Indiana University-Purdue University Indianapolis (IUPUI) / MCTP2 (multiple C2 domain transmembrane containing protein 2) encodes a protein with poorly understood roles in lipid metabolism and lipid droplet biogenesis. Genetic studies previously identified variations in MCTP2 in conjunction with left ventricular outflow tract obstructive forms of congenital heart disease (CHD). This dissertation research aimed to delineate the biomedical significance of Mctp2 by investigating its expression and consequences of its genetic deletion in mouse models. Temporal and spatial expression of Mctp2 was investigated by RT-PCR and in-situ hybridization. A novel isoform, designated as isoform 2 in mice, results from alternative pre-mRNA splicing. Similar levels of Mctp2 isoforms 1 and 2 are present in embryonic tissues, whereas isoform 1 is preferentially expressed in adult tissues with high lipid metabolism. During mouse embryonic development, in-situ hybridization suggests expression of Mctp2 at the gut tube, liver bud and near the pharyngeal arches from E8.5 – E10.5. Given association of MCTP2 with CHD, the biological significance of Mctp2 was addressed using gene trap (GT) and conditional mouse models. Survival of Mctp2 GT mice was dependent on the genetic background strain, suggesting a role for strain-specific modifiers. Conditional knockout of Mctp2 in cardiac progenitor cells displayed no effect on survival. The role of Mctp2 in cardiac development remains to be delineated. The role of Mctp2 in cardiac function was addressed in both mouse models. Initial findings suggest Mctp2 allele dosage effects on the development of heart failure. GT mice lacking one, or both, copies of Mctp2 display cardiac systolic dysfunction, with upregulation of heart failure markers at 50 weeks of age in heterozygotes and increases in cardiac fibrosis in homozygotes. Systemic conditional deletion of Mctp2 did not show heart failure phenotypes using the strain protective from lethality. However, cardiac specific deletion of Mctp2 using the Nkx2.5-Cre driver, a line that is sensitized for cardiac dysfunction, led to decreased ejection fraction and fractional shortening in mice with conditional deletion of both copies of Mctp2 as well as Mctp2 dosage dependent penetrance of cardiac dilation. These studies of knockout mice suggest a role for Mctp2 in maintenance of cardiac function and possible genetic interaction with Nkx2.5. / 2022-02-02

La phospholipase D (PLD) hydrolyse les phospholipides membranaires en libérant leur tête polaire afin de générer l'acide phosphatidique (PA), impliqué dans la signalisation cellulaire. Pour comprendre les propriétés biochimiques des PLDs, les travaux présentés ont été réalisés autour de deux axes. Le premier axe concerne l'expression recombinante et la purification de la PLDa d'Arabidopsis thaliana (AtPLDa) dans la levure Pichia pastoris. La détermination de la séquence N-terminale a révélé que l'AtPLDa est amputée de ses 35 premiers résidus, suggérant ainsi la participation d'un mécanisme de maturation. Cependant, la région N-terminale des PLDs de plantes est homologue au domaine C2, impliqué dans leur interaction Ca2+-dépendante avec la membrane. Afin d'évaluer l'impact d'un tel clivage, les domaines C2 de l'AtPLDa mais également de l'AtPLDß, à titre de comparaison, ont été étudiés sous leur forme entière ou mature. Ainsi, la caractérisation de leur affinité pour les phospholipides, associée à leur modélisation tridimensionnelle, ont permis de démontrer que les différences de régulation par le Ca2+, observées entre les formes entières et mature, provenait de la présence d'une hélice a amphipathique, retirée lors du processus de maturation. Le second axe concerne le développement d'une nouvelle méthode de mesure des activités PLD via le dosage de manière direct, spécifique et continu du PA grâce à la propriété d'amplification de fluorescence par chélation de la 8-hydroxyquinoléine, en présence de Ca2+. Ainsi, ce test apparait adapté pour le suivi de l'inhibition des PLDs et pour l'étude de leur spécificité de substrat, en utilisant des phospholipides naturels avec différentes tête polaires, et à l'échelle d'une microplaque / Phospholipase D (PLD) hydrolyses membrane phospholipids, leading to the formation of free polar headgroup and phosphatidic acid releasing, involved in cell signaling. To understand the biochemical properties of PLDs, this work has been made around two axes. The one first concerns the recombinant expression and purification of the PLDa of Arabidopsis thaliana (AtPLDa) in the yeast Pichia pastoris. The N-terminal sequence of the recombinant AtPLDa has been determined and found to lack its first 35 amino acids, suggesting the involvement of a maturing mechanism. However, plant PLDs exhibit a C2-lipid binding domain at their N-terminal region, which is involved in their Ca2+-dependent membrane targeting. Thus, to assess the impact of such a cleavage, whole and mature-like C2 domains of AtPLDa, as well as of AtPLDß, for the sake of comparison were studied. Thus, the characterization of their affinity for phospholipids, combined with their three-dimensional modeling have demonstrated that the differences observed in their regulation by Ca2+, observed between whole and mature-like forms, originated from the presence of a N-terminus amphipathic a helix, removed during the maturation process. The second axis concerns the development of a novel PLD assay that measure PA in a direct, specific and continuous manner, using the chelation enhanced fluorescence property of 8-hydroxyquinoline in the presence of Ca2+. Thus, this assay appears suitable for monitoring both the inhibition of PLDs as well as their substrate specificity, using natural phospholipids with different polar headgroups, and at a microplate scale