In human astrocytes neurotropic flaviviruses increase autophagy, yet their replication is autophagy-independent

Abstract

Astrocytes, an abundant type of glial cells, are the key cells providing homeostasis in the central nervous system. Due to their susceptibility to infection, combined with high resilience to virus-induced cell death, astrocytes are now considered one of the principal types of cells, responsible for virus retention and dissemination within the brain. Autophagy plays an important role in elimination of intracellular components and in maintaining cellular homeostasis and is also intertwined with the life cycle of viruses. The physiological significance of autophagy in astrocytes, in connection with the life cycle and transmission of viruses, remains poorly investigated. In the present study, we investigated flavivirus-induced modulation of autophagy in human astrocytes by monitoring a tandem fluorescent-tagged LC3 probe (mRFP-EGFP-LC3) with confocal and super-resolution fluorescence microscopy. Astrocytes were infected with tick-borne encephalitis virus (TBEV) or West Nile virus (WNV), both pathogenic flaviviruses, and with mosquito-only flavivirus (MOF), which is considered non-pathogenic. The results revealed that human astrocytes are susceptible to infection with TBEV, WNV and to a much lower extent also to MOF. Infection and replication rates of TBEV and WNV are paralleled by increased rate of autophagy, whereas autophagosome maturation and the size of autophagic compartments are not affected. Modulation of autophagy by rapamycin and wortmannin does not influence TBEV and WNV replication rate, whereas bafilomycin A1 attenuates their replication and infectivity. In human astrocytes infected with MOF, the low infectivity and the lack of efficient replication of this flavivirus are mirrored by the absence of an autophagic response.

Astrocytes Are a Key Target for Neurotropic Viral Infection

“…In the last decade, interest in other cell types, especially astrocytes, has grown considerably ([ 19 , 21 , 36 , 37 , 38 , 39 , 40 ] and references within), but the pathophysiology of viral infection in astrocytes remains poorly understood. However, several lines of evidence support the importance of astrocytes in viral infections of the CNS: astrocytes (i) are one of the first cell types to be infected after viruses cross the BBB; (ii) are almost as numerous as neurons [ 41 ] and have a surface area-to-volume ratio that is fourfold higher than that of neurons [ 42 ]; (iii) express several viral receptors (as discussed in detail later); (iv) are highly metabolically active and are closely coupled metabolically with neurons [ 35 , 43 , 44 , 45 ]; and (v) can produce and release high amounts of viral progeny [ 38 , 40 , 46 ]. The relatively high level of virus production in astrocytes likely relies on the properties of astroglial aerobic glycolysis [ 47 ].…”

“…IFN-I is a crucial anti-viral factor in astrocytes; it restricts viral growth via autocrine and paracrine signalling through the interferon-α/β receptors, as demonstrated after infection of astrocytes with TBEV, JEV, WNV, and ZIKV [ 124 ]. When autophagy was monitored in human astrocytes, induction of autophagy was detected after infection with TBEV and WNV ( Figure 3 ) [ 46 ]. However, inhibition of the formation of autophagic structures does not affect viral replication or the production of infective viruses [ 46 ].…”

“…When autophagy was monitored in human astrocytes, induction of autophagy was detected after infection with TBEV and WNV ( Figure 3 ) [ 46 ]. However, inhibition of the formation of autophagic structures does not affect viral replication or the production of infective viruses [ 46 ]. Thus, in these cells, remodelling of the ER with the unfolded protein response pathway, not autophagic structures, appears to be a decisive factor for flavivirus replication [ 46 ].…”

Immune Functions of Astrocytes in Viral Neuroinfections

“…In addition to their well-recognized roles in sustaining and modulating functioning of neurons in normal physiologic conditions, they also participate in the development and progression of several diseases of the CNS [ 2 , 3 , 4 , 5 ]. An increasing amount of data on viral infections of astrocytes, in conjunction with further knowledge of cellular functions that are modified by virus infections (e.g., upregulation of cytokines, vesicular traffic, and autophagy; Figure 1 ) [ 6 , 7 , 8 , 9 , 10 , 11 ], have led to a whole new perspective on astrocytes in terms of their contribution to CNS diseases. To date, infections of astrocytes have been documented for viruses from different families, including enveloped positive-sense single-stranded RNA viruses (e.g., Flaviviridae, Coronaviridae, Retroviridae, and Togaviridae), enveloped negative-sense single-stranded RNA (e.g., Paramyxoviridae, Rhabdoviridae, and Bunyaviridae), non-enveloped viruses with a single-stranded RNA (Picomaviridae), and enveloped double-stranded DNA viruses (Herpeseviridae).…”

“…However, coronavirus infection does not always produce infectious virions in astrocytes [ 32 , 33 , 34 , 35 , 36 ]. Astrocytes are also susceptible to infection with La Crosse virus and mosquito-only flavivirus mosquito-borne pathogens, although infection of such cells is mostly non-productive [ 11 , 37 ]. The common denominator of infection with neurotropic viruses is the induction of a reactive astrogliosis [ 38 ].…”