| Abstract |
This doctoral thesis was performed at the B.S.R.C. “Alexander Fleming” in Vari, Athens
under the supervision of Professor Charalambos Savakis together with Researcher A’,
Efthimios M.C. Skoulakis. The research study involves a genetic screen employing MiMIC
transposable element. The aim was to identify new genes implicated in habituation of
Drosophila melanogaster.
Habituation is a highly conserved, yet little understood form of behavioral plasticity that
enables salience filtering, by precipitating perceptual changes that attenuate the value of
environmental stimuli. Normal habituation allows animals to ignore/devalue repetitive or
prolonged non-reinforced stimuli and does not involve sensory adaptation or fatigue. It likely
underlies selective attention. Defective habituation is the retention of the value of an
inconsequential stimulus beyond the time typical for the onset of the attenuated response and
is thought to underlie Schizophrenia (SD) as SD patients present characteristic deficits in
devaluing and attenuating responses to repeated stimulation.
Here, we used MiMIC insertions within different genetic loci of Drosophila melanogaster,
to identify genes involved in habituation. Drosophila, as an experimental animal model,
exhibits multiple assets such as its ease of handling, low cost of breeding and availability of
scientific tools that allow molecular and genetic analysis. MiMIC mutants were tested using the
shock habituation experimental paradigm. Several insertion lines that exhibited abnormal
responses were identified. Among them, insertions within the genes Btk, Tau and rut were
further analyzed.
The Btk gene encodes a non-receptor tyrosine kinase. This study showed that footshock
habituation consists of two distinct phases, which both depend on different mushroom body
neurons and Btk is necessary for both of them. Btk is expressed in the mushroom bodies of
Drosophila melanogaster and has distinct roles in different neurons. In the α/β neurons, it
protects from premature habituation, while in the α’/β’ neurons promotes habituation after
repetitive electric footshocks. Thus, elimination of Btk protein in the α/β neurons results in
premature habituation whereas in the α’/β’ neurons it results in defective habituation.
Defective habituation can be restored after administration of the antipsychotic drugs clozapine
and risperidone. Hence, we propose a link between defective habituation and schizophrenia.
A second identified gene, Tau, encodes a protein involved in microtubule stabilization
and is found mainly in axons. It is implicated in human neurodegenerative diseases. According
to our study, elimination of drosophila Tau protein from the α’/ β’ mushroom body neurons
results in defective habituation while its overexpression leads to premature habituation. The third identified gene examined in this thesis is rut, which codes for an adenylate
cyclase. Experiments showed that rut is required within the mushroom bodies for the fly to
ignore the repetitive stimuli during the footshock habituation assay. As for Btk, defective
habituation of rut mutants can be restored after administration of risperidone. Further
experiments are required to clarify rut function.
In conclusion, this study contributed to the identification of new genes implicated in
habituation and shed light on yet poorly known mechanisms. These genes and their
mechanisms of action will certainly lead to a better understanding of the habituation related
diseases of great complexity, such as schizophrenia. Thus the study demonstrated that
Drosophila melanogaster can be a potent model not only for studying habituation as an
endophenotype of schizophrenia and the identification of new implicated genes, but also for
the verification and further study of already known genes related to human diseases.
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Identification of novel genes implicated in habituation of D. melanogaster, using Minos transposable element.
