Polyploidy and liver proliferation

Polyploid Cancer Cell Models in Drosophila

Cells with an abnormal number of chromosomes have been found in more than 90% of solid tumors, and among these, polyploidy accounts for about 40%. Polyploidized cells most often have duplicate centrosomes as well as genomes, and thus their mitosis tends to promote merotelic spindle attachments and chromosomal instability, which produces a variety of aneuploid daughter cells. Polyploid cells have been found highly resistant to various stress and anticancer therapies, such as radiation and mitogenic inhibitors. In other words, common cancer therapies kill proliferative diploid cells, which make up the majority of cancer tissues, while polyploid cells, which lurk in smaller numbers, may survive. The surviving polyploid cells, prompted by acute environmental changes, begin to mitose with chromosomal instability, leading to an explosion of genetic heterogeneity and a concomitant cell competition and adaptive evolution. The result is a recurrence of the cancer during which the tenacious cells that survived treatment express malignant traits. Although the presence of polyploid cells in cancer tissues has been observed for more than 150 years, the function and exact role of these cells in cancer progression has remained elusive. For this reason, there is currently no effective therapeutic treatment directed against polyploid cells. This is due in part to the lack of suitable experimental models, but recently several models have become available to study polyploid cells in vivo. We propose that the experimental models in Drosophila, for which genetic techniques are highly developed, could be very useful in deciphering mechanisms of polyploidy and its role in cancer progression.

Effect of Hepatic Pathology on Liver Regeneration: The Main Metabolic Mechanisms Causing Impaired Hepatic Regeneration

Liver regeneration has been studied for many decades, and the mechanisms underlying regeneration of normal liver following resection are well described. However, no less relevant is the study of mechanisms that disrupt the process of liver regeneration. First of all, a violation of liver regeneration can occur in the presence of concomitant hepatic pathology, which is a key factor reducing the liver's regenerative potential. Understanding these mechanisms could enable the rational targeting of specific therapies to either reduce the factors inhibiting regeneration or to directly stimulate liver regeneration. This review describes the known mechanisms of normal liver regeneration and factors that reduce its regenerative potential, primarily at the level of hepatocyte metabolism, in the presence of concomitant hepatic pathology. We also briefly discuss promising strategies for stimulating liver regeneration and those concerning methods for assessing the regenerative potential of the liver, especially intraoperatively.

Wnt signaling regulates hepatocyte cell division by a transcriptional repressor cascade

As a general model for cell cycle control, repressors keep cells quiescent until growth signals remove the inhibition. For S phase, this is exemplified by the Retinoblastoma (RB) protein and its inactivation. It was unknown whether similar mechanisms operate in the M phase. The Wnt signaling pathway is an important regulator of cell proliferation. Here, we find that Wnt induces expression of the transcription factor Tbx3, which in turn represses mitotic inhibitors E2f7 and E2f8 to permit mitotic progression. Such a cascade of transcriptional repressors may be a general mechanism for cell division control. These findings have implications for tissue homeostasis and disease, as the function for Wnt signaling in mitosis is relevant to its widespread role in stem cells and cancer.

Cell proliferation is tightly controlled by inhibitors that block cell cycle progression until growth signals relieve this inhibition, allowing cells to divide. In several tissues, including the liver, cell proliferation is inhibited at mitosis by the transcriptional repressors E2F7 and E2F8, leading to formation of polyploid cells. Whether growth factors promote mitosis and cell cycle progression by relieving the E2F7/E2F8-mediated inhibition is unknown. We report here on a mechanism of cell division control in the postnatal liver, in which Wnt/β-catenin signaling maintains active hepatocyte cell division through Tbx3, a Wnt target gene. The TBX3 protein directly represses transcription of E2f7 and E2f8, thereby promoting mitosis. This cascade of sequential transcriptional repressors, initiated by Wnt signals, provides a paradigm for exploring how commonly active developmental signals impact cell cycle completion.

Hallmarks of the human intestinal microbiome on liver maturation and function

As one of the most metabolically complex systems in the body, the liver ensures multi-organ homeostasis and ultimately sustains life. Nevertheless, during early postnatal development, the liver is highly immature and takes about 2 years to acquire and develop almost all of its functions. Different events occurring at the environmental and cellular levels are thought to mediate hepatic maturation and function postnatally. The crosstalk between the liver, the gut and its microbiome has been well appreciated in the context of liver disease, but recent evidence suggests that the latter could also be critical for hepatic function under physiological conditions. The gut-liver crosstalk is thought to be mediated by a rich repertoire of microbial metabolites that can participate in a myriad of biological processes in hepatic sinusoids, from energy metabolism to tissue regeneration. Studies on germ-free animals have revealed the gut microbiome as a critical contributor in early hepatic programming, and this influence extends throughout life, mediating liver function and body homeostasis. In this seminar, we describe the microbial molecules that have a known effect on the liver and discuss how the gut microbiome and the liver evolve throughout life. We also provide insights on current and future strategies to target the gut microbiome in the context of hepatology research.

Inhibition of Hedgehog Delays Liver Regeneration through Disrupting the Cell Cycle

Liver regeneration is a complicated biological process orchestrated by various liver resident cells. Hepatic cell proliferation and reconstruction of the hepatic architecture involve multiple signaling pathways. It has been reported that the Hh signal is involved in liver regeneration. However, the signal transduction pathways and cell types involved are ill studied. This study aimed to investigate hedgehog signal response cell types and the specific molecular mechanism involved in the process of liver regeneration. Partial hepatectomy (PH) of 70% was performed on ICR (Institute of Cancer Research) mice to study the process of liver regeneration. We found that the hedgehog signal was activated significantly after PH, including hedgehog ligands, receptors and intracellular signaling molecules. Ligand signals were mainly expressed in bile duct cells and non-parenchymal hepatic cells, while receptors were expressed in hepatocytes and some non-parenchymal cells. Inhibition of the hedgehog signal treated with vismodegib reduced the liver regeneration rate after partial hepatectomy, including inhibition of hepatic cell proliferation by decreasing Cyclin D expression and disturbing the cell cycle through the accumulation of Cyclin B. The current study reveals the important role of the hedgehog signal and its participation in the regulation of hepatic cell proliferation and the cell cycle during liver regeneration. It provides new insight into the recovery of the liver after liver resection.

