Characterization of tick metabolism in response to pathogen infection

Resumen

The obligate intracellular pathogen, Anaplasma phagocytophilum, is the causative agent of life-threatening diseases: granulocytic anaplasmosis in humans and tick-borne fever in animals. In the USA, Europe, Africa and Asia it is an emerging tick-borne pathogen, and every year the number of cases increases. A. phagocytophilum has evolved to use common strategies to establish infection in both vertebrate hosts and tick vectors. Understanding tick-pathogen interactions will help to identify the molecules involved and to develop novel strategies for the control of this pathogen. Chapter I: General introduction. Chapter I is made up of two review papers [Cabezas-Cruz, A., Espinosa, P. J., Alberdi, P., and de la Fuente, J. (2019). Tick-pathogen interactions: the metabolic perspective. Trends Parasitol. 1838, 1–13. doi:10.1016/j.pt.2019.01.006.] and [Alberdi, P., Espinosa, P. J., Cabezas-Cruz, A., and de la Fuente, J. (2016). Anaplasma phagocytophilum manipulates host cell apoptosis by different mechanisms to establish infection. Vet. Sci. 3, 1–11. doi:10.3390/vetsci3030015.] that constitute an introduction for this thesis by reviewing the current understanding of tick metabolism modulation by tick-borne pathogens with focus in the model intracellular bacterium Anaplasma phagocytophilum, and how this bacterium uses different mechanisms to inhibit apoptosis for infection of both vertebrate and invertebrate hosts. This chapter describes how ticks react to pathogens by activating different mechanisms to limit infection, while different pathogens have developed similar strategies to manipulate tick innate immune responses and apoptosis to facilitate infection, multiplication and transmission. Chapter II: Manipulation of metabolic and oxidative stress responses in tick cells infected with Anaplasma phagocytophilum. Chapter II describes how tick cells increase the synthesis of phosphoenolpyruvate (PEP) from tyrosine, a carbon source, to control A. phagocytophilum infection levels [Cabezas- Cruz, A., Espinosa, P. J., Obregón, D. A., Alberdi, P., and de la Fuente, J. (2017). Ixodes scapularis tick cells control Anaplasma phagocytophilum infection by increasing the synthesis of phosphoenolpyruvate from tyrosine. Front. Cell. Infect. Microbiol. 7, 1–16. doi:10.3389/fcimb.2017.00375]. We explored the development of novel strategies for the control of human granulocytic anaplasmosis by targeting some of the enzymes involved in the tyrosine/OAA/PEPCK-M/PEP node. Chapter II also focuses on the important role played by heat shock proteins (Hsp) in tick-pathogen interactions [Espinosa, P. J., Alberdi, P., Villar, M., Cabezas-Cruz, A., and de la Fuente, J. (2018). “Heat Shock proteins in vector-pathogen interactions: The Anaplasma phagocytophilum model,” in Heat Shock Proteins in Veterinary Medicine and Sciences (Springer International), 375– 398. doi:10.1007/978-3-319-73377-7]. The results indicated that at the transcript and protein levels, tick and bacterial Hsp are regulated in response to A. phagocytophilum infection in the tick vector Ixodes scapularis. Finally, the role of oxidative stress during A. phagocytophilum infection of ticks was characterised through the function of different pathways involved in ROS production [Espinosa P. J., Alberdi P., Villar M., Cabezas- Cruz A. and de la Fuente J. The redox metabolic pathways function to limit Anaplasma phagocytophilum infection and mutiplication while preserving fitness in tick vector cells. Submitted]. The results revealed that tick cells increase mitochondrial ROS production to limit infection, whereas A. phagocytophilum inhibits alternative ROS production pathways and apoptosis to preserve cell fitness and facilitate infection. Chapter III: alfa-Gal synthesis and its role in tick-host and tick-pathogen interactions. Chapter II despicts the molecular basis of endogenous alfa-Gal synthesis in ticks [Cabezas- Cruz, A., Espinosa, P. J., Alberdi, P., Šimo, L., Valdés, J. J., Mateos-Hernández, L., Contreras M., Villar Rayo M. and de la Fuente J. (2018). Tick galactosyltransferases are involved in alfa-Gal synthesis and play a role during Anaplasma phagocytophilum infection and Ixodes scapularis tick vector development. Sci. Rep. 8, 1–18. doi:10.1038/s41598- 018-32664-z]. The results suggest that tick galactosyltranferases are involved in vector development, tick-pathogen interactions and possibly the etiology of alfa-Gal syndrome in humans. We hypothesized that the enzymatic activity of b4galt7, a4galt-1 and a4galt-2 is critical for tick feeding. Knockdown using RNAi of the genes encoding for galactosyltransferases was lethal in feeding ticks within 2-3 days from attachment to rabbits. In addition, alfa-Gal production was also found to affect A. phagocytophilum, higher levels of alfa-Gal were associated with lower bacterial infections.