Transcription repressor-mediated control of engulfment receptor expression in Drosophila phagocytes

We previously reported that Drosophila phagocytes enhance their phagocytic activity after apoptotic cell engulfment accompanied by the activation of the transcription repressor Tailless and an increase in the levels of engulfment receptors. We herein investigated the underlying mechanisms. We found that Tailless phosphorylation levels decreased in Drosophila phagocytes following the stimulation with apoptotic cells. Anticipating the involvement of another transcription repressor, we examined the possible involvement of Krüppel, a bibliographically identified repressor whose expression is controlled by Tailless. The level of Krüppel in phagocytes decreased after the stimulation in a Tailless-dependent manner. The RNAi knockdown of Krüppel abrogated increases in the levels of engulfment receptors and phagocytic activity in stimulated phagocytes. The binding of Krüppel to the 5'-upstream regions of genes coding for engulfment receptors was demonstrated. These results suggest the following pathway: Tailless is activated by de-phosphorylation; Krüppel expression is inhibited by Tailless; the transcription of engulfment receptors-encoding genes is augmented due to a decrease of inhibition by Krüppel; and finally phagocytic activity is enhanced.