synMuv B proteins antagonize germline fate in the intestine and ensure C. elegans survival

Previous studies demonstrated that a subset of synMuv B mutants ectopically misexpress germline-specific P-granule proteins in their somatic cells, suggesting a failure to properly orchestrate a soma/germline fate decision. Surprisingly, this fate confusion does not affect viability at low to ambient temperatures. Here, we show that, when grown at high temperature, a majority of synMuv B mutants irreversibly arrest at the L1 stage. High temperature arrest (HTA) is accompanied by upregulation of many genes characteristic of germ line, including genes encoding components of the synaptonemal complex and other meiosis proteins. HTA is suppressed by loss of global regulators of germline chromatin, including MES-4, MRG-1, ISW-1 and the MES-2/3/6 complex, revealing that arrest is caused by somatic cells possessing a germline-like chromatin state. Germline genes are preferentially misregulated in the intestine, and necessity and sufficiency tests demonstrate that the intestine is the tissue responsible for HTA. We propose that synMuv B mutants fail to erase or antagonize an inherited germline chromatin state in somatic cells during embryonic and early larval development. As a consequence, somatic cells gain a germline program of gene expression in addition to their somatic program, leading to a mixed fate. Somatic expression of germline genes is enhanced at elevated temperature, leading to developmentally compromised somatic cells and arrest of newly hatched larvae.     

CDC-25.1 stability is regulated by distinct domains to restrict cell division during embryogenesis in C. elegans

Cdc25 phosphatases are key positive cell cycle regulators that coordinate cell divisions with growth and morphogenesis in many organisms. Intriguingly in C. elegans, two cdc-25.1(gf) mutations induce tissue-specific and temporally restricted hyperplasia in the embryonic intestinal lineage, despite stabilization of the mutant CDC-25.1 protein in every blastomere. We investigated the molecular basis underlying the CDC-25.1(gf) stabilization and its associated tissue-specific phenotype. We found that both mutations affect a canonical beta-TrCP phosphodegron motif, while the F-box protein LIN-23, the beta-TrCP orthologue, is required for the timely degradation of CDC-25.1. Accordingly, depletion of lin-23 in wild-type embryos stabilizes CDC-25.1 and triggers intestinal hyperplasia, which is, at least in part, cdc-25.1 dependent. lin-23(RNAi) causes embryonic lethality owing to cell fate transformations that convert blastomeres to an intestinal fate, sensitizing them to increased levels of CDC-25.1. Our characterization of a novel destabilizing cdc-25.1(lf) intragenic suppressor that acts independently of lin-23 indicates that additional cues impinge on different motifs of the CDC-25.1 phosphatase during early embryogenesis to control its stability and turnover, in order to ensure the timely divisions of intestinal cells and coordinate them with the formation of the developing gut.     

Divided loyalties: transdetermination and the genetics of tissue regeneration

Most tissues contain cells capable of the self-renewal and differentiation necessary to maintain tissue and organ integrity. These somatic stem cells are generally thought to have limited developmental potential. The mechanisms that restrict cell fate decisions in somatic stem cells are only now being understood. This understanding will be important in the clinical exploitation of adult stem cells in tissue repair and replacement. Experiments performed over fifty years ago in Drosophila showed that developmental restriction could be relaxed in the proliferating larval cells that are destined to form the adult fly integument. This phenomenon, called transdetermination, can serve as a model for mechanisms of stem-cell commitment. A recent publication (1) sheds new light on the mechanism of transdetermination by demonstrating that loss of homeotic gene silencing leads to increased frequency of transdetermination. In addition, the authors link a specific signaling pathway induced by tissue regeneration to the relaxation of homeotic gene silencing. The data identify key mechanisms that control developmental homeostasis and cell fate restriction that could be manipulated to make somatic stem-cell engineering possible.     

