Hyd ubiquitinates the NF-κB co-factor Akirin to activate an effective immune response in Drosophila

Activation of inflammatory response is a tightly controlled process that is mediated by NF-kB pathways. Ubiquitinations are known to modulate the mammalian inflammatory response at all levels of the NF-kB pathways, but the mechanism involved in this ubiquitin-mediated modulation of the inflammation is still not fully understood. Here, we used Drosophila genetics to identify E3-ubiquitin ligases involved in the innate immune response and explore their conservation in mammals. To this aim, we conducted an ex-vivo RNA interference screen of the E3 ubiquitin-ligases encoded by the fruit fly genome downstream of the Drosophila IMD pathway. This screen identified Hyperplastic Discs Hyd as acting genetically at the level of the NF-kB cofactor Akirin. Drosophila lacking Hyd failed to express the full set of anti-microbial peptides coding genes and succumbed to immune challenges. Remarkably, HyD-mediated K63-polyubiquitination of Akirin is required for the efficient binding of Akirin to the NF-kB transcription factor Relish. This Hyd-mediated interaction triggers the activation of a subset of Relish target genes, required in-vivo to survive immune challenges. We show further that Urb5, the mammalian homolog of Hyd, is also required in the NF-kB pathway. This study links the action of a E3-ubiquitin ligase to the activation of immune effector genes, deepening our understanding of the involvement of ubiquitination in inflammation and identifying a potential target for the control of inflammatory diseases.