ABCD3 as interface between ER and peroxisomes

The primary aim of this project is to elucidate the biochemical function of the poorly characterized protein ABCD3 and its role in the metabolism of human cells. ABCD3 is a transport protein located in the membrane of intracellular organelles termed peroxisomes and is believed to be responsible for the uptake of lipophilic substances into the peroxisome. Recently, it was described that dysfunctional ABCD3 leads to a human disease that is characterized by an impaired lipid metabolism and leads to death within the first years of life. ABCD3 is further believed to be involved in the pathomechanism of another disease named X-linked adrenoleukodystrophy (X-ALD). Importantly, with an incidence of 1:17,000 in newborns X-ALD represents the most common peroxisomal metabolic disease, but also one of the most abundant monogenetic neurodegenerative diseases. In the course of this project, on the one hand substrates transported by ABCD3 should be identified and on the other hand, the role of this transporter in human diseases, particularly in X-ALD, should be established. The respective studies will be conducted by the use of in vitro cultured human skin cells that were obtained from either affected patients or healthy control subjects. The proposed work will be carried out in the Department of Pathobiology of the Nervous System at the Center for Brain Research of the Medical University of Vienna in cooperation with researchers from Belgian and other Viennese groups. The major research topic of the department are inherited peroxisomal disorders, with a special emphasis on X-ALD.

The aim of this project was to elucidate the function of the previously poorly characterized protein ABCD3 and its role in the metabolism of human cells. ABCD3 is located in the membrane of intracellular organelles termed peroxisomes, and is responsible for the transport of metabolites across their outer membrane. Before the start of this project, ABCD3 had been linked to the transport of various compounds, like branched-chain fatty acids or precursors of bile acid synthesis, but the exact mechanism of this transport had not been fully established. In this work, further substrates transported by ABCD3 should be identified while a special focus was placed on pathways requiring the interaction between peroxisomes and the endoplasmic reticulum (ER). In the course of this project, we were not only able to provide evidence for the transport of several candidate compounds by ABCD3, but could also dissect differences and similarities to the metabolite transport by the related peroxisomal transport protein ABCD1. The latter is of particular relevance as mutations in the ABCD1 gene cause the most common peroxisomal metabolic disorder X-linked adrenoleukodystrophy. The most intriguing finding of our study was that ABCD3, and to a lesser extent also ABCD1, plays an essential role in the metabolism of polyunsaturated fatty acids (PUFAs). By the use of an analytical method newly established with cooperation partners, we were able to reveal the influence of ABCD3 and ABCD1 on the relative distribution of the different PUFAs in human cells. Using the same method, we were also able to show the involvement of ABCD3 and ABCD1 in the biosynthesis of ether lipids, a pathway that also requires the interaction between the ER and peroxisomes. While the project was in progress, a direct physical interaction between the ER and peroxisomes was described, being mediated by the protein ACBD5 on the peroxisomal side. Therefore, we also investigated the influence of this connection on the function of ABCD3 and found that the presence of ACBD5 indeed significantly increases metabolite transport by ABCD3. Overall, our study not only made a significant contribution to the fundamental understanding of the function of the peroxisomal transporter ABCD3, but also revealed an unexpected role of ABCD3 in the metabolism of PUFAs and the biosynthesis of ether lipids. The significance of this result reaches beyond the field of basic peroxisomal research, as polyunsaturated fatty acids as well as ether lipids play an important role in various common neurological, cardiovascular, inflammatory or malignant diseases and therefore could form the basis for new therapeutic approaches.