Stem cell therapy in liver regeneration: Focus on mesenchymal stem cells and induced pluripotent stem cells

The liver has the ability to repair itself after injury; however, a variety of pathological changes in the liver can affect its ability to regenerate, and this could lead to liver failure. Mesenchymal stem cells (MSCs) are considered a good source of cells for regenerative medicine, as they regulate liver regeneration through different mechanisms, and their efficacy has been demonstrated by many animal experiments and clinical studies. Induced pluripotent stem cells, another good source of MSCs, have also made great progress in the establishment of organoids, such as liver disease models, and in drug screening. Owing to the recent developments in MSCs and induced pluripotent stem cells, combined with emerging technologies including graphene, nano-biomaterials, and gene editing, precision medicine and individualized clinical treatment may be realized in the near future.

Circadian Organelles: Rhythms at All Scales

Circadian clocks have evolved in most light-sensitive organisms, from unicellular organisms to mammals. Consequently, a myriad of biological functions exhibits circadian rhythmicity, from behavior to physiology, through tissue and cellular functions to subcellular processes. Circadian rhythms in intracellular organelles are an emerging and exciting research arena. We summarize herein the current literature for rhythmicity in major intracellular organelles in mammals. These include changes in the morphology, content, and functions of different intracellular organelles. While these data highlight the presence of rhythmicity in these organelles, a gap remains in our knowledge regarding the underlying molecular mechanisms and their functional significance. Finally, we discuss the importance and challenges faced by spatio-temporal studies on these organelles and speculate on the presence of oscillators in organelles and their potential mode of communication. As circadian biology has been and continues to be studied throughout temporal and spatial axes, circadian organelles appear to be the next frontier.

Ssu72 Dual-Specific Protein Phosphatase: From Gene to Diseases

More than 70% of eukaryotic proteins are regulated by phosphorylation. However, the mechanism of dephosphorylation that counteracts phosphorylation is less studied. Phosphatases are classified into 104 distinct groups based on substrate-specific features and the sequence homologies in their catalytic domains. Among them, dual-specificity phosphatases (DUSPs) that dephosphorylate both phosphoserine/threonine and phosphotyrosine are important for cellular homeostasis. Ssu72 is a newly studied phosphatase with dual specificity that can dephosphorylate both phosphoserine/threonine and phosphotyrosine. It is important for cell-growth signaling, metabolism, and immune activation. Ssu72 was initially identified as a phosphatase for the Ser5 and Ser7 residues of the C-terminal domain of RNA polymerase II. It prefers the cis configuration of the serine-proline motif within its substrate and regulates Pin1, different from other phosphatases. It has recently been reported that Ssu72 can regulate sister chromatid cohesion and the separation of duplicated chromosomes during the cell cycle. Furthermore, Ssu72 appears to be involved in the regulation of T cell receptor signaling, telomere regulation, and even hepatocyte homeostasis in response to a variety of stress and damage signals. In this review, we aim to summarize various functions of the Ssu72 phosphatase, their implications in diseases, and potential therapeutic indications.

An Engineered Mouse to Identify Proliferating Cells and Their Derivatives

BACKGROUND

Cell proliferation is a fundamental event during development, disease, and regeneration. Effectively tracking and quantifying proliferating cells and their derivatives is critical for addressing many research questions. Cell cycle expression such as for Ki67, proliferating cell nuclear antigen (PCNA), or aurora kinase B (Aurkb), or measurement of 5-bromo-2'-deoxyuridine (BrdU) or 3H-thymidine incorporation have been widely used to assess and quantify cell proliferation. These are powerful tools for detecting actively proliferating cells, but they do not identify cell populations derived from proliferating progenitors over time.

AIMS

We developed a new mouse tool for lineage tracing of proliferating cells by targeting the Aurkb allele.

RESULTS

In quiescent cells or cells arrested at G1/S, little or no Aurkb mRNA is detectable. In cycling cells, Aurkb transcripts are detectable at G2 and become undetectable by telophase. These findings suggest that Aurkb transcription is restricted to proliferating cells and is tightly coupled to cell proliferation. Accordingly, we generated an Aurkb ER Cre/+ mouse by targeting a tamoxifen inducible Cre cassette into the start codon of Aurkb. We find that the Aurkb ER Cre/+ mouse faithfully labels proliferating cells in developing embryos and regenerative adult tissues such as intestine but does not label quiescent cells such as post-mitotic neurons.

CONCLUSION

The Aurkb ER Cre/+ mouse faithfully labels proliferating cells and their derivatives in developing embryos and regenerative adult tissues. This new mouse tool provides a novel genetic tracing capability for studying tissue proliferation and regeneration.

Polyploidy spectrum: a new marker in HCC classification

OBJECTIVES

Polyploidy is a fascinating characteristic of liver parenchyma. Hepatocyte polyploidy depends on the DNA content of each nucleus (nuclear ploidy) and the number of nuclei per cell (cellular ploidy). Which role can be assigned to polyploidy during human hepatocellular carcinoma (HCC) development is still an open question. Here, we investigated whether a specific ploidy spectrum is associated with clinical and molecular features of HCC.

DESIGN

Ploidy spectra were determined on surgically resected tissues from patients with HCC as well as healthy control tissues. To define ploidy profiles, a quantitative and qualitative in situ imaging approach was used on paraffin tissue liver sections.

RESULTS

We first demonstrated that polyploid hepatocytes are the major components of human liver parenchyma, polyploidy being mainly cellular (binuclear hepatocytes). Across liver lobules, polyploid hepatocytes do not exhibit a specific zonation pattern. During liver tumorigenesis, cellular ploidy is drastically reduced; binuclear polyploid hepatocytes are barely present in HCC tumours. Remarkably, nuclear ploidy is specifically amplified in HCC tumours. In fact, nuclear ploidy is amplified in HCCs harbouring a low degree of differentiation and TP53 mutations. Finally, our results demonstrated that highly polyploid tumours are associated with a poor prognosis.

