Microarray analysis of gene expression of mouse hepatocytes of different ploidy

Functional consequences of somatic polyploidy in development

Polyploid cells contain multiple genome copies and arise in many animal tissues as a regulated part of development. However, polyploid cells can also arise due to cell division failure, DNA damage or tissue damage. Although polyploidization is crucial for the integrity and function of many tissues, the cellular and tissue-wide consequences of polyploidy can be very diverse. Nonetheless, many polyploid cell types and tissues share a remarkable similarity in function, providing important information about the possible contribution of polyploidy to cell and tissue function. Here, we review studies on polyploid cells in development, underlining parallel functions between different polyploid cell types, as well as differences between developmentally-programmed and stress-induced polyploidy.

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.

The consequences of tetraploidy on Caenorhabditis elegans physiology and sensitivity to chemotherapeutics

Polyploid cells contain more than two copies of each chromosome. Polyploidy has important roles in development, evolution, and tissue regeneration/repair, and can arise as a programmed polyploidization event or be triggered by stress. Cancer cells are often polyploid. C. elegans nematodes are typically diploid, but stressors such as heat shock and starvation can trigger the production of tetraploid offspring. In this study, we utilized a recently published protocol to generate stable tetraploid strains of C. elegans and compared their physiological traits and sensitivity to two DNA-damaging chemotherapeutic drugs, cisplatin and doxorubicin. As prior studies have shown, tetraploid worms are approximately 30% longer, shorter-lived, and have a smaller brood size than diploids. We investigated the reproductive defect further, determining that tetraploid worms have a shorter overall germline length, a higher rate of germ cell apoptosis, more aneuploidy in oocytes and offspring, and larger oocytes and embryos. We also found that tetraploid worms are modestly protected from growth delay from the chemotherapeutics but are similarly or more sensitive to reproductive toxicity. Transcriptomic analysis revealed differentially expressed pathways that may contribute to sensitivity to stress. This study reveals phenotypic consequences of whole-animal tetraploidy that make C. elegans an excellent model for ploidy differences.

The consequences of tetraploidy on Caenorhabditis elegans physiology and sensitivity to chemotherapeutics

Polyploid cells contain more than two copies of each chromosome. Polyploidy has important roles in development, evolution, and tissue regeneration/repair, and can arise as a programmed polyploidization event or be triggered by stress. Cancer cells are often polyploid. C. elegans nematodes are typically diploid, but stressors such as heat shock and starvation can trigger the production of tetraploid offspring. In this study, we utilized a recently published protocol to generate stable tetraploid strains of C. elegans and compared their physiological traits and sensitivity to two DNA-damaging chemotherapeutic drugs, cisplatin and doxorubicin. As prior studies have shown, tetraploid worms are approximately 30% longer, shorter-lived, and have a smaller brood size than diploids. We investigated the reproductive defect further, determining that tetraploid worms have a shorter overall germline length, a higher rate of germ cell apoptosis, more aneuploidy in oocytes and offspring, and larger oocytes and embryos. We also found that tetraploid worms are modestly protected from growth delay from the chemotherapeutics but are similarly or more sensitive to reproductive toxicity. Transcriptomic analysis revealed differentially expressed pathways that may contribute to sensitivity to stress. Overall, this study reveals the phenotypic consequences of whole-animal tetraploidy in C. elegans.

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.

The Nuclear Receptor PXR in Chronic Liver Disease

Pregnane X receptor (PXR), a nuclear receptor known for modulating the transcription of drug metabolizing enzymes and transporters (DMETs), such as cytochrome P450 3A4 and P-glycoprotein, is functionally involved in chronic liver diseases of different etiologies. Furthermore, PXR activity relates to that of other NRs, such as constitutive androstane receptor (CAR), through a crosstalk that in turn orchestrates a complex network of responses. Thus, besides regulating DMETs, PXR signaling is involved in both liver damage progression and repair and in the neoplastic transition to hepatocellular carcinoma. We here summarize the present knowledge about PXR expression and function in chronic liver diseases characterized by different etiologies and clinical outcome, focusing on the molecular pathways involved in PXR activity. Although many molecular details of these finely tuned networks still need to be fully understood, we conclude that PXR and its modulation could represent a promising pharmacological target for the identification of novel therapeutical approaches to chronic liver diseases.

Single-nucleus RNA-seq2 reveals functional crosstalk between liver zonation and ploidy

Single-cell RNA-seq reveals the role of pathogenic cell populations in development and progression of chronic diseases. In order to expand our knowledge on cellular heterogeneity, we have developed a single-nucleus RNA-seq2 method tailored for the comprehensive analysis of the nuclear transcriptome from frozen tissues, allowing the dissection of all cell types present in the liver, regardless of cell size or cellular fragility. We use this approach to characterize the transcriptional profile of individual hepatocytes with different levels of ploidy, and have discovered that ploidy states are associated with different metabolic potential, and gene expression in tetraploid mononucleated hepatocytes is conditioned by their position within the hepatic lobule. Our work reveals a remarkable crosstalk between gene dosage and spatial distribution of hepatocytes.

Cytokinesis regulators as potential diagnostic and therapeutic biomarkers for human hepatocellular carcinoma

Cytokinesis, the final step of mitosis, is critical for maintaining the ploidy level of cells. Cytokinesis is a complex, highly regulated process and its failure can lead to genetic instability and apoptosis, contributing to the development of cancer. Human hepatocellular carcinoma is often accompanied by a high frequency of aneuploidy and the DNA ploidy pattern observed in human hepatocellular carcinoma results mostly from impairments in cytokinesis. Many key regulators of cytokinesis are abnormally expressed in human hepatocellular carcinoma, and their expression levels are often correlated with patient prognosis. Moreover, preclinical studies have demonstrated that the inhibition of key cytokinesis regulators can suppress the growth of human hepatocellular carcinoma. Here, we provide an overview of the current understanding of the signaling networks regulating cytokinesis, the key cytokinesis regulators involved in the initiation and development of human hepatocellular carcinoma, and their applications as potential diagnostic and therapeutic biomarkers.

Regulation of CAR and PXR Expression in Health and Disease

Pregnane X receptor (PXR, NR1I2) and constitutive androstane receptor (CAR, NR1I3) are members of the nuclear receptor superfamily that mainly act as ligand-activated transcription factors. Their functions have long been associated with the regulation of drug metabolism and disposition, and it is now well established that they are implicated in physiological and pathological conditions. Considerable efforts have been made to understand the regulation of their activity by their cognate ligand; however, additional regulatory mechanisms, among which the regulation of their expression, modulate their pleiotropic effects. This review summarizes the current knowledge on CAR and PXR expression during development and adult life; tissue distribution; spatial, temporal, and metabolic regulations; as well as in pathological situations, including chronic diseases and cancers. The expression of CAR and PXR is modulated by complex regulatory mechanisms that involve the interplay of transcription factors and also post-transcriptional and epigenetic modifications. Moreover, many environmental stimuli affect CAR and PXR expression through mechanisms that have not been elucidated.

