The Significance of Polyploid Hepatocytes During Aging Process

Pathological significance of intranuclear structures in liver biopsy samples

AIM

Glycogenated nuclei (GN) are glycogen deposits within the nuclei and are a frequent pathological finding in metabolic dysfunction-associated steatotic liver disease. This study aimed to investigate the relationship between GN and two morphologically distinct types of intranuclear lipid droplets in liver biopsy specimens and to explore their respective pathological significance.

METHODS

We analyzed 135 liver biopsy specimens. A portion of the liver biopsy specimen was examined using transmission electron microscopy (TEM) to investigate intranuclear lipid droplets in hepatocytes. Nuclear inclusion bodies with clear boundaries and unstained areas on hematoxylin and eosin staining were identified as nuclear glycogen.

RESULTS

TEM revealed nucleoplasmic lipid droplets (nLD) in 65% of liver biopsy specimens and invagination of cytoplasmic lipid droplets into the nucleus in 30% of specimens. In contrast, light microscopy detected GN in 82% of specimens. No significant correlations were observed between the frequencies of the two types of intranuclear lipid droplets and nuclear glycogen levels. A significant positive correlation was observed between the frequency of nLD and transaminase levels. Glycogenated nuclei were frequently observed in liver biopsy specimens from patients with MASLD; however, their frequency did not significantly correlate with the degree of hepatic steatosis. Instead, a significant positive correlation was observed between nuclear glycogen and blood HbA1c levels.

CONCLUSIONS

The two types of intranuclear lipid droplets and nuclear glycogen observed in liver biopsy specimens showed no significant correlation in their formation frequencies, suggesting that they possess distinct pathological significance.

Influencing factors and mechanism of hepatocyte regeneration

As a research hotspot in the field of regenerative medicine, hepatocyte regeneration has great potential in the treatment of liver diseases. This paper comprehensively summarizes the diverse sources of hepatocyte regeneration and its complex influencing factors, and deeply discusses the typical mechanism. According to the existing research, we observed that Wnt signaling pathway and Notch signaling pathway can play a synergistic role in the process of hepatocyte regeneration. So we further analyzed the crosstalk between Wnt and Notch signal pathway and the cross mechanism with TGF-β, YAP/TAZ pathway during regeneration. Despite the remarkable progress in the study of liver regeneration at the cellular and molecular levels, the comprehensive understanding of the fine regulation of influencing factors and the interaction between mechanisms still needs to be deepened. This paper aims to systematically analyze the interaction between influencing factors and classical mechanisms of hepatocyte regeneration by integrating multi-group data and advanced bioinformatics methods, so as to provide feasible ideas for the treatment of liver diseases and lay a solid theoretical foundation for the future development of regenerative medicine. It is believed that focusing on the rational development of innovative means such as inducing gene tendentiousness expression and anti-aging therapy, and in-depth analysis of the complex interactive network between hepatocyte regeneration mechanisms are expected to open up a new road for the development of more effective treatment strategies for liver diseases.

Folliculin depletion results in liver cell damage and cholangiocarcinoma through MiT/TFE activation

Mutations in the tumor suppressor gene Folliculin (FLCN) are responsible for Birt-Hogg-Dube' (BHD) syndrome, a rare inherited condition that predisposes affected individuals to skin tumors, pulmonary cysts, and kidney tumors. FLCN regulates key cellular pathways, including TFEB, TFE3, and mTORC1, which are critical for maintaining cell homeostasis. Loss of FLCN leads to both hyperactivation of mTORC1 and constitutive activation of TFEB and TFE3, contributing to tumorigenesis. While previous studies showed that Flcn liver-specific conditional knockout (FlcnLiKO) mice are protected from developing liver fibrosis and damage upon high-fat diet exposure, the potential role of FLCN loss in liver carcinogenesis remained unexplored. Here, we demonstrate that hepatic loss of FLCN in mice results in cancer associated with inflammation and fibrosis with features of cholangiocarcinoma (CCA). This phenotype emerges in mice over 90-week-old, with a male predominance. Moreover, FlcnLiKO mice are more prone to develop diethylnitrosamine (DEN)- or 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)- induced liver tumors with heterogenous histological features. Notably, depletion of TFE3, but not TFEB, in the liver of FlcnLiKO mice fully rescues the cancer phenotype and normalized mTORC1 signaling, highlighting TFE3 as the primary driver of liver cancer and mTORC1 hyperactivity in the absence of FLCN.

