Hibernation for space travel: Impact on radioprotection

Abstract

Hibernation is a state of reduced metabolic activity used by some animals to survive in harsh environmental conditions. The idea of exploiting hibernation for space exploration has been proposed many years ago, but in recent years it is becoming more realistic, thanks to the introduction of specific methods to induce hibernation-like conditions (synthetic torpor) in non-hibernating animals. In addition to the expected advantages in long-term exploratory-class missions in terms of resource consumptions, aging, and psychology, hibernation may provide protection from cosmic radiation damage to the crew. Data from over half century ago in animal models suggest indeed that radiation effects are reduced during hibernation. We will review the mechanisms of increased radioprotection in hibernation, and discuss possible impact on human space exploration.

Full text

Life Sciences in Space Research 11 (2016) 1–9
Contents lists available at ScienceDirect
Life Sciences in Space Research
journal homepage: www.elsevier.com/locate/lssr
Review Article
Hibernation for space travel: Impact on radioprotection
Matteo Cerri
a , b
, Walter Tinganelli
c
, Matteo Negrini
b
, Alexander Helm
c
, Emanuele Scifoni
c
,
Francesco Tommasino
c , d
, Maximiliano Sioli
b , e
, Antonio Zoccoli
b , e
, Marco Durante
c , ∗
a
Department of Biomedical and Neuromotor Sciences, University of Bologna, Piazza di Porta S.Donato 2, 40126 Bologna, Italy
b
National Institute of Nuclear Physics (INFN), Section of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
c
National Institute of Nuclear Physics (INFN), Trento Institute for Fundamental Physics and Applications (TIFPA), Via Sommarive 14, 38123 Trento, Italy
d
Department of Physics, University of Trento, Via Sommarive 14, 38123 Trento, Italy
e
Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
a r t i c l e i n f o
Article history:
Received 30 July 2016
Revised 2 September 2016
Accepted 6 September 2016
Keywo rds:
Hibernation
Space radiation
Radioprotection
Space exploration
Torpor
a b s t r a c t
Hibernation is a state of reduced metabolic activity used by some animals to survive in harsh environ-
mental conditions. The idea of exploiting hibernation for space exploration has been proposed many years
ago, but in recent years it is becoming more realistic, thanks to the introduction of specific methods to
induce hibernation-like conditions (synthetic torpor) in non-hibernating animals. In addition to the ex-
pected advantages in long-term exploratory-class missions in terms of resource consumptions, aging, and
psychology, hibernation may provide protection from cosmic radiation damage to the crew. Data from
over half century ago in animal models suggest indeed that radiation effects are reduced during hiber-
nation. We will review the mechanisms of increased radioprotection in hibernation, and discuss possible
impact on human space exploration.
©2016 The Committee on Space Research (COSPAR). Published by Elsevier Ltd. All rights reserved.
1. Introduction
Hibernation is a state in which the metabolic activity of the
body is reduced. The drop can be as high as 98%. It is a solu-
tion used by some mammals to survive periods of scarcity of re-
sources. Obligate hibernators are those mammals that seasonally
decrease their metabolism and body temperature regardless of the
surrounding environment. Facultative hibernators enter hibernation
only when experiencing a negative energy balance. Hibernation is
not necessarily linked to a decrease in body temperature, but al-
ways comprises a reduced metabolism ( Heldmaier et al., 2004 ).
Considering the drastic reductions in all the metabolic function,
it is not surprising that during hibernation cell proliferation is
halted and mitochondrial respiration decreases. Hibernation seems
to provide animals with a higher resilience to stress. For instance,
a lethal hemorrhagic shock in a non-hibernator was shown to be
non-lethal in hibernators ( Bogren et al., 2014 ). Cells of hibernators
are able to resist stress better than regular mammalian cells ( Talaei
et al., 2011 ). Serotonin and dopamine are some of the mediators
involved in providing such protection, potentially acting on the
H
2
S pathway ( Dugbartey et al., 2015 ).
Ionizing radiation is a powerful stressor, and therefore it can be
asked whether hibernation also protects from the damage induced
∗Corresponding author.
E-mail address: marco.durante@tifpa.infn.it (M. Durante).
by exposure to high-energy electromagnetic and particle radiation.
Pioneering experiments with X- or γ-rays were performed in
the mid-XX century (see Table 1 and references therein). Most
of the experiments gave strong indications of induced radioresis-
tance during hibernation, yet the interest for humans was very
limited, because of the impossibility to induce hibernation in
non-hibernating animals.
Hibernation is an excellent tool for space travel: it ensures very
low metabolic consumption, and therefore minimizes the need of
storing large amounts of food and liquids for life support systems.
It also relieves most of the psychological problems associated to
isolation in very long missions. Reduced metabolism also mitigate
aging. Hibernation is therefore an ideal solution for very long
missions, exceeding 3–4 years, as it should be for Mars and
beyond and was indeed proposed for exploratory space missions
already many years ago ( Hock, 1960; Cockett and Beehler, 1962 ).
However, the initial enthusiasm, also reflected in so many popular
science fiction works, was hampered by the simple problem that
humans are non-hibernating animals, and inducing torpor with
drugs does not generally reproduce the main features of “real”
hibernation. Only recently, hibernation-like condition (synthetic
torpor) was induced in non-hibernating animals ( Cerri et al., 2013;
Tupone et al, 2013 ). This new opportunity has immediately reno-
vated the interest in hibernation for interplanetary human travel
( Gemignani et al., 2015 ). NASA is studying the design of a
specific habitat to host support crewmembers in hiberna-
http://dx.doi.org/10.1016/j.lssr.2016.09.001
2214-5524/© 2016 The Committee on Space Research (COSPAR). Published by Elsevier Ltd. All rights reserved.