CONCLUSIONS

Our results underline the importance of quantification of cellular and nuclear ploidy spectra during HCC tumorigenesis.

In Vivo Lineage Tracing of Polyploid Hepatocytes Reveals Extensive Proliferation during Liver Regeneration

The identity of cellular populations that drive liver regeneration after injury is the subject of intense study, and the contributions of polyploid hepatocytes to organ regeneration and homeostasis have not been systematically assessed. Here, we developed a multicolor reporter allele system to genetically label and trace polyploid cells in situ. Multicolored polyploid hepatocytes undergo ploidy reduction and subsequent re-polyploidization after transplantation, providing direct evidence of the hepatocyte ploidy conveyor model. Marker segregation revealed that ploidy reduction rarely involves chromosome missegregation in vivo. We also traced polyploid hepatocytes in several different liver injury models and found robust proliferation in all settings. Importantly, ploidy reduction was seen in all injury models studied. We therefore conclude that polyploid hepatocytes have extensive regenerative capacity in situ and routinely undergo reductive mitoses during regenerative responses.

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).

Advances in heart regeneration based on cardiomyocyte proliferation and regenerative potential of binucleated cardiomyocytes and polyploidization

One great achievement in medical practice is the reduction in acute mortality of myocardial infarction due to identifying risk factors, antiplatelet therapy, optimized hospitalization and acute percutaneous coronary intervention. Yet, the prevalence of heart failure is increasing presenting a major socio-economic burden. Thus, there is a great need for novel therapies that can reverse damage inflicted to the heart. In recent years, data have accumulated suggesting that induction of cardiomyocyte proliferation might be a future option for cardiac regeneration. Here, we review the relevant literature since September 2015 concluding that it remains a challenge to verify that a therapy induces indeed cardiomyocyte proliferation. Most importantly, it is unclear that the detected increase in cardiomyocyte cell cycle activity is required for an associated improved function. In addition, we review the literature regarding the evidence that binucleated and polyploid mononucleated cardiomyocytes can divide, and put this in context to other cell types. Our analysis shows that there is significant evidence that binucleated cardiomyocytes can divide. Yet, it remains elusive whether also polyploid mononucleated cardiomyocytes can divide, how efficient proliferation of binucleated cardiomyocytes can be induced, what mechanism regulates cell cycle progression in these cells, and what fate and physiological properties the daughter cells have. In summary, we propose to standardize and independently validate cardiac regeneration studies, encourage the field to study the proliferative potential of binucleated and polyploid mononucleated cardiomyocytes, and to determine whether induction of polyploidization can enhance cardiac function post-injury.

Lessons we can learn from neurons to make cancer cells quiescent

Cancer is a major concern for contemporary societies. However, the incidence of cancer is unevenly distributed among tissues and cell types. In particular, the evidence indicates that neurons are absolutely resistant to cancer and this is commonly explained on the basis of the known postmitotic state of neurons. The dominant paradigm on cancer understands this problem as a disease caused by mutations in cellular genes that result in unrestrained cell proliferation and eventually in tissue invasion and metastasis. However, the evidence also shows that mutations and gross chromosomal anomalies are common in functional neurons that nevertheless do not become neoplastic. This fact suggests that in the real nonexperimental setting mutations per se are not enough for inducing carcinogenesis but also that the postmitotic state of neurons is not genetically controlled or determined, otherwise there should be reports of spontaneously transformed neurons. Here we discuss the evidence that the postmitotic state of neurons has a structural basis on the high stability of their nuclear higher order structure that performs like an absolute tumor suppressor. We also discuss evidence that it is possible to induce a similar structural postmitotic state in nonneural cell types as a practical strategy for stopping or reducing the progression of cancer.

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.

New Cell Cycle Inhibitors Target Aneuploidy in Cancer Therapy

Aneuploidy is a hallmark of cancer. Defects in chromosome segregation result in aneuploidy. Multiple pathways are engaged in this process, including errors in kinetochore-microtubule attachments, supernumerary centrosomes, spindle assembly checkpoint (SAC) defects, and chromosome cohesion defects. Although aneuploidy provides an adaptation and proliferative advantage in affected cells, excessive aneuploidy beyond a critical level can be lethal to cancer cells. Given this, enhanced chromosome missegregation is hypothesized to limit survival of aneuploid cancer cells, especially when compared to diploid cells. Based on this concept, proteins and pathways engaged in chromosome segregation are being exploited as candidate therapeutic targets for aneuploid cancers. Agents that induce chromosome missegregation and aneuploidy now exist, including SAC inhibitors, those that alter centrosome fidelity and others that are under active study in preclinical and clinical contexts. This review explores the therapeutic potentials of such new agents, including the benefits of combining them with other antineoplastic agents.

Polyploidy in tissue homeostasis and regeneration

Polyploid cells, which contain multiple copies of the typically diploid genome, are widespread in plants and animals. Polyploidization can be developmentally programmed or stress induced, and arises from either cell-cell fusion or a process known as endoreplication, in which cells replicate their DNA but either fail to complete cytokinesis or to progress through M phase entirely. Polyploidization offers cells several potential fitness benefits, including the ability to increase cell size and biomass production without disrupting cell and tissue structure, and allowing improved cell longevity through higher tolerance to genomic stress and apoptotic signals. Accordingly, recent studies have uncovered crucial roles for polyploidization in compensatory cell growth during tissue regeneration in the heart, liver, epidermis and intestine. Here, we review current knowledge of the molecular pathways that generate polyploidy and discuss how polyploidization is used in tissue repair and regeneration.

