conclusions:
- DALP and Hic-5 function as phylogenetically conserved regulators of myoblast fate.
- These proteins act upstream of MyoD, serving as molecular switches to either block muscle differentiation or induce apoptosis.
- DALP and Hic-5 are key players in the programmed cell death of muscle cells.