Immediate Early Gene Expression


Part of our laboratory studies how the expression of the HCMV immediate early (IE) genes is regulated during both lytic and latent infections, and how modulation of HCMV IE gene expression might be used to treat lytic infections, or control / clear latent infections.


Our most recent IE gene expression paper:

 

Lee, S.H., Albright, E.R., Lee, J-H., Jacobs, D., and Kalejta, R.F. (2015) Cellular defense against latent colonization foiled by human cytomegalovirus UL138 protein. Sci. Adv. 1(10):e1501164 PMCID: PMC4681346 pdf >publications

A classic IE gene expression paper from our lab:

Saffert, R.T., and Kalejta, R.F., (2006)  Inactivating a cellular intrinsic immune defense mediated by Daxx is the mechanism through which the human cytomegalovirus pp71 protein stimulates viral immediate early gene synthesis. J. Virol. 80 (8), 3863-3871.  PMCID: PMC1440479 pdf   >publications

One of our IE gene expression review articles:

Kalejta, R.F. (2013) Pre-Immediate Early Tegument Protein Functions. In Cytomegaloviruses: From Molecular Pathogenesis to Intervention, Volume 1, Chapter 9; M.J. Reddehase, ed. (Norfolk, UK, Caister Academic Press), pp.141-151; PMCID: none pdf  >publications

 


 

Upon entry into a cell, HCMV can either initiate a productive lytic infection or establish a latent infection where the viral genome is maintained without progeny virion production.  In general, lytic infections are initiated when the virus infects terminally differentiated cells such as fibroblasts, and latent infections are established when the virus infects certain incompletely differentiated cells of the myeloid lineage, such as CD34+ hematopoietic progenitor cells.

Fusion of the virion lipid envelope to the cell membrane during viral entry introduces the virion contents into the cell.  These include not only the double-stranded DNA genome (which is packaged in an icosahedral capsid) but also the proteins found between the capsid and envelope of infectious virions that form a proteinaceous layer known as the tegument.  A critical activity of the tegument is to initiate the lytic replication cycle by activating the expression of the first set of viral lytic phase genes that encode the viral immediate early (IE) proteins.  The most prominent IE proteins (IE1 and IE2) are encoded by a single locus whose transcription is controlled by the Major Immediate Early Promoter (MIEP) and activated by a tegument-delivered viral protein named pp71.

The general mechanism through which pp71 activates IE gene expression is well established, and involves counteracting the effects of a cellular intrinsic immune defense designed to silence the incoming viral genome. Upon entry into the nucleus the viral genome becomes associated with histones, as well as cellular proteins that normally localize to Promyelocytic Leukemia-Nuclear Body (PML-NB) structures. HCMV genomes associated with PML-NBs at very early times after infection display a chromatin structure reminiscent of transcriptionally silent heterochromatin, and viral gene expression is not observed. In cells destined to initiate a lytic infection (such as terminally differentiated fibroblasts), tegument-delivered pp71 traffics to the nucleus and counteracts the PML-NB intrinsic defense.

A major target of pp71 is the cellular Daxx protein, a transcriptional co-repressor found at PML-NBs that silences gene expression through the recruitment of histone deacetylases (HDACs) to targeted promoters. pp71 neutralizes the ability of Daxx to silence HCMV IE gene expression by displacing the co-repressor ATRX, inducing Daxx sumoylation and eventually its proteasome-dependent, ubiquitin-independent degradation. Activation of IE1 expression by pp71 allows this protein to disrupt PML-NB structure, further weakening this intrinsic defense, amplifying IE gene expression, and fully activating the lytic replication cycle.

When experimental latent infections are established in vitro in THP-1 monocytes or primary CD34+ cells, the PML-NB intrinsic defense is not neutralized and Daxx is not degraded because tegument-delivered pp71 remains in the cytoplasm.  Daxx silences viral IE gene expression in latently infected cells in cooperation with the viral UL138 protein, and at least one additional (yet to be identified) viral factor.  Daxx appears to silence viral gene expression through histone deacetylation.  UL138 is a Golgi-localized protein that silences viral gene expression by preventing cellular lysine demethylases (KDMs) and a protein they interact with, CtBP1, from associating with the MIEP where they would remove repressive histone methylation marks (such as H3K23me3 and H3K9me2) thereby activating transcription.