Aryl Hydrocarbon Receptor Promotes Liver Polyploidization and Inhibits PI3K, ERK, and Wnt/β-Catenin Signaling

Aryl hydrocarbon receptor (AhR) deficiency alters tissue homeostasis. However, how AhR regulates organ maturation and differentiation remains mostly unknown. Liver differentiation entails a polyploidization process fundamental for cell growth, metabolism, and stress responses. Here, we report that AhR regulates polyploidization during the preweaning-to-adult mouse liver maturation. Preweaning AhR-null (AhR−/−) livers had smaller hepatocytes, hypercellularity, altered cell cycle regulation, and enhanced proliferation. Those phenotypes persisted in adult AhR−/− mice and correlated with compromised polyploidy, predominance of diploid hepatocytes, and enlarged centrosomes. Phosphatidylinositol-3-phosphate kinase (PI3K), extracellular signal-regulated kinase (ERK), and Wnt/β-catenin signaling remained upregulated from preweaning to adult AhR-null liver, likely increasing mammalian target of rapamycin (mTOR) activation. Metabolomics revealed the deregulation of mitochondrial oxidative phosphorylation intermediates succinate and fumarate in AhR−/− liver. Consistently, PI3K, ERK, and Wnt/β-catenin inhibition partially rescued polyploidy in AhR−/− mice. Thus, AhR may integrate survival, proliferation, and metabolism for liver polyploidization. Since tumor cells tend to be polyploid, AhR modulation could have therapeutic value in the liver.

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AhR is required for liver polyploidization during preweaning-to-adult transition

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INS-R/PI3K/AKT, ERK, Wnt/β-Cat and mTOR are downregulated during liver polyploidization

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Reduced polyploidy relates with enhanced mitochondrial metabolism in AhR-null liver

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Understanding how AhR modulates polyploidy may provide strategies against cancer

Engaging Anaphase Catastrophe Mechanisms to Eradicate Aneuploid Cancers

Cancer cells often have supernumerary centrosomes that promote genomic instability, a pathognomonic feature of cancer. During mitosis, cancer cells with supernumerary centrosomes undergo bipolar cell division by clustering centrosomes into two poles. When supernumerary centrosome clustering is antagonized, cancer cells are forced to undergo multipolar division leading to death of daughter cells. This proapoptotic pathway, called anaphase catastrophe, preferentially eliminates aneuploid cancer cells and malignant tumors in engineered mouse models. Anaphase catastrophe occurs through the loss or inhibition of the centrosomal protein CP110, a direct cyclin-dependent kinase 1 (CDK1) and CDK2 target. Intriguingly, CP110 is repressed by the KRAS oncoprotein. This sensitizes KRAS-driven lung cancers (an unmet medical need) to respond to CDK2 inhibitors. Anaphase catastrophe-inducing agents like CDK1 and CDK2 antagonists are lethal to cancer cells with supernumerary centrosomes, but can relatively spare normal cells with two centrosomes. This mechanism is proposed to provide a therapeutic window in the cancer clinic following treatment with a CDK1 or CDK2 inhibitor. Taken together, anaphase catastrophe is a clinically tractable mechanism that promotes death of neoplastic tumors with aneuploidy, a hallmark of cancer. Mol Cancer Ther; 17(4); 724-31. ©2018 AACR.

Modeling Dynamics and Function of Bone Marrow Cells in Mouse Liver Regeneration

In rodents and humans, the liver can efficiently restore its mass after hepatectomy. This is largely attributed to the proliferation and cell cycle re-entry of hepatocytes. On the other hand, bone marrow cells (BMCs) migrate into the liver after resection. Here, we find that a block of BMC recruitment into the liver severely impairs its regeneration after the surgery. Mobilized hematopoietic stem and progenitor cells (HSPCs) in the resected liver can fuse with hepatocytes, and the hybrids proliferate earlier than the hepatocytes. Genetic ablation of the hybrids severely impairs hepatocyte proliferation and liver mass regeneration. Mathematical modeling reveals a key role of bone marrow (BM)-derived hybrids to drive proliferation in the regeneration process, and predicts regeneration efficiency in experimentally non-testable conditions. In conclusion, BM-derived hybrids are essential to trigger efficient liver regeneration after hepatectomy.

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Bone marrow cell migration after liver hepatectomy is key for liver regeneration

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Migrated bone marrow cells fuse with hepatocytes

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Hybrids are essential for liver regeneration

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Mathematical modeling unveils the hybrid function for liver regeneration

A population of G2-arrested cells are selected as sensory organ precursors for the interommatidial bristles of the Drosophila eye

Cell cycle progression and differentiation are highly coordinated during the development of multicellular organisms. The mechanisms by which these processes are coordinated and how their coordination contributes to normal development are not fully understood. Here, we determine the developmental fate of a population of precursor cells in the developing Drosophila melanogaster retina that arrest in G2 phase of the cell cycle and investigate whether cell cycle phase-specific arrest influences the fate of these cells. We demonstrate that retinal precursor cells that arrest in G2 during larval development are selected as sensory organ precursors (SOPs) during pupal development and undergo two cell divisions to generate the four-cell interommatidial mechanosensory bristles. While G2 arrest is not required for bristle development, preventing G2 arrest results in incorrect bristle positioning in the adult eye. We conclude that G2-arrested cells provide a positional cue during development to ensure proper spacing of bristles in the eye. Our results suggest that the control of cell cycle progression refines cell fate decisions and that the relationship between these two processes is not necessarily deterministic.

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.

The Influence of Extracellular RNA on Cell Behavior in Health, Disease and Regeneration

PURPOSE OF REVIEW

An overview of the role of extracellular RNAs (exRNA) in the regulation of homeostasis, disease progression, and regeneration is provided herein. Several exRNAs have been identified as potential biomarkers for disease and disease progression. In addition, the potential of exRNAs as a therapeutic modality is discussed.

RECENT FINDINGS

Fibrotic diseases of the lung, liver, and heart, among other organs share a number of identical exRNAs which play key roles in disease pathogenesis. Though regeneration is limited to only a few tissues in humans, small RNAs (e.g. microRNA) have been shown to be involved in the regenerative process of tissues such as liver and bone. The regulation of healing versus disease appears to be balanced by small RNAs. Because small RNAs are critical to health, they are being investigated as drug targets in multiple ongoing clinical trials. Preclinical studies suggest that promoting or blocking specific small RNAs can provide a novel therapeutic approach.