Hyperactivated Wnt Signaling Induces Synthetic Lethal Interaction with Rb Inactivation by Elevating TORC1 Activities

Inactivation of the Rb tumor suppressor can lead to increased cell proliferation or cell death depending on specific cellular context. Therefore, identification of the interacting pathways that modulate the effect of Rb loss will provide novel insights into the roles of Rb in cancer development and promote new therapeutic strategies. Here, we identify a novel synthetic lethal interaction between Rb inactivation and deregulated Wg/Wnt signaling through unbiased genetic screens. We show that a weak allele of axin, which deregulates Wg signaling and increases cell proliferation without obvious effects on cell fate specification, significantly alters metabolic gene expression, causes hypersensitivity to metabolic stress induced by fasting, and induces synergistic apoptosis with mutation of fly Rb ortholog, rbf. Furthermore, hyperactivation of Wg signaling by other components of the Wg pathway also induces synergistic apoptosis with rbf. We show that hyperactivated Wg signaling significantly increases TORC1 activity and induces excessive energy stress with rbf mutation. Inhibition of TORC1 activity significantly suppressed synergistic cell death induced by hyperactivated Wg signaling and rbf inactivation, which is correlated with decreased energy stress and decreased induction of apoptotic regulator expression. Finally the synthetic lethality between Rb and deregulated Wnt signaling is conserved in mammalian cells and that inactivation of Rb and APC induces synergistic cell death through a similar mechanism. These results suggest that elevated TORC1 activity and metabolic stress underpin the evolutionarily conserved synthetic lethal interaction between hyperactivated Wnt signaling and inactivated Rb tumor suppressor.     

Notch signaling maintains Leydig progenitor cells in the mouse testis

During testis development, fetal Leydig cells increase their population from a pool of progenitor cells rather than from proliferation of a differentiated cell population. However, the mechanism that regulates Leydig stem cell self-renewal and differentiation is unknown. Here, we show that blocking Notch signaling, by inhibiting gamma-secretase activity or deleting the downstream target gene Hairy/Enhancer-of-split 1, results in an increase in Leydig cells in the testis. By contrast, constitutively active Notch signaling in gonadal somatic progenitor cells causes a dramatic Leydig cell loss, associated with an increase in undifferentiated mesenchymal cells. These results indicate that active Notch signaling restricts fetal Leydig cell differentiation by promoting a progenitor cell fate. Germ cell loss and abnormal testis cord formation were observed in both gain- and loss-of-function gonads, suggesting that regulation of the Leydig/interstitial cell population is important for male germ cell survival and testis cord formation.     

Notch signaling in intestinal homeostasis across species: the cases of Drosophila, Zebrafish and the mouse

Notch signaling has been recently shown to have a fundamental role in stem cell maintenance and control of proper homeostasis in the intestine of different species. Here, we briefly review the current literature on Notch signals in the intestine of Drosophila, Zebrafish and the mouse, and try to highlight conserved and divergent Notch functions across species. Notch signals show a remarkably conserved role in skewing cell fate choices in intestinal lineages throughout evolution. Genetic analysis demonstrates that loss of Notch signaling invariably leads to increased numbers of secretory cells and loss of enterocytes, while gain of Notch function will completely block secretory cell differentiation. Finally, we discuss the potential contribution of Notch signaling to the initiation of colorectal cancer by controlling the maintenance of the undifferentiated state of intestinal neoplastic cells and speculate on the therapeutic consequences of affecting cancer stem cells.     

Mago nashi is essential for spermatogenesis in Marsilea

Spermatogenesis in Marsilea vestita is a rapid process that is activated by placing dry microspores into water. Nine division cycles produce seven somatic cells and 32 spermatids, where size and position define identity. Spermatids undergo de novo formation of basal bodies in a particle known as a blepharoplast. We are interested in mechanisms responsible for spermatogenous initial formation. Mago nashi (Mv-mago) is a highly conserved gene present as stored mRNA and stored protein in the microspore. Mv-mago protein increases in abundance during development and it localizes at discrete cytoplasmic foci (Mago-dots). RNA interference experiments show that new Mv-mago protein is required for development. With Mv-mago silenced, asymmetric divisions become symmetric, cell fate is disrupted, and development stops. The alpha-tubulin protein distribution, centrin translation, and Mv-PRP19 mRNA distribution are no longer restricted to the spermatogenous cells. Centrin aggregations, resembling blepharoplasts, occur in jacket cells. Mago-dots are undetectable after the silencing of Mv-mago, Mv-Y14, or Mv-eIF4AIII, three core components of the exon junction complex (EJC), suggesting that Mago-dots are either EJCs in the cytoplasm, or Mv-mago protein aggregations dependent on EJCs. Mv-mago protein and other EJC components apparently function in cell fate determination in developing male gametophytes of M. vestita.     