2 M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9
Tabl e 1
Hibernation/hypothermia and radioresistance. The table shows the main references for the experiments conducted to test the hybernation induced radioresistance.
Yea r Authors Species Title Key outcome
2015 Cheng et al. Human cells Modulation of radiation-induced
cytogenetic damage in human peripheral
blood lymphocytes by hypothermia
“This indicates that the temperature effect observed in
peripheral blood lymphocytes after irradiation is not related
to a temporary perturbation of the cell cycle. Also, it is not
due to selective elimination of damaged cells by apoptosis”
2014 Lisowska et al. Human cells Effect of hypothermia on radiation-induced
micronuclei and delay of cell cycle
progression in TK6 cells
“…in conclusion the protective effect of hypothermia observed
at the level of cytogenetic damage was not due to a
modulation of cell cycle progression”
2006 Ignat’ev
et al. Rat The effect of hypothermia on the rat
radioresistance
“The cooling of Wistar rats up to 15–19 °C under a condition
hypoxia-hypercapnia increased the radioresistance with a
dose reduction factor (DRF) of 1.4

1973 Musacchia Hamster, Gerbil Hibernation, stress, intestinal functions,
and catecholamine turnover rate in
hamsters and gerbils
“Radioresistance increased in hibernating as wel l as in
hypothermic hamsters. Marked changes in hamster
catecholamine turnover rates we re observed during
acclimatization to high temperature stress”
1972 Barr and Musacchia Ground squirrel Postirradiation and radiation response of
ground squirrels: telemetry surveillance
“The results showed the highest levels of survival among
squirrels irradiated while torpid and that additional increase
in survival may have occurred due to postirradiation
hibernation. No significant effect was noted relating to the
three postirradiation environments”
1971 Musacchia et al. Hamster Radioresistance in hamster during
hypothermic depressed metabolism
induced with helium and low
temperatures
“Increased radioresistance was evident in the metabolically
depressed hypothermic hamsters… hypothermic hamsters
showed increased mean survival times. Dose-reduction
factors were approximately 1.2 2 and 1.31. The increased
radioresistance was examined in light of limited hypox ia and
general metabolic depression”
1969 Barr and Musacchia Thirteen-lined
ground squirrel
The effect of body temperature and
postirradiation cold exposure on the
radiation response of the hibernator
Citellus tridecemlineatus
“Results indicated that squirrels irradiated while hibernating
had higher percentages of survival than squirrels irradiated
while active…complicated rel ationship existed between the
effects of a postirradiation exposure to environments of 5 °C,
13 °C, or 23 °C for 90 days and the effects of irradiating
squirrels while at body temperatures of 5 °C, 13 °C, 23 °C, or
37 °C”
1961 Mraz and Praslicka Mouse Influence of Hypothermia on the survival
of mice after large doses of X-radiation
“…prolonged hypothermia at 22–25 °C, lasting 6–48 h,
after
irradiation also slows down destructive changes in the blood,
spleen and bone marrow. After the ending of hypothermia
and reaching the normal body temperature these changes,
however, develop even more intensely than in controls
subjected to irradiation only and the mean survival-time is
shorter”
1951 Smith and Grenan Marmot Effect
of hibernation upon survival time
following whole-body irradiation in the
Marmot (Marmota monax).
“Increased metabolic rate resulting from thyroid administration
has been found to coincide with an increased mortality in
mice following irradiation. On the other hand, the
administration of antithyroid substances, which reduce the
metabolic ra te, does not alter radiation lethality nor increase
the survival time in irradiated mice”
tion for the mission to Mars ( Bradford et al., 2014 ) ( Fig. 1 ).
ESA has sponsored a dedicated study and included hiberna-
tion in the life sciences roadmap ( Bereiter-Hahn et al., 2015 ).
However, for very long exploratory-class missions, the main
showstopper is generally considered the exposure to galac-
