Glucose-induced period-doubling cascade in the electrical activity of pancreatic β-cells

 In the presence of stimulatory concentrations of glucose, the membrane potential of pancreatic β-cells may experience a transition from periods of rapid spike-like oscillations alternating with a pseudo-steady state to spike-only oscillations. Insulin secretion from β-cells closely correlates the periods of spike-like oscillations. The purpose of this paper is to study the mathematical structure which underlines this transitional stage in a pancreatic β-cell model. It is demonstrated that the transition can be chaotic but becomes more and more regular with increase in glucose. In particular, the system undergoes a reversed period-doubling cascade leading to the spike-only oscillations as the glucose concentration crosses a threshold. The transition interval in glucose concentration is estimated to be extremely small in terms of the rate of change for the calcium dynamics in the β-cells. The methods are based on the theory of unimodal maps and the geometric and asymptotic theories of singular perturbations. Received: 25 October 1996/Revised version: 18 August 1997     

Comparison of functional aspects of the coagulation cascade in human and sea turtle plasmas

Functional hemostatic pathways are critical for the survival of all vertebrates and have been evolving for more than 400 million years. The overwhelming majority of studies of hemostasis in vertebrates have focused on mammals with very sparse attention paid to reptiles. There have been virtually no studies of the coagulation pathway in sea turtles whose ancestors date back to the Jurassic period. Sea turtles are often exposed to rapidly altered environmental conditions during diving periods. This may reduce their blood pH during prolonged hypoxic dives. This report demonstrates that five species of turtles possess only one branch of the mammalian coagulation pathway, the extrinsic pathway. Mixing studies of turtle plasmas with human factor-deficient plasmas indicate that the intrinsic pathway factors VIII and IX are present in turtle plasma. These two factors may play a significant role in supporting the extrinsic pathway by feedback loops. The intrinsic factors, XI and XII are not detected which would account for the inability of reagents to induce coagulation via the intrinsic pathway in vitro. The analysis of two turtle factors, factor II (prothrombin) and factor X, demonstrates that they are antigenically/functionally similar to the corresponding human factors. The turtle coagulation pathway responds differentially to both pH and temperature relative to each turtle species and relative to human samples. The coagulation time (prothrombin time) increases as the temperature decreases between 37 and 15 °C. The increased time follows a linear relationship, with similar slopes for loggerhead, Kemps ridley and hawksbill turtles as well as for human samples. Leatherback turtle samples show a dramatic nonlinear increased time below 23 °C, and green turtle sample responses were similar but less dramatic. All samples also showed increased prothrombin times as the pH decreased from 7.8 to 6.4, except for three turtle species. The prothrombin times decreased, to varying extents, in a linear fashion relative to reduced pH with the rate of change greatest in leatherbacks>greenloggerhead turtles. All studies were conducted with reagents developed for human samples which would impact on the quantitative results with the turtle samples, but are not likely to alter the qualitative results. These comparative studies of the coagulation pathway in sea turtles and humans could enhance our knowledge of structure/function relationships and evolution of coagulation factors.     

Why do good hunters have higher reproductive success?

Anecdotal evidence from many hunter-gatherer societies suggests that successful hunters experience higher prestige and greater reproductive success. Detailed quantitative data on these patterns are now available for five widely dispersed cases (Ache, Hadza, !Kung, Lamalera, and Meriam) and indicate that better hunters exhibit higher age-corrected reproductive success than other men in their social group. Leading explanations to account for this pattern are: (1) direct provisioning of hunters’ wives and offspring, (2) dyadic reciprocity, (3) indirect reciprocity, (4) costly signaling, and (5) phenotypic correlation. I examine the qualitative and quantitative evidence bearing on these explanations and conclude that although none can be definitively rejected, extensive and apparently unconditional sharing of large game somewhat weakens the first three explanations. The costly signaling explanation has support in some cases, although the exact nature of the benefits gained from mating or allying with or deferring to better hunters needs further study. For comments on earlier drafts, I thank Monique Borgerhoff Mulder, Mike Gurven, Ray Hames, Kristen Hawkes, Kim Hill, Robert Kelly, Frank Marlowe, John Patton, and Polly Wiessner. Rebecca Bliege Bird and Douglas W. Bird invited me to collaborate in the Meriam research and (along with Del Passi of Mer) collected the data on Meriam demography. Geoff Kushnick and Matt Wimmer ably assisted with coding and statistical analysis of these data. Eric Alden Smith (PhD 1980, Cornell University) is a professor of anthropology at the University of Washington, Seattle. His research interests include the links between production and reproduction, the ecology and evolution of collective action, and politics in small-scale societies. He has conducted fieldwork among Inuit on Hudson Bay, Meriam in Torres Strait, and Mardu Aborigines in the Australian Western Desert.     

