Biomedical subjects

B O Palsson

Publications and source records attributed to B O Palsson.

At least 19 recordsLinked to original sources

PubMed1992

Optimal selection of metabolic fluxes for in vivo measurement. I. Development of mathematical methods.

The measurement of uptake and secretion rates is often not sufficient to allow the calculation of all internal metabolic fluxes. Measurements of internal fluxes are needed and these additional measurements are used in conjunction with mass-balance equations to calculate the complete metabolic flux map. A method is presented that identifies the fluxes that should be selected for experimental measurement, and the fluxes that can be computed using the mass-balance equations. The criterion for selecting internal metabolic fluxes for measurement is that the values of the computed fluxes should have low sensitivity to experimental error in the measured fluxes. A condition number indicating the upper bound on this sensitivity, is calculated based on stoichiometry alone. The actual sensitivity is dependent on both the flux measurements and the error in flux measurements, as well as the stoichiometry. If approximate physiologic ranges of fluxes are known a realistic sensitivity can be computed. The exact sensitivity cannot be calculated since the experimental error is usually unknown. The most probable value of the actual sensitivity for a given selection of measured fluxes is estimated by selecting a large number of representative error vectors and calculating the actual sensitivity for each of these. A frequency distribution of actual sensitivities is thus obtained giving a representative range of actual sensitivities for a particular experimental situation.

Animals↗

PubMed1992

Optimal selection of metabolic fluxes for in vivo measurement. II. Application to Escherichia coli and hybridoma cell metabolism.

A method of analysis was presented in part I of this series for determining the fluxes in a biochemical network that are the optimal choices for experimental measurement. This algorithm is applied to two important biological models: Escherichia coli and a hybridoma cell line (167.4G5.3). Our results show that potentially poor choices for in vivo measurement of metabolic fluxes exist for both model systems. For the subset of reactions in E. coli that was studied, the condition number of the augmented stoichiometric matrix reveals that a 60-fold amplification of experimental error during computations is possible. The biochemical network of the hybridoma cell is more complex than the E. coli system, and thus results in much larger possible error amplification--up to 100,000-fold. The physiological situations appear to have sensitivities that are less than 1/4 to 1/10 of those estimated by the condition number, and the maximum sensitivities are proportional to the condition number. These maximum sensitivities calculated using estimates of the fluxes and the worst possible error vector are upper bounds on the system's actual sensitivity. By examining the effect of measurement error on the sensitivity, the most probable sensitivity is calculated. These results indicate that an approximate two-fold increase in sensitivity of the E. coli system is likely when the worst set of fluxes are measured rather than the best set. The most likely sensitivity of the hybridoma system can range three orders of magnitude, depending on the set of fluxes that are measured. The propagation of experimental error during computations can be diminished for both systems by increasing the number of flux measurements over and above the minimum number of experimental measurements. The findings from these two model systems indicate that the calculation of the condition number can be a useful method for efficient experimental design, and that the usefulness of this method increases as the order of the system increases.

Animals↗

PubMed1992

Network analysis of intermediary metabolism using linear optimization. I. Development of mathematical formalism.

Analysis of metabolic networks using linear optimization theory allows one to quantify and understand the limitations imposed on the cell by its metabolic stoichiometry, and to understand how the flux through each pathway influences the overall behavior of metabolism. A stoichiometric matrix accounting for the major pathways involved in energy and mass transformations in the cell was used in our analysis. The auxiliary parameters of linear optimization, the so-called shadow prices, identify the intermediates and cofactors that cause the growth to be limited on each nutrient. This formalism was used to examine how well the cell balances its needs for carbon, nitrogen, and energy during growth on different substrates. The relative values of glucose and glutamine as nutrients were compared by varying the ratio of rates of glucose to glutamine uptakes, and calculating the maximum growth rate. The optimum value of this ratio is between 2-7, similar to experimentally observed ratios. The theoretical maximum growth rate was calculated for growth on each amino acid, and the amino acids catabolized directly to glutamate were found to be the optimal nutrients. The importance of each reaction in the network can be examined both by selectively limiting the flux through the reaction, and by the value of the reduced cost for that reaction. Some reactions, such as malic enzyme and glutamate dehydrogenase, may be inhibited or deleted with little or no adverse effect on the calculated cell growth rate.

