Loss of foxo rescues stem cell aging in Drosophila germ line

Adar Regulates Drosophila melanogaster Spermatogenesis via Modulation of BMP Signaling

The dynamic process of Drosophila spermatogenesis involves asymmetric division, mitosis, and meiosis, which ultimately results in the production of mature spermatozoa. Disorders of spermatogenesis can lead to infertility in males. ADAR (adenosine deaminase acting on RNA) mutations in Drosophila cause male infertility, yet the causative factors remain unclear. In this study, immunofluorescence staining was employed to visualize endogenous ADAR proteins and assess protein levels via fluorescence-intensity analysis. In addition, the early differentiation disorders and homeostatic alterations during early spermatogenesis in the testes were examined through quantification of transit-amplifying region length, counting the number of GSCs (germline stem cells), and fertility experiments. Our findings suggest that deletion of ADAR causes testicular tip transit-amplifying cells to accumulate and become infertile in older male Drosophila. By overexpressing ADAR in early germline cells, male infertility can be partially rescued. Transcriptome analysis showed that ADAR maintained early spermatogenesis homeostasis through the bone-morphogenetic-protein (BMP) signaling pathway. Taken together, these findings have the potential to help explore the role of ADAR in early spermatogenesis.

Multifaceted role of mTOR (mammalian target of rapamycin) signaling pathway in human health and disease

The mammalian target of rapamycin (mTOR) is a protein kinase that controls cellular metabolism, catabolism, immune responses, autophagy, survival, proliferation, and migration, to maintain cellular homeostasis. The mTOR signaling cascade consists of two distinct multi-subunit complexes named mTOR complex 1/2 (mTORC1/2). mTOR catalyzes the phosphorylation of several critical proteins like AKT, protein kinase C, insulin growth factor receptor (IGF-1R), 4E binding protein 1 (4E-BP1), ribosomal protein S6 kinase (S6K), transcription factor EB (TFEB), sterol-responsive element-binding proteins (SREBPs), Lipin-1, and Unc-51-like autophagy-activating kinases. mTOR signaling plays a central role in regulating translation, lipid synthesis, nucleotide synthesis, biogenesis of lysosomes, nutrient sensing, and growth factor signaling. The emerging pieces of evidence have revealed that the constitutive activation of the mTOR pathway due to mutations/amplification/deletion in either mTOR and its complexes (mTORC1 and mTORC2) or upstream targets is responsible for aging, neurological diseases, and human malignancies. Here, we provide the detailed structure of mTOR, its complexes, and the comprehensive role of upstream regulators, as well as downstream effectors of mTOR signaling cascades in the metabolism, biogenesis of biomolecules, immune responses, and autophagy. Additionally, we summarize the potential of long noncoding RNAs (lncRNAs) as an important modulator of mTOR signaling. Importantly, we have highlighted the potential of mTOR signaling in aging, neurological disorders, human cancers, cancer stem cells, and drug resistance. Here, we discuss the developments for the therapeutic targeting of mTOR signaling with improved anticancer efficacy for the benefit of cancer patients in clinics.

Fat enough for the winter? Does nutritional status affect diapause?

Many insects enter a dormant state termed diapause in anticipation of seasonal inhospitable conditions. Insects drastically reduce their feeding during diapause. Their reduced nutrient intake is paired with substantial nutrient costs: maintaining basal metabolism during diapause, repairing tissues damaged by adverse conditions, and resuming development after diapause. Many investigators have asked "Does nutrition affect diapause?" In this review, we survey the studies that have attempted to address this question. We propose the term nutritional status, a holistic view of nutrition that explicitly includes the perception, intake, and storage of the great breadth of nutrients. We examine the studies that have sought to test if nutrition affects diapause, trying to identify specific facets of nutritional status that affect diapause phenotypes. Curiously, low quality host plants during the diapause induction phase generally induce diapause, but food deprivation during the same phase generally averts diapause. Using the geometric framework of nutrition to identify specific dietary components that affect diapause may reconcile these contrasting findings. This framework can establish nutritionally permissive space, distinguishing nutrient changes that affect diapause from changes that induce other dormancies. Refeeding is another important experimental technique that distinguishes between diapause and quiescence, a non-diapause dormancy. We also find insufficient evidence for the hypothesis that nutrient stores regulate diapause length and suggest manipulations to investigate the role of nutrient stores in diapause termination. Finally, we propose mechanisms that could interface nutritional status with the diapause program, focusing on combined action of the nutritional axis between the gut, fat body, and brain.