SUMMARY

exRNA can be utilized for both detection and treatment of disease. Natural and synthetic RNA carriers are being investigated as delivery methods for small RNA molecules. Current and future investigations are likely to lead to expanded applications for exRNAs.

p38α regulates actin cytoskeleton and cytokinesis in hepatocytes during development and aging

Background

Hepatocyte poliploidization is an age-dependent process, being cytokinesis failure the main mechanism of polyploid hepatocyte formation. Our aim was to study the role of p38α MAPK in the regulation of actin cytoskeleton and cytokinesis in hepatocytes during development and aging.

Methods

Wild type and p38α liver-specific knock out mice at different ages (after weaning, adults and old) were used.

Results

We show that p38α MAPK deficiency induces actin disassembly upon aging and also cytokinesis failure leading to enhanced binucleation. Although the steady state levels of cyclin D1 in wild type and p38α knock out old livers remained unaffected, cyclin B1- a marker for G2/M transition- was significantly overexpressed in p38α knock out mice. Our findings suggest that hepatocytes do enter into S phase but they do not complete cell division upon p38α deficiency leading to cytokinesis failure and binucleation. Moreover, old liver-specific p38α MAPK knock out mice exhibited reduced F-actin polymerization and a dramatic loss of actin cytoskeleton. This was associated with abnormal hyperactivation of RhoA and Cdc42 GTPases. Long-term p38α deficiency drives to inactivation of HSP27, which seems to account for the impairment in actin cytoskeleton as Hsp27-silencing decreased the number and length of actin filaments in isolated hepatocytes.

Conclusions

p38α MAPK is essential for actin dynamics with age in hepatocytes.

Mechanics and regulation of cytokinesis in budding yeast

Cytokinesis is essential for the survival of all organisms. It requires concerted functions of cell signaling, force production, exocytosis, and extracellular matrix remodeling. Due to the conservation in core components and mechanisms between fungal and animal cells, the budding yeast Saccharomyces cerevisiae has served as an attractive model for studying this fundamental process. In this review, we discuss the mechanics and regulation of distinct events of cytokinesis in budding yeast, including the assembly, constriction, and disassembly of the actomyosin ring, septum formation, abscission, and their spatiotemporal coordination. We also highlight the key concepts and questions that are common to animal and fungal cytokinesis.

Hepatocyte homeostasis for chromosome ploidization and liver function is regulated by Ssu72 protein phosphatase

Hepatocyte chromosome polyploidization is an important feature of liver development and seems to be required for response to liver stress and injury signals. However, the question of how polyploidization can be tightly regulated in liver growth remains to be answered. Using a conditional knockout mouse model, liver-specific depletion of Ssu72 protein phosphatase was found to result in impairment in regulation of polyploidization. Interestingly, the aberrant polyploidization in Ssu72-depleted mice was associated with impaired liver damage response and increased markers of liver injury and seemed to mimic the phenotypic features of liver diseases such as fibrosis, steatosis, and steatohepatitis. In addition, depletion of Ssu72 caused deregulation of cell cycle progression by overriding the restriction point of the cell cycle and aberrantly promoting DNA endoreplication through G2 /M arrest.

CONCLUSION

Ssu72 plays a substantial role in the maintenance of hepatic chromosome homeostasis and would allow monitoring of liver function.

Involvement of Receptor Activator of Nuclear Factor-κB Ligand (RANKL)-induced Incomplete Cytokinesis in the Polyploidization of Osteoclasts

Osteoclasts are specialized polyploid cells that resorb bone. Upon stimulation with receptor activator of nuclear factor-κB ligand (RANKL), myeloid precursors commit to becoming polyploid, largely via cell fusion. Polyploidization of osteoclasts is necessary for their bone-resorbing activity, but the mechanisms by which polyploidization is controlled remain to be determined. Here, we demonstrated that in addition to cell fusion, incomplete cytokinesis also plays a role in osteoclast polyploidization. In in vitro cultured osteoclasts derived from mice expressing the fluorescent ubiquitin-based cell cycle indicator (Fucci), RANKL induced polyploidy by incomplete cytokinesis as well as cell fusion. Polyploid cells generated by incomplete cytokinesis had the potential to subsequently undergo cell fusion. Nuclear polyploidy was also observed in osteoclasts in vivo, suggesting the involvement of incomplete cytokinesis in physiological polyploidization. Furthermore, RANKL-induced incomplete cytokinesis was reduced by inhibition of Akt, resulting in impaired multinucleated osteoclast formation. Taken together, these results reveal that RANKL-induced incomplete cytokinesis contributes to polyploidization of osteoclasts via Akt activation.

Centrosomes and cancer: revisiting a long-standing relationship

Over a century ago, centrosome aberrations were postulated to cause cancer by promoting genome instability. The mechanisms governing centrosome assembly and function are increasingly well understood, allowing for a timely reappraisal of this postulate. This Review discusses recent advances that shed new light on the relationship between centrosomes and cancer, and raise the possibility that centrosome aberrations contribute to this disease in different ways than initially envisaged.

