Mitochondrial and Innate Immunity Transcriptomes from Spodoptera frugiperda Larvae Infected with the Spodoptera frugiperda Ascovirus

Abstract

Ascoviruses are large, enveloped DNA viruses that induce remarkable changes in cellular architecture during which the cell is partitioned into numerous vesicles for viral replication. Previous studies have shown that these vesicles arise from a process resembling apoptosis yet which differs after nuclear lysis in that mitochondria are not degraded but are modified by the virus, changing in size, shape, and motility. Moreover, infection does not provoke an obvious innate immune response. Thus, we used in vivo RNA sequencing to determine whether infection by the Spodoptera frugiperda ascovirus 1a (SfAV-1a) modified expression of host mitochondrial, cytoskeletal, and innate immunity genes. We show that transcripts from many mitochondrial genes were similar to those from uninfected controls, whereas others increased slightly during vesicle formation, including those for ATP6, ATP8 synthase, and NADH dehydrogenase subunits, supporting electron microscopy (EM) data that these organelles were conserved for virus replication. Transcripts from 58 of 106 cytoskeletal genes studied increased or decreased more than 2-fold postinfection. More than half coded for mitochondrial motor proteins. Similar increases occurred for innate immunity transcripts and their negative regulators, including those for Toll, melanization, and phagocytosis pathways. However, those for many antimicrobial peptides, such as moricin, increased more than 20-fold. In addition, transcripts for gloverin-3, spod_x_tox, Hdd23, and lebocin, also antimicrobial, increased more than 20-fold. Interestingly, a phenoloxidase inhibitor transcript increased 12-fold, apparently to interfere with melanization. SfAV-1a destroys most fat body cells by 7 days postinfection, so innate immunity gene transcripts apparently occur in remaining cells in this tissue and possibly other major tissues, namely, epidermis and tracheal matrix. IMPORTANCE Ascoviruses are large DNA viruses that infect insects, inducing a cellular pathology that resembles apoptosis but which differs by causing enormous cellular hypertrophy followed by cleavage of the cell into numerous viral vesicles for replication. Previous EM studies suggest that mitochondria are important for vesicle formation. Transcriptome analyses of Spodoptera frugiperda larvae infected with SfAV-1a showed that mitochondrial transcripts were similar to those from uninfected controls or increased slightly during vesicle formation, especially for ATP6, ATP8 synthase, and NADH dehydrogenase subunits. This pattern resembles that for chronic disease-inducing viruses, which conserve mitochondria, differing markedly from viruses causing short-term viral diseases, which degrade mitochondrial DNA. Though mitochondrial transcript increases were low, our results demonstrate that SfAV-1a alters host mitochondrial expression more than any other virus. Regarding innate immunity, although SfAV-1a destroys most fat body cells, certain immunity genes were highly upregulated (greater than 20-fold), suggesting that these transcripts may originate from other tissues.

Extended in vivo transcriptomes of two ascoviruses with different tissue tropisms reveal alternative mechanisms for enhancing virus reproduction in hemolymph

“…However, it remains unknown how replication continues after their caspase and similar proteins cause nuclear lysis 17 , 18 . Since ascoviruses cause a chronic disease that lasts for weeks, we expanded our previous in vivo transcriptomic studies 22 , 26 by extending our SfAV experiments to three weeks and making these comparative by including TnAV. We compared hemolymph transcriptomes at 7, 14, and 21 days for both viruses with those of the other primary body tissues at the same time points.…”

Early in vivo transcriptome of Trichoplusia ni ascovirus core genes

“…'asco' in Greek meaning 'sac' . Ultrastructural and more recent transcriptome studies indicate that ascoviruses manipulate host-cell mitochondria to provide the energy required for synthesizing the membranes that delimit viral vesicles [2,10,11]. After formation, these vesicles accumulate in the larval haemolymph, turning it milky white and continue to assemble progeny virions.…”

“…Previous transcriptome studies on ascoviruses found that as the infection progresses there are significant expression variations in viral and lepidopteran host genes [10][11][12][13]. All of these studies were conducted in vivo due to a lack of a robust in vitro tissue culture system for ascovirus replication that closely resembles cell pathogenesis in larvae [14].…”

Heliothis virescens ascovirus 3h blocks the cell cycle of Spodoptera exigua fat body cells at G2/M phase by downregulating cyclin B1 and cyclin‐dependent kinase 1

“…The HvAV‐3h‐infected larvae showed reduced feeding, stunted growth, and extended lifespan (S. J. Li et al, 2013, 2016; Yu et al, 2018). A previous study showed that infection by Spodoptera frugiperda ascovirus (SfAV‐1a) moderated the host immune response of S. frugiperda larvae and prolonged the larval developmental period by increasing certain negative regulators of Tolls, melanization, and phagocytosis pathways (Zaghloul et al, 2020). Ascovirus‐infected insect cell lines stopped proliferating (Asgari, 2006; Federici & Govindarajan, 1990; Govindarajan & Federici, 1990; Yu et al, 2019).…”

“…Li et al, 2013Li et al, , 2016Yu et al, 2018). A previous study showed that infection by Spodoptera frugiperda ascovirus (SfAV-1a) moderated the host immune response of S. frugiperda larvae and prolonged the larval developmental period by increasing certain negative regulators of Tolls, melanization, and phagocytosis pathways (Zaghloul et al, 2020).…”