Voltage-gated ion channels are expressed in the Malpighian tubules and anal papillae of the yellow fever mosquito (Aedes aegypti), and may regulate ion transport during salt and water imbalance

Vectors of infectious disease include several species of Aedes mosquitoes. The life cycle of Aedes aegypti, the yellow fever mosquito, consists of a terrestrial adult and an aquatic larval life stage. Developing in coastal waters can expose larvae to fluctuating salinity, causing salt and water imbalance, which is addressed by two prime osmoregulatory organs - the Malpighian tubules (MTs) and anal papillae (AP). Voltage-gated ion channels (VGICs) have recently been implicated in the regulation of ion transport in the osmoregulatory epithelia of insects. In the current study, we: (i) generated MT transcriptomes of freshwater-acclimated and brackish water-exposed larvae of Ae. aegypti, (ii) detected expression of several voltage-gated Ca2+, K+, Na+ and non-ion-selective ion channels in the MTs and AP using transcriptomics, PCR and gel electrophoresis, (iii) demonstrated that mRNA abundance of many altered significantly following brackish water exposure, and (iv) immunolocalized CaV1, NALCN, TRP/Painless and KCNH8 in the MTs and AP of larvae using custom-made antibodies. We found CaV1 to be expressed in the apical membrane of MTs of both larvae and adults, and its inhibition to alter membrane potentials of this osmoregulatory epithelium. Our data demonstrate that multiple VGICs are expressed in osmoregulatory epithelia of Ae. aegypti and may play an important role in the autonomous regulation of ion transport.

Sestrins induce natural killer function in senescent-like CD8+ T cells

Aging is associated with remodeling of the immune system to enable the maintenance of life-long immunity. In the CD8+ T cell compartment, aging results in the expansion of highly differentiated cells that exhibit characteristics of cellular senescence. Here we found that CD27-CD28-CD8+ T cells lost the signaling activity of the T cell antigen receptor (TCR) and expressed a protein complex containing the agonistic natural killer (NK) receptor NKG2D and the NK adaptor molecule DAP12, which promoted cytotoxicity against cells that expressed NKG2D ligands. Immunoprecipitation and imaging cytometry indicated that the NKG2D-DAP12 complex was associated with sestrin 2. The genetic inhibition of sestrin 2 resulted in decreased expression of NKG2D and DAP12 and restored TCR signaling in senescent-like CD27-CD28-CD8+ T cells. Therefore, during aging, sestrins induce the reprogramming of non-proliferative senescent-like CD27-CD28-CD8+ T cells to acquire a broad-spectrum, innate-like killing activity.

Immature particles and capsid-free viral RNA produced by Yellow fever virus-infected cells stimulate plasmacytoid dendritic cells to secrete interferons

Plasmacytoid dendritic cells (pDCs) are specialized in the production of interferons (IFNs) in response to viral infections. The Flaviviridae family comprises enveloped RNA viruses such as Hepatitis C virus (HCV) and Dengue virus (DENV). Cell-free flaviviridae virions poorly stimulate pDCs to produce IFN. By contrast, cells infected with HCV and DENV potently stimulate pDCs via short-range delivery of viral RNAs, which are either packaged within immature virions or secreted exosomes. We report that cells infected with Yellow fever virus (YFV), the prototypical flavivirus, stimulated pDCs to produce IFNs in a TLR7- and cell contact- dependent manner. Such stimulation was unaffected by the presence of YFV neutralizing antibodies. As reported for DENV, cells producing immature YFV particles were more potent at stimulating pDCs than cells releasing mature virions. Additionally, cells replicating a release-deficient YFV mutant or a YFV subgenomic RNA lacking structural protein-coding sequences participated in pDC stimulation. Thus, viral RNAs produced by YFV-infected cells reach pDCs via at least two mechanisms: within immature particles and as capsid-free RNAs. Our work highlights the ability of pDCs to respond to a variety of viral RNA-laden carriers generated from infected cells.

