Transient inactivation of Rb and ARF yields regenerative cells from postmitotic mammalian muscle

Mechanisms and translational applications of regeneration in limbs: From renewable animals to humans

BACKGROUND

The regenerative capacity of organisms declines throughout evolution, and mammals lack the ability to regenerate limbs after injury. Past approaches to achieving successful restoration through pharmacological intervention, tissue engineering, and cell therapies have faced significant challenges.

OBJECTIVES

This review aims to provide an overview of the current understanding of the mechanisms behind animal limb regeneration and the successful translation of these mechanisms for human tissue regeneration.

RESULTS

Particular attention was paid to the Mexican axolotl (Ambystoma mexicanum), the only adult tetrapod capable of limb regeneration. We will explore fundamental questions surrounding limb regeneration, such as how amputation initiates regeneration, how the limb knows when to stop and which parts to regenerate, and how these findings can apply to mammalian systems.

CONCLUSIONS

Given the urgent need for regenerative therapies to treat conditions like diabetic foot ulcers and trauma survivors, this review provides valuable insights and ideas for researchers, clinicians, and biomedical engineers seeking to facilitate the regeneration process or elicit full regeneration from partial regeneration events.

Bleomycin-treated myoblasts undergo p21-associated cellular senescence and have severely impaired differentiation

As we age, the ability to regenerate and repair skeletal muscle damage declines, partially due to increasing dysfunction of muscle resident stem cells-satellite cells (SC). Recent evidence implicates cellular senescence, which is the irreversible arrest of proliferation, as a potentiator of SC impairment during aging. However, little is known about the role of senescence in SC, and there is a large discrepancy in senescence classification within skeletal muscle. The purpose of this study was to develop a model of senescence in skeletal muscle myoblasts and identify how common senescence-associated biomarkers respond. Low-passage C2C12 myoblasts were treated with bleomycin or vehicle and then evaluated for cytological and molecular senescence markers, proliferation status, cell cycle kinetics, and differentiation potential. Bleomycin treatment caused double-stranded DNA breaks, which upregulated p21 mRNA and protein, potentially through NF-κB and senescence-associated super enhancer (SASE) signaling (p < 0.01). Consequently, cell proliferation was abruptly halted due to G2/M-phase arrest (p < 0.01). Bleomycin-treated myoblasts displayed greater senescence-associated β-galactosidase staining (p < 0.01), which increased over several days. These myoblasts remained senescent following 6 days of differentiation and had significant impairments in myotube formation (p < 0.01). Furthermore, our results show that senescence can be maintained despite the lack of p16 gene expression in C2C12 myoblasts. In conclusion, bleomycin treatment provides a valid model of damage-induced senescence that was associated with elevated p21, reduced myoblast proliferation, and aberrant cell cycle kinetics, while confirming that a multi-marker approach is needed for the accurate classification of senescence within skeletal muscle.

The roles of miRNAs in adult skeletal muscle satellite cells

Satellite cells are bona fide muscle stem cells that are indispensable for successful post-natal muscle growth and regeneration after severe injury. These cells also participate in adult muscle adaptation in several capacities. MicroRNAs (miRNAs) are post-transcriptional regulators of mRNA that are implicated in several aspects of stem cell function. There is evidence to suggest that miRNAs affect satellite cell behavior in vivo during development and myogenic progenitor behavior in vitro, but the role of miRNAs in adult skeletal muscle satellite cells is less studied. In this review, we provide evidence for how miRNAs control satellite cell function with emphasis on satellite cells of adult skeletal muscle in vivo. We first outline how miRNAs are indispensable for satellite cell viability and control the phases of myogenesis. Next, we discuss the interplay between miRNAs and myogenic cell redox status, senescence, and communication to other muscle-resident cells during muscle adaptation. Results from recent satellite cell miRNA profiling studies are also summarized. In vitro experiments in primary myogenic cells and cell lines have been invaluable for exploring the influence of miRNAs, but we identify a need for novel genetic tools to further interrogate how miRNAs control satellite cell behavior in adult skeletal muscle in vivo.

Significance of RB Loss in Unlocking Phenotypic Plasticity in Advanced Cancers

Cancer cells can undergo plasticity in response to environmental stimuli or under selective therapeutic pressures that result in changes in phenotype. This complex phenomenon of phenotypic plasticity is now recognized as a hallmark of cancer. Lineage plasticity is often associated with loss of dependence on the original oncogenic driver and is facilitated, in part, by underlying genomic and epigenetic alterations. Understanding the molecular drivers of cancer plasticity is critical for the development of novel therapeutic strategies. The retinoblastoma gene RB1 (encoding RB) is the first tumor suppressor gene to be discovered and has a well-described role in cell-cycle regulation. RB is also involved in diverse cellular functions beyond cell cycle including differentiation. Here, we describe the emerging role of RB loss in unlocking cancer phenotypic plasticity and driving therapy resistance across cancer types. We highlight parallels in cancer with the noncanonical role of RB that is critical for normal development and lineage specification, and the downstream consequences of RB loss including epigenetic reprogramming and chromatin reorganization that can lead to changes in lineage program. Finally, we discuss potential therapeutic approaches geared toward RB loss cancers undergoing lineage reprogramming.

Genomic and Epigenetic Changes Drive Aberrant Skeletal Muscle Differentiation in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS), the most common soft-tissue sarcoma in children and adolescents, represents an aberrant form of skeletal muscle differentiation. Both skeletal muscle development, as well as regeneration of adult skeletal muscle are governed by members of the myogenic family of regulatory transcription factors (MRFs), which are deployed in a highly controlled, multi-step, bidirectional process. Many aspects of this complex process are deregulated in RMS and contribute to tumorigenesis. Interconnected loops of super-enhancers, called core regulatory circuitries (CRCs), define aberrant muscle differentiation in RMS cells. The transcriptional regulation of MRF expression/activity takes a central role in the CRCs active in skeletal muscle and RMS. In PAX3::FOXO1 fusion-positive (PF+) RMS, CRCs maintain expression of the disease-driving fusion oncogene. Recent single-cell studies have revealed hierarchically organized subsets of cells within the RMS cell pool, which recapitulate developmental myogenesis and appear to drive malignancy. There is a large interest in exploiting the causes of aberrant muscle development in RMS to allow for terminal differentiation as a therapeutic strategy, for example, by interrupting MEK/ERK signaling or by interfering with the epigenetic machinery controlling CRCs. In this review, we provide an overview of the genetic and epigenetic framework of abnormal muscle differentiation in RMS, as it provides insights into fundamental mechanisms of RMS malignancy, its remarkable phenotypic diversity and, ultimately, opportunities for therapeutic intervention.

