Nutritional regulation of stem and progenitor cells in Drosophila

Wnt signaling couples G2 phase control with differentiation during hematopoiesis in Drosophila

During homeostasis, a critical balance is maintained between myeloid-like progenitors and their differentiated progeny, which function to mitigate stress and innate immune challenges. The molecular mechanisms that help achieve this balance are not fully understood. Using genetic dissection in Drosophila, we show that a Wnt6/EGFR-signaling network simultaneously controls progenitor growth, proliferation, and differentiation. Unlike G1-quiescence of stem cells, hematopoietic progenitors are blocked in G2 phase by a β-catenin-independent (Wnt/STOP) Wnt6 pathway that restricts Cdc25 nuclear entry and promotes cell growth. Canonical β-catenin-dependent Wnt6 signaling is spatially confined to mature progenitors through localized activation of the tyrosine kinases EGFR and Abelson kinase (Abl), which promote nuclear entry of β-catenin and facilitate exit from G2. This strategy combines transcription-dependent and -independent forms of both Wnt6 and EGFR pathways to create a direct link between cell-cycle control and differentiation. This unique combinatorial strategy employing conserved components may underlie homeostatic balance and stress response in mammalian hematopoiesis.

The Potential Reversible Transition between Stem Cells and Transient-Amplifying Cells: The Limbal Epithelial Stem Cell Perspective

Stem cells (SCs) undergo asymmetric division, producing transit-amplifying cells (TACs) with increased proliferative potential that move into tissues and ultimately differentiate into a specialized cell type. Thus, TACs represent an intermediary state between stem cells and differentiated cells. In the cornea, a population of stem cells resides in the limbal region, named the limbal epithelial stem cells (LESCs). As LESCs proliferate, they generate TACs that move centripetally into the cornea and differentiate into corneal epithelial cells. Upon limbal injury, research suggests a population of progenitor-like cells that exists within the cornea can move centrifugally into the limbus, where they dedifferentiate into LESCs. Herein, we summarize recent advances made in understanding the mechanism that governs the differentiation of LESCs into TACs, and thereafter, into corneal epithelial cells. We also outline the evidence in support of the existence of progenitor-like cells in the cornea and whether TACs could represent a population of cells with progenitor-like capabilities within the cornea. Furthermore, to gain further insights into the dynamics of TACs in the cornea, we outline the most recent findings in other organ systems that support the hypothesis that TACs can dedifferentiate into SCs.

The midgut epithelium of mosquitoes adjusts cell proliferation and endoreplication to respond to physiological challenges

BACKGROUND

Hematophagous mosquitoes transmit many pathogens that cause human diseases. Pathogen acquisition and transmission occur when female mosquitoes blood feed to acquire nutrients for reproduction. The midgut epithelium of mosquitoes serves as the point of entry for transmissible viruses and parasites.

RESULTS

We studied midgut epithelial dynamics in five major mosquito vector species by quantifying PH3-positive cells (indicative of mitotic proliferation), the incorporation of nucleotide analogs (indicative of DNA synthesis accompanying proliferation and/or endoreplication), and the ploidy (by flow cytometry) of cell populations in the posterior midgut epithelium of adult females. Our results show that the epithelial dynamics of post-emergence maturation and of mature sugar-fed guts were similar in members of the Aedes, Culex, and Anopheles genera. In the first three days post-emergence, ~ 20% of cells in the posterior midgut region of interest incorporated nucleotide analogs, concurrent with both proliferative activity and a broad shift toward higher ploidy. In mature mosquitoes maintained on sugar, an average of 3.5% of cells in the posterior midgut region of interest incorporated nucleotide analogs from five to eight days post-emergence, with a consistent presence of mitotic cells indicating constant cell turnover. Oral bacterial infection triggered a sharp increase in mitosis and nucleotide analog incorporation, suggesting that the mosquito midgut undergoes accelerated cellular turnover in response to damage. Finally, blood feeding resulted in an increase in cell proliferation, but the nature and intensity of the response varied by mosquito species and by blood source (human, bovine, avian or artificial). In An. gambiae, enterocytes appeared to reenter the cell cycle to increase ploidy after consuming blood from all sources except avian.

CONCLUSIONS

We saw that epithelial proliferation, differentiation, and endoreplication reshape the blood-fed gut to increase ploidy, possibly to facilitate increased metabolic activity. Our results highlight the plasticity of the midgut epithelium in mosquitoes' physiological responses to distinct challenges.

Insulin signaling in development

Nutrient intake is obligatory for animal growth and development, but nutrients alone are not sufficient. Indeed, insulin and homologous hormones are required for normal growth even in the presence of nutrients. These hormones communicate nutrient status between organs, allowing animals to coordinate growth and metabolism with nutrient supply. Insulin and related hormones, such as insulin-like growth factors and insulin-like peptides, play important roles in development and metabolism, with defects in insulin production and signaling leading to hyperglycemia and diabetes. Here, we describe the insulin hormone family and the signal transduction pathways activated by these hormones. We highlight the roles of insulin signaling in coordinating maternal and fetal metabolism and growth during pregnancy, and we describe how secretion of insulin is regulated at different life stages. Additionally, we discuss the roles of insulin signaling in cell growth, stem cell proliferation and cell differentiation. We provide examples of the role of insulin in development across multiple model organisms: Caenorhabditis elegans, Drosophila, zebrafish, mouse and human.

Inter-organ Wingless/Ror/Akt signaling regulates nutrient-dependent hyperarborization of somatosensory neurons

Nutrition in early life has profound effects on an organism, altering processes such as organogenesis. However, little is known about how specific nutrients affect neuronal development. Dendrites of class IV dendritic arborization neurons in Drosophila larvae become more complex when the larvae are reared on a low-yeast diet compared to a high-yeast diet. Our systematic search for key nutrients revealed that the neurons increase their dendritic terminal densities in response to a combined deficiency in vitamins, metal ions, and cholesterol. The deficiency of these nutrients upregulates Wingless in a closely located tissue, body wall muscle. Muscle-derived Wingless activates Akt in the neurons through the receptor tyrosine kinase Ror, which promotes the dendrite branching. In larval muscles, the expression of wingless is regulated not only in this key nutrient-dependent manner, but also by the JAK/STAT signaling pathway. Additionally, the low-yeast diet blunts neuronal light responsiveness and light avoidance behavior, which may help larvae optimize their survival strategies under low-nutritional conditions. Together, our studies illustrate how the availability of specific nutrients affects neuronal development through inter-organ signaling.

Formation of benign tumors by stem cell deregulation

Within living organisms, stem cells respond to various cues, including to niche signals and growth factors. Niche signals originate from the stem cell's microenvironment and promote the undifferentiated state by preventing differentiation, allowing for stem cell self-renewal. On the other hand, growth factors promote stem cell growth and proliferation, while their sources comprise of a systemic input reflecting the animal's nutritional and metabolic status, and a localized, homeostatic feedback signal from the tissue that the stem cells serve. That homeostatic signal prevents unnecessary stem cell proliferation when the corresponding differentiated tissues already have optimal cell contents. Here, we recapitulate progresses made in our understanding of in vivo stem cell regulation, largely using simple models, and draw the conclusion that 2 types of stem cell deregulations can provoke the formation of benign tumors. Namely, constitutive niche signaling promotes the formation of undifferentiated "stem cell" tumors, while defective homeostatic signaling leads to the formation of differentiated tumors. Finally, we provide evidence that these general principles may be conserved in mammals and as such, may underlie benign tumor formation in humans, while benign tumors can evolve into cancer.

