Cellular autophagy is an intracellular membrane trafficking
pathway which delivers cytoplasmic material to lysosomes where the material is
degraded thereby removing not only damaged organelles -as in the case of
mitophagy- but also as part of the unfolded protein response misfolded proteins
(aggrephagy) or the replication process of viruses (xenophagy). Generally,
autophagy is induced under conditions of cellular stress such as nutrient
withdrawal, DNA damage, or viral infections. During this process, isolation
membranes (the phagosome) derived from the ER are formed and expanded into
double-membrane autophagosomes that engulf the cellular cargo. Autophagosomes
then fuse either with late Endosomes or Lysosomes forming the autolysosome
where the cargo is degraded by lysosomal enzymes. The induction of autophagy is
regulated by mammalian target of rapamycin complex-1 (mTORC1) kinase, that
regulates a complex consisting of unc-51-like kinase (ULK-1), focal adhesion
kinase family interacting protein of 200 kDa (FIP200), Atg13, and Atg101. ULK-1
activates a downstream complex couch as the phsophatidylinositol 3-kinase
(PI3-kinase) complex (Atg6/beclin1-Atg14-Vps15-Vps34) and the Atg12
(Atg12-Atg5-Atg16) system as well as the LC-3 ubiquitin-like conjugation system
that converts LC3-I to LC3-II in a PI3-kinase dependent manner by recruiting
the Atg12-Atg5-Atg16 complex to the phagosome.
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| Outline of autophagy, its enzymes and inhibitors |
In the absence of starvation, mTORC1
inhibitors such as rapamycin or torin can activate the formation of the autophagosome,
whereas PI-3 kinase inhibitors such as Wortmannin can inhibit the recruitment
of Beclin1/Atg14/Vps15/Vps34. In general, the engulfment of cytoplasmic
components is non-specific but ubiquitinylated proteins can be recruited to the
autophagosomes specifically by nuclear or cytosolic p62/Sequestosome-1 (SQSTM1)
thus binding polyubiquitinated proteins to membrane-associated LC3-II.
In contrast to intracellular cargo, extracellular cargo that
is internalized by endocytosis, endocytosed macromolecules are delivered to
lysosomes via endosomes, bypassing the autophagy pathway. This pathway involves
a “kiss and run” as well as a complete fusion event, the latter dependent on
the presence of a target membrane (t-) SNARE complex and a vesicle membrane
(v-) SNARE complex which form a tetrameric transSNARE complex. Late endosomes
can either fuse with other late endosomes (“homotypic fusion”) or other
lysosomes (“heterotypic fusion”), the former requiring Syntaxin-8, VAMP-8, and
Vti-1b and the latter Syntaxin -11, VAMP-7 as well as Vti-1b. Both homotypic
and heterotypic fusion are also dependent on Rab5GTPase, Rab7GTPase, N-ethylmaleimide (NEM)
sensitive factor (NSF) and its soluble associated proteins (SNAPs).
Both the autophagy and the endocytic pathway are not
completely separated but united by a common interlocutor, UV radiation
resistance-associated (UVRAG). UVRAG activates autophagy by associating with
Beclin-1 (thus extending the phagophore) as well as inducing the maturation of
the autophagosome in later stages via binding to C/Vps, and accelerating
endocytic transport by activating Rab7 (thus leading to heterotypic fusion of
the late endosome with lysosomes). In
addition to the role in autophagy and fusion of endosomes with lysosomes, UVRAG
also plays a role in the integrity of the ER and the Golgi as well in the DNA
damage response.
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| UVRAG as the interlocutor between autophagy and homotypic endosome fusion |
Although the role of various autophagy related proteins in viral
infections is well established, the role of UVRAG in particular in mediating
viral entry has been elusive. A potential role for UVRAG in mediating viral
entry can be postulated from the observation that in UVRAG deficient cells cell
surface receptor degradation is downregulated. Since UVRAG overexpression is
known to target viral proteins to autophagosomes (and thus lead to potential
degradation of viral components) and cell surface receptors to lysosomal
degradation, findings that suggest that UVRAG overexpression leads to increased
viral replication of two negative strand RNA viruses -Influenza A and Vesicular
Stomatitis Virus (VSV) - seem at first counterintuitive. Both viruses however
encode proteins that inhibit the autophagy pathway (VSV-G and Influenza Virus
M2 protein) thus counteracting the antiviral autophagy response, either by
inhibiting Akt kinase mediated activation of mTOR (VSV-G) or the degradation of
the autophagosome (Influenza Virus M2). On the other hand, UVRAG seems to be
required for the replication of VSV and Influenza Virus, suggesting that UVRAG
targets internalized viral particles to structures, that prevent them from
being degraded and/or recognised by Pattern Recognition Receptors (PRRs) which
as we have seen in a different post are an essential part of the cellular
antiviral response. By infecting HeLa cells with DiI-labelled VSV, it was shown that viral particles localize to
(acidic) late endosomes in a UVRAG dependent manner. Mutational analysis of
UVRAG determined that UVRAG mediated virus entry is dependent on the
interaction between the C2 and CDD domain of UVRAG and C/Vps as well as Vps-16
and -18. Successful targeting of VSV to endosomes is in addition dependent on
VAMP-8. Indeed, VAMP-8 is recruited to VSV-G and Influenza Virus M protein
positive vesicles. Since the endosome is involved in recognizing viral RNA via Toll-like receptors, it might be interesting to determine if these endosomes
are positive for TLRs or if viral proteins inhibit the antiviral signaling.