Transit Amplifying Cells (TACs): a still not fully understood cell population

Maintenance of tissue homeostasis and tissue regeneration after an insult are essential functions of adult stem cells (SCs). In adult tissues, SCs proliferate at a very slow rate within "stem cell niches", but, during tissue development and regeneration, before giving rise to differentiated cells, they give rise to multipotent and highly proliferative cells, known as transit-amplifying cells (TACs). Although differences exist in diverse tissues, TACs are not only a transitory phase from SCs to post-mitotic cells, but they also actively control proliferation and number of their ancestor SCs and proliferation and differentiation of their progeny toward tissue specific functional cells. Autocrine signals and negative and positive feedback and feedforward paracrine signals play a major role in these controls. In the present review we will consider the generation and the role played by TACs during development and regeneration of lining epithelia characterized by a high turnover including epidermis and hair follicles, ocular epithelial surfaces, and intestinal mucosa. A comparison between these different tissues will be made. There are some genes and molecular pathways whose expression and activation are common to most TACs regardless their tissue of origin. These include, among others, Wnt, Notch, Hedgehog and BMP pathways. However, the response to these molecular signals can vary in TACs of different tissues. Secondly, we will consider cultured cells derived from tissues of mesodermal origin and widely adopted for cell therapy treatments. These include mesenchymal stem cells and dedifferentiated chondrocytes. The possible correlation between cell dedifferentiation and reversion to a transit amplifying cell stage will be discussed.

Current view of liver cancer cell-of-origin and proposed mechanisms precluding its proper determination

Hepatocellular carcinoma and intrahepatic cholangiocarcinoma are devastating primary liver cancers with increasing prevalence in many parts of the world. Despite intense investigation, many aspects of their biology are still largely obscure. For example, numerous studies have tackled the question of the cell-of-origin of primary liver cancers using different experimental approaches; they have not, however, provided a clear and undisputed answer. Here, we will review the evidence from animal models supporting the role of all major types of liver epithelial cells: hepatocytes, cholangiocytes, and their common progenitor as liver cancer cell-of-origin. Moreover, we will also propose mechanisms that promote liver cancer cell plasticity (dedifferentiation, transdifferentiation, and epithelial-to-mesenchymal transition) which may contribute to misinterpretation of the results and which make the issue of liver cancer cell-of-origin particularly complex.

Plasticity of muscle stem cells in homeostasis and aging

We are living longer, but our healthspan has not increased. The goal of regenerative medicine is to increase quality of life through an understanding of the cellular and molecular processes that underlie effective tissue repair in order to restore damaged tissues. The drivers of muscle regeneration are the muscle stem cells that cycle between quiescent and activated states to meet tissue regenerative demands. Here we review recent findings on the role of the niche, or tissue microenvironment, in the modulation of muscle stem cell plasticity and the mechanisms responsible for the drastic loss of stem cell function with aging. These new studies unveil fundamental mechanisms of stem cell plasticity with broad relevance to other tissues and lay the foundation for the development of therapeutic strategies to boost the regenerative potential of aged muscle stem cells.

Axolotl: A resourceful vertebrate model for regeneration and beyond

The regenerative capacity varies significantly among the animal kingdom. Successful regeneration program in some animals results in the functional restoration of tissues and lost structures. Among the highly regenerative animals, axolotl provides multiple experimental advantages with its many extraordinary characteristics. It has been positioned as a regeneration model organism due to its exceptional renewal capacity, including the internal organs, central nervous system, and appendages, in a scar-free manner. In addition to this unique regeneration ability, the observed low cancer incidence, its resistance to carcinogens, and the reversing effect of its cell extract on neoplasms strongly suggest its usability in cancer research. Axolotl's longevity and efficient utilization of several anti-aging mechanisms underline its potential to be employed in aging studies.

Skeletal Muscle Nuclei in Mice are not Post-mitotic

The skeletal muscle research field generally accepts that nuclei in skeletal muscle fibers (ie, myonuclei) are post-mitotic and unable to proliferate. Because our deuterium oxide (D2O) labeling studies showed DNA synthesis in skeletal muscle tissue, we hypothesized that resident myonuclei can replicate in vivo. To test this hypothesis, we used a mouse model that temporally labeled myonuclei with GFP followed by D2O labeling during normal cage activity, functional overload, and with satellite cell ablation. During normal cage activity, we observed deuterium enrichment into myonuclear DNA in 7 out of 7 plantaris (PLA), 6 out of 6 tibialis anterior (TA), 5 out of 7 gastrocnemius (GAST), and 7 out of 7 quadriceps (QUAD). The average fractional synthesis rates (FSR) of DNA in myonuclei were: 0.0202 ± 0.0093 in PLA, 0.0239 ± 0.0040 in TA, 0.0076 ± 0. 0058 in GAST, and 0.0138 ± 0.0039 in QUAD, while there was no replication in myonuclei from EDL. These FSR values were largely reproduced in the overload and satellite cell ablation conditions, although there were higher synthesis rates in the overloaded PLA muscle. We further provided evidence that myonuclear replication is through endoreplication, which results in polyploidy. These novel findings contradict the dogma that skeletal muscle nuclei are post-mitotic and open potential avenues to harness the intrinsic replicative ability of myonuclei for muscle maintenance and growth.

Novel insights into RB1 mutation

Since the discovery of the retinoblastoma susceptibility gene (RB1) decades ago, RB1 has been regarded as a prototype tumor suppressor gene providing a paradigm for tumor genetic research. Constant research has updated the understanding of RB1-related pathways and their impact on tumor and nontumor diseases. Mutation of RB1 gene has been observed in multiple types of malignant tumors including prostate cancer, lung cancer, breast cancer, and almost every familial and sporadic case of retinoblastoma. Even if well-known and long-investigated, the application potential of RB1 mutation has not been fully tapped. In this review, we focus on the mechanism underlying RB1 mutation during oncogenesis. Therapeutically, we have further discussed potential clinical strategies by targeting RB1-mutated cancers. The unsolved problems and prospects of RB1 mutation are also discussed.

Abrogation of Rb Tumor Suppression Initiates GBM in Differentiated Astrocytes by Driving a Progenitor Cell Program

Glioblastoma (GBM) remains lethal with no effective treatments. Despite the comprehensive identification of commonly perturbed molecular pathways, little is known about the disease’s etiology, particularly in early stages. Several studies indicate that GBM is initiated in neural progenitor and/or stem cells. Here, we report that differentiated astrocytes are susceptible to GBM development when initiated by perturbation of the RB pathway, which induces a progenitor phenotype. In vitro and in vivo inactivation of Rb tumor suppression (TS) induces cortical astrocytes to proliferate rapidly, express progenitor markers, repress differentiation markers, and form self-renewing neurospheres that are susceptible to multi-lineage differentiation. This phenotype is sufficient to cause grade II astrocytomas which stochastically progress to GBM. Together with previous findings, these results demonstrate that cell susceptibility to GBM depends on the initiating driver.

