Vitamins A, B, C, D, E, K, folic acid. You certainly know these vitamins. But have you ever heard of thiamine, also called vitamin B1? Jérôme Savocco and Sylvain Nootens and their colleagues in the laboratory of Pierre Morsomme, a biochemistry professor at the Louvain Institute of Biomolecular Science and Technology (LIBST), decided to study the absorption of this vitamin by a yeast, to better understand its assimilation by our cells. Their conclusions have just been published in the scientific journal PLOS Biology.
Vitamins essential to our survival
Thirteen vitamins are essential for humans: A, thiamine (B1), B2, B3, B5, B6, B12, biotin, C, D, E, folic acid and K. These organic molecules, essential to our body’s proper functioning, are classified into two groups:
- Fat-soluble vitamins (including A, D, E and K): they are found in foods containing fat (fats, eggs, meat, etc.). Our body assimilates them into fatty tissue.
- Water-soluble vitamins (including B and C): they are found in vegetables, fruit, cereals, yeast and dairy products. Our body doesn’t store them and quickly eliminates food surpluses, especially via the urine.
All of these vitamins are found throughout our body, but we can’t synthesise them all. Only vitamins D and B3 are synthesised by humans and animals. The others we must absorb via food. This is the case with thiamine, a vitamin produced by bacteria, fungi and plants. In order to understand how thiamine is absorbed by cells, Prof. Morsomme's team studied its absorption by yeast (Saccharomyces cerevisiae) cells.
Yeast as a model
‘Our laboratory specialises in the proteomics of membrane proteins,’ Prof. Morsomme explains. ‘Proteomics studies proteomes, that is, all of the proteins in an organism, tissue, cell, or subcellular organelle. Membrane proteins are found on the cell surface, at the interface between the inside and outside of a cell. Five years ago, we began by studying all the proteins located on yeast cell membrane.’ They chose baker’s yeast, which is used to make wine, bread and beer; it’s used often in biology experiments because it has a lot in common with human and plant cells, and is easy to grow, inexpensive and very well characterised.
Thiamine transporter: a front door
Prof. Morsomme says, ‘We observed that the protein that transports thiamine from the outside to the inside of the yeast behaved interestingly.’ For a cell to function, it must be able to exchange compounds with the outside world. This is the role of membrane transporters: cell surface (plasma membrane) proteins that carry inside or outside everything the cell needs or needs to eliminate. The transporter for thiamine is called Thi7. It acts like the front door (Thi7) of a building (the cell) through which vitamins enter. ‘To study this phenomenon, we put the cells in specific conditions to measure the stability of proteins on the surface.’ The team found that when they placed vitamins in the cell’s environment, the cell controlled their entry by modifying the stability of the transporter. Specifically, ‘when you place many vitamins in the cell’s environment, the Thi7 transporter is degraded to keep too many vitamins from entering.’
Thi7 transporter in detail
Over five years of research, Prof. Morsomme's laboratory team carried out several experiments. They grew the yeast in a normal way, then added many vitamins nearby and timed how long the transporter had been gone. ‘After two hours,’ the professor says, ‘the cell had removed a large majority of surface transporters. This means the cell controls very precisely the amount of thiamine that can enter.’ The team also inactivated cell genes that encode proteins responsible for absorbing and synthesising thiamine, to examine how the cell behaved and evolved with and without the vitamin. This experiment made it possible to affirm that the transporter’s absence was harmful to the cell. ‘In the absence of biosynthesis – production of the vitamin directly by the cells of the living being – when the transporter is absent, the cell dies.’ Another experiment used a fluorescent version of the transporter to better understand its function. ‘The protein/transporter, when it has to be degraded by the cell, enters the cell by endocytosis before being degraded. When we add thiamine outside, all the fluorescence is quickly found inside the cell because the protein has been internalised in order to be degraded. It’s spectacular to see under a fluorescence microscope.’
These scientific experiments led the researchers to certain conclusions. First, if there are very few vitamins in the cell’s environment, the cell will place many transporters on its surface. It’s like installing multiple doors in a building. Conversely, when there are too many vitamins, the cell reduces the number of entry doors and therefore of transporters. For human cells, however, the transporter is crucial, because humans can’t synthesise vitamins. If the transporter doesn’t work, the cell can’t synthesise the vitamin. ‘What’s interesting is that these conclusions can be generalised because the other transporters (of sugars, amino acids, ions, etc.) are also regulated by endocytosis.’
A history of amino acids
To go further, the team tried to understand how, in concrete terms, the cell controls and regulates the number of transporters on its surface. Two things seem to be very important: there must be a sufficient quantity of thiamine inside the cell; and the transporter must be active. The LIBST team modified (mutated) the protein to make it inactive. In this case, the protein is no longer degraded, even if the thiamine accumulates inside the cell through another door. ‘It’s as if the cell doesn’t see the point of removing a door that’s already locked.’
And human cells?
This study made it possible to understand the molecular mechanisms which allow the absorption of nutrients by a yeast cell. What about humans? Humans eat food, in which vitamins and other nutrients provide the energy they need to live. These nutrients enter each of our cells via transporters. Through the LIBST team’s yeast transporter discoveries, we understand better how the cell controls the entry of nutrients. ‘But it remains to be seen whether these results are confirmed in humans. Are transporters regulated the same way? This must be the subject of a new study. We could also extend our research to other transporters (sugar, amino acids, for example) in order to see whether the conclusions match.’
A glance at Pierre Morsomme's bio
Pierre Morsomme earned a master’s degree in bioengineering (1994) and a PhD in agricultural science and bioengineering (1999) from the UCLouvain Faculty of Bioengineering. In 2003, after a postdoctoral fellowship at the Basel (Switzerland) Biozentrum, he became an FNRS research associate. He is currently a professor at UCLouvain, where he teaches courses in cell biology and biochemistry. He leads a Louvain Institute of Biomolecular Science and Technology (LIBST) research team that focuses on biological membrane and membrane transporter function.