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Biomedical subjects

Christopher Rhodes

Publications and source records attributed to Christopher Rhodes.

7 recordsLinked to original sources
PubMed2006

Impact of defined matrix interactions on insulin production by cultured human beta-cells: effect on insulin content, secretion, and gene transcription.

The impact of extracellular matrix on insulin production needs to be understood both to optimize the derivation of functional beta-cells for transplantation and to understand mechanisms controlling islet neogenesis and glucose homeostasis. In this study, we present evidence that adhesion to some common matrix constituents has a profound impact on the transcription, secretion, and storage of insulin by human beta-cells. The integrin-dependent adhesion of fetal beta-cells to both collagen IV and vitronectin induces significant glucose-independent insulin secretion and a substantial reciprocal decline in insulin content. Collagen IV, but not vitronectin, induces comparable responses in adult beta-cells. Inhibition of extracellular signal-regulated kinase activation abrogates matrix-induced insulin secretion and effectively preserves the insulin content of adherent beta-cells. Using real-time PCR, we demonstrate that adhesion of both fetal and adult beta-cells to collagen IV and vitronectin also results in the marked suppression of insulin gene transcription. Based on these findings, we contend that integrin-dependent adhesion and signaling in response to certain matrices can have a significant negative impact on insulin production by primary human beta-cells. Such responses were not found to be associated with cell death but may precede beta-cell dedifferentiation.

Butadienes↗
PubMed2005

Digital microfluidics using soft lithography.

Although microfluidic chips have demonstrated basic functionality for single applications, performing varied and complex experiments on a single device is still technically challenging. While many groups have implemented control software to drive the pumps, valves, and electrodes used to manipulate fluids in microfluidic devices, a new level of programmability is needed for end users to orchestrate their own unique experiments on a given device. This paper presents an approach for programmable and scalable control of discrete fluid samples in a polydimethylsiloxane (PDMS) microfluidic system using multiphase flows. An immiscible fluid phase is utilized to separate aqueous samples from one another, and a novel "microfluidic latch" is used to precisely align a sample after it has been transported a long distance through the flow channels. To demonstrate the scalability of the approach, this paper introduces a "general-purpose" microfluidic chip containing a rotary mixer and addressable storage cells. The system is general purpose in that all operations on the chip operate in terms of unit-sized aqueous samples; using the underlying mechanisms for sample transport and storage, additional sensors and actuators can be integrated in a scalable manner. A novel high-level software library allows users to specify experiments in terms of variables (i.e., fluids) and operations (i.e., mixes) without the need for detailed knowledge about the underlying device architecture. This research represents a first step to provide a programmable interface to the microfluidic realm, with the aim of enabling a new level of scalability and flexibility for lab-on-a-chip experiments.

Computers↗
PubMed2004

Direct imaging shows that insulin granule exocytosis occurs by complete vesicle fusion.

Confocal imaging of GFP-tagged secretory granules combined with the use of impermeant extracellular dyes permits direct observation of insulin packaged in secretory granules, trafficking of these granules to the plasma membrane, exocytotic fusion of granules with the plasma membrane, and eventually the retrieval of membranes by endocytosis. Most such studies have been done in tumor cell lines, using either confocal methods or total internal reflectance microscopy. Here we compared these methods by using GFP-syncollin or PC3-GFP plus rhodamine dextrans to study insulin granule dynamics in insulinoma cells, normal mouse islets, and primary pancreatic beta cells. We found that most apparently docked granules did not fuse with the plasma membrane after stimulation. Granules that did fuse typically fused completely, but a few dextran-filled granules lingered at the membrane. Direct recycling of granules occurred only rarely. Similar results were obtained with both confocal and total internal reflection microscopy, although each technique had advantages for particular aspects of the granule life cycle. We conclude that insulin exocytosis involves a prolonged interaction of secretory granules with the plasma membrane, and that the majority of exocytotic events occur by full, not partial, fusion.

Animals↗
PubMed2004

A role for the malonyl-CoA/long-chain acyl-CoA pathway of lipid signaling in the regulation of insulin secretion in response to both fuel and nonfuel stimuli.

