Human cytomegalovirus (HCMV) is a type of herpesvirus. HCMV infects the majority of individuals, but a healthy immune system is able to suppress a primary infection. Upon initial infection HCMV inhibits an antiviral response through the up-regulation of myeloid cell leukemia-1 protein (MCL-1). The induction of MCL-1 expression is dependent on the activation of the ERK-MAPK pathway rather than viral gene expression.(1) The ERK-MAPK pathway induces proteins such as mitogen-activated protein kinase and extracellular signal-regulated kinase. After the primary infection is controlled HCMV establishes a latent infection in undifferentiated bone marrow precursor cells and monocytes cells.(2) The activation of the ERK-MAPK pathway impacts long-term latency in progenitor cells by priming them from the initial virus encounter and enabling them for future reactivation.(1) When the individual with the latent infection becomes immunocompromised such as during transplants, however, the latent HCMV can become reactivated and have detrimental effects. During viral latency there is limited viral gene expression and no detectable virion production. One viral protein expressed during latency is latency unique natural antigen (LUNA) encoded by the latency associated transcript UL81-82ast. Previous studies have suggested that LUNA is involved in the regulation of HCMV reactivation by suppressing lytic transcription and without LUNA may not be able to enter into latency in order to be reactivated later.(2,3) Another viral protein present during latency is UL138. The UL133-138 is required for HCMV infection in endothelial cells.(4) The function of UL138 in productive and lytic infection is understood, but not much is known about the function of UL138 in latent infection.
Using plasma membrane profiling the authors determined which plasma membrane protein levels were affected by the presence of UL138 during HCMV latency. Multidrug resistance-associated protein-1 (MRP1) was the most dramatically down-regulated of the three proteins affected by the presence of UL138. In fact, in the presence of UL138 MRP1 was undetectable in cells, suggesting that it might be getting degraded. MRP1 is important in multidrug resistance and handling organic anions. It is found in bone marrow progenitor calls and monocytes, as well as mature leukocytes(5). In infected cells, the deletion of UL138 restored the expression of MRP1, indication that UL138 is necessary for MRP1 down-regulation. UL138 is also known to be present during lytic infection in addition to latent infection and high levels of UL138 48 hours after lysis coincides with decreased levels of MRP1. It was determined that MRP1 mRNA levels were not decreased from UL138 expression, so the down-regulation of MRP1 is probably post-transcriptional. MRP1 produces many cytotoxic agents including vincristine. When vincristine was added to HCMV-latent monocytes the number of latent cells decreased as well as the expression of UL138 RNA. This indicates that vincristine caused the death of latently infected cells. In addition to reducing the number of latently infected cells through killing, vincristine also reduced the number of cells in which reactivation of HCMV occurred after differentiation of monocytes to dendritic cells.
What is not understood is why UL138 targets MRP1 for degradation in latent cells. One possible reason is that the reduction of MRP1 also reduced LTC4 which is a substrate of MRP1 and therefore inhibited migration of infected cells and impaired the activation of an immune response. The decreased expression of MRP1 could also inhibit the differentiation of precursor cells until the environment for reactivation was sufficient, therefore maintaining latent infection.
The findings of this paper are important because they provide possible strategies for blocking reactivation of HCMV latent cells. Patients that have to go through transplantations are forced to become immunocompromised so that their immune system will no view the new organ as a foreign body and attack and try to kill it. Becoming immunocompromised is dangerous because it is the suitable environment for HCMV reactivation which is associated with high morbidity and mortality. Treatment of vincristine to latently infected cells decreased the levels of reactivated virus after differentiation. From this study we also understand that we could scan for cells with decreased and non-present MRP1 expression to target for killing before transplantation. Future studies could investigate the relationship between UL138 and Delta-like protein 1 (DLL1), the other plasma membrane protein which was down-regulated by UL138 to determine DLL1 function in maintained HCMV latency.
I had a few critiques of the paper. There is one line in the paper that states, “In the presence of UL138, [35S]-methionine radiolabeled 170-kD MRP1 matured normally through the secretory pathway before rapidly degrading, with a reduction in half-life from 16 to 20 hours to less than 3 hours, suggesting that MRP1 targets UL138 for degradation in the late secretory pathway.” I think that MRP1 and the second UL138 should be reversed in the sentence. I also thought the information about SNARF-1 export in cells where MRP1 was not expressed was not necessary. I would have liked for the authors to have gone more in depth on how UL138 reduced MRP1 expression through lysosomal degradation and why MRP1 was targeted by UL138 for degradation.
Weekes, Michael P., Shireen Y.L. Tan, et al. "Latency-Associated Degradation of the MRP1 Drug Transporter During Latent Human Cytomegalovirus Infection." Science. 340. (2013): 199-202. Web. 3 May. 2013.
1. Reeves, MB, A Breidenstein, and T Compton. "Human cytomegalovirus activation of ERK and myeloid cell leukemia-1 protein correlates with survival of latently infected cells." National Academy of Sciences. 109. (2012): 588-93. Web. 3 May. 2013.
2. O'Connor, CM, and EA Murphy. "A myeloid progenitor cell line capable of supporting human cytomegalovirus latency and reactivation, resulting in infectious progeny." Journal of Virology. 86.18 (2012): 9854-65.
3. Keyes, LR, D Hargett, et al. "HCMV protein LUNA is required for viral reactivation from latently infected primary CD14 cells." Plos One. 7.12 (2012): e52827. Web. 3 May. 2013.
4. Bughio, F, DA Elliott, and F Goodrum. "An endothelial cell-specific requirement for the UL133-UL138 locus of human cytomegalovirus for efficient virus maturation." Journal of Virology. 87.6 (2013): 3062-75. Web. 3 May. 2013.
5. Laupeze, B, L Amiot, et al. "Multidrug resistance protein (MRP) activity in normal mature leukocytes and CD34-positive hematopoietic cells from peripheral blood." Life Sciences. 68.11 (2001): 1323-31. Web. 3 May. 2013.