Systems biology of tissue-specific response to Anaplasma phagocytophilum reveals differentiated apoptosis in the tick vector Ixodes scapularis.

Nieves Ayllón,Pilar Alberdi,Jesús Vázquez,Ruth C Galindo,José De La Fuente,Radek Šíma,Margarita Villar,Alejandro Cabezas-Cruz,Petr Kopáček,Juan A López,Katherine M Kocan

doi:10.1371/journal.pgen.1005120

Abstract

Anaplasma phagocytophilum is an emerging pathogen that causes human granulocytic anaplasmosis. Infection with this zoonotic pathogen affects cell function in both vertebrate host and the tick vector, Ixodes scapularis. Global tissue-specific response and apoptosis signaling pathways were characterized in I. scapularis nymphs and adult female midguts and salivary glands infected with A. phagocytophilum using a systems biology approach combining transcriptomics and proteomics. Apoptosis was selected for pathway-focused analysis due to its role in bacterial infection of tick cells. The results showed tissue-specific differences in tick response to infection and revealed differentiated regulation of apoptosis pathways. The impact of bacterial infection was more pronounced in tick nymphs and midguts than in salivary glands, probably reflecting bacterial developmental cycle. All apoptosis pathways described in other organisms were identified in I. scapularis, except for the absence of the Perforin ortholog. Functional characterization using RNA interference showed that Porin knockdown significantly increases tick colonization by A. phagocytophilum. Infection with A. phagocytophilum produced complex tissue-specific alterations in transcript and protein levels. In tick nymphs, the results suggested a possible effect of bacterial infection on the inhibition of tick immune response. In tick midguts, the results suggested that A. phagocytophilum infection inhibited cell apoptosis to facilitate and establish infection through up-regulation of the JAK/STAT pathway. Bacterial infection inhibited the intrinsic apoptosis pathway in tick salivary glands by down-regulating Porin expression that resulted in the inhibition of Cytochrome c release as the anti-apoptotic mechanism to facilitate bacterial infection. However, tick salivary glands may promote apoptosis to limit bacterial infection through induction of the extrinsic apoptosis pathway. These dynamic changes in response to A. phagocytophilum in I. scapularis tissue-specific transcriptome and proteome demonstrated the complexity of the tick response to infection and will contribute to characterize gene regulation in ticks.

Highlights

Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) is an emerging zoonotic pathogen transmitted by Ixodes ticks of which the major vector species are I. scapularis in the US and I. ricinus in Europe [1]
The mechanism(s) regulating mitochondrial permeability and the release of Cytochrome c during apoptosis are not fully understood, Bcl-2 may acts through the voltage-dependent anion channel or Porin, which in turn may play a role in regulating Cytochrome c release [34]. These results demonstrated that A. phagocytophilum infection results in down-regulation of Porin expression in tick salivary gland but not midgut cells, which did not translate in different protein levels but resulted in the inhibition of Cytochrome c release as the anti-apoptotic mechanism to facilitate bacterial infection (Fig 8C)
All apoptosis pathways described in other organisms were identified in I. scapularis, except for the absence of the Perforin ortholog in the Perforin/Granzyme pathway, and tissue-specific differences were found in the response to A. phagocytophilum infection

Summary

Introduction

Anaplasma phagocytophilum (Rickettsiales: Anaplasmataceae) is an emerging zoonotic pathogen transmitted by Ixodes ticks of which the major vector species are I. scapularis in the US and I. ricinus in Europe [1] This intracellular bacterium infects tick midguts [2] and salivary glands [3] and vertebrate host granulocytes causing human, canine and equine granulocytic anaplasmosis and tick-borne fever of ruminants [4,5,6,7,8]. As shown for other tick-pathogen models [17], information is not available on the tick tissue-specific responses to A. phagocytophilum infection These facts underline the importance of defining strategies by which these bacteria establish infection in the tick vector

Methods

Results

Conclusion