The role of senescence in cellular plasticity: Lessons from regeneration and development and implications for age-related diseases

Senescence is a cellular state which involves cell cycle arrest and a proinflammatory phenotype, and it has traditionally been associated with cellular and organismal aging. However, increasing evidence suggests key roles in tissue growth and regrowth, especially during development and regeneration. Conversely, cellular plasticity-the capacity of cells to undergo identity change, including differentiation and dedifferentiation-is associated with development and regeneration but is now being investigated in the context of age-related diseases such as Alzheimer disease. Here, we discuss the paradox of the role for cellular senescence in cellular plasticity: senescence can act as a cell-autonomous barrier and a paracrine driver of plasticity. We provide a conceptual framework for integrating recent data and use the interplay between cellular senescence and plasticity to provide insight into age-related diseases. Finally, we argue that age-related diseases can be better deciphered when senescence is recognized as a core mechanism of regeneration and development.

A remarkable rodent: Regeneration and reproduction in spiny mice (Acomys)

Although certain organisms are chosen and employed to better understand a specific problem in biology (so-called model organisms), sometimes an animal model reveals its' biomedical importance by happenstance. In many ways, the advent of spiny mice (Acomys) as an emerging model to study regeneration and menstruation stands as a case study in scientific pseudoserendipity (Diaz de Chumaceiro, 1995). As we recount in this chapter, the discovery of these phenotypes, while not entirely accidental, was nonetheless unexpected. In addition to recounting how we uncovered these unusual mammalian traits, we outline recent work by our groups and others that has begun to outline the cellular and genetic mechanisms underlying bonafide mammalian tissue regeneration and a human-like mode of reproduction in spiny mice.

Comprehensive expression analysis for the core cell cycle regulators in the chicken embryo reveals novel tissue-specific synexpression groups and similarities and differences with expression in mouse, frog and zebrafish

The core cell cycle machinery is conserved from yeast to humans, and hence it is assumed that all vertebrates share the same set of players. Yet during vertebrate evolution, the genome was duplicated twice, followed by a further genome duplication in teleost fish. Thereafter, distinct genes were retained in different vertebrate lineages; some individual gene duplications also occurred. To which extent these diversifying tendencies were compensated by retaining the same expression patterns across homologous genes is not known. This study for the first time undertook a comprehensive expression analysis for the core cell cycle regulators in the chicken, focusing in on early neurula and pharyngula stages of development, with the latter representing the vertebrate phylotypic stage. We also compared our data with published data for the mouse, Xenopus and zebrafish, the other established vertebrate models. Our work shows that, while many genes are expressed widely, some are upregulated or specifically expressed in defined tissues of the chicken embryo, forming novel synexpression groups with markers for distinct developmental pathways. Moreover, we found that in the neural tube and in the somite, mRNAs of some of the genes investigated accumulate in a specific subcellular localisation, pointing at a novel link between the site of mRNA translation, cell cycle control and interkinetic nuclear movements. Finally, we show that expression patterns of orthologous genes may differ in the four vertebrate models. Thus, for any study investigating cell proliferation, cell differentiation, tissue regeneration, stem cell behaviour and cancer/cancer therapy, it has to be carefully examined which of the observed effects are due to the specific model organism used, and which can be generalised.

Restoring the Cell Cycle and Proliferation Competence in Terminally Differentiated Skeletal Muscle Myotubes

Terminal differentiation is an ill-defined, insufficiently characterized, nonproliferation state. Although it has been classically deemed irreversible, it is now clear that at least several terminally differentiated (TD) cell types can be brought back into the cell cycle. We are striving to uncover the molecular bases of terminal differentiation, whose fundamental understanding is a goal in itself. In addition, the field has sought to acquire the ability to make TD cells proliferate. Attaining this end would probe the very molecular mechanisms we are trying to understand. Equally important, it would be invaluable in regenerative medicine, for tissues depending on TD cells and devoid of significant self-repair capabilities. The skeletal muscle has long been used as a model system to investigate the molecular foundations of terminal differentiation. Here, we summarize more than 50 years of studies in this field.

Salamander Insights Into Ageing and Rejuvenation

Exhibiting extreme regenerative abilities which extend to complex organs and entire limbs, salamanders have long served as research models for understanding the basis of vertebrate regeneration. Yet these organisms display additional noteworthy traits, namely extraordinary longevity, indefinite regenerative potential and apparent lack of traditional signs of age-related decay or “negligible senescence.” Here, I examine existing studies addressing these features, highlight outstanding questions, and argue that salamanders constitute valuable models for addressing the nature of organismal senescence and the interplay between regeneration and ageing.

Role of Cdkn2a in the Emery-Dreifuss Muscular Dystrophy Cardiac Phenotype

The Cdkn2a locus is one of the most studied tumor suppressor loci in the context of several cancer types. However, in the last years, its expression has also been linked to terminal differentiation and the activation of the senescence program in different cellular subtypes. Knock-out (KO) of the entire locus enhances the capability of stem cells to proliferate in some tissues and respond to severe physiological and non-physiological damages in different organs, including the heart. Emery-Dreifuss muscular dystrophy (EDMD) is characterized by severe contractures and muscle loss at the level of skeletal muscles of the elbows, ankles and neck, and by dilated cardiomyopathy. We have recently demonstrated, using the LMNA Δ8-11 murine model of Emery-Dreifuss muscular dystrophy (EDMD), that dystrophic muscle stem cells prematurely express non-lineage-specific genes early on during postnatal growth, leading to rapid exhaustion of the muscle stem cell pool. Knock-out of the Cdkn2a locus in EDMD dystrophic mice partially restores muscle stem cell properties. In the present study, we describe the cardiac phenotype of the LMNA Δ8-11 mouse model and functionally characterize the effects of KO of the Cdkn2a locus on heart functions and life expectancy.

Study of Natural Longlife Juvenility and Tissue Regeneration in Caudate Amphibians and Potential Application of Resulting Data in Biomedicine

The review considers the molecular, cellular, organismal, and ontogenetic properties of Urodela that exhibit the highest regenerative abilities among tetrapods. The genome specifics and the expression of genes associated with cell plasticity are analyzed. The simplification of tissue structure is shown using the examples of the sensory retina and brain in mature Urodela. Cells of these and some other tissues are ready to initiate proliferation and manifest the plasticity of their phenotype as well as the correct integration into the pre-existing or de novo forming tissue structure. Without excluding other factors that determine regeneration, the pedomorphosis and juvenile properties, identified on different levels of Urodele amphibians, are assumed to be the main explanation for their high regenerative abilities. These properties, being fundamental for tissue regeneration, have been lost by amniotes. Experiments aimed at mammalian cell rejuvenation currently use various approaches. They include, in particular, methods that use secretomes from regenerating tissues of caudate amphibians and fish for inducing regenerative responses of cells. Such an approach, along with those developed on the basis of knowledge about the molecular and genetic nature and age dependence of regeneration, may become one more step in the development of regenerative medicine.

