SPOTLIGHT Saccharopine, a lysine degradation intermediate, is a mitochondrial toxin João Leandro and Sander M. Houten

Saccharopine, a nonproteinogenic amino acid originally isolated from the yeast Saccharomyces cerevisiae, is an intermediate in lysine metabolism. In this issue, Zhou et al. (2019. J. Cell Biol. https://doi​ .org​ ./​ 10​ .1083/​ jcb​ .201807204​ ) show that abnormal accumulation of saccharopine results in defective mitochondrial dynamics and function in worm and mouse models.

Lysine is an essential amino acid in humans. increased plasma lysine concentration (3). SDH mutants. These data convincingly illus- It is primarily catabolized through the sacch- Seminal work by Dancis et al. has established trate that the accumulation of saccharopine aropine pathway in liver mitochondria, lead- that hyperlysinemia is most likely a benign is most likely responsible for the observed ing to the production of acetyl-CoA. The first metabolic variant without clinical signifi- mitochondrial abnormalities. enzyme of the lysine degradation pathway cance (4). In very few cases, hyperlysinemia Mitochondria are dynamic organelles, is α-aminoapidate semialdehyde synthase is accompanied by saccharopinuria (some- changing shape by fusion and fission pro- (AASS). AASS is of particular interest be- times referred to as hyperlysinemia type 2), cesses. They also establish contact with cause it is a bifunctional enzyme that essen- which indicates that there is also abnormal other organelles such as the ER. Impairment tially catalyzes the ε-deamination of lysine excretion of saccharopine in urine. While in of such processes has been associated with (Fig. 1). In the first step, lysine and α-keto- most hyperlysinemia cases the entire AASS several neurological disorders (7). Zhou et glutarate (α-KG) are converted into saccha- enzyme or the LKR domain is defective, al. (6) demonstrated that saccharopine ac- ropine by the lysine-ketoglutarate reductase these saccharopinuria cases are character- cumulation results in impairment of fission domain (LKR; EC 1.5.1.8). Saccharopine is ized by an isolated defect in the SDH domain and ER-dependent mitochondrial tubula- then oxidized to α-aminoapidate semialde- (Fig. 1). Although most experts in metabolic tion, while ongoing fusion is responsible for hyde and glutamate by the saccharopine de- diseases consider both forms hyperlysin- mitochondrial enlargement. Saccharopine hydrogenase domain (SDH; EC 1.5.1.9). emia biochemical phenotypes of question- accumulation also led to mitochondrial The lysine degradation pathway is also able clinical significance (5), an association damage and functional loss, and ultimately clinically relevant, as several defects in this between neurological disease and a pro- impaired growth in adult C. elegans. Using pathway can cause neurometabolic disor- nounced biochemical abnormality remains additional assays including a suppressor ders. For example, the step directly follow- a challenging clinical question, especially if screen in C. elegans, Zhou et al. further- ing AASS in the lysine degradation pathway there is a paucity in available data. more showed that inactivation of genes con- involves the conversion of α-aminoapidate In this issue, Zhou et al. establish a link trolling the levels of mitochondrial lysine semialdehyde into α-aminoapidate acid by between abnormal saccharopine accumu- (slc-25A29) and α-KG (slc-25A18.1, gdh-1, α-aminoapidate semialdehyde dehydroge- lation and mitochondrial dynamics and and idh-2), the precursors of saccharopine, nase (ALDH7A1/antiquitin). Mutations in function (6). By performing a mutagenesis ameliorate the mitochondrial phenotype of ALDH7A1 are a cause of pyridoxine-depen- screen in Caenorhabditis elegans expressing the SDH mutant. dent epilepsy (PDE). In this disease, accu- mitochondria-targeted GFP in the hypoder- To investigate if saccharopine accumu- mulation of α-aminoapidate semialdehyde, mis, they observed abnormal enlargement lation also induces mitochondrial dysfunc- the ALDH7A1 substrate, leads to depletion of of mitochondria in the presence of muta- tion in mammals, Zhou et al. created two pyridoxal phosphate, an essential coenzyme tions of the aass-1 gene, the orthologue of knock-in mouse models. The first model derived from vitamin B6 (1). Importantly, in- human AASS. Importantly, only mutations harbors a mutation in the LKR domain, lead- hibition of the upstream AASS enzyme has in the SDH domain with the consequent ing to complete AASS deficiency, equivalent been proposed as substrate reduction ther- increase in saccharopine and lysine levels to that observed in a previously reported apy for the treatment of PDE (2). Therefore induced mitochondrial enlargement. High hyperlysinemia case (8). The second model understanding of the consequences of AASS lysine levels due to mutations in the LKR do- has a mutation in the SDH domain that leads inhibition is of the utmost importance. main did not cause abnormal mitochondrial to impaired SDH activity with preserved Human genetics has offered a unique morphology. Furthermore, inactivation of LKR function. Whereas the LKR mutant had insight into the consequences of AASS in- aass-1 by RNAi did not lead to mitochondrial no detectable clinical phenotype, the SDH hibition. Mutations in AASS cause hyper- abnormalities, but instead was able to sup- mutant presented with liver disease, devel- lysinemia, a condition characterized by an press the mitochondrial enlargement in the opmental delay, and premature death. This

