1. Axon and synaptic remodelling underlying aging and learning/memory
Learning deficits with age are thought to arise from a progressive loss of synapses and synaptic plasticity, but in vivo evidence has been lacking. We have imaged diverse types of excitatory axons and their boutons in the somatosensory cortex of aged (> 22 months) mice with impaired long-term recognition memory. We have combined two-photon microscopy with behavioural assessment and a novel, computer-based, method to rigorously track the size (i.e., strength) and location of large populations of synaptic boutons over extended periods of time, which we have extensively validated including with correlated two-photon-serial section EM. Axonal arbors and their boutons continue to remodel in the aged brain. Unexpectedly, the aged cortex shows circuit-specific increased rates of axonal bouton addition, elimination and destabilization. Compared to the young adult brain, large (i.e., strong) boutons show 10-fold higher rates of destabilization and 20-fold higher turnover in the aged cortex. Size fluctuations of persistent boutons, believed to encode long-term memories, are larger in the aged brain, while bouton size and density are not affected. Our study suggests that increased synaptic plasticity in specific cortical circuits is a novel mechanism for age-related cognitive decline. Since large boutons are thought to encode long-term memories, and are stabilized at the same rate as in young adults, their increased destabilization suggests that learning deficits in the aged brain arise because of a higher probability of forgetting rather than a failure to learn.
Increased axonal bouton dynamics in the aging mouse cortex.
Grillo, F. W., Song, S., Teles-Grilo Ruivo, L. M., Huang, L., Ge, G., Knott, G. W., Maco, B., Ferretti, V., Thompson, D., Little, G. E., De Paola, V. (2013). Proceedings of the National Academy of Sciences of the United States of America 110, E1514-E1523.
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2. Cellular mechanisms of axon re-growth and degeneration
Major efforts in the field of axon regeneration have so far concentrated on the spinal cord and white matter injury models. As a consequence, whether, how and when axons regenerate in the brain is not fully understood, leaving open basic questions regarding its endogenous repair capacity. For example, can axons regenerate in the absence of glial scar-mediated inhibition? Can axons elongate for distances sufficient to cover the lost output in the highly interconnected grey matter? Does the pattern of re-growth recapitulate developmental axon growth or does it employ a distinct growth program? And, finally, can axons reach the pre-lesion targets and form synapses? These questions are particularly important now that many experimental manipulations can successfully initiate axon regeneration in the adult injured CNS. Here we have used a combination of two-photon microscopy, femtosecond laser micro-surgery and a variety of post-hoc techniques including novel retrospective focused ion beam-electron microscopy. This enabled us, for the first time, to assess the real-time reorganization of injured axons for periods up to a year in the brain of living mice and in the absence of glial scars. We report four main findings: i. Many severed cortical axons in the grey matter can spontaneously regrow within weeks depending on cell type. ii. Axons can extend for distances larger than the amount cut (i.e. distances unseen in the adult cortex) at speeds comparable to peripheral nerves. iii. Injured axons grow monotonically, a unique pattern of growth which surprisingly differs from developmental axon growth. iv. Regenerating axons consistently form new boutons to re-establish the pre-lesion synaptic densities but never reconnect to the original targets. Our data unequivocally provide in vivo evidence for spontaneous axon regeneration in the mature brain and identify key underlying cellular and synaptic mechanisms. Moreover, we provide evidence for a rapid and lasting process of synaptic elimination, which depends on cell type and their recent history, while synapse formation rates are globally unaffected. Recent work in Caenorhabditis elegans and mouse spinal cord has highlighted the power of in vivo optical imaging to dissect the axonal response to injury (Bradke et al., 2012), and we provide the first quantitative analysis of axonal regeneration and synaptic reorganization in the cortex and cerebellum of living mice by combining longitudinal in vivo 2-photon imaging with precise laser micro-surgery.
In vivo single branch axotomy induces GAP-43-dependent sprouting and synaptic remodeling in cerebellar cortex.
Allegra-Mascaro, A. L., Cesare, P., Sacconi, L., Grasselli, G., Mandolesi, G., Maco, B., Knott, G., Huang, L, De Paola, V., Strata, P., Pavone, F. S. (2013). Proceedings of the National Academy of Sciences of the United States of America 110, 10824-10829.
In-vivo single neuron axotomy triggers axon regeneration to restore synaptic density in specific cortical circuits.
Canty, A. J., Huang, L., Jackson, J..S., Little, G. E., Knott, G., Maco, B., De Paola, V. (2013). Nature Communications 4, 2308.
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Synaptic elimination and protection after minimal injury depend on cell type and their prelesion structural dynamics in the adult cerebral cortex.
Canty, A. J., Teles-Grilo Ruivo, L., Nesarajah, C., Song, S., Jackson, J. S., Little, G. E., De Paola, V. (2013). The Journal of Neuroscience 33, 10374-10383.
Featured article (by Anthony Lewis)
Window on the brain – Researchers look into the living brain.
Lewis, A. (2013).