Role of N-cadherin in bone formation

Cell-cell adhesion mediated by cadherins is essential for the function of bone forming cells during osteogenesis. Here, the evidence that N-cadherin is an important regulator of osteoblast differentiation and osteogenesis is reviewed. Osteoblasts express a limited number of cadherins, including the classic N-cadherin. The expression profile of N-cadherin in osteoblasts during bone formation in vivo and in vitro suggests a role of this molecule in osteogenesis. Functional studies using neutralizing antibodies or antisense oligonucleotides indicate that N-cadherin is involved in the control the expression of osteoblast marker gene expression and differentiation. Cleavage of N-cadherin during osteoblast apoptosis also suggests a role of N-cadherin-mediated-cell-cell adhesion in osteoblast survival. Hormonal and local factors that regulate osteoblast function also regulate N-cadherin expression and subsequent cell-cell adhesion associated with osteoblast differentiation or survival. Signaling mechanisms involved in N-cadherin-mediated cell-cell adhesion and osteoblast gene expression have also been identified. Alterations of N-cadherin expression are associated with abnormal osteoblast differentiation and osteogenesis in pathological conditions. These findings indicate that N-cadherin plays a role in normal and pathological bone formation and provide some insight into the process involved in N-cadherin-mediated cell-cell adhesion and differentiation in osteoblasts.     

Pulsatile urea excretion in the gulf toadfish: mechanisms and controls

Opsanus beta expresses a full complement of ornithine–urea cycle (OUC) enzymes and is facultatively ureotelic, reducing ammonia-N excretion and maintaining urea-N excretion under conditions of crowding/confinement. The switch to ureotelism is keyed by a modest rise in cortisol associated with a substantial increase in cytosolic glutamine synthetase for trapping of ammonia-N and an upregulation of the capacity of the mitochondrial OUC to use glutamine-N. The entire day's urea-N production is excreted in 1 or 2 short-lasting pulses, which occur exclusively through the gills. The pulse event is not triggered by an internal urea-N threshold, is not due to pulsatile urea-N production, but reflects pulsatile activation of a specific branchial excretion mechanism that rapidly clears urea-N from the body fluids. A bidirectional facilitated diffusion transporter, with pharmacological similarity to the UT-A type transporters of the mammalian kidney, is activated in the gills, associated with an increased trafficking of dense-cored vesicles in the pavement cells. An 1814 kB cDNA (‘tUT’) coding for a 475–amino acid protein with approximately 62% homology to mammalian UT-A's has been cloned and facilitates phloretin-sensitive urea transport when expressed in Xenopus oocytes. tUT occurs only in gill tissue, but tUT mRNA levels do not change over the pulse cycle, suggesting that tUT regulation occurs at a level beyond mRNA. Circulating cortisol levels consistently decline prior to a pulse event and rise thereafter. When cortisol is experimentally clamped at high levels, natural pulse events are suppressed in size but not in frequency, an effect mediated through glucocorticoid receptors. The cortisol decline appears to be permissive, rather than the actual trigger of the pulse event. Fluctuations in circulating AVT levels do not correlate with pulses; and injections of AVT (at supraphysiological levels) elicit only minute urea-N pulses. However, circulating 5-hydroxytryptamine (5-HT) levels fluctuate considerably and physiological doses of 5-HT cause large urea-N pulse events. When the efferent cranial nerves to the gills are sectioned, natural urea pulse events persist, suggesting that direct motor output from the CNS to the gill is not the proximate control.     

Biclustering microarray data by Gibbs sampling

MOTIVATION: Gibbs sampling has become a method of choice for the discovery of noisy patterns, known as motifs, in DNA and protein sequences. Because handling noise in microarray data presents similar challenges, we have adapted this strategy to the biclustering of discretized microarray data. RESULTS: In contrast with standard clustering that reveals genes that behave similarly over all the conditions, biclustering groups genes over only a subset of conditions for which those genes have a sharp probability distribution. We have opted for a simple probabilistic model of the biclusters because it has the key advantage of providing a transparent probabilistic interpretation of the biclusters in the form of an easily interpretable fingerprint. Furthermore, Gibbs sampling does not suffer from the problem of local minima that often characterizes Expectation-Maximization. We demonstrate the effectiveness of our approach on two synthetic data sets as well as a data set from leukemia patients.     