Influence of Nutrition, Food Supplements and Lifestyle in Hair Disorders

Genetic, androgenic, epigenetic, environmental, lifestyle, diet, and nutritional factors have influence over promoting or regulating inflammation, immunity, and genetic expression predisposing to hair loss. Oxidative stress is a major mediator for various mechanisms of hair loss, including the release of transforming growth factor-β ( TGF-β). Nutrients counter oxidative stress, repair cellular damage, support cellular functions, and restore hair growth. Nutrients can be synergistic or antagonistic. Covert subclinical nutritional deficiencies are common. Higher dose of nutrients does not mean higher efficiency but may reverse the benefits converting an antioxidant to become prooxidant. Nutrients do not work alone but are supported by accessory micronutrients which ensure biological utilization. Providing proper nutritional environment can neutralize free radicals and perpetuate active hair growth cycles.

Articulating the "stem cell niche" paradigm through the lens of non-model aquatic invertebrates

Stem cells (SCs) in vertebrates typically reside in "stem cell niches" (SCNs), morphologically restricted tissue microenvironments that are important for SC survival and proliferation. SCNs are broadly defined by properties including physical location, but in contrast to vertebrates and other "model" organisms, aquatic invertebrate SCs do not have clearly documented niche outlines or properties. Life strategies such as regeneration or asexual reproduction may have conditioned the niche architectural variability in aquatic or marine animal groups. By both establishing the invertebrates SCNs as independent types, yet allowing inclusiveness among them, the comparative analysis will allow the future functional characterization of SCNs.

Segment number threshold determines juvenile onset of germline cluster expansion in Platynereis dumerilii

Development of sexual characters and generation of gametes are tightly coupled with growth. Platynereis dumerilii is a marine annelid that has been used to study germline development and gametogenesis. P. dumerilii has germ cell clusters found across the body in the juvenile worms, and the clusters eventually form the gametes. Like other segmented worms, P. dumerilii grows by adding new segments at its posterior end. The number of segments reflect the growth state of the worms and therefore is a useful and measurable growth state metric to study the growth‐reproduction crosstalk. To understand how growth correlates with progression of gametogenesis, we investigated germline development across several developmental stages. We discovered a distinct transition period when worms increase the number of germline clusters at a particular segment number threshold. Additionally, we found that keeping worms short in segment number, by manipulating environmental conditions or via amputations, supported a segment number threshold requirement for germline development. Finally, we asked if these clusters in P. dumerilii play a role in regeneration (as similar free‐roaming cells are observed in Hydra and planarian regeneration) and found that the clusters were not required for regeneration in P. dumerilii, suggesting a strictly germline nature. Overall, these molecular analyses suggest a previously unidentified developmental transition dependent on the growth state of juvenile P. dumerilii leading to substantially increased germline expansion.

Total segment number predicts the state of germline development and the abundance of germline clusters in Platynereis dumerilii.

Changing environmental conditions or amputating worms do not alter the segment number threshold requirement for germline development.

The vasa‐expressing germ clusters are not required for regeneration in Platynereis dumerilii.

Relish plays a dynamic role in the niche to modulate Drosophila blood progenitor homeostasis in development and infection

Immune challenges demand the gearing up of basal hematopoiesis to combat infection. Little is known about how during development, this switch is achieved to take care of the insult. Here, we show that the hematopoietic niche of the larval lymph gland of Drosophila senses immune challenge and reacts to it quickly through the nuclear factor-κB (NF-κB), Relish, a component of the immune deficiency (Imd) pathway. During development, Relish is triggered by ecdysone signaling in the hematopoietic niche to maintain the blood progenitors. Loss of Relish causes an alteration in the cytoskeletal architecture of the niche cells in a Jun Kinase-dependent manner, resulting in the trapping of Hh implicated in progenitor maintenance. Notably, during infection, downregulation of Relish in the niche tilts the maintenance program toward precocious differentiation, thereby bolstering the cellular arm of the immune response.

Physiological Alterations in Deletion Mutants of Two Insulin-Like Peptides Encoded in Maruca vitrata Using CRISPR/Cas9

Most insect species encode multiple insulin-like peptides (ILPs) that exhibit functional overlaps in mediating physiological processes such as development and reproduction. Why do they need multiple ILPs? To address this question, we tested a hypothesis of the requirement of multiple ILPs by generating mutants lacking individual ILP genes using the CRISPR/Cas9 technology. Two ILPs (ILP1 and ILP2) in the legume pod borer, Maruca vitrata, mediate similar physiological processes such as hemolymph sugar level, larval development, and adult reproduction. Individual knock-out mutants (ΔILP1 and ΔILP2) were generated. They showed successful development from larvae to adults. However, they suffered from high hemolymph sugar levels by enhancing trehalose titers in the hemolymph. The hyperglycemic effect was more evident in ΔILP2 mutants than in ΔILP1 mutants. Both mutants showed increased expression of trehalose-6-phosphate synthase but suppressed expression of trehalase. These mutants also showed altered expression patterns of insulin signaling components. Expression levels of insulin receptor and Akt genes were upregulated, while those of FOXO and Target of rapamycin genes were downregulated in these mutants. These alterations of signal components resulted in significant retardation of immature development and reduced body sizes. ΔILP1 or ΔILP2 females exhibited poor oocyte development. Bromo-uridine incorporation was much reduced at the germarium of ovarioles of these mutants compared with wild females. Expression of the vitellogenin gene was also reduced in these mutants. Furthermore, males of these deletion mutants showed impaired reproductive activities when they mated with wild-type females. These results suggest that both ILPs are required for mediating larval development and adult reproduction in M. vitrata.

Non-autonomous regulation of germline stem cell proliferation by somatic MPK-1/MAPK activity in C. elegans

Extracellular-signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) is a major positive regulator of cell proliferation, which is often upregulated in cancer. However, few studies have addressed ERK/MAPK regulation of proliferation within a complete organism. The Caenorhabditis elegans ERK/MAPK ortholog MPK-1 is best known for its control of somatic organogenesis and germline differentiation, but it also stimulates germline stem cell proliferation. Here, we show that the germline-specific MPK-1B isoform promotes germline differentiation but has no apparent role in germline stem cell proliferation. By contrast, the soma-specific MPK-1A isoform promotes germline stem cell proliferation non-autonomously. Indeed, MPK-1A functions in the intestine or somatic gonad to promote germline proliferation independent of its other known roles. We propose that a non-autonomous role of ERK/MAPK in stem cell proliferation may be conserved across species and various tissue types, with major clinical implications for cancer and other diseases.

The prevailing paradigm is that ERK/MAPK functions autonomously to promote cell proliferation upon mitogen stimulation. Robinson-Thiewes et al. now demonstrate that C. elegans ERK/MAPK acts within somatic tissues to non-autonomously promote the proliferation of germline stem cells. Germline ERK/MAPK is thus dispensable for germline stem cell proliferation.

