Criteria for the appearance of post-tetanic potentiation of transmission at central synapses inHelix aspersa

Summary Long-lasting potentiation of inhibitory post-synaptic potentials occurs at 2 identifiable synapses inHelix brain. It appears only after tetanic stimulation and after a minimum of 20 impulses. Its amplitude and duration depend on the number of stimuli.     

NF-kappaB blockade and oncogenic Ras trigger invasive human epidermal neoplasia

The nuclear factor NF-kappaB and oncogenic Ras can alter proliferation in epidermis, the most common site of human cancer. These proteins are implicated in epidermal squamous cell carcinoma in mice, however, the potential effects of altering their function are uncertain. Whereas inhibition of NF-kappaB enhances apoptosis in certain tumours, blockade of NF-kappaB predisposes murine skin to squamous cell carcinoma. Because therapeutics inhibiting Ras and NF-kappaB pathways are being developed to treat human cancer, it is essential to assess the effects of altering these regulators. The medical relevance of murine studies is limited, however, by differences between mouse and human skin, and by the greater ease of transforming murine cells. Here we show that in normal human epidermal cells both NF-kappaB and oncogenic Ras trigger cell-cycle arrest. Growth arrest triggered by oncogenic Ras can be bypassed by IkappaBalpha-mediated blockade of NF-kappaB, generating malignant human epidermal tissue resembling squamous cell carcinoma. Human cell tumorigenesis is dependent on laminin 5 and alpha6beta4 integrin. Thus, IkappaBalpha circumvents restraints on growth promotion induced by oncogenic Ras and can act with Ras to induce invasive human tissue neoplasia.     

The mismatch repair system is required for S-phase checkpoint activation

Defective S-phase checkpoint activation results in an inability to downregulate DNA replication following genotoxic insult such as exposure to ionizing radiation. This 'radioresistant DNA synthesis' (RDS) is a phenotypic hallmark of ataxia-telangiectasia, a cancer-prone disorder caused by mutations in ATM. The mismatch repair system principally corrects nucleotide mismatches that arise during replication. Here we show that the mismatch repair system is required for activation of the S-phase checkpoint in response to ionizing radiation. Cells deficient in mismatch repair proteins showed RDS, and restoration of mismatch repair function restored normal S-phase checkpoint function. Catalytic activation of ATM and ATM-mediated phosphorylation of the protein NBS1 (also called nibrin) occurred independently of mismatch repair. However, ATM-dependent phosphorylation and activation of the checkpoint kinase CHK2 and subsequent degradation of its downstream target, CDC25A, was abrogated in cells lacking mismatch repair. In vitro and in vivo approaches both show that MSH2 binds to CHK2 and that MLH1 associates with ATM. These findings indicate that the mismatch repair complex formed at the sites of DNA damage facilitates the phosphorylation of CHK2 by ATM, and that defects in this mechanism form the molecular basis for the RDS observed in cells deficient in mismatch repair.     

Genome-wide RNAi analysis of Caenorhabditis elegans fat regulatory genes

Regulation of body fat storage involves signalling between centres that regulate feeding in the brain and sites of fat storage and use in the body. Here we describe an assay for analysing fat storage and mobilization in living Caenorhabditis elegans. By using RNA-mediated interference (RNAi) to disrupt the expression of each of the 16,757 worm genes, we have systematically screened the C. elegans genome for genes necessary for normal fat storage. We identify 305 gene inactivations that cause reduced body fat and 112 gene inactivations that cause increased fat storage. Analysis of the fat-reducing gene inactivations in insulin, serotonin and tubby signalling mutants of C. elegans, which have increased body fat, identifies a core set of fat regulatory genes as well as pathway-specific fat regulators. Many of the newly identified worm fat regulatory genes have mammalian homologues, some of which are known to function in fat regulation. Other C. elegans fat regulatory genes that are conserved across animal phylogeny, but have not previously been implicated in fat storage, may point to ancient and universal features of fat storage regulation, and identify targets for treating obesity and its associated diseases.     

SAP is required for generating long-term humoral immunity

Long-lived plasma cells and memory B cells are the primary cellular components of long-term humoral immunity and as such are vitally important for the protection afforded by most vaccines. The SAP gene has been identified as the genetic locus responsible for X-linked lymphoproliferative disease, a fatal immunodeficiency. Mutations in SAP have also been identified in some cases of severe common variable immunodeficiency disease. The underlying cellular basis of this genetic disorder remains unclear. We have used a SAP knockout mouse model system to explore the role of SAP in immune responses. Here we report that mice lacking expression of SAP generate strong acute IgG antibody responses after viral infection, but show a near complete absence of virus-specific long-lived plasma cells and memory B cells, despite the presence of virus-specific memory CD4+ T cells. Adoptive transfer experiments show that SAP-deficient B cells are normal and the defect is in CD4+ T cells. Thus, SAP has a crucial role in CD4+ T-cell function: it is essential for late B-cell help and the development of long-term humoral immunity but is not required for early B-cell help and class switching.     

Deficiency of Mbd2 suppresses intestinal tumorigenesis

Gene silencing through de novo methylation of CpG island promoters contributes to cancer. We find that Mbd2, which recruits co-repressor complexes to methylated DNA, is essential for efficient tumorigenesis in the mouse intestine. As Mbd2-deficient mice are viable and fertile, their resistance to intestinal cancer may be of therapeutic relevance.     

A conserved RNA-binding protein controls germline stem cells in Caenorhabditis elegans

Germline stem cells are defined by their unique ability to generate more of themselves as well as differentiated gametes. The molecular mechanisms controlling the decision between self-renewal and differentiation are central unsolved problems in developmental biology with potentially broad medical implications. In Caenorhabditis elegans, germline stem cells are controlled by the somatic distal tip cell. FBF-1 and FBF-2, two nearly identical proteins, which together are called FBF ('fem-3 mRNA binding factor'), were originally discovered as regulators of germline sex determination. Here we report that FBF also controls germline stem cells: in an fbf-1 fbf-2 double mutant, germline proliferation is initially normal, but stem cells are not maintained. We suggest that FBF controls germline stem cells, at least in part, by repressing gld-1, which itself promotes commitment to the meiotic cell cycle. FBF belongs to the PUF family ('Pumilio and FBF') of RNA-binding proteins. Pumilio controls germline stem cells in Drosophila females, and, in lower eukaryotes, PUF proteins promote continued mitoses. We suggest that regulation by PUF proteins may be an ancient and widespread mechanism for control of stem cells.     

Quantum control of energy flow in light harvesting

Coherent light sources have been widely used in control schemes that exploit quantum interference effects to direct the outcome of photochemical processes. The adaptive shaping of laser pulses is a particularly powerful tool in this context: experimental output as feedback in an iterative learning loop refines the applied laser field to render it best suited to constraints set by the experimenter. This approach has been experimentally implemented to control a variety of processes, but the extent to which coherent excitation can also be used to direct the dynamics of complex molecular systems in a condensed-phase environment remains unclear. Here we report feedback-optimized coherent control over the energy-flow pathways in the light-harvesting antenna complex LH2 from Rhodopseudomonas acidophila, a photosynthetic purple bacterium. We show that phases imprinted by the light field mediate the branching ratio of energy transfer between intra- and intermolecular channels in the complex's donor acceptor system. This result illustrates that molecular complexity need not prevent coherent control, which can thus be extended to probe and affect biological functions.