Comptes rendus des seances de la Societe de biologie et de ses filiales最新文献

The recent identification of genes involved in the regulation of the longevity of some invertebrates (drosophila, nematodes) as well as the presence of human genes exhibiting homologies to those identified in invertebrates, revived the deterministic theories of aging. It appears however that those mechanisms which were shown experimentally to be involved in aging in vertebrates and in particular in humans belong to the epigenetic mechanisms such as the Maillard reaction and free radical attack in particular. Some diseases which imitate an accelerated aging as Progeria and the Werner syndrome are due to mutations, some of them mutagenic themselves. Reflections based on such arguments concerning reactions with catalysers coded in the genome but with activities escaping strict control, suggest an indirect determinism of phenomena involved directly in cell- and tissue aging. These considerations are illustrated by the example of the elastin-laminin receptor studied in our laboratory. Its sustained activation by circulating elastin peptides appears to be involved in cell and tissue aging.

Ras proteins play the role of molecular switches by conformational change between a GTP and a GDP-bound state. In the yeast Saccharomyces cerevisiae, they are encoded by two partially redundant genes RAS1 and RAS2 with a different pattern of gene expression. They are essential for growth because they are required for the activation of the adenylate cyclase and thus the protein kinase A pathway. Other possible biological functions remains to be established. To achieve their biological function, they need to be processed after their synthesis, they are modified farnesylated and palmitoylated at their C-terminal end at their CaaX box. Palmitoylation, involved in membrane localization, is not essential for growth but required for glucose signaling whereas farnesylation appears to participate in adenylate cyclase activation. In the GTP-bound state ras proteins interact through their conserved effector domain with the adenylate cyclase, the product of the CYR1/CDC35 gene. They also interact with GTPase activating proteins encoded by IRA1 and IRA2. These proteins are specific for yeast ras. It has been shown that Ira2p recognizes specific residues of yeast ras not shared by mammalian ras. The interaction with the guanine nucleotide exchange factor (GEF) of the CDC25 family is enhanced by dominant negative mutations such as RAS2ala22. Using the two hybrid approach, we have showed the key role of position 80 in Ras2p and confirmed the involvement of the a2 helix, the other switching part of ras, in this interaction and the induced effect. As a counterpart we have identified positions in HGRF55 conserved in other GEF involved in ras interaction. The triggering elements of ras activation: the GEF Cdc25p and Sdc25p are limiting components of the ras system. Cdc25p is part of a multimolecular complex associated with the membrane. We have shown that it can form homodimers and heterodimers with Sdc25p. It is an unstable protein containing a cyclin destruction box. Therefore its activity on ras could be regulated by controlling its cellular content.

Rac1 and Rac2 are 92% homologous cytosolic small GTPase proteins. Both Rac1 and Rac2 have been implicated with NADPH oxidase activation in vitro, however, Rac2 is largely predominant in human phagocytes. NADPH oxidase is a plasma membrane enzyme of phagocytes, generating superoxide anions which serve as bactericidal agents. Activation of this multimolecular enzyme, minimally requires assembly at the membrane with flavocytochrome b258 of cytosolic components p47phox, p67phox and Rac proteins. Using the yeast two hybrid system, we provide data demonstrating in vivo interactions between human p47phox, p67phox, and Rac proteins. Rac proteins interact with p67phox in a GTP-dependent manner, but do not interact with p47phox. Moreover, Rac effector site mutants which are known to be inactive in NADPH oxidase lose their interaction with p67phox. Finally, we observe that p67phox interacts six fold better with Rac2 than with Rac1. We also show a strong intracellular interaction between p47phox and p67phox. These results indicate that activated Rac, and particularly Rac2, can regulate superoxide production by NADPH oxidase of phagocytic cells through direct interaction with p67phox subunit. Recently published data suggest that Rac proteins could transduce mitogenic signals in non-phagocytic cells through superoxide production by a phagocytic-related NADPH oxidase enzymatic system which remains to be determined. NADPH oxidase regulation by Rac proteins in phagocytes could then be used as a model to understand the molecular mechanisms underlying Rac functions in various cell types.

