Amoebae, Giant Viruses, and Virophages Make Up a Complex, Multilayered Threesome

Viral infection had not been observed for amoebae, until the Acanthamoeba polyphaga mimivirus (APMV) was discovered in 2003. APMV belongs to the nucleocytoplasmatic large DNA virus (NCLDV) family and infects not only A. polyphaga, but also other professional phagocytes. Here, we review the Megavirales to give an overview of the current members of the Mimi- and Marseilleviridae families and their structural features during amoebal infection. We summarize the different steps of their infection cycle in A. polyphaga and Acanthamoeba castellani. Furthermore, we dive into the emerging field of virophages, which parasitize upon viral factories of the Megavirales family. The discovery of virophages in 2008 and research in recent years revealed an increasingly complex network of interactions between cell, giant virus, and virophage. Virophages seem to be highly abundant in the environment and occupy the same niches as the Mimiviridae and their hosts. Establishment of metagenomic and co-culture approaches rapidly increased the number of detected virophages over the recent years. Genetic interaction of cell and virophage might constitute a potent defense machinery against giant viruses and seems to be important for survival of the infected cell during mimivirus infections. Nonetheless, the molecular events during co-infection and the interactions of cell, giant virus, and virophage have not been elucidated, yet. However, the genetic interactions of these three, suggest an intricate, multilayered network during amoebal (co-)infections. Understanding these interactions could elucidate molecular events essential for proper viral factory activity and could implicate new ways of treating viruses that form viral factories.

The virus of my virus is my friend: ecological effects of virophage with alternative modes of coinfection

Virophages are viruses that rely on the replication machinery of other viruses to reproduce within eukaryotic hosts. Two different modes of coinfection have been posited based on experimental observation. In one mode, the virophage and the virus enter the host independently. In the other mode, the virophage adheres to the virus so both virophage and virus enter the host together. Here we ask: what are the ecological effects of these different modes of coinfection? In particular, what ecological effects are common to both infection modes, and what are the differences particular to each mode? We develop a pair of biophysically motivated ODE models of viral-host population dynamics, corresponding to dynamics arising from each mode of infection. We find that both modes of coinfection allow for the coexistence of the virophage, virus, and host either at a stable fixed point or through cyclical dynamics. In both models, virophage tends to be the most abundant population and their presence always reduces the viral abundance and increases the host abundance. However, we do find qualitative differences between models. For example, via extensive sampling of biologically relevant parameter space, we only observe bistability when the virophage and the virus enter the host together. We discuss how such differences may be leveraged to help identify modes of infection in natural environments from population level data.

Cells and Viruses

Cells are the smallest structural component of all known living organisms capable of self-maintenance and reproduction. Although cells vary greatly in their appearance or size, their structure is basically similar. Even the plant and animal cells show a significant degree of similarity in their overall organization.

There are two types of cells: eukaryotic and prokaryotic. The main difference between them is the method of genetic material storage: in eukaryotic cells — in an isolated nucleus, in prokaryotic cells — directly in the cytoplasm (there is no nucleus). Prokaryotic cells are usually independent (unicellular), while eukaryotic cells are often found in multicellular organisms.

Mimivirus: the emerging paradox of quasi-autonomous viruses

What is a virus? Are viruses alive? Should they be classified among microorganisms? One would expect these simple questions to have been settled a century after the discovery of the first viral disease. For years, modern virology successfully unravelled the huge diversity of viruses in terms of genetic material, replication mechanism, pathogenicity, host infection, and more recently particle structure, planet-wide distribution and ecological significance. Yet, little progress was made in understanding their evolutionary origin(s), as well as the fundamental nature of their relationship with the cellular world. Thanks to the recent studies on Mimivirus and other large DNA viruses, we are now entering a new era where the most basic concepts about viruses are revisited, including their true nature, how fundamentally different they are from cellular microorganisms, and how essential they might have been in the major innovations that punctuated the evolution of life.

Mimivirus and its virophage

Mimivirus, a virus infecting amoebae of the acanthamoeba genus, is the prototype member of the Mimiviridae, the latest addition to the family of the nucleocytoplasmic large DNA viruses, already including the Poxviridae, the Iridoviridae, the Asfarviridae, and the Phycodnaviridae. Because of the size of its particle-a fiber-covered icosahedral protein capsid 0.75 microm in diameter-Mimivirus was initially mistaken for a parasitic bacterium. Its 1.2-Mb genome sequence encodes more than 900 proteins, many of them associated with functions never before encountered in a virus, such as four aminoacyl-tRNA synthetases. These findings revived the debate about the origin of DNA viruses and their possible role in the emergence of the eukaryotic nucleus. The recent isolation of a new type of satellite virus, called a virophage, associated with a second strain of Mimivirus, confirmed its unique position within the virus world. Post-genomic studies are now in progress, slowly shedding some light on the physiology of the most complex virus isolated to date.

Large virus for an even bigger task: can the mimivirus close the gene-therapy vector void?

Gene therapy holds exceptional biotechnological and medical potential, but it has not been able to unite efficient delivery with reliability over the years. Dependable genetic elements are often large and do not, quite simply, fit into the present line of efficient vectors or require therapy combinations to carefully regulate genetic constructs. Recently, however, a discovery in virology – the field of study that has produced the most efficient vectors to date – uncovered a virus with a threefold higher coding capacity than any previously described virus and, thus, can be envisioned to stimulate the development of a new line of vectors, which could combine the transfer of large, stable and reliable genetic elements with the efficiency associated with viruses. However, extensive further research is, required in order to probe the potential of this virus and verify the current hypothesis.