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, 15 (4), 617-26

A Serpin That Regulates Immune Melanization in the Respiratory System of Drosophila

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A Serpin That Regulates Immune Melanization in the Respiratory System of Drosophila

Huaping Tang et al. Dev Cell.

Abstract

Epithelial tissues facing the external environment are essential to combating microbial infection. In addition to providing a physical barrier, epithelial tissues mount chemical defenses to prevent invasion of internal tissues by pathogens. Here, we describe that the melanization reaction implicated in host defense is activated in the respiratory system, the trachea, of Drosophila. Tracheal melanization can be activated by the presence of microorganisms but is normally blocked by Spn77Ba, a protease inhibitor in the serpin family. Spn77Ba inhibits a protease cascade involving the MP1 and MP2 proteases that activates phenol oxidase, a key enzyme in melanin biosynthesis. Unexpectedly, we found that tracheal melanization resulting from Spn77Ba disruption induces systemic expression of the antifungal peptide Drosomycin via the Toll pathway. Such signaling between local and systemic immune responses could represent an alarm mechanism that prepares the host in case a pathogen breaches epithelial defenses to invade internal tissues.

Figures

Figure 1
Figure 1. Serpin Spn77Ba is Required to Prevent Tracheal Melanization
(A) Melanization of larval trachea resulted when Spn77Ba RNAi was broadly activated with the act-Gal4 driver and a UAS construct encoding an inverted repeat of Spn77Ba sequence. (B-D) Spn77Ba RNAi with tracheal-specific btl-Gal4 driver resulted in tracheal melanization in larvae (B), pupae (C), and adults (D). (E) Embryonic lethality caused by a spn77Ba null mutation was rescued by Spn77Ba expression resulting from leaky activity of the hs-Gal4 driver at 18°C. The larval survivors showed tracheal melanization.
Figure 2
Figure 2. Spn77Ba is Expressed by Tracheal Cells and is Localized at Apical Surface of Tracheal Epithelium Where Melanization is Initiated
(A) A Gal4 driver containing 2.2 kbp of DNA from 5’end of Spn77Ba gene was used to activate GFP expression from a UAS-GFP construct. GFP signal is seen in trachea and at lower levels in gut and epidermis. (B) When expressed by transfected S2 cells, HA-tagged Spn77Ba protein was detected mainly in the culture medium by Western blot using anti-HA antibody. (C-D) When Spn77Ba-GFP fusion protein was expressed under control of the da-Gal4 driver, GFP signal (C and D, green) was seen on apical surface of tracheal epithelium, toward luminal side of apical F-actin and co-localized with chitin. F-actin of tracheal cells was stained with Alexa Fluor 633–phalloidin (C’, red) and chitin was visualized with a rhodamine-conjugated chitin-binding probe (D’, red). N: nucleus. (E) Melanization (arrow) resulting from Spn77Ba RNAi can be initially detected between tracheal cuticle and epithelial cells. Cells of tracheal epithelium were visualized by expression of cytoplasmic GFP under control of the btl-Gal4 driver (E’). N: nucleus. (F) Non-melanized and melanized sections of trachea in btl>Spn77Ba-IR larva were compared for integrity of the basement membrane, which was visualized with a vkg-GFP reporter (Morin et al., 2001). F-actin staining with Alexa Fluor 633–phalloidin was used to visualize the epithelial cells.
Figure 3
Figure 3. Spn77Ba Inhibits the MP1/MP2 Protease Cascade that Activates Phenol Oxidase
(A-D) Phenol oxidase (PO) activity was assayed ex vivo using dissected larval trachea incubated in L-DOPA solution. Trachea from wild-type (wt) larva showed no detectable melanization (A), but trachea from btl>Spn77Ba-IR larva showed significant melanization, indicating a high level of PO activity (B). Trachea in which MP1 protease was over-expressed showed weak melanization (C), which was suppressed by simultaneously over-expressing Spn77Ba (D). (E) Spn77Ba regulates tracheal melanization by inhibiting the MP1/MP2 protease cascade. The percentage of adults of various genotypes showing tracheal melanization was measured. (F) Suppression of tracheal melanization rescues semi-lethality caused by Spn77Ba RNAi in trachea. Adult flies of various genotypes were counted within one day of eclosion. Siblings carrying the CyO balancer were regarded as 100% viable. Data represent the average of at least three independent experiments ± SDV.
Figure 4
Figure 4. Tracheal Melanization Induces Drs Expression
