The Salmonella enterica
serovar Typhimurium (S.
Typhimurium) effector proteins SipA, SopA, SopB, SopD and SopE2, translocated through the Salmonella
Pathogenicity Island-1 (SPI-1) encoded type III secretion system (T3SS-1), play important roles in generating an inflammatory response in the bovine ligated ileal loop model. A S.
Typhimurium strain carrying mutations in sipA
causes 60% less inflammatory responsiveness and decreased fluid accumulation relative to wild type infected loops. On the other hand, mutations in the slrP
genes that encode other T3SS-1 effector proteins did not reduce fluid accumulation or the degree of inflammation in bovine ligated ileal loops. The degree of attenuation observed in a sipAsopABDE2
mutant was similar to that of a S.
], a strain deficient for translocating T3SS-1 effector proteins into host cells. These findings suggest that SipA, SopA, SopB, SopD and SopE2 are the major effector proteins required for S.
Typhimurium-induced enteritis in calves.
Typhimurium T3SS-1-translocated effector proteins, SipA, SopA, SopB, SopD and SopE2, induce cytoskeletal rearrangements leading to bacterial internalization [2
]. SopA associates with a host ubiquitin E3 ligase inducing bacterial escape into the cytosol of epithelial cells [3
]. SopE2, is a guanine nucleotide exchange factor [4
], and SopB, is an inositol phosphate phosphatase [5
]. Phosphatidylinositol phosphates (generated by SopB) and activated Rho-family GTPases (produced by SopE2) act in concert to activate WASP/Scar proteins, which in turn recruit the Arp2/3 complex to initiate the formation of new branches on actin filaments [6
]. SopD acts cooperatively with SopB to promote membrane fission and macropinosome formation during the invasion process [7
]. SipA, which is delivered into host cells as quickly as 10 sec after exposure [8
], localizes in the host cell plasma membrane [9
] where it acts as an actin-binding protein [10
Once in contact with host cells, S.
Typhimurium was found to alter cytosolic Ca2+
concentrations within 2 to 4 minutes after infection [11
is an important intracellular messenger involved in many cellular functions, including vesicular trafficking [15
], cytoskeletal rearrangements [16
] and gene expression [17
]. Therefore, changes in intracellular Ca2+
level, depending on the amplitude of transients, may influence different cell signaling pathways [18
Other bacterial pathogens have been found to alter Ca2+
homeostasis in host cells. Although intracellular Ca2+
has been implicated in mediating cytoskeletal rearrangements, the dependence of bacterial invasion on changes in Ca2+
fluxes is controversial. Invasion of Shigella flexneri
in epithelial cells induces an increase in intracellular Ca2+
that is dependent on a functional T3SS, but cytoskeletal rearrangements are also observed in cells with no detectable Ca2+
]. Listeria monocytogenes
and Campylobacter jejuni
induce intracellular Ca2+
changes in host cells [20
]. While chelation of intracellular Ca2+
interferes with C. jejuni
], it has no effect on L. monocytogenes
]. Previous studies suggest that S.
Typhimurium invasion is dependent on the induction of cytosolic Ca2+
], and that chelation of Ca2+
inhibits bacterial internalization [22
]. However, a non-invasive S.
mutant induces cytosolic Ca2+
], indicating that host cell internalization is not a prerequisite for inducing Ca2+
Because Ca2+ is central to many cell signaling events, including the expression of IL-8 and the induction of cytoskeletal changes, and the S. Typhimurium effector proteins SipA, SopA, SopB, SopD and SopE2 induce inflammatory responses and bacterial invasion, we focused our studies on the role(s) of intracellular Ca2+ in S. Typhimurium-induced invasion and the induction of IL-8 expression.