Adenosine Triphosphate↗

PubMed1992

Network analysis of intermediary metabolism using linear optimization. II. Interpretation of hybridoma cell metabolism.

The reaction network of intermediary metabolism in the mammalian cell has been studied using linear optimization. Experimental measurements of metabolite fluxes entering and leaving hybridoma cell line 167.4G5.3 have been used to interpret the interactions of nutrients and the demand for intermediates for growth. We have ascertained the effects of waste production and energy loads on the cell growth rate using linear optimization. This analysis has shown that neither the maintenance demand for ATP nor the antibody production rate limit growth rate at normal experimental conditions. In addition, the cell uses its nutrients for growth with only 57-78% efficiency, due to the large secretion of alanine. The sensitivity of the growth rate with respect to the demand for cofactors and the supply of nutrients is given by the shadow price for each constraint. The shadow prices have shown that amino acids are the limiting nutrients at experimental conditions. The sensitivities of the growth rate to flux through reactions, given by the reduced costs, have shown that flux through the reaction glutamate dehydrogenase may actually slow down cell growth. We have also found that intermediates with lower shadow prices, and thus with lower value to the cell, are the precursors to compounds secreted from the cell. The shadow prices are also a means for comparing the costs of synthesizing various intermediates in terms of the two major nutrients, glucose and glutamine. At anaerobic conditions, glucose and glutamine have similar values to the cell, and the cost to synthesize most intermediates in terms of glucose is identical to the cost in terms of glutamine. At aerobic conditions, glucose is nearly twice as valuable to the cell as glutamine.

Adenosine Triphosphate↗

PubMed1992

The influence of extra-cellular matrix and stroma remodeling on the productivity of long-term human bone marrow cultures.

The stromal cell layer is believed to play an important role in long-term human bone marrow cultures (LTHBMCs). At present, neither the role that the stromal cell extra-cellular matrix (ECM) plays in influencing stroma behavior is well understood nor are the effects of stroma aging. Rapid medium exchanged LTHBMCs were established on surfaces precoated with human natural fibronectin and type 1 rat tail collagen. Although initial adhesion of hematopoietic cells was improved by the presence of both ECMs, the overall progenitor and nonadherent cell productivity was not improved nor did the stroma grow to confluency faster. Thus, the ECMs used did not significantly influence the cell productivity of LTHBMCs. To examine the influence of stromal cell layer aging, conditioned medium was obtained from the first two weeks of LTHBMCs that was subsequently concentrated and used as a medium supplement in a second set of slowly exchanged LTHBMCs. The presence of the concentrated conditioned medium (conCM) enhanced the production of nonadherent cells three-fold compared with control over an eight week culture period. Control cultures that were exposed to conCM after 4 weeks in culture significantly improved their cell productivity during the latter 4 weeks of culture compared with control. The productivity of cultures exposed to conCM for 4 weeks dropped significantly when unsupplemented medium was used for the latter 4 weeks of culture. Interestingly, phytohemagglutin-stimulated leukocyte-conditioned medium stimulated LTHMBCs in a similar fashion, as did conditioned medium from early LTHBMCs. Taken together, these results strongly suggest that the stromal cell layer does produce important factors for active hematopoiesis during its growth to confluence.

Bone Marrow Cells↗

PubMed1992

A Macintosh software package for simulation of human red blood cell metabolism.

We have developed a computer software package for Macintosh to simulate the metabolism and hemoglobin binding affinity of human red blood cell. The model is capable of simulating hemoglobin binding of ligands, metabolite concentrations, and metabolic fluxes at physiological steady state and in response to extracellular parameter variations, such as pH, osmolarity, glucose, and adenine concentrations. The kinetic parameters of enzymes, extracellular conditions, and initial intracellular metabolite concentrations can be specified by the user in order to model a particular situation. The software is use friendly, utilizing menu, window, and mouse to interact with the user. It also provides a pathway map of the red cell, which allows a direct access to enzyme kinetics by clicking the enzymes in the map.

Adenine↗

PubMed1992

Replication of prophage P1 is cell-cycle specific.

