The Human and Mouse SLC25A29 Mitochondrial Transporters Rescue the Deficient Ornithine Metabolism in Fibroblasts of Patients With the Hyperornithinemia-Hyperammonemia-Homocitrullinuria (HHH) Syndrome

Abstract

The hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome is a disorder of the urea cycle (UCD) and ornithine degradation pathway caused by mutations in the mitochondrial ornithine transporter (ORNT1). Unlike other UCDs, HHH syndrome is characterized by a less severe and variable phenotype that we believe may, in part, be due to genes with redundant function to ORNT1, such as the previously characterized ORNT2 gene. We reasoned that SLC25A29, a member of the same subfamily of mitochondrial carrier proteins as ORNT1 and ORNT2, might also have overlapping function with ORNT1. Here, we report that both the human and mouse SLC25A29, previously identified as mitochondrial carnitine/acyl-carnitine transporter-like, when overexpressed transiently also rescues the impaired ornithine transport in cultured HHH fibroblasts. Moreover, we observed that, in the mouse, the Slc25a29 message is more significantly expressed in the CNS and cultured astrocytes when compared with the liver and kidney. These results suggest a potential physiologic role for the SLC25A29 transporter in the oxidation of fatty acids, ornithine degradation pathway, and possibly the urea cycle. Our results show that SLC25A29 is the third human mitochondrial ornithine transporter, designated as ORNT3, which may contribute to the milder and variable phenotype seen in patients with HHH syndrome.

The Human Gene SLC25A29, of Solute Carrier Family 25, Encodes a Mitochondrial Transporter of Basic Amino Acids

“…1-3 and 5 for reviews). The transport properties and kinetic characteristics of recombinant SLC25A29, together with its localization to mitochondria reported before (7,8), demonstrate that this protein is a mitochondrial transporter of basic amino acids with a preference for arginine and lysine. In contrast to the great majority of mitochondrial carriers, which are obligatory exchangers (31-37), SLC25A29 catalyzes a substantial uniport besides exchange of substrates.…”

“…Therefore, SLC25A29 is not the human ornithine transporter isoform 3 (ORNT3) (8). In this respect, it is worth mentioning that the percentage of identical amino acids between SLC25A29 and the two human isoforms of the ornithine carrier (31%) is not much higher than the basic homology existing between the different members of the mitochondrial carrier family.…”

“…The importance of mitochondrial carriers is demonstrated by their wide distribution in all eukaryotes, their role in numerous metabolic pathways and cell functions, and the identification of several diseases caused by alterations of their genes (1)(2)(3)(4)(5)(6). SLC25A29 is localized to mitochondria (7,8), expressed in several tissues, including heart, brain, liver, and kidney (7,8), and induced after partial hepatectomy or fasting (7). In 2003, Sekoguchi et al (7) reported that murine Slc25a29 functions as carnitine/acylcarnitine transporter-like (CACL) because a low palmitoylcarnitine transport activity (pmol/30 min) was found in Escherichia coli and in mitochondria from NIH3T3 cells both expressing SLC25A29.…”

“…In 2003, Sekoguchi et al (7) reported that murine Slc25a29 functions as carnitine/acylcarnitine transporter-like (CACL) because a low palmitoylcarnitine transport activity (pmol/30 min) was found in Escherichia coli and in mitochondria from NIH3T3 cells both expressing SLC25A29. Later, Camacho and RiosecoCamacho (8) concluded that the human and mouse SLC25A29 proteins transport ornithine into mitochondria and designated them ornithine transporter isoform 3 (ORNT3) because their overexpression rescues the defect in radioactive ornithine incorporation into cellular protein in cultured fibroblasts from patients with hyperornithinemiahyperammonemia-homocitrullinuria (HHH) 2 syndrome. As the carnitine/acylcarnitine and ornithine carriers of multiple organisms are distinct subfamilies of mitochondrial carriers, which are characterized by different substrates and different structural hallmarks (5), we decided to perform a more indepth study on the activity of SLC25A29.…”

Heterologous Expression in Yeast of Human Ornithine Carriers ORNT1 and ORNT2 and of ORNT1 Alleles Implicated in HHH Syndrome in Humans

“…Nevertheless, the cellular role(s) of the human ORNT2 protein is not precisely established yet. Another gene of the same family, SLC25A29, encodes a mitochondrial transporter that was also proposed to vicariate the function of ORNT1 and was named ORNT3 (Camacho and Rioseco-Camacho 2009). However, this protein presents less than 50% of sequence similarity and a recent work showed that SLC25A29 transports preferentially lysine and arginine and to a much lesser extent histidine and ornithine when expressed in phospholipid vesicles (Porcelli et al 2014).…”