Oligodendrocyte-myelin glycoprotein is a Nogo receptor ligand that inhibits neurite outgrowth

The inhibitory activity associated with myelin is a major obstacle for successful axon regeneration in the adult mammalian central nervous system (CNS). In addition to myelin-associated glycoprotein (MAG) and Nogo-A, available evidence suggests the existence of additional inhibitors in CNS myelin. We show here that a glycosylphosphatidylinositol (GPI)-anchored CNS myelin protein, oligodendrocyte-myelin glycoprotein (OMgp), is a potent inhibitor of neurite outgrowth in cultured neurons. Like Nogo-A, OMgp contributes significantly to the inhibitory activity associated with CNS myelin. To further elucidate the mechanisms that mediate this inhibitory activity of OMgp, we screened an expression library and identified the Nogo receptor (NgR) as a high-affinity OMgp-binding protein. Cleavage of NgR and other GPI-linked proteins from the cell surface renders axons of dorsal root ganglia insensitive to OMgp. Introduction of exogenous NgR confers OMgp responsiveness to otherwise insensitive neurons. Thus, OMgp is an important inhibitor of neurite outgrowth that acts through NgR and its associated receptor complex. Interfering with the OMgp/NgR pathway may allow lesioned axons to regenerate after injury in vivo.     

P75 interacts with the Nogo receptor as a co-receptor for Nogo,MAG and OMgp

In inhibiting neurite outgrowth, several myelin components, including the extracellular domain of Nogo-A (Nogo-66), oligodendrocyte myelin glycoprotein (OMgp) and myelin-associated glycoprotein (MAG), exert their effects through the same Nogo receptor (NgR). The glycosyl phosphatidylinositol (GPI)-anchored nature of NgR indicates the requirement for additional transmembrane protein(s) to transduce the inhibitory signals into the interior of responding neurons. Here, we demonstrate that p75, a transmembrane protein known to be a receptor for the neurotrophin family of growth factors, specifically interacts with NgR. p75 is required for NgR-mediated signalling, as neurons from p75 knockout mice are no longer responsive to myelin and to each of the known NgR ligands. Blocking the p75-NgR interaction also reduces the activities of these inhibitors. Moreover, a truncated p75 protein lacking the intracellular domain, when overexpressed in primary neurons, attenuates the same set of inhibitory activities, suggesting that p75 is a signal transducer of the NgR-p75 receptor complex. Thus, interfering with p75 and its downstream signalling pathways may allow lesioned axons to overcome most of the inhibitory activities associated with central nervous system myelin.     

Protein-folding location can regulate manganese-binding versus copper- or zinc-binding

Metals are needed by at least one-quarter of all proteins. Although metallochaperones insert the correct metal into some proteins, they have not been found for the vast majority, and the view is that most metalloproteins acquire their metals directly from cellular pools. However, some metals form more stable complexes with proteins than do others. For instance, as described in the Irving-Williams series, Cu(2+) and Zn(2+) typically form more stable complexes than Mn(2+). Thus it is unclear what cellular mechanisms manage metal acquisition by most nascent proteins. To investigate this question, we identified the most abundant Cu(2+)-protein, CucA (Cu(2+)-cupin A), and the most abundant Mn(2+)-protein, MncA (Mn(2+)-cupin A), in the periplasm of the cyanobacterium Synechocystis PCC 6803. Each of these newly identified proteins binds its respective metal via identical ligands within a cupin fold. Consistent with the Irving-Williams series, MncA only binds Mn(2+) after folding in solutions containing at least a 10(4) times molar excess of Mn(2+) over Cu(2+) or Zn(2+). However once MncA has bound Mn(2+), the metal does not exchange with Cu(2+). MncA and CucA have signal peptides for different export pathways into the periplasm, Tat and Sec respectively. Export by the Tat pathway allows MncA to fold in the cytoplasm, which contains only tightly bound copper or Zn(2+) (refs 10-12) but micromolar Mn(2+) (ref. 13). In contrast, CucA folds in the periplasm to acquire Cu(2+). These results reveal a mechanism whereby the compartment in which a protein folds overrides its binding preference to control its metal content. They explain why the cytoplasm must contain only tightly bound and buffered copper and Zn(2+).     