In the present review article, we have attempted to define and class the so called "developmental genes". This designation applies to genes whose activity is involved in lineage determination, in differentiation pathways or in the modes of tissue or organ formation. A special emphasis has been given to genes controlling the body plan in the course of development. Three main subclasses of such genes are described: genes involved in very early development by establishing the anteroposterior or dorso-ventral axes of the embryo (or of the insect larva); segmentation genes; homeotic genes containing an homeodomain (HD) motif and homeotic-related genes (with a "LIM" or "PAX" motif). While classical (HD containing) homeotic genes were initially discovered and studied in drosophila, their general occurrence in many other species, particularly in vertebrates was soon reported. They are characterized phylogenetically speaking, by a high degree of sequence similarities and chromosomal organization, although their number appears much greater in vertebrates than in insects due to successive duplication and chromosomal distribution events. In vertebrates they are essentially involved in the development of the central nervous system. Large varieties of genes with "LIM" or "PAX" motifs have been reported, and many subclasses can be distinguished depending upon the presence or absence of distinct peptidic motifs within the genes under concern. The peptidic domains encoded by the LIM motifs include "Zn finger" containing varieties which are involved in protein-protein interactions. The roles of genes with a LIM motif vary according to the type of sub-variety (organization of the central nervous system; axonal distribution of peripheric nerves at distinct levels of the spinal cord, control of tissue differentiation, etc.). "Pax" genes include a DNA binding domain (encoded by the "paired" motif) with variable sequences and organization, which is sometime in the vicinity of an homeodomain. Generally speaking "Pax" genes behave like controlling elements in the development of the brain or of tissues derived from the neural crest.

Vertebrate limbs are an amazing example of successful adaptation to various environmental conditions. In higher vertebrates, forelimbs help to fly, swim, walk, dig or grasp, yet their basic structure (the sequence and spatial arrangement of bony elements) is always the same. This implies the existence of a unique developmental strategy for building a limb (a limb plan) that imposes early on a basic scheme, on the top of which subsequent species-specific customizations will occur. The description of such a universal limb plan, hence the idea that the genetic and developmental processes that generate this plan are very ancient, has been controversial for about a century. It is worth asking whether recent discoveries of important genes involved in these processes can bring novel arguments to the debate.

This paper reviews the present state of the studies on the repair of a major oxydative lesion on DNA, the 8-oxo-guanine (8-OxoG). This modified base has been proved to be highly mutagenic and therefore implicated in the ethiology of several pathologies. The cloning of the yeast OGG1 gene, a functional homolog of the fpg from bacteria, allowed the isolation of the mammalian homologs. These genes code for 8-OxoG DNA glycosylases/lyases, whose biochemical properties are consistent with their postulated role as the main defence against the genetic instability induced by the presence of 8-OxoG in DNA. This, together with the mutator phenotype of the yeast ogg1 mutant strains, make of the human OGG1 a candidate for a cancer predisposition gene. The localization of this gene to chromosome 3p and other evidences discussed in this paper indicate that OGG1 could be a tumor suppressor gene implicated in lung cancer.

Collagen can be preserved during tens of thousands of years in bones and teeth under favorable conditions. Natural isotopic abundances in carbon (13C/12C) and in nitrogen (15N/14N) of ancient bone and tooth collagen correspond to those recorded during the biogenic synthesis and have not been significantly altered during fossilization. These isotopic abundances are linked to those of the proteic fraction of animal and human diets, and to physiological conditions. Three kinds of applications are made possible through these natural isotopic signatures: determination of subsistence strategies in ancient human populations, determination of the diet of extinct species and the analysis of past environmental changes.