Camptothecin Effectively Regulates Germline Differentiation through Bam-Cyclin A Axis in Drosophila melanogaster

Camptothecin (CPT), first isolated from Chinese tree Camptotheca acuminate, produces rapid and prolonged inhibition of DNA synthesis and induction of DNA damage by targeting topoisomerase I (top1), which is highly activated in cancer cells. CPT thus exhibits remarkable anticancer activities in various cancer types, and is a promising therapeutic agent for the treatment of cancers. However, it remains to be uncovered underlying its cytotoxicity toward germ cells. In this study we found that CPT, a cell cycle-specific anticancer agent, reduced fecundity and exhibited significant cytotoxicity toward GSCs and two-cell cysts. We showed that CPT induced GSC loss and retarded two-cell cysts differentiation in a niche- or apoptosis-independent manner. Instead, CPT induced ectopic expression of a differentiation factor, bag of marbles (Bam), and regulated the expression of cyclin A, which contributed to GSC loss. In addition, CPT compromised two-cell cysts differentiation by decreasing the expression of Bam and inducing cell arrest at G1/S phase via cyclin A, eventually resulting in two-cell accumulation. Collectively, this study demonstrates, for the first time in vivo, that the Bam-cyclin A axis is involved in CPT-mediated germline stem cell loss and two-cell cysts differentiation defects via inducing cell cycle arrest, which could provide information underlying toxicological effects of CPT in the productive system, and feature its potential to develop as a pharmacology-based germline stem cell regulation agent.

Stress-induced reversible cell-cycle arrest requires PRC2/PRC1-mediated control of mitophagy in Drosophila germline stem cells and human iPSCs

Following acute genotoxic stress, both normal and tumorous stem cells can undergo cell-cycle arrest to avoid apoptosis and later re-enter the cell cycle to regenerate daughter cells. However, the mechanism of protective, reversible proliferative arrest, "quiescence," remains unresolved. Here, we show that mitophagy is a prerequisite for reversible quiescence in both irradiated Drosophila germline stem cells (GSCs) and human induced pluripotent stem cells (hiPSCs). In GSCs, mitofission (Drp1) or mitophagy (Pink1/Parkin) genes are essential to enter quiescence, whereas mitochondrial biogenesis (PGC1α) or fusion (Mfn2) genes are crucial for exiting quiescence. Furthermore, mitophagy-dependent quiescence lies downstream of mTOR- and PRC2-mediated repression and relies on the mitochondrial pool of cyclin E. Mitophagy-dependent reduction of cyclin E in GSCs and in hiPSCs during mTOR inhibition prevents the usual G1/S transition, pushing the cells toward reversible quiescence (G0). This alternative method of G1/S control may present new opportunities for therapeutic purposes.

Sci-Seq of Human Fetal Salivary Tissue Introduces Human Transcriptional Paradigms and a Novel Cell Population

