Capn3 depletion causes Chk1 and Wee1 accumulation and disrupts synchronization of cell cycle reentry during liver regeneration after partial hepatectomy

Hepatic depletion of nucleolar protein mDEF causes excessive mitochondrial copper accumulation associated with p53 and NRF1 activation

Copper is an essential component in the mitochondrial respiratory chain complex IV (cytochrome c oxidases). However, whether any nucleolar factor(s) is(are) involved in regulating the mitochondrial copper homeostasis remains unclear. The nucleolar localized Def-Capn3 protein degradation pathway cleaves target proteins, including p53, in both zebrafish and human nucleoli. Here, we report that hepatic depletion of mDEF in mice causes an excessive copper accumulation in the mitochondria. We find that mDEF-depleted hepatocytes show an exclusion of CAPN3 from the nucleoli and accumulate p53 and NRF1 proteins in the nucleoli. Furthermore, we find that NRF1 is a CAPN3 substrate. Elevated p53 and NRF1 enhances the expression of Sco2 and Cox genes, respectively, to allow more copper acquirement in the mDef^loxp/loxp, Alb:Cre mitochondria. Our findings reveal that the mDEF-CAPN3 pathway serves as a novel mechanism for regulating the mitochondrial copper homeostasis through targeting its substrates p53 and NRF1.

Ronidazole Is a Superior Prodrug to Metronidazole for Nitroreductase-Mediated Hepatocytes Ablation in Zebrafish Larvae

The liver plays a very important role in physiological processes of the human body. Liver regeneration has developed into an important area of study in liver disease. The Mtz (metronidazole)/NTR (nitroreductase)-mediated cell ablation system has been widely used to study the processes and mechanisms of liver injury and regeneration. However, high concentrations and toxic side effects of Mtz severely limit the application of the Mtz/NTR system. Therefore, screening new analogs to replace Mtz has become an important means to optimize the NTR ablation system. In this study, we screened five Mtz analogs including furazolidone, ronidazole, ornidazole, nitromide, and tinidazole. We compared their toxicity on the transgenic fish line Tg(fabp10a: mCherry-NTR) and their specific ablation ability on liver cells. The results showed that Ronidazole at a lower concentration (2 mM) had the same ability to ablate liver cells comparable with that of Mtz (10 mM), almost without toxic side effects on juvenile fish. Further study found that zebrafish hepatocyte injury caused by the Ronidazole/NTR system achieved the same liver regenerative effect as the Mtz/NTR system. The above results show that Ronidazole can replace Mtz with NTR to achieve superior damage and ablation effects in zebrafish liver.

Upregulated miR-322-5p regulates cell cycle and promotes cell proliferation and apoptosis by directly targeting Wee1 in mice liver injury

Liver injury from any number of causes (e.g. chemical material, drugs and diet, viral infection) is a global health problem, and its mechanism is not clearly understood. MicroRNAs (miRNAs) expression profiling is gaining popularity because miRNAs, as key regulators in gene expression networks, can influence many biological processes and have also shown promise as biomarkers for disease. Previous studies reported the regulation effects of miRNAs in liver injury, whereas function and molecular mechanisms of miR-322-5p were still unclear. Therefore, our study focused on the biological role of miR-322-5p in carbon tetrachloride (CCl4)-induced liver injury proliferation, apoptosis, and cell cycle. A mouse model of CCl4-induced liver injury was established, and the transcriptomes and miRNAs transcriptomes of 2d and 5d liver tissues after injury were sequenced. The expression of miR-322-5p and the cell cycle genes were detected in liver tissues and Hepa1-6 cell line by miRNA RT-PCR, qRT-PCR. The effects of miR-322-5p on liver cell proliferation, cell cycle and apoptosis were evaluated using MTS assays and flow cytometry analysis. The relationship between miR-322-5p and Wee1 was predicted and confirmed by bioinformatics analysis and a dual luciferase reporter assay. Functional experiments, including an MTS assay and flow cytometric analysis, were performed to study the effects of Wee1. MiR-322-5p was upregulated in injury liver tissues, and downregulated miR-322-5p was proved to inhibit proliferation, apoptosis and arrest cell cycle at G2/M in vitro. The dual-luciferase reporter assay results indicated that miR-322-5p has a binding site at position 285 in the Wee1 3´UTR. The effects of miR-322-5p in proliferation and cell cycle regulation can be abolished by Wee1 through rescue experiments. By directly targeting Wee1 influenced the expression of several cell cycle factors, including Cyclin dependent kinase 1 (Cdk1), cyclin B1 (Ccnb1) and Cell division cyclin 25C (Cdc25C). MiR-322-5p may function as a suppressive factor by negatively controlling Wee1, thus, highlighting the potential role of miR-322-5p as a therapeutic target for liver injury.Abbreviations: ALT: Alanine aminotransferase; AST: Aspartate aminotransferase; GSH: Glutathione, γ-glutamyl cysteinel + glycine; CCl4: Carbon tetrachloride; HE: Haematoxylin and eosin; KEGG: Kyoto Encyclopedia of Genes and Genomes.