tic cosmic radiation ( Durante and Cucinotta, 2008 ). The re-
cent Mars Science Laboratory (MSL) measurements demon-
strate that the average dose-rate in deep space is about
1.8 mSv/day ( Zeitlin et al., 2013 ). Therefore any missions of several
years will exceed cumulative doses of 1 Sv. These recent measure-
ments support previous estimates of the dose in deep space and
confirm that countermeasures are indeed needed for a safe explo-
ration program. Unfortunately, shielding is poorly effective against
cosmic radiation, especially during the cruise where severe mass
constraints have to be imposed ( Durante, 2014 ). Biological coun-
termeasures, such as radioprotective drugs ( Kennedy, 2014 ), are
as yet far from providing practical solutions, and they are indeed
hardly used even in patients treated with radiotherapy for solid
tumors.
Can hibernation reduce the radiation damage? If true, this
effect will greatly expand the benefits of hibernation for space
travel, and make it an essential tool for the safety of the crews. In
this review, we will provide an overview of the past experiments
on hibernation and radiation. Starting from the interpretation of
previous results, we will evaluate potential advantages offered by
the application of new approaches to induce a hibernation-like
state for medical and space applications.
2. Biology of hibernation
2.1. Mammals
The term torpor identify a state of active suppression of
metabolism, usually leading to a reversible and undefended state
of hypothermia. Multiple episodes of torpor are at the base of hi-
bernation, aestivation , in which hibernation takes place during the
summer season, and brumation , that happens in ectothermic or-
ganisms such as reptiles or hibernacula ( Fig. 2 ). All these peculiar
animal states are characterized by the reduction of the metabolic
rate, decrease of oxygen consumption, decrease of heart and res-
piratory rate, reduction of the body temperature in dependence
of the ambient temperature and, at molecular level, gene expres-
sion and protein synthesis abatement ( Table 2 ). The large de-
crease in body temperature, caused by the reduction in metabolic
rate, does not activate homeostatic compensatory measures
( Heldmaier et al., 2004 ). Moreover, hibernators, besides the reduc-

M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9 3
Fig. 1. Mars transfer habitat design by SpaceWorks Enterprises, Inc. (SEI). The habitat can support a crew of 6 in hibernation/stasis for a 180-d ay outbound mission transit,
plus a 500-day aborted surface mission contingency, and 180 -d a y Earth return transit. Robotic manipulator arms are used to manage the crew as needed, and neuromuscular
electric stimulation (NMES) is used to prevent muscle atrophy. Tot al Parenteral Nutrition (TPN) is administered via tunneled central venous catheter in chest. The estimated
weight and volume of the torpor stasis habitat is less than 1/2 of the NASA DRA 5 referenc e habitat for the same Mars mission. Courtesy
of SpaceWork, study available
online at https://www.nasa.gov/content/torpor- inducing- transfer-habitat-for-human-stasis-to-mars.
Fig. 2. Different states of metabolic suppression experienced by different animals: squirrel, snake, frog, bear.
tion in metabolic needs, show other relevant features that may be
exploited in medical sciences, once a safe procedure to induce such
state would be available for humans.
For centuries it has been known that several mammals can
undergo hibernation, e.g. squirrels, hamsters, and bears. Only in
2004, a German research team discovered the first primate able
to undergo hibernation: the fat tailed dwarf lemur Cheirogaleus
medius ( Dausmann et al., 2004 ). The ability to enter a hy-
pometabolic state such as torpor, hibernation or aestivation is an
ancestral trait that was present in early mammals, and that was
later lost in some species. The widespread distribution of hiber-
nators in the mammals family suggest that the gene set necessary
to survive the process is probably common among mammals,
while the regulatory mechanisms may have been lost in non-
hibernators.

