New mechanism found that maintains water balance within cells

An illustration of a biomolecular condensate in a cell

Scientists have discovered a new mechanism that allows cells to maintain their ideal water availability from second to second.

Water is a major constituent of cells and is critical for life because it provides the driving force for proteins to fold  and many biochemical reactions. Biological molecules within the crowded cell interior and conditions outside the cell affect water availability. In order for cells to survive and thrive, water balance often needs to be rapidly restored in response to changes inside the cell and in the external environment.

Rachel Edgar’s group in the Department of Infectious Disease, Imperial College, in collaboration with Emmanuel Derivery’s and John O’Neill’s group’s in the UKRI-MRC Laboratory of Molecular Biology, have discovered a new way that cells reestablish optimal water availability in seconds.

Previously described mechanisms that counteract changes in external salt concentrations, for example, involve large movements of water and ions in or out of the cell over slower timescales (minutes to hours).

The research published in Nature, found that water balance inside the cell is maintained by redistributing proteins into fluid assemblies called biomolecular condensates. Water within the ‘hydration layer’ that surrounds a protein is not available to participate in other cellular processes. Proteins that are disordered constrain more water in their hydration layers compared to compact proteins. The team found that moving disordered proteins into biomolecular condensates liberates water from their hydration layers, and conversely that water is sequestered into hydration layers when the condensates dissolve .  In this way, cells regulate how much ‘free’ water they have – a buffering system that can rapidly restore water balance without movement of water in or out of the cell and disruptive changes to cell volume.

“By working out what drives biomolecular condensation and its function in cells, our research provides the basis for future investigations into these diseases and development of therapeutics." Dr. Rachel Edgar Department of Infectious Disease

Dr. Rachel Edgar from the Department of Infectious Disease said: “This study reframes the way we think about phase-separated biomolecular condensates, which are altered in many chronic diseases including neurodegenerative conditions and cancer. In addition, many viruses form biomolecular condensates as part of their replication cycle, and they are also involved in our responses to stress and infection.”

“By working out what drives biomolecular condensation and its function in cells, our research provides the basis for future investigations into these diseases and development of therapeutics.”

Temperature changes and cell survival

In addition, the scientists found that water becomes very sensitive to small changes in protein concentration and temperature. The mechanism they discovered ensures ideal water availability upon natural fluctuations in temperature, salt concentration and pressure that occur countless times in our bodies every day.

They found that during lower temperatures more water molecules are held in hydration layers, reducing water availability in an equivalent way to increasing external salt concentration or pressure, where ‘free’ water leaves the cell.

As a consequence, when the researchers reduced the salt concentration in the culture medium they found that they could rescue cells from dying during prolonged exposure to cold (0^oC for 24h). In this dilute culture medium, water balance is partially restored as ‘free’ water entering the cell by osmosis offsets the increased amount of water within hydration layers at low temperature. Combining two different stressful conditions, the team could prevent cell death due to their opposing effects on water balance, highlighting its importance for biology.

The implications of discovering human cells can survive prolonged low temperatures when cultured in growth medium with lower salt concentration are wide-ranging.

It could pave the way for potential treatments for hypothermia to developing better cryopreservation protocols that are critical for preserving fertility during IVF.

This work was funded by the Wellcome Trust, the Royal Society, UKRI MRC, the Human Frontier Science Program, a Versus Arthritis, Grifols, the Alpha-1-Foundation, EMBO, Volkswagen ‘Life’ and the DFG Excellence Cluster Physics of Life.

This article has been adapted from materials from the MRC Laboratory of Molecular Biology.