Long-chain polyunsaturated fatty acids (LC-PUFA) are enriched in many neurones and are thought to be required for neuronal development and function. Indeed, alteration of LC-PUFA metabolism causes specific forms of at least two human diseases: mental retardation and retinal macular dystrophy. Furthermore, correlative evidence has linked LC-PUFA dietary intake with normal neonatal brain development. However, the precise role of LC-PUFA in the nervous system remains unclear since neither animals nor cultured neurones devoid of LC-PUFA have been analysed. To understand the role of LC-PUFA we generated a C. elegans strain lacking these fatty acids. We isolated two deletions of the gene
fat-3, which encodes a D6-desaturase, an enzyme essential for LC-PUFA biosynthesis.
fat-3 mutant animals display specific behavioural defects associated with neuronal impairment. In particular, they are defective in reversing direction and show an egglaying phenotype reminiscent of that caused by mutations disrupting the Hermaphrodite Specific Neurones. Using a highly sensitive and quantitative method, we demonstrated that
fat-3 mutant animals are defective in LC-PUFA production. In addition, we showed that the behavioural defects of
fat-3(-) animals are caused by reduced LC-PUFA levels. Expression of
fat-3(+) under the control of the
unc-119 promoter, which drives expression specifically in the entire nervous system, was sufficient to rescue the motility defects caused by loss of
fat-3 activity. Conversely, expression of the
fat-3 coding sequence under the control of the muscle-specific
myo-3 promoter, did not rescue the behavioural defects associated with the
fat-3 mutation. Therefore, LC-PUFA are likely to be required in neurones for normal nervous system function. Deficiency of LC- PUFA caused functional rather than morphological defects in neurones since exogenous application of LC-PUFA rescued the behavioural deficits associated with adult
fat-3(-) animals. In addition, we could not detect any morphological defect in neurones of
fat-3 mutant animals. This suggested that the behavioural defects seen in
fat-3(-) animals derive from reduced neuronal functionality. To test this possibility, we measured the transmission efficiency of serotonergic neuromuscular junctions (NMJs), involved in egg-laying, as well as cholinergic NMJs, involved in body wall muscle contraction and movement. We found that both NMJs release neurotransmitter inefficiently in
fat-3 mutant animals. Since unsaturated fatty acids stimulate membrane fusion in vitro and a specific LC-PUFA induces endocytosis in an in vitro assay, it is likely that LC-PUFA affect neurotransmitter availability at the NMJ by interfering with synaptic vesicle fusion and/or endocytosis.