The iron transporter Transferrin 1 mediates homeostasis of the endosymbiotic relationship between Drosophila melanogaster and Spiroplasma poulsonii

Endosymbionts as hidden players in tripartite pathosystem of interactions and potential candidates for sustainable viral disease management

The convoluted relationships between plants, viruses, and arthropod vectors housing bacterial endosymbionts are pivotal in the spread of harmful plant viral diseases. Endosymbionts play key roles in: manipulating host responses, influencing insect resistance to pesticides, shaping insect evolution, and bolstering virus acquisition, retention, and transmission. This interplay presents an innovative approach for developing sustainable strategies to manage plant diseases. Recent progress in targeting specific endosymbionts through genetic modifications, biotechnological advancements, and RNA interference shows potential for curbing viral spread and disease progression. Additionally, employing synthetic biology techniques like CRISPR/Cas9 to engineer endosymbionts and disrupt crucial interactions necessary for viral transmission in arthropod vectors holds promise for effective control measures. In this review, these obligate and facultative bacterial cruxes have been discussed to elaborate on their mechanistic involvement in the regulation and/or inhibition of tripartite pathways of interactions. Furthermore, we provide an in-depth understanding of endosymbionts' synergistic and antagonistic effects on: insect biology, plant immunity, and virus acquisition and transmission. Finally, we point out open questions for future research and provide research directions concerning the deployment of genetically engineered symbionts to affect plant-virus-vector interactions for sustainable disease management. By addressing existing knowledge gaps and charting future research paths, a deeper comprehension of the role of endosymbionts in plant-virus-vector interactions can pave the way for innovative and successful disease management strategies. The exploration of antiviral therapies, paratransgenesis, and pathogen-blocking tactics using engineered endosymbionts introduces pioneering solutions for lessening the impact of plant viral diseases and green pest management.

In situ detection of ferric reductase activity in the intestinal lumen of an insect

The rise of atmospheric oxygen as a result of photosynthesis in cyanobacteria and chloroplasts has transformed most environmental iron into the ferric state. In contrast, cells within organisms maintain a reducing internal milieu and utilize predominantly ferrous iron. Ferric reductases are enzymes that transfer electrons to ferric ions, either extracellularly or within endocytic vesicles, enabling cellular ferrous iron uptake through Divalent Metal Transporter 1. In mammals, duodenal cytochrome b is a ferric reductase of the intestinal epithelium, but how insects reduce and absorb dietary iron remains unknown. Here we provide indirect evidence of extracellular ferric reductase activity in a small subset of Drosophila melanogaster intestinal epithelial cells, positioned at the neck of the midgut's anterior region. Dietary-supplemented bathophenanthroline sulphate (BPS) captures locally generated ferrous iron and precipitates into pink granules, whose chemical identity was probed combining in situ X-ray absorption near edge structure and electron paramagnetic resonance spectroscopies. An increased presence of manganese ions upon BPS feeding was also found. Control animals were fed with ferric ammonium citrate, which is accumulated into ferritin iron in distinct intestinal subregions suggesting iron trafficking between different cells inside the animal. Spectroscopic signals from the biological samples were compared to purified Drosophila and horse spleen ferritin and to chemically synthesized BPS-iron and BPS-manganese complexes. The results corroborated the presence of BPS-iron in a newly identified ferric iron reductase region of the intestine, which we propose constitutes the major site of iron absorption in this organism.

The endosymbiont Spiroplasma poulsonii increases Drosophila melanogaster resistance to pathogens by enhancing iron sequestration and melanization

Facultative endosymbiotic bacteria, such as Wolbachia and Spiroplasma species, are commonly found in association with insects and can dramatically alter their host physiology. Many endosymbionts are defensive and protect their hosts against parasites or pathogens. Despite the widespread nature of defensive insect symbioses and their importance for the ecology and evolution of insects, the mechanisms of symbiont-mediated host protection remain poorly characterized. Here, we utilized the fruit fly Drosophila melanogaster and its facultative endosymbiont Spiroplasma poulsonii to characterize the mechanisms underlying symbiont-mediated host protection against bacterial and fungal pathogens. Our results indicate a variable effect of S. poulsonii on infection outcome, with endosymbiont-harboring flies being more resistant to Rhyzopus oryzae, Staphylococcus aureus, and Providencia alcalifaciens but more sensitive or as sensitive as endosymbiont-free flies to the infections with Pseudomonas species. Further focusing on the protective effect, we identified Transferrin-mediated iron sequestration induced by Spiroplasma as being crucial for the defense against R. oryzae and P. alcalifaciens. In the case of S. aureus, enhanced melanization in Spiroplasma-harboring flies plays a major role in protection. Both iron sequestration and melanization induced by Spiroplasma require the host immune sensor protease Persephone, suggesting a role of proteases secreted by the symbiont in the activation of host defense reactions. Hence, our work reveals a broader defensive range of Spiroplasma than previously appreciated and adds nutritional immunity and melanization to the defensive arsenal of symbionts.

