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Sustained activation of Akt elicits mitochondrial dysfunction to block Plasmodium falciparum infection in the mosquito host.

Luckhart S, Giulivi C, Drexler AL, Antonova-Koch Y, Sakaguchi D, Napoli E, Wong S, Price MS, Eigenheer R, Phinney BS, Pakpour N, Pietri JE, Cheung K, Georgis M, Riehle M - PLoS Pathog. (2013)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Medical Microbiology and Immunology, School of Medicine, University of California, Davis, Davis, California, United States of America. sluckhart@ucdavis.edu

ABSTRACT
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.

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Atg6 and Atg8 expression levels were reduced in 18 d HM myrAkt relative to NTG An. stephensi.Midguts were dissected from 18 d HM myrAkt and NTG An. stephensi for RNA isolation and quantitative RT-PCR as described in the Methods. The analyses were performed on midgut RNAs from three independent cohorts of An. stephensi. Each data point represents Atg6 or Atg8 expression from one of three biological replicate samples; values were normalized to NTG levels (indicated as 1.0). Means are indicated as bars for each treatment. Data were analyzed by paired Student's t-test (alpha = 0.05) and P values are noted on the graph.
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ppat-1003180-g004: Atg6 and Atg8 expression levels were reduced in 18 d HM myrAkt relative to NTG An. stephensi.Midguts were dissected from 18 d HM myrAkt and NTG An. stephensi for RNA isolation and quantitative RT-PCR as described in the Methods. The analyses were performed on midgut RNAs from three independent cohorts of An. stephensi. Each data point represents Atg6 or Atg8 expression from one of three biological replicate samples; values were normalized to NTG levels (indicated as 1.0). Means are indicated as bars for each treatment. Data were analyzed by paired Student's t-test (alpha = 0.05) and P values are noted on the graph.

Mentions: Although autophagy is required for normal mitochondrial turnover, the accumulation of inclusions suggested (i) stalled autophagy in HT and HM relative to NTG mosquitoes, (ii) normal autophagy overwhelmed by the high increase in damaged mitochondria, and (iii) over-reactive autophagy that could perhaps eliminate normal as well as dysfunctional mitochondria. In support of the first hypothesis, altered Atg6 and Atg8 mRNA expression levels were observed in the midgut of 18 d HM An. stephensi (Fig. 4). Atg6, also known as Beclin-1, is required for the generation of pre-autophagosome structures [58], while Atg8-phosphoethanolamine conjugates and the Atg5–Atg12 complex are essential components of the autophagosomal membrane [59]. In 18 d HM females, expression of Atg8 was significantly reduced relative to NTG females (Fig. 4), while expression of Atg6 showed a similar, non-significant trend, suggesting that autophagosome maturation is decreased in 18 d HM females.


Sustained activation of Akt elicits mitochondrial dysfunction to block Plasmodium falciparum infection in the mosquito host.

Luckhart S, Giulivi C, Drexler AL, Antonova-Koch Y, Sakaguchi D, Napoli E, Wong S, Price MS, Eigenheer R, Phinney BS, Pakpour N, Pietri JE, Cheung K, Georgis M, Riehle M - PLoS Pathog. (2013)

Atg6 and Atg8 expression levels were reduced in 18 d HM myrAkt relative to NTG An. stephensi.Midguts were dissected from 18 d HM myrAkt and NTG An. stephensi for RNA isolation and quantitative RT-PCR as described in the Methods. The analyses were performed on midgut RNAs from three independent cohorts of An. stephensi. Each data point represents Atg6 or Atg8 expression from one of three biological replicate samples; values were normalized to NTG levels (indicated as 1.0). Means are indicated as bars for each treatment. Data were analyzed by paired Student's t-test (alpha = 0.05) and P values are noted on the graph.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3585164&req=5

ppat-1003180-g004: Atg6 and Atg8 expression levels were reduced in 18 d HM myrAkt relative to NTG An. stephensi.Midguts were dissected from 18 d HM myrAkt and NTG An. stephensi for RNA isolation and quantitative RT-PCR as described in the Methods. The analyses were performed on midgut RNAs from three independent cohorts of An. stephensi. Each data point represents Atg6 or Atg8 expression from one of three biological replicate samples; values were normalized to NTG levels (indicated as 1.0). Means are indicated as bars for each treatment. Data were analyzed by paired Student's t-test (alpha = 0.05) and P values are noted on the graph.
Mentions: Although autophagy is required for normal mitochondrial turnover, the accumulation of inclusions suggested (i) stalled autophagy in HT and HM relative to NTG mosquitoes, (ii) normal autophagy overwhelmed by the high increase in damaged mitochondria, and (iii) over-reactive autophagy that could perhaps eliminate normal as well as dysfunctional mitochondria. In support of the first hypothesis, altered Atg6 and Atg8 mRNA expression levels were observed in the midgut of 18 d HM An. stephensi (Fig. 4). Atg6, also known as Beclin-1, is required for the generation of pre-autophagosome structures [58], while Atg8-phosphoethanolamine conjugates and the Atg5–Atg12 complex are essential components of the autophagosomal membrane [59]. In 18 d HM females, expression of Atg8 was significantly reduced relative to NTG females (Fig. 4), while expression of Atg6 showed a similar, non-significant trend, suggesting that autophagosome maturation is decreased in 18 d HM females.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Medical Microbiology and Immunology, School of Medicine, University of California, Davis, Davis, California, United States of America. sluckhart@ucdavis.edu

ABSTRACT
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.

Show MeSH
Related in: MedlinePlus