Forward genetics in C. elegans reveals genetic adaptations to polyunsaturated fatty acid deficiency

Polyunsaturated fatty acids (PUFAs) are essential for mammalian health and function as membrane fluidizers and precursors for signaling lipids, though the primary essential function of PUFAs within organisms has not been established. Unlike mammals who cannot endogenously synthesize PUFAs, <i>C. elegans</i> can de novo synthesize PUFAs starting with the Δ12 desaturase FAT-2, which introduces a second double bond to monounsaturated fatty acids to generate the PUFA linoleic acid. FAT-2 desaturation is essential for <i>C. elegans</i> survival since <i>fat-2</i> null mutants are non-viable; the near-null <i>fat-2(wa17</i>) allele synthesizes only small amounts of PUFAs and produces extremely sick worms. Using fluorescence recovery after photobleaching (FRAP), we found that the <i>fat-2(wa17</i>) mutant has rigid membranes and can be efficiently rescued by dietarily providing various PUFAs, but not by fluidizing treatments or mutations. With the aim of identifying mechanisms that compensate for PUFA-deficiency, we performed a forward genetics screen to isolate novel <i>fat-2(wa17</i>) suppressors and identified four internal mutations within <i>fat-2</i> and six mutations within the HIF-1 pathway. The suppressors increase PUFA levels in <i>fat-2(wa17</i>) mutant worms and additionally suppress the activation of the <i>daf-16</i>, UPR^er and UPR^mt stress response pathways that are active in <i>fat-2(wa17</i>) worms. We hypothesize that the six HIF-1 pathway mutations, found in <i>egl-9</i>, <i>ftn-2</i>, and <i>hif-1,</i> all converge on raising Fe²⁺ levels and in this way boost desaturase activity, including that of the <i>fat-2(wa17</i>) allele. We conclude that PUFAs cannot be genetically replaced and that the only genetic mechanism that can alleviate PUFA-deficiency do so by increasing PUFA levels.