Photo by Likeaduck (Flickr)
by Morven Caie
Underneath its cute and cuddly appearance, the arctic ground squirrel may bear a more serious scientific importance. Found mostly within the Arctic Circle, Alaska and Siberia, the frozen tundra is the home of the Arctic Ground Squirrel, where it can survive the coldest temperature of any mammal. It enters a state of torpor for eight months of the year, evading the cold winter months. Similar to hibernation, the squirrel will not eat, urinate or defecate for this period, but will arouse from torpor every 2-3 weeks for a few hours at a time, raising its core body temperature from -3℃ to around 36℃. This intermittent flow of warm blood pulsing through its body in the arousal state is enough to keep the squirrel alive, even though almost every system in its body is shut down.
Incidentally, the squirrel is subject to another natural phenomenon: supercooling. Despite lacking anti-freeze proteins, present in other organisms such as Arctic fish species, the squirrel’s blood is cleansed so thoroughly that it is completely absent of any particles that would allow ice crystals to form - enabling the blood to remain liquid in sub-zero temperatures).
The arctic ground squirrel will induce arousal by an increased respiration rate, followed by shivering in order to raise its body temperature. Notably, though, it’s the changes in the neurons of the squirrel’s brain that has sparked a fascination for researchers, particularly in the field of Alzheimer’s disease (AD). One of the pathological characteristics of AD is the accumulation of misshapen and hyperphosphorylated tau protein molecules in neurons, which causes destruction of the structural microtubules and prohibits transport of nutrients within the cell and leads to cell death. Whether the build up of the tau proteins in clumps is the cause of the disease, or a side effect, is still unknown.
When the squirrel undergoes torpor, the neurons slowly degrade and lose their neuronal synapses. Effectively, when the squirrel enters the arousal state after weeks of torpor, the brain is similar to that of a human Alzheimer patient. Intriguingly, the squirrel is able to reverse this, and a few hours after arousal the brain not only compensates for synapses lost, but actually produces an excess. A day later, they are ‘pruned’ to pre-torpor numbers, as they are unnecessary. This is similar to a developing mammalian brain. Simultaneously, the tau proteins in the squirrel’s brain appear to dephosphorylate, and after a few hours, they are cleared to a normal physiological level.
If the mechanisms behind this reversal can be determined, and the plasticity of the squirrels’ brains better understood, scientists may have an opportunity to study a brain that is capable of not only surviving extreme conditions, but also of fully recuperating and growing stronger. The clinical applications could potentially involve a therapeutic treatment for the neuronal damage associated with AD and other neurodegenerative disorders.
