Apoptosis, autophagy and unfolded protein response pathways in Arbovirus replication and pathogenesis

Abstract

Arboviruses are pathogens that widely affect the health of people in different communities around the world. Recently, a few successful approaches toward production of effective vaccines against some of these pathogens have been developed, but treatment and prevention of the resulting diseases remain a major health and research concern. The arbovirus infection and replication processes are complex, and many factors are involved in their regulation. Apoptosis, autophagy and the unfolded protein response (UPR) are three mechanisms that are involved in pathogenesis of many viruses. In this review, we focus on the importance of these pathways in the arbovirus replication and infection processes. We provide a brief introduction on how apoptosis, autophagy and the UPR are initiated and regulated, and then discuss the involvement of these pathways in regulation of arbovirus pathogenesis.

Figure 1.

Routes of transmission and human exposure to zoonotic arboviruses. Infectious agents may be transmitted to humans by direct contact with infected animals, mechanical vectors or intermediate hosts. Arboviruses are maintained in mosquito-monkey, mosquito-rodent, mosquito-bird, mosquito-pig, mosquito-horse and mosquito-human cycles. The enzootic cycle occurs in the region where humans intrude into the natural foci of infection. The rural epizootic cycle is involved among domestic animals and mosquitos, and amplified in the presence of intermediate hosts, which result in representing a large reservoir of viruses and severe spillover effect to dead-end hosts. In urban settings, viruses are transmitted between humans and the mosquito vectors in an urban epidemic cycle, using humans for amplification (Ref. 10).

Figure 2.

Global distribution of some of the most important arboviruses. (A) DENV, Dengue virus (Refs 41, 42), (B) YFV, Yellow fever (Refs 43, 44), (C) WNV, West Nile virus (Refs 45, 46, 47), (D) CHIKV, Chikungunya virus (Refs 47, 48), (E) JEV, Japanese encephalitis virus (Ref. 49), (F) VEEV, Venezuelan equine encephalitis virus (Ref. 47), (G) RVFV, Rift valley fever virus (Ref. 50), (H) CCHF, Crimean–Congo haemorrhagic fever (Ref. 51).

Figure 3.

Graphic representation of autophagy. Autophagy is a process for the degradation and recycling of damaged or unnecessary cellular compartments, which has several tightly regulated steps including induction, nucleation, expansion and completion, fusion and degradation. The mTOR is known as the key regulator of autophagy induction and can be suppressed by ULK1, leading to trigger VPS34-Beclin 1-class III PI3-kinase complex. Several different membrane pools contribute to the formation of the phagophore. The Atg proteins (Atg2, Atg9, Atg18) are essential for phagophore formation. The ATG and LC3 conjugation system also contribute in autophagosome membrane formation and elongation. The autophagolysosome then is formed by fusion of the autophagosome with a lysosome to degrade and reuse the compounds. ATG, autophagy-related genes; mTOR, mammalian target of rapamycin.

Figure 4.

Graphic representation of apoptosis signalling pathways. Apoptosis is initiated via two different routes including extrinsic and intrinsic apoptotic pathways. The extrinsic signals are initiated by cell death ligands (e.g. FasL, APO-2L, TRAIL, TNF) and activate FADD and subsequently cleave pro-caspase-8. Cleavage of pro-caspases-8 and -10 initiate activation of caspases-8 and -10, which later can directly trigger effector caspases including caspases-3, -6 and -7. The intrinsic pathway is stimulated via DNA damage. Once DNA damage occurs, p53 is activated and induces apoptosis in a mitochondria-dependent manner. In this pathway, pro-apoptotic and antiapoptotic proteins are up- and down-regulated, leading to release of cytochrome c. Released cytochrome c later can activate caspase 9 which in turn activates caspase-3. FasL, Fas (Apo-1/CD95) ligand; TNF, tumour necrosis factor receptor TRAIL, TNF, tumour necrosis factor receptor.

Figure 5.

Graphic representation of ER stress and virus replication. ER stress is enhanced in the viral infected cells and activates UPR proteins (e.g. PERK, ATF6, and IRE1). Activated PERK leads to induce ATF4 via phosphorylation of eIF2α, causing attenuation of translation and inducing genes encoding CHOP. Upon IRE1 activation, TRAF2 and sXBPmRNA1 splicing are initiated in the cytoplasm, subsequently leading to activation of UPR target genes. The degradation of ATF6 is increased through recruitment of ATF6, a UPR sensor. ATF6 translocates to the Golgi and is cleaved to a nucleus targeting form that promotes expression of UPR-responsive genes. The consequences of UPR activation are necessary for viral replication and pathogenesis. ATF, activating transcription factor; CHOP, C/EBP homologous protein; ER, endoplasmic reticulum; IRE1, inositol-requiring enzyme; PERK, protein kinase RNA like ER kinase; UPR, unfolded protein response.

Figure 6.

Graphic proviral functions of autophagy. There are five possible mechanisms for modulating viral replication by autophagy. Amphisome formation is thought to be beneficial for viral cellular entry and replication. Induction of autophagosome formation is also important for some virus’ replication. Furthermore, viruses initiate autophagy to benefit from lipid droplets as an energy source during viral replication. Free fatty acids are liberated from lipid droplets during autophagy to produce ATP. Viruses also stimulate autophagy to subvert immune responses by selectively degrading key regulatory molecules. Another mechanism is that viruses promote their replication by prolonging cell survival and suppressing cell death. The mechanistic details related to proviral functions of autophagy are discussed in the text.

Figure 7.

Graphic representation of apoptosis and viral replication. Viral infection, in general, can induce both intrinsic and extrinsic apoptotic pathways. Viruses like CHIKV, CCHFV and RVFV initiate extrinsic signals through cell death ligands (e.g. FasL, APO-2L, TRAIL, TNF), causing caspases-8 activation which then triggers caspases-3, -6 and -7). AHSV and WNV directly trigger caspase 3; however, CHIKV targets caspase 9. DENV and WNV affect the intrinsic pathway of apoptosis through stimulation of P53. Once P53 is activated, mitochondria-dependent apoptosis can be activated. Viral infection can also induce PKR and this kinase can affect eIF2a, resulting in activation of effector caspases and initiation of apoptosis. Viruses can also have anti-apoptotic activity. DENV, WNV and JEV trigger survival signalling through PI3K-AKT signalling pathway. PKR can be initiated by Sindbis virus which leads to inhibition of cellular translation through eIF2a phosphorylation, suppressing Mcl-1 biosynthesis. Sindbis virus can regulate 14-3-3 through activation of JNK followed by induction of PKR (for other details see text). AHSV, African horse sickness virus; CHIKV, Chikungunya virus; CCHF, Crimean–Congo haemorrhagic fever virus; DENV, Dengue virus; FasL, Fas (Apo-1/CD95) ligand; JEV, Japanese encephalitis virus; JNK, c-Jun N-terminal kinases; TNF, tumour necrosis factor receptor; TRAIL, TNF-related apoptosis-inducing ligand; PKR, (dsRNA)-activated protein kinase; RVFV, Rift valley fever virus; WNV, West Nile virus.