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

The 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-beta 1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin beta 1 in a fibronectin- and VirB/D4 T4SS-independent manner and that activated integrin beta 1 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 beta 1 during invasome formation. Finally, integrin-beta 1-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.