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1 Department of Anatomy and Physiology, University of Padova, 35131 Padova, Italy; Laboratory of Neuromuscular Plasticity, University of Sciences and Technologies, 59655 Villeneuve d'Ascq, France
2 Laboratory of Neuromuscular Plasticity, University of Sciences and Technologies, 59655 Villeneuve d'Ascq, France
3 Department of Anatomy and Physiology, University of Padova, 35131 Padova, Italy
* To whom correspondence should be addressed. E-mail: carlo.reggiani{at}unipd.it.
In striated muscles Myosin Light Chain 2 (MLC2) phosphorylation regulates calcium sensitivity and mediates sarcomere organization. Little is known about the changes in MLC2 phosphorylation in relation with skeletal muscle plasticity. Here we studied changes in MLC2 phosphorylation in three conditions causing slow-to-fast transitions: i) atrophy induced by 14 days of hindlimb suspension (HS), ii) hypertrophy induced by 14 days of clenbuterol administration (CB) and iii) 14 days of combined treatment (HS-CB). Three variants of the slow (MLC2s) and two variants of the fast MLC2 isoform (MLC2f) were separated using two dimensional electrophoresis and identified with monoclonal and polyclonal antibodies specific for MLC2: their relative proportions were densitometrically quantified. In control Soleus MLC2s predominated over MLC2f (91.4±3.9 vs 8.5±3.9%) and was separated in two spots, the less acidic spot being 73.5±4.3% of the total. All treatments caused the decrease of the less acidic unphosphorylated spot of MLC2s (64.1±5.6% in CB, 62.4±6.8% in HS and 56.4±4.4% in CB-HS), the appearance of a third more acidic variant of MLC2s (representing 3.9-5.9% of total MLC2s), the increase of MLC2f (30.9±3.1% in CB, 23.9±3.3% in HS and 25.3±3.9% in CB-HS), and the phosphorylation of a large fraction of MLC2f (30.4±6.7% in CB, 28.7±6.5% in HS and 21.8.±2.1% in CB-HS). The treatment with alkaline phosphatase or with PP1 phosphatase removed the most acidic spots of both MLC2f and MLC2s. We conclude that in rat skeletal muscles an increase of MLC2 phosphorylation is associated with the slow-to-fast transition regardless whether or not hypertrophy or atrophy develops.
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