Scientists have long known that humans and other animals have a master biological clock situated in a deep brain region called the hypothalamus. In this master clock, cells manufacture timekeeper molecules whose levels oscillate to maintain the 24 hour cycles that define our lives. Yet these same timekeeper molecules are made throughout the brain and body, suggesting undiscovered functions for circadian genes outside the hypothalamus.
New research from investigators at Harvard University reveals that circadian genes play a vital role in mammalian brain development and plasticity— that is, the ability of the brain to learn from, and physically change in response to, environmental experiences. Mice deficient for the core circadian gene ‘Clock’ had significantly delayed and prolonged plasticity in the visual system, as reported online March 19 in Neuron by the lab of Takao K. Hensch, a professor in the Departments of Molecular and Cellular Biology and Neurology at Harvard University and Boston Children’s Hospital, as well as the Center for Brain Science at Harvard. “We found a cell-intrinsic Clock may control the normal trajectory of brain development,” Hensch says.
Visual deprivation through one eye during a critical window of plasticity early in life usually biases vision in favor of the un-deprived eye, but has no effect in adult mice. However, in the Clock-deficient mice, this scenario was reversed, such that visual deprivation through one eye had no effect in juvenile mice but a clear effect in adult mice.
While oscillations in the activity of circadian rhythm genes appear before birth in the hypothalamus, in the cerebral cortex—which develops on a later timescale—these oscillations don’t occur until after birth, explains Yohei Kobayashi, a postdoctoral fellow in the Hensch lab who was the lead author on the study. Looking in the visual cortex specifically, Kobayashi noted that the oscillation of Clock-controlled circadian rhythm genes there is not established until after young mice have opened their eyes. The rhythmicity emerges gradually with age after this developmental milestone—its onset coinciding with the refinement of neural connections in the visual cortex by visual experience. This observation led to the hypothesis that circadian rhythm genes might be involved in timing the developmental windows of plasticity known as critical periods, when environmental experience best ‘gets into’ the brain and tunes neural circuits.
Interestingly, the abnormal temporal profile of visual plasticity in Clock-deficient mice could be avoided when the mice were given diazepam, a drug known to boost the activity of inhibitory neurons which communicate via the neurotransmitter GABA. This finding led the team to ask whether the maturation of specific subsets of neurons in the visual cortex might be disrupted in the mutant mice. After looking at marker genes for a broad range of neural types, Kobayashi found that a subtype of GABA neurons called parvalbumin-cells (PV-cells) was most notably impacted by the loss of Clock. A variety of morphological and physiological tests suggested that PV-cell maturation was significantly delayed in the Clock-deficient mice.
Given that the Hensch group and others have previously demonstrated that PV-cells are key players in orchestrating the onset of critical period of plasticity, these findings strongly suggested that the delay in visual plasticity in Clock-deficient mice is due to the delay in PV circuit maturation. To test this idea, the researchers designed conditional mutant mice, where Clock (or an associated circadian rhythm protein called Bmal1) was deleted only from PV-cells. These mice, just like the systemic Clock-deficient mice, displayed aberrant visual plasticity and PV-circuit maturation—directly implicating the Clock machinery of PV-cells in these processes.
This new research might unite neurobiologists studying circadian rhythms with those studying developmental brain plasticity, Kobayashi notes, and invite inquiries into the role of circadian rhythm genes in other parts of the cerebral cortex, including regions which control cognition and social behaviors. The findings also hold promise for better elucidating the pathology in brain disorders such as autism and schizophrenia. A number of genes associated with mental illness were expressed differently in the PV-cells of control versus Clock-deficient mice. Further, the Hensch group and others have long proposed that neurodevelopmental disorders are associated with timing defects in critical periods of plasticity. Factors that influence circadian rhythms such as sleep deprivation, seasonal changes limiting sunlight exposure, night shift work, etc. have also been suggested to increase the risk of mood disorders. So by implicating circadian rhythm genes in the control of developmental brain plasticity, the new study may help bridge these two sets of hypotheses, indicating translational or clinical value in further exploring the Clock machinery of PV-cells.
Hensch, who also directs the NIMH Silvio Conte Center for Mental Health research at Harvard and is a senior fellow of the Canadian Institute for Advanced Research, says “such breakthroughs in basic neuroscience are needed to drive deeper insight into the etiology of mental illness and novel strategies for correcting them – ideally before they arise.”
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