Hemagglutination activities of purified pertussis toxin and filamentous hemagglutinin against erythrocytes from various animals

In Vivo Models and In Vitro Assays for the Assessment of Pertussis Toxin Activity

One of the main virulence factors produced by Bordetella pertussis is pertussis toxin (PTx) which, in its inactivated form, is the major component of all marketed acellular pertussis vaccines. PTx ADP ribosylates Gα_i proteins, thereby affecting the inhibition of adenylate cyclases and resulting in the accumulation of cAMP. Apart from this classical model, PTx also activates some receptors and can affect various ADP ribosylation- and adenylate cyclase-independent signalling pathways. Due to its potent ADP-ribosylation properties, PTx has been used in many research areas. Initially the research primarily focussed on the in vivo effects of the toxin, including histamine sensitization, insulin secretion and leukocytosis. Nowadays, PTx is also used in toxicology research, cell signalling, research involving the blood–brain barrier, and testing of neutralizing antibodies. However, the most important area of use is testing of acellular pertussis vaccines for the presence of residual PTx. In vivo models and in vitro assays for PTx often reflect one of the toxin’s properties or details of its mechanism. Here, the established and novel in vivo and in vitro methods used to evaluate PTx are reviewed, their mechanisms, characteristics and limitations are described, and their application for regulatory and research purposes are considered.

R468A mutation in perfringolysin O destabilizes toxin structure and induces membrane fusion

Perfringolysin O (PFO) belongs to the family of cholesterol-dependent cytolysins. Upon binding to a cholesterol-containing membrane, PFO undergoes a series of structural changes that result in the formation of a β-barrel pore and cell lysis. Recognition and binding to cholesterol are mediated by the D4 domain, one of four domains of PFO. The D4 domain contains a conserved tryptophan-rich loop named undecapeptide (E₄₅₈CTGLAWEWWR₄₆₈) in which arginine 468 is essential for retaining allosteric coupling between D4 and other domains during interaction of PFO with the membrane. In this report we studied the impact of R468A mutation on the whole protein structure using hydrogen-deuterium exchange coupled with mass spectrometry. We found that in aqueous solution, compared to wild type (PFO), PFO^R468A showed increased deuterium uptake due to exposure of internal toxin regions to the solvent. This change reflected an overall structural destabilization of PFO^R468A in solution. Conversely, upon binding to cholesterol-containing membranes, PFO^R468A revealed a profound decrease of hydrogen-deuterium exchange when compared to PFO. This block of deuterium uptake resulted from PFO^R468A-induced aggregation and fusion of liposomes, as found by dynamic light scattering, microscopic observations and FRET measurements. In the result of liposome aggregation and fusion, the entire PFO^R468A molecule became shielded from aqueous solution and thereby was protected against proteolytic digestion and deuteration. We have established that structural changes induced by the R468A mutation lead to exposure of an additional cholesterol-independent liposome-binding site in PFO that confers its fusogenic property, altering the mode of the toxin action.

Heterotrimeric G protein Gi is involved in a signal transduction pathway for ATP release from erythrocytes

Erythrocytes are reported to release ATP in response to mechanical deformation and decreased oxygen tension. Previously we proposed that receptor-mediated activation of the heterotrimeric G protein G(s) resulted in ATP release from erythrocytes. Here we investigate the hypothesis that activation of heterotrimeric G proteins of the G(i) subtype are also involved in a signal transduction pathway for ATP release from rabbit erythrocytes. Heterotrimeric G proteins G(alphai1), G(alphai2), and G(alphai3) but not G(alphao) were identified in rabbit and human erythrocyte membranes. Pretreatment of rabbit erythrocytes with pertussis toxin (100 ng/ml, 2 h), which uncouples G(i/o) from their effector proteins, inhibited deformation-induced ATP release. Incubation of rabbit and human erythrocytes with mastoparan (Mas, 10 microM) or Mas-7 (1 microM), which are compounds that directly activate G(i) proteins, resulted in ATP release. However, rabbit erythrocytes did not release ATP when incubated with Mas-17 (10 microM), which is an inactive Mas analog. In separate experiments, Mas (10 microM) but not Mas-17 (10 microM) increased intracellular concentrations of cAMP when incubated with rabbit erythrocytes. Importantly, Mas-induced ATP release from rabbit erythrocytes was inhibited after treatment with pertussis toxin (100 ng/ml, 2 h). These data are consistent with the hypothesis that the heterotrimeric G protein G(i) is a component of a signal transduction pathway for ATP release from erythrocytes.