Septins Recognize and Entrap Dividing Bacterial Cells for Delivery to Lysosomes

Abstract

SummaryThe cytoskeleton occupies a central role in cellular immunity by promoting bacterial sensing and antibacterial functions. Septins are cytoskeletal proteins implicated in various cellular processes, including cell division. Septins also assemble into cage-like structures that entrap cytosolic Shigella, yet how septins recognize bacteria is poorly understood. Here, we discover that septins are recruited to regions of micron-scale membrane curvature upon invasion and division by a variety of bacterial species. Cardiolipin, a curvature-specific phospholipid, promotes septin recruitment to highly curved membranes of Shigella, and bacterial mutants lacking cardiolipin exhibit less septin cage entrapment. Chemically inhibiting cell separation to prolong membrane curvature or reducing Shigella cell growth respectively increases and decreases septin cage formation. Once formed, septin cages inhibit Shigella cell division upon recruitment of autophagic and lysosomal machinery. Thus, recognition of dividing bacterial cells by the septin cytoskeleton is a powerful mechanism to restrict the proliferation of intracellular bacterial pathogens.

The history of septin biology and bacterial infection

“…It has been recognised for approximately 10 years that septin assembly is membrane facilitated (Tanaka‐Takiguchi, Kinoshita, & Takiguchi, ), and recent work suggested that septins sense micron‐scale curvature at the plasma membrane (Bridges, Jentzsch, Oakes, Occhipinti, & Gladfelter, ). In agreement with this, a recent study showed that septins are recruited to regions of micron‐scale curvature presented by dividing bacterial cells (Figure [S3]), and cardiolipin (a curvature‐specific phospholipid) promotes septin recruitment to these regions (Krokowski et al, ). Following pharmacological inhibition of bacterial cell division, septins are recruited to bacterial cell poles but fail to assemble into cages, indicating that bacterial cell growth is required for septin cage entrapment.…”

“…Septin cage‐entrapped S. flexneri is shown as dashed white outline. Image adapted with permission from Krokowski et al ()…”

“…Following pharmacological inhibition of bacterial cell division, septins are recruited to bacterial cell poles but fail to assemble into cages, indicating that bacterial cell growth is required for septin cage entrapment. Therefore, bacterial cell division can be viewed as a fundamental danger signal used by the host cell to recognise intracellular bacteria (Krokowski et al, ; Krokowski & Mostowy, ; Van Ngo & Mostowy, ).…”

“…Septins regulate actin dynamics to promote Salmonella Typhimurium entry in host cells (Boddy et al, 2018). Septins recognise micron-scale membrane curvature of dividing S. flexneri for delivery to lysosome, highlighting a new link between cell division and cell-autonomous immunity (Krokowski et al, 2018). Septins and the autophagy machinery can promote the proliferation of intracellular S. flexneri not entrapped in septin cages (Lobato-Márquez, Krokowski, Sirianni, Larrouy-Maumus, & Mostowy, 2019) 3.2 | Enter the Shigella septin cage had no significant impact on bacterial directionality or movement, indicating that septin recruitment is not essential for actin-based motility.…”

“…Septin cage-entrapped S. flexneri is shown as dashed white outline. Image adapted with permission from Krokowski et al (2018Krokowski et al ( ) et al, 2008. Importantly, L. monocytogenes ΔactA and S. flexneri ΔicsA, isogenic mutants unable to polymerise actin tails, both fail to recruit septin (Mostowy et al, 2010).…”