A simple method for ex vivo honey bee cell culture capable of in vitro gene expression analysis

Deformed wing virus coopts the host arginine kinase to enhance its fitness in honey bees (Apis mellifera)

BACKGROUND

Deformed wing virus (DWV) is a major honey bee pathogen that is actively transmitted by the parasitic mite Varroa destructor and plays a primary role in Apis mellifera winter colony losses. Despite intense investigation on this pollinator, which has a unique environmental and economic importance, the mechanisms underlying the molecular interactions between DWV and honey bees are still poorly understood. Here, we report on a group of honey bee proteins, identified by mass spectrometry, that specifically co-immunoprecipitate with DWV virus particles.

RESULTS

Most of the proteins identified are involved in fundamental metabolic pathways. Among the co-immunoprecipitated proteins, one of the most interesting was arginine kinase (ArgK), a conserved protein playing multiple roles both in physiological and pathological processes and stress response in general. Here, we investigated in more detail the relationship between DWV and this protein. We found that argK RNA level positively correlates with DWV load in field-collected honey bee larvae and adults and significantly increases in adults upon DWV injection in controlled laboratory conditions, indicating that the argK gene was upregulated by DWV infection. Silencing argK gene expression in vitro, using RNAi, resulted in reduced DWV viral load, thus confirming that argK upregulation facilitates DWV infection, likely through interfering with the delicate balance between metabolism and immunity.

CONCLUSIONS

In summary, these data indicate that DWV modulates the host ArgK through transcriptional regulation and cooptation to enhance its fitness in honey bees. Our findings open novel perspectives on possible new therapies for DWV control by targeting specific host proteins.

Mechanisms of Pathogen and Pesticide Resistance in Honey Bees

Bees are the most important insect pollinators of the crops humans grow, and Apis mellifera, the Western honey bee, is the most commonly managed species for this purpose. In addition to providing agricultural services, the complex biology of honey bees has been the subject of scientific study since the 18th century, and the intricate behaviors of honey bees and ants, fellow hymenopterans, inspired much sociobiological inquest. Unfortunately, honey bees are constantly exposed to parasites, pathogens, and xenobiotics, all of which pose threats to their health. Despite our curiosity about and dependence on honey bees, defining the molecular mechanisms underlying their interactions with biotic and abiotic stressors has been challenging. The very aspects of their physiology and behavior that make them so important to agriculture also make them challenging to study, relative to canonical model organisms. However, because we rely on A. mellifera so much for pollination, we must continue our efforts to understand what ails them. Here, we review major advancements in our knowledge of honey bee physiology, focusing on immunity and detoxification, and highlight some challenges that remain.

A Novel Method for Primary Blood Cell Culturing and Selection in Drosophila melanogaster

The blood cells of the fruit fly Drosophila melanogaster show many similarities to their vertebrate counterparts, both in their functions and their differentiation. In the past decades, a wide palette of immunological and transgenic tools and methods have been developed to study hematopoiesis in the Drosophila larva. However, the in vivo observation of blood cells is technically restricted by the limited transparency of the body and the difficulty in keeping the organism alive during imaging. Here we describe an improved ex vivo culturing method that allows effective visualization and selection of live blood cells in primary cultures derived from Drosophila larvae. Our results show that cultured hemocytes accurately represent morphological and functional changes following immune challenges and in case of genetic alterations. Since cell culturing has hugely contributed to the understanding of the physiological properties of vertebrate blood cells, this method provides a versatile tool for studying Drosophila hemocyte differentiation and functions ex vivo.

Procedures and potential pitfalls for constructing a bee-infecting RNA virus clone

Viruses are factors that can fluctuate insect populations, including honey bees. Most honey bee infecting viruses are single positive-stranded RNA viruses that may not specifically infect honey bees and can be hazardous to other pollinator insects. In addition, these viruses could synergize with other stressors to worsen the honey bee population decline. To identify the underlying detailed mechanisms, reversed genetic studies with infectious cDNA clones of the viruses are necessary. Moreover, an infectious cDNA clone can be applied to studies as an ideal virus isolate that consists of a single virus species with a uniform genotype. However, only a few infectious cDNA clones have been reported in honey bee studies since the first infectious cDNA clone was published four decades ago. This article discusses steps, rationales, and potential issues in bee-infecting RNA virus cloning. In addition, failed experiences of cloning a Deformed wing virus isolate that was phylogenetically identical to Kakugo virus were addressed. We hope the information provided in this article can facilitate further developments of reverse-genetic studies of bee-infecting viruses to clarify the roles of virus diseases in the current pollinator declines.

From Mosquito Ovaries to Ecdysone; from Ecdysone to Wolbachia: One Woman's Career in Insect Biology

In anautogenous mosquitoes, synchronous development of terminal ovarian follicles after a blood meal provides an important model for studies on insect reproduction. Removal and implantation of ovaries, in vitro culture of dissected tissues and immunological assays for vitellogenin synthesis by the fat body showed that the Aedes aegypti (L.) (Diptera, Culicidae) mosquito ovary produces a factor essential for egg production. The discovery that the ovarian factor was the insect steroid hormone, ecdysone, provided a model for co-option of the larval hormones as reproductive hormones in adult insects. In later work on cultured mosquito cells, ecdysone was shown to arrest the cell cycle, resulting in an accumulation of diploid cells in G1, prior to initiation of DNA synthesis. Some mosquito species, such as Culex pipiens L. (Diptera, Culicidae), harbor the obligate intracellular bacterium, Wolbachia pipientis Hertig (Rickettsiales, Anaplasmataceae), in their reproductive tissues. When maintained in mosquito cell lines, Wolbachia abundance increases in ecdysone-arrested cells. This observation facilitated the recovery of high levels of Wolbachia from cultured cells for microinjection and genetic manipulation. In female Culex pipiens, it will be of interest to explore how hormonal cues that support initiation and progression of the vitellogenic cycle influence Wolbachia replication and transmission to subsequent generations via infected eggs.

Optimization of in vitro culture of honeybee nervous tissue for pesticide risk assessment

The most used pesticides have neurotoxic action on the neurotransmitter system of target and non-targeted insects, such as honeybees. However, honeybees have foremost importance worldwide, which has encouraged the development of tools to evaluate the action of specific pesticide molecules on their nervous system, providing accurate data on damage to their brain. In this sense, our study aimed to optimize in vitro honeybee nervous tissue culture to assess pesticide risks. To this end, six forager honeybee brains were dissected and transferred to different combinations of Leibovitz-15 (L-15) culture medium supplemented with Fetal Bovine Serum (FBS), Hank's Balanced Salt Solution (HBSS), and Insect Medium Supplement (IMS). Nervous tissues were collected after different incubation times (1, 6, 12, and 24 h) for morphology and Kenyon cell analyses. Our results showed that L-15 medium supplemented with HBSS and with HBSS plus FBS were the best media for culturing honey nervous tissue, as they resulted in less tissue spacing and cell disarrangement. Therefore, they may be assessed in future ecotoxicological tests.