An immune-responsive serpin, SRPN6, mediates mosquito defense against malaria parasites

doi:10.1073/pnas.0508335102

An immune-responsive serpin, SRPN6, mediates mosquito defense against malaria parasites

^*Department of Molecular Microbiology and Immunology, Malaria Research Institute, Johns Hopkins Bloomberg School of Public Health, 615 North Wolfe Street, Baltimore, MD 21205; ^‡European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany; ^§Department of Biological Sciences, Imperial College London, Sir Alexander Fleming Building, South Kensington Campus, London SW7 2AZ, United Kingdom; and ^¶Department of Pathology, University of Texas Medical Branch, Galveston, TX 77550

Contributed by Fotis C. Kafatos, September 23, 2005

Abstract

We have functionally analyzed the orthologous SRPN6 genes from Anopheles stephensi and Anopheles gambiae using phylogenetic, molecular, reverse genetic, and cell biological tools. The results strongly implicate SRPN6 in the innate immune response against Plasmodium. This gene belongs to a mosquito-specific gene cluster including three additional Anopheles serpins. SRPN6 expression is induced by Escherichia coli and both rodent and human malaria parasites. The gene is specifically expressed in midgut cells invaded by Plasmodium ookinetes and in circulating and attached hemocytes. Knockdown of SRPN6 expression by RNA interference in susceptible An. stephensi leads to substantially increased parasite numbers, whereas depletion in susceptible An. gambiae delays progression of parasite lysis without affecting the number of developing parasites. However, the An. gambiae SRPN6 knockdown increases the number of melanized parasites in the L3-5 refractory strain and in susceptible G3 mosquitoes depleted of CTL4. These results indicate that AsSRPN6 is involved in the parasite-killing process, whereas AgSRPN6 acts on parasite clearance by inhibiting melanization and/or promoting parasite lysis. We propose that these observed phenotypic differences are due to changed roles of the respective target serine proteases in the two mosquito species.

Footnotes

↵ ∥ To whom correspondence may be addressed. E-mail: k.michel{at}imperial.ac.uk, mlorena{at}jhsph.edu, or kafatos{at}embl-heidelberg.de.
↵ † E.G.A. and S.B.P. contributed equally to this work.
↵ ** M.J.-L. and K.M. contributed equally to this work.
Author contributions: S.H., F.C.K., M.J.-L., and K.M. designed research; E.G.A., S.B.P., A.G., D.L.V., and A.B. performed research; E.G.A., S.B.P., A.B., and S.H. analyzed data; and E.G.A., S.B.P., S.H., F.C.K., M.J.-L., and K.M. wrote the paper.
Conflict of interest statement: No conflicts declared.
Abbreviations: Ag, An. gambiae; As, An. stephensi; hpi, hours postinfection; KD, knockdown; ONN, o'nyong-nyong; qRT-PCR, quantitative RT-PCR.