Each year 50 to 100 million cases of dengue are reported around the world with no vaccine or easy cure (Figure 1)(CDC, 2014). Transmitted through Aedes mosquito bites, dengue has the ability to affect over 40% of the world population (CDC, 2014). The dengue virus, which causes dengue fever, is part of the Flaviviridae virus family and encodes genetic material in single (+) strand RNA. Most RNA viruses replicate in the cytoplasm of the cell, but this leaves them vulnerable to innate, or basic, immune system responses. If the immune system detects foreign RNA in the cytoplasm of a cell, signals will be released leading to cell death to prevent the spread of the invading virus.
Figure 1: Geographic map of dengue fever outbreaks in 2016
with red and pink areas of highest concern and blue regions of
low prevalence (CDC).
To evade immune system detection, dengue viruses replicate in membrane-enclosed structures, like autophagosomes, that are found in the cytoplasm. Autophagosomes are like cellular bubbles, with double layered membranes, that transport cellular contents to lysosomes (another vesicle) for degradation (Yorimitsu & Klionsky, 2005). The isolated replication space acts to hide the virus from immune responses and allows the virus to grow by copying its genome in one area before releasing it from the cell. Previously, dengue viruses have been shown to use autophagosomes for replication and to avoid cell death (Lee et al, 2008). Although preventing cell death may seem beneficial to the body, it actually is better for the infection, as it allows the virus to grow and spread to more cells without programmed cell death to stop this cycle.
Recent work by Datan et al. (2016), proposed a model to explain the upregulation or increase in autophagosome appearance in dengue-infected cells. They showed infection of a cell with dengue virus increased the levels of a signaling protein called calreticulin. High levels of this protein mark cellular endoplasmic reticulum (ER) stress, a condition causing the build up of misfolded or unfolded proteins in the ER lumen (Oslowski & Urano, 2011). The continued accumulation of incorrect proteins can lead to cell death, so the cell tries to reverse this ER stress by initiating autophagosomes to come and remove protein for degradation. Multiple pathways are used to signal autophagosome production, but the PERK pathway was identified as an especially important one since signaling occurs early during ER stress and it increases autophagosome turnover or regeneration. Inhibition of the PERK pathway reduced autophagosome production and dengue replication, although it did not affect the ability for dengue to enter the cell.
|Figure 2: Datan et al. (2016) proposed model of dengue infection and autophagosome signalling pathways.|
Aside from PERK/ER stress induced autophagy, infection alone can cause autophagosome production via activation of ATM kinase. ATM kinase signals autophagosome production very early after viral infection of dengue. Preventing ATM kinase signaling led to decreased dengue protection and replication due to the loss of autophagosomes. Together, ATM kinase and the PERK pathway in ER stress lead to increased autophagosomes, virus protection, and dengue replication. Cells gain the ability to survive dengue infection as the hidden dengue virus in autophagosomes prevents cells from programmed death or other toxic assaults. However, ultimately cell survival allows the dengue virus to continue replicating and spreading from cell to cell. Treatment for dengue infection can thus be targeted at interrupting ATM kinase or the PERK pathway during ER stress (Figure 2). Because of the multiple pathway interaction during ER stress, drugs targeted to PERK pathway inhibition would have a greater impact on autophagosome production and turnover, but then the cell will have a harder time reversing ER stress. Understanding the importance and model of autophagosome-mediated dengue replication is crucial to developing a direct cure or vaccination for this widespread disease. The research by Datan et al. (2016) moves medical advances one step closer to preventing the 22,000 deaths that occur each year from dengue infections (CDC, 2014).
Datan, E., S. G. Roy, G. Germain, N. Zali, J. E. McLean, G. Golshan, S. Harbajan, R. A. Lockshin, and Z. Zakeri. "Dengue-induced autophagy, virus replication and protection from cell death require ER stress (PERK) pathway activation." Cell death & disease 7.3 (2016): e2127.
CDC. “Dengue Epidemiology”. U.S. Department of Health and Human Services. 9 June 2014.
Lee, Ying-Ray, Huan-Yao Lei, Ming-Tao Liu, Jen-Ren Wang, Shun-Hua Chen, Ya-Fen Jiang-Shieh, Yee-Shin Lin, Trai-Ming Yeh, Ching-Chuan Liu, and Hsiao-Sheng Liu. "Autophagic machinery activated by dengue virus enhances virus replication." Virology 374.2 (2008): 240-248.
Oslowski, Christine M., and Fumihiko Urano. "Measuring ER stress and the unfolded protein response using mammalian tissue culture system." Methods in enzymology 490 (2011): 71.
Yorimitsu, T., and Daniel J. Klionsky. "Autophagy: molecular machinery for self-eating." Cell Death & Differentiation 12 (2005): 1542-1552.
Figure 1: CDC 2016 - http://images.medicinenet.com/images/appictures/dengue-fever-s3-outbreaks-cdc-map-2016.jpg
Figure 2: Datan et al. 2016 - Figure 6 from source paper