A comparative perspective on lung and gill regeneration

The ability to continuously grow and regenerate the gills throughout life is a remarkable property of fish and amphibians. Considering that gill regeneration was first described over one century ago, it is surprising that the underlying mechanisms of cell and tissue replacement in the gills remain poorly understood. By contrast, the mammalian lung is a largely quiescent organ in adults but is capable of facultative regeneration following injury. In the course of the past decade, it has been recognized that lungs contain a population of stem or progenitor cells with an extensive ability to restore tissue; however, despite recent advances in regenerative biology of the lung, the signaling pathways that underlie regeneration are poorly understood. In this Review, we discuss the common evolutionary and embryological origins shared by gills and mammalian lungs. These are evident in homologies in tissue structure, cell populations, cellular function and genetic pathways. An integration of the literature on gill and lung regeneration in vertebrates is presented using a comparative approach in order to outline the challenges that remain in these areas, and to highlight the importance of using aquatic vertebrates as model organisms. The study of gill regeneration in fish and amphibians, which have a high regenerative potential and for which genetic tools are widely available, represents a unique opportunity to uncover common signaling mechanisms that may be important for regeneration of respiratory organs in all vertebrates. This may lead to new advances in tissue repair following lung disease.

The PRC2 complex directly regulates the cell cycle and controls proliferation in skeletal muscle

The polycomb repressive complex 2 (PRC2) is an important developmental regulator responsible for the methylation of histone 3 lysine 27 (H3K27). Here, we show that the PRC2 complex regulates the cell cycle in skeletal muscle cells to control proliferation and mitotic exit. Depletions of the catalytic subunit of the PRC2 complex, EZH2, have shown that EZH2 is required for cell viability, suggesting that EZH2 promotes proliferation. We found that EZH2 directly represses both positive and negative cell cycle genes, thus enabling the PRC2 complex to tightly control the cell cycle. We show that modest inhibition or depletion of EZH2 leads to enhanced proliferation and an accumulation of cells in S phase. This effect is mediated by direct repression of cyclin D1 (Ccnd1) and cyclin E1 (Ccne1) by the PRC2 complex. Our results show that PRC2 has pleiotropic effects on proliferation as it serves to restrain cell growth, yet clearly has a function required for cell viability as well. Intriguingly, we also find that the retinoblastoma protein gene (Rb1) is a direct target of the PRC2 complex. However, modest depletion of EZH2 is not sufficient to maintain Rb1 expression, indicating that the PRC2 dependent upregulation of cyclin D1 is sufficient to inhibit Rb1 expression. Taken together, our results show that the PRC2 complex regulates skeletal muscle proliferation in a complex manner that involves the repression of Ccnd1 and Ccne1, thus restraining proliferation, and the repression of Rb1, which is required for mitotic exit and terminal differentiation.

Bach1 promotes muscle regeneration through repressing Smad-mediated inhibition of myoblast differentiation

It has been reported that Bach1-deficient mice show reduced tissue injuries in diverse disease models due to increased expression of heme oxygenase-1 (HO-1)that possesses an antioxidant function. In contrast, we found that Bach1 deficiency in mice exacerbated skeletal muscle injury induced by cardiotoxin. Inhibition of Bach1 expression in C2C12 myoblast cells using RNA interference resulted in reduced proliferation, myotube formation, and myogenin expression compared with control cells. While the expression of HO-1 was increased by Bach1 silencing in C2C12 cells, the reduced myotube formation was not rescued by HO-1 inhibition. Up-regulations of Smad2, Smad3 and FoxO1, known inhibitors of muscle cell differentiation, were observed in Bach1-deficient mice and Bach1-silenced C2C12 cells. Therefore, Bach1 may promote regeneration of muscle by increasing proliferation and differentiation of myoblasts.

Mitochondrial division inhibitor 1 (mdivi-1) increases oxidative capacity and contractile stress generated by engineered skeletal muscle

In skeletal muscle fibers, mitochondria are densely packed adjacent to myofibrils because adenosine triphosphate (ATP) is needed to fuel sarcomere shortening. However, despite this close physical and biochemical relationship, the effects of mitochondrial dynamics on skeletal muscle contractility are poorly understood. In this study, we analyzed the effects of Mitochondrial Division Inhibitor 1 (mdivi-1), an inhibitor of mitochondrial fission, on the structure and function of both mitochondria and myofibrils in skeletal muscle tissues engineered on micromolded gelatin hydrogels. Treatment with mdivi-1 did not alter myotube morphology, but did increase the mitochondrial turbidity and oxidative capacity, consistent with reduced mitochondrial fission. Mdivi-1 also significantly increased basal, twitch, and tetanus stresses, as measured using the Muscular Thin Film (MTF) assay. Finally, mdivi-1 increased sarcomere length, potentially due to mdivi-1-induced changes in mitochondrial volume and compression of myofibrils. Together, these results suggest that mdivi-1 increases contractile stress generation, which may be caused by an increase in maximal respiration and/or sarcomere length due to increased volume of individual mitochondria. These data reinforce that mitochondria have both biochemical and biomechanical roles in skeletal muscle and that mitochondrial dynamics can be manipulated to alter muscle contractility.

Human ARF Specifically Inhibits Epimorphic Regeneration in the Zebrafish Heart

The Alternative Reading Frame (ARF) protein is a tumor suppressor encoded by the Cyclin Dependent Kinase Inhibitor 2A gene in mammals but not lower regenerative vertebrates, and has been previously implicated as a context-sensitive suppressor of regeneration in murine skeletal muscle and humanized ARF-expressing zebrafish fins. This study extends our investigation of the role of ARF in the regeneration of other solid tissues, including the zebrafish heart and the mammalian digit. Heart regeneration after cryoinjury was used to mimic massive myocardial infarction. ARF gene expression was upregulated during the cardiac regenerative process and slowed the rate of morphological recovery. ARF specifically impacts cardiomyocytes, neovascularization, and the endothelial-mesenchymal transition, while not affecting epicardial proliferation. This suggests that in the context of regeneration, ARF is specifically expressed in cells undergoing dedifferentiation. To investigate ARF as a suppressor of epimorphic regeneration in mammalian systems, we also tested whether the absence of ARF was permissive for murine digit regeneration, but found that ARF absence alone was insufficient to significantly alter digit restoration. These findings provide additional evidence that ARF suppresses epimorphic regeneration, but suggests that modulation of ARF alone is insufficient to permit regeneration.

Tumor Milieu Controlled by RB Tumor Suppressor

The RB gene is one of the most frequently mutated genes in human cancers. Canonically, RB exerts its tumor suppressive activity through the regulation of the G1/S transition during cell cycle progression by modulating the activity of E2F transcription factors. However, aberration of the RB gene is most commonly detected in tumors when they gain more aggressive phenotypes, including metastatic activity or drug resistance, rather than accelerated proliferation. This implicates RB controls' malignant progression to a considerable extent in a cell cycle-independent manner. In this review, we highlight the multifaceted functions of the RB protein in controlling tumor lineage plasticity, metabolism, and the tumor microenvironment (TME), with a focus on the mechanism whereby RB controls the TME. In brief, RB inactivation in several types of cancer cells enhances production of pro-inflammatory cytokines, including CCL2, through upregulation of mitochondrial reactive oxygen species (ROS) production. These factors not only accelerate the growth of cancer cells in a cell-autonomous manner, but also stimulate non-malignant cells in the TME to generate a pro-tumorigenic niche in a non-cell-autonomous manner. Here, we discuss the biological and pathological significance of the non-cell-autonomous functions of RB and attempt to predict their potential clinical relevance to cancer immunotherapy.

