An orchestrated program regulating secretory pathway genes and cargos by the transmembrane transcription factor CREB-H.

CREB3 proteins comprise a set of ER-localized bZip transcription factors defined by the presence of a transmembrane domain. They are regulated by inter-compartmental transport, Golgi cleavage and nuclear transport where they promote appropriate transcriptional responses. Although CREB3 proteins play key roles in differentiation, inflammation and metabolism, a general framework relating their defining features to these diverse activities is lacking. We identify unique features of CREB3 organization including the ATB domain, which we show it is essential for transcriptional activity. This domain is absent in all other human bZip factors, but conserved in Drosophila CREBA, which controls secretory pathway genes (SPGs). Furthermore, each of the five human CREB3 factors was capable of activating SPGs in Drosophila, dependent upon the ATB domain. Expression of the CREB3 protein, CREB-H, in 293 cells, upregulated genes involved in secretory capacity, extracellular matrix formation and lipid metabolism and increased secretion of specific cargos. In liver cells, which normally express CREB-H, the active form specifically induced secretion of apolipoproteins, including ApoA-IV, ApoAI, consistent with data implicating CREB-H in metabolic homeostasis. Based on these data and other recent studies, we propose a general role for the CREB3 family in regulating secretory capacity, with particular relevance to specialized cargos.

Trafficking of the bZIP transmembrane transcription factor CREB-H into alternate pathways of ERAD and stress-regulated intramembrane proteolysis.

CREB-H is an ATF6-related, transmembrane transcription factor that, in response to endoplasmic reticulum (ER)-associated stress, is cleaved by Golgi proteases and transported to the nucleus to effect appropriate adaptive responses. We characterize the ER processing and turnover of CREB-H with results which have important implications for ER stress regulation and signalling. We show that CREB-H is glycosylated and demonstrate that both the ER and nuclear forms of CREB-H have short half-lives. We also show that CREB-H is subject to cycles of retrotranslocation, deglycosylation and degradation through the ER-associated degradation (ERAD) pathway. Proteasome inhibition resulted in accumulation of a cytosolic intermediate but additionally, in contrast to inhibition of glycosylation, promoted specific cleavage of CREB-H and nuclear transport of the N-terminal-truncated product. Our data indicate that under normal conditions CREB-H is transported back from the ER to the cytosol, where it is subject to ERAD, but under conditions that repress proteasome function or promote load CREB-H is diverted from this pathway instead undergoing cleavage and nuclear transport. Finally, we identify a cytoplasmic determinant involved in CREB-H ER retention, deletion of which results in constitutive Golgi transport and corresponding cleavage. We present a model where cellular stresses may be sensed at different levels by different members of the basic and leucine zipper domain transmembrane proteins.

Modelling dynamics of the type I interferon response to in vitro viral infection.

Innate immunity is crucial in the early stages of resistance to novel viral infection. The family of cytokines known as the interferons (IFNs) forms an essential component of this system: they are responsible for signalling that an infection is underway and for promoting an antiviral response in susceptible cells. We construct a spatial stochastic model, parameterized by experimental data and informed by analytic approximation, to capture the dynamics of virus-IFN interaction during in vitro infection of Madin-Darby bovine kidney cell monolayers by Herpes simplex virus 1. The dose dependence of infection progression, subsequent monolayer destruction and IFN-beta production are investigated. Implications for in vivo infections, in particular the priming of susceptible cells by IFN-beta during infection, are considered.

Characterization of a potent refractory state and persistence of herpes simplex virus 1 in cell culture.

