Impact of LCA-Associated E14L LRAT Mutation on Protein Stability and Retinoid Homeostasis

Abstract

Vitamin A (all-trans-retinol) is metabolized to the visual chromophore (11-cis-retinal) in the eyes and to all-trans-retinoic acid, a hormone like compound, in most tissues. A key enzyme in retinoid metabolism is lecithin:retinol acyltransferase (LRAT), which catalyzes the esterification of vitamin A. The importance of LRAT is indicated by pathogenic missense and nonsense mutations, which cause devastating blinding diseases. Retinoid-based chromophore replacement therapy has been proposed as treatment for these types of blindness based on studies in LRAT null mice. Here, we analyzed the structural and biochemical basis for retinal pathology caused by mutations in the human LRAT gene. Most LRAT missense mutations associated with retinal degeneration are localized within the catalytic domain, whereas E14L substitution is localized in an N-terminal α-helix, which has been implicated in interaction with the phospholipid bilayer. To elucidate the biochemical consequences of this mutation, we determined LRAT(E14L)’s enzymatic properties, protein stability, and impact on ocular retinoid metabolism. Bicistronic expression of LRAT(E14L) and enhanced green fluorescence protein revealed instability and accelerated proteosomal degradation of this mutant isoform. Surprisingly, instability of LRAT(E14L) did not abrogate the production of the visual chromophore in a cell-based assay. Instead, expression of LRAT(E14L) led to a rapid increase in cellular levels of retinoic acid upon retinoid supplementation. Thus, our study unveils the potential role of retinoic acid in the pathology of a degenerative retinal disease with important implications for the use of retinoid-based therapeutics in affected patients.

Retinylidene chromophore hydrolysis from mammalian visual and non-visual opsins

“…This article contains supporting information ( 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 ).…”

The molecular aspects of absorption and metabolism of carotenoids and retinoids in vertebrates

“…Located in the endoplasmic reticulum, this enzyme adopts a bitopic membrane topology with a transmembrane helix at the C-terminus and an additional amphiphilic domain present at the N-terminus (Fig. 5A) [218,219]. LRAT belongs to an ancient NlpC/P60 thiol peptidase protein superfamily that includes seven representatives encoded in the human genome: two neurological sensory proteins (NSE1-2), and five H-ras-like tumor suppressors (HRASLS1-5) [220,221].…”

LRAT coordinates the negative-feedback regulation of intestinal retinoid biosynthesis from β-carotene

“…We previously observed in cells and tissues that RA concentrations increase with impaired lecithin:retinol acyltransferase (LRAT) function (23,24). LRAT acts downstream in the pathway for retinoid biosynthesis and esterifies retinol (ROL) by transferring an acyl moiety from the sn-1 position of phosphatidylcholine (25,26).…”