Jockusch Small RNAs: Their Diversity, Roles and Practical Uses

1. Auflage 2012
ISBN: 978-0-12-387681-2
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 42, 216 Seiten

Reihe: Advances in Insect Physiology

Advances in Insect Physiology is committed to publishing volumes containing comprehensive and in-depth reviews on all aspects of insect physiology. First published in 1963, these volumes are an essential reference source for invertebrate physiologists, insect neurobiologists, entomologists, zoologists and insect biochemists. This volume is themed on small RNAs and RNAi in insects. - Contains comprehensive and in-depth reviews - Essential reference source for invertebrate physiologists, insect neurobiologists, entomologists, zoologists and insect biochemists - This volume is themed on small RNAs and RNAi in insects

Autoren/Hrsg.

Jockusch, Elizabeth

Leseproben

Advances in Insect Physiology, Vol. 42, No. Suppl C, 2012 ISSN: 0065-2806 doi: 10.1016/B978-0-12-387680-5.00001-X Chapter 1Of Insects and Viruses The Role of Small RNAs in Insect Defence Nicolas Vodovar, Maria-Carla Saleh, Institut Pasteur, Viruses and RNA interference, CNRS URA3015, Paris, France Abstract In the past decade, small RNA pathways have been identified as a major mechanism of gene regulation. From an immunity standpoint, these pathways play a central role either by regulating immune reactions or by acting as immune effectors. In insects, several studies have unravelled the role of RNA interference (RNAi) as an antiviral response and have uncovered a complex relationship between insects and viruses that co-evolve in an ongoing race for supremacy. In this review, we comment on the role of small RNA pathways in insect defence and the exploitation of these same pathways by pathogens. We illustrate the host–pathogen relationship under RNAi constraints using several examples and we discuss future directions in using RNAi as a tool to control insect immunity. Keywords Innate immunity Arthropods RNA interference small RNAs Antiviral immunity Arboviruses 1 Introduction
Host–pathogen interactions can be pictured as an arms race between two adversaries. On the one hand, the pathogen deploys virulence factors to exploit the resources of the host. On the other hand, the host fights back with immune responses to clear the pathogen or at least minimise its deleterious effects. The outcome of this interaction can vary from commensalism to the death of one of the two players, depending on the relative strength of the effectors involved and possible escape mechanisms to limit these effects. There are two types of immune responses, relying on different mechanisms and effectors: innate and adaptive. While the latter is specific to vertebrates, innate immunity is present in all multicellular organisms, including insects. In vertebrates, the activation of the innate immune response is a prerequisite to the activation of adaptive immunity. Two theories have been proposed to model the induction of the innate immune response. The first theory relies on the recognition by Pattern Recognition Receptors (PRRs) of invariant molecular patterns (Pathogen-Associated Molecular Pattern—PAMP) that are present in most, if not all, microbes (Janeway, 1989, 1992). According to the second theory, the ‘theory of danger’, the immune system is elicited by alarm signals sent by injured cells (Matzinger, 1994). These two theories are not mutually exclusive as illustrated by the dual activation of Drosophila immune response to both glucans (PAMP) and virulence factors (alarm signal) upon infection by the entomopathogenic fungi Beauveria bassiana and Metarhizium anisopliae (Gottar et al., 2006). Recently, Polly Matzinger reformulated her initial theory of danger, unifying the recognition by PRRs and the recognition of alarm signals into a single model in which the immune system responds to ‘danger signals’ from different origins and sources (Matzinger, 2007). 1.1 Insects and viruses
Insects, like other organisms, are subject to infection by viruses with RNA or DNA genomes of different structure and polarity. These viruses fall into two distinct classes, depending on the type of host they use during their replication cycle. Viruses from the first class are transmitted from insect to insect, although their host range may not be solely restricted to insects. Among these viruses, some present a strong agronomical impact by affecting domesticated insects. For instance, the Israeli Acute Paralysis Virus (IAPV) is among the pathogens potentially responsible for the honeybee (Apis mellifera) colony collapse disorder, which results in the destruction of the hives (Cox-Foster et al., 2007). Others, like Densovirus (Parvoviridae) have been successfully used to eliminate Galleria mellonella (greater wax moth) infestations in beehives (Lavie et al., 