MicrobiologyBytes

A new study has created a model of what scientists think the first multicellular cooperation might have looked like, showing that yeast cells – in an environment that requires them to work for their food – grow and reproduce better in multicellular clumps than singly.

Researchers found that cells of brewer’s yeast that clumped together were able more effectively to manipulate and absorb sugars in their environment than were similar cells that lived singly. The experiments showed that in environments where the yeast’s sugar food source is dilute and the number of cells is small, the ability to clump together allowed cells that otherwise would have remained hungry and static to grow and divide.

The work used the yeast Saccharomyces cerevisiae, which is commonly used in brewing and bread-making and has long been used by scientists as a model organism for understanding single-celled life. The researchers devised a series of experiments that presented two problems for the yeast cells to solve if they were to take in enough food to grow and divide: the first was how to change their food from an unusable form to a usable form; the second was how to actually take in this food. The researchers put the yeast in a solution of sucrose – table sugar – which is composed of two simpler sugars, glucose and fructose. Yeast lives on sugar, but the sucrose can’t get through the membrane that surrounds the cell. So the yeast makes an enzyme called invertase to chop the sucrose into glucose and fructose, each of which can enter the cell using gate-keeping molecules, called transporters, that form part of the membrane.

Working alone, a single yeast cell in a dilute solution of sucrose would never take in enough glucose and fructose to be able to grow and divide. But by cooperating, clumps of yeast in that same solution might have a chance. With several cells in proximity, all releasing invertase to create smaller sugars, these cooperating yeast cells would increase the density of those sugars near the clump, increasing the chances that each cell could take in enough to grow and divide. Sure enough, when the researchers tested these hypotheses on two strains of yeast, they found that the strain which clumped cells together was growing and dividing, while the yeast cells living alone were not.

Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity. (2011) PLoS Biol 9(8): e1001122. doi:10.1371/journal.pbio.1001122
We use the budding yeast, Saccharomyces cerevisiae, to investigate one model for the initial emergence of multicellularity: the formation of multicellular aggregates as a result of incomplete cell separation. We combine simulations with experiments to show how the use of secreted public goods favors the formation of multicellular aggregates. Yeast cells can cooperate by secreting invertase, an enzyme that digests sucrose into monosaccharides, and many wild isolates are multicellular because cell walls remain attached to each other after the cells divide. We manipulate invertase secretion and cell attachment, and show that multicellular clumps have two advantages over single cells: they grow under conditions where single cells cannot and they compete better against cheaters, cells that do not make invertase. We propose that the prior use of public goods led to selection for the incomplete cell separation that first produced multicellularity.

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Studies using model organisms have pointed to the existence of evolutionarily conserved genes and signaling pathways that regulate life span. Changes in the activity of these genes/pathways have also been implicated in mediating the beneficial effect of calorie restriction, a well recognized intervention that extends the life span from yeast to mammals. Researchers investigated the global gene expression changes and identified genes involved in the metabolism of various kinds of carbon sources that are associated with longevity in the single cell organism, the baker’s yeast Saccharomyces cerevisiae. Although glucose and ethanol are common carbon sources for growth, they also have detrimental pro-aging effects in yeast. Long-lived yeast mutants actively utilize available glucose and ethanol and produce glycerol, which does not adversely affect the yeast life span extension. New findings suggest that this “carbon source substitution” observed in long-lived yeast creates an environment mimicking calorie restriction, which together with the direct regulation of stress resistance systems optimizes life span extension.

Yeast maintained on a glycerol diet live twice as long as normal – as long as yeast cells on a severe caloric-restriction diet. They are also more resistant to cell damage. Many studies have shown that caloric restriction can extend the life span of a variety of laboratory animals. Caloric restriction is also known to cause major improvements in a number of markers for cardiovascular diseases in humans. This study is the first to propose that “dietary substitution” can replace “dietary restriction” in a living species. If you add glycerol, or restrict caloric intake, you obtain the same effect. This is as effective as calorie restriction, yet cells can take it up and utilize it to generate energy or for the synthesis of cellular components.

The researchers investigated the effect of a glycerol diet after discovering that genetically engineered long-lived yeast cells that survive up to 5-fold longer than normal have increased levels of the genes that produce glycerol. In fact, they convert virtually all the glucose and ethanol into glycerol. Notably, these cells have a reduced activity in the TOR1/SCH9 pathway, which is also believed to extend life span in organisms ranging from worms to mice. When the researchers blocked the genes that produce glycerol, the cells lost most of their life span advantage. However, the “glucose to glycerol” switch is believed to represent only one component of the protective systems required for extended survival. This study indicates that glycerol biosynthesis is an important process in the metabolic switch that allows this simple organism to activate its protective systems and live longer. This is a fundamental observation in a very simple system, that at least introduces the possibility that you don’t have to be calorie-restricted to achieve some of the remarkable protective effects of the hypocaloric diet observed in many organisms, including humans. It may be sufficient to substitute the carbon source and possibly other macronutrients with nutrients that do not promote the “pro-aging” changes induced by sugars. Findings using these simple genetic models should help to discover fundamental longevity regulatory mechanisms and identify similar pathways in mammals. Darn useful things, yeasts :-)

Tor1/Sch9-Regulated Carbon Source Substitution Is as Effective as Calorie Restriction in Life Span Extension. PLoS Genet 5(5): e1000467
The effect of calorie restriction (CR) on life span extension, demonstrated in organisms ranging from yeast to mice, may involve the down-regulation of pathways, including Tor, Akt, and Ras. Here, we present data suggesting that yeast Tor1 and Sch9 (a homolog of the mammalian kinases Akt and S6K) is a central component of a network that controls a common set of genes implicated in a metabolic switch from the TCA cycle and respiration to glycolysis and glycerol biosynthesis. During chronological survival, mutants lacking SCH9 depleted extracellular ethanol and reduced stored lipids, but synthesized and released glycerol. Deletion of the glycerol biosynthesis genes GPD1, GPD2, or RHR2, among the most up-regulated in longlived sch9D, tor1D, and ras2D mutants, was sufficient to reverse chronological life span extension in sch9D mutants, suggesting that glycerol production, in addition to the regulation of stress resistance systems, optimizes life span extension. Glycerol, unlike glucose or ethanol, did not adversely affect the life span extension induced by calorie restriction or starvation, suggesting that carbon source substitution may represent an alternative to calorie restriction as a strategy to delay aging.

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