Growth arrest-specific 2 protein family: Structure and function

Members of the growth arrest–specific 2 (GAS2) protein family consist of a putative actin‐binding (CH) domain and a microtubule‐binding (GAR) domain and are considered miniversions of spectraplakins. There are four members in the GAS2 family, viz. GAS2, GAS2L1, GAS2L2 and GAS2L3. Although GAS2 is defined as a family of growth arrest–specific proteins, the significant differences in the expression patterns, interaction characteristics and biological issues or diseases among the different GAS2 family members have not been systemically reviewed to date. Therefore, we summarized the available evidence on the structures and functions of GAS2 family members. This review facilitates a comprehensive molecular understanding of the involvement of the GAS2 family members in an array of biological processes, including cytoskeleton reorganization, cell cycle, apoptosis and cancer development.

Polyploidy in liver development, homeostasis and disease

Polyploidy (or whole-genome duplication) is the condition of having more than two basic sets of chromosomes. Polyploidization is well tolerated in many species and can lead to specific biological functions. In mammals, programmed polyploidization takes place during development in certain tissues, such as the heart and placenta, and is considered a feature of differentiation. However, unscheduled polyploidization can cause genomic instability and has been observed in pathological conditions, such as cancer. Polyploidy of the liver parenchyma was first described more than 100 years ago. The liver is one of the few mammalian organs that display changes in polyploidy during homeostasis, regeneration and in response to damage. In the human liver, approximately 30% of hepatocytes are polyploid. The polyploidy of hepatocytes results from both nuclear polyploidy (an increase in the amount of DNA per nucleus) and cellular polyploidy (an increase in the number of nuclei per cell). In this Review, we discuss the regulation of polyploidy in liver development and pathophysiology. We also provide an overview of current knowledge about the mechanisms of hepatocyte polyploidization, its biological importance and the fate of polyploid hepatocytes during liver tumorigenesis.

Mice With Increased Numbers of Polyploid Hepatocytes Maintain Regenerative Capacity But Develop Fewer Hepatocellular Carcinomas Following Chronic Liver Injury

BACKGROUND & AIMS

Thirty to 90% of hepatocytes contain whole-genome duplications, but little is known about the fates or functions of these polyploid cells or how they affect development of liver disease. We investigated the effects of continuous proliferative pressure, observed in chronically damaged liver tissues, on polyploid cells.

METHODS

We studied Rosa-rtTa mice (controls) and Rosa-rtTa;TRE-short hairpin RNA mice, which have reversible knockdown of anillin, actin binding protein (ANLN). Transient administration of doxycycline increases the frequency and degree of hepatocyte polyploidy without permanently altering levels of ANLN. Mice were then given diethylnitrosamine and carbon tetrachloride (CCl₄) to induce mutations, chronic liver damage, and carcinogenesis. We performed partial hepatectomies to test liver regeneration and then RNA-sequencing to identify changes in gene expression. Lineage tracing was used to rule out repopulation from non-hepatocyte sources. We imaged dividing hepatocytes to estimate the frequency of mitotic errors during regeneration. We also performed whole-exome sequencing of 54 liver nodules from patients with cirrhosis to quantify aneuploidy, a possible outcome of polyploid cell divisions.

RESULTS

Liver tissues from control mice given CCl₄ had significant increases in ploidy compared with livers from uninjured mice. Mice with knockdown of ANLN had hepatocyte ploidy above physiologic levels and developed significantly fewer liver tumors after administration of diethylnitrosamine and CCl₄ compared with control mice. Increased hepatocyte polyploidy was not associated with altered regenerative capacity or tissue fitness, changes in gene expression, or more mitotic errors. Based on lineage-tracing experiments, non-hepatocytes did not contribute to liver regeneration in mice with increased polyploidy. Despite an equivalent rate of mitosis in hepatocytes of differing ploidies, we found no lagging chromosomes or micronuclei in mitotic polyploid cells. In nodules of human cirrhotic liver tissue, there was no evidence of chromosome-level copy number variations.

CONCLUSIONS

Mice with increased polyploid hepatocytes develop fewer liver tumors following chronic liver damage. Remarkably, polyploid hepatocytes maintain the ability to regenerate liver tissues during chronic damage without generating mitotic errors, and aneuploidy is not commonly observed in cirrhotic livers. Strategies to increase numbers of polypoid hepatocytes might be effective in preventing liver cancer.

Alterations in Phosphorylation of Hepatocyte Ribosomal Protein S6 Control Plasmodium Liver Stage Infection

Plasmodium parasites are highly selective when infecting hepatocytes and induce many changes within the host cell upon infection. While several host cell factors have been identified that are important for liver infection, our understanding of what facilitates the maintenance of infection remains incomplete. Here, we describe a role for phosphorylated ribosomal protein S6 (Ser235/236) (p-RPS6) in Plasmodium yoelii-infected hepatocytes. Blocking RPS6 phosphorylation prior to infection decreases the number of liver stage parasites within 24 h. Infected hepatocytes exhibit elevated levels of p-RPS6 while simultaneously abrogating the induction of phosphorylation of RPS6 in response to insulin stimulation. This is in contrast with the regulation of p-RPS6 by Toxoplasma gondii, which elevates levels of p-RPS6 after infection but does not alter the response to insulin. Our data support a model in which RPS6 phosphorylation is uncoupled from canonical regulators in Plasmodium-infected hepatocytes and is relied on by the parasite to maintain infection.

Polyploidy in Cardiomyocytes: Roadblock to Heart Regeneration?

The hallmark of most cardiac diseases is the progressive loss of cardiomyocytes. In the perinatal period, cardiomyocytes still proliferate, and the heart shows the capacity to regenerate upon injury. In the adult heart, however, the actual rate of cardiomyocyte renewal is too low to efficiently counteract substantial cell loss caused by cardiac injury. In mammals, cardiac growth by cell number expansion changes to growth by cardiomyocyte enlargement soon after birth, coinciding with a period in which most cardiomyocytes increase their DNA content by multinucleation and nuclear polyploidization. Although cardiomyocyte hypertrophy is often associated with these processes, whether polyploidy is a prerequisite or a consequence of hypertrophic growth is unclear. Both the benefits of cardiomyocyte enlargement over proliferative growth of the heart and the physiological role of polyploidy in cardiomyocytes are enigmatic. Interestingly, hearts in animal species with substantial cardiac regenerative capacity dominantly comprise diploid cardiomyocytes, raising the hypothesis that cardiomyocyte polyploidy poses a barrier for cardiomyocyte proliferation and subsequent heart regeneration. On the contrary, there is also evidence for self-duplication of multinucleated myocytes, suggesting a more complex picture of polyploidy in heart regeneration. Polyploidy is not restricted to the heart but also occurs in other cell types in the body. In this review, we explore the biological relevance of polyploidy in different species and tissues to acquire insight into its specific role in cardiomyocytes. Furthermore, we speculate about the physiological role of polyploidy in cardiomyocytes and how this might relate to renewal and regeneration.

Residual Diploidy in Polyploid Tissues: A Cellular State with Enhanced Proliferative Capacity for Tissue Regeneration?

A major objective of modern biomedical research aims to promote tissue self-regeneration after injury, obviating the need for whole organ transplantation and avoiding mortality due to organ failure. Identifying the population of cells capable of regeneration, alongside understanding the molecular mechanisms that activate that population to re-enter the cell cycle, are two important steps to advancing the field of endogenous tissue regeneration toward the clinic. In recent years, an emerging trend has been observed, whereby polyploidy of relevant parenchymal cells, arising from alternative cell cycles as part of a normal developmental process, is linked to restricted proliferative capacity of those cells. An accompanying hypothesis, therefore, is that a residual subpopulation of diploid parenchymal cells retains proliferative competence and is the major driver for any detected postnatal cell turnover. In this perspective review, we examine the emerging literature on residual diploid parenchymal cells and the possible link of this population to endogenous tissue regeneration, in the context of both heart and liver. We speculate on additional cell types that may play a similar role in their respective tissues and discuss outstanding questions for the field.

