Cytological evidence for holocentric chromosomes of the silkworms,Bombyx mori and B. mandarina, (Bombycidae,Lepidoptera)

The nature of the centromere and the orientation in meiosis of silkworm chromosomes were investigated using the trivalent of the F₁ hybrid between the wild and domestic silkworm and X-ray-induced aberrant chromosomes as well as normal silkworm chromosomes. The results of the experiments were as follows: (1) Pro-metaphase chromosomes showed no distinct primary constriction even after treatment with hypotonic solution, (2) sister chromatids separated in parallel along the entire length of the chromosome at mitotic anaphase, (3) chiasmata underwent complete terminalization during diakinesis and thus chromosome dyads were always connected end-to-end by a terminal chiasma at metaphase I, (4) radiation-induced aberrant chromosomes were stably transmitted throughout a number of cell generations, and (5) although the homomorphic bivalents generally orientated axially at metaphase I and equatorially at metaphase II, this normal sequence tended to be inverted or modified in the X-ray-induced aberrant chromosomes and in the trivalent of the F₁ hybrid silkworms. These observations may be best interpreted by assuming the holocentric nature of silkworm chromosomes.     

Contribution of membrane-associated prostaglandin E2 synthase to bone resorption

This study initially confirmed that, among prostaglandins (PGs) produced in bone, only PGE(2) has the potency to stimulate osteoclastogenesis and bone resorption in the mouse coculture system of osteoblasts and bone marrow cells. For the PGE(2) biosynthesis two isoforms of the terminal and specific enzymes, membrane-associated PGE(2) synthase (mPGES) and cytosolic PGES (cPGES) have recently been identified. In cultured mouse primary osteoblasts, both mPGES and cyclooxygenase-2 were induced by the bone resorptive cytokines interleukin-1, tumor necrosis factor-alpha, and fibroblast growth factor-2. Induction of mPGES was also seen in the mouse long bone and bone marrow in vivo by intraperitoneal injection of lipopolysaccharide. In contrast, cPGES was expressed constitutively both in vitro and in vivo without being affected by these stimuli. An antisense oligonucleotide blocking mPGES expression inhibited not only PGE(2) production, but also osteoclastogenesis and bone resorption stimulated by the cytokines, which was reversed by addition of exogenous PGE(2). We therefore conclude that mPGES, which is induced by and mediates the effects of bone resorptive stimuli, may make a target molecule for the treatment of bone resorptive disorders.     

Long-Term Acceptance of Major Histocompatibility Complex-Mismatched Cardiac Allograft Induced by a Low Dose of CTLA4IgM Plus FK506

The immunosuppressant FK506 prolongs allograft survival. However, at therapeutic doses it has significant side effects. A fusion protein consisting of the extracellular portion of CTLA4 and the Fc portion of human IgG (CTLA4IgG) also prolongs allograft survival, but large doses of CTLA4IgG are required for the induction of cardiac allograft acceptance. Therefore, we constructed a pentameric form of a new CTLA4 fusion protein, CTLA4IgM. We tested whether low doses of CTLA4IgG or CTLA4IgM in combination with subtherapeutic doses of FK506 can prolong allograft survival in a synergistic fashion. C57BL/6 (H-2^b) neonatal hearts were transplanted to CBA/J (H-2^k) mice in a heterotopic, nonvascularized cardiac allograft model. The findings demonstrate that a combination of low doses of FK506 plus a pentameric form of CTLA4Ig, CTLA4IgM, leads to significant graft survival, while a combination of FK506 plus CTLA4IgG does not.     

Beta‐diversity in temperate and tropical forests reflects dissimilar mechanisms of community assembly

Site‐to‐site variation in species composition (β‐diversity) generally increases from low‐ to high‐diversity regions. Although biogeographical differences in community assembly mechanisms may explain this pattern, random sampling effects can create this pattern through differences in regional species pools. Here, we compared assembly mechanisms between spatially extensive networks of temperate and tropical forest plots with highly divergent species pools (46 vs. 607 species). After controlling for sampling effects, β‐diversity of woody plants was similar and higher than expected by chance in both forests, reflecting strong intraspecific aggregation. However, different mechanisms appeared to explain aggregation in the two forests. In the temperate forest, aggregation reflected stronger environmental correlations, suggesting an important role for species‐sorting (e.g. environmental filtering) processes, whereas in the tropics, aggregation reflected stronger spatial correlations, more likely reflecting dispersal limitation. We suggest that biogeographical differences in the relative importance of different community assembly mechanisms contribute to these striking gradients in global biodiversity.     

