Insights into the ISG15 transfer cascade by the UBE1L activating enzyme

ISG15 accelerates acute kidney injury and the subsequent AKI-to-CKD transition by promoting TGFβR1 ISGylation

Rationale: Acute kidney injury (AKI) has substantial rates of mortality and morbidity, coupled with an absence of efficacious treatment options. AKI commonly transits into chronic kidney disease (CKD) and ultimately culminates in end-stage renal failure. The interferon-stimulated gene 15 (ISG15) level was upregulated in the kidneys of mice injured by ischemia-reperfusion injury (IRI), cisplatin, or unilateral ureteral obstruction (UUO), however, its role in AKI development and subsequent AKI-to-CKD transition remains unknown. Methods: Isg15 knockout (Isg15 KO) mice challenged with bilateral or unilateral IRI, cisplatin, or UUO were used to investigate its role in AKI. We established cellular models with overexpression or knockout of ISG15 and subjected them to hypoxia-reoxygenation, cisplatin, or transforming growth factor- β1 (TGF-β1) stimulation. Renal RNA-seq data obtained from AKI models sourced from public databases and our studies, were utilized to examine the expression profiles of ISG15 and its associated genes. Additionally, published single cell RNA-seq data from human kidney allograft biopsies and mouse IRI model were analyzed to investigate the expression patterns of ISG15 and the type I TGF-β receptor (TGFβR1). Western blotting, qPCR, co-immunoprecipitation, and immunohistochemical staining assays were performed to validate our findings. Results: Alleviated pathological injury and renal function were observed in Isg15 KO mice with IRI-, cisplatin-, or UUO-induced AKI and the following AKI-to-CKD transition. In hypoxia-reoxygenation, cisplatin or TGF-β1 treated HK-2 cells, knockout ISG15 reduced stimulus-induced cell fibrosis, while overexpression of ISG15 with modification capacity exacerbated cell fibrosis. Immunoprecipitation assays demonstrated that ISG15 promoted ISGylation of TGFβR1, and inhibited its ubiquitination. Moreover, knockout of TGFβR1 blocked ISG15's fibrosis-exacerbating effect in HK-2 cells, while overexpression of TGFβR1 abolished the renal protective effect of ISG15 knockout during IRI-induced kidney injury. Conclusions: ISG15 plays an important role in the development of AKI and subsequent AKI-to-CKD transition by promoting TGFβR1 ISGylation.

Mechanisms of USP18 deISGylation revealed by comparative analysis with its human paralog USP41

The ubiquitin-like protein ISG15 (interferon-stimulated gene 15) regulates the host response to bacterial and viral infections through its conjugation to proteins (ISGylation) following interferon production. ISGylation is antagonized by the highly specific cysteine protease USP18, which is the major deISGylating enzyme. However, mechanisms underlying USP18's extraordinary specificity towards ISG15 remains elusive. Here, we show that USP18 interacts with its paralog USP41, whose catalytic domain shares 97% identity with USP18. However, USP41 does not act as a deISGylase, which led us to perform a comparative analysis to decipher the basis for this difference, revealing molecular determinants of USP18's specificity towards ISG15. We found that USP18 C-terminus, as well as a conserved Leucine at position 198, are essential for its enzymatic activity and likely act as functional surfaces based on AlphaFold predictions. Finally, we propose that USP41 antagonizes conjugation of the understudied ubiquitin-like protein FAT10 (HLA-F adjacent transcript 10) from substrates in a catalytic-independent manner. Altogether, our results offer new insights into USP18's specificity towards ISG15, while identifying USP41 as a negative regulator of FAT10 conjugation.