Light Stimuli and Circadian Clock Affect Neural Development in Drosophila melanogaster

Endogenous clocks enable organisms to adapt cellular processes, physiology, and behavior to daily variation in environmental conditions. Metabolic processes in cyanobacteria to humans are under the influence of the circadian clock, and dysregulation of the circadian clock causes metabolic disorders. In mouse and Drosophila, the circadian clock influences translation of factors involved in ribosome biogenesis and synchronizes protein synthesis. Notably, nutrition signals are mediated by the insulin receptor/target of rapamycin (InR/TOR) pathways to regulate cellular metabolism and growth. However, the role of the circadian clock in Drosophila brain development and the potential impact of clock impairment on neural circuit formation and function is less understood. Here we demonstrate that changes in light stimuli or disruption of the molecular circadian clock cause a defect in neural stem cell growth and proliferation. Moreover, we show that disturbed cell growth and proliferation are accompanied by reduced nucleolar size indicative of impaired ribosomal biogenesis. Further, we define that light and clock independently affect the InR/TOR growth regulatory pathway due to the effect on regulators of protein biosynthesis. Altogether, these data suggest that alterations in InR/TOR signaling induced by changes in light conditions or disruption of the molecular clock have an impact on growth and proliferation properties of neural stem cells in the developing Drosophila brain.

Tissue-autonomous immune response regulates stress signaling during hypertrophy

Postmitotic tissues are incapable of replacing damaged cells through proliferation, but need to rely on buffering mechanisms to prevent tissue disintegration. By constitutively activating the Ras/MAPK-pathway via Ras^V12-overexpression in the postmitotic salivary glands (SGs) of Drosophila larvae, we overrode the glands adaptability to growth signals and induced hypertrophy. The accompanied loss of tissue integrity, recognition by cellular immunity, and cell death are all buffered by blocking stress signaling through a genuine tissue-autonomous immune response. This novel, spatio-temporally tightly regulated mechanism relies on the inhibition of a feedback-loop in the JNK-pathway by the immune effector and antimicrobial peptide Drosomycin. While this interaction might allow growing SGs to cope with temporary stress, continuous Drosomycin expression in Ras^V12-glands favors unrestricted hypertrophy. These findings indicate the necessity to refine therapeutic approaches that stimulate immune responses by acknowledging their possible, detrimental effects in damaged or stressed tissues.

Tissues and organs work hard to maintain balance in everything from taking up nutrients to controlling their growth. Ageing, wounding, sickness, and changes in the genetic code can all alter this balance, and cause the tissue or organ to lose some of its cells. Many tissues restore this loss by dividing their remaining cells to fill in the gaps. But some – like the salivary glands of fruit fly larvae – have lost this ability.

Tissues like these rely on being able to sense and counteract problems as they arise so as to not lose their balance in the first place. The immune system and stress responses are crucial for this process. They trigger steps to correct the problem and interact with each other to find a common decision about the fate of the affected tissue. To better understand how the immune system and stress response work together, Krautz, Khalili and Theopold genetically manipulated cells in the salivary gland of fruit fly larvae. These modifications switched on signals that stimulate cells to keep growing, causing the salivary gland’s tissue to slowly lose its balance and trigger the stress and immune response.

The experiments showed that while the stress response instructed the cells in the gland to die, a peptide released by the immune system called Drosomycin blocked this response and prevented the tissue from collapsing. The cells in the part of the gland not producing this immune peptide were consequently killed by the stress response. When all the cells in the salivary gland were forced to produce Drosomycin, none of the cells died and the whole tissue survived. But it also allowed the cells in the gland to grow uncontrollably, like a tumor, threatening the health of the entire organism.

Mapping the interactions between immune and stress pathways could help to fine-tune treatments that can prevent tissue damage. Fruit flies share many genetic features and molecular pathways with humans. So, the next step towards these kinds of treatments would be to screen for similar mechanisms that block stress activation in damaged human tissues. But this research carries a warning: careless activation of the immune system to protect stressed tissues could lead to uncontrolled tissue growth, and might cause more harm than good.

Multi-level analysis of reproduction in an Antarctic midge identifies female and male accessory gland products that are altered by larval stress and impact progeny viability

The Antarctic midge, Belgica antarctica, is a wingless, non-biting midge endemic to Antarctica. Larval development requires at least 2 years, but adults live only 2 weeks. The nonfeeding adults mate in swarms and females die shortly after oviposition. Eggs are suspended in a gel of unknown composition that is expressed from the female accessory gland. This project characterizes molecular mechanisms underlying reproduction in this midge by examining differential gene expression in whole males, females, and larvae, as well as in male and female accessory glands. Functional studies were used to assess the role of the gel encasing the eggs, as well as the impact of stress on reproductive biology. RNA-seq analyses revealed sex- and development-specific gene sets along with those associated with the accessory glands. Proteomic analyses were used to define the composition of the egg-containing gel, which is generated during multiple developmental stages and derived from both the accessory gland and other female organs. Functional studies indicate the gel provides a larval food source as well as a buffer for thermal and dehydration stress. All of these function are critical to juvenile survival. Larval dehydration stress directly reduces production of storage proteins and key accessory gland components, a feature that impacts adult reproductive success. Modeling reveals that bouts of dehydration may have a significant impact on population growth. This work lays a foundation for further examination of reproduction in midges and provides new information related to general reproduction in dipterans. A key aspect of this work is that reproduction and stress dynamics, currently understudied in polar organisms, are likely to prove critical in determining how climate change will alter their survivability.

The Kinase Activity of Drosophila BubR1 Is Required for Insulin Signaling-Dependent Stem Cell Maintenance

As a core component of the mitotic checkpoint complex, BubR1 has a modular organization of molecular functions, with KEN box and other motifs at the N terminus inhibiting the anaphase-promoting complex/cyclosome, and a kinase domain at the C terminus, whose function remains unsettled, especially at organismal levels. We generate knock-in BubR1 mutations in the Drosophila genome to separately disrupt the KEN box and the kinase domain. All of the mutants are homozygously viable and fertile and show no defects in mitotic progression. The mutants without kinase activity have an increased lifespan and phenotypic changes associated with attenuated insulin signaling, including reduced InR on the cell membrane, weakened PI3K and AKT activity, and elevated expression of dFoxO targets. The BubR1 kinase-dead mutants have a reduced cap cell number in female germaria, which can be rescued by expressing a constitutively active InR. We conclude that one major physiological role of BubR1 kinase in Drosophila is to modulate insulin signaling.

Reclaiming Warburg: using developmental biology to gain insight into human metabolic diseases

Developmental biologists have frequently pushed the frontiers of modern biomedical research. From the discovery and characterization of novel signal transduction pathways to exploring the molecular underpinnings of genetic inheritance, transcription, the cell cycle, cell death and stem cell biology, studies of metazoan development have historically opened new fields of study and consistently revealed previously unforeseen avenues of clinical therapies. From this perspective, it is not surprising that our community is now an integral part of the current renaissance in metabolic research. Amidst the global rise in metabolic syndrome, the discovery of novel signaling roles for metabolites, and the increasing links between altered metabolism and many human diseases, we as developmental biologists can contribute skills and expertise that are uniquely suited for investigating the mechanisms underpinning human metabolic health and disease. Here, we summarize the opportunities and challenges that our community faces, and discuss how developmental biologists can make unique and valuable contributions to the field of metabolism and physiology.

