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Peroxisomal β-oxidation is indispensable for retinal integrity: insights from multifunctional protein 2 knockout mice

Boek - Dissertatie

Peroxisomes are multifunctional organelles, present in almost every eukaryotic cell. Their importance is highlighted by the life-threatening diseases caused by peroxisomal dysfunction. Patients with mutations in the central peroxisomal β-oxidation enzyme, multifunctional protein 2 (MFP2) exhibit a diverse clinical image. In the worst case, they present with general multi-organ failure and have a life expectancy of less than one year. Interestingly, retinopathy is a recurrent symptom, even in milder disease presentations. However, to date, the role of peroxisomes in the retina is highly understudied and the mechanisms underlying the retinopathy are still elusive.To gain insight into the normal physiological function of peroxisomes, we first studied the distribution of peroxisomes and proteins involved in peroxisomal lipid metabolism in the retina of wild type mice. In line with earlier observations, we showed the presence of peroxisomes in almost every retinal layer. However, a novel finding was that the peroxisomal lipid metabolizing proteins displayed a differential distribution in the neural retina and retinal pigment epithelium (RPE). This suggests that peroxisomal lipid metabolism, including peroxisomal β-oxidation, plays a specific role in the different retinal cell types.Secondly, we investigated the role of peroxisomal β-oxidation in the retina. Given its role in the metabolism of very long chain (VLCFAs) and poly-unsaturated (PUFAs) fatty acids, we hypothesized that peroxisomal β-oxidation is involved in retinal PUFA homeostasis. The retina is indeed highly endowed with the n-3 PUFA docosahexaenoic acid (DHA, C22:6n-3), which mainly resides in the photoreceptor outer segments (POS). These POS are also relatively enriched with VLC-PUFAs containing more than 30 carbon atoms. Furthermore, it has been shown in other diseases and models that a disruption of the retinal PUFA homeostasis leads to severe retinal deterioration.Hereto, we performed an in-depth characterization of the retinal phenotype of mice lacking MFP2 (Mfp2-/- mice). These mice displayed decreased retinal function and visual acuity. Moreover, we observed a dual phenotype, in which photoreceptors and RPE cells were severely affected. On one hand, MFP2 deficiency caused impaired photoreceptor development, resulting in profoundly shortened outer and inner segments with surprisingly normal ultrastructure. Together with this morphological abnormality, Mfp2-/- retinas exhibited severe changes in gene expression and lipid profile. The latter was characterized by a severe reduction in DHA-containing phospholipids. Although we cannot exclude that the local defect of peroxisomal β-oxidation could underlie this DHA decrease, we found that a hampered systemic supply and retinal traffic were important contributors. Moreover, the VLC-PUFAs showed a peculiar profile, with accumulation of those containing ≥34 carbons. The latter suggests that there is an uncontrollable elongation of retinal PUFAs. On the other hand, we also observed a degenerative component in the retinal phenotype of the Mfp2-/- mice, with a progressive loss of photoreceptors and RPE cell damage. The latter was characterized by a marked protrusion of these cells into the POS layer and loss of the hexagonal morphology. In addition, the MFP2 deficient RPE cells showed a progressive deterioration of the ultrastructure and loss of visual cycle proteins, which led to the conclusion that the RPE cells were undergoing some kind of dedifferentiation process.In conclusion, we established that normal peroxisomal β-oxidation is indispensable for retinal integrity. Moreover, this research laid the foundations for future mechanistic studies, using the retinal phenotype of the MFP2 deficient mice as a starting point. It will be essential to unravel how the observed distorted PUFA profile influences the retinal phenotype and which cells drive the onset of the pathology. Elucidating the underlying mechanisms of the retinopathy in peroxisomal disease patients will hopefully lead to the development of therapies for these blinding diseases.
Jaar van publicatie:2020