Molecular mechanisms linking missense ACTG2 mutations to visceral myopathy

Area of Science:

• Genetics
• Molecular Biology
• Biochemistry

Background:

• Visceral myopathy is a severe condition causing muscle weakness in internal organs.
• Mutations in the smooth muscle gamma-actin (ACTG2) gene are the primary cause of visceral myopathy.
• The precise molecular mechanisms underlying ACTG2 mutation-induced muscle dysfunction remain unclear.

Purpose of the Study:

• To investigate how four common ACTG2 mutations (R40C, R148C, R178C, R257C) affect actin function and protein interactions.
• To correlate the biochemical and structural properties of these ACTG2 mutations with varying disease severities.
• To explore the impact of heterozygous mutations and potential rescue mechanisms.

Main Methods:

• Biochemical assays to assess actin polymerization and interactions with actin-binding proteins.
• Analysis of mutant ACTG2 in 50/50 mixtures with wild-type (WT) ACTG2.
• Cryo-electron microscopy to determine the structural impact of mutations on actin filaments.
• Investigating the effect of smooth muscle tropomyosin isoform Tpm1.4 as a potential therapeutic agent.

Main Results:

• R178C mutation led to premature protein degradation.
• R148C mutation impaired interactions with key actin-binding proteins.
• R40C mutation inhibited actin polymerization, while R257C destabilized actin filaments.
• Heterozygous WT/R40C mutations disrupted leiomodin 1-mediated filament nucleation.
• WT/R257C mutations resulted in filaments susceptible to fragmentation by smooth muscle myosin.
• Cryo-EM revealed disrupted intersubunit contacts in R40C and R257C mutant filaments.
• Tpm1.4 partially rescued the functional defects caused by R40C and R257C mutations.

Conclusions:

• Specific ACTG2 mutations exhibit distinct molecular defects, including degradation, impaired protein interactions, inhibited polymerization, and filament destabilization.
• The observed biochemical and structural alterations in ACTG2 mutants directly correlate with the clinical severity of visceral myopathy.
• Understanding these mutation-specific mechanisms provides insights into disease pathogenesis and potential therapeutic strategies targeting actin dynamics.