Root proliferation in perennial grasses of low and high palatability

Root proliferation of desirable (Stipa clarazii andS. tenuis) and undesirable (S.ambigua)perennial grasses was studied in semiarid rangelands of Central Argentina(40°39S, 62°54W) in 1998. On 17 September, soil coreswereremoved from the edge of the plant, metal structures lined with screen mesh(hereafter called bags) were buried in the holes, and root-free soil was placedinto these structures. Numbers of green tillers and circumference per plant hadpreviously been determined. Since plants were of unequal size among species,root length and root dry weight data are reported on a per green tiller basis.Half of the plants was defoliated to 5 cm stubble height on 17September and/or 12 October, while the other half remained undefoliated(controls). Bags were destructively harvested either 20 days after the firstdefoliation (first sampling) or 56 days after the second defoliation (secondsampling) by digging out soil very carefully around each bag. Roots were washedfrom soil, root length estimated by the line intercept method, root dry weightdetermined after oven-drying, and root length per unit root dry weightcalculated from the two measured variables. Root length and dry weight weremorethan 96% greater on defoliated and undefoliated plants ofS. clarazii than on those of S.tenuisor S. ambigua for both sampling dates. Root length perunitroot dry weight, however, was more than 43% greater (p < 0.05) inS. tenuis than in S. clarazii andS. ambigua during the second sampling. Defoliated plantshada similar root length and root dry weight than undefoliated plants in all threespecies, although plants of S. tenuis defoliated twiceshowed a greater (p < 0.05) root length than undefoliated controls. Rootlength and root dry weight were similar between sampling periods, except onundefoliated plants of S. tenuis which had a greater (p<0.05) root length and root dry weight at the first than at the second sampling.Although root length per unit root dry weight may be greater inS. tenuis than in S. clarazii andS. ambigua, greater root length and dry weight increasesinS. clarazii after defoliation appear determinant incontributing to explain its greater competitive ability and defoliationtolerance when compared with the other two species.Nomenclature of taxa followed.     

Microsite-dependent changes in methanogenic populations in a boreal oligotrophic fen

Wetlands, including peatlands, are the main source of natural methane emission. Well-defined fen microsites have different methane emissions rates, but it is not known whether the methane-producing Archaea communities vary at these sites. Possible horizontal variations of communities, in a natural oligotrophic fen, were analysed by characterizing the methanogens from two well-defined microsites: Eriophorum lawn and Hummock. Community structures were studied at two different layers of the fen, showing, respectively, high and low methane production. The structure of methanogen populations was determined using molecular techniques targeting the 16SrRNA gene and combined denaturing gradient gel electrophoresis (DGGE) and restriction fragment length polymorphism (RFLP) analysis. Results subjected to non-metric multidimensional scaling (MDS), diversity indices calculation and phylogenetic analysis revealed that upper layer communities changed with site while deeper layer communities remained the same. Phylogenetic analyses revealed six different clusters of sequences grouping with only two known orders of methanogens. Upper layers of Hummock were dominated by sequences clustering with members of Methanomicrobiales and sequences dominating the upper part of the Eriophorum lawn were related to members of the order Methanosarcinales. Novel methanogenic sequences were found at both sites at both depths. Vegetation characterizing the microsites probably influences the microbial communities in the layers of the fen where methane is produced.     

