| MicrobiologyBytes: Virology: Baculoviruses | Updated: January 28, 2007 | Search |
The following are some specific examples of how baculoviruses manipulate the biology of the host to enable them to have and effective infection. These are not the only ways in which viruses and hosts interact, but they are some of the best characterised for baculoviruses.
Apoptosis is a cellular response to a cellular "insult" such as UV light, chemical or physical damage or a viral infection. This insult starts a cascade of events which lead to the destruction of the cell. This mechanism is often called "programmed cell death" as it is an innate response of the cell which protects the rest of the organism from a potentially harmful agent. The process of apoptosis is shown in the diagram below.
To overcome the apoptosis response of the infected host baculoviruses have a number of genes which can act at different stages of the apoptosis process. These genes include P35 - which is the best characterised of the baculovirus apoptosis inhibitors. Other inhibitors that have been identified include a number of inhibitors of apoptosis or "IAPs".
It is believed that apoptosis is induced in host cells during the transition of the virus from the early to the late stages of infection. During this transition in an infected cell viral DNA synthesis is initiated and synthesis of of cellular RNA and proteins is shut down. This would provide multiple signals to trigger the start of the apoptotic process. By using drugs to inhibit the synthesis of cellular RNA in in vitro studies and, in separate experiments, causing an extra round of DNA synthesis it has been shown that either of these events on their own is sufficient to induce apoptosis.
Of the identified baculoviral apoptosis inhibitors P35 is the best understood. It has been shown to irreversibly block the caspase cascade of the apoptosis process. Other inhibitors have been identified and most baculoviruses have been shown to carry multiple IAPs.
The discovery of multiple IAPs (different forms) in the baculoviral genome leads to the question of why have more than one IAP? It is postulated that different IAPs may have different sites of action in the host organism and that there may even be IAPs which act in certain cell types, but not others, or alternatively that multiple IAPs are carried to allow a baculovirus to infect multiple species of insect.
The baculovirus EppoMNPV is currently being studied at the University of Otago. This baculovirus has been shown to carry at least 4 different IAPs: IAP-1, IAP-2, IAP-3 and IAP-4. When these were tested (method shown below) in tissue culture it was found that two of them were active in vitro. One interesting observation from the apoptosis inhibition assays was that the IAP-1 was only able to delay the onset of apoptosis, whereas the IAP-2 actually prevented apoptosis. This suggests that the different IAPs may act on different pathways (or at least different stages) in the apoptosis pathway. It is possible that the different IAPs are all active in vivo and may act at different tissues of the host insect. An alternative explanation for the multiple IAPs is that they may all be active, but may only act in certain host species. This remains to be shown experimentally.
Many viruses encode genes which manipulate the life cycle of their host. An example of this in baculoviruses is the egt gene. This gene encodes an ecdysteriod UDP-glucosyltransferase (EGT). This enzyme modifies the host hormone ecdysone to render it inactive.
Ecdysone is an insect moulting hormone. An increase in the levels of this hormone is a signal for the the larvae to either moult or pupate. The viral EGT modifies this hormone by the conjugation of a UDP-sugar group (either UDP-glucose or UDP-galactose). The resulting conjugated hormone is inactive and as a result the insect will not pupate or moult. And overview of this is shown in the following diagram.
The prevention of the larval moult reduces metabolic stress and prolongs the larval life cycle during infection. This in turn leads to an increase in the production of progeny virus within the host. This delay of the moulting of the insect has a negative effect if the baculovirus is to be used as a biocontrol agent. As the infected insects are not subjected to the stress of moulting, they continue to feed. This allows the virus to continue to replicate until the very late genes are activated, which essentially lead to the liquefaction of the host insect. It has been shown experimentally that inactivation or deletion of EGT can reduce the kill time of the baculovirus.
Another role of EGT is that it may increase the chance of an effective infection by the baculovirus. During a larval moult the midgut cells become apoptotic and are sloughed off. By inactivating the moulting hormone the baculovirus prevents the midgut cells sloughing off and preventing an effective infection.
Baculoviruses have a number of other genes which are also involved in the manipulation of host biology. These genes include a number of proteases for manipulation of cell walls and host membranes, as well as genes encoding proteins which arrest cellular protein expression and RNA synthesis at the late and very late stages of a baculoviral infection.
An example of this is chitinase which is postulated to be involved in the breakdown of the peritrophic membrane. This would facilitate the entry of nucleocapsids into the midgut cells during the initial infection. Cathepsin and chitinase are important in the final liquefaction of the host which aids in the dissemination of the progeny virus. Cathepsin is involved in the breakdown of the internal tissues of the insect which facilitates the liquefaction of the insect. The chitinase, in synergy with other proteases, breaks down the chitinous cuticle of the larvae to aid the final dissemination of the progeny virus.
© Kalmakoff & Ward
University of Otago, Dunedin, New Zealand, 2003.