P1 prophage replication during the Escherichia coli division cycle has been analyzed by using the membrane-elution technique to produce cells labelled at different times during the division cycle and scintillation counting for quantitative analysis of radioactive prophage DNA. P1 prophage replicates during a restricted portion of the bacterial division cycle, like the minichromosome, but at a time during the division cycle different than the time at which the minichromosome replicates in the same cell. A high-copy mini-R6K plasmid present in the same cell replicates throughout the division cycle. Over a wide range of growth rates, the P1 prophage replicates approximately one-half generation after the minichromosome replicates. Thus, the mechanisms underlying P1 replication are similar to those for the F plasmid and the chromosome. Replication occurs when some property related to cell size or cell mass reaches a constant value per origin.

Cell Cycle↗

PubMed1992

Replication of the R6K plasmid during the Escherichia coli cell cycle.

The cell-cycle replication pattern of the R6K plasmid has been investigated by using the membrane-elution technique to produce cells labelled at different times during the division cycle and scintillation counting for quantitative analysis of radioactive plasmid DNA. The high-copy plasmid R6K replicates exponentially in a cell-cycle-independent manner. A mini-R6K plasmid deleted for the ori alpha origin of replication also replicates, exponentially in a cell-cycle-independent manner.

Cell Cycle↗

PubMed1992

Can Dexter cultures support stem cell proliferation?

The in vitro culture of mouse bone marrow (Dexter cultures) has allowed a detailed analysis of the biology of murine hematopoiesis. However, attempts to develop stable long-term human bone marrow cultures have been unsuccessful. Available culture systems all have finite and relatively short lifetimes. The reasons for the limited longevity are unknown. Utilizing computer-assisted integration techniques, we have theoretically simulated culture cell production kinetics to help identify factors that may be responsible for culture decay, as well as to suggest possible means of improving culture longevity. The simulation demonstrates that removal of stem cells is a possible mechanism leading to culture decline. Under the standard bone marrow culture conditions, even with a high stem cell renewal rate, the cultures appear to be destined to fail. Thus, the development of proper sampling techniques or improved stem cell retention may be critical to obtain successful long-term cultures.

Animals↗

PubMed1991

In vitro myelopoiesis stimulated by rapid medium exchange and supplementation with hematopoietic growth factors.

We studied the effect of the combination of rapid culture medium exchange with the addition of the human hematopoietic growth factors interleukin-3 (IL-3), granulocyte-macrophage colony-stimulating factor (GM-CSF), and erythropoietin (Epo) on the proliferation and differentiation of human long-term bone marrow cultures (LTBMCs). Individually and in combinations, IL-3, GM-CSF, and Epo were added to the culture medium of LTBMCs that were maintained with 50% medium volume exchange per day. The combination of IL-3 + GM-CSF + Epo generated the most prolific cultures with an order of magnitude increase in nonadherent cell production from weeks 2 through 8 in culture as compared with unsupplemented controls. Under these conditions, the cultures produced as many cells as were inoculated every 2 weeks and led to a greater than 2.5-fold expansion in terms of the number of nonadherent cells produced over a 6- to 8-week period. Furthermore, the LTBMCs produced nonadherent colony-forming unit-GM (CFU-GM) for more than 20 weeks. The rapid medium exchange combined with the addition of human hematopoietic CSFs significantly enhances the proliferation and differentiation of LTBMCs. These results indicate that addition of combinations of hematopoietic CSFs, together with a rapid medium exchange rate, can provide culture conditions that are suitable for the expansion of the progenitor cell pool and perhaps for the increased survival of hematopoietic stem cells in culture. Although these culture conditions still fall short of full reconstitution of functional human bone marrow, they provide an improved approach to hematopoietic cell culture that may permit the expansion and manipulation of progenitor cells in vitro.

Bone Marrow Cells↗

PubMed1991

Rapid medium perfusion rate significantly increases the productivity and longevity of human bone marrow cultures.

