By: Widad Dantoft

Title: Role of POU/Oct transcription factors in Drosophila immunity

Opponent: Dr. Anne Uv, Göteborgs Universitet

Academic dissertation for the Degree of Doctor of Philosophy in Molecular
Biology at Stockholm University to be publicly defended on Friday 12
September 2014 at 10.00 in Magnélisalen, Kemiska övningslaboratoriet,
Svante Arrhenius väg 16 B.

In response to infection, Drosophila melanogaster relies on the ability to mount a robust and potent innate immune response, characterized by induction of cellular and humoral processes. While rapid immune induction is of utmost importance, it is equally important to restrict and repress immune gene expression, where and when it is not needed. The aim of my studies was to elucidate the in vivo role of the Oct1/POU2F1 homolog Nubbin (Nub), an evolutionarily conserved transcription factor, in innate immunity.
The transcription factor Nub-PD, a product of the nub gene, was studied in wild type and
mutant (nub1) flies, as well as in transgenic flies that either overexpress or down-regulate Nub-PD. Infection-studies were conducted after both oral and septic infection. Expression analysis of target genes using quantitative mRNA measurements and microarray analysis of wild type and nub1, reveal that Nub-PD acts as a bona fide repressor of NF-κB/Relish-dependent expression of immune defense genes, e.g. antimicrobial peptides. I show that Nub-PD represses transcription in uninfected flies by binding to oct DNA sequence motifs located upstream of a number of immune
genes. In the event of infection, repression by Nub-PD is alleviated through rapid signal-dependent degradation, in a proteasome- and Imd-dependent manner, allowing initiation of antimicrobial peptide gene expression in both fat body and intestine. Lastly, we show that nub1 mutants also exhibit a shortened lifespan as well as elevated loads of gut bacteria with altered composition. We suggest that the aberrant immune gene activation, elevated bacterial loads, and altered bacterial composition in nub1 can collectively explain the shortened lifespan. In conclusion, this work reveals a novel function of Nub-PD as a negative regulator of immune and stress response genes in vivo. Furthermore, my work sheds light on the importance of Nub-PD in host survival and in supporting a balanced composition of the gut microbiota.

Keywords: Antimicrobial peptides, Drosophila, host-pathogen interaction, immune gene
expression, immune signaling, NF-kB, microbiota, POU/Oct transcription factors, protein
degradation, stress response, transcriptional regulation