An 86 amino acids motif in CAPN3 is essential for formation of the nucleolus-localized Def-CAPN3 complex

Digestive-organ expansion factor (Def) is a nucleolar protein that recruits cysteine proteinase Calpain3 (CAPN3) into the nucleolus to form the Def-CAPN3 complex in both human and zebrafish. This complex mediates the degradation of the tumor suppressor p53 and ribosome biogenesis factor mitotic phosphorylated protein 10 (Mpp10) in nucleolus, demonstrating the importance of this complex in regulating cell cycle and ribosome biogenesis. However, the Def and CAPN3 interacting motifs have yet been identified. In this report, by using a series of truncated or internally deleted human CAPN3 (hCAPN3) derivatives we identify that an essential motif of 86 amino acids (86-aa) (430-515aa) in hCAPN3 for its interaction with human Def (hDef), and this 86-aa motif is highly conserved in zebrafish Capn3b (zCapn3b) and is also required for the interaction between zebrafish Def (zDef) and zCapn3b. We further identify the 2/3 C-terminus of hDef is responsible for mediating the hDef-hCAPN3 interaction, and the corresponding region is conserved for the zDef and zCapn3b interaction. Our results lay the ground to resolve the structure of the Def-CAPN3 complex in the future.

Nucleolus-localized Def-CAPN3 protein degradation pathway and its role in cell cycle control and ribosome biogenesis

The nucleolus, as the 'nucleus of the nucleus', is a prominent subcellular organelle in a eukaryocyte. The nucleolus serves as the centre for ribosome biogenesis, as well as an important site for cell-cycle regulation, cellular senescence, and stress response. The protein composition of the nucleolus changes dynamically through protein turnover to meet the needs of cellular activities or stress responses. Recent studies have identified a nucleolus-localized protein degradation pathway in zebrafish and humans, namely the Def-CAPN3 pathway, which is essential to ribosome production and cell-cycle progression, by controlling the turnover of multiple substrates (e.g., ribosomal small-subunit [SSU] processome component Mpp10, transcription factor p53, check-point proteins Chk1 and Wee1). This pathway relies on the Ca²⁺-dependent cysteine proteinase CAPN3 and is independent of the ubiquitin-mediated proteasome pathway. CAPN3 is recruited by nucleolar protein Def from cytoplasm to nucleolus, where it proteolyzes its substrates which harbor a CAPN3 recognition-motif. Def depletion leads to the exclusion of CAPN3 and accumulation of p53, Wee1, Chk1, and Mpp10 in the nucleolus that result in cell-cycle arrest and rRNA processing abnormality. Here, we summarize the discovery of the Def-CAPN3 pathway and propose its biological role in cell-cycle control and ribosome biogenesis.

All routes lead to Rome: multifaceted origin of hepatocytes during liver regeneration

Liver is the largest internal organ that serves as the key site for various metabolic activities and maintenance of homeostasis. Liver diseases are great threats to human health. The capability of liver to regain its mass after partial hepatectomy has widely been applied in treating liver diseases either by removing the damaged part of a diseased liver in a patient or transplanting a part of healthy liver into a patient. Vast efforts have been made to study the biology of liver regeneration in different liver-damage models. Regarding the sources of hepatocytes during liver regeneration, convincing evidences have demonstrated that different liver-damage models mobilized different subtype hepatocytes in contributing to liver regeneration. Under extreme hepatocyte ablation, biliary epithelial cells can undergo dedifferentiation to liver progenitor cells (LPCs) and then LPCs differentiate to produce hepatocytes. Here we will focus on summarizing the progresses made in identifying cell types contributing to producing new hepatocytes during liver regeneration in mice and zebrafish.