4 M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9
Tabl e 2
Physiological changes induced by hibernation. In bold, the changes that can be exploited by crews of interplanetary missions.
2.2. Molecular features of torpor and hibernation
Torpo r induces many changes in the molecular biology of the
cell in many animals ( Storey and Storey, 2010 ), including primates
( Faherty et al., 2016 ). Gene expression, for instance, is strongly
affected during hibernation in many organs such as the brown
adipose tissue ( Ballinger et al., 2016; Biggar et al., 2014 ), skeletal
muscle ( Anderson et al., 2016 ), the heart ( Vermillion et al., 2015 ),
and the bone marrow ( Cooper et al., 2016 ). Whereas gene ex-
pression studies provide relevant information, proteomics ( Grabek
et al., 2015 ) and epigenetics ( Storey, 2015 ) are opening new pos-
sibility to unravel the complex molecular cascade that regulates
cellular hypometabolic states. So far, gene expression related to
hypometabolic states has been studied only during natural torpor.
Transcriptomic and proteomic changes during synthetic torpor
(see below Cerri, 2017 ) have not been reported yet.
Considering that the main feature of torpor is an active re-
duction in metabolic rate, and therefore in oxygen consumption
( Heldmaier et al., 2004 ), the activity of mitochondria has been a
relevant focus in hibernation research ( Staples and Brown, 2008 ).
The reduction in oxygen consumption could be in fact explained
by some form of inhibition of the mitochondrial function ( Mathers
et al., 2016 ). The overall idea that mitochondria are the target
of the mechanism of metabolic suppression is intriguing, but
not free of objections. For instance, if mitochondrial activity was
to be reduced first, cellular damage would probably arise for
the unbalance between energy demand and energy supply. To
solve such problem, it is possible to refer to the idea of channel
arrest ( Hochachka et al., 198 6 ), according to which a reduction
in cellular energy consumption, for instance by reducing mem-
brane pumps activity, has to precede the reduction in energy
production. If this idea was applied to torpor, the reduction in
mitochondrial activity would not be the first target for metabolic
suppression, but the consequence of a drastic reduction in energy
demand.
2.3. Arousal
Hibernation is interrupted by periodic arousals . In these periods
the increase of temperature and the acceleration of the metabolism
allow the animal to restore almost normal vital and active func-
tions.
The arousal could permit the activation of the immune sys-
tem and subsequently initiate an immune response ( Prendergast
et al., 2002 ) or the periodic arousal may be needed by the animal
to solve a slept-debt, as it occurs in the arctic ground squirrel
( Daan et al., 1991 ). It has been further speculated that the arousal
is needed to restore the energy deposit. The arousal is strictly
dependent on the body temperature and in Cheirogaleus medius it
occurs when the temperature exceeds 30 °C. However, there is a
high energetic cost for the arousal. In this dwarf mammal the in-
creased active thermogenesis necessary for the arousal is avoided
using the temperature of the tropical situation in which they live.
In the case of the Cheirogaleus medius they simply continue to stay
in hibernation even over 30 °and, suspending endogenous ther-
mogenesis and becoming then ectotherm like a reptile, can go over
30 °without any energy consumption ( Dausmann et al., 2004 ).
2.4. Human hibernation
After many reported cases of humans that surviving a pro-
longed low body temperature, it was suggested that humans could
also be facultative hibernators. There are examples of people sur-
viving accident in situations where their whole body temperature
fell to 13.7 °C without any serious damage.

M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9 5
Tabl e 3
Different strategies used to induce an artificial state of torpor. “Synthetic torpor” indicates a state that mimics for the most part the natural features of the torpor occurring
in hibernating species, which would be the ideal condition to translate for humans applications. It is noteworthy that the mouse is a facultative heterotherm, capable to
enter torpor under specific circumstances, and therefore not the best model to be used.
Yea r Species Ibernator Approach Effects Reference
2005 Mouse Yes H
2
S inhalation Induction of a state resembling torpor Blackstone et al.
2008 Rat No H
2
S inhalation No synthetic torpor Lou et al.
2008 Piglet No H
2
S inhalation No synthetic torpor Li et al.
2011 Pig No H
2
S iv No synthetic torpor Drabek et al.
2015 Sheep No H
2
S inhalation No synthetic torpor Haouzi et al.
2006 Mouse Yes 5

-amp systemic Induction hypothermia but with
different physiological characteristic
compared to the state of torpor
( Swoap et al., 2007 )
Zhang et al.
2009 Rats No 5

