A Mouse Model of Stargardt Disease-3 Exhibits Lipid Deficiency but Not an Unfolded Protein/Stress Response

Abstract

Purpose:: A 5-bp deletion in the human elongase of very long chain fatty acids-4 (ELOVL4) gene is a genetic basis of hereditary dominant Stargardt disease-3 (STGD3). Recently, we generated Stgd3 gene-knockin mice that carry the corresponding mutation in the mouse Elovl4 gene. The goal of the current studies was to determine whether this mutation causes an unfolded protein/stress response and/or deficiency of Elovl4 lipid products in Stgd3 mice.

Methods:: Because of neonatal lethality of Stgd3/Stgd3 mice, only the fully developed retinas of adult Stgd3/wt and wt/wt mice were analyzed for the presence of cellular stress markers: increased levels of BIP protein and of the mRNA for a spliced, short form of the XBP1 transcription factor. The BIP protein levels were measured by Western immunoblotting and XBP1 mRNAs by RT-PCR. The lipid content of neonatal Stgd3/Stgd3, Stgd3/wt and wt/wt mouse skin was analyzed using thin layer chromatography and high performance liquid chromatography combined with mass spectrometry. At birth, mouse skin, but not the retina, expresses high levels of Elovl4 mRNA.

Results:: Stgd3/wt mouse retinas showed no increase in levels of either BIP protein or the spliced, short form of XBP1 mRNA compared to the levels in the wt/wt retinas. Lipid analysis of neonatal skin detected, however, a complete absence of acylceramide lipids in Stgd3/Stgd3 mice and a reduction in their levels by a half in Stgd3/wt mice compared to the wt/wt mice.

Conclusions:: Studies of our mouse model of human Stargardt disease-3 did not detect an unfolded protein/stress response in the retinas of Stgd3/wt mice. However, lipid analysis revealed a deficiency of acylceramides in the skin of mice carrying the mutated Elovl4 gene. This deficiency shows a requirement for Elovl4 enzymatic activity during acylceramide synthesis and suggests that the C26-C40 fatty acids, components of acylceramides, are the most likely products of Elovl4. We conclude, therefore, that deficiency of ELOVL4 lipid products, rather than an unfolded protein/stress response, plays a role in pathogenesis of human Stargardt disease-3.

Keywords: transgenics/knock-outs • retinal degenerations: hereditary • lipids