Non-apoptotic regulated cell death in Fuchs endothelial corneal dystrophy

Introduction

Fuchs endothelial corneal dystrophy (FECD) is the leading cause of corneal blindness in developed countries. Corneal endothelial cells in FECD are susceptive to oxidative stress, leading to mitochondrial dysfunction and cell death. Oxidative stress causes many forms of cell death including parthanatos, which is characterized by translocation of apoptosis-inducing factor (AIF) to the nucleus with upregulation of poly (ADP-ribose) polymerase 1 (PARP-1) and poly (ADP-ribose) (PAR). Although cell death is an important aspect of FECD, previous reports have often analyzed immortalized cell lines, making the evaluation of cell death difficult. Therefore, we established a new in vitro FECD model to evaluate the pathophysiology of FECD.

Methods

Corneal endothelial cells were derived from disease-specific induced pluripotent stem cells (iPSCs). Hydrogen peroxide (H2O2) was used as a source for oxidative stress to mimic the pathophysiology of FECD. We investigated the responses to oxidative stress and the involvement of parthanatos in FECD-corneal endothelial cells.

Results

Cell death ratio and oxidative stress level were upregulated in FECD with H2O2 treatment compared with non-FECD control, indicating the vulnerability of oxidative stress in FECD. We also found that intracellular PAR, as well as PARP-1 and AIF in the nucleus were upregulated in FECD. Furthermore, PARP inhibition, but not pan-caspase inhibition, rescued cell death, DNA double-strand breaks, mitochondrial membrane potential depolarization and energy depletion, suggesting that cell death was mainly due to parthanatos.

Conclusions

We report that parthanatos may be involved in the pathophysiology of FECD and targeting this cell death pathway may be a potential therapeutic approach for FECD.

OUP accepted manuscript

Pluripotent stem cell (PSC)-based cell therapies have increased steadily over the past few years, and assessing the risk of tumor formation is a high priority for clinical studies. Current in vivo tumorigenesis studies require several months and depend strongly on the site of grafting. In this study, we report that the anterior eye chamber is preferable to the subcutaneous space for in vivo tumorigenesis studies for several reasons. First, cells can easily be transplanted into the anterior chamber and monitored in real-time without sacrificing the animals due to the transparency of the cornea. Second, tumor formation is faster than with the conventional subcutaneous method. The median tumor formation time in the subcutaneous area was 18.50 weeks (95% CI 10.20-26.29), vs. 4.0 weeks (95% CI 3.34-.67) in the anterior chamber (P = .0089). When hiPSCs were spiked with fibroblasts, the log₁₀TPD50 was 3.26, compared with 4.99 when hiPSCs were transplanted without fibroblasts. There was more than a 40-fold difference in the log₁₀TPD50 values with fibroblasts. Furthermore, the log₁₀TPD50 for HeLa cells was 1.45 and 100% of animals formed tumors at a concentration greater than 0.1%, indicating that the anterior chamber tumorigenesis assays can be applied for cancer cell lines as well. Thus, our method has the potential to become a powerful tool in all areas of tumorigenesis studies and cancer research.

Transplantation of iPSC-derived corneal endothelial substitutes in a monkey corneal edema model

OBJECTIVE

In order to provide regenerative therapy for millions of patients suffering from corneal blindness globally, we derived corneal endothelial cell substitute (CECSi) cells from induced pluripotent stem cells (iPSCs) to treat corneal edema due to endothelial dysfunction (bullous keratopathy).

METHODS AND RESULTS

We developed an efficient xeno-free protocol to produce CECSi cells from both research grade (Ff-MH09s01 and Ff-I01s04) and clinical grade (QHJI01s04) iPSCs. CECSi cells formed a hexagonal confluent monolayer with Na, K-ATPase alpha 1 subunit expression (ATP1A1), tight junctions, N-cadherin adherence junction formation, and nuclear PITX2 expression, which are all characteristics of corneal endothelial cells. CECSi cells can be cryopreserved, and thawed CECSi cell suspensions also expressed N-cadherin and ATP1A1. Residual undifferentiated iPSCs in QHJI01s04-derived CECSi cells was below 0.01%. Frozen stocks of Ff-I01s04- and QHJI01s04-derived CECSi cells were transported, thawed and transplanted into a monkey corneal edema model. CECSi-transplanted eyes significantly reduced corneal edema compared to control group.

CONCLUSION

Our results show a promising approach to provide bullous keratopathy patients with an iPS-cell-based cell therapy to recover useful vision.

