Research Article: Adenylate Cyclase Toxin Promotes Internalisation of Integrins and Raft Components and Decreases Macrophage Adhesion Capacity

Date Published: February 23, 2011

Publisher: Public Library of Science

Author(s): César Martín, Kepa B. Uribe, Geraxane Gómez-Bilbao, Helena Ostolaza, Jérôme Nigou.

Abstract: Bordetella pertussis, the bacterium that causes whooping cough, secretes an adenylate cyclase toxin (ACT) that must be post-translationally palmitoylated in the bacterium cytosol to be active. The toxin targets phagocytes expressing the CD11b/CD18 integrin receptor. It delivers a catalytic adenylate cyclase domain into the target cell cytosol producing a rapid increase of intracellular cAMP concentration that suppresses bactericidal functions of the phagocyte. ACT also induces calcium fluxes into target cells. Biochemical, biophysical and cell biology approaches have been applied here to show evidence that ACT and integrin molecules, along with other raft components, are rapidly internalized by the macrophages in a toxin-induced calcium rise-dependent process. The toxin-triggered internalisation events occur through two different routes of entry, chlorpromazine-sensitive receptor-mediated endocytosis and clathrin-independent internalisation, maybe acting in parallel. ACT locates into raft-like domains, and is internalised, also in cells devoid of receptor. Altogether our results suggest that adenylate cyclase toxin, and maybe other homologous pathogenic toxins from the RTX (Repeats in Toxin) family to which ACT belongs, may be endowed with an intrinsic capacity to, directly and efficiently, insert into raft-like domains, promoting there its multiple activities. One direct consequence of the integrin removal from the cell surface of the macrophages is the hampering of their adhesion ability, a fundamental property in the immune response of the leukocytes that could be instrumental in the pathogenesis of Bordetella pertussis.

Partial Text: Adenylate cyclase toxin (ACT) is an essential virulence factor secreted by Bordetella pertussis, the bacterium that causes whooping cough [1]. This severe childhood disease remains endemic worldwide despite extensive vaccination programmes [2]. ACT is a ≈200 kDa calmodulin-activated adenylyl cyclase toxin [3]–[5] that behaves as an anti-inflammatory and anti-phagocytic factor, facilitating colonization of the respiratory tract by B. pertussis[2], [6]. Upon binding to its cell surface receptor, the αMβ2 integrin [7], ACT becomes an integral membrane protein and inserts its N-terminal adenylyl cyclase domain (AC domain) into the cytosol of the target cell. After binding to calmodulin, ACT–AC raises the intracellular cAMP concentration in host cells cAMP to a pathological level [8], [9]. In addition, ACT can form cation-selective small pores, independent of AC domain translocation, which permeabilise cell membranes at high toxin concentrations [10], [11].

ACT is a key virulence factor secreted by the whooping cough bacterium B. pertussis. The toxin belongs to the so-called RTX family of bacterial proteins that share several features, including a dedicated secretion mechanism, and a strict calcium-binding and post-translational fatty-acylation requirement to exhibit full biological activity. From recent experiments, it may be concluded that two additional features are also shared by members of this family: the induction of rises in intracellular calcium and the involvement of cholesterol-rich membrane domains, at some (maybe several different) stages, in their toxicity mechanisms [13], [19], [30], [37]–[39].