Cardiac troponin T mutations: correlation between the type of mutation and the nature of myofilament dysfunction in transgenic mice.
1. The heterogenic nature of familial hypertrophic cardiomyopathy (FHC) in humans suggests a link between the type of mutation and the nature of patho-physiological alterations in cardiac myocytes. Exactly how FHC-associated mutations in cardiac troponin T (cTnT) lead to impaired cardiac function is unclear. 2. We measured steady-state isometric force and ATPase activity in detergent-skinned cardiac fibre bundles from three transgenic (TG) mouse hearts in which 50, 92 and 6 % of the native cTnT was replaced by the wild type (WT) cTnT, R92Q mutant cTnT (R92Q) and the C-terminal deletion mutant of cTnT (cTnT(DEL)), respectively. 3. Normalized pCa-tension relationships of R92Q and cTnT(DEL) fibres demonstrated a significant increase in sensitivity to Ca2+ at short (2.0 microm) and long (2.3 microm) sarcomere lengths (SL). At short SL, the pCa50 values, representing the midpoint of the pCa-tension relationship, were 5.69 +/- 0.01, 5.96 +/- 0.01 and 5.81 +/- 0.01 for WT, R92Q and cTnT(DEL) fibres, respectively. At long SL, the pCa50 values were 5.81 +/- 0.01, 6.08 +/- 0.01 and 5.95 +/- 0.01 for WT, R92Q and cTnT(DEL) fibres, respectively. 4. The difference in pCa required for half-maximal activation (DeltapCa50) at short and long SL was 0.12 +/- 0.01 for the R92Q (92 %) TG fibres, which is significantly less than the previously reported DeltapCa50 value of 0.29 +/- 0.02 for R92Q (67 %) TG fibres. 5. At short SL, Ca2+-activated maximal tension in both R92Q and cTnT(DEL) fibres decreased significantly (24 and 21 %, respectively; P < 0.005), with no corresponding decrease in Ca2+-activated maximal ATPase activity. Therefore, at short SL, the tension cost in R92Q and cTnT(DEL) fibres increased by 35 and 29 %, respectively (P < 0.001). 6. The fibre bundles reconstituted with the recombinant mutant cTnT(DEL) protein developed only 37 % of the Ca2+-activated maximal force developed by recombinant WT cTnT reconstituted fibre bundles, with no apparent changes in Ca2+ sensitivity. 7. Our data indicate that an important mutation-linked effect on cardiac function is the result of an inefficient use of ATP at the myofilament level. Furthermore, the extent of the mutation-induced dysfunction depends not only on the nature of the mutation, but also on the concentration of the mutant protein in the sarcomere.
Montgomery
D E
DE
Department of Physiology and Biophysics and Program in Cardiovascular Sciences, University of Illinois at Chicago, College of Medicine, Chicago, IL 60612, USA.
Tardiff
J C
JC
Chandra
M
M
D008841:Actin Cytoskeleton / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D024741:Cardiomyopathy, Hypertrophic, Familial / Q000503:physiopathology*; D003902:Detergents; D004734:Energy Metabolism / Q000502:physiology; D066298:In Vitro Techniques; D051379:Mice; D008822:Mice, Transgenic; D009154:Mutation / Q000502:physiology; D009200:Myocardial Contraction / Q000502:physiology; D009206:Myocardium / Q000166:cytology / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D020107:Troponin T / Q000235:genetics* / Q000378:metabolism
D003902:Detergents; D011994:Recombinant Proteins; D020107:Troponin T; D002118:Calcium