Identification of long-lived synaptic proteins by proteomic analysis of synaptosome protein turnover

Significance The majority of cellular proteins undergo rapid degradation and synthesis to minimize the toxic effect to cells and tissues and to guarantee normal cellular functions. It has been appreciated that proteins with longer half-lives exist in certain cells and tissues. Here we identify synaptic long-lived proteins by high-resolution mass spectrometry. In general, synaptic proteins exhibit slower turnover than cytosolic proteins, and synaptic protein turnover from mouse brain is enhanced by enriched environment exposure. Moreover, protein half-lives are dynamically regulated during changes in neuronal activity. These findings demonstrate the existence of long-lived proteins in synapses in the brain and support a potential role for them in synaptic plasticity and learning and memory. Memory formation is believed to result from changes in synapse strength and structure. While memories may persist for the lifetime of an organism, the proteins and lipids that make up synapses undergo constant turnover with lifetimes from minutes to days. The molecular basis for memory maintenance may rely on a subset of long-lived proteins (LLPs). While it is known that LLPs exist, whether such proteins are present at synapses is unknown. We performed an unbiased screen using metabolic pulse-chase labeling in vivo in mice and in vitro in cultured neurons combined with quantitative proteomics. We identified synaptic LLPs with half-lives of several months or longer. Proteins in synaptic fractions generally exhibited longer lifetimes than proteins in cytosolic fractions. Protein turnover was sensitive to pharmacological manipulations of activity in neuronal cultures or in mice exposed to an enriched environment. We show that synapses contain LLPs that may underlie stabile long-lasting changes in synaptic structure and function.