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A stable latent reservoir for HIV-1 in resting CD4+ T cells is the principal barrier to a cure1-3. Curative strategies that target the reservoir are being tested4,5 and require accurate, scalable reservoir assays. The reservoir was defined with quantitative viral outgrowth assays for cells that release infectious virus after one round of T cell activation1. However, these quantitative outgrowth assays and newer assays for cells that produce viral RNA after activation6 may underestimate the reservoir size because one round of activation does not induce all proviruses7. Many studies rely on simple assays based on polymerase chain reaction to detect proviral DNA regardless of transcriptional status, but the clinical relevance of these assays is unclear, as the vast majority of proviruses are defective7-9. Here we describe a more accurate method of measuring the HIV-1 reservoir that separately quantifies intact and defective proviruses. We show that the dynamics of cells that carry intact and defective proviruses are different in vitro and in vivo. These findings have implications for targeting the intact proviruses that are a barrier to curing HIV infection.

The murine oct-3 gene encodes a transcription factor containing a POU-specific domain and a homeodomain. In marked contrast to other homeodomain-encoding genes, oct-3 is expressed in the totipotent and pluripotent stem cells of the pregastrulation embryo and is down-regulated during differentiation to endoderm and mesoderm, suggesting that it has a role in early development. The oct-3 gene is also expressed in primordial germ cells and in the female germ line.

The early mammalian embryo is patterned by signals emanating from extraembryonic and embryonic signalling centres, most notably the anterior visceral endoderm (AVE) and the node, respectively. The AVE is responsible for anterior development, whereas further axis specification depends on the node, the equivalent of Spemann's organizer. Formation of the node, at the anterior primitive streak, depends on expression of the transcription factor HNF3beta (ref. 4). However, both the source and the nature of the signals responsible for inducing the node have been unknown. Here we describe a recessive lethal mutation, arkadia, generated using gene-trap mutagenesis. Mutant embryos establish an AVE but fail to maintain anterior embryonic structures and lack a node. The mutation has disrupted the Arkadia gene, which encodes a putative intracellular protein containing a RING domain. Arkadia is essential for HNF3beta expression in the anterior primitive streak. Analysis with chimaeras, however, shows that Arkadia functions within extraembryonic tissues, revealing that these are required to induce the node. Furthermore, our experiments show that Arkadia interacts genetically with the transforming growth factor (TGF)beta-like factor Nodal, implying that Nodal mediates the function of Arkadia in node induction.

A genetic interference phenomenon in the nematode Caenorhabditis elegans has been described in which expression of an individual gene can be specifically reduced by microinjecting a corresponding fragment of double-stranded (ds) RNA1. One striking feature of this process is a spreading effect: interference in a broad region of the animal is observed following the injection of dsRNA into the extracellular body cavity. Here we show that C. elegans can respond in a gene-specific manner to dsRNA encountered in the environment. C. elegans normally feed on bacteria, ingesting and grinding them in the pharynx and subsequently absorbing bacterial contents in the gut. We find that Escherichia coli bacteria expressing dsRNAs can confer specific interference effects on the nematode larvae that feed on them.

Thrombin is a coagulation protease that activates platelets, leukocytes, endothelial and mesenchymal cells at sites of vascular injury, acting partly through an unusual proteolytically activated G-protein-coupled receptor1–3. Knockout of the gene encoding this receptor provided definitive evidence for a second thrombin receptor in mouse platelets and for tissue-specific roles for different thrombin receptors4. We now report the cloning and characterization of a new human thrombin receptor, designated protease-activated receptor 3 (PAR3). PAR3 can mediate throm-bin-triggered phosphoinositide hydrolysis and is expressed in a variety of tissues, including human bone marrow and mouse megakaryocytes, making it a candidate for the sought-after second platelet thrombin receptor. PAR3 provides a new tool for understanding thrombin signalling and a possible target for therapeutics designed selectively to block thrombotic, inflammatory and proliferative responses to thrombin.

