Single-cell RNA sequencing identifies novel cell types in Drosophila blood

Drosophila has been extensively used to model the human blood-immune system,as both systems share many developmental and immune response mechanisms.However,while many human blood cell types have been identified,only three were found in flies:plasmatocytes,crystal cells and lamellocytes.To better understand the complexity of fly blood system,we used single-cell RNA sequencing technology to generate co mprehensive gene expression profiles for Drosophila circulating blood cells.In addition to the known cell types,we identified two new Drosophila blood cell types:thanacytes and primocytes.Thanacytes,which express many stimulus response genes,are involved in distinct responses to different types of bacteria.Primocytes,which express cell fate commitment and signaling genes,appear to be involved in keeping stem cells in the circulating blood.Furthermore,our data revealed four novel plasmatocyte subtypes(Ppn+,CAH7~+,Lsp~+ and reservoir plasmatocytes),each with unique molecular identities and distinct predicted functions.We also identified cross-species markers from Drosophila hemocytes to human blood cells.Our analysis unveiled a more complex Drosophila blood system and broadened the scope of using Drosophila to model human blood system in development and disease.     

Another notch in stem cell biology: Drosophila intestinal stem cells and the specification of cell fates

Previous work has suggested that many stem cells can be found in microanatomic niches, where adjacent somatic cells of the niche control the differentiation and proliferation states of their resident stem cells. Recently published work examining intestinal stem cells (ISCs) in the adult Drosophila midgut suggests a new paradigm where some stem cells actively control the cell fate decisions of their daughters. Here, we review recent literature((1)) demonstrating that, in the absence of a detectable stem cell niche, multipotent Drosophila ISCs modulate the Notch signaling pathway in their adjacent daughter cells in order to specify the differentiated lineages of their descendants. These observations made in Drosophila are challenging and advancing our understanding of stem cell biology.     

A role for extra macrochaetae downstream of Notch in follicle cell differentiation

The Drosophila ovary provides a model system for studying the mechanisms that regulate the differentiation of somatic stem cells into specific cell types. Ovarian somatic stem cells produce follicle cells, which undergo a binary choice during early differentiation. They can become either epithelial cells that surround the germline to form an egg chamber ('main body cells') or a specialized cell lineage found at the poles of egg chambers. This lineage goes on to make two cell types: polar cells and stalk cells. To better understand how this choice is made, we carried out a screen for genes that affect follicle cell fate specification or differentiation. We identified extra macrochaetae (emc), which encodes a helix-loop-helix protein, as a downstream effector of Notch signaling in the ovary. EMC is expressed in proliferating cells in the germarium, as well as in the main body follicle cells. EMC expression in the main body cells is Notch dependent, and emc mutant cells located on the main body failed to differentiate. EMC expression is reduced in the precursors of the polar and stalk cells, and overexpression of EMC caused dramatic egg chamber fusions, indicating that EMC is a negative regulator of polar and/or stalk cells. EMC and Notch were both required in the main body cells for expression of Eyes Absent (EYA), a negative regulator of polar and stalk cell fate. We propose that EMC functions downstream of Notch and upstream of EYA to regulate main body cell fate specification and differentiation.     

Antioxidants Maintain E-Cadherin Levels to Limit Drosophila Prohemocyte Differentiation

Mitochondrial reactive oxygen species (ROS) regulate a variety of biological processes by networking with signal transduction pathways to maintain homeostasis and support adaptation to stress. In this capacity, ROS have been shown to promote the differentiation of progenitor cells, including mammalian embryonic and hematopoietic stem cells and Drosophila hematopoietic progenitors (prohemocytes). However, many questions remain about how ROS alter the regulatory machinery to promote progenitor differentiation. Here, we provide evidence for the hypothesis that ROS reduce E-cadherin levels to promote Drosophila prohemocyte differentiation. Specifically, we show that knockdown of the antioxidants, Superoxide dismutatase 2 and Catalase reduce E-cadherin protein levels prior to the loss of Odd-skipped-expressing prohemocytes. Additionally, over-expression of E-cadherin limits prohemocyte differentiation resulting from paraquat-induced oxidative stress. Furthermore, two established targets of ROS, Enhancer of Polycomb and FOS, control the level of E-cadherin protein expression. Finally, we show that knockdown of either Superoxide dismutatase 2 or Catalase leads to an increase in the E-cadherin repressor, Serpent. As a result, antioxidants and targets of ROS can control E-cadherin protein levels, and over-expression of E-cadherin can ameliorate the prohemocyte response to oxidative stress. Collectively, these data strongly suggest that ROS promote differentiation by reducing E-cadherin levels. In mammalian systems, ROS promote embryonic stem cell differentiation, whereas E-cadherin blocks differentiation. However, it is not known if elevated ROS reduce E-cadherin to promote embryonic stem cell differentiation. Thus, our findings may have identified an important mechanism by which ROS promote stem/progenitor cell differentiation.     

Wingless signaling regulates the maintenance of ovarian somatic stem cells in Drosophila

Identifying the signals involved in maintaining stem cells is critical to understanding stem cell biology and to using stem cells in future regenerative medicine. In the Drosophila ovary, Hedgehog is the only known signal for maintaining somatic stem cells (SSCs). Here we report that Wingless (Wg) signaling is also essential for SSC maintenance in the Drosophila ovary. Wg is expressed in terminal filament and cap cells, a few cells away from SSCs. Downregulation of Wg signaling in SSCs through removal of positive regulators of Wg signaling, dishevelled and armadillo, results in rapid SSC loss. Constitutive Wg signaling in SSCs through the removal of its negative regulators, Axin and shaggy, also causes SSC loss. Also, constitutive wg signaling causes over-proliferation and abnormal differentiation of somatic follicle cells. This work demonstrates that wg signaling regulates SSC maintenance and that its constitutive signaling influences follicle cell proliferation and differentiation. In mammals, constitutive beta-catenin causes over-proliferation and abnormal differentiation of skin cells, resulting in skin cancer formation. Possibly, mechanisms regulating proliferation and differentiation of epithelial cells, including epithelial stem cells, is conserved from Drosophila to man.     