Ovary ecdysteroidogenic hormone functions independently of the insulin receptor in the yellow fever mosquito, Aedes aegypti

Most mosquito species must feed on the blood of a vertebrate host to produce eggs. In the yellow fever mosquito, Aedes aegypti, blood feeding triggers medial neurosecretory cells in the brain to release insulin-like peptides (ILPs) and ovary ecdysteroidogenic hormone (OEH). Theses hormones thereafter directly induce the ovaries to produce ecdysteroid hormone (ECD), which activates the synthesis of yolk proteins in the fat body for uptake by oocytes. ILP3 stimulates ECD production by binding to the mosquito insulin receptor (MIR). In contrast, little is known about the mode of action of OEH, which is a member of a neuropeptide family called neuroparsin. Here we report that OEH is the only neuroparsin family member present in the Ae. aegypti genome and that other mosquitoes also encode only one neuroparsin gene. Immunoblotting experiments suggested that the full-length form of the peptide, which we call long OEH (lOEH), is processed into short OEH (sOEH). The importance of processing, however, remained unclear because a recombinant form of lOEH (rlOEH) and synthetic sOEH exhibited very similar biological activity. A series of experiments indicated that neither rlOEH nor sOEH bound to ILP3 or the MIR. Signaling studies further showed that ILP3 activated the MIR but rlOEH did not, yet both neuropeptides activated Akt, which is a marker for insulin pathway signaling. Our results also indicated that activation of TOR signaling in the ovaries required co-stimulation by amino acids and either ILP3 or rlOEH. Overall, we conclude that OEH activates the insulin signaling pathway independently of the MIR, and that insulin and TOR signaling in the ovaries is coupled.

The Aquaporin gene family of the yellow fever mosquito, Aedes aegypti

Background

The mosquito, Aedes aegypti, is the principal vector of the Dengue and yellow fever viruses. During feeding, an adult female can take up more than its own body weight in vertebrate blood. After a blood meal females excrete large amounts of urine through their excretion system, the Malpighian tubules (MT). Diuresis starts within seconds after the mosquito starts feeding. Aquaporins (AQPs) are a family of membrane transporters that regulate the flow of water, glycerol and other small molecules across cellular membranes in both prokaryotic and eukaryotic cells. Our aim was to identify aquaporins that function as water channels, mediating transcellular water transport in MTs of adult female Ae. aegypti.

Methodology/Principal Findings

Using a bioinformatics approach we screened genome databases and identified six putative AQPs in the genome of Ae. aegypti. Phylogenetic analysis showed that five of the six Ae. aegypti AQPs have high similarity to classical water-transporting AQPs of vertebrates. Using microarray, reverse transcription and real time PCR analysis we found that all six AQPs are expressed in distinct patterns in mosquito tissues/body parts. AaAQP1, 4, and 5 are strongly expressed in the adult female MT. RNAi-mediated knockdown of the MT-expressed mosquito AQPs resulted in significantly reduced diuresis.

Conclusions/Significance

Our results support the notion that AQP1, 4, and 5 function as water transporters in the MTs of adult female Ae. aegypti mosquitoes. Our results demonstrate the importance of these AQPs for mosquito diuresis after blood ingestion and highlight their potential as targets for the development of novel vector control strategies.

Host-cell interaction of attenuated and wild-type strains of yellow fever virus can be differentiated at early stages of hepatocyte infection

Yellow fever (YF) virus is currently found in tropical Africa and South America, and is responsible for a febrile to severe illness characterized by organ failure and shock. The attenuated YF 17D strain, used in YF vaccine, was derived from the wild-type strain Asibi. Although studies have been done on genetic markers of YF virulence, differentiation of the two strains in terms of host-cell interaction during infection remains elusive. As YF wild-type strains are hepatotropic, we chose a hepatic cell line (HepG2) to study YF virus-host cell interaction. HepG2 cells rapidly produced high titres of infectious viral particles for 17D and Asibi YF strains. However, HepG2 cells were more susceptible to the attenuated 17D virus infection, and only this virus strain induced early apoptosis in these cells. Molecular markers specific for the 17D virus were identified by microarray analysis and confirmed by quantitative RT-PCR analysis. As early as 1h postinfection, three genes, (IEX-1, IRF-1, DEC-1) all implicated in apoptosis pathways, were upregulated. Later in infection (48 h) two other genes (HSP70-1A and 1B), expressed in cases of cellular stress, were highly upregulated in 17D-infected HepG2 cells. The early specific upregulation of these cellular genes in HepG2 cells may be considered markers of the 17D virus. This study on the YF attenuated strain gives a new approach to the analysis of the factors involved in virus attenuation.

