SIFD-Associated TRNT1 Deficiency Unveils Importance of TSPO During Macrophage Antibacterial and Antiviral Responses

Duale Ahmed¹Angelo Slade¹Thet Fatica^1,2Stephen Baird²Krishna Bhattarai¹Thérèse Atallah¹Edana Cassol¹Martin Holcik^1*

• ¹Department of Health Sciences, Faculty of Science, Carleton University, Ottawa, Canada
• ²CHEO Research Institute, University of Ottawa, Ottawa, Ontario, Canada

Mitochondria are fundamental to cellular function, satisfying the biosynthetic/bioenergetic demands of the cell as well as a facilitator of cell signaling. Yet, the wide-ranging roles of mitochondria and its dysfunction make it difficult to understand mitochondrial disease pathology. Congenital sideroblastic anemia, B-cell immunodeficiency, periodic fevers, and developmental delay syndrome (SIFD) is caused by mutations in the tRNA-maturing enzyme TRNT1. SIFD patients suffer from various immunodeficiencies, but little exploration has focused on macrophages, a crucial bridge between the innate and adaptive immune systems. Here, we found that TRNT1 knockdown in murine RAW264.7 cells significantly reduces inflammatory cytokine production following stimulation with two well-characterized TLR ligands mimicking bacterial (LPS) and viral (Poly (I:C)) infections. This reduction in inflammatory capacity was linked to a reduction in their respective mitochondrial reprogramming that was vital in driving their specific effector responses. Bioinformatic analysis of mRNA transcripts enriched in TRNT1-dependent codons identified the mitochondrial translocator protein (TSPO), associated with various mitochondrial functions, as a potential target. TSPO expression is differentially expressed following LPS and Poly (I:C) treatment of TRNT1-deficient cells. TSPO ligand stabilization/activation was not sufficient to recover the inflammatory response. However, TSPO overexpression prior to TRNT1 knockdown restored the diminished inflammatory capacity in cells treated with Poly (I:C) but not LPS. This restoration was linked to heightened recruitment of VDAC to the mitochondrial permeability transition pore, via TSPO, suggesting that the differential mitochondrial reprogramming associated with responses to different pathogens may dictate the potential for reversing SIFD immunodeficiency, thus providing a novel potential therapeutic approach against SIFD.