The Posterior Signaling Center Is an Important Microenvironment for Homeostasis of the Drosophila Lymph Gland

Hematopoiesis is a necessary process for development and immune defense in Drosophila from the embryonic period to adulthood. There are two main stages in this process: the first stage occurs in the head mesoderm during the embryonic stage, and the second occurs in a specialized hematopoietic organ along the dorsal vessel, the lymph gland, during the larval stage. The lymph gland consists of paired lobes, each of which has distinct regions: the cortical zone (CZ), which contains mature hemocytes; the medullary zone (MZ), which contains hematopoietic progenitors; and the posterior signaling center (PSC), which specifically expresses the early B-cell factor (EBF) transcription factor Collier (Col) and the HOX factor Antennapedia (Antp) to form a microenvironment similar to that of the mammalian bone marrow hematopoietic stem cell niche. The PSC plays a key role in regulating hematopoietic progenitor differentiation. Moreover, the PSC contributes to the cellular immune response to wasp parasitism triggered by elevated ROS levels. Two recent studies have revealed that hematopoietic progenitor maintenance is directly regulated by Col expressed in the MZ and is independent of the PSC, challenging the traditional model. In this review, we summarize the regulatory networks of PSC cell proliferation, the controversy regarding PSC-mediated regulation of hematopoietic progenitor differentiation, and the wasp egg infection response. In addition, we discuss why the PSC is an ideal model for investigating mammalian hematopoietic stem cell niches and leukemia.

The Phosphatase CSW Controls Life Span by Insulin Signaling and Metabolism Throughout Adult Life in Drosophila

Noonan syndrome and related disorders are caused by mutations in genes encoding for proteins of the RAS-ERK1/2 signaling pathway, which affect development by enhanced ERK1/2 activity. However, the mutations’ effects throughout adult life are unclear. In this study, we identify that the protein most commonly affected in Noonan syndrome, the phosphatase SHP2, known in Drosophila as corkscrew (CSW), controls life span, triglyceride levels, and metabolism without affecting ERK signaling pathway. We found that CSW loss-of-function mutations extended life span by interacting with components of the insulin signaling pathway and impairing AKT activity in adult flies. By expressing csw-RNAi in different organs, we determined that CSW extended life span by acting in organs that regulate energy availability, including gut, fat body and neurons. In contrast to that in control animals, loss of CSW leads to reduced homeostasis in metabolic rate during activity. Clinically relevant gain-of-function csw allele reduced life span, when expressed in fat body, but not in other tissues. However, overexpression of a wild-type allele did not affect life span, showing a specific effect of the gain-of-function allele independently of a gene dosage effect. We concluded that CSW normally regulates life span and that mutations in SHP2 are expected to have critical effects throughout life by insulin-dependent mechanisms in addition to the well-known RAS-ERK1/2-dependent developmental alterations.

HNF4 Regulates Fatty Acid Oxidation and Is Required for Renewal of Intestinal Stem Cells in Mice

BACKGROUND & AIMS

Functions of intestinal stem cells (ISCs) are regulated by diet and metabolic pathways. Hepatocyte nuclear factor 4 (HNF4) family are transcription factors that bind fatty acids. We investigated how HNF4 transcription factors regulate metabolism and their functions in ISCs in mice.

METHODS

We performed studies with Villin-Cre^ERT2;Lgr5-EGFP-IRES-Cre^ERT2;Hnf4α^f/f;Hnf4γ^{Crispr/Crispr} mice, hereafter referred to Hnf4αγ^DKO. Mice were given tamoxifen to induce Cre recombinase. Mice transgenic with only Cre alleles (Villin-Cre^ERT2, Lgr5-EGFP-IRES-Cre^ERT2, Hnf4α^+/+, and Hnf4γ^+/+) or mice given vehicle were used as controls. Crypt and villus cells were isolated, incubated with fluorescently labeled fatty acids or glucose analog, and analyzed by confocal microscopy. Fatty acid oxidation activity and tricarboxylic acid (TCA) cycle metabolites were measured in cells collected from the proximal half of the small intestine of Hnf4αγ^DKO and control mice. We performed chromatin immunoprecipitation and gene expression profiling analyses to identify genes regulated by HNF4 factors. We established organoids from duodenal crypts, incubated them with labeled palmitate or acetate, and measured production of TCA cycle metabolites or fatty acids. Acetate, a precursor of acetyl coenzyme A (CoA) (a product of fatty acid β-oxidation [FAO]), or dichloroacetate, a compound that promotes pyruvate oxidation and generation of mitochondrial acetyl-CoA, were used for metabolic intervention.

RESULTS

Crypt cells rapidly absorbed labeled fatty acids, and messenger RNA levels of Lgr5⁺ stem cell markers (Lgr5, Olfm4, Smoc2, Msi1, and Ascl2) were down-regulated in organoids incubated with etomoxir, an inhibitor of FAO, indicating that FAO was required for renewal of ISCs. HNF4A and HNF4G were expressed in ISCs and throughout the intestinal epithelium. Single knockout of either HNF4A or HNF4G did not affect maintenance of ISCs, but double-knockout of HNF4A and HNF4G resulted in ISC loss; stem cells failed to renew. FAO supports ISC renewal, and HNF4 transcription factors directly activate FAO genes, including Acsl5 and Acsf2 (encode regulators of acyl-CoA synthesis), Slc27a2 (encodes a fatty acid transporter), Fabp2 (encodes fatty acid binding protein), and Hadh (encodes hydroxyacyl-CoA dehydrogenase). In the intestinal epithelium of Hnf4αγ^DKO mice, expression levels of FAO genes, FAO activity, and metabolites of TCA cycle were all significantly decreased, but fatty acid synthesis transcripts were increased, compared with control mice. The contribution of labeled palmitate or acetate to the TCA cycle was reduced in organoids derived from Hnf4αγ^DKO mice, compared with control mice. Incubation of organoids derived from double-knockout mice with acetate or dichloroacetate restored stem cells.

CONCLUSIONS

In mice, the transcription factors HNF4A and HNF4G regulate the expression of genes required for FAO and are required for renewal of ISCs.

Mercury chloride exposure induces DNA damage, reduces fertility, and alters somatic and germline cells in Drosophila melanogaster ovaries

Mercury exposure has been shown to affect the reproductive system in many organisms, although the molecular mechanisms are still elusive. In the present study, we exposed Drosophila melanogaster Canton-S adult females to concentrations of 0 mM, 0.1 mM, 0.3 mM, 3 mM, and 30 mM of mercury chloride (HgCl₂) for 24 h, 48 h, or 72 h to determine how mercury could affect fertility. Alkaline assays performed on dissected ovaries showed that mercury induced DNA damage that is not only dose-dependent but also time-dependent. All ovaries treated for 72 h have incorporated mercury and exhibit size reduction. Females treated with 30 mM HgCl₂, the highest dose, had atrophied ovaries and exhibited a drastic 7-fold reduction in egg laying. Confocal microscopy analysis revealed that exposure to HgCl₂ disrupts germinal and somatic cell organization in the germarium and leads to the aberrant expression of a germline-specific gene in somatic follicle cells in developing egg chambers. Together, these results highlight the potential long-term impact of mercury on germline and ovarian cells that might involve gene deregulation.

Sex-specific transcriptomic responses to changes in the nutritional environment

Males and females typically pursue divergent reproductive strategies and accordingly require different dietary compositions to maximise their fitness. Here we move from identifying sex-specific optimal diets to understanding the molecular mechanisms that underlie male and female responses to dietary variation in Drosophila melanogaster. We examine male and female gene expression on male-optimal (carbohydrate-rich) and female-optimal (protein-rich) diets. We find that the sexes share a large core of metabolic genes that are concordantly regulated in response to dietary composition. However, we also observe smaller sets of genes with divergent and opposing regulation, most notably in reproductive genes which are over-expressed on each sex's optimal diet. Our results suggest that nutrient sensing output emanating from a shared metabolic machinery are reversed in males and females, leading to opposing diet-dependent regulation of reproduction in males and females. Further analysis and experiments suggest that this reverse regulation occurs within the IIS/TOR network.

