Biomedical subjects

C J Rhodes

Publications and source records attributed to C J Rhodes.

At least 55 records · Page 3Linked to original sources

PubMed1997

On the critical behaviour of simple epidemics.

We show how ideas and models which were originally introduced to gain an understanding of critical phenomena can be used to interpret the dynamics of epidemics of communicable disease in real populations. Specifically, we present an analysis of the dynamics of disease outbreaks for three common communicable infections from a small isolated island population. The strongly fluctuating nature of the temporal incidence of disease is captured by the model, and comparisons between exponents calculated from the data and from simulations are made. A forest-fire model with sparks is used to classify the observed scaling dynamics of the epidemics and provides a unified picture of the epidemiology which conventional epidemiological analysis is unable to reproduce. This study suggests that power-law scaling can emerge in natural systems when they are driven on widely separated time-scales, in accordance with recent analytic renormalization group calculations.

Disease Outbreaks↗

PubMed1997

Epidemic thresholds and vaccination in a lattice model of disease spread.

We use a lattice-based epidemic model to study the spatial and temporal rates of disease spread in a spatially distributed host population. The prevalence of the disease in the population is studied as well as the spread of infection about a point source of infection. In particular, two distinct critical population densities are identified. The first relates to the minimum population density for a epidemic to occur, whilst the second is the minimum population density for long-term persistence to occur. Vaccination regimes are introduced that are used to measure the impact of spatially and nonspatially dependent intervention strategies. Specifically we show how a ring of vaccinated susceptibles, of sufficient thickness, can halt the spread of infection across space.

Animals↗

PubMed1997

The kinetics of immunological responses to microfilariae in a murine host: experimental and mathematical studies.

We present a mathematical model which is used to interpret the dynamics of the immunological response of a mouse host to infection with the filarial worm Onchocerca lienalis. The model mimics changes in worm burden over time post-infection and after reinfection and its behaviour provides a good description of experimental results. Measured production of T-cells and eosinophils is also compared with the predictions of the model. Our results show that the immune response mechanism proposed on the basis of experimental results, involving CD4+ T-cells and eosinophil destruction of the parasite, is supported by the insights gained from the mathematical model. Also, using the parameters estimated to describe the primary infection dynamics, the degree of acquired immunity to secondary infection is also well described by the model. Our analysis highlights the importance of obtaining quantitative measures of the many rate parameters involved in even the simplest interpretations of immunological responses to parasitic infection.

Animals↗

PubMed1997

Muon spectroscopy applied to biological systems: a study of thiyl radicals, RS.

Thiyl radicals (RS.) are formed when positive muons (mu+) are implanted into solutions of thiocarbonyl compounds (C = S) in ethanol, tetrahydrofuran or formamide solvents. A solvent dependence is found, which reflects changing electronic interactions and overall conformations, suggesting that the properties of RS. radicals may be dependent on the polarity (philicity) of the environment in which they are formed.

Animals↗

PubMed1997

Multicomponent spectroscopic investigations of salivary antioxidant consumption by an oral rinse preparation containing the stable free radical species chlorine dioxide (ClO2.).

A multicomponent evaluation of the oxidative consumption of salivary biomolecules by a commercially-available oral rinse preparation containing an admixture of the stable free radical species chlorine dioxide (ClO2.) with chlorite anion (ClO2-) has been investigated using high resolution 1H NMR spectroscopy. The results obtained demonstrated that ClO2. and/or ClO2- present in this preparation effected the oxidative decarboxylation of salivary pyruvate (to acetate and CO2). Experiments conducted on chemical model systems confirmed the oxidative decarboxylation of pyruvate by this oral rinse, and also demonstrated that urate, thiocyanate anion, and the amino acids cysteine and methionine (precursors to volatile sulphur compounds responsible for oral malodour), were oxidatively consumed. The biochemical, periodontal and therapeutic significance of the results are discussed.

