Adenosine deaminase (ADA) is an enzyme present in all organisms that catalyzes the irreversible deamination of adenosine and deoxyadenosine to inosine and deoxyinosine. Both adenosine and deoxyadenosine are biologically active purines that can have a deep impact on cellular physiology; notably, ADA deficiency in humans causes severe combined immunodeficiency. We have established a Drosophila model to study the effects of altered adenosine levels in vivo by genetic elimination of adenosine deaminase-related growth factor-A (ADGF-A), which has ADA activity and is expressed in the gut and hematopoietic organ. Here we show that the hemocytes (blood cells) are the main regulator of adenosine in the Drosophila larva, as was speculated previously for mammals. The elevated level of adenosine in the hemolymph due to lack of ADGF-A leads to apparently inconsistent phenotypic effects: precocious metamorphic changes including differentiation of macrophage-like cells and fat body disintegration on one hand, and delay of development with block of pupariation on the other. The block of pupariation appears to involve signaling through the adenosine receptor (AdoR), but fat body disintegration, which is promoted by action of the hemocytes, seems to be independent of the AdoR. The existence of such an independent mechanism has also been suggested in mammals.

Heterorhabditis bacteriophora is an entomopathogenic nematode (EPN) which infects its host by accessing the hemolymph where it releases endosymbiotic bacteria of the species Photorhabdus luminescens. We performed a genome-wide transcriptional analysis of the Drosophila response to EPN infection at the time point at which the nematodes reached the hemolymph either via the cuticle or the gut and the bacteria had started to multiply. Many of the most strongly induced genes have been implicated in immune responses in other infection models. Mapping of the complete set of differentially regulated genes showed the hallmarks of a wound response, but also identified a large fraction of EPN-specific transcripts. Several genes identified by transcriptome profiling or their homologues play protective roles during nematode infections. Genes that positively contribute to controlling nematobacterial infections encode: a homolog of thioester-containing complement protein 3, a basement membrane component (glutactin), a recognition protein (GNBP-like 3) and possibly several small peptides. Of note is that several of these genes have not previously been implicated in immune responses.

Insect silk is made up of structural fibrous (fibroins) and sticky (sericins) proteins, and contains a few small peptides of hitherto unknown functions. We demonstrate that two of these peptides inhibit bacterial and fungal proteinases (subtilisin, proteinase K and pronase). These ‘silk proteinase inhibitors’ 1 and 2 (SPI 1 and 2) are produced in the middle section of the silk‐secreting glands prior to cocoon spinning and their production is controlled at transcription level. The full length cDNA of pre‐SPI 1 contains 443 nucleotides and encodes a peptide of 76 amino‐acid residues, of which 20 make up a signal sequence. The mature SPI 1 (6056.7 Da, 56 residues) is a typical thermostable Kunitz‐type proteinase inhibitor with Arg in P1 position. The cDNA of pre‐SPI 2 consists of 260 nucleotides and yields a putative secretory peptide of 58 amino‐acid residues. The functional SPI 2 (3993 Da, 36 residues) is a single‐domain Kazal‐type proteinase inhibitor with unique structural features: free segment of the N‐terminus is reduced to a single amino‐acid residue, lack of CysI and CysV precludes formation of the A‐ring and provides increased flexibility to the C‐ring, and absence of several residues around the normal position of CysV shortens and changes the α helix segment of the protein. The structure reveals that the length and arrangement of the B‐ring, including exposure of the P1 residue, and the position of the C‐terminus relative to the B‐loop, are essential for the activity of the Kazal‐type inhibitors.

We describe a protein family in Drosophila containing six adenosine deaminase-related growth factors (ADGFs), which are homologous to a mitogenic growth factor discovered in conditioned medium from cells of a different fly species, Sarcophaga . Closely related proteins have been identified in other animals, and a human homolog is implicated in the genetic disease Cat-Eye Syndrome. The two most abundantly expressed ADGFs in Drosophila larvae are ADGF-A, which is strongly expressed in the gut and lymph glands, and ADGF-D, which is mainly expressed in the fat body and brain. Recombinant ADGF-A and ADGF-D are active adenosine deaminases (ADAs), and they cause polarization and serum-independent proliferation of imaginal disk and embryonic cells in vitro . The enzymatic activity of these proteins is required for their mitogenic function, making them unique among growth factors. A culture medium prepared without adenosine, or depleted of adenosine by using bovine ADA, also stimulates proliferation of imaginal disk cells, and addition of adenosine to this medium inhibits proliferation. Thus ADGFs secreted in vivo may control tissue growth by modulating the level of extracellular adenosine.