Date Published: November 14, 2013
Publisher: Public Library of Science
Author(s): Rollie J. Clem, A. Lorena Passarelli, Richard C. Condit.
The baculoviruses (family: Baculoviridae) are a group of large DNA viruses that infect insects. These viruses are well known for their utility and versatility as gene expression vectors, biological pesticides, and vectors for transduction of mammalian cells –. However, baculoviruses are much more than just useful laboratory tools. The rich and fascinating biology associated with these viruses provides many interesting examples of virus-host interactions and virus modification of host processes.
Baculovirus-infected larvae exhibit several interesting examples of pathogen-modified physiology and behavior. One of these involves inhibition of larval development. Normally, lepidopteran larvae pause feeding periodically to go through larval molts. However, baculovirus-infected larvae do not molt, due to the action of a viral enzyme called ecdysteroid UDP-glucosyltransferase (EGT) . EGT prevents larval molting by inactivating ecdysone, the major insect molting hormone. Infected larvae grow larger than normal before succumbing to a dramatic death worthy of a horror film: infected corpses liquefy, due to expression of viral cathepsin-like protease and chitinase enzymes, and liquefaction aids in release of viral occlusion bodies. The amount of progeny virus is impressive, with upwards of 10 million occlusion bodies produced per milligram of larval tissue.
Baculoviruses are transmitted orally in nature, and the target of primary infection is the larval midgut epithelium. After being consumed, occlusion bodies dissolve in the midgut lumen, releasing the embedded ODV, which infect midgut epithelial cells. Progeny BV bud from the basal surface of the epithelium, cross the basal lamina (see below), and infect most of the remaining tissues of the larva (Figure 1). The process of ODV attachment and entry depends on several viral proteins found in the ODV envelope, referred to as PIFs (per os infectivity factors). Recent results indicate that some of the PIFs form a complex in the ODV envelope, likely interact with an unknown midgut receptor, and mediate fusion with the plasma membrane . Thus the PIFs constitute a novel virus attachment/fusion protein complex. Interestingly, PIFs appear to be ancient genes that are conserved among three related insect virus families, as pif homologs are also found in the nudiviruses and the polydnaviruses, the latter of which form mutualistic relationships with parasitic wasps , .
As with other DNA viruses, baculovirus gene expression occurs as a cascade. Baculovirus gene expression is traditionally divided into three stages, each of which is dependent on expression of the earlier stages: early, late, and very late. The early genes are transcribed by host RNA polymerase II, while late and very late genes are transcribed by the viral RNA polymerase. This unique viral polymerase consists of four subunits that have little homology to other known polymerases. Late and very late promoters are also distinctive and simple, consisting mainly of the tetranucleotide sequence TAAG.
Bacmid technology, which allows baculovirus genomes to be manipulated in bacteria, has provided an important tool to study baculovirus gene function. Investigators have used bacmids to delete many baculovirus genes, especially the so-called core genes (those genes, currently numbering 37, that are conserved in all known baculovirus genomes), which are often essential for replication. While these include the pifs and genes involved in viral DNA replication and late gene transcription, a large number of core genes appear to be required for proper nucleocapsid assembly, even though they are not necessarily structural genes (reviewed in , ). It is not clear why so many genes are required for virion assembly, but some of them are likely involved in processing and packaging of the DNA genome. Further characterization of these genes will define their specific functions in virion assembly or a process prior to assembly that manifests as a defect in virion morphology.
While we have learned a great deal about baculoviruses over the past several decades, there is still much we do not know. It is likely that we have only uncovered the tip of the iceberg in terms of the diversity of baculoviruses in nature, especially those that infect nonlepidopteran insects. Also, even in the AcMNPV genome, roughly half of the genes have no assigned function, meaning that there are probably many more interesting baculovirus accessory genes whose functions are waiting to be discovered. Since BV can bind to and enter many kinds of vertebrate and invertebrate cells, baculovirus host range is more complex than just receptor binding, and the determinants of host range are poorly characterized. Host range manipulation will be important in developing improved biopesticides and gene therapy vectors. Understanding and manipulating baculovirus-host interactions will be facilitated by deriving additional lepidopteran genomic sequences and tools to genetically manipulate these insects. In addition to the PIFs found in ODV, two types of attachment/fusion proteins (F and GP64) are present in BV, and while these have been fairly well studied, host receptor molecules have not been identified for either ODV or BV. The mechanisms of baculovirus DNA replication and capsid assembly are also still largely undefined. Finally, invertebrate antiviral immunity is still a young field, and little is known about immune responses of insects against baculoviruses. Further research will continue to contribute to our general understanding of virus-host interactions, as well as continue to improve the utility of baculoviruses in biotechnology and agriculture.