Multiple pathologies and non-pathological factors can disrupt the function of the non-regenerative human salivary gland including cancer and cancer therapeutics, autoimmune diseases, infections, pharmaceutical side effects, and traumatic injury. Despite the wide range of pathologies, no therapeutic or regenerative approaches exist to address salivary gland loss, likely due to significant gaps in our understanding of salivary gland development. Moreover, identifying the tissue of origin when diagnosing salivary carcinomas requires an understanding of human fetal development. Using computational tools, we identify developmental branchpoints, a novel stem cell-like population, and key signaling pathways in the human developing salivary glands by analyzing our human fetal single-cell sequencing data. Trajectory and transcriptional analysis suggest that the earliest progenitors yield excretory duct and myoepithelial cells and a transitional population that will yield later ductal cell types. Importantly, this single-cell analysis revealed a previously undescribed population of stem cell-like cells that are derived from SD and expresses high levels of genes associated with stem cell-like function. We have observed these rare cells, not in a single niche location but dispersed within the developing duct at later developmental stages. Our studies introduce new human-specific developmental paradigms for the salivary gland and lay the groundwork for the development of translational human therapeutics.

Chemical Genetic Screen in Drosophila Germline Uncovers Small Molecule Drugs That Sensitize Stem Cells to Insult-Induced Apoptosis

Cancer stem cells, in contrast to their more differentiated daughter cells, can endure genotoxic insults, escape apoptosis, and cause tumor recurrence. Understanding how normal adult stem cells survive and go to quiescence may help identify druggable pathways that cancer stem cells have co-opted. In this study, we utilize a genetically tractable model for stem cell survival in the Drosophila gonad to screen drug candidates and probe chemical-genetic interactions. Our study employs three levels of small molecule screening: (1) a medium-throughput primary screen in male germline stem cells (GSCs), (2) a secondary screen with irradiation and protein-constrained food in female GSCs, and (3) a tertiary screen in breast cancer organoids in vitro. Herein, we uncover a series of small molecule drug candidates that may sensitize cancer stem cells to apoptosis. Further, we have assessed these small molecules for chemical-genetic interactions in the germline and identified the NF-κB pathway as an essential and druggable pathway in GSC quiescence and viability. Our study demonstrates the power of the Drosophila stem cell niche as a model system for targeted drug discovery.

Finding Solutions for Fibrosis: Understanding the Innate Mechanisms Used by Super-Regenerator Vertebrates to Combat Scarring

Soft tissue fibrosis and cutaneous scarring represent massive clinical burdens to millions of patients per year and the therapeutic options available are currently quite limited. Despite what is known about the process of fibrosis in mammals, novel approaches for combating fibrosis and scarring are necessary. It is hypothesized that scarring has evolved as a solution to maximize healing speed to reduce fluid loss and infection. This hypothesis, however, is complicated by regenerative animals, which have arguably the most remarkable healing abilities and are capable of scar-free healing. This review explores the differences observed between adult mammalian healing that typically results in fibrosis versus healing in regenerative animals that heal scarlessly. Each stage of wound healing is surveyed in depth from the perspective of many regenerative and fibrotic healers so as to identify the most important molecular and physiological variances along the way to disparate injury repair outcomes. Understanding how these powerful model systems accomplish the feat of scar-free healing may provide critical therapeutic approaches to the treatment or prevention of fibrosis.

Activation of matrix metalloproteinases and FoxO3a in HaCaT keratinocytes by radiofrequency electromagnetic field exposure

As the skin is the largest body organ and critically serves as a barrier, it is frequently exposed and could be physiologically affected by radiofrequency electromagnetic field (RF-EMF) exposure. In this study, we found that 1760 MHz RF-EMF (4.0 W/kg specific absorption rate for 2 h/day during 4 days) exposure could induce intracellular reactive oxygen species (ROS) production in HaCaT human keratinocytes using 2′,7′-dichlorofluorescin diacetate fluorescent probe analysis. However, cell growth and viability were unaffected by RF-EMF exposure. Since oxidative stress in the skin greatly influences the skin-aging process, we analyzed the skin senescence-related factors activated by ROS generation. Matrix metalloproteinases 1, 3, and 7 (MMP1, MMP3, and MMP7), the main skin wrinkle-related proteins, were significantly increased in HaCaT cells after RF-EMF exposure. Additionally, the gelatinolytic activities of secreted MMP2 and MMP9 were also increased by RF-EMF exposure. FoxO3a (Ser318/321) and ERK1/2 (Thr 202/Tyr 204) phosphorylation levels were significantly increased by RF-EMF exposure. However, Bcl2 and Bax expression levels were not significantly changed, indicating that the apoptotic pathway was not activated in keratinocytes following RF-EMF exposure. In summary, our findings show that exposure to 1760 MHz RF-EMF induces ROS generation, leading to MMP activation and FoxO3a and ERK1/2 phosphorylation. These data suggest that RF-EMF exposure induces cellular senescence of skin cells through ROS induction in HaCaT human keratinocytes.