Long-term in vitro human hematopoietic (Dexter) cultures are limited both in their longevity (8-12 weeks) and in their cell production over time. Hematopoiesis in vivo continues unabated throughout life, suggesting that failure to obtain continuous hematopoiesis in vitro is probably due to the failure of culture conditions to accurately reproduce the in vivo state. Since bone marrow cells in vivo exist at high density and are rapidly perfused by plasma components, we examined the effects of the culture medium perfusion rate and inoculum density on longevity and productivity of human bone marrow cultures. Culture efficiency and longevity were relatively independent of the variation in inoculum density from 10(6) to 5 x 10(6) cells per ml but were significantly altered by the medium perfusion rate. Increased culture perfusion was superior to traditional Dexter schedules, with 0.5 medium volume exchange per day (3.5 volume per week) being optimal. The cultures under these conditions demonstrated an increasing rate of cell production from weeks 4 to 10, with the cell production rate doubling approximately every 2 weeks. Following weeks 10-12 in culture, the cell production rate of all cultures decayed. Production of nonadherent progenitor cells was also highest in cultures perfused at 3.5 medium volume exchanges per week, where progenitor cell production was stable from weeks 6 to 18. The nonadherent cells collected were predominantly macrophages by week 19, except for the cultures perfused at a rate of 3.5 volumes per week and seeded at 5 x 10(6) cells per ml, in which production of granulocytes remained high through week 19. Establishment of more optimal perfusion conditions supports the continuous stable generation of progenitor cells over 5 months in culture, suggesting that primitive stem cells are surviving and proliferating in these cultures.

Bone Marrow Cells↗

PubMed1991

The construction of high efficiency human bone marrow tissue ex vivo.

The successful ex vivo reconstruction of human bone marrow is an extraordinarily important basic scientific and clinical goal. Fundamentally, the system is the paradigm of a complex interactive tissue, in which the proliferation and regulated differentiation of one parenchymal cell type (the hematopoietic stem cell) is governed by the surrounding stromal cells. Understanding and reproducing the molecular interactions between bone marrow stromal cells and stem cells in tissue culture models is therefore the critical step in successful bone marrow tissue culture. Clinically, successful reconstruction of human bone marrow would permit the controlled production of mature blood cells for transfusion therapy, and immature bone marrow stem cells for bone marrow transplantation. In approaching the bone marrow culture system, we recognize the critical role that hematopoietic growth factors (HGFs) play in hematopoiesis. Since stromal cells in traditional human bone marrow cultures produce little HGFs, we have begun by asking whether local supplementation of hematopoietic growth factors via genetically engineered stromal cells might augment hematopoiesis in liquid cultures. The results indicate that locally produced GM-CSF and IL-3 do augment hematopoiesis for several weeks in culture. In combination with geometric and dynamic approaches to reconstructing physiological bone marrow microenvironments, we believe that this approach has promise for reconstructing human bone marrow ex vivo, thereby permitting its application to a variety of basic and clinical problems.

Bone Marrow Cells↗

PubMed1991

Culture perfusion schedules influence the metabolic activity and granulocyte-macrophage colony-stimulating factor production rates of human bone marrow stromal cells.

The metabolic function and GM-CSF production rates of adherent human bone marrow stromal cells were investigated as functions of medium and serum feeding rates. A range of medium exchange schedules was studied, ranging from a typical Dexter culture protocol of one weekly medium exchange to a full media exchange daily, which more closely approximates what bone marrow cells experience in situ. Glucose consumption was found to be significantly higher at full daily exchange rate than at any other exchange schedule examined. However, the lactate yield on glucose was a constant, at 1.8 mol/mol, under all conditions considered. Differential serum vs. medium exchange experiment showed that both serum supply and medium nutrients were responsible for the altered behavior at high exchange rates. Glutamine consumption was found to be insignificant under all culture conditions examined. A change in exchange schedule from 50% daily medium exchange to full daily medium exchange after 14 days of culture was found to result in a transient production of GM-CSF and a change in metabolic behavior to resemble that of cultures which had full daily exchange from day one. These results suggest that both stromal cell metabolism and GM-CSF production are sensitive to medium exchange schedules. Taken together, the data presented indicate that attempts to model the function of human bone marrow in vitro may be well served by beginning with medium exchange schedules that more closely mimic the in vivo physiologic state of bone marrow.

Adult↗

PubMed1991

Examination of serum and bovine serum albumin as shear protective agents in agitated cultures of hybridoma cells.