Transcriptomic Dissection of Hepatocyte Heterogeneity: Linking Ploidy, Zonation, and Stem/Progenitor Cell Characteristics

BACKGROUND & AIMS

There is a long-standing debate regarding the biological significance of polyploidy in hepatocytes. Recent studies have provided increasing evidence that hepatocytes with different ploidy statuses behave differently in a context-dependent manner (eg, susceptibility to oncogenesis, regenerative ability after injury, and in vitro proliferative capacity). However, their overall transcriptomic differences in a physiological context is not known.

METHODS

By using microarray transcriptome analysis, we investigated the heterogeneity of hepatocyte populations with different ploidy statuses. Moreover, by using single-cell quantitative reverse-transcription polymerase chain reaction (scPCR) analysis, we investigated the intrapopulational transcriptome heterogeneity of 2c and 4c hepatocytes.

RESULTS

Microarray analysis showed that cell cycle-related genes were enriched in 8c hepatocytes, which is in line with the established notion that polyploidy is formed via cell division failure. Surprisingly, in contrast to the general consensus that 2c hepatocytes reside in the periportal region, in our bulk transcriptome and scPCR analyses, the 2c hepatocytes consistently showed pericentral hepatocyte-enriched characteristics. In addition, scPCR analysis identified a subpopulation within the 2c hepatocytes that co-express the liver progenitor cell markers Axin2, Prom1, and Lgr5, implying the potential biological relevance of this subpopulation.

CONCLUSIONS

This study provides new insights into hepatocyte heterogeneity, namely 2c hepatocytes are preferentially localized to the pericentral region, and a subpopulation of 2c hepatocytes show liver progenitor cell-like features in terms of liver progenitor cell marker expression (Axin2, Prom1, and Lgr5).

[Hepatic polyploidy: Dr Jekyll or Mr Hyde]

Polyploidy (alias whole genome amplification) refers to organisms containing more than two basic sets of chromosomes. Polyploidy was first observed in plants more than a century ago, and it is known that such processes occur in many eukaryotes under a variety of circumstances. In mammals, the development of polyploid cells can contribute to tissue differentiation and therefore possibly a gain of function. Alternately, it can be associated with development of disease such as cancer. Polyploidy can occur because of cell fusion or abnormal cell division. Polyploidy is a common characteristic of the mammalian liver. Polyploidization occurs notably during liver development, but also in adults because of cellular stress. Recent progresses have unraveled the mechanisms and functional consequences of hepatocytes polyploidization during normal and pathological liver growth.

The origins and functions of hepatic polyploidy

Polyploid cells contain more than two homologous sets of chromosomes. The original observations of liver polyploidy date back to the 1940s, but functional roles for polyploid cells are still unclear. Liver polyploidy may influence regeneration, stress response, and cancer, although little evidence has established direct causal links between polyploidy and these biological phenotypes. In this review, we will introduce broad concepts about polyploidy including its distribution in nature and how polyploids form in normal and pathological situations. Then we will examine recent discoveries that have begun to clarify functionality and disease relevance of liver polyploidy. Finally, we will discuss implications and future directions of research about polyploidy in the liver.

Effect of triploidy on liver gene expression in coho salmon (Oncorhynchus kisutch) under different metabolic states

BACKGROUND

Triploid coho salmon are excellent models for studying gene dosage and the effects of increased cell volume on gene expression. Triploids have an additional haploid genome in each cell and have fewer but larger cells than diploid coho salmon to accommodate the increased genome size. Studying gene expression in triploid coho salmon provides insight into how gene expression may have been affected after the salmonid-specific genome duplication which occurred some 90 MYA. Triploid coho salmon are sterile and consequently can live longer and grow larger than diploid congeners in many semelparous species (spawning only once) because they never reach maturity and post-spawning mortality is averted. Triploid fishes are also of interest to the commercial sector (larger fish are more valuable) and to fisheries management since sterile fish can potentially minimize negative impacts of escaped fish in the wild.

RESULTS

The vast majority of genes in liver tissue had similar expression levels between diploid and triploid coho salmon, indicating that the same amount of mRNA transcripts were being produced per gene copy (positive gene dosage effects) within a larger volume cell. Several genes related to nutrition and compensatory growth were differentially expressed between diploid and triploid salmon, indicating that some loci are sensitive to cell size and/or DNA content per cell. To examine how robust expression between ploidies is under different conditions, a genetic/metabolic modifier in the form of different doses of a growth hormone transgene was used to assess gene expression under conditions that the genome has not naturally experienced or adapted to. While many (up to 1400) genes were differentially expressed between non-transgenic and transgenic fish, relatively few genes were differentially expressed between diploids and triploids with similar doses of the transgene. These observations indicate that the small effect of ploidy on gene expression is robust to large changes in physiological state.

CONCLUSIONS

These findings are of interest from a gene regulatory perspective, but also valuable for understanding phenotypic effects in triploids, transgenics, and triploid transgenics that could affect their utility in culture conditions and their fitness and potential consequences of release into nature.

Polyploid Hepatocytes Facilitate Adaptation and Regeneration to Chronic Liver Injury

The liver contains diploid and polyploid hepatocytes (tetraploid, octaploid, etc.), with polyploids comprising ≥90% of the hepatocyte population in adult mice. Polyploid hepatocytes form multipolar spindles in mitosis, which lead to chromosome gains/losses and random aneuploidy. The effect of aneuploidy on liver function is unclear, and the degree of liver aneuploidy is debated, with reports showing aneuploidy affects 5% to 60% of hepatocytes. To study relationships among liver polyploidy, aneuploidy, and adaptation, mice lacking E2f7 and E2f8 in the liver (LKO), which have a polyploidization defect, were used. Polyploids were reduced fourfold in LKO livers, and LKO hepatocytes remained predominantly diploid after extensive proliferation. Moreover, nearly all LKO hepatocytes were euploid compared with control hepatocytes, suggesting polyploid hepatocytes are required for production of aneuploid progeny. To determine whether reduced polyploidy impairs adaptation, LKO mice were bred onto a tyrosinemia background, a disease model whereby the liver can develop disease-resistant, regenerative nodules. Although tyrosinemic LKO mice were more susceptible to morbidities and death associated with tyrosinemia-induced liver failure, they developed regenerating nodules similar to control mice. Analyses revealed that nodules in the tyrosinemic livers were generated by aneuploidy and inactivating mutations. In summary, we identified new roles for polyploid hepatocytes and demonstrated that they are required for the formation of aneuploid progeny and can facilitate adaptation to chronic liver disease.

p38α deficiency restrains liver regeneration after partial hepatectomy triggering oxidative stress and liver injury

p38α MAPK negatively regulates the G₁/S and G₂/M cell cycle transitions. However, liver-specific p38α deficiency impairs cytokinesis and reduces hepatocyte proliferation during cirrhosis and aging in mice. In this work, we have studied how p38α down-regulation affects hepatocyte proliferation after partial hepatectomy, focusing on mitotic progression, cytokinesis and oxidative stress. We found that p38α deficiency triggered up-regulation of cyclins A1, B1, B2, and D1 under basal conditions and after hepatectomy. Moreover, p38α-deficient hepatocytes showed enhanced binucleation and increased levels of phospho-histone H3 but impaired phosphorylation of MNK1 after hepatectomy. The recovery of liver mass was transiently delayed in mice with p38α-deficient hepatocytes vs wild type mice. We also found that p38α deficiency caused glutathione oxidation in the liver, increased plasma aminotransferases and lactate dehydrogenase activities, and decreased plasma protein levels after hepatectomy. Interestingly, p38α silencing in isolated hepatocytes markedly decreased phospho-MNK1 levels, and silencing of either p38α or Mnk1 enhanced binucleation of hepatocytes in culture. In conclusion, p38α deficiency impairs mitotic progression in hepatocytes and restrains the recovery of liver mass after partial hepatectomy. Our results also indicate that p38α regulates cytokinesis by activating MNK1 and redox modulation.

