Biology Reference
In-Depth Information
early. Consistently, TUNEL labeling, expected to reflect caspase-3-mediated acti-
vation of nucleases, occurs by 48 h. In contrast, detection of active caspase-3 or
cleaved substrate (PARP) is apparently variable and occurs consistently only by
96 h. Thus, evaluation of specific steps along the apoptosis pathway may highlight
important viral controls. Nevertheless, a deletion mutant of UL38 in pAD/Cre
induces an apoptotic death that is repaired by growth on pUL38-expressing cells.
pUL38 is sufficient to inhibit death induced by E1B-19-kDA-deficient adenovirus
or thapsigargin, but is ineffective against Fas-mediated apoptosis. Thus, pUL38
protects from intrinsic but not extrinsic death signals. Thus far, little is known of
the pUL38-dependent antiapoptotic mechanism or protein domains required for
function, but these studies will likely be included in future endeavors.
M45 Is a Cell Type-Specific Survival Factor
Betaherpesviruses, including HCMV, encode the UL45 genes that are related by
sequence but not function to ribonucleotide reductase (Chee et al. 1990; Patrone
et al. 2003; Lembo et al. 2004). Viral mutants that disrupt the MCMV M45 gene
induce apoptosis and are growth-restricted in endothelial cells and macrophages
but not fibroblasts, bone marrow stromal cells, or hepatocytes (Brune et al. 2001).
Although early and late genes are expressed, infection induces apoptosis and
infectious progeny are not produced. Assays predictive of apoptosis, including
nuclease activity and phosphatidylserine exposure, implicate this pathway; how-
ever, a direct link between decreased apoptosis and rescued growth has not been
established. Further, M45-dependent survival in apoptosis models or viral repli-
cation has not been demonstrated, and it is unclear how the phenotype of the
mutant virus relates to apoptosis pathways and M45. However, disruption of M45
produces a nonpathogenic virus (Lembo et al. 2004), and further studies will
likely answer these important questions.
Neither replication in endothelial cells or resistance to induced intrinsic
stress requires HCMV UL45 (Hahn et al. 2002). Thus, the intrinsic stresses
revealed by deletion of MCMV M45 are apparently controlled by other viral
factors in HCMV. In contrast, UL45 does increase viral production of the labo-
ratory strain AD169
var
ATCC in fibroblasts following a low multiplicity infec-
tion (Patrone et al. 2003). However, decreased yield does not result from
increased apoptosis. The resistance of HCMV to extrinsic but not intrinsic
apoptosis is also halved, but the contribution of UL45 is unknown and UL45-
expressing fibroblasts remain sensitive to Fas-mediated apoptosis. Considerable
effort has been required to evaluate the potential of UL45 as a ribonucleotide
reductase (Patrone et al. 2003; Lembo et al. 2004). Future studies that define the UL45
function that increases low multiplicity growth and the M45 function that
permits replication in endothelial cells will likely clarify the role of this
perplexing gene.