Department of Genetics and Genomic Sciences and Icahn Institute for Data Science and Genomic Technology, Icahn School of Medicine at Mount Sinai, New York, NY. Correspondence to Sander M. Houten: sander.houten@mssm​ .edu​ . © 2019 Leandro and Houten This article is distributed under the terms of an Attribution–Noncommercial–Share Alike–No Mirror Sites license for the first six months after the publication date (see http://​www​.rupress​.org/​terms/​). After six months it is available under a Creative Commons License (Attribution–Noncommercial–Share Alike 4.0 International license, as described at https://​creativecommons​.org/​licenses/​by​-nc​-sa/​4​.0/​).

Rockefeller University Press https://doi.org/10.1083/jcb.201901033 391 J. Cell Biol. 2019 Vol. 218 No. 2 391–392 Figure 1. Schematic representation of the con- sequences of different mutations in AASS on mitochondrial dynamics and function. (A) The wild type AASS consist of two domains. LKR cat- alyzes the NADPH-dependent​ formation of sac- charopine from lysine and α-KG. SDH catalyzes the NAD-dependent formation of α-aminoapidate semialdehyde from saccharopine. (B) Mutations that produce a total AASS KO or an isolated defect in LKR lead to hyperlysinemia, but without any apparent clinical or mitochondrial consequences. (C) Mutations that cause an isolated defect in SDH lead to hyperlysinemia with saccharopinuria that ultimately produces mitochondrial defects and liver disease in mice. ΔΨm, mitochondrial membrane potential; ALT, plasma alanine aminotransferase; AST, plasma aspartate aminotransferase.

phenotype coincided with mitochondrial de- saccharopinuria with neurological disease, Acknowledgments fects reminiscent of the mitochondrial phe- the study of the SDH mutant mice should The work on genetic disorders of the lysine notype of the C. elegans SDH mutants (6). not only focus on the liver, but also other or- degradation pathway in the Houten labo- How accumulation of saccharopine, gans such as the brain. ratory is supported by the National Insti- an adduct of lysine and α-KG, affects mi- In summary, Zhou et al. demonstrate that tutes of Health’s Eunice Kennedy Shriver tochondrial morphology, dynamics, and saccharopine is a mitochondrial toxin det- National Institute of Child Health and function remains to be elucidated. It might rimental to mitochondrial dynamics and Human Development under award numbers result from a direct effect of saccharopine function (6). This work establishes that R03HD092878 and R21HD088775. on proteins involved in the fusion/fission AASS mutations leading to an isolated defect The authors declare no competing finan- and tubulation machinery, or indirect via of SDH are a new cause of mitochondrial cial interests. inhibition of a protein or enzyme in another dysfunction that can result in a potentially pathway. Notably SDH deficiency could re- harmful condition at least in worms and 1. Mills, P.B., et al. 2006. Nat. Med. 12:307–309. sult in changes of the mitochondrial α-KG/ mice. Therefore hyperlysinemia with sac- 2. Pena, I.A., et al. 2017. Biochim Biophys Acta Mol Basis Dis. glutamate ratio and/or the NADP+/NAD​PH charopinuria in humans warrants careful 1863:121–128. 3. Sacksteder, K.A., et al. 2000. Am. J. Hum. Genet. ratio. However, limiting the mitochondrial observation and potentially a therapeutic 66:1736–1743. production of glutamate and α-KG was intervention. At the same time, this work 4. Dancis, J., et al. 1983. Am. J. Hum. Genet. 35:438–442. shown to rescue the saccharopine-induced confirms the notion that hyperlysinemia 5. Goodman, S.I., and M. Duran. 2014. Biochemical pheno- phenotype, arguing against a prominent due to complete AASS or isolated LKR defi- types of questionable clinical significance. In Physician’s guide to the diagnosis, treatment, and follow-up of role of these metabolites in the observed ciency is a biochemical abnormality without inherited metabolic diseases. Springer-Verlag Berlin phenotypes. Further mechanistic studies clinical consequences, which further paves Heidelberg. 691–705. 6. Zhou, J., et al. 2019. J. Cell Biol. https://doi​ .org​ ./​ 10​ .1083/​ ​ of the worm and mouse models generated the way for the development of LKR inhibi- jcb.201807204.​ by Zhou et al. will undoubtedly yield ad- tion as a potential strategy for the treatment 7. Bertholet, A.M., et al. 2016. Neurobiol. Dis. 90:3–19. ditional insights. Given the association of of PDE and saccharopinuria. 8. Houten, S.M., et al. 2013. Orphanet J. Rare Dis. 8:57.

Leandro and Houten Journal of Cell Biology 392 Saccharopine is a mitochondrial toxin https://doi.org/10.1083/jcb.201901033