1. Clonal evolution mechanisms in NT5C2 mutant-relapsed acute lymphoblastic leukaemia.
Relapsed acute lymphoblastic leukaemia (ALL) is associated with resistance to chemotherapy and poor prognosis. Gain-of-function mutations in the 5′-nucleotidase, cytosolic II (NT5C2) gene induce resistance to 6-mercaptopurine and are selectively present in relapsed ALL. Yet, the mechanisms involved in NT5C2 mutation-driven clonal evolution during the initiation of leukaemia, disease progression and relapse remain unknown. Here Gannie Tzoneva at Columbia University in New York, USA and her colleagues use a conditional-and-inducible leukaemia model to demonstrate that expression of NT5C2(R367Q), a highly prevalent relapsed-ALL NT5C2 mutation, induces resistance to chemotherapy with 6-mercaptopurine at the cost of impaired leukaemia cell growth and leukaemia-initiating cell activity. The loss-of-fitness phenotype of NT5C2+/R367Q mutant cells is associated with excess export of purines to the extracellular space and depletion of the intracellular purine-nucleotide pool. Consequently, blocking guanosine synthesis by inhibition of inosine-5′-monophosphate dehydrogenase (IMPDH) induced increased cytotoxicity against NT5C2-mutant leukaemia lymphoblasts. These results identify the fitness cost of NT5C2 mutation and resistance to chemotherapy as key evolutionary drivers that shape clonal evolution in relapsed ALL and support a role for IMPDH inhibition in the treatment of ALL.
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2. Structures of β-klotho reveal a ‘zip code’-like mechanism for endocrine FGF signaling.
Canonical fibroblast growth factors (FGFs) activate FGF receptors (FGFRs) through paracrine or autocrine mechanisms in a process that requires cooperation with heparan sulfate proteoglycans, which function as co-receptors for FGFR activation. By contrast, endocrine FGFs (FGF19, FGF21 and FGF23) are circulating hormones that regulate critical metabolic processes in a variety of tissues. FGF19 regulates bile acid synthesis and lipogenesis, whereas FGF21 stimulates insulin sensitivity, energy expenditure and weight loss. Endocrine FGFs signal through FGFRs in a manner that requires klothos, which are cell-surface proteins that possess tandem glycosidase domains. Here Sangwon Lee at Yale School of Medicine in New Haven, Connecticut, USA and his colleagues describe the crystal structures of free and ligand-bound β-klotho extracellular regions that reveal the molecular mechanism that underlies the specificity of FGF21 towards β-klotho and demonstrate how the FGFR is activated in a klotho-dependent manner. β-Klotho serves as a primary ‘zip code’-like receptor that acts as a targeting signal for FGF21, and FGFR functions as a catalytic subunit that mediates intracellular signalling. Their structures also show how the sugar-cutting enzyme glycosidase has evolved to become a specific receptor for hormones that regulate metabolic processes, including the lowering of blood sugar levels. Finally, they describe an agonistic variant of FGF21 with enhanced biological activity and present structural insights into the potential development of therapeutic agents for diseases linked to endocrine FGFs.
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3. Structure and mutagenesis reveal essential capsid protein interactions for KSHV replication.