Human neonatal hepatocyte transplantation induces long-term rescue of unconjugated hyperbilirubinemia in the Gunn rat

Crigler-Najjar type 1 disease is a rare inherited metabolic disease characterized by high levels of unconjugated bilirubin due to the complete absence of hepatic uridine diphosphoglucuronate-glucuronosyltransferase activity. Hepatocyte transplantation (HT) has been proposed as an alternative treatment for Crigler-Najjar syndrome, but it is still limited by the quality and the low engraftment and repopulation ability of the cells used. Because of their attachment capability and expression of adhesion molecules as well as the higher proportion of hepatic progenitor cells, neonatal hepatocytes may have an advantage over adult cells. Adult or neonatal hepatocytes were transplanted into Gunn rats, a model for Crigler-Najjar disease. Engraftment and repopulation were studied and compared by immunofluorescence (IF). Additionally, the serum bilirubin levels, the presence of bilirubin conjugates in rat serum, and the expression of uridine diphosphate glucuronosyltransferase 1 family polypeptide A1 (UGT1A1) in rat liver samples were also analyzed. Here we show that neonatal HT results in long-term correction in Gunn rats. In comparison with adult cells, neonatal cells showed better engraftment and repopulation capability 3 days and 6 months after transplantation, respectively. Bilirubinemia decreased in the transplanted animals during the whole experimental follow-up (6 months). Bilirubin conjugates were also present in the serum of the transplanted animals. Western blots and IF confirmed the presence and expression of UGT1A1 in the liver. This work is the first to demonstrate the advantage of using neonatal hepatocytes for the treatment of Crigler-Najjar in vivo.

p27(Kip1) participates in the regulation of endoreplication in differentiating chick retinal ganglion cells

Nuclear DNA duplication in the absence of cell division (i.e. endoreplication) leads to somatic polyploidy in eukaryotic cells. In contrast to some invertebrate neurons, whose nuclei may contain up to 200,000-fold the normal haploid DNA amount (C), polyploid neurons in higher vertebrates show only 4C DNA content. To explore the mechanism that prevents extra rounds of DNA synthesis in these latter cells we focused on the chick retina, where a population of tetraploid retinal ganglion cells (RGCs) has been described. We show that differentiating chick RGCs that express the neurotrophic receptors p75 and TrkB while lacking retinoblastoma protein, a feature of tetraploid RGCs, also express p27^Kip1. Two different short hairpin RNAs (shRNA) that significantly downregulate p27^Kip1 expression facilitated DNA synthesis and increased ploidy in isolated chick RGCs. Moreover, this forced DNA synthesis could not be prevented by Cdk4/6 inhibition, thus suggesting that it is triggered by a mechanism similar to endoreplication. In contrast, p27^Kip1 deficiency in mouse RGCs does not lead to increased ploidy despite previous observations have shown ectopic DNA synthesis in RGCs from p27^Kip1−/− mice. This suggests that a differential mechanism is used for the regulation of neuronal endoreplication in mammalian versus avian RGCs.

Oxidative stress promotes pathologic polyploidization in nonalcoholic fatty liver disease

Polyploidization is one of the most dramatic changes that can occur in the genome. In the liver, physiological polyploidization events occur during both liver development and throughout adult life. Here, we determined that a pathological polyploidization takes place in nonalcoholic fatty liver disease (NAFLD), a widespread hepatic metabolic disorder that is believed to be a risk factor for hepatocellular carcinoma (HCC). In murine models of NAFLD, the parenchyma of fatty livers displayed alterations of the polyploidization process, including the presence of a large proportion of highly polyploid mononuclear cells, which are rarely observed in normal hepatic parenchyma. Biopsies from patients with nonalcoholic steatohepatitis (NASH) revealed the presence of alterations in hepatocyte ploidy compared with tissue from control individuals. Hepatocytes from NAFLD mice revealed that progression through the S/G2 phases of the cell cycle was inefficient. This alteration was associated with activation of a G2/M DNA damage checkpoint, which prevented activation of the cyclin B1/CDK1 complex. Furthermore, we determined that oxidative stress promotes the appearance of highly polyploid cells, and antioxidant-treated NAFLD hepatocytes resumed normal cell division and returned to a physiological state of polyploidy. Collectively, these findings indicate that oxidative stress promotes pathological polyploidization and suggest that this is an early event in NAFLD that may contribute to HCC development.

TALEN-mediated somatic mutagenesis in murine models of cancer

Cancer genome sequencing has identified numerous somatic mutations whose biologic relevance is uncertain. In this study, we used genome-editing tools to create and analyze targeted somatic mutations in murine models of liver cancer. Transcription activator-like effector nucleases (TALEN) were designed against β-catenin (Ctnnb1) and adenomatous polyposis coli (Apc), two commonly mutated genes in hepatocellular carcinoma (HCC), to generate isogenic HCC cell lines. Both mutant cell lines exhibited evidence of Wnt pathway dysregulation. We asked whether these TALENs could create targeted somatic mutations after hydrodynamic transfection into mouse liver. TALENs targeting β-catenin promoted endogenous HCC carrying the intended gain-of-function mutations. However, TALENs targeting Apc were not as efficient in inducing in vivo homozygous loss-of-function mutations. We hypothesized that hepatocyte polyploidy might be protective against TALEN-induced loss of heterozygosity, and indeed Apc gene editing was less efficient in tetraploid than in diploid hepatocytes. To increase efficiency, we administered adenoviral Apc TALENs and found that we could achieve a higher mutagenesis rate in vivo. Our results demonstrate that genome-editing tools can enable the in vivo study of cancer genes and faithfully recapitulate the mosaic nature of mutagenesis in mouse cancer models. Cancer Res; 74(18); 5311-21. ©2014 AACR.

Endocycles: a recurrent evolutionary innovation for post-mitotic cell growth

In endoreplication cell cycles, known as endocycles, cells successively replicate their genomes without segregating chromosomes during mitosis and thereby become polyploid. Such cycles, for which there are many variants, are widespread in protozoa, plants and animals. Endocycling cells can achieve ploidies of >200,000 C (chromatin-value); this increase in genomic DNA content allows a higher genomic output, which can facilitate the construction of very large cells or enhance macromolecular secretion. These cells execute normal S phases, using a G1-S regulatory apparatus similar to the one used by mitotic cells, but their capability to segregate chromosomes has been suppressed, typically by downregulation of mitotic cyclin-dependent kinase activity. Endocycles probably evolved many times, and the various endocycle mechanisms found in nature highlight the versatility of the cell cycle control machinery.