Activation of a Src-JNK pathway in unscheduled endocycling cells of the Drosophila wing disc induces a chronic wounding response

The endocycle is a specialized cell cycle during which cells undergo repeated G / S phases to replicate DNA without division, leading to large polyploid cells. The transition from a mitotic cycle to an endocycle can be triggered by various stresses, which results in unscheduled, or induced endocycling cells (iECs). While iECs can be beneficial for wound healing, they can also be detrimental by impairing tissue growth or promoting cancer. However, the regulation of endocycling and its role in tissue growth remain poorly understood. Using the Drosophila wing disc as a model, we previously demonstrated that iEC growth is arrested through a Jun N-Terminal Kinase (JNK)-dependent, reversible senescence-like response. However, it remains unclear how JNK is activated in iECs and how iECs impact overall tissue structure. In this study, we performed a genetic screen and identified the Src42A-Shark-Slpr pathway as an upstream regulator of JNK in iECs, leading to their senescence-like arrest. We found that tissues recognize iECs as wounds, releasing wound-related signals that induce a JNK-dependent developmental delay. Similar to wound closure, this response triggers Src-JNK-mediated actomyosin remodeling, yet iECs persist rather than being eliminated. Our findings suggest that the tissue response to iECs shares key signaling and cytoskeletal regulatory mechanisms with wound healing and dorsal closure, a developmental process during Drosophila embryogenesis. However, because iECs are retained within the tissue, they create a unique system that may serve as a model for studying chronic wounds and tumor progression.

Spatial transcriptomics of healthy and fibrotic human liver at single-cell resolution

Single-cell RNA sequencing (scRNA-seq) has advanced our understanding of cell types and their heterogeneity within the human liver, but the spatial organization at single-cell resolution has not yet been described. Here we apply multiplexed error robust fluorescent in situ hybridization (MERFISH) to map the zonal distribution of hepatocytes, spatially resolve subsets of macrophage and mesenchymal populations, and investigate the relationship between hepatocyte ploidy and gene expression within the healthy human liver. Integrating spatial information from MERFISH with the more complete transcriptome produced by single-nucleus RNA sequencing (snRNA-seq), also reveals zonally enriched receptor-ligand interactions. Finally, MERFISH and snRNA-seq analysis of fibrotic liver samples identify two hepatocyte populations that expand with injury and do not have clear zonal distributions. Together these spatial maps of the healthy and fibrotic liver provide a deeper understanding of the cellular and spatial remodeling that drives disease which, in turn, could provide new avenues for intervention and further study.

Is the liver resilient to the process of ageing?

The liver's unique regenerative capacity, immunotolerant feature, and polyploidy status distinguish it as a metabolic organ unlike any other in the body. Despite aging, the liver generally exhibits fewer pathological abnormalities than other organs (such as the kidney), maintaining its functions near-normal balanced manner. Subtle changes in the liver, including reduced blood flow, detoxification alterations, pseudo-capillarization, and lipofuscin deposition, may occur with chronological age. Research indicates that carefully selected liver grafts from octogenarian donors can perform well post-transplant, emphasizing instances where age doesn't necessarily compromise liver function. Notably, a recent report suggests that the liver is a youthful organ, with hepatocytes averaging an age of only 3 years. Despite the liver's impressive regenerative capabilities and cellular reserve, a lingering question persists: how does the liver maintain its youthful characteristic amidst the chronological aging of the entire organism? The various adaptive mechanism possibly include:(a) cellular hypertrophy to maintain physiological capacity even before proliferation initiates, (b) the "ploidy conveyor" as a genetic adaptation to endure aging-related stress, (c) sustained telomere length indicative of youthfulness (d) active extracellular matrix remodelling for normal cellular functioning, (e) Mitochondria-Endoplasmic Reticulum based metabolic adaptation and (c) cellular plasticity as fitness mechanisms for healthy aging. However, it is crucial to note that aged livers may have compromised regenerative capacity and chronic liver disease is often associated with declining function due to premature hepatocyte senescence. This review delves into varied cellular adaptations sustaining liver homeostasis with chronological aging and briefly explores the role of accelerated hepatocyte aging as a precursor to chronic liver disease.

Post-death Vesicles of Senescent Bone Marrow Mesenchymal Stromal Polyploids Promote Macrophage Aging and Breast Cancer

Potential systemic factors contributing to aging-associated breast cancer (BC) remain elusive. Here, we reveal that the polyploid giant cells (PGCs) that contain more than two sets of genomes prevailing in aging and cancerous tissues constitute 5-10% of healthy female bone marrow mesenchymal stromal cells (fBMSCs). The PGCs can repair DNA damage and stimulate neighboring cells for clonal expansion. However, dying PGCs in advanced-senescent fBMSCs can form "spikings" which are then separated into membraned mtDNA-containing vesicles (Senescent PGC-Spiking Bodies; SPSBs). SPSB-phagocytosed macrophages accelerate aging with diminished clearance on BC cells and protumor M2 polarization. SPSB-carried mitochondrial OXPHOS components are enriched in BC of elder patients and associated with poor prognosis. SPSB-incorporated breast epithelial cells develop aggressive characteristics and PGCs resembling the polyploid giant cancer cells (PGCCs) in clonogenic BC cells and cancer tissues. These findings highlight an aging BMSC-induced BC risk mediated by SPSB-induced macrophage dysfunction and epithelial cell precancerous transition.