JARID2 and the PRC2 complex regulate the cell cycle in skeletal muscle

JARID2 is a noncatalytic member of the polycomb repressive complex 2 (PRC2) which methylates of histone 3 lysine 27 (H3K27). In this work, we show that JARID2 and the PRC2 complex regulate the cell cycle in skeletal muscle cells to control proliferation and mitotic exit. We found that the stable depletion of JARID2 leads to increased proliferation and cell accumulation in S phase. The regulation of the cell cycle by JARID2 is mediated by direct repression of both cyclin D1 and cyclin E1, both of which are targets of PRC2-mediated H3K27 methylation. Intriguingly, we also find that the retinoblastoma protein (RB1) is a direct target of JARID2 and the PRC2 complex. The depletion of JARID2 is not sufficient to activate RB1. However, the ectopic expression of RB1 can suppress cyclin D1 expression in JARID2-depleted cells. Transient depletion of JARID2 in skeletal muscle cells leads to a transient up-regulation of cyclin D1 that is quickly suppressed with no resulting effect on proliferation, Taken together, we show that JARID2 and the PRC2 complex regulate skeletal muscle proliferation in a precise manner that involves the repression of cyclin D1, thus restraining proliferation and repressing RB1, which is required for mitotic exit and terminal differentiation.

Can laboratory model systems instruct human limb regeneration?

Regeneration has fascinated scientists since well before the 20th century revolutions in genetics and molecular biology. The field of regenerative biology has grown steadily over the past decade, incorporating advances in imaging, genomics and genome editing to identify key cell types and molecules involved across many model organisms. Yet for many or most tissues, it can be difficult to predict when and how findings from these studies will advance regenerative medicine. Establishing technologies to stimulate regrowth of a lost or amputated limb with a patterned replicate, as salamanders do routinely, is one of the most challenging directives of tissue regeneration research. Here, we speculate upon what research avenues the field must explore to move closer to this capstone achievement.

Connective tissue fibroblasts from highly regenerative mammals are refractory to ROS-induced cellular senescence

A surveillance system in mammals constantly monitors cell activity to protect against aberrant proliferation in response to damage, injury and oncogenic stress. Here we isolate and culture connective tissue fibroblasts from highly regenerative mammals (Acomys and Oryctolagus) to determine how these cells interpret signals that normally induce cellular senescence in non-regenerating mammals (Mus and Rattus). While H₂O₂ exposure substantially decreases cell proliferation and increases p53, p21, p16, and p19 in cells from mice and rats, cells from spiny mice and rabbits are highly resistant to H₂O₂. Quantifying oxygen consumption and mitochondrial stability, we demonstrate that increased intracellular H₂O₂ is rapidly detoxified in regenerating species, but overwhelms antioxidant scavenging in cells from non-regenerative mammals. However, pretreatment with N-acetylcysteine (NAC) protects mouse and rat cells from ROS-induced cellular senescence. Collectively, our results show that intrinsic cellular differences in stress-sensing mechanisms partially explain interspecific variation in regenerative ability.

In regenerative animals, how cells respond to injury signals inducing senescence is unclear. Here, the authors show that cells from highly regenerative mammals are resistant to ROS-induced cellular senescence, but non-regenerating species exhibit mitochondrial dysfunction/senescence in response to hydrogen peroxide exposure.

Common themes in tetrapod appendage regeneration: a cellular perspective

Complete and perfect regeneration of appendages is a process that has fascinated and perplexed biologists for centuries. Some tetrapods possess amazing regenerative abilities, but the regenerative abilities of others are exceedingly limited. The reasons underlying these differences have largely remained mysterious. A great deal has been learned about the morphological events that accompany successful appendage regeneration, and a handful of experimental manipulations can be reliably applied to block the process. However, only in the last decade has the goal of attaining a thorough molecular and cellular biological understanding of appendage regeneration in tetrapods become within reach. Advances in molecular and genetic tools for interrogating these remarkable events are now allowing for unprecedented access to the fundamental biology at work in appendage regeneration in a variety of species. This information will be critical for integrating the large body of detailed observations from previous centuries with a modern understanding of how cells sense and respond to severe injury and loss of body parts. Understanding commonalities between regenerative modes across diverse species is likely to illuminate the most important aspects of complex tissue regeneration.

Quiescence Entry, Maintenance, and Exit in Adult Stem Cells

Cells of unicellular and multicellular eukaryotes can respond to certain environmental cues by arresting the cell cycle and entering a reversible state of quiescence. Quiescent cells do not divide, but can re-enter the cell cycle and resume proliferation if exposed to some signals from the environment. Quiescent cells in mammals and humans include adult stem cells. These cells exhibit improved stress resistance and enhanced survival ability. In response to certain extrinsic signals, adult stem cells can self-renew by dividing asymmetrically. Such asymmetric divisions not only allow the maintenance of a population of quiescent cells, but also yield daughter progenitor cells. A multistep process of the controlled proliferation of these progenitor cells leads to the formation of one or more types of fully differentiated cells. An age-related decline in the ability of adult stem cells to balance quiescence maintenance and regulated proliferation has been implicated in many aging-associated diseases. In this review, we describe many traits shared by different types of quiescent adult stem cells. We discuss how these traits contribute to the quiescence, self-renewal, and proliferation of adult stem cells. We examine the cell-intrinsic mechanisms that allow establishing and sustaining the characteristic traits of adult stem cells, thereby regulating quiescence entry, maintenance, and exit.

Tumor Suppressors RB1 and CDKN2a Cooperatively Regulate Cell-Cycle Progression and Differentiation During Cardiomyocyte Development and Repair

RATIONALE

Although rare cardiomyogenesis is reported in the adult mammalian heart, whether this results from differentiation or proliferation of cardiomyogenic cells remains controversial. The tumor suppressor genes RB1 (retinoblastoma) and CDKN2a (cyclin-dependent kinase inhibitor 2a) are critical cell-cycle regulators, but their roles in human cardiomyogenesis remains unclear.

OBJECTIVE

We hypothesized that developmental activation of RB1 and CDKN2a cooperatively cause permanent cell-cycle withdrawal of human cardiac precursors (CPCs) driving terminal differentiation into mature cardiomyocytes, and that dual inactivation of these tumor suppressor genes promotes myocyte cell-cycle reentry.