Characterization of the enzymatic activity of human kallikrein 6: Autoactivation,substrate specificity,and regulation by inhibitors

Human kallikrein 6 (hK6) is a trypsin-like serine protease, member of the human kallikrein gene family. Studies suggested a potential involvement of hK6 in the development and progression of Alzheimer's disease. The serum levels of hK6 might be used as a biomarker for ovarian cancer. To gain insights into the physiological role of this enzyme, we sought to determine its substrate specificity and its interactions with various inhibitors. We produced the proform of hK6 and showed that this enzyme was able to autoactivate, as well as proteolyse itself, leading to inactivation. Kinetic studies indicated that hK6 cleaved with much higher efficiency after Arg than Lys and with a preference for Ser or Pro in the P2 position. The efficient degradation of fibrinogen and collagen types I and IV by hK6 indicated that this kallikrein might play a role in tissue remodeling and/or tumor invasion and metastasis. We also demonstrated proteolysis of amyloid precursor protein by hK6 and determined the cleavage sites at the N-terminal end of the protein. Inhibition of hK6 was achieved via binding to different serpins, among which antithrombin III was the most efficient.     

TIMP-1 inhibits apoptosis in breast carcinoma cells via a pathway involving pertussis toxin-sensitive G protein and c-Src

In addition to inhibiting matrix metalloproteinases, tissue inhibitor of metalloproteinase-1 (TIMP-1) is involved in the regulation of cell growth and survival. To determine its mechanism of action, we investigated effects of TIMP-1 on cell proliferation and survival and signaling pathways induced by TIMP-1 in the human breast carcinoma T-47D cell line. Treatment of T-47D cells with TIMP-1 strongly inhibited apoptosis induced by serum deprivation, but did not affect cell proliferation. TIMP-1 induced phosphorylation of Akt and extracellular signal-regulated protein kinases (ERKs), but pertussis toxin and specific inhibitors of Src family tyrosine kinases, protein tyrosine kinases, and phosphatidylinositol-3 kinase (PI3 kinase) blocked the ability of TIMP-1 to activate Akt and ERKs as well as the anti-apoptotic effect of TIMP-1. We found that TIMP-1 enhanced the kinase activities of c-Src and PI3 kinase and that this enhancement was inhibited by pertussis toxin. Inhibition of ERK activation, however, resulted in a slight decrease of the TIMP-1-induced anti-apoptotic effect. These findings demonstrate that the ability of TIMP-1 to inhibit apoptosis in T-47D cells is mediated by the sequential activation of pertussis toxin-sensitive G protein, c-Src, PI3 kinase, and Akt.     

Phosphorylation-dependent structural alterations in the small hsp30 chaperone are associated with cellular recovery

Small heat shock proteins (hsps) act as molecular chaperones by preventing the thermal aggregation and unfolding of cellular protein; however, the manner by which cells regulate chaperone activity remains unclear. In the present study, we examined the role of phosphorylation on the chaperone function of the Xenopus small hsp30. Both heat stress and sodium arsenite treatment in A6 cells resulted in a rapid activation of p38alpha and MAPKAPK-2. Surprisingly, the association of MAPKAPK-2 with hsp30 and its subsequent phosphorylation were more prevalent during recovery after heat stress. Treatment of A6 cells with SB203580, an inhibitor of the p38 MAP kinase pathway, resulted in a loss of hsp30 phosphorylation. Phosphorylation resulted in the formation of smaller multimeric hsp30 complexes and resulted in a significant loss of secondary structure. Consequently the phosphorylation-induced structural changes severely compromised the ability of hsp30 to prevent the heat-induced aggregation of citrate synthase and luciferase in vitro. We confirmed that the loss of chaperone activity was coincident with an attenuated binding of phosphorylated hsp30 with target proteins. Our data suggest that phosphorylation may be necessary to regulate the post-heat stress molecular chaperone activity of hsp30.     

Membrane incorporation of 22-hydroxycholesterol inhibits chemokine receptor activity

Cell membrane exposure to oxysterols, such as 22-hydroxycholesterol (22-OHC), has previously been shown to induce a suppressive effect on lymphocyte activation. Based on our previous findings that chemokine binding was significantly inhibited by the extraction of membrane cholesterol, we sought to assess the effects of 22-OHC treatment on chemokine ligand-binding and receptor activity. Our results revealed that 22-OHC, but not nonoxidized cholesterol, significantly reduced the binding of both SDF-1alpha and MIP-1beta to human T-cell lines and PBMCs within 1 h of treatment. Incubating the treated cells at 37 degrees C for 1 h reversed a majority of the inhibitory effects on chemokine binding. 22-OHC also inhibited intracellular calcium mobilization and cell migration in response to SDF-1alpha treatment. Interestingly, while the presence of oxysterols in cell membranes significantly inhibits chemokine receptor function, this inhibitory effect does not involve alterations in receptor conformation, expression, or a direct antagonism of chemokine binding. We propose here a novel mechanism for oxysterol-mediated inhibition of chemokine receptor function and the implications for the presence of oxysterols on immune cells.