AKI Clinical Research Progress in 2022 III

Common causes of acute kidney injury (AKI) include renal ischemia-reperfusion, sepsis, and nephrotoxins that damage tubular epithelial cells (TECs), which can cause acute tubular death. As the cell with the largest number and the highest proportion in renal tissue, the injury, and death of TECs are the most critical in the decline of renal function. Its dysfunction and impairment are the initial events in AKI and are directly related to the death of TECs. In the past year, basic research on AKI has made great progress in tubular epigenetics, polyploidization, ferroptosis, pyroptosis, and aging.

AKI and tubular epigenetics

The research progress of epigenetics shows that a variety of epigenetic modifications including acetylation, methylation, and microRNA are involved in the pathogenesis of AKI. Changes in DNA promoter methylation occur in the kidney after ischemia/reperfusion injury, and targeting the epigenetic modification regulation process may contribute to the clinical targeted therapy of AKI patients. Using a vitamin C-deficient mouse model (Gulo knockout mouse), Yu et al. created a comprehensive profile of each cell type in the kidney by single-cell RNA sequencing, whole-genome bisulfite sequencing, and methylated RNA immunoprecipitation sequencing. specific transcription and DNA/RNA methylation profiles. RESULTS: Following vitamin C deficiency, the loss of proximal tubular epithelial DNA hydroxymethylation and DNA hypermethylation preceded tubular necrosis in the kidney, suggesting that vitamin C remodels DNA/RNA epigenetic modifications. Preventive supplementation of antioxidant vitamin C derivatives (APM) promotes DNA demethylation and ameliorates cisplatin-induced acute tubular necrosis. These findings will contribute to the development of vitamin C as an epigenetic therapy for kidney diseases, and provide new insights and ideas for vitamin C as an epigenetic regulator of renal homeostasis1.

AKI and polyploidization of renal tubules

Polyploidization is the process by which normal diploid cells acquire an extra set of chromosomes. It is often a compensatory mechanism for renal tubular cells after acute kidney injury to maintain residual renal function and avoid renal failure. Chiara et al. explored the tubular cell distribution of polyploidy after AKI by DNA content analysis and single-cell RNA-sequencing techniques and subsequently investigated the functional role of tubular polyploidization using various transgenic animal models and drug interventions. The study further confirmed that the polyploidization of renal tubules driven by YAP1 protein is the main survival mechanism for maintaining residual renal function in the early stage of AKI, but with the development of AKI, this survival mechanism promotes the progression of AKI to CKD at the cost of polyploid renal tubular senescence. change. If the small molecule compound CA3 is used to target YAP1/TEAD transcriptional activity in the early stage of AKI, polyploidization of renal tubular cells can be prevented, but the transition from AKI to CKD can be prevented without affecting the renal failure caused by AKI. Antiaging drug treatment (quercetin + dasatinib) blocked AKI-CKD transition by inhibiting tubular polyploidization. These findings identify a potential drug target and provide an important rationale for clinically improving AKI prognosis2.

AKI and renal tubular necrosis

Necrosis of renal tubular cells includes necroptosis, ferroptosis, pyroptosis, etc. Each type of cell necrosis has its own specific molecular regulation mechanism, which is an active process determined by genes. An orderly pattern of death is prevalent in AKI and plays an important role in maintaining tissue homeostasis3.

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Ferroptosis is regulatory necrosis characterized by lipid peroxidation, such as phosphatidylethanolamine (PE) containing arachidonic acid plays an important role in AKI. Renal lipid metabolism studies were performed in a folate-induced AKI (FA-AKI) model by Martin-Saiz et al. Renal lipid metabolism was assessed by imaging mass spectrometry (IMS), and glomeruli, medulla, and cortex were differentiated in control, AKI, and AKI+Ferrostatin-1 (ferroptosis inhibitor)-treated groups. Of the 139 lipids identified in 16 categories, 29 (20.5%) were significantly different between the 48-h control and AKI groups. Total PE and lysine sulfate species decreased, while phosphatidylinositol (PI) species increased in AKI. The mRNA expression levels of Pemt, Pgs1, Cdipt, and Tamm related to lipid metabolism were disturbed in AKI and consistent with the observed lipid changes. Ferrostatin-1 improved AKI and attenuated some AKI-related lipid changes, such as the reduction of PE and soluble sulfate species, without changing the expression of lipid metabolism enzymes. In conclusion, in nephrotoxic AKI, changes in renal lipid composition are associated with changes in gene expression of lipid-metabolizing enzymes, and this pathological process can be partially blocked by Ferrostatin-1.

