Enteropathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 83

Apoptosis is another part of the host innate immune response induced by
bacterial infection; this action aids in the elimination of the infecting pathogen
and promotes a dendritic-cell-mediated immune response. EPEC subverts this
response at multiple stages throughout the process. UV-induced apoptosis is
JNK-dependent and may be inhibited by transient expression of NleD. In addi-
tion, infection with an nleD mutant strain of EPEC induces a greater percentage
of apoptosis compared with wild-type. These results indicate that NleD inhibits
JNK-dependent apoptosis, likely via JNK cleavage ( Baruch et al., 2011 ). EspZ,
a LEE-encoded effector, promotes the survival of epithelial cells by enhancing
the phosphorylation of the focal adhesion kinase (FAK) and thereby may stabi-
lize the epithelium upon infection ( Shames et al., 2010 ). NleH inhibits apoptosis
by preventing pro-caspase-3 cleavage through direct binding to Bax inhibitor-1
(BI-1) protein ( Hemrajani et al., 2010 ). The NleF effector, highly conserved
in EPEC and EPEC, binds the active site of caspase-9 to inhibit its activity
( Blasche et al., 2013 ). For additional information, see 'Apoptosis', below.
Damage
Cytoskeletal rearrangement
Numerous signaling and actin-associated/binding proteins including α-actinin,
ezrin, and myosin light chain II (MLC) are recruited to the site of bacterial inti-
mate adhesion ( Manjarrez-Hernandez et al., 1996 ; Cantarelli et al., 2001 ; Goosney
et al., 2001 ). These proteins are involved in pedestal formation, which leads to loss
of absorptive surface (effacing). EPEC achieves this effect by subverting funda-
mental host cell functions to build focal adhesion-like structures that anchor the
bacteria to the host cytoskeleton. Upon EPEC infection and insertion of Tir into
the host cell membrane, Tir becomes clustered at the site of bacterial attachment,
where it serves as the receptor for intimin. F-actin and other host cytoskeletal
proteins accumulate at this site and generate a pedestal ( Figure 4.2 B) ( Knutton
et al., 1987a,b ; da Silva et al., 1989 ; Finlay et al., 1992 ). The localized F-actin
accumulation can be demonstrated using fluorescent probes, which is the basis
of the diagnostic fluorescent-actin staining (FAS) test ( Figure 4.1 D). Recent
advances have begun to decipher the detailed molecular architecture of the ped-
estals and how EPEC remodels the host cytoskeleton at the surface of the cell.
These studies have revealed that different strains of EPEC use different path-
ways to achieve the same end. For the E2348/69 strain, Tir is phosphorylated
on Y474 by redundant kinases from the Src family, including Fyn and Tec/Abl
family kinases ( Phillips et al., 2004 ; Bommarius et al., 2007 ). This phosphoryla-
tion is essential for actin remodeling and pedestal formation ( Rosenshine et al.,
1996a,b ; Kenny, 1999 ) but it is not required for the membrane insertion of Tir or
intimin binding ( Kenny et al., 1997 ; Gauthier et al., 2000 ). After phosphorylation,
the residues flanking Y474 directly bind to the host SH3/SH2 adaptor protein
Nck ( Gruenheid et al., 2001 ; Campellone et al., 2002 ). Nck recruits the neural-
Wiskott-Aldrich syndrome protein (N-WASP), which activates the actin-related