Helicobacter pylori Employs a Unique Basolateral Type IV Secretion Mechanism for CagA Delivery.

The Helicobacter pylori (Hp) type IV secretion system (T4SS) forms needle-like pili, whose binding to the integrin-ß1 receptor results in injection of the CagA oncoprotein. However, the apical surface of epithelial cells is exposed to Hp, whereas integrins are basolateral receptors. Hence, the mechanism of CagA delivery into polarized gastric epithelial cells remains enigmatic. Here, we demonstrate that T4SS pilus formation during infection of polarized cells occurs predominantly at basolateral membranes, and not at apical sites. Hp accomplishes this by secreting another bacterial protein, the serine protease HtrA, which opens cell-to-cell junctions through cleaving epithelial junctional proteins including occludin, claudin-8, and E-cadherin. Using a genetic system expressing a peptide inhibitor, we demonstrate that HtrA activity is necessary for paracellular transmigration of Hp across polarized cell monolayers to reach basolateral membranes and inject CagA. The contribution of this unique signaling cascade to Hp pathogenesis is discussed.

Subversion of host kinases: a key network in cellular signaling hijacked by Helicobacter pylori CagA.

Helicobacter pylori is a paradigm of persistent pathogens and major risk factor for developing severe diseases including adenocarcinoma in the human stomach. An important bacterial factor linked to gastric disease progression is the cag pathogenicity island-encoded type-IV secretion system (T4SS) effector protein CagA. Translocated CagA undergoes tyrosine phosphorylation at EPIYA-motifs and then activates or inactivates multiple host signaling proteins in a phosphorylation-dependent and phosphorylation-independent fashion. In this way, intracellular CagA acts as a 'masterkey' or 'picklock', which evolved during evolution to hijack key host cell signal transduction functions. Crucial targets of CagA represent a variety of serine/threonine and tyrosine kinases, which control major checkpoints of eukaryotic signaling. Here we review the signal transmission by translocated CagA on multiple receptor kinases (c-Met and EGFR) and non-receptor kinases (Src, Abl, Csk, aPKC, Par1, PI3K, Akt, FAK, GSK-3, JAK, PAK1, PAK2 and MAP kinases), manipulating a selection of fundamental processes in the human gastric epithelium such as cell adhesion, polarity, proliferation, motility, receptor endocytosis, cytoskeletal rearrangements, apoptosis, inflammation and cell cycle progression. This enormous complexity generates a highly remarkable and puzzling scenario during H. pylori infection. The contribution of these signaling pathways to bacterial survival, persistence and gastric pathogenesis is discussed.