By Doug Macron
An international team of researchers this month published data showing that RNAi in C. elegans functions effectively in space, suggesting that the gene-silencing technology could be used to treat spaceflight-associated conditions that affect some astronauts.
And while that is a long-term goal — therapeutic RNAi is still a work in progress on Earth — the findings “open up spaceflight research” by giving astronaut-investigators access to the technology for their experiments, study co-author and University of Nottingham researcher Nate Szewczyk told Gene Silencing News.
“A primary aim of the world's space agencies is to send humans to other planetary bodies such as Mars,” according to the paper, which appeared in PLoS One. “However, preventing attainment of this goal is the frequent occurrence of various pathophysiologic adaptations during spaceflight, which may be detrimental for crew health and mission performance.”
Most notably among these are decreases in skeletal muscle mass to levels that could impair contractile function and rehabilitation and loss of bone mass that are frequently in the range of five to ten percent, the study's authors wrote.
Additionally, indirect measures of metabolism indicate that astronauts experience increased reliance on glucose utilization and decreased fat oxidation as a source of energy, “which may negatively affect physical performance.
“Finally, T lymphocytes harvested and cultured from crew members display a lowered ability to activate in response to mitogens and also increases in chromosomal aberrations,” according to the paper. This suggests “impaired immune function which, if persistent over longer flights, may pose a serious health risk.”
Although popular opinion holds that such conditions are primarily due to the lack of gravity in space, “my feeling is that it has to do with living in a closed environment,” Szewczyk said. “Most of the changes you see across species seem to relate to changes in metabolism … [such as] alterations in insulin signaling, which doesn't sound like it has anything to do with gravity.”
Regardless of their source, these pathophysiologic changes are likely to increase the risks to space explorers during long-term missions, requiring the development of countermeasures. Given its ability to selectively target genes of interest, RNAi may prove a useful technology for doing so, but it was not clear whether it would function in space as effectively as it does on Earth.
Szewczyk's study suggests it could, at least in C. elegans.
Among those working in the field of space exploration, “there are a fair number of reasons for suspecting that RNAi might not work” in space, Szewczyk explained, adding that most have to do with whether RNA metabolism is altered in flight.
“There are past reports of RNA metabolism being altered such that total RNA content is reduced by a factor of two or four in different tissues in space,” he said.
To address this question, the researchers arrested the development of C. elegans at various stages of their development. The worms were then brought onto the International Space Station, which sits in a low Earth orbit, and introduced to a food source to restart their development.
In one experiment, the worms spent eight days in space and were returned to Earth and, when compared to controls on Earth, were found to have retained normal expression of genes encoding for various RNAi-related proteins, including those in the Dicer and Argonaute families. The researchers also found that spaceflight didn't appear to alter the expression of 228 of 232 microRNAs analyzed.
In other experiments, the worms were introduced to bacteria containing double-stranded RNA against either green fluorescent protein and rbx-1, which the researchers have previously found to play a role in C. elegans meiosis, mitotic chromosomal condensation and segregation, and cytokinesis.
“Fluorescent light microscopy on return to Earth demonstrated that in vector controls, GFP-expression levels were comparable between [worms on a four-day] spaceflight and ground controls,” the investigators wrote. “Furthermore, in both spaceflight and ground control conditions, RNAi against rbx-1 induced abnormal embryonic nuclear segregation and arrest of meiotic division.”
The team also showed that using dsRNA against the lysosomal cathepsins asp-4 and asp-6 can prevent degradation in the muscle protein alpha-actin.
“Basically, we showed that [RNAi] could work in multiple tissues, and that it can act on endogenous or exogenous genes,” Szewczyk said.
“It appears therefore that factors unique to spaceflight, for example microgravity and increased radiation exposure do not impair cellular ability to recognize and initiate gene silencing in response to … genetic material,” the study authors concluded. Likewise, “spaceflight does not induce negative regulatory post-transcriptional or post-translational modifications that impair the efficiency of RNAi.”
Still, questions remain unanswered, they noted.
For instance, it is not clear whether similar findings would be obtained for worms treated outside of low Earth orbit, “where the effects of radiation are increased.”
Still, “RNAi presents a potential therapeutic strategy for combating deleterious spaceflight-induced adaptations, for maintained crew health, and mission performance,” they wrote. “Indeed, we show that protein degradation; a key element underlying muscle wasting, can be prevented by the application of RNAi against proteolytic enzymes on Earth and in space.
As such, achieving long-duration explorations of deep space may be made more feasible through the use of RNAi technology to overcome the numerous threats posed to human health by prolonged exposure to the space environment,” they concluded.
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