Longevity Briefs: Mouse Brain Activity Preserved After Freezing

Longevity briefs provides a short summary of novel research in biology, medicine, or biotechnology that caught the attention of our researchers in Oxford, due to its potential to improve our health, wellbeing, and longevity.

The problem:

Cryopreservation is the process of cooling biological tissues to extremely low temperatures such that all chemical reactions are paused, perfectly preserving the tissue indefinitely. This technology (if improved sufficiently) would have immediate applications for organ transplantation. Since organs currently cannot be stored for long, those in need of organ transplants must suffer long waits for an organ to become available. If donor organs could simply be harvested and stored until somebody needed them, these waiting lists would be cut dramatically. A more futuristic application of cryopreservation, of course, is the preservation of corpses or brains in the hope that future technology will one day be capable of reviving them.

The major problem with cryopreserving biological tissue is that the water contained within forms ice crystals that cause severe damage at the cellular level. If the tissue is cooled extremely rapidly (to around -200°C in a matter of seconds), then water molecules will not have enough time to arrange into crystals, resulting in vitrification – instead of becoming ice, water solidifies in a manner more akin to glass. This works for single cells like gametes and very small amounts of tissue, but isn’t possible for organs or larger blocks of tissue, as these require more cooling time). Instead, organs must be injected with chemicals called cryoprotectants, which prevent ice crystals from forming, but are themselves toxic.

Some scientists believe these problems to be practically insurmountable and have also ridiculed the idea of trying to preserve dead humans. However, progress towards better cryopreservation methods is being made at a steady pace, and every few years the possibility of reversible cryopreservation looks a little more credible (though the rather significant challenge of actually resurrecting a dead person remains!) In this study, researchers show that vital electrical functions in mouse brains are preserved after vitrification and rewarming.

The discovery:

Researchers used a proprietary vitrification protocol to preserve slices of mouse hippocampus (a region of the brain important for learning and memory). These were very thin slices – around 350 micrometres thick, roughly three times the thickness of a sheet of paper. The slices were loaded with cryoprotectant solution and then cooled rapidly by a liquid nitrogen-cooled cylinder. They were stored at around -150°C for up to 7 days and then rapidly rewarmed.

Researchers found that after cryopreservation, the slices were still ‘alive’ – hippocampal cells were still consuming oxygen, admittedly at a 22% reduced rate compared to pre-vitrification, which was deemed to be due to the toxicity of the cryoprotectant. This indicated that the mitochondria (the ‘power plants’ of the cell that extract energy from nutrients) were still functional. Perhaps more remarkably, researchers found that electrochemical functions like synaptic transmission (the transmission of electrical signals from one neuron to the next at a synaptic junction) and long term potentiation (the ability of synaptic transmission to strengthen in response to repeated stimulation) were still intact.

Researchers also repeated this experiment with whole mouse brains within skulls, a significantly more complex task in that cryoprotectants had to get into the brain via the circulatory system. This ultimately resulted in the ‘death’ of the majority of the brains. However, one in three brains showed preserved metabolic and electrical functions similar to the slices, though researchers only examined a specific subset of hippocampal cells.

The implications:

This research is significant as it demonstrates the preservation of brain functions after vitrification that had previously not been shown. The most exciting finding is the preservation of long-term potentiation, which is essential for learning and memory. This suggests that as long as the physical structure of the brain can be preserved, its electrical functions (and therefore the cognitive function of the organism) will return even after they have been ‘paused’ via vitrification.

While these findings are exciting, it is important to realise that the vitrification protocol used here would not work for anything larger than a mouse brain. Furthermore, some electrical function being preserved in a specific brain region does not necessarily mean that the brain would be able to function normally in a living mouse. For the moment, this remains a proof-of-concept study, but it does make the very distant ambition of full brain preservation look a lot more credible.