Absorption↗

PubMed1996

A scaling analysis of measles epidemics in a small population.

We present a detailed analysis of the pattern of measles outbreaks in the small isolated community of the Faroe Islands. Measles outbreaks in this population are characterized by frequent fade-out of infection resulting in long intervals when the disease is absent from the islands. Using an analysis of the distribution of epidemic sizes and epidemic durations we propose that the dynamical structure observed in the measles case returns reflects the existence of an underlying scaling mechanism. Consequently the dynamics are not as purely stochastic as is usually thought for epidemiological systems of this sort. We use a lattice-based epidemic model to provide a theoretical estimate of the scaling exponents and show that a conventional compartmental SEIR model is unable to reproduce this result. The methods discussed in this paper are general and represent a novel way to consider the dynamics of any other communicable disease where there is frequent fade-out in the case returns.

Denmark↗

PubMed1996

Insulin-stimulated translocation of GLUT4 glucose transporters requires SNARE-complex proteins.

A major physiological role of insulin is the regulation of glucose uptake into skeletal and cardiac muscle and adipose tissue, mediated by an insulin-stimulated translocation of GLUT4 glucose transporters from an intracellular vesicular pool to the plasma membrane. This process is similar to the regulated docking and fusion of vesicles in neuroendocrine cells, a process that involves SNARE-complex proteins. Recently, several SNARE proteins were found in adipocytes: vesicle-associated membrane protein (VAMP-2), its related homologue cellubrevin, and syntaxin-4. In this report we show that treatment of permeabilized 3T3-L1 adipocytes with botulinum neurotoxin D, which selectively cleaves VAMP-2 and cellubrevin, inhibited the ability of insulin to stimulate translocation of GLUT4 vesicles to the plasma membrane. Furthermore, treatment of the permeabilized adipocytes with glutathione S-transferase fusion proteins encoding soluble forms of VAMP-2 or syntaxin-4 also effectively blocked insulin-regulated GLUT4 translocation. These results provide evidence of a functional role for SNARE-complex proteins in insulin-stimulated glucose uptake and suggest that adipocytes utilize a mechanism of regulating vesicle docking and fusion analogous to that found in neuroendocrine tissues.

1-Methyl-3-isobutylxanthine↗

PubMed1996

Insulin-secreting non-islet cells are resistant to autoimmune destruction.

Transgenic nonobese diabetic mice were created in which insulin expression was targeted to proopiomelanocortin-expressing pituitary cells. Proopiomelanocortin-expressing intermediate lobe pituitary cells efficiently secrete fully processed, mature insulin via a regulated secretory pathway, similar to islet beta cells. However, in contrast to the insulin-producing islet beta cells, the insulin-producing intermediate lobe pituitaries are not targeted or destroyed by cells of the immune system. Transplantation of the transgenic intermediate lobe tissues into diabetic nonobese diabetic mice resulted in the restoration of near-normoglycemia and the reversal of diabetic symptoms. The absence of autoimmunity in intermediate lobe pituitary cells engineered to secrete bona fide insulin raises the potential of these cell types for beta-cell replacement therapy for the treatment of insulin-dependent diabetes mellitus.

Animals↗

PubMed1996

Power laws governing epidemics in isolated populations.

Temporal changes in the incidence of measles virus infection within large urban communities in the developed world have been the focus of much discussion in the context of the identification and analysis of nonlinear and chaotic patterns in biological time series. In contrast, the measles records for small isolated island populations are highly irregular, because of frequent fade-outs of infection, and traditional analysis does not yield useful insight. Here we use measurements of the distribution of epidemic sizes and duration to show that regularities in the dynamics of such systems do become apparent. Specifically, these biological systems are characterized by well-defined power laws in a manner reminiscent of other nonlinear, spatially extended dynamical systems in the physical sciences. We further show that the observed power-law exponents are well described by a simple lattice-based model which reflects the social interaction between individual hosts.

