Abstract
Wistar rats pharmacogenetically selected for a high susceptibility to the dopamine agonist apomorphine (APO-SUS rats) display many differences with their phenotypic counterpart (APO-UNSUS rats), which are remarkably similar to those observed in schizophrenic patients. In this thesis we tried to understand the molecular mechanism underlying the APO-SUS/-UNSUS complex phenotype, which might aid in finding the cause of schizophrenia in human. Microarray expression analyses revealed in APO-SUS rats, relative to -UNSUS rats, a reduced expression of Aph-1b, a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signalling pathways. These differences were caused by different gene copy numbers; APO-SUS rats harbour only one or two gene copies of Aph-1b, whereas APO-UNSUS rats contain three copies. This gene-dosage imbalance was caused by an unequal crossing-over event involving a segmental duplication within the Aph-1b locus. Although the expression levels of the other gamma-secretase components were not different, the gamma-secretase cleavage activity towards several substrates was affected and the Aph-1b genotypes segregated with a number of behavioural phenotypes. Through crossing and genetic re-selection, new rat lines were generated with one, two or three Aph-1b gene copies against otherwise similar genetic backgrounds. These rats revealed, next to affected Aph-1b expression levels throughout development, temporal, tissue- and substrate-specific changes in gamma-secretase cleavage activity. Preliminary studies on transgenic Xenopus laevis expressing Aph-1a under control of the proopiomelanocortin promoter, imply that the manipulation of Aph-1 expression in Xenopus melanotrope cells leads to a modified sensitivity of the dopaminergic system. In conclusion, these studies suggest that a subtle change in the expression of a (neuro)developmentally important protein may spatio-temporally affect diverse signalling pathways, ultimately resulting in a complex phenotype that is generally thought to be caused by multiple affected genes. Furthermore, we implicate the possible involvement of the gamma-secretase enzyme in complex disorders, like schizophrenia.
