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Research Interests
Dr. Johnson's research interest concern dietary effects on mitochrondrial energy production and early programming of that function in developing heart muscle. The goal of this work is to determine mechanisms through which dietary copper supports cardiovascular function and to translate that knowledge into the formulation of dietary copper requirements needed to maintain optimal cardiovascular health.
Mitochondria are the power plants that provide the energy for the cell's various functions. Their energy production involves a linked series of proteins including cytochrome c oxidase that helps generate energy and depends on copper for its activity and is very sensitive to the amount of copper in the diet. Thus, suboptimal copper can impair functions of cardiac cells by reducing their energy states. Reduced cytochrome c oxidase activity also increases the production of highly reactive oxygen radicals that can damage other proteins needed for mitochondrial energy production also impairing cellular energy production. Dr. Johnson is focusing on determining whether these effects of marginal copper status can cause oxidative damage that permanently impairs mitochondrial function and contributes to the development of cardiovascular disease.
Dr. Johnson recently found that marginal copper intake during pregnancy results in reduced cytochrome c oxidase in heart mitochondria of offspring that is not corrected by feeding the pups copper. This perinatal effect leads to increased oxygen radical production in heart mitochondria even after the offspring become adults. This finding lead to a general hypothesis that mitochondrial function in the first generation can be programmed during early development by the maternal diet during pregnancy and/or lactation. Future research will test this hypothesis by studying the effects of copper and other essential nutrients also involved in mitochondrial function (iron, zinc) and protein in maternal diets during pregnancy on the long-term mitochondrial function in the hearts of the first generation. This work will be extended by examining the long-term effects of maternal diet on muscle and adipose mitochondria in the first generation to determine whether the programming of mitochondrial function during early development also contributes to diabetes and obesity.
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Research Accomplishments
Demonstrated that copper deficiency increases the amount of a 170,000 dalton protein in erythrocyte membranes. This protein is associated with the cytoskeleton and indicates that copper may be essential for maintaining normal cytoskeletal structure and function in blood cells.
Demonstrated that interactions between cytoskeletal proteins in platelets following thrombin activation are affected by copper status. Copper deficiency enhanced myosin association with the cytoskeleton and actin polymerization following thrombin activation.
Found that dense granule secretion from thrombin-activated platelets is increased two-fold by copper deficiency in rats. This hypersecretory response is apparently related to changes in the manner by which signals are processed by the protein kinase C-dependent signaling pathway. The specific defect may involve either depressed protein kinase C activity or impaired activation of this enzyme following platelet stimulation with thrombin. Also found that the mobilization of intracellular calcium following thrombin activation and a thrombin-independent calcium transport mechanism are both impaired in platelets by copper deficiency. These findings are the first to indicate that copper nutriture influences an important signal transducing pathway and affects the way ionic calcium, an essential intercellular messenger, is processed.
Found that platelet cytochrome c oxidase is perhaps the most sensitive enzymatic indicator of copper status. The activity of this enzyme was significantly depressed by marginal copper deficiency and may be useful for detecting marginal copper status in humans.
Determined iron and copper requirements for the long-term culture of HL-60 cells in serum-free medium. This model may be useful for studying the biochemical/cellular roles of copper in a cell culture system that does not have the problems, such as complications caused by trace metal contamination and the presence of metal-binding proteins, inherent in using serum in culture medium.
Demonstrated that copper deficiency, because it reduces cytochrome c oxidase activity and impairs mitochondrial energy metabolism, causes an increase in the number of mitochondria that are actively engaged in energy production in platelets. However, it was found that this was insufficient to maintain energy levels needed for cellular function unless GTP was also used for energy production. Because GTP has a role in G-protein-mediated signal transduction, the diversion of GTP into energy production may help explain the altered platelet responses that occur during copper deficiency.
Showed that copper deficiency increased cytochrome P450-dependent monooxygenase activity in rat small intestine. This increase resulted from facilitated electron transfer to cytochrome P450 caused by elevated cytochrome P450 reductase activity rather than from increased cytochrome P450 levels per se. The increase in monooxygenase activity suggests that copper deficiency may affect xenobiotic metabolism in the small intestine.
Found that expression of the alpha, beta, and gamma isoforms of protein kinase C was attenuated in brains of neonatal rats whose mothers were copper-deficient during pregnancy and lactation. Because protein kinase C functions as an important element in the regulation of postnatal brain development, reduced expression of its isoforms in the developing brain may help explain some of the neuropathology, such as ataxia and hypomyelination, that occurs in the offspring of copper-deficient mothers.
Demonstrated that moderate copper deficiency during pregnancy and lactation impairs developmental expression of cytochrome c oxidase in cardiac mitochondria in neonates. Furthermore, the effect of maternal copper deficiency on cardiac cytochrome c oxidase in the offspring was resistant to copper supplementation, persisting up to nine months after copper supplementation was initiated. Also, even after nine months of Cu supplementation, hydrogen peroxide generation by cardiac mitochondria was significantly increased in the offspring of copper-deficient dams. The results of this research show that moderate copper deficiency during pregnancy and lactation may irreversibly alter mitochondrial function in neonates in a manner that increases their risk for developing cardiovascular disease during adulthood.
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