Prostacyclin (prostaglandin I2, PGI2), produced by the blood vessel wall is the most potent known inhibitor of platelet aggregation induced by such stimuli as ADP and thrombin. It binds to a specific platelet receptor and activates adenylate cyclase, raising the cyclic AMP level in platelets. This property can be important because platelets participate in several significant interactions. For example, the interaction with fibrinogen or fibrin contributes to the formation of the haemostatic plug. Intact plasma fibrinogen is required for the aggregation of platelets induced by ADP, and endogenous platelet fibrinogen influences thrombin-induced aggregation. We have therefore studied the effect of prostacyclin on the interaction of fibrinogen with human platelets. We now report that prostacyclin inhibits the mobilisation of specific binding sites ('receptors') for fibrinogen on human platelets and that this effect parallels the inhibition of ADP- or thrombin-induced aggregation. The inhibitory effect of prostacyclin may limit the extent of platelet-fibrinogen interaction in vivo and in extracorporeal circulation.

THE fibrinogen molecule, which comprises 5% of plasma proteins, is involved in enzymatic reactions responsible for formation and dissolution of the fibrin clot1. Thereby, fibrinogen is being transformed by thrombin into fibrin, which is cross linked by glutaminyltransamidase (factor XIII), and ultimately degraded by plasmin. The Aα, Bβ and γ polypeptide chains (nomenclature according to the recommendations of the Committee on Nomenclature of the International Society on Thrombosis and Haemostasis, 1971) of fibrinogen involved in these reactions and the sites of enzymatic attack thereon are defined2–8. In addition to being involved in the enzymatic reactions responsible for formation and dissolution of the fibrin clot, fibrinogen exhibits the ability to interact in a non-enzymatic reaction with pathogenic staphylococci to cause their clumping9. Clumping of staphylococci is used to detect fibrinogen and its derivatives in blood and body fluids10,11. This reaction can detect as little as 0.03 µg human fibrinogen or its derivatives. The mechanism through which human fibrinogen interacts with staphylococci is however, unexplained and the localisation of the binding site(s) for staphylococci remains unknown. We therefore studied localisation of the binding site for staphylococci on human fibrinogen. We now report that the binding site for staphylococci is located on Aα and Bβ chains of human fibrinogen.

GRAM-NEGATIVE bacteria produce endotoxin and cause intravascular coagulation, shock and ultimate death of an estimated 25% of patients with bloodstream infections1,2. The number of blood platelets in such patients decreases3. In animal experiments, endotoxin aggregates platelets, induces release of vasoactive amines and adenine nucleotides, and unmasks the phospholipid procoagulant (platelet factor 3, PF3), which accelerates blood clotting4. To determine the mechanism of endotoxin action in man we studied the endotoxin interaction with human platelets. Here we describe experiments which demonstrate that the endotoxin-sensitive component (receptor) is present on the membrane of human platelets. Two types of changes resulting from endotoxin interaction with this receptor have been noted: selective release of 5-hydroxytryptamine (5-HT) and unmasking of platelet phospholipid (PF3).

NUMEROUS attempts to achieve a photo-sensitized reduction of carbon dioxide by water in vitro are on record1. Various degrees of success have been claimed; but in almost every case the repetition of such experiments in other laboratories has failed and, in any event, the yields originally reported were extremely small. A significant advance, however, appeared to have been made in 1943 by Baur and his co-workers2. After many years of experimentation, Baur developed a two-phase system consisting of glycerol, geraniol, chlorophyll and methylene blue, which on illumination in a carbon dioxide atmosphere was stated to produce formaldehyde and oxygen in a photochemical yield amounting to about 5 per cent of that of natural photosynthesis. Although it is now known that formaldehyde is not an intermediate in natural photosynthesis, the realization of a photosensitized hydrogen-transfer reaction between carbon dioxide and water outside the living cell would be a result of great significance, and a confirmation of Baur's results, which have been widely quoted, therefore appeared desirable.

Interpretation of the Ultraviolet Spectra of Natural Products By Prof. A. I. Scott. (International Series of Monographs on Organic Chemistry, Vol. 7.) Pp. x + 443. (London and New York: Pergamon Press, 1964.) 84s. net.