Enteropathogenic Escherichia coli - Escherichia coli: Pathotypes and Principles of Pathogenesis - page 86

EPEC infection also affects sodium ion transport in Caco-2 cells by
reducing the expression of sodium hydrogen exchanger 3 (NHE3) in an
EspF-dependent manner ( Hodges et al., 2008 ). NHE3 contributes to sodium
ion (Na + ) absorption, verified by the production of watery stool by NHE3-
deficient mice ( Gawenis et al., 2002 ). EPEC also reduces Cl - /HCO 3 - exchange
both in vivo and in vitro ( Gill et al., 2007 ). This effect is mediated by the T3S
effectors EspG and EspG2, which disrupt the microtubular network, alter-
ing the distribution of the membrane targeting Cl - /HCO 3 - exchanger DRA,
and resulting in accumulation of Cl - in the lumen and loss of water. Other
EPEC virulence factors including EspF, Map, Tir, and intimin have been
shown to alter function of SGLT-1, a cotransporter that is responsible for the
majority of fluid uptake in the small intestine ( Dean et al., 2006 ). In a murine
infection, C. rodentium causes the mislocalization of aquaporins from the
cell membrane to the cytoplasm, which correlates to diarrhea-like phenotype
and which is partially dependent on the EspF and EspG effector proteins
( Guttman et al., 2007 ).
Apoptosis
Apoptosis, the programmed death of cells in response to internal or external
stimuli, is an important defense mechanism launched by multicellular organ-
isms where infected cells are induced to die for the benefit of the remaining cells
( Elmore, 2007 ). EPEC is able to manipulate the host cell death pathway by two
distinct mechanisms; intrinsic (mitochondrial- and ER-mediated pathway) and
extrinsic (receptor-mediated pathway) ( Figure 4.2 C) ( Elmore, 2007 ).The intrin-
sic pathway involves the activation of the Bcl-2 family proteins Bak and Bax,
which leads to mitochondrial outer membrane permeabilization and release of
cytochrome C. The cytosolic cytochrome C interacts with apoptosis activating
factor 1 (Apaf-1) and procaspase-9 to activate downstream caspases-3, -6, and -7
( Li et al., 1997 ). This pathway is induced by translocation of the T3S effector
EspF. The N-terminus of EspF (residues 1 to 20) contains the bacterial secretion
signal ( Charpentier and Oswald, 2004 ) and superimposed onto this region is the
organelle-targeting domain ( Nougayrède and Donnenberg, 2004 ; Nagai et al.,
2005 ; Dean et al., 2010a,b ). The mitochondrial targeting of EspF induces apop-
tosis ( Crane et al., 2001 ; Nougayrède and Donnenberg, 2004 ) via disruption of
the mitochondrial membrane potential, causing the release of cytochrome C
and subsequent activation of caspase 9 ( Nougayrède and Donnenberg, 2004 ).
EspF also binds to and depletes Abcf2, an anti-apoptotic factor ( Nougayrède
et al., 2007 ). Another T3S effector, EspG, has been shown to activate the host
cysteine protease calpain inducing rapid cell death and necrosis, a process mod-
ulated by the presence of Tir ( Dean et al., 2010a ). A second EspG homolog,
EspG2, encoded on the EspC pathogenicity island ( Mellies et al., 2001 ), also
homologous to VirA, the Shigella flexneri effector protein, activates calpain
( Elliott et al., 2001 ; Bergounioux et al., 2012 ). Recently, EspH has been shown
to induce caspase-3 activation and this activity is counteracted by translocation