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.     

Inference to the best explanation and Norton's material theory of induction

I argue that we should consider Norton's material theory of induction as consisting of two largely independent claims. First, there is the claim that material facts license inductions - a claim which I interpret as a type of contextualism about induction. Second, there is the claim that there are no universal rules of induction. While a good case can be made for the first claim, I believe that Norton's arguments for the second claim are lacking. In particular, I spell out Norton's argument against the claim that all induction may be reduced to inference to the best explanation, and argue that it is not persuasive. Rejecting this part of Norton's theory does not however require us to abandon the first claim that material facts license inductions. In this way, I distinguish the parts of the material theory of induction we should happily accept from the parts about which we should be more skeptical.     

Systematic generation of high-resolution deletion coverage of the Drosophila melanogaster genome

In fruit fly research, chromosomal deletions are indispensable tools for mapping mutations, characterizing alleles and identifying interacting loci. Most widely used deletions were generated by irradiation or chemical mutagenesis. These methods are labor-intensive, generate random breakpoints and result in unwanted secondary mutations that can confound phenotypic analyses. Most of the existing deletions are large, have molecularly undefined endpoints and are maintained in genetically complex stocks. Furthermore, the existence of haplolethal or haplosterile loci makes the recovery of deletions of certain regions exceedingly difficult by traditional methods, resulting in gaps in coverage. Here we describe two methods that address these problems by providing for the systematic isolation of targeted deletions in the D. melanogaster genome. The first strategy used a P element-based technique to generate deletions that closely flank haploinsufficient genes and minimize undeleted regions. This deletion set has increased overall genomic coverage by 5-7%. The second strategy used FLP recombinase and the large array of FRT-bearing insertions described in the accompanying paper to generate 519 isogenic deletions with molecularly defined endpoints. This second deletion collection provides 56% genome coverage so far. The latter methodology enables the generation of small custom deletions with predictable endpoints throughout the genome and should make their isolation a simple and routine task.     

Cell fusion-independent differentiation of neural stem cells to the endothelial lineage

Somatic stem cells have been claimed to possess an unexpectedly broad differentiation potential (referred to here as plasticity) that could be induced by exposing stem cells to the extracellular developmental signals of other lineages in mixed-cell cultures. Recently, this and other experimental evidence supporting the existence of stem-cell plasticity have been refuted because stem cells have been shown to adopt the functional features of other lineages by means of cell-fusion-mediated acquisition of lineage-specific determinants (chromosomal DNA) rather than by signal-mediated differentiation. In this study we co-cultured mouse neural stem cells (NSCs), which are committed to become neurons and glial cells, with human endothelial cells, which form the lining of blood vessels. We show that in the presence of endothelial cells six per cent of the NSC population converted to cells that did not express neuronal or glial markers, but instead showed the stable expression of multiple endothelial markers and the capacity to form capillary networks. This was surprising because NSCs and endothelial cells are believed to develop from the ectoderm and mesoderm, respectively. Experiments in which endothelial cells were killed by fixation before co-culture with live NSCs (to prevent cell fusion) and karyotyping analyses, revealed that NSCs had differentiated into endothelial-like cells independently of cell fusion. We conclude that stem-cell plasticity is a true characteristic of NSCs and that the conversion of NSCs to unanticipated cell types can be accomplished without cell fusion.     

Indian hedgehog is a major mediator of progesterone signaling in the mouse uterus

The hedgehog family of morphogens are regulators of cell proliferation, differentiation and cell-cell communication. These morphogens have been shown to have important roles in organogenesis, spermatogenesis, stem cell maintenance and oncogenesis. Indian hedgehog (encoded by Ihh) has been shown to be expressed in the uterine epithelium under the control of the steroid hormone, progesterone. Although in vivo and in vitro studies have shown that progesterone achieves its effects on uterine function through epithelial-stromal cross-talk, molecular mediator(s) for this cellular communication pathway have not been elucidated. Using new experimental approaches that ablate Ihh specifically in Pgr-positive uterine cells of the mouse, we demonstrate that Ihh is an essential mediator of Pgr action in the uterus, and expression of this factor is critical in mediating the communication between the uterine epithelium and stroma required for embryo implantation.     

Germline KRAS mutations cause Noonan syndrome

Noonan syndrome (MIM 163950) is characterized by short stature, facial dysmorphism and cardiac defects. Heterozygous mutations in PTPN11, which encodes SHP-2, cause approximately 50% of cases of Noonan syndrome. The SHP-2 phosphatase relays signals from activated receptor complexes to downstream effectors, including Ras. We discovered de novo germline KRAS mutations that introduce V14I, T58I or D153V amino acid substitutions in five individuals with Noonan syndrome and a P34R alteration in a individual with cardio-facio-cutaneous syndrome (MIM 115150), which has overlapping features with Noonan syndrome. Recombinant V14I and T58I K-Ras proteins show defective intrinsic GTP hydrolysis and impaired responsiveness to GTPase activating proteins, render primary hematopoietic progenitors hypersensitive to growth factors and deregulate signal transduction in a cell lineage-specific manner. These studies establish germline KRAS mutations as a cause of human disease and infer that the constellation of developmental abnormalities seen in Noonan syndrome spectrum is, in large part, due to hyperactive Ras.     

Recommendations of the 2006 Human Variome Project meeting

Lists of variations in genomic DNA and their effects have been kept for some time and have been used in diagnostics and research. Although these lists have been carefully gathered and curated, there has been little standardization and coordination, complicating their use. Given the myriad possible variations in the estimated 24,000 genes in the human genome, it would be useful to have standard criteria for databases of variation. Incomplete collection and ascertainment of variants demonstrates a need for a universally accessible system. These and other problems led to the World Heath Organization-cosponsored meeting on June 20-23, 2006 in Melbourne, Australia, which launched the Human Variome Project. This meeting addressed all areas of human genetics relevant to collection of information on variation and its effects. Members of each of eight sessions (the clinic and phenotype, the diagnostic laboratory, the research laboratory, curation and collection, informatics, relevance to the emerging world, integration and federation and funding and sustainability) developed a number of recommendations that were then organized into a total of 96 recommendations to act as a foundation for future work worldwide. Here we summarize the background of the project, the meeting and its recommendations.