SIGNIFICANCE

Mechanisms underlying aging-associated cancer risk remain unelucidated. This work demonstrates that polyploid giant cells (PGCs) in bone marrow mesenchymal stromal cells (BMSCs) from healthy female bone marrow donors can boost neighboring cell proliferation for clonal expansion. However, the dying-senescent PGCs in the advanced-senescent fBMSCs can form "spikings" which are separated into mitochondrial DNA (mtDNA)-containing spiking bodies (senescent PGC-spiking bodies; SPSBs). The SPSBs promote macrophage aging and breast epithelial cell protumorigenic transition and form polyploid giant cancer cells. These results demonstrate a new form of ghost message from dying-senescent BMSCs, that may serve as a systemic factor contributing to aging-associated immunosuppression and breast cancer risk.

Graphic Abstract

The Ploidy State as a Determinant of Hepatocyte Proliferation

The liver's unique chromosomal variations, including polyploidy and aneuploidy, influence hepatocyte identity and function. Among the most well-studied mammalian polyploid cells, hepatocytes exhibit a dynamic interplay between diploid and polyploid states. The ploidy state is dynamic as hepatocytes move through the "ploidy conveyor," undergoing ploidy reversal and re-polyploidization during proliferation. Both diploid and polyploid hepatocytes actively contribute to proliferation, with diploids demonstrating an enhanced proliferative capacity. This enhanced potential positions diploid hepatocytes as primary drivers of liver proliferation in multiple contexts, including homeostasis, regeneration and repopulation, compensatory proliferation following injury, and oncogenic proliferation. This review discusses the influence of ploidy variations on cellular activity. It presents a model for ploidy-associated hepatocyte proliferation, offering a deeper understanding of liver health and disease with the potential to uncover novel treatment approaches.

Epithelial mechanics are maintained by inhibiting cell fusion with age in Drosophila

A characteristic of normal aging and age-related diseases is the remodeling of the cellular organization of a tissue through polyploid cell growth. Polyploidy arises from an increase in nuclear ploidy or the number of nuclei per cell. However, it is not known whether age-induced polyploidy is an adaption to stressors or a precursor to degeneration. Here, we find that abdominal epithelium of the adult fruit fly becomes polyploid with age through generation of multinucleated cells by cell fusion. Inhibition of fusion does not improve the lifespan of the fly, but does enhance its biomechanical fitness, a measure of the healthspan of the animal. Remarkably, Drosophila can maintain their epithelial tension and abdominal movements with age when cell fusion is inhibited. Epithelial cell fusion also appears to be dependent on a mechanical cue, as knockdown of Rho kinase, E-cadherin or α-catenin is sufficient to induce multinucleation in young animals. Interestingly, mutations in α-catenin in mice result in retina pigment epithelial multinucleation associated with macular disease. Therefore, we have discovered that polyploid cells arise by cell fusion and contribute to the decline in the biomechanical fitness of the animal with age.

PRMT5 determines the pattern of polyploidization and prevents liver from cirrhosis and carcinogenesis

Human hepatocellular carcinoma (HCC) occurs almost exclusively in cirrhotic livers. Here, we report that hepatic loss of protein arginine methyltransferase 5 (PRMT5) in mice is sufficient to cause cirrhosis and HCC in a clinically relevant way. Furthermore, pathological polyploidization induced by hepatic loss of PRMT5 promotes liver cirrhosis and hepatic tumorigenesis in aged liver. The loss of PRMT5 leads to hyper-accumulation of P21 and endoreplication-dependent formation of pathological mono-nuclear polyploid hepatocytes. PRMT5 and symmetric dimethylation at histone H4 arginine 3 (H4R3me2s) directly associate with chromatin of P21 to suppress its transcription. More importantly, loss of P21 rescues the pathological mono-nuclear polyploidy and prevents PRMT5-deficiency-induced liver cirrhosis and HCC. Thus, our results indicate that PRMT5-mediated symmetric dimethylation at histone H4 arginine 3 (H4R3me2s) is crucial for preventing pathological polyploidization, liver cirrhosis and tumorigenesis in mouse liver.