"You are what you eat" is a popular saying that can often make scientific sense. Everything an animal eats gets broken down into smaller molecules that fuel the many biological processes required to survive, move and reproduce. However, the food that the sexes need to maximize their fertility may not be exactly the same, as males make lots of small, mobile sperm cells while females create a small number of large eggs. In fruit flies for example, females benefit most from foods that contain lots of protein, while males are more fertile when they eat foods that are rich in carbohydrates. However, it remained unclear how these differences have evolved.

Here, Camus et al. examine the genes that are active in male and female fruit flies which eat a diet rich in either carbohydrates or in proteins. Their experiments showed that both sexes share a large collection of genes which respond to the two diets in the same way. However, the type of food had opposite effects on the activity of certain genes involved in male and female reproduction. When the fruit flies had a protein-rich diet, for example, genes that promoted reproduction got turned on in females, but switched off in males. The opposite pattern was observed when the insects were exposed to carbohydrate-rich diets. Further analyses suggested that these different responses might be linked to a molecular network called IIS/TOR, which is a specific cascade of reactions that responds to nutrient availability.

The findings of Camus et al. suggest that male and female flies produce different signals in reaction to food, which helps them to reproduce when they are able to meet their particular nutritional needs. Armed with a better understanding of the fundamental differences between the sexes, it may be possible to improve research into human health and animal keeping.

Drosophila Mon1 constitutes a novel node in the brain-gonad axis that is essential for female germline maturation

Monensin-sensitive 1 (Mon1) is an endocytic regulator that participates in the conversion of Rab5-positive early endosomes to Rab7-positive late endosomes. In Drosophila, loss of mon1 leads to sterility as the mon1 mutant females have extremely small ovaries with complete absence of late stage egg chambers - a phenotype reminiscent of mutations in the insulin pathway genes. Here, we show that expression of many Drosophila insulin-like peptides (ILPs) is reduced in mon1 mutants and feeding mon1 adults an insulin-rich diet can rescue the ovarian defects. Surprisingly, however, mon1 functions in the tyramine/octopaminergic neurons (OPNs) and not in the ovaries or the insulin-producing cells (IPCs). Consistently, knockdown of mon1 in only the OPNs is sufficient to mimic the ovarian phenotype, while expression of the gene in the OPNs alone can 'rescue' the mutant defect. Last, we have identified ilp3 and ilp5 as critical targets of mon1. This study thus identifies mon1 as a novel molecular player in the brain-gonad axis and underscores the significance of inter-organ systemic communication during development.

Insulin signaling mediates previtellogenic development and enhances juvenile hormone-mediated vitellogenesis in a lepidopteran insect, Maruca vitrata

Background

Insulin/insulin-like growth peptide signaling (IIS) down-regulates hemolymph sugar level and facilitates larval growth in the soybean pod borer, Maruca vitrata. The objective of this study is to determine whether IIS of M. vitrata can mediate ovarian development of adult females.

Results

A pair of ovaries consists of 8 ovarioles, each of which is separated into distal germarium and proximal vitellarium in M. vitrata. In the germarium, oocyte development occurred with active mitotic activity which was visible by incorporating bromodeoxyribose uridine. Previtellogenic development and subsequent vitellogenesis began soon after adult emergence. They continued with increase of female age. Oocyte development was facilitated by up-regulation of vitellogenin (Vg) and Vg receptor (VgR) gene expression. Larval diets significantly influenced on ovarian development of M. vitrata because oocyte development varied with pupal size derived from larvae treated with different nutritional diets. Its ovarian development was dependent on endocrine signal(s) from the head because decapitation soon after adult emergence prevented oogenesis and subsequent vitellogenesis along with marked reduction of Vg and VgR expression. Topical application of juvenile hormone (JH) significantly recovered its ovarian development whereas farnesoic acid (a precursor of JH biosynthesis) or 20-hydroxyecdysone treatment did not. JH stimulated vitellogenesis and choriogenesis, but not previtellogenic development. In contrast, insulin injection to decapitated females stimulated oocyte differentiation and vitellogenesis along with increase of Vg and VgR expression. To further analyze the effect of insulin on ovarian development, expression of four IIS components (InR, FOXO, Akt, and TOR) genes was manipulated by RNA interference. Hemocoelic injection of gene-specific double stranded RNAs significantly reduced their target gene mRNA levels and interfered with ovarian development. An addition of insulin to JH treatment against decapitated females enhanced the gonadotropic effect of JH by stimulating oogenesis.

Conclusions

IIS plays crucial role in mediating previtellogenic development of M. vitrata in response to nutrient signal. It also enhances the gonadotropic effect of JH II on vitellogenesis.

Electronic supplementary material

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Proteomics of Asrij Perturbation in Drosophila Lymph Glands for Identification of New Regulators of Hematopoiesis

Hematopoiesis is the process of differentiation of precursor blood cells into mature blood cells that is controlled by a complex set of molecular interactions. Understanding hematopoiesis is important for the study of hematological disorders. However, a comprehensive understanding of how physiological and genetic mechanisms regulate blood cell precursor maintenance and differentiation is lacking. Owing to simplicity and ease of genetic analysis, the Drosophila melanogaster lymph gland (LG) is an excellent model to study hematopoiesis. Here, we quantitatively analyzed the LG proteome under genetic conditions that either maintain precursors or promote their differentiation in vivo, by perturbing expression of Asrij, a conserved endosomal regulator of hematopoiesis. Using iTRAQ-based quantitative proteomics, we determined the relative expression levels of proteins in Asrij-knockout and overexpressing LGs from 1500 larval dissections compared with wild type. Our data showed that at least 6.5% of the Drosophila proteome is expressed in wild type LGs. Of the 2133 proteins identified, 780 and 208 proteins were common to previously reported cardiac tube and hemolymph proteomes, respectively, resulting in the identification of 1238 proteins exclusive to the LG. Perturbation of Asrij levels led to differential expression of 619 proteins, of which 27% have human homologs implicated in various diseases. Proteins regulating metabolism, immune system, signal transduction and vesicle-mediated transport were significantly enriched. Immunostaining of representative candidates from the enriched categories and previous reports confirmed 73% of our results, indicating the validity of our LG proteome. Our study provides, for the first time, an in vivo proteomics resource for identifying novel regulators of hematopoiesis that will also be applicable to understanding vertebrate blood cell development.

Drosophila Cellular Immunity Against Parasitoid Wasps: A Complex and Time-Dependent Process

Host-parasitoid interactions are among the most studied interactions between invertebrates because of their fundamental interest - the evolution of original traits in parasitoids - and applied, parasitoids being widely used in biological control. Immunity, and in particular cellular immunity, is central in these interactions, the host encapsulation response being specific for large foreign bodies such as parasitoid eggs. Although already well studied in this species, recent data on Drosophila melanogaster have unquestionably improved knowledge of invertebrate cellular immunity. At the same time, the venomics of parasitoids has expanded, notably those of Drosophila. Here, we summarize and discuss these advances, with a focus on an emerging "time-dependent" view of interactions outcome at the intra- and interspecific level. We also present issues still in debate and prospects for study. Data on the Drosophila-parasitoid model paves the way to new concepts in insect immunity as well as parasitoid wasp strategies to overcome it.

