ErbB2 (green) and invading bacteria (red) colocalize.

Neisseria meningitidis, also known as meningococcus, is a pathogen with a nasty habit of forcing normally nonphagocytic endothelial cells to engulf it, allowing the bacterium to penetrate cellular barriers. Hoffmann et al. now demonstrate on page 133 that N. meningitidis activates two distinct signaling pathways in host cells to cause actin cytoskeleton reorganization and engulfment of the bacterium.Adherence of the bacteria to endothelial cells causes massive and specific aggregation of the receptor tyrosine kinase ErbB2, which normally transduces signals by heterodimerization with other ErbB-like receptors. However, during N. meningitidis invasion, ErbB2 forms homodimers that, once autophosphorylated, can activate the src kinase. Inhibiting this process prevents internalization of the pathogen, but does not block the characteristic actin rearrangements induced by the bacterium. Blocking both ErbB2/src signaling and actin polymerization completely prevents bacterial engulfment, indicating that the two pathways are independent, but that both are required for invasion. While it is possible that ErbB2 is a receptor for the bacterial pilus, the data are also consistent with the recruitment of ErbB2 as part of a multiprotein complex that includes an unknown receptor.

In separate work, Bierne et al. (page 101) characterized phagocytosis induced by the pathogen Listeria monocytogenes, and observed actin cytoskeleton remodeling that is apparently mediated by the activation and deactivation of cofilin. ▪