IMPORTANCE

Defensive endosymbiotic bacteria conferring protection to their hosts against parasites and pathogens are widespread in insect populations. However, the mechanisms by which most symbionts confer protection are not fully understood. Here, we studied the mechanisms of protection against bacterial and fungal pathogens mediated by the Drosophila melanogaster endosymbiont Spiroplasma poulsonii. We demonstrate that besides the previously described protection against wasps and nematodes, Spiroplasma also confers increased resistance to pathogenic bacteria and fungi. We identified Spiroplasma-induced iron sequestration and melanization as key defense mechanisms. Our work broadens the known defense spectrum of Spiroplasma and reveals a previously unappreciated role of melanization and iron sequestration in endosymbiont-mediated host protection. We propose that the mechanisms we have identified here may be of broader significance and could apply to other endosymbionts, particularly to Wolbachia, and potentially explain their protective properties.

Two ferrous iron transporter-like proteins independently participate in asexual development under iron limitation and virulence in Beauveria bassiana

Reductive assimilation pathway involves ferric reductase and ferrous iron transporter, which is integral for fungal iron acquisition. A family of ferric reductase-like proteins has been functionally characterized in the filamentous entomopathogenic fungus Beauveria bassiana. In this investigation, two ferrous iron transporter-like proteins (Ftr) were functionally annotated in B. bassiana. BbFtr1 and BbFtr2 displayed high similarity in structure and were associated with the plasma and nuclear membrane. Their losses had no negatively influence on fungal growth on various nutrients and development under the iron-replete condition. Single mutants of BbFTR1 and BbFTR2 displayed the iron-availability dependent developmental defects, and double mutant exhibited the significantly impaired developmental potential under the iron-limited conditions. In insect bioassay, the double mutant also showed the weaker virulence than either of two single disruption mutants. These results suggested that two ferrous iron transporter-like proteins function independently in fungal physiologies under the iron-deficient condition. Intriguingly, a bZIP transcription factor BbHapX was required for expression of BbFTR1 and BbFTR2 under iron-depleted conditions. This study enhances our understanding of the iron uptake system in the filamentous entomopathogenic fungi.

Coordination of host and endosymbiont gene expression governs endosymbiont growth and elimination in the cereal weevil Sitophilus spp

BACKGROUND

Insects living in nutritionally poor environments often establish long-term relationships with intracellular bacteria that supplement their diets and improve their adaptive and invasive powers. Even though these symbiotic associations have been extensively studied on physiological, ecological, and evolutionary levels, few studies have focused on the molecular dialogue between host and endosymbionts to identify genes and pathways involved in endosymbiosis control and dynamics throughout host development.

RESULTS

We simultaneously analyzed host and endosymbiont gene expression during the life cycle of the cereal weevil Sitophilus oryzae, from larval stages to adults, with a particular emphasis on emerging adults where the endosymbiont Sodalis pierantonius experiences a contrasted growth-climax-elimination dynamics. We unraveled a constant arms race in which different biological functions are intertwined and coregulated across both partners. These include immunity, metabolism, metal control, apoptosis, and bacterial stress response.

CONCLUSIONS

The study of these tightly regulated functions, which are at the center of symbiotic regulations, provides evidence on how hosts and bacteria finely tune their gene expression and respond to different physiological challenges constrained by insect development in a nutritionally limited ecological niche. Video Abstract.

