The long-loop recycling (LLR) of synaptic components as a question of economics

The pre- and post-synaptic compartments contain a variety of molecules that are known to recycle between the plasma membrane and intracellular organelles. The recycling steps have been amply described in functional terms, with, for example, synaptic vesicle recycling being essential for neurotransmitter release, and postsynaptic receptor recycling being a fundamental feature of synaptic plasticity. However, synaptic protein recycling may also serve a more prosaic role, simply ensuring the repeated use of specific components, thereby minimizing the energy expenditure on the synthesis of synaptic proteins. This type of process has been recently described for components of the extracellular matrix, which undergo long-loop recycling (LLR), to and from the cell body. Here we suggest that the energy-saving recycling of synaptic components may be more widespread than is generally acknowledged, potentially playing a role in both synaptic vesicle protein usage and postsynaptic receptor metabolism.