Glucocerebrosidases catalyze a transgalactosylation reaction that yields a newly-identified brain sterol metabolite, galactosylated cholesterol

β-Glucocerebrosidase (GBA) hydrolyzes glucosylceramide (GlcCer) to generate ceramide. Previously, we demonstrated that lysosomal GBA1 and nonlysosomal GBA2 possess not only GlcCer hydrolase activity, but also transglucosylation activity to transfer the glucose residue from GlcCer to cholesterol to form β-cholesterylglucoside (β-GlcChol) in vitro β-GlcChol is a member of sterylglycosides present in diverse species. How GBA1 and GBA2 mediate β-GlcChol metabolism in the brain is unknown. Here, we purified and characterized sterylglycosides from rodent and fish brains. Although glucose is thought to be the sole carbohydrate component of sterylglycosides in vertebrates, structural analysis of rat brain sterylglycosides revealed the presence of galactosylated cholesterol (β-GalChol), in addition to β-GlcChol. Analyses of brain tissues from GBA2-deficient mice and GBA1- and/or GBA2-deficient Japanese rice fish (Oryzias latipes) revealed that GBA1 and GBA2 are responsible for β-GlcChol degradation and formation, respectively, and that both GBA1 and GBA2 are responsible for β-GalChol formation. Liquid chromatography-tandem MS revealed that β-GlcChol and β-GalChol are present throughout development from embryo to adult in the mouse brain. We found that β-GalChol expression depends on galactosylceramide (GalCer), and developmental onset of β-GalChol biosynthesis appeared to be during myelination. We also found that β-GlcChol and β-GalChol are secreted from neurons and glial cells in association with exosomes. In vitro enzyme assays confirmed that GBA1 and GBA2 have transgalactosylation activity to transfer the galactose residue from GalCer to cholesterol to form β-GalChol. This is the first report of the existence of β-GalChol in vertebrates and how β-GlcChol and β-GalChol are formed in the brain.

Enhanced vulnerability to oxidative stress and induction of inflammatory gene expression in 3-phosphoglycerate dehydrogenase-deficient fibroblasts

l‐Serine (l‐Ser) is a necessary precursor for the synthesis of proteins, lipids, glycine, cysteine, d‐serine, and tetrahydrofolate metabolites. Low l‐Ser availability activates stress responses and cell death; however, the underlying molecular mechanisms remain unclear. l‐Ser is synthesized de novo from 3‐phosphoglycerate with 3‐phosphoglycerate dehydrogenase (Phgdh) catalyzing the first reaction step. Here, we show that l‐Ser depletion raises intracellular H₂O₂ levels and enhances vulnerability to oxidative stress in Phgdh‐deficient mouse embryonic fibroblasts. These changes were associated with reduced total glutathione levels. Moreover, levels of the inflammatory markers thioredoxin‐interacting protein and prostaglandin‐endoperoxide synthase 2 were upregulated under l‐Ser‐depleted conditions; this was suppressed by the addition of N‐acetyl‐l‐cysteine. Thus, intracellular l‐Ser deficiency triggers an inflammatory response via increased oxidative stress, and de novo l‐Ser synthesis suppresses oxidative stress damage and inflammation when the external l‐Ser supply is restricted.

Aglycon diversity of brain sterylglucosides: structure determination of cholesteryl- and sitosterylglucoside

To date, sterylglucosides have been reported to be present in various fungi, plants, and animals. In bacteria, such as Helicobacter pylori, proton NMR spectral analysis of isolated 1-O-cholesteryl-β-d-glucopyranoside (GlcChol) demonstrated the presence of an α-glucosidic linkage. By contrast, in animals, no detailed structural analysis of GlcChol has been reported, in part because animal-derived samples contain a high abundance of glucosylceramides (GlcCers)/galactosylceramides, which exhibit highly similar chromatographic behavior to GlcChol. A key step in vertebrate GlcChol biosynthesis is the transglucosylation reaction catalyzed by glucocerebrosidase (GBA)1 or GBA2, utilizing GlcCer as a glucose donor. These steps are expected to produce a β-glucosidic linkage. Impaired GBA1 and GBA2 function is associated with neurological disorders, such as cerebellar ataxia, spastic paraplegia, and Parkinson's disease. Utilizing a novel three-step chromatographic procedure, we prepared highly enriched GlcChol from embryonic chicken brain, allowing complete structural confirmation of the β-glucosidic linkage by ¹H-NMR analysis. Unexpectedly, during purification, two additional sterylglucoside fractions were isolated. NMR and GC/MS analyses confirmed that the plant-type sitosterylglucoside in vertebrate brain is present throughout embryonic development. The aglycon structure of the remaining sterylglucoside (GSX-2) remains elusive due to its low abundance. Together, our results uncovered unexpected aglycon heterogeneity of sterylglucosides in vertebrate brain.

