91桃色

Neurological infections; Drosophila; Virus-host interactions; Neurodegeneration; Pathogenesis

Introduction

The influence of pathogenic viruses on human health is constantly changing, and comprehending how viral proteins impact cellular homeostasis in vivo is crucial for understanding disease development and treatment options. The ethical and physiological limitations that hinder the use of human models are not an obstacle in Drosophila (fruit fly) systems, whether for examining pathogenicity or conducting preclinical research. Additionally, it is a highly manipulable system for genetics and therapeutics. Furthermore, Drosophila is beneficial in determining the pathological effect of viral proteins in a complete system. Drosophila has been utilized in research for more than a century, and its use as a versatile model organism has grown significantly [1,2]. This organism has been widely employed to investigate various aspects of human health, such as cancer, neurodegenerative diseases, immune responses, viral infections, and antiviral therapeutics. It’s simple maintenance, distinct phenotypes, and significant homology with about 75% of human disease genes having comparable sequences in Drosophila make it an ideal organism for exploring human health and disease [3-6]. While Drosophila offers many advantages as a model for studying human diseases, including neurological disorders, there are also some notable disadvantages such as differences in brain anatomy and function and the lack of an adaptive immune system. Furthermore, translating dosage effects utilized in treatment from Drosophila to mammalian systems is not straightforward. Nevertheless, the fruit fly is an inexpensive and easyto- manipulate model that can offer preliminary insights that need to be validated in mammalian systems and eventually in clinical trials. Viruses have the ability to infect every form of life [7]. Pathogenic human viruses are constantly evolving in order to evade the immune system, thus putting pressure on host antiviral mechanisms to evolve alongside them. This ongoing competition presents an opportunity to identify viral neuropathological mechanisms and immune evasion strategies that are related to neurological disorders. Drosophila is a versatile model organism that can be utilized to untangle the mystery of human viral infection and comprehend the functioning of antiviral immunity proteins (as recently reviewed in [8,9]). Despite variations in antiviral immunity activation between humans and Drosophila, they share significant conservation of innate immune signaling pathways and proteins [10-13]. As a result, Drosophila has been employed by various research groups to uncover how different types of human viruses operate to cause pathology. The Drosophila antiviral immune system has been extensively investigated and reviewed in other sources [14,15].

The Toll signalling pathway, which was first discovered in Drosophila, is crucial in initiating an innate immune response in humans. Additionally, Toll signalling plays an essential role in the development and plasticity of the central nervous system (CNS), impacting neurite outgrowth, synaptic impulses, and neuron survival or death. In Drosophila, there are nine Toll and Toll-related proteins, while humans have ten Toll-like receptors (TLRs) that mediate Toll signalling. In humans, Toll signalling is triggered by pathogenassociated molecular patterns (PAMPs) directly binding to TLRs. In contrast, classical Toll signalling activation in Drosophila is more complex, with PAMP recognition occurring extracellularly by pattern recognition receptors (PRRs) such as peptidoglycan recognition proteins (PGRP), Gram-negative bacteria binding proteins (GNBP), and the serine protease Persephone. These PRRs detect different types of pathogens, causing the spatzle processing enzyme to cleave spätzle (spz), which then binds to the Toll receptor and triggers a series of events leading to the expression of antimicrobial peptides (AMPs) and cytokines. Toll signalling is increased upon viral infection, and AMPs can activate Drosophila blood cells, hemocytes, to engulf infected cells and strengthen Toll signalling. The Toll pathway plays a leading role in resistance to oral-route-specific viral infections in Drosophila, but not for systemic viral infections. Furthermore, Toll-7 signalling in Drosophila can activate the conserved autophagy pathway, which initiates an antiviral autophagy response during viral infections such as Rift Valley Fever Virus (RVFV) and Vesicular Stomatitis Virus (VSV).

The IMD pathway, which is similar to the mammalian TNFR pathway, activates Relish, an NF -like transcription factor. Gramnegative bacteria can activate the IMD pathway either at the cell membrane or intracellularly via PGRP-LC and PGRP-LE. Upon activation, IMD initiates two distinct signalling branches that later converge. The first branch involves the activation of dTAK1, which then activates Kenny and Ird5, resulting in the phosphorylation of Relish. The second branch involves IMD binding to dFadd, which activates Dredd (the Drosophila caspase-8 homologue), leading to the cleavage of the previously phosphorylated Relish into Rel-49 and Rel-68. The N-terminal of Rel-68 is then translocated into the nucleus and regulates the expression of immune genes such as Attacin and Diptericin, which were identified as antiviral peptides. Another version of the IMD pathway that is not the typical one is important for immunity against viruses and involves dSTING, the Drosophila equivalent of the stimulator of interferon genes in mammals. In mammals, cGAS detects cytosolic dsDNA and generates 2-cGAMP, which then binds to STING, resulting in the nuclear translocation of NF-B and IRF3, which leads to antiviral gene expression. In Drosophila, two cGAS-like receptors, cGLR1 and cGLR2, are present. The activation of cGLR1 occurs through long dsRNA, while cGLR2’s stimulus is currently unknown. Similarly, dSTING is activated by these cyclic dinucleotides, particularly 2-cGAMP generated by cGLR2. As a result, 2cGAMP can defend Drosophila against both DNA and RNA viruses, and this protection is dependent on dSTING. During viral infection, dSTING’s function is to control the expression of antiviral effectors and is positively regulated by dIKKβ and Relish. Goto et al. have identified Nazo as one such effector, which has antiviral properties against Drosophila C Virus and Cricket Paralysis Virus and also restricts Zika virus infection through dSTING-activated autophagy in the Drosophila brain.

