Researchers at the Medical Research Council (MRC) Toxicology Unit have identified a protective mechanism that activates when body temperature is lowered, initiating a process that prevents the loss of brain cells and the connections between them.
The MRC team discovered that this protective process may be defective in neurodegenerative diseases such as Alzheimer’s, contributing to the death of brain cells in these disorders. By simulating the effects of cooling in mice, the scientists have revealed a possible new target for drugs that could protect against neurodegeneration.
It has long been known that during hibernation, where a mammal’s core temperature cools to well below normal body temperature, synapses are depleted. This allows the animal to enter a state of torpor, allowing the animal to survive without nutrition for weeks or months. As the animal comes out of hibernation and warms up, connections between brain cells are reformed and the number of synapses once again rises, restoring normal brain activity.
In humans, hypothermia is known to protect the brain. For example, people have survived hours after a cardiac arrest with no brain damage after falling into icy water. Artificially cooling the brains of babies that have suffered a loss of oxygen at birth is also used to protect against brain damage.
Cooling and hibernation lead to the production of a number of different proteins in the brain known as cold-shock proteins. One of these, RBM3, has been associated with preventing brain cell death, but it has been unclear how it affects synapse degeneration and regeneration. Knowing how these proteins activate synapse regeneration might help scientists find a way of preventing synapse loss, without the need for actual cooling.
In this study, researchers reduced the body temperature of healthy mice to 16-18 degrees Celsius – similar to the temperature of a hibernating small mammal – for 45 minutes. They found that the synapses in the brains of these mice, which do not naturally hibernate, also dismantled on cooling and regenerated on re-warming.
The team then repeated the cooling in mice bred to reproduce features of neurodegenerative diseases (Alzheimer’s and prion disease) and found that the capacity for synapse regeneration disappeared as the disease progressed, accompanied by a disappearance of RBM3 levels.
When the scientists artificially boosted levels of the RBM3 protein they found that this alone was sufficient to protect the Alzheimer and prion mice, preventing synapse and brain cell depletion, reducing memory loss and extending lifespan.
The researchers were therefore able to conclude that RBM3 – and perhaps other cold-shock proteins – affects the ability of neurons to regenerate synapses in neurodegenerative diseases, which is essential to prevent synapse loss during disease progression. The pathway could be a useful target for drugs so that brain cells could be preserved without the need for cooling.
Giovanna Mallucci, who led the research team, said: “We have known for some time that cooling can slow down or even prevent damage to brain cells, but reducing body temperature is rarely feasible in practice: it is unpleasant and involves risks such as pneumonia and blood clots. But, by identifying how cooling activates a process that prevents the loss of brain cells, we can now work towards finding a means to develop drugs that might mimic the protective effects of cold on the brain.”
Hugh Perry, chairman of the MRC’s Neurosciences and Mental Health Board, which funded the research, said: “The neuroprotective pathway identified in this study could be an important step forward. We now need to find something to reproduce the effect of brain cooling. Just as anti-inflammatory drugs are preferable to cold baths in bringing down a high temperature, we need to find drugs which can induce the effects of hibernation and hypothermia.”