A Drosophila melanogaster model of diastolic dysfunction and cardiomyopathy based on impaired troponin-T function

"A Drosophila melanogaster model of diastolic dysfunction and cardiomyopathy based on impaired troponin-T function"

Abstract: Rationale: Regulation of striated muscle contraction is achieved by Ca2+-dependent steric modulation of myosin cross-bridge cycling on actin Ca2+by the thin filament troponin-tropomyosin complex. Alterations in the complex trigger contractile dysregulation and myopathy. For example, mutations between or near residues 112-136 of cardiac troponin-T, the crucial TnT1 tropomyosin-binding region, cause cardiomyopathy. The Drosophila up101 Glu/Lys amino acid substitution localizes just downstream from this phylogenetically conserved sequence.
 

Objective: Using an integrative approach, we sought to determine the molecular trigger of up101 myofibrillar degeneration, to evaluate contractile dysfunction at the cellular level, and to examine the effects of the mutation on the entire Drosophila heart, to elucidate regulatory roles for conserved TnT1 regions and provide mechanistic insight into human cardiomyopathy.
 

Methods and Results: Live video imaging of Drosophila cardiac performance revealed the troponin-T mutation prolongs systole and restricts diastolic dimensions of the heart due to increased numbers of actively cycling myosin cross-bridges. Elevated resting myocardial stiffness, consistent with up101 diastolic dysfunction, was confirmed by an atomic force microscopy-based nanoindentation approach. Direct visualization of mutant thin filaments via electron microscopy and three-dimensional reconstruction resolved destabilized tropomyosin positioning and aberrantly exposed myosin binding sites under low Ca2+ conditions.
 

Conclusions: As a result of troponin-tropomyosin dysinhibition, up101 hearts exhibit cardiac dysfunction and remodeling comparable to that observed during human restrictive cardiomyopathy. Thus, reversal of charged residues about the conserved tropomyosin-binding region of TnT1 may perturb critical intermolecular associations required for proper steric regulation, which likely leads to myopathy in Drosophila and in humans.