Denmark↗

PubMed1996

Persistence and dynamics in lattice models of epidemic spread.

We present a simple epidemic model representing the spread of a communicable disease in a spatially extended host population. The model falls into the general class of techniques which utilise lattice based simulation as a way of incorporating spatial effects. The factors relating to the persistence and dynamics of the disease are investigated. There exists a clear population threshold below which the disease dies out and above which it settles to an endemically stable state. The rate of population mixing is shown to affect this threshold density. Equations which accurately account for the mean-field limit of the model are introduced and the relevance to the epidemiological modelling of measles is discussed.

Communicable Diseases↗

PubMed1996

PKC-dependent stimulation of exocytosis by sulfonylureas in pancreatic beta cells.

Hypoglycemic sulfonylureas represent a group of clinically useful antidiabetic compounds that stimulate insulin secretion from pancreatic beta cells. The molecular mechanisms involved are not fully understood but are believed to involve inhibition of potassium channels sensitive to adenosine triphosphate (KATP channels) in the beta cell membrane, causing membrane depolarization, calcium influx, and activation of the secretory machinery. In addition to these effects, sulfonylureas also promoted exocytosis by direct interaction with the secretory machinery not involving closure of the plasma membrane KATP channels. This effect was dependent on protein kinase C (PKC) and was observed at therapeutic concentrations of sulfonylureas, which suggests that it contributes to their hypoglycemic action in diabetics.

Animals↗

PubMed1996

A novel regulatory mechanism for trimeric GTP-binding proteins in the membrane and secretory granule fractions of human and rodent beta cells.

Recently we described roles for heterotrimeric and low-molecular-mass GTP-binding proteins in insulin release from normal rat islets. During these studies, we observed that a protein with an apparent molecular mass (37 kDa) similar to that of the beta subunit of trimeric GTP-binding proteins underwent phosphorylation in each of five classes of insulin-secreting cells. Incubation of the beta cell total membrane fraction or the isolated secretory granule fraction (but not the cytosolic fraction) with [gamma-32P]ATP or [gamma-32P]GTP resulted in the phosphorylation of this protein, which was selectively immunoprecipitated by an anti-serum directed against the common beta subunit of trimeric G-proteins. Disruption of the alpha beta gamma trimer (by pretreatment with either fluoroaluminate or guanosine 5'(-)[gamma-thio]triphosphate) prevented beta subunit phosphorylation. Based on differential sensitivities to pH, heat and the histidine-selective reagent diethyl pyrocarbonate (and reversal of the latter by hydroxylamine), the phosphorylated amino acid was presumptively identified as histidine. Incubation of pure beta subunit alone or in combination with the exogenous purified alpha subunit of transducin did not result in the phosphorylation of the beta subunit, but addition of the islet cell membrane fraction did support this event, suggesting that membrane localization (or a membrane-associated factor) is required for beta subunit phosphorylation. Incubation of phosphorylated beta subunit with G alpha.GDP accelerated the dephosphorylation of the beta subunit, accompanied by the formation of G alpha-GTP. Immunoblotting detected multiple alpha subunits (of Gi, G(o) and Gq) and at least one beta subunit in the secretory granule fraction of normal rat islets and insulinoma cells. These data describe a potential alternative mechanism for the activation of GTP-binding proteins in beta cells which contrasts with the classical receptor-agonist mechanism: G beta undergoes transient phosphorylation at a histidine residue by a GTP-specific protein kinase; this phosphate, in turn, may be transferred via a classical Ping-Pong mechanism to G alpha.GDP (inactive), yielding the active configuration G alpha.GTP in secretory granules (a strategic location to modulate exocytosis).

Animals↗

PubMed1996

Specific co-ordinated regulation of PC3 and PC2 gene expression with that of preproinsulin in insulin-producing beta TC3 cells.