METHODS AND RESULTS

Directed differentiation of human pluripotent stem cells (hPSCs) into cardiomyocytes revealed that RB1 and CDKN2a are upregulated at the onset of cardiac precursor specification, simultaneously with GATA4 (GATA-binding protein 4) homeobox genes PBX1 (pre-B-cell leukemia transcription factor 1) and MEIS1 (myeloid ecotropic viral integration site 1 homolog), and remain so until terminal cardiomyocyte differentiation. In both GATA4+ hPSC cardiac precursors and postmitotic hPSC-cardiomyocytes, RB1 is hyperphosphorylated and inactivated. Transient, stage-specific, depletion of RB1 during hPSC differentiation enhances cardiomyogenesis at the cardiac precursors stage, but not in terminally differentiated hPSC-cardiomyocytes, by transiently upregulating GATA4 expression through a cell-cycle regulatory pathway involving CDKN2a. Importantly, cytokinesis in postmitotic hPSC-cardiomyocytes can be induced with transient, dual RB1, and CDKN2a silencing. The relevance of this pathway in vivo was suggested by findings in a porcine model of cardiac cell therapy post-MI, whereby dual RB1 and CDKN2a inactivation in adult GATA4+ cells correlates with the degree of scar size reduction and endogenous cardiomyocyte mitosis, particularly in response to combined transendocardial injection of adult human hMSCs (bone marrow-derived mesenchymal stromal cells) and cKit+ cardiac cells.

CONCLUSIONS

Together these findings reveal an important and coordinated role for RB1 and CDKN2a in regulating cell-cycle progression and differentiation during human cardiomyogenesis. Moreover, transient, dual inactivation of RB1 and CDKN2a in endogenous adult GATA4+ cells and cardiomyocytes mediates, at least in part, the beneficial effects of cell-based therapy in a post-MI large mammalian model, a finding with potential clinical implications.

Beyond Trophic Factors: Exploiting the Intrinsic Regenerative Properties of Adult Neurons

Injuries and diseases of the peripheral nervous system (PNS) are common but frequently irreversible. It is often but mistakenly assumed that peripheral neuron regeneration is robust without a need to be improved or supported. However, axonal lesions, especially those involving proximal nerves rarely recover fully and injuries generally are complicated by slow and incomplete regeneration. Strategies to enhance the intrinsic growth properties of reluctant adult neurons offer an alternative approach to consider during regeneration. Since axons rarely regrow without an intimately partnered Schwann cell (SC), approaches to enhance SC plasticity carry along benefits to their axon partners. Direct targeting of molecules that inhibit growth cone plasticity can inform important regenerative strategies. A newer approach, a focus of our laboratory, exploits tumor suppressor molecules that normally dampen unconstrained growth. However several are also prominently expressed in stable adult neurons. During regeneration their ongoing expression “brakes” growth, whereas their inhibition and knockdown may enhance regrowth. Examples have included phosphatase and tensin homolog deleted on chromosome ten (PTEN), a tumor suppressor that inhibits PI3K/pAkt signaling, Rb1, the protein involved in retinoblastoma development, and adenomatous polyposis coli (APC), a tumor suppressor that inhibits β-Catenin transcriptional signaling and its translocation to the nucleus. The identification of several new targets to manipulate the plasticity of regenerating adult peripheral neurons is exciting. How they fit with canonical regeneration strategies and their feasibility require additional work. Newer forms of nonviral siRNA delivery may be approaches for molecular manipulation to improve regeneration.

Skeletal Muscle Progenitor Cell Heterogeneity

Tissue-specific stem cells contribute to adult tissue maintenance, repair, and regeneration. In skeletal muscle, many different mononuclear cell types are capable of giving rise to differentiated muscle. Of these tissue stem-like cells, satellite cells (SCs) are the most studied muscle stem cell population and are widely considered the main cellular source driving muscle repair and regeneration in adult tissue. Within the satellite cell pool, many distinct subpopulations exist, each exhibiting differential abilities to exit quiescence, expand, differentiate, and self-renew. In this chapter, we discuss the different stem cell types that can give rise to skeletal muscle tissue and then focus on satellite cell heterogeneity during the process of myogenesis/muscle regeneration. Finally, we highlight emerging opportunities to better characterize muscle stem cell heterogeneity, which will ultimately deepen our appreciation of stem cells in muscle development, repair/regeneration, aging, and disease.

The silencing of long non-coding RNA ANRIL suppresses invasion, and promotes apoptosis of retinoblastoma cells through the ATM-E2F1 signaling pathway

As one of the most common primary intraocular carcinomas, retinoblastoma generally stems from the inactivation of the retinoblastoma RB1 gene in retinal cells. Antisense non-coding RNA in the INK4 locus (ANRIL), a long non-coding RNA (lncRNA), has been reported to affect tumorigenesis and progression of various cancers, including gastric cancer and non-small cell lung cancer. However, limited investigations emphasized the role of ANRIL in human retinoblastoma. Hence, the current study was intended to investigate the effects of ANRIL on the proliferation, apoptosis, and invasion of retinoblastoma HXO-RB₄₄ and Y79 cells. The lentivirus-based packaging system was designed to aid the up-regulation of ANRIL and ATM expressions or employed for the down-regulation of ANRIL in human retinoblastoma cells. Afterward, ANRIL expression, mRNA and protein expression of ATM and E2F1, and protein expression of INK4b, INK4a, alternate reading frame (ARF), p53 and retinoblastoma protein (pRB) were determined in order to elucidate the regulation effect associated with ANRIL on the ATM-E2F1 signaling pathway. In addition, cell viability, apoptosis, and invasion were detected accordingly. The results indicated that the down-regulation of ANRIL or up-regulation of ATM led to an increase in the expressions of ATM, E2F1, INK4b, INK4a, ARF, p53, and pRB. The silencing of ANRIL or up-regulation of ATM exerted an inhibitory effect on the proliferation and invasion while improving the apoptosis of HXO-RB₄₄ and Y79 cells. In conclusion, the key observations of our study demonstrated that ANRIL depletion could act to suppress retinoblastoma progression by activating the ATM-E2F1 signaling pathway. These results provide a potentially promising basis for the targetted intervention treatment of human retinoblastoma.

CCAAT/Enhancer Binding Protein β inhibits myogenic differentiation via ID3

Myogenesis is regulated by the coordinated expression of muscle regulatory factors, a family of transcription factors that includes MYOD, MYF5, myogenin and MRF4. Muscle regulatory factors are basic helix-loop-helix transcription factors that heterodimerize with E proteins to bind the regulatory regions of target genes. Their activity can be inhibited by members of the Inhibitor of DNA binding and differentiation (ID) family, which bind E-proteins with high affinity, thereby preventing muscle regulatory factor-dependent transcriptional responses. CCAAT/Enhancer Binding protein beta (C/EBPβ) is a transcription factor expressed in myogenic precursor cells that acts to inhibit myogenic differentiation, though the mechanism remains poorly understood. We identify Id3 as a novel C/EBPβ target gene that inhibits myogenic differentiation. Overexpression of C/EBPβ stimulates Id3 mRNA and protein expression, and is required for C/EBPβ-mediated inhibition of myogenic differentiation. Misexpression of C/EBPβ in myogenic precursors, such as in models of cancer cachexia, prevents the differentiation of myogenic precursors and we show that loss of Id3 rescues differentiation under these conditions, suggesting that the stimulation of Id3 expression by C/EBPβ is an important mechanism by which C/EBPβ inhibits myogenic differentiation.