(A-F) Flies carrying a Drs-GFP construct, in which GFP is expressed under the Drs transcriptional control region, were used to assay Drs expression in larvae, pupae and adults with designated genotypes. Note the darkened regions in the two dorsal tracheal trunks where intense melanization excluded GFP signal (B). (G and G’) Drs expression was detected on the trachea that had melanization (arrow) but not the other trachea lacking visible melanization. (H) Quantitative RT-PCR was used to monitor the expression level of Drs in 1-2 day old adult males of various genotypes. Drs expression level in control flies having btl-Gal4 driver alone was defined as 1. (I) Quantitative RT-PCR was used to monitor the expression level of Drs in late third instar larvae of various genotypes. Drs expression level in wild-type larvae was defined as 1. Data represent average of at least three independent experiments ± SDV.
Figure 5
Figure 5. A Product of the Melanization Reaction Induces Drs Expression through the Toll Signaling Pathway
(A-B) Larvae and prepupae with the genotype of T80-Gal4, tubP-Gal80ts, UAS-Spn77Ba-IR, UAS-GFP were used to control the activation of Spn77Ba RNAi at different developmental times. At 18°C, the temperature-sensitive Gal80 prevents Gal4 from activating Spn77Ba RNAi, whereas at 29°C, Gal80 is inactivated, thereby allowing activation of Spn77Ba RNAi. Quantitative RT-PCR was used to monitor Spn77Ba (A) and Drs (B) expression levels. The gene expression levels of wild-type larvae or prepupae at the corresponding developmental stages were used as controls to define 100% and 1 for Spn77Ba and Drs, respectively, in samples 1 to 4. (C and C’) Melanized trachea from act-Gal4/Spn77Ba-IR larva or non-melanized trachea from wild-type larva was transplanted to a recipient fly carrying a Drs-GFP reporter. GFP signal was observed in most flies after transplantation, but the signal persisted for at least 120 hours in flies receiving Spn77Ba trachea while it began fading away 48 hours later in flies receiving wild-type trachea. Recipient flies are shown at 96 hours after transplantation, when 75% and 15% of flies (n>20) receiving Spn77 RNAi and wild-type trachea, respectively, were GFP positive. Wild-type trachea became melanized after transplantation. (D) Quantitative RT-PCR was used to monitor the expression level of Drs in 1-2 day old adult males. Drs expression level in male flies carrying the btl-Gal4 driver alone was defined as 1. The higher than control level of Drs seen in the experiment with the seml mutation may be largely due to the associated yellow (y) mutation, which results in an accumulation of dopachrome intermediates of the melanization reaction (Wittkopp et al., 2003). Data represent average of at least three independent experiments ± SDV.
Figure 6
Figure 6. Tracheal Melanization can be Induced by Microorganisms
(A) Larvae in which Spn77Ba RNAi was activated with btl-Gal4 were raised under 3 different conditions: sterile, normal, or + bacteria. The percentage of larvae and pupae showing no, mild, or severe melanization as seen in (B) was visually recorded. At least 200 larvae and pupae were counted for each condition. (B) Representative examples of pupae with no melanization, mild melanization, and severe melanization. Arrow indicates melanization. (C) Quantitative RT-PCR was used to monitor Drs expression level of prepupae showing either mild or severe melanization when cultured under sterile or non-sterile conditions. Data represent the average of at least three independent experiments ± SDV. (D-G) Tracheal PO activity was assayed ex vivo in the presence or absence of bacteria (see Experimental Procedures). A mixture of E. coli and M. luteus induced in wild-type trachea a high level of PO activity, which was suppressed by either Spn77Ba over-expression or MP1 RNAi. (H and H’) Exposure of wild-type larvae to B. bassiana spores induced melanization on cuticle (arrows) and Drs expression in both trachea and fat body. (I) Cuticle melanization induced by B. bassiana was partially suppressed by Spn77Ba over-expression or MP1 RNAi. The percentage of larvae that showed melanization 24 hours after infection was counted. (J) Drs expression induced by B. bassiana was partially suppressed by Spn77Ba over-expression or MP1 RNAi. The percentage of larvae that showed Drs-GFP expression 24 hours after infection was counted. *: p<0.05 (paired T-test).
Figure 7
Figure 7. Epithelial melanization and the induction of a systemic immune response
Spn77Ba inhibits a protease cascade involving the MP1 and MP2 proteases that activates phenol oxidase (PO), a key enzyme in melanin biosynthesis. Melanization is activated in extracellular space between cuticle lining the tracheal lumen and the apical surface of tracheal cells. The MP1/MP2 cascade also activates melanization in the hemolymph where Spn27A inhibits it. Tracheal melanization induces Drs expression in tracheal epithelium as well as in the fat body. The mechanism for signaling from trachea to fat body involves a yet-to-be-identified product of tracheal melanization that passes through basement membrane to hemolymph, where it activates the Persephone (Psh) protease involved in cleaving Spätzle to generate the ligand for the receptor Toll, which in turn initiates signaling leading to induction of Drs expression.

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