In preparation for winter, the common shrew (Sorex araneus) shrinks its brain by 30 percent to conserve precious energy. Then, come spring, the shrew miraculously 'regrows' its shrivelled brain, neurons fully intact.
Scientists have now traced the evolutionary origins of this rare adaptation and the genes that likely enable it. While fascinating in its own right, this information may also spark new approaches to understanding and treating human brain degeneration.
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The unusual adaptation is known as Dehnel's phenomenon, named for Polish zoologist August Dehnel, who first described the shrew's remarkable brain-shrinking ability, a method of coping with the seasonal energy shortage.
Dehnel's phenomenon is rare, but shrews aren't the only practitioners: European moles (Talpa europaea), common weasels (Mustela nivalis), and stoats (Mustela erminea) also reduce their brain size with the seasons. These mammals all have fast metabolisms, and they don't hibernate, which may explain why they take such drastic measures to reduce their energy needs when food is short.
Ecologist William Thomas, from Stony Brook University in the US, led a study to map the common shrew's entire genome, comparing it to those of other mammals who also exhibit Dehnel's phenomenon to see what genetic tricks they have evolved.
The study builds on the team's previous work, which looked at seasonal changes in gene expression in two parts of the shrew's brain, and determined which sections of DNA were more active – and therefore possibly involved – during these massive bodily shifts.
Putting that data together, the researchers found genes associated with the creation of brain cells were upregulated across multiple species that exhibit Dehnel's phenomenon.
The shrew specifically had amped up expression of VEGFA, a gene associated with blood-brain barrier permeability (which may improve nutrient sensing in the brain). Its genome was also enriched with genes associated with DNA repair and longevity.
Water-regulation genes were also active, supporting theories that shrews achieve reversible loss of brain volume by losing water, not net brain cells.
The findings point to "a finely tuned system that enables common shrews to reversibly regulate brain shrinkage while avoiding the detrimental effects typically associated with neurodegeneration," the researchers write in their published paper.
Cell biologist Aurora Ruiz-Herrera from the Autonomous University of Barcelona adds: "The role of genes related to energy homeostasis and the blood-brain barrier points to possible biomarkers and therapeutic targets for neurodegenerative diseases, always with the necessary caution when extrapolating to humans."
The research was published in Molecular Biology and Evolution.