Rejuvenation of aged progenitor cells by exposure to a young systemic environment

The decline of tissue regenerative potential is a hallmark of ageing and may be due to age-related changes in tissue-specific stem cells. A decline in skeletal muscle stem cell (satellite cell) activity due to a loss of Notch signalling results in impaired regeneration of aged muscle. The decline in hepatic progenitor cell proliferation owing to the formation of a complex involving cEBP-alpha and the chromatin remodelling factor brahma (Brm) inhibits the regenerative capacity of aged liver. To examine the influence of systemic factors on aged progenitor cells from these tissues, we established parabiotic pairings (that is, a shared circulatory system) between young and old mice (heterochronic parabioses), exposing old mice to factors present in young serum. Notably, heterochronic parabiosis restored the activation of Notch signalling as well as the proliferation and regenerative capacity of aged satellite cells. The exposure of satellite cells from old mice to young serum enhanced the expression of the Notch ligand (Delta), increased Notch activation, and enhanced proliferation in vitro. Furthermore, heterochronic parabiosis increased aged hepatocyte proliferation and restored the cEBP-alpha complex to levels seen in young animals. These results suggest that the age-related decline of progenitor cell activity can be modulated by systemic factors that change with age.     

Conformational diversity in a yeast prion dictates its seeding specificity

A perplexing feature of prion-based inheritance is that prions composed of the same polypeptide can evoke different phenotypes (such as distribution of brain lesions), even when propagated in genetically identical hosts. The molecular basis of this strain diversity and the relationship between strains and barriers limiting transmission between species remain unclear. We have used the yeast prion phenomenon [PSI+]4 to investigate these issues and examine the role that conformational differences may have in prion strains. We have made a chimaeric fusion between the prion domains of two species (Saccharomyces cerevisae and Candida albicans) of Sup35, the protein responsible for [PSI+]. Here we report that this chimaera forms alternate prion strains in vivo when initiated by transient overexpression of different Sup35 species. Similarly, in vitro the purified chimaera, when seeded with different species of Sup35 fibres, establishes and propagates distinct amyloid conformations. These fibre conformations dictate amyloid seeding specificity: a chimaera seeded by S. cerevisiae fibres efficiently catalyses conversion of S. cerevisiae Sup35 but not of C. albicans Sup35, and vice versa. These and other considerations argue that heritable prion strains result from self-propagating conformational differences within the prion protein itself. Moreover, these conformational differences seem to act in concert with the primary structure to determine a prion's propensity for transmission across a species barrier.     

Murine leukaemogenesis: monoclonal antibodies to T-cell determinants arrest T-lymphoma cell proliferation

We have proposed a receptor-mediated leukaemogenesis hypothesis wherein T lymphomas would be clones of T cells bearing mitogen-linked surface receptors specific for the envelope determinants of the inducing MuLV. A prediction of the hypothesis is that T-lymphoma proliferation is dependent on continued presentation of MuLV envelope determinants to these cell-surface receptors, and that substances which interfere with receptor-virus interactions should inhibit T-lymphoma proliferation. Rat monoclonal antibodies were raised to the AKR mouse T lymphoma KKT-2, and these antibodies were screened independently for blockade of virus-binding and for cytostatic activity on KKT-2 cells. We report here that those monoclonal antibodies which block virus binding inhibit growth of KKT-2 cells in vitro, whereas monoclonal antibodies which bind to these cells but do not block virus binding are not cytostatic. Three of the four cytostatic antibodies detect determinants on the Thy-1 molecule, while none of the other (noncytostatic) antibodies detect Thy-1. Antibody inhibition of KKT-2 cell growth is precluded by saturation of KKT-2 virus receptors with the inducing leukaemia virus.     

V-J joining of immunoglobulin kappa genes only occurs on one homologous chromosome

In general, heterozygous animal cells express both alleles at a particular locus. The only exceptions are cells of XX genotype after inactivation of one X chromosome, and immunoglobulin-producing cells; in each case only one of the two alleles is expressed in differentiated cells and their progeny. This phenomenon, termed allelic exclusion, has been described for several mammalian species including man and mouse. It has been shown that the variable (V) and constant (C) region genes of immunoglobulins undergo a rearrangement during ontogeny. We wished to test whether allelic exclusion in B cells could be the consequence of V- and C-region rearrangement on one of the two homologous chromosomes only. For that reason we chose to analyse the rearrangement of immunoglobulin light chain genes in normal B lymphocytes isolated on the fluorescence-activated cell sorter. We now present evidence that during normal B-lymphocyte differentiation V-C rearrangement occurs only on one chromosome.     

Cell-fate conversion of lymphoid-committed progenitors by instructive actions of cytokines

The primary role of cytokines in haemato-lymphopoiesis is thought to be the regulation of cell growth and survival. But the instructive action of cytokines in haematopoiesis has not been well addressed. Here we show that a clonogenic common lymphoid progenitor, a bone marrow-resident cell that gives rise exclusively to lymphocytes (T, B and natural killer cells), can be redirected to the myeloid lineage by stimulation through exogenously expressed interleukin (IL)-2 and GM-CSF (granulocyte/macrophage colony-stimulating factor) receptors. Analysis of mutants of the beta-chain of the IL-2 receptor revealed that the granulocyte- and monocyte-differentiation signals are triggered by different cytoplasmic domains, showing that the signalling pathway(s) responsible for these unique developmental outcomes are separable. Finally, we show that the endogenous myelomonocytic cytokine receptors for GM-CSF and macrophage colony-stimulating factor (M-CSF) are expressed at low to moderate levels on the more primitive haematopoietic stem cells, are absent on common lymphoid progenitors, and are upregulated after myeloid lineage induction by IL-2. We conclude that cytokine signalling can regulate cell-fate decisions and propose that a critical step in lymphoid commitment is downregulation of cytokine receptors that drive myeloid cell development.