The Polyploid State Restricts Hepatocyte Proliferation and Liver Regeneration in Mice

The liver contains a mixture of hepatocytes with diploid or polyploid (tetraploid, octaploid, etc.) nuclear content. Polyploid hepatocytes are commonly found in adult mammals, representing ~90% of the entire hepatic pool in rodents. The cellular and molecular mechanisms that regulate polyploidization have been well characterized; however, it is unclear whether diploid and polyploid hepatocytes function similarly in multiple contexts. Answering this question has been challenging because proliferating hepatocytes can increase or decrease ploidy, and animal models with healthy diploid-only livers have not been available. Mice lacking E2f7 and E2f8 in the liver (liver-specific E2f7/E2f8 knockout; LKO) were recently reported to have a polyploidization defect, but were otherwise healthy. Herein, livers from LKO mice were rigorously characterized, demonstrating a 20-fold increase in diploid hepatocytes and maintenance of the diploid state even after extensive proliferation. Livers from LKO mice maintained normal function, but became highly tumorigenic when challenged with tumor-promoting stimuli, suggesting that tumors in LKO mice were driven, at least in part, by diploid hepatocytes capable of rapid proliferation. Indeed, hepatocytes from LKO mice proliferate faster and out-compete control hepatocytes, especially in competitive repopulation studies. In addition, diploid or polyploid hepatocytes from wild-type (WT) mice were examined to eliminate potentially confounding effects associated with E2f7/E2f8 deficiency. WT diploid cells also showed a proliferative advantage, entering and progressing through the cell cycle faster than polyploid cells, both in vitro and during liver regeneration (LR). Diploid and polyploid hepatocytes responded similarly to hepatic mitogens, indicating that proliferation kinetics are unrelated to differential response to growth stimuli. Conclusion: Diploid hepatocytes proliferate faster than polyploids, suggesting that the polyploid state functions as a growth suppressor to restrict proliferation by the majority of hepatocytes.

Hypoxia/reperfusion predisposes to atherosclerosis

Surgical interventions on blood vessels bear a risk for intimal hyperplasia and atherosclerosis as a consequence of injury. A specific feature of intimal hyperplasia is the loss of vascular smooth muscle cell (VSMC) differentiation gene expression. We hypothesized that immediate responses following injury induce vascular remodeling. To differentiate injury due to trauma, reperfusion and pressure changes we analyzed vascular responses to carotid artery bypass grafting in mice compared to transient ligation. As a control, the carotid artery was surgically laid open only. In both, bypass or ligation models, the inflammatory responses were transient, peaking after 6h, whereas the loss of VSMC differentiation gene expression persisted. Extended time kinetics showed that transient carotid artery ligation was sufficient to induce a persistent VSMC phenotype change throughout 28 days. Transient arterial ligation in ApoE knockout mice resulted in atherosclerosis in the transiently ligated vascular segment but not on the not-ligated contralateral side. The VSMC phenotype change could not be prevented by anti-TNF antibodies, Sorafenib, Cytosporone B or N-acetylcysteine treatment. Surgical interventions involving hypoxia/reperfusion are sufficient to induce VSMC phenotype changes and vascular remodeling. In situations of a perturbed lipid metabolism this bears the risk to precipitate atherosclerosis.

Hepatocyte Ploidy Is a Diversity Factor for Liver Homeostasis

Polyploidy, the existence of cells containing more than one pair of chromosomes, is a well-known feature of mammalian hepatocytes. Polyploid hepatocytes are found either as cells with a single polyploid nucleus or as multinucleated cells with diploid or even polyploid nuclei. In this study, we evaluate the degree of polyploidy in the murine liver by accounting both DNA content and number of nuclei per cell. We demonstrate that mouse hepatocytes with diploid nuclei have distinct metabolic characteristics compared to cells with polyploid nuclei. In addition to strong differential gene expression, comprising metabolic as well as signaling compounds, we found a strongly decreased insulin binding of nuclear polyploid cells. Our observations were associated with nuclear ploidy but not with total ploidy within a cell. We therefore suggest ploidy of the nuclei as an new diversity factor of hepatocytes and hypothesize that hepatocytes with polyploid nuclei may have distinct biological functions than mono-nuclear ones. This diversity is independent from the well-known heterogeneity related to the cells' position along the porto-central liver-axis.

Hepatocyte polyploidization and its association with pathophysiological processes

A characteristic cellular feature of the mammalian liver is the progressive polyploidization of the hepatocytes, where individual cells acquire more than two sets of chromosomes. Polyploidization results from cytokinesis failure that takes place progressively during the course of postnatal development. The proportion of polyploidy also increases with the aging process or with cellular stress such as surgical resection, toxic stimulation, metabolic overload, or oxidative damage, to involve as much as 90% of the hepatocytes in mice and 40% in humans. Hepatocyte polyploidization is generally considered an indicator of terminal differentiation and cellular senescence, and related to the dysfunction of insulin and p53/p21 signaling pathways. Interestingly, the high prevalence of hepatocyte polyploidization in the aged mouse liver can be reversed when the senescent hepatocytes are serially transplanted into young mouse livers. Here we review the current knowledge on the mechanism of hepatocytes polyploidization during postnatal growth, aging, and liver diseases. The biologic significance of polyploidization in senescent reversal, within the context of new ways to think of liver aging and liver diseases is considered.

Dynamic zonation of liver polyploidy

The liver is a polyploid organ, consisting of hepatocytes with one or two nuclei each containing 2, 4, 8 or more haploid chromosome sets. The dynamic changes in the spatial distributions of polyploid classes across the liver lobule, its repeating anatomical unit, have not been characterized. Identifying these spatial patterns is important for understanding liver homeostatic and regenerative turnover, as well as potential division of labor among ploidy classes. Here, we use single molecule-based tissue imaging to reconstruct the spatial zonation profiles of liver polyploid classes in mice of different ages. We find that liver polyploidy proceeds in spatial waves, advancing more rapidly in the mid-lobule zone compared to the periportal and perivenous zones. We also measure the spatial zonation profiles of S-phase entry at different ages and identify more rapid S-phase entry in the mid-lobule zone at older ages. Our findings reveal fundamental features of liver spatial heterogeneity and highlight their dynamic changes during development and aging.