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| VSV-G and UVRAG |
It remains to be seen if the infection of cells with
positive strand RNA viruses such as Coronaviruses or Enteroviruses or the
infection with DNA viruses induces the formation of similar structures or if
these are limited to negative sense RNA viruses. As discussed before, positive
strand RNA viruses induce the formation of replication transcription complexes
and induce autophagy via viral nsps. Targeting viral particles to late
endosomes by recruiting UVRAG however might be required early in the replication
cycle prior to the formation of RTCs. In this context it might be possible that
these early (hypothetical) structures are transported to the site of
replication via the cytoskeleton. Indeed, late endosomes have been shown to
be transported towards the perinuclear MTOC in a Rab7GTPase dependent manner
and VSV-G co-localises with acidic endosomal vesicles in the perinuclear
region in a Nocadozole sensitive manner.
Finally, it remains to be seen if the endosomal structures induced by
the interaction between VSV-G and Influenza Virus respectively, can mature into
LC3-II positive autophagosomes via ATG9; it might be possible however that both
VSV-G and Influenza Virus M2 proteins. The finding that the localisation of VSV and Influenza Virus to
Autophagosomes are known to be induced upon entry of various other viruses. Food Mouth and Disease Virus (FMDV) and Vaccinia Virus (VACV) induces the formation of autophagosomes in a PI3-Kinase independent pathway (maybe UVRAG and/or ATG9 dependent?), and Echovirus 7 utilizes the autophagy pathway for its entry by an yet unidentified mechanism (although ATG16L is required) ditto for Dengue Virus. African Swine Fever Virus (AFSV) particles localize to the late endosome, suggesting that UVRAG might be important as well.
In conclusion, UVRAG might be an universal connector between viral entry the induction of autophagy.
Liang, C., Lee, J., Inn, K., Gack, M., Li, Q., Roberts, E., Vergne, I., Deretic, V., Feng, P., Akazawa, C., & Jung, J. (2008). Beclin1-binding UVRAG targets the class C Vps complex to coordinate autophagosome maturation and endocytic trafficking Nature Cell Biology, 10 (7), 776-787 DOI: 10.1038/ncb1740
He S, Ni D, Ma B, Lee JH, Zhang T, Ghozalli I, Pirooz SD, Zhao Z, Bharatham N, Li B, Oh S, Lee WH, Takahashi Y, Wang HG, Minassian A, Feng P, Deretic V, Pepperkok R, Tagaya M, Yoon HS, & Liang C (2013). PtdIns(3)P-bound UVRAG coordinates Golgi-ER retrograde and Atg9 transport by differential interactions with the ER tether and the beclin 1 complex. Nature cell biology, 15 (10), 1206-19 PMID: 24056303
Lamb CA, Yoshimori T, & Tooze SA (2013). The autophagosome: origins unknown, biogenesis complex. Nature reviews. Molecular cell biology, 14 (12), 759-74 PMID: 24201109
Ao X, Zou L, & Wu Y (2014). Regulation of autophagy by the Rab GTPase network. Cell death and differentiation, 21 (3), 348-58 PMID: 24440914
Pirooz SD, He S, Zhang T, Zhang X, Zhao Z, Oh S, O'Connell D, Khalilzadeh P, Amini-Bavil-Olyaee S, Farzan M, & Liang C (2014). UVRAG is required for virus entry through combinatorial interaction with the class C-Vps complex and SNAREs. Proceedings of the National Academy of Sciences of the United States of America, 111 (7), 2716-21 PMID: 24550300
Huotari, J., & Helenius, A. (2011). Endosome maturation The EMBO Journal, 30 (17), 3481-3500 DOI: 10.1038/emboj.2011.286
Cureton, D., Massol, R., Saffarian, S., Kirchhausen, T., & Whelan, S. (2009). Vesicular Stomatitis Virus Enters Cells through Vesicles Incompletely Coated with Clathrin That Depend upon Actin for Internalization PLoS Pathogens, 5 (4) DOI: 10.1371/journal.ppat.1000394
Berryman, S., Brooks, E., Burman, A., Hawes, P., Roberts, R., Netherton, C., Monaghan, P., Whelband, M., Cottam, E., Elazar, Z., Jackson, T., & Wileman, T. (2012). Foot-and-Mouth Disease Virus Induces Autophagosomes during Cell Entry via a Class III Phosphatidylinositol 3-Kinase-Independent Pathway Journal of Virology, 86 (23), 12940-12953 DOI: 10.1128/JVI.00846-12
Diehl N, & Schaal H (2013). Make yourself at home: viral hijacking of the PI3K/Akt signaling pathway. Viruses, 5 (12), 3192-212 PMID: 24351799
Kim C, & Bergelson JM (2014). Echovirus 7 entry into polarized caco-2 intestinal epithelial cells involves core components of the autophagy machinery. Journal of virology, 88 (1), 434-43 PMID: 24155402
Alonso C, Galindo I, Cuesta-Geijo MA, Cabezas M, Hernaez B, & Muñoz-Moreno R (2013). African swine fever virus-cell interactions: from virus entry to cell survival. Virus research, 173 (1), 42-57 PMID: 23262167
Jae LT, Raaben M, Herbert AS, Kuehne AI, Wirchnianski AS, Soh TK, Stubbs SH, Janssen H, Damme M, Saftig P, Whelan SP, Dye JM, & Brummelkamp TR (2014). Virus entry. Lassa virus entry requires a trigger-induced receptor switch. Science (New York, N.Y.), 344 (6191), 1506-10 PMID: 24970085
Baird NL, York J, & Nunberg JH (2012). Arenavirus infection induces discrete cytosolic structures for RNA replication. Journal of virology, 86 (20), 11301-10 PMID: 22875974
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