Phenotypic analyses, protein localization, and bacteriostatic activity of Drosophila melanogaster transferrin-1

Transferrin-1 (Tsf1) is an extracellular insect protein with a high affinity for iron. The functions of Tsf1 are still poorly understood; however, Drosophila melanogaster Tsf1 has been shown to influence iron distribution in the fly body and to protect flies against some infections. The goal of this study was to better understand the physiological functions of Tsf1 in D. melanogaster by 1) investigating Tsf1 null phenotypes, 2) determining tissue-specific localization of Tsf1, 3) measuring the concentration of Tsf1 in hemolymph, 4) testing Tsf1 for bacteriostatic activity, and 5) evaluating the effect of metal and paraquat treatments on Tsf1 abundance. Flies lacking Tsf1 had more iron than wild-type flies in specialized midgut cells that take up iron from the diet; however, the absence of Tsf1 had no effect on the iron content of whole midguts, fat body, hemolymph, or heads. Thus, as previous studies have suggested, Tsf1 appears to have a minor role in iron transport. Tsf1 was abundant in hemolymph from larvae (0.4 μM), pupae (1.4 μM), adult females (4.4 μM) and adult males (22 μM). Apo-Tsf1 at 1 μM had bacteriostatic activity whereas holo-Tsf1 did not, suggesting that Tsf1 can inhibit microbial growth by sequestering iron in hemolymph and other extracellular environments. This hypothesis was supported by detection of secreted Tsf1 in tracheae, testes and seminal vesicles. Colocalization of Tsf1 with an endosome marker in oocytes suggested that Tsf1 may provide iron to developing eggs; however, eggs from mothers lacking Tsf1 had the same amount of iron as control eggs, and they hatched at a wild-type rate. Thus, the primary function of Tsf1 uptake by oocytes may be to defend against infection rather than to provide eggs with iron. In beetles, Tsf1 plays a role in protection against oxidative stress. In contrast, we found that flies lacking Tsf1 had a typical life span and greater resistance to paraquat-induced oxidative stress. In addition, Tsf1 abundance remained unchanged in response to ingestion of iron, cadmium or paraquat or to injection of iron. These results suggest that Tsf1 has a limited role in protection against oxidative stress in D. melanogaster.

Disproportionate investment in Spiralin B production limits in-host growth and favors the vertical transmission of Spiroplasma insect endosymbionts

Insects frequently harbor endosymbionts, which are bacteria housed within host tissues. These associations are stably maintained over evolutionary timescales through vertical transmission of endosymbionts from host mothers to their offspring. Some endosymbionts manipulate host reproduction to facilitate spread within natural populations. Consequently, such infections have major impacts on insect physiology and evolution. However, technical hurdles have limited our understanding of the molecular mechanisms underlying such insect-endosymbiont interactions. Here, we investigate the nutritional interactions between endosymbiotic partners using the tractable insect Drosophila melanogaster and its natural endosymbiont Spiroplasma poulsonii. Using a combination of functional assays, metabolomics, and proteomics, we show that the abundance and amino acid composition of a single Spiroplasma membrane lectin, Spiralin B (SpiB), dictates the amino acid requirements of the endosymbiont and determines its proliferation within host tissues. Ectopically increasing SpiB levels in host tissues disrupts localization of endosymbionts in the fly egg chambers and decreases vertical transmission. We find that SpiB is likely to be required by the endosymbiont to enter host oocytes, which may explain the massive investment of S. poulsonii in SpiB synthesis. SpiB both permits vertical transmission of the symbiont and limits its growth in nutrient-limiting conditions for the host; therefore, a single protein plays a pivotal role in ensuring durability of the interaction in a variable environment.

The Role of Microbiota in Drosophila melanogaster Aging

Intestinal microbial communities participate in essential aspects of host biology, including nutrient acquisition, development, immunity, and metabolism. During host aging, dramatic shifts occur in the composition, abundance, and function of the gut microbiota. Although such changes in the microbiota are conserved across species, most studies remain descriptive and at most suggest a correlation between age-related pathology and particular microbes. Therefore, the causal role of the microbiota in host aging has remained a challenging question, in part due to the complexity of the mammalian intestinal microbiota, most of which is not cultivable or genetically amenable. Here, we summarize recent studies in the fruit fly Drosophila melanogaster that have substantially progressed our understanding at the mechanistic level of how gut microbes can modulate host aging.

The roles of metals in insect-microbe interactions and immunity

Metal ions play essential roles in diverse physiological processes in insects, including immunity and interactions with microbes. Some, like iron, are essential nutrients and therefore are the subject of a tug-of-war between insects and microbes. Recent findings showed that the hypoferremic response mediated by Transferrin 1 is an essential defense mechanism against pathogens in insects. Transferrin 1 and the overall iron metabolism were also implicated in mediating interactions between insects and beneficial microbes. Other metals, like copper and zinc, can interfere with insect immune effectors, and either enhance (antimicrobial peptides) or reduce (reactive oxygen species) their activity. By covering recent advances in the field, this review emphasizes the importance of metals as essential mediators of insect-microbe interactions.