Non-canonical transcriptional regulation of heme oxygenase in Aedes aegypti.

Heme oxygenase (HO) is a ubiquitous enzyme responsible for heme breakdown, which yields carbon monoxide (CO), biliverdin (BV) and ferrous ion. Here we show that the Aedes aegypti heme oxygenase gene (AeHO - AAEL008136) is expressed in different developmental stages and tissues. AeHO expression increases after a blood meal in the midgut, and its maximal transcription levels overlaps with the maximal rate of the further modified A. aegypti biglutaminyl-biliverdin (AeBV) pigment production. HO is a classical component of stress response in eukaryotic cells, being activated under oxidative stress or increased heme levels. Indeed, the final product of HO activity in the mosquito midgut, AeBV, exerts a protective antioxidant activity. AeHO, however, does not seem to be under a classical redox-sensitive transcriptional regulation, being unresponsive to heme itself, and even down regulated when insects face a pro-oxidant insult. In contrast, AeHO gene expression responds to nutrient sensing mechanisms, through the target of rapamycin (TOR) pathway. This unusual transcriptional control of AeHO, together with the antioxidant properties of AeBV, suggests that heme degradation by HO, in addition to its important role in protection of Aedes aegypti against heme exposure, also acts as a digestive feature, being an essential adaptation to blood feeding.

Aedes aegypti ferritin heavy chain homologue: feeding of iron or blood influences message levels, lengths and subunit abundance.

Secreted ferritin in the mosquito, Aedes aegypti, has several subunits that are the products of at least two genes, one encoding a homologue of the vertebrate heavy chain (HCH) and the other the light chain homologue (LCH). Here we report the developmental and organ specific pattern of expression of the ferritin HCH messages and of both subunit types in control sugar-fed mosquitoes, in those exposed to high levels of dietary iron, and after blood feeding. When Northern blots were probed with a HCH cDNA, two bands were observed, representing at least two messages of different sizes that result from the choice of two different polyadenylation sites. Either raising mosquito larvae in an iron-enriched medium, or blood feeding adult female mosquitoes resulted in a marked increase in the HCH message level, particularly of the shorter message. Changes in the amount and length of messages and amount of ferritin subunits were studied over the life span of the mosquito and in different organs of female mosquitoes after blood feeding. The midgut of blood-fed insects is the main site of increased ferritin message synthesis. Ferritin protein levels also increase in midgut, fat body and hemolymph after blood feeding. Ferritin messages and subunits are synthesized in the ovaries and ferritin is found in the eggs. These observations are discussed in terms of translational and transcriptional control of ferritin synthesis and are compared to similar events in the regulation of Drosophila melanogaster ferritin.

Characterization of mitochondrial ferritin in Drosophila.

Mitochondrial function depends on iron-containing enzymes and proteins, whose maturation requires available iron for biosynthesis of iron-sulfur clusters and heme. Little is known about how mitochondrial iron homeostasis is maintained, although the recent discovery of a mitochondrial ferritin in mammals and plants has uncovered a potential key player in the process. Here, we show that Drosophila melanogaster expresses mitochondrial ferritin from an intron-containing gene. It has high similarity to the mouse and human mitochondrial ferritin sequences and, as in mammals, is expressed mainly in testis. This ferritin contains a putative mitochondrial targeting sequence and an epitope-tagged version localizes to mitochondria in transfected cells. Overexpression of mitochondrial ferritin fails to alter both total-body iron levels and iron that is bound to secretory ferritins. However, the viability of iron-deficient flies is compromised by overexpression of mitochondrial ferritin, suggesting that it may sequester iron at the expense of other important cellular functions. The conservation of mitochondrial ferritin in an insect species underscores the importance of this iron-storage molecule.

A heme-degradation pathway in a blood-sucking insect.

Hematophagous insects are vectors of diseases that affect hundreds of millions of people worldwide. A common physiological event in the life of these insects is the hydrolysis of host hemoglobin in the digestive tract, leading to a massive release of heme, a known prooxidant molecule. Diverse organisms, from bacteria to plants, express the enzyme heme oxygenase, which catalyzes the oxidative degradation of heme to biliverdin (BV) IX, CO, and iron. Here, we show that the kissing bug Rhodnius prolixus, a vector of Chagas' disease, has a unique heme-degradation pathway wherein heme is first modified by addition of two cysteinylglycine residues before cleavage of the porphyrin ring, followed by trimming of the dipeptides. Furthermore, in contrast to most known heme oxygenases, which generate BV IXalpha, in this insect, the end product of heme detoxification is a dicysteinyl-BV IXgamma. Based on these results, we propose a heme metabolizing pathway that includes the identified intermediates produced during modification and cleavage of the heme porphyrin ring.