A murine hybridoma cell line (167.4G5.3) was cultured in batch mode using IMDM containing different serum concentrations and bovine serum albumin (BSA). Cell growth and death, metabolism and antibody production were studied in these cultures. The cells were more susceptible to shear in the stationary and in the decline phase of growth as evidenced by higher death rates. Cell growth was best at high serum concentrations with high specific growth and low specific death rates. When BSA was used instead of serum in IMDM, no protective effect was observed. Cell metabolism and monoclonal antibody production rates were not influenced by the level of serum or by BSA. The use of serum in commercial serum-free media (OPTI-MEM) also resulted in no change in both growth and death rates.

Animals↗

PubMed1991

Cell-cycle-specific F plasmid replication: regulation by cell size control of initiation.

F plasmid replication during the Escherichia coli division cycle was investigated by using the membrane-elution technique to produce cells labeled at different times during the division cycle and scintillation counting for quantitative analysis of radioactive plasmid DNA. The F plasmid replicated, like the minichromosome, during a restricted portion of the bacterial division cycle; i.e., F plasmid replication is cell-cycle specific. The F plasmid replicated at a different time during the division cycle than a minichromosome present in the same cell. F plasmid replication coincided with doubling in the rate of enzyme synthesis from a plasmid-encoded gene. When the cell cycle age of replication of the F plasmid was determined over a range of growth rates, the cell size at which the F plasmid replicated followed the same rules as did replication of the bacterial chromosome--initiation occurred when a constant mass per origin was achieved--except that the initiation mass per origin for the F plasmid was different from that for the chromosome origin. In contrast, the high-copy mini-R6K plasmid replicated throughout the division cycle.

Cell Cycle↗

PubMed1990

Metabolic dynamics in the human red cell. Part III--Metabolic reaction rates.

The kinetics of the enzymatic reaction and transport rates that constitute the main metabolic pathways of the red cell metabolism are described. The red cell metabolic model consists of 33 dynamic mass balances that contain 41 enzymatic reaction rate laws and transmembrane processes. Our goal here is two-fold: the development of rate expressions for the individual biochemical reactions of the integrated red cell metabolism, and order of magnitudes estimates to gauge their dynamic properties in terms of response times and action scales of substrates, products and regulators.

Adenosine Monophosphate↗

PubMed1990

Metabolic dynamics in the human red cell. Part IV--Data prediction and some model computations.

The biochemical information on the basic cellular determinants of the human red cell has been compiled into a comprehensive model of the red cell metabolic machinery. This model gives a consistent interpretation--both qualitative and quantitative--of the experimental findings. The predicted steady states and the dynamic states of the model are compared with the experimental findings, and the utility of this model is demonstrated.

2,3-Diphosphoglycerate↗

PubMed1990

Effect of initial cell density on hybridoma growth, metabolism, and monoclonal antibody production.

A murine hybridoma cell line (167.4G5.3) was cultivated in batch mode with varying inoculum cell densities using IMDM media of varying fetal bovine serum concentrations. It was observed that maximum cell concentrations as well as the amount of monoclonal antibody attainable in batch mode were dependent on the inoculum size. Specifically, cultures with lower inoculum size resulted in lower cell yield and lower antibody concentrations. However, in the range of 10(2) to 10(5) cells per ml, the initial cell density affected the initial growth rate by a factor of only 20%. Furthermore, specific monoclonal antibody production rates were independent of initial cell density and the serum concentration. Glutamine was the limiting nutrient for all the cultures, determining the extent of growth and the amount of antibody produced. Serum was essential for cell growth and cultures with initial cell concentrations up to 10(6) cells per ml could not grow without serum. However, when adapted, the cells could grow in a custom-made serum-free medium containing insulin, transferrin, ethanolamine, and selenium (ITES) supplements. The cells adapted to the ITES medium could grow with an initial growth rate slightly higher than in 1.25% serum and the growth rate showed an initial density dependency-inocula at 10(3) cells per ml grew 30% slower than those at 10(4) or 10(5). This difference in growth rate was decreased to 10% with the addition of conditioned ITES medium. The addition of conditioned media, however, did not improve the cell growth for serum-containing batches.

Animals↗