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Troponin I phosphorylation in the normal and failing adult human heart
G S Bodor1, A E Oakeley, P D Allen
1Department of Laboratories, Denver Health Medical Center, Colo, USA.
Insights
In failing hearts, cardiac troponin I (cTnI) has less phosphorylation, increasing calcium sensitivity. This difference in cTnI phosphorylation may impact heart function, potentially leading to adaptive or maladaptive responses.
Area of Science:
- Cardiovascular Biology
- Molecular Cardiology
- Cardiac Muscle Physiology
Background:
- Failing human hearts exhibit increased myofibrillar calcium sensitivity and reduced maximal myofibrillar ATPase activity compared to normal hearts.
- Phosphorylation of cardiac troponin I (cTnI) influences myofilament calcium sensitivity and ATPase activity.
- Specific phosphorylation sites on cTnI modulate these contractile properties.
Purpose of the Study:
- To investigate differences in cTnI phosphorylation between failing and normal human hearts.
- To identify specific cTnI phosphorylation patterns associated with cardiac dysfunction.
Main Methods:
- Western blot analysis of left ventricular myocardium from failing (n=20) and normal (n=24) human hearts.
- Use of monoclonal antibodies (MAbs) specific to cardiac troponin I (cTnI) and its NH2-terminus.
- Assessment of cTnI phosphorylation states (cTnI1 and cTnI2) and the impact of enzymatic and kinase treatments.
Main Results:
- No significant differences in the relative amounts of actin, tropomyosin, and TnI between failing and normal myocardium.
- Two cTnI bands (cTnI1 and cTnI2) were identified; cTnI1 represents the phosphorylated form.
- The percentage of cTnI composed of cTnI1 (phosphorylated cTnI) was significantly greater in normal hearts than in failing hearts (P<.00).
Conclusions:
- A reduced percentage of phosphorylated cTnI (cTnI1) in failing hearts may explain the increased myofibrillar calcium sensitivity.
- This phosphorylation difference could be an adaptive mechanism to compensate for altered calcium transients in heart failure.
- Alternatively, this difference may contribute to adverse outcomes such as ventricular diastolic dysfunction.
Background:
In the failing human heart myofibrillar calcium sensitivity of tension development is greater and maximal myofibrillar ATPase activity is less than in the normal heart. Phosphorylation of the cardiac troponin I (cTnI)-specific NH2-terminus decreases myofilament sensitivity to calcium, while phosphorylation of other cTnI sites decreases maximal myofibrillar ATPase activity.
Methods And Results:
We examined cTnI phosphorylation in left ventricular myocardium collected from failing hearts at the time of transplant (n=20) and normal hearts from trauma victims (n=24). The relative amounts of actin, tropomyosin, and TnI did not differ between failing and normal myocardium. Using Western blot analysis with a monoclonal antibody (MAb) that recognizes the striated muscle TnI isoforms, we confirmed that the adult human heart expresses only cTnI. A cTnI-specific MAb recognized two bands of cTnI, designated cTnI1 and cTnI2, while a MAb whose epitope is located in the cTnI-specific NH2-terminus recognized only cTnI1. Alkaline phosphatase decreased the relative amount of cTnl1, while protein kinase A and protein kinase C increased cTnI1. The percentage of cTnI made up of cTnI1, the phosphorylated form of TnI, is greater in the normal than the failing human heart (P<.00).
Conclusions:
This phosphorylation difference could underlie the reported greater myofibrillar calcium sensitivity of failing myocardium. The functional consequence of this difference may be an adaptive or maladaptive response to the lower and longer calcium concentration transient of the failing heart, eg, enhancing force development or producing ventricular diastolic dysfunction.