-amp systemic No synthetic torpor Zhang et al.
2013 Rats No Inhibition of RPa neurons Synthetic torpor Cerri et al.
2013 Rats No Central adenosine A1 receptor agonist Synthetic torpor Tupone et al.
Mitsutaka Uchikoshi is considered the first human being to
undergo a hibernation-like state following an accident. According
his physicians, the hibernation-like state slowed many of his
organs, yet his brain was protected.
Anna Elisabeth Johansson Bagenholm is a Swedish radiologist
who survived a prolonged extreme hypothermia. Her body tem-
perature was around 14. 4 °C, the arterial blood sample showed
normal serum potassium and oxygenation, the carbon dioxide
(CO
2
) concentration was higher than the standard level while the
acidosis was severely high. A foamy pink fluid streamed from the
endotracheal tube. She even reached the lowest body temperature
ever recorded in a living human, i.e. 13.7 °C ( Gilbert et al., 20 0 0 ).
However, besides small nerve injury and a slight tingling in her
hands, Anna Elisabeth Johansson five months later recovered com-
pletely from this tragic experience and came back to her normal
life and work.
Erika Nordby, a 13 months old Canadian baby left the house
unattended wearing only a diaper. The ambient temperature out-
side was around −24 °C. When Erika was found, she was consid-
ered to be clinically dead. She had no heartbeat, no respiration and
her body temperature was around 16 °C. Her conditions became
normal after that she was placed under a warming blanket. No se-
rious damage remained and her physician suggested that the low
temperature could have brought her to a hibernation-like state.
2.5. Synthetic torpor
Hibernation, torpor, aestivation, hibernation-like state, torpor-
like state, pseudo-torpor, hypometabolism, hypothermia, deep hy-
pothermia, suspended animation, stasis and probably others are
some of the names that are used in the field. They all refer
more or less to a state of hypometabolism that is artificially in-
duced in a species that does not show any torpor or hiberna-
tion. With synthetic torpor we indicate not just an hypometabolic
state, but a state that mimics for the most part the natural fea-
tures of the torpor occurring in hibernating species, which would
be the ideal condition to translate for humans applications ( Cerri,
2017 ). Several methods to induce hibernation-like states are re-
ported in Table 3 . Mimicking hibernation may bring great benefits
also to the clinical practice, in terms of neuroprotection after cere-
bral hypoxia, during major surgery, or to extend the window for
organ transplantation. We also hypothesize that it can be a bene-
fit in radiotherapy, for reducing radiotoxicity in normal tissues (see
Section 3.3 ).
Since hibernation is characterized by a reduction of the
metabolism, a possible method mimicking it would be to lower
the metabolic rate by reducing the cellular energy production. This
approach was tried in mice, and a suspended animation state was
induced by using 5

-AMP ( Zhang et al., 2006 ), or H
2
S ( Blackstone
et al., 2005 ). All these substances reduce or block the energy
produced in every cell by inhibiting the mitochondrial function,
but all these approaches proved ineffective when applied to
non-hibernating mammals ( Haouzi et al., 2008 ). It is noteworthy
that the mouse is a facultative hibernator able to enter torpor
under specific circumstances, and that this could be the reason of
the failure in translating those approaches to non-hibernators. In
other words, the pharmacological treatments given to mice may
have triggered the mechanism of torpor, but may not be effective
in doing so in species that do not have the same underlying
mechanisms.
It could be argued that hibernation is a trait of a specific group
of species, and therefore may not be replicated in non-hibernators.
For instance, flying is a trait of a specific group of mammals
(bats), because they have evolved the biological structure (wings)
to support that trait. But that is not the case of hibernation.
Hibernators are, in fact not a homogeneous group of species, but
as discussed above, they are present in every family of mammals,
including primates. This suggests that the ability to suppress
metabolic rate was probably a trait present in the proto-mammal
(approximately 150 million years ago), an animal that, because
of his homeothermy, was able to be nocturnal to reduce the risk
of predation by dinosaurs. Such ability was critical to improve
the survival chance of the newly-appeared mammal, since it
provided the great energy saving skill, necessary to support the
higher energy expenditure required by homeothermy. The trait
was then lost in many mammals after the extinction of the di-
nosaurs, but remained present in all the species that relied on
energy saving to survive (for instance species living in extreme
climate). From this evolutionary reasoning, it can be argued that
the set of genes and the biochemical regulation necessary to
survive during the hypometabolic bout are shared among mam-
mals, while only the regulatory mechanism was lost in non-
hibernators.
To this day, only two procedures were shown to induce such
conditions in non-hibernating rodents: (i) the inhibition of the
sympathetic premotor neurons within the Raphe Pallidus (RPa)
( Cerri et al, 2013 ); (ii) the activation of the central Adenosine A1
receptors ( Tupone et al., 2013 ). The approach used differs substan-
tially from what was done in mice: instead of acting on the whole
organism metabolism, it inhibits the activity of specific neural
pathways within the central nervous system. The physiology ob-
served during such hibernation-like state strongly resembles spon-
taneous hibernation, in terms of cardiovascular, respiratory and
neuro-physiology, as well as behavior. Fig. 3 shows the decrease in
temperature in a rat following inactivation of the Raphe pallidus.

