Long-chain polyunsaturated fatty acids (LC-PUFAs) are highly enriched in synaptic membranes, including synaptic vesicles, and we have shown that they have critical synaptic functions (Lesa et al., J. Cell Sci, 116, 4965). A picture is emerging that altered synaptic function is at the basis of many brain diseases, including anxiety, mood disorders and dementia. To understand the precise role of LC-PUFAs at synapses, we use C. elegans fat-3 mutants that lack LC-PUFAs. These LC-PUFA-deficient animals are depleted of synaptic vesicles and, as a result, they display striking behavioural phenotypes caused by insufficient release of acetylcholine and serotonin at neuromuscular junctions (their most striking phenotype is their inability to move well; see movies below). These findings suggest that LC-PUFAs are required to maintain a normal pool of synaptic vesicles.

These movies show that fat-3 mutants (left) move considerably less than wild-type animals.

We have recently found that the reduced number of synaptic vesicles at synapses and the impaired neurotransmission observed in fat-3 mutants are caused by defects in synaptic vesicle recycling: the synaptic vesicle protein synaptobrevin is not efficiently retrieved after synaptic vesicles fusion with the presynaptic membrane (Figure 1), and the presynaptic terminals contain abnormally large endosomal-like compartments and synaptic vesicles.

Moreover, the mutants have unusually low levels of the phosphoinositide phosphatase synaptojanin at synaptic release sites (Figure 2) and accumulate the main synaptojanin substrate phosphatidylinositol 4,5-bisphosphate at these sites. Since synaptojanin is essential to ensure normal synaptic vesicle endocytosis, these findings suggest that LC-PUFAs are required for efficient synaptic vesicle endocytosis by modulating synaptojanin localization at sites of release.

Synaptojanin is localized to synaptic membranes by specific localizing proteins. It is likely, therefore, that LC-PUFAs control synaptojanin localization via one or more of these proteins. We have data indicating that LC-PUFAs do not act through the major synaptojanin localizing proteins, endophilin and amphiphysin, suggesting that LC-PUFAs act via novel protein(s).

One of our main goals is to identify these proteins and to elucidate the mechanisms that underly LC-PUFA-mediated synaptic vesicle recycling.