The cytoplasmic cage domain of the mechanosensitive channel MscS is a sensor of macromolecular crowding

Cells actively regulate the macromolecular excluded volume of the cytoplasm to maintain the reciprocal fraction of free aqueous solution that is optimal for intracellular processes. However, the mechanisms whereby cells sense this critical parameter remain unclear. The mechanosensitive channel of small conductance (MscS channel), which is the major regulator of turgor in bacteria, mediates efflux of small osmolytes in response to increased membrane tension. At moderate sustained tensions produced by a decrease in external osmolarity, MscS undergoes slow adaptive inactivation; however, it inactivates abruptly in the presence of cytoplasmic crowding agents. To understand the mechanism underlying this rapid inactivation, we combined extrapolated and equilibrium molecular dynamics simulations with electrophysiological analyses of MscS mutants to explore possible transitions of MscS and generated models of the resting and inactivated states. Our models suggest that the coupling of the gate formed by TM3 helices to the peripheral TM1–TM2 pairs depends on the axial position of the core TM3 barrel relative to the TM1–TM2 shaft and the state of the associated hollow cytoplasmic domain (“cage”). They also indicate that the tension-driven inactivation transition separates the gate from the peripheral helices and promotes kinks in TM3s at G113 and that this conformation is stabilized by association of the TM3b segment with the β domain of the cage. We found that mutations destabilizing the TM3b–β interactions preclude inactivation and make the channel insensitive to crowding agents and voltage; mutations that strengthen this association result in a stable closed state and silent inactivation. Steered simulations showed that pressure exerted on the cage bottom in the inactivated state reduces the volume of the cage in the cytoplasm and at the same time increases the footprint of the transmembrane domain in the membrane, implying coupled sensitivity to both membrane tension and crowding pressure. The cage, therefore, provides feedback on the increasing crowding that disengages the gate and prevents excessive draining and condensation of the cytoplasm. We discuss the structural mechanics of cells surrounded by an elastic cell wall where this MscS-specific feedback mechanism may be necessary.     

MKR mice are resistant to the metabolic actions of both insulin and adiponectin: discordance between insulin resistance and adiponectin responsiveness

Most rodent models of insulin resistance are accompanied by decreased circulating adiponectin levels. Adiponectin treatment improves the metabolic phenotype by increasing fatty acid oxidation in skeletal muscle and suppressing hepatic glucose production. Muscle IGF-I receptor (IGF-IR)-lysine-arginine (MKR) mice expressing dominant-negative mutant IGF-IRs in skeletal muscle are diabetic with insulin resistance in muscle, liver, and adipose tissue. Adiponectin levels are elevated in MKR mice, suggesting an unusual discordance between insulin resistance and adiponectin responsiveness. Therefore, we investigated the metabolic actions of adiponectin in MKR mice. MKR and ob/ob mice were treated both acutely (28 microg/g) and chronically (for 2 wk) with full-length adiponectin. Acute hypoglycemic effects of adiponectin were evident only in ob/ob mice but not in MKR mice. Chronic adiponectin treatment significantly improved both insulin sensitivity and glucose tolerance in ob/ob but not in MKR mice. Adiponectin receptor mRNA levels and adiponectin-stimulated phosphorylation of AMPK in skeletal muscle and liver were similar among MKR, wild-type, and ob/ob mice. Thus MKR mice are adiponectin resistant despite normal expression of adiponectin receptors and normal AMPK phosphorylation in muscle and liver. MKR mice may be a useful model for dissecting relationships between insulin resistance and adiponectin action in regulation of glucose homeostasis.     

Enhanced poly(3-hydroxybutyrate) production in transgenic tobacco BY-2 cells using engineered acetoacetyl-CoA reductase

Highly active mutant of NADPH-dependent acetoacetyl-CoA reductase (PhaB) was expressed in Nicotiana tabacum cv. Bright Yellow-2 cultured cells to produce poly(3-hydroxybutyrate) [P(3HB)]. The mutated PhaB increased P(3HB) content by three-fold over the control, indicating that the mutant was a versatile tool for P(3HB) production. Additionally, the PhaB-catalyzed reaction was suggested to be a rate-limiting step of P(3HB) biosynthesis in tobacco BY-2 cells.     

