Single Cell Studies of Glycolytic Oscillations Using Microfluidics and Optical Tweezers

Abstract: The complex set of reactions in metabolism has been extensively studied in populations with millions of cells, but much information can still be gained by studying the heterogeneous metabolic behaviour in individual cells. The first part of energy metabolism is called glycolysis and is very similar in mammalian cells, such as human cells, and in yeast cells. If yeast cells are exposed to certain concentrations of glucose and cyanide, the concentration of metabolites in glycolysis starts to oscillate. These glycolytic oscillations have been studied since the 1950s in both intact cells and in yeast extracts. In dense cell cultures, glycolytic oscillations are synchronized via the metabolic intermediate acetaldehyde, which rapidly diffuses through the cell membrane. Synchronization is a requirement when oscillations from millions of cells in a population are to be investigated. Such studies will, however, only give information about the population average and the oscillatory behaviour on the single cell level will be unknown. Several questions have for long been unanswered, such as why, as reported, a population of cells loses its oscillations. This could be due to non-oscillatory cells but it might also be an effect of desynchronization of the oscillations. Another question is if there is a large heterogeneity in for instance amplitude and period time of the oscillations between the individual cells. If a population of cells will show complete, partial or no synchronization depends both on the heterogeneity on the single cell level and on the coupling strength between the cells. In this work, these questions have been investigated by combining optical tweezers for cell positioning in arrays with variable cell density, microfluidics for spatial and temporal environmental control and fluorescence microscopy for detection of the responses on the single cell level. It was found that sustained glycolytic oscillations can be induced in isolated yeast cells and hence that a high cell density is not a requirement for oscillations to occur. A large heterogeneity in the oscillatory behaviour could also be seen on the single cell level. This setup enables future studies where important information about metabolism can be gained and where cell-to-cell coupling and synchronization can be further investigated.