There are several methods to investigate the effects of viral infection in Drosophila. While using viruses that infect Drosophila is a straightforward way to study the interaction between pathogen and host, researchers can also infect flies with viruses that typically infect humans or other animals. A more simplified approach involves using transgenic Drosophila that express individual viral genes or proteins to explore how these viral products impact the host’s molecular pathology and behavior. In cases where these models display significant virusinduced phenotypes, they can be used to test the effectiveness of antiviral medications. The following viral models in Drosophila are some examples that demonstrate how studying flies can offer insights into human neurological disorders.

A common method to investigate viral infection is to use Drosophila cell lines for in vitro culture. As discussed in a recent review, there are numerous Drosophila cell lines readily available, and in combination with somatic cell genetics, there is potential to create novel lines. Hemocyte-derived lines are often used to study immunity at the host-pathogen interface due to their innate immune responses similar to those of vertebrate macrophages, which are vital for virus control and brain immunity. Schneider lines are also frequently used to examine viral receptor-ligand interactions and modulation of signalling cascades. The following section highlights instances of the advantages and disadvantages of using Drosophila cell lines to investigate viral neuroinfections.

Discussion

Despite being widely used for studying neurological development, processes, and disorders, Drosophila models have not been extensively utilized as a model for neurological infections. However, due to their ease of genetic manipulation and animal husbandry, they provide a fast and cost-effective way to address this need. Various approaches can be used to study viral infection and immunity in neurological diseases, which can help in understanding both known and emerging pathogens, as well as genome-encoded viruses. Drosophila models also allow for the examination of differences in viral strain or viral proteins in a variety of genetic backgrounds, as well as assessing environmental variables and developmental stages due to the fly’s short lifespan.

Conclusion

Overall, the use of fruit flies as a tool for investigating virus-host interactions has been instrumental in advancing our understanding of the molecular and cellular mechanisms of viral infections and host immune responses. These advantages of neurological infection models in flies must be weighed against the drawback of their less developed brain and immune systems compared to mammals. Studies should emphasise the fact that, depending on the virus under study, some features of pathophysiology in humans cannot be replicated in flies. Future research might lead to the identification of several novel virushost interactions by using Drosophila as a useful model system for brain diseases.

Acknowledgement

Not applicable.

Conflict of Interest

Author declares no conflict of interest.

1. Pistacchi M, Gioulis M, Marsala SZ (2016) Neurol 21:32-33.

, ,

2. Capo F, Wilson A, Di Cara F (2019) Microorganisms 7:336. 

, ,

3. Bajimaya S, Hayashi T, Frankl T, Bryk P, Ward B, et al. (2017) Virology 501:127-135.

, ,

4. Benoit I, Di Curzio D, Civetta A, Douville RN (2022) Cells 11:2685.

, ,

5. Barnes JN, Matzek LJ, Charkoudian N, Joyner MJ, Curry TB, et al. (2012) Front Physiol 3:187.

, ,

6. Thibault ST, Singer MA, Miyazaki WY, Milash B, Dompe NA, et al. (2004) Nat Genet 36:283-287.

, ,

7. Goic B, Vodovar N, Mondotte JA, Monot C, Frangeul L, et al. (2013) Nat Immunol14:396-403.

, ,

8. Pevzner A, Schoser B, Peters K, Cosma N-C, Karakatsani A, et al. (2012) J Neuro l259:427-435.

, ,

9. Matthews I, Chen S, Hewer R, McGrath V, Furmaniak J, et al. (2004) Clin Chim Acta 348:95-99.

, ,

10. Oger J, Frykman H (2015) Clin Chim Acta 449:43-48.

, ,

11. Leite MI, Jacob S, Viegas S, Cossins J, Clover L, et al. (2008) Brain 131:1940-1952.

, ,

12. Hong Y, Zisimopoulou P, Trakas N, Karagiorgou K, Stergiou C, et al. (2017) Eur J Neurol 24: 844-850.

, ,

13. Hoch W, McConville J, Helms S, Newsom-Davis J, Melms A (2001) Nat Med 7: 365-8.

, ,

14. Hewer R, Matthews I, Chen S, McGrath V, Evans M, et al. (2006) Clin Chim Acta 364: 159-166.

, ,

15. Gasperi C, Melms A, Schoser B, Zhang Y, Meltoranta J, et al. (2014) Neurology 82: 1976-1983.

, ,