Experimental yellow fever virus infection in the Golden Hamster (Mesocricetus auratus). I. Virologic, biochemical, and immunologic studies

This report describes the clinical laboratory findings in golden hamsters experimentally infected with yellow fever (YF) virus. An accompanying paper describes the pathologic findings. Following intraperitoneal inoculation of a virulent strain of YF virus, hamsters developed a high-titered viremia (up to 109/mL) lasting 5--6 days and abnormal liver function tests. YF hemagglutination-inhibiting antibodies appeared 4 or 5 days after infection, often while viremia was still present. The mortality rate in YF-infected hamsters was variable, depending on the virus strain and the age of the animals. Clinical and pathologic changes in the infected hamsters were very similar to those described in experimentally infected macaques and in fatal human cases of YF, which indicates that the golden hamster may be an excellent alternative animal model, in place of nonhuman primates, for research on the pathogenesis and treatment of YF and other viscerotropic flavivirus diseases.

Mosquito transferrin, an acute-phase protein that is up-regulated upon infection

When treated with heat-killed bacterial cells, mosquito cells in culture respond by up-regulating several proteins. Among these is a 66-kDa protein (p66) that is secreted from cells derived from both Aedes aegypti and Aedes albopictus. p66 was degraded by proteolysis and gave a virtually identical pattern of peptide products for each mosquito species. The sequence of one peptide (31 amino acids) was determined and found to have similarity to insect transferrins. By using conserved regions of insect transferrin sequences, degenerate oligonucleotide PCR primers were designed and used to isolate a cDNA clone encoding an A. aegypti transferrin. The encoded protein contained a signal sequence that, when cleaved, would yield a mature protein of 68 kDa. It contained the 31-amino acid peptide, and the 3' end exactly matched a cDNA encoding a polypeptide that is up-regulated when A. aegypti encapsulates filarial worms [Beerntsen, B. T., Severson, D. W. & Christensen, B. M. (1994) Exp. Parasitol. 79, 312-321]. This transferrin, like those of two other insect species, has conserved iron-binding residues in the N-terminal lobe but not in the C-terminal lobe, which also has large deletions in the polypeptide chain, compared with transferrins with functional C-terminal lobes. The hypothesis is developed that this transferrin plays a role similar to vertebrate lactoferrin in sequestering iron from invading organisms and that degradation of the structure of the C-terminal lobe might be a mechanism for evading pathogens that elaborate transferrin receptors to tap sequestered iron.

Changes in body fluid compartments, tissue water and electrolyte distribution, and lipid concentrations in rhesus macaques with yellow fever

Rhesus macaques were inoculated subcutaneously with 40 plaque-forming units of yellow fever (YF) virus. To identify pathophysiologic mechanisms of YF, rectal temperatures and evidence of viremia were observed daily; physiologic and biochemical changes were studied on postinoculation day (PID) 5. Marked viremia was detected on PID 2 through 5, and fever was first observed on PID 4. On PID 5, blood and plasma volumes and circulatory K+ values increased, whereas RBC volume, PCV, and plasma cholesterol concentration decreased. Total lipids (mainly triglycerides) accumulated in the liver of inoculated macaques; alterations in hepatic content of water, electrolytes, and trace metals were also observed. Certain parts of the CNS, skeletal muscle, skin, heart, diaphragm, and renal cortex were affected, with changes noticed in water, electrolyte, trace metal, and lipid concentrations. These tissue changes indicated that cellular metabolism was altered and that the transport mechanisms of cell membranes of certain tissues were modified by YF virus or the disease process caused by the virus. Terminal hypoglycemia (57.6 +/- 12.1 mg/dl) was observed. The YF-induced intracellular dehydration of the medulla oblongata at the later stage of illness may depress the cardiovascular and respiratory centers, thus contributing to death of rhesus macaques infected with YF virus.