Herpes simplex virus (HSV) normally undergoes productive cytocidal infection in culture and is thought of as relatively resistant to innate immune responses such as interferon. We previously described an unusual pattern of infection in culture in MDBK cells, which after initial productive infection, surprisingly resulted in progressive suppression of replication and cell recovery. The dominance of the refractory state was due to the inability to suppress interferon production and subsequent paracrine signaling. Here, using a wild-type HSV-1 strain expressing green fluorescent protein (GFP)-VP16, we analyze aspects of long-term HSV persistence resulting from this oscillating refractory state. We show that the gradual suppression of GFP-VP16 expression correlated with a biphasic pattern of accumulation of viral DNA and extracellular virus titers. We quantify virus maintenance in a minor subpopulation of cells during subculture, show the reemergence of virus by infectious center assay, and demonstrate that this required intracellular events over a 24- to 48-h time course. We also demonstrate that conditioned medium (cMed) from infected cells induced a profound shutoff of HSV gene expression at the transcriptional level. Finally, we demonstrate that this suppression was extremely rapid, requiring only 1 h of treatment to essentially abolish HSV immediate-early expression, and surprisingly persisted for almost 2 days after removal of the cMed. These combined effects underpin the oscillating effect both in plaque progression, where infection spreads but is overwhelmed by the accumulation of inhibitory components, enabling cell recovery, and virus maintenance in a subpopulation of cells. These results may be relevant to consider in studies of HSV latency in different animal models.

Characterization of VP22 in herpes simplex virus-infected cells.

We examine biochemical characteristics of the herpes simplex virus (HSV) tegument protein VP22 by gel filtration, glycerol sedimentation, and chemical cross-linking experiments and use time course radiolabeling and immunoprecipitation assays to analyze its synthesis and interaction with other infected-cell proteins. VP22 was expressed as a delayed early protein with optimal synthesis requiring DNA replication. In immunoprecipitation assays, VP22 was found in association with several additional proteins including VP16 and a kinase activity likely to be that of UL13. Furthermore, in sizing chromatography experiments, VP22 was present in several higher-order complexes in infected cells. From gel filtration analysis the major form of VP22 migrated with a molecular mass of approximately 160 kDa, consistent with its presence as a tetramer, or a dimer complexed with other proteins, with a fraction of the protein migrating at larger molecular mass. In vitro-synthesized VP22 sedimented in a size range consistent with a mixture of tetramers and dimers. Short N- or C-terminal deletions resulted in migration almost exclusively as dimers, indicating that VP22, in the absence of additional virus-encoded proteins, could form higher-order assemblies, most likely tetramers, but that both N-and C-terminal determinants were required for stabilizing such assemblies. Consistent with this we found that isolated proteins encompassing either the N-terminal or C-terminal region of VP22 sedimented as dimers, and that the purified C-terminal domain could be cross-linked into dimeric structures. These results are discussed with regard to possible virus and host interactions involved in VP22 recruitment into virus particles.

Suppression of herpes simplex virus 1 in MDBK cells via the interferon pathway.

Herpes simplex virus (HSV) normally undergoes productive infection in culture, causing cell destruction and plaque formation. Here we characterize an unusual pattern of HSV type 1 (HSV-1) infection in MDBK cells which surprisingly results in suppression of replication, cell recovery, and maintenance of virus. Compared to Vero cells, MDBK cells supported a normal productive infection at a high multiplicity with complete cell destruction. At low multiplicity, HSV also showed an identical initial specific infectivity in the two cell types. Thereafter, the progression of infection was radically different. In contrast to the rapid plaque expansion and eventual destruction in Vero monolayers, in MDBK cells, after initial plaque formation, plaque size actually decreased and, with time, monolayers recovered. Using a green fluorescent protein (GFP)-VP16-expressing virus, we monitored infection in live individual plaques. After early stages of intense GFP-VP16 expression, expression regressed to a thin boundary at the edge of the plaques and was completely suppressed by 10 days. Cells lacking expression then began to grow into the plaque boundaries. Furthermore, following media replacement, individual cells expressing GFP-VP16 could be observed reinitiating infection. The results indicated the production of a potent inhibitory component during infection in MDBK cells, and we show the continued and prolonged presence of interferon in the medium, at times when there was no longer evidence of ongoing productive infection. We exploited the ability of V protein of simian virus 5 to degrade Stat1 and prevent interferon signaling. We established MDBK cells constitutively expressing the V protein with the resultant loss of Stat1. In comparison to the parental cells, infection in these cells now progressed at a rapid rate with expanding plaque formation. We believe the conclusions have significant implications for the study of HSV-1 and interferon signaling both in culture and in animal models.