Exercise for your cerebellum
Sara El Jaouhari
Supervisor: Dr. Alanna Watt
Reaching for your alarm clock in the morning, handling a fork, and walking are all examples of everyday activities you could not perform without the small but powerful part of the brain called the cerebellum. This structure in the brain is involved in motor coordination, learning and timing and is vital for our basic motor functions. As we grow, our cerebellum matures and these motor skills develop and fine-tune themselves in a parallel manner.1,2
Recent advances in neuroscience research have shown that in some regions of the brain, particularly those involved in visual and auditory processing, sensory-driven network activity can coordinate development.3,4 This, however, has not been well characterized in motor brain regions such as the cerebellum. Can externally-driven motor activity shape cerebellar development? This was the question I had an opportunity to explore this past year.
We investigated the effects of exercise on Purkinje cell morphology in the cerebellum, specifically by looking for changes in dendritic spines. Dendritic spines, the sites that receive excitatory input in Purkinje cells, can be modified by synaptic activity through changes in their distribution, density and shape.5,6 These morphological changes reflect changes in the organization of the circuit in the cerebellum.7 The plasticity of these spines is believed to be important for the maturation of the cerebellum and the development of motor control, and hence may have some important implications for cerebellar dysfunctions such as ataxia.6,8
Externally-driven motor activity, such as limb movements or running on a treadmill, has been shown to influence activity in the cerebellum. Previous studies have also shown increases in spine density with exercise.9 Hence, we hypothesized that with exercise, we would see an increase in spine density and/or differences in the proportions of certain spine classes. There are several classes of dendritic spines such as Thin, Mushroom, Stubby, and Branched spine types. Thin and mushroom spine types are associated with greater synaptic strength, signifying higher efficiency in transmission of synaptic information than the other spine types.7,10
An increase in spine density would correlate with greater synaptic strength which may serve as a protective function in neurodegenerative diseases. Indeed, one study revealed that with exercise, mice with spinocerebellar ataxia showed significant improvements in overall survival rate.11 It seemed that exercise had beneficial effects in this case, and could possibly have important implications for other neurodegenerative diseases.
In order to modulate cerebellar activity, experimental mice were given access to a running wheel on which they ran extensively without any invasive procedures. Our control mice were deprived of a running wheel and therefore did not exercise. With this activity manipulation, we were able to examine the effect of exercise on the structure of Purkinje cells in the cerebellum. Slices from the third lobule of the cerebellum were stained using immunohistochemistry, and imaged on a confocal microscope. Through the aid of Neurolucida software, we were able to mark and classify dendritic spines on Purkinje cells using images from both conditions (Figure 1). This state-of-the-art software allowed us to compare spine properties such as density and proportions of different spine classes between exercise and control mice, in order to characterize how they are altered by an externally-driven network.
Figure 1 Screenshot image from Neurolucida software showing completed analysis of a sample of dendrites with marked spines. Different colors represent the different classes of spines (i.e. yellow=thin spine, blue=mushroom spine, pink=stubby spine, green= filopodia spine).
Surprisingly, we found no significant differences in spine density or in spine class proportions with exercise, which was not consistent with previous studies that had shown an increase in spine density with exercise.9 We believed this was due to the particular mouse strain we used, C57Bl/6. We initially used this strain because it is the classical transgenic mouse model used in the field of neuroscience research. However, in a recent study where 12 different mouse strains were compared, this strain was the only one that did not display activity-dependent changes in the hippocampus.12 While this prior study did not look at the cerebellum, by extension, it may explain why we did not see any activity-dependent changes in our study. Perhaps the C57Bl/6 strain displays less exercise-dependent brain plasticity overall than is typically seen in most mice. While we did not see activity-dependent changes in the C57Bl/6 strain, our sample sizes were relatively low, which may have also contributed to our lack of significant results.
This past summer, we have continued this project in a different mouse strain, B6D2fl. Previous studies have demonstrated that this strain shows robust activity-dependent changes in the hippocampus which may make it a more typical mouse strain than our initial choice, and therefore may influence its cerebellar plasticity as well.12 Indeed, we found significant differences in the thickness of the molecular layer in certain lobules of the cerebellum between the exercise and control mice. These exciting results not only indicate exercise-dependent plasticity in localized regions in the cerebellum of this new strain, but also reinforce our belief that our initial mouse strain C57Bl/6 exhibits less activity-dependent plasticity in the cerebellum than most mice. The next step in our project is to investigate whether these localized regions in the cerebellum also display activity-dependent changes in spine morphology in Purkinje cells, with the hopes of clarifying our previous findings.
Nevertheless, this study provides valuable insight into how activity influences the organization of the cerebellar circuit and drives cerebellar development. Of course, the ultimate goal of studying the factors that affect cerebellar development is to apply these findings to understanding the human brain and the processes that influence its healthy development. With this mind, studies on the effects of exercise on the cerebellum may one day lead to new avenues of therapeutic intervention in developmental brain diseases.
References
1. Horne, M.K. and E.G. Butler, The role of the cerebello-thalamo-cortical pathway in skilled movement. Prog Neurobiol, 1995. 46(2-3): p. 199-213.
2. Thach, W.T., H.P. Goodkin, and J.G. Keating, The cerebellum and the adaptive coordination of movement. Annu Rev Neurosci, 1992. 15: p. 403-42.
3. Greenough, W.T. and F.R. Volkmar, Pattern of dendritic branching in occipital cortex of rats reared in complex environments. Exp Neurol, 1973. 40(2): p. 491-504.
4. Carmignoto, G. and S. Vicini, Activity-dependent decrease in NMDA receptor responses during development of the visual cortex. Science, 1992. 258(5084): p. 1007-11.
5. Yuste, R. and T. Bonhoeffer, Morphological changes in dendritic spines associated with long-term synaptic plasticity. Annu Rev Neurosci, 2001. 24: p. 1071-89.
6. Matus, A., Actin-based plasticity in dendritic spines. Science, 2000. 290(5492): p. 754-8
7. Koch, C., A. Zador, and T.H. Brown, Dendritic spines: convergence of theory and experiment. Science, 1992. 256(5059): p. 973-4
8. Grutzendler, J., N. Kasthuri, and W.B. Gan, Long-term dendritic spine stability in the adult cortex. Nature, 2002. 420(6917): p. 812-6.
9. Pysh, J.J. and G.M. Weiss, Exercise during development induces an increase in Purkinje cell dendritic tree size. Science, 1979. 206(4415): p. 230-2
10. Koch, C. and A. Zador, The function of dendritic spines: devices subserving biochemical rather than electrical compartmentalization. J Neurosci, 1993. 13(2): p. 413-22.
11. Fryer, J.D., et al., Exercise and genetic rescue of SCA1 via the transcriptional repressor Capicua. Science, 2011. 334(6056): p. 690-3.
12. Clark, P.J., et al., Genetic influences on exercise-induced adult hippocampal neurogenesis across 12 divergent mouse strains. Genes Brain Behav, 2011. 10(3): p. 345-53.