The mechanisms shaping the single-cell transcriptional landscape

Recent technological and computational advances in understanding the transcriptional and chromatin features of single cells have begun answering longstanding questions in the extent and impact of biological heterogeneity. Here, we outline the intrinsic and extrinsic mechanisms that underlie the transcriptional and functional diversity within superficially homogeneous populations, and we discuss how fascinating new studies have afforded novel insight into each mechanism. The studies are chosen in part to include initial reports of novel functional genomics tools where the eventual applications will clearly have profound impact on our understanding the dynamics of cell-to-cell transcriptional variation-from individual cells to whole organisms.

Polyploid titan cells produce haploid and aneuploid progeny to promote stress adaptation

Cryptococcus neoformans is a major life-threatening fungal pathogen. In response to the stress of the host environment, C. neoformans produces large polyploid titan cells. Titan cell production enhances the virulence of C. neoformans, yet whether the polyploid aspect of titan cells is specifically influential remains unknown. We show that titan cells were more likely to survive and produce offspring under multiple stress conditions than typical cells and that even their normally sized daughters maintained an advantage over typical cells in continued exposure to stress. Although polyploid titan cells generated haploid daughter cell progeny upon in vitro replication under nutrient-replete conditions, titan cells treated with the antifungal drug fluconazole produced fluconazole-resistant diploid and aneuploid daughter cells. Interestingly, a single titan mother cell was capable of generating multiple types of aneuploid daughter cells. The increased survival and genomic diversity of titan cell progeny promote rapid adaptation to new or high-stress conditions.

IMPORTANCE

The ability to adapt to stress is a key element for survival of pathogenic microbes in the host and thus plays an important role in pathogenesis. Here we investigated the predominantly haploid human fungal pathogen Cryptococcus neoformans, which is capable of ploidy and cell size increases during infection through production of titan cells. The enlarged polyploid titan cells are then able to rapidly undergo ploidy reduction to generate progeny with reduced ploidy and/or aneuploidy. Under stressful conditions, titan cell progeny have a growth and survival advantage over typical cell progeny. Understanding how titan cells enhance the rate of cryptococcal adaptation under stress conditions may assist in the development of novel drugs aimed at blocking ploidy transitions.

Oxidative stress triggers cytokinesis failure in hepatocytes upon isolation

Primary hepatocytes are highly differentiated cells and proliferatively quiescent. However, the stress produced during liver digestion seems to activate cell cycle entry by proliferative/dedifferentiation programs that still remain unclear. The aim of this work was to assess whether the oxidative stress associated with hepatocyte isolation affects cell cycle and particularly cytokinesis, the final step of mitosis. Hepatocytes were isolated from C57BL/6 mice by collagenase perfusion in the absence and presence of N-acetyl cysteine (NAC). Polyploidy, cell cycle, and reactive oxygen species (ROS) were studied by flow cytometry (DNA, phospho-histone 3, and CellROX(®) Deep Red) and Western blotting (cyclins B1 and D1, and proliferating cell nuclear antigen). mRNA expression of cyclins A1, B1, B2, D1, and F by reverse transcription (RT)-PCR was also assessed. Glutathione levels were measured by mass spectrometry. Here we show that hepatocyte isolation enhanced cell cycle entry, increased hepatocyte binucleation, and caused marked glutathione oxidation. Addition of 5 mM NAC to the hepatocyte isolation media prevented glutathione depletion, partially blocked ROS production and cell cycle entry of hepatocytes, and avoided the blockade of mitosis progression, abrogating defective cytokinesis and diminishing the formation of binucleated hepatocytes during isolation. Therefore, addition of NAC to the isolation media decreased the generation of polyploid hepatocytes confirming that oxidative stress occurs during hepatocyte isolation and it is responsible, at least in part, for cytokinesis failure and hepatocyte binucleation.

FLJ25439, a novel cytokinesis-associated protein, induces tetraploidization and maintains chromosomal stability via enhancing expression of endoplasmic reticulum stress chaperones

Investigation of the mechanisms leading to aneuploidy and polyploidy is critical to cancer research. Previous studies have provided strong evidence of the importance of tetraploidization as an early step in tumorigenesis. In cancer cells, tetraploid cells may contribute to abnormal mitotic progression, which may be associated with cytokinesis failure. Tetraploidy leads to genomic instability due to centrosome and chromosome over-replication. Until now, the mechanism by which cells maintain tetraploid status has been unknown. Here, we identified a novel D box-containing protein, FLJ25439, which displays a dynamic expression profile during mitosis/cytokinesis with the midbody as the most prominent associated structure. To understand the function of FLJ25439, we established stable cell lines overexpressing FLJ25439. FLJ25439-overexpression cells grew slower and displayed a tetraploid DNA content in comparison with diploid parental cells. They also showed aberrant mitosis and dysregulated expression of p53, pRb and p21, suggesting a defect in cell cycle progression. To explore the molecular mechanisms responsible for FLJ25439-induced tetraploidization, we conducted a comparative analysis of the global protein expression patterns of wild type and overexpressors using proteomics and bioinformatics approaches. Protein category profiling indicated that FLJ25439 is involved in pathways related to anti-apoptosis, protein folding, the cell cycle, and cytoskeleton regulation. Specifically, genotoxic-stress- and ER stress-related chaperone proteins greatly contributed to the FLJ25439 overexpression phenotypes. The results of this study pave the way to our further understanding of the role of this novel cytokinesis-related protein in protecting cells from environmental stress and tetraploid formation.

Polyploidization in liver tissue

Polyploidy (alias whole genome amplification) refers to organisms containing more than two basic sets of chromosomes. Polyploidy was first observed in plants more than a century ago, and it is known that such processes occur in many eukaryotes under a variety of circumstances. In mammals, the development of polyploid cells can contribute to tissue differentiation and, therefore, possibly a gain of function; alternately, it can be associated with development of disease, such as cancer. Polyploidy can occur because of cell fusion or abnormal cell division (endoreplication, mitotic slippage, or cytokinesis failure). Polyploidy is a common characteristic of the mammalian liver. Polyploidization occurs mainly during liver development, but also in adults with increasing age or because of cellular stress (eg, surgical resection, toxic exposure, or viral infections). This review will explore the mechanisms that lead to the development of polyploid cells, our current state of understanding of how polyploidization is regulated during liver growth, and its consequence on liver function.

Nuclear double-fluorescent reporter for in vivo and ex vivo analyses of biological transitions in mouse nuclei

Cre-responsive dual-fluorescent alleles allow in situ marking of cell lineages or genetically modified cells. Here we report a dual-fluorescent allele, ROSA nT-nG , which directs nuclear accumulation of tdTomato in Cre-naïve lineages. Cre converts the allele to ROSA nG , which drives nuclear EGFP accumulation. Conditions were established for analyzing marked nuclei by flow cytometry on the basis of red-green fluorescence and ploidy, with a particular focus on liver nuclei. Hydrodynamic delivery of a Cre-expression plasmid was used to time-stamp arbitrary hepatocytes for lineage tracing. The distinct green fluorescence of nuclei from Cre-exposed lineages facilitated analyses of ploidy transitions within clones. To assess developmental transitions in liver nuclei, ROSA nT-nG was combined with the hepatocyte-specific AlbCre transgene, facilitating discrimination between hepatocyte and nonhepatocyte nuclei. Nuclei extracted from postnatal day 2 (P2) livers were 41 % green and 59 % red and reached a stable level of 84 % green by P22. Until P20, green nuclei were >98 % diploid (2N); at P40 they were ~56 % 2N, 43 % 4N, and <1 % 8N; and by P70 they reached a stable distribution of ~46 % 2N, 45 % 4N, and 9 % 8N. In conclusion, ROSA nT-nG will facilitate in vivo and ex vivo studies on liver and will likely be valuable for studies on tissues like muscle, kidney, or brain in which cells are refractory to whole-cell flow cytometry, or like trophectoderm derivatives or cancers in which cells undergo ploidy transitions.