The malonyl-CoA/long-chain acyl-CoA (LC-CoA) model of glucose-induced insulin secretion (GIIS) predicts that malonyl-CoA derived from glucose metabolism inhibits fatty acid oxidation, thereby increasing the availability of LC-CoA for lipid signaling to cellular processes involved in exocytosis. For directly testing the model, INSr3 cell clones overexpressing malonyl-CoA decarboxylase in the cytosol (MCDc) in a tetracycline regulatable manner were generated, and INS(832/13) and rat islets were infected with MCDc-expressing adenoviruses. MCD activity was increased more than fivefold, and the malonyl-CoA content was markedly diminished. This was associated with enhanced fat oxidation at high glucose, a suppression of the glucose-induced increase in cellular free fatty acid (FFA) content, and reduced partitioning at elevated glucose of exogenous palmitate into lipid esterification products. MCDc overexpression, in the presence of exogenous FFAs but not in their absence, reduced GIIS in all beta-cell lines and in rat islets. It also markedly curtailed the stimulation of insulin secretion by other fuel and nonfuel secretagogues. In the absence of MCDc overexpression, the secretory responses to all types of secretagogues were amplified by the provision of exogenous fatty acids. In the presence of exogenous FFAs, the fatty acyl-CoA synthetase inhibitor triacsin C reduced secretion in response to glucose and nonfuel stimuli. The data show the existence of important links between the metabolic coupling factor malonyl-CoA, the partitioning of fatty acids, and the stimulation of insulin secretion to both fuel and nonfuel stimuli.

Acyl Coenzyme A↗
PubMed2003

Detection of sugar syrups in apple juice by delta(2)H per thousand and delta(13)C per thousand analysis of hexamethylenetetramine prepared from fructose.

An improved procedure for determining (13)C and (2)H isotope ratios, using gas chromatography-isotope ratio mass spectrometry (GC-IRMS), has been developed for identifying the addition of low cost commercial sugar syrups to apple juices and related products. Isotopic techniques are commonly used to identify the addition of low cost sugars to fruit juices and are difficult to circumvent as it is not economically viable to change the isotopic ratios of the sugars. The procedure utilizes the derivative hexamethylenetetramine, which is produced through chemical transformation of a sugar degradation product and provides position-specific (13)C and (2)H ratios that relate to the parent sugar molecule. The new procedure has advantages over methods using nitro-sugar derivatives in terms of analysis time and sensitivity. The differences between the delta(2)H per thousand and delta(13)C per thousand values of the 100 authentic apple juices and beet and cane commercial sugar syrups permit their addition to be reliably detected.

Beverages↗
PubMed2002

Transepithelial transport of poly(amidoamine) dendrimers across Caco-2 cell monolayers.

The objective of this study was to investigate the influence of physiochemical parameters (such as size, molecular weight, molecular geometry, and number of surface amine groups) of poly (amidoamine) (PAMAM) dendrimers, on their permeability across Caco-2 cell monolayers. The permeability of a series of PAMAM dendrimers, generations 0-4 (G0-G4), was investigated across Caco-2 cell monolayers in both the apical to basolateral (AB) and basolateral to apical (BA) directions. The influence of PAMAM dendrimers on the integrity, paracellular permeability, and viability of Caco-2 cell monolayers was also monitored by measuring the transepithelial electrical resistance (TEER), mannitol permeability, and leakage of lactate dehydrogenase (LDH) enzyme, respectively. G0, G1 and G2 demonstrated similar AB permeabilities, which were moderate several fold higher than the AB permeability of higher generations. The AB and BA permeability of G0-G4 typically increased with the increase in donor concentration and incubation time. Permeability values are not reported at generations, concentrations or incubation times that the dendrimers were toxic to Caco-2 cells. TEER values decreased and mannitol permeability increased as a function of donor concentration, incubation time, and generation number. LDH results for G3 and G4 indicate that Caco-2 cell viability was reduced with increasing donor concentration, incubation time, and generation number. The appreciable permeability of G0-G2, coupled with their nontoxic effects on Caco-2 cells, suggest their potential as water-soluble polymeric drug carriers for controlled oral drug delivery.

Biocompatible Materials↗