The endoderm specifies the mesodermal niche for the germline in Drosophila via Delta-Notch signaling

Interactions between niche cells and stem cells are vital for proper control over stem cell self-renewal and differentiation. However, there are few tissues where the initial establishment of a niche has been studied. The Drosophila testis houses two stem cell populations, which each lie adjacent to somatic niche cells. Although these niche cells sustain spermatogenesis throughout life, it is not understood how their fate is established. Here, we show that Notch signaling is necessary to specify niche cell fate in the developing gonad. Surprisingly, our results indicate that adjacent endoderm is the source of the Notch-activating ligand Delta. We also find that niche cell specification occurs earlier than anticipated, well before the expression of extant markers for niche cell fate. This work further suggests that endoderm plays a dual role in germline development. The endoderm assists both in delivering germ cells to the somatic gonadal mesoderm, and in specifying the niche where these cells will subsequently develop as stem cells. Because in mammals primordial germ cells also track through endoderm on their way to the genital ridge, our work raises the possibility that conserved mechanisms are employed to regulate germline niche formation.     

Dual role for Insulin/TOR signaling in the control of hematopoietic progenitor maintenance in Drosophila

The interconnected Insulin/IGF signaling (IlS) and Target of Rapamycin (TOR) signaling pathways constitute the main branches of the nutrient-sensing system that couples growth to nutritional conditions in Drosophila. Here, we addressed the influence of these pathways and of diet restriction on the balance between the maintenance of multipotent hematopoietic progenitors and their differentiation in the Drosophila lymph gland. In this larval hematopoietic organ, a pool of stem-like progenitor blood cells (prohemocytes) is kept undifferentiated in response to signaling from a specialized group of cells forming the posterior signaling center (PSC), which serves as a stem cell niche. We show that, reminiscent of the situation in human, loss of the negative regulator of IIS Pten results in lymph gland hyperplasia, aberrant blood cell differentiation and hematopoietic progenitor exhaustion. Using site-directed loss- and gain-of-function analysis, we demonstrate that components of the IIS/TOR pathways control lymph gland homeostasis at two levels. First, they cell-autonomously regulate the size and activity of the hematopoietic niche. Second, they are required within the prohemocytes to control their growth and maintenance. Moreover, we show that diet restriction or genetic alteration mimicking amino acid deprivation triggers progenitor cell differentiation. Hence, our study highlights the role of the IIS/TOR pathways in orchestrating hematopoietic progenitor fate and links blood cell fate to nutritional status.     

小肠干细胞的命运调控

小肠上皮具有快速更新的能力,是研究成体干细胞的理想系统.小肠上皮由绒毛和隐窝两部分组成,而位于小肠隐窝底部的小肠干细胞是其持续更新的源泉.近年来,以Lgr5为代表的小肠干细胞标记物的发现、Lgr5+小肠干细胞的分离培养和多种转基因小鼠模型的出现,极大地促进了对小肠干细胞自我更新和分化调控的研究,使得人们可以更加深入地认识小肠干细胞命运决定的分子机制.本文简要综述了近年来人们对Wnt,BMP,Notch和EGF等信号如何在小肠干细胞命运调控中发挥作用的认识.     

The role of Wingless signaling in establishing the anteroposterior and dorsoventral axes of the eye disc

The posteriorly expressed signaling molecules Hedgehog and Decapentaplegic drive photoreceptor differentiation in the Drosophila eye disc, while at the anterior lateral margins Wingless expression blocks ectopic differentiation. We show here that mutations in axin prevent photoreceptor differentiation and lead to tissue overgrowth and that both these effects are due to ectopic activation of the Wingless pathway. In addition, ectopic Wingless signaling causes posterior cells to take on an anterior identity, reorienting the direction of morphogenetic furrow progression in neighboring wild-type cells. We also show that signaling by Decapentaplegic and Hedgehog normally blocks the posterior expression of anterior markers such as Eyeless. Wingless signaling is not required to maintain anterior Eyeless expression and in combination with Decapentaplegic signaling can promote its downregulation, suggesting that additional molecules contribute to anterior identity. Along the dorsoventral axis of the eye disc, Wingless signaling is sufficient to promote dorsal expression of the Iroquois gene mirror, even in the absence of the upstream factor pannier. However, Wingless signaling does not lead to ventral mirror expression, implying the existence of ventral repressors.     

The Hopscotch Jak kinase requires the Raf pathway to promote blood cell activation and differentiation in<Emphasis Type="Italic"> Drosophila</Emphasis>

Cytokines regulate the development and differentiated functions of hematopoietic cells by activating multiple signaling pathways, including the Jak-Stat pathway, the PI3-kinase pathway, and the Ras/Raf pathway. While the Jak-Stat interaction has been extensively studied, the relationship between this pathway and other cytokine-induced signaling pathways is not fully understood. In Drosophila melanogaster, mutations that result in hyperactivity of the Jak kinase Hopscotch (Hop) cause an activation of the larval blood cell encapsulation response, including blood cell aggregation and differentiation of plasmatocytes into apparent lamellocytes. Here, we demonstrate that Hop requires the activity of the Raf pathway to promote the activation response of larval plasmatocytes, and provide evidence to suggest that the Hop and D-Raf proteins physically interact. We also show that basal level activity of the Raf pathway is required for the accumulation of circulating blood cells.