6 M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9
Fig. 3. Examples of infrared images taken during the procedure of induction of a
synthetic torpor state by inhibition of RPa neurons. Top line: before the inhibition;
middle line: shortly after the beginning of the procedure; bottom line: at the tem-
perature nadir. In the right panel of the bottom line the rat shape is highlighted in
red, since the animals shape is not very visible being in thermal balance with the
environment. Experimental details in Cerri et al., 2013 . (For interpretation of the
references to color in this figure legend, the reader is referred to the web version
of
this article.)
3. Radiation effects during hibernation
3.1. Experiments in hibernating animals
It is generally accepted that hibernation increases resistance
to stress. In particular, many authors provided evidence indicating
a relevant increase in radioresistance. This effect was intensively
studied a few decades ago (1950–60) through animal experiments,
mainly comparing the survival rate of squirrels after providing
variable doses of gamma radiation ( Table 1 ).
Smith and Grenan (1951) reported that lethal damage on ex-
posed hibernating marmots seems to be just delayed after the
arousal, where the same survival fractions are observed for mar-
mots irradiated during or outside hibernation, if the survival time
is considered from the time of the arousal. The result is later con-
firmed by Doull and Dubois (1953 ) for ground squirrels. Since irra-
diation affects mitosis, the radioprotective effect caused by the stall
of the cell cycle while in hibernation may be reduced after arousal.
A mild increase of radiosensitivity was observed by Kuskin and
coworkers ( Kuskin et al., 1959 ), where a small decrease of body
temperature was artificially induced on a population of mice by
means of neuroplegic drugs. Hypothermia induced by neuroplegic
drugs followed by physical cooling of the animal instead did not
show a radioprotective effect. The authors suggested that the pro-
tective effect is not related to the depression of the metabolism
alone but instead to a general depression of bodily processes.
Naturally occurring radioprotection in hibernating ground
squirrels ( citellus tridecemlineatus ) has been reported by Musacchia
and Barr (1968) over a broad range of doses (10–200 Gy). Jaroslow
and colleagues ( Jaroslow et al., 1969 ) reports the study of survival
rates for hibernating squirrels exposed to a lethal dose from a
60
Co source. The highest survival rate is observed for squirrels
aroused from hibernation one day after irradiation. Although the
lack of food and water for longer hibernation periods may account
in part for the difference in survival rates, an increased survival is
observed also with respect to squirrels immediately aroused after
irradiation.
An increased radiosensitivity of ileum crypt cells of ground
squirrels was observed Jaroslow et al. (1976 ) comparing the cell
survival fractions for irradiation during hibernation, arousal, and
euthermic state. In squirrels irradiated 1 h after the initiation
of the arousal, with core temperature in the range of euthermic
controls, the survival was comparable with the one observed for
irradiation during hibernation. The radioprotective effect disap-
pears 3–7 h after irradiation. This study suggests hypoxia as a
possible explanation for the radioprotective effect. The changes in
the mitotic index of ileum cells during hibernation and arousal
may also play a role.
More recent experiments investigated mechanisms of acquired
resistance using in vitro systems. Baird et al. (2011) measured a
suppressed DNA repair response in BJ-hTERT human cells irradi-
ated in hypothermic conditions (irradiation at 13 °C, 20 °C and
30 °C). Hypothermia was observed to interfere with the cell cycle
progression and a low fraction of cells in the S-phase. While repair
of double strand break is temperature-dependent and suppressed
at low temperature, the larger survival fractions for irradiation in
hypothermia suggests that slowly repair of double strand breaks
may not have a large negative impact on the survival of the cell.
Povinelli et al. (2015) reported that even a mild cold stress
has a protective effect on hematopoietic stem and progenitor cells
after both nonlethal and lethal total body irradiation on mice,
prolonging the survival time. Injecting propanolol (non selective
β-blocker) partially reduces the protective effect induced by cold
stress, while it has no effect on mice kept at thermoneutral
temperature. The proposed mechanism operates through the β-
adrenergic signaling pathway, causing the inhibition of the apop-
tosis of hematopoietic stem and progenitor cells in bone marrow.
Taken together, all the experiments support the hypothesis that
hibernation increases radioresistance. It is difficult to pool the
data to give quantitative estimates of the observed effects. Fig. 4
reports a pool of the data on survival to high γ- or X-ray doses.
In the panel (a) we pooled the data relative to ground squirrel,
a hibernating animal, while the panel (b) shows the survival of
rats, a non-hibernator, irradiated in hypothermia. The pooling
of different experimental conditions produce a very significant
spread in the data, but the trend clearly shows the increased
survival of squirrels irradiated during hibernation. LD50 values
were calculated with a probit fit:
M
(
%
)
=
1
2

