Differential expression of tropomyosin during segmental heart development in Mexican axolotl

Translational control of tropomyosin expression in vertebrate hearts

The tropomyosin (TM) gene family produces a set of related TM proteins with important functions in striated and smooth muscle, and nonmuscle cells. In vertebrate striated muscle, the thin filament consists largely of actin, TM, the troponin (Tn) complex (Tn-I, Tn-C and Tn-T), and tropomodulin (Tmod) and is responsible for mediating Ca(2+) control of muscle contraction and relaxation. There are four known genes (designated as TPM1, TPM2, TPM3, and TPM4) for TM in vertebrates. The four TM genes generate a multitude of tissue- and developmental-specific isoforms through the use of different promoters, alternative mRNA splicing, different 3'-end mRNA processing and tissue-specific translational control. In this review, we have focused mainly on the regulation of TM expression in striated muscles, primarily in vertebrate hearts with special emphasis on translational control using mouse and Mexican axolotl animal models.

Expression of tropomyosin in relation to myofibrillogenesis in axolotl hearts

The anatomy, function and embryonic development of the heart have been of interest to clinicians and researchers alike for centuries. A beating heart is one of the key criteria in defining life or death in humans. An understanding of the multitude of genetic and functional elements that interplay to form such a complex organ is slowly evolving with new genetic, molecular and experimental techniques. Despite the need for ever more complex molecular techniques some of our biggest leaps in knowledge come from nature itself through observations of mutations that create natural defects in function. Such a natural mutation is found in the Mexican axolotl, Ambystoma mexicanum. It is a facultative neotenous salamander well studied for its ability to regenerate severed limbs and tail. Interestingly it also well suited to studying segmental heart development and differential sarcomere protein expression due to a naturally occurring mendelian recessive mutation in cardiac mutant gene “c”. The resultant mutants are identified by their failure to beat and can be studied for extended periods before they finally die due to lack of circulation. Studies have shown a differential expression of tropomyosin between the conus and the ventricle indicating two different cardiac segments. Tropomyosin protein, but not its transcript have been found to be deficient in mutant ventricles and sarcomere formation can be rescued by the addition of TM protein or cDNA. Although once thought to be due to endoderm induction our findings indicate a translational regulatory mechanism that may ultimately control the level of tropomyosin protein in axolotl hearts.

Distinct cardiogenic preferences of two human embryonic stem cell (hESC) lines are imprinted in their proteomes in the pluripotent state

Although both the H1 and HES2 human embryonic stem cell lines (NIH codes: WA01 and ES02, respectively) are capable of forming all three germ layers and their derivatives, various lines of evidence including the need to use different protocols to induce cardiac differentiation hint that they have distinct preferences to become chamber-specific heart cells. However, a direct systematic comparison has not been reported. Here we electrophysiologically demonstrated that the distributions of ventricular-, atrial- and pacemaker-like derivatives were indeed different (ratios=39:61:0 and 64:33:3 for H1 and HES2, respectively). Based on these results, we hypothesized the differences in their cardiogenic potentials are imprinted in the proteomes of undifferentiated H1 and HES2. Using multiplexing, high-resolution 2-D Differential In Gel Electrophoresis (DIGE) to minimize gel-to-gel variations that are common in conventional 2-D gels, a total of 2000 individual protein spots were separated. Of which, 55 were >2-fold differentially expressed in H1 and HES2 (p<0.05) and identified by mass spectrometery. Bioinformatic analysis of these protein differences further revealed candidate pathways that contribute to the H1 and HES2 phenotypes. We conclude that H1 and HES2 have predetermined preferences to become ventricular, atrial, and pacemaker cells due to discrete differences in their proteomes. These results improve our basic understanding of hESCs and may lead to mechanism-based methods for their directed cardiac differentiation into chamber-specific cardiomyocytes.