Hyperpolarization induces differentiation in human cardiomyocyte progenitor cells.

In the past years, cardiovascular progenitor cells have been isolated from the human heart and characterized. These cells can differentiate into cardiomyocytes, smooth muscle cells and endothelial cells and are therefore of great value for investigation of the mechanisms that drive progenitor cell function and plasticity, drug testing and, potentially, therapeutical purposes. In this respect, most studies have focused on enhancing differentiation with chemicals or growth factors, or co-culture with other cell types. Although they have revealed important mechanisms, protocols need to be established that exclude the need for such factors when one considers using progenitor cells to repair the human heart. In this study we tested whether we could induce cardiomyogenic differentiation of human cardiomyocyte progenitor cells (CMPCs) by altering their membrane potential. We induced hyperpolarization in CMPCs by either co-culturing them with a K(ir)2.1-overexpressing cell line or by overnight culture in medium containing low potassium concentrations. Hyperpolarization led to increased intracellular calcium concentrations, activation of calcineurin signaling, increased cardiac-specific gene and protein expression levels and, ultimately, to the formation of spontaneously beating cardiomyocytes. Thus, hyperpolarization is sufficient to induce differentiation of CMPCs, thereby revealing a novel mechanism for cardiomyogenic differentiation of heart-derived progenitor cells.

The influence of monoterpene synthase transformation on the odour of tobacco.

Monoterpenes are an important class of terpenoids that are commonly present in plant essential oils. These can be extracted from plants and are used in the flavouring and perfumery industry. Monoterpene synthases are the key enzymes in monoterpene biosynthesis, as they catalyse the cyclisation of the ubiquitous geranyl diphosphate (GDP) to the specific monoterpene skeletons. Tobacco is one of the most studied model plants, it can easily and efficiently be transformed, and is a suitable model to study the release of plant volatiles. Thus, we have isolated monoterpene synthases from lemon, transformed tobacco with these cDNAs and have used human panelists to study the change in fragrance of the transgenic in comparison to the wild type plants. In a triangle test, we found that subjects were capable of smelling significant differences between leaf samples. However, as a result of variability in panel ratings, no significant difference between two sets of transgenic flowers and the wild type tobacco flowers was found for the generated attributes in a descriptive test.

Domain swapping of Citrus limon monoterpene synthases: impact on enzymatic activity and product specificity.

Monoterpene cyclases are the key enzymes in the monoterpene biosynthetic pathway, as they catalyze the cyclization of the ubiquitous geranyl diphosphate (GDP) to the specific monoterpene skeletons. From Citrus limon, four monoterpene synthase-encoding cDNAs for a beta-pinene synthase named Cl(-)betaPINS, a gamma-terpinene synthase named ClgammaTS, and two limonene synthases named Cl(+)LIMS1 and Cl(+)LIMS2 were recently isolated [J. Lücker et al., Eur. J. Biochem. 269 (2002) 3160]. The aim of our work in this study was to identify domains within these monoterpene synthase enzymes determining the product specificity. Domain swapping experiments between Cl(-)betaPINS and ClgammaTS and between Cl(+)LIMS2 and ClgammaTS were conducted. We found that within the C-terminal domain of these monoterpene synthases, a region comprising 200 amino acids, of which 41 are different between Cl(-)betaPINS and ClgammaTS, determines the specificity for the formation of beta-pinene or gamma-terpinene, respectively, while another region localized further downstream is required for a chimeric enzyme to yield products in the same ratio as in the wild-type ClgammaTS. For Cl(+)LIMS2, the two domains together appear to be sufficient for its enzyme specificity, but many chimeras were inactive probably due to the low homology with ClgammaTS. Molecular modeling was used to further pinpoint the amino acids responsible for the differences in product specificity of ClgammaTS and Cl(-)betaPINS.

Monoterpene biosynthesis in lemon (Citrus limon). cDNA isolation and functional analysis of four monoterpene synthases.

Citrus limon possesses a high content and large variety of monoterpenoids, especially in the glands of the fruit flavedo. The genes responsible for the production of these monoterpenes have never been isolated. By applying a random sequencing approach to a cDNA library from mRNA isolated from the peel of young developing fruit, four monoterpene synthase cDNAs were isolated that appear to be new members of the previously reported tpsb family. Based on sequence homology and phylogenetic analysis, these sequences cluster in two separate groups. All four cDNAs could be functionally expressed in Escherichia coli after removal of their plastid targeting signals. The main products of the enzymes in assays with geranyl diphosphate as substrate were (+)-limonene (two cDNAs) (-)-beta-pinene and gamma-terpinene. All enzymes exhibited a pH optimum around 7; addition of Mn(2+) as bivalent metal ion cofactor resulted in higher activity than Mg(2+), with an optimum concentration of 0.6 mm. K(m) values ranged from 0.7 to 3.1 microm. The four enzymes account for the production of 10 out of the 17 monoterpene skeletons commonly observed in lemon peel oil, corresponding to more than 90% of the main components present.

Somatic embryogenesis from Arabidopsis shoot apical meristem mutants.

Zygotic embryos of three Arabidopsis thaliana (L.) Heynh. mutants lacking an embryonic shoot apical meristem (SAM), shoot meristemless (stm), wuschel (wus) and zwille/pinhead (zll/pnh) were used as explants to establish embryogenic cell cultures. Somatic embryos of all three mutants showed the same mutant phenotypes as their zygotic equivalents. These results provide genetic evidence that the developmental program of somatic and zygotic embryos is indistinguishable. They also suggest that a functional SAM is not required for somatic embryogenic cell formation in Arabidopsis.