Troponin I: in sickness and in health-and normal development.

Troponin is the key component of the calciumdependent switch of the contractile apparatus in striated muscle. There are three subunits of troponin: troponin C, a Ca -binding calmodulin-like protein; troponin T, which attaches the complex to tropomyosin, anchoring it to the thin filament as well as having a regulatory role; and troponin I (TnI), named for its ability to inhibit actin-myosin interactions at diastolic levels of Ca . As cytosolic Ca increases in systole, it binds to a regulatory Ca -binding site on TnC, leading to increasing affinity of TnC for TnI and weakening the interactions of TnI and actin. This permits movement of tropomyosin-troponin on the thin filament such that the inhibition of actin-myosin interaction is diminished, increasing the probability of crossbridge cycling and muscle shortening. The ability to inhibit the actin-myosin interaction resides within a 12-amino acid region of the TnI molecule, although its inhibitory function is modulated by other regions of TnI.1 In mature mammals, a different TnI gene is expressed specifically in each of the three types of striated muscle: fast twitch (fsTnI), slow twitch (ssTnI), and cardiac (cTnI). The fetal and neonatal cardiac atria and ventricle also express ssTnI, and there is a gradual downregulation of the mRNA for ssTnI and increased expression of cTnI with maturation.2 This gene switch occurs in all mammalian species studied to date, including human. The tight regulation of this process was made apparent when the cTnI gene was subjected to targeted deletion in mice by Huang et al.3 Rather than persistently maintaining the ssTnI gene expression after its normal developmental stage in the heart, the mice died as this gene was turned off postnatally. What are the advantages of the expression of ssTnI in the fetal heart? One functional measure of the calcium switch in muscle is the rate of ATP hydrolysis by myofilaments (actin-activated myosin or myofibrillar MgATPase activity), which increases as the available Ca increases up to a maximal saturating level. A similar, although not completely analogous, relationship of steady-state isometric tension development can be measured in skinned muscle fibers exposed to varying Ca concentrations. The presence of ssTnI in the myofilament results in a baseline left shift in this relationship, referred to as Ca sensitization.4,5 This is conceivably advantageous in the fetal heart, in which the sarcoplasmic reticulum is poorly developed, and the contractile force is much more dependent on Ca entry through the cell membrane. Another advantage, first noted by Solaro et al6 before the basis of the TnI isoform shift was known, is that myofibrils from immature heart are resistant to the effect of acidosis on the myofibrillar MgATPase-Ca relation. In contrast, the myofibrillar MgATPase activity of myofibrils isolated from mature heart is significantly desensitized to Ca (ie, right-shifted) when exposed to lower pH, which limits contractile force. Finally, it is notable that the ssTnI isoform lacks the 32-residue amino terminal extension present in cTnI. This region of the molecule contains two adjacent serine residues that are targets of protein kinase A (PKA) phosphorylation. Functionally, PKA phosphorylation desensitizes the myofilament to Ca , although it may also affect crossbridge kinetics allowing more rapid cycling of crossbridges.7 The lack of these PKA targets on TnI in the fetus coincides temporally with the lack of sympathetic innervation in immature heart. There are also sites for PKC phosphorylation on TnI, some of which differ between the ssTnI and cTnI isoforms. Alterations in phosphorylation of TnI have been implicated in disease states such as dilated cardiomyopathy in mouse models and end-stage heart failure in humans.8–10 In this issue of Circulation Research, Westfall and colleagues11 use a myocyte gene transfer model to examine an apparent paradox that arose from prior studies reporting the expression of TnI variants in the hearts of transgenic mice. Previously, Fentzke et al5 completely replaced cTnI with ssTnI in adult mouse hearts and found that the mice exhibited some evidence of diastolic dysfunction as well as Ca sensitization of the myofilaments. However, these mice did not have the typical phenotype of hypertrophic cardiomyopathy at the organ level: myocyte disarray and fibrosis were not noted, nor were these hearts hypertrophied. James et al12 then created mice expressing a TnI mutation that had been noted in a pedigree with familial hypertrophic cardiomyopathy (FHC), cTnI R146G, a mutation in the critical inhibitory region of TnI. Although it could not be measured directly, these mice likely had less than full replacement of the native TnI with the mutant, but despite this, two lines with this mutant had early lethality with grossly abnormal hearts, suggesting dose-dependent lethality. The remaining lines demonstrated varying degrees of cardiac fibrosis, myocyte disarray, diastolic dysfunction, and, in the one line in which it was measured, increased Ca sensitization of tension. The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association. From the Division of Cardiology, Department of Pediatrics and Institute of Molecular Cardiobiology, Johns Hopkins University School of Medicine, Baltimore, Md. Correspondence to Anne M. Murphy, Division of Cardiology, Department of Pediatrics and Institute of Molecular Cardiobiology, Johns Hopkins University School of Medicine, Ross 1144, 720 Rutland Ave, Baltimore, MD 21205. E-mail [email protected] (Circ Res. 2002;91:449-450.) © 2002 American Heart Association, Inc.