Phenobarbital induces cell cycle transcriptional responses in mouse liver humanized for constitutive androstane and pregnane x receptors

The constitutive androstane receptor (CAR) and the pregnane X receptor (PXR) are closely related nuclear receptors involved in drug metabolism and play important roles in the mechanism of phenobarbital (PB)-induced rodent nongenotoxic hepatocarcinogenesis. Here, we have used a humanized CAR/PXR mouse model to examine potential species differences in receptor-dependent mechanisms underlying liver tissue molecular responses to PB. Early and late transcriptomic responses to sustained PB exposure were investigated in liver tissue from double knock-out CAR and PXR (CAR(KO)-PXR(KO)), double humanized CAR and PXR (CAR(h)-PXR(h)), and wild-type C57BL/6 mice. Wild-type and CAR(h)-PXR(h) mouse livers exhibited temporally and quantitatively similar transcriptional responses during 91 days of PB exposure including the sustained induction of the xenobiotic response gene Cyp2b10, the Wnt signaling inhibitor Wisp1, and noncoding RNA biomarkers from the Dlk1-Dio3 locus. Transient induction of DNA replication (Hells, Mcm6, and Esco2) and mitotic genes (Ccnb2, Cdc20, and Cdk1) and the proliferation-related nuclear antigen Mki67 were observed with peak expression occurring between 1 and 7 days PB exposure. All these transcriptional responses were absent in CAR(KO)-PXR(KO) mouse livers and largely reversible in wild-type and CAR(h)-PXR(h) mouse livers following 91 days of PB exposure and a subsequent 4-week recovery period. Furthermore, PB-mediated upregulation of the noncoding RNA Meg3, which has recently been associated with cellular pluripotency, exhibited a similar dose response and perivenous hepatocyte-specific localization in both wild-type and CAR(h)-PXR(h) mice. Thus, mouse livers coexpressing human CAR and PXR support both the xenobiotic metabolizing and the proliferative transcriptional responses following exposure to PB.

Epidermal growth factor receptor plays a role in the regulation of liver and plasma lipid levels in adult male mice

Dsk5 mice have a gain of function in the epidermal growth factor receptor (EGFR), caused by a point mutation in the kinase domain. We analyzed the effect of this mutation on liver size, histology, and composition. We found that the livers of 12-wk-old male Dsk5 heterozygotes (+/Dsk5) were 62% heavier compared with those of wild-type controls (+/+). The livers of the +/Dsk5 mice compared with +/+ mice had larger hepatocytes with prominent, polyploid nuclei and showed modestly increased cell proliferation indices in both hepatocytes and nonparenchymal cells. An analysis of total protein, DNA, and RNA (expressed relative to liver weight) revealed no differences between the mutant and wild-type mice. However, the livers of the +/Dsk5 mice had more cholesterol but less phospholipid and fatty acid. Circulating cholesterol levels were twice as high in adult male +/Dsk5 mice but not in postweaned young male or female mice. The elevated total plasma cholesterol resulted mainly from an increase in low-density lipoprotein (LDL). The +/Dsk5 adult mouse liver expressed markedly reduced protein levels of LDL receptor, no change in proprotein convertase subtilisin/kexin type 9, and a markedly increased fatty acid synthase and 3-hydroxy-3-methyl-glutaryl-CoA reductase. Increased expression of transcription factors associated with enhanced cholesterol synthesis was also observed. Together, these findings suggest that the EGFR may play a regulatory role in hepatocyte proliferation and lipid metabolism in adult male mice, explaining why elevated levels of EGF or EGF-like peptides have been positively correlated to increased cholesterol levels in human studies.

Hyperdiploid tumor cells increase phenotypic heterogeneity within Glioblastoma tumors

Here we report the identification of a proliferative, viable, and hyperdiploid tumor cell subpopulation present within Glioblastoma (GB) patient tumors. Using xenograft tumor models, we demonstrate that hyperdiploid cell populations are maintained in xenograft tumors and that clonally expanded hyperdiploid cells support tumor formation and progression in vivo. In some patient tumorsphere lines, hyperdiploidy is maintained during long-term culture and in vivo within xenograft tumor models, suggesting that hyperdiploidy can be a stable cell state. In other patient lines hyperdiploid cells display genetic drift in vitro and in vivo, suggesting that in these patients hyperdiploidy is a transient cell state that generates novel phenotypes, potentially facilitating rapid tumor evolution. We show that the hyperdiploid cells are resistant to conventional therapy, in part due to infrequent cell division due to a delay in the G₀/G₁ phase of the cell cycle. Hyperdiploid tumor cells are significantly larger and more metabolically active than euploid cancer cells, and this correlates to an increased sensitivity to the effects of glycolysis inhibition. Together these data identify GB hyperdiploid tumor cells as a potentially important subpopulation of cells that are well positioned to contribute to tumor evolution and disease recurrence in adult brain cancer patients, and suggest tumor metabolism as a promising point of therapeutic intervention against this subpopulation.

Haplotype assembly in polyploid genomes and identical by descent shared tracts

Motivation: Genome-wide haplotype reconstruction from sequence data, or haplotype assembly, is at the center of major challenges in molecular biology and life sciences. For complex eukaryotic organisms like humans, the genome is vast and the population samples are growing so rapidly that algorithms processing high-throughput sequencing data must scale favorably in terms of both accuracy and computational efficiency. Furthermore, current models and methodologies for haplotype assembly (i) do not consider individuals sharing haplotypes jointly, which reduces the size and accuracy of assembled haplotypes, and (ii) are unable to model genomes having more than two sets of homologous chromosomes (polyploidy). Polyploid organisms are increasingly becoming the target of many research groups interested in the genomics of disease, phylogenetics, botany and evolution but there is an absence of theory and methods for polyploid haplotype reconstruction.

Results: In this work, we present a number of results, extensions and generalizations of compass graphs and our HapCompass framework. We prove the theoretical complexity of two haplotype assembly optimizations, thereby motivating the use of heuristics. Furthermore, we present graph theory–based algorithms for the problem of haplotype assembly using our previously developed HapCompass framework for (i) novel implementations of haplotype assembly optimizations (minimum error correction), (ii) assembly of a pair of individuals sharing a haplotype tract identical by descent and (iii) assembly of polyploid genomes. We evaluate our methods on 1000 Genomes Project, Pacific Biosciences and simulated sequence data.

