Microgravity modifies proteome: Dry immersion model for weightlessness reveals irreversible protein changes

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  • Published: Apr 1, 2011
  • Author: Steve Down
  • Channels: Proteomics
thumbnail image: Microgravity modifies proteome: Dry immersion model for weightlessness reveals irreversible protein changes

Support for space research

Astronauts train for the effects of weightlessness by taking countless flights in the vomit comet, a plane which flies repeatedly in parabolas to induce zero gravity for a few seconds at a time. While it can be a lot of fun, apart from the vomiting, it is difficult to assess the long-term effects of zero gravity in this situation.

As an alternative, the Russian space program introduced a system known as dry immersion which can be carried out in the lab and can model the effects of weightlessness for days at a time. The subject is cradled in an elastic, waterproof cloth in a bath of thermoneutral water, remaining dry while being lightly supported. The force of the support is distributed across the whole body surface, giving a close approximation of microgravity.

Exhaustive comparative studies have shown that dry immersion is the best model of microgravity. It mirrors most closely the development time and extent of known structural and functional motor disorders that occur in real microgravity. In contrast to bed rest, it allows the physiological effects of the lack of a supporting structure for the body, a phenomenon called supportlessness, to be studied.

These induced effects include centralisation of body fluids and hypokinesia, which affect the cardiovascular, muscular and immune systems, among others. It has also been suggested that gene expression is affected and, with it, the proteome.

A team of Russian scientists from the Institute for Biomedical Problems in Moscow suggested that studying the serum proteome would help to understand how the human body adjusts during dry immersion.

Proteins affected by microgravity

Senior reporter N.A. Pakharukova and coworkers L.Kh. Pastushkova, I.M. Larina and A.I. Grigoriev examined the serum from three groups of men who were suspended in a dry immersion bath for seven days. The first group of healthy men was studied without any countermeasures but the second and third groups were subjected to external stimulation.

One group was given low-frequency myostimulation once a day for 30-60 minutes at various points on the body. For the other, the soles of the feet were subjected to mechanical stimulation to mimic walking. Both of these have been reported to reduce the negative effects of dry immersion on muscle function.

Blood was taken from each subject seven days before dry immersion, on the final day of immersion and seven days into recovery. The serum was treated by weak cation exchange and the proteins in the fractions were identified by matrix-assisted laser desorption/ionisation mass spectrometry.

A total of 170 proteins were identified but there were some significant changes between the three groups, affecting different parts of the body.

The level of angiotensin II was markedly decreased on the seventh day of dry immersion. Pakharukova noted that this would lead to accelerated excretion of electrolytes and liquids and a reduction in the volume of circulating plasma, which was confirmed by calculation of the hemocrit values.

Fibrinopeptide A fragment was also reduced, marking a reduction in fibrinolytic activity during dry immersion, whereas reduced high-mass kininogen fragment was indicative of the onset of an inflammatory response. Three other proteins of reduced abundance after immersion were complement C3 fragment, inter-alpha-trypsin inhibitor fragment and serum amyloid A.

Not all proteins were affected the same way. Apolipoprotein CIII was increased after dry immersion and continued to increase in the recovery period, indicating that the levels of triglycerides and cholesterol in the blood were likely to be enhanced.

The serum studies carried out following recovery for seven days showed that none of the induced changes had reverted to their pre-immersion levels, although they showed a wide inter-individual variability. So, the human body does not recover completely from long-term exposure to microgravity.

Neither of the countermeasures, myostimulation or mechanical stimulation, prevented the changes in the proteome from occurring but they did alleviate the effects to some extent for fibrinopeptide A and apolipoprotein CIII, for which the abundance changes were less pronounced.

So, dry immersion affects several systems in the human body that control the inflammatory system, lipid metabolism and the immune system. Two of the current measures thought to alleviate the effects of dry immersion provide only partial protection for the proteins, so other processes need to be found to give complete recovery.

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

 Dry immersion, which is used to simulate weightlessness over periods of several days, induces irreversible changes in the human serum proteome that cannot be alleviated by mechanical or myostimulation, say Russian scientists. The hormonal, immune and lipid systems are all affected

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