Physiological significance of polyploidization in mammalian cells

Programmed polyploidization occurs in all mammalian species during development and aging in selected tissues, but the biological properties of polyploid cells remain obscure. Spontaneous polyploidization arises during stress and has been observed in a variety of pathological conditions, such as cancer and degenerative diseases. A major challenge in the field is to test the predicted functions of polyploidization in vivo. However, recent genetic mouse models with diminished polyploidization phenotypes represent novel, powerful tools to unravel the biological function of polyploidization. Contrary to a longstanding hypothesis, polyploidization appears to not be required for differentiation and has no obvious impact on proliferation. Instead, polyploidization leads to increased cell size and genetic diversity, which could promote better adaptation to chronic injury or stress. We discuss here the consequences of reducing polyploidization in mice and review which stress responses and molecular signals trigger polyploidization during development and disease.

To divide or not to divide: revisiting liver regeneration

The liver has a remarkable capacity to regenerate. Even with surgical removal (partial hepatectomy) of 70% of liver mass, the remnant tissue grows to recover the original mass and functions. Liver regeneration after partial hepatectomy has been studied extensively since the 19th century, establishing the long-standing model that hepatocytes, which account for most of the liver weight, proliferate to recover the original mass of the liver. The basis of this model is the fact that almost all hepatocytes undergo S phase, as shown by the incorporation of radioactive nucleotides during liver regeneration. However, DNA replication does not necessarily indicate the execution of cell division, and a possible change in hepatocyte size is not considered in the model. In addition, as 15-30% of hepatocytes in adult liver are binuclear, the difference in nuclear number may affect the mode of cell division during regeneration. Thus, the traditional model seems to be oversimplified. Recently, we developed new techniques to investigate the process of liver regeneration, and revealed interesting features of hepatocytes. In this review, we first provide a historical overview of how the widely accepted model of liver regeneration was established and then discuss some overlooked observations together with our recent findings. Finally, we describe the revised model and perspectives on liver regeneration research.

Aneuploidy, polyploidy and ploidy reversal in the liver

Polyploidy has been described in the liver for over 100 years. The frequency of polyploid hepatocytes varies by age and species, but up to 90% of mouse hepatocytes and approximately 50% of human hepatocytes are polyploid. In addition to alterations in the entire complement of chromosomes, variations in chromosome copy number have been recently described. Aneuploidy in the liver is pervasive, affecting 60% of hepatocytes in mice and 30-90% of hepatocytes in humans. Polyploidy and aneuploidy in the liver are closely linked, and the ploidy conveyor model describes this relationship. Diploid hepatocytes undergo failed cytokinesis to generate polyploid cells. Proliferating polyploid hepatocytes, which form multipolar spindles during cell division, generate reduced ploidy progeny (e.g., diploid hepatocytes from tetraploids or octaploids) and/or aneuploid daughters. New evidence suggests that random hepatic aneuploidy can promote adaptation to liver injury. For instance, in response to chronic liver damage, subsets of aneuploid hepatocytes that are differentially resistant to the injury remain healthy, regenerate the liver and restore function. Future work is required to elucidate the mechanisms regulating dynamic chromosome changes in the liver and to understand how these processes impact normal and abnormal liver function.

Polyploidy and chromatin remodeling in hepatocytes from insulin-dependent diabetic and normoglycemic aged mice

Changes in polyploidization, chromatin supraorganization, and chromatin accessibility were investigated in hepatocytes collected from adult, nonobese diabetic (NOD) mice with increasing hyperglycemia and compared with adult normoglycemic controls and 56-week-old normoglycemic BALB/c mice. Our goal was to determine the changes in ploidy degrees and chromatin characteristics in mouse hepatocytes that are associated with insulin-dependent diabetes and to detect similarities in these aspects with those verified with aging, with greater accuracy than previous studies. Image analysis of Feulgen-stained nuclei revealed changes in ploidy degrees and chromatin supraorganization. Chromatin accessibility was assessed with micrococcal nuclease (MNase) digestion. Increased polyploidy was associated with increasing levels of glycemia, and this trend toward polyploidy was found even under normoglycemic conditions in NOD mice. Although high degrees of ploidy were also detected in aged BALB/c mice, the magnitude of polyploidy was not the same magnitude as that in the diabetic mice. While there was increased homogeneity of chromatin packaging with increasing polyploidy under conditions of severe hyperglycemia (and even under conditions of normoglycemia) in NOD mice, an inverse relationship was observed in aged BALB/c mice. Chromatin accessibility to MNase increased under severe hyperglycemia and advanced age, but it was much higher in the diabetic mice. In conclusion, although similarities in polyploidy were observed between the hepatocytes from increasingly hyperglycemic adult mice and those from normoglycemic aged mice, the relationship between chromatin remodeling and increases in ploidy degrees was not the same between the hepatocytes of these two groups. These findings demonstrate that strict similarities between diabetes and aging are not always true at the cellular level. This discordance is likely due to differences in the metabolic state of mouse hepatocytes during aging and diabetic conditions consequent to specificities in their gene regulatory programs.

Genome-wide analysis of hepatic gene silencing in hepatoma cell variants

Genome-wide gene expression profiling was carried out on rat hepatoma cells and compared to profiles of hepatoma "variant" cell lines derived via a stringent selection protocol that enriches for rare cells (<1 in 100,000 cells) that fail to drive liver function. Results show 132 genes that are strongly (>5-fold) repressed in each of the four variant cell lines tested. An additional 68 genes were repressed in 3 of 4 variant cell lines. Importantly, several of the repressed genes are members of transcriptional activation pathways, suggesting that they may contribute to maintaining the hepatic phenotype. Ectopic expression of the HNF1A gene in a variant cell line resulted in activation of 56 genes, 37 of which were included in the repressed data set. These data suggest that a high level of reprogramming occurs when hepatoma cells convert to a non-differentiated phenotype, a process that can be partially reversed by the introduction of transcription factors.

Polyploidy and liver proliferation

Organisms containing an increase in DNA content by whole number multiples of the entire set of chromosomes are defined as polyploid. Cells that contain more than two sets of chromosomes were first observed in plants about a century ago, and it is now recognized that polyploid cells form in many eukaryotes under a wide variety of circumstances. Although it is less common in mammals, some tissues, including the liver, show a high percentage of polyploid cells. Thus, during post-natal growth, the liver parenchyma undergoes dramatic changes characterized by gradual polyploidization during which hepatocytes of several ploidy classes emerge as a result of modified cell-division cycles. Liver cell polyploidy is generally considered to indicate terminal differentiation and senescence and to both lead to a progressive loss of cell pluripotency and to a markedly decreased replication capacity. In adults, liver polyploidization is differentially regulated upon loss of liver mass and liver damage. Here we review the current state of understanding about how polyploidization is regulated during normal and pathological liver growth, and detail by which mechanisms hepatocytes become polyploid.

