Loss of ER retention and sequestration of the wild-type ELOVL4 by Stargardt disease dominant negative mutants

PURPOSE: Mutations in ELOVL4, a member of the fatty acid elongase (ELO) family, are responsible for autosomal dominant Stargardt-like macular degeneration. The specific role of ELOVL4 in photoreceptors and the degenerative events induced by dominant ELOVL4 mutations are not well understood. As a first step to identifying possible mechanisms contributing to cellular dysfunction, we transfected HEK293 and COS cells with fluorescent-labeled wild-type and mutant ELOVL4 constructs. Effects of mutant ELOVL4 on interaction with wild-type protein were examined in this in vitro model. METHODS: Wild-type and mutant ELOVL4 proteins including ELOVL4 truncation (270X, a truncated ELOVL4 protein at amino acid position 270) and ELOVL4 5 bp deletion (5bp-del) and ELOVL4 (5A, substituting the ER retention signal, KAKGD, with a five alanine amino acid tract) were expressed as EGFP or DsRed fusion proteins. Cellular localization of these proteins was examined by fluorescence microscopy. ELOVL4 protein aggregates were measured by co-immunoprecipitation and by sucrose gradient centrifugation followed by immunodetection with western blots. To study cellular status of cells expressing mutant ELOVL4 proteins, transfected cells were examined for upregulation of Bip and CHOP, markers for the unfolded protein response (UPR) by western blotting. RESULTS: ELOVL4 mutants were not retained within the ER but were rather mislocalized and formed aggregates. Importantly, when cotransfected with wild-type ELOVL4, the mutants bound to and sequestered the wild-type protein into the aggregates. Expression of ELOVL4 mutants also induced UPR as evidenced by Bip and CHOP expression. CONCLUSIONS: Using this in vitro cell system, we have identified alterations in wild-type ELOVL4 protein localization, aggregate formation, and the induction of cellular stress by the ELOVL4 mutants. We propose that "inactivation" of the wild-type ELOVL4 protein through sequestration to a non-ER compartment by ELOVL4 mutants may play a role in cellular dysfunction.