1 −erf

γ√
π1 −D
LD 50
 
(1)

M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9 7
Fig. 4. Meta-analysis of observed lethality in animals exposed to graded γ- or X-ray doses. Lines are fits with a probit function to lethality of: (a) squirrels in hibernation
(blue) or active (red); (b) rats in hypothermia (blue) or normal temperature (red). Experimental data are mortality data from different published papers. (a), squirrels: circles
are mortality at 30 days from Musacchia et al., 1968 : full circles, Summer hibernation; dashed circles, Winter hibernation; other symbols are mortality rates at 90 days from
Jaroslow et al., 1969 ( ࢞, normal hibernation; ♦ 2 weeks at 5 ºC; ∗20 h at 5 ºC). (b), rats: circles are mortality rates at 30 days
from Ignat’ev et al. (2006) , squares from
Benvenuto and Lewis (1960) . (For interpretation of the references to color in this figure legend, the reader is referred to the web ver sion of this article.)
Tabl e 4
Fitting parameters of the data in Fig. 4 using the probit formula ( Eq. (1 )). The pro-
tective factor is the ratio of the LD50 values. Uncertainties are standard errors of the
fitting parameters calculated using the software package Wolfra m Mathematica
®.
Animal Normal conditions Hibernation/hypothermia Protective factor
γLD50 (Gy) γLD50 (Gy)
Squirrels 4.4 ±1. 2 10. 82 ±0.17 1.1 ±0.4 14.6 ±1.1 1.3 5 ±0.07
Rats 2.3 ±0.5 6.03 ±0.13 3.6 ±2.7 8.6 ±0.4 1. 4 2 ±0.10
where M is the mortality rate, γthe slope parameter, erf the error
function, D the dose, and LD50 the dose corresponding to 50%
mortality. The fitting parameters are reported in Table 4 . Even
considering the large differences in experimental conditions, it
is remarkable that the protective factor (ratio of LD50 values in
hibernation/hypothermia and normal conditions) is around 1.4 for
both squirrels (hibernators) and rats (non-hibernators). This value
is high and makes it interesting for radioprotection. It is close to
the protection factor of the few agents (amifostine and palifermin)
in clinical use as radioprotectors during radiotherapy ( Johnke et al.
2014 ). In a recent study in mice exposed to high X-ray doses and
protected with amifostine (WR 2721) or its polymer complex, the
protection factors calculated from LD50 values were 1.29 and 1.35,
respectively ( Koseva et al., 2014 ).
3.2. Possible mechanisms of radioresistance
Discrepancies are present in the interpretation of the different
experiments. Some may be related to differences in the reactions
of different species but also the differences in the realization of
the experiments (like time in hibernation before and after irradia-
tion, the ambient temperature) may have a non-negligible impact
on the outcome. At the cellular level, it is well known that cells
irradiated in resting state, and left undisturbed for 24 h after irra-
diation, are more resistant than cells re-plated immediately after
exposure. This effect is known as delayed plating, and is generally
attributed to the potentially lethal damage repair (PLDR) in resting
phase ( Marchese et al., 198 7 ). PLDR is also observed in vivo, fol-
lowing exposure of xenografts in mouse ( Little et al., 1973 ) but it
is unclear whether a reduced metabolism grants enhanced protec-
tion from radiation in a whole organism.
The oxygen level during hibernation may contribute to the
enhanced radioresistance. Hypoxia is a well know protective
mechanism, and is arguably the main reason of local control
failure in radiotherapy. In fact, tumors are generally hypoxic com-
pared to the normal tissues, and the ratio of the doses producing
the same cell killing in hypoxic and aerobic atmosphere (oxygen
enhancement ratio, OER) is close to 3 after X-rays ( Wilson and
Hay, 2011 ). The reduction in oxygen metabolism during hiberna-
tion may well provide an additional protection towards radiation
damage. With respect to this latter point, it is important to notice
that differences in the dynamics of awakening may affect the over-
all health status of the animal ( Cerri et al., 2013 ), suggesting that
the previously reported silent damage induced by radiation during
hibernation may somehow be linked to the arousal process, the
phase when oxygen metabolism (and therefore the production of
reactive oxygen species (ROS)) is highest. The role of awakening in
the resulting damage from irradiation is clearly relevant, but still
unclear. For instance, it was also suggested that the protective ef-
fect might occur during the arousal from hibernation, following the
increase of catecholamines that may occur in the arousal process
and could cause severe vasoconstriction ( Prewitt and Musacchia,
1975 ). Similar results were observed by Barr and Musacchia (1972) ,
where different groups of squirrels were kept after irradiation in
environments with three different temperatures (5 °C, 13 °C and
23 °C), without observing any significant difference in terms of
radioresistance between the three groups. The temperature itself
has a sensible effect on the radiosensitivity of many biological sys-
tems. The radiosensitivity is generally reduced under hypothermia,
which is one of the main peculiarities shown by animals during
hibernation. Living in a cold environment has an impact on the ra-
dioresistance of animals (see Sazykina and Kryshev, 2011 for a re-
view). Imbalances in the temperature-dependent cell damage and
repair processes for animals living in cold environment make the
overall picture unclear and the topic is still an active research field.
3.3. Radiotherapy
Radiotherapy is an essential component of the cancer cure. Over
50% of the cancer patients are treated with high-doses of radia-