How mitotic errors contribute to karyotypic diversity in cancer

Aneuploidy is a common feature of cancer cells, and is believed to play a critical role in tumorigenesis and cancer progression. Most cancer cells also exhibit high rates of mitotic chromosome mis-segregation, a phenomenon known as chromosomal instability, which leads to high variability of the karyotype. Here, we describe the nature, nuances, and implications of cancer karyotypic diversity. Moreover, we summarize recent studies aimed at identifying the mitotic defects that may be responsible for inducing chromosome mis-segregation in cancer cells. These include kinetochore attachment errors, spindle assembly checkpoint dysfunction, mitotic spindle defects, and other cell division inaccuracies. Finally, we discuss how such mitotic errors generate karyotypic diversity in cancer cells.

Decidual cell polyploidization necessitates mitochondrial activity

Cellular polyploidy has been widely reported in nature, yet its developmental mechanism and function remain poorly understood. In the present study, to better define the aspects of decidual cell polyploidy, we isolated pure polyploid and non-polyploid decidual cell populations from the in vivo decidual bed. Three independent RNA pools prepared for each population were then subjected to the Affymetrix gene chip analysis for the whole mouse genome transcripts. Our data revealed up-regulation of 1015 genes and down-regulation of 1207 genes in the polyploid populations, as compared to the non-polyploid group. Comparative RT-PCR and in situ hybridization results indeed confirmed differential expressional regulation of several genes between the two populations. Based on functional enrichment analyses, up-regulated polyploidy genes appeared to implicate several functions, which primarily include cell/nuclear division, ATP binding, metabolic process, and mitochondrial activity, whereas that of down-regulated genes primarily included apoptosis and immune processes. Further analyses of genes that are related to mitochondria and bi-nucleation showed differential and regional expression within the decidual bed, consistent with the pattern of polyploidy. Consistently, studies revealed a marked induction of mitochondrial mass and ATP production in polyploid cells. The inhibition of mitochondrial activity by various pharmacological inhibitors, as well as by gene-specific targeting using siRNA-mediated technology showed a dramatic attenuation of polyploidy and bi-nucleation development during in vitro stromal cell decidualization, suggesting mitochondria play a major role in positive regulation of decidual cell polyploidization. Collectively, analyses of unique polyploidy markers and molecular signaling networks may be useful to further characterize functional aspects of decidual cell polyploidy at the site of implantation.

Assessing the potential for AAV vector genotoxicity in a murine model

Gene transfer using adeno-associated virus (AAV) vectors has great potential for treating human disease. Recently, questions have arisen about the safety of AAV vectors, specifically, whether integration of vector DNA in transduced cell genomes promotes tumor formation. This study addresses these questions with high-dose liver-directed AAV-mediated gene transfer in the adult mouse as a model (80 AAV-injected mice and 52 controls). After 18 months of follow-up, AAV-injected mice did not show a significantly higher rate of hepatocellular carcinoma compared with controls. Tumors in mice treated with AAV vectors did not have significantly different amounts of vector DNA compared with adjacent normal tissue. A novel high-throughput method for identifying AAV vector integration sites was developed and used to clone 1029 integrants. Integration patterns in tumor tissue and adjacent normal tissue were similar to each other, showing preferences for active genes, cytosine-phosphate-guanosine islands, and guanosine/cytosine-rich regions. [corrected] Gene expression data showed that genes near integration sites did not show significant changes in expression patterns compared with genes more distal to integration sites. No integration events were identified as causing increased oncogene expression. Thus, we did not find evidence that AAV vectors cause insertional activation of oncogenes and subsequent tumor formation.

Reprogramming factors involved in hybrids and cybrids of human embryonic stem cells fused with hepatocytes

Embryonic stem cells (ESCs) have the potential to reprogram somatic cells into ESC-like cells through cell fusion. In the present study, the potential of human (h)ESC cytoplasts and karyoplasts to reprogram human hepatocytes was evaluated. Green fluorescent protein (GFP) transfected hESCs (ENVY cells) were fused with SNARF-1 (CellTracker)-labeled human hepatocytes using polyethylene glycol (PEG) and fluorescence-activated cell sorting (FACS) to produce hESC-hepatocyte hybrids. Immunocytochemical analysis of ESC markers showed that the hybrids expressed OCT4, TRA-1-60, TRA-1-81, SSEA-4, and GCTM-2. However, SSEA-1, which is typically low or absent on hESCs, was detected on hESC–hepatocyte hybrids. Moreover, reverse transcriptase polymerase chain reaction (RT-PCR) showed that alpha-fetoprotein, which is highly expressed in hepatocytes, was erased in the hybrids. These results indicated that hESCs have the potential to reprogram hepatocyte phenotype to a relatively undifferentiated state, but such hybrid cells are not identical to hESCs. Although hESC–hepatocyte hybrids were aneuploid, they were able to differentiate into embryoid bodies and some types of somatic cells. Furthermore, cybrids of enucleated hESCs and hepatocytes were produced by cell fusion, but the cybrids were unable to self-renew in the same way as hESCs. Presumably, the reprogramming factors are associated with the karyoplast and not the cytoplast of hESCs.

Somatic polyploidy promotes cell function under stress and energy depletion: evidence from tissue-specific mammal transcriptome

Polyploid cells show great among-species and among-tissues diversity and relation to developmental mode, suggesting their importance in adaptive evolution and developmental programming. At the same time, excessive polyploidization is a hallmark of functional impairment, aging, growth disorders, and numerous pathologies including cancer and cardiac diseases. To shed light on this paradox and to find out how polyploidy contributes to organ functions, we review here the ploidy-associated shifts in activity of narrowly expressed (tissue specific) genes in human and mouse heart and liver, which have the reciprocal pattern of polyploidization. For this purpose, we use the modular biology approach and genome-scale cross-species comparison. It is evident from this review that heart and liver show similar traits in response to polyploidization. In both organs, polyploidy protects vitality (mainly due to the activation of sirtuin-mediated pathways), triggers the reserve adenosine-5'-triphosphate (ATP) production, and sustains tissue-specific functions by switching them to energy saving mode. In heart, the strongest effects consisted in the concerted up-regulation of contractile proteins and substitution of energy intensive proteins with energy economic ones. As a striking example, the energy intensive alpha myosin heavy chain (providing fast contraction) decreased its expression by a factor of 10, allowing a 270-fold increase of expression of beta myosin heavy chain (providing slow contraction), which has approximately threefold lower ATP-hydrolyzing activity. The liver showed the enhancement of immunity, reactive oxygen species and xenobiotic detoxication, and numerous metabolic adaptations to long-term energy depletion. Thus, somatic polyploidy may be an ingenious evolutionary instrument for fast adaptation to stress and new environments allowing trade-offs between high functional demand, stress, and energy depletion.

