Background: Apart from its mitochondrial localization, mechanistic details of STAT3 import and assembly in mitochondria remain elusive. Results: Using an in vitro import assay, we show that STAT3 associates with the mitochondrial inner membrane in a GRIM-19-dependent manner. Conclusion: GRIM-19 chaperones the recruitment of STAT3 into mitochondrial inner membrane complexes. Significance: This study identifies a novel function of GRIM-19 and a mechanism for STAT3 import into mitochondria.

Dengue virus infections pose a significant threat to human health at present, which is reported from nearly 140 countries. The genome of this virus encodes three structural and seven non-structural (NS) proteins along with two un-translated regions, one each on both ends. Among them, dengue protease (NS3) plays a pivotal role in polyprotein processing and virus multiplication. NS3 is also known to regulate several host proteins to induce and maintain pathogenesis.Certain viral proteins are known to interact with mitochondrial membrane proteins and interfere with their functions. But the association of a virus-coded protein with the mitochondrial matrix is not known. In this report, by using in silico analysis, we show that NS3pro alone is capable of mitochondrial import; however, dependent on its innate mitochondrial transport signal (MTS). Transient transfection and protein import studies confirm the import of NS3pro to the mitochondrial matrix. Similarly, NS3pro-helicase (1-464 amino acids of NS3) also targets the mitochondria. Intriguingly, reduced levels of the matrix localized GrpEL1, a co-chaperone of mtHsp70, were noticed in NS3pro, NS3pro-helicase expressing, and virus-infected cells. Upon using purified components, GrpEL1 undergoes cleavage, and the sites have been mapped to KR81A and QR 92S. Importantly, the levels of GrpEL1 are seriously compromised in severe dengue infected clinical samples. Our studies provide novel insights into the import of NS3 into host mitochondria and identify a hitherto unknown factor, GrpEL1 as a cleavage target, and thereby providing new avenues for the dengue research and the design of potential therapeutics. IMPORTANCE About 40% of the world's population is at the risk of dengue virus infections. There is no specific drug or potential vaccine for these infections until now. Lack of complete understanding about the pathogenesis is one of the hurdles for developing antivirals for this virus infection. In the present study, we show that the virus-coded protease imports to the mitochondrial matrix, which is the first-ever report with reference to the animal and human viruses. The analysis indicated that the observed mitochondrial import is due to the inherited mitochondrial transport signal. We also show that the matrix localized GrpEL1, a co-chaperone of mtHsp70, is also the substrate of dengue virus protease, as observed in in vitro, ex vivo, virus-infected cells, and dengue virus-infected clinical samples. Hence our studies reveal an essential aspect of the pathogenesis of dengue virus infections, which may aid in developing anti-dengue therapeutics.

Noncanonical functions of several nuclear transcription factors in the mitochondria have been gaining exceptional traction over the years. These transcription factors include nuclear hormone receptors like estrogen, glucocorticoid, and thyroid hormone receptors: p53, IRF3, STAT3, STAT5, CREB, NF-kB, and MEF-2D. Mitochondria-localized nuclear transcription factors regulate mitochondrial processes like apoptosis, respiration and mitochondrial transcription albeit being nuclear in origin and having nuclear functions. Hence, the cell permits these multi-stationed transcription factors to orchestrate and fine-tune cellular metabolism at various levels of operation. Despite their ubiquitous distribution in different subcompartments of mitochondria, their targeting mechanism is poorly understood. Here, we review the current status of mitochondria-localized transcription factors and discuss the possible targeting mechanism besides the functional interplay between these factors.

STAT3, a transcription factor involved in various physiological and pathological processes, is also present in mitochondria. Mitochondrial STAT3 regulates complex I activity and reactive oxygen species (ROS) production, yet the mechanisms governing its translocation to mitochondria remain poorly understood. In this study, we show that rotenone‐induced ROS triggers the Ser727 phosphorylation of STAT3 and its increased mitochondrial localisation. Furthermore, we show that STAT3‐depleted cells display increased ROS levels during rotenone treatment. Targeted expression in mitochondria of wild‐type STAT3 ‒ but not S727A mutant ‒ lowers ROS levels, indicating the importance of Ser727 phosphorylation, both in rotenone‐induced mitochondrial targeting and quenching of ROS levels. Together, our results demonstrate a novel STAT3‐mediated feedback mechanism to maintain redox homeostasis during stress.

Aging leads to the diminished pulsatile secretion of hypothalamic gonadotropin-releasing hormone (GnRH). Kisspeptin (Kp), the upstream regulator of the hypothalamic-pituitary-gonadal (HPG) axis, regulates GnRH synthesis and release through its cognate receptor, G-protein coupled receptor 54 (GPR54). In turn, GnRH regulates GPR54 expression. GnRH administration into the third ventricle has been shown to induce neurogenesis in different brain regions in old age. However, aging-associated changes in hypothalamic and extra-hypothalamic GPR54 expression were unclear. Therefore, the expression levels of GPR54 were evaluated in various brain regions of adult (age, 3-4 months) and old (age, 20-24 months) male Wistar rats in the present study. In the hypothalamus, mRNA and protein levels of Kp and GPR54 were identified to be significantly decreased in old age. Furthermore, GnRH1 expression in the hypothalamus was analyzed to observe the functional consequence of a reduced Kp-GPR54 system in the hypothalamus. It was found that hypothalamic GnRH1 levels were significantly decreased in old age. As GnRH regulates GPR54 levels, GPR54 was examined in extra-hypothalamic regions. GPR54 levels were found to be significantly decreased in the hippocampus and medulla and pons in old-age rats when compared to adult rats. Notably, GPR54 expression was observed in the frontal lobe, cortex, midbrain and cerebellum of adult and old-age rats; however, the difference between the two groups was not statistically significant. To the best of our knowledge, this is the first study that provides the quantitative distribution of GPR54 in different brain regions during aging. Thus, the reduced levels of Kp and its receptor, GPR54 in the hypothalamus could be cumulatively responsible for reduced levels of GnRH observed in old age.