Implications of Polyploidy and Ploidy Alterations in Hepatocytes in Liver Injuries and Cancers

Polyploidy, a condition in which more than two sets of chromosomes are present in a cell, is a characteristic feature of hepatocytes. A significant number of hepatocytes physiologically undergo polyploidization at a young age. Polyploidization of hepatocytes is enhanced with age and in a diseased liver. It is worth noting that polyploid hepatocytes can proliferate, in marked contrast to other types of polyploid cells, such as megakaryocytes and cardiac myocytes. Polyploid hepatocytes divide to maintain normal liver homeostasis and play a role in the regeneration of the damaged liver. Furthermore, polyploid hepatocytes have been shown to dynamically reduce ploidy during liver regeneration. Although it is still unclear why hepatocytes undergo polyploidization, accumulating evidence has revealed that alterations in the ploidy in hepatocytes are involved in the pathophysiology of liver cirrhosis and carcinogenesis. This review discusses the significance of hepatocyte ploidy in physiological liver function, liver injury, and liver cancer.

Polyploidization of Hepatocytes: Insights into the Pathogenesis of Liver Diseases

Polyploidization is a process by which cells are induced to possess more than two sets of chromosomes. Although polyploidization is not frequent in mammals, it is closely associated with development and differentiation of specific tissues and organs. The liver is one of the mammalian organs that displays ploidy dynamics in physiological homeostasis during its development. The ratio of polyploid hepatocytes increases significantly in response to hepatic injury from aging, viral infection, iron overload, surgical resection, or metabolic overload, such as that from non-alcoholic fatty liver diseases (NAFLDs). One of the unique features of NAFLD is the marked heterogeneity of hepatocyte nuclear size, which is strongly associated with an adverse liver-related outcome, such as hepatocellular carcinoma, liver transplantation, and liver-related death. Thus, hepatic polyploidization has been suggested as a potential driver in the progression of NAFLDs that are involved in the control of the multiple pathogenicity of the diseases. However, the importance of polyploidy in diverse pathophysiological contexts remains elusive. Recently, several studies reported successful improvement of symptoms of NAFLDs by reducing pathological polyploidy or by controlling cell cycle progression in animal models, suggesting that better understanding the mechanisms of pathological hepatic polyploidy may provide insights into the treatment of hepatic disorders.

Understanding the Unique Microenvironment in the Aging Liver

In the past decades, many studies have focused on aging because of our pursuit of longevity. With lifespans extended, the regenerative capacity of the liver gradually declines due to the existence of aging. This is partially due to the unique microenvironment in the aged liver, which affects a series of physiological processes. In this review, we summarize the related researches in the last decade and try to highlight the aging-related alterations in the aged liver.

RORα contributes to the maintenance of genome ploidy in the liver of mice with diet-induced nonalcoholic steatohepatitis

Hepatic polyploidization is closely linked to the progression of nonalcoholic fatty liver disease (NAFLD); however, the underlying molecular mechanism is not clearly understood. In this study, we demonstrated the role of retinoic acid-related orphan receptor α (RORα) in the maintenance of genomic integrity, particularly in the pathogenesis of NAFLD, using the high-fat diet (HFD)-fed liver-specific RORα knockout (RORα-LKO) mouse model. First, we observed that the loss of hepatic retinoic acid receptor-related orphan receptor α (RORα) accelerated hepatocyte nuclear polyploidization after HFD feeding. In 70% partial hepatectomy experiments, enrichment of hepatocyte polyploidy was more obvious in the RORα-LKO animals, which was accompanied by early progression to the S phase and blockade of the G2/M transition, suggesting a potential role of RORα in suppressing hepatocyte polyploidization in the regenerating liver. An analysis of a publicly available RNA sequencing (RNA-seq) and chromatin immunoprecipitation-seq dataset, together with the Search Tool of the Retrieval of Interacting Genes/Proteins database resource, revealed that DNA endoreplication was the top-enriched biological process Gene Ontology term. Furthermore, we found that E2f7 and E2f8, which encode key transcription factors for DNA endoreplication, were the downstream targets of RORα-induced transcriptional repression. Finally, we showed that the administration of JC1-40, an RORα activator (5 mg/kg body wt), significantly reduced hepatic nuclear polyploidization in the HFD-fed mice. Together, our observations suggest that the RORα-induced suppression of hepatic polyploidization may provide new insights into the pathological polyploidy of NAFLD and may contribute to the development of therapeutic strategies for the treatment of NAFLD.NEW & NOTEWORTHY It has been reported that hepatic polyploidization is closely linked to the progression of NAFLD. Here, we showed that the genetic depletion of hepatic RORα in mice accelerated hepatocyte polyploidization after high-fat diet feeding. The mechanism could be the RORα-mediated repression of E2f7 and E2f8, key transcription factors for DNA endoreplication. Thus, preservation of genome integrity by RORα could provide a new insight for developing therapeutics against the disease.