In most organisms, glycolysis is the central pathway to make energy available to cells from sugars such as glucose and is therefore key in the Human Metabolism map. Although the glycolytic pathway is well-described in all textbooks, activity and regulation details of all its chemical reactions are not fully known. This understanding is however very important for biotechnological applications, such as sugar fermentation and biofuel applications. It is also of great significance to diseases like diabetes and cancer, in which this pathway is dysfunctional.
One genotype, two phenotypes
When glucose is suddenly offered to cells, glycolysis needs to start up: all ten glycolytic enzymes sequentially start catalyzing chemical reactions to break down the sugars. Hence the name ‘gluco-lysis’. This logistic challenge requires proper regulation, like traffic lights steer traffic in rush hour. In mutants with defective regulation, glycolysis fails to start up properly, causing ‘metabolic traffic jams’ (see figure). While wild-type cells continue to proliferate, mutant cells cannot grow on glucose and ultimately die.
In the transition from low to high glucose, two phenotypes of viable and unviable cells arise. Conditions and mutations change the proportion of cells with one or the other phenotype.
When studying these mutants, the systems biologists discovered – together with colleagues from the VU and the Kluyver Center in Delft – that a tiny fraction (0.1%) of the mutant cells escaped the traffic jam modus and was able to proliferate. This finding raised the suspicion that for this mutant, two phenotypes co-existed. Subsequently, the researchers were able to demonstrate the reverse: a fraction of wild-type cells were also unable to launch glycolysis properly. Hence, for glucose metabolism, two phenotypes exist: the genotype (wild-type or mutant) only affects the probability to show one or the other phenotype.
Johan van Heerden, Bas Teusink and their colleagues further showed that the co-existence of viable and unviable cells in one populations arises from the fundamental design of glycolysis – a design that is key to the function of the pathway as an energy generator. This design however comes with an inherent danger when glucose suddenly becomes available to cells. The glycolytic pathway has to initiate promptly, and this requires proper regulation.
Improper initiation leads to a disturbance in the consecutive reactions, where the first steps of glycolysis carry more flux of metabolites than the downstream steps. In the mutants that contain no traffic light, traffic jams occur in almost all cells, but even in wild-type cells (with a fully functional traffic light) traffic jams happen in 7% of the cells.
30 years of literature
The researchers initially discovered this bi-modal phenomenon in a computer model they developed, that simulates the flow of metabolites through all steps of the glycolytic pathway. They subsequently demonstrated this phenomenon experimentally, both on single-cell and population level. While at the single-cell level, two qualitatively different behaviors co-exist in the same population, most experimental cell biology techniques probe the population average. Hence, the average behavior does not reflect what happens in individual cells! This new perspective allowed the researchers to unify 30 years of literature on malfunctioning yeast mutants.
Teusink explains the applications of this study: “We now know much better how glycolysis is regulated and how that can influence functional and impaired cell state. We can use this knowledge to enhance the share of vital cells for biotechnical applications or increase the fraction of cells in which this pathway is dysfunctional, as would be very interesting for cancer research.”
Read the article LOST IN TRANSITION: startup of glycolysis yields subpopulations of non-growing cells here.