A viroplasm is an inclusion body in a cell where viral replication/assembly occurs. They may be thought of as viral factories in the cell. Very little is understood about the mechanism of viroplasm formation. These appear electron-dense under the electron microscope, most likely due to high amounts of viral RNA in them.
A Viroplasm is a perinuclear or a cytoplasmic large inclusion where viral replication and assembly occur. Viral products and cell elements are confined in viroplasms. The viroplasm formation seems to be caused by the interactions between the virus and the infected cell. There are many viroplasms in one infected cell. Viroplasms appear dense to electron microscopy, they are insoluble.
Synonymous : viral factory, electron-dense inclusion
Groups of viruses that form viroplasms
Viroplasms have been reported in many unrelated groups of Eukaryotic viruses that replicate in cytoplasm, however, viroplasms from plant’s viruses have been less investigated than viroplasms from animal’s viruses.
|I: dsDNA viruses||Poxviridae||Vaccinia virus 
African swine fever virus 
frog virus 
Acanthamoeba polyphaga mimivirus 
|II: ssDNA viruses||Herpesviridae||Herpes simplex virus
|III: dsRNA viruses||Reoviridae||Avian reovirus |
|IV: (+)ssRNA viruses||Togaviridae||Rubella virus |
|V: (−)ssRNA viruses||Rhabdoviridae||Rabies virus |
|VI: ssRNA-RT viruses||Retroviridae||Human immunodeficiency virus |
|VII: dsDNA-RT viruses||Caulimoviridae||Cauliflower mosaic virus |
A high number of other viruses most probably form viroplasms as well, but they are not described or discovered yet.
The viroplasm : structure and formation
Viroplasms are localized in the perinuclear area or in the cytoplasm of infected cells. Viroplasms are formed early in the infection cycle. The number and the size of viroplasms depend of the virus, the virus isolate, hosts species, and the step of infection. For exemple, viroplasms of mimivirus have a similar size as the size of the nucleus of its host, the Amoebae Acanthameba polyphaga. They alter large areas of the infected cells and induce changes in composition and organization of cellular compartments depending on the step of the virus replication cycle. This process involves a number of complex interactions and signaling events between viral and cell factors.
Viroplasms are formed early during the infection, there is an exclusion of host proteins and organelles from the region where the factory will be assembled. At the same time, large amounts of viral structural proteins and viral DNA accumulate, ribosomes, protein-synthesis machinery, and chaperones are recruited. Mitochondria, cytoskeletal components and membrane components frequently participate in the formation of viral factories. Some of the membrane components are used for viral replication while some others will be modified to produce viral envelopes, when the viruses are enveloped. The viral multiplication requires a considerable amount of energy. There are large clusters of mitochondria at the periphery of viroplasms, due to a migration of pre-existing organelles to the periphery of viroplasms. These mitochondria seem to provide the energy required for viral replication, protein synthesis and assembly. The virus factory is often enclosed by a membrane (often derived from the rough endoplasmic reticulum) or by cytoskeletal elements.
In animal cells, virus particles are gathered by the microtubule dependent aggregation of toxic or misfolded protein near the microtubule organizing center (MTOC). So, the viroplasms of animal viruses are generally localized near the MTOC. MTOCs are not found in plant cells. Plant viruses induce the rearrangement of membranes structures to form the viroplasm. This is mostly shown for plant RNA viruses.
Viroplasms are places where the viral replication and assembly take place. These structures concentrate viral components required for the genome replication and the morphogenesis of new virus particles. This wrapping increases the efficiency of these processes. These subcellular domains could also protect viral complexes from the cellular degradation machinery and the viral RNA from silencing. But the viroplasm can also protect host cells and prevent the virus propagation by stocking viral products in a subcellular domain meanwhile the degradation of them by proteases and nucleases.
In the case of the Cauliflower mosaic virus (CaMV), viroplasms improve the virus transmission by the aphid vector. Viroplasms control release of virons when the insect stings an infected plant-cell or a cell near the infected cells.
Viroplasm : product of the coevolution between virus / host ?
Aggregated structures may protect viral functional complexes from the cellular degradation machinery. For example, formation of viral factories of the ASFV viroplasm is very similar to the aggresome formation. An aggresome is a perinuclear site where misfolded proteins are transported and stored by the cell components meanwhile their destruction. The viroplasm could be the product of a coevolution between the virus and its host. It’s possible that a cellular response originally designed to reduce the toxicity of misfolded proteins is exploited by cytoplasmic viruses to improve their replication, the capsid synthesis and the assembly. Alternatively, the activation of host defense mechanisms may involve sequestration of virus components in aggregates to prevent their dissemination, followed by their neutralisation. For example, viroplasms of mammalian viruses contain certain elements of the cellular degradation machinery which might enable cellular protective mechanisms against viral components. Given the co-evolution of viruses with their host cells, changes in cell structure induced during infection are likely to involve a combination of the two strategies.
Use in diagnostic
Presence of viroplasms is used to diagnostic certain viral infections. Understanding the phenomenon of virus aggregation and of the cell response to the presence of virus, and whether viroplasms facilitate or inhibit virus multiplication, may help to develop new therapeutic approaches against virus infections in animal and plant cells.
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Dr. Aman Debbarma(agmc)