Two families of nuclear receptors for retinoic acid (RA) have been characterized. Members of the RAR family (types alpha, beta and gamma and their isoforms alpha 1, alpha 2, beta 1 to beta 4, and gamma 1 and gamma 2) are activated by most physiologically occurring retinoids (all-trans RA, 9-cis RA, 4oxo RA and 3,4 dihyroRA). In contrast, members of the RXR family (types alpha, beta and gamma and their isoforms) are activated by 9cis-RA only. In addition to the multiplicity of receptors, the complexity of retinoid signalling is further increased by the fact that, at least in vitro, RARs bind to their cognate response elements as heterodimers with RXRs. Moreover, RXRs can also bind, in vitro, to some DNA elements as homodimers, and are heterodimeric partners for other nuclear receptors, including TRs, VDR, PPARs and a number of orphan nuclear receptors. To evaluate the functions of the different RARs and RXRs types and isoforms, we have generated null mutant mice by targeted gene disruption in ES cells. As to the functions of RARs, we found that RAR alpha 1 and RAR gamma 2 null mutant mice are apparently normal. Mice deficient in RAR alpha or RAR gamma (i.e., all alpha or gamma isoforms disrupted) show aspects of the post-natal vitamin A deficiency (VAD) syndrome which can be cured or prevented by RA, including post-natal lethality, poor weight gain and male sterility. RAR beta 2 (and RAR beta) null mutants display a retrolenticular membrane which represents the most frequent defect of the fetal VAD syndrome. That these abnormalities were restricted to a small subset of the tissues normally expressing these receptors suggested that some degree of functional redundancy should exist in the RAR family. To test this hypothesis we then generated RAR double null mutants. RAR alpha beta, RAR alpha gamma and RAR beta gamma compound mutants exhibit all the malformations of the fetal VAD syndrome, thus demonstrating that RA is the vitamin A derivative which plays a crucial role at many different stages and in different structures during organogenesis. Interestingly, almost all the structures derived from mesenchymal neural crests cells (NCC) are affected in RAR compound mutants. As to the functions of RXRs, RXR gamma null mutants are viable, fertile and morphologically normal. In contrast, RXR alpha null fetuses display a thin ventricular wall and die in utero from cardiac failure. A myocardial hypoplasia has also been observed in some RAR compound mutants as well as in VAD fetuses. Thus, RXR alpha seems to act as an inhibitor of ventricular cardiocyte differentiation and/or as a positive regulator of their proliferation, and these functions might involve heterodimerization with RARs and activation by RA. RXR beta null mutants are viable but the males are sterile, most probably because of an abnormal lipid metabolism in the Sertoli cells. New abnormalities, absent in RXR alpha mutants, are generated in RXR alpha/RAR (alpha, beta or gamma) compound muta

The role of mevalonic acid (MVA) and its products (isoprenoids) in cell proliferation prompted us to investigate the effect of drugs affecting diverse enzymatic steps of the MVA pathway on rat aorta smooth muscle cell (SMC) proliferation. Competitive inhibitors of HMG-CoA reductase (statins) decreased SMC proliferation in a dose-dependent manner. The inhibitory effect induced by simvastatin 3.5 microM (70% +/- 3.8 decrease) was prevented by addition of 100 microM MVA, (100% +/- 2.3), 10 microM farnesol (F-OH) (85% +/- 1.2) and 5 microM of all-trans geranylgeraniol (GG-OH) (precursor of prenylated proteins) (81% +/- 1.1), but not by 2-cis GG-OH (precursor of dolichols), squalene and ubiquinone. The same inhibitory effect was obtained with 6-fluoromevalonate (1-50 microM), an inhibitor of MVA-PP decarboxylase. Squalestatin 1 (1-25 microM) and NB-598 (1-10 microM), potent squalene synthase and epoxidase inhibitors, respectively, caused a complete inhibition of cholesterol synthesis without affecting SMC proliferation. Finally, BZA-5B (10-50 microM) a specific inhibitor of protein farnesyl tranferase (PFTase), inhibited SMC proliferation in a dose- (10-50 microM) and time-dependent manner, reaching 52% +/- 6.3 inhibition after 9 days, in the presence of 50 microM BZA-5B, without affecting cholesterol synthesis. This effect was partially prevented by mevalonate (76% +/- 3.2) and GG-OH (87% +/- 7.3) but not by F-OH. On the other hand, SMC proliferation was not affected by the closely related compound BZA-7B (93% +/- 4), which does not inhibit PFTase. Taken together, these findings support the involvement of specific isoprenoid metabolites, probably through farnesylated and geranylgeranylated proteins in cell proliferation.

Maintenance of normal brain activity is dependent among other factors on the maintenance of a functional blood-brain barrier (BBB), localised mainly at the capillary wall of cerebral microcirculation. The modifications of the BBB during aging play an important role in cognitive decline with aging as well as in dementias. A review of the experiments of our laboratory over the last decades is presented, on the interaction of endothelial cells with their basement membranes, both together representing a functional unit of BBB. The action of proteolytic enzymes on the basement membrane increases BBB permeability by increasing the transcellular transport activity of endothelial cells. Flavonoid drugs protect BBB from proteolytic activity by interacting with collagen fibers and protecting sensitive peptide bonds from attack by proteolytic enzymes. These drugs enhance also the resynthesis of degraded basement membranes.