Type I phosphatidylinositol kinase makes a novel inositol phospholipid, phosphatidylinositol-3-phosphate

The generation of second messengers from the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdInsP2) by phosphoinositidase C has been implicated in the mediation of cellular responses to a variety of growth factors and oncogene products. The first step in the production of PtdInsP2 from phosphatidylinositol (PtdIns) is catalysed by PtdIns kinase. A PtdIns kinase activity has been found to associate specifically with several oncogene products, as well as with the platelet-derived growth factor (PDGF) receptor. We have previously identified two biochemically distinct PtdIns kinases in fibroblasts, and have found that only one of these, designated type I, specifically associates with activated tyrosine kinases. We have now characterized the site on the inositol ring phosphorylated by type I PtdIns kinase, and find that this kinase specifically phosphorylates the D-3 ring position to generate a novel phospholipid, phosphatidylinositol-3-phosphate (PtdIns(3)P). In contrast, the main PtdIns kinase in fibroblasts, designated type II, specifically phosphorylates the D-4 position to produce phosphatidylinositol-4-phosphate (PtdIns(4)P), previously considered to be the only form of PtdInsP. We have also tentatively identified PtdIns(3)P as a minor component of total PtdInsP in intact fibroblasts. We propose that type I PtdIns kinase is responsible for the generation of PtdIns(3)P in intact cells, and that this novel phosphoinositide could be important in the transduction of mitogenic and oncogenic signals.     

Myeloid leukaemia inhibitory factor maintains the developmental potential of embryonic stem cells

Embryonic stem (ES) cells, the totipotent outgrowths of blastocysts, can be cultured and manipulated in vitro and then returned to the embryonic environment where they develop normally and can contribute to all cell lineages. Maintenance of the stem-cell phenotype in vitro requires the presence of a feeder layer of fibroblasts or of a soluble factor, differentiation inhibitory activity (DIA) produced by a number of sources; in the absence of DIA the ES cells differentiate into a wide variety of cell types. We recently noted several similarities between partially purified DIA and a haemopoietic regulator, myeloid leukaemia inhibitory factor (LIF), a molecule which induces differentiation in M1 myeloid leukaemic cells and which we have recently purified, cloned and characterized. We demonstrate here that purified, recombinant LIF can substitute for DIA in the maintenance of totipotent ES cell lines that retain the potential to form chimaeric mice.     

Multiple liquid crystal phases of DNA at high concentrations

DNA packaging in vivo is very tight, with volume concentrations approaching 70% w/v in sperm heads, virus capsids and bacterial nucleoids. The packaging mechanisms adopted may be related to the natural tendency of semi-rigid polymers to form liquid crystalline phases in concentrated solutions. We find that DNA forms at least three distinct liquid crystalline phases at concentrations comparable to those in vivo, with phase transitions occurring over relatively narrow ranges of DNA concentration. A weakly birefringent, dynamic, 'precholesteric' mesophase with microscopic textures intermediate between those of a nematic and a true cholesteric phase forms at the lowest concentrations required for phase separation. At slightly higher DNA concentrations, a second mesophase forms which is a strongly birefringent, well-ordered cholesteric phase with a concentration-dependent pitch varying from 2 to 10 micron. At the highest DNA concentrations, a phase forms which is two-dimensionally ordered and resembles smectic phases of thermotropic liquid crystals observed with small molecules.     

Brain-derived neurotrophic factor prevents neuronal death in vivo

Developing vertebrate neurons are thought to depend for their survival on specific neurotrophic proteins present in their target fields. The limited availability of these proteins does not allow the survival of all neurons initially innervating a target, resulting in the widely observed phenomenon of naturally occurring neuronal death. Although a variety of proteins have been reported to promote the survival of neurons in tissue culture, the demonstration that these proteins increase neuronal numbers and/or decrease neuronal death in vivo has only been possible with nerve growth factor (NGF). The generalization of the concept that neurotrophic proteins regulate neuronal survival during normal development critically depends on the demonstration that the survival of neurons in vivo can be increased by the administration of a neurotrophic protein different from NGF. We report here that this is the case with brain-derived neurotrophic factor, a protein of extremely low abundance purified from the central nervous system.     

Incubation periods for paediatric AIDS patients

A recent seroprevalence study of newborns indicates that one in 62 children born in New York City has antibodies to the human immunodeficiency virus (HIV). The distribution of incubation periods for paediatric patients is needed to estimate future AIDS case loads from these seroprevalence data. Current estimates of incubation periods for paediatric patients are based on limited data. We use parametric and non-parametric methods to analyse incubation periods for 215 paediatric patients with AIDS whose only known route of infection is maternal. We conclude that incubation periods are longer than previously reported; that there is a distinct knee in the incubation period distribution at seven months which suggests two risk populations; and that there is an increase in incidence which is consistent with exponential growth.     

Neural encoding of individual words and faces by the human hippocampus and amygdala

Patients with lesions in the medial temporal lobe (MTL) of the brain, which includes the hippocampus, amygdala and parahippocampal gyrus, are severely impaired in their ability to remember and recognize words or faces which they saw only a short time ago. These lesions also prevent the effect of word repetition on cortical event-related potentials that are associated with these tasks. We have been able to study the response of individual neurons in the human medial temporal lobe to such delayed recognition tasks in epileptic patients undergoing neurosurgery. We found that some MTL neurons preferentially fired on sight of one particular word from a set of ten words used in a memory task, and others fired in response to one particular face. This stimulus-specific firing was maximal during the time that the neocortical event potentials are most sensitive to stimulus repetition, suggesting that the MTL contributes specific information to the cortex during the retrieval of recent memories.     

Positive selection of CD4+ thymocytes controlled by MHC class II gene products

The mature T-cell antigen receptor repertoire is characterized by lack of reactivity to self-components as well as by preferential reactivity to foreign antigens in the context of polymorphic self-proteins encoded within the major histocompatibility complex. Whereas the former characteristic (referred to as negative selection or tolerance) is associated with intrathymic deletion of T cells expressing T-cell antigen receptor beta-chain variable (V beta) domains, which confer a preferential reactivity to self antigens, the existence of the latter (referred to as positive selection or MHC restriction) has so far only been inferred indirectly from functional studies. We show here that intrathymic deletion of V+beta 6 T cells (reactive with a self-antigen encoded by the Mlsa locus) is controlled by polymorphic MHC class II determinants. Furthermore, in mice lacking expression of Mlsa, the same class II MHC loci control the frequency of occurrence of V+beta 6 cells among mature CD4+ T lymphocytes. These data are direct evidence for positive selection by MHC determinants in the thymus in unmanipulated animals.