Short-term (less than 2 h) glucose stimulation of isolated pancreatic islets specifically increases the biosynthesis of proinsulin and its converting enzymes PC2 and PC3 at the translation level. To determine whether gene expression of PC2 and PC3 was also regulated by longer-term (more than 6 h) glucose stimulation along with that of preproinsulin, studies were performed with the beta TC3 insulin-producing cell line. By Northern blot analysis, glucose maintained PC2 and PC3 mRNA levels in parallel with those of preproinsulin. After 48 h, mRNA levels of preproinsulin, PC2 and PC3 were, respectively, 2.9 (P < 0.05), 3.0 (P < 0.005) and 5.3 (P < 0.001) times greater in the presence of glucose than in beta TC3 cells cultured in the absence of glucose. Glucose-regulated PC2 and PC3 gene expression, like that of preproinsulin, was maximal at glucose concentrations above 5.5 mM. Studies of mRNA stability showed that the half-lives of PC2 (9 h) and PC3 (5 h) mRNA were much shorter than that of preproinsulin mRNA (over 24 h), but little effect of glucose on stability of these mRNAs was observed. Nuclear run-off analysis indicated that transcription of preproinsulin, PC2 and PC3 was modestly induced after 1 h exposure to 16.7 mM glucose. Therefore preproinsulin, PC2 and PC3 mRNA levels in beta TC3 cells were most probably maintained at the level of gene transcription. In contrast, elevation of cyclic AMP by forskolin had no effect on mRNA levels or gene transcription of preproinsulin, PC2 and PC3, despite a cyclic-AMP-induced phosphorylation of the cyclic AMP response element binding protein that correlated with a marked increase in cJun and cFos gene transcription in the same beta-cells. These results suggest that preproinsulin, PC2 and PC3 gene transcription can be specifically glucose-regulated in a mechanism that is unlikely to involve a key role for cyclic AMP. The co-ordinate increase in PC2 and PC3 mRNA levels with that of preproinsulin mRNA in response to chronic glucose represents a long-term means of catering for an increased demand on proinsulin conversion.

Animals↗

PubMed1996

Glucose-regulated translational control of proinsulin biosynthesis with that of the proinsulin endopeptidases PC2 and PC3 in the insulin-producing MIN6 cell line.

In the short term (< 2 h), proinsulin biosynthesis is predominately glucose regulated at the translational level; however, the details at the molecular level behind this mechanism are not well defined. One of the major hindrances for gaining a better understanding of the proinsulin biosynthetic mechanism has been a lack of an abundant source of beta-cells that express a phenotype of regulated proinsulin biosynthesis in the appropriate 2.8-16.7 mmol/l glucose range as defined in normal pancreatic islets. In this study, we demonstrate that in the MIN6 cell line, specific glucose-regulated translational control of proinsulin biosynthesis is present in the appropriate glucose concentration range. In addition to that of proinsulin, the biosynthesis of the two proinsulin conversion endopeptidases, PC2 and PC3, was coordinately glucose regulated in MIN6 cells, whereas that of the exopeptidase, carboxypeptidase H, was unaffected by glucose. Proinsulin, PC2 and PC3 biosynthesis was specifically stimulated over that of total MIN6 cell protein synthesis above a threshold of 4 mmol/l glucose that reached a maximum rate between 8 and 10 mmol/l glucose. Glucose-induced proinsulin, PC2, and PC3 biosynthesis was rapid (occurring after a 20-min lag period but reaching a maximum by 60 min), unaffected by the presence of actinomycin D; and in parallel experiments, stimulatory glucose concentrations did not alter MIN6 cell total preproinsulin, PC2, or PC3 mRNA levels. Thus, short-term (< 2 h) glucose stimulation of proinsulin, PC2 and PC3 biosynthesis in MIN6 cells, like that in isolated islets, was mediated at the translational level. Intracellular signals for mediating glucose-stimulated proinsulin PC2 and PC3 biosynthesis translation in MIN6 cells also appeared to be similar to those in pancreatic islets, requiring glucose metabolism and a supporting role for protein kinase A. However, protein kinase C or a Ca(2+)-dependent protein kinase did not appear to be required for glucose-regulated proinsulin biosynthesis in MIN6 cells, as in islets. MIN6 cells are the first beta-cell line that indicate glucose-regulated proinsulin biosynthesis translation essentially identical to that in differentiated islet beta-cells and will be an important experimental model to better define the mechanism of proinsulin biosynthesis in detail.

