The common theme of our research group is the application of engineering principles to biological materials or systems. Most of the problems we work on are motivated by a desire to understand normal or pathophysiological processes occurring in the body, and their implications for the prevention, diagnosis, or treatment of human disease. Much of the work entails collaboration with physicians, physiologists, and other biological scientists.
One area of focus involves the fundamentals of water
and macromolecule transport in liquid-filled spaces of molecular dimensions.
This is important for understanding mass transfer in body tissues, as well as
for designing membranes or other separation devices. A key objective of our
work is to develop models to predict transport hindrances in porous or fibrous
materials, based on the size, shape, and electrical charge of the permeating
molecule and the nanostructural properties of the material. The theoretical
models are tested using membranes or gels of well-defined structure. They are
used also to interpret experiments that probe the permeability properties of
mammalian kidney capillaries in health and disease. We are endeavoring to relate
the ultrastructure of those capillaries to their transport characteristics,
through detailed analyses of convection and diffusion at the cellular and subcellular
At left, a
schematic of macromolecular transport through the glomerular capillary wall.
Plasma proteins and other macromolecules are selectively filtered from the blood,
across the three layers of the capillary wall, and are collected in primary
urine in the Bowman's space. Our research studies this transport through a variety
of in vitro and in vivo experiments and structure-based models.
At left, a schematic of macromolecular transport through the glomerular capillary wall. Plasma proteins and other macromolecules are selectively filtered from the blood, across the three layers of the capillary wall, and are collected in primary urine in the Bowman's space. Our research studies this transport through a variety of in vitro and in vivo experiments and structure-based models.
Another area of interest is transport and reaction of nitric oxide (NO) in biological systems. It has been shown in recent years that NO is synthesized throughout the body and that it is a key intercellular messenger molecule (e.g., in the regulation of blood pressure). Transient increases in NO synthesis are important also in the response of the immune system to infection, in that the toxicity of NO helps to kill invading microorganisms. However, sustained high levels of NO synthesis (as may occur with chronic infection or inflammation) carries with it the risk of collateral damage to host tissues, including mutational changes that may lead to cancer. To provide insight into the biological effects of NO and of the various reactive NOx species derived from NO, we are studying reaction kinetics and diffusion in aqueous solutions and cell cultures. Using such data, we are developing computational models to predict the consequences of NO synthesis by cells in vivo or in vitro.Figure:
Professor Deen's group has been investigating the fundamental aspects of the production and fate of NO and related nitrogen oxides in biological systems in collaboration with several faculty in the Biological Engineering Division. To assess health risks and design interventions, one needs to know which compounds actually mediate the various harmful effects of excess NO. Underlying that effort is the need to know what their concentrations are near an NO source (e.g., an activated macrophage), and over what distances those concentrations remain elevated. A chemical engineering problem emerges: The concentration fields must reflect the interplay between rates of reaction and diffusion. The modeling problems are made challenging by the complex chemistry, which includes enzymatic sources and sinks for NO, a network of inorganic oxidations that yield other reactive nitrogen oxides, and the interactions of the nitrogen oxides with soluble or structural biomolecules. The geometric complexity of tissues, and even of cell culture systems used to study NO toxicity, adds its own difficulties. Experimentally, few of the compounds of interest are present at measurable concentrations, and their levels must be inferred from analyses of oxidation end products and biomarkers (e.g., trace levels of nitrogen oxide-modified proteins).The work in Professor Deen's group has yielded several recent insights. A reaction-diffusion model developed to describe the fate of NO released by macrophages cultured on plates revealed that some species will be present only in the immediate vicinity of the cells, but others will exist throughout the culture medium. Accordingly, various chemical reactions will be spatially segregated, even in extracellular fluid. Experiments varying the depth of the liquid medium indicated that NO strongly inhibits its own synthesis by macrophages, a phenomenon that has been demonstrated previously with isolated enzymes but not with intact cells. An analysis of the kinetics of NO production and consumption in macrophages suggested that the maximum NO concentration that can be achieved at any cell number density is about 1 M. This was the first indication of a possible upper limit for NO concentrations at sites of inflammation. Knowing better what to mimic, an apparatus was designed to permit "target cells" (cells that do not produce NO) to be exposed to controlled, micromolecular levels of NO for up to several days. In collaboration with Professor Gerald N. Wogan of the Biological Engineering Division, the effects of NO concentration and total dose (area under the concentration curve) were then characterized in terms of cell survival and several types of cellular damage. It was found that there are dual thresholds for NO toxicity. Toxic effects were not seen unless a minimum concentration and a minimum dose were both exceeded. The concentration threshold for the cell lines used was about 0.5 M (about half the inferred physiological maximum). This is the first quantitative demonstration of toxicity thresholds for NO, and is stimulating a new round of experiments to examine the underlying intracellular events. An important objective in the next few years will be to develop reaction-diffusion models for tissues that will provide a rational means to extrapolate kinetic, diffusional, and cell culture data to pathophysiological conditions in the body.
Selected Publications and
"Analysis of the effects of cell spacing and liquid depth on nitric oxide and its oxidation products in cell cultures," Chem. Res. Toxicol., 14, 135-147 (2001), (with B. Chen).
"Concentration polarization in stirred ultrafiltration cells," AIChE J., 47, 1115-1125 (2001), (with S.T. Johnston and K.A. Smith).
"Equilibrium partitioning of flexible macromolecules in fibrous membranes and gels," Macromolecules, 33, 8504-8511 (2000), (with J.A. White).
"Effects of multisolute steric interactions on membrane partition coefficients," J. Colloid Interface Sci., 226, 112-122 (2000), (with M.J. Lazzara and D. Blankschtein).
"Ultrastructural model for size selectivity in glomerular filtration," Am. J. Physiol., 276, F892-F902 (1999), (with A. Edwards and B.S. Daniels).
"Diffusion and reaction of nitric oxide in suspension cell cultures," Biophys. J., 75, 745-754 (1998), (with B. Chen and M. Keshive).
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