Targeting a Single Codon to Rescue Septic Acute Kidney Injury

Sepsis is a life-threatening syndrome characterized by the aberrant systemic immune response to infection, leading to multiple organ failure and patient death. AKI is a common complication in sepsis that contributes to the unacceptably high rate of mortality in affected patients.¹ Unfortunately, there is no effective treatment for preventing or alleviating septic AKI, mostly because of an incomplete understanding of its pathogenesis.

Multiple complex mechanisms, such as inflammation and metabolic changes, dynamically affect different phases of septic AKI.² In animal models of endotoxemia-induced AKI, inflammatory cytokine expression increases in kidney tubular cells within the first few hours, which is followed by a late phase of sepsis with a dramatic decrease in synthesis of most proteins that affect multiple metabolic pathways.³ In this paradigm, transient blockage of protein synthesis in the early phase of septic AKI could be beneficial as it reduces energy consumption and limits the production of inflammatory cytokines. However, persistent shutdown of translation or protein synthesis would prevent kidney repair and recovery from initial injury. As such, restoring translation may promote or accelerate kidney recovery from septic insult and, therefore, be beneficial.

In eukaryotic cells, the integrated stress response (ISR) is a common adaptive pathway that triggers translation shutdown in response to various types of stress, such as viral infection and oxidative stress.⁴ Generally, viralactivation of ISR tunes down global mRNA translation yet upregulates the synthesis of a few selected proteins to maintain cellular homeostasis. A key event in ISR is the inactivation of eIF2, the protein synthesis initiating factor, via phosphorylation of its α-subunit by protein kinases such as PKR that is activated by different stress signals, such as virus-derived dsRNA. Conversely, the phosphatase regulatory subunit PPP1R15A (also termed GADD34), by functioning as a scaffold, facilitates the dephosphorylation of eIF2α and counteracts ISR.⁵ Therefore, the balance between PKR and PPP1R15A dictates the phosphorylation status of eIF2α and associated translation in ribosomes under pathophysiologic conditions (Figure 1).

Figure 1.

Targeting the ISR pathway in septic AKI. In the early phase of sepsis, PKR phosphorylates eIF2α to shut down protein synthesis or translation, whereas in the late phase, eIF2α needs to be dephosphorylated to restore translation for kidney recovery. Inhibition of the ISR pathway, using PKR inhibitors, ISRIB, or genetic modulation of PPP1R15A uORF, may reverse translation shutdown and improve kidney recovery from septic AKI.

In humans, eIF2 signaling is a key feature in the transcriptomic response to both fecal peritonitis and pneumonia, supporting a role of eIF2 in sepsis regardless of the source of infection.⁶ Using animal models of Gram-negative septic AKI, Hato and colleagues³ revealed that aberrant eIF2α activation links the early phase of inflammation to systemic translation shutdown in the later phase of sepsis. Of clinical interest, the integrated stress response inhibitor (ISRIB), a small molecule that potently reverses the effect of eIF2α phosphorylation on protein synthesis, had significant renoprotective effects when administered early in the course of sepsis (Figure 1). However, the therapeutic window of ISRIB observed in that study was narrow, and nonspecific effects of ISRIB were not excluded. Mechanistically, it was elusive how eIF2α phosphorylation is maintained to keep translation shutdown in septic AKI.

In the current issue of JASN,⁷ Hato and colleagues report that excessive eIF2α phosphorylation in the late phase of septic AKI is a result of reduced expression of the counter-regulatory phosphatase Ppp1r15a, leading to global translation shutdown. Using a newly generated knock-in mouse model, mutant cell lines, and an in vivo antisense approach, they show that Ppp1r15a is translationally repressed by the existence of an upstream open reading frame (uORF) in front of the canonical coding sequence. uORF is a sequence within the 5′ untranslated region of a primary mRNA, which can be translated into a short peptide to regulate the translation of the primary mRNA. Remarkably, blockage of the uORF expression by introducing a point mutation in its start codon increased Ppp1r15a expression and mitigated septic AKI after lipopolysaccharide challenge. These findings are both novel and compelling and prove that targeting a single uORF can effectively affect the ISR pathway, restore protein synthesis, and save the kidneys in sepsis. Of clinical significance, this study provides a paradigm for the design of uORF-based therapeutic strategies, which may be used to control the dynamics of gene translation with codon level precision (Figure 1). In addition to unveiling the Ppp1r15a uORF in regulating ISR, the authors also performed sophisticated time-course analysis that captured the grand landscapes of the transcriptome, translatome, proteome, and immunopeptidome across the timeline of septic AKI. This vast information allows for further comprehensive analysis of the dynamic changes in septic AKI to reveal other potential therapeutic targets.

This study also highlights several issues that require further research before attempting a clinical application of targeting uORFs to prevent AKI in sepsis. First, the mechanism whereby the uORF prevents Ppp1r15a expression remains elusive. In this regard, the uORF may encode a micropeptide that prevents Ppp1r15a mRNA translation by stalling ribosomes,⁸ an intriguing possibility awaiting investigation. Second, the impact of Ppp1r15a uORF modulation may be magnitude and context dependent. This study suggests the existence of nonlinear relationships between disease severity and optimal dosing of targeting Ppp1r15a, highlighting the importance of quantitating the uORF-based therapy in a disease-dependent manner. Meanwhile, these findings necessitate further genome-wide association studies to explore the involvement of genetic variants in the susceptibility of septic AKI, considering that a single codon change can sufficiently alter the disease outcome. Third, in addition to ISR, PPP1R15A also regulates other stress response pathways. For example, Gambardella et al.⁹ recently suggested an essential role of PPP1R15A in autophagy, an intrinsic protective mechanism in AKI.¹⁰ In this case, expression of PPP1R15A may restore protein synthesis and promote autophagy to combat AKI. Moreover, the PPP1R15A-eIF2α axis may exert distinct biological functions in various cell types across the sepsis timeline. Therefore, it is critical to determine the cell type-specific strategies with appropriate treatment timing for optimal therapeutic effect. Finally, it is of great interest to investigate the impact of ISR modulation on the long-term outcome of septic AKI, which may lead to novel therapies to prevent or slow down the development of chronic kidney problems in patients who have survived sepsis.

Disclosures

Z. Dong reports Consultancy: DILIsym/RenaSym, Mitobridge; Honoraria: DILIsym/RenaSym; and Advisory or Leadership Role: Associate Editor for American Journal of Physiology—Renal Physiology, Associate Editor for Kidney Diseases, Editorial board member for JASN, Editorial board member for Kidney International, Editorial board member for American Journal of Physiology—Cell Physiology, Board of Directors of Chinese American Society of Nephrology, Adjunct Professorship at The Second Xiangya Hospital of Central South University in China. The remaining author has nothing to disclose.

Funding

Z. Dong was supported by the US Department of Veterans Affairs (5I01BX000319) and the US National Institutes of Health (2R01DK058831, 5R01DK087843). He is also a recipient of the Senior Career Research Scientist award of the US Department of Veterans Affairs (5IK6BX005236).

Author Contributions

Z. Dong conceptualized the study and was responsible for supervision; G. Chen wrote the original draft; and Z. Dong reviewed and edited the manuscript.