FIGURE 2.

CHIKV life cycle in mammalian infected cells. (1) CHIKV cell binding occurs through the interaction of virus E2 protein and a still unknown cellular receptor. Like other alphaviruses, it can enter the cell by clathrin-dependent and independent endocytosis. (2) Once inside the endosome, the acidic environment leads to conformational rearrangement of glycoproteins followed by dissociation of E2-E1 heterodimers and E1 rearrangement into fusogenic homotrimers that induce fusion of viral and endosomal membrane, allowing the release of nucleocapsid into the cytosol. (3) Following uncoating and genomic RNA release, the non-structural proteins are translated as polyproteins denominated P123 and P1234. (4) A replicative complex (RC) formed by uncleaved P123 plus nsP4, the genomic RNA, and several host factors is targeted and anchored at the plasma membrane inducing bulb-shaped invaginations, known as spherules, where RNA synthesis will occur. dsRNA indicates the viral replicative intermediate. nsP1-3 associates with nsP4 in a specific quaternary structure converts the RC into a positive-strand RNA replicase, which synthesizes the viral genomic and subgenomic RNAs. Spherules are internalizate and shape functional large cytopathic vacuoles that bear multiple spherules. (5) Subgenomic RNA (26S) is translated, producing the structural polyprotein (6) E1and E2-E3 (pE2) are translocated into the ER and go through the post-translational process of maturation and glycosylation. (7) Capsid autoproteolysis releases free capsid into the cytoplasm that interacts with genomic RNA, giving origin to the nucleocapsid. (8) The viruses bud out of infected cells through the cell membrane in a pH and temperature-dependent process. (9) CHIKV replication induces ER stress and activates the Unfolded Protein Response (UPR). By non-elucidated mechanisms CHIKV infection also results in oxidative stress, generating Reactive Oxygen Species (ROS) and Reactive Nitrogen Species (NRS). (10) Both ER and oxidative stress can trigger autophagy, a pro-survival signal, in an attempt to preserve cell viability. When CHIKV capsid is produced in the cytoplasm, it can be ubiquitinated and sequestered by adaptor protein SQMT1/p62 into the autophagosomes, leading to capsid degradation in the autophagolisosome. (11) CHIKV is able to trigger NLRP3 inflammasome, starting a signaling cascade that culminates in the activation of the caspase 1, that turns able to cleaves of pro IL-1β and pro IL-18, generating mature cytokines, that will elicit adaptive responses, but also can contribute to pathological inflammatory events such as edema and arthritic disease symptoms.