Developmental changes in passive stiffness and myofilament Ca²⁺sensitivity due to titin and troponin-I isoform switching are not critically triggered by birth

Abstract

. Developmental changes in passive stiffness and myofilament Ca 2ϩ sensitivity due to titin and troponin-I isoform switching are not critically triggered by birth. Am J Physiol Heart Circ Physiol 291: H496 -H506, 2006. First published May 5, 2006; doi:10.1152/ajpheart.00114.2006.-The giant protein titin, a major contributor to myocardial mechanics, is expressed in two main cardiac isoforms: stiff N2B (3.0 MDa) and more compliant N2BA (Ͼ3.2 MDa). Fetal hearts of mice, rats, and pigs express a unique N2BA isoform (ϳ3.7 MDa) but no N2B. Around birth the fetal N2BA titin is replaced by smaller-size N2BA isoforms and N2B, which predominates in adult hearts, stiffening their sarcomeres. Here we show that perinatal titin-isoform switching and corresponding passive stiffness (STp) changes do not occur in the hearts of guinea pig and sheep. In these species the shift toward "adult" proportions of N2B isoform is almost completed by midgestation. The relative contributions of titin and collagen to STp were estimated in force measurements on skinned cardiac muscle strips by selective titin proteolysis, leaving the collagen matrix unaffected. Titin-based STp contributed between 42% and 58% to total STp in late-fetal and adult sheep/guinea pigs and adult rats. However, only ϳ20% of total ST p was titin based in late-fetal rat. Titin-borne passive tension and the proportion of titin-based ST p generally scaled with the N2B isoform percentage. The titin isoform transitions were correlated to a switch in troponin-I (TnI) isoform expression. In rats, fetal slow skeletal TnI (ssTnI) was replaced by adult carciac TnI (cTnI) shortly after birth, thereby reducing the Ca 2ϩ sensitivity of force development. In contrast, guinea pig and sheep coexpressed ssTnI and cTnI in fetal hearts, and skinned fibers from guinea pig showed almost no perinatal shift in Ca 2ϩ sensitivity. We conclude that TnI-isoform and titin-isoform switching and corresponding functional changes during heart development are not initiated by birth but are genetically programmed, species-specific regulated events. heart development; connectin; elasticity; myocardium BIRTH IS A DRAMATIC EVENT in mammalian heart development. With the newborn's first breaths of air, the fetal circulation changes, and cardiac pump function is intensified to keep up with the increased power requirements of the newborn that suddenly lacks placental nurturing and oxygen supply. Heart rate, end-diastolic pressure, stroke volume, and left ventricular (LV) dimensions all increase to meet the metabolic demands of newborn life (4, 26). During perinatal development of myocardium, many sarcomere proteins alter their isoform pattern rather rapidly. Among them are the contractile proteins myosin heavy chain (MyHC) (9, 22, 32, 53) and ␣-actin (10); regulatory proteins, including troponin-I (TnI), troponin-T (TnT), tropomyosin (36,46,(51)(52)(53), and myosin light chain-1 (60); and scaffolding proteins such as myomesin (1) and titin (27,41,43, 56). However, as these studies have usually been performed ...

Nuances of electrophoresis study of titin/connectin

“…These were gradually replaced by adult forms (N2BA-A1, N2BA-A2) with sizes of 3390 kDa and 3220 kDa, respectively (Warren et al 2004). Similar titin isoform transformations were observed in embryonic/neonatal hearts of rat and other mammals and reported by the researchers (Lahmers et al 2004;Opitz et al 2004;Greaser et al 2005;Krüger et al 2006).…”

Differential contribution of cardiac sarcomeric proteins in the myofibrillar force response to stretch

“…The myocardium co-expresses two titin isoforms, the short N2B and the long N2BA isoform. As described previously, guinea pig myocardium expresses mostly the short N2B titin isoform [34]. To establish whether a regional variation in titin was responsible for the difference in stiffness between ENDO and EPI cells, we measured titin content and isoform distribution in both cell types.…”

Mutation that dramatically alters rat titin isoform expression and cardiomyocyte passive tension

“…The isoform ratio changes in rats during development starting with a single large N2BA species at the embryonic stage and proceeding to the adult pattern by 20 days after birth [12][13][14]. The developmental transformation appears to vary between species [15]. The ratio of the N2BA to N2B isoforms affects passive tension in the heart with muscle containing higher N2BA proportions exhibiting lower tension at a given sarcomere length.…”