Effect of diet on the metabolic response to infection: protein-sparing modified fast plus 100 grams glucose and yellow fever immunization

The metabolic response to yellow fever immunization was investigated in four obese subjects who were consuming a protein-sparing modified fast plus 100 g glucose (PSMF + 100 g) for 3 wk. Fasting, 1/2, 1, and 2 h postprandial values for insulin, glucagon, glucose, lactate, beta hydroxybutyrate, acetoacetate, and free fatty acids were assessed before and the 1st, 3rd, and 5th day postimmunization. The hormone and substrate response to dietary protein plus glucose was a substantial rise in insulin (p less than 0.005), glucose (p less than 0.005), and fall in free fatty acid and ketone levels (p less than 0.005). These results are distinctly different from the response to dietary protein ingestion only (PSMF) determined in a previous study which was a slight rise in insulin and glucose and slight fall in ketone bodies, but a substantial rise in glucagon. The only significant (p less than 0.05) change following immunization in the PSMF + 100 g was a rise in serum glucagon on day 1. During the PSMF significant elevations in insulin and lactate and falls in serum ketones were noted postimmunization. The metabolic response to mild infection is dependent on the diet consumed. These differences may have a clinical impact when the infection is severe, but this possibly requires study in an experimental infection of greater severity.

The metabolic response to yellow fever immunization: protein-sparing modified fast

The metabolic response to yellow fever immunization was investigated in five obese patients who were consuming a protein-sparing modified fast for 3 wk. Fasting 1/2, 1, and 2 h postprandial values for insulin, glucagon, glucose, lactate, beta hydroxybutyrate, acetoacetate, and free fatty acids were assessed before and the 1st, 3rd, and 5th day after immunization. The hormone and substrate responses to meat ingestion in patients well adapted to a protein-sparing modified fast and prior to infection was a rise in glucagon, no change in insulin, lactate, or ketone bodies and slight increase in serum free fatty acid and glucose levels. Over the entire period a significant (p less than 0.01) postprandial rise in glucagon was noted. Significant increases in insulin (p less than 0.01) and lactate (p less than 0.01) and falls in beta hydroxybutyrate (p less than 0.01) and acetoacetate (p less than 0.01) were noted in the postinfection period. There was also a significant interaction of diet and infection to increase insulin levels (p less than 0.05). The clinically mild infection produced by yellow fever immunization elicits a metabolic response which thus may be useful to investigate intermediary metabolism in the hospital setting.

Failure of yellow fever immunization to produce a catabolic response in individuals fully adapted to a protein-sparing modified fast

In previous studies, patients consuming a protein-sparing modified fast (PSMF) did not develop the expected increase in nitrogen excretion following incidental infectious illness. To assess the catabolic response with a controlled infection in such patients, 17-D yellow fever vaccine was administered to four young obese adults after 3 weeks of this hypocaloric regimen (PSMF), essentially carbohydrate-free and providing 1.5 g protein/kg ideal body weight. Average daily nitrogen excretion from the third week of the PSMF was compared to values from the week following immunization. Contrary to the experience with malnourished children being nutritionally rehabilitated or young adults fed an adequate calorie, low (0.1 g/kg body weight) protein diet, neither urinary nitrogen excretion nor nitrogen balance was significantly affected in the study group. In three of the four subjects in the group, average nitrogen balance was positive in the week pre-and post-yellow fever immunization. In individuals fully adapted to a PSMF the metabolic consequences of mild infection on nitrogen metabolism were reduced. This may be because the usual catabolic hormones, glucocorticoid and glucagon, which are elicited by infection fail to increase nitrogen excretion in fasting or whenever dietary carbohydrate is excluded.