Availability and Implementation: HapCompass is available for download at http://www.brown.edu/Research/Istrail_Lab/.

Contact:Sorin_Istrail@brown.edu

Supplementary information:Supplementary data are available at Bioinformatics online.

Mechanisms of liver repair following injury

Liver injury caused by a variety of physical or chemical factors is a common disease, and severe or persistent liver injury can ultimately lead to acute liver failure. Its treatment is still a formidable challenge to clinicians. Elucidation of mechanisms underlying liver repair following injury is the cornerstone of treatment of hepatic diseases. Despite many research efforts over the past decades, the mechanisms behind liver repair following injury are still not clear. Recent studies have demonstrated that oval cells and bone marrow stem cells are involved in this complex process. A variety of cells and factors may play a role in different stages of this process. In this paper, we will review mechanisms of liver repair following injury.

The transcription factor FOXM1 (Forkhead box M1): proliferation-specific expression, transcription factor function, target genes, mouse models, and normal biological roles

FOXM1 (Forkhead box M1) is a typical proliferation-associated transcription factor, which stimulates cell proliferation and exhibits a proliferation-specific expression pattern. Accordingly, both the expression and the transcriptional activity of FOXM1 are increased by proliferation signals, but decreased by antiproliferation signals, including the positive and negative regulation by protooncoproteins or tumor suppressors, respectively. FOXM1 stimulates cell cycle progression by promoting the entry into S-phase and M-phase. Moreover, FOXM1 is required for proper execution of mitosis. Accordingly, FOXM1 regulates the expression of genes, whose products control G1/S-transition, S-phase progression, G2/M-transition, and M-phase progression. Additionally, FOXM1 target genes encode proteins with functions in the execution of DNA replication and mitosis. FOXM1 is a transcriptional activator with a forkhead domain as DNA binding domain and with a very strong acidic transactivation domain. However, wild-type FOXM1 is (almost) inactive because the transactivation domain is repressed by three inhibitory domains. Inactive FOXM1 can be converted into a very potent transactivator by activating signals, which release the transactivation domain from its inhibition by the inhibitory domains. FOXM1 is essential for embryonic development and the foxm1 knockout is embryonically lethal. In adults, FOXM1 is important for tissue repair after injury. FOXM1 prevents premature senescence and interferes with contact inhibition. FOXM1 plays a role for maintenance of stem cell pluripotency and for self-renewal capacity of stem cells. The functions of FOXM1 in prevention of polyploidy and aneuploidy and in homologous recombination repair of DNA-double-strand breaks suggest an importance of FOXM1 for the maintenance of genomic stability and chromosomal integrity.

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.

Transformation of somatic cells into stem cell-like cells under a stromal niche

To study the genomic plasticity of somatic cells without ectopic genetic manipulation, we cultured mouse fibroblasts with ovarian cells, embryonic fibroblasts of different strains, and parthenogenetic embryonic stem cells (ESCs). Of 41 trials, cell aggregation resembling nascent ESC colony from inner cell mass was detected in 9 cases (22%), and 6 cases (67%) yielded fibroblast-derived colonies with ESC morphology. Cells used in coculture provided the critical (P=0.0061) inducing factor for the aggregation. These colony-forming fibroblasts (CFFs) showed similar characteristics to those in ESCs and induced pluripotent stem cells (iPSCs), including pluripotency gene expression, in vitro differentiation, and teratoma formation. Furthermore, CFFs produced somatic chimera, although none showed germline chimerism. CFFs had a tetraploid-like karyotype, and their imprinting patterns differed from parthenogenetic ESCs, thereby confirming their nongermline transmissibility. We observed dysregulation of cell cycle-related proteins, as well as both homologous and heterologous recombination of genomic single-nucleotide polymorphisms in CFFs. Our observations provide information on somatic cell plasticity, resulting in stemness or tumorigenesis, regardless of colony-forming cell progenitors in the fibroblast population. The plasticity of somatic genomes under environmental influences, as well as acquisition of pluripotency by cell fusion, is also implicated.

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.

Cellular mechanisms of circadian pacemaking: beyond transcriptional loops

Circadian clocks drive the daily rhythms in our physiology and behaviour that adapt us to the 24-h solar and social worlds. Because they impinge upon every facet of metabolism, their acute or chronic disruption compromises performance (both physical and mental) and systemic health, respectively. Equally, the presence of such rhythms has significant implications for pharmacological dynamics and efficacy, because the fate of a drug and the state of its therapeutic target will vary as a function of time of day. Improved understanding of the cellular and molecular biology of circadian clocks therefore offers novel approaches for therapeutic development, for both clock-related and other conditions. At the cellular level, circadian clocks are pivoted around a transcriptional/post-translational delayed feedback loop (TTFL) in which the activation of Period and Cryptochrome genes is negatively regulated by their cognate protein products. Synchrony between these, literally countless, cellular clocks across the organism is maintained by the principal circadian pacemaker, the suprachiasmatic nucleus (SCN) of the hypothalamus. Notwithstanding the success of the TTFL model, a diverse range of experimental studies has shown that it is insufficient to account for all properties of cellular pacemaking. Most strikingly, circadian cycles of metabolic status can continue in human red blood cells, devoid of nuclei and thus incompetent to sustain a TTFL. Recent interest has therefore focused on the role of oscillatory cytosolic mechanisms as partners to the TTFL. In particular, cAMP- and Ca²⁺-dependent signalling are important components of the clock, whilst timekeeping activity is also sensitive to a series of highly conserved kinases and phosphatases. This has led to the view that the 'proto-clock' may have been a cytosolic, metabolic oscillation onto which evolution has bolted TTFLs to provide robustness and amplify circadian outputs in the form of rhythmic gene expression. This evolutionary ascent of the clock has culminated in the SCN, a true pacemaker to the innumerable clock cells distributed across the body. On the basis of findings from our own and other laboratories, we propose a model of the SCN pacemaker that synthesises the themes of TTFLs, intracellular signalling, metabolic flux and interneuronal coupling that can account for its unique circadian properties and pre-eminence.

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.