Intestinal renewal across the animal kingdom: comparing stem cell activity in mouse and Drosophila

The gastrointestinal (GI) tract renews frequently to sustain nutrient digestion and absorption in the face of consistent tissue stress. In many species, proliferative intestinal stem cells (ISCs) are responsible for the repair of the damage arising from chemical and mechanical aspects of food breakdown and exposure to pathogens. As the cellular source of all mature cell types of the intestinal epithelium throughout adulthood, ISCs hold tremendous therapeutic potential for understanding and treating GI disease in humans. This review focuses on recent advances in our understanding of ISC identity, behavior, and regulation during homeostasis and injury-induced repair, as revealed by two major animal models used to study regeneration of the small intestine: Drosophila melanogaster and Mus musculus. We emphasize recent findings from Drosophila that are likely to translate to the mammalian GI system, as well as challenging topics in mouse ISC biology that may be ideally suited for investigation in flies. For context, we begin by reviewing major physiological similarities and distinctions between the Drosophila midgut and mouse small intestine.

A Neuronal Relay Mediates a Nutrient Responsive Gut/Fat Body Axis Regulating Energy Homeostasis in Adult Drosophila

The control of systemic metabolic homeostasis involves complex inter-tissue programs that coordinate energy production, storage, and consumption, to maintain organismal fitness upon environmental challenges. The mechanisms driving such programs are largely unknown. Here, we show that enteroendocrine cells in the adult Drosophila intestine respond to nutrients by secreting the hormone Bursicon α, which signals via its neuronal receptor DLgr2. Bursicon α/DLgr2 regulate energy metabolism through a neuronal relay leading to the restriction of glucagon-like, adipokinetic hormone (AKH) production by the corpora cardiaca and subsequent modulation of AKH receptor signaling within the adipose tissue. Impaired Bursicon α/DLgr2 signaling leads to exacerbated glucose oxidation and depletion of energy stores with consequent reduced organismal resistance to nutrient restrictive conditions. Altogether, our work reveals an intestinal/neuronal/adipose tissue inter-organ communication network that is essential to restrict the use of energy and that may provide insights into the physiopathology of endocrine-regulated metabolic homeostasis.

Insulin-like peptides of the legume pod borer, Maruca vitrata, and their mediation effects on hemolymph trehalose level, larval development, and adult reproduction

Insulin-like peptides (ILPs) of insects mediate various physiological processes including hemolymph sugar level, immature growth, female reproduction, and lifespan. In target cells of ILPs, insulin/insulin-like growth factor signaling (IIS) is highly conserved in animals. IIS in the legume pod borer, Maruca vitrata (Lepidoptera: Crambidae), is known to be involved in maintaining hemolymph trehalose levels and promoting larval growth. However, ILPs in M. vitrata have not been reported yet. This study predicted two ILP genes of Mv-ILP1 and Mv-ILP2 from transcriptome of M. vitrata. Mv-ILP1 and Mv-ILP2 shared high sequence homologies and domain architecture with Drosophila ILPs. Both ILPs exhibited similar expression patterns in most developmental stages, showing high expression levels in adult stage. In the larval stage, Mv-ILP1 and Mv-IlP2 were expressed mostly in the brain and fat body. However, in the adult stage, both ILP genes were expressed more in the abdomen than those in the head containing the brain. RNA interference (RNAi) of either Mv-ILP1 or Mv-ILP2 during larval stage resulted in significant malfunctioning in regulating hemolymph trehalose titers. RNAi-treated larvae also exhibited significant retardation of larval growth. RNAi treatment in adult stage interfered with the ovarian development of females. These results suggest that Mv-ILP1 and Mv-ILP2 play crucial roles in mediating larval growth and adult reproduction.

High-fat diet disturbs lipid raft/TGF-β signaling-mediated maintenance of hematopoietic stem cells in mouse bone marrow

Despite recent in vivo data demonstrating that high-fat diet (HFD)-induced obesity leads to major perturbations in murine hematopoietic stem cells (HSC), the direct role of a HFD is not yet completely understood. Here, we investigate the direct impact of a short-term HFD on HSC and hematopoiesis in C57BL/6J mice compared with standard diet-fed mice. We detect a loss of half of the most primitive HSC in the bone marrow (BM) cells of HFD-fed mice, which exhibit lower hematopoietic reconstitution potential after transplantation. Impaired maintenance of HSC is due to reduced dormancy after HFD feeding. We discover that a HFD disrupts the TGF-β receptor within lipid rafts, associated to impaired Smad2/3-dependent TGF-β signaling, as the main molecular mechanism of action. Finally, injecting HFD-fed mice with recombinant TGF-β1 avoids the loss of HSC and alteration of the BM’s ability to recover, underscoring the fact that a HFD affects TGF-β signaling on HSC.

High fat diets (HFD) are thought to perturb murine hematopoiesis as a result of obesity. Here the authors find that short-term HFD reduces hematopoietic stem cells (HSC), disrupts lipid rafts and TGF-β1 signalling. Injecting HFD-fed mice with recombinant TGF-β1 can rescue HSC loss.

For Intestinal Homeostasis, You Are What You Eat

Nutrients play a central role in controlling the form and function of the intestinal epithelium. In this issue of Developmental Cell, Mattila et al. (2018) and Obniski et al. (2018) uncover important mechanisms by which Drosophila intestinal stem cells respond to dietary signals, linking nutrients to tissue homeostasis.

Opposing Action of Hedgehog and Insulin Signaling Balances Proliferation and Autophagy to Determine Follicle Stem Cell Lifespan

Egg production declines with age in many species, a process linked with stem cell loss. Diet-dependent signaling has emerged as critical for stem cell maintenance during aging. Follicle stem cells (FSCs) in the Drosophila ovary are exquisitely responsive to diet-induced signals including Hedgehog (Hh) and insulin-IGF signaling (IIS), entering quiescence in the absence of nutrients and initiating proliferation rapidly upon feeding. Although highly proliferative FSCs generally exhibit an extended lifespan, we find that constitutive Hh signaling drives FSC loss and premature sterility despite high proliferative rates. This occurs due to Hh-mediated induction of autophagy in FSCs via a Ptc-dependent, Smo-independent mechanism. Hh-dependent autophagy increases during aging, triggering FSC loss and consequent reproductive arrest. IIS is necessary and sufficient to suppress Hh-induced autophagy, promoting a stable proliferative state. These results suggest that opposing action of diet-responsive IIS and Hh signals determine reproductive lifespan by modulating the proliferation-autophagy balance in FSCs during aging.

Hsp83/Hsp90 Physically Associates with Insulin Receptor to Promote Neural Stem Cell Reactivation

Neural stem cells (NSCs) have the ability to exit quiescence and reactivate in response to physiological stimuli. In the Drosophila brain, insulin receptor (InR)/phosphatidylinositol 3-kinase (PI3K)/Akt pathway triggers NSC reactivation. However, intrinsic mechanisms that control the InR/PI3K/Akt pathway during reactivation remain unknown. Here, we have identified heat shock protein 83 (Hsp83/Hsp90), a molecular chaperone, as an intrinsic regulator of NSC reactivation. Hsp83 is both necessary and sufficient for NSC reactivation by promoting the activation of InR pathway in larval brains in the presence of dietary amino acids. Both Hsp83 and its co-chaperone Cdc37 physically associate with InR. Finally, reactivation defects observed in brains depleted of hsp83 were rescued by over-activation of the InR/PI3K/Akt pathway, suggesting that Hsp83 functions upstream of the InR/PI3K/Akt pathway during NSC reactivation. Given the conservation of Hsp83 and the InR pathway, our finding may provide insights into the molecular mechanisms underlying mammalian NSC reactivation.