Dexamethasone, a widely used immunosuppressant, was recently found to induce ferroptosis by depleting glutathione (GSH). Massenhausen et al. found that dexamethasone significantly decreased the level of GSH in tubular cells and increased the sensitivity of tubular cells to ferroptosis. Further studies found that dexamethasone up-regulated GSH metabolism-regulating protein dipeptidase-1 (DPEP1) in a glucocorticoid receptor (GR)-dependent manner, and the up-regulated DPEP1 could accelerate GSH catabolism. DPEP1 knockout reverses dexamethasone-induced ferroptosis. Ferroptosis inhibitors, the DPEP1 inhibitor cilastatin, or DPEP1 gene knockout ameliorated dexamethasone-induced tubular ferroptosis. This literature reveals that dexamethasone mediates the increase of DPEP1 expression through GR, promotes GSH depletion and renal tubular cell ferroptosis, and inhibiting this process can improve renal tubular cell ferroptosis, which has great clinical therapeutic significance5.

AKI and Repair, Regenerative Therapy

Under the action of injury factors, renal tubular epithelial cells lose their normal polarity, necrosis, or apoptosis, causing a rapid and progressive decline in renal function in a short period. Although kidney tissue cells have a certain ability to regenerate and repair, when the damage factors are severe, complex, or persistent, the kidney undergoes incomplete repair and will gradually develop into chronic kidney disease. Tubular epithelial cells (TECs) are one of the important target cells for the occurrence and progression of AKI and are also the key to AKI repair. In recent years, research on the mechanism, diagnosis, and treatment of renal tubular regeneration and repair after AKI has been increasing, and how to promote renal tubular epithelial cell regeneration and delay chronicity has attracted increasing attention and attention.

Pax2 is an indispensable transcription factor during kidney development. The study found that in the mouse model of ischemia-reperfusion injury (IRI), the number of Pax2 positive cells and the level of Pax2 mRNA were significantly increased after IRI, and the number of ki-67+ cells in renal tubular Pax2 knockout mice was significantly decreased, while TUNEL+ The number of cells was significantly increased; and after 14 days of IRI, the renal interstitial fibrosis of Pax2 knockout mice was aggravated. Cyclin-dependent kinase 4 (CDK4) expression was significantly reduced in Pax2-knockout mice after IRI compared with wild-type mice, and the upregulation of CDK4 expression was suppressed by Pax2 inhibitors. These results suggest that after IRI, reactivated Pax2 in proximal tubular epithelial cells promotes the proliferation of proximal tubular epithelial cells and inhibits renal fibrosis by upregulating the expression of CDK41.

In addition, TECs after AKI can also secrete high levels of GM-CSF, induce the activation of repairing macrophages expressing Arginase-1 (Arg1), stimulate TEC's proliferation, and promote kidney repair. TECs can also express an autophagy regulator, beclin1 protein, to attenuate acute kidney injury, promote antiproliferation and inhibit renal fibrosis.

Since the pathogenesis and repair mechanism of AKI is not fully understood, the current clinical monitoring of AKI relies on urine output and plasma markers. Klocke et al. used flow cytometry to sort kidney cells in urine and established a workflow for urine single-cell RNA sequencing. Analyzing the transcriptome of TECs in urine reflects the process of AKI damage and repair and provides new means and perspectives for AKI target identification, subclassification, and monitoring and intervention of the natural course4.

In terms of AKI treatment, recent studies have confirmed that formoterol, a β2 adrenoceptor agonist, can promote kidney repair after AKI in mice by inducing mitochondrial biogenesis. But formoterol has been limited by its daily dosing requirements and cardiotoxicity. Shi et al. developed nanoparticles containing formoterol, which can be administered to the kidneys, thereby achieving the same therapeutic effect as systemic administration with less frequent dosing and lower total doses, and reducing potential Drug Toxicity5.

Glutamine is the most abundant free amino acid in the body. In the case of infection and tissue damage, it participates in protein biosynthesis, energy metabolism, and scavenging of active oxygen, and is a conditionally essential amino acid. Kidney tubular cells and immune cells are the main "consumers" of glutamine. Thomas et al. found that intravenous glutamine can significantly improve the renal injury and renal function decline induced by ischemia-reperfusion in mice; studies have shown that glutamine induces transcriptome and proteome reprogramming in renal tubular epithelial cells (TECs), reducing Epithelial cell apoptosis, neutrophil recruitment, improved mitochondrial function and oxidative phosphorylation. Meanwhile, the researchers identified glutamine γ-glutamyltransferase 2 (Tgm2) and apoptosis signal-regulating kinase (Ask1) as the main targets of glutamine in apoptosis signaling. Glutamine targets up-regulating the Tgm2-HSP70 signaling pathway inhibits the activation of Ask1 and JNK, and finally reduces the endogenous apoptosis of TEC's mitochondria. The above research results suggest that glutamine drugs that have been used in clinical practice may be used to treat patients with AKI6.

