Date Published: February 28, 2013
Publisher: Public Library of Science
Author(s): Shirley Luckhart, Cecilia Giulivi, Anna L. Drexler, Yevgeniya Antonova-Koch, Danielle Sakaguchi, Eleonora Napoli, Sarah Wong, Mark S. Price, Richard Eigenheer, Brett S. Phinney, Nazzy Pakpour, Jose E. Pietri, Kong Cheung, Martha Georgis, Michael Riehle, David S. Schneider.
The overexpression of activated, myristoylated Akt in the midgut of female transgenic Anopheles stephensi results in resistance to infection with the human malaria parasite Plasmodium falciparum but also decreased lifespan. In the present study, the understanding of mitochondria-dependent midgut homeostasis has been expanded to explain this apparent paradox in an insect of major medical importance. Given that Akt signaling is essential for cell growth and survival, we hypothesized that sustained Akt activation in the mosquito midgut would alter the balance of critical pathways that control mitochondrial dynamics to enhance parasite killing at some cost to survivorship. Toxic reactive oxygen and nitrogen species (RNOS) rise to high levels in the midgut after blood feeding, due to a combination of high NO production and a decline in FOXO-dependent antioxidants. Despite an apparent increase in mitochondrial biogenesis in young females (3 d), energy deficiencies were apparent as decreased oxidative phosphorylation and increased [AMP]/[ATP] ratios. In addition, mitochondrial mass was lower and accompanied by the presence of stalled autophagosomes in the posterior midgut, a critical site for blood digestion and stem cell-mediated epithelial maintenance and repair, and by functional degradation of the epithelial barrier. By 18 d, the age at which An. stephensi would transmit P. falciparum to human hosts, mitochondrial dysfunction coupled to Akt-mediated repression of autophagy/mitophagy was more evident and midgut epithelial structure was markedly compromised. Inhibition of RNOS by co-feeding of the nitric-oxide synthase inhibitor L-NAME at infection abrogated Akt-dependent killing of P. falciparum that begins within 18 h of infection in 3–5 d old mosquitoes. Hence, Akt-induced changes in mitochondrial dynamics perturb midgut homeostasis to enhance parasite resistance and decrease mosquito infective lifespan. Further, quality control of mitochondrial function in the midgut is necessary for the maintenance of midgut health as reflected in energy homeostasis and tissue repair and renewal.
Malaria is one of the greatest public health threats worldwide and is caused by infection with protozoan parasites of the genus Plasmodium that are transmitted by Anopheles mosquitoes. Shortly after an infective bloodmeal is consumed by the female mosquito, which can occur as early as 3 d of age, zygotes form and develop into motile ookinetes in the midgut lumen. Ookinetes must successfully traverse the midgut epithelium to form non-motile oocysts that grow and develop on the outside of the midgut for a minimum of 12 d. Within 14–16 d of ingesting a parasite-containing blood meal (or at 17–19 d post-emergence of the mosquito), oocyst-derived sporozoites invade the salivary glands to yield a mosquito that is infective to humans for the duration of her life. Despite this need for lengthy development, only a small percentage of mosquitoes under natural conditions live long enough to become fully infective –.
Overexpression of a constitutively active Akt targeted to the midgut of An. stephensi inhibited P. falciparum infection and reduced the duration of mosquito infectivity . Here, we have elucidated the mechanism for this process, demonstrating that Akt-dependent anti-parasite resistance is due to early, toxic effects of NO/RNOS followed by sustained mitochondrial dysfunction that cannot be mitigated via biogenesis and/or mitophagy/autophagy. Collectively, these phenomena lead to midgut epithelial damage (Figs. 2, S1; Table 1) and systemic energy deficiencies (as judged by ECP) that would be consistent with a reduction in lifespan and, as a consequence, a reduced infective lifespan. Activation of nuclear factor (NF)-kB-dependent immunity does not contribute to parasite resistance in myrAkt An. stephensi: Pakpour et al.  showed that activation of PI-3K-dependent signaling represses NF-KB activation in response to immune signals in An. stephensi cells in vitro and in vivo. Rather, overwhelming RNOS production with overexpression of myrAkt not only confers resistance to parasite infection, but also adversely impacts host infective lifespan. We would assert that balance and successful resolution of oxidative stress-induced mitophagy and mitochondrial biogenesis are the driving forces behind these phenotypes (Fig. 12) and that genetic manipulation of mitochondrial processes can provide a basis to alter multiple mosquito phenotypes to inhibit malaria parasite transmission.