Microarray data on altered transcriptional program of Phgdh-deficient mouse embryonic fibroblasts caused by ʟ-serine depletion

Inherent ʟ-Ser deficiency culminates in intrauterine growth retardation, severe malformation of multiple organs particularly the central nervous system, and perinatal or early postnatal death in human and mouse. To uncover the molecular mechanisms underlying the growth-arrested phenotypes of l-Ser deficiency, we compared gene expression profiles of mouse embryonic fibroblasts deficient in 3-phosphoglycerate dehydrogenase (Phgdh), the first enzyme of de novo ʟ-Ser synthetic pathway, between ʟ-Ser-depleted and -supplemented conditions. The datasets (CEL and CHP files) from this study are publicly available on the Gene Expression Omnibus repository (accession number GEO: GSE55687).

Adaptive response to l-serine deficiency is mediated by p38 MAPK activation via 1-deoxysphinganine in normal fibroblasts

Reduced availability of l‐serine limits cell proliferation and leads to an adaptation to l‐serine‐deficient environment, the underlying molecular mechanism of which remain largely unexplored. Genetic ablation of 3‐phosphoglycerate dehydrogenase (Phgdh), which catalyzes the first step of de novo l‐serine synthesis, led to diminished cell proliferation and the activation of p38 MAPK and stress‐activated protein kinase/Jun amino‐terminal kinase in mouse embryonic fibroblasts under l‐serine depletion. The resultant l‐serine deficiency induced cyclin‐dependent kinase inhibitor 1a (Cdkn1a; p21) expression, which was mediated by p38 MAPK. Survival of the Phgdh‐deficient mouse embryonic fibroblasts was markedly reduced by p38 MAPK inhibition under l‐serine depletion, whereas p38 MAPK could be activated by 1‐deoxysphinganine, an atypical alanine‐derived sphingoid base that was found to accumulate in l‐serine‐depleted mouse embryonic fibroblasts. These observations provide persuasive evidence that when the external l‐serine supply is limited, l‐serine synthesized de novo in proliferating cells serves as a metabolic gatekeeper to maintain cell survival and the functions necessary for executing cell cycle progression.

Database

Gene Expression Omnibus, accession number GSE55687.

L-Serine Deficiency Elicits Intracellular Accumulation of Cytotoxic Deoxysphingolipids and Lipid Body Formation

L-serine is required to synthesize membrane lipids such as phosphatidylserine and sphingolipids. Nevertheless, it remains largely unknown how a diminished capacity to synthesize L-serine affects lipid homeostasis in cells and tissues. Here, we show that deprivation of external L-serine leads to the generation of 1-deoxysphingolipids (doxSLs), including 1-deoxysphinganine, in mouse embryonic fibroblasts (KO-MEFs) lacking D-3-phosphoglycerate dehydrogenase (Phgdh), which catalyzes the first step in the de novo synthesis of L-serine. A novel mass spectrometry-based lipidomic approach demonstrated that 1-deoxydihydroceramide was the most abundant species of doxSLs accumulated in L-serine-deprived KO-MEFs. Among normal sphingolipid species in KO-MEFs, levels of sphinganine, dihydroceramide, ceramide, and hexosylceramide were significantly reduced after deprivation of external L-serine, whereas those of sphingomyelin, sphingosine, and sphingosine 1-phosphate were retained. The synthesis of doxSLs was suppressed by supplementing the culture medium with L-serine but was potentiated by increasing the ratio of L-alanine to L-serine in the medium. Unlike with L-serine, depriving cells of external L-leucine did not promote the occurrence of doxSLs. Consistent with results obtained from KO-MEFs, brain-specific deletion of Phgdh in mice also resulted in accumulation of doxSLs in the brain. Furthermore, L-serine-deprived KO-MEFs exhibited increased formation of cytosolic lipid bodies containing doxSLs and other sphingolipids. These in vitro and in vivo studies indicate that doxSLs are generated in the presence of a high ratio of L-alanine to L-serine in cells and tissues lacking Phgdh, and de novo synthesis of L-serine is necessary to maintain normal sphingolipid homeostasis when the external supply of this amino acid is limited.