Dedifferentiation, transdifferentiation and cell fusion: in vivo reprogramming strategies for regenerative medicine

Regenerative capacities vary enormously across the animal kingdom. In contrast to most cold-blooded vertebrates, mammals, including humans, have very limited regenerative capacity when it comes to repairing damaged or degenerating tissues. Here, we review the main mechanisms of tissue regeneration, underlying the importance of cell dedifferentiation and reprogramming. We discuss the significance of cell fate and identity changes in the context of regenerative medicine, with a particular focus on strategies aiming at the promotion of the body's self-repairing mechanisms. We also introduce some of the most recent advances that have resulted in complete reprogramming of cell identity in vivo. Lastly, we discuss the main challenges that need to be addressed in the near future to develop in vivo reprogramming approaches with therapeutic potential.

Chemical modulation of cell fates: in situ regeneration

Chemical modulation of cell fates has been widely used to promote tissue and organ regeneration. Small molecules can target the self-renewal, expansion, differentiation, and survival of endogenous stem cells for enhancing their regenerative power or induce dedifferentiation or transdifferentiation of mature cells into proliferative progenitors or specialized cell types needed for regeneration. Here, we discuss current progress and potential using small molecules to promote in vivo regenerative processes by regulating the cell fate. Current studies of small molecules in regeneration will provide insights into developing safe and efficient chemical approaches for in situ tissue repair and regeneration.

Prognostic and clinicopathological value of p16 protein aberrant expression in colorectal cancer: A PRISMA-compliant Meta-analysis

PURPOSE

Several studies have examined the potential role of p16 protein expression as a diagnostic and prognostic biomarker in various cancers. However, it remains unclear whether p16 protein expression is a prognostic and diagnostic factor for colorectal cancer. Therefore, this meta-analysis is conducted to evaluate the associations of p16 protein expression with overall survival (OS) and clinicopathological characteristics of colorectal cancer.

METHODS

According to PRISMA guideline, relevant literatures were identified by searching Medicine, Web of Science, WanFang, and CNKI databases. The pooled hazard ratios (HRs) with 95% confidence intervals (CIs) were extracted from included studies to assess the association between p16 protein expression and OS of patients with colorectal cancer. Other relevant data were extracted to evaluate the correlations of p16 protein expression with risk and clinicopathological characteristics of colorectal cancer. Stata 12.0 software was applied to calculate the strength of association between p16 protein expression and colorectal cancer.

RESULTS

Forty-one studies were included to evaluate the association between p16 protein expression and colorectal cancer. Nine studies involving 1731 patients with colorectal cancer found that there was no association between p16 protein expression and OS of colorectal cancer in the overall analysis (HR = 0.78, 95% CI: 0.55-1.10). However, p16 protein overexpression was significantly associated with a better prognosis in patients with colorectal cancer when cut-off value of p16 protein expression was <10% (HR = 0.23, 95% CI: 0.08-0.66). The results of subgroup analysis based on ethnicity indicated that p16 protein overexpression was a risk factor for the occurrence of colorectal cancer in Caucasians (odds ratio = 28.95, 95% CI: 6.08-137.89), but not in Asians. Furthermore, p16 protein overexpression was significantly associated with the Dukes stage, lymph node metastasis, tumor location, and Tumor Lymph Node Metastasis-stage of colorectal cancer.

CONCLUSIONS

p16 protein overexpression might be a useful biomarker to predict the clinicopathological progress and prognosis of colorectal cancer.

The Impact of Aging on Mechanisms of Mammalian Epimorphic Regeneration

Aging is associated with a significant decline of tissue repair and regeneration, ultimately resulting in tissue dysfunction, multimorbidity, and death. Salamanders possess remarkable regenerative abilities and have been studied with the prospect of inducing regeneration in humans and counteracting regenerative decline with aging. However, epimorphic regeneration, the full replacement of amputated structures, also occurs in mammals. One of the best studied models is digit tip regeneration, which is described for mice, and occurs in humans in a comparable manner. To accomplish regeneration, the amputated digit tip has to undergo three interdependent, overlapping steps: (i) wound healing without formation of a scar; (ii) formation of a blastema, a highly proliferative cell mass; and (iii) spatiotemporally regulated differentiation to generate a pattern similar to the original structure. Aging likely interferes with each of these steps. In this article, we provide an overview of the critical signaling pathways for regeneration, as revealed by investigating mammalian digit regeneration, the possible impact of aging on these pathways, and approaches to induce regeneration in the elderly. We hypothesize that with aging, increased Wnt signaling, NF-κB and tumor suppressor activity, and loss of positional information hampers regeneration. Knowledge about the impact of aging on regenerative mechanisms will enable us to safely activate endogenous regeneration in the elderly, and to generate a regeneration-permissive environment for cell therapies.

Concise Review: Translating Regenerative Biology into Clinically Relevant Therapies: Are We on the Right Path?

Despite approaches in regenerative medicine using stem cells, bio‐engineered scaffolds, and targeted drug delivery to enhance human tissue repair, clinicians remain unable to regenerate large‐scale, multi‐tissue defects in situ. The study of regenerative biology using mammalian models of complex tissue regeneration offers an opportunity to discover key factors that stimulate a regenerative rather than fibrotic response to injury. For example, although primates and rodents can regenerate their distal digit tips, they heal more proximal amputations with scar tissue. Rabbits and African spiny mice re‐grow tissue to fill large musculoskeletal defects through their ear pinna, while other mammals fail to regenerate identical defects and instead heal ear holes through fibrotic repair. This Review explores the utility of these comparative healing models using the spiny mouse ear pinna and the mouse digit tip to consider how mechanistic insight into reparative regeneration might serve to advance regenerative medicine. Specifically, we consider how inflammation and immunity, extracellular matrix composition, and controlled cell proliferation intersect to establish a pro‐regenerative microenvironment in response to injuries. Understanding how some mammals naturally regenerate complex tissue can provide a blueprint for how we might manipulate the injury microenvironment to enhance regenerative abilities in humans. Stem Cells Translational Medicine2018;7:220–231

Temporally distinct transcriptional regulation of myocyte dedifferentiation and Myofiber growth during muscle regeneration

BACKGROUND

Tissue regeneration requires a series of steps, beginning with generation of the necessary cell mass, followed by cell migration into damaged area, and ending with differentiation and integration with surrounding tissues. Temporal regulation of these steps lies at the heart of the regenerative process, yet its basis is not well understood. The ability of zebrafish to dedifferentiate mature "post-mitotic" myocytes into proliferating myoblasts that in turn regenerate lost muscle tissue provides an opportunity to probe the molecular mechanisms of regeneration.