Somatostatin suppresses ghrelin secretion from the rat stomach

Ghrelin is an acylated peptide that stimulates food intake and the secretion of growth hormone. While ghrelin is predominantly synthesized in a subset of endocrine cells in the oxyntic gland of the human and rat stomach, the mechanism regulating ghrelin secretion remains unknown. Somatostatin, a peptide produced in the gastric oxyntic mucosa, is known to suppress secretion of several gastrointestinal peptides in a paracrine fashion. By double immunohistochemistry, we demonstrated that somatostatin-immunoreactive cells contact ghrelin-immunoreactive cells. A single intravenous injection of somatostatin reduced the systemic plasma concentration of ghrelin in rats. Continuous infusion of somatostatin into the gastric artery of the vascularly perfused rat stomach suppressed ghrelin secretion in both dose- and time-dependent manner. These findings indicate that ghrelin secretion from the stomach is regulated by gastric somatostatin.     

The dominant role of the prosegment of prorenin in determining the rate of activation by acid or trypsin: studies with molecular chimeras

Human prorenin activation by acid or trypsin is faster than rat prorenin by two orders of magnitude. No plausible mechanism exists to explain the difference. Two chimeric mutant prorenins were produced in CHO cells. A chimera, hPro/rRen, composed of human prorenin prosegment and rat active renin segment, was activated as fast as wild-type human prorenin at pH 3.3 and 25 degrees C or by trypsin (1 microg/ml). The other chimera, rPro/hRen, composed of rat prorenin prosegment and human active renin segment, was activated as slowly as wild-type rat prorenin at pH 3.3 and 25 degrees C or by trypsin (50 microg/ml). These results indicate that the rate of activation of prorenin is predominantly determined by the N-terminal pro-sequence. Plausible mechanisms are discussed.     

Central effects of neuromedin U in the regulation of energy homeostasis

Neuromedin U (NMU) is a brain-gut peptide whose peripheral activities are well-understood but whose central actions have yet to be clarified. The recent identification of two NMU receptors in rat brain has provided a springboard for further investigation into its role in the central nervous system. Intracerebroventricular administration of NMU to free-feeding rats decreased food intake and body weight. Conversely, NMU increased gross locomotor activity, body temperature, and heat production. NMU, a potent endogenous anorectic peptide, serves as a catabolic signaling molecule in the brain. Further investigation of the biochemical and physiological functions of NMU will help our better understanding of the mechanisms of energy homeostasis.     

β-Lactam Selectivity of Multidrug Transporters AcrB and AcrD Resides in the Proximal Binding Pocket

β-Lactams are mainstream antibiotics that are indicated for the prophylaxis and treatment of bacterial infections. The AcrA-AcrD-TolC multidrug efflux system confers much stronger resistance on Escherichia coli to clinically relevant anionic β-lactam antibiotics than the homologous AcrA-AcrB-TolC system. Using an extensive combination of chimeric analysis and site-directed mutagenesis, we searched for residues that determine the difference in β-lactam specificity between AcrB and AcrD. We identified three crucial residues at the “proximal” (or access) substrate binding pocket. The simultaneous replacement of these residues in AcrB by those in AcrD (Q569R, I626R, and E673G) transferred the β-lactam specificity of AcrD to AcrB. Our findings indicate for the first time that the difference in β-lactam specificity between AcrB and AcrD relates to interactions of the antibiotic with residues in the proximal binding pocket.     

Mitochondrial dysfunction and reduced prostaglandin synthesis in skeletal muscle of Group VIB Ca2+-independent phospholipase A2γ-deficient mice

Group VIB Ca²⁺-independent phospholipase A₂γ (iPLA₂γ) is a membrane-bound iPLA₂ enzyme with unique features, such as the utilization of distinct translation initiation sites and the presence of mitochondrial and peroxisomal localization signals. Here we investigated the physiological functions of iPLA₂γ by disrupting its gene in mice. iPLA₂γ-knockout (KO) mice were born with an expected Mendelian ratio and appeared normal and healthy at the age of one month but began to show growth retardation from the age of two months as well as kyphosis and significant muscle weakness at the age of four months. Electron microscopy revealed swelling and reduced numbers of mitochondria and atrophy of myofilaments in iPLA₂γ-KO skeletal muscles. Increased lipid peroxidation and the induction of several oxidative stress-related genes were also found in the iPLA₂γ-KO muscles. These results provide evidence that impairment of iPLA₂γ causes mitochondrial dysfunction and increased oxidative stress, leading to the loss of skeletal muscle structure and function. We further found that the compositions of cardiolipin and other phospholipid subclasses were altered and that the levels of myoprotective prostanoids were reduced in iPLA₂γ-KO skeletal muscle. Thus, in addition to maintenance of homeostasis of the mitochondrial membrane, iPLA₂γ may contribute to modulation of lipid mediator production in vivo.