Broad phylogenomic sampling improves resolution of the animal tree of life

Long-held ideas regarding the evolutionary relationships among animals have recently been upended by sometimes controversial hypotheses based largely on insights from molecular data. These new hypotheses include a clade of moulting animals (Ecdysozoa) and the close relationship of the lophophorates to molluscs and annelids (Lophotrochozoa). Many relationships remain disputed, including those that are required to polarize key features of character evolution, and support for deep nodes is often low. Phylogenomic approaches, which use data from many genes, have shown promise for resolving deep animal relationships, but are hindered by a lack of data from many important groups. Here we report a total of 39.9 Mb of expressed sequence tags from 29 animals belonging to 21 phyla, including 11 phyla previously lacking genomic or expressed-sequence-tag data. Analysed in combination with existing sequences, our data reinforce several previously identified clades that split deeply in the animal tree (including Protostomia, Ecdysozoa and Lophotrochozoa), unambiguously resolve multiple long-standing issues for which there was strong conflicting support in earlier studies with less data (such as velvet worms rather than tardigrades as the sister group of arthropods), and provide molecular support for the monophyly of molluscs, a group long recognized by morphologists. In addition, we find strong support for several new hypotheses. These include a clade that unites annelids (including sipunculans and echiurans) with nemerteans, phoronids and brachiopods, molluscs as sister to that assemblage, and the placement of ctenophores as the earliest diverging extant multicellular animals. A single origin of spiral cleavage (with subsequent losses) is inferred from well-supported nodes. Many relationships between a stable subset of taxa find strong support, and a diminishing number of lineages remain recalcitrant to placement on the tree.     

An inhibitor of Bcl-2 family proteins induces regression of solid tumours

Proteins in the Bcl-2 family are central regulators of programmed cell death, and members that inhibit apoptosis, such as Bcl-X(L) and Bcl-2, are overexpressed in many cancers and contribute to tumour initiation, progression and resistance to therapy. Bcl-X(L) expression correlates with chemo-resistance of tumour cell lines, and reductions in Bcl-2 increase sensitivity to anticancer drugs and enhance in vivo survival. The development of inhibitors of these proteins as potential anti-cancer therapeutics has been previously explored, but obtaining potent small-molecule inhibitors has proved difficult owing to the necessity of targeting a protein-protein interaction. Here, using nuclear magnetic resonance (NMR)-based screening, parallel synthesis and structure-based design, we have discovered ABT-737, a small-molecule inhibitor of the anti-apoptotic proteins Bcl-2, Bcl-X(L) and Bcl-w, with an affinity two to three orders of magnitude more potent than previously reported compounds. Mechanistic studies reveal that ABT-737 does not directly initiate the apoptotic process, but enhances the effects of death signals, displaying synergistic cytotoxicity with chemotherapeutics and radiation. ABT-737 exhibits single-agent-mechanism-based killing of cells from lymphoma and small-cell lung carcinoma lines, as well as primary patient-derived cells, and in animal models, ABT-737 improves survival, causes regression of established tumours, and produces cures in a high percentage of the mice.     

Ephrin signalling controls brain size by regulating apoptosis of neural progenitors

Mechanisms controlling brain size include the regulation of neural progenitor cell proliferation, differentiation, survival and migration. Here we show that ephrin-A/EphA receptor signalling plays a key role in controlling the size of the mouse cerebral cortex by regulating cortical progenitor cell apoptosis. In vivo gain of EphA receptor function, achieved through ectopic expression of ephrin-A5 in early cortical progenitors expressing EphA7, caused a transient wave of neural progenitor cell apoptosis, resulting in premature depletion of progenitors and a subsequent dramatic decrease in cortical size. In vitro treatment with soluble ephrin-A ligands similarly induced the rapid death of cultured dissociated cortical progenitors in a caspase-3-dependent manner, thereby confirming a direct effect of ephrin/Eph signalling on apoptotic cascades. Conversely, in vivo loss of EphA function, achieved through EphA7 gene disruption, caused a reduction in apoptosis occurring normally in forebrain neural progenitors, resulting in an increase in cortical size and, in extreme cases, exencephalic forebrain overgrowth. Together, these results identify ephrin/Eph signalling as a physiological trigger for apoptosis that can alter brain size and shape by regulating the number of neural progenitors.