Animals↗

PubMed1995

Increased secretory demand rather than a defect in the proinsulin conversion mechanism causes hyperproinsulinemia in a glucose-infusion rat model of non-insulin-dependent diabetes mellitus.

Hyperproinsulinemia in non-insulin-dependent diabetes mellitus (NIDDM) is due to an increased release of proinsulin from pancreatic beta cells. This could reside in increased secretory demand placed on the beta cell by hyperglycemia or in the proinsulin conversion mechanism. In this study, biosynthesis of the proinsulin conversion enzymes (PC2, PC3, and carboxypeptidase-H [CP-H]) and proinsulin, were examined in islets isolated from 48-h infused rats with 50% (wt/vol) glucose (hyperglycemic, hyperinsulinemic, and increased pancreatic proinsulin to insulin ratio), 20% (wt/vol) glucose (normoglycemic but hyperinsulinemic), and 0.45% (wt/vol) saline (controls). A decrease in the islet content of PC2, PC3, and CP-H from hyperglycemic rats was observed. This reduction did not correlate with any deficiency in mRNA levels or biosynthesis of PC2, PC3, CP-H, or proinsulin. Furthermore, proinsulin conversion rate was comparable in islets from hyperglycemic and control rats. However, in islets from hyperglycemic rats an abnormal increased proportion of proinsulin was secreted, that was accompanied by an augmented release of PC2, PC3 and CP-H. Stimulation of the beta cell's secretory pathway by hyperglycemia, resulted in proinsulin being prematurely secreted from islets before its conversion could be completed. Thus, hyperproinsulinemia induced by chronic hyperglycemia likely results from increased beta cell secretory demand, rather than a defect in the proinsulin processing enzymes per se.

Animals↗

PubMed1994

Rab3A effector domain peptides induce insulin exocytosis via a specific interaction with a cytosolic protein doublet.

A key protein involved in the regulated exocytotic mechanism in neuroendocrine cells is the GTP-binding protein, Rab3A. Rab3A is thought to mediate exocytosis by an interaction of its effector domain with a putative effector protein. We demonstrate here that Rab3A effector domain peptides specifically stimulated insulin exocytosis in electroporated insulin-secreting cells (K0.5 activation, 6-8 microM) in a Ca(2+)-independent manner, although in the presence of Ca2+ insulin exocytosis was further potentiated. By using a 125I-radiolabeled photoactivated cross-linking Rab3A effector domain peptide, we identified a cytosolic protein doublet (REEP-1 and REEP-2), which specifically interacted with the Rab3A effector domain. Competitive inhibition studies revealed this protein-protein interaction to be at a concentration equivalent to that required for Rab3A effector domain peptides to trigger insulin exocytosis (Ki, 6-8 microM). Furthermore, under basal secretory conditions REEP-1 and -2 were membrane-associated, but upon stimulation of exocytosis they were released into a cytosolic fraction. Our results suggest that REEP-1 and -2 are part of the regulated exocytotic machinery, and their dissociation upon stimulation of hormone release (likely from a protein complex) may be essential to the mechanism that triggers regulated exocytosis in pancreatic beta-cells.

Amino Acid Sequence↗

PubMed1994

A Kex2-related endopeptidase activity present in rat liver specifically processes the insulin proreceptor.

