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. Author manuscript; available in PMC: 2009 Oct 1.
Published in final edited form as: Dev Cell. 2008 Oct;15(4):617–626. doi: 10.1016/j.devcel.2008.08.017

Figure 7. Epithelial melanization and the induction of a systemic immune response.

Figure 7

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.