Growth differentiation factor 15 (GDF15) is a member of the GDF subfamily and plays an important role in anti-inflammation, anti-proliferation, and anti-tumor. Clinical studies have shown that serum GDF15 levels are elevated in CKD patients and are associated with CKD progression and mortality; at the same time, serum GDF15 levels are correlated with renal interstitial GDF15 mRNA levels, suggesting that GDF15 in the kidney may be released into the blood circulation. However, the role of GDF15 in acute kidney injury and interstitial fibrosis has not been elucidated. Valino-Rivas et al. confirmed that GDF15 can improve acute kidney injury and interstitial fibrosis induced by toxicants (folate or cisplatin) through a variety of transgenic animal models, and injection of GDF15 human-derived recombinant protein does not affect acute kidney injury and interstitial fibrosis. Has a protective effect. Mechanistically, GDF15 promotes the expression of the renoprotective factor Klotho and inhibits the activation of the transcription factor NF-kB. Therefore, GDF15 may be a new drug target, providing a theoretical basis for the clinical treatment of AKI and CKD7.

AKI and COVID-19

COVID-19 is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, which can cause severe multi-organ damage and death. The kidney is one of the main target organs of COVID-19, and acute kidney injury (AKI) is common in critically ill patients with COVID-19. However, the mechanisms by which COVID-19 causes AKI remain largely unknown.

Researchers such as Wang carried out a series of in-depth studies on the pathogenic mechanism of the new coronavirus infection and found that the AKI induced by the SARS-CoV-2 N protein is dependent on the Smad3 protein because the SARS-CoV-2 N protein can interact with Smad3 and Enhanced TGF-β/Smad3 signaling leads to tubular epithelial cell death and AKI via a G1 cell cycle arrest mechanism, revealing a direct role of SARS-CoV-2 N protein in AKI. At the same time, the team found through the study of Smad3 gene knockout mice and specific Smad3 inhibitors that knocking out the Smad3 gene and inhibiting the Smad3 protein can significantly improve SARS-CoV-2 N protein-induced cell death and acute kidney injury. Therefore, using Smad3 protein as an inhibitory target is expected to become a new method for the treatment of acute kidney injury caused by COVID-191.

ACE2 is the main receptor for COVID-19 to enter target cells, and researchers have designed a variety of interventions for this protein to achieve the purpose of treating COVID-19. Hassler et al. demonstrated the preclinical efficacy of a novel soluble ACE2 protein (ACE2-1-618-DDC-ABD) in a lethal mouse model of SARS-CoV-2 infection. Researchers fused human soluble ACE2 variants to albumin-binding domain (ABD) linked by a dimeric matrix hinge-like 4-cysteine dodecapeptide (DDC) to improve the binding ability to SARS-CoV-2. This novel soluble ACE2 protein was then administered intranasally and intraperitoneally, followed by intranasal infection with SARS-CoV-2. The results showed that lung and brain viral titers of SARS-CoV-2 were significantly reduced in animals receiving ACE2-1-618-DDC-ABD, with concomitant kidney damage, compared to high titers in untreated infected controls Visibly lightened. Thus, this novel soluble human ACE2 variant, ACE2-1-618-DDC-ABD, can convert a fatal infection into a milder one, as demonstrated in an animal model of COVID-19 with severe lung and moderate proximal tubular kidney injury Based on its preclinical efficacy, this experiment provides new ideas for the improvement and treatment of COVID-192.

As previously mentioned, ACE2, as the first identified host receptor for SARS-CoV-2, is responsible for the reverse regulation of the renin-angiotensin signaling (RAS) pathway by enzymatically inactivating angiotensin II. Losartan is an angiotensin II receptor blocker that inhibits angiotensin II-mediated internalization of angiotensin ACE2. Based on this background, Rahmani et al. investigated the effect of Ang II and losartan on SARS-CoV-2 infection to provide evidence to support losartan as a possible neoadjuvant therapy for SARS-CoV-2.

Virus-specific RNA sequences participate in host antiviral mechanisms by activating hundreds of IFN-stimulated genes (ISGs), promoting the host antiviral state. The team found that losartan up-regulates antiviral ISGs such as BST2 and IFITM1 through a kidney organoid model, which helps reduce the susceptibility of renal tubular cells to SARS-CoV-2. Therefore, losartan treatment may protect against COVID-19-AKI. This study suggests a potential protective role of RAS inhibitors against SARS-CoV-2 infection3.

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