1965), although the use of viruses for biological control of pests remains limited because of their potential impact on human health. Viruses of the second class are the arthropod-borne viruses (arboviruses). These viruses have the particularity of alternating between hematophagic invertebrate and vertebrate hosts in an obligate fashion. Several insects are responsible for arbovirus transmission to human or cattle, including the mosquitoes Aedes spp. (e.g. Rift Valley Fever Virus, Chikungunya Virus, Dengue Virus, Yellow Fever Virus), Anopheles spp. (e.g. O’nyong’nyong Virus) and Culex spp. (e.g. Rift Valley Fever Virus, Japanese Encephalitis Virus, West Nile Virus); the sand fly Phlebotomus spp. (e.g. Vesicular Stomatitis Virus, VSV) and biting midges in the genus Culicoides (e.g. Bluetongue Virus; for review see Mellor, 2000 and references therein). Importantly, arboviral infections are asymptomatic in insects but responsible for severe incapacitating diseases in mammalian hosts, especially humans, suggesting a complex co-evolutionary process. There is no specific treatment against such diseases and vaccines are available for only two of them (the Yellow Fever and Japanese Encephalitis viruses). The strong impact of arboviruses on human health and economy, the spread of these diseases around the globe due to climate changes and travelling habits, and the emergence of new arboviral diseases make clear the importance of finding new strategies to limit arboviral transmission to mammals. More generally, insects play important roles in human life, both beneficial and detrimental; this prompts a desire for better understanding of their immune system in order to protect those that are valuable to humans and limit those that bring adverse effects. 1.2 Drosophila as a model to study host–pathogen interactions
Upon viral infection, insects mount a distinctive immune response whose hallmark is RNA interference (RNAi). This defence mechanism was originally identified as an antiviral defence in plants (Ratcliff et al., 1997). In insects, most of our knowledge about RNAi and more generally about small RNA pathways come from studies performed in the fruit fly Drosophila melanogaster, which celebrated its hundredth anniversary as a model organism in 2010. Since its introduction in the laboratory by Thomas Morgan to study development, Drosophila has been the source of invaluable contributions to genetics, developmental biology, neuroscience and immunology. For instance, the first mutation described by Morgan in 1910 led to the discovery of sex-linkage, introduced genetics as a science and genetic analysis as a powerful tool in biology. Throughout the century, Drosophila has benefited from a dynamic and collaborative community that rendered this small fly one of the most accomplished multicellular animal models in which to carry out genetic analysis. In addition, the sequence of its genome showed that more than 60% of the genes involved in human genetic diseases are conserved in Drosophila (Bier, 2005). Thus, its affordability, short generation time and amenability to both direct and reverse genetics makes it suitable for intensive analysis and provides an appropriate alternative to vertebrate models when characterizing biological processes. The major breakthrough in immunology that can be attributed to Drosophila is the identification of the Toll (Lemaitre, 2004; Lemaitre et al., 1995b, 1996) and Imd (Lemaitre et al., 1995a) pathways which direct an immune response capable of discriminating and taking the appropriate action against an invading microbe, depending on its class (Lemaitre et al., 1997). The identification of Toll pioneered the discovery of the Toll-like receptors (TLR) in mammals and the subsequent understanding of the mechanisms that govern innate immunity (Rock et al., 1998). More recently, the identification of the RNAi pathway as the major antiviral defence mechanism positioned Drosophila as a central model to study insect antiviral immunity (Galiana-Arnoux et al., 2006; van Rij et al., 2006; Wang et al., 2006; Zambon et al., 2006). In the case of arboviruses, enthusiasm for Drosophila as a study model over genuine vectors such as mosquitoes may be dampened by the fact that it is not a vector for any arboviral disease. However, the ability of several arboviruses to replicate in Drosophila (e.g. Sindbis Virus (SINV): Xiong et al., 1989; West Nile Virus: Chotkowski et al., 2008; Dengue Virus: Sessions et al., 2009; VSV: Mueller et al., 2010) and the quasi-absence of genetic tools available in other insects make Drosophila a very powerful model to study all virus–insect, and more specifically arbovirus–insect, interactions. 2 Generalities about insect defence mechanisms
Insects have developed effective defence mechanisms to protect themselves from infections. These defence...