The role of thioredoxin in the redox regulation of the Tpx1/Pap1 pathway in Schizosaccharomyces pombe.

Abstract

Reactive oxygen species (ROS) can damage cellular components leading to dysfunction and cell death. Paradoxically, ROS, such as hydrogen peroxide, are also essential for a range of metabolic and signalling functions within cells. Given these opposing functions, cells have developed several redox signalling mechanisms to manage ROS within specific homeostatic limits. In bacterial cells, thiol-peroxidases (peroxiredoxins) and other enzymes detoxify ROS, while the antioxidant transcriptional response is induced by transcription factors directly oxidized by ROS. In many eukaryotes, these functions are combined with peroxiredoxins detoxifying ROS as well as activating redox-sensitive transcription factors. The relative benefits and disadvantages of such sensor-mediated redox signalling systems are unknown, and we aimed to understand the logic underlying this signalling mechanism using the Schizosaccharomyces pombe Tpx1/Pap1 pathway. In this pathway, the peroxiredoxin Tpx1 reduces hydrogen peroxide and oxidizes the redox transcription factor Pap1. Following a hydrogen peroxide perturbation, the Pap1 signal profile revealed a biphasic profile with a rapid initial increase followed by a relatively prolonged decrease in Pap1 oxidation. These dynamics were suggestive of an incoherent feedforward loop, and we hypothesized that the Trx1 protein was responsible for the incoherence as it could both dampen and increase the signal by reducing Pap1 and Tpx1, respectively. To test this hypothesis, we analyzed the effect of several oxidants (hydrogen peroxide, tert-butyl hydroperoxide, and diamide) on Pap1 activation to determine if we could selectively modulate signal duration. However, we could not quantitatively delineate the effects of these oxidants on the signal profiles obtained. We, therefore, utilized computational modelling to analyze the Tpx1/Pap1 pathway and found that excess Trx1 reduced Tpx1 faster, preventing the association of Tpx1 and Pap1. On the other hand, insufficient Trx1 allowed for Pap1 to be oxidized over a longer interval which increased the signal duration. Thus, our analysis showed that, in contrast to our hypothesis, Trx1 limitation, rather than incoherence, was responsible for the Pap1 oxidation profile. These results indicate that in the presence of ROS, Trx1 plays a vital role in determining the signal profile of Pap1.