Transcriptomic signature of cell lines isolated from canine mammary adenocarcinoma metastases to lungs

Metastasis is a final step in the progression of mammary gland cancer, usually leading to death. Potentially, a molecular signature of metastasis can be defined via comparison of primary tumors with their metastases. Currently, there is no data in the literature regarding the molecular portrait of metastases in dogs and only few reports regarding human cancer. This is the first report describing the transcriptomic signature of canine cancer metastatic cells. Two adenocarcinoma cell lines isolated from the canine mammary gland (CMT-W1 and CMT-W2) were compared with cell lines isolated from their lung metastases (CMT-W1M and CMT-W2M) with regards to the following cytometric parameters: cell cycle, ploidy, Bcl-2 expression, susceptibility to induced apoptosis, and transcriptomic profile. Cytometric analyses revealed significant differences in cell cycle and antiapoptotic potential between the examined cells. Using oligonucleotide microarrays, we found 104 up-regulated genes in the metastatic cell line CMT-W1M and 21 up-regulated genes in the primary CMT-W1 cell line. We also found 83 up-regulated genes in the CMT-W2M cell line and only 21 up-regulated genes in the CMT-W2 cell line. Among the up-regulated genes in both metastatic cell lines, we found 15 common genes. These differently expressed genes are involved mainly in signal transduction, cell structure and motility, nucleic acid metabolism, developmental processing, and apoptosis (GHSR, RASSF1, ARF1GAP, WDR74, SMOC2, SFRP4, DIAPH1, FSCN1, ALX4, SNX15, PLD2, WNT7B, POU6F2, NKG7, and POLR2F). Seven of them are involved in a cellular pathway dependent on ghrelin via growth hormone secretagogue receptor (GHSR). Our results suggest that this pathway may be essential for mammary cancer cells to have a metastatic potential.

Gene profiling of maternal hepatic adaptations to pregnancy

BACKGROUND

Maternal metabolic demands change dramatically during the course of gestation and must be co-ordinated with the needs of the developing placenta and fetus. The liver is critically involved in metabolism and other important functions. However, maternal hepatic adjustments to pregnancy are poorly understood.

AIM

The aim of the study was to evaluate the influences of pregnancy on the maternal liver growth and gene expression profile.

METHODS

Holtzman Sprague-Dawley rats were mated and sacrificed at various stages of gestation and post-partum. The maternal livers were analysed in gravimetric response, DNA content by PicoGreen dsDNA quantitation reagent, hepatocyte ploidy by flow cytometry and hepatocyte proliferation by ki-67 immunostaining. Gene expression profiling of non-pregnant and gestation d18.5 maternal hepatic tissue was analysed using a DNA microarray approach and partially verified by northern blot or quantitative real-time PCR analysis.

RESULTS

During pregnancy, the liver exhibited approximately an 80% increase in size, proportional to the increase in body weight of the pregnant animals. The pregnancy-induced hepatomegaly was a physiological event of liver growth manifested by increases in maternal hepatic DNA content and hepatocyte proliferation. Pregnancy did not affect hepatocyte polyploidization. Pregnancy-dependent changes in hepatic expression were noted for a number of genes, including those associated with cell proliferation, cytokine signalling, liver regeneration and metabolism.

CONCLUSIONS

The metabolic demands of pregnancy cause marked adjustments in maternal liver physiology. Central to these adjustments are an expansion in hepatic capacity and changes in hepatic gene expression. Our findings provide insights into pregnancy-dependent hepatic adaptations.

Distinct and redundant functions of cyclin E1 and cyclin E2 in development and cancer

The highly conserved E-type cyclins are core components of the cell cycle machinery, facilitating the transition into S phase through activation of the cyclin dependent kinases, and assembly of pre-replication complexes on DNA. Cyclin E1 and cyclin E2 are assumed to be functionally redundant, as cyclin E1-/- E2-/- mice are embryonic lethal while cyclin E1-/- and E2-/- single knockout mice have primarily normal phenotypes. However more detailed studies of the functions and regulation of the E-cyclins have unveiled potential additional roles for these proteins, such as in endoreplication and meiosis, which are more closely associated with either cyclin E1 or cyclin E2. Moreover, expression of each E-cyclin can be independently regulated by distinct transcription factors and microRNAs, allowing for context-specific expression. Furthermore, cyclins E1 and E2 are frequently expressed independently of one another in human cancer, with unique associations to signatures of poor prognosis. These data imply an absence of co-regulation of cyclins E1 and E2 during tumorigenesis and possibly different contributions to cancer progression. This is supported by in vitro data identifying divergent regulation of the two genes, as well as potentially different roles in vivo.

Cardiomyocyte cell cycle control and growth estimation in vivo--an analysis based on cardiomyocyte nuclei

AIMS

Adult mammalian cardiomyocytes are traditionally viewed as being permanently withdrawn from the cell cycle. Whereas some groups have reported none, others have reported extensive mitosis in adult myocardium under steady-state conditions. Recently, a highly specific assay of 14C dating in humans has suggested a continuous generation of cardiomyocytes in the adult, albeit at a very low rate. Mice represent the most commonly used animal model for these studies, but their short lifespan makes them unsuitable for 14C studies. Herein, we investigate the cellular growth pattern for murine cardiomyocyte growth under steady-state conditions, addressed with new analytical and technical strategies, and we furthermore relate this to gene expression patterns.

METHODS AND RESULTS

The observed levels of DNA synthesis in early life were associated with cardiomyocyte proliferation. Mitosis was prolonged into early life, longer than the most conservative previous estimates. DNA synthesis in neonatal life was attributable to bi-nucleation, therefore suggesting that cardiomyocytes withdraw from the cell cycle shortly after birth. No cell cycle activity was observed in adult cardiomyocytes and significant polyploidy was observed in cardiomyocyte nuclei.

CONCLUSION

Gene analyses identified 32 genes whose expression was predicted to be particular to day 3-4 neonatal myocytes, compared with embryonic or adult cells. These cell cycle-associated genes are crucial to the understanding of the mechanisms of bi-nucleation and physiological cellular growth in the neonatal period.

DNA content, chromatin supraorganization, nuclear glycoproteins and RNA amounts in hepatocytes of mice expressing insulin-dependent diabetes

Chromatin supraorganization and extensibility and nuclear glycoprotein content have been reported to change in hepatocytes from mice during development and aging, as well as under starvation and refeeding conditions. In non-obese diabetic (NOD) mice, the expression of insulin-dependent diabetes may be accompanied by metabolic changes in the liver. These changes are likely to be similar to those involved in the aging processes of non-diabetic animals. Therefore, we hypothesized that the chromatin organization, as well as the physical properties and compositions of hepatocyte nuclei would also be affected in NOD mice in the same way as those in aged non-diabetic mice. Nuclear image parameters were evaluated by image analysis of Feulgen-stained preparations. Chromatin extensibility in response to gravity was observed with polarized light after lysis and toluidine blue staining. The Con-A response of nuclear glycoproteins was evaluated with scanning microspectrophotometry. These characteristics were assessed using hepatocyte imprints from female NOD mice after a 28-day period of diabetes expression. Observations and measurements were made in comparison to healthy BALB/c mice. Total RNA amounts were determined for livers of NOD and BALB/c mice. Enhanced polyploidy levels, a decrease in chromatin higher-order packing states, an increased frequency of extended chromatin fiber formation, and deeper Con-A-responsive chromatin areas were observed in the hepatocytes of the NOD mice expressing insulin-dependent diabetes. Reduced amounts of total RNA were also found in the livers of these mice. Our findings for NOD mice expressing insulin-dependent diabetes are consistent with previously reported data for old-aged mice of the inbred strain A/Uni and may reflect changes in transcriptional activities associated with the stressful physiological demands on the liver during the expression of diabetes.