July 11, 2012 | 2
The honeybee brain is dynamic and full of surprises. For instance, much like the human brain, its neurons not only modulate their activity in response to sensory stimuli but also alter their gene and protein expression patterns—changes that in bees are so dramatic as to essentially rewire the brain. And even more remarkable is that this plasticity is strongly influenced by social environment, a feature that was underscored recently by the discovery that bees who changed social roles effectively reversed the aging of their brains.
The reversal, described in terms of recovery of learning ability, occurred when older honeybees reverted from foraging tasks to caring for newborn bees and was linked to increased brain levels of stress response and antioxidant proteins, which serve important cellular maintenance and repair functions. One of the proteins was similar to the mammalian enzyme peroxiredoxin-6 (Prx6). In humans, Prx6 defends against oxidative stress and inflammation associated with Alzheimer disease and Huntington disease, indicating that a better understanding of the molecules involved in brain plasticity and cognitive recovery in honeybees could inform research on dementia and related conditions.
The new findings are especially intriguing for what they suggest about the influence of social environment on cognitive function. Studies in humans have linked strong social relationships with increased likelihood for survival and declining social engagement in mid- to late-life with increasing risk of dementia. However, relatively little is known about the significance of social environment in the context of human cognitive function and aging.
Honeybees are animals with complex social environments, the organization of which—a hierarchy of three distinct castes, including a queen, workers (sexually immature females), and drones (males)—has been likened to socialist governments. The parallels in this regard have drawn much interest. But as recent studies are beginning to reveal, even more striking than the honeybee societal structure are its neurobiological underpinnings, which are strangely human. Or, perhaps, human neurobiology is disquietingly insect-like.
While the honeybee queen lives sequestered away within her hive, where she exerts her dominance by chemical means, workers are sent off to perform the labor that supports colony survival. The division of labor is by life stage (a phenomenon known as temporal polyethism, which is also found in other types of eusocial insects, including the European paper wasp and Pheidole ants). Young nurse bees, about one to two weeks old, perform activities that take place within the nest, such as caring for the queen, nursing (feeding and raising larvae), cleaning, and comb building. Older bees, on the other hand, perform activities outside the nest, namely foraging, in which they find and collect nectar to bring back to the colony.
Forager honeybees undergo rapid physiological senescence and thereby age quickly. Within several weeks of embarking on their first foraging expedition, their wings show signs of wear from extended flight, and long-duration foragers experience modifications in brain protein expression that are consistent with the aging process as it has been observed in other species, particularly Drosophila fruit flies.
But interestingly, cognitive decline is not a function of chronological age in honeybees. Rather, it is related to social role. Compared with nurse bees of the same chronological age, foragers show significant impairments in tactile and olfactory learning after just two weeks of activity outside the nest. And as the new research reveals, foragers that revert to nursing essentially turn back the clock on brain aging. This is intriguing not only for its implications concerning the role of plasticity in undoing the brain aging process but also because it suggests that physical factors, such as wing wear, are the major constraints on a honeybee’s life span.
The transition from foraging to nursing is not an artificial circumstance in honeybee societies. In nature, workers can shift between the different social roles, depending on colony needs. If there are many foragers, for instance, some may revert to nursing and again perform tasks within the nest. As they transition from one role to another, their brains change—the mushroom body (a center for olfactory processing) shrinks or expands, the brain proteome transforms, and even the microRNA transcriptome morphs. The complexity is astounding, made more so by the suggestion that all these modifications essentially function to slow or accelerate the rate at which the brain ages.
Whether brain aging in humans can be reversed through changes in social behavior is unknown and, perhaps, unlikely. Aging is unkind to the human brain. It causes the brain to shrink and its grooved landscape to change, and it litters the once nicely sulcused surface with plaques and tangles of degenerating neurons. The formation of those plaques and tangles is associated with the deterioration of brain function and dementia.
Finding ways to prevent or slow the onset of dementia is of urgent importance, considering that the condition currently is experiencing an unprecedented rate of growth globally, owing largely to increased life expectancy in countries spanning the spectrum of economic income. So, even though, in the words of A.A. Milne, “You never can tell with bees,” they may turn out to be valuable contributors in the effort to stem the advance of dementia.