Actin-based motility and cell-to-cell spread of bacterial pathogens

Abstract

Subversion of the host actin cytoskeleton is a critical virulence mechanism used by a variety of intracellular bacterial pathogens during their infectious life cycles. These pathogens manipulate host actin to promote actin-based motility and coordinate motility with cell-to-cell spread. Growing evidence suggests that the tactics employed by pathogens are surprisingly diverse. Here, we review recent advances suggesting that bacterial surface proteins exhibit divergent biochemical mechanisms of actin polymerization and recruit distinct host protein networks to drive motility, and that bacteria deploy secreted effector proteins that alter host cell mechanotransduction pathways to enable spread. Further investigation into the divergent strategies used by bacterial pathogens to mobilize actin will reveal new insights into pathogenesis and cytoskeleton regulation.

Figure 1. Life cycles of intracellular bacterial pathogens that harness actin-based motility to enable cell-to-cell spread

The cartoon depicts the intracellular life cycles of the pathogens discussed in this review. After invading bacteria are phagocytosed and escape the phagosome, they enter the host cell cytosol, where they polymerize actin using distinct mechanisms and undergo actin-based motility, forming actin comet tails with different filament organizations. R. parkeri, a representative of the SFG of Rickettsia spp., undergo two temporally segregated and biochemically-distinct phases of actin-based motility, as depicted. All of these pathogens also undergo diverse pathways of cell-to-cell spread via protrusion- and vesicle-mediated transfer (for S. flexneri, L. monocytogenes, and Rickettsia spp.), or direct cell-cell fusion (for Burkholderia spp). Actin, red; bacteria, green.

Figure 2. Actin-based motility is regulated by diverse molecular mechanisms

(A) Images of different bacterial pathogens and their associated actin tails in infected host cells. Each image corresponds to one of the three types of host actin polymerization pathways hijacked or mimicked for actin-based motility (Arp2/3, formin-like and Ena/VASP-like). Actin is labeled with phalloidin, red; bacteria, green. Scale bar, 1 µm. (B) A closer look at the molecular mechanisms of actin polymerization at the bacterial surface highlights the impressive coordination between host and bacterial proteins, and reveals critical differences between pathogens. In addition to the bacterial surface proteins that promote actin polymerization (in green), many host proteins regulate the nucleation (Arp2/3 complex, N-WASP, Toca-1), elongation (Eva/VASP, profilin, lamellipodin (Lpd)), protein phosphorylation (CK2-β, Abl, Btk), actin crosslinking (nexillin, α-actinin, T-plastin) and actin dynamics and recycling (capping protein, cofilin). For some pathogens, though, host protein regulators await discovery.

Figure 3. Distinct strategies of bacterial cell-to-cell spread

(A) Host cells in tissues are mechanically coupled together via actin cytoskeletal networks that link together adhesive junctional complexes. Bacteria have evolved diverse strategies to manipulate different junctions to promote spread by either reducing bicellular tight junction (bTJ) or adherens junction (AJ) tension, or spreading primarily at tricellular tight junctions (tTJ). (B) A closer look at the specific stages of spread, with particular emphasis on protrusion initiation, morphology and engulfment. L. monocytogenes secrete InlC into the host cell where it inhibits the Tuba:N-WASP interaction and reduces tension to promote protrusion initiation. Once in a protrusion, the combination of spatially-segregated actin polymerization strategies, along with host membrane-anchoring and motor proteins, help drive the extension of long protrusions into the neighboring cell. In polarized epithelia, S. flexneri protrude predominantly at tricellular junctions. Protrusion initiation may also be regulated by formin- or Ena/VASP-mediated changes to the cortical actin network. Once S. flexneri enter protrusions, the actin network collapses, resulting in VLPs that are enriched in phosphotyrosine and PI(3)P and that are extended into neighboring cells with assistance of myosins. Non-canonical clathrin-mediated endocytosis then promotes engulfment of the protrusion into the neighboring cell. R. parkeri are distinct from the others in that they lose their actin tails before spreading, and generate short protrusions. R. parkeri protrusion engulfment is regulated by the secreted bacterial protein Sca4, which may promote host plasma membrane flexibility by interfering with the vinculin:α-catenin interaction and relaxing vinculin-dependent intercellular tension.