Snx5, as a Mind bomb-binding protein, is expressed in hematopoietic and endothelial precursor cells in zebrafish

Notch signaling has an evolutionarily conserved function for cell fate determination and stem cell maintenance. Previously, we identified a novel component of the Notch signaling pathway in zebrafish, mind bomb, which encodes an E3 ubiquitin ligase essential for Notch signal activation. Further studies showed that Mind bomb(-/-) mouse embryos exhibited pan-Notch phenotypes in various tissues, suggesting that Mind bomb function is conserved in mammals. Therefore we sought to understand the various molecular partners of Mind bomb using yeast two-hybrid screening. In this search we identified Sorting nexin 5 (Snx5) as a novel interacting partner of Mind bomb. Furthermore we demonstrated that Snx5 colocalizes with Mind bomb in early endosomal compartments, suggesting that Snx5 is important for Mind bomb trafficking. In addition, we identified zebrafish orthologue of Snx5 and showed that snx5 is predominantly expressed in hematopoietic and endothelial precursor cells in zebrafish. We also found defects in hematopoiesis and blood vessel development in snx5 morpholino-injected embryos. Taken together, we show that Snx5, a novel interacting partner of Mind bomb, may have an essential role for cell fate determination in early development.     

Wnt signaling controls the stem cell-like asymmetric division of the epithelial seam cells during C. elegans larval development

Metazoan stem cells repopulate tissues during adult life by dividing asymmetrically to generate another stem cell and a cell that terminally differentiates. Wnt signaling regulates the division pattern of stem cells in flies and vertebrates. While the short-lived nematode C. elegans has no adult somatic stem cells, the lateral epithelial seam cells divide in a stem cell-like manner in each larval stage, usually generating a posterior daughter that retains the seam cell fate and an anterior daughter that terminally differentiates. We show that while wild-type adult animals have 16 seam cells per side, animals with reduced function of the TCF homolog POP-1 have as many as 67 seam cells, and animals with reduced function of the β-catenins SYS-1 and WRM-1 have as few as three. Analysis of seam cell division patterns showed alterations in their stem cell-like divisions in the L2-L4 stages: reduced Wnt signaling caused both daughters to adopt non-seam fates, while activated Wnt signaling caused both daughters to adopt the seam fate. Therefore, our results indicate that Wnt signaling globally regulates the asymmetric, stem cell-like division of most or all somatic seam cells during C. elegans larval development, and that Wnt pathway regulation of stem cell-like behavior is conserved in nematodes.     

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.     

The bovine B and C blood group systems are not likely to be the orthologues of human RH: an interesting twist in the comparative map

We tested the hypothesis that either the bovine B or C blood group system is the orthologue of human RH. A comparative linkage mapping strategy was applied, using blood typing and restriction fragment length polymorphism (RFLP) analysis of four loci linked to RH on HSA1; PGD, FGR, ALPL and FUCA1. Four sires with a total of 255 half-sib offspring were used for the linkage analysis. Strong support for linkage between ALPL, FUCA1 and FGR was obtained for all sire families (lod scores >11 for all pairwise comparisons). This new linkage group was assigned to bovine synteny group U17 based on previous somatic cell mapping of the FGR locus. The most favoured order is ALPL—FUCA1—FGR (2·18:1), with ALPL and FGR 5·4 cm and 6·3 cm , respectively, from FUCA1. The B and C blood group systems and PGD were genetically independent of each other and all other markers, indicating that neither B nor C is likely to be the bovine orthologue of human RH. However, given available comparative mapping data, there is some chance that the bovine orthologue of RH is on bovine synteny group U6. Although gene order appears to be conserved with humans, the differences in recombination rates between these three loci in cattle, humans and mice strongly suggest that it is not possible to use human map distances to predict map distances in cattle, making it imperative that bovine gene mappers continue to emphasize adding type I markers to the bovine linkage map.