L-serine deficiency caused by genetic Phgdh deletion leads to robust induction of 4E-BP1 and subsequent repression of translation initiation in the developing central nervous system

Targeted disruption in mice of the gene encoding D-3-phosphoglycerate dehydrogenase (Phgdh) results in embryonic lethality associated with a striking reduction in free L-serine and growth retardation including severe brain malformation. We previously observed a severe impairment in neurogenesis of the central nervous system of Phgdh knockout (KO) embryos and a reduction in the protein content of their brains. Although these findings suggest that L-serine deficiency links attenuation of mRNA translation to severe developmental malformation of the central nervous system, the underlying key molecular event remains unexplored. Here we demonstrate that mRNA of Eif4ebp1 encoding eukaryotic initiation factor 4 binding protein 1 and its protein, 4E-BP1, are markedly induced in the central nervous system of Phgdh KO embryos, whereas a modest induction is observed in the liver. The increase in 4E-BP1 was associated with a decrease in the cap initiation complex in the brain, as shown by lower levels of eukaryotic translation initiation factor 4G bound to eukaryotic translation initiation factor 4E (eIF4E) and increased eIF4E interaction with 4E-BP1 based on 7-methyl-GTP chromatography. eIF4E protein and polysomes were also diminished in Phgdh KO embryos. Induction of Eif4ebp1 mRNA and of 4E-BP1 was reproduced in mouse embryonic fibroblasts established from Phgdh KO embryos under the condition of L-serine deprivation. Induction of Eif4ebp1 mRNA was suppressed only when L-serine was supplemented in the culture medium, indicating that reduced L-serine availability regulates the induction of Eif4ebp1/4E-BP1. These data suggest that elevated levels of 4E-BP1 may be involved in a mechanism to arrest brain development in Phgdh KO embryos.

Brain-specific Phgdh deletion reveals a pivotal role for L-serine biosynthesis in controlling the level of D-serine, an N-methyl-D-aspartate receptor co-agonist, in adult brain

In mammalian brain, D-serine is synthesized from L-serine by serine racemase, and it functions as an obligatory co-agonist at the glycine modulatory site of N-methyl-D-aspartate (NMDA)-selective glutamate receptors. Although diminution in D-serine level has been implicated in NMDA receptor hypofunction, which is thought to occur in schizophrenia, the source of the precursor L-serine and its role in D-serine metabolism in adult brain have yet to be determined. We investigated whether L-serine synthesized in brain via the phosphorylated pathway is essential for D-serine synthesis by generating mice with a conditional deletion of D-3-phosphoglycerate dehydrogenase (Phgdh; EC 1.1.1.95). This enzyme catalyzes the first step in L-serine synthesis via the phosphorylated pathway. HPLC analysis of serine enantiomers demonstrated that both L- and D-serine levels were markedly decreased in the cerebral cortex and hippocampus of conditional knock-out mice, whereas the serine deficiency did not alter protein expression levels of serine racemase and NMDA receptor subunits in these regions. The present study provides definitive proof that L-serine-synthesized endogenously via the phosphorylated pathway is a key rate-limiting factor for maintaining steady-state levels of D-serine in adult brain. Furthermore, NMDA-evoked transcription of Arc, an immediate early gene, was diminished in the hippocampus of conditional knock-out mice. Thus, this study demonstrates that in mature neuronal circuits L-serine availability determines the rate of D-serine synthesis in the forebrain and controls NMDA receptor function at least in the hippocampus.

Characterization of the mitochondrial protein LETM1, which maintains the mitochondrial tubular shapes and interacts with the AAA-ATPase BCS1L

LETM1 is located in the chromosomal region that is deleted in patients suffering Wolf-Hirschhorn syndrome; it encodes a homolog of the yeast protein Mdm38 that is involved in mitochondrial morphology. Here, we describe the LETM1-mediated regulation of the mitochondrial volume and its interaction with the mitochondrial AAA-ATPase BCS1L that is responsible for three different human disorders. LETM1 is a mitochondrial inner-membrane protein with a large domain extruding to the matrix. The LETM1 homolog LETM2 is a mitochondrial protein that is expressed preferentially in testis and sperm. LETM1 downregulation caused mitochondrial swelling and cristae disorganization, but seemed to have little effect on membrane fusion and fission. Formation of the respiratory-chain complex was impaired by LETM1 knockdown. Cells lacking mitochondrial DNA lost active respiratory chains but maintained mitochondrial tubular networks, indicating that mitochondrial swelling caused by LETM1 knockdown is not caused by the disassembly of the respiratory chains. LETM1 was co-precipitated with BCS1L and formation of the LETM1 complex depended on BCS1L levels, suggesting that BCS1L stimulates the assembly of the LETM1 complex. BCS1L knockdown caused disassembly of the respiratory chains as well as LETM1 downregulation and induced distinct changes in mitochondrial morphology.

