01/24/2022
By Lindsey Howland

The Kennedy College of Sciences, Department of Biological Sciences, invites you to attend a master’s thesis defense by Lindsey Howland on "Cardiomyopathy-linked mutation in tropomyosin alters calcium-based regulation of cardiac muscle contraction."

Candidate Name: Lindsey Howland
Defense Date: Monday, Feb. 7, 2022
Time: 11:30 a.m. to 12:30 p.m.
Location: Via Zoom

Thesis/Dissertation Title: Cardiomyopathy-linked mutation in tropomyosin alters calcium-based regulation of cardiac muscle contraction

Advisor: Jeffrey Moore, Biological Sciences, University of Massachusetts Lowell

Committee Members:

  • Nicolai Konow, Biological Sciences, University of Massachusetts Lowell
  • Mathew Gage, Chemistry, University of Massachusetts Lowell

Abstract:
Hypertrophic Cardiomyopathy (HCM) is the leading cause of sudden cardiac death in young adults, affecting 1 in 500 individuals. HCM causes an enlargement of the left ventricular wall, leading to decreased blood flow throughout the body. The cardiac muscle contraction-relaxation cycle is dependent on alterations to intracellular calcium levels. In the absence of Ca2+, tropomyosin (Tpm) remains in the blocked state, preventing the interaction between myosin and actin, therefore inhibiting contraction. When Ca2+ is present, muscle contraction is facilitated by Ca2+ binding to Troponin C, allowing Tpm movement on actin that exposes acto-myosin binding sites thus allowing actomyosin interactions and cardiac muscle contraction. HCM is caused by mutations in cardiac contractile proteins, including the thin filament regulatory protein Tpm. Here we used actin co-sedimentation, viscosity measurements and in vitro motility assays to determine the structural and mechanistic effects of the HCM-linked mutation, Tpm-S215L, on cardiac muscle contraction regulation. Despite being located toward the central region of Tpm, the Tpm mutant S215L shows an increase in Tpm-Tpm bond strength and therefore overall cooperativity of cardiac thin filament activation.

Compared to WT controls, mutant S215L Tpm showed weakened binding to actin in the absence of Ca2+, but binding saturation was achieved with prolonged, 24 hr incubation. Movement of cardiac thin filaments reconstituted with S215L Tpm was fully inhibited at low Ca2+ concentrations but required less calcium to induce activation of thin filament motion. The observed effects suggest that mutation-induced disruption of the blocked-closed equilibrium of Tpm underlies the initial molecular insult that ultimately leads to HCM.