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Hsp83/Hsp90 and its co-chaperone Cdc37 are required for NSC reactivation

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Hsp83 overexpression results in premature NSC reactivation on fed condition

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Hsp83 and Cdc37 physically associate with InR

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Hsp83 and Cdc37 are required for the activation of InR pathway in NSCs

A sisRNA/miRNA Axis Prevents Loss of Germline Stem Cells during Starvation in Drosophila

Animal reproduction responds to nutritional status. During starvation, Drosophila and Caenorhabditis elegans enter a period of reproductive diapause with increase apoptosis, while maintaining a stable pool of germline stem cells (GSCs). How GSCs are protected is not understood. Here, we show that a sisRNA/miRNA axis maintains ovarian GSCs during starvation in Drosophila. Starvation induces the expression of an ovary-enriched sisRNA sisR-2, which negatively regulates GSC maintenance via a fatty acid metabolism gene dFAR1. sisR-2 promotes the expression of bantam, which in turn inhibits the activity of sisR-2, forming a negative feedback loop. Therefore, bantam acts as a buffer to counteract sisR-2 activity to prevent GSC loss during starvation. We propose that the sisR-2/bantam axis confers robustness to GSCs in Drosophila.

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sisR-2 regulates the number of GSCs

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sisR-2 regulates GSC maintenance by repressing dFAR1

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bantam regulates GSC maintenance by repressing sisR-2 activity

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sisR-2/bantam axis protects GSCs from starvation

The effect of age on stem cell function and utility for therapy

During development, stem cells generate all of the differentiated cells that populate our tissues and organs. Stem cells are also responsible for tissue turnover and repair in adults, and as such, they hold tremendous promise for regenerative therapy. Aging, however, impairs the function of stem cells and is thus a significant roadblock to using stem cells for therapy. Paradoxically, the patients who would benefit the most from regenerative therapies are usually advanced in age. The use of stem cells from young donors or the rejuvenation of aged patient-derived stem cells may represent part of a solution. Nonetheless, the transplantation success of young or rejuvenated stem cells in aged patients is still problematic, since stem cell function is greatly influenced by extrinsic factors that become unsupportive with age. This article briefly reviews how aging impairs stem cell function, and how this has an impact on the use of stem cells for therapy.

Diet regulates membrane extension and survival of niche escort cells for germline homeostasis via insulin signaling

Diet is an important regulator of stem cell homeostasis, however, the underlying mechanisms of this regulation are not fully known. Here, we report that insulin signaling mediates dietary maintenance of Drosophila ovarian germline stem cells (GSCs) by promoting the extension of niche escort cell (EC) membranes to wrap around GSCs. This wrapping may facilitate the delivery of BMP stemness factors from ECs in the niche to GSCs. In addition to the effects on GSCs, insulin signaling-mediated regulation of EC number and protrusions controls the division and growth of GSC progeny. The effects of insulin signaling on EC membrane extension are, at least in part, driven by enhanced translation of Failed axon connections (Fax) via Ribosomal protein S6 kinase. Fax is a membrane protein that may participate in Abl-regulated cytoskeletal dynamics and is known to be involved in axon bundle formation. Therefore, we conclude that dietary cues stimulate insulin signaling in the niche to regulate EC cellular structure, probably via Fax-dependent cytoskeleton remodeling. This mechanism enhances intercellular contact and facilitates homeostatic interactions between somatic and germline cells in response to diet.

Iron Homeostasis Controls Myeloid Blood Cell Differentiation in Drosophila

Iron is an essential divalent ion for aerobic life. Life has evolved to maintain iron homeostasis for normal cellular and physiological functions and therefore imbalances in iron levels exert a wide range of consequences. Responses to iron dysregulation in blood development, however, remain elusive. Here, we found that iron homeostasis is critical for differentiation of Drosophila blood cells in the larval hematopoietic organ, called the lymph gland. Supplementation of an iron chelator, bathophenanthroline disulfate (BPS) results in an excessive differentiation of the crystal cell in the lymph gland. This phenotype is recapitulated by loss of Fer1HCH in the intestine, indicating that reduced levels of systemic iron enhances crystal cell differentiation. Detailed analysis of Fer1HCH-tagged-GFP revealed that Fer1HCH is also expressed in the hematopoietic systems. Lastly, blocking Fer1HCH expression in the mature blood cells showed marked increase in the blood differentiation of both crystal cells and plasmatocytes. Thus, our work suggests a relevance of systemic and local iron homeostasis in blood differentiation, prompting further investigation of molecular mechanisms underlying iron regulation and cell fate determination in the hematopoietic system.

Conserved insulin signaling in the regulation of oocyte growth, development, and maturation

Insulin signaling regulates various aspects of physiology, such as glucose homeostasis and aging, and is a key determinant of female reproduction in metazoans. That insulin signaling is crucial for female reproductive health is clear from clinical data linking hyperinsulinemic and hypoinsulinemic condition with certain types of ovarian dysfunction, such as altered steroidogenesis, polycystic ovary syndrome, and infertility. Thus, understanding the signaling mechanisms that underlie the control of insulin-mediated ovarian development is important for the accurate diagnosis of and intervention for female infertility. Studies of invertebrate and vertebrate model systems have revealed the molecular determinants that transduce insulin signaling as well as which biological processes are regulated by the insulin-signaling pathway. The molecular determinants of the insulin-signaling pathway, from the insulin receptor to its downstream signaling components, are structurally and functionally conserved across evolution, from worms to mammals-yet, physiological differences in signaling still exist. Insulin signaling acts cooperatively with gonadotropins in mammals and lower vertebrates to mediate various aspects of ovarian development, mainly owing to evolution of the endocrine system in vertebrates. In contrast, insulin signaling in Drosophila and Caenorhabditis elegans directly regulates oocyte growth and maturation. In this review, we compare and contrast insulin-mediated regulation of ovarian functions in mammals, lower vertebrates, C. elegans, and Drosophila, and highlight conserved signaling pathways and regulatory mechanisms in general while illustrating insulin's unique role in specific reproductive processes.

The matrix protein Tiggrin regulates plasmatocyte maturation in Drosophila larva

The lymph gland (LG) is a major source of hematopoiesis during Drosophila development. In this tissue, prohemocytes differentiate into multiple lineages, including macrophage-like plasmatocytes, which comprise the vast majority of mature hemocytes. Previous studies have uncovered genetic pathways that regulate prohemocyte maintenance and some cell fate choices between hemocyte lineages. However, less is known about how the plasmatocyte pool of the LG is established and matures. Here, we report that Tiggrin, a matrix protein expressed in the LG, is a specific regulator of plasmatocyte maturation. Tiggrin mutants exhibit precocious maturation of plasmatocytes, whereas Tiggrin overexpression blocks this process, resulting in a buildup of intermediate progenitors (IPs) expressing prohemocyte and hemocyte markers. These IPs likely represent a transitory state in prohemocyte to plasmatocyte differentiation. We also found that overexpression of Wee1 kinase, which slows G₂/M progression, results in a phenotype similar to Tiggrin overexpression, whereas String/Cdc25 expression phenocopies Tiggrin mutants. Further analysis revealed that Wee1 inhibits plasmatocyte maturation through upregulation of Tiggrin transcription. Our results elucidate connections between the extracellular matrix and cell cycle regulators in the regulation of hematopoiesis.

