Bartonella henselae engages inside-out and outside-in signaling by integrin β1 and talin1 during invasome-mediated bacterial uptake

Abstract

SummaryThe VirB/D4 type IV secretion system (T4SS) of the bacterial pathogen Bartonella henselae (Bhe) translocates seven effector proteins (BepA-BepG) into human cells that subvert host cellular functions. Two redundant pathways dependent on BepG or the combination of BepC and BepF trigger the formation of a bacterial uptake structure termed the invasome. Invasome formation is a multi-step process consisting of bacterial adherence, effector translocation, aggregation of bacteria on the cell surface and engulfment, and eventually, complete internalization of the bacterial aggregate occurs in an F-actin-dependent manner. In the present study, we show that Bhe-triggered invasome formation depends on integrin-b1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin b1 in a fibronectin-and VirB/D4 T4SS-independent manner and that activated integrin b1 is essential for both effector translocation and the actin rearrangements leading to invasome formation. Furthermore, we show that talin1, but not talin2, is required for inside-out activation of integrin b1 during invasome formation. Finally, integrin-b1-mediated outside-in signaling by FAK, Src, paxillin and vinculin is necessary for invasome formation. This is the first example of a bacterial entry process that fully exploits the bi-directional signaling capacity of integrin receptors in a talin1-specific manner.

Intruders below the Radar: Molecular Pathogenesis of Bartonella spp

“…Consistent with this, the effectorless mutant of B. henselae Houston-1 enters human endothelial cells into BCVs (358). Recent work showed that B. henselae Houston-1 binds ␤1-integrins on host cells and activates them in a fibronectin-independent manner via talin-1 inside-out signaling, indicating that integrins may be a major receptor for B. henselae on host cells (419). This hypothesis is further supported by the finding that an extended, activated conformation of ␤1-integrins on human endothelial cells is necessary for effector translocation and is recruited to the sites of invasome formation.…”

“…The interaction partners of these Bartonella outer membrane proteins on the host cell surface are not known, but it has been suggested that ICAM-1 may be one of them, since this protein is enriched in the tips of host cell membrane protrusions that tightly associate with the bacterial aggregates during invasome formation (120). It is further tempting to speculate that one or more of the host cell binding outer membrane proteins would be the elusive factor of B. henselae Houston-1 that binds ␤1-integrins in the context of invasome formation and is regulated by the BatR/BatS system (419).…”

Bartonella henselaetrimeric autotransporter adhesin BadA expression interferes with effector translocation by the VirB/D4 type IV secretion system

“…BadA mediates adherence to extracellular matrix proteins and ECs under static and dynamic conditions (Riess et al, 2004;Müller et al, 2011) and B. henselae subsequently colonizes ECs via endocytic uptake (Kyme et al, 2005). In absence of BadA, Functional interferences of BadA and the VirB/D4 T4SS in B. henselae 771 B. henselae adheres to ECs via BatR-regulated adhesins that mediate binding to integrin b1 Truttmann et al, 2011b). After colonization of ECs, BepA protects the host cells from apoptosis (Schmid et al, 2006) and the VirB/D4 T4SS also mediates NFkBdependent pro-inflammatory activation (Schmid et al, 2004).…”

Integrin-mediated internalization of Staphylococcus aureus does not require vinculin

“…For example, siRNA screening has revealed that vinculin and other integrin-associated molecules are critical for the formation of the characteristic invasome structure, which mediates entry of the zoonotic pathogen B. henselae (Bh) into endothelial cells [33]. …”