Protein phosphatase activity is necessary for myofibrillogenesis

Area of Science:

• Muscle development
• Cellular biology
• Biochemistry

Background:

• Myofibrillogenesis involves myosin light-chain kinase (MLCK) phosphorylating myosin II's regulatory light chain (RLC).
• Protein phosphatases (PPs) dephosphorylate RLC in adult muscles, but their embryonic role is unknown.
• Rho kinase (RK) influences PP activity in embryonic cardiac and smooth muscle.

Purpose of the Study:

• Investigate the role of protein phosphatase (PP) activity in embryonic myofibrillogenesis.
• Determine the specific PP isoforms involved in regulating RLC phosphorylation during muscle development.
• Clarify the interplay between MLCK and PP activity in myosin assembly.

Main Methods:

• Pharmacological inhibition of PP1 and PP2A using tautomycin (TM), okadaic acid (OA), and fostriecin (FOS).
• Assessment of actin and myosin assembly in skeletal myocytes.
• Knockdown of the myosin-targeting (M) subunit of PP1 using morpholino antisense oligonucleotides.

Main Results:

• Tautomycin inhibited both actin and myosin assembly, suggesting PP1 involvement.
• Okadaic acid and fostriecin did not disrupt assembly, indicating PP2A is not primarily involved.
• Knockdown of the PP1M subunit perturbed myosin organization and reduced somite number.
• Rho kinase inhibition did not affect actin or myosin assembly in skeletal myocytes.

Conclusions:

• Protein phosphatase 1 (PP1) isoforms, particularly PP1M, are critical for embryonic myofibrillogenesis.
• A dynamic balance between MLCK and PP1M activity is essential for proper myosin assembly and muscle development.
• PP1, not PP2A, is the primary phosphatase regulating RLC phosphorylation during embryonic muscle formation.