DAF-18/PTEN signals through AAK-1/AMPK to inhibit MPK-1/MAPK in feedback control of germline stem cell proliferation

Under replete growth conditions, abundant nutrient uptake leads to the systemic activation of insulin/IGF-1 signalling (IIS) and the promotion of stem cell growth/proliferation. Activated IIS can stimulate the ERK/MAPK pathway, the activation of which also supports optimal stem cell proliferation in various systems. Stem cell proliferation rates can further be locally refined to meet the resident tissue's need for differentiated progeny. We have recently shown that the accumulation of mature oocytes in the C. elegans germ line, through DAF-18/PTEN, inhibits adult germline stem cell (GSC) proliferation, despite high systemic IIS activation. We show here that this feedback occurs through a novel cryptic signalling pathway that requires PAR-4/LKB1, AAK-1/AMPK and PAR-5/14-3-3 to inhibit the activity of MPK-1/MAPK, antagonize IIS, and inhibit both GSC proliferation and the production of additional oocytes. Interestingly, our results imply that DAF-18/PTEN, through PAR-4/LKB1, can activate AAK-1/AMPK in the absence of apparent energy stress. As all components are conserved, similar signalling cascades may regulate stem cell activities in other organisms and be widely implicated in cancer.

Adult Muscle Formation Requires Drosophila Moleskin for Proliferation of Wing Disc-Associated Muscle Precursors

Adult muscle precursor (AMP) cells located in the notum of the larval wing disc undergo rapid amplification and eventual fusion to generate the Drosophila melanogaster indirect flight muscles (IFMs). Here we find that loss of Moleskin (Msk) function in these wing disc-associated myoblasts reduces the overall AMP pool size, resulting in the absence of IFM formation. This myoblast loss is due to a decrease in the AMP proliferative capacity and is independent of cell death. In contrast, disruption of Msk during pupal myoblast proliferation does not alter the AMP number, suggesting that Msk is specifically required for larval AMP proliferation. It has been previously shown that Wingless (Wg) signaling maintains expression of the Vestigial (Vg) transcription factor in proliferating myoblasts. However, other factors that influence Wg-mediated myoblast proliferation are largely unknown. Here we examine the interactions between Msk and the Wg pathway in regulation of the AMP pool size. We find that a myoblast-specific reduction of Msk results in the absence of Vg expression and a complete loss of the Wg pathway readout β-catenin/Armadillo (Arm). Moreover, msk RNA interference knockdown abolishes expression of the Wg target Ladybird (Lbe) in leg disc myoblasts. Collectively, our results provide strong evidence that Msk acts through the Wg signaling pathway to control myoblast pool size and muscle formation by regulating Arm stability or nuclear transport.

Symbiont-induced odorant binding proteins mediate insect host hematopoiesis

Symbiotic bacteria assist in maintaining homeostasis of the animal immune system. However, the molecular mechanisms that underlie symbiont-mediated host immunity are largely unknown. Tsetse flies (Glossina spp.) house maternally transmitted symbionts that regulate the development and function of their host's immune system. Herein we demonstrate that the obligate mutualist, Wigglesworthia, up-regulates expression of odorant binding protein six in the gut of intrauterine tsetse larvae. This process is necessary and sufficient to induce systemic expression of the hematopoietic RUNX transcription factor lozenge and the subsequent production of crystal cells, which actuate the melanotic immune response in adult tsetse. Larval Drosophila's indigenous microbiota, which is acquired from the environment, regulates an orthologous hematopoietic pathway in their host. These findings provide insight into the molecular mechanisms that underlie enteric symbiont-stimulated systemic immune system development, and indicate that these processes are evolutionarily conserved despite the divergent nature of host-symbiont interactions in these model systems.

Regulation of the Balance Between Proliferation and Differentiation in Germ Line Stem Cells

In many animals, reproductive fitness is dependent upon the production of large numbers of gametes over an extended period of time. This level of gamete production is possible due to the continued presence of germ line stem cells. These cells can produce two types of daughter cells, self-renewing daughter cells that will maintain the stem cell population and differentiating daughter cells that will become gametes. A balance must be maintained between the proliferating self-renewing cells and those that differentiate for long-term gamete production to be maintained. Too little proliferation can result in depletion of the stem cell population, while too little differentiation can lead to a lack of gamete formation and possible tumor formation. In this chapter, we discuss our current understanding of how the balance between proliferation and differentiation is achieved in three well-studied germ line model systems: the Drosophila female, the mouse male, and the C. elegans hermaphrodite. While these three systems have significant differences in how this balance is regulated, including differences in stem cell population size, signaling pathways utilized, and the use of symmetric and/or asymmetric cell divisions, there are also similarities found between them. These similarities include the reliance on a predominant signaling pathway to promote proliferation, negative feedback loops to rapidly shutoff proliferation-promoting cues, close association of the germ line stem cells with a somatic niche, cytoplasmic connections between cells, projections emanating from the niche cell, and multiple mechanisms to limit the spatial influence of the niche. A comparison between different systems may help to identify elements that are essential for a proper balance between proliferation and differentiation to be achieved and elements that may be achieved through various mechanisms.

Two Independent Functions of Collier/Early B Cell Factor in the Control of Drosophila Blood Cell Homeostasis

Blood cell production in the Drosophila hematopoietic organ, the lymph gland, is controlled by intrinsic factors and extrinsic signals. Initial analysis of Collier/Early B Cell Factor function in the lymph gland revealed the role of the Posterior Signaling Center (PSC) in mounting a dedicated cellular immune response to wasp parasitism. Further, premature blood cell differentiation when PSC specification or signaling was impaired, led to assigning the PSC a role equivalent to the vertebrate hematopoietic niche. We report here that Collier is expressed in a core population of lymph gland progenitors and cell autonomously maintains this population. The PSC contributes to lymph gland homeostasis by regulating blood cell differentiation, rather than by maintaining core progenitors. In addition to PSC signaling, switching off Collier expression in progenitors is required for efficient immune response to parasitism. Our data show that two independent sites of Collier/Early B Cell Factor expression, hematopoietic progenitors and the PSC, achieve control of hematopoiesis.

Drosophila blood as a model system for stress sensing mechanisms

The Drosophila lymph gland is the hematopoietic organ in which stem-like progenitors proliferate and give rise to myeloid-type blood cells. Mechanisms involved in Drosophila hematopoiesis are well established and known to be conserved in the vertebrate system. Recent studies in Drosophila lymph gland have provided novel insights into how external and internal stresses integrate into blood progenitor maintenance mechanisms and the control of blood cell fate decision. In this review, I will introduce a developmental overview of the Drosophila hematopoietic system, and recent understandings of how the system uses developmental signals not only for hematopoiesis but also as sensors for stress and environmental changes to elicit necessary blood responses. [BMB Reports 2015; 48(4): 223-228]

DAF-18/PTEN locally antagonizes insulin signalling to couple germline stem cell proliferation to oocyte needs in C. elegans

During development, stem cell populations rapidly proliferate to populate the expanding tissues and organs. During this phase, nutrient status, by systemically affecting insulin/IGF-1 signalling, largely dictates stem cell proliferation rates. In adults, however, differentiated stem cell progeny requirements are generally reduced and vary according to the spatiotemporal needs of each tissue. We demonstrate here that differential regulation of germline stem cell proliferation rates in Caenorhabditis elegans adults is accomplished through localized neutralization of insulin/IGF-1 signalling, requiring DAF-18/PTEN, but not DAF-16/FOXO. Indeed, the specific accumulation of oocytes, the terminally differentiated stem cell progeny, triggers a feedback signal that locally antagonizes insulin/IGF-1 signalling outputs in the germ line, regardless of their systemic levels, to block germline stem cell proliferation. Thus, during adulthood, stem cells can differentially respond within tissues to otherwise equal insulin/IGF-1 signalling inputs, according to the needs for production of their immediate terminally differentiated progeny.