Novel Strategies to Inhibit Pertussis Toxin

Abstract

Pertussis, also known as whooping cough, is a respiratory disease caused by infection with Bordetella pertussis, which releases several virulence factors, including the AB-type pertussis toxin (PT). The characteristic symptom is severe, long-lasting paroxysmal coughing. Especially in newborns and infants, pertussis symptoms, such as leukocytosis, can become life-threatening. Despite an available vaccination, increasing case numbers have been reported worldwide, including Western countries such as Germany and the USA. Antibiotic treatment is available and important to prevent further transmission. However, antibiotics only reduce symptoms if administered in early stages, which rarely occurs due to a late diagnosis. Thus, no causative treatments against symptoms of whooping cough are currently available. The AB-type protein toxin PT is a main virulence factor and consists of a binding subunit that facilitates transport of an enzyme subunit into the cytosol of target cells. There, the enzyme subunit ADP-ribosylates inhibitory α-subunits of G-protein coupled receptors resulting in disturbed cAMP signaling. As an important virulence factor associated with severe symptoms, such as leukocytosis, and poor outcomes, PT represents an attractive drug target to develop novel therapeutic strategies. In this review, chaperone inhibitors, human peptides, small molecule inhibitors, and humanized antibodies are discussed as novel strategies to inhibit PT.

Keywords: ADP-ribosylation inhibitor; chaperones; human defensins; humanized antibodies; novel inhibitors; pertussis toxin.

Figure 1

Schematic depiction of the cellular uptake and mode of action of PT. PT binds via its B-subunit to the cell surface. PT is endocytosed and follows a retrograde route through the Golgi to the endoplasmic reticulum (ER). The A-subunit PTS1 dissociates from the B-Oligomer and is translocated via the ER-associated degradation (ERAD) pathway into the cytosol. Here, PTS1 ADP-ribosylates inhibitory α-subunits (Gαi) of G-protein coupled receptors (GPCR) thereby inactivating it. Thus, if the respective GPCR is activated, Gαi is no longer able to inhibit the adenylate cyclase (AC) resulting in increased and disturbed cAMP signaling [28,33].

Figure 2

Schematic depiction of different targets of AB-type toxin-inhibitors. AB-type toxins can be targeted in various ways by inhibitors. The toxin-producing bacteria are targeted by antibiotics. The released toxins that act independently of the bacteria can be targeted extracellularly by neutralizing antibodies or peptides. If the toxin already entered the cell, small molecule inhibitors of toxin enzyme activity or of cellular proteins that aid the translocation of the toxin enzyme subunit into the cytosol still can protect the target cell from intoxication. Details are given in the text.

Figure 3

Proposed mechanism for mode of action of chaperones during uptake of PT into cells. PT reaches the ER via a retrograde intracellular transport. ATP binding leads to the release and unfolding of the enzyme subunit PTS1. PTS1 is recognized by the ER-associated degradation pathway and transported into the cytosol. If the activity of chaperones Hsp90 or Hsp70 or peptidyl-prolyl cis/trans isomerases of the cyclophilin or FK506 binding protein family are inhibited by specific pharmacological inhibitors, less PTS1 reaches the cytosol [41,42,43]. (Rad = radicicol, VK112, NIM811 = non-immunosuppressive CsA derivatives; further explanations are given in the text).