Lack of quinine-evoked activity in rat trigeminal subnucleus caudalis

Conflicting reports exist regarding the ability of quinine to activate neurons in the trigeminal system. We used the complementary approaches of single-unit electrophysiology and c-fos immunohistochemistry to investigate whether quinine (100 mM) activates chemonociceptive cells in the brainstem trigeminal subnucleus caudalis (Vc). In electrophysiological experiments, 38 units responded to noxious mechanical, thermal and chemical (200 mM pentanoic acid) stimuli applied to the tongue with an increase in firing rate; none responded to lingual quinine whether the quinine was presented before or after application of pentanoic acid. In the c-fos immunohistochemical experiment, both quinine and water elicited equivalent levels of fos-like immunoreactivity (FLI) in dorsomedial Vc that were significantly lower than the level of FLI evoked by pentanoic acid. These data collectively indicate that quinine does not elicit activity in chemonociceptive Vc neurons.     

Insular bird populations can be saved from rats: a long-term experimental study of white-chinned petrels<Emphasis Type="Italic"> Procellaria aequinoctialis</Emphasis> on Ile de la Possession (Crozet archipelago)

The white-chinned petrel is a subantarctic seabird that requires urgent implementation of conservation measures for the species. At sea, adults suffer heavy mortality due to fisheries' practices. On land, introduced rats prey on chicks at several localities, and we test here if and how rats can be efficiently controlled. Since 1994, we have conducted an intensive rat-control program during each breeding season in a white-chinned petrel colony on Ile de la Possession (150 km²; Crozet archipelago, southern Indian Ocean), which had been monitored since 1986. On the same island, a control white-chinned petrel colony, where no poisoning occurred, was also monitored, and we assessed the seasonal variations of rat abundance. We compared three situations: high rat-poisoning, low rat-poisoning and control conditions without poisoning. Low-poisoning trials performed in our experimental colony between 1988 and 1991 did not lead to higher chick production than for the previous two control years. However, petrel-breeding success was significantly higher when intensive poisoning occurred (50%) than for the previous years (16%). The duration of our study (8 years before intensive poisoning, plus 8 years afterwards), combined with a comparison of petrel annual breeding success between our experimental and control colonies, allowed us to assess more effectively the impact of rats. Forty-one per cent of breeding failures occurring in non-poisoned areas were attributed to rats. We conclude that threatened insular bird populations can be conserved and restored in localities even where total rat eradication is not possible. However, only intensive and repeated (long-term) poisoning will control rats sufficiently.     

Optical tweezers in biology and medicine

Optical trapping techniques provide unique means to manipulate biological particles such as virus, living cells and subcellular organelles. Another area of interest is the measurement of mechanical (elastic) properties of cell membranes, long strands of single DNA molecule, and filamentous proteins. One of the most attractive applications is the study of single motor molecules. With optical tweezers traps, one can measure the forces generated by single motor molecules such as kinesin and myosin, in the piconewton range and, for the first time, resolve their detailed stepping motion.     

Arabidopsis haiku mutants reveal new controls of seed size by endosperm

In flowering plants, maternal seed integument encloses the embryo and the endosperm, which are both derived from double fertilization. Although the development of these three components must be coordinated, we have limited knowledge of mechanisms involved in such coordination. The endosperm may play a central role in these mechanisms as epigenetic modifications of endosperm development, via imbalance of dosage between maternal and paternal genomes, affecting both the embryo and the integument. To identify targets of such epigenetic controls, we designed a genetic screen in Arabidopsis for mutants that phenocopy the effects of dosage imbalance in the endosperm. The two mutants haiku 1 and haiku 2 produce seed of reduced size that resemble seed with maternal excess in the maternal/paternal dosage. Homozygous haiku seed develop into plants indistinguishable from wild type. Each mutation is sporophytic recessive, and double-mutant analysis suggests that both mutations affect the same genetic pathway. The endosperm of haiku mutants shows a premature arrest of increase in size that causes precocious cellularization of the syncytial endosperm. Reduction of seed size in haiku results from coordinated reduction of endosperm size, embryo proliferation, and cell elongation of the maternally derived integument. We present further evidence for a control of integument development mediated by endosperm-derived signals.