Exosomes are vesicles of endocytic origin released by cells into their environment on fusion of multivesicular endosomes with the plasma membrane. They represent a novel mechanism of cell communication by intercellular transfer of proteins, lipids and RNAs. In our laboratory, we are interested in the roles neuronal exosomes could play in the central nervous system. We first showed that mature neurons secrete exosomes and that this is regulated by synaptic glutamatergic activity and by Ca2+ influx. We next demonstrated that the C-terminal part of the tetanus toxin heavy chain can be released in association with neuronal exosomes which can then be taken up by other neurons. Moreover, such a cargo seems to influence the actual fate of the exosome. In order to further examine exosome reuptake, we used exosomes from N2a cells expressing the tetraspanin CD63 fused to the green fluorescent protein, GFP. By incubating cultured hippocampal neurons with GFP-CD63 exosomes, we succeeded in proving that they were found inside the recipient neurons. However, although exosomes are internalized, our results suggest that their traffic is independent of the classical endosomal pathway. We also studied the RNAs contained in the N2a and neuronal exosomes. These were mainly short RNAs (≤ 200 nucleotides) including microRNAs 132 and 138. MicroRNAs are key regulators of gene expression. Their transfer by exosomes could represent a new way for fine regulation with a potentially powerful impact on the nervous system. Neuronal exosomes could play a crucial role in the normal physiology of synapses, by allowing the exchange of RNAs and neurotransmitter receptors between neurons. They could also propagate pathogenic proteins such as tetanus toxin and be involved in neurodegenerative disorders such as Alzheimer's and Creutzfeldt-Jacob's diseases. Altogether, our results pave the way towards the demonstration that exosomes play an important part in the functioning of the central nervous system via their involvement in physiological and pathological processes.