RESULTS

Following subtotal excision of adult zebrafish lateral rectus muscle, dedifferentiating residual myocytes were collected at two time points prior to cell cycle reentry and compared to uninjured muscles using RNA-seq. Functional annotation (GAGE or K-means clustering followed by GO enrichment) revealed a coordinated response encompassing epigenetic regulation of transcription, RNA processing, and DNA replication and repair, along with protein degradation and translation that would rewire the cellular proteome and metabolome. Selected candidate genes were phenotypically validated in vivo by morpholino knockdown. Rapidly induced gene products, such as the Polycomb group factors Ezh2 and Suz12a, were necessary for both efficient dedifferentiation (i.e. cell reprogramming leading to cell cycle reentry) and complete anatomic regeneration. In contrast, the late activated gene fibronectin was important for efficient anatomic muscle regeneration but not for the early step of myocyte cell cycle reentry.

CONCLUSIONS

Reprogramming of a "post-mitotic" myocyte into a dedifferentiated myoblast requires a complex coordinated effort that reshapes the cellular proteome and rewires metabolic pathways mediated by heritable yet nuanced epigenetic alterations and molecular switches, including transcription factors and non-coding RNAs. Our studies show that temporal regulation of gene expression is programmatically linked to distinct steps in the regeneration process, with immediate early expression driving dedifferentiation and reprogramming, and later expression facilitating anatomical regeneration.

E6/E7 oncogenes in epithelial suprabasal layers and estradiol promote cervical growth and ear regeneration

Tissue growth is a common characteristic of carcinogenesis and regeneration. Here we show that suprabasal expression of human papillomavirus (HPV)16 E6/E7 oncogenes in Tg(K6b-E6/E7) mice, similar to that observed in HPV-infected human tissue, and estradiol increased cervical epithelium growth and ear-hole closure efficiency. Oncogenes in combination with estradiol had a significant contribution to the proliferation of suprabasal cells of cervical epithelium that correlated with an increased expression of keratin genes. Remarkably, long-term treatments with estradiol resulted in evident cellular and tissue abnormalities indicative of a precancerous phenotype. Regenerating ear epithelium of transgenic mice also showed increased suprabasal cell proliferation and expression of keratin genes. Unexpectedly, we observed higher ear regeneration efficiency in adult than in young female mice, which was further increased by E6/E7 oncogenes. Supporting a role of estradiol in this phenomenon, ovariectomy and treatment with an estrogen receptor inhibitor caused a significant reduction in regenerative capacity. Our data suggest that Tg(K6b-E6/E7) mice are unique to mimic the initial stages of HPV-mediated cervical carcinogenesis, and ear regeneration could facilitate the elucidation of mechanisms involved.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Intersection of retinoblastoma tumor suppressor function, stem cells, metabolism, and inflammation

The Retinoblastoma (RB) tumor suppressor regulates G₁/S transition during cell cycle progression by modulating the activity of E2F transcription factors. The RB pathway plays a central role in the suppression of most cancers, and RB mutation was initially discovered by virtue of its role in tumor initiation. However, as cancer genome sequencing has evolved to profile more advanced and treatment‐resistant cancers, it has become increasingly clear that, in the majority of cancers, somatic RB inactivation occurs during tumor progression. Furthermore, despite the presence of deregulation of cell cycle control due to an INK4A deletion, additional CCND amplification and/or other mutations in the RB pathway, mutation or deletion of the RB gene is often observed during cancer progression. Of note, RB inactivation during cancer progression not only facilitates G₁/S transition but also enhances some characteristics of malignancy, including altered drug sensitivity and a return to the undifferentiated state. Recently, we reported that RB inactivation enhances pro‐inflammatory signaling through stimulation of the interleukin‐6/STAT3 pathway, which directly promotes various malignant features of cancer cells. In this review, we highlight the consequences of RB inactivation during cancer progression, and discuss the biological and pathological significance of the interaction between RB and pro‐inflammatory signaling.

Advances in Decoding Axolotl Limb Regeneration

Humans and other mammals are limited in their natural abilities to regenerate lost body parts. By contrast, many salamanders are highly regenerative and can spontaneously replace lost limbs even as adults. Because salamander limbs are anatomically similar to human limbs, knowing how they regenerate should provide important clues for regenerative medicine. Although interest in understanding the mechanics of this process has never wavered, until recently researchers have been vexed by seemingly impenetrable logistics of working with these creatures at a molecular level. Chief among the problems has been the very large size of salamander genomes, and not a single salamander genome has been fully sequenced to date. Recently the enormous gap in sequence information has been bridged by approaches that leverage mRNA as the starting point. Together with functional experimentation, these data are rapidly enabling researchers to finally uncover the molecular mechanisms underpinning the astonishing biological process of limb regeneration.

Genome Editing and Muscle Stem Cells as a Therapeutic Tool for Muscular Dystrophies

Purpose of Review

Muscular dystrophies are a group of severe degenerative disorders characterized by muscle fiber degeneration and death. Therapies designed to restore muscle homeostasis and to replace dying fibers are being experimented, but none of those in clinical trials are suitable to permanently address individual gene mutation. The purpose of this review is to discuss genome editing tools such as CRISPR/Cas (clustered regularly interspaced short palindromic repeats/CRISPR-associated), which enable direct sequence alteration and could potentially be adopted to correct the genetic defect leading to muscle impairment.

Recent Findings

Recent findings show that advances in gene therapy, when combined with traditional viral vector-based approaches, are bringing the field of regenerative medicine closer to precision-based medicine.

Summary

The use of such programmable nucleases is proving beneficial for the creation of more accurate in vitro and in vivo disease models. Several gene and cell-therapy studies have been performed on satellite cells, the primary skeletal muscle stem cells involved in muscle regeneration. However, these have mainly been based on artificial replacement or augmentation of the missing protein. Satellite cells are a particularly appealing target to address these innovative technologies for the treatment of muscular dystrophies.

Regeneration and Regrowth Potentials of Digit Tips in Amphibians and Mammals

Tissue regeneration and repair have received much attention in the medical field over the years. The study of amphibians, such as newts and salamanders, has uncovered many of the processes that occur in these animals during full-limb/digit regeneration, a process that is highly limited in mammals. Understanding these processes in amphibians could shed light on how to develop and improve this process in mammals. Amputation injuries in mammals usually result in the formation of scar tissue with limited regrowth of the limb/digit; however, it has been observed that the very tips of digits (fingers and toes) can partially regrow in humans and mice under certain conditions. This review will summarize and compare the processes involved in salamander limb regeneration, mammalian wound healing, and digit regeneration in mice and humans.