TRP75-Mediated STAT3 Activation Promotes Anti-Apoptotic Signaling and Ehrlichia chaffeensis Infection

Ehrlichia chaffeensis is an obligately intracellular bacterium that manipulates mononuclear phagocytes by hijacking host cell signaling pathways to promote infection. In part, E. chaffeensis accomplishes host cell manipulation through the actions of secreted effector proteins, including the tandem repeat proteins (TRPs). The most well-characterized TRP, TRP120, uses multiple short linear motifs (SLiMs) to interact with host proteins and co-opt signaling axes to inhibit apoptosis. Interestingly, other ehrlichial TRPs have been shown to interact with a diverse array of host proteins and harbor predicted SLiM sequences which may mediate these interactions. Previous studies from our laboratory have shown that multiple STAT (Signal Transducer and Activator of Transcription) family members interact with E. chaffeensis effector proteins. However, the functional role of STATs during infection remains poorly understood. Notably, STAT3, a highly immunomodulatory and pro-survival factor, interacts with the E. chaffeensis effector protein TRP75. Additionally, TRP75 contains predicted SLiMs which could mediate interfaces with SH2 and SH3 domains, interaction modules important in STAT activation. In this study, we examined activation of STAT family members and transcription of STAT target genes during E. chaffeensis infection. We observed significant activation of multiple STATs (STAT1, STAT3, STAT5, and STAT6), with STAT3 showing the highest level of activation. Therefore, we further investigated STAT3 activation dynamics and effects of its inhibition on infection. STAT3 phosphorylation and nuclear translocation were detected beginning 48 h post-infection, coinciding with upregulation of STAT3 target genes, including the anti-apoptotic gene MCL1. Pharmacological inhibition of STAT3 significantly reduced MCL1 expression and increased caspase cleavage, implicating STAT3 as a regulator of anti-apoptotic signaling during infection. Furthermore, both pharmacological inhibition and genetic KO of STAT3 significantly reduced bacterial load, highlighting its critical role in supporting infection. Ectopic expression of TRP75 in HEK-293 cells induced STAT3 phosphorylation, demonstrating a specific role for TRP75 in STAT3 activation. Collectively, these findings support a model in which E. chaffeensis exploits STAT3 via the TRP75 effector to activate an anti-apoptotic program and other cellular pathways that promote infection.

Name thesis:degree_name

Human Pathophysiology and Translational Medicine (Doctoral)

Grantor

The University of Texas Medical Branch at Galveston

Year dc:date.issued

2025

Author dc:creator

• Pittner, Nicholas Anthony 1997-

Advisor dc:contributor.advisor

• McBride, Jere (jemcbrid@utmb.edu)

Committee members dc:contributor.committeemember

• Walker, David
• Stevenson-Lerner, Heather
• Ismail, Nahed

Handle dc:identifier.uri

https://hdl.handle.net/2152.3/12891

OAI identifier oai:identifier

oai:utmb-ir.tdl.org:2152.3/12891