The life cycle of the alphaviruses: From an antiviral perspective

Abstract

The alphaviruses are a widely distributed group of positive-sense, single stranded, RNA viruses. These viruses are largely arthropod-borne and can be found on all populated continents. These viruses cause significant human disease, and recently have begun to spread into new populations, such as the expansion of Chikungunya virus into southern Europe and the Caribbean, where it has established itself as endemic. The study of alphaviruses is an active and expanding field, due to their impacts on human health, their effects on agriculture, and the threat that some pose as potential agents of biological warfare and terrorism. In this systematic review we will summarize both historic knowledge in the field as well as recently published data that has potential to shift current theories in how alphaviruses are able to function. This review is comprehensive, covering all parts of the alphaviral life cycle as well as a brief overview of their pathology and the current state of research in regards to vaccines and therapeutics for alphaviral disease.

Keywords: Positive-sense RNA viruses; Viral life cycle; Viral replication; alphaviruses.

Figure 1.

The transmission cycle of encephalitic alphaviruses. A) Alphaviruses are maintained in nature by cycling between a host species, typically a bird or small mammal, and a mosquito vector species. B) Spillover events often occur into livestock, which reach high viral titers and readily transmit the virus to additional vectors. In the case of the New-World viruses this infection almost always leads to death. C) Typically, after infection of livestock, humans that work in close association with these animals can also be infected by vector species. Humans are regarded as dead-end hosts for most alphaviruses. In humans these infections may lead to disease, and, in severe cases, death.

Figure 2.

The replication cycle of alphaviruses. The virion enters a susceptible cell via receptor mediated endocytosis, primarily mediated by clathrin (green) and due to pH changes of the endosome releases its RNA (purple) into the cytoplasm of the host cell. The positive sense genomic RNA is first used by ribosomes to translate the viral nsPs as a polyprotein (yellow = nsP123, green = nsP4). The polyprotein will undergo cleavage events that control the synthesis of the viral RNA species (individual nsPs represented as single green circles). This RNA synthesis occurs in membrane invaginations that are termed spherules. These spherules protect the viral RNA and nsPs from detection by the host cell. Late in infection the structural genes are synthesized (pink = the E proteins, black = capsid). The capsid will form into nucleocapsid cores as it packages the viral RNA, and the glycoproteins are transported to the cell membrane. The nucleocapsid cores translocate to the cellular membrane where they bud off, collecting their envelope and glycoproteins and forming new infectious virions.

Figure 3.

A) The genetic structure of the alphaviruses. The alphaviruses have an approximately 12kb, linear, positive-sense genome. The genome has two open reading frames, the nonstructural and the structural. The nonstructural open reading frame is here displayed in green, and encodes the four nonstructural proteins, which are responsible for replication of the viral RNA. The structural open reading frame is displayed here in yellow and encodes for the E proteins and capsid as well as the 6K and TF proteins. The capsid and E proteins form the structure of the viral particle. B) RNA synthesis of alphaviruses. This RNA synthesis activity is carried out in spherules on the membranes of cellular organelles. After release into the cytoplasm the genomic RNA (purple) is used to synthesize the initial nonstructural polyprotein. nsP2 initially cleaves between nsP3 and 4 leading to nsP123/4 (nsP123 = yellow, nsP4 = green), which synthesizes primarily negative-sense template RNA (red). The protein undergoes rapid cleavage through intermediate states to reach the final replicase complex nsP1/2/3/4 (represented as individual green circles). This complex synthesizes new positive-sense genomic (purple) and subgenomic (blue) RNA and can no longer synthesize negative-sense RNA. The genomic RNA is used to synthesize additional genomes and is packaged into progeny virions. The subgenomic RNA is used to synthesize the structural genes that form the new virions.