8 M. Cerri et al. / Life Sciences in Space Research 11 (2016) 1–9
tion, either alone or in combination with surgery or chemotherapy.
Radiotherapy has recently experienced many advances in method-
ology and techniques and the dose to the tumor is nowadays
delivered with a much higher precision. Highly conformal dose
distributions can be achieved with X-rays using intensity modu-
lated deliver (IMRT), or accelerating charged particles (generally
protons or C-ions) at very high energies ( Thariat et al., 2013 ).
Notwithstanding these excellent technological improvements, ra-
diotherapy is still limited by the normal tissue toxicity. In fact, or-
gan tolerance doses are thresholds to avoid unacceptable morbidity
in the patient. This limits the maximum dose that can be used to
control locally the tumor, and in some cases prevents treatments.
For instance, multiple metastases in close proximity cannot be
treated without exceeding the tolerance dose of the normal organ,
even if their treatment with hypofractionated radiotherapy can
substantially improve the survival of patients with stage IV cancers
( Lo et al., 2011 ).
The torpor-induced reduced metabolic rate suggests that the
tumors will stop growing during hibernation. This was indeed
found in old experiments where human tumors where trans-
planted in hamsters ( Lyman and Fawcett, 1954; Patterson et al.,
1957 ). However, tumors return to normal proliferation as soon as
the body temperature is restored. It is, however, unknown whether
the radioresistance is differentially modulated by hibernation in
normal and tumor tissues. If hypoxia plays a role, it can be ex-
pected that normal tissues (aerobic) will be more protected than
tumors (normally already hypoxic). Hence, a hibernated patient
could be treated, e.g. for multiple metastases, at doses that would
not be acceptable in normal conditions, because the organ dose
would be exceeded. At arousal, the patient will be cured. This is
obviously a speculative hypothesis, and yet so attractive that we
believe it deserves experimental verification.
4. Conclusions
Hibernation might be a promising strategy for putative future
human interplanetary missions. Hibernated astronauts would re-
duce the spaceship’s total energy consumption more than 50%,
they would increase their lifespan, saving then the travelling time.
Furthermore a hibernation-like state would help them with psy-
chological issues resulting from the flight in a narrow spacecraft,
spending much of their time in an “unconscious-like” state, and
would reduce enormously the waste production. Moreover, hiber-
nation may solve another crucial objection for manned space ex-
ploration, i.e. radiation exposure. The old observations pointing to
increased radioresistance in torpor open new, fascinating scenar-
ios. Exposure to cosmic rays is generally acknowledged as the main
health threat in long-term exploratory-class missions. The problem
is particularly critical for the cruise period, when thick shielding of
the spacecraft is impossible. Drugs or dietary supplementary are,
as yet, equally disappointing. Hibernation may represent an ideal
solution. A radioprotective effect of hibernation is observed for
several species and on different irradiation conditions. Our meta-
analysis ( Fig. 4 ) suggests a protective factor around 1.4 , close to
that provided by drugs used in radiotherapy (such as amifostine)
to prevent normal tissue toxicity. The overall mechanisms at the
basis of the increased radioresistance in hibernation or hypother-
mia are poorly understood and need further investigation. Old ex-
periments are limited to natural hibernators exposed to high doses,
with acute effects as main endpoints, generally survival. For space
exploration, new experiments are needed, and they should explore
late effects at low doses. The issue is extremely relevant not only
for space travel, but also for cancer radiotherapy.
Acknowledgments
We thank Juergen Bereiter-Hahn and Alexander Chouker, from
the ESA Topi cal Tea m on hibernation for useful discussions. MD is
on leave from University of Naples Federico II. This work was sup-
ported by the National Institute for Nuclear Physics (INFN) under
grant HIBRAD, CSN5 (2016).
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