Category: Hibernation

How did plants emerge on our planet? Fossil evidence suggests that the first plant-type organisms initially emerged in the form of algae, likely around 1.2 billion years ago. And for more than 700 million years, they remained exclusively marine, thriving while submerged in water (possibly following the famous crab’s advice: “Darling, it’s better down where it’s wetter”). This is especially remarkable because current evidence indicates that the first masses of land emerged above sea level about 3.7 billion years ago. This means that for over 3 billion years, land was completely devoid of plants. However, about 470 million years ago, marine plants began to transition to terrestrial living, with ferns being among the pioneers of these early conquerors of the land.

We can imagine that terrestrial living provided plenty of benefits to these early plants: little competition for space, much more sunlight, and access to mineral nutrients. Perhaps there were also fewer predators. However, with these advantages came the real risk of death from dehydration, which likely promoted the selection of plant variants capable of tolerating prolonged lack of water. It took some lineages of plants a gradual evolution into seed plants around 350 million years ago during the Late Devonian period. The invention of seeds has provided a colossal advantage for plant survival. For many higher plants, seeds can lose up to 95% of their water content and still remain viable, entering a state of dormancy that protects them from desiccation damage. This allows plants to pause their life cycle until the environment is suitable for active growth.

According to current estimates, the emergence of seed plants coincided with (or preceded by) the origin of so called LEA proteins, which is a special class of protective proteins in plant genomes: . These proteins, through a poorly understood mechanism, allow seeds to remain viable in the face of desiccation. We currently know that most known LEA proteins are highly hydrophilic and often non-globular proteins that protect seeds, as well as mature plant tissues, from desiccation damage. Their name stands for Late Embryogenesis Abundant proteins, which stems from their discovery in cotton seeds, where LEA proteins accumulate during the late stages of embryo development. In plants, LEA proteins enable organisms to survive severe and reversible dehydration, a phenomenon termed “anhydrobiosis.” Furthermore, the ability of LEA proteins can be replicated in other organisms. For example, LEA proteins can confer desiccation tolerance when recombinantly expressed in mammalian cells. In one particular study, mammalian skin cells were shown to become almost completely nonviable (with only 1% of cells alive) after 4 hours of air drying. However, more than 58% of these cells survived if they expressed the protein dehydrin from maize, the cold acclimation protein WCOR410 from wheat, and LEA protein group 3 from brine shrimp.

The evolutionary origin of LEA proteins in nature has likely been driven by the spread of plants from marine to terrestrial environments, which occurred around 470 million years ago during the Ordovician period. As plants transitioned to land, they faced the risk of desiccation and gradually evolved into seed plants, around 350 million years ago during the Late Devonian period. According to current estimates, it is during this time that LEA proteins emerged as small, highly hydrophilic proteins that protect seeds (but also mature plant tissues) from desiccation damage.

Although “canonical” LEA proteins originate from plants, they appear to have spread to other species via horizontal gene transfer. As a result, LEA are present not only in plants but also in other eukaryotes and many bacteria. In tested species, LEA proteins help cells survive harsh environmental conditions, such as desiccation, cold shock, and possibly many other unfavorable circumstances.

In E. coli, there are at least five LEA proteins, but we understand the biology of just one of them, the RMF protein. This protein binds to ribosomes in metabolically inactive (stationary or transiently stressed) cells and prevents ribosomes from being degraded by cellular nucleases and proteases. This allows E. coli to remain inactive with a lower risk of digesting their essential molecular machinery, making it possible for these bacteria to survive prolonged states of metabolic inactivity and recover when conditions improve or when the cells adapt to a new environment.

When we talk about hibernation, we typically mean surviving winter times. However, many organisms use hibernation to survive heat, not cold.