The insulin proreceptor is cleaved by limited proteolysis post-translationally at an Arg-Lys-Arg-Arg site to generate its mature alpha- and beta-subunit form. An 35S-labelled insulin proreceptor substrate preparation and a 15-mer peptide substrate that mimics the amino acid sequence around and including the insulin proreceptor processing site (IRP-peptide) has revealed an endopeptidase activity that catalyses insulin proreceptor cleavage in a rat liver subcellular fraction. Under optimal conditions, normal 35S-labelled insulin proreceptor substrate processing by this fraction was quantitative. This fraction was not able to process an 35S-labelled insulin proreceptor variant substrate (where the Arg-1 of the tetrabasic cleavage site had been replaced by Ala-1), similarly to previous in vivo observations, suggesting that this endopeptidase activity has physiological relevance. Biochemical characterization of the insulin proreceptor/IRP-peptide processing revealed this rat liver endopeptidase activity to have a broad pH range (> 70% maximal activity between pH 5.5 and 10.0) and a pH optimum of pH 8-10. It was Ca(2+)-dependent activity, maximally active between 0.5 and 5 mM Ca2+ and half-maximally activated between 50 and 90 microM Ca2+. Endoproteolytic activity was not inhibited by group-specific inhibitors of serine-, cysteinyl or aspartyl proteinases or by 1,10-phenanthroline; however, EDTA and 1,2-cyclohexanediaminetetraacetic acid did inhibit the activity, but this was accounted for by Ca2+ chelation. The IRP-peptide substrate assay enabled measurement of an apparent Km of 22 microM and a Vmax of 18.6 pmol/min for this endopeptidase activity. These biochemical characteristics suggest that insulin proreceptor processing endopeptidase activity to be a legitimate member of the Kex2-related proprotein convertase family. Immunoblotting detected furin and PACE4 proteins (both members of this family) to be present in the rat liver subcellular fraction containing insulin proreceptor processing activity. Since the biochemical characteristics of the insulin proreceptor processing endopeptidase activity mostly resembled those of furin activity, it is likely that insulin proreceptor proteolytic maturation can be catalysed by furin in the liver.

3T3 Cells↗

PubMed1994

What beta-cell defect could lead to hyperproinsulinemia in NIDDM? Some clues from recent advances made in understanding the proinsulin-processing mechanism.

Pancreatic beta-cell dysfunction is a characteristic of non-insulin-dependent diabetes mellitus (NIDDM). An aspect of this dysfunction is that an increased proportion of proinsulin is secreted, but an actual beta-cell defect that leads to hyperproinsulinemia is unknown. Nevertheless, an impairment in beta-cell proinsulin conversion mechanism has been suggested as the most likely cause. Insulin is produced from its precursor molecule, proinsulin, by limited proteolytic cleavage at two dibasic sequences (Arg31, Arg32 and Lys64, Arg65). Two endopeptidase activities catalyze this cleavage: PC2 and PC3. PC2 endopeptidase cleaves predominately at Lys64, Arg65, and PC3 endopeptidase cleaves at Arg31, Arg32. The recent identification and characterization of these endopeptidases has enabled a better understanding of the human proinsulin-processing mechanism. In particular, experimental evidence suggests that the majority of human proinsulin processing is sequential. PC3 cleaves proinsulin first to generate a proinsulin conversion intermediate that is the preferred substrate of PC2. Both PC2 and PC3 activities are influenced by Ca2+ and pH, but the more stringent Ca2+ and pH requirements of PC3 suggest it as the most likely enzyme to regulate proinsulin conversion, as well as initiate it. When an increased demand is placed on the proinsulin-processing mechanism by a glucose-stimulated increase in proinsulin biosynthesis, there is a coordinate increase in PC3 biosynthesis (but not in PC2). This supports PC3 as the key endopeptidase that regulates proinsulin processing. In this perspective, the current concepts of the enzymology and regulation of proinsulin conversion at a molecular level are reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