Kaposi’s sarcoma-associated herpesvirus (KSHV) causes Kaposi’s sarcoma, a cancer that commonly affects patients with AIDS and which is endemic in sub-Saharan Africa. The KSHV capsid is highly pressurized by its double-stranded DNA genome, as are the capsids of the eight other human herpesviruses. Capsid assembly and genome packaging of herpesviruses are prone to interruption and can therefore be targeted for the structure-guided development of antiviral agents. However, herpesvirus capsids—comprising nearly 3,000 proteins and over 1,300 Å in diameter—present a formidable challenge to atomic structure determination and functional mapping of molecular interactions. Here Xinghong Dai at University of California, Los Angeles (UCLA) in Los Angeles, California, USA and his colleagues report a 4.2 Å resolution structure of the KSHV capsid, determined by electron-counting cryo-electron microscopy, and its atomic model, which contains 46 unique conformers of the major capsid protein (MCP), the smallest capsid protein (SCP) and the triplex proteins Tri1 and Tri2. Their structure and mutagenesis results reveal a groove in the upper domain of the MCP that contains hydrophobic residues that interact with the SCP, which in turn crosslinks with neighbouring MCPs in the same hexon to stabilize the capsid. Multiple levels of MCP–MCP interaction—including six sets of stacked hairpins lining the hexon channel, disulfide bonds across channel and buttress domains in neighbouring MCPs, and an interaction network forged by the N-lasso domain and secured by the dimerization domain—define a robust capsid that is resistant to the pressure exerted by the enclosed genome. The triplexes, each composed of two Tri2 molecules and a Tri1 molecule, anchor to the capsid floor via a Tri1 N-anchor to plug holes in the MCP network and rivet the capsid floor. These essential roles of the MCP N-lasso and Tri1 N-anchor are verified by serial-truncation mutageneses. Their proof-of-concept demonstration of the use of polypeptides that mimic the smallest capsid protein to inhibit KSHV lytic replication highlights the potential for exploiting the interaction hotspots revealed in our atomic structure to develop antiviral agents.
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4. Monitoring T cell–dendritic cell interactions in vivo by intercellular enzymatic labeling.
Interactions between different cell types are essential for multiple biological processes, including immunity, embryonic development and neuronal signalling. Although the dynamics of cell–cell interactions can be monitored in vivo by intravital microscopy, this approach does not provide any information on the receptors and ligands involved or enable the isolation of interacting cells for downstream analysis. Here Giulia Pasqual at The Rockefeller University in New York, USA and his colleagues describe a complementary approach that uses bacterial sortase A-mediated cell labelling across synapses of immune cells to identify receptor–ligand interactions between cells in living mice, by generating a signal that can subsequently be detected ex vivo by flow cytometry. They call this approach for the labelling of ‘kiss-and-run’ interactions between immune cells ‘Labelling Immune Partnerships by SorTagging Intercellular Contacts’ (LIPSTIC). Using LIPSTIC, they show that interactions between dendritic cells and CD4+ T cells during T-cell priming in vivo occur in two distinct modalities: an early, cognate stage, during which CD40–CD40L interactions occur specifically between T cells and antigen-loaded dendritic cells; and a later, non-cognate stage during which these interactions no longer require prior engagement of the T-cell receptor. Therefore, LIPSTIC enables the direct measurement of dynamic cell–cell interactions both in vitro and in vivo. Given its flexibility for use with different receptor–ligand pairs and a range of detectable labels, they expect that this approach will be of use to any field of biology requiring quantification of intercellular communication.
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5. Regulation of embryonic haematopoietic multipotency by EZH1.
All haematopoietic cell lineages that circulate in the blood of adult mammals derive from multipotent haematopoietic stem cells (HSCs). By contrast, in the blood of mammalian embryos, lineage-restricted progenitors arise first, independently of HSCs, which only emerge later in gestation. As best defined in the mouse, ‘primitive’ progenitors first appear in the yolk sac at 7.5 days post-coitum. Subsequently, erythroid–myeloid progenitors that express fetal haemoglobin, as well as fetal lymphoid progenitors, develop in the yolk sac and the embryo proper, but these cells lack HSC potential. Ultimately, ‘definitive’ HSCs with long-term, multilineage potential and the ability to engraft irradiated adults emerge at 10.5 days post-coitum from arterial endothelium in the aorta-gonad-mesonephros and other haemogenic vasculature. The molecular mechanisms of this reverse progression of haematopoietic ontogeny remain unexplained. Linda T. Vo at Boston Children’s Hospital in Boston, Massachusetts, USA and her colleagues hypothesized that the definitive haematopoietic program might be actively repressed in early embryogenesis through epigenetic silencing, and that alleviating this repression would elicit multipotency in otherwise lineage-restricted haematopoietic progenitors. Here they show that reduced expression of the Polycomb group protein EZH1 enhances multi-lymphoid output from human pluripotent stem cells. In addition, Ezh1 deficiency in mouse embryos results in precocious emergence of functional definitive HSCs in vivo. Thus, they identify EZH1 as a repressor of haematopoietic multipotency in the early mammalian embryo.
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