Virus infections in infant mice causing persistent impairment of turnover of brain catecholamines

Newborn mice were inoculated with attenuated Coxsackie type B4 virus. Three-to-4-day old mice were infected with yellow fever virus vaccine. A number of mice survived the acute infections. Some of these demonstrated residual neurological symptoms, some showed recovery from symptoms while others survived the infection without revealing symptoms of disease. Determinations of dopamine, noradrenaline, 5-hydroxytryptamine, homovanillic acid and 5-hydroxyindoleacetic acid in the inoculated brains indicated an imparied turnover of neurotransmitters. Subnormal concentrations of catecholamines and homovanillic acid were encountered in the acutely-infected mice as well as among the survivors. Failure to synthesize catecholamines was observed not only in mice demonstrating symptoms of disease or in animals which recovered from their infection but also among a proportion of the mice which never demonstrated neurological symptoms. In contrast, 6-week-old Swiss albino mice infected with West Nile virus showed no effect on the turnover of brain monoamines either in acutely infected mice or in animals which survived the acute infection. Herpes simplex virus infection of 3-week-old mice induced during the acute infection an increased release of neurotransmitters. When these mice were "cured" of the infection by increasing the environmental temperature the elevated turnover of monoamine metabolism was normalized. Two months later there were no differences in concentrations of catecholamines or homovanillic acid between infected animals or uninfected controls. Thus, persistent impairment of brain monoamine metabolism was induced in mice infected when very young. The possible importance of the observations, in particular the findings of an impaired turnover after subclinical infection, is discussed.

Accelerated host metabolism of L-thyroxine during acute infection: role of the leukocyte and peripheral leukocytosis

Metabolism of thyroid hormones is accelerated during acute infection in man and in experimental animals. The pathogenetic mechanisms mediating this phenomenon are uncertain, but activated leukocytes of the infected host have been implicated as potentially important sites of hormone degradation. The present studies were conducted in an attempt to assess the in vivo contribution of leukocytes and peripheral leukocytosis to the enhancement of L-thyroxine (T4) clearance seen during infection, and to evaluate further the possible roles of fever and of changes in the extracellular binding of T4. In 10 rhesus monkeys inoculated with virulent yellow fever (YF) virus, peripheral disposal of T4 was significantly accelerated (2-fold) during the febrile phase of the illness. This experimental viral infection was not accompanied by neutrophilic leukocytosis nor by detectable changes in serum free T4 levels, suggesting that neither an increased circulating neutrophil mass nor diminished extracellular binding of T4 contributed appreciably to the increase in metabolism of T4. A pathogenetic role for fever in the enhancement of T4 degradation was not specifically excluded in these infected monkeys. However, the failure of T4 turnover to increase during other febrile infections, such as that which followed inoculation of monkeys with Venezuelan equine encephalitis virus, indicates that the acceleration of peripheral disposal of T4 seen in infection is not a simple concomitant of fever. In monkeys with bacterial sepsis and in those inoculated iv with zymosan particles, T4 turnover was similarly accelerated in the absence of detectable changes in serum free T4 levels, suggesting a pathogenetic role for enhances cellular uptake and metabolism of hormone. However, in these monkeys deiodination of T4 by leukocytes did not appear to account for the increase in T4 disposal. During sepsis and following zymosan administration, T4 turnover was markedly increased in both intact monkeys with a neutrophilic leukocytosis and in those with irradiation-induced neutropenia. Therefore, the cellular sites of increased T4 degradation during infection remain uncertain. Fixed tissue macrophages serve as the major site of clearance of YF virus, circulating bacteria and zymosan particles. Accordingly, a relationship between activation of these phagocytic cells and the acceleration of T4 metabolism seemed possible but was not established by our studies.

A clinicopathological study of human yellow fever

During an epidemic of yellow fever in the Jos Plateau area of Nigeria, 9 adult males with clinically diagnosed yellow fever were studied by haematological, biochemical, virological, serological, and liver biopsy methods. The ages of the patients ranged from 20 to 55 years and the duration of illness was 3-62 days. No virus was isolated from any patient but all patients should biochemical evidence of severe hepatocellular damage. Leucopenia was a feature of the late acute stage of the disease. Five sera had antibodies to yellow fever at titres greater than 1: 32, 3 of them being monospecific for yellow fever. The classical histological features of yellow fever were present only in the acute or late acute stages, when complement-fixation tests may be negative. With convalescence and the production of complement-fixing antibodies in high titres, the histological features resembled those of a persisting nonspecific hepatitis. In an endemic area, the histological features of yellow fever will depend on the stage of the disease and a picture of nonspecific hepatitis would not exclude yellow fever in the absence of confirmation from serological tests.