Cyclic nucleotide signalling in innate immunity

Immune function in animals counters the possible effects of infection. In insects, an ancient form of immunity exists, which forms the template for human and animal immune systems. Studies conducted in the tiny fruit fly, Drosophila, using the insect analogue of the kidney (Malpighian tubule), have shown that this tissue itself can sense an immune attack, and can confer immune status on the whole animal. This suggests that peripheral tissue such as kidney may be more important than previously thought in immune surveillance for the whole body. How does the tubule mount an immune defense? We have evidence that a simple molecule, cyclic GMP, regulates gene expression of a key set of anti-microbial defence peptides in the tubule. This can affect the immune status of the entire animal. The studies described in the application will allow us to work out how cGMP can set the level of the immune state. As cGMP also exists and works in a similar way in humans as in the fly, it is possible that we may uncover new modulators of immunity by working out how such molecules work in Drosophila. This may have important consequences for both insect pest control strategies (many insects are major vectors of human disease) and also in the assessment of potential new therapeutic drugs, that act to control levels of these small molecules in various disease conditions.

Innate immunity is an ancient defence against microbial attack, which in insects is thought to originate mainly in the fat body. We have recently shown that the fluid-transporting Malpighian (renal) tubule of D. melanogaster is an autonomous immune-sensing tissue, which uses the nitric oxide (NO) signalling pathway (MeGettigan et al., Ins Biochem. Mol. Biol. 2005). More recently, we have identified a novel role for cyclic nucleotides in the immune response: our unpublished tubule microarray dataset, validated by Q-PCR, show that cGMP modulates the expression of antimicrobial peptides (AMPs). We have thus uncovered a novel and powerful role of tubules in immune sensing and animal survival via a mechanism that involves cGMP. We also show that differential regulation of AMPs occurs via both cGMP and cAMP. We aim to unravel the potential role(s) of cyclic nucleotides on epithelial immune gene expression; and the role of such modulation of immune function on the whole animal by: 1: fully characterising cGMP/cAMP modulation of the Imd and Toll pathways; 2: understanding the contribution of other significant immune tissue to the modulation of immune status by cGMP ; 3: delineation of a novel range of transcriptional targets for cGMP and 4. defining the role of downstream effectors of cGMP ie., cGMP-dependent protein kinases on immune status. Techniques used include: Q-PCR, survival studies, generation of transgenic flies; use of reporter gene assays both in cultured cells and in transgenic flies.