Recent studies suggest that central nervous system (CNS) synapses persist
for many weeks, months and even lifetimes, yet little is
known on the mechanisms that allow these structures to persist for so
long despite the many deconstructive processes acting at biological
systems and neurons in particular. As a step toward a better
understanding of synaptic maintenance we set out to examine some of the
deconstructive and reconstructive forces acting at individual CNS
synapses. To that end we studied the molecular dynamics of several
presynaptic and postsynaptic cytomatrix molecules. Fluorescence
recovery after photobleaching (FRAP) and photoactivation experiments
revealed that these molecules are continuously incorporated into and lost
from individual synaptic structures within tens of minutes.
Moreover, these dynamics can be accelerated by synaptic activity.
Finally, we find that synaptic molecules are continuously exchanged
between nearby synaptic structures at similar rates and that these rates
greatly exceed the rates at which synapses are replenished with molecules
arriving from somatic sources. Our findings indicate that the dynamics of
key synaptic matrix molecules may be dominated by local protein exchange
and redistribution, whereas protein synthesis and degradation serve to
maintain and regulate the sizes of local, shared pools of these proteins.
The nature of these dynamics raises intriguing questions as to how
synapses manage to maintain their
individual, use-dependent structural and functional characteristics over
long durations.
