AUTHOR=Kumar Manmohan , Sharma Shagun , Haque Munira , Kumar Jai , Hathi Umesh Prasad Sah , Mazumder Shibnath 

TITLE=TLR22-Induced Pro-Apoptotic mtROS Abets UPRmt-Mediated Mitochondrial Fission in Aeromonas hydrophila-Infected Headkidney Macrophages of Clarias gariepinus

JOURNAL=Frontiers in Immunology

VOLUME=13

YEAR=2022

URL=https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2022.931021

DOI=10.3389/fimmu.2022.931021

ISSN=1664-3224

ABSTRACT=<p>Toll-like receptors (TLRs) are epitomized as the first line of defense against pathogens. Amongst TLRs, TLR22 is expressed in non-mammalian aquatic vertebrates, including fish. Using headkidney macrophages (HKM) of <italic>Clarias gariepinus</italic>, we reported the pro-apoptotic and microbicidal role of TLR22 in <italic>Aeromonas hydrophila</italic> infection. Mitochondria act as a central scaffold in the innate immune system. However, the precise molecular mechanisms underlying TLR22 signaling and mitochondrial involvement in <italic>A. hydrophila</italic>-pathogenesis remain unexplored in fish. The aim of the present study was to investigate the nexus between TLR22 and mitochondria in pro-apoptotic immune signaling circuitry in <italic>A. hydrophila</italic>-infected HKM. We report that TLR22-induced mitochondrial-Ca<sup>2+</sup> [Ca<sup>2+</sup>]<sub>mt</sub> surge is imperative for mtROS production in <italic>A. hydrophila</italic>-infected HKM. Mitigating mtROS production enhanced intracellular bacterial replication implicating its anti-microbial role in <italic>A. hydrophila</italic>-pathogenesis. Enhanced mtROS triggers <italic>hif1a</italic> expression leading to prolonged <italic>chop</italic> expression. CHOP prompts mitochondrial unfolded protein response (UPR<sup>mt</sup>) leading to the enhanced expression of mitochondrial fission marker <italic>dnml1</italic>, implicating mitochondrial fission in <italic>A. hydrophila</italic> pathogenesis. Inhibition of mitochondrial fission reduced HKM apoptosis and increased the bacterial burden. Additionally, TLR22-mediated alterations in mitochondrial architecture impair mitochondrial function (ΔΨ<sub>m</sub> loss and cytosolic accumulation of cyt <italic>c</italic>), which in turn activates caspase-9/caspase-3 axis in <italic>A. hydrophila</italic>-infected HKM. Based on these findings we conclude that TLR22 prompts mtROS generation, which activates the HIF-1α/CHOP signalosome triggering UPR<sup>mt</sup>-induced mitochondrial fragmentation culminating in caspase-9/-3-mediated HKM apoptosis and bacterial clearance.</p>