If you visit the Chihuahuan Desert—one of the most fascinating places on our planet for anyone interested in biodiversity—you may find one of its most resilient residents, a moss-like plant commonly known as the Resurrection Plant. Here are some remarkable and mind-boggling facts about it:

• Its actual growth period lasts only a small fraction of the year, primarily occurring during the wet months (~2-3 months per year) when conditions are optimal for growth.
• For the rest of the year, this plant remains dormant: when the rains stop, it dries out and shrinks into a tight ball that weighs only about 3% of its initial weight. So, technically, this plant turns into viable but dormant hay.
• In this dry form, it can remain dormant for up to several years, effectively suspending its growth until it receives sufficient moisture. When rehydrated, it quickly resumes growth, often in under an hour.
• This plant species arose about 250 million years ago (which makes it approximately a thousand times older than humans) and is called Selaginella lepidophylla, where Selaginella refers to selāgō—a Latin name for a type of juniper—and lepidophylla means “scaly” (from lepidus) and “leaf” (from phyllon). This name highlights the plant’s unique feature of having scaly leaves that enable it to survive in extremely dry conditions.

Thus, when you think about dormancy next time, imagine not only a bear’s den but also a hot, dry desert.

A superpower we take for granted.

Scientists say it is impossible for a human body to go without food for longer than 8 days, with some optimists claiming that up to 21 days is sometimes possible. Even the Bible considers 40 days of fasting the ultimate feat of bodily strength. Yet what’s not possible for a human — even the legendary Christ — appears to be fairly mundane for creatures that are comparable to us in size, anatomy and lifespan. These creatures are bears.

Bears have a remarkable knack for starvation. Unlike humans, they can enjoy their breakfast in mid-September to have their lunch only in April! I guess if they would be into academic research or the literary career, it would be easy for them to implement this golden rule by Kingsley Amis: “A bad review may spoil your breakfast, but you shouldn’t allow it to spoil your lunch.”

— Hey bear, a bad review may spoil your breakfast, but you shouldn’t allow it to spoil your lunch.
— Easy as pie, pal. Just give it to me in September (proceeds to reading comments by Reviewer 2, yawns, and falls asleep).

So, what’s the trick? What makes bears so good in their ability to cope with starvation? The answer is hibernation.

All animals are equal? You’ve got to be kidding. Hibernation. We sort of know what hibernation is, but really — not quite. When I was a kid, my understanding of hibernation — mainly learned from Fozzie Bear — was plain and simple: a long nap. Over the years, however, I learned that the actual state of hibernation is much deeper and more complex than usual sleep. Although we do not fully understand this process, we now know that hibernation has many unique metabolic signatures that distinguish it from ordinary sleep, and mechanisms of hibernation appear to be wildly distinct in different species. This suggests that the ability to hibernate has evolved many times in the history of life on Earth, with many species inventing their own unique ways of hibernating.

For instance, when grizzly bears hibernate, their body temperature decreases from 38 to 31°C; their heart rate drops from 40-50 to just 8-19 beats per minute; their respiration decreases from 6-10 to just about 1 breath per minute; their circadian clocks get shut off; and their metabolism not only slows down but changes in such a way that bears do not release bodily waste for several weeks. Instead, they turn their urine into protein through a urea recycling process, along with other metabolic changes that allow them to survive cold, despondent winters.

But in other species, like Arctic ground squirrels, the process of hibernation looks strikingly different. These animals undergo the process of “supercooling,” in which their average body temperature drops to -3°C, with only their heart and brain remaining slightly warmer. Unlike most other animals, these squirrels can somehow prevent the nucleation of ice crystals in their bodies. This allows them to remain alive and soft, yet technically frozen, when the air temperature drops to -26°C. Just imagine if we, humans, would have such a skill!

Even a more impressive example of hibernation can be found in smaller animals, ironically known as water bears or tardigrades. While actively growing tardigrades are highly sensitive to various stressors, including heat and cold, things totally change as they hibernate. When these caterpillar-looking creatures encounter starvation or stress, they rapidly purge most of the water from their bodies. This “dry hibernation” allows these (not-so-much-)water bears to survive prolonged stress for decades and possibly much longer. In this state, tardigrades can survive exposure to extreme environments, including temperatures as low as -200°C and as high as 150°C, or high doses of radiation, or life in the open space outside our planet.

