Biology Reference
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than that of rats infected with wild type virus (Beisser et al. 1998). Additionally,
whereas wild type virus can be recovered from salivary glands of both MCMV-
infected mice and RCMV-infected rats, their respective M33 and R33 deletion
mutant counterparts remained undetectable in these organs throughout infection
(Davis-Poynter et al. 1997; Beisser et al. 1998). Other knockout CMVs have been
generated that had lower salivary gland replication rate phenotypes (Manning et al.
1992; Xiao et al. 2000; Kaptein et al. 2004), but the M33/R33 deletion mutant
studies indicated an absolute requirement of the MCMV M33 and RCMV R33
vGPCR genes for salivary gland tropism in vivo.
Deletion of M78 and R78 from the MCMV or RCMV genome, respectively,
resulted in mutant strains that were attenuated both in vitro and in vivo (Beisser et al.
1999; Oliveira and Shenk 2001). In contrast, deletion of UL78 from the HCMV
genome resulted in a mutant strain that was neither attenuated in cell culture nor in
renal artery explant culture (Michel et al. 2005). Yet, the expression of U51-specific
siRNA in HHV-6-infected cells resulted in significantly lower levels of viral replica-
tion than the expression of control siRNA in infected cells (Zhen et a. 2005). These
results indicate that the vGPCRs encoded by M78, R78 and U51 may fulfill different
roles in replication than those encoded by HCMV UL78.
Generation of mutants in which the US28 gene was deleted from the HCMV
genome has been instrumental in confirming the US28-specific signaling activities
of infected cells. Deletion of US28 resulted in a phenotype in which infected cells
were no longer able to bind RANTES and mobilize intracellular Ca
2+
(Vieira et al.
1998). Infected smooth muscle cells were no longer capable of chemokinesis or
chemotaxis (Streblow et al. 1999). This clearly demonstrated a role for US28 in the
dissemination of HCMV within the host, by enabling infected cells to navigate
through tissue by chemotaxis. Cells infected with US28 deletion mutant virus were
no longer capable of sequestering RANTES and MCP-1 (Bodaghi et al. 1998;
Randolph-Habecker et al. 2002), allowing more monocytes to be attracted by the
medium from cell cultures infected with US28-deleted virus than by medium for
cultures infected with wild type HCMV (Randolph-Habecker et al. 2002). Additionally,
cells infected with US28 deletion mutant virus showed an increase in IL-8 secretion.
These phenotypes indicate that HCMV US28 has an anti-inflammatory effect, which
could ensure persistence of the infection.
Localization of CMV vGPCRs
Chemokine receptors are, subsequent to their synthesis, transported to the outer
cellular membrane by intrinsic GPCR domains. At the outer membrane, they can
interact with both extracellular chemokines and intracellular membrane-bound
heterotrimeric G proteins to establish a signaling bridge. Of all CMV vGPCRs
detected in either infected or transfected cells, only the vGPCRs encoded by
MCMV M33 and RCMV R33 were clearly localized at the outer cellular membrane
(Waldhoer et al. 2002; Casarosa et al. 2003). All other CMV vGPCRs were