Molecular basis of reproductive senescence: insights from model organisms

PURPOSE

Reproductive decline due to parental age has become a major barrier to fertility as couples have delayed having offspring into their thirties and forties. Advanced parental age is also associated with increased incidence of neurological and cardiovascular disease in offspring. Thus, elucidating the etiology of reproductive decline is of clinical importance.

METHODS

Deciphering the underlying processes that drive reproductive decline is particularly challenging in women in whom a discrete oocyte pool is established during embryogenesis and may remain dormant for tens of years. Instead, our understanding of the processes that drive reproductive senescence has emerged from studies in model organisms, both vertebrate and invertebrate, that are the focus of this literature review.

CONCLUSIONS

Studies of reproductive aging in model organisms not only have revealed the detrimental cellular changes that occur with age but also are helping identify major regulator proteins controlling them. Here, we discuss what we have learned from model organisms with respect to the molecular mechanisms that maintain both genome integrity and oocyte quality.

Autophagy and its role in regeneration and remodeling within invertebrate

Background

Acting as a cellular cleaner by packaging and transporting defective proteins and organelles to lysosomes for breakdown, autophagic process is involved in the regulation of cell remodeling after cell damage or cell death in both vertebrate and invertebrate. In human, limitations on the regenerative capacity of specific tissues and organs make it difficult to recover from diseases. Comprehensive understanding on its mechanism within invertebrate have strong potential provide helpful information for challenging these diseases.

Method

In this study, recent findings on the autophagy function in three invertebrates including planarian, hydra and leech with remarkable regenerative ability were summarized. Furthermore, molecular phylogenetic analyses of DjATGs and HvATGs were performed on these three invertebrates compared to that of Saccharomyces cerevisiae, Caenorhabditis elegans, Drosophila melanogaster, Mus musculus and Homo sapiens.

Results

In comparison with Scerevisiae, C elegans, D melanogaster, M musculus and human, our analysis exhibits the following characteristics of autophagy and its function in regeneration within invertebrate. Phylogenetical analysis of ATGs revealed that most autophagy-related genes (ATGs) were highly similar to their homologs in other species, which indicates that autophagy is a highly conservative biological function in both vertebrate and invertebrate. Structurally, almost all the core amino acids necessary for the function of ATG8 in mammal were observed in invertebrate HvATG8s and DjATG8s. For instance, ubiquitin-like domain as a signature structure in each ATG8, was observed in all ATG8s in three invertebrates. Basically, autophagy plays a key role in the regulation of regeneration in planarian. DjATG8-2 and DjATG8-3 associated with mTOR signaling pathway are sophisticated in the invertebrate tissue/organ regeneration. Furthermore, autophagy is involved in the pathway of neutralization of toxic molecules input from blood digestion in the leech.

Conclusions

The recent investigations on autophagy in invertebrate including planarian, hydra and leech suggest that autophagy is evolutionally conserved from yeast to mammals. The fundamental role of its biological function in the invertebrate contributing to the regeneration and maintenance of cellular homeostasis in these three organisms could make tremendous information to confront life threatening diseases in human including cancers and cardiac disorders.