Thus, hibernation is a superpower: when organisms hibernate, they can do things impossible in their ordinary sleep or alert state: being able to enter a state of hibernation allows organisms to survive in some of the most extreme conditions known to science. No wonder, we, humans, would gain a lot if we could understand this process better and learn how to induce hibernation at a desired time in a target organism, including our own bodies. Putting gravely ill people or astronauts to sleep would be one immediate application of such a hibernation by design.

More common than you think. What is even more important about hibernation is how common it occurs in nature. Contrary to popular belief, hibernation is not limited to large animals or “weird” insects. In fact, hibernation is routinely exercised by practically all known microorganisms, including most studied bacteria.

One reason we know little about the hibernation of microorganisms merely stems from semantics. When bacteria hibernate, we usually do not call them ‘hibernating’. Instead, we call them ‘dormant’ or ‘persistent’. Semantics frequently screws us up. Remeber George Carlin’s ‘Soft language’? While not deliberate or malicious, this use of different terms for hibernation of animals as opposed to hibernation of germs creates a common and wrong perception that hibernation is rare, and the ability to hibernate is limited to just a handful of animal species that need to survive harsh winters.

Instead, we actually know that when bacteria face starvation or stress, including exposure to certain drugs and therapies, they undergo a conceptually similar transformation to the one observed in grizzly bears, squirells or tardigrades: some bacteria lose their water, some shut off most (but not all) of their metabolism, some produce special anti-freeze agents that prevent formation of ice crystals when bacteria are exposed to sub-freezing temperatures. This allows bacteria to survie in hostile environments for a very long time. For example, recent studies of the Greenland ice sheet have identified and ‘resurrected’ a specimen of the bacterium Chryseobacterium greenlandensis that are 120,000 years old! And it is possible that 120,000 years is very far from the actual survival limit for a hibernating microorganism.

This example goes to show that hibernation — frequently viewed as something exotic — is in fact one of the most common survival strategies of modern microorganisms. What is even more important: at least in the microbial world, the process of hibernation shows that life can be put on hold, paused, or at least marginally suspended for thousands and thousands of years without loss of viability. Thus, a typical germ can easily outperform Briar Rose, making bacteria extremely successful in surviving not only starvation and stress but also clinical drugs that we use to cure bacterial infections.

When organisms hibernate, their molecules can hibernate too. What happens inside a living cell when an organism hibernates? It was believed until recently that when a living cell encounters environmental stress, it ceases most of its metabolic activities and its key enzymes remain vacant and idle until conditions improve. However, it has become clear in recent years that organisms, from bacteria to humans, can employ a special mechanism to protect their key enzymes from degradation or unwanted activity under conditions of stress. This mechanism is based on putting their enzymes into a state akin hibernation of animals, known as molecular hibernation. Typically, molecular hibernation involve enzyme association with a specialized inhibitor/partner that occupies the active site of this enzyme.

One example of hibernating enzymes includes protasome. Proteasome is an enzyme complex that catalyzes the degradation of proteins. In some species of animals, the activity of proteasome decreases during hibernation, allowing the animal to conserve energy. This is accomplished by the binding of an inhibitor called lactacystin to the active site of proteasome, which prevents it from catalyzing the degradation of proteins.

Another example is calcineurin, an enzyme that catalyzes the dephosphorylation of target proteins in response to calcium signals. In some species of animals, the activity of calcineurin decreases during hibernation, allowing the animal to conserve energy. This is accomplished by the binding of an inhibitor called cyclosporin A, FK506, and Tacrolimus to the active site of calcineurin, which prevents it from catalyzing the dephosphorylation of target proteins.