Identification of a novel protein MICS1 that is involved in maintenance of mitochondrial morphology and apoptotic release of cytochrome c

Mitochondrial morphology dynamically changes in a balance of membrane fusion and fission in response to the environment, cell cycle, and apoptotic stimuli. Here, we report that a novel mitochondrial protein, MICS1, is involved in mitochondrial morphology in specific cristae structures and the apoptotic release of cytochrome c from the mitochondria. MICS1 is an inner membrane protein with a cleavable presequence and multiple transmembrane segments and belongs to the Bi-1 super family. MICS1 down-regulation causes mitochondrial fragmentation and cristae disorganization and stimulates the release of proapoptotic proteins. Expression of the anti-apoptotic protein Bcl-XL does not prevent morphological changes of mitochondria caused by MICS1 down-regulation, indicating that MICS1 plays a role in maintaining mitochondrial morphology separately from the function in apoptotic pathways. MICS1 overproduction induces mitochondrial aggregation and partially inhibits cytochrome c release during apoptosis, regardless of the occurrence of Bax targeting. MICS1 is cross-linked to cytochrome c without disrupting membrane integrity. Thus, MICS1 facilitates the tight association of cytochrome c with the inner membrane. Furthermore, under low-serum condition, the delay in apoptotic release of cytochrome c correlates with MICS1 up-regulation without significant changes in mitochondrial morphology, suggesting that MICS1 individually functions in mitochondrial morphology and cytochrome c release.

Inactivation of the 3-phosphoglycerate dehydrogenase gene in mice: changes in gene expression and associated regulatory networks resulting from serine deficiency

D-3-Phosphoglycerate dehydrogenase (Phgdh) is a necessary enzyme for de novo L-serine biosynthesis. Mutations in the human PHGDH cause serine deficiency disorders characterized by severe neurological symptoms including congenital microcephaly and psychomotor retardation. We showed previously that targeted disruption of Phgdh in mice causes overall growth retardation with severe brain microcephaly and leads to embryonic lethality. Here, amino acid analysis of Phgdh knockout (KO) mouse embryos demonstrates that free serine and glycine concentrations are decreased markedly in head samples, reflecting the metabolic changes of serine deficiency found in human patients. To understand the pathogenesis of serine deficiency disorders at the molecular level, we have exploited this animal model to identify altered gene expression patterns using a microarray technology. Comparative microarray analysis of the Phgdh KO and wild-type head at gestational day 13.5 revealed an upregulation of genes involved in transfer RNA aminoacylation, amino acid metabolism, amino acid transport, transcriptional regulation, and translation, and a downregulation of genes involved in transcription in neuronal progenitors and muscle and cartilage development. A computational network analysis software was used to construct transcriptional regulatory networks operative in the Phgdh KO embryos in vivo. These observations suggest that Phgdh inactivation alters transcriptional programs in several regulatory networks.

An RNAi screen for mitochondrial proteins required to maintain the morphology of the organelle in Caenorhabditis elegans

Mitochondria are dynamic organelles that frequently divide and fuse together, resulting in the formation of intracellular tubular networks. In yeast and mammals, several factors including Drp1/Dnm1 and Mfn/Fzo1 are known to regulate mitochondrial morphology by controlling membrane fission or fusion. Here, we report the systematic screening of Caenorhabditis elegans mitochondrial proteins required to maintain the morphology of the organelle using an RNA interference feeding library. In C. elegans body wall muscle cells, mitochondria usually formed tubular structures and were severely fragmented by the mutation in fzo-1 gene, indicating that the body wall muscle cells are suitable for monitoring changes in mitochondrial morphology due to gene silencing. Of 719 genes predicted to code for most of mitochondrial proteins, knockdown of >80% of them caused abnormal mitochondrial morphology, including fragmentation and elongation. These findings indicate that most fundamental mitochondrial functions, including metabolism and oxidative phosphorylation, are necessary for maintenance of the tubular networks as well as membrane fission and fusion. This is the first evidence that known mitochondrial activities are prerequisite for regulating the morphology of the organelle. Furthermore, 88 uncharacterized or poorly characterized genes were found in the screening to be implicated in mitochondrial morphology.