Metabolic Control over mTOR-Dependent Diapause-like State

Regulation of embryonic diapause, dormancy that interrupts the tight connection between developmental stage and time, is still poorly understood. Here, we characterize the transcriptional and metabolite profiles of mouse diapause embryos and identify unique gene expression and metabolic signatures with activated lipolysis, glycolysis, and metabolic pathways regulated by AMPK. Lipolysis is increased due to mTORC2 repression, increasing fatty acids to support cell survival. We further show that starvation in pre-implantation ICM-derived mouse ESCs induces a reversible dormant state, transcriptionally mimicking the in vivo diapause stage. During starvation, Lkb1, an upstream kinase of AMPK, represses mTOR, which induces a reversible glycolytic and epigenetically H4K16Ac-negative, diapause-like state. Diapause furthermore activates expression of glutamine transporters SLC38A1/2. We show by genetic and small molecule inhibitors that glutamine transporters are essential for the H4K16Ac-negative, diapause state. These data suggest that mTORC1/2 inhibition, regulated by amino acid levels, is causal for diapause metabolism and epigenetic state.

Germline stem cell aging in the Drosophila ovary

The age-related decline of adult stem cells leads to loss of tissue homeostasis and contributes to organismal aging. Though the phenotypic hallmarks of aging are well-characterized at the organ or tissue level, the molecular processes that govern stem cell aging remain unclear. This review seeks to highlight recent research in stem cell aging in the Drosophila ovary and connect the discoveries in the fly to ongoing questions in stem cell aging.

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.

Endocrine regulation of female germline stem cells in the fruit fly Drosophila melanogaster

Germline stem cells (GSCs) are critical for the generation of sperms and eggs in most animals including the fruit fly Drosophila melanogaster. It is well known that self-renewal and differentiation of female D. melanogaster GSCs are regulated by local niche signals. However, little is known about whether and how the GSC number is regulated by paracrine signals. In the last decade, however, multiple humoral factors, including insulin and ecdysteroids, have been recognized as key regulators of female D. melanogaster GSCs. This review paper summarizes the role of humoral factors in female D. melanogaster GSC proliferation and maintenance in response to internal and external conditions, such as nutrients, mating stimuli, and aging.

mTOR: A Cellular Regulator Interface in Health and Disease

The mechanistic target of Rapamycin (mTOR) is a ubiquitously-conserved serine/threonine kinase, which has a central function in integrating growth signals and orchestrating their physiologic effects on cellular level. mTOR is the core component of differently composed signaling complexes that differ in protein composition and molecular targets. Newly identified classes of mTOR inhibitors are being developed to block autoimmune diseases and transplant rejections but also to treat obesity, diabetes, and different types of cancer. Therefore, the selective and context-dependent inhibition of mTOR activity itself might come into the focus as molecular target to prevent severe diseases and possibly to extend life span. This review provides a general introduction to the molecular composition and physiologic function of mTOR complexes as part of the Special Issue “2018 Select Papers by Cells’ Editorial Board Members”.

PPARβ/δ: Linking Metabolism to Regeneration

In contrast to the general belief that regeneration is a rare event, mainly occurring in simple organisms, the ability of regeneration is widely distributed in the animal kingdom. Yet, the efficiency and extent of regeneration varies greatly. Humans can recover from blood loss as well as damage to tissues like bone and liver. Yet damage to the heart and brain cannot be reversed, resulting in scaring. Thus, there is a great interest in understanding the molecular mechanisms of naturally occurring regeneration and to apply this knowledge to repair human organs. During regeneration, injury-activated immune cells induce wound healing, extracellular matrix remodeling, migration, dedifferentiation and/or proliferation with subsequent differentiation of somatic or stem cells. An anti-inflammatory response stops the regenerative process, which ends with tissue remodeling to achieve the original functional state. Notably, many of these processes are associated with enhanced glycolysis. Therefore, peroxisome proliferator-activated receptor (PPAR) β/δ—which is known to be involved for example in lipid catabolism, glucose homeostasis, inflammation, survival, proliferation, differentiation, as well as mammalian regeneration of the skin, bone and liver—appears to be a promising target to promote mammalian regeneration. This review summarizes our current knowledge of PPARβ/δ in processes associated with wound healing and regeneration.