SAM, Histones, and Stem Cells: Mouse embryonic stem cells require threonine for growth and express large amounts of the enzyme that catalyzes the first step in threonine metabolism. To find out what is so important about threonine in these cells, Shyh-Change et al. (p. 222 , published online 1 November; see the Perspective by Sassone-Corsi ) monitored changes in metabolism by mass spectrometry in induced pluripotent stem cells. Threonine was required to maintain cellular concentrations of S-adenosylmethionine (SAM), a substrate used for protein methylation. Restriction of threonine inhibited methylation of histones, which is characteristic of chromatin in embryonic stem cells. Thus, altered metabolism of threonine and methionine in stem cells may be linked to epigenetic changes that influence genetic reprogramming and decisions of stem cells to proliferate or differentiate.

Thrombocytes are the nucleated equivalent of platelets in nonmammalian vertebrates such as the zebrafish, Danio rerio. We have cloned zebrafish CD41 cDNA (αIIb, glycoprotein IIb [GPIIb]) and its promoter and have generated transgenic zebrafish lines with green fluorescent protein (GFP)–tagged thrombocytes. CD41 mRNA transcripts appeared 42 hours after fertilization (hpf) by reverse-transcriptase–polymerase chain reaction (RT-PCR) and at 48 hpf in circulating hematopoietic cells. Flow sorting of thrombocytes from the mesonephros of adult CD41-GFP zebrafish showed a GFPhigh subset, which had the morphologic appearance of mature thrombocytes, and a GFPlow subset with an immature appearance, suggesting that they may be thrombocyte precursors. Confocal laser microscopy of embryos 40 and 48 hpf also showed a nonmobile population of GFP+ cells in a discrete area between the dorsal aorta and caudal vein. Production of circulating thrombocytes was inhibited by the injection of antisense morpholinos for the stem-cell transcription factor scl and c-mpl, the receptor for thrombopoietin. The nonmobile pool of GFP+ cells was abolished by scl knockdown and partially inhibited by c-mpl knockdown. These studies have shown that it is possible to identify thrombocytes, thrombocyte precursors, and, possibly, early hematopoietic stem cells in zebrafish embryos and track their proliferation and maturation.

An RNA interference screen of 30 gene candidates has identified Lin28, a negative regulator of let-7 microRNA processing, as a potentially key regulator of primordial germ cell development, the process in the developing embryo that selects the cells destined to produce sperm and eggs. In addition, Lin28 levels are elevated in primary human germ cell tumours, suggesting that it may also be implicated in germ cell malignancy. In order to investigate the earliest molecular mechanisms of germ cell specification, mouse embryonic stem cells were differentiated into putative primordial germ cells (PGCs) in vitro. The use of inhibitory RNAs to then screen candidate genes for effects on the development of these cells demonstrates a genetic pathway for PGC specification involving Lin28, a negative regulator of let-7 microRNA processing. The rarity and inaccessibility of the earliest primordial germ cells (PGCs) in the mouse embryo thwart efforts to investigate molecular mechanisms of germ-cell specification. stella (also called Dppa3) marks the rare founder population of the germ lineage1,2. Here we differentiate mouse embryonic stem cells carrying a stella transgenic reporter into putative PGCs in vitro. The Stella+ cells possess a transcriptional profile similar to embryo-derived PGCs, and like their counterparts in vivo, lose imprints in a time-dependent manner. Using inhibitory RNAs to screen candidate genes for effects on the development of Stella+ cells in vitro, we discovered that Lin28, a negative regulator of let-7 microRNA processing3,4,5,6, is essential for proper PGC development. Furthermore, we show that Blimp1 (also called Prdm1), a let-7 target and a master regulator of PGC specification7,8,9, can rescue the effect of Lin28 deficiency during PGC development, thereby establishing a mechanism of action for Lin28 during PGC specification. Overexpression of Lin28 promotes formation of Stella+ cells in vitro and PGCs in chimaeric embryos, and is associated with human germ-cell tumours. The differentiation of putative PGCs from embryonic stem cells in vitro recapitulates the early stages of gamete development in vivo, and provides an accessible system for discovering novel genes involved in germ-cell development and malignancy.

The growing knowledge of ferroptosis has suggested the role and therapeutic potential of ferroptosis in cancer, but has not been translated into effective therapy. Liver cancer, primarily hepatocellular carcinoma (HCC), is highly lethal with limited treatment options. LIFR is frequently downregulated in HCC. Here, by studying hepatocyte-specific and inducible Lifr-knockout mice, we show that loss of Lifr promotes liver tumorigenesis and confers resistance to drug-induced ferroptosis. Mechanistically, loss of LIFR activates NF-κB signaling through SHP1, leading to upregulation of the iron-sequestering cytokine LCN2, which depletes iron and renders insensitivity to ferroptosis inducers. Notably, an LCN2-neutralizing antibody enhances the ferroptosis-inducing and anticancer effects of sorafenib on HCC patient-derived xenograft tumors with low LIFR expression and high LCN2 expression. Thus, anti-LCN2 therapy is a promising way to improve liver cancer treatment by targeting ferroptosis.