Uptake of lithium carmine by sinusoidal endothelial and Kupffer cells of the rat liver: new insights into the classical vital staining and the reticulo-endothelial system

Sinusoidal cells in the rat liver were studied in vivo and in vitro using the original vital staining with lithium carmine, which has contributed much to the development of the concept of the reticulo-endothelial system. Immunohistochemical and electron-microscopic studies revealed that the dye-incorporating cells were sinusoidal endothelial cells, Kupffer cells, and monocytes. The endothelial cells took up much more dye than did the Kupffer cells and bulged largely into the sinusoidal lumen. Electron microscopy revealed that small particles of lithium carmine were associated with coated vesicles of endothelial cells and ruffled membranes of Kupffer cells. In the endothelial cells, these particles were present in various concentrations within vacuolated structures and condensed in the lysosomes forming large aggregates of lithium carmine lumps. These lumps showed crystalline structures, within which the size of the individual particle was up to 30 nm in width and 50 nm in length. A few endothelial cells containing abundant dye underwent degeneration, and some were taken up by Kupffer cells. Liver endothelial cells isolated from lithium carmine-administered rats endocytosed fluorescence-labeled collagen. Isolated endothelial cells from normal rat liver, when cultured with lithium carmine, did not take up any dye, and their endocytosis of formaldehyde-treated albumin was inhibited dose-dependently. We conclude that in the liver, endothelial cells, but not Kupffer cells, predominantly take up lithium carmine. Furthermore, we propose the existence of a generalized cell system based on its vital staining capacity.     

12-O-tetradecanoyl phorbol-13-acetate (TPA) stimulates somatostatin release from isolated perfused rat stomach

Effect of TPA (12-O-tetradecanoyl phorbol-13-acetate), a potent tumor promoter, on immunoreactive somatostatin release was investigated using the isolated perfused rat stomach. TPA at the concentration as low as 20nM significantly stimulated the somatostatin release from isolated perfused rat stomach. The integrated net output of somatostatin induced by TPA was dose-dependent in a range of 5 - 50nM TPA. Since TPA is known to activate C-kinase specifically at a low dose (less than 20nM), these findings suggest that C-kinase system may be involved in the regulation of somatostatin release in rat stomach.     

Human parathyroid hormone related protein fragment-(1-34) had glucose-6-phosphate dehydrogenase activity on distal convoluted tubules in cytochemical bioassay

In the present study, the action of parathyroid hormone related protein (PTHrP) on glucose-6-phosphate dehydrogenase (G6PD) activity of the distal convoluted tubules was examined utilizing cytochemical bioassay (CBA). Recently full amino acid residues of human PTHrP (hPTHrP), one of the causative agents of HHM, was identified based on the cDNA clone using BEN cells. We synthesized hPTHrP-(1-34) and examined the effect of this protein on G6PD activities on the distal convoluted tubules, and compared its bioactivity to that of human parathyroid hormone (hPTH)-(1-84). hPTHrP-(1-34) stimulated G6PD activity in a log linear fashion with equivalent activity to that of hPTH-(1-84) on a molar basis. Conclusively, we found that PTHrP act on distal convoluted tubules similar to hPTH.     

The activation of cAMP-dependent protein kinase is directly linked to the regulation of osteoblast proliferation (UMR-106) by parathyroid hormone.

In order to characterize the direct involvement of cAMP in the change of osteoblast proliferation by parathyroid hormone (PTH), we employed the diastereoisomers of adenosine 3',5'-cyclic phosphorothioate, Sp-cAMPS and Rp-cAMPS, which have been recently shown to act directly as agonist and antagonist, respectively in the activation of cAMP-dependent protein kinase (PKA). Dibutyryl cAMP (dbcAMP) and cholera toxin as well as human(h) PTH-(1-34) significantly inhibited [3H]thymidine incorporation (TdR) in osteoblastic osteosarcoma cells, UMR-106. Sp-cAMPS (10(-6)-10(-4) M) inhibited TdR in a dose-dependent manner. Although Rp-cAMPS (10(-6)-10(-4) M) itself did not affect TdR, it significantly blocked dbcAMP-, cholera toxin- and Sp-cAMPS-induced suppression of TdR. Moreover, Rp-cAMPS (10(-6)-10(-4) M) dose-dependently antagonized hPTH-induced suppression of TdR. Present studies first indicated that the activation of PKA is directly linked to the change of osteoblast proliferation by PTH.